Heterotopic Bidentate NHC Ligands and Their Late Transition Metal Complexes-Synthesis and Catalysis

In this thesis, the cationic rhodium (I) complexes bearing with hemilabile NHC bidentate ligands have been developed for the service as homogeneous catalysts using in the reactions such as hydrosilylation, hydroformylation, conjugate addition, and cycloaddition is studied.
The cationic rhodium (I) complexes bearing NHC bidentate ligands in the form of [Rh(COD)LN-C]+X- or in the form of [Rh(COD)LS-C]+X- are successfully synthesized via transmetalation of silver NHC complex to cationic rhodium (I) metal source. The rhodium carbene carbon of the cationic rhodium (I) complexes (4a-4i) show 13C NMR resonance in 176.0-177.0 ppm and JRh-C = 53.0-54.0 Hz. The cationic rhodium (I) pyrimidyl NHC complexes display excellent catalytic activity for the hydrosilytion of acetophenones.(98% yield)

The proton NMR of 4a shows the broad peaks in the COD and the singlet peak of the methylene protons of the thioether side arm at room temperature. To study the phenomenon in the solution containing 4a, the variable temperature NMR spectra are measured. According to the Gutowsky-Holm relationship24 and Eyring equation, the free energy of the sulfur inversion can be calculated in 48.61 KJmol-1.
The rhodium (I) thioether NHC complexes can be effective catalysts for hydroformylation of aromatic or aliphatic olefins, but the selectivity of linear/branch aldehydes is fair (the ratio of linear/branch is almost 1). By adding phosphines, we can tune the ratio of linear/branch to be all branched aldehyde but the conversion is low (25% conversion). We can reduce the pressure of syn gas (H2/CO=1/1) to 300 psi instead of high pressure (1000 psi). The conversion of the hydroformylation can be quantitative and the ratio of the linear/branched aldehyde can be 0.93-1.23.
The rhodium (I) thioether NHC complexes are efficient catalysts for the conjugate addition of boronic acids to enones. The catalyst loading was reduced to 0.5 mol% instead of the 3 mol% catalyst loading. The electron deficient or electron rich aryl boronic acids cannot retard the reaction. In spite of the bulky substituent such as o-methoxyphenyl boronic acid, the yield of Michael addition product was obtained in quantitative yield (98%). If thiol replaced the boronic acid, the thia- Michael addition can also be excellent yied (95%) by using rhodium thioether carbene catalyst.
[2+2+2] Cycloaddition of DEAD or DMAD can be achieved in aqueous solution by using rhodium (I) thioether carbene catalyst. The DEAD and diyne can be different alkyne moiety and cyclotrimerize to form the cyclic benezene derivative.
Pd (II) complexes bearing bidentate pyrimidyl-N-heterocyclic carbene ligands in the form of [LN-C]PdCl2 (LN-C = 2-pyrimidyl-imidazolylidene-NR, R = Me (5a), PhCH2 (5b), 2,6-Me2C6H3 (5d), 2,4,6-Me3C6H2 (5c)) have been synthesized and structurally characterized. The pyrimidyl-NHC ligand can facilitate these complexes for Suzuki-Miyaura cross coupling of aryl bromides and boronic acids.