Imidazole Phosphines: Synthesis, Reaction Chemistry, and Their Use in Suzuki C,C Cross-Coupling Reactions

A straightforward consecutive synthesis methodology for the preparation of phosphino imidazoles 1-(4-PR₂-C₆H₄)-4,5-Me₂-1H-C₃HN₂ (4a, R = C₆H₅; 4b, R = ^cC₆H₁₁) and 1-(4-PR₂-C₆H₄)-2-PR′₂-4,5-Me₂-1H-C₃N₂ (R = C₆H₅: 6a, R′ = C₆H₅; 6b, R′ = ^cC₆H₁₁; 6c, R′ = ^cC₄H₃O; R = ^cC₆H₁₁: 6d, R′ = C₆H₅; 6e, R′ = ^cC₆H₁₁; 6f, R′ = ^cC₄H₃O) is presented. Phosphino imidazoles 6a–f were reacted with [PdCl₂(SEt₂)₂] (7), giving [Pd­(1-(4-PR₂-C₆H₄)-2-PR′₂-4,5-Me₂-1H-C₃N₂)­Cl₂]₂. Single crystals of [Pd­(1-(4-P­(C₆H₅)₂-C₆H₄)-2-P­(^cC₄H₃O)₂-4,5-Me₂-1H-C₃N₂)­Cl₂]₂ (8) suitable for single-crystal X-ray structure analysis could be obtained by using the synthesis-cum-diffusion strategy, confirming the formation of a neutral 18-membered Pd₂P₄ cycle with two trans-configurated palladium centers. A Pt₄P₄ cyclic compound, accessible by molecular recognition, was obtained via treatment of [Pt­(dppf)­(CC-C₆H₄-4-P­(C₆H₅)₂)₂] (dppf = 1,1′-bis­(diphenylphosphino)­ferrocene) (11) with [PtCl₂(SEt₂)₂] (12). The structure of 13 in the solid state was confirmed by crystal structure determination, proving the formation of a neutral molecular square composed of Pt­(dppf) and PtCl₂ corner units and 4-(C₆H₅)₂P-C₆H₄-CC linkers. In addition, compounds 6a–f were applied in the palladium-promoted Suzuki cross-coupling of 2-bromotoluene with phenylboronic acid using potassium carbonate as base. All in situ generated phosphino imidazole palladium species showed high catalytic activity at which the diphosphino systems featuring phenyl and cyclohexyl groups achieved the best results. Additionally, phosphine 6d was applied in the coupling of 4-chlorotoluene with phenylboronic acid and in the synthesis of sterically hindered biaryls under mild reaction conditions, showing an excellent performance. In comparison with other catalytically active species, equal or higher productivities were obtained using lower catalyst loadings and lower temperatures.