Reversed redox generation of silyl radicals in a four-electrode flow-through EPR spectroelectrochemical cell

Abstract

A flow-through four-electrode EPR spectroelectrochemical cell was developed which allowed the observation of silyl radical formation in apparently multielectron electrochemical processes, in which these species could not be detected directly because of the high driving force of their further reduction/oxidation leading to non-paramagnetic products. Silyl radicals thus generated were characterized by spin trapping with alpha-phenyl-N-tert-butyl nitrone (PBN), intramolecular spin trapping or by direct detection. The overall multielectron process is realized in the first, generating, compartment of the cell and the ionic species formed are then transformed into the corresponding radicals in the second compartment via a one-electron redox process in the opposite direction, e.g. two-electron reductions of Ph(3)SiCl or Et(3)SiCl followed by one-electron oxidation of the resulting Ph(3)Si(-) or Et(3)Si(-) anions (+2e/-e process). These radical species were then identified as their secondary paramagnetic products or by their spin trapping with PBN. Using (2-[cyclohex-3-enyl]ethyl)dimethyl chlorosilane in this process, the formation of the silicon-centered radical and its intramolecular addition across the internal double bond were evidenced by spin trapping. The reduction of electrophilic silicon intermediates issued from the oxidation of Ph(3)SiSiPh(3) (-2e/+2e process) resulted in Ph(3)Si* radicals trapped with PBN. The reduction of the electrochemically prepared persistent dication of a stable disilene, thiatetrasilacyclopentene, allowed generation of a disilene cation radical characterized by EPR.

Decatungstate‐Photocatalyzed Si−H/C−H Activation in Silyl Hydrides: Hydrosilylation of Electron‐Poor Alkenes

“…This was caused by the trapping of the PhMe 2 Si . radical with a N =14.7 G; a H =5.73 G (Figure a), in fair agreement with previous reports . In addition, a small amount of C‐centered radicals (<10 %) was detected in the spectrum.…”

Nucleophilic displacement versus electron transfer in the reactions of alkyl chlorosilanes with electrogenerated aromatic anion radicals

“…space is such that the silyl radicals 5 (with own reduction potentials E  -0.5…-1.4 V [17,18]) are instantaneously transformed into silyl anions, which is consistent with the results of Corriu [12].…”

“…this latter also agrees with the generation of silyl radicals from the oxidation of silyl anions at close potentials, as shown by real-time EPR-spectroelectrochemistry [18]. Thus this fundamental limitation precludes not only the intermediacy of silyl radicals and radical-based reactions in the direct cathodic reduction of R 3 SiCl compounds but also the use of many electrophilic reagents susceptible to react with R 3 Si -anions because these electrophiles undergo own reduction at less negative potentials.…”

Complexes of germanium chlorides (RnGeCl4-n, n=0-3; R=Me, Ph) with redox active N-hetero­aromatic ligands: an electrochemical study

“…The spectroelectrochemical cell was a three-electrode version of that described in Zeitouny et al (2009). 2,2-Diphenyl-1-picrylhydrazyl (DPPH) was used for calibrating g factors.…”