On the reduction of α,β-unsaturated (group 6) carbene complexes by NaBH4

Chromium and tungsten styryl Fischer carbene complexes 12 and 15 were transformed into Z-vinyl ether 13 and E-allyl ether 14 by NaBH4 reduction in EtOH. Deuterium labeling experiments demonstrate that the reaction occurs by the 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 general and has been applied to a series of alpha,beta-unsaturated alkoxy and aminocarbene complexes. In the case of chromium and tungsten alkynyl carbenes 38 and 39, NaBH4 reduction exclusively yields E-allyl ether 14. The intermediacy of an allenyl complex 41 obtained after the 1,3-rearrangement of the metal center is confirmed by deuterium labeling experiments. (C) 2000 Elsevier Science Ltd. All rights reserved.