UNSATURATED NEOPENTYL COMPOUNDS . EFFECT OF METHYL SUBSTITUTION ON SOLVOLYTIC COURSE OF HOMOALLENIC BROSYLATES

The acetolysis products of a series of 2,2-dimethyl-3,4-pentadienyl p-bromobenzenesulfonates with methyl substituents at C-3 and/or C-5 have been determined at 40 or 55°. In the absence of a methyl substituent at C-3, the starting brosylates yield rearranged allenic products exclusively: 2-methyl-4,5-hexadien-2-yl acetate, 2-methyl-2,4,5-hexatriene, and/or 2-methyl-1,4,5-hexatriene with one or two methyl substituents at C-5. When the starting brosylate has a methyl substituent at C-3, the acetolysis also yields cyclobutane-ring-containing acetates and hydrocarbons, as well as conjugated and/or cross-conjugated rearranged acyclic nonallenic trienes. The effect of methyl substituents on the acetolysis course of these homoallenic neopentyl-type brosylates is compared with similar effects reported previously for the solvolysis of other homoallenic arenesulfonates which exhibit π-electron participation and it is concluded that the initial intermediate is a cyclopropylcarbinyl-type vinyl ca ion in each case. It is suggested that the nonallenic acyclic trienes which are observed when the starting neopentyl-type homoallenic brosylate also contains a methyl group at C-3 result from an unusual 1,4-hydride shift in the initial gem-dimethyl-cyclopropylcarbinyl-type vinyl cation which is followed by a 1,5-proton shift and/or proton loss and in some cases by a 1,5- or 1,7-sigmatropic hydrogen shift. © 1969, American Chemical Society. All rights reserved.