Selectivity enhancement in functionalization of C–H bonds: A review

Abstract

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).

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.…”

“…C(1) : C(2) : C(3) : C(4) = 1.0 : 6.7 : 6.6 : 6.1. It can be seen that hydrogen atoms in position 4 posess lower activity, aparently due to some sterical hindrance [50,57,60]. The bond-selectivity parameter (1° : 2° : 3°; the relative normalized reactivities of hydrogen atoms at the primary, secondary and tertiary carbons) in the oxidation of methylcyclohexane (1.0 : 6.7 : 17.5) is close to the corresponding values found for the systems oxidizing alkanes with hydroxyl radicals (see, for example, Refs.…”

“…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.…”