Stereochemistry hydrodimerization

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... 

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) ... 

Useful information concerning the stereochemistry of hydrodimerization comes from a study of the electrochemical behavior of taws-2,2,6,6-tetra-... 

There is insufficient information on the stereochemistry of the experimentally less simple hydrodimerization of aliphatic carbonyl compounds in a protic... 

This section concerns the classical hydrodimerization of alkenes activated by electron-withdrawing substituents, as in Eq. (1). The literature in this area is extensive and this chapter cannot be exhaustive. The focus will be on typical reactions and general conclusions, which may serve as guidelines for further work. Special emphasis will be put on the effect of reaction conditions on the mechanisms, product selectivity, and stereochemistry. Section II.A deals with the monoactivated alkenes, that is, structures of the type 1 where R and R" are H, alkyl, or aryl Sec. II.B deals with intramolecular coupling reactions where two identically activated alkenes are linked together within the same molecule. The reactions of alkenes activated by two electron-withdrawing groups either in a, a- or in a,yS-positions, are treated in Sec. II.C. 

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 stereochemistry of the cathodic reaction of carbonyl compounds has been extensively studied from synthetic and mechanistic aspects. In this section the reaction is discussed in three parts hydrogenation to alcohols, hydrodimerization to pinacols, and cross-dimerization with unsaturated compounds. 

In the hydrodimerization of 2,3-dimethylindanone, 11 hydrodimers, including 8 stereoisomers, were analyzed [150]. The stereochemistry of hydrodimers formed from 2-methylindenone in an oxygen-saturated DMF solution would also be complicated, but it is known only that the stereochemistry around the central C-C bond is a threo form [151]. Another complication in the stereochemistry of a hydrodimer was observed in the dimerization of benzoin at a mercury-plated copper cathode in a basic aqueous ethanol solution A single stereoisomeric hydrodimer was formed [152]. 

The stereochemistry of hydrodimerization of aliphatic carbonyl compounds has very rarely been studied. It was reported that an aliphatic ketone gave an almost equal quantity of the isomeric hydrodimers at a mercury cathode [160]. 

In the isotope experiment, the stereochemistry of the insertion is different, depending on whether a-H or a-D agostic interaction is involved. If an isotope effect is found, agostic transition states are indicated. For example, Kraule-dat and Brintzinger [69a,b] have shown that in hydrodimerization of deuter-ated 1-hexene with methylaluminoxane (MAO)-activated zirconocene dichloride (Cp2ZrCl2) the product distribution is consistent with an a-agostic assisted insertion (Eq. 2.24). 

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