Formation of 3-Methylene-1,4-cyclohexadienes from α,α,α',α'-Tetraaryl-1,4-benzenedimethanols by the Intramolecular Migration of an Aryl Group

Article abstract of DOI:10.1246/bcsj.70.1413

By reactions of diethyl p-phthalate with 2,6-dimethoxyphenyllithium, ΦLi, a tri-substituted compound, p-HOCΦ₂C₆H₄C(O)Φ (1), or tetra-substituted 1,4-benzenedimethanol, p-HOCΦ₂C₆H₄CΦ₂OH (2a), were obtained. Compound 1 further reacted with aryllithium, ArLi, to give the 1,4-benzenedimethanols, p-HOCΦ₂C₆H₄CArΦOH (Ar = 4-MeC₆H₄ 2b, 4-MeOC₆H₄ 2c). When 2a was treated with acid in dimethyl sulfoxide, 3-(2,6-dimethoxybenzoyl)-3-(2,6-dimethoxyphenyl)-6-[bis(2,6-dimethoxyphenyl) methylene]-1,4-cyclohexadiene, Φ₂C chemical equation presented (4a) was obtained in quantitative yield under a variety of conditions. The reaction is understood to involve an initial formation of carbenium ion, [p-Φ₂C⁺-C₆H₄CΦ ₂OH] (3a), of which the positive charge must be partly shared with the other end of the p-phenylene ring, where one of the neighboring Φ-groups migrates. Reactions in acetone resulted to give 4a or dark-violet crystals of (2′,6′-dimethoxy-4-biphenylyl)bis(2,6-dimethoxyphenyl)carbenium perchlorate, [(4-ΦC₆H₄)CΦ₂]ClO₄, depending on the reaction conditions. Reactions in methanol, ethanol, or in hot 2-propanol resulted to give the reduced compound, (4-ΦC₆H₄)Φ₂CH. The 1,4-benzenedimethanols, 2b and 2c, also reacted with perchloric acid in dimethyl sulfoxide to form the 3-methylene-1,4-cyclohexadiene derivatives, Φ₂C chemical equation presented (Ar = 4-MeC₆H₄ 4b, 4-MeOC₆H₄ 4c), respectively. Evidence is given by the ¹H NMR spectrum, which shows the formation of intermediate carbenium ions 3a - c.