Dinuclear Triazole-Derived Janus-Type N‑Heterocyclic Carbene Complexes of Palladium: Syntheses, Isomerizations, and Catalytic Studies toward Direct C5-Arylation of Imidazoles

2016-01-26T12:08:35Z (GMT) by Shuai Guo Han Vinh Huynh
The dipalladium triazolidine-diylidene complex <i>all-trans</i>-[PdBr<sub>2</sub>(CH<sub>3</sub>CN)]<sub>2</sub>(μ-ditz) (<b>1</b>) (ditz = 1,2,4-trimethyltriazolidine-3,5-diylidene) was synthesized via <i>in situ</i> deprotonation of the precursor salt with a basic metal precursor. Ligand replacements of <i>all</i><b>-</b><i>trans</i><b>-1</b> with monodentate or chelating phosphines afforded the dicarbene-bridged complexes <i>all-cis</i>-[PdBr<sub>2</sub>(PPh<sub>3</sub>)]<sub>2</sub>(μ-ditz) (<b>2</b>) and [PdBr­(DPPP)]<sub>2</sub>(μ-ditz)­Br<sub>2</sub> (<b>3</b>), respectively. Bromido substitution of <i>all-cis</i><b>-2</b> gave tetra-acetato complex <i>all-cis</i>-[Pd­(CH<sub>3</sub>COO)<sub>2</sub>(PPh<sub>3</sub>)]<sub>2</sub>(μ-ditz) (<b>4</b>) with retention of the configuration as the predominant product. In addition, monopalladium triazolin-5-ylidene complexes <i>trans</i>-[PdBr<sub>2</sub>(CH<sub>3</sub>CN)­(tazy)] (<b>6</b>, tazy =1,4-dimethyltriazolin-5-ylidene), <i>cis</i>-[PdBr<sub>2</sub>(PPh<sub>3</sub>)­(tazy)] (<b>7</b>), [PdBr­(DPPP)­(tazy)]­Br (<b>8</b>), and <i>cis</i>-[Pd­(CH<sub>3</sub>COO)<sub>2</sub>(PPh<sub>3</sub>)­(tazy)] (<b>9</b>) were also synthesized as the respective mononuclear equivalents for comparison. A comparative catalytic study revealed the general superiority of dinuclear complexes <b>1</b>–<b>4</b> over their respective mononuclear counterparts <b>6</b>–<b>9</b> in the direct C5-arylation reaction of 1-methylimidazoles. Overall, mixed dicarbene/diphosphine complex <b>3</b> showed the best catalytic performance.