D2-symmetric dimer of 1,1′-binaphthyl and its chiral π-conjugated carbodianion

The synthesis of a chiral, nonracemic pi-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 pi-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 degrees, 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 degrees 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 Naf 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.