- - - -- - - - Reduction Studies Using the Polymer

Bound EDA-borane Rearrent

Borane reagents such a . diborane, borane-methyl sulfide, borane-THF and

amine-borane reagents have prominent role in synthetic organic The

stability and selecrivity of'amine-borane reagents are well pronounced. In temls of

reducing action. the? range between the electrophilic diborane and nucleophilic

sodium borohydride. I'he primary and secondary amine-borane reagents have been

shown to be mild. efficient, versatile, and reactive reducing agents for aldehydes and

ketones. l;vetr though tertiary amine-borane reagents such as trimethyl amine-borane

and pyridine-horane were reported to be less reactive, the 1' and 2' arnine-borane

reagents werc iounti rt1 reduce aldehydes and ketones rapidly and in good yields'ii.

T'hc reductio~l 01. hcnzaldehyde with t-butylamine-borane reagents in CHCI{ at O'C

\vithin I O rnir, has already been reported2".

I'he IIX 01' airrino alcohol-boranc reagent in synthetic organic chemistry

ir iilready i\~.ll ~st;~bli ihed"~. The amino alcohol-borane reagent reduces

Reduction Studies

benzaidehycle to be~iryl alcohol within 30 min at room temperature72. When

anchored to a pol!n~rr support, the stability and selectivity of the borane

reagents were increascd P<~lyvinyl pyridine-borane232, polypropylene sulfide-

horane'" and polymer supported amino alcohol-borane reagents7* were reported

to have chemoselectivity in the reduction of aldehydes. Easy reaction work up

and less contamination of the product are advantages of reactions using these

reagents. We have prepared ethylenediamine-borane reagent on crosslinked

polystyrene resins and investigated its utility as a reducing agent.

4.1 Reduction Studies Using Polymer Bound EDA-borane

4.1.a Reduction of aldehydes

l h e reductioti reactions with the polymer bound ethylenediarnine-borane

reagents were carried out at room temperature. Using this polymeric reagent

henzaldehyde was reduced to henzyl alcohol. The reduction of benzaldehyde

was donc with 2 molar excess of the polymeric reagent. The suspension of the

polymer hound reagent (le) in DCM was shaken with benzaldehyde. The extent of

the reactlon was hllo\ved by I'LC. Complete conversion of henzaldehyde in the

reaction mixture w a i ohsetl;cd after 10 h. No product formation in the reaction

mixture was obscr\cd Ille IR and UV spectra of the reaction mixture showed

colnplct~. conversion o i hcnyaldehyde. Product formation on polynler beads as

borate cyter was reported in the case of polyvinylpyridine-borane reagent.

Sollcni \\as cvap~l-ntcd from the reaction mixture and the resin after swelling in

I I l l . n,i. stlaket~ i \ ~ t t i 1b1 IICI. The product irr the filtrate was extracted with

Reduction Studies

ethcl r11.1eti over sodium sulfate and the solvent was evaporated. The product

l~er~/!l ,~lcohol obtained was confirmed by TLC, IR and IJV spectroscopic

~ i e 1 1 1 . Renraldehydes having various substituents were reduced with the

pol?nicr~c EDA-borane reagents derived from DVB-PS. BDDMA-PS and

HI>OI) \-PS rehlns. The results are summarized in Table 4.l.a, 4.1.b and 4.l.c.

Tahle 4.la Reduction o f aldehydes using polymer bound EDA-borane ( le) derlved from DVB-PS resin

l r l - -

I !

No. , Substrate

~ ~~ ~ --

llenzaldehyde -~

2-nitrobenzaldehyde ~

.i-cyanobenzaldehyde

1 Cinnarnyl alcohol 1 18.0 1 88.5 I

Time m for Yield of

I 11 methoxqbenzaldehyde I p-rnethoxybenzyl alcohol I I S 5

Tahle J.lb. Reduction of aldehydes using polymer bound EDA-borane (2e1) derrbed from HDODA-PS resin

- -- I - -

!

