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.