Unlocking Lewis acidity via the redox non-innocence of a phenothiazine-substituted borane.

We describe a new approach to enhancing Lewis acidity, through the single electron oxidation of a borane with a pendant phenothiazine. This results in the formation of a persistent radical cation with increased electrophilicity. Computational and experimental studies indicate this radical cation significantly enhances the Lewis acidity and catalytic activity compared to its neutral analog. These results illustrate the viability of this approach in turning on the Lewis acidity of relatively inert boranes.

Probing the Impact of Solvent on the Strength of Lewis Acids via Fluorescent Lewis Adducts.

Various methods have been developed to measure the strength of a Lewis acid. A major challenge for these measurements lies in the complexity that arises from variable solvent interactions and perturbations of Lewis acids as their reaction environment changes. Herein, we investigate the impact of solvent effects on Lewis acids for the first time as measured by the fluorescent Lewis adduct (FLA) method. The binding of a Lewis acid in various solvents reveals a measurable dichotomy between both polarity and donor ability of the solvent. While not strictly separable, we observe that the influence of solvent polarity on Lewis acid unit (LAU) values is distinctly opposite to the influence of donor ability. This dichotomy was confirmed by titration data, illustrating that solvation effects can be appropriately and precisely gauged by the FLA method.

A boron dipyrromethene (BODIPY) based probe for selective passive sampling of atmospheric nitrous acid (HONO) indoors.

People spend up to 90% of their time indoors, and yet our understanding of indoor air quality and the chemical processes driving it are poorly understood, despite levels of key pollutants typically being higher indoors compared to outdoors. Nitrous acid (HONO) is a species that drives these indoor chemical processes, with potentially detrimental health effects. In this work, a BODIPY-based probe was synthesized with the aim of developing the first selective passive sampler for atmospheric HONO. Our probe and its products are easily detected by UV-Vis spectroscopy with molar extinct coefficients of 37 863 and 33 787 M-1 cm-1, respectively, and a detection limit of 14.8 ng mL-1. When protonated, the probe fluoresces with a quantum yield of 33%, which is turned off upon reaction. The synthesized BODIPY probe was characterized using NMR and UV-Vis spectroscopy. Products were characterized by UV-Vis and ultra high-resolution mass spectrometry. The reaction kinetics of the probe with nitrite was studied using UV-Vis spectroscopy, which had a pseudo-first-order rate of k = 7.7 × 10-4 s-1. The rapid reaction makes this probe suitable for targeted ambient sampling of HONO. This was investigated through a proof-of-concept experiment with gaseous HONO produced by a custom high-purity calibration source delivering the sample to the BODIPY probe in an acidic aqueous solution in clean air and a real indoor air matrix. The probe showed quantitative uptake of HONO in both cases to form the same products observed from reaction with nitrite, with no indication of interferences from ambient NO or NO2. The chemical and physical characteristics of the probe therefore make it ideal for use in passive samplers for selective sampling of HONO from the atmosphere.

Allenic phosphonium borate zwitterions via a phosphonium allenylidene intermediate.

We describe the synthesis of alkynyl phosphanes of the type R2P-C[triple bond, length as m-dash]C-C(OCH3)Ph2 (R = Ph, Cy) and investigate their transformation to geminally substituted phosphonium borato-allene zwitterions upon their reaction with B(C6F5)3. The mechanism for this transformation was studied experimentally and by density functional theory computations (DFT), suggesting the intermediacy of an unsaturated 3-coordinate phosphonium electrophile akin to a methylene phosphonium cation.

The synthesis, properties, and reactivity of Lewis acidic aminoboranes.

The evolution of frustrated Lewis pair chemistry has led to significant research into the development of new Lewis acidic boranes. Much of this has focused on modifying aryl substituents rather than introducing heteroatoms bound to boron. We recently reported that bis(pentafluorophenyl)phenothiazylborane (1) could be used as a Lewis acid catalyst for the heterolytic dehydrocoupling of stannanes. In this work, we synthesize and characterize a family of Lewis acidic aminoboranes and explored their reactivity with various Lewis bases as well as their efficacy as catalysts for stannane dehydrocoupling and hydrosilylation. Quantum chemical calculations were undertaken to understand the origins of the Lewis acidity and the most Lewis acidic aminoborane (5) was found to be an effective catalyst even in coordinating solvents such as water or acetonitrile, suggesting the amino substituent provides a level of protection against competing donors.

Bis(pentafluorophenyl)phenothiazylborane - an intramolecular frustrated Lewis pair catalyst for stannane dehydrocoupling.

We synthesized a novel Lewis acidic aminoborane containing a phenothiazyl substituent and demonstrated its potential to catalytically promote the dehydrocoupling of tin hydrides. The observed reactivity would imply a homolytic frustrated Lewis pair type mechanism, however computational analysis suggests a heterolytic mechanism for this reaction. This result represents one of the first frustrated Lewis pair systems to dehydrocouple stannanes in a heterolytic fashion.

