Abnormal Mesoionic Carbene Silver Complex: Synthesis, Reactivity, and Mechanistic Insight on Oxidative Fluorination

A silver-catalyzed intramolecular
amination of alkynl-imine substrates
has been extensively studied to build various isoquinoline derivatives
efficiently. However, most of these transformations are limited to
hydroamination, and the related oxidative reaction is quite rare.
Importantly, the mechanistic details are still unknown, which retarded
further progress in the field. In this work, a novel abnormal mesoionic
carbene silver complex (MIC)nAg­(I) was
isolated and fully characterized as the key intermediate. Further
investigation on the oxidative transformation of the silver complex
reveals that successful oxidative halogenation could be achieved with
NXS (X = Cl, Br, and I), as well as F+ reagent. Surprisingly,
the fluorination reaction occurred in the presence of both strong
(SelectFluor) and weak (NFSI) fluorinating reagents, although the
F-Py-type reagent, whose oxidative potential lies between, is ineffective.
Further mechanistic studies disclosed that (1) from kinetic data,
the (MIC)­Ag­(I) complex was proved to be the reactive intermediate
in the fluorination reaction, and pyridyl-oxazoline (Pyox) ligand
could significantly improve this transformation; (2) from DFT calculation
results, two different mechanistic pathways were suggested to be involved,
a metathesis process in the case of NFSI promoted by the chelation
of sulfonyl group toward the silver center and a redox process in
the case of SelectFluor due to its strong oxidative potential.