Frustrated Lewis Pairs: from dihydrogen activation to asymmetric catalysis

Stereocontrol via Propeller Chirality in FLP-Catalyzed Asymmetric Hydrogenation

Utilization of chiral frustrated Lewis pairs as catalysts in enantioselective hydrogenation of unsaturated molecules represents a promising approach in asymmetric synthesis. In our effort to improve our current understanding of the factors governing the stereoselectivity in these catalytic processes, herein we examined the mechanism of direct hydrogenation of aromatic enamines catalyzed by a binaphthyl-based chiral amino-borane. Our computational analysis reveals that only one particular conformer of the key borohydride reaction intermediate can be regarded as a reactive form of this species. This borohydride conformer has a well-defined chiral propeller shape, which induces facial selectivity in the hydride transfer to pro-chiral iminium intermediates. The propeller chirality of the reactive borohydride conformer is generated by the axially chiral binaphthyl scaffold of the amino-borane catalyst through stabilizing π-π stacking interactions. This new computational insight can be readily used to interpret the high degree of stereoinduction observed for these reactions. We expect that the concept of chirality relay could be further exploited in catalyst design endeavors.

Structure and reactivity studies of the aluminum analogue of Piers' borane [HAl(C6F5)2]3

The aluminum analogue of Piers' borane, [HAl(C₆F₅)₂]₃1, is prepared on a gram-scale. Density functional theory (DFT) calculations reveal 1 has a higher fluoride ion affinity (FIA) than Piers' borane, while the Al-H moiety proved to be a strong hydride donor, reacting with alcohol and terminal alkyne to give the corresponding dehydrogenative products 3 and 4. Hydroalumination product 5 was prepared via reaction of 1 with aldehyde. In addition, 1 catalyzes the hydrosilylation of alkynes and alkenes.

Taming PH3: State of the Art and Future Directions in Synthesis

Appetite for reactions involving PH₃ has grown in the past few years. This in part is due to the ability to generate PH₃ cleanly and safely via digestion of cheap metal phosphides with acids, thus avoiding pressurized cylinders and specialized equipment. In this perspective we highlight current trends in forming new P–C/P–OC bonds with PH₃ and discuss the challenges involved with selectivity and product separation encumbering these reactions. We highlight the reactivity of PH₃ with main group reagents, building on the early pioneering work with transition metal complexes and PH₃. Additionally, we highlight the recent renewal of interest in alkali metal sources of H₂P^– which are proving to be useful synthons for chemistry across the periodic table. Such MPH₂ sources are being used to generate the desired products in a more controlled fashion and are allowing access to unexplored phosphorus-containing species.

Tuning Activity and Selectivity during Alkyne Activation by Gold(I)/Platinum(0) Frustrated Lewis Pairs

Introducing transition metals into frustrated Lewis pair systems has attracted considerable attention in recent years. Here we report a selection of three metal-only frustrated systems based on Au(I)/Pt(0) combinations and their reactivity toward alkynes. We have inspected the activation of acetylene and phenylacetylene. The gold(I) fragments are stabilized by three bulky phosphines bearing terphenyl groups. We have observed that subtle modifications on the substituents of these ligands proved critical in controlling the regioselectivity of acetylene activation and the product distribution resulting from C(sp)-H cleavage of phenylacetylene. A mechanistic picture based on experimental observations and computational analysis is provided. As a result of the cooperative action of the two metals of the frustrated pairs, several uncommon heterobimetallic structures have been characterized.

Evidence for Genuine Bimetallic Frustrated Lewis Pair Activation of Dihydrogen with Gold(I)/Platinum(0) Systems

A joint experimental/computational effort to elucidate the mechanism of dihydrogen activation by a gold(I)/platinum(0) metal-only frustrated Lewis pair (FLP) is described herein. The drastic effects on H₂ activation derived from subtle ligand modifications have also been investigated. The importance of the balance between bimetallic adduct formation and complete frustration has been interrogated, providing for the first time evidence for genuine metal-only FLP reactivity in solution. The origin of a strong inverse kinetic isotopic effect has also been clarified, offering further support for the proposed bimetallic FLP-type cleavage of dihydrogen.

Reactivity of a gold(i)/platinum(0) frustrated Lewis pair with germanium and tin dihalides

The rich reactivity of tetrylene dihalides towards a metallic FLP: phosphine exchange reactions and formation of homo and heterobimetallic compounds.

