Cycloaddition of Ethylene to Butadiene

Figure 13-2. Reaction path for the cycloaddition of ethylene to butadiene.

Application of CM theory to explain pericyclic reactions was first attempted by Epiotis and coworkers (Epiotis, 1972, 1973, 1974 Epiotis and Shaik, 1978b Epiotis et al 1980). The following analysis is a much-simplified treatment of that approach. Let us compare, therefore, the CM analysis for the [4 + 2] allowed cycloaddition of ethylene to butadiene to give cyclohexene with the [2 + 2] forbidden dimerization of two ethylenes to give cyclobutane. For simplicity only the suprafacial-suprafacial approach is considered, although this simplification in no way weakens the argument. 

Table 13-2. Computed activation (AEa) and reaction energies (AEr) for the concerted gas-phase cycloaddition of ethylene to Irans-butadiene [kcal/mol]. The HF and DFT calculations were performed with the 6-311+G(d,p) basis set and include zero-point vibrational contributions.

An additional point of interest concerns the behaviour of homo and lumo orbitals of reactants in allowed reactions. Fukui (1970, 1975) has pointed out that the frontier-orbital gap actually narrows as the reaction proceeds. This has been confirmed computationally for the cycloaddition of ethylene and butadiene (Townshend et al., 1976), and contrasts with what one might expect based on a static homo-lumo interaction. Such an interaction causes the energy gap between resultant orbitals to widen, as indicated in Fig. 29. 

The simplest of all Diels-Alder reactions, cycloaddition of ethylene to 1,3-butadiene, does not proceed readily. It has a high activation energy and a low reaction rate. Substituents such as C=0 or C=N, however, when directly attached to the double bond of the dienophile, increase its reactivity, and compounds of this type give high yields of Diels-Alder adducts at modest temperatures. 

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... 

The two cycloaddition reactions we have discussed so far illustrate some of these points. The [2+2] cycloaddition is forbidden. However, olefins can dimerize to make cyclobutanes. It is just that the reaction is not concerted, but rather involves a biradical intermediate. The [4+2] cycloaddition is allowed, but in fact the concerted cycloaddition of ethylene and butadiene requires high temperature and pressure. It does occur by a concerted allowed path, but the activation barrier is high. 

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 ... 

On orbital symmetry grounds, the addition of ethylene to ethylene with ring closure (cycloaddition) should be thermally forbidden. If one compares this reaction with the reaction of trimethylene with approaching ethylene and butadiene (Fig.4), it is readily seen that, the A level being below the S level in trimethylene, the behaviour with respect to cycloaddition to olefins is reversed, that is, trimethylene is essentially an anti-ethylene structure. This principle can be generalized for instance (16) ... 

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. 

A synthetically useful pressure-induced increase of diastereoselectivity was also found for normal Diels-Alder reactions such as the cycloaddition of the phenyl butadienes (93a-c) with the dicyano ethylenes (94-97) to yield the ds-adducts 98a-d, lOOa-d and 102a-d as well as the trous-adducts 99a-d, lOla-d and 103a-d (Scheme 8.24) [54]. 

Flow can we understand these simple results Could we have predicted them a priori It turns out that the r on of the potential surface that corresponds to the 2 + 2 addition is virtually identical to the 2 + 2 cycloaddition of two ethylenes. During the initial phase of the reaction four of the electrons in benzene become passive in a butadiene-like structure. But this might have been one s intuitive guess anyway. Further, one can find synchronous structures, both transition states and conical intersections, for the direct addition of ethylene to form the meta- and para- structures but it transpires that these are quite h h in energy. 

To develop the concepts related to understanding all pericyclic reactions, we will study two prototype reactions, shown in Eqs. 15.1 and 15.2. Both are cycloadditions, reactions in which two (or more) molecules combine to make a new ring system. We will develop the nomenclature more fully below, but for now it is convenient to refer to the dimerization of ethylene to give cyclobutane as a [2-1-2] cycloaddition, and the combination of butadiene and ethylene to give cyclohexene as a 4-l-2] cycloaddition. We assume a pericyclic transition state, and that the two partners approach each other in a symmetrical fashion, forming a symmetrical cyclic transition state, and then go on to product. "Symmetrical" means that the trajectory for approach of the reactants and the geometry of the transition state have a particular symmetry element, such as a o plane, C axis, S axis, etc. We will refer to this symmetry element in many of the theoretical models used to analyze pericyclic reactions. 

Unsaturated compoimds are inherently reactive because transformation of a it bond to a a bond provides ca. 20 kcal/mol of energy. This energy could be used to power a variety of chemical transformations. The Diels-Alder reaction is a classical example. Cycloaddition of 1,3-butadiene with ethylene gives cyclohexene with an exchange of two n bonds in the reactants for two o bonds in the product. Each chain extension of the radical polymerization of ethylene is sustained by the... 

Table 10.1. Optimized bond lengths (A) of stationary points on the PES of reaction of [4 + 2]-cycloaddition of 1,3-butadiene to ethylene according to MINDO/3 [18] and ab initio [28] calculations (latter in parentheses)...

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]. 

Diels-Alder Reaction In the manner similar to cycloaddition of ethylene Diels-Alder reaction can be analysed which involves Jt-molecular orbitals of butadiene and ethylene. 

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 reac tivity 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 m 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 permit bond formation between the ends of the diene system and the two carbons of the dienophile double bond because the necessary orbitals overlap m phase with each other Cycloaddition of a diene and an alkene is said to be a symmetry allowed reaction... 

How do orbital symmetry requirements relate to [4tc - - 2tc] and other cycloaddition reactions Let us constmct a correlation diagram for the addition of butadiene and ethylene to give cyclohexene. For concerted addition to occur, the diene must adopt an s-cis conformation. Because the electrons that are involved are the n electrons in both the diene and dienophile, it is expected that the reaction must occur via a face-to-face rather than edge-to-edge orientation. When this orientation of the reacting complex and transition state is adopted, it can be seen that a plane of symmetry perpendicular to the planes of the... 

Fig. 11.9. Symmetry properties of ethylene, butadiene, and cyclohexene orbitals with respect to cycloaddition.

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]... 

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