Palladium selenolate macrocycles of diphosphines as efficient catalysts for Sonogashira reactions

Three Pd(II) complexes of 4-pyridylselenolate [Pd(P^∩P)(4-Sepy)]_n(OTf)_n, namely, the previously reported P^∩P = dppe (1) and dppf (2) complexes and a new dtbpf (1,1′-bis(di-tert-butylphosphino)ferrocene) derivative (3), were investigated as catalysts for Sonogashira reactions. For 1 and 2, dinuclear and tetranuclear N^∩Se-bridged macrocycles were observed through NMR spectra and X-ray structures, while for 3, only a dinuclear Se-bridged complex was observed from the NMR data. DFT calculations of 3 support the long-wavelength band maxima at 466 nm, which is very close to the experimental value of 461 nm. The catalytic activity of the three materials and previously reported complexes [Pd(Xantphos)(4-Spy)](OTf) (4) and mononuclear precursor complexes of dppf was tested for Sonogashira coupling reactions using phenyl acetylene and various aryl bromides. Comparisons under optimized conditions showed a high turnover number (TON) of up to 950 with 95% yield for 2, while 1 gave TONs of only 840 (84%) and 800 (80%) for 4. Compound 3 seemed to be rather inactive. Upon further optimization of catalyst 2, a TON of 500 000 was attained with 50% yield.