Asymmetric induction and racemization in allenic sulphoxides

Menthyl sulphinates with Grignard reagents derived from α-acetylenic halides give mixtures of allenic and acetylenic sulphoxides. The extent of asymmetric induction in the allene groups is compared with that obtained in a simple alkyl sulphoxide by use of 1-methylheptylmagnesium bromide. Absolute configurations are assigned to the two chiral centres in the sulphoxides; that of the sulphinyl group follows from that of the sulphinate ester, and that of the allene system is derived from the sign of rotation of the allenic sulphone obtained by oxidation. Allenic sulphoxides undergo racemisation of the sulphinyl group under conditions in which the chirality of the allene system is unaffected. The activation parameters for the process in which the sulphinyl group is racemised provide evidence for sulphoxide–sulphenate equilibration, and this conclusion is supported by interception, with an amine, of an intermediate sulphenate ester.