Exclusive Formation of Crystalline sp-9-(o-Isopropylphenyl)fluorene from its ap Rotamer in Solution

Reduction of ap-9-(o-isopropylphenyl)-9-fluorenol provided 9-(o-isopropylphenyl)fluorene shown to be the sp and ap rotamers in solution by ¹H NMR. Evaporation of the solvent yielded sharp-melting crystals identified by X-ray diffraction as sp-9-(o-isopropylphenyl)fluorene (C₂₂H₂₀) exclusively. A solution of these crystals redissolved in the same solvent again contained the sp and ap rotamers in the same ratio as before. The X-ray structure shows a moderately strained molecule in which the phenyl and fluorenyl rings are almost perpendicular and isopropyl-phenyl and phenyl-fluorenyl bonds are slightly distorted. While these effects do not preclude this rotamer from forming the more stable crystal lattice, they are apparently responsible for lowering its barrier of rotation sufficiently to effect the observed equilibrium in solution between it and the more strained ap rotamer.

-1