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

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.