Product

Benzyl alcohol

2-nitrobenzyl alcohol

4-cyanobenzyl alcohol

90

1 / Time for I Yield of /

complete reaction

(h ) 10

5.5

6.5

I I3en~dldehqde Benzyl alcohol - - 6 1 9 0 1

isolated product

( % )

88 pp

97

97

I

No. , Substrate Product

i i.lilorobenzaIdehyde ~ - --

1 iittrcibe~~raldchyde . ~~ ~- p~

1~ii1etlioxy henraldrhyde -~ ~ -

2 1 1 itrobenzaidehyde - ~~ I

'.il~ci laldehydc

I ( ~ ~ ~ t i a ~ ~ ~ a l d c l i y l e -

1 ~>; \~~oben / ;~ l ( l eh)~ le - - .. - -

- - - (h)

4-Chlorobenzyl alcohol

4-nitrobenzyl alcohol

4-methoxy benzyl alcohol

2-nitrobenzyl alcohol 99

Salcyl alcohol 5 96 -~ -

Cinna~nyl alcohol 12 95 -- - - -.

4-cyanobenzyl alcoliol ..

99 -~ - - -

(Yo)

complete reaction

isolated product

Reduction Studies

Table 4.1~: Reduction of aldehydes using polymer bound EDA-borane derived from BDDMA-PS resin (resin 3e)

r - Time for complete Yield of isolated Substrate reaction aroduct

4.1.b Reduction of ketones

The reduction of acetophenone was also done with the polymer bound EDA-

borane reagents. The reaction was found to be very slow as compared to the

reduction of benzaldehyde. Alter 20 h 30% conversion was obtained. In the reduction

of Cnitroacetophenone also the rate was slow. 50% conversion was obtained after

20 h. In the reduction of benzophenone only 10% conversion was observed even

after 50 h. We also attempted the reduction of bend with the polymer bound borane

reagent. The reaction was very slow and very low conversions were obtained.

4.2 Mechanism of Reduction Using Polymer Bound EDA-borane

Several rnechanjs~ns were suggested for the carbonyl reductions involving

the amine-borane reagents, either a direct attack of the amine-borane or prior

'5-1.258 dissociation to free BH3- . The two possible mechanisms suggested by Brown

and Murray were given below.

Reduction Studies

1 Direct attack

2 Prior dissociation

One suggestion is the formation of an adduct of amine-borane with the

carbonyl compound and the other possibility is the decomposition of amine-

borane to free BH3 and its combination with the carbonyl compound. The

possible mechanism suggested in the case of reduction with polymer bound

EDA-borane is the initial formation of an unstable intermediate of the carbonyl

group with the mine-borane and simultaneous hydride transfer, giving the product

alcohol. The reaction path suggested in this case is as given in Scheme 4.1.

RCHO ~ H ~ . B H ~ ---+ WH~.BH~.OCHZR

Scheme 4.1: Mechanism of reduction ofaldehyde with polymer bound amine-borane

I he possibility of prior dissociation of amine-borane moiety was not

supported by the observations in the reduction of benzaldehyde. The polymeric

EDA-borane was reacted with excess benzaldehyde for 10 h. After the reaction

Reduction Studies

the IR spectrum of the polymer beads showed the B-N stretching band at

1150 crn ' . From this observation it was well evident that prior dissociation of

amine-borane reagents was not happened in this case. The product benzyl

alcohol was obtained only after acid hydrolysis of borate ester bound on polymer

beads. ln the case of substituted aldehydes some of the product was formed in the

reaction mixture prior to hydrolysis. In this case the product formation in the reaction

mixture was observed by TLC and W spectrophotometric analysis. This may be due

to the instability of the borate intermediate offered by the steric hindrance of the

substituents, which facilitated the cleavage of the alcohol from the polymeric reagent.

