II. SOLID-PHASE POLYMERIC ANALOGUES OF CHLORAMINE-T AND
BROMAMINE-T: PREPARATION AND USE AS SYNTHETIC - REAGENTS
The diverse na tu re of t h e chemistry of
N-halogeno-N-metallo' r eagen t s is a consequence of t h e i r
abili ty t o a c t as a source of <a> halonfum cations, <b>
hypohalite species, <c> N-anions which act as
bases and nucleophiles and <d> nitrinoids i n limited
cases. A s a resul t , these reagents r e a c t with a
surprising range of functional groups ef fec t ing an
a r r ay of molecular transformations 173,174
Historically t h e important early developments i n
t h e area stemmed from t h e synthesis of chloramine-T
and related aryl sulphonamide derivatives.
The p resen t section deals with t h e
preparation and synthet ic applications of
polystyrene-supported analogues of chloramine-T and
bromamine-T <N-chloroA-bromo-N-sodiopolystyrene-
sulphonamides> which have proved t he i r efficiency as
conventional low-molecular oxidising and halogenating
reagents. The sec t ion also describes t h e comparison
of t h e new reagen t with o ther reported polymeric
reagen t s and t h e analogous low-molecular weight
reagents. The various reaction parameters which could
affect t h e course and t h e extent of t h e synthet ic -
reactions a r e also analysed.
R e s u l t s and Discussion
11. i. PI-eparatioll of N-Halo-N-sodiopolystyrene-
s u l p h o n a m i d e Resins (10 11)
The polymeric analogues of N-chloro and
H-bromo-p- toluenesulphonamides w e r e prepared from
commercially available 2% divinylbenzene-crosslinked
polystyrene beads by a four-step polymer analogous
reaction (Scheme II.l>. F i r s t s t e p w a s t h e
preparation of polystyrenesulphonic acid <7> which is
a w e l l known cation exchange resin. This w a s done by
a t h e action of concentrated sulphuric acid on t h e
poly<styrene-co-divinylbenzene) beads in presence of
silver sulphate as catalyst. I t is b e t t e r t o pre-swell
t h e poly<styrene-co-divinylbenzene) beads in methylene
chloride t o reduce t h e possibility of disintegration of
t h e product. A f t e r t h e completion of t h e sulphonation
reaction, t h e sulphonated product remains suspended
in a fairly concentrated solution of sulphuric acid.
The removal of acid w a s done by slow dilution method.
The sulphonic acid group capacity was estimated by t h e
175 excess back t i t r a t i o n method suggested by Kunin .
The res in w a s found t o have a capacity of 5-5.3 -
mequiv of S 0 3 H group per gram. This corresponds t o a
Hz SO, SOCI,
Ag2SO' -
b
DMF
Scheme 11.1 P r e p a r a t i o n 6f polys ty rene s u p p o r t e d chloramine-T a n d bromamine-T
f unctionalisation of 92-98% of t h e para-position o f
benzene r ings i n t h e polymer. The IR spectrum of t h e
resul tant r e s in showed peaks a t 1410 cm1 <SO2 asym.)
and Ii70 cm-I <SO2 sytn.) which confirms t h e introduction
of S O H group i n t h e polymer. 3
Various methods of preparing
chlorosulphonated polystyrenes have been
reported 176-178. H e r e w e have t r i ed two methods f o r
t h e preparation of r e s in <a>. The first w a s t o use
thionyl chloride in presence of dimethylformamide
<Dm>. DMF can be used both as a ca ta lys t as w e l l
as a swelling solvent. Benzene w a s used as t h e
solvent in t h i s case. In t h e second method,
chlorosulphonic acid was used instead of thionyl
chloride and solvent used w a s chloroform. The procedure
involves t h e s t i r r i n g of t h e sulphonic acid r e s in <7> i n
chloroform under reflux f o r overnight. The t o t a l
chlorine in t h e product res in w a s est imated by
179 modified Volhard's method . The chlorine con ten t
w a s in t h e range 4.5 - 5.0 mequiv of chlorine pe r gram
of t h e res in corresponding t o 84 - 94% conversion.
The presence of some unconverted sulphonic acid
groups has some advantages. The sulphonyl
chloride group although reactive, is hydrophobic
and t h e presence of hydrophilic sulphonic acid group
might fac i l i ta te t h e f u r t h e r conversion of acid
chloride t o amide using ammonia. The IR spectrum of
t h e resin <8> showed peaks a t 1170 cm-I and 1370 cm-I
which are charac te r i s t i c of sulphonyl chlorides.
The sulphonamide res in <9> w a s obtained
from t h e r e s in (8) by t rea tment with concentrated -
aqueous ammonia solution.' Tetrahydrof w a n w a s used f o r
pre-swelling t h e resin. The conversion of acid
chloride t o amide w a s complete as evidenced by t h e
absence of any residual chlorine. The est imation of
nitrogen gave a value of 7.5% which corresponds t o
5.35 mequiv of -S02NH2 group/g of t h e resin. IR
spectrum showed character is t ic absorption band a t
1365 c c l .
The conversion of sulphonamide r e s in <9>
t o N- chloro-N-sodiopolysty~~enesulphonamide (10) and
N-bromo- N-sodiopolystyrenesulphonamide <ii> w e r e
a t ta ined by t r e a t m e n t with NaOCl and NaOBr solution.
