Regioselective Photoreduction of Zinc(ii) Porphyrins to Give Chlorins

Abstract

The ascorbic acid/organic base photoreduction of zinc(II) porphyrins was investigated. It was established that certain substituents can direct the photoreduction to the site of the macrocycle to which they are attached. For example, zinc(II) vinylporphyrins (8, 12, 16, 20) are photoreduced with cis stereochemistry on the ring bearing the vinyl group to give the corresponding chlorins. Zinc(II) acetylporphyrins (22, 24) were likewise reduced to chlorins such that cis-hydrogenation took place on the ring bearing the acetyl group. Zinc(II) formylporphyrins 33 also appear to reduce at the ring bearing the formyl group. When the zinc(II) acrylic porphyrin 28 was photoreduced, reduction did take place at the ring bearing the acrylic side chain, but migration of the acrylate double bond was very rapid, and the product isolated was the corresponding porphyrin propionate 30. Reduction of a zinc(II) porphyrin 35 bearing both a vinyl group and a nuclear carboxylic ester took place at the ring bearing the carboxylic ester. The reaction provides a general method for regioselective synthesis of chlorins from zinc(II) porphyrins without any evidence of formation of over-reduction products characteristic of many other procedures for formation of chlorins from porphyrin precursors.

Syntheses and Functionalizations of Porphyrin Macrocycles

“…On the other hand, trans -chlorins can be obtained from the reduction of Fe(III) β-substituted porphyrins with sodium metal in isopentyl alcohol [313, 314]. Porphyrins can also be photo-reduced by light in the presence of amines, ascorbic acid, and other compounds [315-320]. Regioselective photoreductions to cis -chlorins are achieved with ascorbic acid in the presence of diazabicyclo[2.2.2]octane (DABCO) as catalyst.…”