Thermal rearrangements of 2,2-diphenylmethylenecyclopropanes to indenes

Tetrahedron

Letters

No.113

PP. 845-848,

THERMAL REARRANGEMENTS

Pergamon Press.

1970.

OF 2,2-DIPHENYLMETHYLENECYCLOPROPANES

2 Maitland Jones,Jr. ,Michael E.Hendrick3 Department of Chemistry Princeton University Princeton, New Jersey, 08540 (Received

Printed in Great Britain.

in USA 15 January

1970; received

TO INDENESI

John C. Gilbert,James R.Butler' Department of Chemistry University of Texas Austin, Texas 78712 in LK for publication

31 January

1970)

Notice has been twice taken of the thermal conversion of vinylmethyleneto methylenecyclopentenes. 596 We have come upon a close

cyclopropanes

relative of this process enecyclopropanes the penultimate

in the thermal rearrangement

to indenes.

of diphenyldiazomethane

in allene.'

(3-j could be accomplished

a degassed

shift masks

methylenecyclopentenes.

2,2-Diphenylmethylenecyclopropane

indene

of 2,2-diphenylmethyl-

In our examples a final hydrogen

carbon tetrachloride

with the positions

of the methyl

(l_)was synthesized Rearrangement

by the irradiation

of 1 to 2-methyl-3-phenyl-

either by gas-chromatography8 solution.

or by heating

Indene 33 is known, as is the indene

and phenyl groups interchanged.

9-11

The

distinction between the two rests both on a comparison of melting point and 12 and an additional independent synthesis from 2-methyl-1-indanone. spectra The structural

course of the rearrangement

2,2-Diphenylmethylenecyclopropanes exocyclic

double bond also rearrange

specificity

in so doing.

the l-position

is formed.

is thus assured.

4. and T-which bear methyl groups on the to indenes and exhibit a remarkable

Only indene having the added methyl groups at Thus 4 gives only 5 and not 6_, and 7. gives 8_

846

NO.11

and not 9_. Inspection of the nmr spectra fixes the methyl

substitution

at

the 1 and 2 positions,

and the ultraviolet spectra require that the phenyl 13 group occupy the 3 position. As little as l-Zig of compounds 6 and 9_ could easily have been detected. rearrangements the apparent relative

This specificity is not mirrored in the 6 and may be associated with

studied by Shields and Billups, instability

to 1.

of 2,2-diphenyl-3-methylmethylenecyclopropane

(12)

Isomers 1 and @

are related, at least formally, through 16,17 rearrangement.

a methylenecyclopropane

&

135O)

&

not 5

&

1350q&$$

not

There is a surfeit of potential mechanisms cannot yet make definitive intermediacy

Moreover,

between

adduct.

The elusiveness

as it is not expected

the gain in molecular

tion of a new benzene

The

stabilization

ring probably

to

of ?-might have been

to be a particularly

reactive diene.

associated with the forma-

serves as a thermodynamic

force for the further conversion of 2. to 3. the analogous

for these changes, and we the alternatives.

of 2_ seems assured, although we have as yet been unable

trap it as a Diels-Alder anticipated,

distinctions

driving

No such stabilization

attends

hydrogen

shift in the simpler systems studied previously, 6 and there it does not occur. The hydrogen shift required to convert 2 to 3, although of the allowed 18 is sterically difficult, and it is perhaps surprising that

1,5 variety, it occurs

as rapidly

as it does.

We tentatively

suggest that the hydrogen

shift may proceed not in a direct manner but rather via a conducted tour of four swoops, each a 1,5 shift. crucial

In this scheme the phenyl group provides

link in the transportation

of the migrating

Some support for this suggestion derives methylenecyclopropane, phenyl ring necessary

hydrogen

from the observation

which would give an intermediate to consummate

a

to its goal.

that 2-phenyl-

tetraene lacking the

the tour, does not rearrange

to an indene

even when heated to 190° for one hour. The detailed mechanism of intermediates,

a concerted process l-l_and 12 aswell

observed

for the formation of 2_ can proceed

the most attractive (22) resembling

as the concerted

of which are 11 - and 12.

a Cope rearrangement

through a variety Alternatively,

is possible.

reaction 13 - are capable of explaining

facts, including the specificity of the indene formation.

ments designed

Both

to permit differentiation

between

the

Experi-

the various mechanisms

are

in progress.

&~*g~?nt_.

M. J . , Jr. and M. E. H. are indebted to Professor Peter P.

Gaspar and Messrs. conversations.

Thomas M. Gorrie and Ronald H. Levin for enlightening