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Updated: Apr 27, 2026

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
Published on: April 19, 2019
Synthesis and reactivity of carbachlorins and carbaporphyrins
1Department of Chemistry, Illinois State University , Normal, Illinois 61790-4160, United States.
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.
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A key factor in assessing the reactivity of the acid derivatives is the basicity of the substituent or the leaving group. The lower the basicity of the leaving group, the higher the reactivity of the derivative. The basicity of the leaving group follows this order:
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