PHOTOCYCLIZATION OF SUBSTITUTED ALLYL RADICALS AND PROPERTIES OF THE RESULTING CYCLOPROPYL RADICALS

The photochemical transformations of the substituted allyl radicals =S i(CH2)(m)CH=CH=CH2 (m = 0, 1, or 3) and =SiO(CH2)(m)CH=CH=CH2 (m = 0-3 ) grafted onto a silica surface were studied. The cyclization of the a llyl-type radicals resulting in the formation of the corresponding P-s ubstituted cyclopropyl radicals =Si(CH2)(m)-c-Pr-. or =SiO(CH2)(m)-C-P r-. (c-Pr = cyclopropyl) was found to be the primary photochemical pro cess occurring upon photolysis at lambda greater than or equal to 370 nm. Further transformations of the resulting cyclopropyl radicals depe nd crucially on the m value. In the case of m = 0 or 1, the thermal is omerization 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 become s the predominating process. This reaction results in the correspondin g alkyl-type radicals =SiCH(CH2)(m-1)-c-Pr-. or =SiOCH(CH2)(m-1)-c-Pr formation. It was shown that the presence of electron-withdrawing subs tituents in beta-position resulted in the decrease in the thermal stab ility of the cyclopropyl-type radicals, The experimental data obtained were compared with the results of quantum-chemical calculations.