Formation of macrocyclic ring systems by carbonylation of trifunctional P/B/B frustrated Lewis pairs

The trifunctional P/B/B frustrated Lewis pairs 11a-c featuring bulky aryl groups at phosphorus [Dmesp (a), Tipp (b), Mes* (c)] react with H2 by heterolytic hydrogen splitting followed by cleavage of HB(C6F5)2 to give the zwitterionic six-membered heterocyclic PH phosphonium/borate products 14a-c. Compounds 11a,b react with carbon monoxide by means of a Lewis acid induced CO insertion/rearrangement sequence that eventually results in the formation of the macrocyclic dimers 17a,b. The respective carbonylation reaction of the Mes*P/B/B FLP gives the macrocyclic trimer 18c. The new products were characterized spectroscopically and by X-ray diffraction and the reaction mechanism was analyzed by DFT calculations.

Lewis and Brønsted basicity of phosphine-diazomethane derivatives

The compounds EtOC(O)CHNN(PR₃) (R = Ph 1, Cy 2, tBu 3) were prepared via the reactions of the diazomethane and a phosphine and their reactivity has been explored.

Formation and reactions of active five-membered phosphane/borane frustrated Lewis pair ring systems

The five-membered P/B FLP ring systems featuring very bulky Tipp or Mes* aryl groups at phosphorus undergo addition reactions with organic π-reagents.

Halogenated triphenylgallium and -indium in frustrated Lewis pair activations and hydrogenation catalysis

The Lewis acids Ga(C₆F₅)₃, In(C₆F₅)₃ and Ga(C₆Cl₅)₃ are prepared and their Lewis acidity has been probed experimentally and computationally. The species Ga(C₆F₅)₃ and In(C₆F₅)₃ in conjunction with phosphine donors are shown to heterolytically split H₂ and catalyse the hydrogenation of an imine. In addition, frustrated Lewis pairs (FLPs) derived from Ga(C₆F₅)₃ and In(C₆F₅)₃ and phosphines react with diphenyldisulfide to phosphoniumgallates or indates of the form [tBu₃PSPh][PhSE(C₆F₅)₃] and [tBu₃PSPh][(μ-SPh)(E(C₆F₅)₃)₂] (E = Ga, In). The potential of the FLPs based on Ga(C₆F₅)₃, In(C₆F₅)₃ and Ga(C₆Cl₅)₃ and phosphines is also shown in reactions with phenylacetylene to give pure or mixtures of the products [tBu₃PH][PhCCE(C₆X₅)₃] and R₃P(Ph)C=C(H)E(C₆X₅)₃ A number of these species are crystallographically characterized. The implications for the use of these species in FLP chemistry are considered.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

Reaction of the thermo-labile triazenide Na[tBu3SiNNNSiMe3] with CO2: formation of the imido carbonate (tBu3SiO)(Me3SiO)C[double bond, length as m-dash]N-SitBu3 and carbamine acid (tBu3SiO)CONH2

The thermo-labile triazenide Na[tBu₃SiNNNSiMe₃] was prepared by the reaction of Me₃SiN₃ with Na(thf)₂[SitBu₃] in pentane at -78 °C. Treatment of Na[tBu₃SiNNNSiMe₃] with an excess of carbon dioxide in pentane at -78 °C yielded the imido carbonate (tBu₃SiO)(Me₃SiO)C[double bond, length as m-dash]N-SitBu₃ and the carbamine acid (tBu₃SiO)CONH₂ along with other products. From the reaction solution we could isolate the imido carbonate (tBu₃SiO)(Me₃SiO)C[double bond, length as m-dash]N-SitBu₃ and carbamine acid (tBu₃SiO)CONH₂. At first single crystals of the carbamine acid (tBu₃SiO)CONH₂ (triclinic, space group P1[combining macron]) were grown from this solution at room temperature. A second crop of crystals were obtained by concentrating the solution. The second charge consisted of the imido carbonate (tBu₃SiO)(Me₃SiO)C[double bond, length as m-dash]N-SitBu₃ (monoclinic, space group P2₁/n).

