Syntheses, Structural Aspects, Solution Behavior, and Catalytic Utility of Cyclopalladated N,N ',N ''-Triarylguanidines [kappa(2)(C,N))Pd(Pyrazole)(2)X] (X = Br, OC(O)CF3, and PF6) in Suzuki-Miyaura Coupling Reactions of Aryl Bromides

摘要

The reactions of six-membered cyclopalladated N,N',N ''-triarylguanidines [kappa(2)(C,N)Pd(mu-X)](2) (3-7) with 2 equiv of pyrazole (pzH) and 3,5-dimethylpyrazole (3,5-dmpzH) in CH2Cl2 at RT for 24 h afforded a new class of cyclopalladated N,N',N ''-triarylguanidines, [kappa(2)(C,N)Pd(PzH)(2)X] (Ar = 2-MeC6H4; X = Br; PzH = 3,5-dmpzH (8), pzH (9); X = OC(O)CF3; PzH = 3,5-dmpzH (10); Ar = 2-(MeO)C6H4; X = Br; PzH = 3,5-dmpzH (11); OC(O)CF3 (12); Ar = Ph; X = OC(O)CF3; PzH = 3,5-dmpzH (13)) in good yields. The reaction of 8 with NH4PF6 in CH2Cl2 at RT for 24 h afforded [kappa(2)(C,N)Pd(3,5-dmpzH)(2)(PF6)] (14) in 83% yield. Complexes 8-14 were characterized by elemental analyses, IR, NMR (H-1, C-13, and F-19) spectroscopic techniques, and conductivity measurements. Molecular structures of six-membered cyclopalladated N,N',N ''-triphenylguanidine [kappa(2)(C,N)Pd(mu-OC(O)CF3)](2)PhMe (7PhMe), 8, 10, 12, and 14 were determined by single-crystal X-ray diffraction, which revealed transoid in-in (7PhMe), beta-out (10 and 14), and a-out (12) conformations. The eight-membered "[Pd(kappa N-1-PzH)(2)X]" rings in 8, 10, 12, and 14 that consist of a pair of NHX hydrogen bonds are hitherto unknown in palladacyclic chemistry. VT H-19 NMR (CD2Cl2, 400 MHz) spectra of 10 revealed the presence of a mixture of three species, while VT F-19 NMR (CD2Cl2, 376.2 MHz) spectra revealed the presence of a mixture of seven species at 213 K. The presence of more than one solution species of 10 was explained by invoking the presence of a mixture of conformers and intermediates formed between a pair of related conformers. The catalytic efficacy of 8-14 in Suzuki-Miyaura coupling reactions of various aryl bromides with PhB(OH)(2) was evaluated, and these cyclopalladated guanidines showed a wide substrate scope in the coupling reactions with low catalyst loadings (1.00, 0.1, and 0.01 mol %) under mild reaction conditions.