synthesis of acetylenes, allenes and cumulenes || acetylenes, allenes and cumulenes by elimination...

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E:/Archive files/4188-Brandsma/Printer-Files/4188-Chapter-10.3d

10Acetylenes, Allenes and Cumulenes by

Elimination Reactions

10.1 SURVEY OF METHODS

Part of Chapter 3 deals with the synthesis of acetylenic and some cumulenic

derivatives by 1,1-, 1,2-, 1,4- and 1,6-elimination of hydrogen halide, alcohol or

thiol from suitable substrates under the influence of strongly basic alkali

amides or alkyllithium. The product in the reaction mixture is a metallated

acetylene or cumulene, from which the free acetylene or cumulene can be

obtained by addition of a suitable proton donor.

The methods in the present chapter, all concerned with elimination reactions,

afford the free acetylene, allene or cumulene directly. The following subdivision

is made.

10.1.1 1,1-Elimination of hydrogen halide with simultaneousmigration of an organic group

The formation of diphenylacetylene [1] by treatment of Ph2C¼CHCl with

strong bases has little significance as a preparative method for disubstituted

acetylenes. The 1,1-elimination of hydrogen chloride from 2-chloro-

N,N,N1,N1-tetraalkyl-1,1-ethylenediamines (chloroketene aminals) under the

influence of strong bases gives access to the interesting bis(N,N-dialkylami-

no)acetylenes. Like in the formation of diarylacetylenes, the initial carbenoid

loses halide with concomitant migration of a dialkylamino group [2].

203

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10.1.2 Double 1,2-elimination of hydrogen halide from geminalor vicinal dihalogen compounds: RC(Cl2)Me; RCH2CHCl2;RCH(Br)CH2Br or Cl; RCH(Br)CH(Br)R or Cl

Geminal and vicinal dihalogen compounds can be converted into acetylenes

by treatment with potassium hydroxide in the presence of a phase-transfer

catalyst [3]. Since potassium t-butoxide in DMSO causes rapid isomerisation

of 1-alkynes to 2-alkynes under the required conditions, this reagent–solvent

system is unsuitable for the preparation of 1-alkynes, HC�CCH2R (cf. [4]).

10.1.3 Elimination of hydrogen halide from halo-olefinic compounds

Ethoxyacetylene is readily obtained by warming the (E)/(Z)-mixture of

1-bromo-2-ethoxyethene, BrCH¼CHOEt, with powdered potassium hydro-

xide. Only the (Z)-isomer, which is usually formed as a major product from

1,2-dibromo-1-ethoxyethane, BrCH2CH(Br)OEt, and N,N-diethylaniline,

reacts [5]. Another example is the elimination of hydrogen iodide by heating

1-alkyl-1-iodo-2-nitroethenes, RC(I)¼CHNO2, with KOH [6]. For the pre-

paration of cyclooctyne from 1-bromo-1-cyclooctene, lithium diisopropyla-

mide (LDA) appears to be the base of choice [7].

10.1.4 Tele-eliminations of hydrogen chloride from 1,4-dichlorobutene, 1,4-dichlorobutyne and 1,6-dichlorohexadiyne

Vinylacetylene (1-buten-3-yne) is formed by heating (E)-1,2-dichloro-2-butene

with aqueous KOH in the presence of a phase-transfer catalyst [8]. The

formation of butadiyne [9] from 1,4-dichloro-2-butyne and KOH in a

water–DMSO mixture probably proceeds through chlorobutatriene,

ClCH¼C¼C¼CH2. This extremely unstable compound can be isolated by

204 10. ELIMINATION REACTIONS

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heating dichlorobutyne with powdered KOH in a high vacuum and trapping in

a strongly cooled receiver [10]. Hexatriyne can be prepared in a surprisingly

high yield by treating 1,6-dichloro-2,4-hexadiyne with potassium t-butoxide in

THF at very low temperatures [10]. This highly unstable compound has been

obtained as a dilute solution in an organic solvent in moderate yields by the

reaction of the dichloride with sodamide in liquid ammonia [11].

10.1.5 Tele-elimination of alcohol or thiol from acetylenic,allenic or cumulenic derivatives

Potassium t-butoxide dissolved in THF eliminates alcohol from N,N-dialkyl-4-

alkoxy-2-butyn-1-amines, R2NCH2C�CCH2OR, with formation of N,N-

dialkyl-3-buten-1-yn-1-amines [12]. Two possible pathways are presented.

4-Alkylthio-1-buten-3-ynes, RSC�CCH¼CH2, are obtained in an analo-

gous way from the reaction between 1,4-bis(alkylthio)-2-butynes,

RSCH2C�CCH2SR, and t-BuOK in liquid ammonia [13]. The oxygen analo-

gues 4-alkoxy-1-buten-3-ynes, ROC�CCH¼CH2, result from treatment of

1,4-dialkoxy-1,2-butadienes, ROCH¼C¼CHCH2OR, with alkyllithium [10].

Butatrienyl ethers, R2C¼C¼C¼CHOMe, are formed from 1,1-substituted

1-alkoxy-4-methoxy-2-butynes and sodamide in liquid ammonia [14].

The formation of 1-ethoxy-1,5-hexadien-3-yne, H2C¼CHC�CCH¼CHOEt,

from MeCH¼CHC�CCH(OEt)2 and the 1:1 complex t-BuOK–t-BuOH

in DMSO may be considered as a result of a 1,6-elimination of ethanol and

subsequent base-catalysed isomerisation of the intermediary cumulenic ether

1-ethoxy-1,2,3,5-hexatetraene, H2C¼CHCH¼C¼C¼CHOEt [10].

10.1 SURVEY OF METHODS 205

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10.1.6 1,2- and 1,4-Dehalogenation using zinc powder

Allene, H2C¼C¼CH2, is obtained in a high yield by treatment of 2,3-dichloro-

1-propene, H2C¼C(Cl)CH2Cl, with zinc powder in ethanol–water mixtures

[15]. Di- and trifluoroallenes have also been prepared by this dehalogenation

method.

Butatriene and some volatile homologues are trapped in strongly cooled recei-

vers by heating 1,4-dichloro-2-butyne, 1,4-dichloro-2-pentyne or 4-chloro-

1-alkyl-2-butynyl methanesulphinates, ClCH2C�CCH(R)OS(¼O)Me, with

zinc powder in vacuum [16,17].

10.2 EXPERIMENTAL SECTION

Notes

1. For the preparation of alkali amides in liquid ammonia and alkyllithium

reagents see Chapter 2.

2. Reactions in organic solvents or in liquid ammonia at temperatures below its

boiling point are generally carried out under inert gas.

10.2.1 3,3-Dimethyl-1-butyne starting from 3,3-dimethyl-1-butene

Scale: 0.20 molar; Apparatus: 1-litre two-necked, round-bottomed flask con-

nected to a 40-cm Vigreux column, condenser and receiver; on the other neck

of the flask is placed a dropping funnel

10.2.1.1 Procedure

1,2-Dibromo-3,3-dimethylbutane (0.20 mol, obtained by addition of bromine

to a mixture of 0.22 mol of 3,3-dimethyl-1-butene (commercially available) and

75 ml of Et2O at <–40 �C, followed by thorough removal of Et2O under

reduced pressure) is added over 5–10 min to a solution of 0.50 mol (large

excess) of t-BuOK in 140 ml of DMSO. During this addition the flask is

gently swirled to effect homogenisation. A vigorous reaction takes place and


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part of the t-butylacetylene is collected in the single receiver cooled below

–10 �C. The flask is heated for 30 min in a bath at 100 �C. After cooling to

30 �C, the dropping funnel is replaced with a stopper and the apparatus is

evacuated (Figure 1.10). Small amounts of dissolved t-butylacetylene condense

in the receiver, which is cooled at –70 �C. Redistillation of the contents of the

receiver gives pure t-butylacetylene, bp 38–40 �C, in >70% yield.

