Versatile Zirconium Reductants and Carbon-Carbon Coupling Agents Selectively Accessible from the 2:1 Molar Aggregate of n-Butyllithium and Zirconium(IV) Salts

In previous studies of transition metal alkyls the 2:1 molar aggregate of n-butyllithium and zirconium(IV) salts has been found to react both with benzylic hydrocarbons and aromatic carbonyl derivatives in diverse and useful ways. In the present study the reactions of the aggregates, 2nBuLi center dot ZrE4 (E = Cl, OEt), with benzaldehyde have involved carbometallation, hydrometallation and reductive dimerization (paths 1-3) in THF and were selectively achievable by temperature control alone. First, at -78 degrees C benzaldehyde underwent carbolithiation to give upon hydrolysis 1-phenyl-1-pentanol. However, short-term reaction times and prompt D2O quenching revealed that with Zr(OEt)(4) both benzaldehyde and 1-phenyl-1-pentanol were deuteriated, consistent with the presence of a phenyl(lithioxy)carbene intermediate. The observed dimerization of benzaldehyde to benzyl benzoate by lithium 2,2,6,6-tetramethylpiperidide is also consistent with such a phenyl(lithioxy)carbene intermediate. Second, at 25 degrees C the 2nBuLi center dot ZrE4 aggregate reduced benzaldehyde exclusively to benzyl alcohol, which observation is consistent with the formation of the hydrozirconating agent, H2ZrE2. Third, heating the aggregate at reflux and subsequent reaction with benzaldehyde produced solely the reduced dimer, 1,2-diphenyl-1,2-ethanediol with high stereoselectivity: E = Cl, rac/meso of 93:7 and E = OEt, rac/meso of 100:0. The proposed mechanism involves the formation of ZrE2, the epizirconation of benzaldehyde and the insertion of the second benzaldehyde into the zirconaoxacyclopropane under steric control. Finally, the high selectivity in hydrozirconation and reductive dimerization shown by 2nBuLi center dot ZrE4 appears at this time to be superior to that attainable with analogous titanium or hafnium aggregates.