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

Abstract

The photochemical transformations of the substituted allyl radicals ⋮Si(CH2)mCHCHCH2 (m = 0, 1, or 3) and ⋮SiO(CH2)mCHCHCH2 (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.