Phosphine-Mediated Aza-MBH Reactions

Catalyst 119a was also tested for the addition of various activated aromatic aldi-mines with MVK and EVK at low temperature in THF as solvent (Table 5.16). 

The use of molecular sieves increased chemical yields. The N-mesyl and SES-activated aldimines afforded products in similar selectivity, while Ns derivatives decomposed under the reaction conditions. The replacement of MVK by EVK resulted in a diminished reactivity, with longer reaction times and lower chemical yields. 

Cyclic enones such as cyclopentenone or cyclohexanone were inert under the above-mentioned conditions. The reaction of aryl aldimines to cyclic enones can be mediated, however, with the more nucleophilic phosphine 119f [101], although the ee-value of the reactions is usually low (Table 5.17). 

Asymmetric organocatalytic Morita-Baylis-Hillman reactions offer synthetically viable alternatives to metal-complex-mediated reactions. The reaction is best mediated with a combination of nucleophilic tertiary amine/phosphine catalysts, and mild Bronsted acid co-catalysts usually, bifunctional chiral catalysts having both nucleophilic Lewis base and Bronsted acid site were seen to be the most efficient. Although many important factors governing the reactions were identified, our present understanding of the basic factors, and the control of reactivity and selectivity remains incomplete. Whilst substrate dependency is still considered to be an important issue, an increasing number of transformations are reaching the standards of current asymmetric reactions. 

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