Radical anions, enone/enolate

Reduction of a conjugated enone to a saturated ketone requires the addition of two electrons and two protons. As in the case of the Birch reduction of aromatic compounds, the exact order of these additions has been the subject of study and speculation. Barton proposed that two electrons add initially giving a dicarbanion of the structure (49) which then is protonated rapidly at the / -position by ammonia, forming the enolate salt (50) of the saturated ketone. Stork later suggested that the radical-anion (51), a one electron... 

Reversible electron addition to the enone forms the radical anion. Rate determining protonation of the radical anion occurs on oxygen to afford an allylic free radical [Eq. (4b) which undergoes rapid reduction to an allylic carbanion [Eq. (4c)]. Rapid protonation of this ion is followed by proton removal from the oxygen of the neutral enol to afford the enolate ion [Eq. (4c)]. 

Enones may react either as ketones (cf. Chapter 10) or as activated alkenes thus giving pinacols, y6,y6 -coupling, or mixed coupling products. Another feature of enone reduction is that the radical anions, A, in the presence of proton donors are protonated at oxygen in a fast process, and the resulting enol radical, B , is more difficult to reduce than the neutral substrate (Sec. II.A. 1). Radical anions derived from other activated double bonds tend to protonate at carbon in the presence of proton donors, and the resulting radical is more easily reduced than the neutral substrate (Schemes 1 and 3). 

As a consequence, hydrogenation competes to a much smaller extent with coupling in the reduction of enones than in the reduction of other activated double bonds, since the enol radical also participates in coupling reactions. Overall 1-F reduction is therefore observed in most experimental conditions. Whether the coupling takes place at the radical anion stage, A , or at the radical stage, B , depends on the acidity of the medium. 

Transfer of an electron to the conjugated Ti-system of the enone furnishes a radical anion, which on protonation (t-BuOH) followed by transfer of a second electron affords an enolate ion. Its protonation on workup gives the saturated ketone, or it may be alkylated prior to workup to form a new C-C bond. The regiospecific generation of enolate ions from a,P-unsaturated ketones is an important tool in carbon-carbon-bond-forming reactions. 

The influence of the classical anomeric effect and quasi-anomeric effect on the reactivity of various radicals has been probed. The isomer distribution for the deu-teriation of radical (48) was found to be selective whereas allylation was non-selective (Scheme 37). The results were explained by invoking a later transition state in the allylation, thus increasing the significance of thermodynamic control in the later reactions. Radical addition to a range of o -(arylsulfonyl)enones has been reported to give unexpected Pummerer rearrangement products (49) (Scheme 38).A mechanism has been postulated proceeding via the boron enolate followed by elimination of EtaBO anion. 

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