We have presented new amino-NHC Ni-Al complex mediated para C-H bond activation for pyridine and quinolin, and isolated for the first time the intermediate structure of a bimetallic eta(2),eta(1)-pyridine nickel aluminum complex prior to its C-H activation, which serves as key evidence for bimetallic catalysis.

Treatment of 2,4-dien-1-als with allylsilanes and PPh(3)AuSbF(6) (3 mol %) led to formation of 1,4-bis(allyl)cyclopentenyl products; this gold catalyst is superior to commonly used Lewis acids according to catalyst screening. Such gold-catalyzed deoxygenative cyclizations are compatible with various oxygen-, amine-, sulfur-, hydrogen-, and carbon-based nucleophiles. The value of this new catalysis is demonstrated by the diverse annulations of 2,4-dien-1-als with electron-rich alkenes and arenes, providing an easy access to complicated cyclopentenyl frameworks. Structural analysis of annulation products reveals evidence for the participation of Nazarov cyclization. This deoxygenative cyclization is extensible to a tandem intramolecular cyclization/nucleophilic addition cascade, giving polycyclic carbo- or oxacyclic compounds with controlled stereochemistry. This new gold catalysis is applied to a short synthesis of natural compounds of the brazilane family, including brazilane, O-trimethyl-, and O-tetramethyl brazilane.

The mono-(2) and bis(amino-NHC) (3) nickel halide complexes can be prepared in moderate yield by reacting Ni(PPh 3 ) 2 X 2 with the amino pendant linked NHC 1, [C{(MES)N(CHCH)-N(CH 2 CH 2 NHtBu)}], or its lithium bromide carbene adduct 1-LiBr. The mono(amino-carbene) nickel complexes (2) are paramagnetic species bearing a tetrahedrally distorted Ni center featuring a bidentate coordination mode via NHC and amine dative bonds. In contrast, the bis(amino-carbene) nickel complexes (3) are proton NMR active, consisting of a square-planar geometry with two trans NHC moieties. Both complexes 2 and 3 contain nickel-carbene bonds that are comparable to the literature value. Two forms of isomeric products for 3 are observed in the solution structure, but only solid structural syn-trans isomers are obtained, despite numerous attempts using various solvents with different polarity. On the basis of the powder X-ray analysis and various spectroscopy and theoretical results, we have ruled out a cis-trans isomerization, but confirmed a syn-anti conformational equilibrium exhibited in the solution of 3. The variable-temperature NMR for complexes 3 are consistent with restricted rotation about the nickel-carbene carbon bond. Plots of K eq vs 1/T are obtained with calculated ΔH°of 6.03 and 7.61 kJ/mol and ΔS°of 16.1 and 24.1 J/mol 3 K in toluene-d 8 for 3-Br and 3-Cl, respectively, indicating that syn-anti conformers of 3 are more stable. The outcome of the theoretical calculation is in line with the experimental result.