| Abstract
| - Laser flash photolyses of 2-, 3-, and 4-diazoacetylpyridines 8 give the corresponding pyridylketenes 7 formed by Wolff rearrangements, as observed by time-resolved infrared spectroscopy, with ketenylabsorptions at 2127, 2125, and 2128 cm-1, respectively. Photolysis of 2-, 3-, and 4-8 in CH3CN containingn-BuNH2 results in the formation of two transients in each case, as observed by time-resolved IR and UVspectroscopy. The initial transients are assigned as the ketenes 7, and this is confirmed by IR measurementsof the decay of the ketenyl absorbance. The ketenes then form the amide enols 12, whose growth anddecay are monitored by UV. Similar photolysis of diazoacetophenone leads to phenylketene (5), whichforms the amide enol 17. For 3- and 4-pyridylketenes and for phenylketene, the ratios of rate constants foramination of the ketene and for conversion of the amide enol to the amide are 3.1, 7.7, and 22, respectively,while for the 2-isomer the same ratio is 1.8 × 107. The stability of the amide enol from 2-7 is attributed toa strong intramolecular hydrogen bond to the pyridyl nitrogen, and this is supported by the DFT calculatedstructures of the intermediates, which indicate this enol amide is stabilized by 12.8 kcal/mol relative to thecorresponding amide enol from phenylketene. Calculations of the transition states indicate a 10.9 kcal/molhigher barrier for conversion of the 2-pyridyl amide enol to the amide as compared to that from phenylketene.
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