10.2.2 Phenylacetylene starting from styrene

Scale: 0.50 molar; Apparatus: 1-litre three-necked, round-bottomed flask,

equipped with a dropping funnel, a mechanical stirrer and a thermometer-

outlet combination, which for the dehydrohalogenation is replaced with a

reflux condenser.

10.2.2.1 Procedure

Freshly distilled styrene (0.50 mol) and dichloromethane (250 ml) are placed in

the flask. Bromine (� 0.52 mol, just sufficient to give a persisting brown solu-

tion at the end) is added dropwise, while keeping the temperature below

–20 �C. The equipment is then removed and stoppers are placed on the two

outer necks. The solvent is thoroughly removed on a rotary evaporator. The

flask is then equipped as indicated above and a solution of 1.5 mol (excess) of

sodium ethoxide in 550 ml of absolute ethanol (Note) is added over 15 min to

the solid. The temperature rises to 60 �C or higher. When the evolution of heat

has ceased, the mixture is heated under reflux for an additional period of 3 h.

The resulting suspension is then cooled to rt and poured into 2.5 litre of water.

Six to eight extractions with small portions of pentane (if Et2O is used,

relatively large amounts are needed in the first extractions) are carried out.

The combined organic solutions are washed three times with cold dilute

hydrochloric acid and subsequently dried over MgSO4. The greater part of

the solvent is distilled off at atmospheric pressure. Distillation of the remaining

liquid gives phenylacetylene, bp 40 �C/15 Torr, in �70% yield. The residue

consists mainly of 1-(1-bromovinyl)benzene, PhC(Br)¼CH2.

Diphenylacetylene, PhC�CPh, can be prepared in an excellent yield from

1-(1,2-dibromo-2-phenylethyl)benzene, PhCHBrCHBrPh, and ethanolic potas-

sium hydroxide.

10.2 EXPERIMENTAL SECTION 207

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Heteroarylacetylenes, e.g. 2-thienyl–C�CH, can be prepared from the

olefins by a procedure similar to the one described above.

Note

If 96% ethanol is used, the second elimination of HBr proceeds sluggishly and

much PhC(Br)¼CH2 is isolated.

10.2.3 1-Butyne from 1,2-dibromobutane by thephase-transfer method

Scale: 0.50 molar; Apparatus: 2-litre round-bottomed, three-necked flask,

equipped with a dropping funnel, a mechanical stirrer and a reflux condenser

connected to a cold trap (–78 �C); all connections are made gas-tight; a stirrer

of the type displayed in Figure 1.2, is used

A simple method for the preparation of simple 1-alkynes consists of heating a

1,2-dibromo compound with an excess of powdered potassium hydroxide at

80 �C or higher temperatures in the presence of catalytic amounts of tetraoctyl-

ammonium halide and pinacol [3]. The alkoxide of pinacol formed with KOH is

extracted by the phase-transfer catalyst (p.t.c.) into the organic phase where the

double dehydrohalogenation takes place. The elimination can also be carried out

with aqueousKOH and the phase-transfer catalyst, but the rate of elimination is

then much lower. In the procedures with powdered KOH, an inert solvent is

used, e.g. petroleum ether. This has a double function in that it serves as a

conductor of heat supplied by the bath, and it ensures sufficient mixing of the

halogen compound with the KOH. For less volatile acetylenes, e.g. ethynyl-

benzene, PhC�CH (bp � 140 �C at atmospheric pressure), a lower-boiling

petroleum ether fraction (bp <100 �C) should be used, while in the case of

volatile acetylenes the halogen compound is diluted with higher boiling

solvents. The choice of the solvent depends also on the temperature required

for a smooth dehydrohalogenation. For example, 1,1-dichloro-3,3-dimethyl-

butane, t-BuCH2CHCl2, and 2,2-dichloro-3,3-dimethylbutane, t-BuCCl2Me,

are converted only partially at temperatures below 120 �C, making a high

boiling (bp >150 �C) solvent necessary. For the preparation of ethynyl-

cyclohexane from 1-(2,2-dichloroethyl)cyclohexane, c-C6H11CH2CHCl2, the


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use of paraffin oil might be considered. The solid KOH-phase-transfer method

cannot be applied to synthesise thermally less stable acetylenes, e.g. ethoxyace-

tylene, EtOC�CH. Moreover, the starting compound 1,2-dibromo-1-ethox-

yethane, EtOCHBrCH2Br, is likely to undergo other reactions. Neither can

acetylenes that easily undergo base-catalysed isomerisation reactions be pre-

pared by this method.

10.2.3.1 Procedure

Freshly and finely machine-powdered potassium hydroxide (85% technical

grade, 6 mol) and high-boiling petroleum ether (bp >170 �C, 250 ml) are

placed in the flask. Methyl trioctylammonium chloride (75% aqueous solution,

7 g) and anhydrous pinacol (7 g) are added with vigorous stirring. The suspen-

sion is heated to � 90 �C (temperature of the oil bath) and 1,2-dibromobutane

(0.50 mol, obtained by addition of bromine at <–40 �C to a mixture of

1-butene and Et2O followed by thorough removal of this solvent under reduced

pressure) is added dropwise over 20 min. A temporary vigorous reflux (pre-

sumably a mixture of 2-bromo-1-butene, EtC(Br)¼CH2, and 1-bromo-1-

butene, EtCH¼CHBr, is formed) is observed. The temperature of the bath is

gradually raised over 30 min to 120 �C. During this period the intensity of

reflux decreases while 1-butyne begins to condense in the cold trap. When

the reflux has practically stopped, the temperature of the bath is raised to

140–145 �C and kept at this level for an additional 1 h. The dropping funnel

is then replaced with a gas inlet tube, reaching to the middle of the flask.

Nitrogen (200 ml/min) is then passed through the flask for 10 min. Finally,

the trap containing the 1-butyne is connected to another trap cooled at –78 �C

and then placed in a water bath which is gradually warmed (initial temperature

15 �C) to ca. 50 �C. Pure 1-butyne is obtained in >70% yield.

10.2.4 3,3-Dimethyl-1-butyne from 1,1-dichloro-3,3-dimethylbutaneby the phase-transfer method


equipped with a dropping funnel, a mechanical stirrer (Figure 1.2) and a

40-cm Vigreux column, connected to a condenser and receiver, cooled in an

ice-water bath.


