Symmetry orbital ethylene

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) ... 

Hoffmann was the first to apply the concept of orbital symmetry to the cycloaddition of carbenes to olefins. This concept, which is based on EH-calculations was demonstrated for the [H-2]-cyclo-addition of triplet methylene to ethylene. 

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). 

In the refined Criegee mechanism615,616 that includes orbital symmetry considerations as well,617 all three steps are stereospecific. Ozonation of cis- and trans-[1,2-2H2] ethylene, for instance, gave the exo and endo isomers according to Eqs. (9.110) and (9.111) (the cis-exo and cis-endo ozonides from the cis compound, and the trans ozonide form the trans isomer) 602... 

The treatment of the UPS of the tr-allyl complex (CsHs)2Nb(C3Hs) is similar to that of the ethylene complexes. One significant difference between the ethylene and tr-allyl complexes, however, is that the symmetry is reduced to C2 in the latter. The correlations between the appropriate orbital symmetries in the C2v and Cs point groups are indicated in Table XIV. 

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]. 

Fig. 12. Orbital symmetry correlation for the Diels-Alder process, cis-s-butadiene + ethylene.

Fig. 14. Orbital and state symmetry correlations for the dimerization of ethylene. The orbital symmetry designations in the upper diagram are with respect to the planes of symmetry tjyt and < ...

The assumption of a definite location for the negative charge on the j8-carbon requires that 59a and 60a both be on the lowest energy path for substitution and isomerization. That this location should be formed anti derives from orbital symmetry, i.e. microscopic reversibility suggests that anti entry of one halide and syw-departure of the other are improbable. We deduce, therefore, that the substitutions in (142) require at least three elementary steps, e.g. cis-1 - 59a -> 60b -v cis-2 isomerizations with exchange require at least three steps, e.g. cis-l 59a —> 59b - trans-2 isomeriza-... 

As a test of the method, we examined the dimerization of ethylene. The first and second excited states are Az and Alt respectively (Fig. 14). For a four-center square species, the normal vibrations which could lead to dimerization are (Alg) and vz (Blg) (Herzberg, 1945). Accordingly, dimerization would be possible but difficult, since the symmetry of vx matches that of the second excited state. This conclusion agrees with the results of orbital-symmetry arguments. 

Let us now apply this method to a specific example. Consider the ethylene molecule with D2h symmetry. As can be seen from the character table of the L 2h point group (Table 6.4.2), this group has eight symmetry species. Hence the molecular orbitals of ethylene must have the symmetry of one of these eight representations. In fact, the ground electronic configuration for ethylene is... 

So the eight pairs of electrons of this molecule occupy delocalized molecular orbitals lag to 1 3U, while the first vacant orbital is l g- Note that the names of these orbitals are simply the symmetry species of theZ)2h point group. In other words, molecular orbitals are labeled by the irreducible representations of the point group to which the molecule belongs. So for ethylene there are three filled orbitals with Ag symmetry the one with the lowest energy is called lag, the next one is 2ag, etc. Similarly, there are two orbitals with Z iu symmetry and they are called lb u and 2bi . All the molecular orbitals listed above, except the first two, are illustrated pictorially in Fig. 6.4.2. By checking the >2h character table with reference to the chosen coordinate system shown in Fig. 6.4.2, it can be readily confirmed that these orbitals do have the labeled symmetry. In passing, it is noted that the two filled molecular orbitals of ethylene not displayed in Fig. 6.4.2, lag and l iu, are simply the sum and difference, respectively, of the two carbon Is orbitals. 

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. ... 

In examining the cycloalkenes, one must first recognize that a double bond has considerable inherent strain. For example, the dimerization of ethylene to give cyclobutane is fairly exothermic (—18 kcal mol" ) and if there were a way to readily overcome orbital symmetry restrictions, cyclobutane would be a very common reagent. However, in the following, we will take the conventional view that ethylene is unstrained. Then, in comparing cycloalkanes and cycloalkenes it is helpful to define olefinic strain (OS) as the difference in strain between the alkene and the corresponding alkane ... 

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-... 

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