Orbital symmetry, ethylene reaction

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 ... 

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... 

The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the same reaction types discussed in Chapter 11 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2ti + 2ti] cycloaddition of two alkenes can serve as an example. This reaction was classified as a forbidden thermal reaction (Section 11.3) The correlation diagram for cycloaddition of two ethylene molecules (Fig. 13.2) shows that the ground-state molecules would lead to an excited state of cyclobutane and that the cycloaddition would therefore involve a prohibitive thermal activation energy. 

Thermal dimerization of ethylene to cyclobutane is forbidden by orbital symmetry (Sect 3.5 in Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume). The activation barrier is high E =44 kcal mof ) [9]. Cyclobutane cannot be prepared on a preparative scale by the dimerization of ethylenes despite a favorable reaction enthalpy (AH = -19 kcal mol" ). Thermal reactions between alkenes usually proceed via diradical intermediates [10-12]. The process of the diradical formation is the most favored by the HOMO-LUMO interaction (Scheme 25b in chapter Elements of a Chemical Orbital Theory ). The intervention of the diradical intermediates impfies loss of stereochemical integrity. This is a characteric feature of the thermal reactions between alkenes in the delocalization band of the mechanistic spectrum. 

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) ... 

The concept of the conservation of orbital symmetry can be extended to intermolecular cycloaddition reactions which occur in a concerted manner. The simplest case is the dimerization of ethylene molecules to give cyclobutane, the 2n + 2je cycloaddition. The proper geometry for the concerted action would be for the two ethylene molecules to orient one over the other. Two planes of symmetry are thereby set up -perpendicular to the molecular plane bisecting the bond axes oy-parallel to the molecular plane lying in between the two molecules (Figure 8.10). 

Orbital Symmetry Conservation in Bimolecular Cycloadditions. The cycloaddition reactions of carbonyl compounds to form oxetanes with ethylenes, as well as those of enones and their derivatives to form cyclobutanes, are examples of reactions which originate from triplet excited states and lead in the first step to biradical intermediates. Such reactions are of course not concerted, and they show little or no stereo-specificity. 

The alkene metathesis reaction was unprecedented - such a non-catalysed concerted four-centred process is forbidden by the Woodward-Hoffmann rules - so new mechanisms were needed to account for the products. Experiments by Pettit showed that free cyclobutane itself was not involved it was not converted to ethylene (<3%) under the reaction condition where ethylene underwent degenerate metathesis (>35%, indicated by experiments involving Di-ethylene) [10]. Consequently, direct interconversion of the alkenes, via an intermediate complex (termed a quasi-cyclobutane , pseudo-cyclobutane or adsorbed cyclobutane ) generated from a bis-alkene complex was proposed, and a detailed molecular orbital description was presented to show how the orbital symmetry issue could be avoided, Scheme 12.14 (upper pathway) [10]. 

Consider the reaction of acetoxycyclopentadiene with ethylene. In the conformation shown, the oxygen sp2 lone pair (cf. p. 200) has the correct symmetry to interact with the ethylene n orbital in the transition state leading to the anti isomer. This is a four-electron combination, so it is destabilizing. However, at the same time, the oxygen p lone pair interacts favorably with the ethylene n. Furthermore, a rotation of the ace-toxy group around the OC bond will diminish the overlap of the sp2 lone pair with k and increases its overlap with n. Both lone pairs can then interact with n the stabilizing interaction dominates and the anti isomer should be preferred. Experimentally, Winstein et al.32 isolated only this isomer. 

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. 

This [2 + 2] cycloaddition requires the HOMO of one of the ethylenes to overlap with the LUMO of the other. Figure 15-20 shows that an antibonding interaction results from this overlap, raising the activation energy. For a cyclobutane molecule to result, one of the MOs would have to change its symmetry. Orbital symmetry would not be conserved, so the reaction is symmetry-forbidden. Such a symmetry-forbidden reaction can occasionally be made to occur, but it cannot occur in the concerted pericyclic manner shown in the figure. 

Inspection of this correlation diagram immediately reveals that there is a problem. One of the bonding orbitals at the left correlates with an antibonding orbital on the product side. Consequently, if orbital symmetry is to be conserved, two ground state ethylene molecules cannot combine via face-to-face approach to give a ground-state cyclobutane, or vice versa. This concerted reaction is symmetry forbidden. ... 

The orbital correlation diagram for the concerted dimerization of ethylene to form cyclobutane or for the reverse reaction, the fragmentation of cyclobutane into two ethylenes, may be obtained most easily by applying the principle of conservation of orbital symmetry. A mirror plane perpendicular to the molec-... 

We are now equipped to consider a pericyclic reaction to see how best we can accommodate the principle of conservation of orbital symmetry. For this, we shall associate the relevant reactant orbitals and the product orbitals with a certain symmetry element. Let us first consider the n2 + n2 [2 + 2] reaction of two simple ethylene molecules to form cyclobutane as shown below. We create two a bonds in the product at the expense of two n bonds in the reactants. The energy level... 

Opposite page) Molecular orbital picture diagrams rationalizing the Diels-Alder reaction between 1,3-butadiene and ethylene (top) using A) the conservation of orbital symmetry (Woodward and Hoffmann) B) frontier molecular orbital theory (Fukui). 

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 Woodward-Hoffmann (W-H) rules are qualitative statements regarding relative activation energies for two possible modes of reaction, which may have different stereochemical outcomes. For simple systems, the rules may be derived from a conservation of orbital symmetry, but they may also be generalized by an FMO treatment with conservation of bonding. Let us illustrate the Woodward-Hoffmann rules with a couple of examples, the preference of the 4 + 2 over the 2 + 2 product for the reaction of butadiene with ethylene, and the ring-closure of butadiene to cyclobutene. 

As early as in 1938, Evans et al. recognized that the six electrons that are delocalized over six carbon atoms in the transition state of the Diels-TUder reaction exhibit some analogy to the six it electrons in benzene [145-147]. The (hypothetical) symmetric transition state of the [2-l-2]cycloaddition of ethylene includes 4 electrons, and resembles the antiaromatic it system of cyclobutadiene. (Figure 13.18) Evans concluded that the Diels-Ttlder reaction is favored by aromatic stabilization of its transition state, although the [2-l-2]cydoaddition is not Zimmerman [148-150] and Dewar [151,152] later generahzed the Evans principle and developed a method to predict the stereochemistry of pericycHc reactions [139,153] that can be apphed as an alternative to the conservation of orbital symmetry of Woodward and Hoffmami [154]. 

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