Photo-on-demand Phosgenation Reactions with Chloroform Triggered by Cl₂ upon Irradiation with Visible Light: Syntheses of Chloroformates, Carbonate Esters, and Isocyanates

Abstract

Chloroform (CHCl 3 ) upon bubbling with O 2 containing ³2% Cl 2 underwent oxidative photochemical conversion to phosgene (COCl 2 ) when exposed to a white LED light. Cl 2 may serve as a visible light-responsive radical initiator in the radical chain reactions of CHCl 3 and O 2 . This photochemical reaction allowed both in situ and stepwise one-pot phosgenation reactions, which are simple and safe methods, without expensive apparatus. It can be applied to a variety of conventional organic syntheses using phosgene and phosgene oligomers.

Photo-On-Demand Synthesis of α-Amino Acid N-Carboxyanhydrides with Chloroform

“…In support of this proposed mechanism, commercially available l -Phe·HCl was found to provide the corresponding NCA in 70% yield (Table , entry 1I). Thus, the reaction takes a longer time at higher temperature compared with other phosgenation reactions reported previously. − For safe handling of the reaction, the generated amount of COCl 2 should be regulated by ON–OFF cycles and/or intensity of the photo-irradiation, and the vaporized COCl 2 and generated HCl must be trapped with alcohol and alkaline water.…”

“…For example, carbonate esters and their intermediate chloroformates were synthesized in high yields only upon irradiation by UV-C light of CHCl 3 solutions containing an alcohol with or without an organic base . Furthermore, we recently reported that these photochemical reactions occurred with visible light in the presence of a small amount of Cl 2 . To cause the reactions, there were threshold concentrations of alcohol and base in the CHCl 3 solution (generally [CHCl 3 ] ≫ [base] > [alcohol]).…”

“… 20 Furthermore, we recently reported that these photochemical reactions occurred with visible light in the presence of a small amount of Cl 2 . 22 To cause the reactions, there were threshold concentrations of alcohol and base in the CHCl 3 solution (generally [CHCl 3 ] ≫ [base] > [alcohol]). However, surprisingly, we found in this study that photochemical oxidation of CHCl 3 occurred even in the presence of a larger amount of CH 3 CN, which can increase the solvent polarity.…”

Flow Photo-On-Demand Phosgenation Reactions with Chloroform

“…We expect that it will contribute to the sustainable organic synthesis capable of reducing chemical waste and CO 2 emissions and will be used mainly in industry . For the academic purpose, however, it is preferable to use the in situ batch photo-on-demand phosgenation reactions reported mainly by our group for safety reasons. − …”

“…As an alternative on-demand method to generate COCl 2 , we previously reported oxidative photochemical conversion of CHCl 3 to COCl 2 upon exposure to UV light with a batch reaction system. − Although the reaction proceeds without chemical reagents, its efficiency with respect to the amount of CHCl 3 has not been clarified quantitatively. Herein, we studied a novel flow photochemical oxidation of CHCl 3 under precise control of the flow rates of CHCl 3 and O 2 and found that the reaction occurred efficiently in the gas phase in a short time to give COCl 2 in over 96% yield (Scheme b) .…”