7‐(Dimethylamino)tricyclo[5.2.2.0 1,6 ]undecene Derivatives from β‐Cyclohexenyl β‐Dimethylamino‐Substituted α,β‐Unsaturated Fischer Carbenes

When heated in pyridine at 80 °C, pentacarbonyl[(2 E )‐3‐cyclohexenyl‐3‐(dimethylamino)‐1‐ethoxy‐2‐propen‐1‐ylidene]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 × 10 8 s −1 . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)