synthesis of acetylenes, allenes and cumulenes || halogenation of acetylenes

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E:/Archive files/4188-Brandsma/Printer-Files/4188-Chapter-09.3d

9Halogenation of Acetylenes

9.1 METHODS FOR THE DIRECT INTRODUCTION OF HALOGEN

The principal methods for the preparation of 1-halogeno-1-acetylenes from the

corresponding 1-alkynes are represented in the following equations [1].

A general method for 1-chloro-1-alkynes is the reaction of a metallated acet-

ylene with an arenesulphonyl halide in an organic solvent [1,2]. Sodium alky-

nylides have been most often used, but it has been shown [13] that also the

lithium compounds give satisfactory results. Working with the lithium com-

pounds is more convenient, as their solubility is better.

A useful variant of the sulphonyl chloride method is chlorination with

N-chlorosuccinimide in organic solvents [5]. 1-Bromo-1-acetylenes can be pre-

pared analogously from lithium alkynylides or alkynyl Grignard reagents and

N-bromosuccinimide [6]. A disadvantage of the halosuccinimide method is the

very thick suspension of the metal succinimide formed in this reaction. This

makes it necessary to use relatively large volumes of solvents. In this respect

the bromination with cyanogen bromide is more convenient.

In many cases, the free halogens dissolved in a suitable solvent such as Et2O

(for Br2 or I2), THF (for I2) or strongly cooled pentane (for Cl2) can be used.

Generally, the iodinations give the highest yields, as there is little competition

of addition of I2 to the unsaturated system. The halogenations with Cl2, Br2 and

I2 are carried out most conveniently with the lithium alkynylides [3,4].

Subsequent addition of Cl2 and Br2 to the triple bond can, in many cases, be

191

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avoided by a sufficiently slow addition of the solution of the halogen

to a strongly cooled solution or suspension of the lithiated alkyne in Et2O

or THF.

1-Halogeno-1-alkynes are also formed from alkynes and aqueous solutions

of hypohalites. In these reactions, alkynylide anions are transient intermedi-

ates. The conversions give good results when the acetylene has a relatively

low pK (e.g. Aryl�C�CH and RC�CC�CH) or good solubility in water

(e.g. 2-methyl-3-butyn-2-ol, HC�CC(Me)2OH). The hypohalite method has

found most applications in the synthesis of bromoacetylenes [7,8].

Finally, the peculiar formation of 1-iodo-1-alkynes from iodine and acety-

lenes with relatively low (<25) pK values in liquid ammonia should be men-

tioned [9]. The most likely occuring intermediates are acetylide ‘anions’

formed in very low concentrations from the acetylene and the base ammonia.

The conversions proceed very slowly and iodination of the lithiated alkynes in

the same solvent is undoubtedly a superior method.

9.2 EXPERIMENTAL SECTION

Note: All reactions are carried out under inert gas.

Warnings

1. Especially haloalkynes with a conjugated unsaturated system or hetero-

substituents are unstable and may undergo vigorous decomposition upon

heating. Even during storage at low temperatures extensive deterioration

may occur. It therefore has little sense to prepare large amounts for use

over a long period. Structurally simple haloalkynes such as 1-iodo-1-propyne,

MeC�CI, and 1-bromo-1-butyne, EtC�CBr, have been stored for several

months at –20 �C without deterioration.

2. In view of possible physiological effects, contact with the skin and inhalation

of vapours should be avoided.

9.2.1 1-(2-Chloroethynyl)benzene from lithium phenylacetylideand benzenesulphonyl chloride

192 9. HALOGENATION OF ACETYLENES

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Scale: 0.20 molar; Apparatus: Figure 1.1, 500 ml, at a later stage the thermo-

meter is replaced with a reflux condenser.

9.2.1.1 Procedure [13] (cf. [2])

Phenylacetylene (0.20 mol) is added over a few seconds to a mixture of 0.25

mol of (commercially available) lithium amide (Note 1) and 200 ml of THF,

then a solution of 1.5 g of t-BuOK in 5 ml of THF (Note 2) is added and the

mixture is warmed to �40 �C. The mixture is stirred for 30 min at 40–50 �C

(occasional cooling may be necessary). The evolution of ammonia gradually

sets in, while a coarse suspension of PhC�CLi solvated by NH3 is formed.

