It is well known that the electrochemical reduction of 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) yields the corresponding N-heterocyclic carbene (NHC). This carbene is quite unstable and its main use is as an organocatalyst in umpolung reactions of aldehydes. Due to its instability, NHC quantification can be indirectly carried out through its transformation into the corresponding thione, by reaction with elemental sulfur. The cathodic generation is carried out in a divided cell, as NHCs readily oxidise at the anode. The need for high amounts of supporting electrolyte is one of the major drawbacks of batch electrochemistry, which can be addressed by using flow electrolysis cells with small interelectrode gaps. In flow cells the distance between the two electrodes can decrease to such an extent that sometimes no supporting electrolyte is needed, offering additional benefits such as simplification of product isolation. Moreover, flow electrochemistry can provide higher rates of conversion compared to batch. Only one paper in the literature reports NHC generation by cathodic reduction of an imidazolium salt, in a undivided cell, in which the NHC oxidation problem is overcome by the use of a sacrificial anode (Cu), which releases Cu+ ions to form NHC-Cu complexes. In this paper we present the first electrochemical generation of NHC carried in a divided flow cell with NHC accumulation and subsequent use as organocatalyst. We demonstrated not only the NHC accumulation (indirectly by reaction with elemental sulfur), but also the possibility to use this NHC in two classical umpolung reactions of cinnamaldehyde: its cyclodimerization and its oxidative esterification.
Keywords: Flow Electrochemistry; N-heterocyclic carbene; Breslow intermediate; oxidative esterification; cathodic reduction
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Rocco, D.; Folgueiras-Amador, A. A.; Brown, R. C. D.; Feroci, M. Beilstein Arch. 2022, 202243. doi:10.3762/bxiv.2022.43.v1
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