Oxidative Coupling of α,ω - Di(cyclopentadienyl)alkyl-diides

Abstract

The Cu^II-induced oxidative coupling of α,ω-di(cyclopentadienyl)alkyl-diides 6 (n = 2 to 5) – as well as of 1,1,2,2-tetramethyl derivatives 10 and 13 of 6a (n = 2) – has been investigated in view of pertinent questions concerning the 'coupling mode' (e.g. intramolecular vs. intetmolecular coupling), as well as regioselectivity and stereoselectivity of the coupling reaction. As far as the 'coupling mode' is concerned, intermolecular coupling 6 ? 8 strongly dominates over intramolecular coupling 6 → 7 (Scheme 2) and the yields of intramolecular coupling products strongly decrease from 7% (7a, n = 2) to 1% (7b, n = 3) to traces (7c, 7d, n = 4,5). It is interesting to note that intramolecular coupling may be considerably enhanced by replacing the H-atoms of the CH₂CH₂ bridge of 6a (n = 2) by Me groups (see 10 → 11 and 13 →14 + 15, (Scheme 4). As far as regioselectivity is concerned, intramolecular coupling of 1,2-di(cyclopentadienyl)ethane-diides, 6a, 10, and 13, proceeds as a clean 2,2'-coupling of the cyclopentadiene rings within the limits of NMR identification. Furthermore, the couplings 6a → 7a and 10 → 11 (as well as 13 → 14 + 15) proceed stereoselectively to give the C₂-symmetrical cyclohexanes 7a and 11 with a fixed chair conformation.