Electron-Flow in Diels-Alder Reactions

Electrostatic interactions play a significant role in determining the rates of Diels-Alder reactions. 

Compare electrostatic potential maps for the following Diels-Alder transition states cyclopentadiene+ethene, cyclopentadiene+acrylonitrile and cyclopentadiene+ tetracyanoethylene, with those of reactants cyclopentadiene, ethene, acrylonitrile and tetracyanoethylene. Are electrons transferred from diene to dienophile in the transition states (relative to reactants) or vice versa For which reaction is the transfer the greatest The least Quantify your conclusion by measuring the total charge on the diene and dienophile components in the three transition states. 

Calculate activation energies for the three Diels-Alder reactions (energy of transition state - sum of energies of reactants). Which reaction has the smallest energy barrier Which has the largest energy barrier Do your results parallel the measured relative rates of the same reactions (see table at left)  

Is there a correlation between activation energy and the magnitude of charge transfer between diene and dienophile components in the transition state Explain. 

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Oxabicyclo[3.2.0]hepta-l,4,6-triene (289), a planar Sn-electron analog of 4, has been prepared by flow pyrolysis of 288 (both cis and trans) in approximately 10% yield (>95% purity) 289 is an extremely sensitive compound, polymerizing instantaneously on exposure to oxygen. In solution, where it is stable for several days, it slowly dimerizes to give the known compound 291 the pentacyclic intermediate 290 is possibly involved. In Diels-Alder reactions, 289 behaves like an olefin with cyclopentadiene it reacts immediately to give 292. Hydrogenation occurs at the same site. ... 

Recently pyrimidine substituted alkynes 476 were subjected to intramolecular inverse-electron-demand hetero-Diels-Alder reaction with extrusion of HCN affording fused fluorinated pyridines 478. The reaction proceeds at high temperatures in sealed tubes for small amount of the starting materials [256] or as scalable flow process [257] (Scheme 89)... 

The arrows may be drawn in a clockwise or counterclockwise direction to show the flow of electrons in a Diels-Alder reaction. 

On this basis, let us examine the [4 + 2] cycloaddition of 1,3-butadiene and ethylene, the simplest example of the Diels-Alder reaction. The electronic configurations of these compounds—and of dienes and alkenes in general—have been given in Fig. 29.5 (p. 931) and Fig. 29.6 (p. 932). There are two combinations overlap of the HOMO of butadiene ( 2) with the LUMO of ethylene (tt ) and overlap of the HOMO of ethylene (tt) with the LUMO of butadiene ( 3). In either case, as Fig. 29.20 shows, overlap brings together lobes of the same phase. There is a flow of electrons from HOMO to LUMO, and bonding occurs. 

Using the Diels-Alder reaction as an example, pushing arrows are used to show electron flow, but they are drawn in a circle, and the direction of the arrows can arbitrarily be shown as clockwise or anticlockwise. All the bonds are made and broken in a single step. This is called a concerted reaction. 

The curly arrows can be drawn in either direction. Here they are drawn so as to imply a clockwise movement of electrons, but the arrows could equally well have been drawn anti-clockwise. There is no absolute sense to the direction in which the electrons flow. Similarly, there is no absolute sense in which the hydrogen atom that moves from one carbon atom to the other in the ene reaction is a hydride shift, as seems to be implied by the curly arrows, or a proton shift, as it would seem to be if the arrows were to have been drawn in the opposite direction. In other words, neither component can easily be associated with the supply of electrons to any of the new bonds. The curly arrows therefore have a somewhat different meaning from those used in ionic reactions. In this they resemble somewhat the curly arrows used to show resonance in benzene, where the arrows show where to draw the new bonds and which ones not to draw in the canonical structure, but in the drawing of arrows interconnecting resonance structures there is neither a sense of direction nor even an actual movement. The analogy between the resonance of benzene and the electron shift in the Diels-Alder reaction is not farfetched, but it is as well to be clear that one, the Diels-Alder reaction, is a reaction, with starting materials and a product, and the other, resonance in benzene, is not. 

No Mechanism reactions were also extremely accommodating as regards electron-pushing , a favorite pastime of many organic chemists at that time. In reactions that could be presumed to have some polar character, the directionality of charge transmission could be deduced in a more or less straightforward manner. [9] In contrast, the flow of electrons in the course of the Diels-Alder reaction, for example, could be variously depicted as ... 

Earlier we used curved arrows to show the flow of electrons that takes place in the process of bond breaking and bond forming in the Diels-Alder reaction. As discussed, these reactions involve a four-carbon diene and a two-carbon dienophile and are termed [4 + 2] cycloadditions. We can write similar electron-pushing mechanisms for the dimerization of ethylene by a [2 + 2] cycloaddition to form cyclobutane and for the dimerization of butadiene by a [4 + 4] cycloaddition to form 1,5-cyclooctadiene. 

In a Diels-Alder reaction, the electron density flows from the HOMO of the diene to the LUMO of the dienophile. 

Pericyclic reactions involve the continuous flow of electrons in cyclic transition states (TS) by breaking and making of bonds in a concerted process, without formation of an intermediate. Hence, these reactions are known as concerted reactions [1]. These reactions are insensitive to solvent polarity and free radical initiators or inhibitors. These reactions are activated by heat (thermal) or light (photochemical). Detailed study of the mechanisms of these reactions by Woodward and Hoffmann [2] predicted that these reactions occur by the maintenance of symmetry properties of the orbitals of reactant(s) and product(s). The Diels-Alder reaction is a typical example. 

Assuming that all of the bonds are formed in the same step and only tt electrons are involved, we can use the Htickel Tr-electron approximation to explore the process from a molecular orbital perspective and need examine only those orbitals of the reactants that are directly involved in bond formation. These are called the frontier orbitals and are usually the HOMO of one reactant and the LUMO of another. For the Diels-Alder reaction they are the HOMO of the diene and the LUMO of the dienophile and are as shown in Figure 10.10. The choice of this HOMO-LUMO combination is made to be consistent with the experimental fact that electron-withdrawing groups on the dienophile increase its reactivity and suggest that electrons flow from the diene to the dienophile. Notice that the symmetry properties of these two orbitals are such as to permit the in-phase overlap necessary for a bond formation between the diene and dienophile. In MO terms, the Diels-Alder reaction is classified as symmetry-allowed. 

The dimerization of cyclopentadiene occurs primarily through an endo transition state, as is typical for Diels-Alder reactions, (a) In the reactants, draw red and blue shaded lobes for the orbitals that have favorable secondary interactions in the diene and dienophile, causing the preference for an endo transition state, (b) Using bond-line formulas, draw curved arrows to show the flow of electrons that leads to product formation, and draw a three-dimensional formula for the product, (c) The reaction produces a racemic mixture. Show how the reactants align in three dimensions to form each enantiomer. 

Let us now examine the Diels-Alder cycloaddition from a molecular orbital perspective. Chemical experience, such as the observation that the substituents that increase the reactivity of a dienophile tend to be those that attract electrons, suggests that electrons flow from the diene to the dienophile during the reaction. Thus, the orbitals to be considered are the HOMO of the diene and the LUMO of the dienophile. As shown in Figure 10.11 for the case of ethylene and 1,3-butadiene, the symmetry properties of the HOMO of the diene and the LUMO of the dienophile pennit bond fonnation between the ends of the diene system and the two carbons of the dienophile double bond because the necessary orbitals overlap in phase with each other. Cycloaddition of a diene and an alkene is said to be a symmetry-allowed reaction. 

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