Anionic N<i>‐</i>Heterocyclic Carbenes That Contain a Weakly Coordinating Borate Moiety

Kronig, Sabrina; Theuergarten, Eileen; Daniliuc, Constantin G.; Jones, Peter G.; Tamm, Matthias

doi:10.1002/ange.201108813

Cited by 45 publications

(25 citation statements)

References 111 publications

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“…This complex is reminiscent of the single-component zwitterionic Au-tht catalyst supported by an anionic N-heterocyclic carbene [12] that was recently reported by Tamm and coworkers. [13] However, in our case, the charged group is bound directly to the coordinating atom and is therefore much closer to the Au center. Interestingly, when the reaction was performed in THF, a solvent in which LiCl is soluble, THT was liberated, and an unusual anionic complex 3 (LiCl) was isolated (3 (LiCl) can also be prepared by the treatment of isolated 3 (THT) with LiCl in THF).…”

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confidence: 67%

Perhalogenated Carba‐closo‐dodecaborate Anions as Ligand Substituents: Applications in Gold Catalysis

et al. 2013

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The success of modern transition-metal catalysis is largely due to the availability of a diverse range of ligand frameworks. One of the most universal aspects of ligand design is the strategic attachment of bulky substituents to influence the activity of catalysts. Sterically demanding substituents kinetically protect the active metal center and, at the same time, promote substrate exchange and low coordination, two processes necessary for turnover. Bulky ligand substituents also influence the selectivity of the metal center for product formation and substrate consumption. For these purposes, the most common groups appended to ancillary ligands are alkyl and aryl groups, such as adamantyl or 2,6-diisopropylphenyl. Far less common is the use of dicarba-closo-dodecaborane (C 2 B 10 ) clusters [1] as surrogates for alkyl or aryl groups. [2] The three-dimensional aromatic nature of these species and their icosahedral shape lend them properties akin to those of both alkyl and aryl groups. However, owing to the hydridic nature of the BÀH vertices, ligands bearing these substituents tend to undergo undesirable B À H activation reactions, such as cyclometalation. [3] It is this tendency for such intramolecular reactions to occur that has limited the synthetic utility of dicarba-closo-dodecaborane-bearing ligands in the area of catalysis.Interestingly, complexes that contain ligands functionalized with related carba-closo-dodecaborate anions (CB 11 À ) [4] directly bound to the coordinating atom through the carborane cage carbon atom have not been reported. [5] Although similar in size to their neutral "dicarba" cousins (C 2 B 10 ), isoelectronic (CB 11 À ) clusters have significantly different properties. The negative charge is delocalized over all of the 12 cage atoms, and as a result, the anion is very weakly coordinating. This weak coordination ability can be enhanced by the substitution of some or all of the B À H vertices for alkyl or halo groups. In the case of alkyl substitution, the cluster becomes more reactive towards substitution and oxidation. On the other hand, exhaustive halogenation of the boron vertices of the cluster introduces a blanket of electronwithdrawing substituents that enhances the inherent weak coordination ability of the anion and also confers upon these molecules exceptional inertness. It has been demonstrated that perhalogenated carborane counteranions are sufficiently unreactive that they can form isolable salts with potent oxidants, such as C 60 +[6] and CH 3 + . [7] Thus, these clusters can be far more inert than even simple hydrocarbons: CH 3 + abstracts hydrides from n-alkanes with loss of methane at room temperature. It is these properties that have generated increasing interest [8] in the use of perhalogenated carborane anions in silylium catalysis [8i,l] and related processes. [8f] The anion of choice for most applications is the HCB 11 Cl 11 À anion (1), since it is readily accessible [8g] and arguably the most inert carborane anion (Scheme 1).We are interested in using the HCB 1...

