Osmium-mediated C-H and C-C bond cleavage of a phenolic substrate: p-quinone methide and methylene arenium pincer complexes
Résumé
The diphosphine 2,4,6-(CH3)3-3,5-(iPr2PCH2)2C6OH (1) reacts with [OsCl2(PPh3)3] in presence of an excess of triethylamine to yield the isomeric para-quinone methide derivatives [Os{4-(CH2)-1-(O)-2,6-(CH3)2-3,5-(iPr2PCH2)2C6}(Cl)(H)(PPh3)] (2 and 3), which differ in the positions of the mutually trans hydride and chloride ligands. Complex 2 reacts with CO to afford the dicarbonyl species [Os{1-(O)-2,4,6-(CH3)3-3,5-(iPr2PCH2)2C6}(Cl)(CO)2] (4), which results from hydride insertion into the quinonic double bond. Protonation of 2 and 3 leads to the formation of the methylene arenium derivative [Os{4-(CH2)-1-(OH)-2,6-(CH3)2-3,5-(iPr2PCH2)2C6}(Cl)(H)(PPh3)][OSO2CF3] (5 a). The diphosphine 1 reacts with [OsCl2(PPh3)3] at 100 °C under H2 to afford [Os{1-(OH)-2,6-(CH3)2-3,5-(iPr2PCH2)2C6}(Cl)(H2)(PPh3)] (6), a PCP pincer complex resulting formally from C(sp2)C(sp3) cleavage of the CCH3 group in 1. CC hydrogenolysis resulting in the same complex is achieved by heating 2 under H2 pressure. Reaction of the diphosphine substrate with [OsCl2(PPh3)3] under H2 at lower temperature allows the observation of a methylene arenium derivative resulting from CH activation, [Os{4-(CH2)-1-(OH)-2,6-(CH3)2-3,5-(iPr2PCH2)2C6}(Cl)2(H)] (7). This compound reacts with PPh3 in toluene to afford the ionic derivative [Os{4-(CH2)-1-(OH)-2,6-(CH3)2-3,5-(iPr2PCH2)2C6}(Cl)(H)(PPh3)]Cl (5 b). X-ray diffraction studies have been carried out on compounds 2, 3, 4, 5 b, 6, and 7, which allows the study of the structural variations when going from methylene arenium to quinone methide derivatives.