Cycloaddition-dual activation

Chiral Lewis acid-catalyzed cycloaddition-dual activation... 

Chiral Lewis Add-Catalyzed Cycloaddition-Dual Activation... 

J. Li, Y. Liu, C. Li, X. Jia, Adv. Synth. Catal. 2011, 353, 913-917. Silver hexafluoroantimonate-catalyzed three-component [2-I-2-I-1] cycloadditions of allenoates, dual activated olefins, and isocyanides. 

SCHEME IJS Asymmetric cycloadditions of cyclic carbonyl ylides by monoactivation and dual activation approaches. 

A-Acyl aminophosphines catalysed the 4 -i- 2-cycloaddition of a-substituted allenoates with dual-activated olefins at -18 °C with up to 97% ee. A key step in the reaction is the formation of a zwitterion between the catalyst and the allenoates. ... 

Diels-Alder cycloaddition reactions of iminium ions (Sect. 2.1.1) could take place with either the C=N bond or the C=C bond acting as the dienophile. This potential dual reactivity was investigated by Zora using the AMI semi-empirical method [231]. The results showed not only the preferred C=C reactivity (activation barrier is 4.20 kCal mol" lower than for reaction with the C=N bond) but also suggested that the reaction was stepwise. 

Consistent with reported data, free energies of activation of the cycloaddition, of the reverse reaction, and of the rearrangement to the a, p-unsaturated amide, do not differ considerably. Hence, a slight modification of the glycal moiety, or even a change in the solvent could shift the reaction towards the desired cycloadduct, or subsequently towards the unsaturated amide. Polar solvents, such as acetonitrile, acted in a dual manner, causing acceleration of the reaction rate and a shift of the equilibrium towards the cycloadducts. 

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