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 k20°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 106 were demonstrated to indicate a change of reaction mechanism.