7‐(Dimethylamino)tricyclo[5.2.2.01,6]undecene Derivatives from β‐Cyclohexenyl β‐Dimethylamino‐Substituted α,β‐Unsaturated Fischer Carbenes

Abstract

AbstractWhen heated in pyridine at 80 °C, pentacarbonyl[(2E)‐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 × 108 s−1. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)