Direct electrochemical generation of organic carbonates by dehydrogenative coupling

Organic carbonates are an important source for polycarbonate synthesis. However, their synthesis generally requires phosgene, sophisticated catalysts, harsh reaction conditions, or other highly reactive chemicals. We present the first direct electrochemical generation of mesityl methyl carbonate by C–H activation. Although this reaction pathway is still challenging concerning scope and efficiency, it outlines a new strategy for carbonate generation.

Spectroscopy and spectrometry: 1 H NMR and 13 C NMR were recorded at 25 °C by using a Bruker Avance II 400 or a Bruker Avance III HD 400 (Analytische Messtechnik, Karlsruhe, Germany). Chemical shifts (δ) are reported in parts per million (ppm) relative to TMS as internal standard or traces of CHCl3 ( 1 H 7.26 ppm, 13 C 77.16 ppm) in the corresponding deuterated solvent. [2] Mass spectra and high resolution mass spectra were obtained by using a Agilent 6545 Q-ToF MS apparatus employing ESI+.

Instrumentation and Electrode Materials:
The undivided cells are made of Teflon and were used for the screening experiments. [3] For the electrochemical reactions the following electrode materials were used: glassy carbon (SIGRADUR ® G, HTW, Thierhaupten, Germany), boron-doped diamond (DIACHEM ® , 15 μm boron-doped diamond layer on 3 mm silicon support (geometrical surface area does not include additional 60% surface area arising from flatness imperfection of the surface), CONDIAS GmbH, Itzehoe, Germany), isostatic graphite (SIGRAFINE ® V2100, SGL Carbon, Bonn /Bad Godesberg, Germany) and platinum (99.9% Pt, ÖGUSSA GmbH, Wien, Austria). As power source a multi-channel galvanostat was employed. This setup can be commercially purchased from IKA-Werke GmbH & CO KG, Staufen, Germany, as IKA Screening System.

Synthesis protocols
Tetrabutylammonium methyl carbonate S3 29.3 g (22.5 mmol) of a commercially available 21 wt% solution of tetrabutylammonium methoxide in methanol was placed in a round-botton flask and equipped with a gas injection tube. Carbon dioxide (4.5 purity) was passed through the solution vigorously for four hours. The solvent was removed under reduced pressure. For complete removal of residual methanol, the resulting viscous oil was evaporated by short path distillation in high vacuum. The resulting solid was dried overnight with high vacuum and yielded 5.2 g (16.4 mmol; 73%) colorless material. Analytical results were in accordance with the reported ones.

Mesityl methyl carbonate
In an undivided 25 mL beaker-type glas cell, a solution of 2 mmol mesitylene and 2 mmol tetrabutylammonium methyl carbonate (635 mg) in 20 mL acetonitrile (278 µL) is electrolyzed with a current density of 3 mA/cm 2 and an electrode distance of 0.4 cm until 4.5 F charge is applied. Boron-doped diamond (surface area is 5 cm 2 ) the geometrical is used as anode and cathode material. After electrolysis, the electrolysis mixture is placed in a separation funnel and separated with 50 mL ethyl acetate and 50 mL water. The organic layer is washed with 50 mL water and the dried with MgSO4. The solvent is removed under reduced pressure and the product is separated by shortpath distillation. 71 mg product (0.37 mmol, 18%) are obtained as colorless liquid. 1

Parameter screening and scope studies General procedure A for electrolysis in 5 mL Teflon cells for parameter screening and scope studies
Undivided 5 mL Teflon electrolysis cells were used. A solution of 0.5 mmol arene and 0.5 mmol tetrabutylammonium methyl carbonate in 5 mL acetonitrile is electrolyzed, until a defined amount of charge are applied. The detailed electrolysis parameters depend on the parameter screened.
For GC-MS analysis, approximately 0.5 mL of the electrolysis mixture is passed through a short silica plug with ethyl acetate as eluent and analyzed by GC-MS.
For quantification by NMR, the electrolysis mixture is placed in a separation funnel and separated with 20 mL water and 20 mL ethyl acetate. The organic layer is washed with S4 20 mL water and dried with MgSO4. The solvent is removed under reduced pressure. A defined amount of 2,4,6-triiodophenol is added (5-15 mg). The whole mixture is dissolved in CDCl3 and a proton NMR spectrum is recorded with an enhanced relaxation delay time of 20 s for quantification.

Cyclovoltammetric studies
The cyclic voltammogramm shows the oxidation potential of mesitylene at 1.75 V in a NBu4BF4 solution in acetonitrile (Figure 1; black curve). In the tetrabutylammonium methyl carbonate containing electrolye, a electrolyte decomposition starting at approximately 1.4 V can be observed (Figure 2; red curve)). Upon addition of mesitylene, decomposition is suppressed and a slight increase of current can be observed as an overlap of mesitylene and electroyte oxidation (Figure 2; black curve). This behaviour implies that mesitylene is oxidized first.