Electrochemically reduced titanocene dichloride as a catalyst of reductive dehalogenation of organic halides

Abstract

We have studied a reaction between the reduced form of titanocene dichloride (Cp 2TiCl 2) and a group of organic halides: benzyl derivatives (4-X C 6H 4CH 2Cl, X = H, NO 2, CH 3; 4-X C 6H 4CH 2Br, X = H, NO 2, PhC(O); 4-X C 6H 4CH 2SCN, X = H, NO 2) as well as three aryl halides (4-NO 2C 6H 4Hal, Hal = Cl, Br; 4-CH 3O-C 6H 4Cl). It has been shown that the electrochemical reduction of Cp 2TiCl 2 in the presence of these benzyl halides leads to a catalytic cycle resulting in the reductive dehalogenation of these organic substrates to yield mostly corresponding toluene derivatives as the main product. No dehalogenation has been observed for aryl derivatives. Based on electrochemical data and digital simulation, possible schemes of the catalytic process have been outlined. For non-substituted benzyl halides halogen atom abstraction is a key step. For the reaction of nitrobenzyl halides the complexation of Ti(III) species with the nitro group takes place, with the electron transfer from Ti(III) to this group (owing to its highest coefficient in LUMO of the nitro benzyl halide) followed by an intramolecular dissociative electron redistribution in the course of the heterolytic C Hal bond cleavage. The results for reduced titanocene dichloride centers immobilized inside a polymer film showed that the catalytic reductive dehalogenation of the p-nitrobenzyl chloride does occur but with a low efficiency because of the partial deactivation of the film due to the blocking of the electron charge transport between the electrode and catalytic centers.