Diels–Alder reactions of pyrrolo[3,4-b]porphyrins Wei Liu, Frank R. Fronczek, M. Grac ¸a H. Vicente and Kevin M. Smith * Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA Received 22 July 2005; revised 24 August 2005; accepted 25 August 2005 Available online 13 September 2005 Abstract—In the presence of excess dimethylacetylene dicarboxylate (DMAD), nickel(II) pyrrolo[3,4-b]porphyrins undergo both Diels–Alder cycloaddition and Michael addition in toluene to give two bis-adducts, identified as compounds 4 and 5; the reaction can be accelerated by the addition of Lewis or Brønsted–Lowry acids. Refluxing the reaction mixture in 1,2,4-trichlorobenzene (220 °C) leads to a nickel(II) monobenzoporphyrin 2 as the main product. The structure of compound 4 was confirmed by X-ray crystallography. Ó 2005 Elsevier Ltd. All rights reserved. Porphyrin systems bearing fused aromatic rings have been the subject of intense research over the past several years. Since benzoporphyrins were discovered in petro- leum and related deposits, 1–6 literally dozens of novel synthetic approaches to aroporphyrins have been de- scribed. 7 Novel examples also include fused picenopor- phyrins, 8–11 metallocenoporphyrins 12 and pyrrolo[3,4-b]- porphyrins. 13–16 In 1998, Knapp et al. 16 showed that nickel(II) N-tert- butoxycarbonyl-pyrroloporphyrins underwent cyclo- addition to give a N-substituted bicycloadduct. Shortly thereafter, we reported 17 that copper(II) and also nickel- (II) pyrrolo[3,4-b]porphyrins 1 undergo Diels–Alder cycloaddition reactions with dimethyl acetylene-dicarb- oxylate (DMAD) to give eventually benzoporphyrins 2 by way of the adduct 3. Both 2 and 3 were isolated 17 and fully characterized, and an X-ray structure of 2 was obtained. At that time, we were puzzled by the deamination required to transform 2 into 3 and pro- posed a Ôdienophile-induced deaminationÕ for which there is literature precedent. 18 A bis-DMAD adduct, featuring multiple methoxyl resonances in its 1 H NMR spectrum, was also isolated but no structure was pro- posed for it at that time. We now report on the structure of no less than two bis-DMAD-Diels–Alder adducts from Diels–Alder reactions between 1 and DMAD, and clarify the mechanistic processes by which the deamination takes place. N N N N (3) Ph Ph Ph Ph Ni NH N N N N Ph Ph Ph Ph Ni N H CO 2 Me CO 2 Me (1) N N N N Ph Ph Ph Ph Ni (2) CO 2 Me CO 2 Me N N N N Ni N Ph Ph Ph Ph CO 2 Me CO 2 Me CO 2 Me CO 2 Me (4) (5) N N N N Ni Ph Ph Ph Ph N CO 2 Me MeO 2 C CO 2 Me CO 2 Me * * * * 0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2005.08.135 Keywords: Diels–Alder cycloadditions; Pyrrolo[3,4-b]porphyrins; Benzo- porphyrins; Mechanism; Michael addition. * Corresponding author. Tel.: +1 225 578 7442; fax: +1 225 578 3458; e-mail: kmsmith@lsu.edu Tetrahedron Letters 46 (2005) 7321–7324 Tetrahedron Letters