STEPWISE MECHANISM FOR OXIDATIVE ADDITION OF BROMINE TO ORGANOSELENIUM(II) AND ORGANOTELLURIUM(II) COMPOUNDS

The oxidative-addition reactions of bromine (5 x 10(-4) or 5 x 10(-5) M) to phenyl selenide (1), phenyl telluride (2), 2,6-di-tert-butylselenopyran-4-one (3), 2,6-di-tert-butyltelluropyran-4-one (4), 2,6-diphenyltelluropyran-4-one (5), 4-(dicyanomethylidene)-2,6-di-tert-butylselenopyran (6), and 4-(dicyanomethylidene)-2,6-di-tert-butyltelluropyran (7), (1-5 at 5 x 10(-5) M, 6 and 7 at 1 x 10(-5) M) in carbon tetrachloride were monitored by stopped-flow spectroscopy over the temperature range 281.8-307.7 K. Compounds 1, 2, 4, 5, and 7 gave oxidative-addition products 8-12, while compounds 3 and 6 gave no detectable reaction. Kinetic analysis showed three discrete reactions: an initial fast, second-order (first-order in both bromine and substrate) reaction to give an association complex followed by two consecutive first-order processes to give the final products. Single-crystal, X-ray crystallographic analysis of 14 [from the addition of bromine to 2-((dimethylamino)methyl)phenyl phenyl telluride] was indicative of an ionic structure. The addition of tetra-n-butylammonium bromide to 8 and 9 gave the corresponding diphenyl chalcogenides 1 and 2, respectively, and Br3-. Heating crystals of 8 under nitrogen gave melting and gas evolution. The residue was identified as diphenyl selenide (1). The data are consistent with multiple, reversible steps in oxidative addition involving entropy-controlled association, ionic dissociation and recombination, and slow conversion of a product mixture of kinetic control to a product mixture of thermodynamic control.