Nucleophilic Substitution vs Ring Opening – Dual Reactivity of Benzo[b]phosphol-3-yl Triflates toward Alkyl Grignard Reagents

A new method for the synthesis of 3-alkylbenzo[b]phosphole oxides involving nucleophilic substitution through an addition–elimination mechanism has been developed. The reaction proceeds without a metal catalyst and yields a library of previously unavailable 3-alkylbenzo[b]phosphole oxides in yields of up to 90%. This method enables regioselective functionalization at position 3 in the benzophosphole ring, providing the possibility for independent functionalization at both the 3-position and 4–7 positions (benzoring). A competitive ring-opening process that yields 2-ethynylphenyl(alkyl)phosphine oxides and subsequent processes that lead to cyclic side products were also investigated. Mechanistic proposals for all of these processes are presented.