Silver(I)-Catalyzed Insertion of Carbene into Alkane C–H Bonds and the Origin of the Special Challenge of Methane Activation Using DFT as a Mechanistic Probe

Abstract

The argentate trinuclear cluster Ag3(μ2-3,5-(CF3)2PyrPy)3 (3,5-(CF3)2PyrPy = 2,2′-pyridylpyrrolide– ligand) catalytically promotes the insertion of the carbene of ethyl diazoacetate at room temperature into the C–H bond of a series of alkanes ranging from ethane to hexane, as well as branched and cyclic hydrocarbons. In addition to experimental studies, we also present theoretical studies elucidating the mechanism to C–H activation and functionalization by the transient silver carbene monomer (3,5-(CF3)2PyrPy)Ag(CHCO2Et). On the basis of DFT studies, formation of the silver carbene complex was found to be rate-determining for alkane substrates such as ethane and propane. On the other hand, DFT studies on methane, a substrate that we failed to activate, revealed that carbene insertion into the C–H bond was overall rate-determining. Theoretical analysis of charge flow also shows that the change from separated reagents to the TS involves charge flow from alkane to the silver carbene carbon with the bridging H behaving as a conduit. KIE studies using cyclohexane as a substrate suggest that the product-determining step involves only modest C–H bond lengthening, which can be also represented as a very early transition state with respect to C–H insertion of the carbene.

Catalytic Functionalization of Methane and Light Alkanes in Supercritical Carbon Dioxide

“…[26] The hydrocarbon (ethane, propane, n-butane, or iso-butane, large excess, see the Supporting Information for exact amounts) was reacted with EDA (0.5 mmol) in scCO 2 give the insertion products shown in Scheme 4. This time was enough to achieve full consumption of EDA (> 98 %).…”

Two Amphoteric Silver Carbene Clusters

“…[1,2] Thei ntervention of electrophilic silver carbene intermediates in this reaction is highly likely,e ven though rigorous proof is still missing, not least because of the presumably very low stability of such intermediates. [4] As imilar ambiguity surrounds even more recent reports on silver carbene chemistry, [5,6] including applications to the CÀHa ctivation of alkanes [7][8][9] as well as the activation of the CÀXb ond of certain alkyl halides: [10] Once again, the intervention of electrophilic silver carbenes is plausible and probable; [11] yet, it has been prudently mentioned in the literature that free carbenes afford similar products or product distributions. [4] As imilar ambiguity surrounds even more recent reports on silver carbene chemistry, [5,6] including applications to the CÀHa ctivation of alkanes [7][8][9] as well as the activation of the CÀXb ond of certain alkyl halides: [10] Once again, the intervention of electrophilic silver carbenes is plausible and probable; [11] yet, it has been prudently mentioned in the literature that free carbenes afford similar products or product distributions.…”

Catalytic functionalization of low reactive C(sp3)–H and C(sp2)–H bonds of alkanes and arenes by carbene transfer from diazo compounds

“…It is worth mentioning that the trinuclear silver catalyst 17 has also been reported to incorporate carbene units from EDA into ethane and propane without the need of scCO 2 , using pressure of the alkane over a solution of the catalyst and the diazo reagent. 28 More recently, the copper and silver catalysts Tp 39 Interestingly, an unprecedented observation has been found with the copper-based system regarding the regioselectivity. As inferred from data shown in Scheme 4, the various copper catalysts reported for this transformations are inactive or very low active toward the primary sites of hexane.…”