Dissociation-recombination mechanism

The above value for R = H corresponds to an intermolecular process, either assisted by the solvent (in solution) or by other NH-pyrazole molecules (in solution and in the solid state) while metal migrations are probably intramolecular (the bigger the metal, the easier) and those of COR correspond (for R = NHR ) to a dissociation-recombination mechanism. Minkin [quoted in 96MI(15)339] suggests that a tautomeric process should... 

JCS(P1)2673 89H1121]. As proven by crossover experiments, interconversion of the tautomers occurs intermolecularly, an ion pair 37c being the intermediate in the dissociation-recombination mechanism of the rearrangement. 

Several authors " have suggested that in some systems voids, far from acting as diffusion barriers, may actually assist transport by permitting a dissociation-recombination mechanism. The presence of elements which could give rise to carrier molecules, e.g. carbon or hydrogen , and thus to the behaviour illustrated in Fig. 1.87, would particularly favour this mechanism. The oxidant side of the pore functions as a sink for vacancies diffusing from the oxide/gas interface by a reaction which yields gas of sufficiently high chemical potential to oxidise the metal side of the pore. The vacancies created by this reaction then travel to the metal/oxide interface where they are accommodated by plastic flow, or they may form additional voids by the mechanisms already discussed. The reaction sequence at the various interfaces (Fig. 1.87b) for the oxidation of iron (prior to the formation of Fe Oj) would be... 

It is generally accepted that the aza-[l,2]-Wittig rearrangement proceeds via a radical dissociation-recombination mechanism, whereas the aza-[2,3]-rearrangement proceeds... 

Significantly faster is the solid-state racemization of the thioglycolic acid derivative 95 (Scheme 9). A solid sample lost its specific rotation upon standing from -42.3° to -36.9° in 2 months and to -13.9° after 14 months. It is noted that such racemization was more rapid when the acid was exposed to light [57]. A heterolytic dissociation/recombination mechanism appears suitable. 

A-Dialkylaminomethylbenzotriazoles usually exist in the crystalline state solely as the N(l) isomers, but in solution they form equilibrium mixtures of the N(l) (209, 210) and N(2) (211) isomers <75JCS(Pl)l 181, 87JCS(Pi)2673). The N(l) and N(2) isomers are of nearly equal stability in nonpolar solvents and in the gas phase (2 1 ratio on statistical grounds). Polar solvents favor the 1- and 3-substituted forms over the 2-substituted, and conversely substituents at positions four and seven favor the 2-substituted form. The interconversion of these N(l) and N(2) isomers proceeds intermolecularly, as demonstrated by cross-over experiments, by a dissociation-recombination mechanism involving the formation of intermediate iminium cations and the benzotriazole anion. 

The [1,2]-Wittig Rearrangement is a carbanion rearrangement that proceeds via a radical dissociation-recombination mechanism. The lithiated intermediate forms a ketyl radical and a carbon radical, which give an alcoholate after fast recombination within the solvent cage ... 

Br or I by NMR, conductometric, stereochemical, and kinetic studies (256). In particular, it was demonstrated that silylation of optically active alkyl thiophosphonate with a stoichiometric amount of Me3SiBr leads to racemic silyl esters and the rate of optical rotation decay is equal to the rate of silyl ester formation (256) in spite of the fact that no bond to phosphorus was cleaved. Thus, the reaction must involve an achiral transient species [Eq. (58)]. This result is also proof of fast exchange of silyl groups between the complex and the silane, which occurs via the dissociation-recombination mechanism. 

Kinetically, the fluxional process is first order in tetramer. Thomas and coworkers discussed three mechanisms 1) a dissociation-recombination of dimers, 2) unfolding of the tetramer into an eight-membered ring with alternating a-carbons and lithiums and 3) a concerted center to edge rotation of three of the alkyl groups. For different reasons, many authors have preferred the dissociation-recombination mechanism in which the transition structure is a very loose tetramer. 

NMR data show a dissociation/recombination mechanism in which the... 

The dissociation/recombination mechanism was further confirmed by studying the ligand facial exchange in [La(III)(N03)3 18C6] with the value for k of 35 5 s 1 which is close to k i of 50 s 1. The exchange equilibrium may be written as ligand facial exchange ... 

Scheme 2.24 Free radical dissociation-recombination mechanism...

The migrating group migrates with nearly complete retention of configuration. Thus, the reaction is intramolecular and concerted. However, there is evidence which supports the free radical dissociation-recombination mechanism (Scheme 3.27). 

Scheme 46. Intramolecular vs. dissociation/recombination mechanisms for the VCB rearrangement.

The [1,2]-Meisenheimer rearrangement most likely proceeds via a hemolytic dissociation-recombination mechanism, whereas the [2,3]-Meisenheimer rearrangement is a concerted sigmatropic process that goes through a five-membered envelopelike transition state. 

This view was based on a kinetic analysis of the Meisenheimer rearrangement of the related amine oxide where a radical dissociation-recombination mechanism was proposed 131>. 

An interesting example of chemistry involving a radical dissociation-recombination mechanism is the 1,2-Wittig rearrangement [17]. The general scheme for the Wittig rearrangement is shown in Eq. (12). 

This type of carbanion rearrangement is recognised to proceed by means of a radical dissociation-recombination mechanism. In 1983, Mazaleyrat and Wel-vart reported the kinetic resolution of a racemic binaphthyl ether, which involves an enantioselective variant of the [1,2]-Wittig rearrangement [Eq.(2)j [3]. 

At ambient temperatures, the following consecutive rearrangements via a homolytic dissociation-recombination mechanism take place [43] ... 

A previously reported process has been generalized to provide a flexible and directed synthesis of azoxyalkanes (Scheme 130). The Z-E thermal isomerization of aliphatic azodioxy-compounds (dimeric nitroso-alkanes) has been shown to occur exclusively by a dissociation-recombination mechanism. 

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