Dynamic Effects on the Periselectivity, Rate, Isotope Effects, and Mechanism of Cycloadditions of Ketenes with Cyclopentadiene

Abstract

The cycloadditions of cyclopentadiene with diphenylketene and dichloroketene are studied by a combination of kinetic and product studies, kinetic isotope effects, standard theoretical calculations, and trajectory calculations. In contrast to recent reports, the reaction of cyclopentadiene with diphenylketene affords both [4 + 2] and [2 + 2] cycloadducts directly. This is surprising, since there is only one low-energy transition structure for adduct formation in mPW1K calculations, but quasiclassical trajectories started from this single transition structure afford both [4 + 2] and [2 + 2] products. The dichloroketene reaction is finely balanced between [4 + 2] and [2 + 2] cycloaddition modes in mPW1K calculations, as the minimum-energy path (MEP) leads to different products depending on the basis set. The MEP is misleading in predicting a single product, as trajectory studies for the dichloroketene reaction predict that both [4 + 2] and [2 + 2] products should be formed. The periselectivity does not reflect transition state orbital interactions. The 13 C isotope effects for the dichloroketene reaction are well-predicted from the mPW1K/6−31+G** transition structure. However, the isotope effects for the diphenylketene reaction are not predictable from the cycloaddition transition structure and transition state theory. The isotope effects also appear inconsistent with kinetic observations, but the trajectory studies evince that non-statistical recrossing can reconcile the apparently contradictory observations. B3LYP calculations predict a shallow intermediate on the energy surface, but trajectory studies suggest that the differing B3LYP and mPW1K surfaces do not result in qualitatively differing mechanisms. Overall, an understanding of the products, rates, selectivities, isotope effects, and mechanism in these reactions requires the explicit consideration of dynamic trajectories.Selectivity in cycloadditions may take many forms, e.g., endo/exo stereoselectivity, regioselectivity, facial stereoselectivity, and diene/dienophile role selectivity. When two distinct formally allowed processes are possible, as in the [4 + 2] versus [6 + 4] cycloadditions of cyclopentadiene with tropone, 1 their differentiation is referred to as periselectivity. The underlying framework within which chemists usually understand any of these forms of selectivity is transition state theory (TST). The preferred product would be that involving the lowest-energy transition state, and the degree of selectivity would be determined by the relative energies for separate transition states. Even when there is no enthalpic barrier, reactivity and selectivity can be discussed in terms of free-energy barriers. 2 Qualitative theories of selectivity such as FMO theory may be thought of as a simplified surrogate for TST, easing the task of predicting which cycloaddition barrier is lowest in energy.singleton@mail.chem.tamu.edu. Publisher's Disclaimer: This PDF receipt will only be used as the basis for generating PubMed Central (PMC) documents. PMC ...

The Study of Reactive Intermediates in Condensed Phases

“…Even when the branching of reaction paths does occur at a local PES minimum, classical molecular dynamics (MD) simulations have shown that one cannot rely on the validity of the statistical approximation for gas-phase reactions. 15,16,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] The Figure 1. Schematic depiction of a PES with a bifurcating reaction path.…”

Bifurcations on Potential Energy Surfaces of Organic Reactions

“…[59] Again, the ratelimiting transition-state geometry provides a good predictor of periselectivity, but there is no simple way to quantitatively predict branching ratios with dynamics simulations.…”

Conceptual, Qualitative, and Quantitative Theories of 1,3‐Dipolar and Diels–Alder Cycloadditions Used in Synthesis

“…[7] Singleton and co-workers have recently noted that because FMO theory utilizes ground state electronic configurations to describe reactivity, it may predict incorrect trends if the electronic configuration changes significantly near the transition state of the reaction. [8] Recently, Spino et al noted that in Diels-Alder cycloadditions involving electron-deficient dienes, FMO theory effectively predicts reactivity patterns of different dienophiles in normal electron demand reactions but failed to predict reactivities in inverse electron demand reactions. [9] In inverse electron demand reactions, unsymmetrical dienophiles are more reactive than symmetrical dienophiles.…”