Synthesis, Characterization, and Catalytic Application of Mononuclear and Dendritic Cationic CuI Iminopyridine‐Ligated Complexes in Aryl Iodide Hydroxylation

Abstract

A series of mononuclear (C1–C4) and dendritic G1 (DC1–DC4) and G2 (DC5–DC8) cationic CuI iminopyridine complexes of the general formula [Cu{(R‐C5H3N)CH=N(nPr)‐κ2‐N,N}2][BF4] (C1: R = 6‐Me; C2: R = H; C3: R = 6‐Br; C4: R = C4H3) and [DAB‐Gx‐PPI‐(Cu{(R‐C5H3N)‐κ2‐N,N}y)z][BF4]n [DAB: 1,4‐diaminobutane; PPI: poly(propyleneimine); for G1: x = 1, y = 4, z = 2, n = 2; for G2: x = 2, y = 8, z = 4, n = 4; DC1/DC5: R = 6‐Me; DC2/DC6: R = H; DC3/DC7: R = 6‐Br; DC4/DC8: R = C4H3] have been prepared and characterized by a range of spectroscopic and analytical techniques. The mononuclear and dendritic complexes were found to be active catalysts for the hydroxylation of 4‐iodotoluene to p‐cresol in DMSO/H2O mixtures. A positive dendritic effect on catalytic activity was observed. Furthermore, our catalyst system was found to be active for the hydroxylation of 4‐iodotoluene in neat water. The active catalyst could be recycled twice before catalyst deactivation was observed. HRTEM analysis revealed that catalyst deactivation arose as a result of metal agglomeration. A series of poisoning experiments provided evidence for the mediation of hydroxylation by a homogeneous active species for both classes of pre‐catalysts, and a radical‐trapping experiment in combination with our experimental observations provided evidence that the reaction proceeds through a similar mechanism to that reported for Cu‐catalyzed halide exchange.