Photo-on-Demand Phosgenation Reactions with Chloroform for Selective Syntheses of N-Substituted Ureas and Isocyanates

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.