Alkenes association constants

The correlation between the apparent association constants, K. which are derived from the competitive rates on Pt and reductions by diimide indicates that structural changes in the alkene generally have parallel effects on these reactions, Fig. 2. Because the diimide reduction is essentially free of steric effects, this effect is liable to account for some of the differences which are observed in extended groups of compounds. The small range of individual reactivities on Pt, which are zero order in alkene, can be understood in that the variation in structure which increases the driving force towards... 

An explanation for this difference in selectivity of the Ni catalysts is suggested by the studies of Okamoto et al. who correlated the difference in the X-ray photoelectron spectra of various nickel catalysts with their activity and selectivity in hydrogenations (ref. 28,29). They find that in individual as well as competitive hydrogenations of cyclohexene and cyclooctene on Ni-B, cyclooctene is the more reactive while the reverse situation occurs on nickel prepared by the decomposition of nickel formate (D-Ni). On all the nickel catalysts the kinetically derived relative association constant favors cyclooctene (ref. 29). The boron of Brown s P-2 nickel donates electrons to the nickel metal relative to the metal in D-Ni. The association of the alkene with the metal is diminished which indicates that, in these hydrocarbons, the electron donation from the HOMO of the alkene to an empty orbital of the metal is more important than the reverse transfer of electron density from an occupied d-orbital of the metal into the alkene s pi orbital. 

The relative equilibrium constants for the adsorption of alkenes on the surfaces of platinum metals are not available from static measurements. Accordingly, the kinetically derived constants have been compared with the association constants of alkenes with metal ions in solution. Of these, the measurements by Tolman are the most instructive. ... 

In catalysis the excess of a phosphine ligand is often necessary because it preserves the active species in the medium [2a]. However, it retards to some extent the co-ordination of the alkene to the metal center. Recent studies, performed by Monflier and coworkers, have shown that the water-soluble TPPTS ligand could reduce the rate of the reaction by another effect. Indeed, TPPTS can be included partially in the cyclodextrin hydrophobic cavity [53,54] NMR measurements, observation by UV-visible spectroscopy and circular dichroism, as well as scanning tunneling microscopy are consistent with a 1 1 inclusion complex in which the phosphorus atom would be incorporated into the torus of the /S-CD. NMR investigations carried out on (m-sulfonatophenyl)diphenylphosphine have shown that a phenyl group is incorporated [55]. Thus, the phosphorus ligand could modify the association constant of the alkene with the cyclodextrin so that the mass transfer between the two phases could be decreased. 

Kinetic studies have been performed on the individual steps occurring in the NO3 and OH initiated oxidation of VOCs. The studied reactions include essentially reactions of NO3 with alkenes, di-alkenes and dimethyl sulfide (DMS), reactions of NO3 with intermediate peroxy radicals (HO2, CH3O2, C2H5O2) and reactions of OH with methane and oxygenated VOCs (ethers, alcohols). The rate constants for these reactions have been measured, and mechanistic information has been determined. The experimental methods used were discharge-flow reactors coupled with mass spectrometry, electron paramagnetic resonance (EPR), laser-induced fluorescence (LIF) analysis and the laser photolysis associated with LIF analysis. The discharge-flow LIF and laser photolysis LIF experiments have been especially developed for these studies. 

A mechanism is now proposed for Mizoroki-Heck reactions involving Pd(OAc)2 as precursor associated with PPh3 (Scheme 1.22). From the rate constants of the main steps given in Scheme 1.22, it appears that, for comparable iodobenzene and styrene concentrations, the overall carbopalladation (complexation/insertion of the alkene) from PhPd(OAc)(PPh3)2... 

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