Lewis acid and hydrogenation

Alkyl cyanides with electron withdrawing substituents are more reactive and cyclotrimer-ize under both acidic and basic conditions as well as at high pressure (Table 14). Complexes of Lewis acids and hydrogen halides are particularly valuable catalysts. Two mechanisms for the acid-catalyzed cyclotrimerization have been postulated. Grundmann et al. suggest... 

Detailed studies of systems involving aluminium-based Lewis acids and hydrogen halides are scarce. Fontana and Kidder investigated the polymerisation of propene initiated by the pair aluminium bromideTiydrt n bromide. The cocatalytic role of the latter acid was clearly proved since no polymerisation could be detected in its absence. The dependence of the rate of polymerisation upon the cocatalyst concentration and the induction periods observed make this system similar to those in which stannic chloride induces the polymerisation of olefins in the presence of variable quantities of water (see Sect. IV-C-3-b). With relatively large quantities of added hydrogen bromide, addition of this acid to the mcmomer to give fso-propyl bromide must have constituted an important side reaction. 

Ciufolini et al. developed carbonyl-ene reactions catalyzed by the 1 1 complex of Yb(fod)3 and acetic acid [19]. 2-Methoxypropene reacted with a variety of aldehydes under the conditions used, providing the protected alcohols in good yields (Eq. 9). Addition of acetic acid was essential —the reaction did not proceed with Yb(fod)3 alone. Addition of silica gel to the reaction mixture was found to enhance the rate of the reaction and to make the reaction clean, although the use of silica gel was not mandatory. Double activation of the aldehydes as a result of coordination to the Yb Lewis acid and hydrogen-bonding with the acidic hydrogen of the acetic acid was proposed for the reaction (Fig. 2). 

In this respect, both free nitrile molecules and their complex salts with acidic catalysts can take part in the formation of the triazinc.146 Hydrogen chloride, sulfuric acid, Lewis acids and complexes of Lewis acids and hydrogen halides are generally used as catalysts the complexes usually have the best catalytic potential.147-150... 

A variation of the reaction in which the HCO- group is substituted onto the benzene ring of a phenol uses hydrogen cyanide rather than carbon monoxide. Typically, a mixture of zinc cyanide and hydrochloric acid is used, to give zinc chloride (which acts as a Lewis acid) and hydrogen cyanide. The electrophile in this case is protonated hydrogen cyanide ... 

The fact that a Lewis acid is able to accept an electron pair means that it must have either a vacant, low-energy orbital or a polar bond to hydrogen so that it can donate H+ (which has an empty7 Is orbital). Thus, the Lewis definition of acidity includes many species in addition to H+. For example, various metal cations, such as Mg2+, are Lewis acids because they accept a pair of electrons when they form a bond to a base. We ll also see in later chapters that certain metabolic reactions begin with an acid-base reaction between Mg2+ as a Lewis acid and an organic diphosphate or triphosphate ion as the Lewis base. 

This chapter and the following two chapters survey the properties of the elements and their compounds in relation to their locations in the periodic table. To prepare for this journey through the periodic table, we first review the trends in properties discussed in earlier chapters. We then start the journey itself with the unique element hydrogen and move on to the elements of the main groups, working from left to right across the table. The same principles apply to the elements of the d and f blocks, but these elements have some unique characteristics (mainly their wide variety of oxidation states and their ability to act as Lewis acids), and so they are treated separately in Chapter 16. 

In addition to standard catalytic hydrogenolysis, methods for transfer hydrogenolysis using hydrogen donors such as ammonium formate or formic acid with Pd-C catalyst are available.216 The Cbz group also can be removed by a combination of a Lewis acid and a nucleophile for example, boron trifluoride in conjunction with dimethyl sulfide or ethyl sulfide.217... 

One of the problems encountered when dealing with the interaction of Lewis acids and bases in a quantitative way is in evaluating the role of the solvent. Bond energies in molecules are values based on the molecule in the gas phase. However, it is not possible to study the interaction of many Lewis acids and bases in the gas phase because the adducts formed are not sufficiently stable to exist at the temperature necessary to convert the reactants to gases. For example, the reaction between pyridine and phenol takes place readily in solution as a result of hydrogen bonding ... 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

The hydrogen ion accepts the lone pair of electrons from the ammonia to form the ammonium ion. The hydrogen ion, because it accepts a pair of electrons, is the Lewis acid. The ammonia, because it donates a pair of electrons, is the Lewis base. This reaction is also a Brpnsted-Lowry acid-base reaction. This illustrates that a substance may be an acid or a base by more than one definition. All Brpnsted-Lowry acids are Lewis acids, and all Brpnsted-Lowry bases are Lewis bases. However, the reverse is not necessarily true. 

The principal factors affecting solvent-ion interactions can be classified as ion-dipole, Lewis acid-base, hydrogen-bonding, solvent structural, and steric. The solvent obviously plays a major part in these interactions. Therefore, to interpret trends in conductance data, bulk solvent properties such as viscosity and dielectric constant should be considered. Table 1 lists selected physical properties for a number of organic solvents. 

Adduct formation does not require formation of a hydrogen bond, and other acceptor atoms (or molecules) are known (e.g., transition metal cations in general, SbCls, h, and so on). They are collectively called Lewis acids, and they react with electron pair donors, that are collectively called Lewis bases. 

A boron analog - sodium borohydride - was prepared by reaction of sodium hydride with trimethyl borate [84 or with sodium fluoroborate and hydrogen [55], and gives, on treatment with boron trifluoride or aluminum chloride, borane (diborane) [86. Borane is a strong Lewis acid and forms complexes with many Lewis bases. Some of them, such as complexes with dimethyl sulfide, trimethyl amine and others, are sufficiently stable to have been made commercially available. Some others should be handled with precautions. A spontaneous explosion of a molar solution of borane in tetrahydrofuran stored at less than 15° out of direct sunlight has been reported [87]. 

This review will concentrate on metal-free Lewis acids, which incorporate a Lewis acidic cation or a hypervalent center. Lewis acids are considered to be species with a vacant orbital [6,7]. Nevertheless, there are two successful classes of organocatalysts, which may be referred to as Lewis acids and are presented in other chapter. The first type is the proton of a Brpnsted acid catalyst, which is the simplest Lewis acid. The enantioselectivities obtained are due to the formation of a chiral ion pair. The other type are hydrogen bond activating organocatalysts, which can be considered to be Lewis acids or pseudo-Lewis acids. 

Although the mechanism of the platinum catalysis is by no means completely understood, chemists do know a lot about how it works. It is an example of a dual catalyst platinum metal on an alumina support. Platinum, a transition metal, is one of many metals known for its hydrogenation and dehydrogenation catalytic effects. Recently bimetallic platinum/rhenium catalysts are now the industry standard because they are more stable and have higher activity than platinum alone. Alumina is a good Lewis acid and as such easily isomerizes one carbocation to another through methyl shifts. 

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