Palladium(II) Complexes of (2,6‐Dibenzhydryl‐4‐methylphenyl)diphenyl‐phosphane: Synthesis, Structural Studies, and Catalytic Arylation of Imidazoles Under Aerobic Conditions

Abstract

The synthesis and structural studies of palladium(II) complexes of sterically demanding mono‐phosphine Ar*PPh2 (1) (Ar*=2,6‐dibenzhydryl‐4‐methylphenyl) are described. The reactions of 1 with [Pd(COD)Cl2] and [Pd(η3‐C3H5)Cl]2 in 2 : 1 molar ratios afforded mononuclear complexes trans‐[PdCl2{(PPh2Ar*)‐κ1‐P}2] (2) and [Pd(η3‐C3H5)Cl{(PPh2Ar*)‐κ1‐P}] (3), respectively. Both the complexes have been fully characterized through NMR spectroscopy, elemental analyses, and single‐crystal X‐ray analysis. The molecular structures of 2 and 3 showed C−H⋅⋅⋅π interactions between methine hydrogen of one of the Ph2CH groups with the centroid of one of the phenyl rings of the PPh2 group. Complex 2 also showed intramolecular π⋅⋅⋅π stacking interactions between the phenyl rings of Ph2CH and PPh2 groups. In addition, complex 2 also showed C−H⋅⋅⋅Pd anagostic interaction between the methine hydrogen of one of the Ph2CH groups and the palladium center. The palladium complex 2 is found to be an efficient catalyst for the direct C‐5 arylation of imidazole derivatives under aerobic conditions.

Organosulphur and organoselenium ligands in designing metal‐based catalytic systems for C—H arylation of imidazole

“…Such ligands are either carbene precursors or compounds having nitrogen, oxygen and/or phosphorus donors in their framework. 33,[36][37][38][39]42,56,57,72 Generally, many such systems are not known to possess the advantage of wide substrate scope. They require harsh reaction conditions.…”

“…Complexes (e.g., 14, 15, and 16) catalyze C H arylation of imidazole and its derivatives. 36,38,39 The complex, 14 is used at 4.0 mol% concentration to catalyze (Scheme 31) a reaction between 1-methyl-1H-imidazole-4-carbonitrile and ethyl 4-bromo-S C H E M E 2 8 Effect of donor site (sulphur or selenium) on the ligand. 66 S C H E M E 2 9 Effect of binding mode of ligand with metal on catalytic performance.…”

“…It can catalyze the reactions between various aryl iodide(s) and 1-methyl-1H-imidazole. It is used at 3.0 mol% concentration and gives the product in 15%-71% yield when the reaction is carried out at 150 C for 24 h. 38 The 16 is an interesting complex in which palladium is coordinated (Scheme 33) with carbene and phosphorus donor site of the ligand. 36 When it is applied as the catalyst in high concentration, it works at relatively high (140 C) temperature and give the product in moderate yield (68%) in long reaction time of 18 h. 36 On the other hand, a structurally similar complex 9 has a palladium selenium bond.…”

“…[28][29][30][31][32][33][34][35] The most commonly used ligands with palladium are sterically bulky electron rich phosphine or carbene ligands. [36][37][38][39] The main demerits are that the reactions are not highly regioselective (in context of the two reaction sites, i.e., C-2 and C-5 positions) and require the catalyst(s) in very high concentration. 40 Moreover, high reaction temperature ($140 C) and long reaction time 30,41 along with limited substrate scope, 30,40,42 low functional group tolerance, 30,40,42 and requirement of inert atmosphere are the features making one look forward for better alternatives.…”

Unraveling Ultralong Phosphorescence in Ar2PO(H): n(O) → σ*(P-C) Transitions in Ar2PO(H) Stabilize Triplet States Better than n(P) → σ*(P-C) in Ar3P

“…Recently, phosphorus-based organic luminophores have been widely explored for optoelectronic applications owing to their unique geometry and scope for chemical modification via oxidation of the phosphorus center. − In this context, triarylphosphine oxides have enormous applications in catalysis, materials, medicines, etc. − However, the luminescence properties of these molecules are seldom reported in the literature. According to El-Sayed’s rule, the spin-forbidden intersystem crossing from S 1 → T n can be facilitated via n → π* transition in the presence of heteroatoms containing at least a lone pair of electrons .…”