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.

Frustrated Lewis pair chemistry: development and perspectives.

The articulation of the notion of “frustrated Lewis pairs” (FLPs), which emerged from the discovery that H2 can be reversibly activated by combinations of sterically encumbered Lewis acids and bases, has prompted a great deal of recent activity. Perhaps the most remarkable consequence has been the development of FLP catalysts for the hydrogenation of a range of organic substrates. In the past 9 years, the substrate scope has evolved from bulky polar species to include a wide range of unsaturated organic molecules. In addition, effective stereoselective metal-free hydrogenation catalysts have begun to emerge. The mechanism of this activation of H2 has been explored, and the nature and range of Lewis acid/base combinations capable of effecting such activation have also expanded to include a variety of non-metal species. The reactivity of FLPs with a variety of other small molecules, including olefins, alkynes, and a range of element oxides, has also been developed. Although much of this latter chemistry has...

Frustrated Lewis Pairs

BACKGROUND Since the work of Sabatier 100 year ago, chemists have turned almost exclusively to metals to activate H 2 by weakening or cleaving its central bond. This paradigm changed with a 2006 report of a metal-free molecule that reversibly activated H 2 across sterically encumbered Lewis acidic boron and Lewis basic phosphorus sites. Shortly thereafter, similar reactions were mediated by systems described as “frustrated Lewis pairs” (FLPs) that were derived from simple combinations of electron donors and acceptors in which steric demands precluded dative bond formation. The variety of such systems has since been expanded to include a wide range of donors and acceptors. Moreover, FLP reactivity has been shown to result when equilibria governing the formation of Lewis acid-base adducts provides access to free acid and base. Mechanistic studies have demonstrated that the FLP activation of H 2 proceeds via a mechanism directly analogous to the Piers mechanism for borane-mediated hydrosilylation of ketones, first described in 1996. Nonetheless, the discovery of these metal-free reactions of H 2 has prompted considerable interest in this concept and its application to various chemical systems. ADVANCES The application of FLP reactivity with H 2 to metal-free hydrogenation catalysis rapidly led to reductions of polar substrates. Over the past decade, the range of reducible substrates has been expanded to a variety of unsaturated compounds, including imines, enamines, olefins, polyaromatics, alkynes, ketones, and aldehydes. Efforts have also extended this technology to asymmetric hydrogenations, with a number of recent systems achieving high selectivity. Early on, it was recognized that the reactivity of FLPs was not limited to H 2 . FLPs have shown the capacity to capture and react with a variety of small molecules, including olefins, alkynes, CO 2 , SO 2 , NO, CO, N 2 O, and N -sulfinyltolyllamines ( p -tol)NSO ( p -tol, para -tolyl). This has led to metal-free strategies for CO and CO 2 reduction and SO generation and new avenues to transient, persistent, or stable radicals. FLP chemistry has been further extended to new strategies in synthetic organic chemistry, including FLP-mediated approaches to hydroamination, hydroboration, cyclization, and boration reactions. Because transition metals may also be acidic or basic, the reactivity of FLP systems in which one or both of the constituents are metal centers has been reported. Further, metal components can also be ancillary fragments for ligand-based FLP chemistry, or they can act as a scaffold, allowing the cooperative action of an FLP and a metal center on a substrate. In related developments, the notion of FLPs has been applied to the design of model systems for the active sites of the [Ni-Fe], [Fe-Fe], or [Fe] hydrogenase enzymes. Reaching beyond main-group and organic chemistry into polymers and materials chemistry, FLP catalysts have been used to prepare lactone-derived polymers, cyanamide oligomers, and Te-containing heterocycles for applications in photoactive materials. In addition, heterogeneous FLP hydrogenation catalysts have emerged, and the concept also provides a new mechanistic perspective on CO 2 reduction on the surface of indium oxide nanocrystals. OUTLOOK Applications of FLP chemistry to metal-free reductions, asymmetric hydrogenations, C–C bond formation, and C–H bond functionalization are continuing to evolve. Such advances offer strategies for reduced costs and the elimination of toxic contaminants that will undoubtedly garner interest from the synthetic chemistry communities in academia and industry. The expanding range of main-group and transition metal–based FLPs continues to demonstrate the generality of this concept and its broadening utility. However, the innovative synthetic strategies, reactivity, and new perspectives derived from the application of this simple concept to other areas of chemistry are perhaps the most exciting prospect.

