Synthetic protocols for the nitration of corroles

Abstract

The modification of peripheral positions of corroles by introduction of nitro groups is an important functionalization of this macrocycle. The nitro substituent strongly influences the corrole behavior leading to the preparation of macrocycles with different properties, which can be of interest for their exploitation as catalysts, sensing layers in chemical sensors or in the field of supramolecular chemistry. In the last few years we have developed different routes for the β-nitration of the corrole ring, and we report here novel synthetic protocols which can allow the formation of tri- and tetranitro derivatives, as demonstrated by X-ray analysis. In all of the methodologies used, the presence of isocorrole species as reaction intermediates was established, which regenerated the corresponding corrole by metal insertion.

β-Nitro-5,10,15-tritolylcorroles

“…12 In the case of TFA/NaNO 2 or HCl/NaNO 2 systems, the formation of β-nitroisocorroles was observed, but these were converted in a second step into the corresponding aromatic corrole complexes by cobalt insertion. 13 Our hypothesis for the nitration reaction mechanism is that the first step requires an oxidant species for the generation of the corrole α-cation radical, which is then attacked by nitrite to afford the final product. However, corrole free bases demonstrate extreme sensitivity under oxidizing reaction conditions, leading to a quite facile conversion of the macrocycle into the corresponding isocorrole species.…”

“…9 The reaction afforded β-amino-β-nitro derivatives, which can have great synthetic value for the elaboration of new chromophores, as well as extended aromatic β-fused architectures. To date β-nitrocorrole derivatives of copper, 9 germanium, 10 gallium, 11 silver, 12 cobalt, 13 and iron 14 have been prepared by using a variety of nitrating systems and reaction conditions, while all attempts to nitrate the corrole free-base failed, mainly because of the lability of corroles under these substitution conditions. This was evidenced under a variety of reaction conditions with the formation of isocorrole species.…”

Copper β-trinitrocorrolates

“…All these aspects prompted us to study the nitration reaction in depth and, to date, we have provided several synthetic methodologies, applying various nitrating systems to achieve β-nitro-derivatives of Cu [9b], Ge [10], Ag [9a], Fe [11] and Co [12] corrolates. Very recently, we have reported the preparation of β-nitro-5,10,15-tritolylcorroles [9c], which represents the first example of selective β-nitration on the corrole free base and potentially opens the way to novel complexes of β-nitrocorroles upon metal insertion.…”

“…We have recently reported the possibility to achieve tri- and tetra-nitro-derivatives of corroles, using TFA/NaNO 2 as the nitrating system [12]. In this case, the extreme lability of corrole in acidic media caused both the degradation of the starting material and the concomitant formation of the nitroisocorrole species, which was converted into the aromatic form by metalation with cobalt salt and PPh 3 .…”

Synthesis and functionalization of β-alkyl-meso-triarylcorroles

“…The nitration is the first step to prepare new substrates useful for so-called "click chemistry" [12,13]. For this last reaction, different conditions have been mainly optimized [14,15] for the preparation of the 3-NO 2 , and 3,17-(NO 2 ) 2 derivatives, either using the free base or Cu complexes, even though a wide range of differently mono or polynitrated products have been obtained by changing the reaction conditions [16,17].…”