| Abstract |
Titanium amine bis(phenolate) complexes of general formulas [Ti(L)Cl(THF)], [Ti(L)X-2] and [\Ti(L)Cl\(2)-mu-O] (X = Cl, (OPr)-Pr-i) were assessed as catalysts of PhCHO pinacol coupling. In optimal conditions (7.5 mol\% catalyst load, 55 degrees C and 16 h of reaction time in THF) and TMSCI as additive, [Ti(L-3)Cl-2] (11) is the most efficient catalyst giving 100\% conversion and 95\% yield in diol with a ratio DL:meso= 57:43 and 27\% ee. Using acidic additives, lower diol yields but higher diasteriomeric ratios are obtained. No significant improvements are observed when p-substituted benzaldehydes are used. The reduction of the Ti(IV) complexes to Ti(III) species is identified by EPR and a stoichiometric reaction between a [Ti-III(L1)Cl(THF)] (6) and benzaldehyde shows that the pinacol coupling occurs with a conversion in diol of 72\%. Different D,L ratios obtained in the absence or in the presence of reducing metals indicate that different Ti(III) catalysts may form, namely mono-and trinuclear species. (C) 2015 Elsevier B.V. All rights reserved. |