The reduction of substituted aldehydes with polymer bound EDA-borane

reagent was also carried out. Aromatic carbonyl compounds with electron

donating and electron releasing substituents were reduced with the polymeric

reagent. Substrates with electron withdrawing substituents were found to be reduced

at faster rates. In the reduction of nitrobenzaldehyde and cyanobenzaldehyde

increased reaction rates were observed. The electron withdrawing groups reduce the

electron density at the carbonyl carbon and facilitated the hydride transfer to the

carbonyl carbon. In the case of p-methoxybenzaldehyde, decreased reaction rate

was observed as compared to that of benzaldehyde. It can be attributed to the

presence of electron releasing methoxy group which increases the electron density at

the carbonyl carbon and decreases the rate of hydride transfer. In the reduction of

substituted acetophenones and benzaldehydes with morpholineborane, T. C. Wolf

and H. C. Kelley reported the rate accelerating effect of electron withdrawing

Reduction Studies

substitutents and explained that hydride transfer might be more important in the

259,260 formation of activated complex for reduction of substituted aromatic ketones .

[Jnsaturated aldehyde like cinnamaldehyde was reduced to cinnmyl alcohol,

without affecting the double bond. It is reported in the case of low molecular weight

compounds that since the mine-borane reagents are stable compounds, there is no

possibility of fornation of free BH3 which is needed for the hydroboration of olefinic

bonds2h 1-26) . The hydroboration with mine-borane reagents are reported only at

elevated temperature.

4.3 Chemoeelectivity of the Polymer Bound EDA-borane Reagent ( I p . I ( ' *

The polymer-bound EDA-borane reagent acted as a selective reducing P ? r e i . .

agent for aldehyde in the reduction of 1:l molar mixture of benzaldehyde and b' ' '

acetophenone. An equimolar mixture of benzaldehyde and acetophenone was

reacted with 2 molar excess of the polymeric reagent suspended in dichloromethane.

Complete reduction of benzaldehyde was observed within 6 h in the case of

HDODA-PS resin (2el). The reaction was continued for another 5 h in the case of

HDODA-PS, the polymer was filtered and treated with 1M HCl and the

product was isolated from the organic phase. Only benzyl alcohol was obtained

as the product, leaving the acetophenone unreacted. The results are summarized

in Table 4.2.

Reduction Studies

Table 4.2: Reduction of 1 :1 molar mixture of benzaldehyde and acetophenone with the polymer bound EDA-borane

Reaction time (h) Percentage of Percentage of Benzaldehyde Acetophenone

4.4 Effect of Nature of Crosslinking Agent on the Reduction Reaction

The reduction of benzaldehyde and substituted benzaldehydes were

carried out with EDA-borane reagents (le, 2e1 and 3e) derived from DVB-PS

HDODA-PS and BDDMA-PS resins respectively and the results obtained are

compared (Table 4.1). The reagent (2e1) derived from HDODA-PS resin is

found to reduce aldehydes in shorter time interval than the reagents ( l e and 3e)

prepared from DVB-PS and BDDMA-PS resins. This can be explained as a

result of high solvation of the polymer support which may be attributed to

increased chain mobility of the polymer backbone resulting from the

hydrophilic flexible HDODA and BDDMA crosslinks. This enhances the

diffusion of soluble reagents throughout the polymer matrix. The rigid,

hydrophobic. less solvated matrix of the DVB-PS may restrict the penetration of

soluble reagents. Since, better results were obtained with HDODA-PS resin, in the

preparative stages of the polymeric reagent and in the reduction of aldehydes, the

Reduction Studies

study is more concentrated in the case of HDODA-PS supported reagent and a

comparison has made with the reagent derived from DVB-PS support. -pK "

Ihe studies of reduction of aldehyde with polymer bound EDA-borane

reagent was done in a quantitative way by following the reduction of 2-nitro

benzaldehyde spectrophotometrically. Standard solutions of 2-nitrobenzaldehyde

(2 mgirnL) and 2-nitrobenzyl alcohol (2 mg1mL) were prepared. Mixtures of these

solutions at different concentrations were prepared and absorptions of these solutions

(con 0.04 mg/mL) were noted at l- 253 nm. Working curve is drawn with

absorptions against concentrations. Using this curve quantitative conversion of

2-nitrobenzaldehyde with the polymer bound EDA-borane was determined.

