Norbornadiene, metal-catalyzed dimerization

Very recently, Fraser et al. (85a) proved, by isolating and analyzing the intermediate, that a similar reaction mechanism is operative in the transition-metal-catalyzed dimerization of norbornadiene. 

The metal-catalyzed dimerization of norbornadiene was discovered in 1961 by two independent groups of workers (3, 4) and since then has received considerable attention by others. The various possible dimers are shown in Table II. Excluding structures in which the cyclobutane ring is fused, trans, 14 dimers (XXIII-XXXVI) are possible, of which seven have been isolated. The structures of six are now rather well established. Initially (3) the structure of one dimer was considered to be (XXXVII). In the meantime NMR measurements (44) and synthetic studies (45, 46) have shown that (XXXVII) is (XXV), the liquid exo-trans-endo dimer. Pyrolysis of both (XXV) and (XXIII) (m.p. 67-68°) yields cyclopentadiene and a mixture of bicyclo[4.2.1]nona-2,4,7-triene (XXXVIII) and exo-tricyclo[4.2.1.02 ]nona-3.7-diene (XXXIX) (46). The exo-trans-endo isomer (XXV) decomposes more... 

Intermediate metallacyclopentanes are also implicated in transition metal-catalyzed alkene cycloadditions to form cyclobutanes and the corresponding cycloreversions, e.g. dimerization of norbornadiene (73JA597) and rearrangements of cubane and other cyclo-butanoid hydrocarbons (78JA2573). 

A number of transition metal-catalyzed reactions may involve symmetry-forbidden ligand transformations. Bicyclo[2.2.1]hepta-2,5-diene (norbornadiene) (IX) undergoes smooth dimerization to cyclobutane derivatives (X) in the presence of zero-valent Fe (15), Ni (16,17), and Co (18) catalysts. This interesting chemistry... 

A more detailed consideration of the Woodward-Hofimann postu-ulates for olefinic systems in the presence of a transition metal indicates that the thermally forbidden dimerization of two ethylene molecules to cyclobutane becomes allowed if the orbitals of the olefins can interact symmetrically with the dxt and dyz orbitals of the transition metal catalyst (53). One would consequently also expect transition metal complexes to catalyze the conversion of quadricyclene (IV) back to norbornadiene. This has been reported to be the case (54). The reactions leading to the formation of VI, XXX, and XXXI are examples of processes in which thermally allowed sigmatropic reactions become subject to catalysis by transition metal complexes. The catalysts thus display the dual role of removing symmetry restrictions and of generally lowering activation energies. 

A number of cycloaddition and dimerization reactions of norbornadiene catalyzed by transition metal complexes has been described. The mechanism of these reactions was discussed from an empirical and phenomenological viewpoint. Several of the reactions have previously... 

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