Selection rule 1, 5-sigmatropic shift

So it is the number of electrons and not the number of atoms which determines the selection rule. Therefore, the selection rules for hydrogen migration in thermal sigmatropic shifts can be summarized as follows as given in the table ... 

TABLE 6.3 Selection Rules for Sigmatropic Hydrogen Shifts... 

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. 

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. 

TABLE 3.1 Selection rules for the sigmatropic shift of hydrogen by FMO method. 

The selection rules for the sigmatropic shift of carbon by FMO method are given in the Table 3.3, where n is zero or an integer. 

TABLE 3.4 Selection rules for the sigmatropic alkyl shifts by PMO method. 

Experimentally found selection rules for the borotropic migrations correspond to the sigmatropic shifts with inversion on the migrating atom according to the orbital symmetry principles [43]. This in turn corresponds to the electronic configuration of the boron atom ideal for the formation of a Moebius type transition state. 

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. 

TABLE 5.4 Selection Rules for Allowed Sigmatropic Shifts... 

Fig. 6.6. Thermal [1, 3]-sigmatropic shift with inversion at migratory centre. Selection Rule for Sigmatropic Rearrangement (FMO Method) ...

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