FLASH PHOTOLYTIC GENERATION AND INVESTIGATION OF SHORT-LIVED REACTIONINTERMEDIATES - A CASE-STUDY

The advantage of adding more structure-diagnostic information to the s imple detection of flash photolytically generated transient species by changes in UV-visible light absorbance is illustrated by a case study involving the mandelic acid keto-enol system. An early report based o n preliminary evidence proposed that flash photolysis of phenyldiazoac etic acid produces the enol of mandelic acid by hydration of phenylhyd roxyketene, itself generated by a photo-Wolff reaction of the diazo ac id. Further examination, however, shows that this is only a minor rout e, and that the major pathway is a new enol-forming reaction involving what appears to be hydration of a carboxycarbene formed by dediazotiz ation of the diazo compound. Hydration of phenylhydroxyketene is never theless the reaction by which mandelic acid enol is generated when est ers of benzoylformic acid are the flash photolysis substrates. These m echanisms, and also identification of the enol as a tranisent species, are supported by detailed arguments involving acid-base catalysis, so lvent isotope effects, and the use of oxygen-18 as a tracer. The work produces a keto-enol equilibrium constant for the mandelic acid system , pK(E) = 16.19, and also acidity constants of the enol ionizing as an oxygen acid, pK(a)(E) = 6.39, and the keto isomer ionizing as a carbo n acid, pK(a)(K) = 22.57. The bearing of these results on the enzyme-c atalyzed racemization of mandelic acid is discussed. (C) 1998 John Wil ey & Sons, Ltd.