Diels-Alder Reaction 1,3-butadiene with ethylene, concerted

Thus, according to the MINDO/3 calculations, the Diels-Alder reaction is not a concerted process. Table 10.1 lists the geometry characteristics, calculated by this method, which correspond to the most important stationary points on the PES of the reaction of [4 -f 2]-cycloaddition of 1,3-butadiene to ethylene, compared with the results of the ab initio calculations [28]. 

Cycloaddition involves the combination of two molecules in such a way that a new ring is formed. The principles of conservation of orbital symmetry also apply to concerted cycloaddition reactions and to the reverse, concerted fragmentation of one molecule into two or more smaller components (cycloreversion). The most important cycloaddition reaction from the point of view of synthesis is the Diels-Alder reaction. This reaction has been the object of extensive theoretical and mechanistic study, as well as synthetic application. The Diels-Alder reaction is the addition of an alkene to a diene to form a cyclohexene. It is called a [47t + 27c]-cycloaddition reaction because four tc electrons from the diene and the two n electrons from the alkene (which is called the dienophile) are directly involved in the bonding change. For most systems, the reactivity pattern, regioselectivity, and stereoselectivity are consistent with describing the reaction as a concerted process. In particular, the reaction is a stereospecific syn (suprafacial) addition with respect to both the alkene and the diene. This stereospecificity has been demonstrated with many substituted dienes and alkenes and also holds for the simplest possible example of the reaction, that of ethylene with butadiene ... 

The chemical reactions through cyclic transition states are controlled by the symmetry of the frontier orbitals [11]. At the symmetrical (Cs) six-membered ring transition state of Diels-Alder reaction between butadiene and ethylene, the HOMO of butadiene and the LUMO of ethylene (Scheme 18) are antisymmetric with respect to the reflection in the mirror plane (Scheme 24). The symmetry allows the frontier orbitals to have the same signs of the overlap integrals between the p-or-bital components at both reaction sites. The simultaneous interactions at the both sites promotes the frontier orbital interaction more than the interaction at one site of an acyclic transition state. This is also the case with interaction between the HOMO of ethylene and the LUMO of butadiene. The Diels-Alder reactions occur through the cyclic transition states in a concerted and stereospecific manner with retention of configuration of the reactants. 

The Diels-Alder reaction is the best known and most widely used pericyclic reaction. Two limiting mechanisms are possible (see Fig. 10.11) and have been vigorously debated. In the first, the addition takes place in concerted fashion with two equivalent new bonds forming in the transition state (bottom center, Fig. 10.11), while for the second reaction path the addition occurs stepwise (top row, Fig. 10.11). The stepwise path involves the formation of a single bond between the diene (butadiene in our example) and the dienophile (ethylene) and (most likely) a diradical intermediate, although zwitterion structures have also been proposed. In the last step, ring closure results with the formation of a second new carbon carbon bond. Either step may be rate determining. 

The behavior described above has been verified by experiment and calculation on numerous substituted dienes and dienophiles. For example Fig. 10.13 shows results for 2°-D isotope effects on Diels-Alder reactions of 2-methyl-butadiene with cyano-ethylene and 1,1-dicyano-ethylene. The calculated and experimental isotope effects are in quantitative agreement with each other and with the results on (butadiene + ethylene). In each case the excellent agreement between calculated and observed isotope effects validates the concerted mechanism and establishes the structure of the transition state as that shown at the bottom center of Fig. 10.11 and the left side of Fig. 10.12a. 

As a consequence of the concerted mechanism, the Diels-Alder reaction is also stereoselective, implying that the relative configuration of the groups of the reactants is retained. Besides the numerous examples of heterosubstituted compounds,521,522 this was also proved by 1,3-butadiene and ethylene labeled with deuterium [Eqs. (6.88) and (6.89)] 531... 

Figure 15-19 shows that the HOMO of butadiene has the correct symmetry to overlap in phase with the LUMO of ethylene. Having the correct symmetry means the orbitals that form the new bonds can overlap constructively plus with plus and minus with minus. These bonding interactions stabilize the transition state and promote the concerted reaction. This favorable result predicts that the reaction is symmetry-allowed. The Diels-Alder reaction is common, and this theory correctly predicts a favorable transition state. 

In addition to conventional ab initio methods, techniques based on the density functional theory (DFT) have also been used to study the Diels-Alder reaction between butadiene and ethylene . With these kinds of methods, a concerted mechanism through a symmetric transition state is also predicted. Several kinds of density functionals have been used. The simplest one is based on the Local Density Approach (LDA), in which all the potentials depend only on the density. More sophisticated functionals include a dependence on the gradient of the density, such as that of Becke, Lee, Yang and Parr (BLYP). 

These experimental secondary deuterium KIEs observed in Diels-Alder reactions have been compared with the respective theoretical KIEs for the stepwise mechanism involving a diradical intermediate (equation 88a) and for concerted synchronous and asynchronous mechanisms (equation 88b) for the Diels-Alder reaction of butadiene with ethylene . ... 

Houk KN, Lin YT, Brown FK. Evidence for the concerted mechanism of the Diels-Alder reaction of butadiene with ethylene. J Am Chem Soc 1986 108 554-556. 

