Stereochemistry of Reactions Involving Rotationally Restricted, Sterically Hindered Cations, Radicals, and Anions: 9-Fluorenyl Systems

Abstract

A study of the stereochemical pathways of reactions involving rotationally restricted, sterically hindered cations, radicals, and anions has been undertaken utilizing chiral 9-(o-tert-butylphenyl)fluorenes. Previous reports of studies with these or related achiral compounds contained erroneous or equivocal conclusions. This study shows that (+)-sp-9-(o-tert-butylphenyl)-9-methoxy-2-methylfluorene, treated with Tf(2)O-CHCl(3) to form 100% of the 9-cation, then with NaOMe-MeOH, provided 29% of re-formed substrate (configurational retention) and 71% of the (-)-sp enantiomer (inversion). The same substrate treated with HI-CHCl(3) was converted into the 9-radical, which was rapidly reduced, affording 100% isolation of (-)-sp-9-(o-tert-butylphenyl)-2-methylfluorene (inversion). Treatment of the latter with n-BuLi-THF provided the 9-anion which, on acidification, yielded 100% of the enantiomeric (+)-sp-9-(o-tert-butylphenyl)-2-methylfluorene (inversion). The substrates in these reactions were the thermodynamically favored sp rotamers. Inversion directly produced the higher energy nonenantiomeric ap rotamers, which rapidly rotated into the sp products that were enantiomeric with the substrates. These results are explained by the rotational restriction and partial steric hindrance by the tert-butyl group to the original face of the sp(3) antiaromatic 9-cation (4n pi electrons), and the rotational restriction and extensive blockage to the original face of the sp(2) nonaromatic 9-radical (4n + 1 pi electrons) and aromatic (4n + 2 pi electrons) 9-anion. The barrier to rotation in some of the ortho-substituted 9-arylfluorenes is great enough to allow their sp and ap rotamers to be detected coexisting in solution, although their crystals were composed exclusively of one. Rotational restriction and steric hindrance at the 9-position have a large influence on the pK(a) values of these fluorenes and can offset the classic electronic effects of the substituents.