These methods are analogous t o t h e preparat ion of
180 chloramine-T . The act ive halogen con ten t s w e r e
determined by iodometric t i t r a t ion . The
polystyrene-supported chloramine-T <lo> contained upto
5 mequiv of ac t ive chlorine/g of t h e res in whereas
t h e active bromine con ten t w a s upto 3.5 mequiv i n
t h e case o f r es in (11). The
chlorosulphonation and t h e conversion of t h e sulphonyl
chloride t o sulphonamide and then t o N-chlorinated
sulphonamides have been reported in t h e case of ion-
181 exchange resins by Nakamura . Such species have
\ -
17' Results o f also be,en used as bactericidates .
elemental analds of t h e N- chloro-N-sodio-and
N-bromo- N-sodiopolyst~enesulphonamide s are given i n
t h e table 11.1.
Table 11.1. A n a l y t i c a l details of resin <lo> and C i l >
........................................................ Resin % Sulphur % Nitrogen Active halogen
Polymeric b r o m i n e - T and chloramine-T . are completely insoluble in organic and aqueous
solvent systems. However they swell considerably i n
solvents like chloroforln and carbon
tetrachloride. The character isat ion of t h e samples
by NMR spec t ra l technique w a s impossible due t o t h e
insolubility of t h e samples in deuterated solvents.
The sulphonamide resin C 9 > did not undergo degradation
on t rea tment with NaOCl and NaOBr solutions.
The polymeric chloramine-T and bromamnine-T w e r e found
t o be s t ab le under ordinary laboratory conditions
and under hydrolytic conditions. S t i r r ing with w a t e r
f o r a period of upto 25 h did not show any
considerable reauction in capacity of t h e res in <Table
II.2>. The resins r e t a i n t h e bead form charac te r i s t i c
of t he styrene-divinylbenzene copolymers prepared by
suspension copolymerisation. They are easily f i l t e rab le
through s in tered g lass filters of medium porosity. The
res in can be s t o r e d f o r months without any appreciable
loss in capacity.
Table 11.2. Capacity of resin <ll) after s t i r r i n g in w a t e r
S t i r r ing Capacity of resin time<h> mequiv of Br/g ...................................
11.2. Syn the t i c Reactions us ing N-Chloro/N-Bromo-N-sodio-
polystyrel~esulphonamides
Chloramine-T and bromamine-T are w e l l
established synthet ic r eagen t s in preparative organic
cl emi is try 173'f74. They have been used f o r effect ing a
number of molecular t ransformations like oxidation
of alcohols q d a - halogenation of ketones. The
polyCN-halo-N-sodio> resins w e r e used here f o r t h e
oxidation of alcohols, a -halogenation of
ketones, N-halogenation of amides and also olefinic
addition react ions <Scheme II.2>.
a. Oxidation of Alcohols
N-Bromo-N-sodiopolystyrenesulphonamide r e s in
w a s found t o easily oxidise primary and secondary
alcohols under mild conditions t o t h e corresponding
carbonyl compounds in 90 - 100% yield. The procedure
f c ~ t h e oxidation involved s t i r r i n g of a suspension of
t h e res in with t h e s u b s t r a t e dissolved in su i table a
srplvent. ' The react ion w a s followed by th in layer
chromatography and when complete conversion w a s
ohserved by tlc, t h e res in w a s removed by f i l t r a t ion ,
w23shed with solvent and with w a t e r . The organic
layer was separated, dried over sodium sulphate and .
evaporated t o afford t h e oxidised product. The
product w a s f u r t h e r purified by crystal l isat ion
arld/or chromatography. The d i f ferent alcohols
oxidised, t h e yields and conditions of oxidation
are given i n table 11.3. The effect of
temperature, solvent, presence of acid catalyst and - -
varying duration w e r e investigated t o find ou t t h e
optimum conditions f o r t h e most ef fec t ive reaction. The
Table 11.3. Oxidat ion of alcohols using polymeric bromamine-T '%
........................................................ Alcohol Reaction Product Yield mp/<bp>
period<h> X OC ........................................................
i-Phenyl ethanol 24b ~ce tophenone 97 (200)
Benzoin gb Benzil 100 94
Benzhydrol qb Benzophenone 98 47
Cyclohexanol lea Cyclohexanone 83 (154)
11-Butyl alcohol iob n-Butyraldehyde 80 <74>
Benzyl alcohol db Benzaldeh yde 75 <ITS>
{Solvent: chloroform; 3-fold molar excess reagent>
a: react ions a t room tempera ture b: react ions a t refluxing temperature
oxidation of benzoin t o benzil with polymeric
bromamine-T. w a s followed by spectrophotometry. .
Chloroform w a s found t o be t h e be s t solvent and a
five-f old molar excess of t h e reagent gave 100% -
conversion within 2 h. Wetting of t h e r e s in with
dilute sulphuric acid catalysed all t h e oxidation
r6:actions. The react ion occurred most eff icient ly a t
t h e refluxing tempera ture of t h e solvent.
The oxidation react ions w e r e very sluggish
with N-chloro r e s in <lo> compared t o t h e N-bromo
res in <it>. Even t h e use of a large excess CS-fold>
of t h e res in yielded only 20% conversion even after 12
h i n t h e case of oxidation of benzoin. This is in
1i11e with t h e observat ions made in t h e case of
oxidation react ions using poly<N-bromoacrylamide>
and 30 poly<N-chloroacrylasnide> . But in t h e case
a
of the Po~~s ty rene-suppor ted analogues of t-butyl
hypochlorite and hypobromite resins, chloro r e s i n was
5'/ found t o be more ac t ive than bromo res in . Thus
i t appears t h a t t h e na tu re of t h e oxidising species
at.tached t o t h e polymer suppor t is t h e predominant
f atitor in deciding t h e react iv i ty under t he se cases.
