21-Telluraporphyrins. 2. Catalysts for Bromination Reactions with Hydrogen Peroxide and Sodium Bromide

meso-Tetraphenyl-21-telluraporphyrins 3 and 4 act as catalysts for bromination reactions with H₂O₂ and NaBr. Oxidation of telluraporphyrin 3 with excess H₂O₂ follows pseudo-first-order kinetics with a second-order rate constant of 0.85 ± 0.02 M^-1 s^-1 at 20.5 °C in 1:1 CH₂Cl₂/ethanol. Bromination of 4-pentenoic acid in two-phase mixtures of CH₂Cl₂ and pH 6 phosphate buffer with a stoichiometric amount of NaBr in the presence of a 50% excess of H₂O₂ follows second-order kinetics as monitored by the loss of 4-pentenoic acid. Reactions of 4-pentenoic acid and 1,3,5-trimethoxybenzene with excess H₂O₂ and excess NaBr follow pseudo-first-order kinetics and are pseudo-first-order in bromide in two-phase mixtures. Brominations of 4-pentenoic acid were conducted with initial concentrations of 0.2 M 4-pentenoic acid, 1.5 M H₂O₂, and 1.0 M in NaBr in a two-phase system of CH₂Cl₂ and pH 6 phosphate buffer. Brominations of 1,3,5-trimethoxybenzene were conducted with initial concentrations of 0.2 M 1,3,5-trimethoxybenzene, 2.5 M H₂O₂, and 1.8 M in NaBr in a two-phase system of CH₂Cl₂ and pH 6 phosphate buffer. For both systems, telluraporphyrins 3 and 4 were added as catalysts at 0.2 mol % of substrate. Values of k_cat for 3 were (4.0 ± 0.1) × 10^-2 M^-2 s^-1 for bromination of 4-pentenoic acid and (3.4 ± 0.2) × 10^-2 M^-2 s^-1 for bromination of 1,3,5-trimethoxybenzene. Turnover numbers for 3 were ≥2000 for bromination of 4-pentenoic acid.