Preparation of cobalt-containing bulky monodentate phosphines with electron-withdrawing/donating substituents on bridged arylethynyl and their applications in Suzuki coupling reactions

Several cobalt-containing bulky monodentate phosphines (μ-PPh2CH2PPh2)Co2(CO)4(μ,η-(tBu)2PCtriple bond; length of mdashC(C6H4R)) (4a: R=H; 4b: R=p-F; 4cp: R=p-CF3; 4cm: R=m-CF3; 4d: R=p-OMe) were prepared from the reactions of (tBu)2PCtriple bond; length of mdashC(R–C6H4) (2a: R=H; 2b: R=p-F; 2cp: R=p-CF3; 2cm: R=m-CF3; 2d: R=p-OMe) with Co2(CO)6(μ-PPh2CH2PPh2) 3. Further reactions of 4a, 4b, 4cp, 4cm, and 4d with Pd(OAc)2 yielded unique palladium complexes (μ-PPh2CH2PPh2)Co2(CO)4(μ,η-(tBu)2PCtriple bond; length of mdashC(C6H3R)-κC1)Pd(μ-OAc) (5a: R=H; 5b: R=p-F; 5cp: R=p-CF3; 5cm: R=m-CF3; 5d: R=p-OMe, respectively). The strong electron-withdrawing substituents, –F and –CF3, assist the ortho-metalation process during the formation of 5b, 5cp, and 5cm. The more positively charged palladium center in 5b (or 5cp, 5cm) enhances the probability for PhB(OH)3− to attack the metal center and the rate of reduction thereafter. DFT studies on the charges of these palladium centers support this assumption. The enhancement of the reaction rates of the Suzuki–Miyaura cross-coupling reactions using 5b, 5cp, and 5cm is thereby attributed to this effect.