Extraordinary Mechanism of the Diels–Alder Reaction: Investigation of Stereochemistry, Charge Transfer, Charge Polarization, and Biradicaloid Formation

Abstract

The Diels-Alder reaction between 1,3-butadiene and ethene is investigated from far-out in the entrance channel to the very last step in the exit channel thus passing two bifurcation points and extending the range of the reaction valley studied with URVA (Unified Reaction Valley Approach) by 300% compared to previous studies. For the first time, the pre- and postchemical steps of the reaction are analyzed at the same level of theory as the actual chemical processes utilizing the path curvature and its decomposition into internal coordinate or curvilinear coordinate components. A first smaller charge transfer to the dienophile facilitates the rotation of gauche butadiene into its cis form. The actual chemical processes are initiated by a second larger charge transfer to the dienophile that facilitates pyramidalization of the reacting carbon centers, bond equalization, and biradicaloid formation of the reactants. The transition state is aromatically stabilized and moved by five path units into the entrance channel in line with the Hammond-Leffler postulate. The pseudorotation of the boat form into the halfchair of cyclohexene is analyzed. Predictions are made for the Diels-Alder reaction based on a 11-phase mechanism obtained by the URVA analysis.

Does Confinement Always Lead to Thermodynamically and/or Kinetically Favorable Reactions? A Case Study using Diels–Alder Reactions within ExBox+4and CB[7]

“…The thermochemical results for the reactiona re presented in Table 1. The reactioni nvolves af ree-energy activation barrier (DG°)o f27.32 kcal mol À1 at 298.15 K. Notably, the computed activation energy (zero-point energyc orrected) barrier( DE°)o f2 2.95 kcal mol À1 is in reasonable agreement with those obtained in other studies, [50,51] thereby confirming that the chosen level of theory is reasonably reliable. With this background in mind, let us consider the concerned reaction inside the two hosts.E xBox + 4 is composed of four pyridinium rings and four phenyl rings with methylene groups acting as ChemPhysChem 2017ChemPhysChem , 18,2162ChemPhysChem -2170 www.chemphyschem.org bridges.The considered C 2v symmetric structure has an approximate diameter of 9.42 and 14.86 in the vertical and horizontal directions,r espectively.I ti se xpected that reactants as well as TS structures of the guestsw ould interactw ith the host through p-p stacking interactions and this, in turn, should play an extremelyi mportant role in the overall thermodynamics of the studied reaction.…”

“…In accordance with the Woodward-Hoffmann rules [26] for the thermalc ycloadditionr eactions, it is expected that the reaction between 1,3-butadiene (gauche conformation has been considered) (BD) and ethylene (ET) to yield cyclohexene (CHEX)s hould proceed through the supra-supra mode of interaction. It is to be noted that the mechanism involved in the considered reaction has been extensively debated previously [50] and multireferencew ave-function based computations [51] reveal that the synchronous, concerted mechanism should be favoredf rom an energetic point of view over the other possibility involving ab iradical intermediate. Therefore, we have only considered the concerted mechanism in this work.…”

The Molecular Structure of gauche‐1,3‐Butadiene: Experimental Establishment of Non‐planarity

“…[7] The eventual consensus is that the reaction proceeds via as yn-chronous transition state within aconcerted mechanism. [8] For all the diversity of Diels-Alder transition state possibilities, [9,10] butadiene is believed to adopt ap lanar structure in the lowest, aromatically stabilized, pericyclic transition state, [11] no doubt because orbital considerations suggest ap lanar cis conformation for optimal overlap with the psystem of the dienophile. [12] It is therefore surprising that the planarity of cis-butadiene itself remains at opic of interest [13] and even controversy.A lthough the planarity of trans-butadiene is wellestablished and attributed to conjugation between the two double bonds,s teric interaction between the inner terminal hydrogens of cis-butadiene is ac ompeting factor that favors an on-planar gauche conformation.…”

Dieter Cremer's contribution to the field of theoretical chemistry

“…In this way, each curvature peak can be associated with a particular chemical event. Often, only a few vibrational modes at a given path position s contribute to the curving of the reaction path, which makes the analysis of larger chemical reactions feasible . The second important milestone was the improvement of the path following procedure by Hratchian and Kraka, which allows to follow a chemical reaction far into the entrance and exit channel, where often decisive events happen.…”

“…A typical example is the Diels‐Alder reaction between 1,3‐butadiene and ethylene, shown in Figure . According to the URVA analysis, the important events preparing the reactants for the cycloaddition occur already in the prechemical phases. A first, smaller charge transfer to the dienophile facilitates the rotation of gauche butadiene into its cis form.…”