Disproportionation Dissolving metal reductions

Reduction of Ketones and Enones. Although the method has been supplanted for synthetic purposes by hydride donors, the reduction of ketones to alcohols in ammonia or alcohols provides mechanistic insight into dissolving-metal reductions. The outcome of the reaction of ketones with metal reductants is determined by the fate of the initial ketyl radical formed by a single-electron transfer. The radical intermediate, depending on its structure and the reaction medium, may be protonated, disproportionate, or dimerize.209 In hydroxylic solvents such as liquid ammonia or in the presence of an alcohol, the protonation process dominates over dimerization. Net reduction can also occur by a disproportionation process. As is discussed in Section 5.6.3, dimerization can become the dominant process under conditions in which protonation does not occur rapidly. 

The electrochemical behavior of titanium species has been investigated in an acidic EMICI-AICI3 ionic liquid [32]. Titanium dichloride, TiCl2, dissolves up to 60 and 170 mmol dm at 80°C in the acidic ionic liquids of 60.0 and 66.7 mol% AICI3, respectively. The divalent titanium species, Ti(ll), is less stable in the weakly acidic ionic liquid, leading to the disproportionation into insoluble titanium trichloride and metallic Ti. Metallic Ti is impossible to obtain probably because of the large overpotential of the electrodeposition of metallic Ti whose reduction potential is... 

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