On the reduction of alpha,beta-unsaturated (group 6) carbene complexes by NaBH4

Chromium and tungsten styryl Fischer carbene complexes 12 and 15 were trans formed into Z-vinyl ether 13 and E-allyl ether 14 by NaBH4 reduction in EtO H. Deuterium labeling experiments demonstrate that the reaction occurs by t he initial addition of the hydride to the carbene carbon atom, followed by a 1,3-rearrangement of the M(CO)(5) fragment. The process could involve the participation of an eta(3)-allyl chromium intermediate. The reaction is ge neral and has been applied to a series of alpha,beta-unsaturated alkoxy and aminocarbene complexes. In the case of chromium and tungsten alkynyl carbe nes 38 and 39, NaBH4 reduction exclusively yields E-allyl ether 14. The int ermediacy of an allenyl complex 41 obtained after the 1,3-rearrangement of the metal center is confirmed by deuterium labeling experiments. (C) 2000 E lsevier Science Ltd. All rights reserved.