THE STEREOCHEMISTRY OF THE METHYLENECYCLOPROPANE REARRANGEMENT: EVIDENCE AGAINST THE ORTHOGONAL-ALLYLIC INTERMEDIATE

Article abstract of DOI:10.1016/S0040-4020(01)91259-1

The rearrangements of two optically pure derivatives of Feist's acid was studied: that of dimethyl trans-methylenecyclopropane-2,3-dicarboxylate (TRANS-diester) and trans-2,3-dicyano-methylenecyclopropane (TRANS-dinitrile).The optical purity and configuration of the products, methyl(Z)- and (E)-2-carbomethoxycyclopropylideneacetate (SYN- and ANTI-diesters) and (Z)- and (E)-2-cyanocyclopropylideneacetonitrile (SYN- and ANTI-dinitriles), establish that the rearrangements occur with predominant, but not exclusive, inversion of configuration at the migrating center.Investigation of the interconversions of SYN- and ANTI- diesters and dinitriles reveal that racemic product is not obtained, as would be expected from an orthogonal-allylic diradical intermediate, but that the enantiomer corresponding to antarafacial migration is slightly favored.All of the stereochemical results are explicable by application of the Doering-Sachdev diradical transition state hypothesis.