Monosubstituted, Anionic Imidazolyl Ligands from N−H NHC Precursors and Their Activity in Pd‐Catalyzed Cross‐Coupling Reactions

Abstract

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Experiment and Theory of Bimetallic Pd-Catalyzed α-Arylation and Annulation for Naphthalene Synthesis

“…For example, when ligand 1 was mixed with PdCl 2 in methanol, only a monomeric Pd complex 2 was obtained, where Pd was inserted between the P–C bond of the ligand (Figure a). We recently demonstrated that this new N–H N-heterocyclic carbene–palladium complex is highly active in palladium-catalyzed Suzuki–Miyaura reactions . In contrast, we found that if ligand 1 was first coordinated with a boron Lewis acid to form 3 , formation of a bimetallic Pd­(II) complex 4 was observed upon addition of PdCl 2 in dioxane (Figure b).…”

2-Phosphinoimidazole Ligands: N–H NHC or P–N Coordination Complexes in Palladium-Catalyzed Suzuki–Miyaura Reactions of Aryl Chlorides

“…Tolman electronic parameter comparisons between this ligand and other common NHC ligands showed that the anionic imidazolyl ligand of 2a was significantly more donating in Suzuki–Miyaura cross-couplings. The v CO value was shifted by >40 cm –1 for DFT-calculated Ni­(CO) 3 (NHC) structures . Unfortunately, diarylphosphine complexes like 2a had very low reactivity toward cross-couplings with aryl chloride substrates.…”

“…The absence of methanol in the reactions leads to no product formation with 5b (entry 5) and 39% conversion with the mixture of 2c + 5c (entry 6). Previously we have found that methanol is essential for formation of the N–H NHC complex to occur, which suggests that the N–H NHC form of the t -butyl catalyst ( 2b ) may actually be present in the reaction. The fact that the mixture of 2c + 5c does give some product in the absence of methanol further supports our hypothesis that the N–H NHC form of the catalyst is important for efficient catalysis.…”

“…Recently, we reported the synthesis of monosubstituted N–H N -heterocyclic carbene (NHC) complexes of palladium and demonstrated their high activity in Suzuki–Miyaura reactions with aryl bromides . N–H NHC complexes and the corresponding imidazolyl-type ligands are known to enable efficient catalysis in cross-coupling and other classes of reactions by serving as bifunctional catalysts .…”

Effect of Precatalyst Oxidation State in C–N Cross-Couplings with 2-Phosphinoimidazole-Derived Bimetallic Pd(I) and Pd(II) Complexes

“…For the Buchwald–Hartwig reaction, we see only a slight shift in the 31 P NMR spectra for complex 1 and a small amount of free ligand (3.5:1 ratio of 1 to free ligand). The phosphorus shift for complex 1 is likely the result of chloride exchange . In addition, the mass spectrum (MS) of the crude reaction mixture shows mainly dimeric palladium species, suggesting that catalyst breakdown to a monometallic species is not particularly preferred.…”