Cβ(sp2)-H bond activation of α,β-unsaturated ketones promoted by a hydride-elongated dihydrogen complex: formation of osmafuran derivatives with carbene, carbyne, and nh-tautomerized α-substituted pyridine ligands

Abstract

Complex [OsH(η2-H2)(η2-CH 2=CH-o-C5H4N)(PiPr3) 2]BF4 (1) reacts with methyl vinyl ketone in the absence of solvent to give [OsH{CHCHC(O)CH3}2(PiPr 3)2]BF4 (2), which can be described as two osmafurans joined by a common [OsH(PiPr3) 2]+ fragment. The hydride ligand of 2 is fairly acidic. Thus, its treatment with sodium methoxide produces the deprotonation of the metal center to give Os{CHCHC(O)CH3}2(PiPr 3)2 (3). The reaction is not reversible; the protonation of 3 with HBF4·OEt2 leads to [Os{CHCHC(O)CH 3} {=CHCH2C(=O)CH3} (PiPr 3)2]BF4 (4), which in dichloromethane is unstable and evolves into [Os{CHCHC(O)CH3} Cl{≡CCH=C(CH 3)OH} (PiPr3)2]BF4 (5), containing an enolcarbyne ligand. In the solid state the anion of 5 is associated with the OH-hydrogen of the enol, by means of an H⋯F hydrogen bond. In dichloromethane the H⋯F hydrogen bond is broken and DFT calculations suggest that the OH-hydrogen atom forms an H⋯Cl hydrogen bond with the chlorine ligand. In the absence of solvent, complex 1 reacts with benzylideneacetophenone and benzylideneacetone to give [Os{C(Ph)CHC(O)R} (η2-H2){κ-C-[HNC5H3Et]} (PiPr3)2]BF4 (R = Ph (6), CH 3 (10)), containing a NH-tautomerized 2-ethylpyridine ligand. Complexes [Os{C(Ph)CHC(O)R}(η2-H2){ (CH 3CN)}(PiPr3)2]BF4 (R = Ph (8), CH3 (H)) and [Os{C6H4C(O)CH=CHPh (η2-H2) {(CH3CN)}(PiPr 3)2]BF4 (9) have been also isolated and characterized. The X-ray structures of 2, 5, and 6 are reported. © 2008 American Chemical Society.