The reaction force constant as an indicator of synchronicity/nonsynchronicity in [4+2] cycloaddition processes

Abstract

A variety of experimental and computational analyses support the concept that a chemical reaction has a transition region, in which the system changes from distorted states of the reactants to distorted states of the products. The boundaries of this region along the intrinsic reaction coordinate ξ, which includes the traditional transition state, are defined unambiguously by the minimum and maximum of the reaction force F(ξ), which is the negative gradient of the potential energy V(ξ). The transition region is characterized by the reaction force constant κ(ξ), the second derivative of V(ξ), being negative throughout. It has recently been demonstrated that the profile of κ(ξ) in the transition region is a sensitive indicator of the degree of synchronicity of a concerted reaction: a single κ(ξ) minimum is associated with full or nearly full synchronicity, while a κ(ξ) maximum (negative) between two minima is a sign of considerable nonsynchronicity, i.e. a two-stage concerted process. We have now applied reaction force analysis to the Diels-Alder cycloadditions of the various cyanoethylenes to cyclopentadiene. We examine the relative energy requirements of the structurally- and electronically-intensive phases of the activation processes. We demonstrate that the variation of κ(ξ) in the transition region is again indicative of the level of synchronicity. The fully synchronous cycloadditions are those in which the cyanoethylenes are symmetrically substituted. Unsymmetrical substitution leads to minor nonsynchronicity for monocyanoethylene but much more - i.e. two stages - for 1,1-dicyano- and 1,1,2-tricyanoethylene. We also show that the κ(ξ) tend to become less negative as the activation energies decrease.

An experimental and theoretical study of the hetero Diels–Alder reactions between (E)-2-aryl-1-cyano-1-nitroethenes and ethyl vinyl ether: one-step or zwitterionic, two-step mechanism?

“…It should be noted at this point that in all cases, profiles of relationship between intrinsic reaction coordinate and reaction force constant is different as in the case one-stage one-step reaction, and similar to that in the case of two-stage one-step reaction [37].…”

Revisiting the seemingly straightforward hydrogen cyanide/hydrogen isocyanide isomerisation

“…It has been demonstrated that the profile of κ(ξ ) in the transition region can be a sensitive diagnostic of mechanistic features, such as the degree of synchronicity in reactions in which several bonds are broken and/or formed [21,30,31]. A fully or nearly fully synchronous process has V(ξ ), F(ξ ) and κ(ξ ) resembling those shown in Figure 1; in particular, κ(ξ ) has just a single minimum between α and γ .…”

“…A fully or nearly fully synchronous process has V(ξ ), F(ξ ) and κ(ξ ) resembling those shown in Figure 1; in particular, κ(ξ ) has just a single minimum between α and γ . An example is the Diels-Alder reaction between cyclopentadiene and 1,2-dicyanoethylene [31]. However, if there is significant nonsynchronicity, e.g.…”

“…However, if there is significant nonsynchronicity, e.g. cyclopentadiene plus 1,1-dicyanoethylene, then κ(ξ ) changes markedly in Downloaded by [Florida Atlantic University] at 14:47 18 November 2014 the transition region: it now has a negative maximum, with minima on both sides of it [31].…”

Revisiting the concept of the (a)synchronicity of diels‐alder reactions based on the dynamics of quasiclassical trajectories

“…These concepts establish temporal relationships between two chemical events such as the formation/cleavage of chemical bonds and/or electron transfer processes. However, geometric parameters, such as, the (a)symmetry of the bond distances being formed or broken in the transition state (TS), are usually used to ascertain the concertedness and synchronicity of an elementary step . It should be emphasized that relationships between inherently temporal concepts and spatial quantities may not be unique or generally valid.…”