Photo-on-Demand In Situ One-Pot Synthesis of Carbonate Esters from Tetrachloroethylene

The present study reports a novel one-pot synthesis of carbonate esters with photo-oxidized tetrachloroethylene (TCE). Acyclic and cyclic alkyl carbonate esters could be synthesized through base-promoted condensation reactions of alcohols with the photo-oxidized TCE that was prepared by irradiation with UV-C or visible light under O2 or O2/Cl2 (∼4%) bubbling, respectively. Cyclic carbonate esters could also be synthesized from a solution of TCE and the ethylene glycol derivative by irradiation of UV-C light under O2 bubbling. With respect to the reaction mechanism, the photochemical oxidation of TCE mainly provides the highly toxic and corrosive trichloroacetyl chloride (TCAC), which then reacts in situ with the alcohol to give the corresponding trichloroacetic acid ester (TCAE). The subsequent intermolecular or intramolecular base-catalyzed condensation reaction of TCAE with or without the addition of alcohol leads to elimination of CHCl3 in the corresponding carbonate ester. The present reactions enable the in situ one-pot synthesis of a variety of alkyl carbonate esters under mild conditions without the direct handling of TCAC. This is beneficial in terms of safety, cost, and reduced environmental impact.

Photo-on-Demand In Situ Synthesis of N-Substituted Trichloroacetamides with Tetrachloroethylene and Their Conversions to Ureas, Carbamates, and Polyurethanes

N-substituted trichloroacetamides (NTCAs), which serve as blocked isocyanates, were synthesized in ∼97% yields by in situ photo-on-demand trichloroacetylation of amines with tetrachloroethylene (TCE). The reactions were performed by photo-irradiation of TCE solutions containing an amine under O₂ bubbling over 70 °C with a low-pressure mercury lamp. TCE underwent photochemical oxidation to afford trichloroacetyl chloride having high toxicity and corrosivity, which then reacts in situ with the amine to afford NTCA. Compared with conventional NTCA synthesis with hexachloroacetone, the present reaction has the advantage of being widely applicable to a variety of amines, even those with low nucleophilicity such as amides, fluorinated amines, and amine HCl salts. NTCAs could be converted to the corresponding N-substituted ureas and carbamates through base-catalyzed condensation with amines and alcohols, respectively, with the elimination of CHCl₃. The reaction may proceed by the initial formation of isocyanate and its subsequent addition reaction with the amine or alcohol. This photochemical reaction also enables the synthesis of fluorinated NTCAs, which accelerate the reactions, and realizes the synthesis of novel fluorinated chemicals including polyurethanes.

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

Amino acid N-carboxyanhydrides (NCAs) are conventionally synthesized from α-amino acids and phosgene. The present study reports in situ photo-on-demand phosgenation reactions of amino acids with CHCl₃ for synthesizing NCAs. A series of NCAs were obtained on a gram scale upon photo-irradiation of a mixture solution of CHCl₃ and CH₃CN containing an amino acid at 60-70 °C under O₂ bubbling. This method presents a safe and convenient reaction controlled by light without special apparatuses and reagents.