UBC Theses and Dissertations
Stereochemistry of the reaction of molecular oxygen with alkyl free radicals Schindel, Wesley Gerald
In order to obtain chemical evidence concerning the configuration of free alkyl radicals, 1-(carbo-t-butylperoxy)-cis- and trans-1,4-dimethylcyclohexane and exo- and endo-2-(carbo-t-butylperoxy)-2-methylnorbornane have been synthesized, and their decomposition rates and products determined. The cis- and trans-cyclohexyl peresters undergo thermal decomposition at 60° in cumene at essentially the same rates (cis isomer ∆H⁺ = 27.5 kcal/ mole, ∆S⁺ = 5.07 eu; trans isomer ∆H⁺ = 27.8 kcal/mole, ∆S⁺ = 5.75 eu) and with direct formation of 1,4-dimethylcyclohexyl radicals, carbon dioxide, and t-butoxy radicals. The products, cis- and trans-1,4-dimethylcyclohexane, 1,4-dimethylcyclohexene, and 4-methylmethylenecyclohexane, formed from the 1,4-dimethylcyclohexyl radicals in degassed cumene, are the same from both precursors. In the presence of oxygen in 1,2-dimethoxyethane (glyme) or 25 ml Freon TF:1 ml glyme, both the cis and trans peresters gave identical yields (after reduction) of trans-1,4-dimethylcyclohexanol (58%) and cis-1,4-dimethylcyclohexanol (42%) at oxygen pressures from 1 to approximately 600 atm. A common intermediate is thus indicated. In cumene at 75°, exo-2-(carbo-t-butylperoxy)-2-methylnorbornane decomposes 6.6 - 6.7 times faster than its endo isomer. The activation parameters are ∆H⁺ = 27.9 kcal/mole, ∆S⁺ = 6.31 eu, and ∆H⁺ = 30.3 kcal/mole, ∆S⁺ = 9.33 eu, respectively. The products, exo- and endo-2-methylnorbornane, 2-methyl-2-norbornene, and 2-methylenenorbornane, resulting from 2-methyl-2-norbornyl radicals in degassed cumene, are essentially the same from either isomeric source. The same ratio of exo- to endo-2-methyl-2-norbornyl hydroperoxide (15%:85%) is formed from either perester in the presence of oxygen at pressures up to -600 atm. Unlike the apparently unique case of distinctly different cis- and trans-9-decalyl radicals, the alkyl radicals from the decomposition of isomeric norbornyl and cyclohexyl peresters have a single configuration or they invert much faster than they react with oxygen. The results suggest that if radicals with a stereochemical "memory" of their origin exist in these systems, their lifetimes are of the order of 10⁻¹⁰ seconds or less. For the 1,4-dimethylcyclohexyl radical, hydrogen atom transfer and reaction with oxygen were found to occur preferentially (82% and 58%, respectively) from an axial direction. On the other hand, cumene and oxygen react with the 2-methyl-2-norbornyl radical predominantly from an exo direction (90% and 85%, respectively). The carbonylation of tetrahydrolinalool, and several synthetic routes to 1-alkyl-4-t-butylcyclohexanecarboxylic acids also have been explored.
Item Citations and Data