Access to Benzo- and Naphtho-Azaphospholes via C-H Bond Activation of Aryl-Substituted Isonitriles

Differently substituted phenyl isonitriles (with C-H bonds in ortho-position) and naphthyl isonitriles were reacted with the cyclic biradical [⋅P(μ-N-Ter)₂ P⋅] (1). Insertion of the isonitrile formed a cyclic five-membered biradical [⋅P(NTer)₂ C(R)P⋅] (2R, R=phenyl, naphthyl) in the first step, followed by C-H activation at the aryl substituent, resulting in novel azaphospholes (5R), which could be isolated and fully characterized. The formation of the azaphospholes can be prevented by the addition of a second equivalent of isonitrile, which causes the blocking of the radical centers in 2R by adduct formation (3R). Quantum mechanical calculations showed that a significant increase in the aromaticity of the benzo- and naphtho-azaphospholes is one of the driving forces for the activation process leading to the formation of thermodynamically favored azaphospholes. Targeted activation of C-H bonds using biradical systems represents a new synthetic approach to generate benzo- and naphtho-azaphospholes.