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

The silylenes Si(tBu2 bzam)R (tBu2 bzam=N,N'-bis(tertbutyl)benzamidinate; R=mesityl, CH2 SiMe3 ) attack the Ccarbene atom of the Fischer alkynyl(ethoxy)carbene complex [W(CO)5 {C(OEt)C2 Ph}] to give, after a striking rearrangement, zwitterionic σ-allenyl complexes in which the original carbene C atom forms part of the allene C3 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.