Formation of iminium ions from the condensation of chiral secondary or primary amines with α,β-unsatd. aldehydes or ketones can be used as an effective platform for the acceleration of a wide variety of catalytic asym. cycloaddn. and conjugate addn. reactions. The reversible formation of the active iminium ion species simulates the π-electronics and equil. dynamics traditionally assocd. with Lewis acid activation of α,β-unsatd. carbonyl compds. lowering the energy level of the LUMO assocd. with the π-system and activating subsequent reaction. Importantly, these iminium ion catalyzed processes offer the opportunity to conduct reactions in the presence of both moisture and air greatly adding to the practicality and general applicability of the chem. described. Proposed catalytic cycles and transition state models for the induction of asymmetry provide reliable and robust predictive tools for the outcome of reactions and high functional group tolerance suggests this class of transformation will have broad application in the arena of synthetic org. chem. as the area matures. This review describes the rapid expansion of iminium ion catalysis over recent years from its conceptual introduction to the development of a whole new arsenal of highly practical and effective methods with which to approach challenging and fundamental bond construction processes.
|Name||Topics in Current Chemistry|
- iminium catalysis
- amine catalysts