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

Abstract

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...

Carba‐closo‐dodecaborate Anions with Cluster Carbon–Phosphorous Bonds

“…The isopropyl-substituted phosphane derivative [1-iPr 2 P-closo-1-CB 11 H 11 ] -(1c) is easily accessible, similar to the chlorinated analogue [1-iPr 2 P-closo-1-CB 11 Cl 11 ] -that was shown to be a valuable ligand for gold catalyzed hydroaminations of alkynes. [18] Contrary to [1-iPr 2 P-closo-1-CB 11 Cl 11 ] -, 1c was found to be sensitive towards oxygen with [1-iPr 2 (O)P-closo-1-CB 11 H 11 ] -(1d) being the oxidation product.…”

“…[18] In contrast, the P-C bond lengths of 1c are close to the ones that have been reported for 1-iPr 2 P-2-Mecloso-1,2-C 2 B 10 H 10 [d(C cluster -P) = 1.876(6) Å, d(P-C iPr ) = 1.876(8) and 1.863(7) Å] [34] and d(C-P) as well as d(P-O) of 1d are similar to those of the related ortho-carborane…”

“…[1-R-closo-1-CB 11 X 11 ] -(R = functional group; X = H, Hal, alkyl), [5,[7][8][9][10][11][12][13][14][15][16][17] as well. [1] Surprisingly, only a few examples for {closo-1-CB 11 } derivatives with a C cluster -P bond have been described, so far: [1-iPr 2 P-closo-1-CB 11 Cl 11 ] -, [18] [1-Ph 2 P-closo-1-CB 11 H 11 ] -, its oxidation product [1-Ph 2 (O)P-closo-1-CB 11 H 11 ] -, [4] and [1-tBu 2 P-7,8,9,10,11,12-Br 6 -closo-1-CB 11 H 5 ] -. [11] The latter anion was characterized by NMR spectroscopy, only, because of its high sensitivity towards oxygen.…”

A Highly Efficient and Broadly Applicable Cationic Gold Catalyst

“…Catalyst loadings in the ppm range have been reported for a narrow set of gold-catalyzed reactions. [2] Notable examples include the [(NHC)Au I ]-catalyzed (NHC = N-heterocyclic carbene) alkyne hydration, reported by Nolan and co-workers; [3] the [(NHC)Au I ]-catalyzed intramolecular addition of diol to alkyne, reported by Hashmi and co-workers; [4] the hydroamination of alkynes with a hyperhalogenated carba-closo-dodecaborate anionic ligand, reported by Lavallo and co-workers; [5] and the ester assisted hydration of alkynes catalyzed by small gold clusters, reported by Corma and co-workers. [6] In some exceptional cases, even higher turnovers have been achieved but at the cost of employing relatively high temperatures (e.g.…”

Fusing N‐Heterocyclic Carbenes with Carborane Anions

“…[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).…”