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Steric effects

About: Steric effects is a research topic. Over the lifetime, 16112 publications have been published within this topic receiving 319615 citations. The topic is also known as: steric hindrance.


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Journal ArticleDOI
TL;DR: 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 in development of FLP catalysts for the hydrogenation of a range of organic substrates.
Abstract: 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...

807 citations

Journal ArticleDOI
TL;DR: This review presents the development of a more general method to determine the steric parameter of organometallic ligands and two case studies are presented: the tertiary phosphines and the N-heterocyclic carbenes.

798 citations

Journal ArticleDOI
TL;DR: The most dramatic finding from FLP chemistry was the discovery that FLPs can activate H2, thus countering the long-existing dogma that metals are required for such activation, and the development of new metal-free catalytic processes are described.
Abstract: CONSPECTUS: Frustrated Lewis pair (FLP) chemistry has emerged in the past decade as a strategy that enables main-group compounds to activate small molecules. This concept is based on the notion that combinations of Lewis acids and bases that are sterically prevented from forming classical Lewis acid-base adducts have Lewis acidity and basicity available for interaction with a third molecule. This concept has been applied to stoichiometric reactivity and then extended to catalysis. This Account describes three examples of such developments: hydrogenation, hydroamination, and CO2 reduction. The most dramatic finding from FLP chemistry was the discovery that FLPs can activate H2, thus countering the long-existing dogma that metals are required for such activation. This finding of stoichiometric reactivity was subsequently evolved to employ simple main-group species as catalysts in hydrogenations. While the initial studies focused on imines, subsequent studies uncovered FLP catalysts for a variety of organic substrates, including enamines, silyl enol ethers, olefins, and alkynes. Moreover, FLP reductions of aromatic anilines and N-heterocycles have been developed, while very recent extensions have uncovered the utility of FLP catalysts for ketone reductions. FLPs have also been shown to undergo stoichiometric reactivity with terminal alkynes. Typically, either deprotonation or FLP addition reaction products are observed, depending largely on the basicity of the Lewis base. While a variety of acid/base combinations have been exploited to afford a variety of zwitterionic products, this reactivity can also be extended to catalysis. When secondary aryl amines are employed, hydroamination of alkynes can be performed catalytically, providing a facile, metal-free route to enamines. In a similar fashion, initial studies of FLPs with CO2 demonstrated their ability to capture this greenhouse gas. Again, modification of the constituents of the FLP led to the discovery of reaction systems that demonstrated stoichiometric reduction of CO2 to either methanol or CO. Further modification led to the development of catalytic systems for the reduction of CO2 by hydrosilylation and hydroboration or deoxygenation. As each of these areas of FLP chemistry has advanced from the observation of unusual stoichiometric reactions to catalytic processes, it is clear that the concept of FLPs provides a new strategy for the design and application of main-group chemistry and the development of new metal-free catalytic processes.

781 citations

Journal ArticleDOI
09 Dec 2016-Science
TL;DR: The surprising chemistry of so-called frustrated Lewis pairs (FLPs), which cannot form their natural complex together are reviewed, as well as efforts to extend this concept to asymmetric hydrogenations and its application to various chemical systems.
Abstract: 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.

745 citations

Journal ArticleDOI
28 Jun 2012-Nature
TL;DR: A class of easily removable nitrile-containing templates that direct the activation of distal meta-C–H bonds (more than ten bonds away) of a tethered arene that overrides the intrinsic electronic and steric biases as well as ortho-directing effects with two broadly useful classes of arene substrates.
Abstract: Functionalization of unactivated carbon-hydrogen (C-H) single bonds is an efficient strategy for rapid generation of complex molecules from simpler ones. However, it is difficult to achieve selectivity when multiple inequivalent C-H bonds are present in the target molecule. The usual approach is to use σ-chelating directing groups, which lead to ortho-selectivity through the formation of a conformationally rigid six- or seven-membered cyclic pre-transition state. Despite the broad utility of this approach, proximity-driven reactivity prevents the activation of remote C-H bonds. Here we report a class of easily removable nitrile-containing templates that direct the activation of distal meta-C-H bonds (more than ten bonds away) of a tethered arene. We attribute this new mode of C-H activation to a weak 'end-on' interaction between the linear nitrile group and the metal centre. The 'end-on' coordination geometry relieves the strain of the cyclophane-like pre-transition state of the meta-C-H activation event. In addition, this template overrides the intrinsic electronic and steric biases as well as ortho-directing effects with two broadly useful classes of arene substrates (toluene derivatives and hydrocinnamic acids).

707 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023942
20221,917
2021346
2020292
2019296
2018307