Pericyclic reactions suprafacial process

Predict the product of the following pericyclic reaction. Is this [5,5) shift a suprafacial or an antarafacial process ... 

This chapter examines reactions that involve molecular rearrangements and cycloadditions. The use of these terms will not be restricted to concerted, pericyclic reactions, however. Often, stepwise processes that involve a net transformation equivalent to a pericyclic reaction are catalyzed by transition metals. The incorporation of chiral ligands into these metal catalysts introduces the possibility of asymmetric induction by inter-ligand chirality transfer. The chapter is divided into two main parts (rearrangements and cycloadditions), and subdivided by the standard classifications for pericyclic reactions e.g., [1,3], [2,3], [4-1-2], etc.). The latter classification is for convenience only, and does not imply adherence to the pericyclic selection rules. Indeed, the first reaction to be described is a net [1,3]-suprafacial hydrogen shift, which is symmetry forbidden if concerted. 

We may further extend the analysis of pericyclic reactions by considering that a single p orbital, denoted by the symbol m, can be a participant in a pericyclic reaction. In this analysis, one lobe of the p orbital makes up the top face of a one-atom n system, while the other lobe makes up the bottom face. The participation of a single p orbital is suprafacial if both cycloaddition processes involve only one of the two lobes of the p orbital, and it is antarafacial if the cycloaddition involves both. We may thus predict that the conrotatory opening of the cyclopropyl anion to an allyl anion (Figure 11.72) should take place via an -F 2 ] pathway. Conversely, the opening of the cation would be a -F 2 ] process, giving the opposite stereochemistry in the product." ... 

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. 

Transformation of a carbon-carbon triple bond to a carbon-carbon double bond (equation 1) can proceed through a variety of mechanisms. If the reaction takes place in one step (equation 2), the reaction is called pericyclic and, because in the product the atoms or groups X and Y are always on the same side of the double bond, the addition is a suprafacial one. In principle, it is also possible that X and Y are donated by different molecules (equation 3). Transition state a leads to a suprafacial addition product and transition state b to an antarafacial one. These termolecular processes would require the simultaneous encounter of three reacting species, which is not likely to happen. In most cases the addition... 

Similar to the previous pericyclic processes, the sigmatropic reactions can also proceed by two alternative mechanisms. The first of them, where the migrating atom or group remains on the same side of the nodal plane of the n system during the whole process, is called suprafacial, whereas for the other, antarafacial mechanism, the migration of the group from one side of the nodal plane to the other is typical. 

Finally, the last entry in Table 6.5 is, like the first, another example of a cheletropic reaction. This time, in contrast to the first, a diene (rather than a monoene) and sulfur (IV) dioxide (rather than the more reactive carbene) are undergoing an orbital symmetry allowed (A + t[2s) pericyclic process. This allowed reaction occurs with retention of symmetry since (although not shown in Table 6.5) the geometry of the substituted diene is retained in the product. Thus, as shown in Equation 6.57, the diene, (2E,4E)-hexadiene (c Figure 4.42), enters the reaction in a suprafacial fashion and, as expected for a disrotatory process (Chapter 4), produces the d5 -2,5-dihydrothiophene-l,l-dioxide ... 

Occasionally, though, you will run across a more exotic pericyclic process, and will want to decide if it is allowed. In a complex case, a reaction that is not a simple electrocyclic ringopening or cycloaddition, often the basic orbital symmetry rules or FMO analyses are not easily applied. In contrast, aromatic transition state theory and the generalized orbital symmetry rule are easy to apply to any reaction. With aromatic transition state theory, we simply draw the cyclic array of orbitals, establish whether we have a Mobius or Hiickel topology, and then count electrons. Also, the generalized orbital symmetry rule is easy to apply. We simply break the reaction into two or more components and analyze the number of electrons and the ability of the components to react in a suprafacial or antarafacial manner. 

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. 

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