Biphenylene dimers, which may be considered as "macrocyclic" 2 × 2 fragments of net 1 and dimeric fragments of a double-stranded polymer, 5, are prepared. X-ray crystallography on two polymorphs of the dimer 9 reveals that its bond alternation is analogous to that in biphenylene; the tetraphenylene part of 9 has approximately C 2h symmetry with a small dihedral angle (<12°) between the benzene rings in the adjacent biphenylenes. Cyclic voltammetry of 9 gives two reversible waves corresponding to a radical anion and a dianion; the lithium and potassium salts of these anions, 9 •-,M + (M ) Li, K) and 9 2-,2M + (M ) Li, K), were generated and characterized by ESR, NMR, and UV-vis spectroscopies. The spectral data for the dianions are compatible with classical structures; no evidence for "in-plane aromaticity" is found. For polymer 5, a localized band and large density of states near the Fermi level are found, using extended Hu ¨ckel theory calculations.

The synthesis of a chiral, nonracemic π-conjugated system with D 2 point group of symmetry and the corresponding chiral carbodianion is described. Cu(II)-mediated oxidation of (R)-2,2′-dilithio-1,1′-binaphthyl (prepared from (R)(+)-2,2′-dibromo-1,1′-binaphthyl) gives the corresponding dimer, (R)(+)-3, which is a chiral π-conjugated derivative of o-tetraphenylene, a saddle shaped molecule. When partially resolved 2,2′dibromo-1,1′-binaphthyl is used, amplification of the enantiomeric excess (ee) is observed in the coupling product (3). The dihedral angles between the naphthalene moieties in (R)(+)-3 are about 70 °, as determined by the X-ray crystallography. However, CD and UV spectra, and cyclic voltammetric data of (R)(+)-3 are consistent with small, but far from negligible, conjugation (or homoconjugation) between the naphthalene moieties. The melting points of (R)(+)-3 and its racemate differ by more than 200 °C. Reaction of (R)(+)-3 with Li or Na in tetrahydrofuran (THF) gives the corresponding chiral carbodianions 3 2-,2M + (M ) Li and Na). NMR spectroscopy indicates two non-equivalent naphthalene moieties. UV-vis, ESR, NMR, and electrochemical studies show almost complete disproportionation of the radical anion to the carbodianion in THF with Na + as counterions. The free energy barrier for racemization in carbodianion 3 2-,2Na + is lowered by at least 25 kcal/mol compared to that in (R)(+)-3. However, carbodianions 3 2-,2M + (M ) Li and Na) remain configurationally stable at room temperature. Solid 3 2-,2Na + is a diamagnetic insulator.

2 VCH Verlugs~esdlschuftft mbH, D-69451 Wemheim. 1997 O570-0833/97/3605-0490 3 15 OO+ 25 0 Angm Cheni Int Ed Engl 1997,36, Ao 5 COMMUNICATIONSThe development of efficient enantioselective methodologies for the construction of tetra-o-phenylenes and less sterically constrained biphenylene dimers is in progress in our laboratory.

Durch symmetrische Anellierung von zwei funktionalisierten Tetra‐o‐phenylenen wurde doppelhelicales Octaphenylen 1 mit einer 360°‐Windung der Doppelhelix hergestellt. Solche π‐konjugierten Systeme mit intrinsischer Chiralität sind für die Entwicklung einer neuen Generation organischer Materialien von Interesse.

No abstract