Symmetry-forbidden reaction

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... 

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. 

WHAT IS SYMMETRY ALLOWED AND SYMMETRY FORBIDDEN REACTION ... 

The details of symmetry forbidden reaction will be clear when we study the cycloaddition of ethene described later. 

In the above example although the ground state orbital of cyclobutene, o, correlates with the ground state orbitals of butadiene /b the n orbital of the former does not correlate the /2 orbital of the latter, but rather it correlates with the /3 which is an excited state. So in such a case the thermal transformation by disrotatory processes will be a symmetry forbidden reaction. 

Similarly the first excited state of butadiene V1V2V3 is correlated with a high energy upper excited state G27tc of cyclobutene. Thus a photochemical conrotatory process in either direction would be a symmetry forbidden reaction. 

What is Symmetry Allowed and Symmetry Forbidden Reactions 33... 

Williams, D.H. Hvistendahl, G. Kinetic Energy Release in Relation to Symmetry-Forbidden Reactions. J. Am. Chem. Soc. 1974, 96, 6753-6755. 

A symmetry-forbidden reaction can take place, but it requires much more energy to drive it ... 

Additional evidence for a stepwise pathway is provided by the fact that a two-step Diels-Alder reaction is observed, in which a formal [2 + 2] reaction gives a vinylcyclobutane (64) which then rearranges to the formal [4 + 2] product (Scheme 41, An = P-CH3OC6H4)118. It has been shown that orbital symmetry control does not operate in these reactions Symmetry-allowed and symmetry-forbidden reactions may take place with equal facility depending upon the conditions119. It has also been shown that the obtention of formally [4 + 2] or [2 + 2] products depends on many factors, including solvent and whether it is the diene or the dienophile which is ionized120. 

Another type of symmetry forbidden reaction has been defined by Woodward and Hoffmann 8> and involves a change in symmetry of the occupied molecular orbitals during the reaction. Certain materials may act as a catalyst in this case, if they can form a reaction intermediate which either circumvents the symmetry rule 9> or lowers the energy of activation, 0>. 

If a cycloaddition produces an overlap of positive-phase orbitals with negative-phase orbitals (destructive overlap), antibonding interactions are generated. Antibonding interactions raise the activation energy, so the reaction is classified as symmetry-forbidden. The thermal [2 + 2] cycloaddition of two ethylenes to give cyclobutane is a symmetry-forbidden reaction. 

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. 

Fig. 39 llustration of (a) a symmetry-allowed HT process and (b) a symmetry-forbidden HT process. Both reactions take place within a dimeric ethene radical cation complex. Both dimers possess C2v symmetry. For the symmetry-forbidden reaction, (a), the two ethene molecules lie in perpendicular planes consequently, the reactant and product have different electronic state symmetries, Bx and B2, respectively, and Vel is therefore zero. For the allowed process, (b), the two ethene groups lie in parallel planes and both reactant and product have identical state symmetries, Bp, thus, Vel is non-zero. For non-adiabatic HT, where Vel is very small ( 25 cm-1), the allowed and forbidden processes have nearly identical free energies of activation. 

Let us go through the same steps for a symmetry-forbidden reaction, the ln s+A] cycloaddition 6.141. We first draw the reaction and put in the curly arrows—the orbitals are evidently the n and n of each of the n bonds. There are two symmetry elements maintained this time—a plane like that in the Diels-Alder reaction, bisecting the n bonds, but also another between the two reagents, which reflect each other through that plane. 

What is Symmetry Allowed and Symmetry Forbidden Reactions Woodward Hoffmann Rule o Bonds involved in Cycloaddition Reactions Some more Examples of 2 + 2 Cycloadditions Photochemical Cycloadditions 2 + 3 Cycloadditions 2, 1 Cycloaddition... 

Kraka, E. Wu, A. Cremer, D. Mechanism of the Diels-Alder reaction studied with the united reaction vaUey approach mechanistic differences between symmetry-allowed and symmetry-forbidden reactions, 7. Phys. Chem. A 2003,107, 9008-9021. 

Our interest will be centered primarily on the metal s role in catalyzing symmetry-forbidden reactions. This, as we shall point out, is a special case and one that might best be introduced in contrast to other kinds of metal-assisted ligand transformations. Pursuing this approach, we shall consider generally the transformation of a ligand A to some ligand B. 

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. 

In this chapter we have attempted to describe rather broad principles regarding the catalysis of symmetry-forbidden reactions. Attention was perhaps narrowly focused on one type of restricted transformation, namely [2 -1-2 ] cycloaddition. This choice of reaction was somewhat arbitrary, but not entirely. The chemistry associated with these reactions appears at this time much broader and more interesting than what has appeared from the catalysis of other types of S5onmetry-forbidden reactions. There are, however, conspicuous exceptions which will be discussed briefly here. 

One of the first and perhaps most interesting examples of a metal-assisted, symmetry-forbidden reaction was Reppe s synthesis of cyclo-octatetraene from acetylene 34). in a careful study of this system, Schrauzer proposed a concerted mechanism in which the four a bonds of the cyclo-octatetraene are essentially formed simultaneously 35). He proposed an octahedral complex (54) with four acetylene hgands fitted to adjacent ligand coordination positions, spatially defining the incipient cyclooctatetraene. 

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