From B(C₆F₅)₃ to B(OC₆F₅)₃:  Synthesis of (C₆F₅)₂BOC₆F₅ and C₆F₅B(OC₆F₅)₂ and Their Relative Lewis Acidity

Abstract

The pentafluorophenyl esters of bis(pentafluorophenyl)borinic acid (C 6 F 5 ) 2 BOC 6 F 5 (2) and pentafluorophenylboronic acid C 6 F 5 B(OC 6 F 5 ) 2 (3) have been prepared and characterized by multinuclear NMR and X-ray analysis. VT NMR studies have shown that restricted rotation around the B-O bond in 2 occurs below 193 K, corresponding to ∆G q ) 35 kJ/mol for this process. This low barrier and the random torsion angles around the B-O bonds observed in the solid state structures of compounds 2, 3, and B(OC 6 F 5 ) 3 (4) suggest that these torsion angles are not related to pπ-pπ interactions between boron and oxygen, but more likely a consequence of the extensive intermolecular F-π interactions seen in the solid state structures. The Lewis acidity of 2, 3, and 4 has been compared with B(C 6 F 5 ) 3 (1), using various Lewis bases. All compounds 1-4 appear to be strong Lewis acids, whereby 4 interacts more strongly with hard bases whereas 1 binds more strongly to softer bases.

Hydrogen Bonding and Lewis Acid–Base Interactions in the System Bis(pentafluorophenyl)borinic Acid / Methanol

“…[30] At low temperature, the two aryl rings became nonequivalent, so two 19 F signals are observed in each of the ortho, para, and meta regions (see Figures S1-S4 in Supporting Information). [19] This behavior, analogous to that previously observed for 1 m [8] and other esters of diarylborinic acids, [28,31] arises from the hindered rotation around the B-O bond, as further confirmed by the scalar and dipolar couplings observed, at 183 K, between the hydrogen atoms of the methoxy group and the ortho fluorine atoms of only one perfluorinated ring. Band-shape analysis of the para 19 F signals in the temperature range 197-281 K provided the kinetic parameters for the rotation of the OMe group around the Ar 2 B-O bond.…”

“…[7h, [26][27][28][29] As mentioned above, methyl borinate is formed even in the absence of an external water scavenger, upon treatment of borinic acid with small amounts of MeOH, at 183 K.…”

Sulfur and SeleniumActivation by Frustrated NHC/B(C6F5)3 Lewis Pairs; Conformational Flexibility of Products

“…However, the signal of the saturated complex 3b at 37.2 ppm is only moderately high-field shifted, indicating a weaker coordination of the B(C 6 F 5 ) 3 moiety at sulfur. The resonances in the 11 B NMR spectra show in comparison to related thioborates (≈ -10 ppm) [12,23] a distinct downfield shift consistent with the known ability of soft donors to form only weak adducts with Lewis acids [24]. The resonances of the former carbene carbon atom in the 13 C NMR spectra appear for 3a at 152.0 ppm and for 3b at 180.9 ppm, and thus are minimally shifted in contrast to the thiones (2a: 160.6 ppm; 2b: 183.6 ppm; [25]).…”

An Axially Chiral, Electron‐Deficient Borane: Synthesis, Coordination Chemistry, Lewis Acidity, and Reactivity

“…[6c, [32][33][34][35] We decided to employ the Gutmann-Beckett method and, in analogy to the work of Britovsek et al, [32] measured the 31 P NMR shifts of 1:1 adducts of 3 and Et 3 PO and Ph 3 PO in Table 1, columns 2 and 4). The Dd values for BA C H T U N G T R E N N U N G (C 6 F 5 ) 3 were then obtained from an identical series of experiments (Table 1, columns 5 and 6).…”

“…For example, non-fluorinated BPh 3 is not even interacting with Ph 3 PO. [32] The promising Lewis acidity of borane 3 encouraged us to test whether it would be sufficiently reactive in catalytic SiÀ H bond activation. [5] We chose the dehydrogenative Si-O coupling of 7 (rac-8!rac-10, Scheme 4, upper) and the carbonyl reduction with 7 (9!rac-10, Scheme 4, lower) as model reactions.…”