Rational Synthesis of 2‑Bromoporphyrins and 2,12-Dibromoporphyrins

Bromoporphyrins were
prepared by the metal-mediated self-condensation
of brominated 1-formyl­dipyrro­methanes. Depending on the
conditions, Mg­(II)-2,12-dibromo­porphyrin and Mg­(II)-2-bromoporphyrin
could be obtained in up to 11% and 17% isolated yield, respectively.
Zn­(II) was also a viable templating metal. The positions of the bromine
substituents were confirmed by 2D-NMR spectroscopic analysis and the
X-ray crystal structure of a derivative. Suzuki and Sonogashira reactions
of the bromoporphyrins yielded 2-substituted or 2,12-disubstituted
porphyrins with red-shifted absorption and emission spectra. This
method provides access to the minimalist core of β-mono- and
β,β′-disubstituted porphyrins from readily available
starting materials.