T h e na ture of t h e polymer suppor t a f f ec t s largely t h e
ex ten t of all t h e polymer-analogous conversions. The
oxidation react ions using N-bromo-N-sodio r e s in w a s
f r e e from any side reactions. The carbonyl compounds
w c r e t h e only products in such cases. In t h e case of
alcohols which have a -hydrogen atoms, a - -halogenations w e r e observed a t elevated
temperature, only a f t e r complete conversion of t h e
alcohol t o carbonyl compounds.
The possibility of a free radical
n~cchanism w a s investigated. Free radical i n i t i a t o r s
were added and t h e durations f o r complete conversion
w c r e noted i n t h e case of benzoin t o benzil
conversion. Free radical i n i t i a t o r s have no effect
011 t h e t i m e required f o r complete conversion. This
sugges ts t h a t t h e react ion occurs by ionic mechanism
rd the r than a f r e e radical mechanism.
b, a -Halorenation of Ketones Olefinic Addition
The polymeric bromamine-T w a s found t o
convert ketones t o a -bromoketones and olefinic
cornpounds t o dibromo compounds in good yields. The
detai ls of t h e halogenation react ions are given in
table 11.4.
b e 11.4. Brominat ion reactions us ing N-bromo-N- sodiopolystyrenesulphonamide resin
- - -
~ompou&d Reaction Product Yield mp/<bp>
period<h> % OG
Acetophenone 1 4 ~ Phenacylbromide 93 48
Acetone eb ~romoacetone' 48 <135>
Ethylmethyl eb i-Bromo-2- butanone 50 (129) ketone
Cyclohexanone loa 2-Bromocyclohexanone 65 134
Styrene loa Styrenedibromide 80 52
trans-Stilbene Stilbenedibromide 72 240
<Solvent: chloroform; $-fold molar excess>
a: reaction a t room temperature.
,. b: reaction a t 1,efluxing temperature.
The procedure f o r bromination is t h e s a m e as
t l ~ a t f o r oxidations. In all these react ions chloroform
w a s used as t h e solvent and a three-fold excess o f
reagent w a s used. The resins w e r e made w e t with
2mL 2N sulphuric acid before use. The a-bromnination
reactions require elevated temperature while olefinic
addition react ions occur a t room temperature. The
ylelds w e r e 50-90% eventhough no t as high as in
tile case of oxidation reactions.
c N-Chlorination of Amides Usine: Polvmeric Chloramine-T
Benzamide, ace+.amide, phthalimide and
s~~ccinimide on t r ea tmen t with chloramine-T r e s i n
afforded t h e corresponding N-chloro derivatives i n 50
- 60% yield. The procedure f o r N-chlorination is t h e
same as in t h e case of oxidation and
a- brominations. A l l t h e reac t ions w e r e car r ied o u t
at room temperature. The yields w e r e less
compared t o a-halogenation and olefinic
brominations. N-Bromination of amides and imides
uxing N-bromo r e s in failed under t h e same
conditions. This w a s in con t r a s t t o t h e r e s u l t s
obtained i n t h e case of oxidation of alcohols t o
carbonyl compounds using t he se reagents. In t h e s e
react ions bromo r e s in w a s found to, be more reac t ive t h a n
chloro resin. The detai ls of t h e N-chlorination o f
arnides and imides are given in table 11.5.
d. Reactions of Unsaturated Alcohols with Polvmeric
Bromamine-T
Reactions w e r e carr ied ou t t o f ind o u t
wl~ether t h e r e is any select ivi ty in t h e react ion of
polymeric bromamine-T with s u b s t r a t e s containing
hydroxyl and unsatura ted functions.
Table 11.5. N-Chlorination of amides and imides us ing polymeric chloramine-T
Compound Reaction Product Yield mp Analytical da ta
Benzamlde 16 N-chloro 63 118 M a s s m/e 155<~+>
benzamide 157<M+2>,105 <C6 H5 GO>, 77
<C6 H5 >, 35, 37, 28
'H-NMR :6 7.5<58>
+ Phthallnlfde 14 N-chloro 58 185 Mass m/e : 181 <M >
phthalimide 183CM+2>, 84
N: 9.3%.
Succinlmide 12 N-chloro 65 143 M a s s m/e :133<M>
succinimide 135<M+2>, 84
acetamide
i. Cinnamyl alcohol
The N-bromo resin effectively oxidises
alcohols t o corresponding carbonyl compounds and
also a f f e c t s olefinic bromination. In t h e
~.t?action of cinnamyl alcohol with N-bromo resin, when
a two-fold molar excess of t he bromo resin was used,
the p r o d u c t obtained w a s 2,3-dibromo-3-phenyl
propanol. Yield: 51%. mp: 8 4 . ~ , IR: 3380 c m - ' < 0 ~ > , 'H - NMK<CDC13>: 6 7.4<S,5H a r o m a t i c pl.otons>, 4.3<d,2H>, '
4.7<d,lH>, 5.3<d,iH>; MS. d e 276<M-H20>.
Z1S<M-HBr), 77<C6Hg>; Anal. Calcd. for C H OBr2: 9 10
C: 36.74%, H: 3.4%. Found: C: 36.92%. H: 3.82%.
