Woodward-Hoffmann rules cheletropic reaction

In the present chapter, however, because the problem is considered from a retrosynthetic point of view, we will distinguish only between heterolytic and homolytic disconnections -to which we will refer to as "retro-annulations"- and concerted or "pericyclic (or cheletropic) cycloreversions". In the same way that Woodward-Hoffmann rules [2] apply to pericyclic reactions, the Baldwin rules [3] may be said to apply to heterolytic as well as to homolytic "monotopic" annulations (see Table 6.1). Although in the preceding Chapter (see 5.5) we have already described some radical "monotopic" annulations, later on in this Chapter (see 6.1.3) and mainly in Chapter 7 we will refer to some new methods, syntheses and strategies which have been developed recently. 

The application of the Woodward-Hoffmann rules to cheletropic reactions is not straightforward. In the [2+1] cycloaddition of singlet carbenes to alkenes, the stereochemistry of the alkene is preserved in the product, so the alkene must react suprafacially. The Woodward-Hoffmann rules suggest that the carbene component of this thermal, four-electron reaction must react antarafacially. However, what this means for a species lacking a 77 system is difficult to interpret. 

In the non-linear approach (b) the carbene orbitals are interacting in an antarafacial manner, and the polyene orbitals can interact either suprafacially or antarafacially as before. The disrotatory process this time relates to a Mobius system, and the conrotatory process to a Hiickel system. The nonlinear cheletropic reaction with conrotation will be preferred for the case (m +2) = 4n + 2), and with disrotation for the case (rn + 2)- 4n. Theffi conclusions are in full agreement with those obtained using the general Woodward-Hoffmann rule. 

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