The conversion is completed by heating the mixture under reflux for an

additional 45 min. The evolution of ammonia has then ceased and the sus-

pension has virtually dissolved (a faint turbidity due to a small amount of

PhC�CK remains). Traces of dissolved ammonia are removed by distilling

off �50 ml of THF from the solution (rotary evaporator). The solution is

cooled to –20 �C and a mixture of 0.22 mol of benzenesulphonyl chloride and

60 ml of THF is added over 30 min, while keeping the temperature of the

mixture between –20 and –10 �C. A yellowish-brown, thick suspension is

formed. The temperature is subsequently allowed to rise to 15 �C. When

heat is no longer evolved, the temperature is raised to 35 �C and maintained

at this level for an additional 15 min. Water (300 ml) is then added with

vigorous stirring. After separation of the layers, four to six extractions with

pentane are carried out. The combined organic solutions are dried over

MgSO4 and subsequently concentrated under reduced pressure (keeping the

bath temperature below 35 �C, Note 3). The remaining dark liquid is sub-

jected to a careful distillation through a 40-cm Vigreux column. After a small

first fraction of phenylacetylene, the chloroalkyne distils and a small viscous

residue remains. Redistillation gives 1-(2-chloroethynyl)benzene, bp 64 �C/

15Torr, in �65% yield.

1-(2-Chloroethynyl)cyclohexene, 1-c-C6H9–C�CCl, bp 75 �C/15 Torr, is

obtained in �70% yield from 1-ethynylcyclohexene , LiNH2 and PhSO2Cl

by a similar procedure.

Notes

1. BuLi in hexane–THF also can be used for the metallation of PhC�CH

(Chapter 3, exp. 3.9.4).

2. This base is added to activate the amide [10].

3. When the solvent is distilled off too quickly, lower yields are obtained.

9.2 EXPERIMENTAL SECTION 193

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9.2.2 1-Bromo-1-alkynes from alkynyllithium and bromine

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

A good method for preparing 1-bromo-1-alkynes, RC�CBr, in which

R represents an alkyl or cycloalkyl group consists of adding bromine to a

strongly cooled solution of the lithiated alkyne in Et2O or THF. Yields are

generally high. The method is not suitable for the preparation of the volatile

1-bromopropyne because its isolation requires a time-consuming distillation.

The bromoalkyne can undergo subsequent addition of bromine. This reac-

tion can be avoided by adding bromine at a sufficiently low rate and low

temperature.

9.2.2.1 Procedure

Bromine (0.20 mol, dried by shaking with 96% H2SO4 and subsequent distil-

lation) is added dropwise over 30 min to an efficiently stirred solution of

0.20 mol of the 1-lithiated acetylene in Et2O or Et2O–hexane (prepared from

the acetylene and a solution of BuLi�LiBr in Et2O or in Et2O–hexane, respec-

tively, Chapter 3, exp. 3.9.4) with cooling below –60 �C. The reaction mixture

(at low temperatures a salt suspension) is then poured into 500 ml of ice water

and the flask rinsed with water and Et2O. After shaking and separation of the

layers, two extractions with Et2O are carried out. The combined organic solu-

tions are dried over MgSO4, after which most of the solvent is distilled off at

atmospheric pressure (Note 1) under inert gas. The remaining liquid is distilled

in vacuo. 1-Bromo-1-butyne, EtC�CBr, bp �50 �C/200 Torr, 1-bromo-3,3-

dimethyl-1-butyne, t-BuC�CBr, bp 70 �C/150 Torr, and 1-bromo-1-hexyne,

n-BuC�CBr, bp 66 �C/50 Torr, are obtained in greater than 70% yields (Note 2).

Notes

1. In the case of 1-bromo-1-butyne, EtC�CBr, the Et2O (hexane cannot be

used as co-solvent) is slowly distilled off through an efficient column. The

temperature of the heating bath is raised to �110 �C in the last stage of

the distillation.

2. A small, high-boiling residue, presumably the addition product of Br2 and

the bromoalkyne is left behind.

3,1-Enynes, RCH¼CHC�CH, may be brominated by a similar procedure.

1-Bromo-3-hexen-1-yne, EtCH¼CHC�CBr (bp �30 �C/10 Torr), is obtained


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in �65% yield from a mixture of (E)- and (Z)-3-hexen-1-yne, HC�

CCH¼CHEt. There is a considerable residue. In general, bromination with

hypobromite is the preferred method for 1-bromo-3-alken-1-ynes, RCH¼

CHC�CBr, and 1-bromo-1,3-diynes, RC�CC�CBr.

