Correlation of Ionization Potentials and HOMO Energies versus Relative Reactivities of Cl₂, of Br₂, and of I₂ with Representative Acyclic Alkenes. Comparison with Other Additions to Alkenes

Abstract

Plots of log k(rel) versus IP or versus HOMO for the title reactions are presented; similarities and differences among the reactions are discussed. The Cl2 and Br2 data each show a single line of correlation with positive slope for all alkenes regardless of the steric requirements; increasing substitution at the double bond increases the reaction rate, indicating an electrophilic reaction. Each plot of the I2 data calculated for adsorption exhibits a natural separation into groups of similarly substituted alkenes, in which increased substitution reduces the rate. Within each group, a good-to-excellent correlation is observed, with a lower IP generally corresponding to a higher relative rate. The results indicate that the relative magnitude of the steric requirements about the double bond is similar to that of the electronic effects in iodination. Plot shapes for iodination are compared to those of other reactions, such as hydroboration, oxymercuration, complexation with Ag+, and complexation with MeHg+.

Alkene selenenylation: A comprehensive analysis of relative reactivities, stereochemistry and asymmetric induction, and their comparisons with sulfenylation

“…The necessity for separate fitting of the sterically similar groups, seen in this study of selenenylation, is in perfect agreement with previously observed correlations (log k rel versus IE or HOMO plots) reported for other types of addition reactions to alkenes [72–73 77]. On the other hand, it is remarkably different from the single line fit of positive slope, which has been obtained in analogous studies of arenesulfenylation [72].…”

“…The near-zero slope is probably due to the rate constants being dependent upon both the first step, involving electrophilic attack on C=C, and the second step, which is nucleophilic attack on the bridged intermediate by Cl − . As we demonstrated before, in this type of plot, electrophilic reactions give positive slopes [72–73 77], and nucleophilic reactions show negative slopes [76,78]. The combination of the opposing effects in the two steps would be expected to lead to a net canceling-out effect in the plot, giving the near-horizontal line obtained in this work.…”

“…The plots with multiple trend lines for PhSeCl addition to the C=C bond indicate that steric effects are significantly more important [72–73 77–78]. For reactions each involving a bridged, 3-membered cyclic stationary point, a plot with a single trend line indicates that steric effects are of low importance and points to a rate-determining transition state preceding the formation of a cyclic intermediate.…”

“…Thus, plotting log k rel values against IEs or HOMO energies of alkenes allows a comparison of the significance of steric and electronic effects in the rate-determining step of the reaction. In particular, additions to alkenes, that proceed via 3-membered intermediates (or transition states), display plots with one trend line if the reaction rate is predominantly dependent upon electronic effects [72–76], and multiple trend lines if both steric and electronic effects are important [72–73 77–78]. In this work, we apply these established relationships for a detailed analysis of the steric and electronic effects in the addition of PhSeCl to a series of representative alkenes and compare these findings with the corresponding data reported for the analogous sulfenylation reactions.…”

Bis-N-heterocyclic Carbene Palladium(IV) Tetrachloride Complexes: Synthesis, Reactivity, and Mechanisms of Direct Chlorinations and Oxidations of Organic Substrates

“…73b Equally, electrophilic alkene functionalizations with 3-membered transition states, such as epoxidations or halogenations, also display a strong preference for internal alkenes. 73b, 75 A picture emerges in which chloronium intermediate is formed via a ligand-mediated transfer of Cl + from an intermediate LPd IV Cl 3 + ( A or B , depending on the precursor 7 or 7·Cl − ) to the alkene; from the viewpoint of the alkene this reaction may be considered an ‘S N 2-type’ attack to a Cl-ligand in A or B . While our reactions have not been extended to catalytic applications yet, it is apparent that these direct Pd IV -based alkene functionalizations diverge from the paradigm in Scheme 1 in which Pd II and a O 76 -, N 46j, 77 -, or C 78 -nucleophile are added across the olefin in the activation 79 step followed by Pd II /Pd IV oxidation 6b, 19a, 80 and the C-X bond forming event (‘functionalization’) on the oxidation state +4; LPd IV Cl 4 has the ability to bypass otherwise necessary interactions between the olefin and Pd II .…”