Dendrimeric organochalcogen catalysts for the activation of hydrogen peroxide: Origins of the "dendrimer effect" with catalysts terminating in phenylseleno groups

Several scenarios were evaluated to explain the large "dendrimer effect" observed in the bromination of cyclohexene with H₂O ₂ and NaBr catalyzed by the addition of Frechét-type dendrimers terminating in -O(CH₂)₃SePh groups. Although phenylseleninic acid was an efficient catalyst for the oxidation of NaBr with H₂O₂, first-order rate constants for the selenoxide elimination were too small to produce PhSeO₂H at a rate sufficient to explain the rates of catalysis and no dendrimer effect was observed in the rates of selenoxide elimination. An induction period was observed using 1-SePh as a catalyst for the oxidation of Br- with H₂O₂. The addition of preformed selenoxide 1-Se(=O)Ph gave immediate catalysis with no induction period. However, rates of oxidation of the selenides with H ₂O₂ under homogeneous or biphasic conditions or with t-BuOOH under homogeneous conditions were too slow to account for the rates of catalysis, and no dendrimer effect was observed in the rates of oxidation. The primary oxidant for converting selenides to selenoxides was "Br+" produced initially by the uncatalyzed background reaction of H₂O ₂ with NaBr and then produced catalytically following formation of selenoxide groups. Autocatalysis is observed, and the rate of oxidation increases with the number of SePh groups. Autocatalysis is the source of the large dendrimer effect observed with the SePh series of catalysts.