Substitution reactions associative

The substituted radical cations [Me2S.. SMe2], [Et2S. .SEt2] and [Et2S.. SMe2] (Fig. 4) have been studied by lilies, McKee and co-workers using a combined experimental/theoretical approach [127-129]. Mass spectrometry experiments on the gas-phase association reactions... 

The reactions of H and F with CH3S are exothermic by 259 and 393 kJ mol-1, respectively, and either could be responsible for formation of CH2S(A3S) [7], Methylated sulfur compounds produce intense emission from HCF, while the corresponding ethyl-substituted compounds produce only trace or no HCF [6]. Emission from HFf was much stronger for the ethyl-substituted than for the methyl-substituted sulfides [6], This suggests that methyl radicals, formed perhaps in Reaction (34), lead to the formation of HCF. Emission of HCF has been identified in F2/CH4 flames [70] where it is attributed to the association reaction... 

BIMOLECULAR ASSOCIATIVE AND SUBSTITUTION PROCESSES 16. Aldol reactions continued... 

In recent years there has been a tendency to assume that the mechanisms of substitution reactions of metal complexes are well understood. In fact, there are many fundamental questions about substitution reactions which remain to be answered and many aspects which have not been explored. The question of associative versus dissociative mechanisms is still unresolved and is important both for a fundamental understanding and for the predicted behavior of the reactions. The type of experiments planned can be affected by the expectation that reactions are predominantly dissociative or associative. The substitution behavior of newly characterized oxidation states such as copper-(III) and nickel (III) are just beginning to be available. Acid catalysis of metal complex dissociation provides important pathways for substitution reactions. Proton-transfer reactions to coordinated groups can accelerate substitutions. The main... 

Redox reactions usually lead, however, to a marked change in the species, as reactions 4-6 indicate. Important reactions involve the oxidation of organic and metalloprotein substrates (reactions 5 and 6) by oxidizing complex ions. Here the substrate often has ligand properties, and the first step in the overall process appears to be complex formation between the metal and substrate species. Redox reactions will often then be phenomenologically associated with substitution. After complex formation, the redox reaction can occur in a variety of ways, of which a direct intramolecular electron transfer within the adduct is the most obvious. 

Of course the Co CNHj) breaks down rapidly in acid into Co + and 5NHJ. Precursor complex formation, intramolecular electron transfer, or successor complex dissociation may severally be rate limiting. The associated reaction profiles are shown in Fig. 5.1. A variety of rate laws can arise from different rate-determining steps. A second-order rate law is common, but the second-order rate constant is probably composite. For example, (Fig. 5.1 (b)) if the observed redox rate constant is less than the substitution rate constant, as it is for many reactions of Cr +, Eu +, Cu+, Fe + and other ions, and if little precursor complex is formed, then = k k2kz ). In addition, the breakdown of the successor complex would have to be rapid k > k 2). This situation may even give rise to negative (= A//° +... 

Pseudorotation is well established in 5-coordinated species involving the main group elements and is best described by the Berry mechanism which interconverts two trigonal bipyramids via a square pyramid. Its operation here is difficult to reconcile with the highly stereospecific nature of substitution in Pt(II). Nevertheless, the mechanism has had substantial support. It may very well be that (a) is favored by polar solvents and that (b) is prevalent in nonpolar media. The associated reaction profiles are shown in Fig. 7.11. 

Acidic dehydration of alcohols, to give an alkene is also associated with substitution reaction to give an ether. 

Figure 14. Plots of apparent bimolecular rate constants for the association reaction of protonated acetone with acetone (and deuterium-substituted variants) as a function of neutral acetone (acetone-d ) pressure in the FTICR cell at a temperature of 47 °C.

Figure 92 (a) Structural mechanism for the hydroxylation of monophenolic substrates by oxytyrosinase (b) reaction coordinate diagram for associative ligand substitution at the copper site of tyrosinase... 

For an inner-sphere reaction there are necessarily more steps since both association and substitution must precede electron transfer. Intermediates like (H20)5CruClCoUI(NH3)54+ and (H20)5CrinClCoII(NH3)54 shown in Scheme 2 are often referred to as the precursor and successor complexes since they precede or follow the electron transfer step. 

Pd and Ni catalysts with the structural effects on reductions with diimide (diazene) (ref. 6) and the equilibrium constants for the association of substituted ethylenes with a Ni(0) complex (ref. 7). These particular reactions were chosen because of our perception of their relation to the mechanisms of catalytic hydrogenation, and the insightful analysis of the relationship between structure and reactivity provided by the authors of these studies. 

The combination of surface-associated reactants with surface-bound H-atoms, occasionally leads to poor photoinduced hydrogenation of the reactant and parallelly to inhibition of H2-evolution. For such systems, tailored bifunctional heterogeneous catalysts have been developed [141], where cooperative catalytic effects are observed in the photohydrogenation reactions. Substitution of ethylene by acetylene, C2H2, in the photosystem composed of Ru(bpy) +/MV2+/Na2EDTA and the Pt colloid results in inefficient hydrogenation of acetylene to ethylene,

[Pg.184]

The distribution function /( ) of the initially formed species resulting from the association reaction is obtained by applying the principle of detailed balancing to the reverse decomposition process, and has the form of eq. (23). When eq. (6) is substituted for k(the specific rate constant for the reverse decomposition process), then... 

N-donor induced disproportionation of [Fe(CO)3(PR3)2]+ (R = Me, Bu, Cy, Ph) as well as halide induced disproportionation of [M(CO)3(PCy3)2]+ (M = Fe, Ru, Os) has been interpreted in terms of nucleophilic attack being rate determining.103 104 The rate data led to the conclusion that the reactivity of these 17-electron complexes is only weakly dependent on the metal, and the suggestion was made that periodic trends in 17-electron systems are generally attenuated in comparison to those for 18-electron analogues. However, it was noted previously that W > Cr by ca. 106 1 for substitution in [CpM(CO)3]. A direct comparison of the rate of associative ligand substitution at a 17-electron center as a function of the metal for a complete triad (Cr, Mo, W) was reported for the reaction in Eq. (20).14... 

This pathway is energetically favored by the gain in aromaticity of the six-membered ring in the -intermediate, a stabilization not available to Cp. It should be noted, however, that this indenyl effect is specific to an associative mechanism substitution reactions of RuCl(rf-ind)(PPh3)2, which proceed via a dissociative pathway, are only one order of magnitude faster than those of RuCl(Cp)(PPh3)2.6... 

Kinetic studies at different phosphine concentrations indicated that the substitution reactions occur totally by a dissociative mechanism, while the ring expansion reaction is by an associative mechanism. The associative reaction could proceed by an 18-electron transition state involving either a bent NO or an rj to rj ring slippage mechanism. The bent NO mechanism seems more likely, because the ring slippage mechanism is known to result in the formation of oxocyclobutenyl product with ring expansion and because the isoelec-tronic cobalt complex does not react by a parallel associative pathway. 

Ligand substitution reactions of square-planar complexes most often occur by associative reaction sequences. An example of square-planar organometallic complexes that will illustrate this reactivity is trani -Ir(Cl)(CO)(PPh3)2 (frequently referred to as Vaska s Complex), see Vaska s Complex). This complex undergoes rapid ligand substitution with CO, PR3, and... 

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