Advances in the synthesis and reactivity of group 6 metal allenyls

Abstract

A class of scarcely known allenyl metal complexes are those of the type [M(CO) 5 {C(OR) = C = CR′R″}] (M = group 6 metal). These complexes have been proposed as intermediates in reactions of Fischer alkynylcarbene complexes with nucleophiles, but they had seldom been isolated. The isolation of stable group 6 metal allenyl complexes by addition of NHCs and heavier tetrylenes to chromium(0) and tungsten(0) (Fischer) alkynyl(alkoxy)carbene complexes has reactivated this classical and important field of stoichiometric organometallic chemistry. NHC-derived group 6 metal allenyl complexes show an unprecedented reactivity in radical and photochemical reactions. The use of heavier tetrylenes in reactions with group 6 metal alkynyl Fischer carbene complexes has opened a new field of research, in which unprecedented reactivities beyond the well-stablished 1,4-addition have been encountered. Thus, the nucleophilic attack of the heavier tetrylene to the carbene Cα atom promoted the sequential migration of the ethoxy group to the E (E = Si, Ge) atom, the transfer of an amidinato N atom to the Cα atom and, finally, a 1,3-shift of the M(CO) 5 to form novel group 6 metal allenyl complexes. The chemistry discussed in this article opens ways to use other carbenes and heavier carbene analogues as reagents towards group 6 metal alkynyl Fischer carbene complexes, probably leading to novel and interesting results.