Norbornadiene Subject

The photosensitized cyclization of norbornadiene to quadricyclane, 18, has been known for some time.270 When barralene, 19, is subjected to photosensitization, all three double bonds are involved and compound 20, which Zimmermann has named semibullvalene, results.271 Irradiation of dibenzobarralene, 21, in acetone produces the dibenzo analog of semibullvalene, 22, in high yield.272 Since there is appreciable interaction between the double bonds in both norbornadiene and barralene, these reactions may be special cases. 

Tetranitromethane adds across various non-conjugated dienes to give cyclic products. The products with norbornadiene and cycloocta-1,5-diene are subject to spontaneous... 

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. 

Franzus et al.tQC examined the diimide reduction of 7-oxygenated norbornadienes with the expectation of obtaining the corresponding syn-norbomene. Since diimide reductions proceed by cis addition and are subject to steric approach control, the 7-substituent should inhibit formation of the exocyclic transition stale with the syn... 

Sugar-derived nitrile oxides 94, generated from corresponding nitroalkanes 93 using tolylene diisocyanate and triethylamine, were reacted with norbornadiene 95 to afford isoxazolino norbornene derivatives 96 containing sugar residues (Scheme 27) [121]. These were subsequently subjected to... 

Similarly, when benzene solutions of the benzannulated norbornadienes 77 and 78 are subject to irradiation in the presence of acetophenone, the predominant products are the tetracyclic compounds 79 (ca. 52%) and 80 (ca. 90%), respectively. " Such outcomes, when considered in conjunction with those detailed in the preceding few paragraphs, lead to the general conclusion that electron donors avoid positioning themselves in conjugation with the carbinyl centers of the cyclopropyldicarbinyl diradicals (cf. 4), while electron-withdrawing groups lead to stabilization when so situated. 

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