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confidence: 67%

Perhalogenated Carba‐closo‐dodecaborate Anions as Ligand Substituents: Applications in Gold Catalysis

et al. 2013

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“…Generally, [AuCl] complexes are inactive in catalysis and dehalogenation is required to activate the complex, especially when using phosphines as a ligand. [51][52][53][54] Surprisingly, using the M 12 L 24 nanospheres, conversion of the substrate was seen at local concentrations of 0.27 M and higher. Moreover, it was observed that the activity of the catalyst increases with the local gold catalyst concentration.…”

Section: L 24 Cagesmentioning

confidence: 93%

“…Dehalogenation of the gold complexes with a silver salt led to degradation of the palladium based Pd 12 L 24 nanosphere, a problem that was not encountered for the more stable platinum based Pt 12 L 24 nanospheres. [55] The more stable Pt 12 L 24 cages also provided a higher tolerance towards functional groups, so the substrate scope for gold-catalyzed transformations could be enlarged (Scheme 12b). The catalytic activity increased when higher local gold concentrations were present in the spheres, and higher yields 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 in [4 + 2] cycloaddition reactions were obtained compared to the free mononuclear gold catalyst [Au(PPh 3 )X].…”

Section: L 24 Cagesmentioning

confidence: 99%

Gold Catalysis in (Supra)Molecular Cages to Control Reactivity and Selectivity

2018

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Gold catalysis has experienced a tremendous development over the past decades, and is nowadays widely used in organic synthesis to perform chemical transformations of π‐bond‐containing molecules. Catalyst development has been based mostly on ligand development and counter‐ion strategies. More recently, the encapsulation of gold catalysts in (supra)molecular cages was explored as a new way to control selectivity and reactivity of gold catalysts. In this review, we describe the cages that have been employed as hosts for gold complexes, along with their impact on the catalytic performance. Covalent and supramolecular approaches to encapsulate single metal complexes will be described and the impact on the catalytic performance will be discussed. Also, recent strategies to pre‐organize multiple metal centers will be discussed.

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“…[12] We envisioned synthesizing a normal NHC that would be rendered dianionic, as a result of containing two negatively charged and weakly coordinating [13] N-carboranyl substituents ( Figure 1). [12] We envisioned synthesizing a normal NHC that would be rendered dianionic, as a result of containing two negatively charged and weakly coordinating [13] N-carboranyl substituents ( Figure 1).…”

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confidence: 99%

“…The 13 C NMR spectrum of 8 shows a symmetrical imidazolium ring with a distinct low-field quaternary carbene resonance (196.9 ppm) as well as a signal for the equivalent hypercoordinate carborane carbons (81.4 ppm). We rationalized that the observed competitive deprotonation reactions might be explained, in part, by kinetic factors resulting from the sterically demanding anionic carborane substituents and the size of the employed base (LDA).…”

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confidence: 99%

Fusing N‐Heterocyclic Carbenes with Carborane Anions

El‐Hellani

Lavallo

2014

Angewandte Chemie

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Here we describe the fusion of two families of unusual carbon-containing molecules that readily disregard the tendency of carbon to form four chemical bonds, namely Nheterocyclic carbenes (NHCs) and carborane anions. Deprotonation of an anionic imidazolium salt with lithium diisopropylamide at room temperature leads to a mixture of lithium complexes of C-2 and C-5 dianionic NHC constitutional isomers as well as a trianionic (C-2, C-5) adduct. Judicious choice of the base and reaction conditions allows the selective formation of all three stable polyanionic carbenes. In solution, the so-called abnormal C-5 NHC lithium complex slowly isomerizes to the normal C-2 NHC, and the process can be proton-catalyzed by the addition of the anionic imidazolium salt. These results indicate that the combination of two unusual forms of carbon atoms can lead to unexpected chemical behavior, and that this strategy paves the way for the development of a broad new generation of NHC ligands for catalysis.

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Anionic N‐Heterocyclic Carbenes That Contain a Weakly Coordinating Borate Moiety

Cited by 45 publications

References 111 publications

Perhalogenated Carba‐closo‐dodecaborate Anions as Ligand Substituents: Applications in Gold Catalysis

Perhalogenated Carba‐closo‐dodecaborate Anions as Ligand Substituents: Applications in Gold Catalysis

Gold Catalysis in (Supra)Molecular Cages to Control Reactivity and Selectivity

Fusing N‐Heterocyclic Carbenes with Carborane Anions

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