A series of novel N,N′-bisaryl bis­(NHC) (NHC = N-heterocyclic carbene) precursors 5a–e, 12a–d, and 13a–c and their palladium complexes 14a–e, 15a–d, and 16a,b with (1R,2R)-cyclohexene and (1R,2R)-diphenylethylene linkers have been designed, synthesized in good yields, and fully characterized by NMR, HRMS, and elemental analysis. The X-ray single crystal analysis of 15a showed that these complexes adopted a slightly distorted square-planar geometry around the Pd­(II) center. These N,N′-bisaryl-bis­(NHC)-Pd complexes exhibit excellent catalytic activity in asymmetric aryl–aryl cross-coupling reactions of arylboronic acids and aryl halides. The axial chiral biaryl compounds could be achieved in high yields (up to 74%) and good enantioselectivities (up to 60% ee) within 24 h. The results show that for this kind of bis­(NHC) palladium catalysts the structure characteristics of the chiral linkers have decisive effect on the enantioselectivities of cross-coupling reactions.

A series of chiral bis(benzimidazolium) salts 10-19 with (1R,2R)-cyclohexene, (1R,2R)-diphenylethylene and (aR)-binaphthylene linkers have been designed and synthesized in 30-94% yield. Ten chiral bis(NHC) palladium complexes 20-28 have been synthesized and characterized by NMR, HRMS, elemental analysis and further confirmed by X-ray single crystal analysis. These bis(NHC)-Pd complexes showed obviously different catalytic properties in the asymmetric Suzuki-Miyaura coupling reactions. The (1R,2R)-cyclohexene-bridged bis(NHC)-Pd complex, (R,R)-23, achieved the highest yield of 90%, while complex (aR)-28, with a binaphthylene linker, showed the best enantioselectivity of 60 ee%. The structural analysis of these complexes suggested that such difference of catalytic performance has a close relationship with their coordination surroundings around metal centres.

The series of chiral 1,2-cyclohexane-bridged bis-N-heterocyclic carbene ligand precursors H2[(1R,2R)-(1a–i)]Br with different substituent groups and their neutral and cationic diaqua palladium complexes, namely {Pd[(1R,2R)-(1a–i)]Br2} (2a–i), {Pd[(1R,2R)-(1a)]X2} (X = Cl (3), OAc (4-OAc), OC(O)CF3 (4-OCH(O)CF 3 )), and {Pd[(1R,2R)-(L OMe )](OH2)2}X2 (X = OTf (5-OTf), SbF6 (5-SbF 6 )) have been prepared in moderate to good yields. These chiral palladium complexes were fully characterized by elemental analysis, high-resolution mass spectra, 1H and 13C NMR, and optical rotation determinations. The crystal structures of the chiral complexes 2a and 5-OTf were further confirmed to adopt a distorted-square-planar coordination geometry around the palladium center. The obtained chiral NHC-Pd compounds were able to catalyze the asymmetric Suzuki–Miyaura couplings of aryl halides with arylboronic acids in good yields (up to 96%) and moderate enantioselectivities (up to 64% ee). The coligand and steric effects were studied carefully. The coligands, including Br–, Cl–, AcO–, CF3COO–, and water molecules, have little influence on the catalytic results. However, a strong steric effect of the two aromatic substituents R on the enantiocontrol has been proved in the catalytic asymmetric Suzuki–Miyaura coupling reaction. The highest enantioselectivity of 64% ee could be achieved under the standard reaction conditions.

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Measurements are reported on the kinetics of the lysozyme-catalysed hydrolysis of several beta-aryl di-N-acetylchitobiosides, some of which have been synthesized for the first time. The catalytic rate constants (k(cat.)) at 45 degrees yield a curved Hammett plot (concave up) and the plot of DeltaH(double dagger) versus DeltaS(double dagger) has a sharp break. Substrates with electron-withdrawing groups exhibit a kinetic deuterium isotope effect (k(H) (cat.)/k(D) (cat.)), whereas those with electron-donating groups show no such isotope effect. The results suggest the operation of different mechanisms for the two types of substrates.