Dienes, hybridization 3 + 2 -cycloaddition

Ene reactions tended to occur as alternative reaction pathways to [2 + 4] cycloaddition, especially when sterically bulky silenes had substituents on the sp2-hybridized carbon atom and dimethylbutadiene served as the diene component. In the ene reactions studied the silene acted as the enophile as often as it acted as the ene. 

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. 

The Diels-Alder reactivity of 1,3,4-oxadiazoles as dienes was investigated with AMI semiempirical and hybrid density-functional methods. The validity and usefulness of the inertia principle for the qualitative evaluation of reactivity in Diels-Alder reactions is presented. The reactivity of 1,1-dimenthene to Diels-Alder cycloaddition is poor as a result of the difficulty of the diene adopting a planar conformation. The Diels-Alder transition states of dienes having conjugating substituents at C(2) or C(3) have been investigated to determine the reason for the unexpected high diene reactivity. Differences in rates of Diels-Alder reactions have been used as experimental indicators of synchronous or asynchronous transition states when non-symmetrical diene 2-(trimethylsilyloxy)cyclohexa-l,3-diene reacts with symmetrical ethylenic dienophiles. ... 

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