The facile and clean oxidative coupling reaction of pyrroles with azoles has been achieved using the recyclable hypervalent iodine(III) reagents having adamantane structures. These iodine(III) reagents could be recovered from the reaction mixtures by a simple solid-liquid separation, i.e., filtration, for reuse.Key words oxidative coupling; hypervalent iodine reagent; recycle; pyrrole; azole; C-N bond formationThe N-pyrrolyl azoles are important fragments in the molecules of biological systems or in many pharmaceuticals, insecticides, and functional materials, especially, fluorescent dyes. [1][2][3][4] In spite of this interest, the preparations of N-aryl azoles are severely restricted because the nitrogen heteroaromatics are sometimes not a good substrate to use in the traditional arylation methods such as the Ullmann coupling, [5][6][7][8][9][10][11] since the pyrrole-based metal and halide compounds are sometimes unstable. Katritzky et al. reported the synthesis of pyrrolyl-benzotriazoles by the Mannich condensation of o-phthalaldehyde and 2,5-dimethoxy-2,5-dihydrofuran with primary amines, 12) but this method was not effective for pyrroles, and the desired products were produced only in less than 10% yields. As a new approach, the oxidative C-H bond functionalization strategies of pyrroles [13][14][15][16][17][18][19][20][21][22] might be expected to be an attractive methodology for enabling rapid access to these molecules. Recently, we have developed a new hypervalent iodine-induced oxidative coupling of pyrroles at the N 1 -positions of the azoles. 23) In 2014, Chen and colleagues reported the oxidative coupling method of 1,2,3-triazoles with pyrroles using N-iodosuccinimide (NIS) as an oxidant, in which the reactions preferentially occurred at the N 2 -position of the 1,2,3-triazoles.24) However, these methods produced stoichiometric quantities of iodoarene or succinimide as a waste product after the reaction (Chart 1).Hypervalent iodine reagents have received much attention due to their mild oxidation abilities and low toxicity, easy handling, as alternatives to toxic heavy metal reagents. [25][26][27][28][29][30][31][32] For developing a new coupling for heteroaromatic compounds, we have been engaged in the study of the oxidative C-H bond arylations using hypervalent iodine reagents that utilize the new reactivities of the heteroaromatic iodonium intermediates. [33][34][35][36][37] As mentioned, we developed a hypervalent iodineinduced oxidative coupling of pyrroles at the N 1 -positions of the azoles.23) This new method is versatile in allowing the reactions under metal-free and mild conditions, and tolerates a diverse series of functionalities of the pyrroles and azole coupling partners. We envisioned that a recycling version of this oxidative coupling should become feasible and provide an eco-friendly synthetic method (Chart 2) because a number of recent studies have demonstrated the utility of the recyclable hypervalent iodine reagents.
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Results and DiscussionThe use of recyclable hypervalent ...