C–H functionalization: thoroughly tuning ligands at a metal ion, a chemist can greatly enhance catalyst's activity and selectivity

Abstract

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.

New Trends in Oxidative Functionalization of Carbon–Hydrogen Bonds: A Review

“…We proposed and developed a simple method for analysis of alkyl hydroperoxides generated in various alkane oxidations in solutions [11][12][13][247][248][249][250][251][252]). A chemist can find that, if the direct injection of a reaction sample into the chromatograph gave comparable amounts of cyclohexanol and cyclohexanone ( Figure 16, Figure 17a), the reduction of the sample with PPh 3 prior to GC analysis led to the noticeable predominance of the alcohol (Figure 17b).…”

“…This review is devoted to functionalization of carbon-hydrogen bonds [1][2][3][4][5][6][7][8][9][10][11][12][13] in various organic compounds, which is a very important process both from academic and practical point of view. Replacing hydrogen atoms in carbon-hydrogen compounds by functional groups X leads to valuable functionalized C-X compounds (Scheme 1a).…”

High Catalytic Activity of Heterometallic (Fe₆Na₇and Fe₆Na₆) Cage Silsesquioxanes in Oxidations with Peroxides

“…Moreover, cyclohexane gives minimum number of oxidation products which are easily identified by the GC method. As demonstrated previously in other oxidations [19,[59][60][61][62][63][64][65][66][67][68], if the direct injection of a reaction sample into the chromatograph gave comparable amounts of cyclohexanol and cyclohexanone, the reduction of the sample with PPh3 (or certain sulfides) prior to GC analysis in many cases led to the noticeable predominance of the alcohol (Figure 8). The comparison of the results obtained before and after the reduction clearly indicated that cyclohexyl hydroperoxide was formed as the main primary product.…”

“…In order to determine concentrations of all cyclohexane oxidation products the samples of reaction solutions after addition of nitromethane as a standard compound were in some cases analyzed twice (before and after their treatment with PPh3) by GC (LKhM-80-6 instrument, columns 2 m with 5% Carbowax 1500 on 0.25-0.315 mm Inerton AW-HMDS; carrier gas argon) to measure concentrations of cyclohexanol and cyclohexanone. This method (an excess of solid triphenylphosphine is added to the samples 10-15 min before the GC analysis) was proposed by one of us earlier [19,[59][60][61][62][63][64][65][66][67][68]. Attribution of peaks was made by comparison with chromatograms of authentic samples.…”

C-Homoscorpionate Oxidation Catalysts—Electrochemical and Catalytic Activity

“…Indeed, oxidation of alkanes has been the object of considerable research [69][70][71][72][73][74][75][76][77][78], but still constitutes a serious challenge to modern chemistry, owing to the high inertness of these substrates. Currently, alkanes are mainly applied as fuels but it would be desirable to direct their application to the synthesis of organic products of a high added value.…”