Secondary α-Deuterium Kinetic Isotope Effects Signifying a Polar Transition State in the Thermolysis of Ring-Substituted tert-Butyl Phenylperacetates
摘要:
Several ring-substituted tert-butyl phenylperacetates (YC6H4CH2CO3But) and their deuterated versions (YC6H4CD2CO3But) were prepared (Y: p-OCH3, P-CH3, p-H; and p-NO2). Thermolyses at 80 degrees C in CDCl3 showed excellent first-order kinetics. The rates have been measured as k(YH) x 10(4) and k(YD) x 10(4) s(-1): 11.9 and 9.20 (p-OCH3), 2.64 and 2.22 (p-CH3), 1.06 and 0.93 (p-H), 0.164 and 0.156 (p-NO2). Hammett correlations were derived to yield rho(YH)(+) = -1.17 and rho(YD)(+) = -1.12. However, better Hammett plots were obtained with three points (p-OCH3, p-CH3, and p-H) showing rho(YH)(+) = -1.35 and rho(YD)(+) = -1.28. SDKIE was calculated as 1.293 (p-OCH3), 1.189 (p-CH3), 1.140 (p-H), and 1.051 (p-NO2), showing substantial substituent effects. Values of k(YH)/k(YD) for p-NO2 showed little temperature dependence. Hammett correlations and SDKIE were derived from the same kinetic entity that is the band cleavage.
Using Secondary α Deuterium Kinetic Isotope Effects To Determine the Symmetry of S<sub>N</sub>2 Transition States
作者:Kenneth Charles Westaway、Thuy Van Pham、Yao-ren Fang
DOI:10.1021/ja962088f
日期:1997.4.1
The secondary α deuterium and heavy atom kineticisotopeeffects found for two different SN2 reactions suggest that the magnitude of secondary α deuteriumkineticisotopeeffects can be determined by the length of only the shorter (stronger) reacting bond in an unsymmetrical SN2 transition state rather than by the usual nucleophile−leaving group distance. Although this means the interpretation of these