Enthalpy versus entropy in chlorocarbene/alkene addition reactions.

We report the first measured activation parameters for the additions of CCl2 and CClF to simple alkenes and demonstrate the existence of enthalpic barriers for CCl2 additions to cyclohexene and 1-hexene. With these two alkenes, additions of PhCCl are "dominated" by entropic contributions to deltaG++ and additions of CCl2 display comparable contributions of deltaH++ and deltaS++, while CClF additions feature dominant enthalpic contributions to deltaG++. Entropic factors, however, control the additions of all three carbenes to the highly reactive alkene, tetramethylethylene.

Chlorofluorocarbene: first UV observation of a dihalocarbene in solution.

Chlorofluorocarbene (ClCF), generated by laser flash photolysis of chlorofluorodiazirine, absorbs at 368 nm in pentane. Absolute rate constants are reported for ClCF additions to several alkenes and pyridine. ClCF is less reactive toward alkenes than CCl(2), and it does not react rapidly with oxygen.