Main-Group Compounds Selectively Oxidize Mixtures of Methane, Ethane, and Propane to Alcohol Esters

Abstract

Much of the recent research on homogeneous alkane oxidation has focused on the use of transition metal catalysts. Here, we report that the electrophilic main-group cations thallium(III) and lead(IV) stoichiometrically oxidize methane, ethane, and propane, separately or as a one-pot mixture, to corresponding alcohol esters in trifluoroacetic acid solvent. Esters of methanol, ethanol, ethylene glycol, isopropanol, and propylene glycol are obtained with greater than 95% selectivity in concentrations up to 1.48 molar within 3 hours at 180°C. Experiment and theory support a mechanism involving electrophilic carbon-hydrogen bond activation to generate metal alkyl intermediates. We posit that the comparatively high reactivity of these d(10) main-group cations relative to transition metals stems from facile alkane coordination at vacant sites, enabled by the overall lability of the ligand sphere and the absence of ligand field stabilization energies in systems with filled d-orbitals.

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

“…These products would of course require additional workup steps to yield the desired oxygenated products [76]. The same group also reported a high yield of EtTFA (73% yield, 91% selectivity) at a reaction temperature of 150˝C when using a perfluoroarene iodine (III) complex in TFAA/HTFA as solvent (Equation (2)) [77].…”

“…The conversion of ethane to ethyl-esters has been recently reported by Periana and co-workers [76][77][78] who adopt an electrophilic activation approach, at temperatures of ca. 180˝C.…”

“…These reactions yield the ethyl-ester products; EtTFA and EG(TFA) 2 at selectivities of ca. 70% and 30% respectively [76,78] with the Tl process proceeding according to Equation (1).…”

Arene C–H activation using Rh(i) catalysts supported by bidentate nitrogen chelates

“…[10][11][12] Numerous studies have sought to understand and improve the Shilov catalyst, [13][14][15][16][17][18] and several electrophilic late transition metal and main group reagents have been developed for light alkane functionalization. [19][20][21][22][23][24][25] These electrophilic catalysts are often inhibited by Lewis bases and, as a result, typically require superacidic media (e.g., oleum). The use of strong acid (HX) can lead to the formation of RX, and the electron withdrawing group "X" can protect the functionalized product from over-oxidation (e.g., MeOSO 3 H formation in H 2 SO 4 ); but, product extraction from strong acids can be problematic.…”

Electrophilic RhI catalysts for arene H/D exchange in acidic media: Evidence for an electrophilic aromatic substitution mechanism

“…In addition, these electrophilic catalysts are readily poisoned by water (or other Lewis bases) and can suffer from product inhibition [33]. More recently, main group elements and transition metal complexes have been shown to activate and functionalize hydrocarbons in less acidic, trifluoroacetic acid [35][36][37][38][39][40][41].…”