Long-range coupling in esters.

Tetrahedron

Letters

No.34,

pp.

3767-3770,

1968.

Pergamon

LONG-RANGE COUPLING N.V. Department

of

Organic

Riggs

Chemistry, N.S.W.

(S.M.V.

on

leave

from

protons

of

the

confirmed

proton

alkyl

several

formate

of

splitting

those

t5J)

than

conformation

as

in

that,

besides

rotation

of

Varanasi-5,

c/s

the

alkyl

and

formates

5J -Bx

previously

being

India)

is

coupled

= 0.5

reported,

was previously

c/s

and

reported

(1)

signals

of

the

a-

and

8-

We have

that

not

methyl

to

(I).

add

by spin-decoupling)

methyl-proton

the

d-bonds

the

latter

plane

in

a variety

of

esters

esters

has

couplings IL,

I la_,

are group

p.m.r.

conditions as to

in

acetates

or

I lb.

sufficiently equivalence

W t4J)

~-TI maximum

of

the

has

tert-butyl

to

occur,

we have

between

a and

acetate

are

a’-protons

each

been

the

at

to

(4).

Such

within

3767

by

quartets,

rotation each

group

of

(2).

too

of

the

making of

formate

at

conformations

smal I to

0-Ca, as

in mechanism -77’

shows

that

hinder

observed

reflected

and,

1~ (R=H, and

Ca-CB,

more

(3)

couplings by either

where

protons,

or

presence

= 5 kcaI/mole

Couplings about

The

into often

spectroscopy

ethyl

gauche

are

c4J)

protons

four-bond

being

barriers

for

divided

systems,

allylic

primarily

the

rotation

c5J)

infrared

the

spectrum

temperature.

free

system

detected

R’=CH3),

in

occurs

be transmitted

barrier

room

extended-W

coupling

character,

(R=H,

may be broadly

as

unsaturated

might

lb

or

interactions

The microwave

= 2 kcaI/mole time-scale

planar

double-bond

conformation

of allow

partial

present,

in

cases,

the

and

stereospecific,

through

with

methyl

bonds

strongly

90°

versa)

under

usually

communicated

symmetrical

on the

C-C

simple

= 0.9

constants

through

in

Ia,

the

and -vice

manifested

of

to

five-bond

R’=CH3;

.-methyl

in

conformations

significantly

being

are

primarily

bond

and

Armidale,

accepted for publication 28 May 1968)

(confirmed

close

0-Acyl

formates

New England,

University,

coupling

systems;

angles

as the

Hindu

a number

e.g_.

Britain.

Australia.

five-bond

couplings

dihedral one

of

it

communicated

or

homoallylic

University

Although

esters

in Great

c/s.

primarily

saturated,

‘Lx

Verma

c/s.

non-formate 0.25

in

coupling

= 0.4

Long-range those

the

resolveable

certain

(X)

group,

5J -Bx

shows

observed of

formyl

Printed

IN ESTERS.

S.M.

2351,

Banaras

(Received in UK 16 May 1968;

The

and

Press.

and

for

rotation are carbonylin

No.34

3768

la:

planar-w

allylic

lb:

4&X

R :.

4LX

H

P

pJR H--d X

H-

:

i-i

H extended

W 5J_x

III:

extended

W 5J -Bx

symmetry values

a particular There

are

in

the

becomes

function

transmission of

the

ester

signals If

very

of

then

small,

the

group

planar-w being

to

the

but or

long-range

residence

coupling

mechanisms,

through

homoal lyl

ic

5-?oX

IV

time-averages.

mechanism

to

I lb:

multiplet

may be as-yet-undescribed

ascribed the

HX

I la:

appropriate observed

.: ;\

due to

all

that

long-range

extended-W

maintain

time

the

coupled in

proton

groups,

a conformation

mechanism couplings

d-bond

systems

necessary

degree

(la_, of

conducive

will here

and

not

be observed.

discussed lla_,

planarity;

or

to

may be

III), in

any

case

o-s

interaction

(homoallylic)

may not

be invoked

to

account

for

five-bond

couplings

in

f ormates. Other have

expected

ethyl

formate

a result

of

diformate groups

effects

are

decreasing (I:

R=H,

essentially

fixes

R=R’=H)

acetate

(II:

