Mechanistic Esr Study in the Solid State: Radical Reactions in Crystals of BIS(11-BROMOUNDECANOYL) Peroxide and its Analogues.

Radical reactions, initiated by UV photolysis of single crystals of bis(11-bromoundecanoyl) peroxide (11-BrUP) and its analogues at liquid nitrogen temperature, were studied by ESR spectroscopy. Structures of radicals and radical pairs were determined by interpretation of their ESR spectra, and radical reaction mechanisms have been established. The low-temperature photolysis initially produces radical pairs that upon annealing abstract neighboring hydrogens to give different radical pairs. These continue to abstract hydrogens and generate other paramagnetic species. In all, at least five different radicals and twelve reaction intermediates were identified. Species observed included 10-bromo-1-decyl, 9-bromo-1-methylnonyl and radicals resulting from hydrogen abstractions from the following positions in the peroxide: alpha, delta and unspecified carbon between varepsilon and theta. In most cases, they were observed as radical pairs. Regiochemical specificity of the hydrogen abstractions was shown by deuterated 11-BrUP and its analogues. Stereospecifically deuterated peroxide, (S)-alpha-d _1-(S)-alpha^ '-d_1-bis(13-bromotridecanoyl) peroxide ((S)-alpha-d_1 -(S)-alpha^' -13-BrTP), gives two diastereomeric crystals; one in the space group P4_12_1 2 and another in its enantiomorph P4_3 2_12. The resultant diastereomers demonstrated the stereochemical specificity of hydrogen abstractions, thus confirming the abstraction mechanisms suggested by analysis of the crystal structure. It has been found that the crystal lattice determines which reaction intermediates are observed as well as their geometries, but the primary isotope effect can change the reaction pathway in specifically deuterated crystals. The structures of paramagnetic intermediates and radical reaction mechanisms were mainly determined with a variety of deuterations in 11-BrUP and its analogues. Selective deuteration of asymmetric long-chain diacyl peroxide such as 11-bromoundecanoyl lauroyl peroxides, which pack in the same local structure as the symmetric peroxides, was particularly helpful for investigating radical reactions by ESR. A secondary radical alpha to the carbonyl group is photofragmented to primary radicals at liquid nitrogen temperature by photolysis with light to the red of 380nm. One of these primary radicals was identified by its characteristic splitting pattern. The conversion appears to be intramolecular. These primary radicals convert to a secondary radical by intermolecular hydrogen abstraction.