Hydrogen participation, and non-chair transition states, in the solvolysis of 2-alkylcyclohexyl tosylates : Evidence from kinetic deuterium isotope effects

Tetr&&w

Letters N0.43, PP. 3739-3744, 1969.

Pergnmon Prees.

Printed

in Great Britain.

PARTICIPATION, AND NON-CRAIR TRANSITION STATES,

HYDRGGBN

IR TRE SOLVOLYSIS OF 2-ALEYLCYCLOREXYL TOSYLATES : EVIDENCEFROMKINETIC DEUTERIUHISOTCPE EFFbpcIs 1. Tichy, J. Bapela, and J. Sicher Institute of Organic Chemistry and Biochemistry, Czechoslovak Academy of Science, Prague (Received in UK 1 August 1969; accepted for publication 15 August 1969)

Recently four

we reported

stereoisomeric

and product

studies

on the solvolysis

4-tert-butyl-2-methylcyclohexyl

As in some previous

(ll).

rate

studies

(2)

it

tosylates

was found

that

(I

of the

- IV, x = 1)

the solvolytic

behaviour

of compounds I - IV does not depend on the axial

or equatorial

character

of the tosyloxy

group

trans

to the vicinal

rates

and a greater

group

methyl.

but rather

on whether

In the former

amount of products

case

this

(compounds

which could

and in the latter

rearrangement

were found.

participation

of the hydrogen

-isomers (III

to account

(1).

for

-methylcyclohexyl not

(compounds

The results on C(2)

This hypothesis,

the high ratio tosylates

so to the reaction

high

by

d4’:

in that

solvolysis

is directly

of the isomer 3739

and IV) lower

in the reaction

which follows

of the (31,

III

II,

for,

rates

and less

by postulating of both the

*cis”-

of the “transv-isomers

that rates

applicable if

Me

IV

were rationalized

I and II but no such participation

and IV)

I and II)

III

II

rearrangement,

or

have ariaen

Buti: But-htTs Me xH I

iscis

proposed of cis-

by Winstein to trans-2-

to the reaction

hydrogen

participation

of I;

No.43

3740

requires

an anti-periplanar

necessary

to postulate

in a non-chair Further from rate

arrangement

a transition

evidence

data

on the course

on the deuterium

be separated

and IV (x = 2). “cis”

isomers

acetate

starting

aluminium hydride

data,

The rates

procedure

(6).

TABLE 1

Rates

a mixture

(x = 21, reduction

and after

individually,

by fusion

alcohols

in Table

(40.0°C,

I II

IV

compounds III

with

1, are corrected using

for

lithium

content

kl

a conductometric

and kh/kD Values

(sec’l)

for

kH’kD

%I

2.775

+ 0.006

. 1O-4

2H

1.333

+ 0.003

. 10-4

1Ii

2.636 + 0.002

. 1O-4

2Ii

1.343

+ 0.003

. 10’4

1H

1.690

2 0.017

. 1O-6

2H

1.418

2 0,.030 . 1O-6

1Ii

1.804

+ 0.005

. 1O-5

2Ii

1.567

2 0.001

. 1O-5

greater

incomplete

I - IV

Tosylate

II

which

with tetrabutylammonium

had a deuterium

70% ethanol)

(4) ;

the corresponding

acetates

I

exists

These

alcohols,

gave

into

were measured in 70% ethanol,

of Solvolysis

Tosylates

tosylation

of the cyclohexyl All

as given

I - IV (x = 2).

of the two “trane”

respectively,

and tosylation.

than 90%; the rate

-

ring

of I - IV was now sought

derivatives

These were converted,

subsequent

deuteration.

is here

from 2-methyl-4-tert-butylcyclohexene

by chromatography,

I and II

(51,

in which the six-membered

of the aolvolyeis

labelled

compound with B2D6 afforded

could

state

it

conformation.

compounds were synthesised this

of the C-H and C-OTa bonds,

2.08

1.96

1.19

1.15

the

NO.43

The values unity, II

of kh/kD for

being

are 2.08

1.19

III

and 1.96,

previously

(7)

assumption

of

step

for

for

compounds of

a different

compounds III is

deuterirrm vacant

for

ia greatest

in the ground

retardation

replacement

when the CB-D bond is

anti-parallel

for

chair

form;

as fast

is fast

drawn (1)

regarding

arrangement

is attainable

transition

state.

their

only

The values compatible

the

of kh/kD with the

mechanism of solvolyaia.

this

VII

interpretation

has to be dealt

the solvolysis

(VI and VII,

x = 1) and found

to that

of

other

of the cis-isomer

of the trans-isomer

2-methyl,

if

exists

i32

investigated

solvolysis

is not

a.F

against

relative

as that

corresponding

II it

by

to the developing

VI

(8)

the

determining

of hydrogen

and IV such an arrangement

in isomer

d - A- OTs d Ii

tosylate

that

III

the compounds I - IV are thus entirely

previously

An objection

(b)

our previous

one hand and the

I the C-O vs C-D anti-periplanar

V

isomers

I and

found

in the rate

on the

through

Me Me

hexyl

involvement,

from

for

to vahea

These data confirm

the type V.

by way of a non-chair

now determined

and Reeves

greatly

the values

closely

the compounds I and II

isomer

state

reacts

Me -

by contraat,

corresponding

For the iaomera

In the

conclusions

IV;

do not differ

and IV on the other.

p-orbital.

molecule

for

isomers

mode of hydrogen

assumed that

attainable.

and 1.15

respectively,

of the reaction,

It

the two vtrans”

2-isopropyl

of cia(a)

that

VI proceeds

the reaction tosylates only

of both and

two to three

the kcis/ktrans

and 2-cyclohexyl

Goering

and trana-2-tert-butylcyclo-

2-alkylcyclohexyl

VII whereas

with.

