Evidence for Nonstatistical Dynamics in the Wolff Rearrangement of a Carbene

Abstract

Two 13C-labeled isomers of the formal Diels−Alder adduct of acetylmethyloxirene to tetramethyl 1,2,4,5-benzenetetracarboxylate have been synthesized. Flash vacuum thermolysis of these adducts leads to various isotopic isomers of acetylmethylketene, the ratios of which have been determined by NMR. The surprising finding that the principal product comes from methylpyruvoyl carbene rather than its more stable isomer diacetylcarbene is explained by MPWB1K density functional calculations, which show that the reactant probably undergoes a unimolecular rearrangement to a norcaradiene derivative prior to its fragmentation. Coupled-cluster calculations on the methylpyruvoyl carbene show that it is capable of undergoing three unimolecular isomerizations. The fastest is 1,2-acetyl migration to give acetylmethylketene directly. The next is rearrangement via acetylmethyloxirene to diacetylcarbene and thence by Wolff rearrangement to acetylmethylketene. The least-favorable reaction is degenerate rearrangement via 1,3-dimethyl-2-oxabicyclo[1.1.0]butan-4-one (the epoxide of dimethylcyclopropenone). The combined experimental and computational results indicate that Wolff rearrangement of the diacetylcarbene occurs with a 2.5:1 ratio of the methyl groups despite the fact that they are related by a twofold axis of symmetry in the carbene. Preliminary molecular dynamics simulations are consistent with this conclusion. Taken together, the results suggest that the Wolff rearrangement is subject to the same kind of nonstatistical dynamical effects detected for other kinds of thermally generated reactive intermediates.

Competition Between Concerted and Stepwise Dynamics in the Triplet Di‐π‐Methane Rearrangement

“…Such nonstatistical reaction dynamics have been reviewed in detail 7b,c. Molecular dynamics studies by Reyes and Carpenter, and Hase and co‐workers, as well as others have demonstrated the critical role of nonstatistical effects in reactions involving singlet biradicals,7a, 8 radical cations,9 and carbenes 10. Singleton and co‐workers have described nonstatistical pathways in alkene hydroborations,11 Diels–Alder reactions,12 and enyne–allene cyclizations 13.…”

Quantum chemical and experimental investigations on inhibitory behavior of amino–imino tautomeric equilibrium of 2-aminobenzothiazole on steel corrosion in H2SO4 solution

“…Although another way to calculate rates is Table 1 The thermodynamic parameters calculated by BPW91 method with 6-31G** basis set, energies of frontier molecular orbitals and global reactivity descriptors of 2-ABT, 2-IBT and 2-ABTH + structures. molecular dynamics, it has been shown that reacting molecules sometimes move on to a plateau region or a bifurcated region of the surface and then head toward products in directions determined by their internal motions [44]. Also, rigorous calculations of rate constants needs to utilize a specialized program based on RRKM (Rice-Ramsperger-Kassel-Marcus) theory [45,46].…”

Organic Reaction Dynamics

“…114 The paper reads like a detective story, chronicling hypotheses, tests, wrong steps, the fortuitous guidance offered by a referee, corrections, and a final summary. 114 The paper reads like a detective story, chronicling hypotheses, tests, wrong steps, the fortuitous guidance offered by a referee, corrections, and a final summary.…”

“…Carpenter 114 was intrigued by the possibility of dynamical effects in carbene chemistry. He chose to explore the Wolff rearrangement taking 100 into 101.…”