The alpha-effect in cyclic secondary amines: new scaffolds for iminium ion accelerated transformations

Tomkinson, Nick and Porzelle, Achim and Cavill, Julie L. and Elliott, Richard L. and Evans, Gareth and Gibbs, Timothy J. K. and Jones, Ian L. and Platts, James A (2009) The alpha-effect in cyclic secondary amines: new scaffolds for iminium ion accelerated transformations. Tetrahedron, 65. p. 9961. ISSN 0040-4020 (https://doi.org/10.1016/j.tet.2009.10.009)

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Abstract

Five-membered secondary amine heterocycles containing an α-heteroatom were prepared and shown to be ineffective as catalysts for the iminium ion catalysed Diels–Alder reaction between cinnamaldehyde and cyclopentadiene. Their six-membered counterparts proved to be highly active catalysts. In stark contrast, the catalytic activity observed when comparing the non α-heteroatom cyclic amines proline methyl ester and methyl pipecolinate showed the five-membered ring amine was significantly more active. Concurrent density functional theoretical calculations suggest a rationale for the observed trends in reactivity, highlighting that LUMO activation through an iminium ion intermediate plays a key role in catalytic activity.

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