Enones, 367. cyanide anion

Phase transfer, also called crown ether cataly, is also useful to promote the 1,4-addition of the cyanide anion to a conjugated enone. 

An especially elegant use of superoxide anion, in combination with dioxygen, is as both an EGB and as an epoxidation reagent [52]. In this way the ROJ species, formed from the carbanion R , reacts in situ with an enone, which is converted into the epoxide (Scheme 15). Specifically, the carrier (PhCHoCN), which has been dubbed an auxiliary carbon acid, is deprotonated by the superoxide anion, and the resulting anion reacts with O2. The Ph2C(CN)OJ species thus formed reacts in situ with an enone, which is converted into the epoxide with elimination of cyanide anion from the carrier and concomitant formation of ketone (Scheme 15). Another possible auxiliary carbon acid is MeCH(C02Et)2 and, apart from the example in Scheme 15, 4,4-dimethyl-2-cyclohexen-l-one, 4-methylpent-3-en-2-one, and trans-cha -cone (PhCH=CHCOPh) may be converted [52] into the corresponding epoxides in >80% yield. 

Alternatively, the expedient cyanide (or fluoride) ion desilylation of acylsilanes (235), in the presence of a,3-enones or a, 3-unsaturated esters, affords -y-dicarbonyl adducts 182 even bis(trimethylsilyl)ketone (236), a formyl anion equivalent, adds to 2-cyclohexen-l-one to afford 3-formylcyclohexanone in moderate yields.182 ... 

Except for the well-documented conjugate additions of diethylaluminum cyanide,92 triethylaluminum-hydrogen cyanide and Lewis acid-tertiary alkyl isonitriles,93 examples of Lewis acid catalyzed conjugate additions of acyl anion equivalents are scant Notable examples are additions of copper aldimines (233),94, 94b prepared from (232), and silyl ketene acetals (234)940 to a,(3-enones which afford 1,4-ketoal-dehydes (235) and 2,5-diketo esters (236), respectively (Scheme 37). The acetal (234) is considered a glyoxylate ester anion equivalent. 

Trimethylsilyl cyanide is a useful reagent for the preparation of /8-amino alcohols,2 a-amino nitriles,3 and a-trimethylsiloxyacrylo-nitriles4 from the corresponding ketones, imines, and ketenes. The reagent adds rapidly to the carbonyl of aldehydes at 25°C,2 and the resulting adducts have proven useful precursors for the preparation of carbonyl anion synthons.5 Enones give exclusively the products derived from 1,2-addition.2... 

Sodium borohydride adsorbed on alumina is a new reagent combination for the reduction of aldehydes and ketones to alcohols in aprotic solvents such as ether. Cyanoborohydride anion, [BHsCN] , supported on an anion-exchange resin containing quaternary ammonium groups has been found in preliminary results to be a successful and convenient reagent for, inter alia, the reduction of enones to allylic alcohols. Ease of work-up and retention of cyanide by the resin are obvious advantages. Carbonyl compounds can also be reduced to alcohols by tributyltin hydride in a cyclohexane-silica gel slurry aldehydes are selectively reduced in the presence of ketones and products are obtained by simple elution from the silica. Adsorption, and consequent polarization, of the carbonyl group by the silica seems to be important in the mechanism of this mild, non-basic, process. 

The addition at the p-position of an a,p-unsaturated carbonyl compound is described as conjugate addition, Michael addition, or 1,4-addition. It can be exemplified by the reaction in Figure 17.57, the addition of a cyanide ion to an enone. Students are often puzzled by the term 1,4-addition. Despite the way we usually write this mechanism, with the enolate anion being... 

Can we use any of the acyl anion equivalents that we met in Section 20.2.2 to make 1,4-difunc-tionalized compounds This should work if, instead of adding them directly to a carbonyl compound, we add them to an a,p-unsaturated species. Thus, we can add cyanide to an enone (Figure 20.30), and the product can be hydrolyzed to a carboxylic acid, reduced to an amine, or otherwise elaborated. Remember that simple nucleophiles such as cyanide undergo conjugate addition to enones under conditions of thermodynamic control. 

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