Mechanism 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) ... 

Formally related reactions are observed when anthracene [210] or arylole-fines [211-213] are reduced in the presence of carboxylic acid derivatives such as anhydrides, esters, amides, or nitriles. Under these conditions, mono- or diacylated compounds are obtained. It is interesting to note that the yield of acylated products largely depends on the counterion of the reduced hydrocarbon species. It is especially high when lithium is used, which is supposed to prevent hydrodimerization of the carboxylic acid by ion-pair formation. In contrast to alkylation, acylation is assumed to prefer an Sn2 mechanism. However, it is not clear if the radical anion or the dianion are the reactive species. The addition of nitriles is usually followed by hydrolysis of the resulting ketimines [211-213]. 

The fact that the iR drop is a smaller problem for UMEs compared to microelectrodes has another straightforward advantage, the substrate concentration can be increased substantially this makes the performance of electroanalytical studies under conditions similar to industrial conditions possible. For instance, the industrially important hydrodimerization of acrylonitrile to adiponitrile takes place at high concentrations in aqueous medium in the presence of tetraalkylammonium salts that form an aprotic medium in the vicinity of the electrode surface. The mechanism consists of a dimerization reaction of the radical anions of acrylonitrile formed upon reduction of acrylonitrile in the aprotic tetraalkylammonium layer, followed by protonation of the dimer in the aqueous phase (Eq. 87). However, at low to moderate concentrations of acrylonitrile, a change in mechanism occurs in favor of a two-electron reduction of acrylonitrile to propionitrile (Eq. 88). 

Some of the most successful applications of LSV and CV are concerned with the study of the kinetics and mechanisms of the reactions of electrode generated intermediates and a large share of the electrochemical literature deals with this aspect of voltammetry [8,9,13-38,72]. The majority of electrochemical reactions include radical ions as the primary intermediates, and the reaction schemes describing the conversion of a substrate A to products are typically composed of one or two one-electron transfers and one or two chemical steps. The examples include cathodic hydrogenations, (-l-2e , +2H ") (see Chapter 6) and hydrodimerizations (-l-e , -t-H ) (see Chapter 21), and anodic additions (—2e , - -2Nu ) (see Chapter 24), dehydrodimerizations (—e , —H ) (see Chapter 22), and substitutions (—2e , +Nu , —H ) (see Chapter 24), where Nu is nucleophile)... 

Hydrodimerization of activated alkenes is a well-established process. 7r-Electron-deficient heteroaromatic compounds activate a double bond similarly to a cyano or car-bethoxy group, and in accordance with that analogy vinylpyridines can be hydrodimer-ized. 4-Vinylpyridine [380] forms l,4-bis-(4-pyridyl)butane in 82% yield on electrolysis in a mildly alkaline solution containing methyltriethylammonium / -toluenesulfonate and some DMF. The mechanism is discussed in Chapter 21. 

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. 

Lactones such as coumarin and its derivatives (14) undergo hydrodimerization even in acidic hydroxylic media. When the supporting electrolyte contains Li, coumarin (14a) gives the 4,4 -hydrodimer in 90% yield in a methanol/acetate buffer (pH 5) [79]. The mechanism of the reduction of 4-methylcoumarin (14b) in DMF, MeOH, and Me0H/H20 has been examined [73]. In all cases the mechanism of dimerization was found to be of the RR type, again with a considerable increase in the observed second-order rate constant in going from aprotic to protic solvents (Table 3), [73]. 

The formal kinetics and diagnostic criteria for the different mechanistic pathways for intramolecular hydrodimerizations under steady-state conditions (LSV, RDE and polar-ography) have been established [115-117]. For the RS-type mechanisms it is normally assumed that the cyclized radical anion (or the neutral, cyclic radical formed by subsequent protonation) is more easily reduced than the starting material. For the RR-type mechanisms AE (the difference in reduction potential of the two electroactive groups) is normally assumed to equal the statistical difference expected for two electronically isolated groups in the same molecule. 

Some fundamental studies were made earlier. It was found that the hydrodimerization of an O, j6-unsaturated ketone (enone) was at least partially stereospecific, with the d, I dimer formed from the /ranj-enone and mainly the meso dimer formed from the cw-enone. Kanetsuna and Nonaka [50] also examined both the stereoselectivity and stereospecificity of the hydrodimerization in detail. It was indicated that either the reaction mechanism or the stereochemical course is changed by the nature (protic or aprotic) of the reaction media. Particularly, the latter is greatly influenced by the manner of adsorption of an anion radical intermediate on the cathode. It was also postulated to explain the experimental results that the radical anion is adsorbed at the )6-position, where the radical p electron is delocalized and the adsorption is affected by the nature of the media. 

This hydrodimerization process has been very thoroughly studied by Baizer and coworkers and others.5 A number of examples have been performed and the yields are usually >90%. Because variations in yield with many conditions, e.g., solvent, cathode material, have been studied and the mechanism has been elucidated, there are many possibilities for performing this reaction in high yield for other substrates. One of the most interesting applications is intramolecular coupling.6... 

The mechanism of hydrodimerization has been elucidated8 and illustrates some general considerations. 

Hydrodimerization is a special case of telomerization, where a (solvent) molecule A-B (the telogen, e.g., HzO) reacts with n molecules of an unsaturated molecule M (the taxogen) to yield oligomers or polymers of relatively low molecular mass (Eqs. 15 and 16). An important special case is the Kuraray 1-octanol process resulting from the products of Eq. (17) by subsequent hydrogenation. This industrially relevant reaction includes hydrodimerization of 1,3-butadiene [17]. Efficient catalysts are palladium-phosphine complexes, e.g., Pd2+/TPPMS (TPPMS = p(Qh4 -m-SO Na+)(C6H5)2). Little is as yet known on mechanisms. 

To illustrate this technique, consider the dimerization of the acrylonitrile anion radical (AN") in DMF (26). The electroreductive hydrodimerization of AN is used commercially to produce adiponitrile [(ANH)2], a precursor in Nylon production. The proposed reaction mechanism, an E1.C2 reaction [Section 12.1.1(b)], is... 

The complete study of the mechanism of the hydrodimerization of acrylonitrile is beyond the capability of the instrumental methods presently available. This is for three principal reasons (i) The concentration of electroactive species generally... 

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. 

The mechanism of hydrodimerization is, however, likely to vary with both the electrolysis conditions and the structure of the olefin. Hence it may be inapprppnate to seek similarities in mechanism particularly between acrylonitrile and the more substituted molecules which give stable anion radicals. The high selectivity of the hydrodimerization of acrylonitrile would indicate that the desired reaction route is very favourable compared to all competitive pathways. 

Despite many attempts to determine the mechanism for the hydrodimerization of acrylonitrile, surprisingly little is known with certainty. The study of the mechanism presents several difficulties ... 

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