Light-driven transition-metal-free direct decarbonylation of unstrained diaryl ketones via a dual C–C bond cleavage

The cleavage and formation of carbon−carbon bonds have emerged as powerful tools for structural modifications in organic synthesis. Although transition−metal−catalyzed decarbonylation of unstrained diaryl ketones provides a viable protocol to construct biaryl structures, the use of expensive catalyst and high temperature (>140 oC) have greatly limited their universal applicability. Moreover, the direct activation of two inert C − C bonds in diaryl ketones without the assistance of metal catalyst has been a great challenge due to the inherent stability of C − C bonds (nonpolar, thermo-dynamically stable, and kinetically inert). Here we report an efficient light-driven transition-metal-free strategy for decarbonylation of unstrained diaryl ketones to construct biaryl compounds through dual inert C − C bonds cleavage. This reaction featured mild reaction conditions, easy-to-handle reactants and reagents, and excellent functional groups tolerance. The mechanistic investigation and DFT calculation suggest that this strategy proceeds through the formation of dioxy radical intermediate via a single-electron-transfer (SET) process between photo-excited diaryl ketone and DBU mediated by DMSO, followed by removal of CO2 to construct biaryl compounds.

General Information: All reagents and solvents were purchased from commercial sources (Alfa, Acros, Aldrich, TCI and Combi-Blocks) and used without further purification unless otherwise stated. 1 H and 13 C NMR spectra were taken on Agilent 600, Bruker 400 or 500 MHz spectrometer. Chemical shifts of 1 H NMR spectra were reported using either residual solvent signal of CDCl3 (δ = 7.26 ppm) or TMS (δ = 0.00 ppm) as internal standard. Chemical shifts of 13 C NMR spectra were reported using residual solvent signal of CDCl3 (δ = 77.16 ppm) as internal standard. The peak patterns are indicated as follows: s, singlet; d, doublet; dd, doublet of doublet; t, triplet; q, quartet; m, multiplet. The coupling constants, J, are reported in Hertz (Hz). All reactions were monitored by thin-layer chromatography (TLC). Column chromatography was performed on silica gel (200-300 mesh) and visualized with ultraviolet light. EI-MS was obtained from the Agilent GC-MS system. All solvents were purified and dried by standard techniques.

General experimental procedure for decarbonylation of unstrained diaryl ketones
In a 15 mL quartz tube charged with a magnetic stir-bar, were added sequentially ketones (0.1 mmol, 1 equiv) and t-BuOK (14 mg, 0.125 mmol, 1.25 equiv). The tube was then evacuated and backfilled with argon three times. DBU (30 μL, 0.2 mmol, 2 equiv), DMSO (1.5 mL) and H2O (80 μL) were added by microsyringe and syringe. Then the tube was placed in a UV reactor 1 at room temperature and the mixture was stirred for 36 h. 10 mL water was added to quench the reaction, and the mixture was extracted with EtOAc (5 mL × 4). The combined organic solvent was washed with brine, dried with Na2SO4, and then concentrated under reduced pressure. The residues were purified by preparative TLC on silica gel eluting with hexane : EtOAc (300:1-20:1) to afford the product.  Then the tube was placed in a UV reactor at room temperature and the mixture was stirred for 36 h.
The results showed that all reactions with metal additives were not as efficient as those without metal. The above three experimental results show that this is a transition metal-free strategy and t-BuOK /DBU combination plays a very important role in the reaction process.
The tube was then evacuated and backfilled with argon three times. DBU (1.8 mL, 12 mmol, 2 equiv), DMSO (60 mL) and H2O (2 mL ) were added by syringe. Then the flask was placed in a UV reactor at S4 room temperature and the mixture was stirred for 72 h. 20 mL water was added to quench the reaction, and the mixture was extracted with EtOAc (15 mL × 4). The combined organic solvent was washed with brine, dried with Na2SO4, and then concentrated under reduced pressure. The residues were purified by column chromatography on silica gel (eluent: hexanes : EtOAc = 50:1) to afford the 4,4'-difluoro-1,1'-biphenyl product (0.51 g, 45% yield).
iii. 18  In a round-bottom flask charged with a magnetic stir-bar, bromine (6.7 mL, 130 mmol) was added dropwise to an ice-cooled solution of dimethyl sulfide (9.5 mL, 130 mmol) in 120 mL carbon tetrachloride and the mixture was continuously stirred for 2 h. The yellowish orange crystals were formed which were filtered out and washed with cold carbon tetrachloride. The solid (27 g) was recrystallized from carbon tetrachloride, and BDMS was obtained as yellow solid.
2) Preparation of DMS 18 O: In a round-bottom flask charged with a magnetic stir-bar, were added dry triethylamine (25 mL, 180 mmol), 18

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CO2 and DMS detection: In a 15 mL quartz tube charged with a magnetic stir-bar, were added sequentially bis(4-fluorophenyl)methanone (21.8 mg, 0.1 mmol, 1 equiv) and t-BuOK (14 mg, 0.125 mmol, 1.25 equiv). The tube was then evacuated and backfilled with argon three times. DBU (30 μL, 0.2 mmol, 2 equiv), DMSO (1.5 mL) and H2O (80 μL) were added by microsyringe and syringe. Then the tube was placed in a UV reactor at room temperature and the mixture was stirred for 36 h. The gas phase inside the flange was analyzed using a valve syringe and gas chromatography mass spectrometer (GC-MS). S8 equiv), DMSO (1.5 mL) and H2O (80 μL) were added by microsyringe and syringe. Then the tube was placed in a UV reactor at room temperature and the mixture was stirred for 36 h. The gas phase inside the flange was analyzed using a valve syringe and gas chromatography mass spectrometer (GC-MS). The results show that there is no CO in the gas phase. We further used the EL-USB-CO detector to detect the trace amount of CO (2:00-7:00 pm continuous testing for 5 h), and also no CO was detected.   To account for the excited-state energy of the photocatalyst, the Gibbs free energy (ΔGeT) can be S10 calculated using the Rehm−Weller equation: where E ox (D) is the oxidation potential of the donor molecule, E red (A) is the reduction potential of the acceptor molecule, E * (D or A) is the singlet or triplet excited-state energy of the excited donor or acceptor molecule and ΔECoulombic (our calculation process can be ignored) is a measure of the interaction between charged ions in the dielectric constant of the solvent in which the reaction is performed. [3] = 0.58 V vs. Fc + /Fc [4]  The possible mechanism of CO2 release is shown in Figure 4 and Figure  Archive file "Cartesian-Coordinates.zip" contains the Cartesian coordinates for all computed species.
The archived Gaussian output for IRC of the oxygen transfer step "RS0-IRC.zip" is also attached as Supplementary Information.