R=H,

acetate-methyl (in

R’=Ph)

parentheses) I lb.

irradiation,

so the

methyl

esters.

to

show 5J_X

ethereal of the

oxygen

0.25

c/s

dipolar

dominant methyl (I

c/s)

is

as

interpretation

in benzhydryl,

cyclohexyl,

following

carbony

shifts

for

methyl

is

fumarate*,

benzyl of

decreasing

the

values

of

preference

for

small

even

into

account

the

acid

are

esters

consistent alcohols

IV sufficiently

for

chlorine

and

the

observed

5&,x

In the

and the small.

last

steric

We have

not

to

signals

are

failure

to

ethylene

Isobutyrate,

mono-

malonate,

is

examined coupling

resolve

separated

observe

case,

effect

unable

been

long

the

we have

our

(6).

with (5).

by

residue

it

by only

five-bond

and di-acetates, lactate,

or

those of

reached coupling

on the

tartrate’,

constants

basis

and 2.2

C/s.

couplings

and cinnamate.’

and secondary

in acetate

and

Five-bond

acetates,

acetoacetate,

methyl

acid.

and methyl

BBS-trichloroethyl

series,

fluoroacetic

methylene

explain

two ester

between

repulsion,

but

to

the

bromoacetate.

phenylacetate

the

conclusions

would

of

between

in conformation

indetectably

toxicity

as

ethylene

half-height

substituents

in methyl

5Lx

c/s),

for

indetectably

interaction

by steric

which

extreme

esters:

primary

I - 13C and alkyl-aH

offset

may be extended

The conformational acylation

the

since

maleate,

di-O_acetyItartrate*,

for

c/s

c/s),

signal

increasing Is

(0.95

c/s),

at

(0.85

negligible.

interactions

, and 0.20

of

5Lax

resides dipolar

(0.20

sIgnal>

introducing

of

R’=Ph)

In the

R’=CH5)

order

lla_ is

apparently

in methyl

and

upon

R=H,

in width

R=R’=CH5),

Similar

dichloroacetate

impracticable

R=H,

expected

observed

may be somewhat

of

(II:

in conformation

indicated.

because

R’=CH2OCHO).

the

R=R’=H)

repulsion

(R=H,

benzyl-methylene

(I:

formyl-proton

polar

the

as a result

cyanoacetate

This

in the

perhaps

may be present

irradiation

and

are

The

by decrease

(II:

chloroacetate

c/s,

fluoroacetate

0.15

acetate

interaction in methyl

of

isopropyl

time

Ia

detected

irradiation

lb.

(I:

one would

otherwise

formate

formate

because

acetate

parallel

Methyl

in methyl

c/s,

methyl

benzyl

presumably as

conformational:

series,

conformation

ethyl

(= 0.1

effects

atoms

for

on the

and

conformation

residence

= 0.27

c/s),

do In fact For

Complementary of

(0.85

c/s),

upon

in the

4LX

singlet,

the

(0.25

signal

conformation

of

R’=CH5)

a sharp

(II:

superimposed

preference

however

acetate

‘sax

values

increasing

is

doubtless

of

between

No.34

3770

A sample Other

samples

measured Varian

of

pure

were

for

neat

HA-60-IL

tert-butyl

commercial Iiqulds

or

(or

for-mate prepared

was kindly by

literature

deuterochloroform

spectrometer

purchased

provided methods.

solutions

with

a grant

by Professor

from

of

P.m.r.

those

the

Lauder.

spectra

marked

Australlan

I.A.

*)

at

were 30°

Research

Committee. REFERENCES. I.

Dora

G.

de

2.

S.

Sternhell,

3.

H.

Laato

4.

J.M.

5.

C.C.J.

6.

G.J.

Kowalewski

and

Riveros

-Rev. R. and

Culvenor, Karabatsos,

and Pure --

Isotalo, E.B.

V.J.

and @. Acta --

Wilson,

Tetrahedron N.

Hsi,

Kowalewski, Chem. them. Jr, Letters

and

C.E.

&

-stand. J.

-Mo1.w. I5 &

2119

&.

1966,

1091. Jr,

93

(1964).

(1964).

Chem.

Orzech,

3

4_&

(1967). 4605

Tetrahedron

(1967).

Letters,

1966,

4639.

on a

Grants