Infrared Multiphoton Isomerization Reactions of Alkenes and Dienes

The infrared multiphoton laser-induced unimolecular isomerization reactions of several simple alkenes, conjugated alkenes, and pentadienes have been investigated. Excitation of (E)-2-butene, pentene, and hexene results in contrathermodynamic E _→Z isomerization and fragmentation, the isomerization/fragmentation ratio decreasing with increasing chain length and increasing laser fluence. These results are in qualitative agreement with RRKM calculations for average reactant energies of 75–85 kcal/mol. In addition to products of C-C homolysis observed with all three alkenes, 1-butene and 1,3-butadiene are formed from (E)-2-butene. (E)-Crotononitrile undergoes laser-induced isomerization without fragmentation, resulting in quantitative conversion to the Z isomer. In contrast, (E)-methyl crotonate undergoes both isomerization and fragmentation, while (E)-ethyl crotonate undergoes essentially quantitative elimination of ethylene. Irradiation of (E)- or (Z)-l,3-pentadiene at low laser fluences results exclusively in E ⇆Z isomerization resulting in steady-state isomer ratios which depend upon the relative magnitude of the single photon cross sections of the two isomers. At higher fluences, both isomers are converted to cyclopentadiene and trace amounts of 1,4-pentadiene. Irradiation of 1,4-pentadiene results in efficient isomerization to (E)- and (Z)-l,3-pentadiene which reacts further to yield cyclopentadiene. The factors which govern these and related infrared multiphoton reactions are discussed.