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...
Designing oxidation catalysts based on CH activation with reduced, low oxidation state species is a seeming dilemma given the proclivity for catalyst deactivation by overoxidation. This dilemma has been recognized in the Shilov system where reduced PtII is used to catalyze methane functionalization. Thus, it is generally accepted that key to replacing PtIV in that system with more practical oxidants is ensuring that the oxidant does not over-oxidize the reduced PtII species. The “Periana-Catalytica” system, which utilizes (bpym)PtIICl2 in concentrated sulfuric acid solvent at 200 °C, is a highly stable catalyst for the selective, high yield oxy-functionalization of methane. In lieu of the over-oxidation dilemma, the high stability and observed rapid oxidation of (bpym)PtIICl2 to PtIV in the absence of methane would seem to contradict the originally proposed mechanism involving CH activation by a reduced PtII species. Mechanistic studies show that the originally proposed mechanism is incomplete and that while CH activation does proceed with PtII there is a solution to the over-oxidation dilemma. Importantly, contrary to the accepted view to minimize PtII overoxidation, these studies also show that increasing that rate could increase the rate of catalysis and catalyst stability. The mechanistic basis for this counterintuitive prediction could help to guide the design of new catalysts for alkane oxidation that operate by CH activation.
General. Unless otherwise noted, all reactions and manipulations were performed in a M-Braun circulating Argon atmosphere glovebox or using standard Schlenk techniques. Glassware was dried in an oven at 150 °C before use. Unless otherwise noted, reagents were purchased from commercial suppliers and used without further purification. Neutral alumina was used in chromatography unless otherwise noted. Diethyl ether, THF and benzene were distilled from sodium/benzophenone ketyl under Argon prior to use. Hexanes and pentane were dried with P 2 O 5 under Argon and kept refluxing under flow of Argon. CH 2 Cl 2 was purified from stabilizer by stirring with concentrated H 2 SO 4 for 4 hours, mixture was separated, organic phase was neutralized with KHCO 3 , dried with MgSO 4 and P 2 O 5 , and refluxed under flow of Argon for 3 days. Organic acids were distilled from P 2 O 5 . Deuterated solvents were degassed by freezing, evacuating, and thawing (3x), and were then dried over 4 Å sieves and stored under Argon.Unless otherwise indicated, NMR spectra were obtained using a Variant Mercury-400 MHz
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