Part I. The radical chemistry of geminal bis-azoalkanes. Part II. The radical chemistry of vicinal bis-azoalkanes. Part III. Homolysis of a weak carbon-carbon bond
Engel, Paul S.
Doctor of Philosophy
Part I. The radical chemistry of geminal bis-azoalkanes. Two geminal bis-azoalkanes have been employed to generate carbon centered radicals adjacent to azo substituents. The activation free energies for C-N bond homolysis of 2,2-bis-(phenylazo)propane (1) and 2-methylazo-2-phenylazopropane (2) were 8 kcal/mol lower than for the corresponding model compounds 3-methyl-3-phenylazo-1-butene (4) and 3-methyl-3-methylazo-1-butene (5). This large rate enhancing effect of an $\alpha$-phenylazo substituent is ascribed to the high energy of azo functional groups. 2-(Phenylazo)-2-propyl radical was easily trapped by good hydrogen donors such as thiophenol to afford a hydrazone. The interconversion of geminal bis-azoalkane 2 and tetrazene 3 by low temperature photolysis has been observed. Part II. The radical chemistry of vicinal bis-azoalkanes. Three vicinal bis-azoalkanes have been synthesized by oxidation of hydrazones. In thermolysis, substituents on the azo group dramatically influence the bond homolysis pattern, which varies from pure C-C homolysis when the substituent is phenyl to pure C-N homolysis in the case of a t-butyl substituent. Phenylazo accelerates $\beta$ C-C bond homolysis by approximately 3.2 kcal/mol more than the methylazo group. This result supports the delocalized $\pi$ electronic structure of the 2-(phenylazo)-2-propyl radical. The photochemical behavior of vicinal bis-azoalkanes resembles that of mono azoalkanes except for 9, which underwent both C-C and C-N bond homolysis. $\beta$-Azo radicals from stepwise C-N bond homolysis have been clearly trapped. Part III. Homolysis of a weak C-C bond. The formal para-recombination product of 2-methoxycarbonylpropyl and trityl radicals (53) was readily air oxidized to hydroperoxide through a radical chain mechanism. The weak C-C bond in 53 was cleaved homolytically in thermolysis, photolysis, and triplet sensitized photolysis generating methyl isobutyrate radical and triphenylmethyl radical. This result rules out McElvain's concerted thermolysis mechanism. Triphenylmethyl radical underwent intramolecular cyclization under the photolysis conditions. The facile aromatization of 53 through a radical mechanism was observed.