Titanium and Zirconium Mediated or Catalyzed Umpolung Reactions

This review presents current strategies of using titanium and zirconium reagents to achieve umpolung of carbonyl and imine functionalities. The reactions of organometallic compounds, including those that are equivalent to acyl anions, α-amino carbanions, α-hydroxy carbanions, and homoenolates, are among the most powerful methods to form carbon-carbon bonds. The remarkable chemoselectivity, regioselectivity, and stereoselectivity observed in many cases once more demonstrate great synthetic application of organotitanium and organozirconium compounds.