Sodium borohydride reaction with unsaturated carbonyl

Varma reported a facile and rapid method for the reduction of aldehydes and ketones to the respective alcohols, using alumina-supported sodium borohydride and microwave irradiation under solvent-free conditions. Aldehydes tend to react at room temperature, while for the reduction of ketones, short microwave irradiation of 30-180 s was applied to produce the corresponding alcohols in 62-92% yield. With unsaturated carbonyl compounds, reduction at the conjugated C=C bond might occur as a side reaction under these conditions (Scheme 4.9)26. 

For the more reactive nucleophiles, where addition is essentially irreversible, whether 1,2-addition or 1,4-addition occurs depends on the relative rates of addition to the two electrophilic sites, the carbonyl carbon and the /3-carbon. Lithium aluminum hydride usually gives predominantly 1,2-addition and provides a useful way to reduce the carbonyl group of an a,/3-unsaturated compound. Sodium borohydride, on the other hand, often gives a mixture of 1,2-addition and the completely reduced product, where 1,4-addition followed by 1,2-addition has occurred. Thus, the reaction of 2-cyclohexenone with lithium... 

Under appropriate experimental conditions, hydrogen telluride, phenyl tellurol, and sodium hydrogen telluride perform the selective reduction of double bonds of a,/3-unsaturated carbonyl systems.89 90 This reaction with a,j3-unsaturated ketones complements the cerium trichloride/sodium borohydride method, which reduces the carbonyl group, keeping the carbon-carbon double bond intact92 (Scheme 18). 

It has been found possible to construct the 1,4-benzothiazine ring by direct interaction of bis-(2-aminobenzene) disulfide (96) with carbonyl compounds.139,140 The reaction is most efficient when conducted under a nitrogen atmosphere with a 1 1 ratio of reactants otherwise, the principle products are benzothiazoles. While reduction of the benzothiazine 97 with sodium borohydride gives a stable dihydro derivative, the unsaturated benzothiazines themselves were prone to autoxidation, giving rise to benzothiazoles and benzothiazine sulfoxides.141... 

Sodium borohydride is a nucleophile that you have seen reducing simple aldehydes and ketones to alcohols, but it will also do conjugate addition reactions. Which of the alternatives actually takes place depends on the reactivity of the C=0 group. NaBH4 usually reacts with a,P-unsaturated aldehydes to give alcohols by direct addition to the carbonyl group. 

The oxy-Cope rearrangement of hexa-l,5-dien-3-ols to give Se-unsaturated carbonyl compounds often gives low yields because of the high temperatures that are needed to elfect the reaction. However, in the presence of mercury tri-fluoroacetate, the reaction proceeds at room temperature and the required Se-unsaturated carbonyls (93) are obtained bydemercuration of the intermediates (92) with sodium borohydride in good yield ( 60%). Nakai and his co-workers... 

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