In situ spectroelectrochemical study of the reductive coupling of benzaldehyde catalyzed by Ti(III) complexes in the presence of Mg(II)

The UV–vis spectroelectrochemical technique has been used to visualize for the first time, the formation of a trimetallic intermediate Ti(III)Mg(II)Ti(III) during the reductive coupling of benzaldehyde in the presence of electrogenerated Ti(III) and MgCl 2 . This intermediate is supposed to induce the high diastereoselectivity of the reaction. We have built a special thin layer cell adapted from the scheme proposed by A. Neudeck and L. Dunsch [‘J. Electroanal. Chem. 370 (1994) 17; J. Electroanal. Chem. 386 (1995) 135’] which allows the simultaneous recording of the cyclic voltammogram and of a great quantity of UV–vis spectra at a frequency of 1 spectrum per second by means of a UV–vis diode array spectrometer. The intermediate complex has been characterized by its UV–vis spectrum ( λ max =287, 321 and 370 nm). Its formation needs the simultaneous presence of electrogenerated Ti(III), MgCl 2 and benzaldehyde in the spectroelectrochemical cell. The global chemical reaction rate constant of Cp 2 TiCl 2 with ArCHO in the presence of MgCl 2 has been determined by cyclic voltammetry and spectroelectrochemistry. Both techniques are in good agreement and provide the same value, k =410±127 M −3/2 s −1