13C NMR studies on the relative reactivity of isocyanate groups of isophorone diisocyanate isomers

Model reactions of isophorone diisocyanate (IPDI ( 1 )) with ethanol were examined using ¹³C NMR technique to locate the chemical shift of different urethane and isocyanate groups in the products. The reactivity of the secondary and primary isocyanate group of the IPDI isomers is catalyst-dependent. In the presence of Lewis acids (e.g., dibutyltin dilaurate) the secondary NCO group is more reactive, while in the presence of Lewis bases (e.g., triethylamine) it is the primary one. This effect allowed the chemical shift assignment of urethane- and isocyanate carbonyl C atoms in adducts formed by reaction of IPDI with ethanol in mole ratio NCO:OH = 2:1. In ethyl carbamates from the main isomer the chemical shift δ = 155,93 ppm is assigned to the carbonyl C atom of the urethane group at the secondary C atom, the signal at δ = 157,07 ppm to the primary urethane group. Furthermore, it was possible to assign the signal at δ = 122,12 ppm to the primary NCO group and the signal at δ = 122,98 ppm to the secondary NCO group of isophorone diisocyanate.