Unexpected dipivaloylation of 9-lithiated fluorene: formation of 1-(fluoren-9-ylidene)-2,2-dimethylpropyl pivalate.

Treatment of 9-lithiated fluorene with pivaloyl chloride provided ap-9-pivaloylfluorene, (1), the major product, and a minor product ultimately identified as the title compound, C(23)H(26)O(2), (2). The latter was also formed directly, but slowly, from 9-lithiated-(1) treated with pivaloyl chloride. Although (1) exists exclusively as its less sterically restricted ap rotamer, its sp(2)-hybridized anion sterically impedes reaction at the 9-position from either face. While 9-lithiated-(1) is exclusively, but slowly, 9-methylated with methyl iodide, reaction with pivaloyl chloride, also slow, leads only to the O-acylated product, (2). The protons of the tert-butyl-C=C moiety approach a proton on the fluorene ring to well within the sum of their van der Waals radii, resulting in significant molecular compression, strain and distortion. For example, distortion in the moiety C=C(O)(C) is exhibited by the enlargement of C=C-C angle to 130.6 (2) degrees at the expense of the corresponding 'equivalent' C=C-O angle, which is compressed to 116.46 (19) degrees.