A Novel Addition Mechanism for the Reaction of “Frustrated Lewis Pairs” with Olefins

Mechanistic Insight Into the Reactivity of Frustrated Lewis Pairs: Liquid-State NMR Studies

Frustrated Lewis pairs (FLPs) have been widely investigated as promising catalysts due to their metal-free feature and ability to activate small molecules. Over the last few years, the structure, dynamics and interactions between the Lewis centers and their effects on the reactivity with different substrates have been studied. Nuclear magnetic resonance (NMR) is a powerful tool in studying the reaction intermediates, kinetics and mechanism of frustrated Lewis pairs (FLPs). Various NMR experiments have been applied to precisely determine the association or cooperativity of FLPs and one or two-dimensional spectra were obtained. Herein, insights coming from NMR spectroscopy for FLPs are presented, the structure and reactivity of FLPs in solution are described, and their effects on the kinetics and mechanism of different substrates are also illustrated in this review.

Computational Design of Frustrated Lewis Pairs as a Strategy for Catalytic Hydrogen Activation and Hydrogenation Catalyst

Catalytic hydrogenation is one of the most important reaction types commonly used in chemistry and chemical industry. Recently, there has been significant interest in developing a metal-free hydrogenation catalyst to avoid the problems caused by using heavy transition metal catalysts. On the basis of the advances of metal-free hydrogen activation with frustrated Lewis pairs (FLPs, e.g. tBu₃P/B(C₆F₅)₃) which often uses boron as a Lewis acid center, we computationally explored the prospect for phosphorus(V) and sulfur(VI) as Lewis acid centers to construct FLPs for hydrogen activation and hydrogenation. We found out that the proposed FLPs with P(V)- or S(VI)-centered Lewis acid can also activate H₂ with a mechanism similar to that used by the conventional FLPs. A heterolytic cleavage of H-H is achieved when electrons are donated simultaneously from the σ orbital of H₂ to the empty orbital of the Lewis acid center and from the lone-pair orbital of the Lewis base center to the σ* orbital of H₂. The multiple C-H···F hydrogen bonds further aid the association of the pairs for H₂ activation. Some of our designed FLPs possess kinetics and thermodynamics for developing hydrogenation catalysts. This computational exploration could inspire experimental development of a new type of FLPs with P(V) or S(VI) or a Lewis acid partner for FLPs for reversible H₂ activation.

A Novel Reactivity of Phosphanylalumane (>P-Al<): Reversible Addition of a Saturated Interelement Bond to Olefins

The reversible addition of olefins to a phosphanylalumane, P-Al single-bond species, was investigated. The P-Al bond added to ethylene and relatively small terminal alkenes (propylene and hex-1-ene) at room temperature to give the corresponding alkene adducts. Heating the terminal alkene adducts released the corresponding alkenes and regenerated the P-Al bond, but no release of ethylene was observed even under vacuum conditions. The reactivity of ethylene adduct as a new saturated C₂ vicinal P/Al-based FLP was also investigated. The ethylene adduct was found to undergo complexation with nitriles to give the corresponding nitrile adducts to the Al center, which retained the ethylene tether as in the case of the corresponding P/B-based FLP. However, the reactivity of ethylene toward CO₂ and benzaldehyde differed from that of the P/B system giving the corresponding adducts.

Observation of Borane-Olefin Proximity Interaction Governing the Structure and Reactivity of Boron-Containing Macrocycles

While attractive interactions between borane and olefin have been postulated to trigger various boron-mediated organic transformations, proximity structures of these functional groups, other than the formation of weak van der Waals complexes, have never been directly observed. Here we show that a close intramolecular borane-olefin interaction operates in macrocyclic systems containing borane and olefinic groups obtained by multi-step 1,2-carboboration between a strained alkyne and 9-borafluorene derivatives. Depending on Lewis acidity of the borane moiety and the size of the macrocycles, the magnitude of interaction changes, resulting in different reaction modes. The whole picture of the multi-step reactions has been revealed experimentally with theoretical supports. The present finding may not only provide a deeper understanding of the fundamental boron-mediated interaction but also lead to the development of new organic transformations involving molecular activation by boranes.

