Monitoring electrochemical reactions by ¹³C NMR spectroscopy

Abstract

The anodic conversion of 2,4,6-tri-tert-butylphenol in methanol, leading to 2,4,6-tris( l,l-dimethylethyl)-4-methoxy-2,5-cyclohexadien-l-one, has been followed by two types of 13C NMR experiments: (a) a static procedure, inserting the electrodes directly into a rotating NMR tube and (b) a continuous flow procedure, combining electrochemical and NMR flow cells. During electrolysis the resolution of the 13C NMR spectra decreased in the static procedure owing to magnetic field gradients. In the flow procedure the resolution was comparable with that of routine spectra.

Design and applications of an in situ electrochemical NMR cell

“…The first irreversible peak at À2.1 V was due to the reduction of 9-chloroanthracene to its radical anion form, which had a certain lifetime and was able to diffuse away from the electrode [56]. This diffusion is very important as it efficiently avoided the direct reduction of the relatively easily reducible radical Ar Å formed in step (2). Therefore, even though Ar Å is thought to more easily reduced [13,50], two successive reduction peaks were observed.…”

“…The formation of Ar À can occur either at the electrode surface (3) or by homogeneous electron transfer with the initially generated radical anion ArCl ÅÀ (4). Reactions (1)-(3) constitute an Electrochemical-Chemical-Electrochemical (ECE) mechanism and reactions (1), (2) and (4) are known as a DISP1 (first disproportionation) pathway [55]. The anthracene anion is a strong base and will be readily protonated by any proton donor available in solution, such as residual water (5a) or supporting electrolyte (5b).…”

“…The overall electrochemical reaction of 9-chloroanthracene (ArCl) in non-aqueous media involves two electrons, which are transferred in (1) and (3), respectively. The reduction starts with a one-electron heterogeneous transfer at the electrode surface (1) and forms a radical anion ArCl ÅÀ , which decomposes to yield the anthracene radical (Ar Å ) and the chloride anion (Cl À ) in step (2). The neutral radical Ar Å formed from the carbon-chlorine bond cleavage is, in general, more easily reducible (i.e., has a more positive reduction potential) than the parent ArCl molecule and can be further reduced to anthracene anion (Ar À ).…”

“…A flow cell arrangement [1,2] avoids the inherent incompatibility between electrochemistry and NMR; however, it is not truly in situ as the electrochemical reaction occurs outside the NMR radio frequency (rf) region. Both experimental arrangements described in the literature required a large solution volume and might not exclude oxygen during experimentation.…”

Review of advances in coupling electrochemistry and liquid state NMR

“…In 1987, Albert et al [11] present the first on line association of electrochemical device and a NMR flow cell dealing with the comparison to static in tube synthesis. It consists in two parallel circuits pumping and repulsing in the same flask.…”