Frustrated Lewis Pair Chemistry Research Articles

Research Progress on Frustrated Lewis Pairs Chemistry

Research Progress on Frustrated Lewis Pairs Chemistry

Bench-stable frustrated Lewis pair chemistry: fluoroborate salts as precatalysts for the C-H borylation of heteroarenes.

While the organotrifluoroborate group is commonly used as a leaving group in cross-coupling reactions, we now show that their high stability can be used to protect the Lewis acidic moieties of frustrated Lewis pair catalysts. Indeed, the air and moisture-stable trifluoro- and difluoroborate derivatives of bulky (tetramethylpiperidino)benzene are shown to be conveniently converted to their dihydroborane analogue which is known to activate small molecules. An efficient synthesis route to these stable and convenient precatalysts, their deprotection chemistry and their benchtop use for the dehydrogenative borylation of heteroarenes is presented.

Hydroalumination of alkynyl-aminophosphines as a promising tool for the synthesis of unusual phosphines: P-N bond activation, a transient phosphaallene, a zwitterionic AlP2C2 heterocycle and a masked Al/P-based frustrated Lewis pair.

Treatment of the new alkynyl-chlorophosphine, Mes-P(Cl)-C ≡ C-CMe3, with LiNR2 afforded various unprecedented aminophosphines, Mes-P(NR2)-C ≡ C-CMe3, which showed a fascinating diversity in their reactivity towards H-Al(t)Bu2. NMe2 and NEt2 derivatives yielded the hydroalumination products Mes-P(NR2)-C(Al(t)Bu2) = C(H)-CMe3 which have an Al-N and an activated P-N bond. Elimination of aluminium amide yielded the transient 3H-phosphaallene, Mes-P = C = C(H)-CMe3, which finally afforded a five-membered AlP2C2 heterocycle with an Al-P bond and two exocyclic C = C bonds. This heterocycle is directly formed with the sterically shielded (i)Pr2N- and dimethylpiperidinophosphines. A unique R2AlH adduct resulted from the NPh2 and N(SiMe3)2 substituted phosphines. It may be viewed as an Al/P-based frustrated Lewis pair (FLP) which coordinates an R2AlH moiety. The heterocyclic AlP2C2 compound is formed in the final step of this reaction. Hydroalumination with Et2AlH yielded [Mes-P(H)-C(AlEt2) = C(H)-CMe3]2 which features an Al2P2C2 heterocycle and two Al-P bonds. This dimer resembles the class of hidden or masked FLPs which show a reactivity similar to uncoordinated FLPs. Its unique structural motif is a P-H bond which may result in a new type of FLP chemistry.

Frustrated Lewis Pair Chemistry Derived from Bulky Allenyl and Propargyl Phosphanes.

The dimesitylpropargylphosphanes mes2 P-CH2 -C≡C-R 6 a (R=H), 6 b (R=CH3 ), 6 c (R=SiMe3 ) and the allene mes2 P-C(CH3 )=C=CH2 (8) were reacted with Piers' borane, HB(C6 F5 )2 . Compound 6 a gave mes2 PCH2 CH=CH(B(C6 F5 )2 ] (9 a). In contrast, addition of HB(C6 F5 )2 to 6 b and 6 c gave mixtures of 9 b (R=CH3 ) and 9 c (R=SiMe3 ) with the regioisomers mes2 P-CH2 -C[B(C6 F5 )2 ]=CRH 2 b (R=CH3 ) and 2 c (R=SiMe3 ), respectively. Compounds 2 b,c underwent rapid phosphane/borane (P/B) frustrated Lewis pair (FLP) reactions under mild conditions. Compound 2 c reacted with nitric oxide (NO) to give the persistent FLP NO radical 11. The systems 2 b,c cleaved dihydrogen at room temperature to give the respective phosphonium/hydridoborate products 13 b,c. Compound 13 c transferred the H(+) /H(-) pair to a small series of enamines. Compound 13 c was also a metal-free catalyst (5 mol %) for the hydrogenation of the enamines. The allene 8 reacted with B(C6 F5 )3 to give the zwitterionic phosphonium/borate 17. The -PPh2 -substituted mes2 P-propargyl system 6 d underwent a typical 1,2-P/B-addition reaction to the C≡C triple bond to form the phosphetium/borate zwitterion 20. Several products were characterized by X-ray diffraction.

ChemInform Abstract: Main Group Lewis Acids in Frustrated Lewis Pair Chemistry: Beyond Electrophilic Boranes

AbstractReview: 107 refs.

ChemInform Abstract: Frustrated Lewis Pair Chemistry: Development and Perspectives

AbstractReview: [198 refs.

