Nonlinear cheletropic reaction

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. 

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. 

The reverse reaction is a linear cheletropic extrusion of SO2 with conrotatory twisting of the terminal methylenes. With cyclic triene SO2 forms only the 1,4 adduct. In this case the antarafacial addition to the triene is geometrically impossible and the alternative non-linear n s + (O s process does not compete with the concerted 71 5 + (O s linear cheletropic addition to a diene component. So a nonlinear cheletropic addition containing eight electrons becomes highly unfavorable. This is illustrated by the fact that SO2 is eliminated 60,000 times more slowly (even at 180°) from (c) than from (b). In (b) + (ip s, retro process is allowed. 

Nonlinear (left) and linear (right) cheletropic reaction. 

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