Symmetry controlled reactions

As pericyclic reactions are largely unaffected by polar reagents, solvent changes, radical initiators, etc., the only means of influencing them is thermally or photochemically. It is a significant feature of pericyclic reactions that these two influences often effect markedly different results, either in terms of whether a reaction can be induced to proceed readily (or at all), or in terms of the stereochemical course that it then follows. Thus the Diels-Alder reaction (cf. above), an example of a cycloaddition process, can normally be induced thermally but not photochemically, while the cycloaddition of two molecules of alkene, e.g. (4) to form a cyclobutane (5), 

This type of reaction, whether it involves the cyclisation of a polyene, as here, or the ring-opening of a cyclic compound to form a polyene, is known as an electrocyclic reaction. 

Whether a particular reaction proceeds via a single step, concerted pathway, or in more than one step via a biradical or bipolar 

So far as AH+ is concerned, it would seem reasonable to suppose that the favoured pathway for a particular reaction would be that I one in which the greatest degree of residual bonding is maintained in the T.S. Maintenance of bonding implies maintenance of orbital overlap, and it is therefore necessary to establish the conditions that i ensure the maintenance of such overlap. To do this we have to ( consider a property of atomic and molecular orbitals not yet referred to, namely phase. 

We have already seen (p. 2) that the individual electrons of an atom can be symbolised by wave functions, and some physical analogy can be drawn between the behaviour of such a wave-like electron and the standing waves that can be generated in a string fastened at both ends—the electron in a (one-dimensional) box analogy. The first three possible modes of vibration will thus be (Fig. 12.1)  

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The thermolysis of cycloalkenes is often a more facile process than for the cycloalkenes. Cyclobutene undergoes thermolysis at 175 °C and yields butadiene in an orbital symmetry controlled reaction as shown by stereochemical studies of... 

Thermal extrusion of a sulfur atom is the most common thermal reaction of a thiepin. The mechanism of this thermal process involves two orbital symmetry controlled reactions (69CC1167). The initial concerted step involving a reversible disrotatory electrocyclic rearrangement is followed by a concerted cheleotropic elimination of sulfur (Scheme 29). Similar aromatization reactions occur with thiepin 1-oxides and thiepin 1,1-dioxides, accompanied by the extrusion of sulfur monoxide and sulfur dioxide respectively. Since only a summary of the major factors influencing the thermal stability of thiepins was given in Section... 

These considerations are borne out by results observed for 1,2- and 3,4-diphenyl derivatives in solution (Sect. 4.4). Schuster and coworkers detected the 1,2-diphenyl species 91 by optical spectroscopy at room temperature and found that it was thermally stable under these conditions [268]. In contrast, Miyashi and coworkers found that the 3,4-diphenyl derivative, under comparable conditions, suffered ring opening [266], Remarkably, this reaction occurs with the same stereochemistry as the orbital symmetry controlled reaction of the neutral precursor. 

The available experimental information does not warrant any definitive distinction between these four possibilities. However, some indirect evidence allows for some choices. Regardless of the 66 kcal/mol of strain embodied in the bicyclobutane ring, it has been shown to be a relatively stable system in the gas phase or in hydrocarbon solvents. Nevertheless, when bicyclobutane is heated at 200 C two of the peripheral C-C bonds are broken, while the central bond remains intact during the purely thermal, symmetry controlled reactions. This relative stability has been associated with the predominantly rr character of this... 

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