For the bromination of enynes and diynes of the types

RCH¼CHCH2C�CH (4-en-1-ynes) and RC�CCH2C�CH (1,4-diynes), the

reaction of the Grignard derivative with N-bromosuccinimide or cyanogen bro-

mide, BrC�N, seems the best method since these conditions involve the least

risk of an isomerisation of the skipped system. This method is also

recommended for the bromination of hydrocarbons with systems

C¼C(CH2)nC�CH, having n>2.

9.2.3 Bromination of acetylenes with aqueous hypobromite


The bromination with alkali hypobromite in aqueous solution gives good

results with (hetero)arylacetylenes, 3,1-enynes, RCH¼CHC�CH, and 1,3-

diynes, RC�CC�CH; all acetylenes that are more acidic than acetylenes in

the aliphatic or cycloaliphatic series with an isolated triple bond. For the con-

jugated systems the hypobromite method is superior to the reaction of metal-

lated acetylenes with bromine. Various acetylenic alcohols also are brominated

smoothly, which can be explained in part by their better solubility in water.

Since in the case of primary and secondary ethynyl alcohols, oxidation of

the alcohol can occur, the use of an excess of hypobromite should be avoided.

The best procedure is dropwise addition of less than an equivalent amount of

hypobromite to a mixture of alcohol and water. If the bromoalkyne to be pre-

pared is not too volatile, small amounts of THF or dioxane may be added

to effect a better solubility of the alkyne in the aqueous phase. Addition of

a co-solvent may be desired also when the starting compound is a solid

(e.g. 1-ethynylcyclohexanol).

9.2.3.1 Procedure

Bromine (80 g) is added to a vigorously stirred solution of 75 g (excess) of

potassium hydroxide in 200 ml of water with cooling between –5 and 0 �C. A

pale yellow solution is formed. This solution of KOBr should be used without

delay.


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4-Bromo-2-methyl-3-butyn-2-ol, BrC�CC(Me)2OH, bp 70 �C/15 Torr, is

obtained in greater than 90% yield by adding 2-methyl-3-butyn-2-ol,

HC�CC(Me)2OH (0.30 mol), over 15 min to the solution of KOBr

(corresponding with a large excess), while keeping the temperature between

10 and 20 �C (occasional cooling). After an additional 15 min at rt the mixture

(turbid aqueous phase and lower layer) is extracted four times with Et2O.

The organic solutions are dried over MgSO4, Et2O is removed under reduced

pressure, and the remaining liquid distilled off through a 20-cm Vigreux

column.

4-Bromo-3-butyn-2-ol, BrC�CCH(Me)OH, (undistilled), is obtained in

�85% yield as a viscous liquid after removal of the Et2O from the extracts.

Of the KOBr solution 60% (corresponding with �0.3 mol) is added dropwise

over 30 min to a mixture of 0.32 mol (slight excess) of 3-butyn-2-ol,

HC�CCH(Me)OH, and 40 ml of water. During this addition, the temperature

of the mixture is kept between 5 and 10 �C. After an additional 15 min (at

10 �C) four extractions with Et2O are carried out. The organic solutions are

dried (without preceding washing) over MgSO4.

1-(2-Bromoethynyl)benzene, PhC�CBr, (undistilled), is obtained in almost

100% yield by vigorously agitating during 3 h under N2 a mixture of 0.25 mol

of phenylacetylene and the KOBr solution (corresponding to a large excess)

described above. The reaction is carried out in a 500-ml flask insulated by

cotton wool. The initial temperature is �20 �C. After 2.5 h, the nD of the

upper layer (interruption of stirring) has become �1.605 and the tempera-

ture 31 to 33 �C. After stirring for 3 h, the product is isolated by

adding 200 ml of ice water and extracting four times with Et2O. The com-

bined organic solutions are dried over MgSO4 and subsequently concentrated

in vacuo.

Other ArC�CBr and 2-(2-bromoethynyl)thiophene, 2-Thienyl-C�CBr (bp

�50 �C/0.06 Torr) can be prepared in a similar way.

