Mechanistic Investigation into the Acetate-Initiated Catalytic Trimerization of Aliphatic Isocyanates: A Bicyclic Ride

Abstract

The acetate-initiated aliphatic isocyanate trimerization to isocyanurate was investigated by state-of-the-art analytical and computational methods. Although the common cyclotrimerization mechanism assumes the consecutive addition of three equivalents of isocyanate to acetate prior to product formation, we found that the underlying mechanism is more complex. In this work, we demonstrate that the product, in fact, is formed via the connection of two unexpected catalytic cycles, with acetate being only the precatalyst. The initial discovery of a precatalyst activation by quantum chemical computations and the resulting first catalysis cycle were corroborated by mass spectrometric and NMR experiments, thereby additionally revealing a catalyst migration to the second catalytic cycle. These results were further confirmed by computations, completing the full mechanistic understanding of this catalytic system. Identification of a side product with undesired properties for final coating applications allows for process optimization in the chemical industry.

Role of Acetate Anions in the Catalytic Formation of Isocyanurates from Aromatic Isocyanates

“…Therefore, forming olefin 6′ does not end the catalytic isocyanurate formation, but instead changes the catalytically active species from the deprotonated amide to deprotonated urea in agreement with findings for aliphatic isocyanates. 41 Then, we studied the trimerization reaction experimentally in THF and toluene at room temperature to verify our mechanistic hypothesis. For experiments, we chose phenyl and p-tolyl isocyanates as model substrates and tetrabutylammonium acetate (TBAA) as catalyst because of their high solubility in both solvents.…”

“…While in the catalytic cycle on the right, the active species cannot undergo similar catalyst migrations as previously, the allophanate isocyanate intermediate 3′ is interestingly predicted to reversibly form the cyclized anion 3″ , which we consider to lead to the formation of electron-poor N-heterocyclic olefin 6′ in agreement with recent findings in the cyclotrimerization of aliphatic isocyanates. 41 The total reaction yields one molar equivalent of hydroxyl anions, but we assume the reaction to proceed via first protonating 3′ by a trace amount of proton sources and then fragmenting water. Water and hydroxyl anions are well known to lead to the formation of urea first by hydrolyzing isocyanate to carbamic acid and forming an aromatic amine after fragmentation of CO 2 .…”

“…Therefore, forming olefin 6′ does not end the catalytic isocyanurate formation, but instead changes the catalytically active species from the deprotonated amide to deprotonated urea in agreement with findings for aliphatic isocyanates. 41 …”

“… 13 , 19 , 40 Recently, Siebert et al also suggested that the catalytically active species originating from acetate anions changes several times during the cyclotrimerization of aliphatic isocyanates. 41 The exact species that catalyze cyclotrimerization of aromatic isocyanates, however, has not been explicitly characterized so far, which hampers the development of new PIR catalysts suitable for large-scale polyurethane production.…”

“…Most studies consider the catalytically active species to be the acetate anion itself, , but, on the other hand, Hoffman’s early experimental work indicated that acetate anions are quickly converted to acetanilide when reacting with aromatic isocyanates, which in turn could potentially act as the anionic catalysts in the active cycle . Further, as the acetanilide anion can be expected to deprotonate urethane, allophanate, urea, and biuret groups in the PU matrix depending on their relative acidities, and their corresponding anions have been shown to be active PIR catalysts, the catalytically active species is expected to change several times during the polymerization. ,, Recently, Siebert et al also suggested that the catalytically active species originating from acetate anions changes several times during the cyclotrimerization of aliphatic isocyanates . The exact species that catalyze cyclotrimerization of aromatic isocyanates, however, has not been explicitly characterized so far, which hampers the development of new PIR catalysts suitable for large-scale polyurethane production.…”