A Biphilic Phosphetane Catalyzes N-N Bond-Forming Cadogan Heterocyclization via P~Ⅲ/P~Ⅴ=O Redox Cycling

摘要

A small-ring phosphacycle, 1,2,2,3,4,4-hexa-methylphosphetane, is found to catalyze deoxygenative N-N bond-forming Cadogan heterocyclization of ο-nitrobenzaldimines, ο-nitroazobenzenes, and related substrates in the presence of hydrosilane terminal reductant. The reaction provides a chemoselective catalytic synthesis of 2H-indazoles, 2H-benzotriazoles, and related fused heterocyclic systems with good functional group compatibility. On the basis of both stoichiometric and catalytic mechanistic experiments, the reaction is proposed to proceed via catalytic P~Ⅲ/P~Ⅴ=O cycling, where DFT modeling suggests a turnover-limiting (3+1) cheletropic addition between the phosphetane catalyst and nitroarene substrate. Strain/distortion analysis of the (3+1) transition structure highlights the controlling role of frontier orbital effects underpinning the catalytic performance of the phosphetane.