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10.2.4.1 Procedure (for Introduction see exp. 10.2.3)

Freshly machine-powdered KOH (85%, 9 mol) and high-boiling petroleum

ether (200 ml, bp >170 �C/760 Torr) are placed in the flask. Stirring is started

and the suspension is heated to 130 �C (oil bath). Methyl trioctylammonium

chloride or tetraoctylammonium chloride (70% aqueous solution, 10 g) and

anhydrous pinacol (10 g) are added, followed by 0.50 mol of 1,1-dichloro-

3,3-dimethylbutane (cf. Chapter 3, exp. 3.9.25, the same procedure as described

for other geminal dichlorides is followed). t-Butylacetylene begins to distil out

after 15–30 min. The temperature of the bath is gradually raised over 1 h to

155 �C and maintained at 160 �C for an additional 1 h. When distillation has

stopped, the suspension (part of the KOH may form a liquid layer on

the bottom of the flask) is allowed to cool. The distillate is redistilled through

a 40-cm Vigreux column to give 3,3-dimethyl-1-butyne, bp 38–40 �C, in �60%

yield.

An additional 5–10% yield of the acetylene may be obtained by evacuating

the apparatus (after cooling to below 40 �C) and collecting the volatile product

in a receiver cooled at –78 �C (Figure 1.10).

Isopropylacetylene (bp � 28 �C) can be prepared by a similar procedure.

10.2.5 Ethoxyacetylene from 1-bromo-2-ethoxyethene andpotassium hydroxide

Scale: 0.50 molar; Apparatus: 1-litre round-bottomed flask connected to a

distillation system consisting of a 30-cm Vigreux column, condenser and recei-

ver, cooled at –78 �C (Figure 1.10).

10.2.5.1 Procedure

Freshly machine-powdered potassium hydroxide (85% technical grade, 3.0

mol) is placed in the flask. 1-Bromo-2-ethoxyethene (0.50 mol, rich in the

(Z)-isomer, Chapter 3, exp. 3.9.23) is added in one portion and the mixture

is immediately shaken or swirled (by hand) to form a homogeneous slurry and

subsequently connected to the column of the distillation apparatus. The flask is

heated in an oil bath at � 120 �C. A very exothermic reaction starts after a

short time and the greater part of the ethoxyacetylene passes over.

Fifteen minutes after this distillation has stopped, the system is evacuated

(water-aspirator pressure) without external heating. The remainder of the


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ethoxyacetylene and some (E)-1-bromo-2-ethoxyethene condense in the

strongly cooled receiver. After warming the receiver to rt, sufficient magnesium

sulphate is added in small portions, with shaking, to just form a coagulate with

the moisture. The almost clear liquid is decanted from the drying agent and

subsequently distilled at normal pressure through a 30-cm Vigreux column.

Ethoxyacetylene, bp 52 �C/760 Torr, is obtained in excellent yields (calculated

on the amount of (Z)-isomer present in the mixture of isomers).

10.2.6 Vinylacetylene from (E)-1,4-dichloro-2-butene bythe phase-transfer method

Scale: 0.50 molar (for Apparatus see exp. 10.2.3)

10.2.6.1 Procedure

Freshly machine-powdered KOH (85%, 300 g) and high-boiling petroleum

ether (200 ml, bp >150 �C) are placed in the flask. Stirring is started and 5 g

of methyl trioctylammonium chloride (75% aqueous solution; tetraoctylam-

monium chloride also may be used) and 5 g of pinacol are added (with tem-

porary removal of the dropping funnel). The mixture is heated for 30 min in an

oil bath at 120 �C, then 1,4-dichloro-2-butene (E-isomer, 0.50 mol) is added

dropwise over 25 min. Nitrogen (� 100 ml/min) is passed through the appara-

tus. The vinylacetylene condenses in two traps cooled at –78 �C. After the

addition the temperature of the bath is gradually raised over 30 min to

135 �C. Stirring and introduction of N2 (� 100 ml/min) are continued for

another 1 h. The traps are successively connected to an empty one (cooled at

–78 �C) and then placed in a water bath at rt. The temperature of the bath is

gradually raised to 50 �C. A small amount of 1-chloro-1,3-butadiene may

remain in one of the traps. The yield of pure (>95%) vinylacetylene is usually

greater than 70%.

Vinylacetylene can also be prepared in high yields by slow addition (over

45 min) of (E)-1,4-dichloro-2-butene (0.45 mol), to a vigorously stirred mixture

of 3 g MeNþOct3Cl–, 250 g of KOH and 250 ml of water, heated at 100 �C

(bath temperature). N2 is slowly (100 ml/min) introduced both during and

for 30 min after the addition of dichlorobutene. The contents of the cold

traps are then ‘redistilled’ as described above.


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10.2.7 Butadiyne from 1,4-dichloro-2-butyne and KOHin a water–DMSO mixture


equipped with a combination of dropping funnel and gas inlet tube, an efficient

gas-tightmechanical stirrer (Figure 1.2) and a combination of a thermometer and

an efficient reflux condenser. The top of the condenser is via two tubes (20 cm

long) filled with lumps of CaCl2 connected to two traps cooled at –78 �C. Both

traps contain 50 g of dry THF (or any other solvent, e.g. MeOH, Et2O; traps þ

solvent are weighed). The inlet tube in the traps dips 0.5 cm below the surface of

the THF (Note 1). All connections are made gas-tight.

1,4-Dichloro-2-butyne reacts sluggishly with concentrated aqueous KOH at

70 �C, because it is slightly soluble in the aqueous phase. If a small amount of

the phase-transfer catalyst MeNOct3Cl (Aliquat) is present, however, the

double elimination of hydrogen chloride proceeds smoothly at that tempera-

ture. Addition of a sufficient amount of DMSO instead of Aliquat causes an

increase of the solubility of dichlorobutyne and the effect is similar to that

obtained with Aliquat. It seems useful to explain some other experimental con-

ditions. The slow introduction of nitrogen into the apparatus serves to trans-

port butadiyne to the cold traps. A second function of nitrogen is to dilute the

gaseous diyne (the estimated bp at 760 Torr is between 10 and 20 �C), and thus

to diminish the danger of (explosive) decomposition. It seems essential to pass

the nitrogen through the aqueous reaction mixture. In this manner, butadiyne is

helped to escape from the aqueous phase. The first elimination product is the

extremely unstable chlorobutatriene, ClCH¼C¼C¼CH2. A too quick addition

of dichlorobutyne results in a too high concentration of this cumulene and con-

sequently in the formation of polymer. If the flow of N2 is too rapid, the vola-

tile cumulene is swept out of the solution and forms a layer of brown polymer

in the upper part of the flask and in the condenser. The results obtained with

the DMSO–water mixture (yields up to 95%) are markedly better than that

obtained with phase-transfer catalysis. In the latter case much amorphous

black or brown material is formed, while diacetylene is obtained in yields up

to �65% [10].

10.2.7.1 Procedure [9]

In the flask is dissolved 130 g of 85% KOH in 200 ml of water. DMSO (40 ml)

is added. N2 is introduced at a rate of � 200 ml/min. The solution is heated up


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to 72 �C (internal) and 0.50 mol of 1,4-dichloro-2-butyne (Chapter 20,

exp. 20.1.6) is added dropwise over 30 min, while maintaining the temperature

between 70 and 75 �C. Butadiyne condenses in the THF. After completion of

the addition, the brown reaction mixture is brought to 95 �C and held at this

temperature for an additional 15 min. The total weight increase of the traps

corresponds to yields up to �90% (Notes 2 and 3).

Notes

1. If the tube is ending above the level of the THF, some diacetylene may

condense as white leaves in the upper, cooled part of the first trap.

