Substitution activation parameters

Activation parameters and reaction mechanism in octahedral substitution. T. W. Swaddle, Coord. Chem. Rev., 1914,14, 217-268 (231). 

A more detailed study of the hydration of phenylacetylene, 9a, and three substituted phenylacetylenes, p-methoxy 10, p-methyl 11, and p-chlorophenyl-acetylene 12, in aqueous sulfuric acid containing 5% ethanol has been carried out by Noyce and co-workers (19,20). The hydration obeys general acid catalysis and gives a linear Hq dependence. The slopes for the logarithm of the observed rate constants versus Ho and the activation parameters for the hydration of these phenylacetylenes are summarized in Table II. 

Exchange and some substitutions in Ni(CO)4 have been shown ° to proceed by first-order processes with virtually the same activation parameters, consistent with a rate-determining dissociation. In the series Co(CO)3NO through Mn(CO)... 

On this basis = 0.0170 sec , = 0.645 sec , and K = 0.739 mole.P at 25 °C. The corresponding activation parameters were determined also by Es-penson. By a method involving extrapolation of the first-order rate plots at various wavelengths to zero time, the absorption spectrum of the intermediate was revealed (Fig. 1). Furthermore, the value of K obtained from the kinetics was compatible with that derived from measurements on the acid dependence of the spectrum of the intermediate. Rate data for a number of binuclear intermediates are collected in Table 2. Espenson shows there to be a correlation between the rate of decomposition of the dimer and the substitution lability of the more labile metal ion component. The latter is assessed in terms of the rate of substitution of SCN in the hydration sphere of the more labile hydrated metal ion. 

In inert systems such as technetium and rhenium, ligand substitution reactions-including solvolysis-proceed under virtually irreversible conditions. Thus, the nature of the reaction center, the nature of the leaving group, and the nature and position of the other ligands in the complex affect the rates and activation parameters in a complicated manner. Most substitution reactions take place via interchange mechanisms. This is not too surprising when the solvent is water - or water-like - and where, in order to compete with the solvent,... 

This gives tautomeric mixtures119 when the tert-butyl group is removed. The methyl ether has been used to obtain 3-hydroxy-2-carbonyl derivatives in the selenophene series.120 The unsubstituted 2-hydroxyselenophene system has been prepared by hydrogen peroxide oxidation of 2-selenophene-boronic acid.121 However, in the 5-methyl-substituted system deboronation became such an important side reaction that 5-methyl-2-hydroxyselenophene had to be prepared by acid-catalyzed dealkylation of 5-methyl-2-fert-butoxy-selenophene. Both 2-hydroxy- and 5-methyl-2-hydroxyselenophene exist mainly as 3-selenolene-2-ones (93) and for the 5-methyl derivative it was possible to isolate the / ,y-unsaturated form (92) and follow the tautomeric isomerization. The activation parameters thus obtained were compared with those for the corresponding furan and thiophene systems. 

The inhibited unimolecular decomposition of symmetrically di-substituted benzoyl peroxides into radicals also obeys the Hammett rho-sigma relationship. Unfortunately, no extensive activation parameter data are available. The effect of the substituent changes on the rates at the single temperature has been explained in terms of dipole-dipole repulsion in the peroxide.122... 

Table 7. Experimental Activation Parameters for Ring Expansion of Fluoro- and Methyl-Substituted Phenylnitrenes...

The mechanism of substitution at these centers can conveniently be probed through CO exchange at the cis-[M(CO)2X2] anions (M =Rh, Ir X = Cl, Br or I). The rate law for these exchanges, and the activation parameters shown in Table IX, suggest the operation of a limiting A mechanism (265). 

The use of 77-acceptors can also achieve a higher rate of substitution. In our group we were able to compare the substitution behavior of the Pt(II) aqua complexes of ethylenediamine and 1,10-phenanthro-line, and although the reactivity of the phenanthroline complex is —102 higher than the ethylenediamine complex, the activation parameters strongly indicate that an associative mechanism is operative (65). 

By analysis of nmr behavior Smallcombe and Caserio (1971) were also able to evaluate the rate+s of several other rapid substitution reactions (45)—(46) involving both Me2SSMe and the alkyldialkylthiosulfonium ion Me (SMe)2. The rate constants (at 40°) and activation parameters for these processes are shown below. Note that the rates of all these reactions are very fast. Comparison of k4 and k4S (or of k 4S and k4t) shows that, as would be... 

Rappoport and Topol investigated the displacement of the halogen of bromo- and chloromethylenemalonates (287 X= Br, Cl) by several substituted anilines and that of the brosyloxy group of (4-nitrophenyl)(4-bromo-phenylsulfonyloxy)methylenemalonate (289) by morpholine and piperidine, in acetonitrile. A rate-determining nucleophilic addition of the amines was suggested as the mechanism for these reactions. Activation parameters (AH, AS ) were determined [72JCS(P2)1823]. 

In order to find out whether captodative substitution of a methyl radical can lead to persistency, the rate of disappearance by bimolecular selfreaction was measured for typical sterically unhindered captodative radicals (Korth et al., 1983). The t-butoxy(cyano)methyl radical, t-butylthio(cyano)-methyl radical and methoxy(methoxycarbonyl)methyl radical have rate constants for bimolecular self-reactions between 1.0 x 10 and 1.5 X 10 1 mol s Mn the temperature range —60 to - -60°C. The dilTusion-controlled nature of these dimerizations is supported by the Arrhenius activation parameters. Thus, it has to be concluded that there is no kinetic stabilization for captodative-substituted methyl radicals. On the other hand, if captodative-substituted radicals are encountered which are kinetically stabilized (persistent) or which exist in equilibrium with their dimers, then other influences than the captodative substitution pattern alone must be added to account for this phenomenon. 

Table 4.12 Activation Parameters for Substitution in Some Square-Planar Complexes in Water at 25 °C Refs. 149-152.

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