BArF complex

In general, the choice of counteranion has a minor effect on catalyst performance, with typical examples being selected from BF4, OTD, PFg, or BARF-. In one example, however, it was noted that [(R.R)-Et-DuPhos Rh CODJOTf gave superior selectivity for the reduction of / -/ disubstituted a-dehydroamino acid derivatives than the corresponding BARF complex when performed in a range of solvents, including supercritical carbon dioxide [39]. 

The data for the insertion rate of CO into a palladium-methyl bond for dppp as the ligand has been studied with considerably more precision by Brookhart and co-workers [24], The kinetics were studied in the temperature range between 191 and 210 K for a reaction similar to that of Figure 12.6, i.e. the starting material was the CO adduct of the methylpalladium(dppp)+BArF complex. A AG of 62 kJ.mol"1 was observed and since AS was close to zero, a half-life time of 10 s is calculated for the CO-adduct at 235 K, much shorter than the value for dppe given above (150 s). 

The [Ir(cod)2]BARF complex also showed high catalytic activity in the hydrogena-tive coupling of alkyne with aldimines to lead to reductive couphng products, aUyl amines [69]. 

The [Mn(CO)3(P)2(CH2Cl2)][BArF] complex, 23, also catalyzes reaction of phenol with triethylsilane, presumably by a mechanism similar to that proposed for the Fe system (Scheme 9). The ratio of silane to the catalyst was about 24 1, and a slight deficiency of phenol was added at —78°C in an NMR tube reaction. XH NMR spectra recorded from... 

Handling, Storage, and Precautions readily oxidizes to the phospholane oxide upon exposure to atmospheric oxygen. The corresponding (allyl)Ni(BARF) complex is stable upon storage at room temperature for several days under a nitrogen atmosphere. 

With one exception, the same Ir complexes precursors as for Rh are used (compare Fig. 3) neutral or cationic bis-diene Ir(l) complexes with Cl or BF4 as anion, respectively, preformed diene-diphosphine complexes, and [cp Ir(in) Cl2]2 for transfer hydrogenations. The exception are the cationic [Ir(cod)P N)]X (X=BF4, SbFs and BARF) complexes depicted in Fig. 16 which are the catalyst of choice for the highly enantioselective hydrogenation of unfunctionalized C=C bonds [108]. The choice of X is important for good catalyst stability and BARF is the preferred anion. 

The ionic iridium(III) carbene complex (47) is prepared from the reaction of IrHCl(03SCF3)-(CO)(PPh3)2] with [RC=NMe](03SCF3).61 Addition of Na(barf) (barf = B(3,5-C6H3 (CF3)2)4) to [Ircp (PMe3)(CH3)(0S02CF3)] in CH2C12 yields the structurally determined species... 

This complex is not the actual catalyst for the hydrovinylation, but needs to be activated in the presence of a suitable co-catalyst. The role of this additive is to abstract the chloride ion from the nickel centre to generate a cationic allyl complex that further converts to the catalytically active nickel hydride species. In conventional solvents this is typically achieved using strong Lewis acids such as Et2AlCl. Alternatively, sodium or lithium salts of non-coordinating anions such as tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate (BARF) can be used to activate hydrovinylation... 

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]. 

The above reactions are rather slow because the insertion reactivity is directly related to the rate of dissociation of the THF ligand, which is also present as the solvent. The most reactive cationic complexes are therefore base-free, i. e. they do not contain coordinating solvents or ligands. Logically, anion coordination is a problem in such systems. Ion pairing reduces the reactivity of the cationic zirconocenes, but in the presence of B(C6F5)4, RB(C6F5)3 (R = H, Me), or BARF", this interaction is only weak. 

A very reactive Lewis acid is obtained when the complex [(EBTHI)Zr(Me)2] is converted in situ to a dicationic species by protonation with the acid H-BARF (BARF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) in the presence of the Diels—Alder substrate oxazolidi-none [88] (Scheme 8.48). The dicationic species is stabilized through coordination by the oxazolidinone and by diethyl ether (derived from the acid etherate employed). The catalyst loading in the Diels—Alder reaction could be lowered to 1 mol% (Zr) and the reaction still... 

The C02-philic perfluoroalkyl-substituted (R,S)-3-H F -BINAPHOS ligand [34] was successfully applied to enantioselective hydrogenation in the inverted SCCO2/H2O system. The complex [Rh(cod)2]BARF was chosen as metal source and the active catalyst was formed in situ. Using the same procedure as above, similar activities and more than 98% ee were obtained consistently over five subsequent cycles in the hydrogenation of methyl 2-acetamido acrylate. The results demonstrate the potential of the inverted SCCO2/H2O system for asymmetric synthesis of chiral biologically active products. 

An iridium(I) complex with the l,2-bis(tcrt-butylmethylphosphino)ethane (4) and tetrakis(3,5-bis(trifluoromethyl)phenyl)borate as the counter anion catalyzes the hydrogenation of several acyclic aromatic Ai-arylimines under atmospheric hydrogen pressure at room temperature, giving the desired chiral amines with high-to-excellent enantioselectivities (up to 99%, Fig. 6) [19]. The authors also tested (S )-BINAP (Fig. 1) and (/ )-Ph-PHOX (PHOX = 2-[2-(diphenylphosphino) phenyl]-4,5-dihydrooxazole) hgands with lower enantioselectivities [19]. Both steric and electronic properties of the ligand and the combination with the BArF anion are in the base of the efficacy of this catalytic system. On the other hand, attempted hydrogenations of Ai-(2,2,2-trifluoro-l-phenylethylidene)aniline and M-(l,2,2-trimethyl-propylidene)aniline under the same conditions resulted in... 

The surface is initially covered by the BARF(-) anion and by an anilinium(+) cation and a floating cation is grafted here the coordination of the floating cation to surface oxygens should be avoided. Indeed results indicate the complexity of the surface. 

While most superoxo complexes—in contrast to peroxo compounds— have been assigned a bent, end-on coordination mode [9], the superoxide ligand of Tp Cr(02)Ph was suggested to exhibit the more unusual side-on (r] ) coordination [10]. The reactivity of the complex did not allow for the determination of its molecular structure however, close analogs could be isolated, crystallized and structurally characterized by X-ray diffraction. For example, the reaction of [Tp Cr(pz H)]BARF (pz H = 3-tert-butyl-5-methylpyrazole, BARF = tetrakis(3,5-bis(trifiuoromethyl)phenyl)borate) with O2 produced the stable dioxygen complex [Tp Cr(pz H)( ] -02)]BARF (Scheme 3, bottom), which featured a side-on bound superoxide ligand (do-o = 1.327(5) A, vo-o = 1072 cm ) [11]. Other structurally characterized... 

High catalytic activities have been achieved by the PYRPHOS- [18], PPCP [20], BICHEP- [21], Et-DuPHOS-Rh [19] complexes among others, allowing the reaction with a substrate-to-catalyst molar ratios (S/C) as high as 50,000. With a [2.2]PHANEPHOS-Rh complex, the reaction proceeds even at -45°C [27], Supercritical carbon dioxide, a unique reaction medium, can be used in the DuPHOS and BPE-Rh-catalyzed hydrogenation [43], A highly lipophilic counteranion such as tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF) or trifluoromethanesulfonate is used to enhance the solubility of the cationic Rh complexes. Under the most suitable reaction conditions of 102 atm of carbon dioxide, 1 atm of hydrogen, and 22°C, a-amino acid derivatives are produced with up to 99.7% ee. 

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