Remote C-H Pyridylation of Hydroxamates through Direct Photoexcitation of O-Aryl Oxime Pyridinium Intermediates.

Abstract

Herein, we report an efficient strategy for the remote C-H pyridylation of hydroxamates with excellent ortho-selectivity by designing a new class of photon-absorbing O-aryl oxime pyridinium salts generated in situ from the corresponding pyridines and hydroxamates. When irradiated by visible light, the photoexcitation of oxime pyridinium intermediates generates iminyl radicals via the photolytic N-O bond cleavage, which does not require an external photocatalyst. The efficiency of light absorption and N-O bond cleavage of the oxime pyridinium salts can be modulated through the electronic effect of substitution on the O-aryl ring. The resultant iminyl radicals enable the installation of pyridyl rings at the γ-CN position, which yields synthetically valuable C2-substituted pyridyl derivatives. This novel synthetic approach provides significant advantages in terms of both efficiency and simplicity and exhibits broad functional group tolerance in complex settings under mild and metal-free conditions.