The Cu"-induced oxidative coupling of a,w -di(cyclopentadienyl)alkane-diides 6 (n = 2-5) has been shown to proceed mainly by an intermolecular pathway to give polymers 8, while the yield of intramolecular coupling 6 +7 strongly decreases with increasing number n of C-atoms of the alkyl chain (Scheme 3 ) . For 11 = 2, intramolecular coupling may be considerably enhanced by replacing the H-atoms of the CH,CH, bridge of 6a (n = 2) by Me groups. In this case, intramolecular couplings 11 + 20 (Scheme 7) and 22 +23 + 24 (Scheme 8) are accomplished with a total yield of 59% and 54%, respectively. All the intramolecular couplings investigated so far proceed stereoselectively to give the C2-symrnetrical cyclohexanes 7a, 20 and 23 with a fixed chair conformation. These results are easily explained, if a conformational equilibrium E F?. F is operative in which large substituents R are assumed to enhance the gauche-conformation F which is the favored conformation 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, respectively (Scheme 9).
The CuII-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.
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ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
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