Solvent-Free Condensation of Pyrrole and Pentafluorobenzaldehyde: A Novel Synthetic Pathway to Corrole and Oligopyrromethenes Zeev Gross,* ,† Nitsa Galili, † Liliya Simkhovich, † Irena Saltsman, † Mark Botoshansky, † Dieter Bla 1 ser, ‡ Roland Boese, ‡ and Israel Goldberg* ,§ Department of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel, Institute of Inorganic Chemistry, Essen UniVersity, 45117 Essen, Germany, and School of Chemistry, Tel AViV UniVersity, Tel AViV 69978, Israel chr10zg@tx.technion.ac.il Received June 15, 1999 ABSTRACT The solvent-free condensation of pyrrole and pentafluorobenzaldehyde (and to a lesser extent other electron-poor aldehydes as well) leads to a variety of products, of which three have been isolated and fully characterized. The two main products (11% each) are an open-chain pentapyrrole and corrole, a tetrapyrrolic macrocycle. The most obvious synthetic pathway for the preparation of porphyrins is the cyclocondensation of an aldehyde and pyrrole (Scheme 1, left side). Landmarks in this aspect are the 1935 Rothmund procedure and the 1986 contribution by Lindsey and co-workers. 1 The Lindsey procedure currently allows the preparation of a large variety of porphyrins in reasonable yields. 2 It consists of mixing equimolar amounts of pyrrole and the appropiate aldehyde together with a catalytic amount of acid in a deaerated inert solvent for about 1 h, followed by treatment of the same solution by a substituted quinone. The intermediate formed prior to the oxidation step is a hexahydroporphyrin, more commonly known as porphyrinogen. Interestingly, the last common intermediate in the biosynthesis of all naturally occurring tetrapyrrolic macrocyclessporphyrin in hemes, chlorin in chlorophylls, and corrine in Vitamin B 12 sis also a por- phyrinogen (uroporphyrinogen III). Another class of cyclic tetrapyrroles are the corroles, which share with corrine an identical ring skeleton and with porphyrins their aromaticity. Porphyrins and corroles have also many other properties in common, 3,4 but corrole’s research is much less developed because of problems in their synthesis. For example, the first meso-aryl-substituted corroles were reported as late as 1993, 5 more than 50 years after meso-tetraphenylporhyrin. 6 Also, all the procedures described up to 1998 require the prepara- tion of at least one (usually many) nonobvious and unstable precursor. Because of the increasing interest in expanded, contracted, and isomeric porphyrins, 3a we have explored a new approachs † Technion. ‡ Essen University. § Tel Aviv University. (1) Lindsey, J. S. Metalloporphyrins Catalyzed Oxidations; Montanari, F., Casella, L., Eds.; Kluwer: Dordrecht, 1994; pp 49-86 and references therein. (2) Li, F. R.; Yang, K. X.; Tyhonas, J. S.; MacCrum, K. A.; Lindsey, J. S. Tetrahedron 1997, 53, 12339. Wagner, R. W.; Johnson, T. E.; Lindsey, J. S. Tetrahedron 1997, 53, 6755. ORGANIC LETTERS 1999 Vol. 1, No. 4 599-602 10.1021/ol990739h CCC: $18.00 © 1999 American Chemical Society Published on Web 07/22/1999