A series of metalloporphyrin dimers as Tröger’s
bases 1 or spiro-Tröger’s bases 2 was
prepared starting from five different C
4-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
2
was proven by electron diffraction.
The reactions of 2-naphthylamine and methyl 6-amino-2-naphthoate with formalin and paraformaldehyde were studied experimentally, spectrally, and by quantum chemical calculations. It was found that neither the corresponding aminals nor imines were formed under the described conditions but could be prepared and spectrally characterized at least in situ under modified conditions. Several of the previously undescribed intermediates and by-products were isolated or at least spectrally characterized. First principle density functional theory (DFT) calculations were performed to shed light on the key aspects of the thermochemistry of decomposition and further condensation of the corresponding aminals and imines. The calculations also revealed that the electrophilicity of methanal was significantly greater than that of ordinary oxo-compounds, except for perfluorinated ones. In summary, methanal was not behaving as the simplest aldehyde but as a very electron-deficient oxo-compound.
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