Secondary Deuterium Isotope Effects on Formyl Transfer Reactions between Sulfur and Oxygen Nucleophiles

Article abstract of DOI:10.1021/ja00528a041

Secondary deuterium isotope effects have been measured for formyl and deuterioformyl transfer between oxygen and sulfur nucleophiles at 25 deg C in aqueous solution.Most data were obtained on p-nitrophenyl formate and deuterioformate, and on p-nitrothiophenyl formate and deuterioformate.The nucleophiles that were used are a series of aromatic and aliphatic thiol anions and oxy anions having a broad range of pK_a values.The value of k_D/k_H is larger for oxy anion attack (ca. 1.22) on esters than for thiol anion attack (ca. 1.00).The effects on k_D/k_H of changing from an aryl to an alkyl anion nucleophile are discussed.No measurable effect on k_D/k_H is observed with changing nucleophile basicity, suggesting little "Hammond postulate" type of change in transition-state structure.This lack of change is found even in regions were β_nuc varies substantially for oxy anions.Data are presented which suggest that the use of absolute values of k_D/k_H for determining transition-state bond orders can give values much different than other indexes of transition-state structure.The possibility that changes in frequencies other than C-H stretching and bending modes may result in a variation in k_D/k_H is considered.