Pericyclic reactions sigmatropic shifts

With these descriptors in hand, we can look at the generalized orbital symmetry rule. There is a definite binary nature to the theory of pericyclic reactions. For cycloadditions, [2-f2] is forbidden (all suprafacial), whereas [4-F2] is allowed (all suprafacial). Continuing with the series, [6+2] is forbidden, and [8+2] is allowed. We will also encounter patterns in the other kinds of pericyclic reactions presented electrocyclic reactions, sigmatropic shifts, etc. Based on patterns such as these. Woodward and Hoffmann proposed the following rule for all pericyclic reactions ... 

Hiickel-type systems (such as Hilcfcel pericyclic reactions and suprafacial sigmatropic shifts) obey the same rules as for sigma electron. The rationale for this observation is clear If the overlap between adjacent p-electron orbitals is positive along the reaction coordinate, only the peraiutational mechanism can... 

The ubiquitous and reversible formation of radical cations in photoelectrochemical transformations allows pericyclic reactions to take place upon photocatalytic activation since the barriers for pericyclic reactions are often lower in the singly oxidized product than in the neutral precursor. For example, ring openings on irradiated CdS suspensions are known in strained saturated hydrocarbons [176], and formal [2 -I- 2] cycloadditions have been described for phenyl vinyl ether [ 177] and A-vinyl carbazole [178]. The cyclization of nonconjugated dienes, such as norbomadiene, have also been reported [179]. A recent example involves a 1,3-sigmatropic shift [180]. 

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. 

Two pericyclic reactions a sigmatropic shift and a cycloaddition in one reaction scheme. Suggested solution... 

Abstract The integration of conservation of orbital symmetry and the orbital overlap effect serves as a powerful tool to reliably predict the stereochemical course of pericyclic reactions as exemplified in this chapter. The orbital overlap factor has been discussed with a variety of examples such as the thermal fragmentations of cyclopropanated and cyclobutanated r .v-3,6-dimethyl-3,6-dihydropyridazine, and [1,5] sigmatropic shifts in c/.v-2-alkenyl-1 -alkylcyclopropanes and civ-2-alkenyl-1 -alkylcyclobutanes. 

Interest in these reactions has continued until today because the course and the stereochemistry of many chemical reactions could now be explained and predicted by simple theoretical concepts based on the properties and interactions of the molecular orbitals involved. Pericyclic reactions have found wide application in synthesis, including that of natural products5. Pericyclic reactions involving sigmatropic hydrogen shifts are also observed in biological systems in nature the photochemical and thermal interconversions of compounds related to the vitamin D complex constitute important examples6-8. 

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