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.