Pericyclic processes, thermally allowed

Within the isolobal formalism, the conversion of 47 to 48 is a symmetry-allowed process, if it were to proceed as a concerted reaction (50). Structure 47 represents a transoid-2-meta.Wa-1,3-butadiene. In the bonding description, complex 48 represents formally a 1-metalla-bicyclo[1.1.0]butane. Therefore, the conversion of 47 to 48 represents a thermally allowed, concerted [ 2a + 2S] ring closure, in analogy to the pericyclic ring opening of bicyclo[1.1.0]butanes to give trans,trans-, 3-butadienes. 

The closed and open forms, 4 and 5, respectively, represent the formal starting and end points of an electrocyclic reaction. In terms of this pericyclic reaction, the transition state 6 can be analysed with respect to its configurational and electronic properties as either a stabilized or destabilized Huckel or Mobius transition state. Where 4 and 5 are linked by a thermally allowed disrotatory process, then 6 will have a Hiickel-type configuration. Where the process involves (4q + 2) electrons, the electrocyclic reaction is thermally allowed and 6 can be considered to be homoaromatic. In those instances where the 4/5 interconversion is a 4q process, then 6 is formally an homoantiaromatic molecule or ion. 

The dipolar intermediate 35 is pivotal in further reactions. The formation of 36 requires a 1,2-hydrogen shift and formally at least such a shift would be a thermally forbidden pericyclic reaction. An alternative to the forbidden process would involve a thermally allowed 1,5-shift to give, for example, 37... 

Because organoboranes and organoalanes form relatively stable intermediate complexes with various substrates as a prelude to final product formation, it seems permissible to try to extend the scope of the Woodward-Hoffmann principle to the reorganization pathways of such complexes. Thus, it would be useful, for example, to consider whether the chemical behavior of an allylic aluminum system complexed with a ketone (3) might resemble the thermally allowed [3, 3] sigmatropic rearrangement (4). The value of viewing the collapse of such complexes as potential pericyclic processes will become evident in Section IV,C, where the interplay of kinetic versus thermodynamic control on ketone insertions into carbon-metal bonds is discussed. 

According to DFT calculations, the mechanism of this reaction is stepwise [73] and consists of a tandem [2+2] cycloaddition-cycloreversion sequence (Scheme 2.2). Both processes take place via supra-supra mechanisms that are thermally allowed because of the interplay between tt fx and lone pair localized orbitals. The corresponding transition structures TSl and TS2 are nonaromatic as indicated by the low negative values of the nucleus-independent chemical shifts (NICS), thus discarding true pericyclic processes [74,75]. In one special case, a highly hindered 1,3,2-/l -oxaphosphazetidine INT (Scheme 2.2) reaction intermediate was charac-... 

We noted in Chapter 15 that, for the most part, the orbital symmetry rules are not directly applicable to photochemistry. However, some photochemical reactions of simple tt systems do give products that are consistent with expectations based on orbital symmetry, although this does not prove that these are concerted, pericyclic processes, The photochemical selection rules for pericyclic reactions are opposite of those for thermal pericyclic reactions. For example, there are many examples of [1,3] and [1,7] sigmatropic shifts that appear to go by the photochemically "allowed" suprafacial-suprafacial pathway Eqs. 16.22 and 16.23 show two (recall that the thermal reactions would be suprafacial-antarafacial). These reactions occur upon direct irradation, while sensitized photolysis produces products more consistent with biradical-type reactions. 

While photocycloadditions are typically not concerted, pericyclic processes, our analysis of the thermal [2+2] reaction from Chapter 15 is instructive. Recall that suprafacial-suprafacial [2+2] cycloaddition reactions are thermally forbidden. Such reactions typically lead to an avoided crossing in the state correlation diagram, and that presents a perfect situation for funnel formation. This can be seen in Figure 16.17, where a portion of Figure 15.4 is reproduced using the symmetry and state definitions explained in detail in Section 15.2.2. The barrier to the thermal process is substantial, but the first excited state has a surface that comes close to the thermal barrier. At this point a funnel will form allowing the photochemical process to proceed. It is for this reason that reactions that are thermally forbidden are often efficient photochemical processes. It is debatable, however, whether to consider the [2+2] photochemical reactions orbital symmetry "allowed". Rather, the thermal forbiddenness tends to produce energy surface features that are conducive to efficient photochemical processes. As we will see below, even systems that could react via a photochemically "allowed" concerted pathway, often choose a stepwise mechanism instead. 

