Mechanism of a Diels-Alder reaction. Butadiene and its dimers

Abstract

Synthesis and kinetic study of (I?)-( +)-1 ,l-dideuterio-2-(cyclohex-3-enyl)ethene (labeled 4vinylcyclohexene) has revealed new reactions of racemization and deuterium exchange, the former of which fixes the heat of formation of cis,trans-(and/or cis,cis) octa-3,6-diene-l,7-diyl diradical more firmly than heretofore. Redetermination of the kinetic parameters for the thermal reactions of cycloocta-l,5-diene establishes the heat of formation of the cleavage of the trans,trans (or, less probably, the cis,cis) diradical to butadiene to be 81.1 5 2.0 kcal/mol, in good agreement with that (82.3 kcal/mol) deduced for the same reaction in the transformations of trans-divinylcyclobutane investigated by Hammond and DeBoer. In relation to the transition state for the Diels-Alder reaction of butadiene with itself to give Cvinylcyclohexene, the cleavage of the diradical appears, within the present limitations of experimental results, to be somewhat higher in energy. The mechanism of this Diels-Alder reaction is tentatively concluded to be concerted, perhaps two stage, but not two step.ontroversy over the mechanism of the Diels-Alder C reaction has centered on the alternatives of a onestep, concerted process and a two-step, unconcerted process. The former, allowed by orbital symmetry,2 envisages the simultaneous formation of both bonds (A),3 whereas the latter involves the rate-determining formation of one bond, followed by a closure of the second bond more rapid than internal rotation (B).435To these widely disputed alternatives, Woodward and Katz have added a third, a merger of the two mechanisms into a two-stage process (C)I (Figure 1).One generally applicable approach, particularly appropriate to the study of mechanism among thermal reorganizations, involves the placement of the interrelated isomeric elements of the system on an energy surface with a view t o including or excluding certain

Evaluation of 1,3-butadiene dimerization and secondary reactions in the presence and absence of oxygen

“…The asynchronous two-stage mechanism is a combination of the two earlier mechanisms. Years of controversy over which mechanism is most dominant has not reached an end, although it is likely to be an asynchronous two-stage mechanism [11].…”

“…1) is suggested based on previous work [11][12][13] to predict a reaction mechanism and calculate themokinetic parameters.…”

Butadiene

“…Simple alkenes like ethylene are poor dienophiles resulting in sluggish reactions. Substituted olefins, XÀ À ÀC ¼ ¼ CÀ À ÀX 0 , are more reactive when X and/or X 0 are C ¼ ¼ C, Ar, COOR, COOH, COH, COR, COCl, CN, halogens, and many other electron-withdrawing substituents (112)(113)(114)(115)(116). A compilation of the reaction parameters between butadiene and C2ÀC4 olefins in the temperature range of 510-7508C has been published (117).…”

Active space and basis set effects in CASPT2 models of the 1,3‐butadiene‐ethene cycloaddition and the 1,3‐butadiene dimerization

“…[1][2][3][4][5][6][7] Previous isotope studies indicate a preference for the concerted mechanism, as supported by the Woodward and Hoffmann rules for pericyclic reactions, [8] however, the formation of a small quantity of a second product in which the isotope labels are scrambled suggests another mechanism involving biradical intermediates. [2,9,10] The biradical transition states consist of the stepwise and asynchronous biradical pathways where the stepwise involves the formation of one bond followed by the diene or dienophile rearranging itself such that the second bond can form. The asynchronous pathway is reminiscent of the concerted pathway, however, one bond forms leading to a reactive intermediate from where the second bond forms via a second, low-energy transition state.…”

“…The asynchronous pathway is reminiscent of the concerted pathway, however, one bond forms leading to a reactive intermediate from where the second bond forms via a second, low-energy transition state. [10] These mechanisms continue to be studied across different systems as new computational methods are developed and more computational power becomes available, allowing for higher correlated levels of theory to be applied to these systems. [11][12][13][14] Previous work on Diels-Alder reactions have used sophisticated complete active space (CAS) self-consistent field (SCF) calculations to account for static correlation in models of the transition states of the butadiene-ethene and butadienebutadiene cycloadditions, however, it has been shown that dynamic correlation is necessary to obtain reaction barriers that are consistent with experimental results.…”