Bi l t t h e use of a f o u r - f o l d excess of resin
r e s u l t e d in both olefinic a d d i t i o n as wel l as the
o x i d a t i o n of alcohols t o c a r b o n y l groups. T h u s
23 -d ib romo-3-pheny l propanal was isolated in 81%
yield. IR: 1690 c m -i <-CHO>; 'H - NMR<CDC13>: 6
9.3<d,lH,-CHO>, 7.4<s,SH, a r o m a t i c protons>, 4.2<d,IH>,
4.7<t,iH>; MS. d e 21ICM-HBr>, 193, 171<C6H5CHBr>,
7'/<C6H51 Anal. Calcd. for C H OBr2; 9 8 C: 36.98%. H:
2.7%; Found C: 37.5496, H: 2.9%. 4
O n e a d v a n t a g e of using t h i s b r o m o resin is
t h a t the required product can be obtained in better
p u l - i t y and yield. W i t h b r o m i n e w a t e r or l o w - m o l e c u l a r
bromamine-T as the reagent, t h e product is a m i x t u r e of
2 3 - d i b r o m o - 3 - p h e n y l propanol, 2,3-dibromo-3-phenyl
propanal a n d 2,3-dibromo-3-phenyl propionic a c i d . T h e
preferential reaction could arise f r o m the s l o w reaction
rate.
i i . Dibenzosuberenol
Dibenzosuberenol contains secondary alcoholic \ -
group and a double bond. Same product w a s obtained when
both equimolar and large excess of reagent was used.
The detailed product analysis revealed t h a t both
oxidation and bromination of double bond have taken
place. The product obtained w a s dibenzosuberenone
dibromfde i n 31% yield with equimolar reagent and t h e
yield was 94% when four-fold excess w a s used. mp:
IIW *c. IRCKBr): 3050, 1640, 1590 -1 lH cm ;
NMR<CDCIS>: 6 5.6Cs,2H>, 7.28<m, 6H>, 7.6B<m, 2H>; MS.
ni/e 366<M>, 368<M+2>, 285CM-HBr>, 206CBase peak).
iii. 3- Hexyne-2,s- diol
3-Hexyne-2,s-diol contains a t r ip le bond
arsd two secondary alcoholic groups. The product
oblained using polymeric bromamine-T reagent w a s
dibromo-3-hexene-2,s- diol . The product was obtained
1x1 3 2 % yield when equimolar reagent w a s used. mp:
208 OC; IH - NMR<DMSO>: 6 : SCq, 2H>, 1.3<d, 6H>; MS. \ -
n ~ / e : 2 7 4 < M > , 276<M+2>, 256<M- H20>; Anal. Calcd. f o r
C H 0 B r C: 26.195, H:3.3%; Found: C: 26.295, H: 3.6%. 6 1 0 2 2
11.3. Ef f ec t o f React ion Conditions on the C o u r s e of
Oxidation React ions Using N-Bromo-N-sodio-
yolystyrenesulphonamide R e s i n <It>
Polymeric reaction takes place smoothly only
wl~en t h e r e is an effect ive interact ion between t h e
reagent function a t tached t o t h e macromolecular
matrix, t h e s u b s t r a t e and t h e react ion medium. This i n
t u r n depends on t h e nature of t h e functional
CI'OUP, degree of f unctionalisation, solvent and 4
temperatune. Influence of t h e concentration of t h e
reagent function, solvent, temperature and crosslink
density on react iv i ty of N-bromo res in w a s studied
using t h e oxidation of benzoin t o benzil as a model
rt-action. For th is , s tandard solutions of
benzoin and benzil of d i f ferent concentrations
were prepared in chloroform and absorbance w e r e
measured a t 390 run. A calibration curve w a s drawn by
plotting absorbance against concentration. The
reactions w e r e done by varying t h e conditions o r
parameters, under investigation. The ex ten t of r eac t ion
was followed by determining t h e concentration of benzil , -
at different intervals: The react ivi ty w a s found t o
depend very much upon t h e conditions. In polymeric
reactions, even if t h e bulk concentration o f t h e
react ive function is low, t h e ef fec t ive local
concentration w i l l be high enough t o cause s u b s t r a t e
conversions.
a. Effect of Concentration of Reagent Function
The effect of concentration of reagen t
function on reac t iv i ty of N-bromo res in i n t h e
oxidation of benzoin t o benzil w a s s tudied by
conducting t h e reaction a t d i f fe ren t
reagent-to-substrake ra t ios . Molar r a t i o s used w e r e
15, 21, 3:1, 4:1, 5:i and 6:l. The e x t e n t s of
reaction under t h e s e conditions are given i n table
11.6. A l l reac t ions w e r e done in chloroform solvent a t
refluxing temperature. The maximum conversion obtained
when molar equivalent of t h e reagent used was 33%. The
reaction does no t go t o completion even if t h e duration
of reaction is extended. The percentage of benzil w a s
doubled after 5 h, when t h e r a t i o (subs t ra te : reagent>
was changed f rom 1:2 t o 1:3. 100% Conversion w a s
Table 11.6. Effec t of concen t r a t i on o f r e a g e n t function on the r e a c t i v i t y of N-bromo resin -
Reagent: % of benzil formed after Subs t ra te
1 h 2 h 3 h 4 h 5 h
-
obtained after 5 h when t h e r a t i o w a s 1:3 while only 55%
conversion w a s obtained i n t h e case of 1:Z molar ra t io .
The duration required f o r 100% conversion w a s reduced t o
2 h from 5 h when t h e molar r a t i o w a s increased from 1:3
t o 1:s. A f u r t h e r increase in molar concentration has
not much effect on t h e react iv i ty as is evident from t h e
I-esults in table 11.6. The changes are represented
graphically in figure. 11.1.
