Inversion Barriers of ortho,ortho'-Bridged BiphenylsThe syntheses and spectroscopic properties of the ortho,ortho'-bridged biphenyls 2-19 are described. In the case of 5, 15, and 16 the inversion barrier is determined from the temperature dependence of the NMR spectra (lineshape analysis), in the case of 12,13, and 14 from the temperature of coalescence, and in the case of 2a. 3b, 7. l?, 18, and 19 from the rate of racemization of the optically active compounds. The experimental inversion barriers, including data from the literature, are compared with those calculated using the extended force field (MM2erw). The conformational behavior of the title systems is significantly affected upon transformation into the corresponding dianion salts. The relevant energy terms are discussed.

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The angle of twist of the biphenyl unit is almost halved on going from 1 to 2. On the other hand, according to the UV spectra it should be greater in 3 than in 1. Particularly striking in the case of 3 is a symmetry lowering by charge localization in the dianion, so that an uncharged butadiene bridge can remain bent. As a result of the charge localization the central bridgehead atoms in 32e do not have the highest charge density; reaction with Me2SO4 leads exclusively to formation of 4.