Orbital symmetry analysis

The prediction on the basis of orbital symmetry analysis that cyclization of eight-n-electron systems will be connotatoiy has been confirmed by study of isomeric 2,4,6,8-decatetraenes. Electrocyclic reaction occurs near room temperature and establishes an equilibrium that favors the cyclooctatriene product. At slightly more elevated temperatures, the hexatriene system undergoes a subsequent disrotatory cyclization, establishing equilibrium with the corresponding bicyclo[4.2.0]octa-2,4-diene ... 

Cheletropic processes are defined as reactions in which two bonds are broken at a single atom. Concerted cheletropic reactions are subject to orbital symmetry analysis in the same way as cycloadditions and sigmatropic processes. In the elimination processes of interest here, the atom X is normally bound to other atoms in such a way that elimination gives rise to a stable molecule. In particular, elimination of S02, N2, or CO from five-membered 3,4-unsaturated rings can be a facile process. 

In some cases, steric interactions can prevent unimolecular reactions. Tetrahe-drane (18) has been the subject of a number of studies, and the conclusion is that, if formed, it would rapidly decompose to form two molecules of acetylene. However, tetra-tert-butyltetrahedrane (19) is a quite stable substance, and on heating rearranges to tetra-tert-butylcyclobutadiene. An orbital symmetry " analysis of the cleavage of tetrahedrane to acetylene indicates that it involves a torsional motion that in the case of the tert-butyl substituted derivative would bring the tert-butyl groups very close to each other. As a result, this mode of reaction is not possible, and the compound is relatively stable. 

An orbital symmetry analysis showed that a transition state for nucleophilic addition formed from a (4n + 1) cation-radical, e.g., 18, and a halide ion would involve orbital interactions energetically unfavorable in symmetry terms and thus prejudice such a transition state in favor of electron transfer to which similar restrictions do not apply. On the other hand, 19 is isoelec-tronic with the anthracene anion-radical and is thus a (4n + 3) species. The transition state for addition of halide becomes energetically favorable in symmetry terms in this instance. 

While the orbital symmetry analysis of the arene-alkene cycloadditions addresses their allowedness , it does not specify a sequence for bond formation. It was recognized, however, that these cycloadditions could proceed stepwise and that in the specific case of the meta cycloaddition, three temporally distinct pathways are possible (Scheme 2) (a) a fully concerted path (A) where all bonds are formed simulta-... 

Orbital symmetry analysis of the 1,5-sigmatropic shift of hydrogen leads to the opposite conclusion. The relevant frontier orbitals in this case are the hydrogen Is and 1/ 3 of the pentadienyl radical. The suprafacial mode is allowed, whereas the antarafacial mode is forbidden. The suprafacial shift corresponds to a geometrically favorable six-membered ring. 

Although orbital symmetry provides a starting point for analyses of photochemical reactions of conjugated dienes and polyenes, experimental studies have identified a number of additional facets of the problem, some of which have to do with the fundamental assumptions of the orbital symmetry analysis. One of the underlying... 

Diels-Alder reaction, 382, 392—393 of benzyne, 931-932 orbital symmetry analysis of, 388—390 Dienes. See also Alkadienes... 

Orbital symmetry analysis of the hexatriene-cyclohexadiene system leads to the conclusion that the disrotatory process will be favored. This basis set orbitals for the conrotatory and disrotatory transition states are shown below ... 

The question posed in the preceding paragraphs as to the need for a reevaluation of the concept of allowedness , can be dismissed as a non-problem as long as it is taken as axiomatic that, for an orbital symmetry analysis to be of any use, the symmetry elements [retained along the pathway] must bisect bonds made or broken in the process . In contrast to the allowed conrotatory cyclization of butadiene to cyclobutene, in which the C2 axis bisects a newly formed cr bond, the only bond bisected by the axis in its conversion to bicyclobutane is the one between C2 and C3, which is essentially single in both the reactant and the product. 

An orbital symmetry analysis can be carried out reliably and unambiguously only in the global symmetry of the reacting system, i.e. a symmetry point group common to the reactant and product. 

Most organic molecules are not highly symmetrical. Does the proposed restriction of orbital symmetry analysis to the global synunetry of the reactant and product exclude their reactions from consideration, or can the approach be extended so as to take them into account ... 

Considerations of this kind, that were not emphasized in connection with the unimolecular reactions dealt with in the preceding chapter, attain crucial importance when the geometric requirements of cycloadditions and cycloreversions are compared. Like the isomerizations previously discussed, cycloreversions are unimolecular a non-totally symmetric vibrational motion that may be called for by the correspondence diagram will ordinarily be opposed by a restoring force. Cycloadditions, at least the prototypical ones, are bimolecular the two reactants can approach each other in a variety of ways, their reorientation in space costing no energy at all. It then becomes reasonable to ask how the conclusions which may be reached by the orbital symmetry analysis depend on the initial geometry assumed for the approach of the reactants towards one another. 

