Activation parameters for the additions of arylhalocarbenes to alkenes

By use of nanosecond laser flash photolytic methods, absolute rate constants have been obtained for the additions of p(trifluoromethy1)phenylbromocarbene (CF3C6H4CBr), phenylbromocarbene (PhCBr), phenylchlorocarbene (PhCCI), phenylfluorocarbene (PhCF), and p-methoxyphenylfluorocarbene (CH30C6H4CF) to tetramethylethylene (TME) and 1 -hexene. The rate constants ranged from 1.7 X IO9 (CF3C6H4CBr/TME) to 5.0 X lo4 L/(M.s) (CH30C6H4CF/l-hexene); classical structure/reactivity relations were obeyed. Determination of the rate constants at temperatures between 3 15 and 200 K gave E,, log A, AH*, AS', and AG* for the 10 reactions. All of the reactions were dominated by AS'. AG* ranged from 5.0 to 11 kcal/mol for the fastest and slowest reactions. Negative values of E, were observed for some of the faster reactions at higher temperatures. These results are analyzed in terms of single-step addition reactions where AG* is dominated by AS' and, alternatively, in terms of a two-step mechanism proceeding through a carbene-alkene complex. Nearly 20 years ago, Skell and Cholod used variable-temper- ature relative rate measurements to dissect the contributions of differential activation entropies and enthalpies in the additions of CC12 to alkenes.' Although AAG* varied with alkene structure in an understandable way, it was the entropic term, rather than the enthalpic term, that usually paralleled the rates and was the dominant contributor to AAG*; only with the less reactive l-al- kenes did enthalpy dominate. Application of the Hammond postulate led to suggestions that more reactive carbenes and al- kenes traversed earlier, looser, entropy-dominated transition states, whereas less reactive carbenes and alkenes reacted via later, tighter, enthalpy-controlled transition states.*s2 More recently, the experiments of Giese3 and the theoretical studies of Houk4 have strongly emphasized the role of entropy in carbene/alkene addition reactions. The applicability of na- nosecond laser flash photolysis to the determination of absofute rate constants for (singlet) arylhalocarbene/alkene addition re- actionsS now provides an opportunity to directly examine Skell's suggestions,' as well as certain of Houk's prediction^.^ At present, sensitivity considerations have restricted laser flash photolytic studies to carbenes that contain an aromatic chromo- phore.6 Within that restriction, and on the basis of our previous re~earch,~ it was clear that the widest reactivity range of readily available singlet carbenes would be afforded by p-(trifluoro- methyl)phenylbromocarbene7 (p-CF3C6H4CBr) and p-methoxy- phenylfluorocarbene (p-CH30C6H,CF), representing (relatively) reactive and unreactive arylhalocarbenes, respectively. In order to provide continuity in the reactivity spectrum, phenylfluoro- carbene (PhCF),Sd,e phenylchlorocarbene (PhCC1),Sa,b,e and phenylbromocarbene (PhCBr)we were also included in our study. Kinetic results for these latter three carbenes were available from previous Tetramethylethylene and 1-hexene were chosen as (relatively) reactive and unreactive substrates, respectively. It was our objective to determine the absolute rate constants and activation parameters for all of the carbene/alkene addition re- actions with this set of reactants and substrates. Results Precursors. Each of the five carbenes was generated by pho- tolysis of the appropriate 3-aryl-3-halodiazirine, 1-5. Bromo- diazirines 4 and 5 and chlorodiazirine 3 were prepared directly by hypobromite or hypochlorite Graham oxidation of the ap- propriate ben~amidine.~~~ The fluorodiazirines, 1 and 2, were derived from the corresponding bromodiazirines (available from Graham oxidations) by fluoride/bromide exchange reactions with