Click reactions of 2 H-azaphosphirene chromium and molybdenum complexes and a surprisingly facile access to a 2 H-1,4,2-diazaphosphole derivative

Abstract

Ring expansion reactions of 2 H-azaphosphirene chromium and molybdenum complexes 1a, b with dimethyl cyanamide, triflic acid, and, subsequently at ambient temperature, with triethylamine gave a mixture of the respective 2 H-1,4,2-diazaphosphole complex 2a, b and the non-ligated heterocycle 3. If the deprotonation with NEt 3 was carried out at low temperature, the selective formation of complexes 2a, b was observed, which were isolated in excellent yields and fully characterized (including single-crystal X-ray crystallography). Experimental and computational results revealed that the P, Cr and P, Mo bonds of 2 H-1,4,2-diazaphosphole complexes are significantly weakened upon N-protonation of the heterocyclic ligand. When mixtures of 1a, b, TfOH, and Me 2NCN were warmed to ambient temperature, the primarily formed N-protonated of 2 H-1,4,2-diazaphosphole complexes 4a, b could be observed by 31P NMR spectroscopy. The latter underwent decomplexation to give the N-protonated free ligand 5, which could be isolated and characterized by multinuclear NMR experiments. The neutral non-ligated heterocycle 3 was isolated from a one-pot reaction of 1b with TfOH and Me 2NCN by adding NEt 3 to a solution of intermediately formed 5.