Functionalization of the corrole ring: the role of isocorrole intermediates

Abstract

Bromination of 3-nitro-5,10,15-triarylcorrole selectively provides two regioisomers, depending on the reaction pathway. An isocorrole species is the key intermediate to drive the reaction towards the 2-Br-17-nitro regioisomer.It is impressive to note the attention that, over the last decade, corrole has received among porphyrinoids. While this consideration certainly arises from the intriguing chemistry shown by this macrocycle, which is unusual compared with the parent porphyrin congeners, it is obvious that these developments have been made possible by progress in synthetic routes leading to triarylcorroles. [1][2][3][4][5][6] The advances achieved in the synthetic chemistry of such a macrocycle allow the preparation of a large variety of corrole derivatives, which have concomitantly opened the way for their practical applications in different fields, ranging from sensors to clinical studies. [7][8][9] For these applications it is of paramount importance to develop methodology for diverse functionalization of corroles with peripheral substituents. It is also noteworthy that the functionalization of the preformed corrole framework is a relatively under-developed area. 10,11 Different factors might explain this developmental problem in the corrole field: while from one side the instability of corrole could hamper some functionalization routes, from the other side the lower symmetry of corrole compared with porphyrin could lower the yields of the desired functionalization, because in principle a complex mixture of different regioisomers could be obtained. 10 A further obstacle to the functionalization of the macrocycle has been the lack of simple protocols for the demetallation of corrole complexes. Some of these problems have been solved in the last few years, such as the admirable regioselectivity showed by corrole for substitution, and the definition of some route for the demetallation of Cu and Mn metal derivatives. [12][13][14][15][16] The problem of corrole instability under reaction conditions for functionalization pathways usually leads to the formation of isocorrole by-products, 17,18 formed because of the facile oxidation of the corrole ring, as has been observed, for example, in the case of nitration reactions. 19 However, this feature should not be an unavoidable drawback but rather a tool to widen the possible pathways for corrole functionalization. To define synthetic routes for the introduction of different substituents on the peripheral positions of meso-triarylcorroles, we decided to use as starting material the 3-nitro-5,10,15-tris(4-methylphenyl)corrole, 3- NIH Public Access Author ManuscriptChem Commun (Camb). Author manuscript; available in PMC 2011 April 14. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript NO 2 -ttcorrH 3 , which can be obtained in good yields from 5,10,15-tris-(4-methylphenyl)corrole ttcorrH 3 20 using AgNO 2 as the nitrating agent, 21 followed by reductive demetallation with DBU; 19 bromination of corrole was selected as the test reaction...

Macrocyclic Transformations from Norrole to Isonorrole and an N‐Confused Corrole with a Fused Hexacyclic Ring System Triggered by a Pyrrole Substituent

“…Thec haracteristic signal at d = 68.3 ppm in the 13 CNMR spectrum supports the proposed isonorrole structure with an sp 3 -hybridized meso-carbon atom. As ar esult of the disruption of the conjugation in L2,t he pyrrolic bÀHs ignals lie in the range of d = 5.8-7.0 ppm, reflecting its nonaromatic character.…”

“…Upon transformation of L2 to L3 and subsequently to L4, typical spectral features of an N-confused corrole [6c] were evident in the 1 Ha nd 13 CNMR spectra (see the Supporting Information). In the 1 HNMR spectrum of L3 ( Figure S7), aresonance signal for an outer NH unit appears at lower field (d = 13.20 ppm) and two internal NH signals appear at relatively high field at d = 8.52 and 4.56 ppm.…”

“…Thefused tricyclic unit (rings D-F) are lying approximately perpendicular to the tripyrrin with an interplanar angle of 72.18 8,w hich is more severely distorted with respect to that of norrole (36.48 8) [6c] because of the presence of the sp 3 -hybridized meso-carbon atom. Based on the similarities between L2-Br and L2 in the 1 Ha nd 13 CNMR spectra ( Figures S1-S6), ad istorted isonorrole-like structure was proposed for compound L2.…”

“…Different from the regular isocorroles, [13] isonorrole L2 can be considered as anew class of tetrapyrrolic macrocycle,and it showed further reactivity for ring-fusion reactions. [14] Thes ingly and doubly N-C Ar -fused N-confused corroles (L3 and L4)w ere subsequently generated by double skeletal rearrangement of NÀC linkages (specifically the NÀC a and NÀC meso bonds).…”

Synthetic protocols for the nitration of corroles

“…The usefulness of the nitro group comes from the possibility to further modify the porphyrinoid macrocycle in a selective way [18, 19]; indeed the ability of this substituent to drive the attack of another functional group on corrole has been reported, making it possible to prepare functionalized compounds not achievable by straightforward synthetic pathways; a second interesting capability of –NO 2 is its reduction to the amino group, which could be the first step toward exploitation of aminocorroles and the construction of covalently linked porphyrinoid moieties.…”