On the association of frustrated Lewis pairs in ionic liquids: a molecular dynamics simulation study

Sterically hindered frustrated Lewis pairs (FLPs) have the ability to activate hydrogen molecules, and their reactivity is strongly determined by the geometric parameters of the Lewis acids and bases. A recent experimental study showed that ionic liquids (ILs) could largely improve the effective configuration of FLPs. However, the detailed mechanistic profile is still unclear. Herein, we performed molecular dynamics (MD) simulations to reveal the effects of ILs on the structures of FLPs, in particular, the association of Lewis acids and bases. For this purpose, mixed systems were adopted consisting of the ILs [C_nmim][NTf₂] (n = 6, 10, 14), [C₆mim][PF₆] and [C₆mim][CTf₃] and the typical FLP (tBu)₃P/B(C₆F₅)₃ for MD simulations. Radial distribution functions (RDFs) results show that toluene competes with (tBu)₃P to interact with B(C₆F₅)₃, resulting in a relatively low effective (tBu)₃P/B(C₆F₅)₃ complex, while [C₁₀mim][NTf₂] shows less competition with (tBu)₃P, which increases the amount of effective FLPs. Spatial distribution functions (SDFs) results show that toluene forms a continuum solvation-shell, which hinders the interactions between (tBu)₃P and B(C₆F₅)₃, while [C₁₀mim][NTf₂] leaves relatively large empty spaces, which are accessible for (tBu)₃P or B(C₆F₅)₃ molecules, resulting in higher probabilities of effective FLP structures. Lastly, we find that the longer alkyl chain length of [C_nmim]⁺ cations, the higher the amount of effective (tBu)₃P/B(C₆F₅)₃ pairs, and the anion [CTf₃]^- shows negative effects, for which even less effective (tBu)₃P/B(C₆F₅)₃ pairs have been found compared to those of toluene.

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.

Chemoselective Borane-Catalyzed Hydroarylation of 1,3-Dienes with Phenols

A B(C₆ F₅ )₃ -catalyzed hydroarylation of a series of 1,3-dienes with various phenols has been established through a combination of theoretical and experimental investigations, affording structurally diverse ortho-allyl phenols. DFT calculations show that the reaction proceeds through a borane-promoted protonation/Friedel-Crafts pathway involving a π-complex of a carbocation-anion contact ion pair. This protocol features simple and mild reaction conditions, broad functional-group tolerance, and low catalyst loading. The obtained ortho-allyl phenols could be further converted into flavan derivatives using B(C₆ F₅ )₃ with good cis diastereoselectivity. Furthermore, this transformation was applied in the late-stage modification of pharmaceutical compounds.

N-Geminal P/Al Lewis Pair-Alkyne Dipolar Cycloaddition to the Zwitterionic C2PNAl-Heterocyclopentene

The N-geminal P/Al Lewis pair [Ph₂PN(2,6-iPr₂C₆H₃)AlEt₂]₂ (1) has been prepared and studied for reaction with a series of alkynes. The reaction of 1 with RC≡CR yielded zwitterionic C₂PNAl-heterocyclopentene [Ph₂ ^⌜PN(2,6-iPr₂C₆H₃)AlEt₂](CR=^⌝CR) (R = Me (2), Ph (3)); with PhC≡CEt produced two isomers, [Ph₂ ^⌜PN(2,6-iPr₂C₆H₃)AlEt₂](CPh=^⌝CEt) (4a) and [Ph₂ ^⌜PN(2,6-iPr₂C₆H₃)AlEt₂](CEt=^⌝CPh) (4b); and with other alkynes generated [Ph₂ ^⌜PN(2,6-iPr₂C₆H₃)AlEt₂](CR¹=^⌝CR²) (R¹, R² = CO₂Et, Ph (5); SiMe₃, Ph (6); PPh₂, Ph (7); SiMe₃,H (8); H, EtO (9)). Natural bond orbital analysis of the charge separation of the C≡C bond of alkynes was carried out, and then, the electronic matching interaction mode between the combined Lewis acid (AlEt₂) and base (PPh₂) groups of 1 and the C≡C bond of such alkynes was discussed. Reactions of 1 with alkene, nitrile, and carbodiimide molecules were also carried out, and cycloaddition compounds 10-12 were produced.

Contrasting Frustrated Lewis Pair Reactivity with Selenium- and Boron-Based Lewis Acids

The activation of π-bonds in diynyl esters has been investigated by using soft and hard Lewis acids. In the case of the soft selenium Lewis acid PhSeCl, sequential activation of the alkyne bonds leads initially to an isocoumarin (1 equiv PhSeCl) and then to a tetracyclic conjugated structure with the isocoumarin subunit fused to a benzoselenopyran (3 equiv PhSeCl). Conversely, the reaction with the hard Lewis acidic borane B(C6 F5 )3 initiates a cascade reaction to yield a complex π-conjugated system containing phthalide and indene subunits.

Reversible [4 + 2] cycloaddition reaction of 1,3,2,5-diazadiborinine with ethylene

Under ambient conditions, a [4 + 2] cycloaddition reaction of 1,3,2,5-diazadiborinine 1 with ethylene afforded a bicyclo[2.2.2] derivative 2, which was structurally characterized.