2. Lower yields are obtained when dichlorobutyne is added too quickly.

3. The solution can be stored for at least 3 days without deterioration at

–20 �C in a well-sealed bottle.

10.2.8 Hexatriyne from 1,6-dichloro-2,4-hexadiyne and t-BuOK intetrahydrofuran

Scale: 0.10 molar; Apparatus: Figure 1.1, 1 litre; stirrer: Figure 1.2

Hexatriyne has been obtained in a moderate yield from the reaction of 2,6-

dichloro-2,4-hexadiyne with alkali amide in liquid ammonia [11]. The com-

pound is extremely unstable and the crystalline substance, isolated from the

organic solution, readily explodes [10]. Generation under conditions similar

to those applied for the preparation of butadiyne (exp. 10.2.7) is likely to

give only decomposition products. In the procedure described below hexa-

triyne is obtained in surprisingly high yields. In analogy with the elimination

of hydrogen chloride from 1,4-dichloro-2-butyne it may be assumed that the

first elimination gives chlorohexapentaene, ClCH¼C¼C¼C¼C¼CH2. This

compound probably does not accumulate but undergoes a rapid tele-elimina-

tion of HCl to give hexatriyne. Since the elimination reaction is strongly

exothermic, it is easy to follow the progress of the reaction. The t-butylalco-

hol formed in the elimination forms the much less reactive 1:1 complex with

t-BuOK, therefore a large excess of base is used. Hexatriyne is expected to be

much more ‘acidic’ than acetylene and part of the compound possibly

remains in solution (as a ‘complex’ with KOH) when, after addition of

water to the reaction mixture, the base is not neutralised. As extraction


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solvent high-boiling petroleum ether is used. THF and t-butylalcohol are

removed by repeated washing of the solution with cold dilute hydrochloric

acid. Hexatriyne is subsequently isolated by evacuation while gradually rais-

ing the temperature of the petroleum ether solution. Although it is not diffi-

cult to collect the compound as a solid in the strongly cooled receiver, it

seems safer to put a (weighed) amount of an inert organic solvent with a

comparable volatility (e.g. heptane) in the receiver prior to carrying out the

evacuation.

10.2.8.1 Procedure

A mixture of 0.10 mol of 1,6-dichloro-2,4-hexadiyne (Chapter 20, exp. 20.1.7)

and 150 ml of THF is cooled to –90 �C. A solution of 0.40 mol of t-BuOK in

150 ml of THF is added dropwise over 25 min with vigorous stirring and

occasional cooling in a bath with liquid N2 to maintain this low temperature.

Care should be taken that the THF does not solidify on the bottom of the

flask. Should it do so, the addition should be interrupted until the THF has

melted. After completion of the addition the mixture is stirred for an addi-

tional 30 min at –65 to –70 �C. High-boiling petroleum ether (bp >190 �C, 150

ml) is then added to the dark reaction mixture, followed by 200 ml of 2 N

hydrochloric acid. The layers are separated as soon as possible (Note 1) and

the aqueous layer extracted twice with 30-ml portions of petroleum ether. The

combined organic solutions are washed ten to fifteen times with 150-ml por-

tions of cold (–10 �C) 2 N HCl in order to remove the THF and t-BuOH

thoroughly. After drying for a few minutes over MgSO4, the brown solution

is transferred into a 1-litre round-bottomed flask, which is equipped for a

vacuum distillation (Figure 1.10). The receiver (250-ml round-bottomed

flask) is filled with 50 g of cold (<–50 �C) heptane (Note 2), after which this

system is evacuated by means of the water aspirator. The extract is gradually

warmed until the solvent (bp � 70 �C/15 Torr) begins to reflux in the top of the

column (40-cm Vigreux). During this operation a small amount of petroleum

ether co-distils. The evacuation operation is therefore repeated with the con-

tents of the receiver. In the beginning no heating is applied, at a later stage the

flask is placed in a bath at 10–15 �C. A mixture of hexatriyne and heptane is

collected in the pre-weighed receiver (cooled at –78 �C). This contains up to

�56 g of heptane solution, corresponding to a yield of � 85% of hexatriyne.

Notes

1. All operations should be carried out under N2, without delay.

2. If no heptane or other solvent is used, the triyne is collected as a white solid

in the receiver, turning violet under the influence of (traces of) oxygen. In


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view of the explosive character of the triyne, we do not recommend its

isolation in an undiluted state.

10.2.9 Cyclooctyne from 1-bromo-1-cyclooctene andlithium diisopropylamide

Scale: 0.25 molar (BuLi and diisopropylamine); Apparatus: Figure 1.1, 1 litre

Cyclooctyne has been prepared in low yield by dehydrohalogenation of

1-bromo-1-cyclooctene with sodamide in an organic solvent [18]. We have

carried out the reaction with sodamide in liquid ammonia and also obtained

low yields [10]. Isomerisation to 1,2-cyclooctadiene under the influence of

the base probably is a serious subsequent reaction. This allene is a very

short-living compound and dimerises, giving rise to a considerable amount

of high-boiling residue. Butyllithium is neither suitable as dehydrohalogenating

agent, since it adds very smoothly to cyclooctyne. Moreover, interaction

between 1-bromocyclooctene and butyllithium gives rise to bromine–lithium

exchange. Lithium diisopropylamide generally shows little tendency to attack

on bromine or to add across multiple bond systems, but is a potent proton

abstractor. In our procedure for cyclooctyne [7], a solution of LDA in THF

and hexane is successfully used to convert the readily available 1-bromo-

1-cyclooctene into cyclooctyne. The subsequent base-induced isomerisation

to 1,2-cyclooctadiene can be largely suppressed by using a large excess

of 1-bromo-1-cyclooctene. This can easily be separated from cyclooctyne by

fractional distillation.

Cyclooctyne has a reasonable thermal stability. It can be stored for at least

one month at –20 �C with only slight polymerisation.

10.2.9.1 Procedure

THF (100 ml) is added to a solution of 0.25 mol of butyllithium in 159 ml of

hexane, cooled below –35 �C. Subsequently 0.25 mol of diisopropylamine is

added with cooling below 0 �C. The obtained solution is cooled to –40 �C and

0.50 mol of 1-bromo-1-cyclooctene (see below) is added over a few minutes.

The temperature of the mixture is allowed to rise over 1.5 h to þ15 �C and

maintained at this level for an additional 1.5 h. The resulting brown solution is

then poured into a mixture of 1 litre of ice water and 30 g of 36% hydrochloric


[13.1.2004–9:58pm] [203–228] [Page No. 216]


acid. After vigorous shaking, the layers are separated (Note). The organic layer

is subsequently washed five times with cold (0 �C) 2 N HCl in order to remove

most of the THF. The original aqueous layer and the washings are combined

and then extracted twice with small portions of pentane. The combined organic

solutions are washed with water, dried over MgSO4 and subsequently concen-

trated in a water-aspirator vacuum (bath temperature not higher than 25 �C).

Careful distillation of the remaining liquid through an efficient column gives

cyclooctyne, bp 48 �C/16 Torr, in at least 70% yield. Most of the excess of

1-bromo-1-cyclooctene (bp 85 �C/15 Torr) is recovered. There is only a small

high-boiling residue.

