Crystal and Molecular Structure of the Minor Cyclo-Dimerization Product of 8,8-Dimethylisobenzofulvene

Abstract

8,8-Dimethylisobenzofulvene undergoes spontaneous cyclo-dimerization to give a mixture of two isomers of composition C₂₄H₂₄, The minor isomer crystallizes in the orthorhombic space group P_bca with a 16.815(8), b 16.975(8) and c 13.058(6)Å p_obs 1.13(3)g cm^-3, p_ealc 1.13 g cm^-3 for Z 8. The structure was solved from diffractometer data by direct methods and was refined by full-matrix least squares to final residuals R 0.040, R_w 0.046 for 885 observed data. The stereochemistry of the minor dimer is established, contrary to theoretical predictions, as the exo-isomer. There is evidence of intramolecular steric strain particularly demonstrated by the elongated C-C single bonds, 1.590(7) and 1.570(7) Å, which are formed in the dimerization process.