Under ambient conditions, a [4 + 2] cycloaddition reaction of 1,3,2,5-diazadiborinine 1 with ethylene afforded a bicyclo[2.2.2] derivative 2, which was structurally characterized. The cyclization process was found to be reversible, and thus retro-[4 + 2] cycloaddition reproduced 1 quantitatively, concomitant with the release of ethylene. Compound 1 reacted regio-selectively and stereo-selectively with styrene derivatives and norbornene, respectively, and these processes were found to be reversible too. Computational studies determined the reaction pathways which were consistent with the regio-selectivity observed in the reaction of styrene, and the reaction was suggested to be essentially concerted but highly asynchronous.

A computational and conceptual DFT study on the mechanism of hydrogen activation by novel frustrated Lewis pairs

A computational and conceptual density functional theory (DFT) study on the mechanism of molecular hydrogen activation by a set of three frustrated Lewis pairs (FLPs) was performed at the ωB97X-D/6-311G(d,p) level of theory. A reduced model and other two prototypes derived from experimental data, based on the donor nitrogen and acceptor boron atoms, were used. Analysis based on the energy results, geometries and the global electron density transfer at the TSs made it possible to obtain some interesting conclusions: (i) despite the well-known very low reactivity of molecular hydrogen, the catalytic effectiveness of the three FLPs produces reactions with almost unappreciable activation energies; (ii) the reactions, being exothermic, follow a one-step mechanism via polarised TSs; (iii) there are neither substituent effects on the kinetics nor on the thermodynamics of these reactions; (iv) the activation of molecular hydrogen seems to be attained when the N-B distance in the FLP derivatives is around 2.74 Å; and (v) the proposed FLP model is consistent with the behaviour of the experimental prototypes. Finally, the ability of the three FLPs as efficient catalysts was evaluated studying the hydrogenation of acetylene to yield ethylene.

Application of a nucleophilic boryl complex in the frustrated Lewis pair: activation of H-H, B-H and C=C bonds with B(C6F5)3 and boryl-borate lithium

The frustrated Lewis pair comprised of B(C6F5)3 and a boryl-borate lithium salt Li[pinBB(Ph)pin] can efficiently activate dihydrogen, pinacolborane and ethylene at ambient temperature. Theoretical studies suggest that the nucleophilic sp(2) boryl moiety of Li[pinBB(Ph)pin] plays different roles in these reactions.

Computational study of metal free alcohol dehydrogenation employing frustrated lewis pairs

The catalysis of acceptorless alcohol dehydrogenation (AAD) is an important area of research. Transition metal-based systems are known to be effective catalysts for this reaction, but developing metal free catalytic systems would lead to highly desirable cheaper and greener alternatives. With this in mind, this computational study investigates design strategies than can lead to metal free frustrated Lewis pairs (FLPs) that can be employed for AAD catalysis. A careful study of 36 different proposed FLP candidates reveals that several new FLPs can be designed from existing, experimentally synthesized FLPs that can rival or be even better than state-of-the-art transition metal-based systems in catalyzing the alcohol dehydrogenation process.

Applications of pentafluorophenyl boron reagents in the synthesis of heterocyclic and aromatic compounds

Recently, main group reagents have attracted a lot of attention in bond-forming reactions in organic synthesis. This article highlights the use of pentafluorophenyl substituted boron reagents in their reactions with C=C and C≡C π-bonds for the synthesis of heterocyclic and aromatic compounds. These cyclisation reactions fall into four general classes although there is some overlap between classes and often combinations of these different types of reactivity are observed in the formation of the final heterocyclic product: (i) 1,2- (and 1,4-) additions of nucleophile and Lewis-acidic boron centre, (ii) 1,1-carboboration, (iii) carbocation rearrangements and (iv) cycloaddition chemistry. In addition, the prospect of using such boron reagents catalytically in the synthesis of aromatic compounds such as oxazoles and dibenzopentalene derivatives is emphasised.

Computational studies of lewis acidity and basicity in frustrated lewis pairs

Computational studies that characterize the effects of Lewis acidity/basicity on FLP formation and reactivity are reviewed. Formation of the FLP encounter complex "cage" depends on Lewis acidities and basicities of substituent "external" atoms, and their abilities to interact intramolecularly. Computations indicate that these interactions are worth 9-18 kcal mol⁻¹ for partly fluorinated FLPs such as (F5C6)3B···P(t-Bu)3, and less for less fluorinated species such as (H5C6)3B···P(t-Bu)3. Reactivity within the cage depends on the "classical" Lewis acidities/basicities of the internal atoms. Energetics here fall into the range of 5-50 kcal mol⁻¹; the larger the value, the greater the ability of the FLP to capture or split a substrate. In several cases the computationally predicted reaction barriers differ little with internal Lewis acidity/basicity, indicating that the rate-determining step involves the substrate entering the cage rather than attack by the Lewis acid/base atoms. In others, barriers vary sizably with Lewis acidity/basicity, indicating the opposite. In one case it was shown that these effects cancel, such that the three component barriers are identical for a range of substituted Lewis acid FLP components.