Flow Rhodaelectro-Catalyzed Alkyne Annulations by Versatile C–H Activation: Mechanistic Support for Rhodium(III/IV)

Abstract

A flow-metallaelectro-catalyzed C–H activation was realized in terms of robust rhodaelectro-catalyzed alkyne annulations. To this end, a modular electro-flow cell with a porous graphite felt anode was designed to ensure efficient turnover. Thereby, a variety of C–H/N–H functionalizations proved amenable for alkyne annulations with high levels of regioselectivity and functional group tolerance, viable in both an inter- or intramolecular manner. The electro-flow C–H activation allowed easy scale up, while in-operando kinetic analysis was accomplished by online flow-NMR spectroscopy. Mechanistic studies suggest an oxidatively induced reductive elimination pathway on rhodium­(III) in an electrocatalytic regime.

New Redox Strategies in Organic Synthesis by Means of Electrochemistry and Photochemistry

“… 40 For example, Ackermann developed electrocatalytic sp 2 C–H activation reactions toward C–O, C–N, and C–C formation, in which turnover of the transition metal catalyst (e.g., Co, Ni, Cu, Rh) is achieved directly on an anode. 41 A similar strategy has also been independently employed by Sanford, 42 Lei, 43 Xu, 44 Mei, 45 Waldvogel, 46 et al for “oxidant-free” oxidative functionalization of C–H bonds.…”

Electrochemical Access to Aza‐Polycyclic Aromatic Hydrocarbons: Rhoda‐Electrocatalyzed Domino Alkyne Annulations

“…The merger of metal catalysis and electrosynthesis, metalla‐electrocatalysis, has been identified as a powerful strategy towards sustainable synthesis, which was substantiated by electrochemical metal‐catalyzed C−H functionalizations in recent years . Very recently, the groups led by Ackermann and Xu disclosed a new catalytic regime, namely anodic oxidation‐induced reductive elimination, where the electricity not only functions as the terminal oxidant, but is also responsible for anodic oxidation‐induced reductive elimination in rhodium‐catalyzed C−N and C−P bond formations. These findings may lead to new catalytic activities and transformations, which are not possible with common chemical oxidants.…”

Electrochemically Enabled Intramolecular and Intermolecular Annulations of Alkynes

“…In the meantime, a set of flow-metallaelectrocatalyzed alkyne annulations resulting from versatile CÀ H activations and displaying excellent levels of positional selectivity, chemoselectivity and regioselectivity, was realized by the same group. [41] The flowrhodaelectro-catalyzed CÀ H activation showed broad regioselectivity and excellent tolerance of substituent groups (Scheme 30).…”