Aluminium complex, chiral

Chiral aluminium complexes have been used as catalysts for inverse electron-demand 1,3-dipolar cycloadditions of alkenes with nitrones, and the first contribution to this field was pubhshed in 1999 (344). The chiral AlMe-BEMOL (BINOL = 2,2 -bis(diphenylphosphino)-l,l -binaphthyl) complexes 235 were excellent catalysts for the reaction between nitrone 225a and vinyl ethers 232 (Scheme 12.68). The diastereo- and enantioselectivities are highly dependent on the chiral ligand. An exo/endo ratio of 73 27 was observed, and the exo-product was... 

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. 

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. 

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). ... 

Chiral aluminium complexes are widely used as hard chiral Lewis acids due to their cheapness, easy preparation, and high reactivity. Impressive achievements have been obtained in asymmetric cyanohydrin synthesis,... 

The Diels-Alder reaction has not escaped the attention of chemists interested in asymmetric synthesis. A binaphthyl-based chiral aluminium complex (30) catalyses the hetero Diels-Alder cycloaddition between benzaldehyde and the Danishefsky diene (29), providing the dihydropyrone (32) with excellent selectivity after acid hydrolysis of the initial adduct (31). 0 2] (We will see further applications of chiral binaphthyl ligands in section 6.4.)... 

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]. 

Duxbury JP, Cawley A, Thomton-Pett M et al (1999) Chiral aluminium complexes as phospho-transfta catalysts. Tetrahedron Lett 40 4403-4406... 

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... 

Several different butadienes and aliphatic aldehydes were used with good success. An interesting approach for this transformation is the in situ complexa-tion of one enantiomer of the aluminium complex employing chiral ketones and thus allowing the remaining enantiomer to be utilized as a Lewis acid for the asymmetric synthesis. 

Solvent free sulfide oxidation (Figure 2.19) has been performed using a chiral aluminium(salalen) complex. Enantioselectivity and yields were found to be... 

Several new chiral modifications of lithium aluminium hydride have been reported, including those formed by reaction with chiral secondary benzylamines (14), with diols such as (15) derived from D-mannitol, or with terpenic glycols such as (16). These complexes reduce phenyl alkyl ketones to optically active phenyl carbinols, and enantiomeric excesses of up to 50% have been observed in the case of reagents derived from (14). However, in the diol complexes, believed to have structures of the type shown in (17), lower chiral selectivity is observed, e.g. up to ca. 12% in the case of (15), or up to an optical yield of 30% with an ethanol-modified complex of (16). Better results have been reported with the chiral diamine complex (18), derived originally from L-proline, which reduces acetophenone in 92% optical yield. Asymmetric induction with reagents in this class (i.e. derivatives of lithium aluminium hydride) is usually low in the reduction of aliphatic ketones, but a complex of UAIH4 and the amino-alcohol (19) has been shown to reduce... 

The addition of highly basic NaOt-Bu to chiral aluminium(m) lithium(i) bis(binaphtholate) 70 was also effective, and the enantioselective Michael reaction of cyclic enones with Homer-Wadswoth-Emmons reagents, such as 71, proceeded at room temperature (Scheme 2.41). Complex 70 itself did not promote the reaction even at 50 °C. 

Diastereoselective complexation of a chiral [antagonist] ketone with a racemic aluminium Lewis acid catalyst effectively removes one enantiomer of the latter, leaving the uncomplexed antipode free to function as a chiral Lewis acid. E Asym. heterodiene synthesis. 0.1 eq. D-3-bromocamphor, 1.05 eqs. startg. siloxydiene, and benzaldehyde added sequentially to 0.1 eq. of the racemic aluminium complex in degassed methylene chloride at —78°, stirred for 3 h, then subjected to acidic work-up (2S,3S)-product. Y 78% (e.e. 82%, upgraded to > 98% by one recrystallization with ca. 60% recovery). F.e.s. K. Maruoka, H. Yamamoto, J. Am. Chem. Soc. Ill, 789-90 (1989). 

In 1991, the Inoue group developed the first example of asymmetric cyano-silylation of aldehydes catalysed by the aluminium complexes (Scheme 19.1)." With chiral acyclic dipeptide ester 1 or a-amino amide 2 containing a phenolic Schiff base as the ligand, the silylated cyanohydrins were afforded in 66-92% yields and 37-71% enantiomeric excess. 

The Maruoka group reported the asymmetric hetero-ene reaction of commercially available 2-metho3qq3ropene with aldehydes catalysed by chiral organoaluminium complex 41. The aluminium catalyst was prepared in situ by the treatment of (i )-2,2 -bis(trifluoromethanesulfonylamino)-l,l -binaphthyl with one equivalent of trimethylaluminium in dichloromethane at refluxing temperature for 1 h (Scheme 19.50). ... 

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