Iminium catalysts enantioselective organocatalytic

The first report of an enantioselective organocatalytic [3+2] cycloaddition between nitrones and a,P-nnsaturated aldehydes was reported by MacMillan and co-workers who showed that iminium ion activation was effective in this reaction (Scheme 8) [64], After a survey of seven catalysts the imidazolidinonium salt 12 HC10 emerged as the most efficient system. The reactions were conducted in a mixture of nitromethane and water at -20 °C in the presence of 20 mol% catalyst... 

Enantioselective organocatalytic cyclopropanations have been performed using directed electrostatic activation conditions.164 Using a new class of iminium catalysts, cyclopropanation has been conducted with enals but not electron deficient alkenes, such as unsaturated nitrile, nitro, or alkylidene malonate systems. 

Kunz RK, MacMillan DWC (2005) Enantioselective organocatalytic cyclopro-panations. The identification of a new class of iminium catalyst based upon directed electrostatic activation. J Am Chem Soc 127 3240-3241 Lacour J, Hebbe-Viton V (2003) Recent developments in chiral anion mediated asymmetric chemistry. Chem Soc Rev 32 373-382 Li X, List B (2007) Catalytic asymmetric hydrogenation of aldehydes. Chem Commun 17 1739-1741... 

The first enantioselective organocatalytic 1,3-dipolar cycloaddition of acyclic nitrones with acrolein and crotonal-dehyde has been reported <2000JA9874>. In particular, the reversible formation of iminium ions from a,/3-unsatu-rated aldehydes and the enantiopure imidazolidinone 535 provided ( A-4-formylisoxazolidines in high yields and ees (Equation 86). A polymer-supported version of catalyst 535 was also prepared <2004EJ0567>. The catalytic performance of various chiral pyrrolidinium salts in the cycloaddition of 1-cycloalkene-l-carboxaldehydes was also evaluated <2003EJO2782>. 

In 2000, MacMillan and his co-workers presented the first enantioselective organocatalytic 1,3-dipolar cycloaddition of nitrones 187 and a,p-unsaturated aldehydes 28 (dipolarphiles) to afford the e do-(45)-isoxazolidine adducts 188, Scheme 3.59 [75]. With the LUMO-lowering activation of a,p-unsaturated aldehydes 28 and enforced formation of (fi)-iminium isomer, the HClO -salt of catalyst 30 effectively promote cycloaddition of the dipolarphile. In addition, e do-cycloaddition effectively alleviated nonbonding interaction between the nitrone phenyl group and the neopentyl methyl substituent on the catalyst framework. Later, in 2002, Karlsson and Hbgberg reported the organocatalytic enantioselective 1,3-dipolar cycloaddition of... 

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