Catalytic Synthesis of Bi- and Teraryls in Aqueous Medium with Palladium(II) Complexes of 2-(Pyridine-2-ylmethylsulfanyl)benzoic Acid

• Published in: European journal of inorganic chemistry 2015-01, Vol.2015 (3), p.520-526
• Main Authors: Prakash, Om, Joshi, Hemant, Sharma, Kamal Nayan, Sharma, Alpesh Kumar, Singh, Ajai K.
• Format: Article
• Language: English
• Subjects: Bonding; C-C coupling; Catalysis; Catalysts; Chemical reactions; Joining; Nanoparticles; NMR spectroscopy; Palladium; S ligands
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201403003

Suzuki–Miyaura coupling reactions of phenylboronic and 1,4‐phenylenediboronic acid with ArBr to form bi‐ and teraryls have been efficiently catalyzed by the air‐ and moisture‐insensitive complexes [PdCl2L1] [1, L1 = 2‐(pyridine‐2‐ylmethyl)sulfanylbenzoic acid; 0.05–0.5 mol‐%] and [Pd(L2‐H)2] (2, L2‐H = 2‐pyridin‐2‐yl‐benzo[b]thiophen‐3‐ol; 0.01–0.5 mol‐%). The complexes were formed by the reaction of [Pd(CH3CN)2Cl2] with L1, and the catalysis proceeds in water for 1. The loading of 0.1–0.5 mol‐% of Pd is very promising for the coupling reactions to form teraryls. The COOH group imparts solubility to 1 in water. Ligand L2 is formed by the unprecedented cyclization of L1 in the course of the complexation reaction. The yield of 2 increases with the reaction time (5 h: 70 %; 24 h: 85 %). The two complexes and L1 have been characterized by NMR spectroscopy (1H and 13C{1H}). The single‐crystal structures of 1 and 2 have been solved, and the Pd–S, Pd–N, and Pd–O bond lengths are 2.269(2), 1.999(6)–2.057(2), and 1.9787(17) Å, respectively. Nanoparticles (NPs) with a narrow size distribution (ca. 3.0–5.5 nm for 80–85 % of the particles) formed at the start of these reactions and appear to be important for the catalytic coupling. Poisoning experiments and a two‐phase test have shown that the catalysis is largely homogeneous and involves [Pd0–PdII] processes. The bond lengths and angles calculated by DFT are consistent with the experimental ones. Complexes [PdCl2L1] [1, L1 = 2‐(pyridine‐2‐ylmethyl)sulfanylbenzoic acid] and [Pd(L2‐H)2] (2, L2‐H = 2‐pyridin‐2‐yl‐benzo[b]thiophen‐3‐ol) catalyze Suzuki coupling (1 in water) reactions to form bi‐ and teraryls. L2 forms by an unprecedented cyclization of L1. Pd nanoparticles (NPs) generated in situ play a role in the catalysis, which is a combination of homogeneous and heterogeneous processes.