Unexpected Zwitterionic Allenyls from Silylenes and a Fischer Alkynylcarbene: A Remarkable Silylene-Promoted Rearrangement.

The silylenes Si(tBu bzam)R (tBu bzam=N,N'-bis(tertbutyl)benzamidinate; R=mesityl, CH SiMe ) attack the C atom of the Fischer alkynyl(ethoxy)carbene complex [W(CO) {C(OEt)C Ph}] to give, after a striking rearrangement, zwitterionic σ-allenyl complexes in which the original carbene C atom forms part of the allene C fragment and also of a Si-C-N-C-N five-membered ring after insertion into a Si-N bond of the original amidinatosilylene. These remarkable allenyl products, which contain two stereogenic groups, are selectively formed as single diastereomers.