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

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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 CO2‐protected imidazol‐2‐ylidenes and imidazolin‐2‐ylidenes were prepared. Selected single‐crystal X‐ray 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(CH3COO)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 activity. Even more important, the Mg‐ and Zn‐derived NHC complexes as well as the CO2‐protected imidazol‐2‐ylidenes displayed pronounced latent behavior combined with high catalytic activity in selected cases, rivaling and exceeding the industrially relevant latent catalyst phenylmercury neodecanoate. This allows for creating one‐component PUR systems with significant pot lives. Pseudo‐second‐order kinetics were found for all Mg‐ and Zn‐based NHC complexes as well as for all CO2‐protected NHCs indicating fast precatalyst decomposition prior to (poly)urethane formation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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