When heated in pyridine at 80°C, pentacarbonyl[(2E)-3-cyclohexenyl-3- (dimethylamino)-1-ethoxy-2-propen-1-yl-idene]chromium (1-Cr) and -tungsten (1-W) complexes undergo 6π-electrocyclization and subsequent reductive elimination to yield the cyclohexane-annelated cyclopentadiene 6, which equilibrates by 1,5-hydrogen shift with its more reactive isomer 7. The latter molecule is efficiently trapped in Diels-Alder reactions with various alkynes 2 and styrenes 8 to give the cyclohexane-annelated norbornadiene 3 and norbornene derivatives respectively, with high regio- and diastereoselectivity in 15-91% yields. The enol ether moiety in compound 3 is particularly easily hydrolyzed, probably due to the through-space interaction between the two double bond moieties, so that the norbornenone derivatives 4 are isolated in all but one case after chromatographic purification. In this reaction, the tungsten complex 1-W consistently gave lower yields than 1-Cr. Arylalkynes with strongly electron-withdrawing groups and minimal steric congestion are particularly suitable reaction partners for 7. The formation of bis-cycloadducts 10 from suitable diynes largely depends upon the steric and electronic properties of the corresponding alkynyl-substituted mono-cycloaddition products of type 4 or 11. With the fluorenyl-substituted tricycle 4w in acetonitrile at 25°C, but not with 4y, dual fluorescence is observed, with a red-shifted unstructured additional emission band with a dipole moment of 32 D, originating from intramolecular charge transfer. The molecule 2y has an exceptionally large radiative rate constant of 13 x 108 s-1.
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