Fig. 11. I. Effec t of molar excess o f reagent on reac t iv i ty of polymeric bromamine-T (benzoin to benzil conversion)
The reaction was carried out at 30, 40, 50
and 6 0 ' ~ . Only 36% conversion was observed after 4 h
a t room tempel.ature whereas there w a s iOO%
Table 11.7. Effect of tempera ture on t h e r e a c t i v i t y of N-bromo r e s i n
Temperature % Conversion a f t e r t i m e
c I h 2 h 3 h 4 h
30 15 22 30 36
<Solvent: chloroform; 3-fold molar excess reagent)
conversion a t 6 0 * ~ during t h e same period. The rate
of reaction almost doubled when the temperature w a s
ir~creased from 50 t o 60 OC. (Table 11.7. * The
temperature a f f ec t s not only t h e r a t e of reaction but
also t h e product of reaction. In the case of
oxidation of a-phenyl ethanol using polymeric
bromamine-T t h e product obtained w a s
acetophenone at room temperature. But a t refluxing
temperature t h e a-bromination reaction also occurred
giving phenacyl bromide. The dependence of t h e rate of
reaction on the reaction temperature is represented
graphically in figure 11.2.
Fig. 11. 2. E f f e c t of temperature on reactivity of polymeric bromamine-T (benzoin to benzil conversion)
c. Concentration of Acid
The reaction does not take place without
catalysing by acid. But the concentration of acid
uscd did not influence the conversion rate much. The
results are given in table 11.8.
'I'able 11.8. E f f e c t of concentration of ac id on the ox ida t ion of benzoin t o benzi l
Concentration % conversion after
of acid used<N> - 1 h 2 h 3 h 4 h
(Solvent: chloroform; 3-fold molar excess reagent)
d. Solvent
, The react ions using crosslinked polymeric
reagents are heterogenous i n nature, taking place i n two
dist inct phases. If t h e reaction has t o take
pl-ce, s t rong in te rac t ions with t h e s u b s t r a t e i n
solution and t h e solid polymeric reagent must occur.
The compatibility of t h e two phases is an important
f a c to r favouring t h e reactions. Eventhough t h e
reagent appears t o be a solid, insoluble i n
solvents, i t expands i n 'good' solvents and behaves
Table 11. 9. Effect of solvent on the reactivity of N- bromo resin on oxidation of benzoin to benzil
-
Solvent % conversion after t i m e
Dioxan 12 26 39 52
Acetonitrile 17 33 48 61
Benzene 15 30 45 58
TtlF 10 25 38 50
Cyclohexane 42 66 74 83
Chloroform 54 70 83 100
a- a t r u e gel. This makes t h e functional groups
attached t o t h e polymer exposed t o t h e organic phase.
T h u s solvents which are capable of swelling t h e
polymer network are found t o be sui table f o r carrying
out t h e oxidation reaction. The solvents used are
benzene, tetrahydrofuran, cyclohexane, ace toni t r i le
and chloroform. Among these solvents, chloroform
wa- found t o be t h e most sui table solvent
CTable.II.9).
chlorciform r cyclohexane i acetonitrile 6 benzene
1 2 3
Time ( hl
Fig. 11. 3. E f f e c t of s o l v e n t on r e a c t i v i t y of p o l y m e r i c bromamine-T (benzoin to benzil conversion)
e. Crosslink Density
The ease of chemical modification o f a
resin and the level of success in its subsequent
application as a reagent depend substantially on the
physical p roper t i e s of t h e res in itself. A
f unctionalised polymeric suppor t must - possess a
s t r u c t u r e which permits adequate diffusion of
t h e reagen t i n t o react ive sites, a phenomenon
which depends on t h e ex ten t of swelling o r solvation,
Table 11.10. P r e p a r a t i o n of styrene-co-divinyllenze~~e polymers o f d i f f e r e n t cross l ink d e n s i t y
Crosslink Styrene Divinyl- Yield Yield density<%> <g> benzene<g> 6 %
t he e f fec t ive pore size and pore volume and t h e
chemical and mechan ica l s tabi l i ty of t h e resins
under t h e react ion conditions. These in t u r n depend
upon t h e degree of crosslinking of t h e resins and t h e
conditions employed f o r t h e preparation of t h e resin.
Polystyrene-supported N-bromo r e s in s w e r e prepared with
3, 6, 10, 15 and 20% DVB-crosslinkili6. by suspension
copolymerisation of s ty rene and divinylbenzene a t
appropriate concentrations. The detai ls of t h e - - -
preparat ion of t h e various differently crosslinked
polystyrene resins are given in table 11. 10.
i. Extent of Functionalisation
The polyCstyrene-co-divinylbenzene) samples
of d i f fe ren t crosslink densities w e r e functionalised
t o give polymeric bromamine-T. The e x t e n t
of functionalisation was studied i n each s t e p
u~rder identical conditions and i t w a s found t o decrease
a- t h e crosslinking increased, in all t h e cases.
Table 11. 11. E f f e c t o f cross l ink d e n s i t y on extent of f unc t iona l l sa t ion
-- - -
DYB -SO H capacity 3 -S02CI
N-Bromo res in
n~ole % rnequi v/g n~equiv/g Active bronline
1x1 the case of sulphonation a capacity of 5.1 mequiv of
-S03H/g of the resin w a s obtained using 3% crosslinked - resin while under the same conditions the capacity w a s
decreased t o 0.81 mequiv/g in the case of the 20%
crosslinked resin <Table II.Il>. Similar trends w e r e
crosslink density
Fig. 11. 4. E f f e c t of crosslink density on e x t e n t of f u~rctionalisation
observed in subsequent transformations also. Thus t h e
c~ipaci ty of 3 meqdv - of bromine/g of t h e res in i n t h e
case of 3% DVB-crosslinked polystyl-ene decreased t o 0.4
ntequiv/g CNg.II.4).
ii. Extent Elf: Reaction
Conversions of benzoin t o benzil by
differently crosslinked N-bromo resins w e r e
followed in f ive solvents. In all t h e react ions,
a three-fold molar excess of t h e reagent w a s used.