In each of the two pericyclic processes so far considered, the course of the reaction was unambiguous. An interesting distinction arises in cases where there is a choice between two stereochemistries for a given pericyclic reaction, one corresponding to a thermally allowed process with a transition state which is aromatic in the ground state, the other to a photochemically allowed process with a transition state which is antiaromatic in the ground... 

The first insight on the mechanism of pericyclic reactions was provided by Woodward and Hoffmann in their famous monograph The Conservation of Orbital Symmetry. The basic idea is that reactions occur readily (are thermally allowed) when there is congruence between the orbital symmetry characteristics of the reactants and the products, while they occur with difficulty (are thermally forbidden) when that congruence does not pertain. In short, orbital symmetry is maintained in concerted reactions. This has been proved by Pearson by means of perturbation theory. While the Woodward and Hoffmann rules determine which reactions are allowed and which are forbidden, they do not establish what the real mechanism of the process is. Although this initial view of pericyclic reactions has been very much debated, it forms the basis of the important progress made in the understanding of these reactions. 

The cis relationship between methyl and ring junction methine hydrogen atom in 7 indicates a disrotatory process. Spirotricyclic product 8 was also isolated in 17% yield from this reaction, presumably as a consequence of a cascade of two pericyclic reactions. A thermally allowed [1,7]-hydrogen shift leads from 6 to triene 9 that undergoes 6ti-disrotatory ring closure to 8. In both photochemical as well as thermal electrocyclizations, there may be multiple... 

For the various reactions in Fig. 4.3 only the 0 2jand 4 interactions require summation. The total of such terms must be odd for a thermal pericyclic reaction and even for a photochemical process. Hence, all of the reactions shown are thermally allowed processes excepr the [,2 + 2J cyclo-addition which is an allowed photochemical reaction. 

The fleeting cis, trans azonine rotamers (86) and (87) were shown through cycloaddition (see following section) to intermediate the thermal isomerization of (88) to (89) (73JOC1959) and were conjectured to cyclize, respectively, to these [5.2.0] frames via symmetry-allowed 4a processes. Further, the thermal isomerization of the urethane analog of (88) and low-temperature irradiation of cyclooctatetraene epoxide (85a) to frans-fused [4.3.0] frames (91a) and (91b) respectively were hypothesized to arise by internal 6s pericyclization of a fleeting mono-frans (f,c,c,c) heteronin frame (90) (75PAC(44)69l). 

The photochemistry of alkenes, dienes, and conjugated polyenes in relation to orbital symmetry relationships has been the subject of extensive experimental and theoretical studyThe analysis of concerted pericyclic reactions by the principles of orbital symmetry leads to a complementary relationship between photochemical and thermal reactions. A process that is forbidden thermally is allowed photochemically and vice versa. The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the reaction types discussed in Chapter 10 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2Tr- -2Tr] cycloaddition of two alkenes, which was classified as a forbidden thermal reaction (see Section 10.1), can serve as an example. The correlation diagram (Figure 12.17) shows that the ground state molecules would lead to a doubly excited state of cyclobutane, and would therefore involve a prohibitive thermal activation energy. 

An oft-cited dichotomy is that if a reaction is thermally forbidden, it is photochemically allowed and vice versa. In fact, photochemical [2 + 2] cycloadditions are well known (see Chapter 16), and other examples of thermally forbidden processes that proceed photochemically can be found. A justification for this binary aspect of pericyclic reactions can be gleaned from the orbital or state correlation diagrams. Figure 15.4 shows a direct correlation between the first excited states of reactants, produced by photolysis, and products for the... 

As such, we will not consider photochemical processes in this chapter, deferring such topics to Chapter 16, which is devoted entirely to photochemistry. When we make tables to present rules for various types of reactions, describing them as allowed or forbidden, we will only be addressing thermal conversions. The photochemical part of such tables has always been redundant you just reverse the thermal predictions. However, on a more basic level we feel that predictions about photochemical reactions based on the level of analysis presented in this chapter are risky and fail to take into account the many subtleties of photochemistry. If you want to consider a photochemical pericyclic reaction, it is best to consider it in the context of the entire field of photochemistry, rather than as the opposite of a thermal process. 

Vitamin D2 is produced by two pericyclic reactions. One of them is photochemicaUy initiated the second thermally initiated. The first step is a photochemical electrocyclic reaction in which a cyclohexadiene of the B ring is isomerized to a triene. The reaction involves six k electrons and is the reverse of the photochemical cyclization reaction discussed in Section 28.4. Thus, by the principle of microscopic reversibility, this photochemicaUy allowed ring opening involving a 4 +2 71 system must occur by a conrotatory process. 

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