Selection rules cheletropic reactions

Cheletropic reactions are cyclizations - or the reverse fragmentations - of conjugated systems in which the two newly made o bonds terminate on the same atom. However, a cheletropic reaction is neither a cycloaddition nor a cycloreversion. The reason is that the chelating atom uses two AOs whereas in cycloadditions, each atom uses one and only one AO. Therefore, Dewar-Zimmerman rules cannot apply to cheletropic reactions. Selection rules must be derived using either FO theory or correlation diagrams 38 The conjugated fragment39 of 4n + 2 electron systems reacts in a disrotarory (conrotarory) mode in linear (nonlinear) reactions. In 4n electron systems, it reacts in a disrotarory (conrotarory) mode in nonlinear (linear) reactions. 

This is no longer true for cheletropic reactions. When applying the selection rules, one must always consider that the chelating fragment X contributes two electrons. Erroneous conclusions can be made otherwise. Thus, in the cyclopropanation reaction, which should be considered a four-electron cheletropic reaction, only one n bond is broken. In the formation of diazene 32, a six-electron cheletropic reaction, butadiene uses two double bonds and N2 one lone pair and one n bond the two components thus employ a total of eight electrons ... 

If X always contributes two electrons, its chemical nature should be unimportant. This is contradicted by experimental results. Whereas fragmentations of diazenes give good yields and are stereospecific,41 heating of nitrosopyrroline 33 gives, in addition to polymers, only traces of butadiene and N20.42 It can be then be expected that the validity of the selection rules is better for pericyclic than for 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). 

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. 

The most important cheletropic reaction is the addition of singlet carbenes to olefins to make cyclopropanes. Only singlet carbenes will be considered here the pericyclic selection rules cannot be applied to triplet states. The electronic structure of a singlet carbene involves an empty p orbital and a roughly sp hybrid that has two electrons (see, for example, the two lone pair orbitals of the water molecule in Appendix 3). We know from Chapter 10 that singlet carbenes add stereospecifically to olefins, and that the olefin stereochemistry is retained in the cyclopropane product. As such, in the present context, the reaction would be described as suprafacial on the olefin. 

Table Selection Rules for thermal cheletropic Reactions...

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