Synthesis and reactivity of carbachlorins and carbaporphyrins.

Abstract

Acid-catalyzed condensation of 3-ethoxymethylenecyclopentene-1-carbaldehyde with a tripyrrane, followed by oxidation with aqueous ferric chloride solutions, afforded moderate yields of a carbachlorin. This porphyrinoid exhibited a porphyrin-like UV-vis spectrum with a slightly intensified peak at 650 nm. The proton NMR spectrum showed that the carbachlorin is highly diatropic, and this has been confirmed by nucleus independent chemical shift (NICS) calculations. Oxidation of the carbachlorin with DDQ gave the first example of a carbaporphyrin that is unsubstituted on the carbocyclic ring. Reaction of the carbachlorin with silver(I) acetate gave the corresponding silver(III) organometallic complex. When the carbachlorin was refluxed with methyl iodide and potassium carbonate in acetone, the 22-methyl derivative was formed. Treatment of the N-alkylation product with palladium(II) acetate afforded an unstable palladium(II) carbachlorin that was partially converted into a palladium(II) carbaporphyrin via an oxidation-methyl group migration process. Improved yields of the carbaporphyrin complex were obtained when the reaction mixture was stirred with aqueous ferric chloride solutions. These results open up the field of carbachlorin and carbaporphyrin chemistry for further study.