Osmium(0)-Catalyzed C–C Coupling of Ethylene and α‑Olefins with Diols, Ketols, or Hydroxy Esters via Transfer Hydrogenation

Osmium­(0) complexes derived from Os₃(CO)₁₂ and XPhos (2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl) catalyze the C–C coupling of α-hydroxy esters <b>1a</b>–<b>1i</b>, α-ketols <b>1j</b>–<b>1o</b>, or 1,2-diols <i>dihydro</i>-<b>1j</b>–<b>1o</b> with ethylene <b>2a</b> to form ethylated tertiary alcohols <b>3a</b>–<b>3o</b>. As illustrated in couplings of 1-octene <b>2b</b> with vicinally dioxygenated reactants <b>1a</b>, <b>1b</b>, <b>1i</b>, <b>1j</b>, <b>1k</b>, <b>1m</b>, higher α-olefins are converted to adducts <b>4a</b>, <b>4b</b>, <b>4i</b>, <b>4j</b>, <b>4k</b>, <b>4m</b> with complete levels of branched regioselectivity. Oxidation level independent C–C coupling is demonstrated by the reaction of 1-octene <b>2b</b> with diol <i>dihydro</i>-<b>1k</b>, α-ketol <b>1k</b>, and dione <i>dehydro</i>-<b>1k</b>. Functionalized olefins <b>2c</b>–<b>2f</b> react with ethyl mandelate <b>1a</b> to furnish adducts <b>5a</b>–<b>8a</b> as single regioisomers. The collective data, including deuterium labeling studies, are consistent with a catalytic mechanism involving olefin–dione oxidative coupling to form an oxa-osmacyclopentane, which upon reductive cleavage via hydrogen transfer from the secondary alcohol reactant releases the product of carbinol <i>C</i>-alkylation with regeneration of the ketone. Single-crystal X-ray diffraction data of the dinuclear complex Os₂(CO)₄(O₂CR)₂(XPhos)₂ and the trinuclear complex Os₃(CO)₁₁(XPhos) are reported. These studies suggest increased π-backbonding at the stage of the metal–olefin π-complex plays a critical role in facilitating alkene–carbonyl oxidative coupling, as isostructural ruthenium(0) complexes, which are weaker π-donors, do not catalyze the transformations reported herein