Stereochemical aspects of the Pummerer reaction. Diastereotopic selectivity in the deprotonation of oxysulfonium cations

The diastereomeric RS/SR- and RR/SS-benzyl-α-d methyl sulfoxides have been subjected to the Pummerer reaction with acetic anhydride and with trifluoroacetic anhydride. The ethoxysulfonium fluoroborate derivatives of these sulfoxides have been subjected to the Pfitzner-Moffatt reaction with sodium hydride in tetrahydrofuran. The deuterium contents of the starting materials and of the Pfitzner-Moffatt product have been determined by mass spectrometry. The deuterium contents of the α-acyloxybenzyl methyl sulfoxides formed in the Pummerer reaction have been determined by 1 Hmr spectroscopy. The resulting data permit the calculation of the diastereotopic selectivities and isotope effects associated with the deprotonation of oxysulfonium cations. The selectivities are found to be lower by a factor of about 10 than those observed in the formation of the benzyl-α-d methyl sulfoxides by hydrogen-deuterium exchange of benzyl methyl sulfoxide. Nevertheless, the existence of selectivity in the Pummerer reactions rules out a sulfurane and a dication as sole product-determining intermediates. The factors contributing to this low selectivity are discussed.