Solvent weakly-coordinating

Upon protonation of 3.6 at 80 C in CH2CI2 or PhCl, new chiral Lewis acids 3.7 and 3.8 were formed with solvent weakly coordinated via the chlorine centre (Scheme 3.3). ... 

N-Alkylpyrroles may be obtained by the Knorr synthesis or by the reaction of the pyrrolyl metallates, ie, Na, K, and Tl, with alkyl haUdes such as iodomethane, eg, 1-methylpyrrole [96-54-8]. Alkylation of pyrroles at the other ring positions can be carried out under mild conditions with allyhc or hensylic hahdes or under more stringent conditions (100—150°C) with CH I. However, unless most of the other ring positions are blocked, poly alkylation and polymerisation tend to occur. N-Alkylation of pyrroles is favored by polar solvents and weakly coordinating cations (Na", K" ). More strongly coordinating cations (Li", Mg " ) lead to more C-alkylation. 

By contrast, in the more weakly coordinating solvent Et20, Me bridges and /X3-OR bridges can... 

Ionic liquids with wealdy coordinating, inert anions (such as [(CF3S02)2N] , [BFJ , or [PFg] under anhydrous conditions) and inert cations (cations that do not coordinate to the catalyst themselves, nor form species that coordinate to the catalyst under the reaction conditions used) can be looked on as innocent solvents in transition metal catalysis. In these cases, the role of the ionic liquid is solely to provide a more or less polar, more or less weakly coordinating medium for the transition metal catalyst, but which additionally offers special solubility for feedstock and products. 

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. 

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

When the reaction is carried out in heptanol [61], the particles are monodisperse in size (3 nm), well dispersed in the solvent, and adopt the hep structure of bulk rutheniiun. They can be isolated and re-dissolved in various solvents, including d -THF for NMR analysis. In this case, it is clear that coordinated heptanol is present at the surface of the particles and acts as a weakly coordinating ligand. In this case, the presence of surface hydrides was demonstrated by NMR techniques. 

Finally, the term steric stabihzation coifid be used to describe protective transition-metal colloids with traditional ligands or solvents [38]. This stabilization occurs by (i) the strong coordination of various metal nanoparticles with ligands such as phosphines [48-51], thiols [52-55], amines [54,56-58], oxazolines [59] or carbon monoxide [51] (ii) weak interactions with solvents such as tetrahydrofuran or various alcohols. Several examples are known with Ru, Ft and Rh nanoparticles [51,60-63]. In a few cases, it has been estab-hshed that a coordinated solvent such as heptanol is present at the surface and acts as a weakly coordinating ligand [61]. 

Under other reaction conditions, the product can result from thermodynamic control. Aldol reactions can be effected for many compounds using less than a stoichiometric amount of base. In these circumstances, the aldol reaction is reversible and the product ratio is determined by the relative stability of the various possible products. Thermodynamic conditions also permit equilibration among the enolates of the nucleophile. The conditions that lead to equilibration include higher reaction temperatures, protic or polar dissociating solvents, and the use of weakly coordinating cations. Thermodynamic conditions can be used to enrich the composition in the most stable of the isomeric products. 

Again, weak coordination is expected for ether ligands to zinc, however, the larger number of structurally characterized examples is at least partially attributable to the frequent use of solvents such as diethyl ether or tetrahydrofuran which may provide additional ligands to the metal center. 

The zinc teflate Zn(OTeF5)2 has been synthesized and characterized by IR and 19F NMR spectroscopy. The interaction with a number of weakly coordinating solvents was studied and the single-crystal X-ray structures determined for nitrobenzene derivatives [Zn(OTeF5)2 (PhN02)2]2 and Zn(0TeF5)2(PhN02)3.428... 

P(OEt), P(0 Pr) ) in n-heptane solution. There was previous evidence for the production of two intermediates tentatively identified as W(CO) L(S) (S = solvent), with L and S cis and trans to each other (49, 0). It has now proved possible to obtain firm identification of these two species in room temperature solution from their V(C-O) absorption spectra. It could be proved that their structures were indeed cis and trans W(CO) L by generating exactly the same species in low-temperature matrices. One noteworthy feature of the experiments is that, even with such a weakly coordinating solvent as heptane, the cis and trans intermediates do not interconvert on a millesecond timescale [.53]. 

The colorless tetramer Ccu(py)l] is fairly stable only in non- or weakly coordination solvents such as benzene, CH Cl, or acetone. At room temperature in solution this copper complex shows an intense red photoluminescence ( 0.04 a f nax = nm (54). The emitting state is a metal-centered 3d 4s excited state which is strongly modified by Cu(I)-Cu(I) interaction in the tetramer. This consists of a (Cul) cubane core. 

In contrast, when boron trifluoride etherate is substituted for the free boron trifluoride, only a trace of the hydrocarbon is formed, even after weeks of reaction.143 The unique effectiveness of boron trifluoride gas in promoting these reductions is believed to be due to several factors, including the ability of the coordinatively unsaturated boron center to rapidly and tightly coordinate with oxygen centers and to the thermodynamically favorable creation of a Si-F bond.1 A slight pressure of boron trifluoride gas must be maintained over the surface of the solution throughout the reaction because boron trifluoride has only limited solubility in the weakly coordinating dichloromethane solvent. 

However, unlike most conventional solvents, many ionic liquids combine high solvating power for polar catalyst complexes (polarity) with weak coordination (nucleophilicity) [38], It is this combination that enables a biphasic reaction mode with these ionic liquids even for catalyst systems which are deactivated by water or polar organic solvents. 

Fig. 9. Composition (5a, X12 = H 5d, X12 = OCH3) and ORTEP (5a) structures of the black p-oxo-bridged diiron(IV) complex rapidly formed in a high yield from la,d and 02 in CH2C12 or other weakly coordinating solvents. The ellipsoids are drawn at the 50% probability level. From Ref. (38).

Effect of Solvent and Base on the Ruthenium Carbonyl/Tri-methylamine System. Solvent plays an important role in the rate of hydrogen production. The ideal solvents are tetrahydrofuran, diglyme, and dimethoxyethane. Alcohols are only slightly less effective. Apparently the solvent must be miscible with water, promote ion formation, and be capable of weakly coordinating with the coordinately unsaturated species formed in the course of the reaction. 

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