Electrocyclization sigmatropic rearrangement

Classify the following reactions as electrocyclizations, sigmatropic rearrangements, cycloadditions, etc., and give the correct symbolism for the electrons involved in each concerted process. Some of the reactions proceed by two sequential processes. 

Keywords a,p-Unsaturated aldehydes, ( )-l-phenyl-2-(2,2,2-trifluoroethylidene), PTSA, toluene, room temperature, electrocyclic/sigmatropic rearrangement, 3-trifluoromethyl-l,4-dihydropyridazines... 

A pericyclic reaction is one that takes place in a single step through a cyclic transition state without intermediates. There are three major classes of peri-cyclic processes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. The stereochemistry of these reactions is controlled by the symmetry of the orbitals involved in bond reorganization. 

Keywords Diels-Alder reactions, dipolar cycloadditions, electrocyclic reactions, ene reactions, pericyclic reactions, sigmatropic rearrangements... 

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 421). Electrocyclic and sigmatropic rearrangements are treated at 18-27-18-35. Double-bond migrations have also been accomplished photochemically, and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts. In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the nwtal hydride addition-elimination mechanism ... 

Finally, it must be emphasised that where, in any of the electrocyclic reactions, cycloadditions or sigmatropic rearrangements considered... 

Orbital correlation diagrams are useful for cycloadditions and electrocyclic reactions but not for sigmatropic rearrangements since no element of symmetry is preserved. 

Rearrangement reactions (Miscellaneous) (see also Electrocyclic rearrangements, Sigmatropic rearrangements, Wittig rearrangement)... 

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