Use of Trifluoromethyl Groups for Catalytic Benzylation and Alkylation with Subsequent Hydrodefluorination.

The electrophilic organofluorophosphonium catalyst [(C6F5)3PF][B(C6F5)4] is shown to effect benzylation or alkylation by aryl and alkyl CF3 groups with subsequent hydrodefluorination, thus resulting in a net transformation of CF3 into CH2-aryl fragments. In the case of alkyl CF3 groups, Friedel-Crafts alkylation by the difluorocarbocation proceeded without cation rearrangement, in contrast to the corresponding reactions of alkyl monofluorides.

Electrophilic Fluorophosphonium Cations in Frustrated Lewis Pair Hydrogen Activation and Catalytic Hydrogenation of Olefins.

The combination of phosphorus(V)-based Lewis acids with diaryl amines and diaryl silylamines promotes reversible activation of dihydrogen and can be further exploited in metal-free catalytic olefin hydrogenation. Combined experimental and density functional theory (DFT) studies suggest a frustrated Lewis pair type activation mechanism.

Synthesis and Lewis acidity of fluorophosphonium cations.

A series of fluorophosphonium salts, [R3PF][X] (R = alkyl or aryl; X = FB(C6F5)3, [B(C6F5)4]), have been prepared by reactions of phosphine/borane frustrated Lewis pairs (FLPs) with XeF2 or difluorophosphoranes with [Et3Si][B(C6F5)4]. As the substituents bound to phosphorus become increasingly electron withdrawing, the corresponding fluorophosphonium salts are shown to be increasingly Lewis acidic. Calculations were also performed to determine the relative fluoride ion affinities (FIA) of these fluorophosphonium cations.

Hydrosilylation of ketones, imines and nitriles catalysed by electrophilic phosphonium cations: functional group selectivity and mechanistic considerations.

The electrophilic phosphonium salt, [(C6 F5 )3 PF][B(C6 F5 )4 ], catalyses the efficient hydrosilylation of ketones, imines and nitriles at room temperature. In the presence of this catalyst, adding one equivalent of hydrosilane to a nitrile yields a silylimine product, whereas adding a second equivalent produces the corresponding disilylamine. [(C6 F5 )3 PCl][B(C6 F5 )4 ] and [(C6 F5 )3 PBr][B(C6 F5 )4 ] are also synthesised and tested as catalysts. Competition experiments demonstrate that the reaction exhibits selectivity for the following functional groups in order of preference: ketone>nitrile>imine>olefin. Computational studies reveal the reaction mechanism to involve initial activation of the Si-H bond by its interaction with the phosphonium centre. The activated complex then acts cooperatively on the unsaturated substrate.

Reactivity of an intramolecular fluorophosphonium fluoroborate.

The reactions of the intramolecular frustrated Lewis pair-adduct Ph(2) PC(p-Tol)=C(C(6) F(5))B(C(6)F(5))2 (CNtBu) with XeF(2) gave Ph(2)P(F)C(p-Tol)=C(C(6)F(5))B(F)(C(6)F(5))(2)(3). This species reacts with two equivalents of Al(C(6)F(5))(3)⋅C(7)H(8) producing the salt, [Ph(2)P(F)C(p-Tol)=C(C(6)F(5))B(C(6)F(5))(2)][F(Al(C(6)F(5))(3))(2)] (4), whereas reaction with HSiEt(3)/B(C(6)F(5))(3) gave Ph(2) P(F)C(p-Tol)=C(H)B(C(6)F(5))(3) (5). The photolysis of 3 resulted in aromatization affording the phenanthralene derivative Ph(2) P(F)C(p-Tol(o-C(6)F(4)))=CB(F)(C(6)F(5))(2) (6).

Metal-free transfer hydrogenation of olefins via dehydrocoupling catalysis.

A major advance in main-group chemistry in recent years has been the emergence of the reactivity of main-group species that mimics that of transition metal complexes. In this report, the Lewis acidic phosphonium salt [(C6F5)3PF][B(C6F5)4] 1 is shown to catalyze the dehydrocoupling of silanes with amines, thiols, phenols, and carboxylic acids to form the Si-E bond (E = N, S, O) with the liberation of H2 (21 examples). This catalysis, when performed in the presence of a series of olefins, yields the concurrent formation of the products of dehydrocoupling and transfer hydrogenation of the olefin (30 examples). This reactivity provides a strategy for metal-free catalysis of olefin hydrogenations. The mechanisms for both catalytic reactions are proposed and supported by experiment and density functional theory calculations.

Olefin isomerization and hydrosilylation catalysis by Lewis acidic organofluorophosphonium salts.

