Aluminium complexes, 1,3-dipolar cycloadditions

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... 

The enantioselective inverse electron-demand 1,3-dipolar cycloadditions of nitrones with alkenes described so far are catalyzed by metal complexes that favor a monodentate coordination of the nitrone, such as boron and aluminium complexes. However, the glyoxylate-derived nitrone 256 favors abidentate coordination to the catalyst, and this nitrone is an interesting substrate, since the products that are obtained from the reaction with alkenes are masked ot-amino acids (Scheme 12.81). 

In general the inverse electron-demand Diels-Alder reaction is carried out using stoichiometric amounts of Lewis acid "catalysts (SnCU, TiCU, TiCl2(OR)2, MAD (methylaluminium bis(2,6-di-fert-butyl-4-methylphenoxide) and MAPh (methyl-aluminium bis(2,6-diphenylphenoxide) [15]) at low temperatures (-90 to 0 °C). Before the [3 + 2] cycloaddition with an electron-poor alkene can take place, the first-formed nitronate has to be separated from the Lewis acid catalyst by an aqueous work-up and chromatography [13, 16]. Probably complexation of the Lewis acid catalyst to the nitronate dipole inactivates the dipole and hinders the 1,3-dipolar cycloadditions from taking place [17]. 

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