Synthetic and structural studies of phosphine coordinated boronium salts

The reaction of BrH₂B·SMe₂ with primary and secondary phosphines affords a range of boronium salts of the form [H₂B(PR₃)₂]Br (PR₃ = PHCy₂1; PHPh₂, 2; PH₂Cy, 3), which have been fully characterised including solid-state determinations. Reactions of bulky tertiary phosphines, e.g., PCy₃ and PPh₃, with BrH₂B·SMe₂ do not proceed beyond the phosphine-stabilised bromoborane adducts, however, the smaller tertiary phosphine PMe₂Ph readily proceeds to form [H₂B(PMe₂Ph)₂]Br (4). The formation of the unsymmetrical boronium salts [H₂B(PH₂Cy)(PHCy₂)]Br (5) and [H₂B(PHCy₂)(PHPh₂)]Br (6) was observed by in situ NMR spectroscopy, however, the compounds were found to spontaneously disproportionate to their respective homophosphine boronium cations, even on prolonged storage in solution at -78 °C. Di- and triphosphines were found to form ring-closed boronium salts to afford [H₂B(κ²-P,P'-diphosphine)]Br (diphosphine = dppe, 7; dcpe, 8; dmpe, 9; dppf, 10; dppf with [AsF₆]^- counterion, 11; amphos, 12). The analogous methodology with Br₂HB·SMe₂ proved less generally applicable due to an accessible decomposition pathway being available to boronium salts bearing primary and secondary phosphines, leading to the formation of phosphonium salts, although [BrHB(dcpe)]Br (13), [BrHB(PMe₂Ph)₂]Br (14) and [BrHB(amphos)]Br (15) were synthesised with varying degrees of success. Reaction of BrH₂B·SMe₂ with diphars afforded the boronium salt [H₂B(κ²-P,P'-diphars)]Br (16), which featured two pendant arsine arms. Similarly, triphos was found to react with BrH₂B·SMe₂ to give [H₂B(κ²-P,P'-triphos)]Br (17), which featured a pendant phosphine arm. Substitution of the bromide counter anion with either hexafluoroarsenate or hexfluoroantimonate anions revealed weak hydrogen bonds between the P-H bonds of the boronium cations and the anions, that appeared through NMR studies to be retained in solution (where hydrogen bonding order was determined to be Br^- > [SbF₆]^-/[AsF₆]^-). This was further demonstrated by comparison of solid-state structures and solution NMR data of 1 with [H₂B(PHCy₂)₂][SbF₆] (18), 4 with [H₂B(PMe₂Ph)₂][AsF₆] (19) and 17 with [H₂B(triphos)][AsF₆] (20).

Chiral carbene-borane adducts: precursors for borenium catalysts for asymmetric FLP hydrogenations

The carbene derived from (1R,3S)-camphoric acid was used to prepare the borane adduct with Piers' borane 7. Subsequent hydride abstraction gave the borenium cation 8. Adducts with 9-BBN and the corresponding (1R,3S)-camphoric acid-derived carbene bearing increasingly sterically demanding N-substituents (R = Me 9, Et 10, i-Pr 11) and the corresponding borenium cations 12-14 were also prepared. These cations were not active as catalysts in hydrogenation, although 9-11 were shown to undergo carbene ring expansion reactions at 50 °C to give species 15-17. The IBOX-carbene precursors 18 and 19 derived from amino alcohols (S)-valinol and (S)-tert-leucinol (R = i-Pr, t-Bu) were used to prepare borane adducts 20-23. Reaction of the carbenes 1,3-dimethylimidazol-2-ylidene (IMe), 1,3-di-iso-propylimidazol-2-ylidene (IPr) 1-benzyl-3-methylimidazol-2-ylidene (IBnMe), 1-methyl-3-phenylimidazol-2-ylidene (IPhMe) and 1-tert-butyl-3-methylimidazol-2-ylidene (ItBuMe) with diisopinocampheylborane (Ipc₂BH) gave chiral adducts: (IMe)(Ipc₂BH) 24, (IPr)(Ipc₂BH) 25, (IBnMe)(Ipc₂BH) 26, (IPhMe)(Ipc₂BH) 27, and (ItBuMe)(Ipc₂BH) 28. Triazolylidene-type adducts including the (10)-phenyl-9-borabicyclo [3.3.2]decane adduct of 1,3,4-triphenyl-1H-1,2,3-triazolium, rac-29 and the 9-BBN derivative of (S)-2-amino-2'-methoxy-1,1'-binaphthalene-1,2,3-triazolium 34a/b were also prepared. In catalytic studies of these systems, while several species were competent catalysts for imine reduction, in general, low enantioselectivities, ranging from 1-20% ee, were obtained. The implications for chiral borenium cation catalyst design are considered.

The Asymmetric Piers Hydrosilylation

An axially chiral, cyclic borane decorated with just one C6F5 group at the boron atom promotes the highly enantioselective hydrosilylation of acetophenone derivatives without assistance of an additional Lewis base (up to 99% ee). The reaction is an unprecedented asymmetric variant of Piers' B(C6F5)3-catalyzed carbonyl hydrosilylation. The steric congestion imparted by the 3,3'-disubstituted binaphthyl backbone of the borane catalyst as well as the use of reactive trihydrosilanes as reducing agents are keys to success.

Nitro-redox reactions at a frustrated borane/phosphane Lewis pair

The unsaturated 1,4-P/B-FLPs reduced nitrobenzene to nitrosobenzene which was directly trapped by an allylboration reaction to give the seven-membered B-O-P compounds and . The FLP reacted analogously with trans-β-nitrostyrene. The products were characterized by X-ray diffraction.

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.