http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
study of some reactions containing radical intermediates
The reduction of trityl halides by lithium dialkylamide bases in THF produces high yields of triphenylmethane and a small amount of trityl dimer. Studies were conducted to investigate the mechanism of this reaction. Kinetic studies were conducted for the reaction of trityl chloride with lithium diisopropylamide and lithium tert-butylethylamide in THF at -78 °C, and these studies suggest that the reaction is first order in both monomeric base and trityl chloride. Deuterated amide bases were used for inter- and intramolecular kinetic isotope effect studies which demonstrated that a rate determining formation of a reaction complex preceded a hydrogen atom transfer step from the amide base to the trityl species. These results were coupled with other kinetic information to formulate a scheme for the reactions. The reactions proceed by a rapid dissociation of trityl halide which leads to the formation of a triytl-THF oxonium cation in the basic THF solvent. The amide base is present in a slow equilibrium between monomeric and dimeric forms, but the monomer is the reactive species. An associative complex is formed by a rate limiting electron transfer process between the monomeric base and the trityl-THF oxonium ion. Within this reactive complex, a fast hydrogen atom transfer between aminyl radical and trityl radical produces the observed triphenylmethane product. The hydrogen atom transfer step competes to a small extent with radical escape from the reaction complex, which ultimately leads to the production of trityl dimer. The acyl derivatives of N-hydroxypyridine-2-thione are used as radical precursors for obtaining kinetic information for reactions which proceed through radical intermediates. This method was used to measure rate constants for the ring opening of the trans-(2-phenylcyclopropyl)carbinyl radical from -48 to 25 °C in toluene. An Arrhenius function was found to be log k = 13.7 - 3.24/2.3 RT, and a rate constant for the ring opening was calculated to be 1.9x10^11 s^-1 at 25 °C.