Trienes cycloaddition with sulfur dioxide

Another anomalous cycloaddition is the insertion of a carbene into an alkene. 6-Electron cheletropic reactions (p. 28) are straightforward allowed pericyclic reactions, which we can now classify with the drawings 3.47 for the suprafacial addition of sulfur dioxide to the diene 2.179 and its reverse. Similarly, we can draw 3.48 for the antarafacial addition of sulfur dioxide to the triene 2.180 and its reverse. The new feature here is that one of the orbitals is a lone pair, which is given the letter co to distinguish it from o- and n-bonds, with suprafacial and antarafacial defined by the drawings 3.45 and 3.46, which apply to all sp3 hybrids and p orbitals, filled or unfilled. 

An anomalous cycloaddition is the insertion of a carbene into an alkene. Some cheletropic reactions are straightforwardly allowed pericyclic reactions, which we can illustrate with the drawing 6.127 for the suprafacial addition of sulfur dioxide to a diene, and with the drawing 6.128 for the 8-electron antarafacial addition of sulfur dioxide to a triene. The problem comes with the insertion of a carbene into a double bond, which is well known to be stereospecifically suprafacial on the alkene with singlet electrophilic carbenes [see (Section 4.6.2) page 149]. This is clearly a forbidden pericyclic reaction if it takes place in the sense 6.129. 

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See also in sourсe #XX -- [ , ]

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