Low Energy Dissociation Processes of Ionized Cyclohexene:  A Theoretical Insight^,

Abstract

The major dissociation reactions of the cyclohexene radical cation, 1, lead to the cyclopentenyl ion by methyl loss and to ionized 1,3-butadiene after elimination of C 2 H 4 . These two reactions are also observed during the Diels-Alder reaction between ionized butadiene and ethylene in the gas phase. The energetic and mechanistic aspects of the methyl loss process from the cyclohexene radical cation or the reaction between ionized butadiene and ethylene are discussed with the help of molecular orbital calculations at the B3LYP/6-311+G(3df,2p)// B3LYP/6-31G(d) levels. Methyl loss is demonstrated to result from successive 1,2-hydrogen shifts and ringcontraction/ring-opening steps involving, as a crucial intermediate, ionized bicyclo[1,3,0]hexane rather than the distonic ion [CH 2 CH 2 CHCHCHCH 2 ] •+ (one of the open forms of ionized cyclohexene). This latter one is however involved during the direct and retro Diels-Alder reactions. The CH 3 and C 2 H 4 loss rate curves of the cyclohexene ion are calculated using the Rice-Ramsperger-Kassel-Marcus (RRKM) equation and the molecular orbital calculation results. These estimations allow understanding of the experimental observations concerning dissociations of the cyclohexene radical cation, 1, and the collision complex formed between ionized butadiene and ethylene.

Reactive atomistic simulations of Diels-Alder reactions: The importance of molecular rotations

“…3 While it is often believed to be a concerted, synchronous reaction involving an aromatic TS governed by the Woodward-Hoffmann rules, 17 experiments and calculations show that this is not true for all cases. [7][8][9][10][11][18][19][20] In principle, one can think of three possible mechanisms (see Scheme 1): (a) concerted, (b) stepwise with a short-lived intermediate, and (c) stepwise with a long-lived intermediate whose lifetime allows for the rotation around a C-C bond. Note that when the system cannot rotate around a C-C bond, as in cases (a) and (b), the reaction is stereo-selective and only the s-cis conformer of the diene will yield the cyclic Diels-Alder product.…”

Vacuum ultraviolet photoionization mass spectrometric study of cyclohexene

“…The authors concluded that the neutral excited cyclohexene molecule dissociates to form 1,3‐butadiene and ethylene as major products, accompanying with a minor amount of cyclopentenyl radical and methyl radical. More recently, Bouchoux et al theoretically discussed the low energy dissociation of ionized cyclohexene leading to the formation of cyclopentenyl ion. They proposed that cyclohexene ion has undergone successive 1,2‐hydrogen shifts and other rearrangements prior to methyl loss according to molecular orbital calculations.…”

A computational study of the Diels-Alder reactions between 2,3-dibromo-1,3-butadiene and maleic anhydride

“…In this case, both paths are interconnected. Note that Bouchoux et al have already found a closure of a 6-membered ring into a 5-membered ring when they explored the evolution of cationic cyclohexene [33]. The system can eliminate H 2 (TS5 + -P2 + ) and HBr (TS9 + -P3 + ) at different points along the isomerization paths (M5 + , M9 + ).…”

“…As no efficient deactivation was possible in the gas phase, the product fragmented under the experimental conditions [31]. Subsequent computational studies explored the possible fragmentation pathways of the Diels-Alder product in order to interpret the experimental findings [32,33].…”