Time-resolved resonance Raman and density functional theory investigation of iodocyclopropanation and addition reactions with alkenes after ultraviolet photolysis of iodoform

Abstract

A time-resolved resonance Raman spectroscopic investigation is reported for the ultraviolet photolysis of CHI 3 in pure cyclohexene and mixed cyclohexane/cyclohexene solvents. The ICHI-I species is observed in pure cyclohexane solvent. Upon addition of cyclohexene, the ICHI-I species lifetime is reduced and new bands from an I 2:cyclohexene complex are observed. Density functional theory computations show that ICHI-I and CHI 2 species have similar barriers of reaction toward addition to the C=C of ethylene. The addition reaction of ICHI-I with ethylene results in formation of an iodocyclopropane and I 2 molecule while addition of CHI 2 results in initial formation of a diiodopropyl radical intermediate. Ultraviolet photolysis of CHI 3 in the presence of cyclohexene is known to produce a reasonable yield of iodonorcarane product and some addition reaction products. We present a mechanism for the iodocyclopropanation reaction that is consistent with both experimental and theoretical characterization of reaction intermediates formed after ultraviolet photolysis of CHI 3. We briefly discuss the concentration dependence of the time-resolved resonance Raman spectra and photochemistry in relation to the competition between the reaction of the ICHI-I and CHI 2 species with the C=C bond of olefins.