Unexpected dipivaloylation of 9‐lithiated fluorene: formation of 1‐(fluoren‐9‐ylidene)‐2,2‐dimethylpropyl pivalate |
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Authors: | Paul D Robinson Hisham G Lutfi Yuqing Hou Cal Y Meyers |
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Abstract: | 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, C23H26O2, (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 sp2‐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)° at the expense of the corresponding `equivalent' C=C—O angle, which is compressed to 116.46 (19)°. |
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