Bronsted acids binary systems

Scheme 3.38 Relay catalysis by ruthenium complex/Bronsted acid binary system.

This is no reliable evidence of any solute acting as a Bronsted acid in HSO3F. So enhancement of acidity has always been achieved by addition of Lewis acids. The first potential acids investigated in HSO3F were binary fluorides. As discussed immediately below in Sec. 11.3.2.1, addition of S03 to fluorides dissolved in HSO3F was subsequently found to enhance acidity due to insertion of S03 into the metal-fluorine bond. More recently, Lewis acids of the HS03F system, i.e. binary fluorosulfates, have been studied. 

It has been established from these studies that the different catalytic properties of transition metal oxides (chromium, cobalt) on zirconium dioxide are attributed to their different acidic properties determined by TPDA and IR-spectroscopy. The most active catalyst is characterized by strong acidic Bronsted centers. The cobalt oxide deposited by precipitation on the zirconium-containing pentasils has a considerable oxidative activity in the reaction N0+02 N02, and for SCR-activity the definite surface acidity is necessary for methane activation. Among the binary systems, 10% CoO/(65% H-Zeolite - 35% Z1O2)... 

Abstract This chapter presents the design and analysis of the microscopic features of binary solvent systems formed by ionic liquids, particularly room temperature ionic liqnids with molecular solvents. Protic ionic liquids, ethylammonium nitrate and l-n-butyl-3-methylmidazohum (bmim)-based ILs, were selected considering the differences in their hydrogen-bond donor acidity. The molecular solvents chosen were aprotic polar (acetonitrile, dimethylsulphoxide and MA(-dimethylformide) and protic (different alcohols). The empirical solvatochromic parameters n, a and P were employed in order to analyse the behaviour of each binary solvent system. The study focuses on the identification of solvent mixtures of relevant solvating properties to propose them as new solvents . Kinetic study of aromatic nucleophilic substitution reactions carried out in this type of solvent systems is also presented. On the other hand, this is considered as a new approach on protic ionic liquids. Ethylammonium nitrate can act as both Bronsted acid and/or nucleophile. Two reactions (aromatic nucleophilic substitution and nncleophilic addition to aromatic aldehydes) were considered as model reactions. 

A special consideration related to BAN and its binary mixtures with molecular solvents allowed us to demonstrate that this protic IL can act both as Bronsted acid and nucleophile. The selected model reaction constitutes a suitable example of how the microscopic feature of a reactive system can be modified by adding aliquots of a protic IL to a molecular solvent. Moreover, the whole reactive system can be modulated with the aim not only to promote acid-catalysed reactions but also to generate nucleophilic species in situ. In this sense, the design of IL can be formulated according to the particular requirement of a reactive system. 

Differences in coordination number become significant in mixed oxide systems, where we find many examples of enhanced Lewis or Bronsted acidity, even for binaries such as Si02-Mg0. However, the best known and possibly most remarkable system of this type is silica-alumina. Typical materials have acid titres of the order 1 milliequivalent/g, but only a small proportion of the acid sites show — Hq values of 3 or more. 

Electrochemical measurements using protic or Bronsted acidic ILs as electrolytes have not received as high a research effort as observed for binary or ternary IL systems. Nevertheless, the proton mobility within these systems results in simplification of reference electrodes available for use. 

Very recently, a binary catalyst system composed of an achiral Lewis base and a chiral Bronsted acid for the enantioselective bromocycloetherification of 5-arylpentenols 28 was described by Denmark et al. (Scheme 43.6) (15). The combination of the achiral Lewis base Ph3P=S and chiral phosphoric acid 10 afforded good enantioselectivities for the cychzation of (Z)-configured 5-arylpentenols to form bromomethyltetrahydrofurans. 

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