Halides reductive dimerization

Orientation effects by the electrode surface have been invoked in a large number of other cases (for reviews of the stereochemistry of electrode processes, see Eberson and Homer, 1973 Fry, 1972a), for example in halide reduction (for a review, see Casanova and Eberson, 1973), formation of dimers in the Kolbe reaction (Hawkes et al., 1973), reduction of systems with double and triple bonds (Horner and Roder, 1969), and anodic coupling of 1-methylcorypalline... 

Reduction of halides. Benzylic and allylic halides are dimerized in good yield by the complex of copper metal and cyclohexylisonitrile (Wurtz reaction). Secondary halides are dimerized in low yield, and primary halides do not react. The system is less active than Cu(I) salts and resembles potassium graphite. 

In the reductive dimerization of methyl cinnamate to a cyclopentanone [Eq. (5)], similar yields are found at the cathode [42] and with metals (sodium, THE, and TBAI, —78°C) [40]. Because of the potential selective conversion at the electrode, halides can be reduced at the cathode to carbanions in the presence of carbonyl compounds, which are reduced at more cathodic potentials. This way labile carbanions can be obtained and reacted under conditions in which the same species generated by a metalorganic route would decompose. Eor example, trichlorobromoalkane can be cathodically converted in the presence of aldehydes to a dichloromethyl anion 0°C [route a, Eq. (6)] and be trapped to form a dichlorotetrahydrofuran, but for the metallorganic route [route b, Eq. (6)] a reaction temperature of — 110°C is necessary [43]. 

Reaction with Sulfonyl Halides. Arylsulfonyl halides undergo reductive dimerization to form the corresponding disulfides (eq 35). Alkylsulfonyl halides, however, undergo this process under somewhat more vigorous conditions. Although sulfones generally do not react with TMS-I, certain cyclic sulfones are cleaved in a manner analogous to lactones. ... 

Use of strongly anionic nucleophiles such as alkylmagnesium halides and lithium alkyls leads to reductive dimerization or displacement of the olefin. It is quite possible however that alkylation of olefin-iron cations (IX) would be feasible with alkyl derivatives of zinc, cadmium, or tin reagents. Alkylation of dienyliron cations with dialkylzinc and dialkylcadmium reagents has recently been demonstrated (Section III,A,4). Alkylation of olefin-iron cations can be achieved under mild conditions with cr-bonded allyliron compounds [Eqs. (9) and (10), (Rosan et ai, 1973)]. 

Although alkylmagnesium halides lead to reductive dimerization of the dienyl cations, synthetically useful alkylations and arylations have recently been achieved using zinc and cadmium reagents [Eqs. (41) and (42) (Birch and Pearson, 1975)]. 

A synthetic sequence has been discovered in which aldehydes which are not branched at the a-position undergo formal reductive dimerization to give symmetrical olefins (e.g. Scheme 14). At the key step, a-stannylalkyl halides, which are stable and easily handled, smoothly couple on treatment with n-butyl-lithium under very mild conditions (-78 to 0 °C). Furthermore, the difference in reactivity between a-stannylalkyl iodides and chlorides enabled a moderate yield of a cross-coupled olefin from two different aldehydes to be obtained, a-Branched aldehydes exhibit a different pattern of reactivity, furnishing terminal olefins instead of coupled products (Scheme 15), probably for steric reasons. 

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