Conformational control of stereochemistry in the non-concerted, degenerate rearrangement of a vinylcyclopropane

The stereochemistry of the degenerate, thermal vinylcyclopropane rearrangement of (−)-2,2,3-trideuterio-Δ3-thujene has been elucidated. Determination of partitioning among the three products of enantiomerization and rearrangement to (+) and (−) 5,5,6-trideuterio-Δ3-thujene reveals that the major path is forbidden by orbital symmetry and is therefore non-concerted, while the minor paths are equally divided between a forbidden and an allowed path. The high degree of stereoselectivity of this non-concerted rearrangement is explained by a small barrier to conformational equilibration. The rate of the racemization is given by the expression log k (sec−1) = 14·33-43·4/2·303 RT.