The 6-(difluoromethyl)phenanthridine unit is a highly
attractive
fluoroalkyl-substituted planar nitrogen heterocycle in pharmaceutical
and agrochemical research. In this paper, we report that difluoromethylborates
can be used as a source of difluoromethyl radicals for isonitrile
insertion, leading to 6-(difluoromethyl)phenanthridines. Tuning the
aryl substituents in the difluoromethylborates and oxidizing reagents
enabled the synthesis of 6-(difluoromethyl)phenanthridines through
the generation of difluoromethyl radical and spontaneous intramolecular
cyclization of the CF2H-imidoyl radical intermediates.
The presence of difluoromethyl radicals was experimentally confirmed,
and the reaction mechanisms including imidoyl radical and prompt cyclization
reactions could be supported theoretically. Furthermore, we obtained
valuable information about the imidoyl radical intermediate by performing
transverse-field muon spin rotation (TF-μSR) measurements of
2-isocyano-4′-methoxy-1,1′-biphenyl and using density
functional theory (DFT) calculations to interpret the spectra. Muonium,
a simple free radical, preferentially adds to the carbon atom of the
isonitrile unit, yielding the corresponding imidoyl radical. The temperature
dependence of the muon hyperfine coupling constant and the spin relaxation
of the muoniated radical signal are compatible with the intramolecular
cyclization of biaryl-substituted imidoyl radicals on the μs
time scale.