Computational study of iminium ion formation: effects of amine structure

Evans, Gareth J. S. and White, Katherine and Platts, James A. and Tomkinson, Nicholas C. O. (2006) Computational study of iminium ion formation: effects of amine structure. Organic and Biomolecular Chemistry, 4 (13). pp. 2616-2627. ISSN 1477-0520

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Abstract

D. functional calcns. are used to explore the formation of iminium ions from secondary amines and acrolein and the subsequent reactivity of the resulting iminium ions. After establishing a feasible profile for this reaction in simulated exptl. conditions, we focus on the effect of variation in amine structure on calcd. barriers. This anal. shows that incorporation of a heteroatom (N or O) in the α-position to the reactive amine results in significantly reduced energy barriers, as does an electron-withdrawing group (carbonyl or thiocarbonyl) in the β-position. Electron d. anal. is used to monitor reactions at a detailed level, and to identify important intermol. interactions at both min. and transition states. Barriers to reaction are linked to calcd. proton affinities of secondary amines, suggesting that the relative ease of protonation-deprotonation of the amine is a key property of effective catalysts. Moreover, barriers for subsequent Diels-Alder reaction of iminium ions with cyclopentadiene are lower than for their formation, suggesting that formation may be the rate detg. step in the catalytic cycle.