4.5 Extent of Conversion with Time

The extent of conversion of 2-nitrobenzaldehyde with the polymer

bound EDA-borane reagent at different time intervals was determined by

measuring the optical density of diluted solutions of reaction mixture at 1-max

253 nm. The concentration of solution comesponding to the absorbance was

noted fiom the working curve and the percentage conversion was calculated at

each time interval. The time course of reduction with EDA-borane derived

from HDODA-PS and DVB-PS resins were studied. Fig.4.1 shows the extent

of reaction with time.

Reduction Studies

I I 4 20 40 60 80 100 120

Time in Minutes

Figure 4.1: Extent of reduction of Znitrobenzaldehyde

4.6 Effect of Degree of Crosslinking on the Reactivity of the Polymer Bound EDA-borane Reagent

In the case of crosslinked polymers due to their insolubility, the

accessibility of the functional groups is diffusion controlled. Crosslink density

of the polymer is a major factor which controls the diffusion of solvent and

substrate molecules into the polymer ma t~ ix"~ . Hence the reactivity of a

functional polymer is very much dependent on the crosslink density of the

polymer. I'he effect of' higher percentage of crosslinking on the reactivity of

polymer bound EDA-borane reagent was studied by carrying out the reduction

of 2-nitrobenzaldehyde with the reagents (2el-2e6) from 2,4,6,8,12 and 20%

HDODA crosslinked polystyrene resins. The reaction was done with 2 molar

excess of reagent resin swollen in DCM. After 3 h the percentage of conversion

in each case was determined spectrophotometrically. In the case of reaction

with 2% crosslinked resin (2er) 100% conversion was observed within 3 h and

Reduction Studies

85% conversion in the case of resin 2e2 having 4% crosslinking and the

percentage of conversion was found to decrease further in the case of resins

(2e3-2e6) of higher degrees of crosslinking (6,8,12 and 20%). The extent of

conversion was found to decrease with increase in crosslinking. The results

show that reactivity and degree of crosslinking bear an inverse relationship.

The reason is that with the increase in the degree of crosslinking, the bridging

between the polymer chains become more frequent and the mobility of the polymer

chains will be red~ced"~. As a result, the rate of d i f i i o n of soluble substrates into

the polymer matrix decreases which in tum results in decreased reaction rates. The

decrease in reactivity with increase in crosslinking was more pronounced at higher

percentages of crosslinking. In the case of 20% crosslinked resin (2%) only 5%

conversion was obtained. The results are depicted in Fig. 4.2.

\ 0 ,

0 5 10 15 20

%of corsslinking

Fig. 4.2: Effect of degree of crosslinking on the reduction

Reduction Studies

4.7 Effect of Solvent on the Reactivity of Polymer Bound EDA- Borane

Solvent plays a prominent role in reactions using polymer supported

reagents. The reaction will take place at considerable rates in solvents which are

compatible with the polymer matrix. In the reduction reactions using polystyrene

bound EDA-borne, DCM was found to be a good solvent in which the reduction of

aldehydes occurred in quiet considerable rates. In order to investigate the

reactivity of the polymeric reagent in other solvents, reduction was carried out

in a variety of solvents commonly used in organic reactions. The reduction of

2-nitrobenzaldehyde with polymer bound EDA-borane (le and 2el ) was carried

out in solvents DCM, THF, toluene, CHCI,, DMF and NMP. The reactions

were followed by TLC and spectrophotometric analysis. The order of solvents in

the increasing order of reaction rates in the case of HDODA-PS resins was

DCM 2 toluene THF > NMP > DMF and in the case of DVB-PS resin the order

was toluene > DCM > THF > NMP > DMF. Swelling studies showed that both

DVB-PS and HDODA-PS resins exhibit better swelling in DCM, toluene and THF

than in DMF. The reactivity of a gel 'type polymer is very much influenced by

solvation. In good solvents, the polymer chains expand to a larger extent and the

anchored hctional groups are more exposed to the substrate molecules dissolved in

the solvent. If the compatibility of the polymer with the solvent is less, the swelling of

the polymer matrix will be comparatively less. In poor swelling solvents the polymer

chains get coiled and the reactive sites near the crosslinks are not readily available.

