Rate-limiting dissociation

Comments on the thermal nitration of enol silyl ethers with TNM. The strikingly similar color changes that accompany the photochemical and thermal nitration of various enol silyl ethers in Table 2 indicates that the preequilibrium [D, A] complex in equation (15) is common to both processes. Moreover, the formation of the same a-nitroketones from the thermal and photochemical nitrations suggests that intermediates leading to thermal nitration are similar to those derived from photochemical nitration. Accordingly, the differences in the qualitative rates of thermal nitrations are best reconciled on the basis of the donor strengths of various ESEs toward TNM as a weak oxidant in the rate-limiting dissociative thermal electron transfer (kET), as described in Scheme 4.40... 

Each of these electron-transfer reactions is followed by rate-limiting dissociation of the iron protein, allowing access of H+ to the active site. 

The associative mechanism involves rapid binding of the nucleophile to the tetrahedral Fe site, followed by rate-limiting dissociation of the leaving group. 

Few mechanistic studies on ligand replacement have been reported. Nanda and Tobe many years ago studied the displacement of Cl by Y = Br-, NCS- in [Co(Cl)3 MeAs(dmap) ], and the independence of rate on the nature and concentration of Y" suggested rate-limiting dissociation. Also it appears that [CoX(solv)(As—As)2]2+ species rapidly undergo anation by X- (halide, pseudohalide). The rate is very much faster than for the corresponding amine complexes. 

The additions of aryl halides to PdL2 complexes of Q-phos derivatives were recently reported by Barrios-Landeros and Hartwig179. The addition to Phi, PhBr and PhCl takes place through distinct mechanistic pathways (Scheme 5). Iodobenzene reacts by associative displacement of a phosphine, bromobenzene reacts by rate-limiting dissociation of phosphine, and chlorobenzene reacts by reversible dissociation of phosphine, followed by oxidative addition. 

Braude ei al. studied the dependence of rates of acid-catalyzed isomerization of of-ethynyl-y-methylallyl alcohoF and a-phenyl-y-methylallyl alcohol on the composition of mixed solvents such as aqueous ethanol and aqueous dioxane. If allowance is made for the effect of solvent on the concentration of the reactive conjugate acid of the alcohol, these data show that the rate of isomerization of the conjugate acid is nearly independent of solvent composi-tion as would be expected for a unimolecular rate-limiting dissociation of the conjugate acid to a solvated carbonium ion, Kwart and Herbig reached a... 

Delocalized highly polarizable anionic transition states occur in certain elimination reactions, too. The breakdown of erythro-dibromophenylpropanoic acid 32 provides an example which can serve to test the influence of the tetrahedral ammonium hosts upon two alternative competing reaction pathways. The investigations of Groven-stein and Cristol revealed that 32 may either undergo rate limiting dissociation of the benzylic carbon-bromine bond which ultimately leads to the formation of... 

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