Tetrahedron Letters No. 49, pp 4345 - 4348, 1977. Pergamon Press. Printed in Great Britain.

THE DEGENERATE THERMAL REARRANGEMENT OF BICYCLO[3.3.l]NONA-X.6-DLENE

Roger Bishop la , William Parker

lb , a.nd Ian Watt

*1c

Department of Chemistry, University of Stirling, Stirling, Scotland.

(Received in UK 96 September 1977; accepted for plblication 13 October 1977)

A recent report by Ja.penga, Kool a.nd Klumpp’ on the thermolysis of bicyclo[3.2.1 jocta-2,6-diene (1,

and bicyclo[3.2.2]nona-2,6-diene (2), prompts us to report our related experiments with bicyclo[3.3. lj-

nona-2,6-diene @3.

&l &? R&R t;;; -

Attempted thermolysis of 3a by heating at 3OO’C (sealed tube, 24br) or by passing it through a hot -

silica tube at 590’ in a strea.m of nitrogen (contact time ca.. 30sec), resulted only in the recovery of un- -

changed s4. Use of higher temperatures (650’) in the flow system ga.ve extensive charring and low

recovery of volatile materia.1, whose g. 1. c. analysis showed a.t least four new compounds. The n. m. r.

spectrum revealed the presence of aromatic species.

The possibility that the recovered diene had undergone a degenerate rearra.ngement wa.8 investigated

using the specifically 3,7-deuteriated derivative 3b. - This diene was prepared from the known 3,3,7,7-

tetradeuteriated bicyclo[3.3.l]nonane-2,6-dione (4)5 by the sequence of reactions indicated. m

DBU = 1,5-diazabicyclo[5.4.Ojundec-5-ene 3b -

The labelled diene thus prepared, vmax . (Ccl4 solution) 2250cm -1

(olefinic C-D stretch), was identica.1

in its g. 1. c. beha.viour to authentic unlabelled materia13. The 220 MHz n. m. r. spectra of 3a a.nd 2 allowed -

unambiguous assignment of all protons (see TABLE6) and confirmed the specific labelling of 3b a.t the 3- -

and 7- positions.

4345

4346 No* 49

TABLE : 220 MHz n. m. r. spectra of m

(CC14 solution).

Signal (3a)

I 1.65 (2H)

% 9

Hs H5 H y3b- 4p a H4cl

3

al H2

I I Integrals (-+ 0.05m Pa.ttern (3a.) Protons

2 (cold) 3b (pyrolysed) - I I

t; JI,9 3.2Hz H9x2 2.00 2.00

dd; J 4o, 48

17.5Hz, J5, 48 4.5Hz H46+H86

2.00 2.00

dm; J 48,4a

17.5Hz (Wh,2 10Hz) H4a+H8a

1.97 1.99

m (Wh,2 11.5Hz) H1+H5

2.00 1.58

dq; J2,3 10.5Hz, J40 3 4.6Hz, JI 3 1.6Hz H3+H7

0.18 0.62 ,

dq; J3 2 10.5Hz, Jl,24.5Hz, J4,,,21.9Hz H2+H6

1.90 1.90 9

Pyrolysis of 3b as before (sea.led tube, 350°, - 24hr) ga.ve only unchanged sta.rting material, but the diene

recovered from pyrolysis in the flow system at 590-600’ showed evidence of degenerate rearrangement.

The 220 MHz n. m. r. spectrum of the recovered diene showed over 300% enha.ncement of the signals from

the 3- and 7- positions, with a corresponding decrea.se in signal from the bridgehead l- a.nd 5- positions.

The other signals were unchanged in strength (see TABLE). The i. r. spectrum (Ccl4 solution) of the

recovered diene showed a. new band at 2140 cm -1

(a.liphatic C-D stretch) a.nd mass spectral a.nalysis showed

that there wa.s no loss of deuterium from the sa.mple during pyrolysis.

These observa.tions are consistent with specific exchange of label between the olefinic 3- and 7- posi-

tions and the bridgehead l- and 5- positions. Although there are other possibilities7, the most likely

expla.nation is that this process is a further exa.mple of a. therma. 1.3 carbon shift reaction’. Thus,

cleavage of a. 1,s (or 4,5) bond and formation of a new bond between C3 and C8 (or between C4 and C7) would

regenerate the diene fra.mework (see FIGURE). In the absence of any isotopic preference’, such shifts

would be expected to distribute the deuterium label equa.lly between the olefinic (3 a.nd 7) positions and the

bridgehea.d (1 and 5) sites.

