Sigmatropic reaction selection rules

Let us see the migration of a hydrogen atom and how the selection rules have been put forward to explain the formation of an imaginary transition state in a sigmatropic reaction. 

The symmetry and the corresponding selection rules of sigmatropic reactions have been studied.113... 

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. 

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. 

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. ... 

As is the case for other pericyclic reactions, the selection rules for a thermal [i, ] sigmatropic reaction are reversed for the photochemical reaction. If irradiation of a 1,5-hexadiene produces the electronically excited state of one and only one of the two allyl components, then the HOMO of one component is (/f3, and the HOMO of ihe other component is suprafacial-suprafacial reaction (Figure 11.46) is forbidden (as is the antarafacial-antar-afacial pathway), but the antarafacial-suprafacial and suprafacial-antarafacial pathways are allowed (Figure 11.47). Analysis of higher sigmatropic reactions shows that the selection rules also reverse with the addition of a carbon-carbon double bond to either of the n systems. Thus, the [3,5] sigmatropic reaction is thermally allowed to be suprafacial-antarafacial or antarafacial-suprafacial and photochemically allowed to be suprafacial- suprafacial or antarafacial-antarafacial. Two of these reaction modes are illustrated in Figure 11.48. 

Because the symmetries of linear polyene systems alternate in a regular way as each additional carbon-carbon double bond is added to either of the fragments in the transition structures, the selection rules for sigmatropic reactions may be generalized as follows ... 

The selection rules for sigmatropic reactions are powerful predictors of thermal rearrangements. They explain how some reactions can occur ster-eospecifically and with surprisingly low activation energies, while others do not. For example, cfs-l,2-divinylcyclopropane (44) undergoes a [3,3] sigmatropic rearrangement to cis,cis-l,4-cycloheptatriene (45) with a half-life of... 

A similar analysis of such systems has led to the formulation of selection rules that state that if a sigmatropic reaction of the order [i,j] (for hydrogen migration i = 1) has i + j = 4n + 2, then thermal reaction is suprafacial and photochemical reaction will be antarafacial. However, for those cases in which i + j = 4n, the rales are reversed and the thermal reactions are antarafacial while the photochemical reaction will be suprafacial. The selection rules for the sigmatropic shift of hydrogen by FMO method are given in Table 3.1. 

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... 

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. 

Woodward, R. B., Hoffmann, R. (1965). Selection rules for sigmatropic reactions. Journal of the American Chemical Society, 87, 2511. 

The selection rules for sigmatropic reactions of neutral molecules are summarized in Table 5.4. 

Electrocyclic and Sigmatropic reactions can be considered in terms of cycloadditions. It has been shown that it is possible if we consider sigma bond as a component of cycloaddition. Selection rules for cycloadditions can be applied to electrocyclic as well as sigmatropic reactions. Involvement of obond in these reactions is possible in following manner ... 

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