Reduction of carbonyl and nitro functionalities

The reductions of ketones, aldehydes, carboxylic acids and acyl derivatives are fundamental reactions in organic synthesis and the use of aqueous media has allowed high regio- and stereoselective processes to be performed under mild conditions. 

Carbonyl compounds are quantitatively reduced regio- and stereo-selectively by NaBH4 at room temperature in aqueous solutions containing glycosidic amphiphiles such as methyl-p-D-galactoside, dodecanoyl-p-D-maltoside, L-arabinose and sucrose [63]. a,P-Unsaturated ketones give 1,2-reduction to corresponding allylic alcohols and cyclohexanones furnish the thermodynamically more stable alcohol. The observed stereodifferentiation can be attributed to hydrophobic interactions between amphiphilic carbohydrates and lipophilic substrates. 

Carrying out the reaction in organic solvents (i-PrOH, DCM, THF), lower conversions were observed. 

The reductions of benzaldehyde and p-tolualdehyde with various types of Raney alloys (Ni-Al, Co-Al, Cu-Al, Fe-Al) in 10% aqueous NaOH afford the corresponding benzyl alcohols in 17-83% yields [66]. Benzoic and /7-toluic acids were obtained as by-products, which suggests that the Cannizzaro reaction is competitive. In aqueous NaHCOs and under US irradiation, the reaction is accelerated and the alcohol is the highly prevalent product. 

The effect of hydrotropes on the crossed Cannizzaro reaction rate of benzaldehydes with aqueous formaldehyde was investigated [67]. In the presence of polyethyleneglycol-200 the reaction rate of w-phenoxybenzalde-hyde increases more than six-fold and the w-phenoxybenzyl alcohol is also obtained with higher selectivity. The enhancement of the reaction rate of m-bromobenzaldehyde is lower than that observed for m-phenoxybenzaldehyde but the reaction selectivity is higher. p-Anisaldehyde and p-chlorobenzalde-hyde show an intermediate behavior. The cationic surfactants cetylpyri-dinium chloride and cetyltrimethylammonium bromide (CTABr) favor the Cannizzaro reaction instead of the crossed Cannizzaro one. 

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Reductions of carbonyl and nitro functionalities via inclusion complexes and by Sml2-H20 systems are illustrated in the pertinent sections 6.8.2 and 6.8.5. 

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