D-2-symmetric dimer of 1,1 '-binaphthyl and its chiral pi-conjugated carbodianion

The synthesis of a chiral, nonracemic pi-conjugated system with D-2 point g roup 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 sadd le shaped molecule. When partially resolved 2.2'-dibromo-1,1'-binaphthyl is used, amplification of the enantiomeric excess (ee) is observed in the cou pling 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 co nsistent with small, but far from negligible, conjugation (or homoconjugati on) between the naphthalene moieties. The melting points of (R)(+)-3 and it s 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 a lmost 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 t hat in (R)(+)-3. However, carbodianions 3(2-),2M(+) (M = Li and Na) remain configurationally stable at room temperature. Solid 3(2-),2Na(+) is a diama gnetic insulator.