Note

Cyclooctyne has a very strong, unpleasant odour. The odour of the aqueous

layers disappears upon shaking with a small amount of bromine.

10.2.9.2 Procedure for 1-bromo-1-cyclooctene

Bromine (0.81 mol, slight excess) is added dropwise to a mixture of 250 ml of

dichloromethane and 0.80 mol of freshly distilled cyclooctene with cooling

between –40 and –60 �C. The solvent is thoroughly removed on the rotary

evaporator. Et2O (350 ml) is added to the remaining liquid and the solution

is cooled to � –10 �C. A solution of 1.6 mol (excess, Note) of t-BuOK in 160 ml

of THF is added over 30 min, while maintaining a temperature of � –10 �C.

After an additional 2.5 h at 0 to 5 �C, 500 ml of ice water is added and three

extractions with Et2O are carried out. The organic solution is washed twice

with saturated aqueous ammonium chloride, dried over magnesium sulphate

and then concentrated in a vacuum. Careful distillation of the residue through

a 40-cm Vigreux column gives 1-bromo-1-cyclooctene, bp 90 �C/18 Torr, in at

least 70% yield. The first fraction contains some cyclooctyne.

Note

The t-BuOH produced in the elimination forms a 1:1 complex with t-BuOK,

which is much less reactive.

10.2.10 t-Butylnitroacetylene from 2-iodo-3,3-dimethyl-1-nitro-1-butene and potassium hydroxide


[13.1.2004–9:58pm] [203–228] [Page No. 217]


Scale: 0.05 molar; Apparatus: 250-ml two-necked (vertical necks!), round-

bottomed flask, equipped with an evacuable dropping funnel and a very

short (� 5 cm) Vigreux column, connected to a short condenser and a single

receiver cooled in a bath at –50 �C or lower (cf. Figure 1.10). B24 glass joints

should be used.

The preparation of the unstable and extremely base-sensitive nitroacetylenes,

RC�CNO2, from the readily available 2-iodonitroalkenes and solid potassium

hydroxide was described in 1969 [6]. 2-Iodo-3,3-dimethyl-1-nitro-1-butene,

t-BuC(I)¼CHNO2, reacts smoothly with powdered KOH in Et2O at

20–30 �C, but no trace of the nitroalkyne can be isolated. In the original pro-

cedure, the vapour of the iodo compound is led over KOH pellets in a tube

heated at � 100 �C. The reaction is carried out at low pressure and the

vapour of the nitroalkyne is condensed in a strongly cooled receiver. In this

way, the contact time of the nitroalkyne with the base is extremely short.

The best results are obtained with the t-butyl derivative, which in fact is the

only representative that can be prepared in reasonable yield. The compound

is stored best at –20 �C or lower temperature as a dilute solution in a (volatile)

organic solvent. Distillation of a sample (� 5 g) which had been allowed to

stand for 24 h at rt, gave only � 70% recovery [10].

Our procedure is a variant of the one published. Although our yields are

somewhat lower, the procedure can be completed within 1 h. In the literature

procedure, the iodo compound is added over several hours.

10.2.10.1 Procedure (cf. [6])

In the flask are placed 50 g of KOH pellets and 20 ml of dry paraffin oil

(this is added for better conduction of the heat from the bath) and in the

dropping funnel 0.05 mol of 2-iodo-3,3-dimethyl-1-nitro-1-butene (see below).

The apparatus is evacuated by means of a mercury diffusion pump (pressure

preferably � 0.01 Torr) and the flask is heated in a bath at 100–120 �C

(at higher bath temperatures the KOH ‘melts’ during the addition of the

iodo compound). After heating for 15 min at the temperature indicated, the

addition of the iodo compound is started. The reaction is very fast and is

accompanied by rather strong foaming. After completion of the addition

(over 15 to 20 min), heating and evacuation are continued for an additional

10 min, then nitrogen is admitted. Pentane (50 ml) is added to the contents of

the receiver. After warming to rt, the yellow organic solution is separated from

the small amount of water and subsequently dried over a small amount of

MgSO4. The clear organic solution is decanted from the MgSO4 (which is

rinsed with some pentane). The liquid remaining after concentration of the

combined pentane solutions in vacuo, is distilled as quickly as possible through


[13.1.2004–9:58pm] [203–228] [Page No. 218]


a 20-cm Vigreux column. t-Butylnitroacetylene bp � 50 �C/10 Torr, is obtained

in yields varying between 50 and 75%.

10.2.10.2 Procedure for 2-iodo-1-nitro-t-butylethene

t-Butylacetylene (0.30 mol, exps. 10.2.1 and 10.2.4), dichloromethane (75 ml)

and iodine (0.20 mol, 51 g) are placed in the flask. A solution of 0.20 mol

(excess) of dinitrogen tetroxide in 50 ml of dichloromethane (Note) is added in

� 20 equal portions over 1.5 h at 25–30 �C. The brown reaction mixture is kept

for � 30 h at rt, then the excess of N2O4 is removed by evacuation. The

remaining brown liquid is dissolved in 100 ml of Et2O and the solution is

shaken with 100 ml of an aqueous solution of 20 g of Na2S2O3 in order to

remove the excess of iodine. After drying the solution and removing the Et2O

under reduced pressure, the remaining yellow liquid is distilled through a short

column to give the product, bp � 80 �C/0.5 Torr, as an (E)/(Z) mixture in

� 80% yield.

Note

N2O4 is commercially available in lecture bottles, but presumably can be made

by dissolving copper in nitric acid with simultaneous introduction of air.

10.2.11 N1,N1,N2,N2-Tetraalkyl-1-acetylenediamines from2-chloro-N,N,N1,N1-tetraalkyl-1,1-ethylenediaminesand potassium amide

Scale: 0.30 molar; Apparatus: Figure 1.1, 1 litre, no thermometer is used

N1,N1,N2,N2-Tetraalkyl-1-acetylenediamines, R2NC�CNR2 (R ¼ Me or

Et), have been prepared from chloroketene aminals, ClCH¼C(NR2)2, with

sodamide in an organic solvent [2]. N1,N1,N2,N2-Tetramethyl-1,2-acetylenedia-

mine, Me2NC�CNMe2, was obtained in a one-pot procedure by treating 1,1,3-

trichloroethylene, Cl2C¼CHCl, with sodamide in liquefied dimethylamine [2].

In our procedure for the N1,N1,N2,N2-tetraalkyl-1-acetylenediamines [19],

which is based on the published ones, the 1-chloro-2,2-ethylenediamines are

dehydrochlorinated by potassium amide in liquid ammonia. Under these

conditions (polar medium, good solubility of the base) the conversions into

the ynediamines proceed very smoothly and the work up is simple. After


[13.1.2004–9:58pm] [203–228] [Page No. 219]


evaporation of the ammonia the products are isolated by extraction of the

remaining salt mass with dry Et2O. Contact with water must be avoided,

since the ynediamines are extremely water-sensitive.

