The Four Classes of Pericyclic Reactions

Cycloadditions are characterised by two components coming together to form two new a bonds, one at each end of both components, joining them together to form a ring, with a reduction in the length of the conjugated system of orbitals in each component (Fig. 6.1a). Cycloadditions like the Diels-Alder reaction are by far the most abundant, varied, featureful and useful of all pericyclic reactions. They are inherently reversible, and the reverse reaction is called a retro-cycloaddition or a cycloreversion. 

Cheletropic reactions are a special group of cycloadditions or cycloreversions in which the two a bonds are made or broken to the same atom. Thus sulfur dioxide adds to butadiene 6.1 to give an adduct 6.2, for which the sulfur has provided a lone pair to one of the a bonds and has received electrons in the formation of the other. As far as the sulfur dioxide is concerned, it is an oxidative addition, changing the sulfur from SIV to SVI. The reaction is readily reversible on heating, and so sulfur dioxide can be used to protect dienes, allowing the adduct 6.2 to be heated in the presence of dienophiles as a convenient (liquid) source of butadiene. 

Another quite different looking sigmatropic reaction is the Mislow rearrangement 6.11 6.12, which is invisible because it is thermodynamically unfavourable, but the ease with which it takes place explains why allyl sulfoxides, with a stereogenic centre at sulfur, racemise so much more easily than other sulfoxides.660 Here, one end of the C S bond moves from the sulfur (S- F) to the oxygen atom (0-2 ) and the other end moves from C-1 to C-3. This is therefore called a [2,3]-shift, the bond marked in bold moving two atoms at one end and three at the other. 

Group transfer reactions make up the fourth category they have few representatives, with ene reactions by far the most common. Stripped to their essence, ene reactions have the form 6.13 6.14, but in practice 

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