Concurrent pathway and unexpected products in the CuAAC reaction of ethyl prop-2-ynyl methylphosphonate with aromatic azides

The CuI-mediated click reaction of aromatic azides, containing the carboxyl moiety in the ortho position to the azido group, with ethyl prop-2-ynyl methylphosphonate proceeded via a concurrent pathway whereby the formation of acrylamides predominated over "classical" cycloaddition products 1,2,3-triazoles. The reaction appears to be specific toward the ortho-carboxyl-substituted aromatic azides, while aryl azides with other substituents do not give the "reduction" products, but form the expected click products: (1-aryl-1H-1,2,3-triazol-4-yl)methyl ethyl methylphosphonates. The reaction mechanism is discussed.