Weakly coordinating anions

Carboranes as anew class of weakly coordinating anions for strong electrophiles, oxidants, and superacids 98ACR133. 

The first successful hydrogenation reactions in ionic liquids were studied by the groups of de Souza [45] and Chauvin [46] in 1995. De Souza et al. investigated the Rh-catalyzed hydrogenation of cyclohexene in l-n-butyl-3-methylimidazolium ([BMIM]) tetrafluoroborate. Chauvin et al. dissolved the cationic Osborn complex [Rh(nbd)(PPh3)2][PFg] (nbd = norbornadiene) in ionic liquids with weakly coordinating anions (e.g., [PFg] , [BFJ , and [SbF ] ) and used the obtained ionic catalyst solutions for the biphasic hydrogenation of 1-pentene as seen in Scheme 5.2-7. 

Rh(nbd) (PPh3)2][PFg] (nbd = norbornadiene) in ionic liquids with weakly coordinating anions. 

An example of a biphasic, Ni-catalyzed co-dimerization in ionic liquids with weakly coordinating anions has been described by the author s group in collaboration with Leitner et al. [12]. The hydrovinylation of styrene in the biphasic ionic liq-uid/compressed CO2 system with a chiral Ni-catalyst was investigated. Since it was found that this reaction benefits particularly from this unusual biphasic solvent system, more details about this specific application are given in Section 5.4. 

The fact that ionic liquids with weakly coordinating anions can combine, in a unique manner, relatively high polarity with low nucleophilicity allows biphasic catalysis with highly electrophilic, cationic Ni-complexes to be carried out for the first time [26]. 

Attempted synthesis of RhY(CO)(PPh3)2 in undried solvents (Y = a weakly coordinating anion, e.g. BF4, C104, S03CF3) leads to... 

Preparations of heterocyclic thiosemicarbazone complexes with cobalt(II) salts of weakly coordinating anions (e.g., perchlorate and tetrafluoroborate) often result in cobalt(III) complexes due to air oxidation. The first 2-acetylpyridine thiosemicarbazone cobalt(III) complex was the diamagnetic [Co(15b-H)2]C104... 

Figure 34 A polymer-supported metallocene catalyst (51) with a weakly coordinating anion, [B(C6F5)4] , produced from lightly cross-linked, chloromethylated polystyrene beads for olefin polymerization. (Adapted from ref. 75.)...

However, the practical, direct synthesis of functionalized linear polyolefins via coordination copolymerization olefins with polar monomers (CH2 = CHX) remains a challenging and industrially important goal. In the mid-1990s Brookhart et al. [25, 27] reported that cationic (a-diimine)palladium complexes with weakly coordinating anions catalyze the copolymerization of ethylene with alkylacrylates to afford hyperbranched copolymers with the acrylate functions located almost exclusively at the chain ends, via a chain-walking mechanism that has been meticulously studied and elucidated by Brookhart and his collaborators at DuPont [25, 27], Indeed, this seminal work demonstrated for the first time that the insertion of acrylate monomers into certain late transition metal alkyl species is a surprisingly facile process. It spawned almost a decade of intense research by several groups to understand and advance this new science and to attempt to exploit it commercially [30-33, 61]. 

The catalyst precursors (cationic Ir-COD complexes with weakly coordinating anions such as BArF) are air- and moisture-stable, and are therefore easy to handle. It is possible to store them for several months under air. Two of the most versatile catalysts (ThrePHOX catalysts, 12a and 12b) recently became commercially available [33]. 

Interest in these compounds—mainly for use as weakly coordinating anions—has grown rapidly in the last decade. Many synthetic routes to CBnHi2 and its derivatives are shown in Table 17 (Figures 4 and 5). The structures of CBnH12 and its derivatives determined experimentally are summarized in Table 18. The table shows one column for cations where the anions are regarded as discrete entities within the crystal lattice and the other for cations where the anions are viewed as a fragment of a molecule or polymer in the solid state. 

On the basis of these results we embarked on a systematic study on the synthesis of vinyl cations by intramolecular addition of transient silylium ions to C=C-triple bonds using alkynyl substituted disila alkanes 6 as precursors.(35-37) In a hydride transfer reaction with trityl cation the alkynes 6 are transformed into the reactive silylium ions 7. Under essentially nonHnucleophilic reaction conditions, i.e. in the presence of only weakly coordinating anions and using aromatic hydrocarbons as solvents, the preferred reaction channel for cations 7 is the intramolecular addition of the positively charged silicon atom to the C=C triple bond which results in the formation of vinyl cations 8-10 (Scheme 1). 

For reviews on weakly coordinating anions see a) C. Reed, Acc. Chem. 

The most active and selective catalysts for both the copolymerisation process and for the apparently simpler ethene carbonylation to monocarbonylated products MP or DEK are cationic square planar Pd(II) complexes in which the metal centre is czs-coordinated by a bidentate P - P ligand, by a Ugand involved in the initial step of the catalysis or in the process of forming the product and with the fourth vacant site coordinated by CO or ethene or a keto group of the growing chain or MeOH (or H2O, always present in the solvent even when not added on purpose) or even by a weakly coordinating anion. 

Chloride versus weakly coordinating anions alkylphosphines... 

It was Ayusman Sen [8] who discovered in 1982 that the use of weakly coordinating anions and phosphines as the ligands together with palladium yielded much more stable and active catalysts for the formation of polyketone from CO and ethene in alcoholic solvents. Cationic palladium-(triphenylphosphine)2(BF4)2 gave a mixture of oligomers having methoxy ester... 

After extensive experimentation, a simple solution for avoiding catalyst deactivation was discovered, when testing an Ir-PHOX catalyst with tetrakis[3,5-bis (trifluoromethyl)phenyl]borate (BArp ) as counterion [5]. Iridium complexes with this bulky, apolar, and extremely weakly coordinating anion [18] did not suffer from deactivation, and full conversion could be routinely obtained with catalyst loadings as low as 0.02 mol% [19]. In addition, the BArp salts proved to be much less sensitive to moisture than the corresponding hexafluorophosphates. Tetrakis (pentafluorophenyl)borate and tetrakis(perfluoro-tert-butoxy)aluminate were equally effective with very high turnover frequency, whereas catalysts with hexafluorophosphate and tetrafluoroborate gave only low conversion while reactions with triflate were completely ineffective (Fig. 1). 

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