Palladium-ADC complexes as efficient catalysts in copper-free and room temperature Sonogashira coupling

The metal-mediated coupling between cis-[PdCl2(CNR1)(2)] [R-1 = cyclohexyl (Cy) 1, t-Bu 2, 2,6-Me2C6H3 (Xyl) 3, 2-Cl-6-MeC6H3 4] and hydrazones H2NNCR2R3 [R-2, R-3 = Ph 5; R-2, R-3 = C6H4(OMe-4) 6; R-2/R-3 = 9-fluorenyl 7; R-2 = H, R-3 = C6H4(OH-2) 8] provided carbene complexes cis-[PdCl2{C(N(H)N=CR2R3)N(H)R-1}(CNR1)] (9-24) in good (80-85%) yields. Complexes 9-24 showed high activity [yields up to 99%, and turnover numbers (TONs) up to 3.7 x 10(4)] in the Sonogashira coupling of various aryl iodides with a range of substituted aromatic alkynes without the need of copper co-catalyst. The catalytic procedure runs at 80 degrees C for 1 h in EtOH using K2CO3 as a base. No formation of homocoupling or acetylene decomposition products was observed. Designed copper-free Sonogashira system can also run at room temperature giving target products with yields up to 87% and TONs up to 87. (C) 2014 Elsevier B.V. All rights reserved.