Mechanistic studies of chain reactions involving bromo compounds.

Abstract

Chain reaction mechanisms have been the focus of this thesis. Two systems, dehydrobromination of ring-substituted $\alpha$-bromoacetophenones in alcohols and 1,5-hydrogen atom transfer reactions involved with o-bromo-p-methoxy ethers, have been studied using different approaches. Ring substituted $\alpha$-bromoacetophenones react with alcohols in a chain reaction leading to the corresponding acetophenone, HBr, and the carbonyl compound from oxidation of the alcohol. Two different mechanisms, involving hydrogen or electron transfer by ketyl radicals, have been proposed in order to accommodate the unusual selectivities of these reactions. By studying the efficiency of isotope incorporation from deuterated alcohols, it has been possible to determine the relative contributions from both mechanisms. A mechanistic study involving 1,5-hydrogen atom transfer for a newly introduced protecting/radical translocating group is described. The absolute rate constant for the radical translocation (1,5-hydrogen atom transfer) following halogen abstraction of 2-bromo-4-methoxy-1-(2-phenylethoxy) benzene (27) by Et$\sb3$Si$\sp\cdot$ radicals has been examined using laser flash photolysis techniques. (Abstract shortened by UMI.)