Chiral N-Heterocyclic Carbene Borane Complexes: Synthesis and Structural Analysis

Abstract

A new family of chiral N-heterocyclic carbene borane complexes was synthesized starting from their corresponding imidazolium salts. The complexes were fully characterized. X-ray crystal structures of the complexes were obtained

Chiral N-Heterocyclic Carbene Gold Complexes: Synthesis, Properties, and Application in Asymmetric Catalysis

“…The bond lengths of Au–C NHC and Au–Cl perfectly correlate with those of the literature known NHC–gold chloride complexes, and the two ligands are coordinated in a linear gometry . We previously pointed out the role of minimization of allylic strain in setting the stereocontrol elements in place in the NHC–Pd and NHC–borane complexes. − The same structural features were observed in the Au(Cl)(NHC) complexes. Thus, the smallest substituent (H) at the stereogenic center is coplanar with the Au–C NHC bond.…”

“…More recently, excellent results were obtained using ligand precursor 7 in Pd-catalyzed asymmetric coupling of the C–H bond of an unactivated methylene group, leading to the formation of highly enantioenriched 2-substituted and 2,3-disubstituted indolines. , Monodentate ligands were required in this reaction, and the successful application bodes well for Au(I) catalysis. Our hypothesis that the appropriate chiral environment at the reaction center originates from the minimization of allylic strain, which sets the stereocontrol elements of the catalyst in place and brings about high asymmetric induction, was confirmed by the analysis of the solid-state structure of Pd–NHC , and NHC–BH 3 complexes. Moreover, highly thermally stable Pd–NHC systems significantly extend the range of successful applications in catalysis. ,,,, To have a broader window for these ligands in homogeneous asymmetric catalysis, we explored their scope in asymmetric gold catalysis.…”

“…The aryl plane is fixed by an alignment of the aryl o -substituent ( o -OMe group in the case of complex 13 and the second aromatic ring of the naphthyl moiety in the case of complex 16 ) with the C–H bond of the stereogenic center. This fixes the aryl plane in space. − …”

“…In complex 19 , minimization of the allylic strain forces coplanarity of the hydrogen atoms at the stereogenic centers with the C NHC –Au bond. Furthermore, o -OMe groups on the phenyl rings become coplanar with the C(stereogenic center)–H bond. − …”

Chiral N-Heterocyclic Carbene Ligand Enabled Nickel(0)-Catalyzed Enantioselective Three-Component Couplings as Direct Access to Silylated Indanols

“…We started our investigations by evaluating a variety of different established chiral NHC-scaffolds (Figure ) using available benzaldehyde, norbornene, and triisopropylsilane for the model transformation (Table ). Kündig ligand L1 , C 1 -symmetric L2 developed by us, and Glorius’ IBiox L3 furnished 3aa in low yields and selectivity (entries 1–3). In contrast, Grubbs’ C 2 -symmetric imidazolidin-2-ylidene L4 gave silylated indanol 3aa in almost quantitative yield, with exclusive diastereoselectivity and promising enantioselectivity of 71.5:28.5 (entry 4).…”