Well-Defined Amidate-Functionalized N-Heterocyclic Carbene -Supported Rare-Earth Metal Complexes as Catalysts for Efficient Hydroboration of Unactivated Imines and Nitriles

Abstract

Four amidate-functionalized N-heterocyclic carbene (NHC) rare-earth metal amido complexes [(κ²-N,O-κ¹-L)₂REN(SiMe₃)₂] (L = 1-(C₆H₅C═ONCH₂CH₂)-3-(CH₃)₃C₆H₂(N(CH)₂NC)) [RE = Er (1), Y (2), Dy (3), Gd (4)] were synthesized by one-pot reactions of 2 equiv of (1-(C₆H₅C═ONHCH₂CH₂)-3-(CH₃)₃C₆H₂-(N(CH)₂NCH))Br (H₂LBr) with 5 equiv of KN(SiMe₃)₃ followed by treatment with 1 equiv of RECl₃ in tetrahydrofuran at -40 °C. These complexes were fully characterized, and their catalytic activities toward hydroboration of unactivated imines and nitriles were investigated, and it was found that these complexes displayed excellent activities as well as remarkable functional group compatibility for imine and nitrile substrates such as halo-, alkyl-, hydroxyl-, N, N-dimethylamino-, and nitro- substituents. Among those, the chemoselectivity for this reaction among the common unsaturated functional groups was achieved in the order C═O ≫ C═N > C≡N > CO₂Et > C═C in the current catalytic system, which may facilitate their further application in synthetic chemistry.