Kinetic studies of alkenes

Kinetic studies of alkene-phosphorus pentachloride reactions in benzene show the effects of substituents when the double bond is terminal.88 When the alkene is conjugated, the standard work-up conditions (using sulphur dioxide) produce alk-1-enylphosphonyl dichlorides (103), instead of 2-chloroalkylphosphonyl dichlorides (104).87... 

Several kinetic studies of alkene isomerizations catalyzed by transition-metal coordination complexes in homogeneous solutions have been reported. 

As noted above, most kinetic studies of alkene radical cations in solution have focused on aryl-substituted systems since they have convenient optical propenies and have been extensively studied by other techniques. Radical cations are frequently identified on the basis of their characteristic UV-visible absorptions and the comparison of their spectra to those obtained in matrices at low temperature." However, a number of other diagnostic tests are also commonly employed to identify these intermediates. For example, their kinetic behavior as a function of solvent nucleophilicity or added nucleophiles is analogous to that of other electrophilic species. Thus, reaction with nucleophiles such as azide and halide ions provides support for the assignment of a transient to a radical cation, although it will not serve to eliminate a carbocation intermediate. More useful in the latter respect is the method of generation of the transient since PET does not in general lead to the formation of carbocations. Quenching of the observed transient with a more easily oxidized... 

Alkenes.—Kinetic studies of alkene isomerization, catalysed by transition-metal complexes, are rare. One recent example is isomerization of hex-l-ene, catalysed by Co(N2)(PPh3)3, where the rate is proportional to the square of the catalyst concentration this suggests hydrogen migration in a dinuclear intermediate. The activation enthalpy for this isomerization is 10.1 kcal mol , the activation entropy being - 21 cal deg mol . Isomerization... 

A kinetic study of the hydroformylation has been carried out306 and the mechanism proposed by Wilkinson (Scheme 16) was extended to express both the associative and dissociative modes of alkene coordination298 and the formation of n- and iso-butyraldehydes. High-pressure IR spectroscopy using CO and either H2 or D2 has confirmed the formation of [RhH(CO)2(PPh3)2] in these reactions.307... 

Based on a kinetic study of the epoxidation of alkenes by m-chloroperbenzoic acid, Equation (62),... 

A kinetic study of the epoxidation showed the reaction to be of the first order with respect to both alkene and dioxirane174. A large steric effect was observed in the epoxidation of certain m/fnmv-dial kyIal kenes the cis compounds were found to exhibit reactivities one order of magnitude higher than the corresponding trans isomers. This large effect reflects a repulsive interaction between the substituents of the olefin and the dioxirane in the transition state (equation 25)9. 

The molecular ions of alkenes have a prevalence for rapid randomisation of their constituent hydrogen and carbon atoms. Specific reaction sequences by which the atoms are randomised can be identified through careful consideration of the kinetics of decompositions at the shortest times (< lOOps) and many FIK studies of alkenes have been made with this objective in mind [223]. 

When a cis phosphine geometry is enforced by the use of a chelating diphosphine, hydride transfer to coordinated alkene is very rapid. Detailed kinetic studies of H-transfer in the system... 

If concentrations of carbenium ions are too low to be observed directly, they must be detected indirectly in kinetic studies of the racemiza-tion of optically active dormant species, ligand exchange and/or detailed studies of the effect of substituents, solvent and salts. Some of the most convincing and elegant work in this area was presented in Chapter 2 using primarily benzhydryl derivatives. As discussed in the next section, correlations between ionization rates and equilibrium constants, rates of solvolysis and rate constants of electrophilic addition can be interpolated and in some cases extrapolated to cationic polymerizations of alkenes to evaluate the reactivities of various active species and the dynamics of their isomerization. 

The epoxidation of alkenes by sodium hypochlorite in the presence of manganese porphyrins under phase-transfer conditions has been thoroughly studied. Kinetic studies of this reaction revealed a Michaelis-Menten rate equation. As in Scheme 12, the active oxidant is thought to be a high-valent manganese( V)-oxo-porphyrin complex which reversibly interacts with the alkene to form a metal oxo-alkene intermediate which decomposes in the rate determining step to the epoxide and the reduced Mn porphyrin. Shape selective epoxidation is achieved when the sterically hindered complex Mn(TMP)Cl is used as the catalyst in the hypochlorite oxidation. ... 

A kinetic study of the reaction of acyclic, (28), and cyclic, (29) and (30), phosphonates with / -substituted benzaldehydes using ethoxide as the base has been carried out. The results show the reactions to be third order, indicate a build up of negative charge in the transition state of the rate determining step (p = +2), and show an acceleration in the case of five-membered cyclic phosphonates of about 20 X over the acyclic analogues. An analogous study of phosphinates and phosphine oxides gives similar results and shows that phosphinates form alkenes approximately 35 x faster than phosphine oxides in spite of similar p/lTa values for the a-protons in each compound. 

A real example following the H2-decoordination mechanism is provided by the selective hydrogenation of 1-alkynes to alkenes with the ruthenium(II) complex [(PP3)RuH(Ti2-H2)]BPh4 [PP3 = P(CH2CH2PPh2)3l [25]. A kinetic study of the hydrogenation of phenylacetylene at ambient or sub-ambient pressure showed that the reaction is first-order in catalyst concentration, second order in H2 pressure and independent of substrate concentration. At very low 1-alkyne concentration (<0.12 M), a first order dependence with respect to the substrate was observed. 

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