The reaction of enantioenriched (S)-(+)-1,3-dimethylallene (13DMA) with 1,1-dichloro-2,2-difluoroethene (1122) produces the two cycloadducts 3 and 4 whose enantiomeric excesses (ee's) have been determined via the tertiary alcohols, formed by hydroboration, and via the dibromides by the use of chiral NMR chemical shift reagents. The major cycloadduct 3 is formed, retaining a higher percentage of the ee of the starting 13DMA (> 30%) than does the minor cycloadduct 4 (> 10%). The results of molecular modeling calculations on the conformations of the two reactants in approaching the activated complexes for diradical intermediate formation, and on models for the anti,anti and anti,syn diradical intermediates, provide for a most reasonable interpretation of the results involving one major continuous, low-energy reaction pathway via the anti,syn diradical intermediate leading to cycloadduct formation. The reaction of enantioenriched (S)-(+)-13DMA with 1,1-diphenylethene (DPE) does not lead to detectable formation of (2 + 2) cycloadducts, but does result in the racemization of 13DMA. The racemization of 13DMA is proposed to occur via the reversible formation of the achiral diradical intermediate 21 and the chiral diradial intermediate 22.
