OXIDATIVE COUPLING OF ALPHA,OMEGA-DI(CYCLOPENTADIENYL)ALKANE-DIIDES

The Cu-II-induced oxidative coupling of alpha,omega-di(cyclopentadieny l)alkane-diides 6 (n = 2-5) has been shown to proceed mainly by an int ermolecular pathway to give polymers 8, while the yield of intramolecu lar coupling 6-->7 strongly decreases with increasing number n of C-at oms of the alkyl chain (Scheme 3). Far n = 2, intramolecular coupling may be considerably enhanced by replacing the H-atoms of the CH2CH2 br idge of 6a (n = 2) by Me groups. In this case, intramolecular coupling s 11-->20 (Scheme 7) and 22-->23 + 24 (Scheme 8) are accomplished with a total yield of 59% and 54%, respectively. All the intramolecular co uplings investigated so far proceed stereoselectively to give the C-2- symmetrical cyclohexanes 7a, 20 and 23 with a fixed chair conformation . These results are easily explained, if a conformational equilibrium E reversible arrow F is operative in which large substituents R are as sumed to enhance the gauche-conformation F which is the favored confor mation for intramolecular couplings. Bridged dihydropentafulvalenes 20 and 23 are quantitatively rearranged to the thermodynamically favored bridged pentafulvenes 27 and 28 under base or acid catalysis, respect ively (Scheme 9).