The generally contrathermodynamic stereochemistry, as well as the disparate substituent, solvent and isotope effects, are consistent with the zwitterionic mechanism illustrated in Fig. 6.8 [56], in which the orbital symmetry analysis plays a small but essential role. It is assumed that substitutional desymmetriza-tion is insufficient to destroy the essential symmetry of the tt orbitals, which sets the reactants on a path in which the four interacting C atoms are initially coplanar, but are induced by orbital symmetry conservation to bond along the diagonal. The preferred direction of approach and the stereochemical consequences then follow directly. 

If the reactants and product are set up in C, all of the occupied MOs would correlate across the diagram. Alternatively, a correspondence diagram, in which the reactants are set up in C2V and the product is anasymmetrized to that symmetry point group, would show that formal desymmetrization of the pathway to C, - i.e. to the true molecular symmetry of the product -is called for. The methylene bridge of cyclopentadiene is innocuous so, as it turns out, is the bridging carbonyl group in tropone. It will become evident from subsequent examples, however, that the presence of heteroatoms and/or multiple bonds can make a substantial difference to the conclusions drawn from an orbital symmetry analysis. 

An interestng attempt was made by Ramsey [20] to determine the nature of the reactive excited state of a cyclic trisilane by means of an orbital symmetry analysis of its photofragmentation. The question addressed was whether the relevant excitation might be to a low-lying 3d orbital rather than to an antibonding valence orbital, as is usually assumed. 

It can be seen that state correlations provide, in a sense, a justification for the simpler orbital symmetry analysis. Clearly, the origin of the avoided crossing seen in the state correlation of the [2+2] cycloaddition can be traced back to the original orbital correlation diagram. It will generally be true that the predictions from an orbital correlation will carry over to the state correlation diagram. Thus, it is rare that practicing chemists construct a state correla-... 

A. Orbital symmetry analysis of the butadiene-cyclobutene interconversion. B. The lines connecting the reactant and product orbitals for the conrotatory process are given in black, while the correlation for the disrotatory process is given in dashed color. 

By now you should anticipate that exactly the opposite conclusions are reached for the hexatriene-cyclohexadiene interconversion. Indeed, a full orbital symmetry analysis, given as an Exercise at the end of the chapter, leads to the conclusion that the disrotatory process is orbital symmetry allowed, whereas the conrotatory process is forbidden. Furthermore, a state correlation analysis constructed along the lines of Figures 15.4 and 15.5 supports the conclusions of the orbital symmetry analysis. Again, we leave this as an Exercise at the end of the chapter. 

Electrocyclic reactions are not really different from cycloadditions. Figure 20.27 compares the equilibration of 1,3-butadiene and cyclobutene with the 2 + 2 dimerization of a pair of ethylenes. The only difference is the extra o bond in butadiene, and this bond is surely not one of the important ones in the reaction—it seems to be just going along for the ride. Why should its presence or absence change the level of detail av able to us through an orbital symmetry analysis It shouldn t, and in fact, it doesn t. 

FIGURE 20.52 An orbital symmetry analysis shows that the thermal Cope rearrangement is allowed. 

A firm understanding of concerted cycloaddition reactions developed as a result of the formulation of the mechanism within the framework of molecular orbital theory. Consideration of the molecular orbitals of reactants and products revealed that in some cases a smooth transformation of the orbitals of the reactants to those of products is possible. In other cases, reactions that appear feasible if no consideration is given to the symmetry and spatial orientation of the orbitals are found to require high-energy transition states when the orbitals are considered in detail. (Review Section 11.3 of Part A for a discussion of the orbital symmetry analysis of cycloaddition reactions.) These considerations permit description of various types of cycloaddition reactions as allowed or forbidden and permit conclusions as to whether specific reactions are likely to be energetically feasible. In this chapter, the synthetic applications of cycloaddition reactions will be emphasized. The same orbital symmetry relationships that are informative as to the feasibility of a reaction are often predictive of the regiochemistry and stereochemistry of the process. This predictability is an important feature for synthetic purposes. Another attractive feature of cycloaddition reactions is the fact that two new bonds are formed in a single reaction. This can enhance the efficiency of a synthetic process. 

A few years later, in 1963, Cotton and coworkers determined the structure of CsReCLj/ which was seen to correspond to a triangular cluster (Fig. 3) [12]. They noted very short Re-Re distances (2.47 A) and carried out a molecular orbital symmetry analysis and concluded that There are just six bonding orbitals which are filled by the 12 electrons, thus accounting for the experimentally observed diamagnetism. If only implicitly, they were stating that there were Re-Re double bonds in the anionic cluster. 

One additional feature that distinguishes the frontier orbital method from orbital symmetry analysis is its ability to separately analyse triplet state processes (Fukui, 1971) in photochemical reactions. However, it is notably difficult in excited state reactions, even if they are stereo ecific, to be sure of concertedness. Certain stereo ecific photochemical cyclo-addition reactions, hitherto thought to be concerted, have recently been re-interpreted as radical cage recombination reactions. The triplet states of many imsaturated moleculesf are lower in energy than the corresponding singlet states because... 

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