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Our interest in the selective reducing properties of cyanoborohydride coupled with the recent attention accorded the reduction of a,β-unsaturated systems1 prompts this report of the capability of cyanoborohydride for the facile and selective reduction of certain conjugated double bonds to the corresponding saturated derivatives.
The reduction of alkenes conjugated with strong electron-withdrawing groups such as esters,5 nitriles,5 sulfonate esters,5 or nitro groups7 has been observed with borohydrides or lithium aluminum hydride.8 Thus, α,β-unsaturated esters are reduced by borohydride to the saturated derivatives if an additional cyano or ester is located at the a position.5 Furthermore, conjugated cyano esters are often reduced to the saturated cyano alcohols.5d,e The highly electronegative cyano group evidently renders the normally resistant carbethoxy substituent electrophilic enough to suffer attack by borohydride. Recently, lithium tri-sec-butylborohydride (L-Selectride) at low temperatures has been found to effectively reduce conjugated carbonyl compounds, including esters, to the saturated derivatives,9 Finally, Dittmer6a has reported reduction of the double bond in the strained ring thiete 1,1-dioxide with borohydride; acetylenic sulfones, however, apparently are reduced clearly to trans α,β-unsaturated sulfones.6b
Sodium cyanoborohydride is such an extremely non-aggressive reducing agent that even normally sensitive groups such as aldehydes and ketones are effectively reduced only when the electrophilicity of the carbonyl is increased by protonation.4,10 Even under acidic conditions, however, other carbonyl derivatives, including esters, acids, and amides, remain unmolested. We envisioned that the reactivity of carbon-carbon double bonds of α,β-unsaturated carbonyl systems might be susceptible to activation by protonation and consequently enable the selective conversion of such systems to the saturated derivatives without affecting other functional groups. This note describes the successful realization of this conception using NaBH3CN in acidic ethanol at ambient temperature. The general procedure utilized was mild and convenient. The substrate, a 10% mole excess of NaBH3CN, and a small quantity of bromocresol green were stirred in ethanol and concentrated HCl added dropwise until the solution was acidic as indicated by a color change to yellow.
Additional HCl was added as required to maintain the solution acidity. After an appropriate time period, usually 1 h, the products were isolated by dilution. with water followed by filtration or extraction with ether. Table I presents results for a variety of structural types. As evident, the method appears suitable for conjugated derivatives which are activated by a nitro group (entry 14) or by two α-positioned electron-withdrawing substituents including ester, cyano, lactone, ketone, or amide in varying combinations. Singly substituted double bonds as in ethyl cinnamate (entry 16) are resistant and aryl substitution enhances the reduction rate (entry 15). The method is quite selective in that other functional groups are unaffected including amido (entries 7, 10, 13), aromatic and aliphatic nitro (entries 4-6, 9, 14, 17) or cyano (entries 8-13) moieties, esters (entries 1-12, 15, 16), lactones (entries 17-19), or aryl ketones (entries 17-19). Furthermore, in contrast to analogous reductions with NaBH4,5d,e cyano esters are not further reduced to the corresponding cyano alcohols. The use of acid, although not essential, results in higher yields (compare entries 1 and 3), ostensibly by rapid protonation of initially produced stable α carbanions before side reactions can intervene. This is evidenced by the relatively high yield of 1-methyl-2-phenylnitroethane obtained (entry 14) compared. to previous investigations7a,7b coupled with the absence of dimeric Michael products which are concomitantly produced with other hydride reagents.7a,11
NaBH3CN was obtained from Aldrich Chemical Co. and used without purification. Starting materials were either obtained commercially or prepared by standard procedures.12
General Reduction Procedure
The general procedure utilized is presented in the text and is described below for the reduction of 6-nitro-3-benzoylcoumarin.
A slurry of 6-nitro-3-benzoylcoumarin (2.95 g, 10 mmol), NaBH3CN (0.69 g, 11 mmol), and a small amount of bromocresol green indicator in 25 ml of ethanol was magnetically stirred while concentrated HCl was added dropwise until the color changed to yellow. Periodically, additional HCl was added in order to maintain the yellow color. After 1.5 h the reaction mixture was diluted with ca. 150 ml of water and cooled and the resulting while needles were filtered and dried under vacuum (2.54 g, 86%). The ir and NMR indicated complete reduction of the double bond.