C–C Bond Formation (Part 1) by Addition Reactions: through Carbometallation Mediated by Group 4–7 Metals

Abstract

Organometals containing group 4–7 metals can undergo various types of carbometallation reactions with alkenes, alkynes, allenes, and higher dienes, enynes, diynes, and so on. In cases where organometallic reagents either are or can be readily made coordinatively unsaturated, they can undergo facile, concerted carbometallation reactions. Such reactions may be acyclic or cyclic processes. The latter may also include those involving metal–carbene and metal–carbine carbometallation reactions that are not discussed here. The cyclic carbometallation reactions involving metallacyclopropanes and metallacyclopropenes have indeed been dominant with the group 4–7 transition metals. Among these metals, only group 4 metals used in conjunction with alkylaluminum compounds have been shown to readily and extensively undergo controlled single-stage acyclic carbometallation reactions. Among them, the Zr-catalyzed carboalumination, especially methylalumination, of alkynes and the Zr-catalyzed asymmetric carboalumination of alkenes have been extensively developed into reactions of considerable synthetic utility. Among cyclic carbometallation reactions, the Ti- and Zr-promoted cyclic carbometallation reactions have been extensively developed into synthetically useful processes, despite the fact that the majority of them are still stoichiometric in these metals. Although Ti and Zr share many common features, many significant differences have also been noted. In this sense, these two metals are complementary to each other. Some other noteworthy and promising reactions include the Cr-catalyzed carboalumination of chiral propargyl electrophiles to give allenes, the acylmanganation, and the Mn-catalyzed carbomagnesation. The acyclic–cyclic dichotomy observed widely with organotransition metals will continue to play a fundamentally significant role in the development of additional carbometallation reactions of synthetic usefulness with the group 4–7 transition metals.