Reactions in microemulsion media. Borohydride reduction of mono- and dicarbonyl compounds

Microemulsions were prepared at 26° from mixtures of hexanes (O), a 50: 1 (w/w) solution (W) of 0.1 M KOH-NaBH₄, and a 1.23:1 (w/w) mixture (S) of hexadecyltrimethylammonium bromide (HTABr) and 1-butanol. A pseudoternary phase map contained a significant microemulsion (μE) region, and μE's A and B (60:35:5 and 20:10:70 S:W:O, respectively) were used for reduction of several monocarbonyl compounds benzophenone (1a), benzaldehyde (2a), acetophenone (3a), and 1-phenyl-1-octadecanone (4a)], an α,β-unsaturated ketone trans-4-phenyl-3-buten-2-one (6a)], and a diketone 4-(4'-benzoylphenyl)-2-butanone (7a)]at 26°. For comparison purposes, reductions were also performed in aqueous 2-propanol (2-PrOH A and 2-PrOH B) prepared by the substitution of 2-propanol for the S and O components of μE's A and B. Generally, the reductions were slightly faster in the microemulsion media than in the corresponding aqueous 2-propanol media. The significantly slower reduction of 4a relative to that of 3a in μE B indicated that the interphase is the reactive site. With enone 6a, the influence of microemulsions on the competition between 1,2- and 1,4-reduction was determined. In μE's A and B there was 8% and ll% 1,4-reduction, respectively, whereas in 2-PrOH A and B there was only a trace. With diketone 7a, the reactivity of the aromatic carbonyl group relative to that of the aliphatic carbonyl group increased on going from 2-PrOH A and B to μE's A and B, respectively. For the sodium borohydride reduction of ketones, microemulsion catalysis is more effective than phase transfer catalysis or the use of a tetraalkylammonium borohydride in a hydrocarbon solvent.