Interaction of t-butyllithium and triphenylmethylenephosphoranes

NMR experiments show that a directed ortho-metalation occurs at one of the aromatic rings of triphenylmethylenephosphoranes (Ph3P-CHR, R=H, Me) when the phosphorus ylide is treated with 1 equiv. t-BuLi. If pre-coordination of t-BuLi is inhibited by a sterically demanding and electron-withdrawing substituent (R=SiMe3) no metalation is observed.