http://science.sciencemag.org/content/sci/354/6317/aaf7229.full.pdf

The broadening reach of frustrated Lewis pair chemistry

Der Begriff “frustrierte Lewis-Paare” (FLPs) bezeichnet Kombinationen von Lewis-Sauren und Lewis-Basen in Losung, die aufgrund sterischer oder elektronischer Faktoren an der Bildung eines stabilen Addukts gehindert werden. Die verbleibende Lewis-Aziditat und -Basizitat ermoglicht den Zugang zu neuen kooperativen Reaktionen mit zugesetzten Substraten. Die Fixierung und Aktivierung von kleinen Molekulen durch FLPs fuhrte zur Entdeckung einer Vielzahl von neuen Reaktionen und ungewohnlichen Reaktionswegen. Die Aktivierung von Diwasserstoff und die anschliesende Umsetzung in metallfreien katalytischen Hydrierungsreaktionen ist ein haufig beobachtetes Merkmal von FLPs. In diesem Aufsatz wird der aktuelle Stand dieses jungen Forschungsgebietes der Chemie beschrieben und die zukunftige Ausrichtung des sich rasch weiterentwickelnden Konzepts der frustrierten Lewis-Paare betrachtet.

Chemie frustrierter Lewis‐Paare: Entwicklung und Perspektiven

The development of new or more sustainable, active, efficient, controlled, and selective polymerization reactions or processes continues to be crucial for the synthesis of important polymers or materials with specific structures or functions. In this context, the newly emerged polymerization technique enabled by main-group Lewis pairs (LPs), termed as Lewis pair polymerization (LPP), exploits the synergy and cooperativity between the Lewis acid (LA) and Lewis base (LB) sites of LPs, which can be employed as frustrated Lewis pairs (FLPs), interacting LPs (ILPs), or classical Lewis adducts (CLAs), to effect cooperative monomer activation as well as chain initiation, propagation, termination, and transfer events. Through balancing the Lewis acidity, Lewis basicity, and steric effects of LPs, LPP has shown several unique advantages or intriguing opportunities compared to other polymerization techniques and demonstrated its broad polar monomer scope, high activity, control or livingness, and complete chemo- or...

Polymerization of Polar Monomers Mediated by Main-Group Lewis Acid-Base Pairs.

Classical and frustrated Lewis pairs (LPs) of the strong Lewis acid (LA) Al(C6F5)3 with several Lewis base (LB) classes have been found to exhibit exceptional activity in the Lewis pair polymerization (LPP) of conjugated polar alkenes such as methyl methacrylate (MMA) as well as renewable α-methylene-γ-butyrolactone (MBL) and γ-methyl-α-methylene-γ-butyrolactone (γ-MMBL), leading to high molecular weight polymers, often with narrow molecular weight distributions. This study has investigated a large number of LPs, consisting of 11 LAs as well as 10 achiral and 4 chiral LBs, for LPP of 12 monomers of several different types. Although some more common LAs can also be utilized for LPP, Al(C6F5)3-based LPs are far more active and effective than other LA-based LPs. On the other hand, several classes of LBs, when paired with Al(C6F5)3, can render highly active and effective LPP of MMA and γ-MMBL; such LBs include phosphines (e.g., PtBu3), chiral chelating diphosphines, N-heterocyclic carbenes (NHCs), and phosphazene superbases (e.g., P4-tBu). The P4-tBu/Al(C6F5)3 pair exhibits the highest activity of the LP series, with a remarkably high turn-over frequency of 9.6 × 104 h−1 (0.125 mol% catalyst, 100% MMA conversion in 30 s, Mn = 2.12 × 105 g mol−1, PDI = 1.34). The polymers produced by LPs at RT are typically atactic (PγMMBL with ∼47% mr) or syndio-rich (PMMA with ∼70–75% rr), but highly syndiotactic PMMA with rr ∼91% can be produced by chiral or achiral LPs at −78 °C. Mechanistic studies have identified and structurally characterized zwitterionic phosphonium and imidazolium enolaluminates as the active species of the current LPP system, which are formed by the reaction of the monomer·Al(C6F5)3 adduct with PtBu3 and NHC bases, respectively. Kinetic studies have revealed that the MMA polymerization by the tBu3P/Al(C6F5)3 pair is zero-order in monomer concentration after an initial induction period, and the polymerization is significantly catalyzed by the LA, thus pointing to a bimetallic, activated monomer propagation mechanism. Computational study on the active species formation as well as the chain initiation and propagation events involved in the LPP of MMA with some of the most representative LPs has added our understanding of fundamental steps of LPP. The main difference between NHC and PR3 bases is in the energetics of zwitterion formation, with the NHC-based zwitterions being remarkably more stable than the PR3-based zwitterions. Comparison of the monometallic and bimetallic mechanisms for MMA addition shows a clear preference for the bimetallic mechanism.

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Mallory G. John

Papers

Lewis pair polymerization by classical and frustrated Lewis pairs: acid, base and monomer scope and polymerization mechanism