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A quantitative reactivity scale for electrophilic fluorinating reagents.

ABSTRACT: Electrophilic N-F fluorination agents underpin the introduction of fluorine in aliphatic systems across drug and academic research. The choice of N-F reagent is currently determined through empirical experimentation in the absence of quantitative values for electrophilicities. Here we report an experimentally-determined kinetic reactivity scale for ten N-F fluorinating reagents, including Selectfluor™, NFSI, Synfluor™ and several N-fluoropyridinium salts, in CH₃CN. The reactivity scale, which covers eight orders of magnitude, employs para-substituted 1,3-diaryl-1,3-dicarbonyl derivatives to measure relative and absolute rate constants. The para-substituted 1,3-diaryl-1,3-dicarbonyl scaffold delivers a convenient, sensitive spectrophotometric reporter of reactivity that also led to the discovery of a unique form of tautomeric polymorphism.

SUBMITTER: Rozatian N

PROVIDER: S-EPMC6263395 | biostudies-literature | 2018 Dec

REPOSITORIES: biostudies-literature

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A quantitative reactivity scale for electrophilic fluorinating reagents.

Rozatian Neshat N Ashworth Ian W IW Sandford Graham G Hodgson David R W DRW

Chemical science 20180914 46

Electrophilic N-F fluorination agents underpin the introduction of fluorine in aliphatic systems across drug and academic research. The choice of N-F reagent is currently determined through empirical experimentation in the absence of quantitative values for electrophilicities. Here we report an experimentally-determined kinetic reactivity scale for ten N-F fluorinating reagents, including Selectfluor™, NFSI, Synfluor™ and several <i>N</i>-fluoropyridinium salts, in CH<sub>3</sub>CN. The reactivi ...[more]

PMID: 30595834

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Project description:The reactivity of mononuclear metal-hydroperoxo adducts has fascinated researchers in many areas due to their diverse biological and catalytic processes. In this study, a mononuclear cobalt(III)-peroxo complex bearing a tetradentate macrocyclic ligand, [CoIII(Me3-TPADP)(O2)]+ (Me3-TPADP = 3,6,9-trimethyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane), was prepared by reacting [CoII(Me3-TPADP)(CH3CN)2]2+ with H2O2 in the presence of triethylamine. Upon protonation, the cobalt(III)-peroxo intermediate was converted into a cobalt(III)-hydroperoxo complex, [CoIII(Me3-TPADP)(O2H)(CH3CN)]2+. The mononuclear cobalt(III)-peroxo and -hydroperoxo intermediates were characterized by a variety of physicochemical methods. Results of electrospray ionization mass spectrometry clearly show the transformation of the intermediates: the peak at m/z 339.2 assignable to the cobalt(III)-peroxo species disappears with concomitant growth of the peak at m/z 190.7 corresponding to the cobalt(III)-hydroperoxo complex (with bound CH3CN). Isotope labeling experiments further support the existence of the cobalt(III)-peroxo and -hydroperoxo complexes. In particular, the O-O bond stretching frequency of the cobalt(III)-hydroperoxo complex was determined to be 851 cm-1 for 16O2H samples (803 cm-1 for 18O2H samples), and its Co-O vibrational energy was observed at 571 cm-1 for 16O2H samples (551 cm-1 for 18O2H samples; 568 cm-1 for 16O22H samples) by resonance Raman spectroscopy. Reactivity studies performed with the cobalt(III)-peroxo and -hydroperoxo complexes in organic functionalizations reveal that the latter is capable of conducting oxygen atom transfer with an electrophilic character, whereas the former exhibits no oxygen atom transfer reactivity under the same reaction conditions. Alternatively, the cobalt(III)-hydroperoxo complex does not perform hydrogen atom transfer reactions, while analogous low-spin Fe(III)-hydroperoxo complexes are capable of this reactivity. Density functional theory calculations indicate that this lack of reactivity is due to the high free energy cost of O-O bond homolysis that would be required to produce the hypothetical Co(IV)-oxo product.

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A quantitative reactivity scale for electrophilic fluorinating reagents.

Publications

A quantitative reactivity scale for electrophilic fluorinating reagents.

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