Preparation and Characterization of Metalloporphyrin Tröger's and Spiro-Tröger's Base Derivatives

A series of metalloporphyrin dimers as Tröger's bases 1 or spiro-Tröger's bases 2 was prepared starting from five different C₄-symmetry porphyrin derivatives substituted in meso-positions by Ph, 3-MeO-Ph, 4-MeO-Ph, 3,4-(MeO)₂-Ph, or 3,5-(MeO)₂-Ph. Free-base porphyrins were converted to metalloporphyrins, which were subsequently nitrated with nickel(II), copper(II), or zinc(II) nitrate to give β-nitrometalloporphyrins. These were further reduced to β-aminometalloporphyrins and treated with a methanal equivalent under acidic conditions to selectively obtain Tröger's base 1, spiro-Tröger's base 2, or a mixture of both, in yields up to 41% of 1 and 45% of 2 depending on the reaction conditions used. The ratio of 1 to 2 was influenced by the methanal equivalent used, the strength of the acid, and, above all, the solvent. The presence of a metal ion within the porphyrin core and the use of a chlorinated solvent were found to be essential for the formation of spiro-Tröger's base 2. The molecular structure of spiroTB 2a-Ni was proven by electron diffraction.