Time taken for complete reduction of 2-nitrobenzaldehyde with EDA-borane reagent

(111 I II>OT)A-13 :rntl I)VU-I'S resins i n tli il>lrnt solvents \\,ere IIOICLI hq 1 '1 ,(.' a11t1

'l'imc for IUU'%/o convcrsioo (11) So lve~~ts

1<cagc11 t resill I c l i c n g c ~ ~ t ~.csio 2c1 ~ ~ - - ~~ ~ ~~-p ----

I)(: M 5.5 .> p-~~

ppp~ . .. -- .- -. -

loluene )

-- -~

NMI' I I (1 ~~ ~

9 p~ ~ -~

0 3 ~

10 3 . - --

'lire exlcnt of ~.cnctiol~ in dill>~-cnt solvents after 3 11 \\.as dcrcrlnined

s1)cctropliolo111ctric:ll~ illld ~ C S L I I I S ;IIK sIic~\v11 in Fig 4.3.

DCM Toluene THF N M P DMF

Solvents

Reduction Studies

4.8 Effect of Temperature

Temperature is a crucial factor which affects the rate and yield of

reactions. In the reduction of benzaldehyde and acetphenone with morpholine-

borane reagent, the rate of reaction increases with increase in temperaturezs9

The rate of the reaction in the reduction of benzaldehyde with morphpoline-

borane reagent was 0.905 x 1o3k/1 rnol-IS-' at 24 .9 '~ and it was increased to

2.21 x 1 0 ~ ~ 1 mol- '~- ' as the temperature was elevated to 34.8'~. The effect of

temperature on the reduction of aldehydes with the polymer bound EDA-borane

reagent was studied by carrying out the reduction of 2-nitrobenzaldehyde with

HDODA-PS bound EDA-borane in toluene at different temperatures at

27,37,47,57 and 67 '~ . The reaction was carried out with 2 molar excess of the

polymeric reagent and time taken for complete reduction of 2 nitrobenzaldehyde

was noted by TLC. The reaction rate was high as the temperature was

increased. At 2 7 ' ~ 3 h was taken for complete reduction. As the temperature

was increased to 3 7 " ~ , reaction was completed within 1 h and 50 min. At 4 7 ' ~ ,

complete reduction of 2-nitrobenzaldehyde was achieved after 1 h. As the

temperature was further increased, the time taken for completion of reaction

was further reduced. The time for 100% conversion was 35 min at 6 7 ' ~ . The

effect of temperature on reactivity was depicted in Fig. 4.4.

Reduction Studies

10 20 30 40 50 60 70 80

Temperature

Figure 4.4: Effect of temperature on reaction

4.9 Presence of Catalyst

In the case of reduction of carbonyl compounds with low molecular

weight amine-borane reagents, increased reaction rates in the presence of or

Lewis acid has been reported264. Brown et al. have reported that the reduction

of cyclohexanone with amine-borane reagent in the presence of acetic acid

proceeded at a faster ratezs8. The amine-borane reagents are stable and hence less

sensitive to acetic acids265. This makes it possible to carry out reductions in acetic

acid solution. Increased reaction rates were reported in the reduction of carbonyl

compounds with polyvinylpyridine-borane in the presence of Lewis acid

catalyst ~~3-etherate '~ ' . To study the effect of acid catalyst on the reduction of

Reduction Studies

aldehydes with polymer bound EDA-borane reagent, the reduction of 2-

nitrobenzaldehyde was carried out in the presence of acetic acid after swelling

the reagent resin (2el) in dichloromethane. The extent of reaction was

monitored by TLC. The time for complete reduction of aldehyde was noted. In

the presence of acetic acid, the reduction of 2-nitrobenzaldehyde was completed

within 50 min where as in the absence of acid catalyst 3 h was taken for the

complete conversion. Several aldehydes were reduced in the presence of acid

with the polymeric reagents l e and 2el The reactions take place in faster rates

in the presence of acid catalyst. The results are given in Tables 4.4 and 4.5

Studies of low molecular weight amine-borane reagents have put forward

several mechanisms to explain the increased rate of reduction with arnine-

borane in the presence of acid260. They are given below.

a) Activation of the reducible group by protonation

The carbonyl group of aldehydes can be activated by co-ordination with

protonic acid or Lewis acid, decreasing the electron density of the carbonyl

carbon, making it more reactive towards the hydride reagent

b) Activation of BH, amine complex by protonation or association with

l.cwis acids

Reduction Studies

Here the proton or Lewis acid could be associated with the -BH3 moiety

or it could be associated with the amine moiety.

