Preparation and Fate of Cubylcarbinyl Radicals

摘要

The cubylcarbinyl radical has been generated from cubylcarbinyl bromide and fromthe N-hydroxy-2-pyridinethione ester of cubylacetic acid under various conditions favoring hydrogen-atom transfer to the radical. Only when selenophenol in high concentration is used as the hydrogen donor is any methylcubane formed. Otherwise the cubylcarbinyl radical rearranges. There is no evidence of a 1.2-shift into the homocubyl system. Instead, one, two, or three bonds of the cubane nucleus cleave, leading to a variety of olefinic products. For the most part, these have been characterized. A mechanistic scheme accounting for their formation is presented; sequential Sigma bond breaking occurs regioselectively, favoring processes in which there is good overlap between the radical orbital and that of the breaking bond. The distribution of products is shown to depend qualitatively on the time the radical intermediates are let live, that is, on the concentration and effectiveness of the hydrogen atom transfer agent. Cubane and many substituted cubanes are very stable compounds, indeed remarkably so considering the extreme strain energy of the skeleton. There is no symmetry-allowed pathway for ring opening.