Main Group Lewis Acids in Frustrated Lewis Pair Chemistry: Beyond Electrophilic Boranes

Abstract The advent of frustrated Lewis pair (FLP) chemistry almost a decade ago was based on the reactivity of sterically encumbered combinations of electrophilic boranes and phosphines with hydrogen. Since that time the chemistry has broadened dramatically in terms of reactivity and applications. Nonetheless, the majority of the work has continued to exploit the borane B(C6F5)3. In this review, we describe FLP chemistry that has developed by employing alternatives to this Lewis acid. Lewis acids derived from group 13, 14, and 15 based systems are described, thus demonstrating a growing area in the main group reactivity based on FLPs.

Frustrated Lewis pair chemistry: development and perspectives.

Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases in solution that are deterred from strong adduct formation by steric and/or electronic factors. This opens pathways to novel cooperative reactions with added substrates. Small-molecule binding and activation by FLPs has led to the discovery of a variety of new reactions through unprecedented pathways. Hydrogen activation and subsequent manipulation in metal-free catalytic hydrogenations is a frequently observed feature of many FLPs. The current state of this young but rapidly expanding field is outlined in this Review and the future directions for its broadening sphere of impact are considered.

ChemInform Abstract: Frustrated Lewis Pairs: From Concept to Catalysis

AbstractReview: 109 refs.

Cover Picture: Kinetic and Thermodynamic Study of the Reduction of 1,1‐Diphenylethylene by a Thermally Frustrated Diethyl Ether‐BCF Lewis Pair (Isr. J. Chem. 2/2015)

The cover picture shows the time dependent heat release for the catalytic reduction of diphenylethylene by the frustrated Lewis pair (FLP) composed of diethyl ether and tris(pentafluorophenyl borane), first reported by Hounjet and co-workers (Angew. Chem. Int. Ed. 2013, 52, 7492). The modified Calvet calorimeter vessels described by Karkamkar et al. (Dalton Trans. 2013, 42, 615) provide an experimental approach to simultaneous measure of enthalpic driving force and rate of reduction in FLP catalysis. Read more on page 196, and in the articles on FLP chemistry in this special issue. The authors gratefully acknowledge Dr. Jonathan M. Darmon (PNNL) for designing the cover picture.

α-CH acidity of alkyl-B(C6F5)2 compounds - the role of stabilized borata-alkene formation in frustrated Lewis pair chemistry.

Alkyl-B(C6F5)2 boranes are markedly α-CH-acidic. Using DFT we have calculated the pK a-values of a series of examples. Typically, (C6F5)2B-CH3 [pK a (calcd) = 18.3 in DMSO, 16.2 in dichloromethane] is almost as CH-acidic as cyclopentadiene. However, this α-CH-B(C6F5)2 acidity is in most cases not sufficient to allow for internal proton transfer in vicinal phosphane/borane frustrated Lewis pairs (FLPs). An exception is the slightly endergonic (+0.3 kcal mol-1) tautomerization of the in situ generated indane derived 1,3-P/B FLP 6 to its zwitterionic borata-alkene/phosphonium isomer 7, which was successfully trapped by Piers' borane [HB(C6F5)2] to yield the stable product 8. The pronounced α-CH[B] carbanion stabilization can be used synthetically. We have found that the dienyl borane E-H2C[double bond, length as m-dash]C(Me)CH[double bond, length as m-dash]CHB(C6F5)2 undergoes clean, thermally induced 1,4-hydrophosphination reactions with HPR2 (R: phenyl, mesityl, or t-butyl) reagents. α-CHB(C6F5)2 carbanion (i.e. borata-alkene) stabilization in the respective intermediates probably plays a decisive role in these reactions.

From unsuccessful H2-activation with FLPs containing B(Ohfip)3 to a systematic evaluation of the Lewis acidity of 33 Lewis acids based on fluoride, chloride, hydride and methyl ion affinities.

