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
cm�1, 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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