Photocyclization of Substituted Allyl Radicals and Properties of the Resulting Cyclopropyl Radicals

Abstract

The photochemical transformations of the substituted allyl radicals ⋮Si(CH2) m CHCHCH2 (m = 0, 1, or 3) and ⋮SiO(CH2) m CHCHCH2 (m = 0−3) grafted onto a silica surface were studied. The cyclization of the allyl-type radicals resulting in the formation of the corresponding β-substituted cyclopropyl radicals ⋮Si(CH2) m -c-Pr• or ⋮SiO(CH2) m -C-Pr• (c-Pr = cyclopropyl) was found to be the primary photochemical process occurring upon photolysis at λ ≥ 370 nm. Further transformations of the resulting cyclopropyl radicals depend crucially on the m value. In the case of m = 0 or 1, the thermal isomerization of the cyclopropyl-type radicals yielding the parent allyl-type radicals is the main reaction pathway at room temperature. For m = 2 or 3 the reaction of intramolecular hydrogen atom transfer becomes the predominating process. This reaction results in the corresponding alkyl-type radicals ⋮SiCH(CH2) m -1-c-Pr• or ⋮SiOĊH(CH2) m -1-c-Pr formation. It was shown that the presence of electron-withdrawing substituents in β-position resulted in the decrease in the thermal stability of the cyclopropyl-type radicals. The experimental data obtained were compared with the results of quantum-chemical calculations.

Low-temperature reactions in the chemistry of nanosystems

“…16 A method for stabilisation and spectroscopic investigation of radicals and short-lived intermediates on an activated silica gel surface has been proposed. 17,18 The second approach is based on co-condensation of reactant vapours on cooled surfaces at rates that exclude collisions between the reactant molecules in the gas phase (molecular beam condensation). 19 Possible modifications of this technique include, e.g., condensation of particles of the same type; layer-by-layer condensation of particles of different types; co-condensation of particles of different types; co-condensation of reactive species with a large excess of inert component; co-condensation of reactive species with a third component (an activator or initiator).…”