1-Bromo-3-hepten-1-yne, n-PrCH¼CHC�CBr ((E)/(Z)–mixture), is pre-

pared in a way similar to that described for PhC�CBr. After 4 to 5 h, the

nD of the upper layer has reached a maximum (�1.51). The mixture is then

extracted with very small portions of pentane. The combined organic solutions

are concentrated in vacuo (bath temperature <25 �C) after drying over

MgSO4. The yield of the bromoenyne (undistilled, satisfactory purity) is at

least 75%. Distillation (bp �58 �C/15 Torr) gives the pure bromoenyne.

In the case of bromination of 1,3-diynes, RC�CC�CH (R¼Me or higher

alkyl), the diyne may be diluted with a small amount of pentane (�50% v/v)

and stirring (at 25–30 �C) is continued for a few hours. The reaction can be

followed by determining the nD of the mixture of diyne and pentane.

Distillation of these bromodiynes seems risky.


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9.2.4 1-Bromo-1-propyne and 1-bromo-1-butyne from the1-alkynes and potassium hypobromite


9.2.4.1 Procedure [11]

Bromine (75 g) is added over 15 min to an efficiently stirred solution of 70 g of

potassium hydroxide in 150 ml of water, while keeping the temperature

between 0 and –5 �C. High-boiling petroleum ether (30 ml, bp>170 �C)

cooled to 0 �C is added and the air in the flask is completely replaced by

inert gas. After cooling the mixture to 0 �C, a cold (–25 �C) solution of

0.30 mol of propyne or 1-butyne in 90 ml of petroleum ether is added in

four equal portions over 15 min with vigorous stirring. After addition of the

last portion the reaction is monitored by measuring the refractive index of the

upper layer, stirring being temporarily stopped. The temperature of the reac-

tion mixture gradually rises to rt. Stirring is continued for an additional 30 min,

when nD has become maximal. Water (150 ml) is added and the layers are

separated. The organic solution is transferred into a 1-litre round-bottomed

flask and 10 g of magnesium sulphate is added. After vigorous shaking, the

flask is equipped for a vacuum distillation (10 to 20 Torr), the receiver being

cooled in a bath at –70 �C (Figure 1.10). The flask is evacuated and heated until

the petroleum begins to pass over (�50 �C/10 Torr). Repetition of this opera-

tion with the contents of the receiver, now not allowing the petroleum to pass

over, gives pure 1-bromo-1-propyne and 1-bromo-1-butyne in high yields.

1-Bromo-1-pentyne and 1-bromo-1-hexyne can be prepared in good yields

by stirring the 1-alkynes with an excess of KOBr in water until nD has reached

its maximal value. In these cases no solvent is used. The bromination of the

higher homologues (exp. 9.2.2) by this procedure proceeds too slowly because

of their decreased solubility in the aqueous phase.

9.2.5 Bromination of lithiated acetylenes with cyanogen bromide



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A convenient and quick way to prepare 1-bromo-1-alkynes consists of

adding an ethereal solution of cyanogen bromide to a strongly cooled solution

of a metallated (Li or MgX) acetylene in an organic solvent (Et2O or THF) [3].

The method seems very general and excellent yields can be obtained provided

that the ethereal solution of BrC�N (which is prepared from an aqueous

solution) is dried well. The advantage of this method over the introduction

with elemental bromine is that CC double bonds in enynes do not react.

9.2.5.1 Procedure

A solution of cyanogen bromide in �150 ml of Et2O (Note 1), prepared on a

0.25 molar scale (large excess) from bromine and potassium cyanide is added

over 15 min to a solution of 0.10 mol of the lithiated acetylene (Chapter 3, exp.

3.9.4) in 63 ml of hexane and 80 ml of THF (Note 2). During this addition the

temperature is maintained between –70 and –50 �C. After an additional period

of 10–15 min (at –30 to –40 �C) water (200 ml) is added with vigorous stirring

to the white suspension. After separation of the layers, three extractions with

Et2O are carried out. The combined organic solutions are dried over MgSO4,

after which the solvent is removed in vacuo (if the volatility of the product

allows). The remaining liquid is carefully distilled through a 30-cm Vigreux

column.

1-Bromo-1-octyne, bp 90 �C/15 Torr, and 1-(2-bromoethynyl)cyclohexene,

c-C6H9C�CBr, bp 50 �C/0.5 Torr, are obtained in � 80% yields. Distillation

of thermally unstable bromoalkynes, e.g. RC�CC�CBr, should not be

carried out.

