Graduate Thesis Or Dissertation

 

Intramolecular bifunctional catalysis of the racemization of phenyl benzenethiosulfinate Public Deposited

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  • Catalysis of the racemizationof optically active phenyl benzenethiolsulfinate, (+)-I, by (a) methylthioacetic acid (II) and (b) a -methylthio-m-toluic acid (III) has been investigated in aqueous dioxane and anhydrous acetonitrile at 39.6°C. Added II did not cause any increase in the rate of racemization of (+)-I in either solvent. In aqueous dioxane containing added perchioric acid, III, acting solely as a nucleophile, catalyzed the racemization of (+)-I via a process that was first-order in (+)-I, first-order in perchloric acid and first-order in III but no catalysis by III alone was observed in the absence of perchloric acid in aqueous dioxane. Since the mechanism of the acid and nucleophile catalyzed racemization has been previously worked out, no attempt was made to study the third-order process further. In acetonitrile, however, (+)-I was found to undergo racemization due to catalysis by III alone. Under these conditions the racemization followed clean second-order kinetics - -first-order in (+)-1 and first-order in III, Since equimolar mixtures of benzoic acid and benzyl methyl sulfide showed no catalysis of the racemization under these conditions, catalysis of the racemization by III under these conditions must involve intramolecular bifunctional catalysis. The mechanism shown in Chart I is proposed for this process and is in accord with the fact that such catalysis can be observed in acetonitrile but not in aqueous dioxane where hydrogen-bonding with the solvent greatly reduces the concentration of the key intermediate complex (+)-A. The initial necessary formation of a complex between (+)-I and III is followed by rate-determining attack by the sulfide part of III on the sulfenyl sulfur of (+)-I accompanied by the complete proton transfer from the carboxylic acid group of III to the sulfinyl group of (+)-I. Some further aspects of the racemization of (+)-I as catalyzed by III in acetonitrile are discussed with the purpose of showing that the process is formally analogous to simple enzyme-catalyzed processes.
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