Mechanistic borderline

Some reactions of a given substrate under a given set of conditions display all the characteristics of Sn2 mechanisms other reactions seem to proceed by SnI mechanisms, but cases are found that cannot be characterized so easily. There seems to be something in between, a mechanistic borderline region. At least two broad theories have been devised to explain these phenomena. One theory holds that intermediate behavior is caused by a mechanism that is neither pure Sn I nor pure Sn2, but some in-between type. According to the second theory, there is no intermediate mechanism at all, and borderline behavior is caused by simultaneous operation, in the same flask, of both the SnI and Sn2 mechanisms that is, some molecules react by the SnI, while others react by the Sn2 mechanism. 

In reactions of mechanistic borderline, the reaction pathway may not follow the minimum energy path, but the reaction proceeds via unstable species on the PES. In other cases, the reacting system remains on the IRC but does not become trapped in the potential energy minimum. In some cases, intermediates are formed in reactions that should be concerted, whereas in other reactions a concerted TS gives an intermediate. Thus, the question of concerted versus stepwise appears too simple and the definition of concerted and stepwise reactions becomes unclear. In some reactions, the post-TS dynamics do not follow IRCs, and path bifurcation gives two types of products through a common TS. 

The mechanistic borderline between E2 and ElcB mechanisms has been studied under various conditions.1,2 The mechanism of the elimination reaction of 2-(2-fluoroethyl)-1-methylpyridinium has been explored explored by Car-Parrinello molecular dynamics in aqueous solution.3 The results indicated that the reaction mechanism effectively evolves through the potential energy region of the carbanion the carbon-fluoride bond breaks only after the carbon-hydrogen bond. 

A. R. Katritzky, B. E. Brycki, Nucleophilic Substitution at Saturated Carbon Atoms. Mechanisms and Mechanistic Borderlines Evidence from Studies with Neutral Leaving Groups, J. Rhys. Org. Chem. 1988, 1, 1-20. 

Some reactions of a given substrate under a given set of conditions display all the characteristics of Sn2 mechanisms other reactions seem to proceed by SnI mechanisms, but cases are found that cannot be characterized so easily. There seems to be something in between, a mechanistic borderline region. At least two broad... 

Taft and Cannell also determined thiocyanate isothiocyanate ratios for the deamination of benzhydrylamine and for the isobutyl fraction in the deamination of isobutylamine. The values of [RNCS] [RSCN] were 9 1 and 5-3 1, respectively. It is interesting that values for the benzhydryl- and t-butyl-amines bracket that for isobutylamine. It is most unlikely that the isobutyl cation is of intermediate selectivity, and the results may suggest that the reaction of primary alkyldiazonium ions is sufficiently near the mechanistic borderline for so strong a nucleophile as thiocyanate to force an Sif2 displacement on the isobutyldiazonium ion. 

This chapter is intended to focus on the mechanistic borderline between a sequential PCET pathway and a one-step HAT pathway and also the effects of metal ions on HAT reactions as well as overall two-electron and two-proton processes in relation to the borderline between the outer-sphere and inner-sphere ET pathways. 

MECHANISTIC BORDERLINE BETWEEN ONE-STEP HAT AND SEQUENTIAL PCET... 

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