40 YEARS OF UMPOLUNG IN ORGANOMETALLIC CHEMISTRY - FROM CARBANIONIC NUCLEOPHILES TO METALLIC ELECTROPHILES

The evolution of mechanistic insight into the nature of organometallic reactions over the last forty years is recounted from the personal pe rspective of a chemist trained in the empirical tradition of organomet allic chemistry. Starting from the viewpoint prevailing in the 1950s o f organometallics as potential carbanionic nucleophiles, this investig ator has become persuaded by his researches with Group 13 organometall ics of the merits of treating these reagents as organometallic electro philes. The profound effects that a tricoordinate boron center can exe rt on the structure and reactivity of boracyclopolyenes is a telling i llustration of such boron electrophilicity operating in an intramolecu lar fashion. The elucidation of the mechanisms of both the carbalumina tion and the hydroalumination of olefins and acetylenes has adduced co gent evidence for the rate-determining step being the electrophilic at tack of tricoordinate aluminum on the carbon-carbon Ti electron cloud of the substrate. Finally, in an investigation of the molecular basis for Ziegler-Natta catalysis, the Breslow-Natta soluble catalyst for th e polymerization of ethylene, Cp(2)TiCl(2)-R(n)AlCl(3-n), was examined in detail and compelling evidence has been adduced that the active ca talyst site is the solvent-separated ion-pair, [Cp(2)TiR](+) [R(n)AlCl (4-n)](-). Here again, the polymerization :reaction is initiated by an organometallic electrophile, indeed by an even more powerful cationic electrophile. The net effect of these studies has been an Umpolung in the manner with which this chemist and many of his colleagues view or ganometallic reaction mechanisms.