Brpnsted plots biphasic

A change in mechanism [at pH pX of (9)] from ElcBi to ElcB is also believed to account for the biphasic Brpnsted plots (/3i 0.7, P2 0) and associated entropy... 

Biphasic Brpnsted plots were obtained for the pyridinolysis of aryl dithiocyclopen-tanecarboxylates (38 n = 0, X=S) in MeCN at 333 K, with a change in slope at pXj, = 5.2. This was attributed to a change in the rate-determining step from breakdown to formation of a zwitterionic tetrahedral intermediate with increasing basicity of the pyridine nucleophile. Aminolysis of both aryl dithiocyclohexanecarboxylates (38 ... 

The biphasic Hammett and Brpnsted plots found for the 5 2 reaction between 3-and 4-substituted pyridines and anilines and (2i ,4f ,55)-(+)-2-chloro-3,4-dimethyl-5-phenyl-l,3,2-oxazaphospholidine 2-sulfide in CH3CN at 5 C is attributed to a change from a backside to a frontside attack by the nucleophile. The Hammett p = -6.l5 and Brpnsted = 1.11 found for strongly basic pyridines was attributed to a frontside attack by the nucleophile, while the Hammett p = 4.73 and Brpnsted fl = -0.75 found with the weakly basic pyridines was thought to be for a backside attack by the nucleophile. The rate constants, AH and AS values, and the isokinetic temperature where 8 AH = T... 

Anti-cancer drugs that are sulfamate esters, ROSO2NH2, appear to act by inhibition of sulfatases. Now, kinetic studies of the aminolysis of p-nitrophenyl sulfamate (109) by secondary alicyclic amines in MeCN at 310 K are reported that model the enzyme reaction. The Brpnsted-type plot was biphasic, the break point at w 18.2 more or less corresponding to the pKa of the ester (109) (17.8). The proposed mechanism (Scheme 30) involves a sequential double deprotonation of (109) leading first, via (110), to the sulfenamine (112) and at higher basic strength to (111) and thence to a novel anionic sulfenamine (113), the products in each case being an V, V-dialkylsulfamide (114) and p-nitrophenol.114... 

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