Solid-State NMR as a Spectroscopic Tool for Characterizing Phosphane–Borane Frustrated Lewis Pairs
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Frustrated Lewis pair
Boranes
J-coupling
We herein explore whether tris(aryl)borane Lewis acids are capable of cleaving H
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The conjugated dienamine 4 selectively adds Piers' borane [HB(C 6 F 5 ) 2 ] to give the enamine/borane system 5 , which features a boratirane structure by internal enamine carbon Lewis base to boron Lewis acid interaction. Compound 5 behaves as a C/B frustrated Lewis pair and undergoes typical addition reactions to benzaldehyde, several nitriles and to sulfur dioxide. This article is part of the themed issue ‘Frustrated Lewis pair chemistry’.
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Oxidation reactions are rarely encountered in frustrated Lewis pair (FLP) chemistry. We describe the selective reaction of an amine/borane FLP with molecular oxygen. The trimethylene-bridged amine/borane frustrated Lewis pair was prepared by hydroboration of N-allyl-tetramethylpiperidine with Piers' borane [HB(C6F5)2]. The intramolecular N/B system undergoes a variety of typical FLP reactions. It is a very active hydrogen splitting reagent at ambient conditions. Its reaction with dioxygen takes place rapidly at ambient conditions to give a tetrahydroisoxazolium/borate salt. A possible pathway of its formation is proposed and supported by DFT calculations.
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Sterically encumbered Lewis acid and Lewis base combinations do not undergo the ubiquitous neutralization reaction to form "classical" Lewis acid/Lewis base adducts. Rather, both the unquenched Lewis acidity and basicity of such sterically "frustrated Lewis pairs (FLPs)" is available to carry out unusual reactions. Typical examples of frustrated Lewis pairs are inter- or intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C(6)F(5))(2) components. Many examples of such frustrated Lewis pairs are able to cleave dihydrogen heterolytically. The resulting H(+)/H(-) pairs (stabilized for example, in the form of the respective phosphonium cation/hydridoborate anion salts) serve as active metal-free catalysts for the hydrogenation of, for example, bulky imines, enamines, or enol ethers. Frustrated Lewis pairs also react with alkenes, aldehydes, and a variety of other small molecules, including carbon dioxide, in cooperative three-component reactions, offering new strategies for synthetic chemistry.
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Abstract In recent years, borane-based frustrated Lewis pairs have proved to be efficient hydrogenation catalysts and they have become an alternative to transition-metal-based systems. The hydrogen activation by classic FLPs leads to a protonated Lewis base and a borohydride. Consequently, hydrogenations catalyzed by classic FLPs consist of stepwise hydride transfer reactions and protonations (or vice versa). More recently, systems that operate via an initial hydroboration have allowed the substrate scope for FLP-catalyzed hydrogenations to be extended. In this review, hydrogenations of organic substrates catalyzed by borane-based frustrated Lewis pairs are discussed. Emphasis is given to the mechanistic aspects of these catalytic reactions. 1 Introduction 2 FLP-Catalyzed Hydrogenation of Polarized Double Bonds 2.1 Hydrogenation of Michael Acceptors by FLPs 2.2 Asymmetric Hydrogenation of Polarized Double Bonds 2.3 Hydrogenation of Arenes and N-Heterocycles 3 Hydrogenation of Unactivated Olefins and Alkynes 3.1 Hydrogenation of Olefins and Alkynes by an Initial Hydroboration 4 Summary and Outlook
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Reactions of tris( ortho -carboranyl)borane with Lewis bases reveals only small bases bind. The tremendous bulk and Lewis acidity is leveraged in frustrated Lewis pair Si–H cleavage with a wider range of Lewis bases and greater efficacy than B(C 6 F 5 ) 3 .
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The coexistence of a strong Lewis acid and a Lewis base in solution, the so-called frustrated Lewis pairs, has led to the discovery of the metal-free hydrogen activation. From then on, this observation has inspired all kinds of chemists to develop more examples. The metal-free hydrogenation has been the so far most studied application of frustrated Lewis pair chemistry, and highly efficient methodologies for a number of unsaturated substrates have been developed. This chapter starts with a brief introduction to frustrated Lewis pair chemistry. The second and third parts discuss the mechanism of the hydrogen activation and the influence of the nature of the Lewis acid and the Lewis base on this process. These factors do not only influence the hydrogen activation but also have severe impact on catalytic transformations. These features are exemplified in the fourth part of the chapter by the discussion of frustrated Lewis pair-catalyzed hydrogenations of a number of substrate classes.
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Treatment of the Lewis acid B(C6F5)3 with the Lewis base 2,6-dimethylpiperidine (DMP) resulted in the formation of the classical Lewis acid·base adduct DMP-B(C6F5)3, 1a, which was anticipated to undergo thermal dissociation to the "unquenched" Lewis centers. The free Lewis pair was able to form a frustrated Lewis pair (FLP), which induced heterolytic splitting of H2, affording the ionic product [DMPH][HB(C6F5)3], 1b. FLPs, derived from B(C6F5)3 and the bulky Lewis bases 2,2,6,6-tetramethylpiperidine (TMP) and 1,2,2,6,6-pentamethylpiperidine (PMP), could also heterolytically activate H2, affording the salts [TMPH][HB(C6F5)3], 2, and [PMPH][HB(C6F5)3], 3, respectively. In a VT NMR study the TMP/B(C6F5)3 reaction was studied in greater detail, trying to trace intermediates. The supposed most prominent intermediate, the TMP/H2/B(C6F5)3 complex, could, however, not be detected. The combination of B(C6F5)3 with the even more sterically demanding Lewis base 1-ethyl-2,2,6,6,-tetramethylpiperidine (Et-TMP) displayed FLP reactivity with H2, but required the high temperature of 110 °C, forming [2,2,6,6-(CH3)4C5H6NH(CH2CH3)][HB(C6F5)3], 4a. In the absence of H2 the combination of B(C6F5)3 and Et-TMP generated at room temperature a mixture of 4a and [2,2,6,6-(CH3)4C5H6N═CHCH2-B(C6F5)3], 4b. 4b was formed via consecutive hydride and proton abstractions with Et-TMP as the base, generating 4a. 2,4,6-Tri-tert-butylpyridine (TTBP), exhibiting reduced Lewis basicity as compared to piperidine derivatives, showed FLP reactivity with B(C6F5)3, which gave in the presence of H2 the [TTBPH][HB(C6F5)3], 5, salt as the only product after several hours. The steric demand of the Lewis bases was evaluated by aid of DFT calculations on borane adducts, which roughly correlated with the reaction temperature of H2 splitting. 1a, 1b, 3, 4a, and 4b were studied by single-crystal X-ray diffraction analyses.
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