John P. Richard

Concurrent stepwise and concerted substitution reactions of 4-methoxybenzyl derivatives and the lifetime of the 4-methoxybenzyl carbocation

The rates of reaction of 4-methoxybenzyl chloride (1), pentafluorobenzoate, and 3,5-dinitrobenzoate in 50:50 (v/v) trifluoroethanol/water are zero order in the concentration of azide ion. These reactions give good yields of the azide adduct from trapping of the 4-methoxybenzyl carbocation intermediate, and there is also strong common chloride ion inhibition of the reaction of 4-MeOArCH 2 Cl. The yields of the azide and solvent adducts give the rate constant ratio k az /k s =25 M −1 , and analysis of the chloride common ion effect gives k Cl /k s =9 M −1 , for partitioning of the carbocation intermediate. The reaction of azide ion with the carbocation intermediate serves as a diffusion-controlled «clock» for the reactions of solvent and chloride ion. There is a solvent-induced change in mechanism and in the less ionizing solvents of 80:20 (v/v) and 70:30 (v/v) acetone/water; the reaction of azide ion with 1 produces the azide adduct by both trapping of the carbocation intermediate and by a concerted bimolecular substitution reaction

The stereochemical course of thiophosphoryl group transfer catalyzed by adenosine kinase

Adenosine kinase was partially purified form beef liver and used to catalyze the conversion of (γR)ATPγS,γ18O and adenosine to ADP and AMPαS,α18O. The configuration at phosphorus in AMPαS,α18O was established by subjecting it to stereospecific phosphorylation to (αS)ATPαS,α18O and showing that only the nonbridging oxygen bonded to the α-P was enriched with 18O. The configuration at α-P in AMPαS,α18O was therefore S, and the transfer of the [18O]thiophosphoryl group occurred with inversion of configuration.