The Iconoclastic Dynamics of the 1,2,6-Heptatriene Rearrangement

Abstract

CASSCF(8,8)/6-31G* and AM1-SRP direct dynamics trajectory calculations have been run on the rearrangement of 1,2,6-heptatriene to 3-methylene-1,5-hexadiene. They show that the experimental results of Roth et al. on this reaction can be explained without the need to invoke a concerted, pericyclic mechanism. Instead, bifurcation occurs at the transition state for conversion of the reactant to 2-methylenecyclohexane-1,4-diyl. Some trajectories leaving the transition state do enter the PES local minimum for the intermediate, but others, differing only in the phases of the real-frequency vibrational modes, bypass the intermediate and proceed over a second transition state to the product. A significant fraction of the trajectories that do enter the biradical local minimum proceed on to the product in <500 fs, despite the fact that the minimum is some 12 kcal/mol deep at the CASSCF level. This nonstatistical behavior seems to be due in part to a resonance between key C-H bending and C-C stretching vibrations in the intermediate.

Bifurcations on Potential Energy Surfaces of Organic Reactions

“…Dynamics simulations using a reparameterized semiempirical molecular orbital (MO) model (AM1-SRP) fit to CASSCF stationary points showed that 17 % of the trajectories follow a "concerted" pathway. [36] Houk and co-workers have identified a similar type of bifurcation on the PES for the rearrangement of 6-methylenebicyclo[3.2.0]hept-2-ene, which yields a diradical intermediate or 5-methylenenorbornene. [37] Bifurcations also exist on the PESs for the rearrangements of cis-bicyclo[6.1.0]nona-2,4,6-triene, 9,9-dicyanobicyclo-[6.1.0]nona-2,4,6-triene, [38] cis-1,2-divinylcyclobutane, and cis-1,2-divinylcyclo-propane.…”

Prediction of mode specificity in 1,3-dipolar cycloadditions using the Sudden Vector Projection model

“…In particular, these authors systematically explored the reversed post-transition state dynamics of the reactions of the prototypical 1,3-dipoles 1-9 ( Figure 1) with the dipolarophiles ethene (ethylene) and ethyne (acetylene), and discovered that, in addition to the relative translation of the reactants, excitation in the 1,3-dipole bending modes has a substantial promotional effect on the reactions. This interesting discovery [12,19] is one of many recent examples suggesting that dynamics plays an important role in organic reactions [20][21][22][23][24][25][26].…”

“…This is presumably due to difficulties associated with dynamical simulations of organic reactions, which often involve relatively large numbers of atoms for which multi-dimensional potential energy surfaces are difficult if not impossible to construct. Even direct dynamics calculations, which calculate the forces on-the-fly and thus require no potential energy surfaces [15], are computationally demanding, so that only a small number of trajectories can be generated [21][22][23][24][25][26]. As an increasing body of evidence has demonstrated [17,18,29,30], however, mode specificity is largely controlled by properties of the transition state.…”

Dynamic Effects on [3,3] and [1,3] Shifts of 6‐Methylenebicyclo[3.2.0]hept‐2‐ene

“…At this level of theory, the TS-12 x saddle point lies at a low value of R (3,8), and the transition vector has a component in the negative R(3,8) direction. At the same time, the flattening of the diradical potential energy well shifts TS-23 x toward larger values of R(3,8), positioning it almost directly in front of TS-12 x.…”

“…Both transition states involve the formation of a bond between C3 and the proximal allyl terminus in 2. [8] From a given intermediate 2, the two products 3 that can be formed are thus epimers with respect to the substituents on that carbon. TS-23 n is favored slightly and places the pseudo-E substituent in the 6x position in the product; TS-23 x places the same substituent in the 6n position.…”