S_N2 and E2 Branching of Main-Group-Metal Alkyl Intermediates in Alkane CH Oxidation: Mechanistic Investigation Using Isotopically Labeled Main-Group-Metal Alkyls

Abstract

The main-group-metal alkyl compounds trialkyltin and dialkylthallium have been utilized to investigate the mechanism of functionalization of monoalkyl thallium and lead species, proposed to be putative intermediates in alkane (RH) functionalization, formed via CH activation of alkanes (methane, ethane, and propane) using electrophilic Tl(III) and Pb(IV) in trifluoroacetic acid (HTFA). Two different organometallic transalkylation methods were used to generate the putative intermediates in situ. The results herein strongly support a mechanism of CH activation to generate a main-group-metal alkyl intermediate which undergoes reductive functionalization to generate the products, R-TFA, and the reduced metal salt. In the case of ethane there are two products, ethyl trifluoroacetate (EtTFA) and 1,2-bis(trifluoroacetoxy)ethylene glycol (EG(TFA) 2 ), observed in the reaction mixture that are proposed to form in parallel from a common intermediate, EtTl(TFA) 2 . The alkyl transfer studies herein strongly support the simultaneous formation of both species from this intermediate. Furthermore, studies conducted using regiospecifically isotopically labeled diethylthallium salts strongly support an S N 2 functionalization from EtTl(TFA) 2 to give EtTFA (and reduced Tl(TFA)) and an E2 elimination (also from EtTl(TFA) 2 ) to generate ethylene, which instantly reacts with an additional 1 equiv of Tl(TFA) 3 to generate EG(TFA) 2 .

A PdIII Sulfate Dimer Initiates Rapid Methane Monofunctionalization by H Atom Abstraction

“…A common mechanistic paradigm for methane functionalization by metal ions in strong acid solvents is electrophilic C–H activation. ,− In this mechanism, methane binds to an open coordination site on the metal and is deprotonated by an ancillary ligand or solvent to generate a M–CH 3 intermediate (Scheme , top). This mechanism is generally invoked for late- or post-transition metals that form strong metal–carbon bonds .…”

“…(Top) A general catalytic cycle for electrophilic methane functionalization by homogeneous metal ions in strong acids. 5,[11][12][13][14][15][16][17][18][19] The strongly electron-withdrawing ester group, X, "protects" the product from further oxidation, and the product, CH3X, can be subsequently hydrolyzed to yield methanol. (Bottom) Structure of Pd III 2 and its methane functionalization reactions.…”

Iodine-Catalyzed Selective Functionalization of Ethane in Oleum: Toward a Direct Process for the Production of Ethylene Glycol from Shale Gas

“…Based on these findings, we propose a reaction pathway (also see Scheme 2D) similar to a mechanism that has recently been proposed for the stochiometric oxidation of alkanes with main group elements in a nonoxidizing acidic environment. 38 It involves an electrophilic C−H activation step yielding an iodo-ethyl species which in turn undergoes an elimination reaction that results in the formation of ethylene. Ethylene further reacts to the bisulfate ester of iodoethanol, in which the labile C−I-bond is cleaved upon nucleophilic attack of the bisulfate anion to give EBS as the final product.…”