Reduction of ketones with polydibenzo-18-crown-6-borohydride

Vol.32No.19.pi Briti

TeurheQonLataa F’rinted in Gny

215Wlao, 1991

oo4o-4039191 $3.00+ .oa

PergMvrnPre& plc

RRLRJCTIONOF KETONES WITH WLYDIBENP)-18_CROUN-~~RO~R~E A Sarkar*

B R Rao

and

Department of Applied Chemistry, Indian School of Mines, Dhanbad-826004, INDIA.

a condensation polymer of dibenzo18-crown-6 and formaldehyde has been used as support for borohydride ions in The reagent has been found to the reduction of few cyclic and acyclic ketones. be more stereoselective than dibenzo-18-crown-6-borohydride in the reduction of 4-J-butylcyclohexanone has been converted exclusively (100%) to cyclic ketones. trans-4-J-butylcyclohexanol.

Abstract: Polydibenzo-18-crown-6-(P-DB-18-C-6).

Supported

borohydrides

are

increasingly

being

used

in

syntheti;

organi;

chemistry . Apart from inert materials of inorganic origin such as alumina , clay , 3 polymeric anion exchangers have slso been used as support for boroetc., 4*5 and the resultant reagent used for reduction of a variety of organic hydride.

silica

functional some

groups!

However,

selectivity

in

in the reductions Apart

the

far

the

the

reduction

of

borohydride from

the

axial

attack

aided

nucleophile

like

on the

carbonyl

hindrance

The 6

(3.0

ml1

to

capacity the

in

dry

remove of

reagent

free

reagent

any

possesses

stereoselectivity

was benzene sodium

a

polymer

of

from

exactly

that

the

the

It

grafted

anion exchanged

equatorial

that

gmup side.

of more stable

out

‘naked

borohydride,

and stereoselectivity

was felt

carbonyl

is grafted

us to carry

P-DB-18-C-6

general)

reagent’,

borohydride

prompted

such

(in

supported

alcohols 10

such

P-DE18-C-6-

the

cyclohexanone

of

This

in

is

so

in case

because

the

of cyclohexa-

product

stability

control

, although

the

probabilities

of

an axial/equatorial

attack

in

the

steric

equal

(particularly

absence

for

a small

of any

on C3/C5). found

pulverised

dilute

this

hardly

when sodium

concerned.

bomhydride

BH6_ equivalent with

are

called

are

using

ketones

attack

than

so

substituents

reagent

though

has

on resin-bound

of

in the formation the

naked

of finely

gl

an

of

was perceived

ketones side

group

fmm

a mixture

by

that

it

bornhydride’

reductions

prefer

resulting

is

found

polydibenzo-18-crown-6’.

an improvement

cyclic axial

of It

of

might

more nones,

be

rates

been

groups7’,

advantages

‘naked

moieties

would

as

usual

investigations.

borohydride’ as

the

ether

has some

ketones5.

of obtaining

crown

present

of cyclic

fmm

the possibility on the

it

reducing

to

be

sodium (15

quite

ml).

The

bomhydride, of

sulphuric

the acid

stable.

bomhydride prepared dried

reagent, was 2157

It

(0.7 resin

in vacua determined

found

was

prepared

by

stirring

g) and polydibenzo-lfkcrownwas and by

to be (1.92

washed stored

with over

decomposing

meq/g).

THF (10 P205. a

part

The of

2158

Ketones reduced

by

and

resin

the

by

(cyclic

following (2.2

that faster

reductions

in

than

on carrying

ketones

in

benzene.

formed

in

the

locked

(80.8%)

found

usually

of equatorial

in reduction

with

ciated

with

resting

observation

in reduction

a mixture

following

were

Dn the

carried

other of

the

the

alcohol

(cis)

which

formation

of

may be mentioned

with benzene as solvent Table

other

equatorial

here,

1: Reduction

of axial

i.e.

of ketones

of equatorial in the same

attack

which

of greater results

alcohol/stable

alcohol.

of equatorial

than that obtained

which

is greater

(75%).

the reductions

percentage

improved

cyclohexanone,

with formation

1) was

cyclohexanones

in ethanol,

borohydride

In

case

percenobtained

were assoAnother

alcohol

inteformed

in ethanol.

with P-DB-la-C-6-BHq

Ethanol

Entry

of

ketones,

higher

(table

temperature

to the stereochemical

cyclic

is always

ketones

followed

in polymer

observed

room

a-Methyl

proceeded

is opposite

In all

at

were

(2 meq)

was

procedures

was

reductions

sodium

These

ketone

4-t-butylcyclohexanone,

with

the reduction

LiAIH4/NaBH4.

a preferential

reduction

the

reaction

The percentages

cyclic

In fact,

It out

hand,

underwent a preponderance in

of

standard GC.

stereoselectivity.

reductions

and benzene.

40°- 5O’C. The

NMR and

which

in benzene.

of 3,3,5-trimethylcyclohexanone tages

at

to 100% trans-4-J-butylcyclohexanol.

is not conformationally than

in ethanol

which

by

by

greater

out the reductions

was reduced

isolated

the

by

in

was stirred

characterised

was accompanied

(stabler)

solvent

of

ethanol

in benzene alcohol

were

were reduced

procedure

BH;)

The products,

reactions

the

a simple meq of

TLC monitoring.

supported

and acyclic)

Benzene

Time h

Yield %

6

100

89.6

24

95

100

22

90

53.9

56

85

57

5

85

76.7

22

80

80.8

12

88

72.5

48

82

75

96’

60

20+

96*

62

23.8+

4

95

18

90

26

90

55

85

20

75

50

70

26

92

66

88

reduction

+ Endo

1. 4-t-butylcyclo-

% of equatorial alcohol

Time h

Yield %

% of equatorial alcohol

hexanone 2. 3,3,5_trimethylcyclohexanone 3. I-methylcyclohexanone 4. 3-methylcyclohexanone 5. Camphor 6. Cyclohexanone 7.

