Diketones cathodic reduction

Under different conditions (in aqueous electrolyte) the selectivity of the cleavage reaction may be perturbed by the occurrence51-53 of a dimerization process. Thus, while the major process remains the two-electron reductive pathway, 20% of a dimer (y diketone) may be isolated from the cathodic reduction of PhC0CH2S02CH3. The absence of crosscoupling products when pairs of / -ketosulphones with different reduction potentials are reduced in a mixture may indicate that the dimerization is mainly a simple radical-radical coupling53 and not a nucleophilic substitution. 

Lipkin et al., 1963). If some atoms of the system of double bonds are different from carbon, additional possibilities open up, such as in the capture of the intermediate in the cathodic reduction of 1,3-diketones (22) to give acetates of cyclopropanediols (Curphey et al., 1969). 

Armand and Boulares carried out an extensive study on the cathodic reduction of five 1,3-dione derivatives in 50 % Me0H-H20 at mercury (Table 4). Each dione exhibited one 2e-wave which was found to be pH dependent in the pH range 1-8, by being shifted cathodically in more basic solutions ( — 1 V to —1.5 V, respectively). It was found that the 2e polarographic wave corresponds to a reduction of the j5-diketones to a 1,2-dihydroxycyclopropane (10). The following mechanism was suggested for the reduction in acidic or neutral protic media. 

Tafel-type rearrangement was found to occur in the cathodic reduction of p-diketones. This is in agreement with the involvement of cyclopropanediol intermediates in this reduction ... 

The role of the alkali metal in complex metal hydride reductions is critical, - os and complexing this metal with a macrocyclic co-ordinator curtails the reaction. A full report on the stereoselectivity of the cathodic reduction of cycloalkanoncs to cycloalkanols has been published. The electrochemical reduction of 1,3-diketones yields cyclopropane-1,2-diols (27), often in good yields. ... 

As mentioned in Section 1.13.2.2 (Clemmensen reduction), 1,3-diketone derivatives give rise to ring contraction products via cyclopropanediols, which can be trapped as acetates. On electrolysis using a Hg cathode in the presence of acetic anhydride, the 1,3-diketone (18) afforded a mixture of stereoisomers (19) in 33% yield, and a similar electrolysis of (56) gave (57) in 71% yield (equation 29). ... 

Cyclohexenone is reduced either by sodium amalgam [138] or electrochemically in acetonitrile containing tetrabutylammonium fluoroborate [139] and affords mainly the tail-to-tail diketonic hydrodimer. Some 10-26% of the head-to-tail hydrodimer has been detected in the mixture of products from reduction at a mercury cathode in aqueous ethanol under both acidic and basic conditions [140]. Substituents on the 3- and 4-positions, as in XXXIII, cause formation of some head-to-tail XXXV and head-to-head hydrodimers XXXVI (see Table 9). The product ratios depended on the concentration of water in acetonitrile used as solvent. Other work discussed later indicates that the presence of metal ions that can coordinate with alkoxy groups also changes these product ratios. 

Aliphatic esters may be reduced to the corresponding alcohol [Eq. (11)], provided a magnesium cathode is used [47]. Interestingly, reduction of aliphatic esters at a magnesium cathode in aprotic solvents [48,49] leads to coupling resulting in the diketone ... 

In a reaction similar to that shown above for aliphatic esters [Eq. (13)], it has been reported that aromatic esters may also be reduced to the corresponding diketone provided the reduction is carried out using a cadmium cathode and magnesium anode [60]. Earlier work demonstrated that phenyl benzoate anion radical cleaves to form phenolate in good yield and presumably benzoyl radical, which leads to dibenzoyl [16]. Nitro-substituted phenyl benzoates are similarly cleaved on reduction to give phenolate [62,63]. 

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