The solvents w e r e chloroform, benzene, cyclohexane,
te trahydrofuran and acetonitrile. In all t h e solvents
t h e react iv i ty w a s found t o decrease as t h e
ex ten t of crosslinking increased. The variat ion of
t h e relat ive react iv i ty of t h e N-bromo resins in
different solvent s y s t e m s as a function of degree
of crosslinking is depicted' i n f igures 11.5-9.
I n t h e various solvent systems studied, t h e
behavioural p a t t e r n s of t h e differently crosslinked
N-bromo res ins w e r e almost s i m i l a r . For t h e oxidation
react ion of benzoin t o benzil chloroform w a s t h e b e s t
solvent. Chloroform, comparably polar, w a s able t o
s w e l l t h e res in matrix and i t could efficiently
pt-metrate through t h e pore s t r u c t u r e of t h e r e s in s o
t l a a t t h e reac t ive sites could be made accessible t o t h e
s u b s t r a t e s d make t h e react ion easy. This w t h e , - case even a t high degree of crosslinking. Thus with
chloroform as t h e solvent, a comparably good value of
30% conversion w a s achieved after 4 h using t h e
20% crosslinked res in as against t h e almost - complete conversion using 3% crosslinked r e s i n
(Figure 11.5). From t h e f igure it is seen t h a t more
than 50% conversion was achieved f o r 3% crosslinked
res in after I h. A s t h e crosslink density increases,
t h e react ivi ty drastically decreases. The percentage
of benzil obtained w a s only 10% f o r t h e 20% crosslinked
res in after 1 h.
When t h e solvent was changed t o benzene, a A
highly non-polar solvent, t h e relat ive r eac t i v i t y
decreased considerably <Figure 11.6). T h i s was t r u e
both f o r 3 and 20% resins. When a 3% crosslinked r e s i n
w a s used here, only 58% conversion w a s obtained even
after 4 h of reaction. For 20% crosslinked r e s in t h e
percentage conversion w a s only 20.
With cyclohexane as t h e solvent, a
fairly good conversion rate w a s obtained, althogh n o t
high as in t h e case of chloroform (Figure 11.7). 42%
conversion is noticed after 1 h with 3% crosslinked
~.esin. But t h e ex t en t of conversion w a s negligible \ -
- with 20% res in during t h e same period. A f t e r 4 h
with 3% resin, about 83% conversion w a s achieved, w h i l e
only 17% conversion w a s observed i n t h e case of 20%
cs.osslinked resin.
Time ( h l
Fig. 11.5. Ef fec t of c ross l fnk d e n s i t y o n r e a c t i v i t y of polymeric bromamine-T (benzoin to benzil convers ion f n chloroform>
When acetoni t r i le and THF w e r e used as
solvents t h e reac t iv i ty w a s f u r t h e r decreased (Figure - 11.8 % 11.9). Only negligible amount of product w a s
formed during t h e ini t ia l s t a g e s of t h e react ion
both f o r 3 and 20% resins. 50-6095 conversions
w e r e observed in t he se solvents using 3% crosslinked
. 3 O/o DVB A 6 s J I
1 10 2 1 l*
& 1 5 2 * "
X 2 0 " "
Fig. 11.6. E f f e c t o f c ross l ink dens i t y on r e a c t i v i t y of pelynleric Lromamine-T (benzoin t o benz i l cunve~.sion in benzene)
resin after 4 h. For the 20% crosslinked resin
percentage conversion remained below 10% even after 4 h. -
r o o t * 3 % D V B A 6 99
C .- 0 N C
$ 60- - 0
C 0 .- Y? L O . a, > C 0 U
20.
Time (h)
Pig. 11.7. Effect of crosslink density on react ivi ty of polymeric bromamine-T (benzoin to benzil conversion in cyclohexane)
From the foregoing discussion i t is
seen that chloroform exhibited the good qualities of a
s~, lvent for the polyme~c system both at low and high
ex ten t of crosslinking. A s t h e degree of crosslinking
irmreases, eventhough, t h e non-polar charac ter of -
t h e polymer matrix increases, a highly non-polar
solvent like benzene w a s n o t suitable f o r carrying
ou t t h e reaction. A highly polar solvent like THF o r
acetoni tr i le failed t o give good resul ts . Thus with
Fig. 11.13. E f f e c t of c ro s s l i nk dens i t y on r e a c t i v i t y of polymeric bromamine-T <benzoin t o benzi l convers ion i n acetonitrile)
100
80
- 3% DVB
A 6 O/O 9*
- 1 10% 71
Time ( h )
4
Fig. 11.9. E f f e c t of c ross l ink d e n s i t y on r e a c t i v i t y o f polymeric bromamine-T (benzoin t o benz i l colaversion in THF>
polymeric bromamine-T, t h e non-polar character of t h e
polystyz*ene matrix showed predominance over t h e
polar react ive function in directing t h e react ion
a t low level of crosslinking. A t higher degrees of
crosslinking a balance between t h e non-polar na tu r e
of t h e polymer-backbone and t h e ionic charac te r of
t h e react ive function has been achieved paving t h e - way f o r chloroform t o act as a b e t t e r solvent.
