SPECTROSCOPY AND ABSOLUTE REACTIVITY OF KETENES IN ACETONITRILE STUDIED BY LASER FLASH-PHOTOLYSIS WITH TIME-RESOLVED INFRARED DETECTION

Laser flash photolysis with time-resolved infrared detection of transi ents (LFP-TRIR) has been used to study the IR spectroscopy and reactiv ity of a number of substituted ketenes, prepared by the 308-nm photoly sis of alpha-diazocarbonyl precursors in acetonitrile solution at room temperature. The correlation of the experimental ketene asymmetric st retching frequency to the Swain-Lupton field (F) and resonance (R) eff ect substituent parameters was unsatisfactory, whereas the correlation to the inductive substituent parameter (sigma(1)) of Charton gave exc ellent results. This suggests that the asymmetric stretching frequency of substituted ketenes depends mainly on the inductive (i.e., field) effect of the substituents. The mechanism and kinetics of the reaction s of these ketenes with various amines in acetonitrile were also studi ed. An intermediate species identified as either zwitterionic ylide or amide enol formed in the nucleophilic addition of the secondary amine to the C-alpha of the ketene is observed by TRIR. The decay of this s pecies is assisted by the amine and is concomitant with the formation of an amide, the final product of the reaction. Our kinetic data on ke tene amine reactions show a general trend, indicating a much higher re activity (ca. 3 orders of magnitude difference in the corresponding ra te constants) of secondary amines compared with that of tertiary amine s. Secondary diethylamine shows reactivity similar to those observed f or primary amines, while secondary piperidine seems to be, in general, somewhat more reactive. The observed trend is rationalized in terms o f the steric effects exerted by both amine and ketene substituents. Ou r data on para-substituted phenyl ketenes provide support for the nega tive charge development on the ketene moiety in the transition state, with electron-withdrawing substituents accelerating and electron-relea sing substituents slowing down the addition reaction.