Conjugate addition activation modes

To end this chapter, other cascade processes initiated by organocatalytic conjugate additions proceeding via other types of activation mode not covered in the previous sections will be presented. 

Although organocatalytic intramolecular conjugate additions have been reported (see Sect 1.3.1.3), the Michael acceptor is almost always either a ketone or an aldehyde, since the coimnon mode of activation for these systems is through iminium ion... 

With respect to the catalysts employed in conjugate additions, a big collection of efficient stable and environmentally friendly natural or newly designed chiral organocatalysts has already been developed. These catalysts are usually cheap to prepare and readily accessible in a range of quantities. They fall into four major classes Lewis bases, Lewis acids, Brpnsted bases, and Brpnsted acids [If]. The identification of the generic modes of activation of these catalysts has been crucial to the success of organocatalysis. 

Complementary to the enamine-based reactions, the diarylprolinol silyl ether catalysts can be applied for activation of a,p-unsaturated aldehydes toward conjugate additions through iminium-ion-induced processes. Mechanistically, this conceptually different activation mode draws close parallels to the enamine chemistry (Scheme 2.4) [5]. The secondary aminocatalyst condenses with the enal to form... 

The mode of iminium activation of carbonyl compounds is the domain of many and various cycloadditions and conjugate additions. As discussed in the... 

The latter mode of activation, in which a charged tetraaminophosphonium salt acts as a Bronsted acid, was realized by the same research group in 2009. Enanti-oselective aza-Michael reaction of 2,4-dimethoxyaniline with nitroalkenes afforded the conjugate addition products in excellent yields and high enantiomeric excesses (Scheme 10.70) [173, 174]. Similarly, chiral diaminooxaphosphonium salts have been used as Br0nsted acids in the asymmetric protonation reactions of ketene disilyl acetals [175]. 

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