D₂-Symmetric Dimer of 1,1‘-Binaphthyl and Its Chiral π-Conjugated Carbodianion

The synthesis of a chiral, nonracemic π-conjugated system with D₂ 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.

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