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Part I
Activation and Functionalization of Carbon Single Bonds and of Small Molecules
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Chapter 1
Organometallic Complexes as Catalysts in Oxidation of C–H Compounds
Georgiy B. Shul'pin
Department of Kinetics and Catalysis, Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia
1.1 Introduction
Organometallic (i.e., containing π or σ metal–carbon bonds) derivatives of transition metals are known as
excellent catalysts in reactions that do not involve the insertion of oxygen atoms [1]. They are used in (selected examples of recent publications are given) hydrogen/deuterium (H/D) exchange [2a], dehydrogenation [2b–e], homogeneous syngas conversion [2f], hydrosilylation [2g], carbonylation [2h], and homogeneous water gas shift reaction [2i]. In other recent works, complex [
], where
and
,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene, was used as a catalyst in the racemization of chiral alcohols [2j], neutral
-arene ruthenium complexes with monodentate P-donor ligands found to catalyze the transfer hydrogenation reaction [2k].
Organometallic catalysts were employed in oxidation reactions of some organic compounds. Thus, complex
is a precatalyst for olefin epoxidation [3a], heterodinuclear ruthenium–iron complexes showed high activity for the catalytic oxidation of secondary alcohols with
tert-butyl hydroperoxide to give ketones in aqueous media [3b]. In contrast, organometallic complexes were very rarely used as (pre)catalysts in oxygenation reactions of aromatic and saturated hydrocarbons [4a–e] (the latter can be called
noble gases of organic chemistry because of their known inertness).
In various C–H oxygenation reactions, organometallic complexes can play the role of precatalyst. Compounds bearing carbon–metal bonds can also be some of intermediate compounds in the catalytic cycles. In this chapter, we discuss reactions in which an organometallic complex catalyzes the insertion of oxygen atoms into C–H bonds of hydrocarbons or other organic compounds. The focus will be made on the author's own works.
1.2 Oxygenation Reactions with Oxidants other than Peroxides
The first example of a metal-catalyzed oxygen atom insertion into the C–H bond was the reaction found by Shilov and Shteinman and their coworkers in 1972 (for reviews, see References 1h and 5). These authors demonstrated that
ion could catalyze H/D exchange in methane in a
COOD solution and, if
is added, the latter oxidizes methane to methanol (Shilov chemistry). The catalytic cycle in which
-methyl complexes of platinum(II) and platinum(IV) are involved is shown in
Fig. 1.1.
Later, Periana and coworkers proposed (2,2′-bipyrimidyl)platinum(II)dichloride as a catalyst (“Periana system”; see a recent review [4d]). Fuming sulfuric acid is the oxidant in this case. A simplified scheme of the catalytic cycle is shown in
Fig. 1.2. It can be seen that some intermediates contain
-methyl-platinum bonds.
Complexes containing the fragment
(
is pentamethylcyclopentadienyl) are active precatalysts in the C−H oxidation of
cis-decalin and cyclooctane. Ceric ammonium nitrate was a sacrificial oxidant and water was the oxygen source (
Fig. 1.3). Calculations using the Density functional theory (DFT) method showed that the C–H oxidation of
cis-decalin by
(ppy)(Cl) (
-phenylpyridine) follows a direct oxygen insertion mechanism on the singlet potential energy surface [6]. The authors proposed that some of intermediate species contain the
ring coordinated to the iridium ion. The authors also made a general conclusion: oxidation catalysis by organometallic species can be hard to interpret because of the possibility that the real catalyst is an oxidation product of the precursor.
Indeed, organometallic precatalysts can be transformed during an induction period into catalytically active species that do not contain metal–carbon bonds. For example, molybdenum [7a] and tungsten [7b] carbonyls catalyze aerobic photooxygenation of cyclohexane to cyclohexyl hydroperoxide (primary product) and cyclohexanol and cyclohexanone (Fig. 1.4). The proposed mechanism is shown in Fig. 1.5. It includes ...
