Computational Study of Catalytic Urethane Formation

Abstract

Polyurethanes (PUs) are widely used in different applications, and thus various synthetic procedures including one or more catalysts are applied to prepare them. For PU foams, the most important catalysts are nitrogen-containing compounds. Therefore, in this work, the catalytic effect of eight different nitrogen-containing catalysts on urethane formation will be examined. The reactions of phenyl isocyanate (PhNCO) and methanol without and in the presence of catalysts have been studied and discussed using the G3MP2BHandHLYP composite method. The solvent effects have also been considered by applying the SMD implicit solvent model. A general urethane formation mechanism has been proposed without and in the presence of the studied catalysts. The proton affinities (PA) were also examined. The barrier height of the reaction significantly decreased (∆E₀ > 100 kJ/mol) in the presence of the studied catalysts, which proves the important effect they have on urethane formation. The achieved results can be applied in catalyst design and development in the near future.

Keywords: DFT; catalyst-free; catalysts; composite method; proton affinities; urethane formation.

Figure 1

Chemical structures of the studied catalysts. 1—1,8-diazabicyclo[5,4,0]undec-7-ene; 2—1-(3-aminopropyl)imidazole; 3—1-methylimidazole; 4—N,N-dimethylcyclohexanamine; 5—1,4-dimethylpiperazine; 6—1,4-diazabicyclo[2.2.2]octane; 7—N,N,N′,N″,N″–pentamethyldiethylenetriamine; 8—N,N-dimethyl-1,3-propanediamine. The catalytic nitrogen-containing groups which are considered in the calculations are highlighted with red circles.

Scheme 1

Schematic representation of the general catalyst-free reaction of isocyanates and alcohols. R—reactants, RC—reactant complex, TS—transition state, and P—product.

Figure 2

Three-dimensional structures of the reactant complex (RC), transition state (TS), and product (P) in the reaction of methanol and phenyl isocyanate, which are used as a model of catalyst-free urethane formation. The structures have been optimized at the BHandHLYP/6-31G(d) level of theory in acetonitrile at 298.15 K and 1 atm.

Figure 3

Energy profile (zero-point corrected) of the phenyl isocyanate and methanol reaction calculated using the G3MP2BHandHLYP composite method in acetonitrile, using the SMD implicit solvent model at 298.15 K and 1 atm.

Scheme 2

Schematic representation of the proposed general reaction mechanism of isocyanates and alcohols in the presence of nitrogen-containing catalysts.

Figure 4

Optimized structures along the reaction pathway between phenyl isocyanate and methanol in the presence of catalyst 1, calculated at the BHandHLYP/6-31G(d) level of theory in acetonitrile at 298.15 K and 1 atm. RC—reactant complex, TS—transition state, IM—intermediate, and PC—product complex.

Figure 5

Energy profiles (zero-point corrected, ∆E₀) of the catalyzed urethane formation reactions calculated with the G3MP2BHandHLYP composite method in acetonitrile, using the SMD implicit solvent model at 298.15 K and 1 atm.