Positive halogens from halides and hydrogen peroxide with organotellurium catalysts

Abstract

The oxidations of sodium bromide, sodium chloride, and sodium iodide to positive halogen with hydrogen peroxide in two-phase systems of dichloromethane and pH 6 phosphate buffer were catalyzed by organotellurium catalysts 1-3. The positive halogens were trapped by cyclohexene for bromine and chlorine to give mixtures of the 1,2-dihalocyclohexane (4) and 2-halocyclohexanol (5). For the bromination (4a)/hydrobromination (5a) of cyclohexene, unoptimized turnover numbers of 1010 mol of product per mole of catalyst for 1, 960 for 2, and 820 for 3 were measured with 4a/5a ratios of 55:45, 53:47, and 52:48, respectively. In cyclohexane, the turnover number for 1 was 150 and the 4a/5a ratio was 68:32. In the uncatalyzed process and in the reaction of aqueous bromine with cyclohexene, the 4a/5a ratio is 55:45 in dichloromethane and 67:33 in cyclohexane. The relative rates of catalysis for equimolar amounts of 1-3 were nearly identical to the relative second-order rate constants for oxidation of the organotellurium compounds with hydrogen peroxide, which suggests that oxidation of the catalyst is the rate-determining step of the process. Stopped-flow studies indicated a rapid reaction (k = 22.5 +/- 0.3 M(-1) s(-1) for iodide and 13.9 +/- 0.5 M(-1) s(-1) for bromide) between halide and oxidized 3 to regenerate catalyst 3. Relative rates of catalysis with 0.1 mol % of 1-3 (relative to cyclohexene) were 4.6 for 1, 2.0 for 2, 1.0 for 3, and 0.11 for the control reaction with no catalyst at 296.1 +/- 0.1 K. Oxidation of chloride with hydrogen peroxide with 1 as a catalyst was much slower but the unoptimized turnover number was 100 with a 4b/5b ratio of 7:93 (10:90 in the uncatalyzed process) in a two-phase cylohexane/aqueous system. Oxidized 3 reacts rapidly with both sodium chloride and sodium bromide to give products from oxidative addition of halogen to the catalyst. Stronger Te-Cl bonds relative to Te-Br bonds slow down the release of the Te(II) state of the catalyst. Positive iodine from catalysis with 1 was trapped by 4-pentenoic acid to give iodomethyl lactone 6.