Nucleophilicity parameters of pyridinium ylides and their use in mechanistic analyses.

Kinetics of the reactions of pyridinium, isoquinolinium, and quinolinium ylides with diarylcarbenium ions, quinone methides, and arylidene malonates (reference electrophiles) have been studied in dimethylsulfoxide solution by UV-vis spectroscopy. The second-order rate constants (log k2) were found to correlate linearly with the electrophilicities E of the reference electrophiles, as required by the linear free-energy relationship log k(20°C) = sN(N + E) (J. Am. Chem. Soc. 2001, 123, 9500), which allowed us to derive the nucleophile-specific parameters N and sN for these ylides. Pyridinium substitution is found to have a similar effect on the reactivity of carbanionic reaction centers as alkoxycarbonyl substitution. Agreement between the rate constants measured for the 1,3-dipolar cycloadditions of pyridinium, isoquinolinium, and quinolinium ylides with acceptor substituted dipolarophiles (arylidenemalononitrile and substituted chalcone) and those calculated from E, N, and sN shows that the above correlation can also be employed for predicting absolute rate constants of stepwise or highly unsymmetrical concerted cycloadditions. Deviations between calculated and experimental rate constants by a factor of 10(6) were demonstrated to indicate a change of reaction mechanism.