Aluminium chiral salenAl complexes

Highly enantioselective catalytic conjugate addition of A-heterocycles, namely purines, benztriazole, benzimidazole, and 5-phenyltetrazole, to a,ft-unsaturated ketones and imides has been attained with chiral, salen-type (Jacobsen) aluminium complexes as catalysts.142... 

Subsequently, the authors used sodium azide, an inexpensive and easily handled hydrazoic acid precursor, in combination with concentrated aqueous hydrochloric acid, in the conjugate addition to a,p-unsaturated ketones. Chiral bimetallic aluminium(salen) complex (5, 5 )-[(salen)Al]20 ent-4 proved efficient (Scheme 19.32). 

Subsequently, the Feng group developed an enantioselective cyanosilylation of ketones by a catalytic double-activation catalyst system composed of chiral (J ,J )-salen 16-triethylaluminium complex and N-oxide 17 (Scheme 19.10). High catalytic turnovers (200 for aromatic ketones, 1000 for aliphatic ones) with high enantioselectivity (up to 94% enantiomeric excess for aromatic ketones, up to 90% enantiomeric excess for aliphatic ones) were achieved under mild reaction conditions. Based on the control experiments, a double-activation model was suggested (Scheme 19.10). The chiral aluminium complex performed as a Lewis acid to activate the ketone, whereas the N-oxide acted as a Lewis base to activate trimethylsilyl cyanide and form an isocyanide species. The activated nucleophile and ketone attracted and approached each other, and so the transition state was formed. The intramolecular transfer of cyanide to the carbonyl group gives the product cyanohydrin O-TMS ether. 

In 1999, the Jacobsen group reported the first examples of highly enantio-selective aza-Michael addition catalysed by chiral aluminium complex 27 of (5,5)-(salen) (Scheme 19.31). With hydrazoic acid as the nitrogen-based nucleophile, a series of ot,p-unsaturated imides with different p-substituents underwent the addition, affording the azide adducts in excellent yields and enantioselectivities. The products could be transformed into p-amino acids. 

In the catalytic enantioselective Strecker reaction, chiral aluminium complexes, especially aluminium-salen-based catalysts and aluminium-binaphthol-based catalysts have been widely used, and great achievements have been obtained. In 2010, the Li group reported a highly enantioselective Strecker reaction of achiral IV-phosphonyl imines by using primary free L-phenglycine 42 as the catalyst and diethylaluminium cyanide as the nucleophile. This work also presented the novel use of nonvolatile and inexpensive diethylaluminium cyanide in asymmetric catalysis (Scheme 19.51). ... 

The base-catalysed hydrophosphonylation of aldehydes or imines (Pudovik reaction) [58] as a convenient method was widely used for the synthesis of 1-hydrox-yalkylphosphonates. Since the pioneering work of Shibuya [50] and Spilling [51] was reported, much attention has been devoted to developing enantioselective catalysts for the synthesis of chiral 1 -hydroxy alkylphosphonates. Chiral aluminium complexes were shown to be more effective chiral catalysts [59-62]. Based on the success of using A1 (salen) and A1 (salcyen) as asymmetric catalysts, Al-Schiff base complexes [63, 64] have been developed to catalyze the asymmetric addition reaction of dial-kylphosphonates and aldehydes. Tridentate Schiff base metal complexes, such as vanadium, chromium, and iron [65], have been successfully applied in many asymmetric synthetic reactions. We noticed that Al(III) complexes could catalyse the asymmetric Pudovik reaction and these ligands could be easily synthesized [66-70]. 

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