TETRAHEDRON LETTERS Tetrahedron Letters 44 (2003) 1207–1210 Pergamon Microwave-assisted synthesis of corroles James P. Collman* and Richard A. Decre ´au Stanford University, Department of Chemistry, Stanford, CA 94305 -5080, USA Received 18 September 2002; revised 7 December 2002; accepted 9 December 2002 Abstract—The recently developed Gross’s method for the synthesis of corroles has been modified and successfully applied to the preparation of new free base tris-aryl- and tris-pyrimidyl-corroles using solvent-free conditions and microwave irradiation. Compared to conventional heating, the microwave technique afforded an increase in corrole yields of ca. 30% and led to noticeably cleaner reaction mixtures. It is demonstrated that short reaction times and high temperatures are required to afford optimum yields. © 2003 Elsevier Science Ltd. All rights reserved. Corroles are porphyrin analogues which lack one meso carbon bridge. Recently several synthetic methods have been reported 1a–d,2a–c which allow corroles to be obtained in reasonable quantities and to be used for a wide range of applications as catalysts. 3a–g The solvent- free method developed by Gross on solid support is quite straightforward, it has been developed for elec- tron-withdrawing aldehydes and in fact only tris-penta- fluorophenylcorrole 4 was obtained in 8–11% yield after heating. 1a,b We report here the application of this methodology to other electron-withdrawing benzyl- aldehydes as well as to one pyrimidyl-aldehyde. Fur- thermore, several reports have shown that using microwave irradiation (MW) instead of conventional heating (H) raises yields, enhances reaction rate, reduces thermal degradation by-products for a wide range of organic reactions 4 including cyclocondensation reactions leading to tetraarylporphyrins 5a,b and phthalocyanines. 6a–d In an effort to improve corrole yields, conventional heating was replaced by microwave irradiation. Indeed the microwave dielectric heating effect is an efficient heating source for thermally demanding organic reactions, such as corrole synthesis. Herein we report the first method to synthesize corroles under microwave irradiation, and the first application of Gross’s method to other kinds of aldehydes. New corroles 1–3 and 5–7 whose structures are depicted in Figure 1, and known ones like tris-pentafluorophenyl corrole 4 1a,b and tris-pyridyl-corrole 8 1d have been syn- thesized following two different heating procedures 7,8 and characterized by 1 H, 19 F NMR, HRMS and UV– vis. 9 In Table 1, yields of 1–8 are compared either when the reaction was carried on under the precise conditions described by Gross using conventional heating (entry H) or after using microwave irradiation (entry MW). Entry c gives the resulting increase of yield. Previously Gross showed that 4 would be obtained in 8–11% yield after 1 min–4 h under conventional heating at 100°C; 1a,b whereas we show here that 13–15% of isolated yield are obtained after only 2 min of microwave irradiation (ca. 47% increase). As pointed out by Gross, the reproducibility of this solvent-free synthesis on alumina in terms of isolated yields varies because of variation in the effectiveness of stirring. With a series of less reactive fluorinated benzaldehydes we showed that a similar range of yield increase is observed. Indeed, tetrafluorobenzaldehyde led to cor- role 3 which went from 10.5% (H) to 13% (MW) (ca. 28% increase). However other aldehyde precursors did not react spontaneously with pyrrole at room tempera- ture and require thermal or microwave activation. Thus trifluorobenzaldehyde led to corrole 2 in 6.7% (H) which increased to 8.9% (MW, ca. 33% increase). An experiment led to 12% which corresponds to 80% yield increase. Similarly p -fluorobenzaldehyde led to corrole 1 in 1.5% (H) and reached 2.0–2.5% (MW, 30% increase). We accounted for the observation Gross pre- viously made on two precursors, 2,6-difluorobenzalde- hyde and pentafluorobenzaldehyde, 1b by examining a range of aldehydes of different degrees of fluorination here, that the yield of corroles 1–4 is dependent upon the percentage of electron-withdrawing F atoms of the aldehydes (Table 1). Keywords : corrole synthesis; microwave irradiation. * Corresponding author. Tel.: (650) 725-0283; fax: (650) 725-0259; e-mail: jpc@stanford.edu 0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved. PII:S0040-4039(02)02792-2