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 .