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Chemoselective isocyanide insertion into the N–H bond using iodine–DMSO: metal-free access to substituted ureas

Bora, Porag, Bez, Ghanashyam
Chemical communications 2018 v.54 no.60 pp. 8363-8366
Lewis bases, aromatic amines, catalysts, chemical bonding, chemoselectivity, metals, polymerization, toxicity
Insertion of isocyanides into the N–H bond gives access to many medicinally important and structurally diverse complex nitrogen-containing heterocycles. Although the transition metal catalyzed isocyanide insertion into the N–H bond is very common, polymerization of isocyanides in the presence of a transition metal and their strong coordination with metals are the common drawbacks. On the other hand, the inertness of most of the isocyanides towards amines in the absence of a metal catalyst has stymied the growth of the metal-free approach for isocyanide insertion into amines. As a result, only a handful of metal catalysed methods with limited substrate scopes have been reported for the synthesis of ureas via isocyanide insertion into amines and no metal-free version has been reported yet. Interestingly, chemoselective isocyanide insertion into amines has not been reported in the literature. We employed the I₂–DMSO reagent system for the chemoselective synthesis of ureas, where isocyanides react with aliphatic amines only, while aromatic amines need a nucleophilic activator (DABCO) to facilitate the formation of ureas. This method gave direct and chemoselective access to ureas by evading the commonly used yet toxic isocyanates.