An efficient one-pot synthesis of aliphatic diisocyanate from diamine and aiphenyl carbonate

•A green-chemical process was without using toxic phosgene or any metal catalysts.•The key to our present success lies in identifying the most suitable solvent DPE.•We have combined the synthesis and thermolysis into a one-pot two-stage NPR process.•This safe approach potentially can be adapted for industrial scale production of ADIs.

A non-phosgene route (NPR) of making aliphatic diisocyanates has been developed through synthesis and thermolysis of diphenyl N,N′-alkenylbiscarbamates (diphenyl biscarbamates). Four diphenyl biscarbamates were prepared readily by carbonylation of three aliphatic diamines and one aryl aliphatic mixed diamine with the carbonylation reagent diphenyl carbonate (DPC) in 1,2-diethoxyethane (EGDEE) under mild conditions. Pure diphenyl biscarbamates were isolated in 85–98% for each case. Thermolysis of individual diphenyl biscarbamate in diphenyl ether solution at 240–255 °C readily cracked them into four respective diisocyanates and recovering phenol. In this study, 1,12-dodecamethylene diisocyanate (DDI) gave the highest isolation yields (84%) through fractionation. The yields of 1,6-hexamethylene diisocyanate (HDI), 1,4-butamethylene diisocyanate (BDI) and 1-isocyanato-4-(isocyanatomethyl)benzene (4-IBPI) were slightly lower (76%) with by-products consisting of isocyanate oligomers. More notable than high yields, we succeeded in developing a one-pot two-stage NPR process in diphenyl ether solution by first synthesis of biscarbamate followed immediately by thermolysis in the same solution. Excellent yields of DDI (80%) and isophorone diisocyanate (86%) along with HDI (71%) were prepared in the one-pot synthesis. Thus, a general green-chemical process has been developed for aliphatic diisocyanate without using toxic phosgene or any undesirable metal catalysts.