Osmium-mediated C-H and C-C bond cleavage of a phenolic substrate: p-quinone methide and methylene arenium pincer complexes

The diphosphine 2,4,6(CH3),-315-(iPr(2)PCH(2))(2)C6OH (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-(iPr(2)PCH(2))(2)C-6}(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(iPr(2)PCH(2))(2)C-6](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)-(CH4)-1-(OH)-2,6-(CH3)(2)-3,5(iPr(2)PCH(2))(2)C-6}(Cl)(H)(PPh3)]- [OSO2CF3] (5a). The diphosphine I reacts with [OSCl2(PPh3)(3)] at 100 degrees C under H-2 to afford [Os{1-(OH)-2,6-(CH3)2-3,5-(iPr(2)pCH(2))(2)C-6}(Cl)(H-2)- (PPh3)] (6), a PCP pincer complex resulting formally from C(sp(2))-C(sp(3)) cleavage of the C-CH3 group in 1. CC hydrogenolysis resulting in the same complex is achieved by heating 2 under H-2 pressure. Reaction of the diphosphine substrate with [OsCl2(PPh3)(3)] under H-2 at lower temperature allows the observation of a methylene aremum derivative resulting from C-H activation, [Os{4-(CH2)-1-(OH)-2,6(CH3)(2)-3,5-(iPr(2)pCH(2))(2)C-6}(Cl)(2)(H)] (7). This compound reacts with PPh3 in toluene to afford the ionic derivative [Os[4-(CH2)-l-(OH)-2,6-(CH3)(2)-3,5-(iPr(2)PCH(2))(2)C-6}(Cl)(H)(PPh3)]Cl (5b). X-ray diffraction studies have been carried out on compounds 2, 3, 4, 5b, 6, and 7, which allows the study of the structural variations when going from methylene arenium to quinone methide derivatives.