I'he studies of reduction of substituted benzaldehydes with polymer

bound EDA-borane showed that the reactivity of the reagent was higher in the

reduction of substrates with electron deficient substituents and lower in the case

of electron donating substituents. The ease of reaction was found to be

inversely proportional to the electron density at the point of attack. These

results suggest that in the presence acids, the protonation of aldehydes causes

increased electron deficiency at the carbon centre and there by making the

hydride transfer to the carbonyl carbon more easy. The results of reduction in

the presence of acetic acid with the polymeric reagent are given in Tables 4.4

and 4.5.

Table 4.4: Comparison of reduction of aldehydes in the presence and absence of acid with EDA-borane bound on HDODA-PS resin

~ -.-- ~

Duration of reaction in the Duration of reaction in Substrates presence of acid catalysts the absence of acid

(h) catalyst (h) ~

3.75 -- - - ~

6 .

2-nitrobenzaldehyde > I * --

3

4-cyanobenzaldehyde 2 4

Reduction Studies

Table 4.5: Comparison of reduction of aldehydes in the presence and absence of acid with EDA-borane bound on DVB-PS resin

4.10 Effect of Concentration of Polymeric Reagent

- Duration of reaction in

Substrates the presence of acid catalysts (h)

Benzaldehyde 8.0

One of the advantages of using polymeric reagents is that it can be used

Duration of reaction in the absence of acid

catalyst (h)

10

in excess to drive a reaction to completion without causing difficulties in

separation. The excess reagent and spent reagent can be separated from the

product by simple filtration. Increased reaction rate and yield were achieved by

using excess polymeric reagent. This effect was investigated by studying the

reduction of 2-nitrobenzaldehyde with different molar concentrations of the

polymer bound EDA-borane reagent. The reaction was done with 1 :1, 1 :2, 1 :3,

1 :4 and 1 : 5 molar ratios of substrate and reagent. The percentage conversion in

each case was determined spectrophotometrically after 1 h. In the case of

reaction with equimolar amount of reagent the percentage conversion was only

20% after 1 h. With 2 molar and 3 molar excess of reagent the percentage

conversions obtained were 42% and 55% respectively. No further increase in

reaction rate was observed after 3 molar excess of reagent. The results are

summarised in Table 4.6. In the case of reduction using 4 molar excess and

Reduction Studies

5 molar excess of reagent the percentage conversions obtained were same as in

the case with 3molar excess of reagent.

Table 4.6: Effect of molar concentration of polymeric reagent the extent of reaction

In the case of I : 1 molar ratio of substrate and reagent the reaction was

not completed even after 10 h.

Substrate : reagent molar ratio

1:l

1.2

4.11 Regeneration and Recyclability of the Polymer Supported EDA-borane Reagent

Percentages of conversion

20%

42%

One of the unique advantages of the polymeric reagents is their

regenerability and recyclability. The polymeric EDA-borane fulfil the

requirement of being regenerable. The polymeric reagent was regenerated by

washing the spent resin with different solvents and then treating with HCI

followed by sodium borohydride.

Reduction Studies

Scheme 4.2: Regeneration of the polymer bound amine-borane reagent from the spent resin

'The regenerated polymeric reagent was characterised by IR spectrum

which showed the B-H stretching bands at 2390 cm-', 2280 cm-' and B-N

stretching band at 11 50 cm-I. The capacity of the reagent resin was determined

after each recycling. No loss in the capacity of the polymeric reagent was

observed atter recycling. The capacity of the resin was the same as that of the

fresh resin even after recycling for 5 times. The results were summarised in

Table 4.7. The efficiency of the regenerated resin was checked by performing

the reduction of' aldehydes and comparing its reactivity with that of the fresh

resin. l h e time for 100% reduction of aldehydes with the fresh resin and the

recycled resin was the same.

