Dimerization, oxidative Hydrodimerization

Before discussing what is known concerning the stereochemistry of hydrodimerization, it will be useful to discuss probable mechansims of hydrodimerization. It appears that a different mechanism is followed in acid than in neutral or alkaline media. In acid the first step is a one-electron reduction of the pro-tonated starting material to form a neutral radical, which then dimerizes. Thus for mesityl oxide (128) ... 

In an EC2j process, the initial ET step is followed by a second-order irreversible homogeneous reaction. For example, the feedback mode of SECM was employed to study the reductive hydrodimerization of the dimethyl fumarate (DF) radical anion [22]. The experiments were carried out in solutions containing either 5.15 or 11.5 mM DF and 0.1 M tetrabutylammonium tetrafluoroborate in A,A,-dimethyl form amide (DMF). The increase in the feedback current with increasing concentration of DF indicated that the homogeneous step involved in this process is not a first-order reaction. The analysis of the data based on the EC2 theory yielded the fc2 values of 180M-1 s-1 and 160M-1 s-1 for two different concentrations. Another second order reaction studied by the TG/SC mode was oxidative dimerization of 4-nitrophenolate (ArO-) in acetonitrile [23]. In this experiment, the tip was placed at a fixed distance from the substrate. The d value was determined from the positive feedback current of benzoquinone, which did not interfere with the reaction of interest. The dimerization rate constant of (1.2 0.3) x 108 M x s-1 was obtained for different concentrations of ArO-. 

The effect of the quaternary ammonium salt depends also on the substrate and on the electrode material [38]. Addition of tetraethylammonium salts thus sharply increases the yield of dimer product from acrylonitrile, has little effect upon the hydrodimerization of methyl acrylate, and does not affect the hydrodimerization of mesityl oxide. 

Promising results have been reported by various laboratories since 1990 on catalysis in molten salts, notably for catalytic hydrogenation, hydroformylation, oxidation, alkoxycarbonylation, hydrodimerization/telomerization, oligomerization, and Trost-Tsuji coupling [113]. A continuous-flow application to the linear dimerization of 1-butene on an ionic-liquid nickel catalyst system reached activities with TON > 18000 [116]. 

The reductive hydrodimerization of activated olefins was considered as a good test system for initial SECM studies, due to the fact that the dimerization rate constant could be tuned via the substituent activating the olefinic double bond." " For the relatively slow dimerization of the dimethyl fumarate (DF) radical anion, a large UME (25 jam diameter Pt electrode) was employed so that the value of K2 (Equation 7.19) was in the range where the SECM response would be sensitive to the kinetics. Experiments were carried out with solutions of 5.15 and 11.5mM DF in dimethylformamide (DMF), containing 0.1 M tetrabutylammoniumtetrafluoroborate as a supporting electrolyte. The oxidation of -tetramethyl-l,4-phenylenediamine (TMPD), included in solutions at a con-... 

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