Unusual Approaches to Organophosphorus Compounds: The Surprising Reactivity of Bis(methylene)phosphoranes and Related Phosphoranylidene Carbenoids

The reactivity of the isomeric LiX-phosphoranylidene carbenoids E-, Z-[P]C(X)Li ([P] =
Mes*PCTms2; X = Cl, Br) as well as the synthesis and reactivity of the corresponding
E-[P]C(F)Li carbenoid was examined. The carbenoids exhibit a pronounced carbanion
character, and differences in their reactivity mainly stem from the configuration of the
carbenoid centers and/or the nature of the halogen substituent. Reaction of the dibromo-substituted bis(methylene)phosphorane, [P]CBr2, with an excess of n-BuLi proceeded
through a dilithio intermediate ([P]CLi2), which was trapped with MeI and H2O. Endo-methylated bis(methylene)phosphoranes ([P]C(Me)X) exhibit a surprising acidity that,
depending on the substitution pattern, induces rearrangement and/or elimination reactions.
Rearrangement reactions accompanied by the loss of LiX afford a phosphirene that was
used as a ligand in a nickel complex. Fluoro-substituted bis(methylene)phosphoranes
([P]C(F)R) show weak interactions with an adjacent γ-silyl group, leading to configurationally stable Z-systems and exhibit carbenoid character in the case of the α-silyl-substituted
bis(methylene)phosphorane.