Conjugate addition, site-selectivity

Site-Selective Conjugate Addition Through Catalytic Generation of Ion-Pairing Intermediates... 

Conjugate addition to a,p,y,5-unsaturated carbonyl compounds is one of the simplest reactions involving site-selectivity. Because two reactive sites, the P- and 8-positions relative to the carbonyl group, often have similar reactivity, nucleophiles can frequently attack both sites. Several catalysts effectively discriminate between two such sites, although it is generally difficult to understand the origin of the observed selectivity. 

In contrast to the extensive use of nitroolefins in asymmetric Michael additions [10], the vinylogous analogues, nitrodienes and nitroenynes, are less frequently utilized in the conjugate addition of enolate equivalents, despite the significant synthetic utility of the resulting products which possess carbonyl, nitro, and olefin functionalities. Likewise, studies on the hetero-Michael reaction with these acceptors have been very limited, possibly because of apprehension of the site-selectivity issue [11]. 

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