Methylation of a Benzene Ring as a Chemical Signal. Marked Changes in the Pattern of Temperature Dependence of the Selectivity in Oxidation of a Pair of Associating Thiols

Temperature dependence of the selectivity [the ratio of an unsymmetrical disulfide to symmetrical one] has been studied in oxidation of a pair of associating thiols, each with the recognition site [C6H4N(CH3)2 and R1], with O2 in 80% (v/v) acetonitrile–20% water in the range of −10 to 70 °C. The selectivity for R1=C6H4CH3 shows a minimum at 35 °C, whereas for R1=C6H5 the selectivity exhibits a maximum at 35 °C; for R1=C6H4C2H5 plots of the selectivity against temperature shows a sigmoid-like curve. The selectivity in oxidation of a pair of non-associating thiols, 2-(dimethylamino)ethanethiol and a para-substituted benzenethiol, remains practically unaltered with temperature in the same solvent. Though thiol-disulflde exchange reaction proceeds slowly below 35 °C, the selectivity in the exchange differs entirely from that in the oxidation. It has been found that changes in hydrophobicity with R1 and temperature cannot explain dramatic alterations in the selectivity with R1 and temperature.