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Pure Appl. Chem. 76(11), 1953-1966, 2004

Pure and Applied Chemistry

Vol. 76, Issue 11

Pyridylketenes. Structure reactivity effects in nucleophilic and radical addition

A. D. Allen, A. V. Fedorov, K. Najafian, T. T. Tidwell, and S. Vukovic

Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada

Abstract: 2-, 3-, and 4-Pyridylketenes have been generated in CH3CN by photochemical Wolff rearrangements and identified by their ketenyl absorption in the infrared at 2127, 2125, and 2128 cm1, respectively. Reaction of these pyridylketenes with n-BuNH2 results in the formation of intermediate amide enols from the 3- and 4-pyridylketenes, which are then converted to the corresponding pyridylacetamides. However , 2-pyridylketene forms a long-lived 1,2-dihydropyridine intermediate stabilized by an intramolecular hydrogen bond, and this is converted to the 2-pyridylacetamide with a rate constant 107 less than those for the conversion of the amide enols from the 3- and 4-pyridylketenes to amides. Hydration of the pyridylketenes results in the formation of an acid enol intermediate in the case of the 3-isomer, while the 2- and 4-isomers form longer-lived dihydropyridines. The pyridylketenes react with the stable free radical tetramethylpiperidinyloxyl (TEMPO,TO) forming 1,2-diaddition products ArCH(OT)CO2T.

* Plenary lecture presented at the 5th Florida Heterocyclic Conference, Gainesville, FL, USA, 7-10 March 2004. Other presentations are published in this issue, pp. 1933-1983.

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