Molecular structures of three diastereoisomers of 3,3′-di-t-butyl-1,1′-spirobi-indan
Abstract
Crystal structures of the three diastereoisomers (+)-(1R,3S,3′S)-(I), (±)-rel-(1R,3R,3′R)-(II), and (±)-rel-(1R,3R,3′S)–(III) of the title compound, C25H32, have been determined by X-ray analysis. Crystals of (I) are orthorhombic, space group P212121, a= 12.412(1), b= 14.842(1), c= 11.287(1)Å, Z= 4. Crystals of (II) are monoclinic, space group P21/c, a= 16.230(1), b= 11.719(1), c= 11.400(1)Å, β= 110.23(1)°, Z= 4. Crystals of (III) are monoclinic, space group P21/c, a= 13.220(1), b= 13.877(1), c= 11.897(1)Å, β= 109.81(1)°, Z= 4. The structures were solved by the direct method and refined by a block-diagonal least-squares technique to R= 0.032 for (I)(1 455 reflections), 0.034 for (II)(2 146), and 0.044 for (III)(2 437). The relative configuration between the chiral centres in each isomer was as expected from 1H n.m.r. data. The cyclopentene rings are all in envelope conformations, and the tertiary butyl groups adopt quasi-equatorial orientations in agreement with 1H n.m.r. data except the one in (II), which takes a quasi-axial orientation. The dihedral angles between the two benzene ring planes are 69.5(1), 91.4(1), and 94.9(1)° for (I)–(III), respectively. Calculation for the optimal structure using the MM2 program showed that the discrepancy in the form of (II) can be ascribed to a crystal packing effect.