Stereochemical features of the (2+2) cycloaddition reactions of chiral allenes. V. Cycloaddition of 1‐tert‐butyl‐3‐methylallene with 1,1‐dichloro‐2,2‐difluoroethene

Abstract

AbstractThe (2 + 2) cycloaddtion reactions of 1‐tert‐butyl‐3‐methylallene (tBMA) with radicophiles were investigated. The attempted cycloaddition reactions with N‐phenylmaleimide, acrylonitrile and methyl acrylate produce only (4 + 2) cycloadducts of 1‐tert‐butyl‐1,3‐butadiene which is formed by the more rapid [1.3] hydrogen sigmatropic rearrangement of the tBMA. The (2 × 2) cycloaddition of tBMA with 1,1‐dichloro‐2,2‐difluoroethene (1122) occurs more rapidly than does the sigmatropic rearrangement, and produces a mixture of the four cycloadducts 1–4. The cycloaddition of 1122 with enantioenriched tBMA produces one cycloadduct (3) in which ca 91% of the enantiomeric excess (ee) of the tBMA is transferred to the cycloadduct. The other three cycloadducts are formed retaining much less of the ee of the starting tBMA. The results are interpreted on the basis of molecular modeling calculations carried out on the 1122–1,3‐dimethylallene system reported previously. It is suggested that cycloadduct 3 is formed by essentially only one continuous minimum‐energy reaction pathway, while cycloadducts 2 and 4 are formed by two competitive minimum‐energy reaction pathways which result in the formation of cycloadducts possessing opposite absolute configurations. The combined contributions of the two competitive pathways result in much lower overall degrees of transfer of the ee of the tBMA to the diradical intermediates and cycloadducts.