Isolation and Racemization Mechanism of Optically Active Benzylmethylphenyltelluronium Salts

Diastereomeric benzylmethylphenyltelluronium (1S)‐(+)‐ and (1R)‐(–)‐camphor‐10‐sulfonates were synthesized and resolved by fractional recrystallization. The absolute configurations of the isomers were determined by X‐ray crystallographic analysis. Enantiomerically pure telluronium salts with achiral counteranions, such as tetrafluoroborate, 4‐chlorobenzenesulfonate, bromide, iodide, and tetraphenylborate, were also obtained by anion‐exchange reactions of the separated diastereomers. The optically active telluronium salts possessing counteranions with a high nucleophilicity, such as bromide and iodide, were found to racemize in solution. The racemization of the telluronium salts was found to proceed either through the formation of a tellurane by the nucleophilic addition of a counteranion to the tellurium atom followed by decomposition into methyl phenyl telluride and benzyl halide or by several pseudorotations of the tellurane. Enantiomeric benzylmethylphenyltelluronium salts were isolated with several counteranions. The racemization was found to proceed either through the formation of a tellurane followed by decomposition into achiral methyl phenyl telluride and benzyl halide or by several pseudorotations (Φ) of the tellurane intermediate.