Silver-mediated fluorination of potassium aryltrifluoroborates with Selectfluor®

Mechanism of the Aryl-F Bond-Forming Step from Bi(V) Fluorides

In this article, we describe a combined experimental and theoretical mechanistic investigation of the C(sp2)-F bond formation from neutral and cationic high-valent organobismuth(V) fluorides, featuring a dianionic bis-aryl sulfoximine ligand. An exhaustive assessment of the substitution pattern in the ligand, the sulfoximine, and the reactive aryl on neutral triarylbismuth(V) difluorides revealed that formation of dimeric structures in solution promotes facile Ar-F bond formation. Noteworthy, theoretical modeling of reductive elimination from neutral bismuth(V) difluorides agrees with the experimentally determined kinetic and thermodynamic parameters. Moreover, the addition of external fluoride sources leads to inactive octahedral anionic Bi(V) trifluoride salts, which decelerate reductive elimination. On the other hand, a parallel analysis for cationic bismuthonium fluorides revealed the crucial role of tetrafluoroborate anion as fluoride source. Both experimental and theoretical analyses conclude that C-F bond formation occurs through a low-energy five-membered transition-state pathway, where the F anion is delivered to a C(sp2) center, from a BF4 anion, reminiscent of the Balz-Schiemann reaction. The knowledge gathered throughout the investigation permitted a rational assessment of the key parameters of several ligands, identifying the simple sulfone-based ligand family as an improved system for the stoichiometric and catalytic fluorination of arylboronic acid derivatives.

Dissecting transmetalation reactions at the molecular level: C-B versus F-B bond activation in phenyltrifluoroborate silver complexes

The gas-phase unimolecular reactions of the silver(i) complex [Ag(PhBF₃)₂]^-, formed via electrospray ionisation mass spectrometry of solutions containing the phenyltrifluoroborate salt and AgNO₃, are examined. Upon collision induced dissociation (CID) three major reaction channels were observed for [Ag(PhBF₃)₂]^-: Ph^- group transfer via a transmetalation reaction to yield [PhAg(PhBF₃)]^-; F^- transfer to produce [FAg(PhBF₃)]^-; and release of PhBF₃^-. The anionic silver product complexes of these reactions, [LAg(PhBF₃)]^- (where L = Ph and F), were then mass-selected and subjected to a further stage of CID. [PhAg(PhBF₃)]^- undergoes a Ph^- group transfer via transmetalation to yield [Ph₂Ag]^- with loss of BF₃. [FAg(PhBF₃)]^- solely fragments via loss of BF₄^-, a reaction that involves Ph^- group transfer from B to Ag in conjunction with F^- transfer from Ag to B. Density functional theory (DFT) calculations on the various competing pathways reveal that: (i) the overall endothermicities govern the experimentally observed product ion abundances; (ii) the Ph^- group and F^- transfer reactions proceed via late transition states; and (iii) formation of BF₄^- from [FAg(PhBF₃)]^- is a multistep reaction in which Ph^- group transfer from B to Ag proceeds first to produce a [FAgPh(BF₃)]^- complex in which the BF₃ moiety is initially weakly bound to the ipso-carbon of the phenyl group and then migrates across the linear [FAgPh]^- moiety from C to Ag to F yielding [PhAg(BF₄)]^-, which can then dissociate via loss of PhAg.

Synthesis of new fluorescent molecules having an aggregation-induced emission property derived from 4-fluoroisoxazoles

Fluorescent molecules based on a fluorinated isoxazole scaffold were synthesized and investigated for their photochemical properties. The introduction of a fluorine substituent into 3,5-diarylisoxazoles led to an increase of fluorescence intensity and exhibited a redshift in the emission intensity. α-Fluorinated boron ketoiminates (F-BKIs) were also synthesized via a ring-opening reaction of 4-fluoroisoxazoles and exhibited highly fluorescent luminescence and aggregation-induced emission (AIE), showing promise as a new fluorophore.

A New Alkylation of Aryl Alcohols by Boron Trifluoride Etherate

The ethylation of aryl alcohols by an ethyl moiety of boron trifluoride etherate is described. The reaction proceeded cleanly and afforded good yields of the corresponding aryl ethyl ethers. It tolerated the presence of functional groups such as aryl, alkyl, halogens, nitro, nitrile, and amino. However, the presence of amino or nitro groups ortho to a hydroxyl group of an aryl compound drastically reduced the yields of the anticipated products due to the chelation of the aforementioned functional groups with boron trifluoride etherate. A nitrogen atom in the aromatic ring system, as exemplified by hydroxypyridine and 8-hydroxyquinoline, completely inhibited the reaction. Resorcinol, hydroquinone, and aryl alcohols with aldehyde functions decomposed under the reaction conditions.

Lewis acid promoted fluorine-alkoxy group exchange reactions for the synthesis of 5-alkoxy-4,4-difluoroisoxazoline systems

Lewis acid promoted substitution of fluorine yields novel 5-alkoxylated 4,4-difluoroisoxazolines via S_N1 type processes.

Exposing the Origins of Irreproducibility in Fluorine NMR Spectroscopy

Fluorine chemistry has taken a pivotal role in chemical reaction discovery, drug development, and chemical biology. NMR spectroscopy, arguably the most important technique for the characterization of fluorinated compounds, is rife with highly inconsistent referencing of fluorine NMR chemical shifts, producing deviations larger than 1 ppm. Herein, we provide unprecedented evidence that both spectrometer design and the current unified scale system underpinning the calibration of heteronuclear NMR spectra have unintentionally led to widespread variation in the standardization of ¹⁹ F NMR spectral data. We demonstrate that internal referencing provides the most robust, practical, and reproducible method whereby chemical shifts can be consistently measured and confirmed between institutions to less than 30 ppb deviation. Finally, we provide a comprehensive table of appropriately calibrated chemical shifts of reference compounds that will serve to calibrate ¹⁹ F spectra correctly.