“Freezing-in” as norcaradiene valence tautomer is possible in the case of 1,6-methano[10]annulene not only by acceptor substituents on the methylene carbon atom but also by suitable peri-bridges. The prognosis based on force field calculations that 2,10-etheno-1, 6-methano[10]annulene exists as valence tautomer 1, whereas 2,10-ethano-1,6-methano[10]annulene—the product of the diimine reduction of 1—retains the annulene structure, finds impressive experimental confirmation.
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Arnz et al. (1991) studied this question.
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