Unsaturated aldehyde, conjugate addition reactions

Unimolecular, 373 Unsaturated, 174 Unsaturated aldehyde, conjugate addition reactions of,... 

Cuprates formed from organomagnesium reagents are acceptable reagents for the conjugate addition reaction with a, 3-unsaturated aldehydes.81 However, these cuprates are highly dependent upon the choice of copper(I) salt. With CuBr-Me2S, a dramatic increase in 1,4- 1,2-selectivity was reported over... 

Conjugate addition reaction of a mixed dialkylzinc reagent to an unsaturated aldehyde preparation of ethyl (Moxomethyljdecanoate28... 

Two conjugate addition reactions involving a terminal alkyne and an a,p-unsaturated aldehyde lead to 2,3-dihydrothiopyran-4-ones 38 bearing different substituents at the 2- and 6-positions <06TL5095>. 

Thioamide enolates may be prepared by conjugate addition of organometallics to a, -unsaturated thioamides. Reaction of these enolates with aldehydes affords anti aldols, often in excellent dia-stereomeric excess (equation 103 Table 15). It is believed that the conjugate addition reactions provide (Z)-enolates, via a cyclic, six-centered transition state. The anti stereochemistry observed in the aldol reactions of these (Z)-enolates would result from a boat-like, chelated transition state. The transition state has boat-like character to avoid a serious gauche interaction between R and the bulky secondary alkyl group in the thioamide enolate. Several of the intermediate enolates in this study e.g. Table 15,... 

The a,(3-unsaturated ester (8), aldehyde (9), and nitro (10) compounds participate as electrophiles in a number of useful conjugate addition reactions. Copper-catalyzed addition of Grignard reagents provides access to aryl butanoic acid derivatives substituted with an oxetane (equation 1 in scheme 13.7). The ester, aldehyde, and nitro electrophiles also undergo mild Rh-catalyzed additions of aryl and vinyl boronic acids (equation 2 through equation 4 in scheme 13.7). Interestingly, the unsaturated aldehyde participates readily in an amine conjugate addition to afford oxetane substituted 3-amino-acetaldehyde derivatives. 

So far we have shown you conjugate additions mainly of a,P-unsaturated aldehydes and unsaturated a,P-ketones. You won t be at all surprised to learn, however, that unsaturated acids, esters, amides, and nitriles—in fact all carboxylic acid derivatives—can also take part in conjugate addition reactions. Two examples, an amide and an ester, are shown on the right below. But notice how the selectivity of these reactions depends on the structure of the unsaturated compound compare the way butyllithium adds to this a,P-unsaturated aldehyde and a,P-unsaturated amide. Both additions are irreversible, and BuLi attacks the reactive carbonyl group of the aldehyde, but prefers conjugate addition to the less reactive amide. Similarly, ammonia reacts with this acyl chloride to give an amide product that derives from direct... 

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 silicon works by reacting with the negatively charged intermediate in the conjugate addition reaction to give a silyl enol ether—a type of molecule we met in Chapter 20. Here is a possible mechanism for a reaction between Bu2CuLi and an a, 3-unsaturated aldehyde in the presence of MeaSiCl. The silyl enol ether simply hydrolyses to the ketone at the end of the reaction. 

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