Organofluorophosphonium salts of the formula [(C6F5)(3-x)Ph(x)PF][B(C6F5)4] (x = 0, 1) exhibit Lewis acidity derived from a low-lying σ* orbital at P opposite F. This acidity is evidenced by the reactions of these salts with olefins, which catalyze the rapid isomerization of 1-hexene to 2-hexene, the cationic polymerization of isobutylene, and the Friedel-Crafts-type dimerization of 1,1-diphenylethylene. In the presence of hydrosilanes, olefins and alkynes undergo efficient hydrosilylation catalysis to the alkylsilanes. Experimental and computational considerations of the mechanism are consistent with the sequential activation and 1,2-addition of hydrosilane across the unsaturated C-C bonds.

Lewis acidity of organofluorophosphonium salts: hydrodefluorination by a saturated acceptor.

Prototypical Lewis acids, such as boranes, derive their reactivity from electronic unsaturation. Here, we report the Lewis acidity and catalytic application of electronically saturated phosphorus-centered electrophilic acceptors. Organofluorophosphonium salts of the formula [(C6F5)(3-x)Ph(x)PF][B(C6F5)4] (x = 0 or 1; Ph, phenyl) are shown to form adducts with neutral Lewis bases and to react rapidly with fluoroalkanes to produce difluorophosphoranes. In the presence of hydrosilane, the cation [(C6F5)3PF](+) is shown to catalyze the hydrodefluorination of fluoroalkanes, affording alkanes and fluorosilane. The mechanism demonstrates the impressive fluoride ion affinity of this highly electron-deficient phosphonium center.

Combinations of ethers and B(C6F5)3 function as hydrogenation catalysts.

It works ether way: Labile adducts of dialkyl ethers with the electrophilic borane B(C6F5)3 are shown to scramble HD to H2 and D2 and catalyze the hydrogenation of 1,1-diphenylethylene.

The Lewis acidity of fluorophosphonium salts: access to mixed valent phosphorus(III)/(V) species.

Oxidative fluorination of the electron-deficient phosphine Ph(2)P(C(6)F(5)) using XeF(2), followed by fluoride ion abstraction from the resulting difluorophosphorane Ph(2)P(F)(2)(C(6)F(5)), produces electrophilic fluorophosphonium salts [Ph(2)P(F)(C(6)F(5))][X] (X = FB(C(6)F(5))(3) or O(3)SCF(3)). Variable temperature NMR spectroscopic analysis of [Ph(2)P(F)(C(6)F(5))][FB(C(6)F(5))(3)] demonstrates a fluxional process attributed to fluoride ion exchange between B(C(6)F(5))(3) and [Ph(2)P(F)(C(6)F(5))](+), suggesting that these species have comparable Lewis acidities. This exchange can also be illustrated by adding phosphine Ph(3)P to [Ph(2)P(F)(C(6)F(5))][FB(C(6)F(5))(3)] at ambient temperature to produce Ph(2)P(F)(2)(C(6)F(5)) and Ph(3)P-B(C(6)F(5))(3), while heating this mixture results in thermally induced para-substitution of Ph(3)P at the C(6)F(5) group of the phosphonium ion to generate [Ph(3)P(C(6)F(4))P(F)(2)Ph(2)][FB(C(6)F(5))(3)]. Such frustrated Lewis pair reactivity also can be exploited by reacting [Ph(2)P(F)(C(6)F(5))][O(3)SCF(3)] with silylphosphine Ph(2)PSiMe(3) to afford the unique mixed-valent salt [Ph(2)P(C(6)F(4))P(F)Ph(2)][O(3)SCF(3)], which upon the addition of fluoride is converted to Ph(2)P(C(6)F(4))P(F)(2)Ph(2). XeF(2) reacts with [Ph(2)P(C(6)F(4))P(F)Ph(2)][O(3)SCF(3)] at ambient temperature, producing equal proportions of the dicationic salt [Ph(2)P(F)(C(6)F(4))P(F)Ph(2)][O(3)SCF(3)](2) and the bis(difluorophosphorane) Ph(2)P(F)(2)(C(6)F(4))P(F)(2)Ph(2), the latter of which can then be quantitatively converted to the former by adding one equiv of Me(3)SiO(3)SCF(3).

Chloro- and phenoxy-phosphines in frustrated Lewis pair additions to alkynes.