The possibility of obtaining frustrated Lewis pairs (FLPs) suitable for H2-activation based on the Lewis acid B(Ohfip)3 1 (Ohfip = OC(H)(CF3)2) was investigated. In this context, the crystal structure of 1 as well as the crystal structure of the very weak adduct 1·NCMe was determined. When reacting solutions of 1 with H2 (1 bar) and selected phosphanes, amines, pyridines and N-heterocyclic carbenes, dihydrogen activation was never observed. Without H2, adduct formation with 1 was observed to be an equilibrium process, regardless of the Lewis base adduct. Thus, the thermodynamics of H2 activation of 1 in comparison with the well-known B(C6F5)3 was analyzed using DFT calculations in the gas phase and different solvents (CH2Cl2, ortho-difluorobenzene and acetonitrile). These investigations indicated that FLP chemistry based on 1 is considerably less favored than that with B(C6F5)3. This is in agreement with control NMR experiments indicating hydride transfer from [H-B(Ohfip)3](-) upon reaction with B(C6F5)3, giving [H-B(C6F5)3](-) and B(Ohfip)3 in toluene and also MeCN. Induced by these unsuccessful reactions, the Lewis acidity towards HSAB hard and soft ions was investigated for gaining a deeper insight. A unified reference system based on the trimethylsilyl compounds Me3Si-Y (Y = F, Cl, H, Me) and their respective ions Me3Si(+)/Y(-) calculated at the G3 level was chosen as the anchor point. The individual ion affinities were then assessed based on subsequent isodesmic reactions calculated at a much less expensive level (RI-)BP86/SV(P). This method was validated by systematic calculations of smaller reference systems at the frozen core CCSD(T) level with correlation effects extrapolated to a full quadruple-ζ basis. Overall, 33 common and frequently used Lewis acids were ranked with respect to their FIA, CIA, HIA and MIA (fluoride/chloride/hydride/methyl ion affinity).

ChemInform Abstract: Computational Design of Metal‐Free Molecules for Activation of Small Molecules, Hydrogenation, and Hydroamination

AbstractReview: 176 refs.

ChemInform Abstract: Solid‐State NMR as a Spectroscopic Tool for Characterizing Phosphane‐Borane Frustrated Lewis Pairs

AbstractReview: 65 refs.

ChemInform Abstract: Frustrated Lewis Pair Chemistry of Carbon, Nitrogen and Sulfur Oxides

AbstractReview: 99 refs.

Frustrated Lewis Pairs: An Elegant Concept for Catalysis

The last decade has witnessed an intense increase in research in the area of frustrated Lewis pairs (FLP). This conceptually simple idea has demonstrated to be extremely fruitful from a synthetic point of view. In fact, FLP chemistry has been able to offer complementary approaches to transformations traditionally in the realm of metal catalysis. This Cluster contains a series of articles that present the most recent achievements in this field, including new concepts and applications.

ChemInform Abstract: Metal‐Free Hydrogenation of Unsaturated Hydrocarbons Employing Molecular Hydrogen

AbstractReview: [41 refs.

An Electrochemical Study of Frustrated Lewis Pairs: A Metal-Free Route to Hydrogen Oxidation

Frustrated Lewis pairs have found many applications in the heterolytic activation of H2 and subsequent hydrogenation of small molecules through delivery of the resulting proton and hydride equivalents. Herein, we describe how H2 can be preactivated using classical frustrated Lewis pair chemistry and combined with in situ nonaqueous electrochemical oxidation of the resulting borohydride. Our approach allows hydrogen to be cleanly converted into two protons and two electrons in situ, and reduces the potential (the required energetic driving force) for nonaqueous H2 oxidation by 610 mV (117.7 kJ mol–1). This significant energy reduction opens routes to the development of nonaqueous hydrogen energy technology.

Metal‐Free Hydrogenation of Unsaturated Hydrocarbons Employing Molecular Hydrogen

The metal-free activation of hydrogen by frustrated Lewis pairs (FLPs) is a valuable method for the hydrogenation of polarized unsaturated molecules ranging from imines, enamines, and silyl enol ethers to heterocycles. However, one of the most important applications of hydrogenation technology is the conversion of unsaturated hydrocarbons into alkanes or alkenes. Despite the fast development of the FLP chemistry, such reactions proved as highly challenging. This Minireview provides an overview of the basic concepts of FLP chemistry, the challenge in the hydrogenation of unsaturated hydrocarbons, and first solutions to this central transformation.

Borata–alkene derivatives conveniently made by frustrated Lewis pair chemistry

Two (aryl)PXY starting materials (aryl = mesityl or 2,4,6-triisopropylphenyl; X,Y = Cl, Br) were reacted with lithiated conjugated enynes (derived from 2-methylbutenyne or 1-ethynylcyclohexene) to yield the respective (aryl)bis(enynyl)phosphanes. Their reaction with HB(C6F5)2 gave the heterocyclic five-membered zwitterionic borata-diene compounds containing the aryl group and one unchanged enynyl substituent at phosphorus. The borata-alkene products were thought to arise from a two step process of regioselective alkyne hydroboration followed by an internal phosphane attack on the boryldiene unit. Three examples of the ring-closed borata-alkene type products were characterized by X-ray diffraction.