High Yield C-Derivatization of Weakly Coordinating Carborane Anions

Abstract

Unlike the "parent" carborane anion CHB 11 H 11 − , halogenated carborane anions such as CHB 11 H 5 Br 6 − can be readily C-functionalized in high yield and purity, enhancing their utility as weakly coordinating anions.B-halogenated icosahedral carborane anions such as CHB 11 H 5 X 6 − and CHB 11 X 11 − (X = halogen) (Figure 1) are particularly useful members of a class of exceptionally inert, weakly coordinating anions 1-5 whose versatility might be further tailored by suitable C-derivatization chemistry. Long-chain hydrocarbon "tails" formed by C-alkylation should lower the lattice energies of salts, increase their solubilities in low dielectric solvents and allow better exploitation of reactive cations in catalysis. Applications in surfactant chemistry can also be envisioned. Similarly, C-fluorocarbon tails should improve the solubility of carborane ion pairs in fluorocarbon solvents, where catalytic applications have been reported. 6 Attachment of a carborane anion to a polymer has allowed exploitation of immobilization chemistry in cationselective sensor technology. 7 C-arylation takes advantage of the unique scaffold of a carborane anion in rigid rod supramolecular chemistry. 8,9 Despite these promising applications, the C-derivatization chemistry of carborane anions has progressed rather slowly. Most work has been performed on the "parent" carborane anion, CHB 11 H 11 − , but is frequently hampered by modest yields and difficult separations from starting material. Being ionic rather than neutral, the chromatographic separation of different carborane anions is not trivial on a synthetic scale. The activation of CHB 11 H 11 − via Clithiation with butyl lithium appears to be essentially quantitative 10 but the partial regeneration of starting material during subsequent reactions with electrophiles is common, despite careful control of conditions. Alkylation of 1-Li-CB 11 H 11 − with alkyl halides gives mixed results. While the yields for methylations are frequently quite high, 2,11-14 they drop to 63% for ethylation of CHB 11 H 11 − and are even lower for most other alkylations, silylations, phosphinations and metallations. 2,10 The yields of C-monohalogenated products, 1-X-CB 11 H 11 − , have been raised to 81-96% by careful attention to conditions but chromatography purification is still recommended for most derivatives. 11 We now report that when these C-functionalization reactions are performed on an already halogenated carborane anion such as CHB 11 H 5 Br 6 − , rather than on the unfunctionalized parent CHB 11 H 11 − , isolated yields are generally excellent and compound purity is sufficiently high

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

“…[1,2] For example {closo-1-CB 11 } clusters with ethynyl groups have been used for the stabilization of unusual {Au I } 4 clusters. [3] Especially for the cluster carbon atom the selective functionalization is well developed and a number of different substituents have been introduced.…”

“…[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.…”

“…[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. [11] In contrast, many examples for dicarba-closo-dodecaboranes with one or two C cluster -P bonds and applications of these compounds have been reported in the literature.…”

“…[11] The latter anion was characterized by NMR spectroscopy, only, because of its high sensitivity towards oxygen. [11] In contrast, many examples for dicarba-closo-dodecaboranes with one or two C cluster -P bonds and applications of these compounds have been reported in the literature. [19][20][21][22][23][24][25] Most of these functionalized dicarba-closo-dodecaboranes contain dialkyl-or diarylphosphanyl substituents or related groups, e.g.…”

“…In addition, the reactive anions [1-Cl 2 P-closo-1-CB 11 Hal 11 ] -(Hal = F (2), Cl (3), Br (4), I (5)) were prepared and characterized by multinuclear NMR spectroscopy in solution.…”