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The intensely colored pyridine adducts 2, which were obtained from the haloboranes 1 via the THF adducts can be described as methylene boranes whose electron-deficiency is balanced out intramolecularly. The double bond character of the CB bond manifests itself for example in the different chemical shifts of the two benzene moieties of the fluorenylidene skeleton R2C. The shift observed for C9, however, also suggests great importance for the resonance structure A (Aryl = 2,4,6-triisopropyl-, 2,3,5,6-tetramethyl-, 2,4,6-trimethylphenyl).
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Lukasch et al. (1988) studied this question.
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