Notes

1. The procedure in Organic Synthesis [12], in which the KC�N is added over

2 h and the BrCN is isolated by steam distillation, is modified. To a

vigorously stirred mixture of 0.25 mol of bromine and 40 ml of water is

added over 10 min a solution of 0.25 mol of potassium cyanide in 50 ml of

water. During this addition the reaction mixture is cooled between 0 and

10 �C. The white suspension of cyanogen bromide is subsequently

extracted three times with small portions (total amount � 120 ml) of

Et2O. The colourless extract is first shaken (without washing) at 0 �C

with a relatively small amount of potassium carbonate, then decanted

and subsequently vigorously shaken (with cooling at 0 to –10 �C) with a

small portion of phosphorus pentoxide. The solution is decanted from the

syrupy mass and shaken with a second small portion of P2O5 (which now

remains suspended; if not, the procedure is repeated).


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2. The reaction with cyanogen bromide presumably can also be carried out

successfully with RC�CLi in Et2O–hexane mixtures or with alkynylmag-

nesium halides (at higher temperatures) in Et2O or THF.

9.2.6 1-Iodo-1-alkynes from 1-alkynyllithium and iodine in organicsolvents

Scale: 0.20 molar; Apparatus: Figure 1.1, 1 litre

9.2.6.1 Procedure

Finely powdered iodine (0.20 mol, the dropping funnel is replaced with a

powder funnel) or a saturated solution of 0.20 mol of iodine in Et2O or

THF is added over 15 to 30 min to a solution or suspension of 0.20 mol of

the 1-alkynyllithium (Chapter 3, exp. 3.9.4) in a mixture of Et2O and hexane or

THF and hexane with cooling between –15 and –30 �C. After this addition the

cooling bath is removed and the temperature is allowed to rise to � 0 �C

(suspensions of RC�CLi may react more slowly). Water (200 ml) is then

added with vigorous stirring, and, after separation of the layers, the aqueous

layer is extracted with Et2O (small amounts of iodine can be removed with an

aqueous Na2S2O3 solution). The organic solution is dried over MgSO4 and

subsequently concentrated in vacuo, followed by distillation of the remaining

liquid. 1-Iodo-1-heptyne, n-BuC�CI, bp 60 �C/10 Torr, is obtained in >80%

yield.

Volatile iodoacetylenes (bp<40 �C/10 Torr) can best be prepared by using

Et2O as the only solvent. The lithium alkynylide is generated from the acety-

lene and BuLi �LiBr in Et2O (Chapter 2, exp. 2.3.6 and Chapter 3, exp. 3.9.4).

For another useful procedure for volatile iodoacetylenes see below.

Acetylenic Grignard derivatives in Et2O or THF also give 1-iodo-1-alkynes

upon addition of iodine at –10 to –20 �C.

9.2.7 1-Iodo-1-alkynes from 1-alkynyllithium and iodinein liquid ammonia

Scale: 0.20 molar; Apparatus: Figure 1.1, 1 litre


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In the presence of water, iodine reacts with ammonia to give explosive NI3 as

a black precipitate. In anhydrous liquid ammonia at –33 �C (or at lower tem-

peratures) practically no conversion takes place, however. This appears most

convincingly from the preparation of aryl- or heteroaryl iodoacetylenes in

excellent yields by stirring a mixture of equimolar amounts of iodine and the

acetylene in liquid ammonia for several hours [9]. For the less acidic alkylace-

tylenes, this method has no practical significance because very long reaction

times are necessary. A much quicker procedure is to add iodine as a solution

in Et2O or THF to an ammoniacal solution of the lithium alkynylide, cooled to

below –33 �C. This reaction is extremely fast and generally gives iodoacetylenes

in excellent yields. The volatile 1-iodo-1-propyne, for example, can be prepared

by adding an ethereal solution of iodine to a solution of propynyllithium in

ammonia cooled to below –60 �C

9.2.7.1 Procedure

A solution of 0.20 mol of alkynyllithium in � 250 ml of liquid ammonia is

prepared as described in Chapter 3, exp. 3.9.3. 1-Propynyllithium and 1-buty-

nyllithium can best be prepared by dropwise addition (over 20 min) of the 1,2-

dibromoalkanes (0.20 mol, cf. Chapter 3, exp. 3.9.21) to a suspension of an

excess of LiNH2 (0.65 mol) in � 400 ml of liquid ammonia (Chapter 2,

exp. 2.3.1). The solutions in ammonia are cooled to below –60 �C (occasional

cooling in a bath with liquid N2), while N2 is passed through the flask

(0.2 litre/min). A solution of 0.25mol (excess) of iodine in Et2O (� 300 ml)

