Addition of bromine gas to crystalline dibenzobarrelene: An enantioselective carbocation rearrangement in the solid state

Tetrahedron Letters,Vol.29,No.l3,pp Printed in Great Britain

1485-1488,1988

0040-4039/88 $3.00 + .OO Pergamon Press plc

ADDITION OF BROMINE GAS TO CRYSTALLINE DIBENZOBARRELENE: AN ENANTIOSELECTIVE CARBOCATION REARRANGEMENT IN THE SOLID

STATE

Miguel Garcia-Garibay, John R. Scheffer,* James Trotter’ and Fred Wireko Department of Chemistry, University of British Columbia, Vancouver, V6T lY6, Canada Abstract. Treatment of chiral crystals of 2,3:5,6-dibenzobicyclo[2.2.2]octa-2,5,7-triene (dibenzobarrelene, 1) with bromine vapor results in the formation of the rearranged product ~-8-bromo-~-4-bromo2,3:6,7-dibenzobicyclo[3.2.l]octa-2,6-diene (2) in up to 8% enantiomeric excess.

The situation motions

but is frozen

several

studies

high optical chiral

the product

optical

that molecular

the enantioselectivity

question

activity.

of such crystals

in such reactions where

the chiral

influence

molecular

this question

was the dominant

factor

of the crystal

of a solid state reaction

can lead to products is to what extent

to the disymmetric

In one study

conformation

conformational

in the solid state is well documented,l

reactions

as opposed

owing to rapid

Several

compound; the chiral

a recent

environment

chosen

Gas-solid

-addition bromination

product

Under

was evident. of starting

After material

product

dibenzobarrelene

has shown

was the reaction bromine

in solution

A second

would occur

dessicator.

including

of crystalline

if formed, is known7

with bromine

and the product

would be chiral.

in

symmetry.5

dibenzobarrelene

to afford

and hiahl

crystallizes

from C2v molecular

are well established,2ap36

in a

(l), the title

that dibenzobarrelene

only very slightly

to dibenzobarrelene,

(20 - 100 mg) single powdered,

alone would be

that crystallizes

of

Interestingly,

the carbocationic

svn-8-bromo-~-4-bromo-2,3:6,7-dibenzobicyclo-bicyclo[3.2.I]octa-2,6-diene

is also chiral.

thoroughly

study

deviating

involving

of dibenzobarrelene

state bromination Large

crystallographic

for study

reactions

of bromine

rearrangement (2), which

X-ray

are known4

it

optical

llv rinid

such compounds

space group C2 while

The process vapor.

the overall

was addressed,2a*3

in inducing

of a compound

and of

conformation,

space group even

svmmetrical.

minor

An interesting environment,

in solution

conformation

that chemical

One way of assessing

to study

is achiral

in a homochiral

lattice

was concluded activity.

a molecule

have shown

activity.2

crystal

determines

chiral

in which

point

with or without crystals

and then exposed

these conditions, two hours,

therefore,

to excess bromine

only a slight

the solid

(1) were grown

indicated

Isolation

1485

from ethanol,

vapor at room temperature

discoloration

of one major

5% yield.lO

was to see whether

rearrangement.8

of dibenzobarreleneg

gas chromatography

and the formation

in approximately

of interest.

volatile

and liquefaction the nearly product

complete

of the crystals consumption

in 85-90% yield

and characterization

in a

plus one

of the major

1486

Br

2

product

showed

bromination

it to be identical

in every

of dibenzobarrelene

on large single

crystals

of dibenzobarrelene,

into a reddish-brown

liquid.

considerably

reduced

yield of dibromide

byproducts.

This mixture

In order

‘H NMR chiral rotation optically proved methods converted

shift

measurements active,

to be ill-suited

the extent

reagent

for shift 3.7

into bromo-acetate

ppm indicated

shift

reagent

was a rapid analysis

induction

by

was carried

out

of the sample indicated

a

of a large number

of

of 2 from

In one experiment, Integration

were racemic.

and therefore a sample

racemic.

H

optical a 2, however,

by known

of 2 with [cx]D = 17.7’ was by NMR at 400 MHz with 0.2 eq methyl

8%; a second

excess of 3%.

