Electrochemical Properties and Reactions of Organoboronic Acids in the Presence of Fluoride Ions

Abstract

Electrochemical analyses of alkyl, phenyl, and thienylboronic acids were comparatively studied by cyclic voltammetric measurements in the absence and presence of fluoride ions, and we found remarkable negative shift of their oxidation potentials in the presence of fluoride ions compared with those in the absence of fluoride ions. Such negative shift seems to be derived from the formation of negative charged boron‐ate complex with fluoride ions. Anodic fluorination of 2‐acetyl‐5‐thienylboronic acid was carried out to provide mono‐ and trifluoro products eliminating a boryl group in moderate yields. These results clearly indicate that a boryl group with fluoride ions is a good electroauxiliary.

General Access to C-Centered Radicals: Combining a Bioinspired Photocatalyst with Boronic Acids in Aqueous Media

“…Interestingly, increased amounts of 3-hydroxypyridine were found in the case of less effective flavin photocatalysts. In line with these results, we suspected that 3-hydroxypyridine might act as a poison through competitive oxidation ( E p ox ≈ 0.9 V vs SCE in H 2 O pH 7.4) in lieu of the less readily oxidized 3-pyridineboronic acid (for reference, phenylboronic acid is E p ox = 2.55 V vs SCE and potassium phenyltrifluoroborate is E p ox = 1.95 V vs SCE) . Indeed, adding 10 mol % or 1 equiv of 3-hydroxypyrdine to our standard reaction mixture resulted in no product formation and no consumption of pyridine-3-boronic acid.…”

“…In line with these results, we suspected that 3hydroxypyridine might act as a poison through competitive oxidation (E p ox ≈ 0.9 V vs SCE in H 2 O pH 7.4) 30 in lieu of the less readily oxidized 3-pyridineboronic acid (for reference, phenylboronic acid is E p ox = 2.55 V vs SCE and potassium phenyltrifluoroborate is E p ox = 1.95 V vs SCE). 31 Indeed, adding 10 mol % or 1 equiv of 3-hydroxypyrdine to our standard reaction mixture resulted in no product formation and no consumption of pyridine-3-boronic acid. At this point, we gathered that modifying the pH of the reaction and changing the organic cosolvent could ameliorate background aryl−alcohol formation and enhance reaction efficiency.…”

“…We hypothesized that the diminished yield could be due to the limited nucleophilicity of the 3-pyridyl radical. To examine this theory, we subjected the more readily oxidized sodium trihydoxy­(pyridine-3-yl)­borate salt to our reaction conditions. , The yield of the reaction was comparable. Employing the more nucleophilic radical precursor, 6-methoxy-3-pyridineboronic acid ( 1 ), afforded a significant increase in reaction efficiency to 26% yield.…”

“…Singlet lumiflavin has been shown to oxidize aromatic compounds ≤2.0 V vs SCE via SET . Aromatic, heteroaromatic, and aliphatic boronic acids often have potentials ≥ 2.0 V vs SCE, leading to an unfavorable electron tranfer . Interestingly, exothermic coordination of a molecule of water to the boronic acid (Scheme , iv, −2.7 kcal mol –1 ) is predicted to facilitate a favorable PCET or HAT event, but not SET, from singlet lumiflavin (Scheme , v, −18 kcal mol –1 ).…”

The selective electrochemical fluorination of S-alkyl benzothioate and its derivatives

“…Based on these facts, we successfully achieved the first anodic fluorination of sulfides having various EWGs at their α-position using Et 3 N·3HF [ 9 – 10 ]. Since then, we have systematically studied the electrochemical fluorination of numerous organic compounds, heterocycles, and macromolecules by using various fluoride salts such as Et 3 N· n HF ( n = 3–5) and Et 4 NF· n HF ( n = 3–5) [ 11 – 19 ].…”