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The Alkaline Hydrolysis of Sulfonate Esters: Challenges in Interpreting Experimental and Theoretical Data
Sulfonate ester hydrolysis has been
the subject of recent debate,
with experimental evidence interpreted in terms of both stepwise and
concerted mechanisms. In particular, a recent study of the alkaline
hydrolysis of a series of benzene arylsulfonates (Babtie et al., <i>Org. Biomol. Chem.</i> <i>10</i>, <b>2012</b>, 8095) presented a nonlinear Brønsted plot, which was explained
in terms of a change from a stepwise mechanism involving a pentavalent
intermediate for poorer leaving groups to a fully concerted mechanism
for good leaving groups and supported by a theoretical study. In the
present work, we have performed a detailed computational study of
the hydrolysis of these compounds and find no computational evidence
for a thermodynamically stable intermediate for any of these compounds.
Additionally, we have extended the experimental data to include pyridine-3-yl
benzene sulfonate and its <i>N</i>-oxide and <i>N</i>-methylpyridinium derivatives. Inclusion of these compounds converts
the Brønsted plot to a moderately scattered but linear correlation
and gives a very good Hammett correlation. These data suggest a concerted
pathway for this reaction that proceeds via an early transition state
with little bond cleavage to the leaving group, highlighting the care
that needs to be taken with the interpretation of experimental and
especially theoretical data