J. Appl. Cryst.の発刊に際して

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

As two coexisting and fast-growing research fields in modern synthetic chemistry, the merging of organocatalysis and C–H bond functionalization is well foreseeable, and the joint force along this line has been demonstrated to be a powerful approach in making inert C–H bond functionalization more viable, predictable, and selective. In this review, we provide a comprehensive summary of organocatalysis in inert C–H bond functionalization over the past two decades. The review is arranged by types of inert C–H bonds including alkane C–H, arene C–H, and vinyl C–H as well as those activated benzylic C–H, allylic C–H, and C–H bonds alpha to the heteroatom such as nitrogen and oxygen. In each section, the discussion is classified by the explicit organocatalytic mode involved.

Organocatalysis in Inert C–H Bond Functionalization

Hydrogenase enzymes efficiently process H2 and protons at organometallic FeFe, NiFe, or Fe active sites. Synthetic modeling of the many H2ase states has provided insight into H2ase structure and mechanism, as well as afforded catalysts for the H2 energy vector. Particularly important are hydride-bearing states, with synthetic hydride analogues now known for each hydrogenase class. These hydrides are typically prepared by protonation of low-valent cores. Examples of FeFe and NiFe hydrides derived from H2 have also been prepared. Such chemistry is more developed than mimicry of the redox-inactive monoFe enzyme, although functional models of the latter are now emerging. Advances in physical and theoretical characterization of H2ase enzymes and synthetic models have proven key to the study of hydrides in particular, and will guide modeling efforts toward more robust and active species optimized for practical applications.

/pdf/hydrogenase-enzymes-and-their-synthetic-models-the-role-of-2sg9pjphaf.pdf

Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides.

The bond activation chemistry of B(C6F5)3 and related electron-deficient boranes is currently experiencing a renaissance due to the fascinating development of frustrated Lewis pairs (FLPs). B(C6F5)3's ability to catalytically activate Si–H bonds through η1 coordination opened the door to several unique reduction processes. The ground-breaking finding that the same family of fully or partially fluorinated boron Lewis acids allows for the related H–H bond activation, either alone or as a component of an FLP, brought considerable momentum into the area of transition-metal-free hydrogenation and, likewise, hydrosilylation. This review comprehensively summarises synthetic methods involving borane-catalysed Si–H and H–H bond activation. Systems corresponding to an FLP-type situation are not covered. Aside from the broad manifold of CX bond reductions and CX/C–X defunctionalisations, dehydrogenative (oxidative) Si–H couplings are also included.

/pdf/a-unified-survey-of-si-h-and-h-h-bond-activation-catalysed-1ix9iq1qpj.pdf

A unified survey of Si–H and H–H bond activation catalysed by electron-deficient boranes

B(C6F5)3-Catalyzed Synthesis of Benzofused-Siloles

Facile Arylation of Four-Coordinate Boron Halides by Borenium Cation Mediated Boro-desilylation and -destannylation.

N-Me-Benzothiazolium salts are introduced as a new family of Lewis acids able to activate Si-H σ bonds. These carbon-centred Lewis acids were demonstrated to have comparable Lewis acidity towards hydride as found for the triarylboranes widely used in Si-H σ-bond activation. However, they display low Lewis acidity towards hard Lewis bases such as Et3 PO and H2 O in contrast to triarylboranes. The N-Me-benzothiazolium salts are effective catalysts for a range of hydrosilylation and dehydrosilylation reactions. Judicious selection of the C2 aryl substituent in these cations enables tuning of the steric and electronic environment around the electrophilic centre to generate more active catalysts. Finally, related benzoxazolium and benzimidazolium salts were found also to be active for Si-H bond activation and as catalysts for the hydrosilylation of imines.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/chem.201604613

N-methyl-benzothiazolium salts as carbon Lewis acids for Si-H σ bond activation and catalytic (de)hydrosilylation.

Whilst hydrogen is a potentially clean fuel for energy storage and utilisation technologies, its conversion to electricity comes at a high energetic cost. This demands the use of rare and expensive precious metal electrocatalysts. Electrochemical-frustrated Lewis pairs offer a metal-free, CO tolerant pathway to the electrocatalysis of hydrogen oxidation. They function by combining the hydrogen-activating ability of frustrated Lewis pairs (FLPs) with electrochemical oxidation of the resultant hydride. Here we present an electrochemical–FLP approach that utilises two different Lewis acids – a carbon-based N-methylacridinium cation that possesses excellent electrochemical attributes, and a borane that exhibits fast hydrogen cleavage kinetics and functions as a “hydride shuttle”. This synergistic interaction provides a system that is electrocatalytic with respect to the carbon-based Lewis acid, decreases the required potential for hydrogen oxidation by 1 V, and can be recycled multiple times.

/pdf/metal-free-electrocatalytic-hydrogen-oxidation-using-53eyjwinu4.pdf

Metal-free electrocatalytic hydrogen oxidation using frustrated Lewis pairs and carbon-based Lewis acids

Frustrated Lewis pair (FLP) chemistry enables a rare example of alkyne 1,2-hydrocarbation with N-methylacridinium salts as the carbon Lewis acid. This 1,2-hydrocarbation process does not proceed through a concerted mechanism as in alkyne syn-hydroboration, or through an intramolecular 1,3-hydride migration as operates in the only other reported alkyne 1,2-hydrocarbation reaction. Instead, in this study, alkyne 1,2-hydrocarbation proceeds by a novel mechanism involving alkyne dehydrocarbation with a carbon Lewis acid based FLP to form the new C-C bond. Subsequently, intermolecular hydride transfer occurs, with the Lewis acid component of the FLP acting as a hydride shuttle that enables alkyne 1,2-hydrocarbation.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/ange.201705100

Frustrated Lewis Pair Mediated 1,2‐Hydrocarbation of Alkynes

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