X-ray crystallographic studies of racemic and optically active 4, 4’-dimethyl-1, 1’-binaphthyl

In contrast to 1,1'-binaphthyl, racemic 4,4'-dimethyl-1,1'-binaphthyl does not undergo spontaneous resolution upon heating from room temperature to just below the melting point. Optically active dimethyl binaphthyl may be obtained by seeding the racemic melt site optically active naphthidine. The crystal structures of both the racemic and optically active dimethyl binaphthyls were solved in the hope of understanding the above observations. The racemate crystallizes in the monoclinic space group C2/c with cell parameters a=13.225, b=10.768, c= 11.572 Å and β=114.040. There are four molecules per unit-cell; two have the B and two have the S configuration. The structure was solved using direct methods and refined to R=0.074. There is a 3° head in the plane of the naphthalene residues, which are cis-oriented with an angle of 68° between them. The optically active form belongs to one of the tetragonal space groups P4₁2₁2 or P4₃2₁2 with cell parameters a = b = 8.3031 and c = 23.706 Å. Direct methods sere used to solve the structure and the final R was 0.060. There are four molecules per unit-cell of identical configuration, but it could not be determined whether this was R or S. The naphthalene residues show a 2.7° bend and are also cis-oriented, but with an angle of 80° between them. Bond lengths and angles are consistent with values previously reported for 1,1'-binaphthyl and naphthalene. The racemate packs somewhat more efficiently and perhaps for this reason it is slightly more stable than the optically active form- It is difficult however to explain the difference in behaviour between the methylated and unmethylated binaphthyls on the basis of these results alone. Further studies would include the crystal structures of optically active 1,1'-binaphthyl and various naphthidines.Science, Faculty ofChemistry, Department ofGraduat