Electrocatalytic dehalogenation of α-bromoketones in the presence of Cp2TiCl2

Abstract

Processes of direct and electrocatalytic (in the presence of electrochemically reduced Cp 2 TiCl 2 ) reduction of three α bromoketones containing the C(sp 3 )-Br or C(sp 2 )-Br bond, viz., 2 bromo and 2,6 dibromo 4,4 dimethylcyclohexa 2,5 dien 1 ones and α bromo acetophenone, were studied by cyclic voltammetry and preparative electrolysis. In all cases, the dissociative electron transfer proceeds via the concerted mechanism. Preparative electrolysis of these α bromoketones in the presence of Cp 2 TiCl 2 affords the reductive debromination prod ucts in 40-80% yield at low cathodic potentials (-0.85 V vs. Ag/AgCl/KCl). In the case of 2,6 dibromo 4,4 dimethylcyclohexa 2,5 dien 1 one in the potentiostatic regime, only one bromine atom can be eliminated selectively.In most cases, the direct electrochemical reduction of alkyl and vinyl halides occurs at rather high negative po tentials 1 and, therefore, the dehalogenation of compli cated polyfunctional compounds is usually nonselective. The use of non direct electrocatalytic reduction (media tor redox catalysis) makes it possible to decrease consid erably the potential of the reaction, which can be substan tial for selectivity of the process.It is known 2 that in the nature many dehalogenation processes proceed via the reduction mechanism involving the catalysts containing the compounds of Co, 3,4 Ni, 5-7 Cu, 8 and other metals. We have previously shown 9,10 that the electrochemically reduced titanocene dichloride (Cp 2 TiCl 2 ) can serve as a catalyst for the reductive dehalogenation of benzylic halides, whereas aryl halides usually undergo no dehalogenation. The present work con tinues these studies and is devoted to the elucidation of the applicability of this approach to cleavage of the C-Hal bond of other types. α Bromoketones containing the C(sp 3 )-Br or C(sp 2 )-Br bond, which is activated by the adjacent carbonyl group, namely, α bromoaceto phenone (1), 2,6 dibromo (2) and 2 bromo 4,4 di methylcyclohexa 2,5 dien 1 one (3), were chosen as ob jects for the study. The latter two compounds are of spe cial interest, because they represent the structural frag ment of discorhabdine molecules C and E (4 and 5, re spectively), which are present in the Lafrunculia sea sponge. It is known 11 that the antimicrobial properties of natural discorhabdines are due to the ability of the C-Hal bond to cleave. This cleavage proceeds presumably via the reduction mechanism. Therefore, it was interesting to reveal whether the electrocatalytic dehalogenation of simi lar compounds is possible in the presence of the electro chemically reduced titanocene dichloride and whether this process is a model of reactions involving discorhabdines in the nature or not. ExperimentalCyclic voltammograms (CV curves) were recorded using an IPC Win potentiostat at the stationary Pt electrode (d = 5 mm) against Bu 4 NBF 4 in anhydrous organic solvents (THF, MeCN) at 20 °С in a special electrochemical cell switched on according