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Hydration of Pyridylketenes:  Formation of Acid Enol and Dihydropyridine (Eneaminone) Transients

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journal contribution
posted on 08.12.2004, 00:00 by Annette D. Allen, Andrei V. Fedorov, Thomas T. Tidwell, Sinisa Vukovic
2-, 3-, and 4-Pyridylketenes 4 formed in water by photochemical Wolff rearrangements using flash photolysis undergo rapid hydration forming transient intermediates observed by UV spectroscopy. 3-Pyridylketene (3-4) formed the acid enol intermediate 3-10 which was converted to the acid 3-11, and phenylketene gave similar behavior. 4-Pyridylketene (4-4) reacted with a similar initial rate constant of 5.0 × 104 s-1 for decay of an absorption at 275 nm, with concomitant formation of a strong absorption at 370 nm with the same rate constant. The intermediate absorbing at 370 nm decayed with a lifetime 2.4 × 103 fold longer than that of the ketene, and is identified as 4-(carboxymethylene)-1,4-dihydropyridine (4-13), resulting from conjugate 1,6-addition of H2O to 4-4. 2-Pyridylketene (2-4) underwent hydration with a similar rate constant of 1.1 × 104 s-1 forming a transient with a UV absorption with maxima at 310 and 380 nm that decayed with biexponetial kinetics, with rate constants slower than the rate of formation by factors of 5.2 and 110, respectively. These results are interpreted as indicating the presence of two species, namely Z- and E-2-(carboxymethylene)-1,2-dihydropyridines (2-13), resulting from conjugate 1,4-addition of H2O to 2-4. The identifications of the 1,2- and 1,4-(carboxymethylene)dihydropyridines 2- and 4-13 were confirmed by comparison of their UV spectra with those of the corresponding N-methyl derivatives. The amination of 2-pyridylketene in CH3CN was reinvestigated, and spectroscopic evidence, computational studies, and preparation of the N-methyl analogue demonstrated formation of the 1,2-dihydropyridine Z-2-8f as the long-lived intermediate.