If. 4. Recyclabili ty of the Spen t Res ins
One major consideration in t h e use o f t h e
polymer-supported reagents is t h e possibility of
recycling and r euse of t h e spen t reagents. The
spen t polymeric reagen t s from t h e oxidation o r
halogenation s t e p can be regenerated i n a single
sLcp without any loss of act ivi ty f o r subsequent
Table 11.12. Regenera t ion of the N-bromo resin
- Number of capacity cycles Cmequiv/g>
reactions. For this, t h e spent resins obtained from
differ'ent reac t ions w e r e collected toge ther and washed
with t h e solvent used t o r'emove any residual organic
s u b s t r a t e o r product. The washed polymer w a s
t r e a t e d with sodium hypobr'omite solution as described
in t h e original preparation of N-bromo-N-
sodiop~lystyrenesulphonamide. The r e s u l t s are given i n
table 11.12. The res in r'etafned t h e bead form and
f i lterabflity and t h e swelling character is t ics upto 5
cycles under t h e s e recycling conditions.
11.5 Prepa ra t i on of N-Benzyl, N-Ethyl a n d N-Methyl
S u b s t i t u t e d N-Bromopolystyrenesulphonamide R e s i n s
and S y n t h e t i c Transformat ions U s i n g These
R e a g e n t s - A Comparative Study
N-Benzyl, N-methyl and N-ethyl polystyrene-
sulphonamides w e r e prepared from polystyrenesulphonyl
chloride r e s in s <8> by t rea tment with appropr ia te
primary amines, iq benzylamine, m e t h y h i n e and
ethylamine <Scheme II.3>.
These sulphonamide res ins w e r e converted
t o t h e corresponding N-bromo res ins by t r ea tmen t with
sodium hypobromite solution <Scheme 11.4). The ac t ive
halogen contents w e r e estimated by iodometric t i t r a t i o n
as in t h e case of polymeric brornamine-T resin. The \
capacity obtained' w a s maximum in t h e case of methyl
subst i tu ted res in (15) C3.01 mequiv/gl. The
bromine capacity obtained in t h e case of ethyl
subst i tu ted res in (16) w a s 2.8 mequiv/g of res in while
t h a t in t h e case of benzyl res in (173 w a s 2.5 mequiv/g
of resin.
Scheme 11. 3. P r e p a r a t i o n of N-subst i tuted polys tyrene- sulphonamide resins
Synthetic transformations w e r e carr ied o u t
using t he se reagents . These res ins also need acid as
catalyst f o r oxidation reactions. The procedure f o r
oxidation is also t h e same as that f o r t h e
N-bromo-N-sodio resin. The reaction efficiency of - these reagents w a s compared with t h a t of res in (11).
For t h i s purpose oxidation of benzoin and benzhydrol
were carried out. The react ions w e r e done i n
chloroform, using a three-fold molar excess of t h e
reagent. The react ions w e r e followed by t l c at half
an hour intervals of t i m e . The resu l t s are given in
table II.i3.
Scheme 11. 4. P repa ra t i on of N-substituted-N-bromo resins
I t can be seen from t h e table t h a t t h e
oxidation efficiency in terms of t i m e f o r complete
conversion is reduced on t h e introduction of methyl,
ethyl and benzyl subst i tuents . The decrease is
n~ilximum in t h e case of benzyl subst i tut ion where
t h e t i m e f o r complete conversion of benzoin t o
henzil is increased from 6 h t o 9 h and from 4 h t o 6 h
r t h e case of conversion of benzhydrol t o - b ~ z o ~ g e n o n e . The yield obtained is no t a f fec ted
Table 11. 13. Comparison of ox ida t ion efficiencies of mv-.thyl. e t h y l a n d benzyl s u b s t i t u t e d P e s i n s wi th polymeric bromamine-T
Alcohol Product -
Reaction Yield t i r n e <h> %
<a> with polymeric bromamine-T -- -
Benzoin B e n z i l 6.0
Bcnzhydrol Benzophenone 4.0
<b> with N-metha subs t i tu ted resin --
Benzoin Benz i l d
Bcrlzhydrol Benzophenone 5.0
<c> with N-etha subs t i tu ted resin ---
Bcrlzoin Benzil
Benzhydrol Benzophenone
<d> with N-benzvl subs t i tu ted resin --
Benzoin Benzil 9.0 94
Benzhydrol Benzophenone 6.0 93
appreciably by t he se subst i tut ions. Methyl and ethyl
subs t i tu ted reagen t s are more ef f ic ient than benzyl
' subst i tu ted reagent eventhough t h e diffe1;ence in , t h e
I-eactivity between t h e s e resins is no t grea t .
The course of oxidation react ions,
using different N-bromo resins, of a-phenyl
e than01 in dichloromethane w a s followed
spectrophotometricaIly. The r e s u l t s are given i n
table 11.14. The observations obtained i n t h i s
c&-e are also consis tent with t h a t in t h e case of
benzoin and benzhydrol. The conversion percentage
obtained within a given period of t i m e is maximum in
Table 11.14. Conversion pe rcen t age o f a-phenyl ethanol us ing d i f f e r e n t N-bromo resins
. T~me in minutes
Resin
N-sodio res in <ti> 15 28 39 51 63 76
N-methyl res in <IS> 9 11 22 32 35 38
N-ethyl res in <16> 9 11 19 25 30 33
N-benzyl res in <IT> 6 10 16 21 26 30
. (Solvent: dichloromethane; 3-fold molar excess reagent)
t he case of N-sodio resin while t h a t is minimum in
t h e case of N-benzyl substituted resins. -
11. 6. Comparison of Polystyrene-Supported Bromamine-T
w i t h Other Polymer-Supported Halogen Containing
Reagents *
Many halogen containing polymeric reagents
have been reported f o r oxidation and halogenation
reactions. These include polystyrene-supported
hypohalite reagents 57 <18>, poly N-haloacrylamide 62
<19), N-bromoacetamido polystyrene 62 (20) and
polyvinylpyrrolldone-bromfne complexes84 (21).
