Metal oxide Lewis acid-base reactions

The coordination chemistry of boron was reviewed some time ago and the structure and properties of compounds of the general formula BX3 L, where X and L can be one of a wide variety of substituents and electron pair donors, respectively (15). Indeed, the reactions of tricoordinate boron compounds in general are thought to proceed via addition of the reaction partner in a Lewis acid-base reaction to yield a tetracoord-inate intermediate that then undergoes further reaction. Stable tetra-coordinate boron compounds are subject to ligand displacement reactions for which a variety of mechanisms obtain (16). The coordination chemistry of transition metals is vast and includes not only structimal facts (17) but considerable information on the mechanistic behavior of these species as well (18). In our brief comparison we will restrict ourselves to low oxidation state chemistry and group 16 metals (19). 

Here, the Lewis base CaO donates an electron pair (one of the lone pairs of the oxygen atom) to the Lewis acid (CO2) to form a coordinate covalent bond in the CO ion. Similar Lewis acid-base reactions can be written for other acid-base anhydride pairs. Sulfur trioxide, for example, reacts with metal oxides to form sulfates ... 

The impressive sulfur-based reactivity of square planar nickel complexes containing tetradentate N2S2 ligands has been known for many years. Interest has recently resurfaced because of the discovery of similar donor sites in metalloproteins that bind nickel, iron, and cobalt.The iV,iV -bis(mercaptoethyl)-l,5-diazacyclooc-tane ligand H2(BME-D ACO) and its nickel complex have been particularly useful in establishing the scope of S-based reactivity with electrophiles as displayed in the reaction summary shown in Scheme 1." The fundamental features of this reactivity include templated macrocycle production, S-oxygenation as contrasted to oxidation, Lewis acid/base adduct formation, metal-ion capture, and the synthesis of heterodi- and polymetallic complexes. ... 

In 1997, the first truly catalytic enantioselective Mannich reactions of imines with silicon enolates using a novel zirconium catalyst was reported [9, 10]. To solve the above problems, various metal salts were first screened in achiral reactions of imines with silylated nucleophiles, and then, a chiral Lewis acid based on Zr(IV) was designed. On the other hand, as for the problem of the conformation of the imine-Lewis acid complex, utilization of a bidentate chelation was planned imines prepared from 2-aminophenol were used [(Eq. (1)]. This moiety was readily removed after reactions under oxidative conditions. Imines derived from heterocyclic aldehydes worked well in this reaction, and good to high yields and enantiomeric excesses were attained. As for aliphatic aldehydes, similarly high levels of enantiomeric excesses were also obtained by using the imines prepared from the aldehydes and 2-amino-3-methylphenol. The present Mannich reactions were applied to the synthesis of chiral (3-amino alcohols from a-alkoxy enolates and imines [11], and anti-cc-methyl-p-amino acid derivatives from propionate enolates and imines [12] via diastereo- and enantioselective processes [(Eq. (2)]. Moreover, this catalyst system can be utilized in Mannich reactions using hydrazone derivatives [13] [(Eq. (3)] as well as the aza-Diels-Alder reaction [14-16], Strecker reaction [17-19], allylation of imines [20], etc. 

Several metal oxides (either acidic or alkaline) have also been investigated for urea alcoholysis [228, 229], with PG finding PC product yields in excess of 90% for ZnO, PbO, and MgO. In such studies, the results obtained coupled with the results of thermal programmed desorption (TPD) and Fourier transform infrared (FTIR) analyses, indicated that catalysts with appropriate acid and base properties were required for the synthesis of CCs. These results confirmed the reports of Aresta et al. [94] and Ball et al. [39], who previously had investigated the reaction of primary and secondary alcohols with urea to form carbonate. These authors found the reaction to proceed in two steps, with a combination of a weak Lewis acid and a Lewis base improving the carbonate formation. 

In acid/base, or donor/acceptor, reactions, bonding results form the overlap of filled orbitals on the "donor" and empty orbitals on the "acceptor". Surface cations are generally Lewis acids and act as electron acceptors, while surface O ions are Lewis bases and can donate electrons to acceptor adsorbates. In lower oxides of the transition metals (i.e., in which the cations are in an oxidation state lower than their maximal valency), cations may also be able to donate electrons in an acid/base reaction. Although one talks of donating and accepting electrons in acid/base reactions, the electrons are in no sense free, and there is no actual electron transfer involved. This type of bonding can be either molecular or dissociative. 

Transition metals have a particular tendency to form complex ions because they have more than one oxidation state. This property allows them to act effectively as Lewis acids in reactions with many molecules or ions that serve as electron donors, or as Lewis bases. For example, a solution of cobalt(II) chloride is pink because of the presence of the Co(H20)6 ions (Figure 16.8). When HCl is added, the solution turns blue as a result of the formation of the complex ion CoCl4 ... 

During typical surface complex formation, or ligand exchange, the surface hydroxyl group on the hydrous oxide exchanges with a similar Lewis base electron pair donor in the solution. Adsorption of either protons or hydroxide ions is interpreted in terms of an acid-base reaction at the oxide surface, i.e., the surface hydroxyl group is either protonated or deprotonated. The adsorption of ligands (anions and weak acids) on a metal-oxide surface can also be compared with complex formation reactions in solution, e.g. ... 

Enantioselective vanadium and niobium catalysts provide chemists with new and powerful tools for the efficient preparation of optically active molecules. Over the past few decades, the use of vanadium and niobium catalysts has been extended to a variety of different and complementaiy asymmetric reactions. These reactions include cyanide additions, oxidative coupling of 2-naphthols, Friedel-Crafts-type reactions, pinacol couplings, Diels-Alder reactions, Mannich-type reactions, desymmetrisation of epoxides and aziridines, hydroaminations, hydroaminoalkylations, sulfoxida-tions, epoxidations, and oxidation of a-hydroxy carbo) lates Thus, their major applications are in Lewis acid-based chemistiy and redox chemistry. In particular, vanadium is attractive as a metal catalyst in organic synthesis because of its natural abundance as well as its relatively low toxicity and moisture sensitivity compared with other metals. The fact that vanadium is present in nature in equal abundance to zinc (albeit in a more widely distributed form and more difficult to access) is not widely appreciated. Inspired by the activation of substrates in nature [e.g. bromoperoxidase. 

Another example of dissociative adsorption by concerted action of a Lewis acid-base couple is It bonding of alkenes on metal oxides, as in reaction (IX). However, dissociative adsorption of water on ideally dehydroxylated oxide surfaces is by far the most important example to cite, in view of the formation of surface hydroxy groups and their tremendous impact on the reactivity of most oxides under ambent or close to ambient conditions [reaction (X)]. 

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