Fluorinated piperidines find wide applications, most
notably in
the development of novel therapies and agrochemicals. Cyclization
of alkenyl N-tosylamides promoted by BF3-activated aryliodine(III) carboxylates is an attractive strategy
to construct 3-fluoropiperidines, but it suffers from selectivity
issues arising from competitive oxoaminations and the inability to
easily modulate the reactions diastereoselectivity. Herein, we report
an itemized optimization of the reaction conditions carried out on
both cyclic and acyclic substrates and outline the origins of substrate-
and reagent-based stereo-, regio-, and chemoselectivity. Extensive
mechanistic studies encompassing multinuclear NMR spectroscopy, deuterium
labeling, rearrangements on stereodefined substrates, and careful
structural analyses (NMR and X-ray) of the reaction products are performed.
This revealed the processes and interactions crucial for achieving
controlled preparation of 3-fluoropiperidines using I(III) chemistry
and has provided an advanced understanding of the reaction mechanism.
In brief, we propose that BF3-coordinated I(III) reagents
attack CC to produce the corresponding iodiranium(III) ion,
which then undergoes diastereodetermining 5-exo-cyclization.
Transiently formed pyrrolidines with an exocyclic σ-alkyl-I(III)
moiety can further undergo aziridinium ion formation or reductive
ligand coupling processes, which dictate not only the final product’s
ring size but also the chemoselectivity. Importantly, the selectivity
of the reaction depends on the nature of the ligand bound to I(III)
and the presence of electrolytes such as TBABF4. Reported
findings will facilitate the usage of ArI(III)-dicarboxylates in the
reliable construction of fluorinated azaheterocycles.