| Abstract
| - The potential energy surfaces for stereomutation of spiropentane (1) and cis- and trans-1,2-dimethylspiropentanes (4 and 7) have been explored, using ab initio methods. In diradicals 2, 5, and 6, whichare formed by cleavage of the peripheral bond between C-1 and C-2 in the spiropentanes, the weakly electron-donating cyclopropane ring results in the lowest energy pathway for stereomutation of all three being computedto be conrotatory. A larger preference for double over single rotation is computed in 4 than in 7, in agreementwith the experimental results reported in 1980 by Gajewski and Chang. However, in contrast to the assumptionmade by these authors, our calculations find that the s-cis-methyl conformation in diradical 6 is lower inenergy than the s-trans-methyl conformation in diradical 5, and moreover, 6 is statistically favored over 5 bya factor of 2. Thus, double rotation is both computed and found to be preferred by more in the stereomutationof 4 than of 7 because 4 undergoes conrotatory opening to 6, the lower energy diradical. A long-range attractionbetween the s-cis methyl group at C-1 and the nonbonding p-π AO at C-3 in 6 is shown to contribute tostabilizing this diradical.
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