10.2.11.1 Procedure

(Note 1) A solution of 0.35 mol of potassium amide in 350 ml of liquid ammo-

nia is prepared as described in Chapter 2 (it is not necessary to filter the

solution of KNH2). The 1-chloro-2,2-ethylenediamine [20] (0.30 mol, see

below) is added over 20 min with efficient stirring. The reaction with the

methyl derivative is faster than that of the ethyl derivative (difference in solu-

bility). After an additional 10 min, the ammonia is removed by placing the

flask in a water bath at 30 �C. In the last stage of this operation, 150 ml of a 1:1

mixture of Et2O and pentane is added to the salt slurry (Note 2). Warming is

continued until the flow of ammonia vapour has become very faint. After

cooling to rt, the supernatant organic solution is carefully decanted from the

salt, which is rinsed four times with small portions of the Et2O–pentane

mixture. The combined organic solutions are concentrated in a water-aspirator

vacuum (in the case of R¼Me, the bath temperature should not exceed 20 �C).

The remaining liquid is distilled through a 30-cm Vigreux column.

N1,N1,N2,N2-Tetramethyl-1-acetylenediamine, bp � 40 �C/20 Torr (collected

in a single receiver, cooled in an ice- water bath, Figure 1.10), and

N1,N1,N2,N2-tetraethyl-1-acetylenediamine, bp 78 �C/15 Torr, are obtained in

>70% yields.

Notes

1. The ynediamines are extremely water-sensitive and anhydrous conditions

must be maintained throughout the experiments.

2. The salt may contain minute particles of potassium. Cleaning of the flask

with water should only be carried out when the organic solvent has com-

pletely evaporated or removed by passing a vigorous flow of nitrogen

through the flask.

10.2.11.2 Procedure for 2-chloro-N,N,N1,N1-tetraalkyl-1,1-ethylenediamines [20]

a. In a 1-litre three-necked, round-bottomed flask, equipped with a

mechanical stirrer and a reflux condenser, is placed a solution of


[13.1.2004–9:58pm] [203–228] [Page No. 220]


1,1,2-trichloroethene (0.20 mol) in Et2O (120 ml). The solution is cooled to

�90 �C and a suspension or solution of lithium dialkylamide [prepared in a

separate flask by addition at –40 �C of BuLi (0.20 mol) in hexane (126 ml)

to a mixture of the amine (0.20 mol) and Et2O (l00 ml)] is added over

� 5 min. During this addition, the temperature of the reaction mixture

containing dichloroacetylene is maintained below –70 �C by occasional

cooling in a bath with liquid nitrogen. Subsequently, the dialkylamine

(0.40 mol) is added and the contents of the flask are warmed to 30 �C

using a water bath. After stirring the suspension for 30 min at this tem-

perature, it is cooled to 20 �C and then subjected to suction filtration (G-2

sintered-glass funnel covered with a thin layer of anhydrous potassium

carbonate). The solid on the filter is rinsed well with dry Et2O. The filtrate

is concentrated in vacuo, using a water bath at 25–30 �C. Subsequent dis-

tillation of the remaining liquid gives the compounds ClCH¼C(Cl)NR2,

R¼Me, bp � 30 �C/15 Torr, and R ¼ Et, bp � 30 �C/0.5 Torr, in 70–80%

yields.

In view of the sensitivity of these compounds towards oxygen and

moisture, the appropriate techniques are applied during their isolation.

The compounds should be stored in well-closed bottles at low tempera-

tures.

b. A mixture of ClCH¼C(Cl)NR2, R¼Et (0.20 mol) and diethylamine

(0.30 mol) is placed in a 250-ml three-necked, round-bottomed flask, pro-

vided with a nitrogen inlet, a mechanical stirrer and a reflux condenser.

The mixture is heated under reflux while salt separates from the solution.

An additional amount of diethylamine (0.30 mol) is added in three equal

portions over 1 h. After an additional 5 to 6 h the mixture is cooled to

rt and Et2O (50 ml) is added. Suction filtration through a sintered-glass

funnel, followed by concentration of the filtrate in vacuo and distil-

lation gives 2-chloro-N,N,N1,N1-tetraethyl-1,1-ethylenediamine [20],

ClCH¼C[NEt2]2, bp � 55 �C/0.3 Torr, in � 80% yield.

10.2.11.3 Preparation of 2-chloro-N,N,N1,N1-tetramethyl-1,1-ethylenediamine [20], ClCH¼C[NMe2]2

1. From ClCH¼C(Cl)NMe2

In a 500-ml three-necked, round-bottomed flask provided with a nitrogen inlet,

a thermometer, a mechanical stirrer and a gas outlet, is placed a solution of

ClCH¼C(Cl)NMe2 (0.20 mol) in dry Et2O (50 ml). The solution is cooled to

–10 �C and a suspension or solution of lithium dimethylamide (0.22 mol) in

Et2O and hexane, prepared as described above, is added over 5 min.


[13.1.2004–9:58pm] [203–228] [Page No. 221]


Subsequently, liquefied dimethylamine (0.40 mol) is added, and the mixture is

warmed to 30 �C. This temperature is maintained for an additional 6 h. After

cooling, the contents of the flask to rt, the salt mass is filtered off on a G-3

sintered-glass funnel and rinsed with dry Et2O. Concentration of the filtrate in

vacuo followed by distillation affords 2-chloro-N,N,N1,N1-tetramethyl-1,1-

ethylenediamine, bp �50 �C/10 Torr, in 70–75% yield.

2. From trichloroethene, sodamide and dimethylamine

In a 3-litre round-bottomed flask is placed 1 litre of anhydrous liquid ammo-

nia. After adding ferric nitrate hydrate (100 mg), the flask is swirled manually

and sodium (3 g) is introduced in 0.5-g pieces. After the blue colour of the

dissolved sodium has disappeared and a grey solution has formed, further

sodium (43 g) is introduced (total amount 2.0 mol). The flask is swirled

occasionally. After all sodium has converted into amide, the flask is placed

in a water bath at 40 �C in order to remove the excess of ammonia. The last

traces of ammonia are removed by evacuation (water aspirator). The solid is

scratched from the glass wall by means of a curved spatula, after which the

flask is again evacuated. During this operation the flask is shaken vigorously

in order to break down the lumps of sodamide. The powder is transferred to

a 1-litre three-necked, round-bottomed flask, provided with a mechanical

stirrer, a dropping funnel combined with a gas inlet and a dry-ice condenser

filled with acetone and solid carbon dioxide (a so-called cold finger).

Anhydrous liquefied dimethylamine (240 ml) is placed in the flask and a

slow stream of nitrogen is passed through the apparatus. 1,1,2-

Trichloroethene (0.30 mol) is added dropwise over 10 min with stirring at

a moderate rate, care being taken that the suspension is not swept into the

upper part of the flask. The dropping funnel is then replaced with a combi-

nation of a thermometer and a gas inlet. The temperature indicates 6–7 �C

just after all trichloroethene has been added. The rate of the reflux from the

condenser increases gradually, while the temperature of the mixture drops

(ammonia liberated in the initial reaction escapes from the solution, con-

denses and returns with a temperature of ca –78 �C). After 1 to 1.5 h the

temperature of the mixture has reached a minimum of � –11 �C. Stirring is

continued for another 2.5 h, then a 1:1 mixture of dry Et2O and pentane (400

ml) is added, the cold finger is removed and the flask is placed in a water

bath at 45 �C. A gas outlet is placed on the flask. When the volume of the

mixture has decreased to � 350 ml, the solid material is filtered off on a

sintered-glass funnel and rinsed well with dry Et2O. Paraffin oil (50 ml) is

then added to the brown solution and the solvent is removed by evacuation,

using a water bath at 30–35 �C (the bulk of the solvent may first be removed

on the rotary evaporator). Subsequently, the product is distilled from the


[13.1.2004–9:58pm] [203–228] [Page No. 222]


paraffin oil at a pressure lower than 0.5 Torr, using a 20 to 30-cm Vigreux

column. The product, which is collected in a single receiver cooled at –70 �C

(Figure 1.10), is carefully redistilled (bp � 50 �C/10 Torr) through a 40-cm

Vigreux column. The yield is �70%.

