Ab initio calculations on spiropentane stereomutations lead to a reinterpretation of the experimental results

The potential energy surfaces for stereomutation of spiropentane (1) and ci s- and trans-1,2-dimethylspiropentanes (4 and 7) have been explored, using ab initio methods. In diradicals 2,5, and 6, which are formed by cleavage o f the peripheral bond between C-1 and C-2 in the spiropentanes, the weakly electron-donating cyclopropane ring, results in the lowest energy pathway f or stereomutation of all three being computed to be conrotatory. A larger p reference for double over single rotation is computed in 4 than in 7, in ag reement with the experimental results reported in 1980 by Gajewski and Chan g. However, in contrast the assumption made by these authors, our calculati ons find that the s-cis-methyl conformation in diradical 6 is lower in ener gy than the s-trans-methyl conformation in diradical 5, and moreover, 6 is statistically favored over 5 by a factor of 2. Thus, double rotation is bot h computed and found to be preferred by more in the stereomutation of 4 tha n of 7 because 4 undergoes conrotatory opening to 6, the lower energy dirad ical. A long-range attraction between the s-cis methyl group at C-1 and the nonbonding p-pi AO at C-3 in 6 is shown to contribute to stabilizing this diradical.