ABSTRACT
Here, we report a novel photo-on-demand in situ phosgenation reaction that crosses three phases of a heterogeneous solution of chloroform (CHCl3) and aqueous NaOH containing an aryl alcohol or amine. This reaction system enables the safe, convenient, and inexpensive synthesis of carbonate esters, polycarbonates, and N-substituted ureas from aryl alcohols, aryl diols, and primary/secondary amines, respectively, on a practical scale and with good yield. The photochemical oxidation of CHCl3 to phosgene (COCl2) occurs upon irradiation with UV light from a low-pressure mercury lamp of both the gas and liquid phases of the reaction system under O2 bubbling of the vigorously stirred sample solution. The following reaction mechanisms are suggested: The aryl alcohol reacts in situ with the generated COCl2 at the interfaces of the organic/aqueous phases and aqueous/gas phases, in competition with the decomposition of COCl2 due to hydrolysis. Nucleophilicity and hydrophilicity are enhanced by the formation of aryl alkoxide ion through the reaction with NaOH, whereas the reaction of amine proceeds through neutralization of the generated HCl by the aqueous NaOH.
ABSTRACT
Two new reaction processes involving the in situ oxidative photochemical conversion of CHCl3 to COCl2 allowed selective syntheses of N-substituted ureas and isocyanates from amines. (I) A CHCl3 solution containing an amine and an organic base under O2 bubbling provided the urea derivative under exposure to UV light generated from a low-pressure mercury lamp at 20-40 °C. (II) A two-step reaction involving the oxidative photodecomposition of CHCl3 at lower temperatures and subsequent sequential injections of an amine and organic base into the sample solution provided the isocyanate in high yield. The reaction processes of (I) and (II) capitalize on the solution conditions of [COCl2] < [amine] and [COCl2] > [amine], respectively, to result in 1:2 and 1:1 reactions. In general, isocyanates, especially aromatic and haloalkyl ones, readily undergo hydrolysis in the presence of an organic base. However, with the advantage of synthesizing the isocyanates in CHCl3 solvent, direct addition of monoalcohols and diols to the as-prepared sample solution containing the diisocyanate allowed the one-pot syntheses of biscarbamates and polyurethanes, respectively. The reactions developed in this study are simple, safe, and inexpensive methods of synthesizing N-substituted ureas and isocyanates, and derivatives of isocyanates such as carbamates and polyurethanes. The present new methods can replace current synthetic methods using COCl2 in both academia and industry.
