DOI: 10.1002/adsc.202000198
Abstract
The electrochemical synthesis of N,N'-disubstituted benzimidazolones from ureas through an intramolecular anodic dehydrogenative N?H/C?H coupling has been developed. The reaction undergoes under the undivided electrolysis conditions and obviates the need for any catalysts and chemical oxidants.
1. Reaction Optimization
2. Substrate Scope
3. CV Experiments
Figure 1. Cyclic voltammograms recorded in n‐Me4NBF4/MeOH (0.1?M): (a) blank, (b) 1?a (0.006?M), (c) NaOMe (0.015?M), (d) 1?a (0.006?M)+NaOMe (0.015?M).
4.Proposed Catalytic Cycle
Conclusions
In summary, we have disclosed a catalyst‐free electrochemical intramolecular dehydrogenative N?H/C?H annulation reaction of functionalized ureas at constant current in an undivided cell. This electrolysis protocol allows for the synthesis of N,N’‐disubstituted benzimidazolones promoted only by a base, with no need for transition‐metal catalysts and/or excess chemical oxidants. Thus, it provides a green and powerful alternative to present approaches to benzimidazolones. Such catalyst‐free electrosynthetic methods are currently being investigated for the construction of other heterocycles in our laboratory.
Experimental Section
A 10?mL three‐necked round‐bottom flask was charged with a urea 1 (0.1?mmol), Me4NBF4 (0.1?mmol), and NaOMe (0.1?mmol). The flask was equipped with a condenser, a RVC plate (100?PPI) anode (1.2?cm×1?cm×1?cm) and a platinum plate (1?cm×1?cm) cathode and then flushed with argon. MeOH (6?mL) was added with a syringe. The constant current (2?mA) electrolysis was carried out at 60?°C (oil bath temperature) until complete consumption of 1. The reaction mixture was cooled to room temperature, followed by addition of water (10?mL) and ethyl acetate (10?mL). The phases were separated and the aqueous phase was extracted with ethyl acetate (3×20?mL). The combined organic solution was dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure. The residue was chromatographed through silica gel eluting with ethyl acetate/petroleum ether to give the desired product 2.
Reference
https://onlinelibrary.wiley.com/doi/full/10.1002/adsc.202000198
