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Synthesis of an Unsymmetrical Carbodiphosphorane Through Skeletal Rearrangement of a Cyclopropylidene Precursor

Depending on the reaction conditions, the methylation of 1,1-bis(diphenylphosphino)cyclopropane (1) with CH3I or FSO3CH3 yields the monophosphonium salt 2 and the bisphosphonium salt 3, respectively. Double deprotonation of the cation in 3b does not afford an ylide with two terminal ylide functions but the unsymmetrical carbodiphosphorane 4. The formation of 4 occurs via nucleophilic attack of a terminal ylide function at the adjacent phosphonium centre in the primary ylide intermediate.