Forty 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 perspective of a chemist trained in the empirical tradition of organometallic chemistry. Starting from the viewpoint prevailing in the 1950s of organometallics as potential carbanionic nucleophiles, this investigator has become persuaded by his researches with Group 13 organometallics of the merits of treating these reagents as organometallic electrophiles. The profound effects that a tricoordinate boron center can exert on the structure and reactivity of boracyclopolyenes is a telling illustration of such boron electrophilicity operating in an intramolecular fashion. The elucidation of the mechanisms of both the carbalumination and the hydroalumination of olefins and acetylenes has adduced cogent evidence for the rate-determining step being the electrophilic attack of tricoordinate aluminum on the carbon-carbon π-electron cloud of the substrate. Finally, in an investigation of the molecular basis for Ziegler-Natta catalysis, the Breslow-Natta soluble catalyst for the polymerization of ethylene, Cp₂TiCl₂-R_nAlCl_3−n, was examined in detail and compelling evidence has been adduced that the active catalyst site is the solvent-separated ion-pair, Cp₂TiR]⁺ R_nAlCl_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 organometallic reaction mechanisms.