The surprising formation of structurally distorted ap-9-(o-tert-butylphenyl)-9-methylthiofluorene and its facile homolysis into sp-9-(o-tert-butylphenyl)-3-methylthiofluorene and sp-9-(o-tert-butylphenyl)fluorene

The continued study of rotationally restricted 9-(o-tert-butylphenyl)fluore nes has provided surprising results. Treatment of sp-9-(o-tert-butylphenyl) -9-fluorenol (1) with ethanol or methanol under acidic conditions affords s p-9- (o-tert-butylphenyl)-9-ethoxyfluorene (2b) and sp-9-(o-tert-butylpheny l)-9-methoxyfluorene (3b), respectively, but similar treatment with methane thiol converts 1 into the rotamerically opposite ap-9-(o-tert-butylphenyl)- 9-methylthiofluorene (4a). While all three products reflect reaction with i nversion at C-9, 2b and 3b reflect subsequent rotation, which is not the ca se with 4a. X-ray diffraction shows 4a to be highly distorted and strained, but apparently favored thermodynamically over its sp rotamer. Homolysis of 4a is observed at room temperature, and at elevated temperatures accounts for the formation of sp-9-(o-tert-butylphenyl)fluorene (6b) and sp-9-(o-ter t-butylphenyl)-3-methylthiofluorene (7c) as major products. X-ray diffracti on shows 6b and 7c to be virtually devoid of distortion. Methylation of 6b via its anion also proceeds with inversion without rotation to form ap-9-(o -tert-butylphenyl)-9-methylfluorene (4c). Dynamic NMR unexpectedly showed t hat in these ap configurations 9-CH3 (of 4c) has a greater bulk effect than 9-CH3S (of 4a) in forcing the o-tert-butyl group into the fluorene plane.