NHX
N-Halo-N-sodiopolystyrenesulphonamide
reagen t s w e r e found t o have g r e a t e r capacity (3-5 - nlequiv/g> than , supported hypohalite
r eagen t s C 1 8 > C2-3 mequiv/g>. Polystyrene-supported
hypohalite r e s in s w e r e prepared from suppol-ted
t-alcohol res ins by t rea tment with hypohalite
solutions. The hypohalite reagents w e r e used f o r
oxidation of alcohols, N-halogenation of amides and
a-halogenation of ketones. In all these cases
hypochlorite r e s in w a s found t o be more react ive than
hypobromite resins. In t h e case of N-halo-N-sodio
polystyrenesulphonaddes, N-bromo res in is more
react ive than N-chloro resin. The react ions using
polystyrene supported hypohalites w e r e no t a f fec ted
by t h e presence o f catalytic amount of acid.
Polystyrene- supported hypohalite reagents are very
much less reac t ive than t h e supported bromamine-T.
For example, a three-f old molar excess of
hypochlorite r es in i n chloroform solvent requires
27 h f o r complete conversion for t h e oxidation of
benzoin t o benzil while t h e t i m e required is 6 h
fo r t h e N-bromo-N-sodiopolystyrenesulphonan~ide
resin. Similar observations have been made in t h e
cctse of N-halogenation of amides and a-halogenation of
carbonyl compounds.
N- Halopolyacrylamide <19> crosslinked
wiLh divinylbenzene is another important class of
polymeric oxidising reagents. ,They are obtained
by t h e hypohalite t r e a tmen t of
DVR-crosslinked polyacrylamides. The capacity obtained
is much higher than polymeric bromamine-T <7
nrcquiv/g> . But one s tr iking similarity of t h i s
reagent with t h e reagent repor ted in this t h e s i s is
t h e higher reac t iv i ty of N- bromopolyacrylamide than
N-chlompolyacrylamide. The N-bromopolyacrylamide s
w e r e found t o convert primary and secondary alcohols t o
corresponding carbonyl compounds, olefins t o dibromo
derivat ives and carbonyl compounds t o the i r a-bromo
derivatives in high yields. A t t h e s a m e t i m e
reac t ions using N-chloro polyacrylamide are sluggish
anad t h e yields repor ted a%e very poor. The
react iv i ty of N-bromopolyacrylamide is no t much
d i f fe ren t from t h a t of polymeric brotnamine-T. The time
required f o r t h e complete conversion of bemoin t o
benzil is 5 h when a thee- fo ld molar excess o f t h e
reagent i n chloroform w a s used. The N- halo
pnlyacrylamides are repor ted t o be incapable of N-
halogenation of amides. Eventhough polymeric bromamine-
T is n o t ef fec t ive in N-halogenation reaction,
pulymeric chloramine-T converts amides t o t h e i r N-
clrloro derivatives. In t h e case of polymer-supported
hypohalite r eagen t s both hypochlorite and hypobromite
resins are eff ic ient3 in t h e ,, of N-halo
derivatives from t h e i r respective amides.
N-Bromoacetamido polystyrenes w e r e
prepared and t h e i r reac t iv i ty w a s compared with t h a t of
N-bromopolyacrylamide- Eventhough t h e reac t ive
species are t h e same, t h e react iv i ty of
N-bromoacetamido polystyrene reagent is very much less
than t h a t of polyacrylamide derivative. T h i s has been
ascribed t o t h e difference in t h e polar na tu re of t h e
polymer matrix used as support. The act ive halogen
capacity is also found t o be less 0 mequiv/g>.
The react iv i ty of N-bromoacetamido reagent is very
much less than t h a t of, polymeric bromamine-T.
Polyvinylpyrrolidone-bromine complexes C21>
have been prepared by t h e bromination
of polyvinylpyrrolidone. DVB-crosslinked polyvinyl-
pyrrolidone-bromine complexes are less react ive than
polymeric bromamine-T. T h i s reagent requires wetting
with w a t e r f o r t h e oxidation react ions t o t ake place.
This is eff ic ient i n oxidation of alcohols and double
bond bromine addition but not f o r N-b~omnination of
ketones o r N-brominatfon of amides.
The above-mentioned investigations reveal
t h a t t h e polymeric bromamine-T fulfills t h e
requirements of an eff icient polymeric solid-phase
reagent f o r t h e oxidation and brornination of
organic subs t r a t e s . The reagent has t h e advantages
of increased shelf-life, operational simplicity,
possibility of 1-egeneration and re-use. The polymeric
chloramine-T can be used f o r t h e N-chlorination of
atnides and imides. The react ivi ty of t he se N- halo
res ins can be changed by introducing I N-aUcyl
subst i tuents . The react ivi ty and efficiency of
polymeric bromamine-T is b e t t e r than many of t h e
repor ted polymeric oxidising and halogenating reagents .