Reactivity of Low Valent Magnesium(I) Complexes with Epoxides and Episulfides

Chemical C-H functionalisation is traditionally the domain of potent organolithium bases that require cryogenic conditions, whereas biologically inspired C-H functionalisation can be executed by transition metal-oxo species. Blending these chemical and biological approaches, this work exploits magnesium-oxo complexes for selective C-H deprotonation at ambient temperature. Overall, three low valent magnesium(I) dimers, [^DippNacnacMg]₂ 1, [^DepNacnacMg]₂ 2 and [^MesNacnacMg]₂ 3, react with two equivalents of propylene oxide to generate dinuclear Mg complexes of general formula [NacnacMg(OH)(OCH₂CHCH₂)MgNacnac] (Nacnac = HC(C(CH₃)NAr)₂). Isolation of oxo-bridged [^DippNacnacMg-O-Mg^DippNacnac] coupled with deuterium labeling studies shows that the reaction proceeds via epoxide deoxygenation, followed by deprotonation and rearrangement of a second equivalent of the epoxide to generate the alkoxide. A range of terminal and internal epoxides was investigated (propylene oxide, butylene oxide, isobutylene oxide, and cyclohexene oxide). The β-C-H position was selectively deprotonated in each case, as confirmed by NMR spectroscopy and X-ray diffraction studies. Exchanging epoxides for episulfides instead generated disulfide-bridged species, showcasing the difference in Brønsted basicity between Mg-O and Mg-S functional groups. Overall, this work demonstrates the unharnessed potential of main group metal-oxo complexes for selective C-H deprotonation.