Organometallic Synthesis, Molecular Structure, and Coloration of 2,7-Disubstituted 1,8-Bis(dimethylamino)naphthalenes. How Significant Is the Influence of “Buttressing Effect” on Their Basicity?
journal contributionposted on 26.12.2003, 00:00 by Alexander F. Pozharskii, Oksana V. Ryabtsova, Valery A. Ozeryanskii, Alexander V. Degtyarev, Olga N. Kazheva, Gregory G. Alexandrov, Oleg A. Dyachenko
On treatment of 2,7-dilithio- (6a) and 2,7-bis(bromomagnesio)- (6b) naphthalenes with a number of electrophiles, new “proton sponge” derivatives 7a−e,g, containing iodo, methylthio, trimethylsilyl, methyl, n-butyl, and ethoxycarbonyl groups in ortho-positions to dimethylamino groups, have been synthesized. The investigation of their molecular structure, spectral characteristics, and basicity reveals that the latter is mainly determined by two groups of factors: (1) the polar effect of ortho-substituents and (2) the so-called “buttressing effect” representing the complex combination of various sterical interactions of ortho-substituents with dimethylamino groups in corresponding bases and cations. A contradictory directionality of these interactions strongly reduces the importance of the buttressing effect in the case of compounds with bulky ortho-substituents, which is most clearly displayed in the absence of any relationship between the size of ortho-substituents and the basicity. On the contrary, for proton sponges having in ortho-positions the electron-donating groups with lesser steric demands, both the buttressing and the polar effects act in the same direction. This is the reason for the exceptionally high basicity of 2,7-dimethoxy- (2) and tetrakis(dimethylamino)- (3a) naphthalenes. It has been found that bis(trimethylsilyl)-bis(dimethylamino)naphthalene 7d has an in-out configuration of the NMe2 groups in the solid that is the first example of its kind in the series of naphthalene proton sponges. The origin of a yellow coloring of 2,7-disubstituted proton sponges is also discussed.