Hydrodimerization of Activated Olefins

Intramolecular hydrodimerization of activated olefins has been exploited for elegant one-step cyclizations and heterocouplings (Fig. 53) [263, 278, 279]. 

Cathodic hydrodimerization of active olefins and carbonyl compounds. 

In addition to the cathodic hydrodimerization of activated olefins [see 3.2.1.1], the electrosyntheses of substituted benzaldehydes are among the few electroorganic reactions which are carried out on a large scale industrially. 

In addition to the cathodic hydrodimerization of activated olefins and the Kolbe reaction, the anodic dimerization of CH-acidic compounds is another possibility for the electrochemical C—C coupling. Monsanto 281 > has used the anodic dimerization of malonates in a laboratory synthesis of intermediates for useful sequestrants and detergency builders. 

Anodic dimerization of electron-rich olefins, the mirror image process to cathodic hydrodimerization of activated olefins (Sect. 12.2), affords a one-step synthesis for substituted butanes(Eq. (175) ) 268)( dienes (Eq. (176) ) 26S precursors of polyenes (Eq. (177)) 36,385 and 1,4-dicarbonyl compounds (Eq. (178)) 35>36). 

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-... 

A wide variety of activated olefins (126) undergo reductive electrochemical dimerization to compounds of structure 127 (electrolytic hydrodimerization) 129 i. While the product 127 is capable of existing in either dl or meso modifications, relatively little attention has been paid to the stereochemistry of hydrodimers... 

Baizer and coworkers established the most brilliant industrial electroorganic synthesis of the hydrodimerization of acrylonitrile to adiponitrile. They extended this hydrodimerization to a variety of activated olefins and in some cases [41 3] paid attention to the stereochemistry of products. However, their stereochemical data were not enough to discuss the stereochemical course of the reaction. Afterward, an attempt was made to provide a working hypothesis in the hydrodimerization of cinnamates by considering an orientated adsorption of radical anion intermediates on a cathode surface, but this was not persuasive because of a lack of experimental data on the stereochemistry of both the starting olefins and products. Recently Utley and coworkers [44-46] have reported stereochemical data of hydrodimers derived from a variety of cinnamic acid esters with chiral alcohol components. 

The large number of synthetically useful intermolecular hydrodimerizations and intramolecular cyclizations of activated olefins to complex carbon skeletons involves in most cases radical anions as key intermediates [152]. 

The hydrodimerization of non-activated olefins at a platinum electrode affords the corresponding d, /-isomeric products in excess ( > 80%) [57]. 

While the hydrodimerization of acrylonitrile to adiponitrile is the only one to have been exploited commercially, it should be recognized that, using the conditions described above, hydrodimerization is a general reaction for activated olefins, i.e. olefins with an electron-withdrawing group on the double bond. The reaction... 

In another approach to the study of the mechanism, many investigators have looked at the hydrodimerization of other activated olefins, particularly those with two or more substituents (e,g. methyl cinnamate, diethyl fumarate) whose anion radicals, are much more stable. With such olefins the mechanism of hydrodimerization may be studied by cyclic voltammetry or with a rotating ring-disc electrode. It is, however, never clear whether the chemistry of stable anion radicals will be analagous to the unstable acrylonitrile anion radical. 

---