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 dimethano-bridged anthracene and pentacene derivs. I and II lead to the (1:1) Diels - Alder adducts III and IV, resp., and to the 9,10-dihydronaphthalene derivs. V and VI, resp. These compds. are intensely colored, most likely due to a charge-transfer absorption. Photolysis of the (1:1) adducts III and IV produces the corresponding cyclooctatetraene derivs. , 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 derivs. V and VI were elucidated by the use of high pressure; this allowed the (2:1) adducts VII and VIII to be detected as intermediates in the reaction of DCA with III or IV to give V and VI, resp. A degenerate rearrangement consisting formally of a [1,5] vinyl shift in V or VI could be detected by their temp.-dependent 1H NMR spectra. The mechanisms of the [1,5]-sigmatropic shift reactions in V and IX have been examd. at the Becke3LYP/6-31G* level of theory. Stepwise mechanisms are predicted, and the calcd. activation barrier is in good agreement with the measured DH.thermod.. The finding that the irreversible rearrangement of V accompanied by the elimination of HCN to give the azulene deriv. proceeds only on heating of V to 80 Deg in a polar solvent is good evidence for a polar mechanism in this case.
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