Table 4.7: Capacity of the polymeric reagent after recycling

1 Number of recycling I Capacity of the resin in mmol/g /

Reduction Studies

4.12 Effect of Monofunctional Amines on the Reactivity of the Polymer Bound Amine-borane

Double binding of ethylenediamine was observed in the case of

ethylenediaminomethyl resin, which caused additional crosslinking of the polymer

chains. Swelling studies of the polymers at different stages of functionalisation

showed a decrease in the swelling of aminated resin (Fig. 3.11). Chloromethyl resins

and unfunctionalised resin have almost the same swelling in solvents like

dichloromethane, tetrahydrofuran etc. In the case of ethylenediaminomethyl resin,

considerable decrease in swelling was observed. This may be due to the

additional crosslinking caused by the double binding. The decreased solvation

may cause decreased reactivity of the polymeric reagent. Since there is no

possibility of double binding in the case of monofunctional amines, we have

attempted to prepare polymer bound mine-borane reagents using monofunctional

amines. The monofunctional primary, secondary and tertiary mine-boranes were

prepared from chloromethyl resins derived from 2% HDODA-polystyrene

copolymer.

4.12.1 Preparation of aminomethyl polystyrene (2c')

To prepare polymer bound primary amine-borane reagent, first we

attempted to prepare aminomethyl polystyrene resin. Aminomethyl polystyrene

was prepared by Gabriel phthalimide synthesis. The chloromethyl resin (2b1)

was converted to phthalimidomethyl resin by heating with potassium phthalimide in

DMF at 110"~. The pltthalimidon~ethyl resin was converted to aminomethyl resin by

Reduction Studies

refluxing with hydmzine hydrate in ethanol. (Scheme 4.3). The aminomethyl resin

obtained was collected by filtering and washing.

Potassium

co

Scheme 4.3: Preparation of aminomethyl resin

The aminated resin on heating with ninhydrin solution showed blue

colour of the beads indicating the presence of primary amine groups. The

amino group capacity of the resin was estimated by picric acid method and the

value obtained was 3.96 mmol/g.

4.1X.2 Preparation of ethylaminomethyl polystyrene

f i e polymer bound 2' amine-borane was prepared from chloromethyl resin

(2bn) by treating with excess ethylamine in DMFfpyridine mixture (Scheme 4.4).

CHi-CHl-NH) DMF / Pyridine

CH2-NH-CH2-CHj

2b1 2c"

Scheme 4.4: Preparation of ethylaminomethyl resin

Reduction Studies

'fhe amino group capacity of the resin was estimated by picric acid

method and the value was found to be 3.7 mmollg.

4.11.3 Preparation of diethylaminomethyl polystyrene

f h e polymer bound diethylaminomethyl resin was prepared by treating

chloromethyl resin with excess diethylamine according to the same procedure

used in the preparation of ethylenediaminomethyl resin and ethylaminomethyl

polystyrene resins.

e - C H 2 c l DMF C2H5)2 1 Pyridine NH CH2-N+CzH~h -

261 2c"'

Scheme 4.5: Preparation of diethylaminomethyl polystyrene resin

The residual chlorine of the aminated resin (2~"') was estimated by

Volhard's titrimetric method and it was found to be 0.5 mmol/g. The hindrance

due to the bulky diethylamino groups retards the possibility of complete

conversion. The amino group capacity of the resin was determined by titration

method and the value was found to be 3 mmollg.

