CO₂ and Sn^II Adducts of N‐Heterocyclic Carbenes as Delayed‐Action Catalysts for Polyurethane Synthesis

Abstract

Catalytic rivals: Both CO(2)-protected tetrahydropyrimidin-2-ylidene-based N-heterocyclic carbenes (NHCs) and Sn(II)-1,3-dimesitylimidazol-2-ylidene, as well as Sn(II)-1,3-dimesitylimidazolin-2-ylidene complexes (example displayed), have been identified as truly latent catalysts for polyurethane (PUR) synthesis rivaling all existing systems both in activity and latency.A series of CO(2)-protected pyrimidin-2-ylidenes as well as 1,3-dimesitylimidazol-2-ylidene and dimesitylimidazolin-2-ylidene complexes of Sn(II) have been prepared. Selected single-crystal X-ray structures are reported. The new compounds were investigated for their catalytic behavior in polyurethane (PUR) synthesis. All compounds investigated showed excellent catalytic activity, rivaling the industrially most relevant catalyst dibutyltin dilaurate. Even more important, all compounds displayed pronounced latent behavior, in selected cases rivaling and exceeding the industrially relevant latent catalyst phenylmercury neodecanoate both in terms of latency and catalytic activity. This allows for creating one-component PUR systems with improved pot lifetimes. Pseudo-second-order kinetics were found for both CO(2)-protected tetrahyropyrimidin-2-ylidenes and for [SnCl(2)(1,3-dimesityldihydroimidazol-2-ylidene)], indicating a fast pre-catalyst decomposition prior to polyurethane formation. 1,3-Di(2-propyl)tetrahydropyrimidin-2-ylidene was additionally found to be active in the cyclotrimerization of various isocyanates, offering access to a broad variability in polymer structure, that is, creating both urethane and isocyanurate moieties within the same polymer.

CO2, Magnesium, Aluminum, and Zinc Adducts of N‐Heterocyclic Carbenes as (Latent) Catalysts for Polyurethane Synthesis

“…Compounds 5 and 6 were accessible in high yields (89 and 90 %) by reaction of the corresponding CO 2 -protected NHCs [17][18][19] 1,3-dimesitylimidazolium-2-carboxylate (3) [20] and 1,3-dimesityl-3,4,5,6-tetrahydropyridinium-2-carboxylate, [21] respectively, with anhydrous ZnCl 2 . Compounds 2 and 4 were prepared in 93 and 70 % yield by reaction of the corresponding imidazolium salt with potassium tert-butoxide and KH, respectively, followed by reaction with dry CO 2 .…”

“…The distance C(1)-C(2) is 153.8(3) pm and is thus almost identical to the one found in other CO 2 adducts of NHCs. [21] For our investigations on the activity of compounds 1-9 as potential (pre)catalysts for PUR synthesis, we focused on a technically relevant mixture of a polymeric hydroxyacrylate and trimeric hexamethylene diisocyanate (HMDI, RNCO/OH, 1:1). As already reported, [21] all compounds may be regarded as either CO 2 -, Mg II -, Al III -, or Zn II -protected NHCs, which, upon thermal initiation, should be able to generate the corresponding free carbene.…”

“…[21] For our investigations on the activity of compounds 1-9 as potential (pre)catalysts for PUR synthesis, we focused on a technically relevant mixture of a polymeric hydroxyacrylate and trimeric hexamethylene diisocyanate (HMDI, RNCO/OH, 1:1). As already reported, [21] all compounds may be regarded as either CO 2 -, Mg II -, Al III -, or Zn II -protected NHCs, which, upon thermal initiation, should be able to generate the corresponding free carbene. Because free NHCs are both highly nucleophilic and basic, they should also catalyze polyurethane (PUR) formation through proton abstraction form the alcohol to thus form the more nucleophilic alcoholate.…”

Mechanochemical Reactions Reporting and Repairing Bond Scission in Polymers

“…Because of their nucleophilicity they show high reactivity towards various substrates but they can also be masked in thermally labile precursors [67]. NHCÀmetal complexes have been applied as thermally latent catalysts in the preparation of a number of polymers, such as poly(urethane), poly(methyl methacrylate), poly(caprolactone) and poly (amide) [68][69][70][71][72].…”

Imidazol(in)ium‐2‐carboxylates as N‐Heterocyclic Carbene Precursors for the Synthesis of Second Generation Ruthenium Metathesis Catalysts

“…[19] Recently, carbon dioxide adducts of NHCs were successfully employed for the synthesis of several organometallic complexes [20] and in organocatalysis. [21,22] From all the methods investigated so far for generating NHCs in situ, the decarboxylation of imidazol-A C H T U N G T R E N N U N G (in)ium-2-carboxylate betaines is the most convenient and versatile one (Scheme 1). Indeed, NHC·CO 2 zwitterions are easily obtained by bubbling carbon dioxide into a free carbene solution, followed by solvent evaporation.…”