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

The silylenes Si(tBu2bzam)R (tBu2bzam=N,N′‐bis(tertbutyl)benzamidinate; R=mesityl, CH2SiMe3) attack the Ccarbene atom of the Fischer alkynyl(ethoxy)carbene complex [W(CO)5{C(OEt)C2Ph}] 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. A striking stereoselective rearrangement, which ends in unprecedented zwitterionic allenyls having one end of the allene C3 fragment inserted into a Si−N bond, takes place in reactions of amidinatosilylenes with a Fischer alkynylcarbene.