Dibromide

2 was converted

of the acetate

excess of approximately

preliminary

the solid state brominations

brominations

studies,

in the solid state brominations,

Encouragingly,

were undertaken.

gave an enantiomeric

2 formed

further.

that samples

Eu(hfc)3.

transformation

2 along with the formation of asymmetric

reagent

dibromide

of this material

3 ([a]D = 10.9’) and analyzed

an enantiomeric

solid state bromination

there

those from solution

into a-acetate

of the chiral

studies

had shown

whereas

to the rearranged

When the solid state bromination

Gas chromatographic

was not studied

to investigate

respect

in solution.7

signals run from

As expected,

at 3.06 and 3.18 a different

solution-derived

3 was

1487

We conclude, important

therefore.

in bringing

reaction

is carried

product

optical

that while

about

activity.

Similar in chiral

molecular

chemical

out also plays a significant

are aware,

our results

are the first

such as these

have obvious

and we are continuing

our studies

of their

is undoubtedly the medium

role and may, if chiral,

conclusions

molecules

conformation

transformations,

have been drawn

solvents1 1 and chiral

symmetrical Results

a chiral

enantioselective

inclusion

implications

for the prebiotic

the

lead to

for chemical

reactions

complexes,12

but as far as we

kind for homomolecular

in this fascinating

bv itself

very

in which

chiral

origins

single

of optical

of

crystals. activity,13

field.

Acknowledgement. Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Financial support by the Natural Sciences and Engineering Research Council of Canada is also gratefully acknowledged.

1. (a) A.C.D. Newman and H.M. Powell, ,I. Chem. Sop 3747 (1952); (b) E. Havinga, B’ h’ elm. 13, 171 (1954); (c) R.E. Pincock adh K.R. Wilson L A . Chem. Sot 93, 1291 (1971); (d) R.E. Pincock, R.R. Perkins, A.S. Ma and K.R. Wilson, Science.?74, 1018 (l&l). 2. (a) B.S. Green, M. Lahav and D. Rabinovich, Acc.~hem. Evans, M. Garcia-Garibay, N. Omkaram, J.R. Scheffer, J. Trotter &,q., 108, 5648 (1986). 3.

D. Rabinovich

and 2. Shakked.

M. Garcia-Garibay,

J.R. Scheffer,

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Acta Crvst.. B. 30, 2829 (1974).

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and”F.

J. Trotter

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I.G. Dinulescu,

M. Avram,

Gh.D. Mateescu

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Chem. In&, 2023 (1964).

8. While photochemical rearrangements in the solid state are well established (review: J.R. Scheffer, M. Garcia-Garibay and 0. Nalamasu in “Organic Photochemistry,” A. Padwa. Ed., Marcel Dekker, New York, 1987, pp 249-347). crystalline phase carbocation rearrangements are extremely rare. One example is reported by G.I. Borodkin, SM. Nagy, V.I. Mamatyuk, MM Shakirov and V.G. Shubin, J.e m un., 1533 (1983). 9. Dibenzobarrelene was prepared Tetrahedron, 28, 303 1 (1972). 10. The spectra epimer of dibromide

by the method

of H.P. Figeys

and A. Dralanis,

of the minor product are consistent with a structure that is the C(4) 2, namely ~-8-bromo-~-4-bromo-2,3:6,3-dibenzobicyclo[3.2.l]octa-2,6-diene.

1488

11. J.D. Morrison and H.S. Mosher, “Asymmetric Society, Washington, D.C., 1976, Ch. 10. 12. (a) V. Ramamurthy, Chem. Sot.. Chem. Commun.,

Tetrahedro& 1429 (1986).

Organic

Reactions,”

American

Chemical

42, 5753 (1986); (b) F. Toda and K. Tanaka,

L

13. (a) B.S. Green and L. Heller, Science, 185, 525 (1974); (b) L. Addadi and M. Lahav in “Origins of Optical Activity in Nature,” D.C. Walker, Ed., Elsevier, New York, 1979, Ch. 14. (Received

in

USA 11 December

1987)