II. SOLID-PHASE POLYMERIC ANALOGUES OF CHLORAMINE-T AND BROMAMINE-T: PREPARATION AND USE AS SYNTHETIC - REAGENTS The diverse nature of the chemistry of N-halogeno-N-metallo' reagents is a consequence of their ability to act as a source of <a> halonfum cations, <b> hypohalite species, <c> N-anions which act as bases and nucleophiles and <d> nitrinoids in limited cases. As a result, these reagents react with a surprising range of functional groups effecting an array of molecular transformations 173,174 Historically the important early developments in the area stemmed from the synthesis of chloramine-T and related aryl sulphonamide derivatives. The present section deals with the preparation and synthetic applications of polystyrene-supported analogues of chloramine-T and bromamine-T <N-chloroA-bromo-N-sodiopolystyrene- sulphonamides> which have proved their efficiency as conventional low-molecular oxidising and halogenating reagents. The section also describes the comparison of the new reagent with other reported polymeric reagents and the analogous low-molecular weight
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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
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