Three-atom electrocyclizations

On the basis of state correlation diagrams we may occasionally make predictions at variance with those based on orbital correlation diagrams, especially where radical species are involved. Consider the three atom electrocyclic process (34) with reference to Table 3 and Fig. 9. The most... 

ELECTROCYCLIC REACTIONS OF IONIC SPECIES 2.5.1 Three-Atom Electrocyclizations... 

Pericyclic reactions are commonly divided into three classes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. An electrocyclic reaction forms a sigma bond between the end atoms of a series of conjugated pi bonds within a molecule. The 1,3-butadiene to cyclobutene conversion is an example, as is the similar reaction of 1,3,5-hexatriene to form 1,3-cyclohexadiene ... 

You may have trouble seeing that the last example of an electrocyclic reaction is a two-electron and not a four-electron reaction. The new a bond is formed between the termini of a three-atom -n system. That three-atom system contains two electrons. The lone pairs of O are not part of the three-atom tt system, so they are not included in the electron count of the electrocyclic reaction. 

Let us consider the reaction of the standard silylating agent B/Si with AN (314) containing at least three carbon atoms. For simplicity, all processes will be described as electrocyclic schemes, although many of them evidently involve nonconcerted reactions. 

In the dyad, because of the symmetry of the 3-phenylazetine ring in BC-53 (Scheme 29), insertion of sulfur between ring positions C-l and C-4 or CC-1 and C-2 leads to the same compound, 5-phenylthiazole (53). Similarly, insertion of a sulfur atom between N3 and C4 or N3 and C2 leads to 4-phenylisothiazole (55). Accordingly, because of this symmetry, only a dyad results. This symmetry is removed, however, in the case of 2-deuterio-5-phenylthiazole (53-2d). Thus, irradiation of 53-2d resulted in the formation of three isomeric products (Scheme 30), viz., 4-deuterio-5-phenylthiazole (53-4d), 5-deuterio-4-phenylisothiazole (55-5d), and 3-deuterio-4-phenylisothiazole (55-3d). D euterium 1 abelling h as t hus e xpanded t he d yad i nto a tetrad. These results are entirely consistent with the electrocyclic ring closure - heteroatom migration mechanism shown in Scheme 31 <94JA2292>. 

Chapter 6 (Photochromism of dihydroindolizines and related systems). The dihydro-, tetrahydro- and hexahydroindolizine compounds involving one, two or three nitrogen atoms make up another family of compounds that exhibits a generally thermoreversible photochromic system based on 1,5-electrocyclization reactions (Scheme 6). Photobleaching is sometimes superimposed on the thermal back reaction. 

The Alder ene reaction is like a Diels Alder reaction in which one Jt-bond in the diene has been replaced by a C-H bond 121. It does not therefore form a ring and does not fit easily into any of the three classes of pericyclic reaction (cycloaddition, electrocyclic, and sigmatropic). Since a hydrogen atom is transferred from one component to the other it is best described as a group transfer reaction.21 The regioselectivity is determined by the interaction 123 with the Jt-bond of the ene (the HOMO) with the LUMO of the enophile. ... 

The Rules for the stereospecificity apply only to pericyclic reactions which are concerted. The Rules apply neither to non-concerted reactions nor to those that are not pericyclic. Pericyclic reactions are those which involve a monocyclic transition state having a conjugated array of interacting orbitals, one per atom. Three types of pericyclic processes have been recognized electrocyclic reactions, cycloadditions, and sigmatropic shifts. 

The equilibration of 3,7,7-trimethylcycloheptatriene at 300 (gas phase) with the 1,7,7- and 2,7,7-isomers occurs by way of an interesting sequence of electrocyclic changes and [1,5] carbon shifts, (Equation 6.97). This type of circulatory carbon atom shift in the three-membered ring also has been observed for other 7,7-disubstituted cyclohepatrienes. 

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