4.12.4 Preparation of Polymer bound 1: 2' and 3' amine-boranes from 2c', 2c" and 2c"'

Ihr polymer bound lo, 2' and 3' amine boranes were prepared fi-om the

corresponding aminated resins according to the same procedure used in the

preparation of polystyrene bound ethylenediamine borane reagents. The aminated

Reduction Studies

reslns 2c', 2c" and 2c"' were first converted to the amine hydrochloride resins by

treating with HCI. The amine hydrochloride resins were transformed to the

corresponding mine-boranes (Ze', 2eW, 2e"') by the reaction with sodium

borohydride. The polymer bound amine-borane was collected by thorough washing

with DMF, water and methanol. The polymer supported amine-boranes were

characterised by IR spectra

The IR spectnun of resin 2e' (Fig. 4.6) showed B-H stretching band at 2392

cm-' and B-N stretching band at 1 170 cm-'. The IR spectrum of resin 2e" (Fig. 4.7)

showed B-H stretching bands at 2363 crn-' and B-N stretching band at 1164 crn-I

respectively. The IR spectrum of resin 2e"' (Fig. 4.8) showed characteristic bands

of B-H stretching at 2370 and 2310 cm-' andthat of B-N stretching at 1164.5 cm-I.

1 5 1 I moo 3000 1000 m lorn 500 400

Fig. 4.6: IR spectrum of the resin 2e'

Reduction Studies

m -4

4000 1000

Fig. 4.7: IR spectrum of resin 2e"

Fig. 4.8: IR spectrum of resin 2e"'

N o unreacted chloride was found in the resin. On treating the resin with

AgNO3 solution no turbidity was observed. The available borane reagent

function in the resins were estimated and the values are given in Table 4.8.

Reduction Studies

Table 4.8: Borane capacities of resins 2e', 2e", 2e"'

4.12.5 Reduction of aldehydes using 2e',2en and Ze"'

Resins

2e'

The reduction of benzaldehyde and substituted benzaldehydes were ,> ~

carried out using the polymer bound lo, 2' and 3' amine-borane reagents (2e: S', :

Borane capacity available for a reduction

2.84mmollg

2e" and 2e"'). The reduction procedures used were the same as that in the case

of polymer bound EDA-borane reagents. The time taken for complete reduction

2e"

2e"' ~.L--

of aldehydes were noted by TLC. The polymer bound primary and secondary

arnine-borane reagents were found to reduce aldehydes at faster rates than the

2.92mmollg

polymer bound EDA-borane reagent. The polymer bound primary mine-borane

reagent was found to reduce benzaldehyde with 100% conversion within 4 h

and in the case of secondary mine-borane reagent the duration of complete

reduction of benzaldehyde was 5 h. But in the case of polymer bound 3' mine-

borane reagent, no reduction of benzaldehyde and 2-nitrobenzaldehyde was

observed with 2 molar excess of reagent at room temperature. As the reagent

concentration was increased to 4 molar excess, benzaldehyde was reduced to benzyl I!'

., e alcohol at 5 0 " ~ ' within 8.5 h. The results are summarized in Table 4.9.

Reduction Studies

Table 4.9: Reduction of aldehydes using resins 2e' and 2e"

She increased reaction rates in the case of resins 2e' and 2e" compared to

that of polymer bound EDA-borane reagent (2e) may be due to the effect of

structure and chemical nature of amine. In the case of monofunctional amines,

the possibility of double binding and additional crosslinking in the resin are

completely eliminated which can be attributed to the increased reactivity of 2e'

and 2e". Moreover primary amine-borane reagents are more reactive than

secondary and tertiary mine-borane reagents. The stability of the amine-borane

reagents is inversely related to its reactivity. The 3Oamine-borane compounds are

more stable than primary and secondary mine-borane compounds. The low

reactivity of 3' amine-borane reagents was reported in the case of low moiecular

weight compounds also2s8. Polymer bound 3' amino alcohol-borane reagent

was found to reduce aldehyes at very slow rate. Regarding the case of polymer

bound 3' amine-borane reagent, the increased stability of the reagent and the

steric effect due to bulky alkyl groups retards the reaction rate.

1 R e y de I Nitrobenzaldehyde ~~

4-Cyanobenzaldehyde __ I

Duration of reaction

Resin 2e' (h)

4.5

1.5

2.0

Resin 2e" (h)

5

2.5

2.75