Asymmetric Phospha-Michael-Additions

The conjugate addition of phosphorus nucleophiles of various oxidation states and in neutral or metallated form constitutes an efficient and well-known method for C—P bond formation [30]. In the case of phosphanes as nucleophiles especially, the corresponding phosphane-borane adducts have been used in 1,4-additions to Michael acceptors. Following the idea to use a chirally modified phosphorus nucleophile in asymmetric Michael additions to aromatic nitroalkenes, we synthesized the new enantiopure phospite 45 starting from TADDOL (44) with nearly quantitative yield. Due to the C2 symmetry, of the 

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Tan also found that guanidine 21, acting as a base to activate the o [3], X [3] tautomers of diaryl phosphine oxides, catalyzes the asymmetric phospha-Michael reachon of aryl nitroalkenes (Scheme 5.42) [76]. He later employed 21 to realize highly enantioselective Michael additions of dithiomalonate and 3-keto thioesters with a range of acceptors, including maleimides, cyclic enones, furanone, and acyclic 1,4-dicarbonylbutenes [77]. 

Numerous guanidine-catalysed asymmetric Michael reactions and its related variants such as aza-Michael, oxa-Michael, phospha-Michael, sulfa-Michael have been reported in the literature. A nonexhaustive selection of conjugate addition reactions that is relevant to green chemistry will be presented. Glycine imines are commonly employed in Michael additions. They are protected a-amino acids and must he deprotected if an amino acid derivative is desired (Scheme 23.5). The large molecular mass of the imine group then makes waste generation a problem. 

The asymmetric Michael addition represents one of the most powerful methods for the formation of C-X (X = C, N, O, S) bonds in organic synthesis. Chiral aluminium complex-catalysed asymmetric Michael additions will be introduced on the basis of different nucleophiles, including carbon-, nitrogen-, ojygen-, sulfur-, and phospha-based nucleophiles. 

Asymmetric Michael Addition with Phospha-based Nucleophiles... 

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