No* 49 4347

FIGURE : Degenera.te rearra.ngement of Bicyclo[J. 3.l]nona-2,6-diene.

D It is evident (see TABLE) that full equilibration of the label ha.s not tazkenplace under the reaction con-

ditionsl’. The observed distribution corresponds to only 47% equilibration, giving a. ra.te constant for the

exchange process of 3. 10 -2 -1

set . The activation energy must lie between 60 and 65 kcal. mol -1

. This

value would be consistent with a mechanism involving homolysis of a. 1,8-bond to generate a diradical species

5.

Both radical centres are formally allylic, but the stereochemica.1 constraints imposed by the molecular

framework of 2 are such that, while the cyclohexenyl centre can benefit from allylic stabilisation (ca. 13

-1 12 kcal. mol ) , the migrating centre would be formed as a ‘perpendicular’ ally1 radical in the transition

state and would not be stabilised. An estimation using Benson’s group increments 13

, predicts that the con-

version of Jj to 5 should be endothermic by 66 kca.1. mol -1

.

Further discussion of the mechanism requires determination of accurate activa.tion pa.ra.meters, and of

the stereochemical fate of the migrating centre.

Acknowledgement. R. B. a.nd I. W. acknowledge the award of Science Resea.rch Council Fellowships at

Stirling.

We thank the Applications Laboratory of Perkin-Elmer for the 220 MHz spectra.

4348 No. 49

REFERENCES AND NOTES.

Present address: School of Chemistry, University of New South Wales, Kensington, N. S. W. 2033, Australia.

1 (a)

04

(C)

2.

3.

7.

8.

9.

10.

11.

12.

Deceased 9 Ma.y 1977.

Address correspondence to this author at present address: Depa.rtment of Chemistry, University of Manchester, Manchester M 13 9PL, England.

J. Japenga, M. Kool a.nd G. W. Klumpp, Tetrahedron Letters, (1974), 3805.

J.K.A. Clarke, E. McMahon, J. B. Thomson a.nd B. Zeeh, J. Organometallic Chem., (1971), 31, 283. Mass spectra.1 analysis of the deuteriated material showed it to be d2 86%: dl 3%; d0 11%.

Apparatus similar to tha.t described by R.F. C. Brown a.nd R. K. Solly, Austral. J. Chem., (1966), 19, 1045.

J. P. Scha.effer and J. C. Cla.rk, J. Org. Chem., (1965), 30, 1337.

‘High Resolution NMR Spectroscopy’, E. D. Becker, Aca.demic Press, N.Y., (1969). Coupling constants are in good a.greement with those expected from dihedral angles taken from mole- cular models.

For example, a dyotropic rearrangement involving the Cg methylene group; see M. T. Reetz, Angew. Chemie Internat. Edn., (1972), 11, 129.

For some exa.mples of 1,3-carbon shift rea.ctions see: (a) J. Japenga., M. Kool a.nd G. W. Klumpp, Tetrahedron Letters, (1975), 1029. (b) J. A. Berson, P. B. Derva.n, R. Ma.lherbe and J. A. Jenkins, J. Amer. Chem. Sot., (1976), 98, 5937. (c) J. A. Berson a.nd J. M. Janusz, J. Amer. Chem. Sot., (1974), s, 5939. (d) J. J. Ga.jewski, L. K. Hoffma.n and C. N. Shih, J. Amer. Chem. Sot., (1974), E, 3705. (e) D. Hasselman, Angew. Chemie Internat. Edn., (1975), 2, 257.

A small preference of the deuterium for the (a.llylic) bridgehea.d position is to be expected. See (a) A. Streitwieser, R. H. Jagow, R. C. Fa.hey a.nd S. Suzuki, J. Amer. Chem. Sot., (1958), @, 2326. (b) K. Humski, R. Malojcic, S. Borcic and D. E. Sunko, J. Amer. Chem. Sot., (1970), 92, 6534. (c) I. Gla.tt and A. Yogev, J. Amer. Chem. Sot., (1976), 2, 7088. In estima.tion of the rate constant we have assumed there is no isotopic preference.

Resubjection of the pyrolysate to the reaction conditions gave further changes in the integral ratios, but lack of material precluded experimental study of the complete approach to equilibrium.

‘Chemical Kinetics’, K. J. Laidler, McGraw-Hill, (1969).

W. von E. Doering and G. H. Beasley, Tetrahedron, (1973), 29, 2231.

13. ‘Thermochemical Kinetics’, 2nd edn., S. W. Benson, J. Wiley, (1976).