(Top) The Eyring plot for the conversion of 1.0 mM 1 to 2 in
CH3CN in the presence of 10 equiv. HPyr+. Temperature
range: −15 to +15 °C. Error bars show one standard deviation from
at least three independent measurements. (Bottom) Correlation of
kobs (monitored at 720 nm) with the acidities of various
substituted pyridinium tosylates. Conditions: 1.0 mM 1 in CH3CN,
10 °C, 10 equiv. pyridinium, 2,6-lutidinium, or 2,4,6-collidinium tosylate.
kobs of 3-bromopyridinium at 10 °C was extrapolated
from an Eyring plot collected at a temperature range of -30 to 0 °C (Figure S2).