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

Abstract

A series of magnesium, aluminum, and zinc complexes of imidazol-2-ylidene-, imidazolin-2-ylidene-, and tetrahydropyrimidin-2-ylidene-derived N-heterocyclic carbenes (NHCs) was prepared. In addition, both symmetrical and unsymmetrical CO 2 -protected imidazol-2-ylidenes and imidazolin-2-ylidenes were prepared. Selected single-crystal Xray structures are reported. All compounds were investigated for their catalytic behavior in (poly)urethane (PUR) synthesis. Out of nine different compounds, the dimeric complex [Zn(CH 3 COO) 2 (1,3-dimesitylimidazol-2-ylidene)] 2 (7) proved to be the most active one, rivaling the industrially most relevant catalyst dibutyltin dilaurate in terms of catalytic ac-

Synthesis and Organocatalytic Applications of Imidazol(in)ium‐2‐thiocarboxylates

“…Thus, the C-O distance in SIMes·COS is significantly shorter than in SIMes·CO 2 [1.2173(16) vs. 1.2277(17) Å]. [32] Conversely, the C-S bond length increases from 1.662(17) to 1.6761(14) Å when switching from SIMes·CS 2 [19] to SIMes·COS. These data reveal a strengthening of the CO bond and a weakening of the CS bond in the COS adduct compared with the corresponding carboxylate and dithiocarboxylate inner salts.…”

“…Altogether, these data are in line with those recorded previously for other salts or zwitterions featuring the 1,3-dimesitylimidazolinium cation. [19,31,32] Like most NHC·CO 2 and NHC·CS 2 betaines investigated so far by XRD, [10] SIMes·COS displays an almost perpendicular arrangement between its cationic and anionic moieties with a N1-C1-C6-S1 torsion angle of 94.96(14)° ( Figure 1). Through-space attractive Coulomb interactions between the imidazol(in)ium C1 carbon atom and the lone pair of electrons on the thiocarboxylate moiety might explain this conformation.…”

Synthesis, characterization and catalytic activity of stable [(NHC)H][ZnXY2] (NHC =N-Heterocyclic carbene, X, Y = Cl, Br) species

“…They have shown to be particularly active in reactions involving the fixation of CO 2 , such as the methylation of amines [3], or the synthesis of cyclic carbonates from epoxides [4]. In addition, they are employed in the synthesis of polyurethanes [5] and in the polymerization of D,L-lactide [6]. To date, the synthesis of Zn(II)-NHC complexes involves the addition of a free carbene to a zinc salt.…”

“…To date, the synthesis of Zn(II)-NHC complexes involves the addition of a free carbene to a zinc salt. The carbene can either be a priori isolated or generated in situ from an appropriate precursor (NHC salt or NHC-CO 2 adduct) [3][4][5][6][7]. Alternatively, the NHC salt can be directly reacted …”

Recent advances in neutral and anionic N-heterocyclic carbene – betaine interconversions. Synthesis, characterization, and applications

“…123 Here, examples of the title compounds will be presented. A comprehensive survey about the interesting subject of NHC adduct formation with trivalent group 13 compounds, [124][125][126][127][128] however, is beyond the scope of this review. The frustrated carbene-borane Lewis pair 170a·B(C 6 F 5 ) 3 formed the aNHC-borane adduct 172 129 which is more stable than the corresponding nNHC-borane adduct.…”