Although alkanes are undoubtedly much less reactive than other organic compounds including unsaturated hydrocarbons, the number of known alkanes reactions is large. In this section we will briefly survey the main types of hydrocarbon transformations that occur without the participation of metal complexes. These reactions are presented for comparison to the metal complex activation reactions.
Recent works devoted to the oxygenations of saturated, aromatic as well as olefinic hydrocarbons are surveyed, and the emphasis is made on the author's own publications. Both soluble metal complexes and solid metal compounds catalyze oxidative transformations of hydrocarbons. Hydrogen peroxide, alkyl peroxides, and peroxy acids were used in these reactions as oxidants. The catalytic systems often include obligatory co-catalysts, for example, nitrogen-containing bases or acids (inorganic, carboxylic or amino acids).
This brief essay consists of a few "exciting stories" devoted to relations within a metal-complex catalyst between a metal ion and a coordinated ligand. When, as in the case of a human couple, the rapport of the partners is cordial and a love cements these relations, a chemist finds an ideal married couple, in other words he obtains a catalyst of choice which allows him to functionalize C-H bonds very efficiently and selectively. Examples of such lucky marriages in the catalytic world of ions and ligands are discussed here. Activity of the catalyst is characterized by turnover number (TON) or turnover frequency (TOF) as well as by yield of a target product. Introducing a chelating N,N- or N,O-ligand to the catalyst molecule (this can be an iron or manganese derivative) sharply enhances its activity. However, the activity of vanadium derivatives (with additionally added to the solution pyrazinecarboxylic acid, PCA) as well as of various osmium complexes does not dramatically depend on the nature of ligands surrounding metal ions. Complexes of these metals are very efficient catalysts in oxidations with H2O2. Osmium derivatives are record-holders exhibiting extremely high TONs whereas vanadium complexes are on the second position. Finally, elegant examples of alkane functionalization on the ions of non-transition metals (aluminium, gallium etc.) are described when one ligand within the metal complex (namely, hydroperoxyl ligand HOO(-)) helps other ligand of this complex (H2O2 molecule coordinated to the metal) to disintegrate into two species, generating very reactive hydroxyl radical. Hydrogen peroxide molecule, even ligated to the metal ion, is perfectly stable without the assistance of the neighboring HOO(-) ligand. This ligand can be easily oxidized donating an electron to its partner ligand (H2O2). In an analogous case, when the central ion in the catalyst is a transition metal, this ion changing its oxidation state can donate an electron to the coordinated H2O2 fragment. This provokes the O-O bond rupture in the hydrogen peroxide molecule as is assumed for the role of Fe(2+) ions in the Fenton system.
The selectivity is an extremely important characteristic of a chemical reaction. This review deals mainly with supramolecular and nano-chemical approaches to the problem of selectivity enhancement in various functionalizations of C-H compounds. Enzyme mimics is a very fruitful method to achieve the predominant formation of desirable products and isomers. By obstructing the approach of certain C-H bonds of a substrate to the active catalytic centre we simultaneously increase the relative reactivity of other fragments. This can be done by creating steric hindrance around the active centre. Spatial restrictions can be made if we place the catalyst into a nano-cavity. We can achieve discrimination in reactivity of different C-H bonds if we allow certain fragments to approach closely the active centre. In order to do this chemists use coordination of the catalyst to some groups of the substrate with the participation of relatively strong binding (chelate control) or relatively weak forces (molecular recognition).
The reactions of saturated hydrocarbons and compounds modelling them with metal complexes, leading to the cleavage of the C-H and C-C bonds, are examined. In particular, processes which result in the formation of organometallic derivatives are described and the mechanism of the oxidation of alkanes by enzymes and their chemical models is discussed. The bibliography includes 355 references.
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