DOMINANT DOUBLE ROTATION IN THE THERMALLY INDUCED 1,2,4-TRIMETHYLSPIROPENTANE GEOMETRIC ISOMERIZATION

The four 1, 2, 4-trimethylspiropentanes interconvert at 561.7 K with first-order rate constants by reversible C1-C2bond fission. Double inversion occurs as fast as the two single-inversion processes in all cases. A factor analysis reveals that a correction for generation or destruction of 1, 4-proximal methyl relationships allows deduction of a substantial preference for double over single inversion. Specifically, if there is no kinetic preference for generating cis over trans 1, 2-dimethyl relationships, double inversion is favored over single inversion by a factor of 2.95 ± 0.10 in every case. Since the cis compounds should undergo double inversion slower than trans compounds if conrotation is the dominant stereorotation mode and vice versa if disrotation is dominant, it is concluded that the two trans isomers undergo conrotatory double inversion and the cis isomers undergo disrotatory double inversion. This is attributed to dynamical effects and not simply to the energy surface since the same biradical species is common to both the trans and cis reactants. The product distribution is unaffected by changes in pressure from 15 Torr to the liquid phase. © 1979, American Chemical Society. All rights reserved.