Sulfur Transfer Versus Phenyl Ring Transfer in the Gas Phase: Sequential Loss of CH 3 OH and CH 3 O–P=O from Protonated Phosphorothioates

Abstract

Collisional activation fragmentation of protonated phosphorothioates leads to skeletal rearrangement and formation of aryl sulfenylium cation (R-PhS⁺) via successive eliminations of CH₃OH and CH₃O–P=O. To better understand this unusual fragmentation reaction, isotope-labeling experiments and density functional theory (DFT) calculations were carried out to investigate two mechanistic pathways. In route 1, a direct intramolecular transfer of the R-phenyl group occurs from the oxygen atom to the sulfur atom on thiophosphoryl to form methoxyl S-(3-methyl-4-methylsulfanyl-phenyl) phosphonium thiolate (a4), which subsequently dissociates to form the m/z 169 cation. In route 2, the sulfur atom of the thiophosphoryl group undergoes two stepwise transfer (1,4-migration to the ortho-carbon atom of the phenyl ring followed by 1,2-migration to the ipso-carbon atom) to form an intermediate isomer, which undergoes the subsequent dissociation to form the m/z 169 cation. DFT calculations suggested that route 2 was more favorable than route 1 from the point view of kinetics.

Graphical Abstract

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