Pyrazolylborate-zinc alkoxide complexes. 1. Basic properties, methylations, and heterocumulene insertions

While a solution of TpPh,MeZn-OH in methanol contains only traces of TpPh,MeZn-OMe, according to the equilibrium constant K = 5.8 x 10(-4), the reactions of TpPh,MeZn-OH with the electronegative alcohols trifluoroethanol and hexafluoro-2-propanol easily yield TpPh,MeZn-OCH2CF3 and TpPh,MeZn-OCH(CF3)2. The extremely hydrolytically sensitive TpPh,MeZn-OR complexes, with R = Me, Et, i-Pr, and CH2CH2F, as well as TpCum,MeZn-OR, with R = Me and i-Pr, are accessible from the Tp*Zn-hydride complexes and the corresponding alcohol. Alkylations with methyl iodide have revealed the high nucleophilicity of TpPh,MeZn-OMe by conversion to dimethyl ether and TpPh,MeZn-I. This conversion occurs rapidly not only with pure TpPh,MeZn-OMe but also with TpPh,MeZn-OH (as such or in the presence of methanol) and with TpPh,MeZn-OCOOMe. A relation of the Tp*Zn-alkoxides to the function of the zinc enzyme alcoholdehydrogenase exists in the reaction of TpPh,MeZn-OCH(CH3)2 with aromatic aldehydes, which yields acetone and the corresponding benzyl oxides TpPh,MeZn-OH2Ar. The heterocumulenes carbon dioxide, carbon disulfide, isothiocyanates, and one isocyanate are inserted into the Zn-OR bonds, yielding one alkyl carbonate complex (TpPh,MeZn-OC(O)OMe), two xanthogenate complexes (TpPh,MeZn-SC(S)OR), three iminothiocarbonate complexes (TpPh,MeZn-SC(NR')OR), and one alkyl carbamate complex (TpPh,MeZn-NR-COOMe). All insertion reactions can be described by a common mechanism involving a four-center intermediate in which the most basic heteroatom of the heterocumulene is attached to zinc.