Acid with noncoordinating anion

Perchloric acid, a superacid, is one of the strongest Bronsted-Lowiy acids. Its pKa is -10. It provides strong acidity without interference from potential nucleophiles such as sulfate or chloride that complicate the use of sulfuric and hydrochloric acids. Other acids of noncoordinating anions, such as fiuoroboric acid and hexafluorophosphoric acid are susceptible to hydrolysis, whereas perchloric acid is not. Despite hazards associated with the explosiveness of its salts, the acid is often preferred in certain syntheses. For similar reasons, it is a useful eluent in ion-exchange chromatography. 

Reactions of P-halogen-NHPs with Lewis acids leading to halide abstraction, or metathetic replacement of the halide by a nucleofugic, noncoordinating anion form by far the most important routes to access phosphenium ions in general [51, 52] and also 1,3,2-diazaphospholenium cations in particular. As Lewis acid assisted P-X... 

The first hydration step was promoted by Bronsted acids containing weakly or noncoordinating anions. In the second step, an intramolecular hydrogen transfer in the secondary alcohol was catalyzed by ruthenium(III) salts with chelating bipyridyl-type ligands. The possible complexation of the latter with the diene did not inhibit its catalytic activity in the allylic rearrangements, under acid-catalyzed hydration conditions. 

These are readily prepared by reaction of the lanthanide oxide or carbonate with the acid. Salts of noncoordinating anions most often crystallize as salts [Ln(OH2)9]X3 (X e.g. bromate, triflate, ethyl sulfate, tosylate). These contain the [Ln(OH2)9] + ion (Figure 4.1), even for the later lanthanides, where in aqueous solution the eight-coordinate species [Ln(OH2)8] predominates. 

The use of palladium and ruthenium as halogen-free carbonylation catalysts has been studied intensively by Shell. The catalysts were principally designed for the carbonylation of olefins in the presence of alcohols in order to yield carboxylic esters [26], but work also well for the synthesis of carboxylic acids or anhydrides. The latter are formed when the reaction is conducted in an acid as a solvent [27]. The palladium systems typically consist of palladium acetate, tertiary phosphines, and strong acids such as mineral acids or acids with weak or noncoordinating anions such as p-toluenesulfonic acid. Remarkable activities are achieved when aromatic phosphines that carry pyridines as substituents are... 

Both selectivity and activity of HMF carbonylation are influenced by the TPPTS/ Pd molar ratio, maximum efficiency being observed for a TPPTS/Pd ratio of 6 1. Interestingly, acids with weakly or noncoordinating anions afford mainly carbonylation, those with strongly coordinating anions reduction. 

The catalysts were cationic palladium-phosphine systems prepared from palladium acetate, an excess of triphenylphosphine (PPh3), and a Bronsted acid of a weakly or noncoordinating anion (e.g., p-tosylate (OTs ) methanol was used as both the solvent and a reactant. An unexpected change in selectivity was observed upon replacement of the excess of PPh3 by a stoichiometric amount of the hidentate l,3-bis(diphenylphosphino)propane (dppp). Under the same conditions, these modified catalysts led to perfectly alternating ethylene/CO copolymers with essentially 100% selectivity [Eq. (2)] [12-14]. 

In solution, AX5 species are known to be much stronger Lewis acids (electron pair acceptors) than AX3 species.The AX6 anions, particularly the fluorides, are noted for their stability, being commonly used as noncoordinating ligands with highly reactive cations. 

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