Potentiostatically Controlled Olefin Metathesis

Abstract

A Ru(II) complex supported by an N-heterocyclic carbene annulated to a redox-active naphthoquinone (NQ) was interrogated using a range of potentiodynamic and potentiostatic electrochemical techniques. The complex exhibited two redox processes, one of which was attributed to the Ru(II)|Ru(III) couple (E 1/2 = +1.10 V vs a saturated calomel electrode) and the other to the NQ|NQ − couple (E 1/2 = −0.62 V). Using potentiostatic coulometry or bulk electrolysis, the application of a fixed negative potential (−0.95 V) to electrodes placed in a dichloromethane solution containing the complex resulted in a reduction reaction. The complex was quantitatively reduced within minutes, as determined by coulometry, and subsequently oxidized to its initial, neutral form through the application of a relatively positive potential (+0.34 V) over similar periods of time. The interconversion process was found to be reversible and used to modulate a series of ring-closing metathesis and ring-opening metathesis polymerization reactions. While relatively high activities were observed when the neutral form of the catalyst was employed, the reaction rates were attenuated upon in situ, potentiostatic reduction. Toggling between relatively negative or positive potentials enabled the aforementioned olefin metathesis reactions to be switched between fast and slow states.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

“…More recently, Bielawski and co-workers reported a method for potentiostatic control over Ru-based olefin metathesis with a modified Hoveyda–Grubbs catalyst containing a redox-active quinone built into NHC ligand ( 96.2 ) (Scheme ). Electrochemical reduction of the quinone to its radical anion deactivates the catalyst toward olefin metathesis by stabilizing the ruthenacyclobutane resting state of the catalytic cycle and thus suppressing the retro-[2 + 2] cycloaddition step. This strategy was applied to the ring closing metathesis ( 96.3 ) and ROMP ( 96.4 ), with approximately a 7-fold decrease in relative reaction rates upon single-electron reduction.…”

Advances in Polymerizations Modulated by External Stimuli

“…In 2020, Bielawski and colleagues introduced their NQ–NHC ligand onto the Grubbs–Hoveyda catalyst to obtain 61 (Scheme ), whose ROMP activity could be modulated electrochemically . Catalyst 61 exhibited high ROMP activity toward COD ( k obs neutral = 4.58 × 10 –4 s –1 ), whereas application of a fixed negative potential (−0.95 V vs SCE) resulted in quantitative reduction of this complex to give 61 red , which exhibited greatly decreased polymerization rates ( k obs reduced = 6.49 × 10 –5 s –1 , k obs neutral / k obs reduced = 7.1).…”

Implementation of Cooperative Designs in Polarized Transition Metal Systems—Significance for Bond Activation and Catalysis

“…To succeed, it is necessary to obtain a detailed picture of the interactions taking place at a molecular level. In such a way, precise molecular control over the bond making and breaking processes can be developed . This article is a step in this direction.…”