Selection rule sigmatropic

Scheme 11.2. Generalized Selection Rules for Sigmatropic Processes...

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. 

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 6.1 Selection rules for sigmatropic hydrogen migration...

Table 6.3 Selection rules for sigmatropic change by FMO method...

Besides the selection rules, which are based on the conservation of orbital symmetry, for sigmatropic rearrangements of order [ij] it is possible to demonstrate that the following correlations will always apply ... 

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

The selection rules for sigmatropic rearrangements were deduced in this manner (Woodward R. B., Hoffmann R., /. Am. Chem. Soc., 1965, 87, 2511). 

The selection rules for sigmatropic rearrangements were derived for open-chain systems. A priori, it is not obvious that they also apply to cyclic systems. Using the PMO method, show that the following sigmatropic migrations21 are allowed ... 

The Woodward-Hoffmann selection rules for sigmatropic rearrangements are summarized in Table 8.2. 

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. 

TABLE 6.3 Selection Rules for Sigmatropic Hydrogen Shifts... 

Termmology of Cycloadditions Selection Rules for Cycloadditions Secondary Interactions Cycloadditions of Charged Species Sigmatropic Rearrangements Termmology... 

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. 

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