HYPOSTROPHENE - AN AM1 STUDY OF STRUCTURE, MECHANISM OF DEGENERATE COPE REARRANGEMENT, AND COMPARISONS WITH BULLVALENE AND OTHER C10H10 ISOMERS

Hypostrophene (1) has been observed to undergo a degenerate Cope rearrangement similar to that well known to occur in bullvalene 3, another (CH)(10) isomer. Neither the activation energy nor the detailed molecular structure of 1 has been determined experimentally. At elevated temperatures 1 rearranges to 4, still another (CH)(10) isomer. We present geometry-optimized AM1 calculations for 1, its Cope rearrangement transition state structure 1', and the related isomer 4. We compare these results with those of similar calculations, already in the literature, for other (CH)(10) isomers, such as pentaprismane (2), bullvalene (3), and [10]annulene (5). The Cope rearrangement activation energy of 1 turns out to be larger than that for 3, in agreement with the observations that the rearrangement of 3 is fast compared to 1. Finally, we combine several pieces of data from experiment and theory to give consistent estimates of strain energies of the (CH)(10) isomers 1-5.