Electrocyclic reactions selection rules

Thermal and photochemical cycloaddition reactions always take place with opposite stereochemistry. As with electrocyclic reactions, we can categorize cycloadditions according to the total number of electron pairs (double bonds) involved in the rearrangement. Thus, a thermal Diels-Alder [4 + 2] reaction between a diene and a dienophile involves an odd number (three) of electron pairs and takes place by a suprafacial pathway. A thermal [2 + 2] reaction between two alkenes involves an even number (two) of electron pairs and must take place by an antarafacial pathway. For photochemical cyclizations, these selectivities are reversed. The general rules are given in Table 30.2. 

The well-known selection rules proposed by Woodward and Hoffman25 to predict the stereochemical course of electrocyclic reactions can be viewed as emphasizing the symmetry requirements for electronic coupling of final and initial states. The rules are expressed in terms of rotatory motions required to convert one electronic state into another, so the matrix element is really vibronic rather than pure electronic. In terms of this paper, it appears that Woodward and Hoffman have identified necessary rotation properties of the perturbation operator. 

The situation is reversed for the tt2s + n4s addition. Figure 11.16 illustrates this case now the bonding orbitals all transform directly to bonding orbitals of the product and there is no symmetry-imposed barrier. As with the electrocyclic processes, the correlation diagrams illustrate clearly the reason for the striking difference observed experimentally when the number of electrons is increased from four to six. The reader may verify that the 4s + 4s reaction will be forbidden. Each change of the total number of electrons by two reverses the selection rule. 

With this model, we need only apply the method already used to derive the selection rules for electrocyclic reactions (p. 53). From the Coulson equations, we can deduce that in the in conrotatory cyclization of pentadiene, the MO generates a destabilizing C5-C4 secondary interaction, a stabilizing and Fg a destabilizing interaction. The absolute values of these contributions rise steadily because the terminal coefficients increase from Fg to Fg. Therefore, the sign of their sum is given by the HOMO contribution. If R is an attractor, the HOMO is Fg and rotation inwards is favored. If R is a donor, the HOMO is 4T and rotation inwards is disfavored. As the Coulson equations are valid only for polyenes, these conclusions are correct insofar as R can be modeled by a carbon 2p orbital. It follows that the Rondan-Houk theory works better for conjugative than for saturated substituents. 

The selection rules for conrotatory electrocyclic reactions are the opposite of those just listed that is, for a molecule with an even number of electron pairs, conrotation is thermally allowed, and for a molecule with an odd number of electron pairs, conrotation is photochemically allowed. 

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. 

To apply the selection rules for electrocyclic reactions, count the number of tt electrons in the open-chain polyene. 

The selection rules can be applied to charged species as well as to neutral molecules. The only requirement is that the reaction be a concerted process involving electrons in overlapping p orbitals. For example, the conversion of a cyclopropyl cation to the allyl cation can be considered as a tt -electrocyclic process. For this process, the selection rules predict a disrotatory process. 

For sigmatropic reactions, as for electrocyclic reactions and cycloadditions, the course of reaction can be predicted by counting the number of electrons involved and applying the selection rules. A comprehensive rationalization of all the stereochemical aspects of these reactions requires application of the frontier orbital or orbital symmetry approaches, and, at this point, we will content ourselves with pointing out the salient features of the more common reactions of this class. 

Electrocyclic transformations, in fact, proceed with high stereospecificity dictated by the number of n electrons in the open-chain it system (XIX). The reaction course taken by an electrocyclic transformation follows that in which the highest occupied molecular orbital in XIX has maximum bonding character throughout the transformation. The symmetry of this orbital, therefore, dictates the course of transformation and is the basis of the Woodward-HoiFmann selection rules (i). Consider, for example, the interconversion of butadiene (XXI) and cyclobutene (XXII). 

We have seen that the stereochemistry of an electrocyclic reaction depends on the mode of ring closure, and the mode of ring closure depends on the number of conjugated 7T bonds in the reactant and on whether the reaction is carried out under thermal or photochemical conditions. What we have learned about electrocyclic reactions can be summarized by the selection rules listed in Table 29.1. These are also known as the Woodward-Hoffmann rules for electrocyclic reactions. 

The rules in Table 29.1 are for determining whether a given electrocyclic reaction is allowed by orbital symmetry. There are also selection mles to determine whether cycloaddition reactions (Table 29.3) and sigmatropic rearrangements (Table 29.4) are... 

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. ... 

We have developed a different set of selection rules for electrocyclic, sigma-tropic, cycloaddition, and other concerted reactions, but the fundamental principle—the conservation of orbital symmetry—is the same in all cases. Now we will see that all of these reactions can be considered to be variants of cycloaddition reactions. To do so, we must first note that a cr bond can participate in a cycloaddition process, just as can a n bond, with the following provisions ... 

Thus, we reach the same conclusions as described earlier by using the orbital correlation diagram method. For convenience, the selection rules by this approach to electrocyclic reactions are tabulated in Table 2.2. 

In reaction dynamics the PJT may be responsible for stereoselectivity, because of the selection rules for vibronic coupling matrix elements. Via these relaxation matrix elements the Wigner-Eckart theorem is at the basis of the Woodward-Hoffmann ruies [12]. We shall not discuss these rules in generai, but consider some simple illustrations, related to electrocyclic reactions. Take as a simple example the ring ciosure of cw-butadiene, as iiiustrated in Fig. 6.3. The relevant occupied orbitals are... 

Treatment for the Rate of Bimolecular, Gas Phase Reactions , if The symmetry rules allowing some reactions and forbidding others were first proposed by Robert B. Woodward and Roald Hoffmann in two letters to the editor Stereochemistry of Electrocyclic Reactions and Selection Rules for Sigmatropic Reactions , Journal of American Chemical Society, 87 (1965) 395, 2511 as well as by Kenichi Fukui and Hiroshi Fujimoto in an article published... 

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