Thermal cheletropic reactions, selection rules

The selection rules for the thermal cheletropic reactions are given in Table 5.1, where m is the number of electrons in the TT-system and n is an integer including zero. 

TABLE 5.1 Selection rules for thermal cheletropic reactions. 

Table Selection Rules for thermal cheletropic Reactions...

An important example of cheletropic reactions is the reversible insertion of a singlet carbene into a carbon—carbon double bond to give a cyclopropane derivative. Only singlet carbenes will be considered here, as the pericyclic selection rules cannot be applied to triplet states. Singlet carbenes add thermally to alkenes in a concerted manner, therefore, the geometry of the alkene is preserved in the product, i.e., the reaction is stereospecific. Hence, in the present context, the reaction is described as suprafacial on the olefin. Therefore, the reaction of -but-2-ene and Z-but-2-ene with singlet carbene gives trans- and di-l,2-dimethylcyclopropane, respectively (Scheme 5.1). 

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