Synthesis of Titanium and Vanadium Diamine Bis(Phenolate) Complexes. Application of Ti Catalysts to the Pinacol Coupling Reaction

Diamine bis(phenolate) ligand precursors 6,6'-(2-(dimethylamino)ethylazanediyl) bis(methylene)bis(2,4-di-tert-butylphenol) (H2L, 1) and (S)-6,6'-((1-ethylpyrrolidin-2-yl) methylazanediyl) bis(methylene)-bis(2,4-di-tert-butylphenol) (H2L, 2) were used to prepare titanium and vanadium complexes. The compounds, with formulas [VOL(OPr)] (3), [TiLCl(THF)] (4), [TiLCl]2(μ-O) (5), [TiL(OPr)2] (6) and [TiLCl2] (7) were characterized by NMR and elemental analysis, and the molecular structures of 3 and 7 were obtained by single crystal X-ray diffraction. The reactivity of the titanium complexes [TiLCl2] and [TiLCl]2(μ-O) towards the pinacol coupling of benzaldehyde was tested and optimized in relation to the following parameters: i) catalyst precursor ii) metallic reductant (Mg, Sn, Zn, Mn), iii) catalyst to benzaldehyde proportion (from 1% to 20%), iv) reaction temperature (between 0oC and 55oC) and v) solvent (THF and toluene). It was found that the best results were obtained using [TiLCl2] and Mn/TMSCl in THF at 55oC. This system gave complete conversion of benzaldehyde to its pinacol product PhCH(OH)CH(OH)Ph with a diastereomeric excess of 71%, when the ratio [Ti]:benzaldehyde is 15%, and of 67 % when it is 7,5%.