Phosphino imidazoles and imidazolium salts for Suzuki C–C coupling reactions

Abstract

The consecutive syntheses of imidazoles 1-(4-X-C₆H₄)-4,5-R₂-^cC₃HN₂ (3a, X = Br, R = H; 3b, X = I, R = Me; 3c, X = H, R = Me; 5, X = Fc, R = H; 7, X = CCFc, R = H; 9, X = C₆H₅, R = Me; Fc = Fe(η⁵-C₅H₄)(η⁵-C₅H₅)), phosphino imidazoles 1-(4-X-C₆H₄)-2-PR′₂-4,5-R₂-^cC₃N₂ (11a–k; X = Br, I, Fc, FcCC, Ph; R = H, Me; R′ = Ph, ^cC₆H₁₁, ^cC₄H₃O), imidazolium salts [1-(4-X-C₆H₄)-3-R′′-4,5-R₂-^cC₃HN₂]I (16a; X = Br, R = H, R′′ = n-Bu; 16b, X = Br, R = H, R′′ = n-C₈H₁₇; 16c, X = I, R = Me, R′′ = n-C₈H₁₇, 16d, X = H, R = Me, R′′ = n-C₈H₁₇) and phosphino imidazolium salts [1-C₆H₅-2-PR′₂-3-n-C₈H₁₇-4,5-Me₂-^cC₃N₂]PF₆ (17a, R′ = C₆H₅; 17b, R′ = ^cC₆H₁₁) or [1-(4-P(C₆H₅)₂-C₆H₄)-3-n-C₈H₁₇-4,5-Me₂-^cC₃HN₂]PF₆, (20) and their selenium derivatives 1-(4-X-C₆H₄)-2-P(Se)R′₂-4,5-R₂-^cC₃N₂ (11a-Se–f-Se; X = Br, I; R = H, Me; R′ = C₆H₅, ^cC₆H₁₁, ^cC₄H₃O) are reported. The structures of 11a-Se and [(1-(4-Br-C₆H₄)-^cC₃H₂N₂-3-n-Bu)₂PdI₂] (19) in the solid state were determined. Cyclovoltammetric measurements were performed with the ferrocenyl-containing molecules 5 and 7 showing reversible redox events at E⁰ = 0.108 V (ΔE_p = 0.114 V) (5) and E⁰ = 0.183 V (ΔE_p = 0.102 V) (7) indicating that 7 is more difficult to oxidise. Imidazole oxidation does not occur up to 1.3 V in dichloromethane using [(n-Bu)₄N][B(C₆F₅)₄] as supporting electrolyte, whereas an irreversible reduction is observed between −1.2–−1.5 V. The phosphino imidazoles 11a–k and the imidazolium salts 17a,b and 20, respectively, were applied in the Suzuki C–C cross-coupling of 2-bromo toluene with phenylboronic acid applying [Pd(OAc)₂] as palladium source. Depending on the electronic character of 11a–k, 17a,b and 20 the catalytic performance of the in situ generated catalytic active species can be predicted. As assumed, more electron-rich phosphines with their higher donor capability show higher activity and productivity. Additionally, 11e was applied in the coupling of 4-chloro toluene with phenylboronic acid showing an excellent catalytic performance when compared to catalysts used by Fu, Beller and Buchwald. Furthermore, 11e is eligible for the synthesis of sterically hindered biaryls under mild reaction conditions. C–C Coupling reactions with the phosphino imidazolium salts 17b and 20 in ionic liquids [BMIM][PF₆] and [BDMIM][BF₄] were performed, showing less activity than in common organic solvents.