Dichotomous mechanistic behavior in Narasaka-Heck cyclizations: Electron rich Pd-catalysts generate iminyl radicals

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Abstract

Pd-catalyzed cyclizations of oxime esters with pendant alkenes are subject to an unusual ligand controlled mechanistic divergence. Pd-systems modified with electron deficient phosphines (e.g. P(3,5-(CF3)2C6H3)3) promote efficient aza-Heck cyclization, wherein C-N bond formation occurs via alkene imino-palladation. Conversely, electron rich ligands, such as P(t-Bu)3, cause deviation to a SET pathway and, in these cases, C-N bond formation occurs via cyclization of an iminyl radical. A series of mechanistic experiments differentiate the two pathways and the scope of the hybrid organometallic radical cyclization is outlined. This study represents a rare example in Pd-catalysis where selection between dichotomous mechanistic manifolds is facilitated solely by choice of phosphine ligand.

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Race, N. J., Faulkner, A., Shaw, M. H., & Bower, J. F. (2016). Dichotomous mechanistic behavior in Narasaka-Heck cyclizations: Electron rich Pd-catalysts generate iminyl radicals. Chemical Science, 7(2), 1508–1513. https://doi.org/10.1039/c5sc04037j

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