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.