Cu(II)-silsesquioxanes as efficient precatalysts for Chan-Evans-Lam coupling

Two cage copper(II)silsesquioxanes, namely, tricopper complex (PhSiO1.5)(8)(CuO)(3)(TMEDA)(2)*(MeCN)(3) Cu-1 and hexacopper complex (MeSiO1,5)(12)(CuO)(6)(Py)(6)*TMEDA Cu-2 were synthesized (in 30% and 16% yield, respectively) via TMEDA-assisted reactions (TMEDA = tetramethylethylenediamine). Structural features of both products were established by X-ray diffraction study. Complexes Cu-1 and Cu-2, along with some Cu(II)-silsesquioxanes reported earlier, were evaluated for the first time as precatalysts for the homogeneous Chan-Evans-Lam (CEL) O-arylation of benzoic acids (the highest yield of phenylbenzoic ester was 84%). Observed catalytic activity was superior to reference catalyst Cu(OAc)(2)*H2O. Finally, an application of coppersilsesquioxanes allowed to observe the first example of C-O CEL coupling under microwave activation characterized by (i) reduced loading of copper, (ii) low amount of added base, (iii) minimization of the time of reaction. (C) 2019 Elsevier B.V. All rights reserved.