Mechanism of Thermal Deazetization of 2,3-Diazabicyclo[2.2.1]hept-2-ene and Its Reaction Dynamics in Supercritical Fluids

Abstract

The mechanism of thermal deazetization of 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) is examined both theoretically and experimentally in this paper. The theoretical work involves CASPT2 ab initio MO calculations using the 6-31G(d) basis set. These calculations are shown to give relative enthalpies for the stationary points on the potential energy surface that are within ±3 kcal/mol of the experimental values. Surprisingly, the transition structure for the rate-limiting step is found to correspond to synchronous C−N cleavage, leading to the formation of the cyclopentane-1,3-diyl biradical. The alternative mechanism, involving stepwise C−N bond scission, is found to have an activation enthalpy that is at least 7 kcal/mol higher. The synchronous mechanism is in apparent conflict with earlier calculations and experiments, all of which seemed to favor the stepwise pathway. A resolution of this disagreement is proposed. The well-known preference for inversion of stereochemistry in the deazetization of 2,3-diazabicyclo[2.2.1]hept-2-ene-exo,exo-5,6-d 2 (DBH-d 2 ) is proposed to involve nonstatistical dynamical effects. This mechanism is similar to, but not identical with, one first proposed by Allred and Smith over 30 years ago. The deazetization of DBH-d 2 in supercritical propane is studied experimentally. The pressure dependence of the reaction stereochemistry is shown to be qualitatively and quantitatively consistent with the dynamic model, but not to be consistent with any mechanisms that would require separate reaction paths across the potential energy surface. In supercritical carbon dioxide, the pressure dependence is found not to be fit by any simple model. Calculations suggest that this may be because CO2 forms weak complexes (binding energies of approximately 2 kcal/mol for each CO2) with the cyclopentane-1,3-diyl intermediate. The calculations also suggest that similar complexes can be expected between CO2 and simple monoradicals.

The Study of Reactive Intermediates in Condensed Phases

“…These surfaces have often proven not to be consistent with the ad hoc mixedmechanism explanations described in Section 1.3. 16,17 In addition, the calculations have shown the common occurrence of phenomena not previously considered in the mechanistic principles summarized above. Prominent among such phenomena are reaction-path bifurcations, which constitute branch-points in a mechanism that are not associated with the local minima on the PES normally considered to be prerequisites for the existence of intermediates.…”

“…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.…”

Synthesis of Symmetric and Unsymmetric Secondary Amines from the Ligand-Promoted Ruthenium-Catalyzed Deaminative Coupling Reaction of Primary Amines

“…In support of this notion, Singleton and co-workers showed that the observation of most pronounced carbon isotope effect has been a definitive tool for establishing the rate-limiting step for both C–C and C–O bond forming reactions. 19c While C–N bond cleavage has been found to be the turnover limiting step for a number of chemical and biochemical coupling reactions, 20 very few carbon kinetic isotope effect measurements have been reported for the catalytic coupling reactions of nitrogen compounds. Eubanks and co-workers measured pronounced carbon isotope effects in Hofmann elimination reaction of para -substituted (2-phenylethyl)-trimethylammonium bromides, which indicated an E2 mechanism involving C–N bond cleavage step.…”

Competition between Reaction and Intramolecular Energy Redistribution in Solution: Observation and Nature of Nonstatistical Dynamics in the Ozonolysis of Vinyl Ethers

“…In such cases, experimental observations such as the selectivity in product formation can be demonstrably inconsistent with statistical expectations, 2 but the selectivity may often be understood, at least qualitatively, by the idea of “dynamic matching.” Such nonstatistical dynamic effects have been proposed to be important in many reactions. 3,4,5,6,7 …”