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 1a–1i, α-ketols 1j–1o, or 1,2-diols dihydro-1j–1o with ethylene 2a to form ethylated tertiary
alcohols 3a–3o. As illustrated in
couplings of 1-octene 2b with vicinally dioxygenated
reactants 1a, 1b, 1i, 1j, 1k, 1m, higher α-olefins
are converted to adducts 4a, 4b, 4i, 4j, 4k, 4m with complete
levels of branched regioselectivity. Oxidation level independent C–C
coupling is demonstrated by the reaction of 1-octene 2b with diol dihydro-1k, α-ketol 1k, and dione dehydro-1k. Functionalized
olefins 2c–2f react with ethyl mandelate 1a to furnish adducts 5a–8a 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 C-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.