Stepwise Mechanism for Oxidative Addition of Iodine to Organotellurium(II) Compounds As Observed by Stopped-Flow Spectroscopy

The reaction of iodine with a series of diorgano tellurides was monitored by stopped-flow spectroscopy. For dihexyl telluride (1), diphenyl telluride (2), di-p-anisyl telluride (3), and phenyl 2-((dimethylamino)methyl)phenyl telluride (4), an initial “fast” reaction was first order in substrate and second order in iodine. For 2,6-di-tert-butyltelluropyran-4-one (5), the “fast” reaction was second order overall and first order in iodine. The “fast” reaction is actually two reactions: the addition of iodine to the tellurium atom to form an η¹-R₂Te-I₂ complex followed by the addition of a second iodine to form presumably an η¹-R₂Te-I₄ complex. The first reaction is faster than the second leading to a rapid preequilibrium and apparent inverted Arrhenius behavior in the temperature dependence of the rate constants. The “fast” reaction(s) is followed by a “slow” reaction, which is first order overall and independent of iodine concentration. The rate constant for the “slow” reaction increases with increasing solvent polarity, which is consistent with a dissociative process leading to ionic intermediates.