Urethane Formation with an Excess of Isocyanate or Alcohol: Experimental and Ab Initio Study

Abstract

A kinetic and mechanistic investigation of the alcoholysis of phenyl isocyanate using 1-propanol as the alcohol was undertaken. A molecular mechanism of urethane formation in both alcohol and isocyanate excess is explored using a combination of an accurate fourth generation Gaussian thermochemistry (G4MP2) with the Solvent Model Density (SMD) implicit solvent model. These mechanisms were analyzed from an energetic point of view. According to the newly proposed two-step mechanism for isocyanate excess, allophanate is an intermediate towards urethane formation via six-centered transition state (TS) with a reaction barrier of 62.6 kJ/mol in the THF model. In the next step, synchronous 1,3-H shift between the nitrogens of allophanate and the cleavage of the C–N bond resulted in the release of the isocyanate and the formation of a urethane bond via a low-lying TS with 49.0 kJ/mol energy relative to the reactants. Arrhenius activation energies of the stoichiometric, alcohol excess and the isocyanate excess reactions were experimentally determined by means of HPLC technique. The activation energies for both the alcohol (measured in our recent work) and the isocyanate excess reactions were lower compared to that of the stoichiometric ratio, in agreement with the theoretical calculations.

Connecting Gas-Phase Computational Chemistry to Condensed Phase Kinetic Modeling: The State-of-the-Art

“…All authors agreed that the addition of a hydroxy group to the C=N was more preferable and showed the importance of autocatalysis. Application of the hybrid SMD model by Cheikh et al [ 311 ] showed that isocyanates may also participate in the autocatalytic pathway for urethane formation.…”

Heterobimetallic complexes composed of bismuth and lithium carboxylates as polyurethane catalysts – alternatives to organotin compounds

“…So far, mechanistic studies on metal complex catalyzed PU reaction have been performed mainly for organotin catalysts, 58,[60][61][62][63][64][65] leaving critical questions still unanswered. It remains for example unclear what factors determine the selectivity and speed of the isocyanate-hydroxyl reaction when catalyzed by tin, zirconium, bismuth, or zinc catalysts.…”

“…Note that the effective rate of the urethan bond forming reaction can be influenced by various phenomena such as catalysis, autocatalysis, solvent effects and molecular interactions. 63,[66][67][68][69] Computations reveal that cyclic, tetrameric bismuth tricarboxylate clusters form thermodynamically driven by a free energy difference of −40 and −18 kJ mol −1 at 25°C and 60°C, respectively (Fig. 6).…”

Reaction Mechanisms and Rate Constants of Auto‐Catalytic Urethane Formation and Cleavage Reactions

“…Moreover, Raspoet et al also showed that auto-catalytic mechanisms are favorable compared to non-catalytic mechanisms in agreement with other studies discussed above. Recently, Cheikh et al [38] used more sophisticated quantum chemical methods to study autocatalytic urethane formation. The authors computed paths in energy, enthalpy and free energy based on the G4MP2 [53] protocol and the SMD solvation model.…”