Han B, Khasnavis SR, Nwerem M, Bertagna M, Ball ND, Ogba OM. Calcium Bistriflimide-Mediated Sulfur(VI)-Fluoride Exchange (SuFEx): Mechanistic Insights toward Instigating Catalysis.
Inorg Chem 2022;
61:9746-9755. [PMID:
35700314 PMCID:
PMC9241145 DOI:
10.1021/acs.inorgchem.2c01230]
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Abstract
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We report a mechanistic
investigation of calcium bistriflimide-mediated
sulfur(VI)–fluoride exchange (SuFEx) between sulfonyl fluorides
and amines. We determine the likely pre-activation resting state—a
calcium bistriflimide complex with ligated amines—thus allowing
for corroborated calculation of the SuFEx activation barrier at ∼21
kcal/mol, compared to 21.5 ± 0.14 kcal/mol derived via kinetics
experiments. Transition state analysis revealed: (1) a two-point calcium-substrate
contact that activates the sulfur(VI) center and stabilizes the leaving
fluoride and (2) a 1,4-diazabicyclo[2.2.2]octane additive that provides
Brønsted-base activation of the nucleophilic amine. Stable Ca–F
complexes upon sulfonamide formation are likely contributors to inhibited
catalytic turnover, and a proof-of-principle redesign provided evidence
that sulfonamide formation is feasible with 10 mol % calcium bistriflimide.
We report a computational and experimental
mechanistic study
of sulfur(VI)-fluoride exchange mediated by a calcium salt. Ca2+ activates the substrate via two critical Lewis acid−base
interactions. Stable fluoride-ligated Ca2+ complexes are
formed upon sulfonamide formation, and computations indicate that
heat and/or fluoride scavengers will facilitate regeneration of the
active Ca2+ species. These calculations guided an experimental
redesign, thus demonstrating improved catalytic efficiency using 10
mol % Ca2+.
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