Michael acceptors conjugate enolate anion addition

The base removes the most acidic hydrogen, and the resulting enolate anion undergoes a conjugate addition with the Michael acceptor ... 

Addition of the a,3-unsaturated anion (21) to the Michael acceptor (22), in which either alkylation or 1,4-addition is possible, affords only the Michael product. Internal alkylation of the interme ate ester enolates leads to cyclopropyl derivatives (equation 7). Terpenoid polyenes are prepared through conjugate addition of the lithiat protected cyanohydrins (23) to dienyl sulfoxide (24 equation 8). ... 

The term Michael addition has been used to describe 1,4- (conjugate) additions of a variety of nucleophiles including organometallics, heteroatom nucleophiles such as sulfides and amines, enolates, and allylic organometals to so-called Michael acceptors such as a,p-unsaturated aldehydes, ketones, esters, nitriles, sulfoxides, and nitro compounds. Here, the term is restricted to the classical Michael reaction, which employs resonance-stabilized anions such as enolates and azaenolates, but a few examples of enamines are also included because of the close mechanistic similarities. 

The actual umpolung reaction that allows a ketene dithioacetal to function as if it had an electrophilic carbon a to a carbonyl is achieved by conjugate addition of nucleophiles to the ketene dithioacetal, followed by hydrolysis. Both sulfide and sulfoxide ketene derivatives can be used. Conjugate addition of an ester enol-ate derived from ferf-butyl acetate (secs. 9.2, 9.4.B, 9.7.A) to the ketene dithioacetal [CH2=C(SOMe)2] gave the stable dithioacetal anion (382). Conversion of the dithioacetal to the bis(sulfoxide) enhanced the ability of that species to function as a Michael acceptor. Subsequent transformation of 382 gave the aldehyde-ester (383).370... 

Michael Reaction (Section 19.8A) A Michael reaction involves the addition of a weakly basic nucleophile to a carbon-carbon double bond made electrophilic by conjugation with the carbonyl group of an aldehyde, a ketone, or an ester or with a nitro or cyano group. The mechanism of the Michael reaction involves initial formation of an enolate anion in base and attack of the enolate nucleophile at the )3-carbon of the Michael acceptor to create a new resonance-stabilized enolate anion intermediate that is protonated on oxygen to create an enol and regenerate the base then tautom-erization to the keto form completes the reaction. The base is catalytic in the Michael reaction. 

Why do stabilized anions undergo 1,4- rather than 1,2-addition to Michael acceptors 1,2-Addition occurs, but is reversible with relatively stable anionic nucleophiles, because it leads to a relatively high-energy alkoxide. Conjugate addition is favored thermodynamically because it produces a resonance-stabilized enolate ion. 

A Michael addition consists of the addition of the enolate of an active-methylene compound, the anion of a nitroalkane, or a ketone enolate to an acceptor-substituted alkene. Such Michael additions can occur in the presence of catalytic amounts of hydroxide or alkoxide. The mechanism of the Michael addition is shown in Figure 13.67. The addition step of the reaction initially leads to the conjugate base of the reaction product. Protonation subsequently gives the product in its neutral and more stable form. The Michael addition is named after the American chemist Arthur Michael. 

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