Concerted reactions symmetry forbidden

Figure 10 12 shows the interaction between the HOMO of one ethylene molecule and the LUMO of another In particular notice that two of the carbons that are to become ct bonded to each other m the product experience an antibondmg interaction during the cycloaddition process This raises the activation energy for cycloaddition and leads the reaction to be classified as a symmetry forbidden reaction Reaction were it to occur would take place slowly and by a mechanism m which the two new ct bonds are formed m separate steps rather than by way of a concerted process involving a sm gle transition state... 

Symmetry forbidden reaction (Section 10 14) Concerted re action in which the orbitals involved do not overlap in phase at all stages of the process The disrotatory ring opening of cyclobutene to 1 3 butadiene is a symmetry forbidden reaction... 

According to the Woodw ard-Hofmann rules the concerted thermal [2n + 2n] cycloaddition reaction of alkenes 1 in a suprafacial manner is symmetry-forbidden, and is observed in special cases only. In contrast the photochemical [2n + 2n cycloaddition is symmetry-allowed, and is a useful method for the synthesis of cyclobutane derivatives 2. 

Let us consider two ethylene molecules approaching each other in such a way that the top of one n system interacts with the bottom of the other. The HOMO of one n system must be matched with the LUMO of the other as shown. In both the cases the phase relationships at one end of the system are wrong for bond formation. So a concerted process in which both new G bonds are formed simultaneously is not possible and the reaction will be termed a symmetry-forbidden reaction. 

In this way Hoffmann and Woodward have established the following simple rule a concerted m + n cycloaddition will be symmetry allowed in the ground state and symmetry forbidden in the excited state if m + n = 4q + 2 (q = 0, 1, 2...) if m + n = 4q the reaction will be symmetry allowed in the excited state and symmetry forbidden in the ground state. This rule applies to ms + ns cycloadditions and to ma + na processes. 

Although thermal [2 + 2] cycloadditions are forbidden as concerted reactions by the orbital symmetry conservation rules the same structural features which promote intermolecular cy-cioadditions will also promote intramolecular reactions. In addition, the proximity between two alkene moieties dictated by the tether length and rigidity would make these processes entropically favorable. A few reports have documented thermal intramolecular cycloadditions to cyclopropenes and activated alkenes. The thermal Cope rearrangement of allylcyclopropenes apparently proceeds by a two-step mechanism in which intramolecular [2 + 2] adducts have been observed.72-73... 

We have emphasized that the Diels-Alder reaction generally takes place rapidly and conveniently. In sharp contrast, the apparently similar dimerization of olefins to cyclobutanes (5-49) gives very poor results in most cases, except when photochemically induced. Fukui, Woodward, and Hoffmann have shown that these contrasting results can be explained by the principle of conservation of orbital symmetry,895 which predicts that certain reactions are allowed and others forbidden. The orbital-symmetry rules (also called the Woodward-Hoffmann rules) apply only to concerted reactions, e.g., mechanism a, and are based on the principle that reactions take place in such a way as to maintain maximum bonding throughout the course of the reaction. There are several ways of applying the orbital-symmetry principle to cycloaddition reactions, three of which are used more frequently than others.896 Of these three we will discuss two the frontier-orbital method and the Mobius-Huckel method. The third, called the correlation diagram method,897 is less convenient to apply than the other two. 

Symmetry requirements for concerted reductive elimination of dialkyls have been considered (314), and for trialkylgold(III) species reductive elimination from a trigonal, three-coordinate intermediate was found to be symmetry forbidden. Solvent participation or the involvement of T-shaped species, however, was suggested as possible. Charge transfer to the high-oxidation state gold(III) center and reductive elimination from such a charge transfer state was proposed as an alternative reaction pathway. 

Schmidt81 has commented that symmetry-forbidden, concerted modes of reaction may be quite accessible if the reaction path is short, the force constants in reactant and product are both small, and the anharmonicities large. He cites a case where a forbidden reaction occurs readily with an activation energy of only 25 kJ mol-1. 

According to the Woodward-Hoffmann rules, the concerted [2S + 2J cycloaddition with two alkenes is photochemically symmetry-allowed, but is symmetry-forbidden at the ground state [9]. Photochemical [2 + 2] cycloaddition, in which one of two alkene partners is electronically excited, has been applied to the synthesis of cage hydrocarbons [10]. In such transformations, the intramolecular version of the reaction is particular efficient. The transformation of compound 1, in which two... 

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. 

A symmetry-forbidden reaction. The HOMO and LUMO of two ethylene molecules have different symmetries, and they overlap to form an antibonding interaction. The concerted [2 + 2] cycloaddition is therefore symmetry-forbidden. 

The statement a chemical reaction is symmetry allowed or symmetry forbidden, should not be taken literally. When a reaction is symmetry allowed, it means that it has a low activation energy. This makes it possible for the given reaction to occur, though it does not mean that it always will. There are other factors which can impose a substantial activation barrier. Such factors may be steric repulsions, difficulties in approach, and unfavorable relative energies of orbitals. Similarly, symmetry forbidden means that the reaction, as a concerted one, would have a high activation barrier. However, various factors may make the reaction still possible for example, it may happen as a stepwise reaction through intermediates. In this case, of course, it is no longer a concerted reaction. 

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 four T-electrons of the two ethene double bonds must redistribute themselves to the a-oibitals of the newly formed cyclobutane single bonds. As it turns out in this case, the requirement of symmetry conservation does not allow two ground-state ethene molecules to combine in a concerted reaction to form a ground-state cyclobutane molecule The reaction is "forbidden." The only allowed interactions are destabilizing. 

A symmetry-forbidden reaction can be switched to an allowed reaction in a number of ways. One of the more interesting mechanisms is that in which the actual forbidden transformation takes place on the coordination sphere of a transition metal. The ligand transformation here is concerted, the S5unmetry restrictions having been removed by the metal. The metal s role in this process has been described briefly in an earher communication by this author with J. H. Schachtschneider 3), and in more detail in a broader treatise 2). The description will not be repeated here instead, the subject will be approached from a different point of view, one that focuses attention on the coordinate bond and its relationship to the forbidden-to-allowed process. 

The question of stepwise vs. concerted processes in the catalysis of symmetry-forbidden reactions becomes more important when the postulated forbidden-to-allowed process itself is questioned. Certainly, stepwise processes in catalysis are common, and have been known to intervene in catal5dic reactions for some time. So to establish that an overall chemical transformation A B proceeds through distinct steps e.g., A - X - - B) is not, in itself, chemically significant. To suggest, however, that a chemical transformation A - B can proceed smoothly on the coordination sphere of a transition metal, and essentially nowhere else, introduces the possibihty of novel chemistry, not encountered in or outside of catalysis. The mere observation that a symmetry-forbidden reaction A - B proceeds in the presence of a metal, does not, in itself, mean that the symmetry-restricted transformation A B proceeded in a concerted manner on the coordination sphere of that metal. Here, the question of stepwise vs. concerted takes on new significance, for it bears on the possible existence of a special kind of chemical transformation. 

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