The structures of 5-methyl-5-phenyl-5H-dibenzo[b,f] silepin and its saturated analog

The structures of 5-methyl-5-phenyl-5H-dibenzo[b,f] silepin (I) and 5-methyl- 5-phenyl-1O,11-dihydro-5H-dibenzo [b,f] silepin (II) have been determined from three-dimensional X-ray data collected by counter methods. I crystallizes in the orthorhombic space group Pnam with a 7.596(3), b 18.102(5) and c 12.190(2) Å; observed and calculated densities (Z = 4) are 1.17 and 1.18 g cm^?3, respectively. II crystallizes in the monoclinic space group $\text{P}\text{2}{}_1\text{c}$ with a 11.115(3), b 7.920(3), c 20.765(5) Å and β 111.71(2)°; observed and calculated densities (Z = 4) are 1.17 g cm^?3. Anisotropic refinement of nonhydrogen atoms, with hydrogen atoms included at fixed ideal locations, gave conventional R-factors of 4.5% (I) and 5.0% (II). Compound I exhibits the boat conformation for the tricyclic framework and is located on a crystallographically required mirror plane. Com- pound II has the expected folded boat conformation. The torsion angle about the 10,11-bond is 0.0° for I, a crystallographic symmetry requirement, and 89.9° for II. Mean SiC bond distances are 1.863 Å(I) and 1.875 Å(II). The dihedral angles between the planar benzo groups are 129.7° (I) and 137.2° (II); introduc- tion of unsaturation at the 10,11-position decreases the dihedral angle in the tri- cyclic system, i.e., the tricyclic system is more bent.