Influence of the halogen in titanocene halide promoted reductions

Ti-III species generated in tetrahydrofuran by a Zn-based reduction of Cp2TiX2 (X = Cl, Br, and I) have been identified by means of cyclic voltammetry and kinetic measurements to be a mixture of Cp2TiX, (Cp2TiX)(2), and Cp2Ti+. The distribution of these species is found to be dependent on the halogen in that Cp2Ti+ is formed in quite substantial amounts for X = Br and I, but not at all for X = Cl. In all three solutions Cp2TiX and (Cp2TiX)(2) are present in appreciable amounts characterized by a dimerization equilibrium constant of (1-3) x 10(3) M-1. The reactivity of (Cp2TiX)(2) is larger than or comparable to that of Cp2TiX and Cp2Ti+, as assessed in their reactions with benzyl chloride and benzaldehyde. In addition, the rate data are essentially independent of the halogen, and no correlation is observed with the standard potentials determined in cyclic voltammetry for the different Ti-IV/Ti-III-based redox pairs. This indicates high inner-sphere character of the electron transfer processes. Analysis of the diastereoselectivities found for the hydrobenzoin formation in the titanocene chloride promoted pinacol couplings of benzaldehyde shows high dl:meso ratios of 97:3 with a decreasing trend as X is changed from Cl to Br and I. This effect is suggested to be due to stereoelectronic interactions between the titanocene moieties in the developing Ti-IV-bound pinacolate of the transition state.