The ethylene-butadiene cycloaddition is a good example to illustrate that symmetry allowedness does not necessarily mean that the reaction occurs easily. This reaction has a comparatively high activation energy, 144 kJ/mol [7-7]. A large number of quantum-chemical calculations has been devoted to this reaction with conflicting results (for recent references, see Ref. [7-18]). It seems, however, that the concerted nature of the prototype Diels-Alder reaction is well established. The reason for the relatively high activation energy is that substantial distortion must occur in the reactants before frontier orbital interactions can stabilize the product. 

MOs, while tlie two 7t c orbitals lead to the tt and tt MOs. In the initial stage of (he dimerization, the interaction between two ethylencs is weak so that 7t+ and tt. lie far below the n+ and tt levels, so that only 7t+ and rr are occupied. Of the a orbitals of cyclobutane described earlier, only those related to the tt., 7t1 and nl levels by symmetry are shown in Figure 11.1. Not all the occupied MOs of the reactant lead to occupied orbitals in the product. In particular, tt. correlates with one component of the empty set in cyclobutane. The tt+ combination ultimately becomes one component of the filled set in cyclobutane. So the reaction is symmetry forbidden. The reader should carefully compare the correlation diagram for ethylene dimerization here with the Ho + O2 reaction in ITgure 5.8. flie two correlation diagrams are very similar, as they should be, since in this instance the spatial dfstributions of tt and n " are similar to those of and respectively, in H2. These two reactions are probably the premier examples of symmetry-forbidden reactions. A related symmetry-allowed example is the concerted cycloaddition of ethylene and butadiene, the Diels-Alder reaction. We shall not cover the orbital symmetry rules for organic, pericyclic reactions. There are several excellent reviews that the reader should consult.But it should be pointed out that the orbital symmetry rules have stereochemical implications in terms of the reaction path and products formed. The development of these rules by Woodward and Hoffmann... 

Because of the relatively large number of atoms involved, only a few ab initio studies have been performed and only on the simplest Diels-Alder reaction, the addition of ethylene to 1,3-butadiene. The first ab initio studies were those of Burke et al. and of Townshend et a/. Burke et al. have carried out extensive calculations on the concerted approach of this reaction at the SCF level with an STO-3G basis set and have also recalculated several points on the hypersurface using a medium-size basis set (7s/3p). Townshend et al. have studied not only the concerted but also the two-step approach using an SCF treatment with limited Cl at the STO-3G level and have recalculated the energy pathways with an extended 4-3IG basis. However, even though these studies already involved a very signiflcant computational effort, they were performed without complete optimization and characterization of the critical points. 

The heats of reaction of tetracyanoethylene with a number of dienes have been determined calorimetrically and the heats of formation of the adducts calculated. Careful investigation of dimerization of butadiene to give 4-vinyl-cyclohexene and of the retro-Diels-Alder reaction suggests that the [4 + 2] cycloaddition is a concerted process, and similar analysis of the elimination of ethylene from (377) suggested a concerted reaction. 

One of the best tests for DFT is probably the Diels-Alder reaction (see Figure 3) of butadiene and ethylene. It has only been recently that the controversy over whether the reaction undergoes a concerted or stepwise mechanism has led to consensus, The reaction mechanism of butadiene and ethylene is concerted, i,e, the reaction involves a transition state with two equal C-C distances of approach between the two reactants. However, the activation barrier for the stepwise mechanism is only a few kcal mol higher, Goldstein et al, have shown that the B3-LPY method is capable of direct comparison of concerted versus stepwise mechanisms. Table 18 illustrates the DFT and ab initio predictions of the geometrical parameters d, R, and Ri, as shown in Figure 3, and the activation energy for the transition state involved in... 

In a definitive study of butadiene s reaction with l,l-dichloro-2,2-difluoio-ethylene, Bartlett concluded that [2+4] adducts of acyclic dienes with fluorinated ethylenes are formed through a mixture of concerted and nonconcerted, diradical pathways [67] The degree of observed [2+4] cycloaddition of fluorinated ethylenes IS related to the relative amounts of transoid and cisoid conformers of the diene, with very considerable (i.e., 30%) Diels-Alder adduct being observed in competition with [2+2] reaction, for example, in the reaction of 1,1 -dichloro-2,2-difluoro-ethylene with cyclopentadiene [9, 68]... 

It was Woodward and Hoffmann who first introduced organic chemists to the idea that so-called frontier orbitals (the HOMO and LUMO) often provide the key to understanding why some chemical reactions proceed easily whereas others do not. For example, the fact that the HOMO in cw-1,3-butadiene is able to interact favorably with the LUMO in ethylene, suggests that the two molecules should readily combine in a concerted manner to form cyclohexene, i.e., Diels-Alder cycloaddition. 

An ab initio MO computational study of the Diels-Alder addition of ethylene to butadiene reveals a symmetrical transition state and a concerted reaction profile. Extended Htickel MO calculations on the endo.exo product distribution in [4 + 2] reactions of cyclopentadiene indicate, within the range of intermolecular distances investigated, that the ewdo-orientation is energetically favoured. A study of the relationship between isomer distribution and solvent polarity in the reactions of cyclopentadiene with acrylonitrile and with methyl acrylate reveals that the endo-adduct. dicyclopentadiene and exo-adduct rdicyclopentadiene ratios are increased linearly with the empirical solvent polarity factor The favoured endo-... 

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