Pressure-induced cycloadditions of dicyanoacetylene to strained arenes: The formation of cyclooctatetraene, 9,10-dihydronaphthalene, and azulene derivatives; A degenerate [1,5] sigmatropic shift - Comparison between theory and experiment

The reaction of dicyanoacetylene (DCA) with the partially hydrogenated dime thano-bridged anthracene and pentacene derivatives 3 and 18 lead to the (1: 1) Diels-Alder adducts 5 and 19, respectively, and to the 9,10-dihydronapht halene derivatives 6 and 21, respectively. These compounds are intensely co lored, most likely due to a charge-transfer absorption. Photolysis of the ( 1:1) adducts 5 and 19 produces the corresponding cyclooctatetraene derivati ves 7 and 8, which are not planar despite the torsional constraints caused by the fusion of the eight-membered ring to norbornane and norbornene units . The mechanisms of formation of the dihydronaphthalene derivatives 6 and 2 1 were elucidated by the use of high pressure; this allowed the (2:1) adduc ts 11 and 20 to be detected as intermediates in the reaction of DCA with 5 or 19 to give 6 and 21, respectively. A degenerate rearrangement consisting formally of a [1,5] vinyl shift in 6 or 21 could be detected by their temp erature-dependent H-1 NMR spectra. The mechanisms of the [1,5]-sigmatropic- shift reactions in 6 and 24 have been examined at the Becke3LYP/6-31G* leve l of theory. Stepwise mechanisms are predicted, and the calculated activati on barrier is in good agreement with the measured Delta H-not equal. The fi nding that the irreversible rearrangement of 6 accompanied by the eliminati on of HCN to give the azulene derivative 14 proceeds only on heating of 6 t o 80 degrees C in a polar solvent is good evidence for a polar mechanism in this case.