Molecular Simulation of Potential Energies, Steric Changesand Substituent Effects in Urethane Formation Reactionsfrom Isocyanates

Abstract

Urethane resins show peculiar coatings, adhesives and cushions. The urethane formations from isocyanates with alcohols etc. are accompanied by carbamic acids, the esters, and their decomposed compounds for material performances. The steric and energy changes in the basic reaction processes of isocyanates、and the catalytic reaction by triethylamine were speculated by MOPAC-PM6 simulation, and the simulations compared with them by other methods. Thus, K. C. Frischs' analysis that the reactivities between substituted phenylisocyanates and an alcohol are related to Hammett substituent constants,σ (R = 0.97), was correlated with Mulliken's electronegativity, (IP + EA)/2 by PM6 method (R = 0.97). The reactions were speculated to proceed via hydrogen-bondings and six-membered ring 1:2 complexes of isocyanate with alcohol (water or amine) and by the activation energy of Ea = 4∼13 kcal mol⁻¹. The higher reactivity is pointed out by the calculated smaller Ea value. The calculations including PM6-D3H4 were also effective for the reactivity of X-NCO. The 4 molecules' catalytic reaction of PhNCO with 2MeOH by Et₃N was speculated to be accelerated from Ea = 7.23 kcal mol⁻¹ to Ea = 3.92 kcal mol⁻¹. The transition state structure showed the easy proton transfer by Et₃N. Hydrolyses of carbamic acid and the methyl carbamate ester were compared for urethane properties, and the decomposition of the acid to diphenylurea via aniline etc. and the stability of the ester were speculated by the calculated Ea and transition state structures.

The catalytic reaction with 2MeOH for the urethane. IRC data of catalytic urethane formation via the complex (Bcom) from PhNCO and 2MeOH by Et3N.