Abstract
The secondary 18 O and 15 N isotope effects have been measured for phosphoryl-transfer reactions of the phosphodiester 3,3-dimethylbutyl p-nitrophenyl phosphate under conditions of acid and alkaline hydrolysis, cleavage by β-cyclodextrin, and cleavage by snake venom phosphodiesterase. Isotopic labeling and other experiments show that these reactions proceed by an S N 2(P) mechanism. The secondary 18 O isotope effect measures changes in transition state bonding to the nonbridge oxygen atoms in the central phosphoryl group in these S N 2 reactions. The 15 N isotope effects measure transition-state bond cleavage to the leaving group p-nitrophenol. The isotope effect data indicate that weaker nucleophiles lead to a more associative transition state with respect to the central phosphoryl group (decreased bonding to the nonbridge oxygens) while the extent of bond cleavage to the leaving group is affected little
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