Benzophenone

8. Acetophenone 9.

Fluorenone * Incomplete

A tentative

explanation

for the slower

reaction

rate and higher

stereoselectivity

2159

in

benzene

more

solvated

axial out

may in

ethanol

than

in

crown-bomhydride benzene.

Fig.Ib

on to the carbonyl

ethanol.

is

more

Moreover,

group is slightly the

possibility

Na+- P-DB-18-C-6 likely

to

cannot

diminish

the

stereoselectivity

of

formation

the

reagent

be

effect.

is

more exposed

relatively

on the of

[Fig. phenyl

of

cyclic

Ia

It appears and

group

advantage

ketones

18-crown-6-BHq Table

‘1 supports

case attack

this

with

hypothesis

become

BH; from

is carried

P-DB-18-C-6-BH4the by

liberated

BH:

following

the

of

the

those

due

to

stereochemical obtained

to a certain

fmm extent

outcome in the reduction

only

on the basis

the borohydride

group

DE18-C-6-bomhydride

a buckling

of formaldehyde

Entry

group results

reduction (Table

of cyclic

effect

caused

with different for

reduction

with dibenzo-

2). ketones.

1

Ethanol 0 trans

Yvent I I Benzene % trans

89.6

100

75

76.7

80.8

74

44.1

41

48

I Toluene*k % trans

hexanone 4-methylcyclohexanone 3,8,5-trimethylcyclohexanone ** Results ,

as reported

the

However,

in the normal

probably

of condensation

comparison

2 : Stereochemical

I-J-butylcyclo-

an axial

the

cannot be explained

occurs

our reagent

of

to

comparatively greater 13 dibenzo-18-crown-6-BH4 and exclusive

vis-a-vis

This

A

of

to us that in P-DB-18-C-6-BHg,

as a result

using

of

likely

when the reduction

In that

reduction.

than the same group

Ib).

dibenzo-crown.

out.

is

approach

dissociation

ruled

of trans-4-t-butylcyclohexanol

of a solvent complex

impeded

of

same mode as in case of sodium bomhydride

units

As a result,

Fig. Ia side

(100%)

complex

n

BHq and

ion

The

proposed.

u

in

into

be

by Matsuda et al.

ll.

2160

The

trend

of

stereochemical

the same trend

(though

percentages

equatorial

smaller the

of

effective

size

valid

reason

only

not have

improved

are 2.283

and 2.387

improved) the

in

has

been

reducing

this

to such

case

with our reagent follows 11 by Matsuda et al . Formation of greater by

in non-polar

the

(the

It appears

nature of the BH; ion in the present

reductions

rationalised

agent also,

an extent

at 2O’C).

of

as observed

alcohol

of

outcome

solvent

percentage

dielectric

those

of

toluene.

equatorial

constants

to us that probably

reagent the axial

workers

of

as

due

If that alcohol

benzene

to was

would

and toluene

due to the more exposed

attack

is more facile.

Acknorledgementk The fruiftul

authors

are

thankful

to

Prof.

Nasipuri

D.

of

I.I.C.B.,

Calcutta,

for

a

discussion.

References and Notest 1.

F.Hodosan

2.

A.Sarkar,

and N.Serban, B.R.Rao

Rev.

and

Roumaine,

M.M.Konar,

Chim,

14,121

Synthetic

(1969).

Comm,

19(13

6

14).

2313-2320

(1989). 3.

P.Tundo,

J.Chem.Soc.

Chem. Comm, 815 (1977).

4.

H. W. Gibson and F.C. Bailey,

5.

R.O.Hutchins,N.R.Natale

6.

A.R.Sande,M.H.Jagdale,R.B.Mane

7.

A.Nag,A.Sarkar.S.K.Sarkar

8.

A.McKillbp

9.

Polydibenzo-18-crown-6 tions :

J. Chem .Soc. Chem .Comm, 815(1977).

and I.M.Taffer,

and S.K.Palit,Synthetic

and D.Y.Young

Synthesis,

401-481

has been used

for

K.P.Janzen.

E.Blasius.

J.Chem.Soc.Chem.Comm.

and M.M.Salunkhe,

H.Klotz,

1086 (1978).

Tet.Lett.,25.3501(1984). Comm.17,1007(1987).

(1979).

reactions

under phase

A.Toussaint,

Macromol.

transfer

condi-

Chem.183,

140X-

1411 (1982). 10.

Felkin’s

model

the equatorial

axial

control. is

is

transfer 471-473

strain

is held

accepted

responsible

in place

for impeding

of product

development

Even then,

difficult

attack

metals

the torsional

is now generally

the exclusive formation of --trans-4-t-butylcycloexplain. It is probable that a kinetic preference for 12 (Fukui effect) , e.g., in the reduction of ketones with dissolving

(stability) hexanol

in which attack

to

operating

mechanism

here (See

and

the

Nasipuri

reduction

and Saha.

goes Indian

through Journal

a

single

electron

of Chemistry,

29B.

(1990).

11.

T.Matsuda

12.

K. Fukui.

and K.Koida, Theory

West Berlin),

of

1975.

(Received in UK21 February 1991)

Bull.Chem.

orientation

Sot.,

Japan,

and stereoselection,

46.2259-2260 Chap. 7.7

(1973). (Springer

Verlag ,