Pericyclic reactions selection rules

These selection rules are summarized in Table 30.4, thereby giving you the ability to predict the stereochemistry of literally thousands of pericyclic reactions. 

Having described by means of correlation diagrams the nature of forbidden and allowed reactions of two particular pericyclic types, we wish to develop a notation that will permit us to state a generalized selection rule summarizing the conclusions of the pericyclic theory. 

Since all possibilities have been enumerated, we have demonstrated that for any simply connected pericyclic reaction the two statements of the selection rules are equivalent and must necessarily make the same predictions. 

In this chapter we shall illustrate the application of the selection rules to particular examples of pericyclic reactions. The chemical literature provides a wealth of illustrative examples, many collected in review articles.1... 

The ene reaction has proved to be particularly powerful in synthesis when carried out intramolecularly. The usual increase in rate for an intramolecular reaction allows relatively unreactive partners to combine. Thus the diene 6.13 gives largely (14 1) the cis disubstituted cyclopentane 6.15 by way of a transition structure 6.14. It is important to recognize that the selective formation of the ci j-disubstituted cyclopentane has nothing to do with the rules for pericyclic reactions. It is a consequence of the lower energy when the trimethylene chain spans the two double bonds in such a way as to leave the hydrogen atoms on the same side of the folded bicyclic structure. This... 

Therefore, the coefficients at the terminal atoms rise steadily, reaching a maximum in the HOMO and the LUMO, and then decline. These properties will be useful for the derivation of the selection rules of pericyclic reactions. 

If X always contributes two electrons, its chemical nature should be unimportant. This is contradicted by experimental results. Whereas fragmentations of diazenes give good yields and are stereospecific,41 heating of nitrosopyrroline 33 gives, in addition to polymers, only traces of butadiene and N20.42 It can be then be expected that the validity of the selection rules is better for pericyclic than for cheletropic reactions. 

The so-called aromaticity rules are chosen for comparison, as they provide a beautiful correspondence with the symmetry-based Woodward-Hoffmann rules. A detailed analysis [92] showed the equivalence of the generalized Woodward-Hoffmann selection rules and the aromaticity-based selection rules for pericyclic reactions. Zimmermann [93] and Dewar [94] have made especially important contributions in this field. 

Each of these theoretical approaches leads to the same predictions regarding reaction conditions and stereochemistry. For a wide range of reactions, the selection rules can be used empirically, based on a simple method of electron counting, without regard to their theoretical basis. The selection rules for pericyclic reactions relate three features ... 

Note Reactions involving a cyclic transition state are not always concerted, and the selection rules and their stereochemical consequences apply only to concerted pericyclic processes. Indeed, failure to conform to the selection rules is usually taken as proof that a reaction does not proceed by a concerted mechanism. On the other hand, failure to react may simply mean that the reaction is symmetry-allowed, but does not occur because the thermodynamics is unfavorable. This point will be discussed further in the next section. 

The following sections present an empirical approach to applying the selection rules. The chapter continues with a basic introduction to the analysis of symmetry properties of orbitals and the application of orbital correlation diagrams to the relatively simply cyclobutene-butadiene interconversion it concludes with some examples of the frontier orbital approach to pericyclic reactions. 

Like other pericyclic reactions, electrocyclic reactions may be initiated either thermally or photochemically. The selection rules enable us to correlate the stereochemical relationship of the starting materials and products with the method of activation required for the reaction and the number of tt electrons in the reacting system. 

In a pericyclic reaction, the pathway predicted by the selection rules is the one that allows maximum orbital overlap along the reaction pathway, including the transition state. Maximum orbital overlap corresponds to the path of minimum energy and is achieved if the orbitals involved are similar in energy and if the symmetry of the orbitals is maintained throughout the reaction path. 

A concerted [1,3] thermal shift of hydrogen to convert 7-31 to 7-32 is not a reasonable mechanism because such shifts are ruled out by the selection rules for pericyclic reactions (see Chapter 6). 

The stereospecific ring-opening reactions of cyclopropyl derivatives have played a key role in establishing selection rules for pericyclic processes . Extensive and conclusive evidence has been presented in support of DePuy s initial postulation that disrotatory ringopening and C-X bond heterolysis are synchronous processes, and all kinetic , stereochemical and theoretical findings lend credence to the DePuy-Hoffmann-Wood ward rule Substituents on the same side of the 3-membered ring as... 

Sigmatropic shifts represent another important class of pericyclic reactions to which the Woodward-Hoffmann rules apply. The selection rules for these reactions are best discussed by means of the Dewar-Evans-Zimmerman rules. It is then easy to see that a suprafacial [1,3]-hydrogen shift is forbidden in the ground state but allowed in the excited state, since the transition state is isoelectronic with an antiaromatic 4N-HQckel system (with n = 1), in which the signs of the 4N AOs can be chosen such that all overlaps are positive. The antarafacial reaction, on the other hand, is thermally allowed, inasmuch as the transition state may be considered as a Mobius system with just one change in phase. 

Nevertheless, frontier orbital theory, for all that it works, does not explain why the barrier to forbidden reactions is so high. Perturbation theory uses the sum of all filled-with-filled and filled-with-unfilled interactions (Chapter 3), with the frontier orbitals making only one contribution to this sum. Frontier orbital interactions cannot explain why, whenever it has been measured, the transition structure for the forbidden pathway is as much as 40 kJ mol 1 or more above that for the allowed pathway. Frontier orbital theory is much better at dealing with small differences in reactivity. We shall return later in this chapter to frontier orbital theory to explain the much weaker elements of selectivity, like the effect of substituents on the rates and regioselectivity, and the endo rule, but we must look for something better to explain why pericyclic reactions conform to the Woodward-Hoffmann rules with such dedication. 

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. 

The selection rules that determine the outcome of electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements are summarized in Tables 29.1, 29.3, and 29.4, respectively. This is still a lot to remember. Fortunately, the selection rules for all pericyclic reactions can be summarized in one word TE-AC. ... 

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