Ring cleavage rearrangement of methyl-sunbstituted cyclopropylmethylboranes

Article abstract of DOI:10.1016/0022-328X(88)80667-3

2,2,3-Trimethyl- and 2,3-dimethylenecyclopropane were prepared and hydroborated using various borane reagents.Changes in the borane solutions as a result of heating were studied by NMR and by oxidation to alcohols.Ring-cleavage rearrangement reactions were observed, analogous to rearrangements previously found for cyclopropylmethyl Grignard reagents and for unsubstituted cyclopropylmethylboranes.Methyl substitution slows the rearrangement involving cleavage of the bond to the substituted ring carbon, and has an especially large effect when cis to the metallomethyl group.In the 2,2,3-trimethylcyclopropylmethyl system, the ring-cleavage rearrangement does not go to completion, but instead approaches an equilibrium in which a substantial concentration of the trans isomer remains.Rearrangement in the 2,3-dimethylcyclopropylmethyl system occurs with retention of configuration at the ring carbon to which the boron migrates.The results of the present work suggest a mechanistic reinterpretation of the carboboration of bicyclobutane.