cyclohexyl

ratio

for

tosylatea

timea the varies

No.43

3742 They concluded

from 60 to 90. hydrogen

participation

in the

(VI1 and consider.ed

that,

-2-alkylcyclohexyl participation

that

methyl

this

for

The relatively cyclohexyl

a factor small

greater

value

indicate

of the ratio

a consequence

hexyl

rates

features will

equatorial

boat

form is

-deuterated

been synthesised;

to raise

eolvolysis

we think pair

in the cis-isomer

- relative If,

required

the unusually

VI,

VII does not

VI but rather

to the other

state

group

(1,101,

converting

clearly

do destabilize

of the

equatorial

then

structural

between is

is

trans-2-alkylcyclo-

the chair

interaction

by

that

as has been suggested,

for

rates

of the solvolssis

a feature

the chair

through

rate

into

a boat

the vicinal of this relative

measurements

kind to a

to tert-butylcyclohexene

by oxidation

aluminium hydride, (Table

followed

and subsequent

(13)

afforded

the

from which the reduction

by chromatographic

2) show that

on CB-

(x = 2) have therefore

trans-2-tert-butylcyclohexanol

by lithium

data

and trans-2-tert-butyl-

from data on

The two compounds VI and VII

was obtained

The rate

inter-

(12).

cis-isomer

isomers.

alternative

the tert-butyl

The syn-clinal

B2Dd addition

deuterated

We suggest

to one another;

point,

transition

arguments may be tested

substrates.

appropriately

as follows.

such interactions

available

group’.

equatorial

as VII the

acceleration

and the tosyloxy

that

The above

for

acceleration

solvolysis.

Evidence

cis-

in both VI as well

respect

the second

large

the enerav

tert-butyl

with an adjacent

of both cis-

particiuation

by way of a boat

accelerate

(11).

This

which lower

toeylate

to see why hydrogen

the following

that

with

k,.is/ktrans

- arises

react

through

not be any in other

butyl

may be estimated

As for

of hydrogen

VII.

tosylates

tosylates

this

of an unusually

trans-isomer

no eoceleration

“difficult

reactivity

from the fact

(9)

than 5.

absence

being

and propose

solvolysis

result

derivgtives

ie

should

but not tertiary

C-O and C-CHJ bonds are “syn-axialn cholestane

there

of VI and VII.

high

tosylate

it

in increased

or isopropyl

the rates

there

either,

result

that

of cis-2-tert-butylcyclohexyl

consequently,

argument is not valid

pretation

(b)

reaction

tosylates, should

(c&-placed)

from

the values

of the ketone

separation of kh/kD cl.24

of the for

No.45

TABLE

3743

2

Rates of Solvolysis (25.0°C, 79% ethanol) and kH/kD Values for the Tosylates VI and VII Tosylate

VI

VII

kl

(sd)

lli

3.242 + 0.004 . 1O-4

2Ii

1.474 + 0.002 . 10'4

1H

8.170 2 0.016 . 1O-5

*H

6.598 + 0.010 . 1O-5

kH'kD

2.20

1.24

trans-isomer VII and 2.20 for the cis-isomer VI) indeed follow the pattern believed to be indicative of hydrogen participation in the cis-isomer (14). The present results thus lend further support to the view (1,lO) that equatorial tosylates undergo solvolysia via a ncn-chair transition state and that solvolysis of cis-2-alkylcyclohexyl tosylates is accelerated by hydrogen participation (1,3).

REFERENCE9

(1)

M. PBnkovB, J.

Sicher, M. Tichy, and M.C. Whiting, J. Chem. Sot. B

1968, 365. (2) (3)

cf. W. Hiickel and M. Hanack, Angew. Chem. 79, 555 (1967). == S. Winstein, B.K. Morse, E. Grunwald, H.W. Jones, J. Copse, D. Trifan,

(4)

and H. Marshall, J. Amer. Chem. Sot. 74, 1127 (1952). == This compound was prepared from 5-tert-butylcyclohex-1-ene carboxylic acid; 5. Tichi, J. Hapala, J. Sicher, to be published.

(5)

M. Tichj, unpublished.

(6) (7)

J. Hapala, Thesis, Prague 1969; L. Hladek, Chem. listy 60, 238 (1966). == S. Winstein and J. Tskahashi, Tetrahedron 2, 316 (1958).

(8)

H.L. Goering and R.L. Reeves, J. Amer. CheH. Sot. If, 4931 (1956).

(9)

S. Nishida, J. Amer. Chem. Sot. 82, 4290 (1960). z= (10) V.J. Shiner and J.G. Jewett, J. Amer. Chem. Sot. 87, 1382 (1965); == W.H. Saunders, Jr. and K.T. Finley, J. Amer. Chem. Sot. 87, 1384 (1965). ==

3744

No.43

(11)

cf. also recent finding that trans-2-tert-butylcyclohexanol is less stable than its cis-epimer:D.J. Pasto and R.D. Rao, J. Amer. Chem. &. 91, 2790 (1969).

(12)

E.J. Gabe and D.F. Grant, Acta Crystallonranhica15, 1074 (1962); == R.D. Stolow, T. Groom, and D.I. Lewis, Tetrahedro?Letters 1969, 913.

(13)

N.C.G. Campbell,J.R.P. Clarke, R.R. Hill, P. Oberhhali, J.H. Parish, R.M. Southam, and M.C. !qhiting,J. Chem. Sot. B 1968, 349.

(14)

The somewhathigher values of kR/kD found for VI and VII when compared with I - IV are probably a consequenceof the lower temperaturesemployed in the solvolysisof the former compounds.