Selective Oxidation of Methane to Methanol Catalyzed, with CH Activation, by Homogeneous, Cationic Gold

Abstract

Some of the most efficient homogeneous catalysts for the lowtemperature, selective oxidation of methane to functionalized products employ a mechanism involving CÀH activation [1] with an electrophilic substitution mechanism. Several such systems have been reported based on the cations Hg II , [2] Pd II , [3] and Pt II . [4] These catalyst systems typically operate by two general steps that involve: A) CÀH activation by coordination of the methane to the inner sphere of the catalyst (E n+ ) followed by cleavage of the CÀH bond by overall electrophilic substitution to generate E n+ ÀCH 3 intermediates, and B) oxidative functionalization involving redox reactions of E n+ À CH 3 to generate the desired oxidized product CH 3 X.[4a]Consequently, efficient catalysts that follow this pathway would be expected to be "soft", highly electrophilic species that form relatively strong covalent bonds to carbon atoms and that are also good oxidants.We considered that gold cations could be uniquely efficient electrophilic catalysts for methane conversion because, as shown in the conceptual catalytic cycle (Scheme 1), [2, 4] This situation is not common, and in most catalytic systems based on "soft", redox-active electrophiles only one oxidation state of the redox couple is active for CÀH activation. Thus, we sought to explore the catalytic chemistry of gold cations for the oxidation of methane. To our knowledge, while gold complexes have been reported to facilitate free-radical reactions of alkanes with peroxides in low yields, [5] no homogeneous gold catalysts that operate by heterolytic CÀH activation and oxidative functionalization have been reported for the selective functionalization of alkanes. This is possibly because of the strong propensity for irreversible formation of gold metal, and any attempts to develop redox catalysis based on homogeneous Au cations must address this issue.In strong acid solvents such as triflic or sulfuric acid, Au III cations (generated by dissolution [6] of Au 2 O 3 ) react with methane at 180 8C to selectively generate methanol (as a mixture of the ester and methanol) in high yield (Table 1, entries 1 and 2). As expected, the irreversible formation of metallic gold is very evident after these reactions and, unlike reactions with Hg II , [2] Pt II , [4d] and Pd II [3a] that are catalytic in 96 % H 2 SO 4 , only stoichiometric reactions (turnover numbers (TONs) < 1) are observed with Au III [Eq. (1)]. Soluble cationic gold is essential for these reactions as no methanol is observed under identical conditions without added Au III ions (entry 3), or in the presence of metallic gold (entry 4) which is not dissolved in hot H 2 SO 4 .Consistent with the known nobility of gold metal, no methanol formed when SO 3 or persulfate (K 2 S 2 O 8 ) were added as possible oxidants of metallic gold (entries 5 and 6). We considered the use of Se VI ions as a more suitable oxidant. Se VI ions are a more powerful oxidizing agent than S VI ions (E o = 1.5 V SeO 4 2À /H 2 SeO 3 , E o = 0.17 V SO 4 2À /H 2 S...

Carbon–Oxygen Bond Formation via Organometallic Baeyer–Villiger Transformations: A Computational Study on the Impact of Metal Identity

“…23,24 Among the more promising catalysts for hydrocarbon partial oxidation are electrophilic late transition metal systems. [1][2][3]9,14,15 For example, heating aqueous solutions of Pt II salts with hydrocarbons and an external oxidant produces alcohol or alkyl chloride compounds (the Shilov system). 1,12,25 The most commonly invoked pathway for Shilov catalysts involves initial alkane C−H activation to generate a Pt II −alkyl intermediate, oxidation of the Pt II −alkyl to produce a Pt IV −alkyl species, nucleophilic attack by water or chloride on the hydrocarbyl ligand, and dissociation of the functionalized product.…”

An Overview of Recent Advances of the Catalytic Selective Oxidation of Ethane to Oxygenates

“…Whilst a number of homogeneous catalytic systems have been reported for the activation of methane, the catalytic oxidation of ethane using homogeneous catalysts has rarely been studied [50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65].…”

Oxidative Coupling Reactions with Gold

“…In fact, theo retical calculations show that the energies of activation for the conversion via dis proportionation for both Au(i) and Au(iii) are rather low. [18] This has been used to convert various Au complexes [19] to aryl acetylenes by reducing the Au(iii) to the corresponding Au(i) species (Scheme 1). [20,21] Such aryl complexes can also be prepared via direct C-H bond activation with an Au(iii) complex.…”