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A Mechanistic Alternative for the Thermal Antara-Antara Cope Rearrangements of Bicyclo [ 3.2.01hepta-2,6-dienes and Bicyclo [4.2.01octa-2,7-dienes1 John E. Baldwin* and Mark S. Kaplan
Contribution from the Department of Chemistry, University of Oregon, Eugene, Oregon 97403. Received November 16, 1970 Abstract: Thermai rearrangements near 190" of various l-methoxy-2-oxobicyclo[3.2.0]hepta-3,6-dienes to 3methoxy-2-oxobicyclo[3.2.0]hepta-3,6-dienes having substituents at positions consistent with a Cope rearrangement formalism were first observed by Miyashi, Nitta, and Mukai in 1967. Similar rearrangements are now reported for l-ethoxybicyclo[3.2.O]hepta-3,6-diene, 3-ethoxybicyclo[3.2.O]hepta-2,6-diene, and the degenerate rearrangement of a deuterium-labeled bicyclo[4.2.0]octa-2,7-diene. The degenerate rearrangement of 3,7-dideuteriobicyclo[3.3.0]octa-2,6-diene does not occur even at 450" for 85 min; 1,5-dimethylbicyclo[3.3.0]octa-2,6-dienefails to isomerize at 350" for 11 hr to 3,7-dimethylbicyclo[3.3.0]octa-2,6-diene.These results are rationalized in terms stressing the decisively important mechanistic role played by the cyclobutene moiety in those systems which isomerize. The rearrangements are held to occur through conrotatory electrocyclic opening of the cyclobutene ring system, giving a cis,trans,cis monocyclic triene as intermediate. which then reverts to starting material or closes in the other direction to give the "Cope rearrangement" product.
T
hermal rearrangements of several bicyclo[3.2.0]heptadienones were examined by Miyashi, Nitta, and Mukai2in 1967.
rangement. In a prespective attempting to empnasize the three-dimensional relationships in the molecular system suffering rearrangement, the structure 6 may be seen to yield product 7 in a one-step concerted fashion4only when the C1-C5 bond is cleaved in a suprafacial disrotatory manner. Thus the process might be con.2, ,2,] cycloreaction or, equivsidered a [,2, alently, an antara-antara [3,3] sigmatropic or antaraantara Cope rearrangement.
+
\
2
R
3
1
a,R=H
+
OCHB
b,R = C a H , c, R = iC3H7
When l a , lb, or IC was heated in chloroform or methanol at 190") it was converted into a mixture of the corresponding isomers 2 and 3. Rearrangement of the 3,7-dideuterio derivative 4 gave only the 1,4dideuterio product 5, thus ruling out two possible mechanisms and favoring the Cope pathway. 6
\
iC3H7
4
/
D
5
Analogs without the C1 methoxy substituent showed no kinetically significant tendency to undergo a similar rearrangement. Orbital symmetry considerations and an acute sense of stereochemical requirements led Woodward and Hoffmann to view these rearrangements as examples of the symmetry-allowed but normally sterically unattainable trans-trans or antara-antara Cope rear(1) Supported initially by Public Health Service Research Grant No. GM-16576, and completed with funds from National Science Foundation Grant No. GP 9259, the Cities Service Oil Co., the Du Pont Co., and the Petroleum Research Fund, administered by the American Chemical Society. (2) T. Miyashi, M. Nitta, and T. Mukai, Tetrahedron Lett., 3433 (19671. - , (3) R. B. Woodward and R. Hoffmann, Angew. Chem., In?.Ed. Engl., 8, 781 (1969).
7
Were this interpretation of the rearrangement correct, kinetic comparisons between suitably analogous systems such as hydrocarbons constrained to suffer Cope rearrangement in the normal supra-supra mode or in the antara-antara mode would give a valuable quantitative response to the question : to what extent can an orbital symmetry possibility for a reaction make up for an intrinsically poor orbital overlap situation? Similar sorts of comparisons between geometrical isomers of the doubly bridged [ 14lannulene derivatives 8 and 9 have been made; temperature-dependent nmr studies indicate that the nearly planar ir configuration of 8 behaves as an aromatic Huckel system, while the isomer 9 having relatively poor overlap between orbitals on C1, C14,and C13 and on C8, C7, and Cs behaves as a mixture of valence isomers, equibrating only slowly
\ - -
(4) J. E. Baldwin and R. H. Fleming, Fortschr. Chem. Forsch., 15 (31, 281 (1970).
Baldwin, Kaplan
Thermal Antara-Antara Cope Rearrangements
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at lower temperatures and undergoing reactions typical of a p~lyolefin.~
Scheme I
CH,
CH2
11
0 8
9
+
Both 8 and 9 correspond formally to the (4n 2) rule, but the overlap differences are sufficient to cause strikingly dissimilar physical and chemical characteristics. For rearrangements wherein different types of orbital symmetry allowed processes may be postulated, we expect in the same way overlap considerations to cause marked variations in reaction kinetics.6 There being apparently no other examples of antaraantara Cope rearrangements, and given the importance of the theoretical issue, further experimentation and analysis seemed warrented. Results Tests for Degenerate Rearrangements of Bicyclo[3.3.0]0cta-2,6-dienes.~ Our initial attempts to uncover new examples of "antara-antara Cope rearrangements" were within the bicyclo[3.3.0]octa-2,6-diene series. This system, being somewhat more flexible than a bicyclo[3.2.0]heptadienone, might, we imagined, make possible a more facile reaction; the orbitals on each end of the Cope system could approach one another without causing great angular distortions. The D2 activated complex 10 would not necessitate a prohibitive strain energy.
13
12
1.NaH,CS,
14
15
3. 180-220°
16
The isomeric ketones 12 and 13 could be separated by glpc. Both showed molecular ion and M - CO peaks in their mass spectra at mje 122 and 94, and carbonyl infrared bands at 1735 and 1730 cm-', respectively. In nmr spectra, both showed two vinyl protons at 6 5.7 ppm and an allylic methine proton at 3.5 as a broad multiplet; ketone 12 had many peaks in the 3.0-1.7-ppm region (7 H), while 13 had three protons at 2.65 and four methylene protons at 2.15 PPmAttempted deuterium exchange with ketone 12 in basic aqueous tetrahydrofuran led to incorporation of three deuterons: the 7 H upfield multiplet was reduced in relative intensity to 4.2 H. We interpret the exchange data in terms of product 17 and the homoenolization phenomenon uncovered by Nickon and coworkers. lo
UD2 17
H
H
"H 10
First, the unlabeled bicyclo[3.3.0]octa-2,6-dienewas synthesized according t o Scheme I. This route proved successful and was easily adapted to the preparation of labeled analogs. It depended, except for the diazomethane ring-expansion step, on earlier work by Roberts and Gorham;8 the product 16 had infrared and nmr spectral properties in accord with the l i t e r a t ~ r e . ~ ( 5 ) E. Vogel, lecture in Houston, Tex., Nov 13, 1968; Proc. Robert A . Welch Found. Conf. Chem. Res., 12, 215 (1969); E. Vogel, U. Haberland, and H. Gunther, Angew. Chem., 82, 510 (1970); E. Vogel, A. Vogel, H.-I