or THF (250 ml) is then added over � 15 min with efficient stirring, while

keeping the temperature between –50 and –70 �C (for the volatile 1-iodo-1-

alkynes with bp<50 �C/15 Torr, Et2O should be used). After an additional

15 min (at � –50 �C), the reaction mixture is cautiously poured onto 500 g of

finely crushed ice, contained in a 2 to 3-litre wide-necked conical flask. The

reaction flask is rinsed with a small amount of ice water. A solution of 20 g of

Na2S2O3 in 150 ml of water is then added to the mixture. After melting of the

ice (some warming may be applied) and vigorous shaking, the layers are sepa-

rated. The aqueous layer is extracted three to five times with small portions of

pentane (this gives a better separation than Et2O). The combined organic

solvents are dried over MgSO4, after which the greater part of the solvent is

removed. In the cases of 1-iodo-1-propyne, MeC�CI, 1-iodo-1-butyne,

EtC�CI and 1-iodo-1-pentyne, n-PrC�CI, the Et2O is distilled off at atmo-

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

ture below 100 �C. In the other cases the solvent can be removed using a rotary

evaporator. The remaining liquid is carefully distilled through a 40-cm

Vigreux column under a reduced pressure, appropriate for the volatility of


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the product: 1-Iodo-1-propyne, MeC�CI, bp� 50 �C/100 Torr, and 1-iodo-1-

heptyne, C5H11C�CI, bp 78 �C/10 Torr, are obtained in excellent yields.

9.2.8 1-(2-Chloroethynyl)benzene from phenylethynyllithiumand chlorine


In analogy with the reaction of lithiated acetylenes with bromine, chloroalk-

ynes can be prepared by chlorination of metallated acetylenes at low tempera-

tures with free chlorine. Since the solvents Et2O and THF are readily attacked

by chlorine, introduction of gaseous chlorine seems a risky operation. In the

procedure for 1-(2-chloroethynyl)benzene, a solution of chlorine in dichloro-

methane (prepared by diluting liquid chlorine with strongly cooled dichloro-

methane) is added at a low temperature to a solution of lithium

phenylacetylide in Et2O and hexane. Presumably an aliphatic or alicyclic acet-

ylene RC�CH (R ¼ alkyl or cycloalkyl) with a sufficiently higher bp than that

of hexane, can be chlorinated in a similar way. For chloroalkynes containing

double bonds or other chlorine-sensitive groups, the procedure of exp. 9.2.1

seems better. Instead of lithiated acetylenes, Grignard derivatives can be

employed. Since chlorine can react with MgBr2 to give free bromine, the acet-

ylenic Grignard derivatives have to be prepared with alkylmagnesium

chlorides.

9.2.8.1 Procedure (cf. [4])

A solution of 0.20 mol of lithium phenylacetylide in 126 ml of hexane and

150 ml of Et2O (Chapter 3, exp. 3.9.4) is cooled to below –60 �C. A mixture of

0.20 mol of chlorine and 30 ml of dry dichloromethane (prepared just before,

by adding the strongly cooled solvent to the liquid chlorine) is added by pour-

ing from the cold trap, over � 10 min to the vigorously stirred solution, which

is kept below –70 �C. Occasional cooling in a bath with liquid N2 allows this

quick addition. After an additional 2 min, the mixture is hydrolysed, the layers

separated and the organic solution dried over MgSO4. After evaporation of the

solvent in vacuo, the remaining liquid is distilled through a 30-cm Vigreux

column to give 1-(2-chloroethynyl)benzene in greater than 80% yield

(cf. exp. 9.2.1). Since lithium alkynylides that are more strongly basic than


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PhC�CLi, react easily with CH2Cl2, it is better to add the chlorine

portionwise as a solution in strongly cooled pentane or hexane (<–50 �C) in

these cases.

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


synthesis of acetylenes, allenes and cumulenes || halogenation of acetylenes

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