10.2.12 4-Ethoxy-1-buten-3-yne from 1,4-diethoxy-1,2-butadieneand butyllithium

Scale: 0.20 molar; Apparatus: 500-ml flask, Figure 1.1

10.2.12.1 Procedure [10]

To a solution of 0.20 mol of 1,4-diethoxy-1,2-butadiene (see Chapter 17, exp.

17.2.11) in 50 ml of dry Et2O is added with cooling at –60 to –70 �C a solution

of 0.20 mol of butyllithium (Chapter 2, exp. 2.3.6, Note 1) in 250 ml of Et2O.

The addition takes 30–40 min. The temperature of the reaction mixture is then

allowed to rise gradually to –20 �C over 30 min, after which the brown reaction

mixture is poured into 200 ml of water. After shaking, the upper layer is

separated and the aqueous layer is extracted four times with small portions

of Et2O. The combined ethereal solutions are washed with water and dried

over magnesium sulphate. The greater part of the Et2O is distilled off at

normal pressure through a 40-cm Vigreux column, keeping the bath tempera-

ture below 65 �C (Note 2). The distillation flask is then cooled to 20–30 �C

and the remaining Et2O is removed in a water-aspirator vacuum, keeping

the receiver immersed in a bath at –10 �C (Figure 1.10). The volatile 4-

ethoxy-1-buten-3-yne passes over between 20 and 35 �C/12 Torr, and is

obtained in �65% yield. A smal1 amount (� 5%) of ethoxybutatriene,

EtOCH¼C¼C¼CH2, may be present.

Notes

1. Butyllithium in hexane cannot be used since separation of the volatile

enyne ether from the hexane would result in serious losses of product.

2. At higher temperatures some of the product might decompose into ethene

and vinylketene.


[13.1.2004–9:58pm] [203–228] [Page No. 223]


10.2.13 N,N-Dimethyl-3-buten-1-yn-1-amine from N,N-dimethyl-4-methoxy-2-butyn-1-amine and t-BuOK

Scale: 0.20 molar; Apparatus: Figure 1.1, 500 ml

10.2.13.1 Procedure

A solution of 0.40 mol of t-BuOK(100 mol% excess) in 75 g of dry THF is

heated at 50 �C. N,N-Dimethyl-4-methoxy-2-butyn-1-amine (0.20 mol, Chapter

13, exp. 13.2.3) is added in one portion. The temperature rises in a few minutes

to 60 �C or higher and a thick precipitate is formed. The thermometer and

outlet are replaced with a reflux condenser and the mixture is heated under

reflux for 30 min. It is then cooled to 30 �C and 100 ml of dry, redistilled

pentane is added. The solid material is filtered off on a sintered-glass funnel

and, after some pressing, rinsed with pentane. The pentane and the greater

part of the THF and t-BuOH are distilled off from the filtrate through a 40-cm

Vigreux column (bath temperature not higher than 110 �C) in a slow

stream of nitrogen. When the distillation has stopped, the residue in the

distillation flask is cooled to 30–40 �C and the distillation is continued at 40

Torr, using an efficient column. After THF and t-BuOH have distilled, N,N-

dimethyl-3-buten-1-yn-1-amine passes over between 45 and 55 �C. The yield is

at least 70%.

N,N-Diethyl-3-buten-1-yn-1-amine, H2C¼CHC�CNEt2, bp 49 �C/15 Torr,

is obtained in 75% yield by heating a 2:1 molar mixture of t-BuOK and

N,N-diethyl-4-methoxy-2-butyn-1-amine (Chapter 13, exp. 13.2.3) in THF

under reflux for 30 min and subsequently carrying out the dry work-up

described above.

Note

Aqueous work-up and extraction with Et2O also can be carried out but will

take longer. In the proposed procedure, the distilled reaction product is col-

lected in a cooled receiver. Without cooling the required pressure of 10–15 Torr

cannot be attained because of the presence of volatile components in the

reaction mixture.


[13.1.2004–9:58pm] [203–228] [Page No. 224]


10.2.14 4-Methylthio-1-buten-3-yne from 1,4-bis(methylthio)-2-butyne and t-BuOK

Scale: 0.20 molar (dichlorobutyne); Apparatus: Figure 1.1, 1 litre, no dropping

funnel; stirrer Figure 1.2.

10.2.14.1 Procedure

In the flask is placed 500 ml of liquid ammonia. Sodium (0.42 mol) is intro-

duced in 1-g pieces. After 15 min (Note 1), 0.20 mol of dimethyl disulphide

(commercially available) is introduced in 1- or 2-ml portions by means of a

pipette or syringe. During the addition, which is carried out over � 20 min

(vigorous reaction) the mixture is stirred vigorously. The temperature of the

reaction mixture is kept between –35 and –45 �C. After completion of the

addition the blue colour should have disappeared completely (if not, a few

more drops of dimethyl disulphide are added). 1,4-Dichloro-2-butyne

(0.20 mol) (Chapter 20, exp. 20.1.6) is then added over 15 min with vigorous

stirring and cooling between –35 and –45 �C. After 10 min, 0.30 mol of pow-

dered t-BuOK is introduced very rapidly (within 3 min) with vigorous stirring

(Note 2). Three minutes later, 75 g of finely crushed ice is added within 1 min

(Note 2). The greater part of the ammonia is then removed by placing the flask

for � 30 min in a water bath at 50 �C. To the remaining mixture are added

300 ml of ice water, then five extractions with redistilled pentane are carried

out. The combined extracts are washed three times with water and dried over

magnesium sulphate. The greater part of the pentane is distilled off at normal

pressure through a 40-cm Vigreux column. The remaining liquid is distilled

through the same column to give 4-methylthio-1-buten-3-yne, bp 34 �C/Torr,

in �75% yield.

Notes

1. If the sodium is not allowed to dissolve completely, the pieces of sodium

may be covered with sodium methanethiolate during the addition of

dimethyl disulphide and it takes at least 1 h for all of the sodium to be

converted.


[13.1.2004–9:58pm] [203–228] [Page No. 225]


2. The potassium methanethiolate formed in the elimination readily adds to

the enyne sulphide and rapid working is therefore necessary. Addition of

ice prior to the work-up causes inactivation of the potassium methanethio-

late by solvation.

10.2.15 Propadiene from 2,3-dichloro-1-propene and zinc in ethanol

Scale: 1.0 molar; Apparatus: 1-litre three-necked round-bottomed flask with a

dropping funnel, a gas-tight mechanical stirrer and a very efficient condenser.

The top of the condenser is connected to a cold trap (–80 �C).