The reaction of tBu(C(6)H(4)O(2))P, with the borane B(C(6)F(5))(3) gives rise to NMR data consistent with the formation of the classical Lewis acid-base adduct tBu(C(6)H(4)O(2))P(B(C(6)F(5))(3)) (1). In contrast, the NMR data for the corresponding reactions of tBu(C(20)H(12)O(2))P and Cl(C(20)H(12)O(2))P with B(C(6)F(5))(3) were consistent with the presence of equilibria between free phosphine and borane and the corresponding adducts. Nonetheless, in each case, the adducts tBu(C(20)H(12)O(2))P(B(C(6)F(5))(3)) (2) and Cl(C(20)H(12)O(2))P(B(C(6)F(5))(3)) (3) were isolable. The species 1 reacts with PhCCH to give the new species tBu(C(6)H(4)O(2))P(Ph)C=CHB(C(6)F(5))(3) (4) in near quantitative yield. In an analogous fashion, the addition of PhCCH to solutions of the phosphines tBu(C(20)H(12)O(2))P, tBuPCl(2) and (C(6)H(3)(2,4-tBu(2))O)(3)P each with an equivalent of B(C(6)F(5))(3) gave rise to L(Ph)C=CHB(C(6)F(5))(3) (L = tBu(C(20)H(12)O(2))P 5, tBuPCl(2)6 and (C(6)H(3)(2,4-tBu(2))O)(3)P 7). X-Ray data for 1, 2, 6 and 7 are presented. The implications of these findings are considered.

Reactions of substituted pyridines with electrophilic boranes.

The lutidine derivative (2,6-Me(2))(4-Bpin)C(5)H(2)N when combined with B(C(6)F(5))(3) yields a frustrated Lewis pair (FLP) which reacts with H(2) to give the salt [(2,6-Me(2))(4-Bpin)C(5)H(2)NH][HB(C(6)F(5))(3)] (1). Similarly 2,2'-(C(5)H(2)(4,6-Me(2))N)(2) and (4,4'-(C(5)H(2)(4,6-Me(2))N)(2) were also combined with B(C(6)F(5))(3) and exposed to H(2) to give [(2,2'-HN(2,6-Me(2))C(5)H(2)C(5)H(2)(4,6-Me(2))N][HB(C(6)F(5))(3)] (2) and [(4,4'-HN(2,6-Me(2))C(5)H(2)C(5)H(2)(2,6-Me(2))N] [HB(C(6)F(5))(3)] (3), respectively. The mono-pyridine-N-oxide 4,4'-N(2,6-Me(2))C(5)H(2)C(5)H(2)(2,6-Me(2))NO formed the adduct (4,4'-N(2,6-Me(2))C(5)H(2)C(5)H(2)(2,6-Me(2))NO)(B(C(6)F(5))(3)) (4) which reacts further with B(C(6)F(5))(3) and H(2) to give [(4,4'-HN(2,6-Me(2))C(5)H(2)C(5)H(2)(2,6-Me(2))NO)B(C(6)F(5))(3)] [HB(C(6)F(5))(3)] (5). In a related sense, 2-amino-6-CF(3)-C(5)H(3)N reacts with B(C(6)F(5))(3) to give (C(5)H(3)(6-CF(3))NH)(2-NH(B(C(6)F(5))(3))) (6). Similarly, the species, 2-amino-quinoline, 8-amino-quinoline and 2-hydroxy-6-methyl-pyridine were reacted with B(C(6)F(5))(3) to give the products as (C(9)H(6)NH)(2-NHB(C(6)F(5))(3)) (7), (C(9)H(6)N)(8-NH(2)B(C(6)F(5))(3)) (8) and (C(5)H(3)(6-Me)NH)(2-OB(C(6)F(5))(3)) (9), respectively; while 2-amino-6-picoline, 2-amino-6-CF(3)-pyridine, 2-amino-quinoline, 8-amino-quinoline and 2-hydroxy-6-methyl-pyridine react with ClB(C(6)F(5))(2) to give the species (C(5)H(3)(6-R)NH)(2-NH(ClB(C(6)F(5))(2))) (R = Me (10), R = CF(3) (11)) (C(9)H(6)NH)(2-NH(ClB(C(6)F(5))(2))) (12), (C(9)H(6)N)(8-NH(2)ClB(C(6)F(5))(2)) (13) and (C(5)H(3)(6-Me)NH)(2-OClB(C(6)F(5))(2)) (14), respectively. In a similar manner, 2-amino-6-picoline and 2-amino-quinoline react with B(C(6)F(5))(2)H to give (C(5)H(3)(6-Me)NH)(2-NH(HB(C(6)F(5))(2))) (15) and (C(9)H(6)NH)(2-NH(HB(C(6)F(5))(2))) (16). The corresponding reaction of 8-amino-quinoline yields (C(9)H(6)N)(8-NHB(C(6)F(5))(2)) (17). In a similar fashion, reaction of 2-amino-6-CF(3)-pyridine resulted in the formation of (18) formulated as (C(5)H(3)(6-CF(3))N)(2-NH(B(C(6)F(5))(2)). Finally, treatment of 15 with iPrMgCl gave (C(9)H(6)N)(2-NH(B(C(6)F(5))(2))) (19). Crystallographic studies of 1, 2, 4, 6, 7, 10, 11, 12 and 15 are reported.