Fast and efficient method for reduction of carbonyl compounds with NaBH4 /wet SiO2 under solvent free condition

Abstract

As reduções de compostos carbonilados estruturalmente diferentes, como aldeídos, cetonas, enais α,β-insaturados e enonas, α-dicetonas e acyloins foram realizadas eficientemente com borohidreto de sódio, na presença de SiO 2 úmido (30% m/m), sem solvente. As reações ocorreram em temperatura ambiente ou a 75-80 ºC, tendo-se obtido excelentes rendimentos dos produtos correspondentes. A redução quimiosseletiva de aldeídos na presença de cetonas foi obtida com êxito, usando-se este sistema de redução.Reduction of structurally different carbonyl compounds such as aldehydes, ketones, α,β-unsaturated enals and enones, α-diketones and acyloins were accomplished efficiently by sodium borohydride in the presence of wet SiO 2 (30% m/m) under solvent free condition. The reactions were performed at room temperature or 75-80 ºC with high to excellent yields of the corresponding products. The chemoselective reduction of aldehydes over ketones was achieved successfully with this reducing system. Keywords: reduction, NaBH 4 , wet SiO 2 , solvent free IntroductionReduction is one of the frequently used reactions in organic synthesis and a vast variety of reducing agents have been introduced for this achievement. 1 Among the powerful and mild reducing agents which have been developed for the reduction of functional groups, LiAlH 4 and NaBH 4 are the commonly used reagents in synthetic organic laboratories. Lithium aluminum hydride is an exceedingly powerful reducing agent capable of reducing practically all-organic functional groups. Consequently, it is quite difficult to apply this reagent for the selective reduction of multifunctional molecules. On the other hand, sodium borohydride is relatively mild reducing agent, primarily used for the reduction of reactive functional groups in protic solvents. Consequently the rate of reductions is sometimes slow and a relatively low chemoselectivity is accompanied with the reactions.To control the reducing power of NaBH 4 , hundreds of substituted boron hydrides have been reported in chemical literature and many of them are commercially available now. In fact, the progress in this field has been realized by: (i) substitution of the hydride(s) with other groups which may exert marked steric and electronic influences upon the reactivity of the substituted complex ion, 2 (ii) variation in the alkali metal cation and metal cation in the complex hydride, 3 (iii) concurrent cation and hydride exchange, 4 (iv) use of ligands to alter behavior of the metal hydrides, 5 (v) combination of borohydrides with metal, metal salts, Lewis acids, mixed solvent systems or some other agents, 6,7 (vi) changing of the cation to quaternary and phosphonium borohydrides, 8 and finally (vii) use of polymers and solid beds for supporting the hydride species. 9 In this context, we extensively reviewed the applications of modified hydroborate agents in organic synthesis. 5,10 On the other hand, the economical demands and the existing state of environment have generated a need for paradigm shift to perform chemical reac...

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