10.2.15.1 Procedure

In the flask are placed 120 g of powdered zinc (Merck), 250 ml of 96% ethanol

and 40 ml of water, in the dropping funnel 1.0 mol of 2.3-dichloro-1-propene

(commercially available). The mixture is stirred at a rate such that the zinc

powder is suspended completely. After heating the mixture up to � 80 �C

(gentle refluxing of the ethanol), the bath is removed and dropwise addition

of the dichloride is started. The addition, which is carried out at a rate such

that the ethanol gently refluxes, takes � 1.5 h. The conversion is completed by

heating the mixture under reflux for an additional 30 min. The trap is con-

nected to an empty one cooled at –80 �C and is subsequently placed in a water

bath at 5 �C. When the greater part of the allene has evaporated and the

evolution of gaseous allene has subsided, the bath temperature is increased

to � 35 �C. The remaining mixture of ethanol and dichloropropene is dis-

carded. The yield of pure propadiene is � 85%.

Note

Allene has a bp of –34.5 �C. Because of the high volatility, precise weighing of

amounts for small-scale procedures (0.05–0.10 mol) is difficult. It is therefore

better to prepare a stock solution by adding a sufficient amount (50–100 g) of

the pre-cooled (–50 �C) reaction solvent (mostly THF) to a fixed amount of

allene (e.g. 0.50 mol). The required amount of allene can be obtained by

weighing part of the solution.


[13.1.2004–9:58pm] [203–228] [Page No. 226]


10.2.16 Butatriene from 1,4-dichloro-2-butyne and zinc indimethylsulphoxide

Scale: 0.20 molar; Apparatus: 1-litre flask with a dropping funnel, a gas-

tight mechanical stirrer and a very efficient reflux condenser; the top of the

condenser is connected with a trap. A tube containing anhydrous CaCl2 is

placed between the trap and the water aspirator. The connection with the

trap is made in such a way that the cumulene vapour can enter the large

annular space (the long inner tube being connected to the water aspirator)

of the trap.

10.2.16.1 Procedure

The flask is charged with 70 ml of dry DMSO, 35 g of zinc powder and 10 g of

sodium iodide. In the dropping funnel is placed 0.20 mol of 1,4-dichloro-2-

butyne (Chapter 20, exp. 20.1.6). After the system has been evacuated to 10–20

Torr, stirring is started and the flask is heated until the DMSO begins to reflux.

The trap is immersed in liquid nitrogen and the dichlorobutyne is added over

15 min from the dropping funnel. The reaction is vigorous and occasional

cooling may be necessary in order to moderate refluxing. After the addition,

heating under reflux (occasional heating) is continued for 20 min. Nitrogen is

then admitted and the trap containing the solid butatriene is placed in a bath

with dry ice and acetone. The yield of pure butatriene is generally higher than

90%. The compound should be used directly after its preparation. If diluted

with an inert organic solvent it can be stored for 12–24 h at –80 �C under

nitrogen. If the undiluted compound is warmed to � 0 �C, it may explode

violently.

10.2.17 1,2,3-Pentatriene from 1,4-dichloro-2-pentyne andzinc in dimethylsulphoxide


[13.1.2004–9:58pm] [203–228] [Page No. 227]


Scale: 0.20 molar; Apparatus: 500-ml three-necked, round-bottomed flask, pro-

vided with a dropping funnel, a gas-tight mechanical stirrer and an efficient

reflux condenser. A trap is placed between the condenser and the water aspira-

tor in such a way that during the reaction the vapour of the product enters the

large annular space of the trap. A drying tube is placed between the trap and

the water aspirator.

10.2.17.1 Procedure

In the flask are placed 70 ml of dry DMSO, 40 g of powdered zinc and 8 g of

sodium iodide, and in the dropping funnel 0.20 mol of 1,4-dichloro-2-pentyne

(Chapter 20, exp. 20.1.6). The system is evacuated by means of the water

aspirator and the trap is then placed in liquid nitrogen. The flask is heated

until the DMSO begins to reflux in the lower part of the condenser. The

heating bath is removed and the dichloride is added dropwise over 10 min

(Note 1). After the addition, the mixture is heated for 45 min under gentle

reflux. Pure nitrogen is then admitted and the trap is placed in a bath at –75 �C.

The NMR spectrum (Note 2) shows the product to be reasonably pure. Traces

of DMSO (swept along with the cumulene) are sometimes present. The increase

in weight of the trap corresponds to a yield of � 70%. The cumulene poly-

merises completely within a few hours at rt, but can be stored without change

at –80 �C under nitrogen for 12–24 h.

Notes

1. The heat developed by the reaction is just enough to cause gentle refluxing,

provided that the mixture is not stirred too vigorously. If refluxing stops

during the addition, external heating must be applied.

2. Traces of oxygen will induce polymerisation of the cumulene. The NMR

tube must therefore be filled with nitrogen before bringing the sample in it.

Low-temperature NMR gives the most representative results.

REFERENCES

1. P. Fritsch, W. P. Buttenberg and H. Wiechell, Liebigs Ann. Chem. 279, 319, 324, 337 (1894).

2. S. Y. Delavarenne and H. G. Viehe, Chem. Ber. 103, 1209 (1970); L. Rene, Z. Janousek and

H. G. Viehe, Synthesis 645 (1982).

3. E. V. Dehmlov and R. Thieser, Tetrahedron 3569 (1986).

4. J. A. P. Thyman, Synth. Commun. 5, 21 (1975).

5. J. F. Arens, in Advances in Organic Chemistry, Methods and Results Interscience Publ., New

York, 1962, Vol. 2, 117.

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6. V. Jager and H. G. Viehe, Angew. Chem., Int. Edn. 9, 273 (1969).

7. L. Brandsma and H. D. Verkruijsse, Synthesis 290 (1978).

8. H. D. Verkruijsse and L. Brandsma, Synth. Commun. 20, 3355 (1990).

9. H. D. Verkruijsse and L. Brandsma, Synth. Commun. 21, 657 (1991).

10. Unpublished observations and results from the author’s laboratory.

11. F. Bohlmann, Chem. Ber. 86, 657 (1953); J. B. Armitage, C. L. Cook, E. R. H. Jones and M. C.

Whiting, J. Chem. Soc. 2010 (1952).

12. W. Verboom, R. H. Everhardus, H. J. T. Bos and L. Brandsma, Recl. Trav. Chim., Pays-Bas

98, 508 (1979).

13. R. H. Everhardus and L. Brandsma, Synthesis 359 (1978).

14. P. P. Montijn, J. H. van Boom, L. Brandsma and J. F. Arens, Recl. Trav. Chim., Pays-Bas 86,

115 (1967).

15. H. N. Cripps and E. F. Kiefer, Org. Synth. 42, 12 (1962).

16. P. P. Montijn, L. Brandsma and J. F. Arens, Recl. Trav. Chim., Pays-Bas 86, 126 (1987);

M. Maurer, H. Hopf, Angew. Chem. 99, 687 (1976).

17. H. Kleijn, H. Westmijze, A. Schaap, H. J. T. Bos and P. Vermeer, Recl. Trav. Chim., Pays-Bas

86, 129 (1987).

18. G. Wittig and H.-L. Dorsch, Liebigs Ann. Chem. 711, 46 (1968).

19. L. Brandsma and H. D. Verkruijsse, Synth. Commun. 21, 811 (1991).

20. R. van der Heiden and L. Brandsma, Synthesis 76 (1987).


synthesis of acetylenes, allenes and cumulenes || acetylenes, allenes and cumulenes by elimination...

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