Construction of building blocks for extended porphyrin arrays by nitration of porphyrin-2,3-diones and quinoxalino[2,3-b]porphyrins

Abstract

Nitration of metal(II) porphyrin-2,3-diones 3–5 and quinoxalino[2,3-b]porphyrins 13–15 with nitrogen dioxide gave a mixture of β-pyrrolic functionalised nitro-porphyrin isomers in relative abundance of 7-nitro- > 12-nitro- > 8-nitro. The large selectivity for reaction at the 7-position over the adjacent 8-position (>5 to 1 in the diones 3–5 and about 3 to 1 in the quinoxalinoporphyrins 13–15) is especially striking. This selectivity results mainly from electronic effects and is consistent with a mechanism involving a porphyrin π-cation radical intermediate. A key step incorporated into the isomer separation sequence was to make use of the different chromatographic polarity of metalated compounds compared to unmetalated compounds coupled with the observation that introduction of a nitro group to the 7-position of free-base porphyrin-dione greatly increases its rate of metalation while introduction of the nitro group at the 12-position greatly decreases its rate of metalation, relative to the unsubstituted parent. Thus demetalation of zinc(II) nitro-porphyrin-dione isomers, which are difficult to separate, allows for highly selective remetalation of the 7-isomer and its very easy separation from the unmetalated 12-nitro-porphyrin-dione. These nitrated compounds are useful building blocks for more elaborate systems.