Osmium(0)-Catalyzed C–C Coupling of Ethylene and α‑Olefins with Diols, Ketols, or Hydroxy Esters via Transfer Hydrogenation
journal contributionposted on 31.08.2016, 00:00 by Boyoung Y. Park, Tom Luong, Hiroki Sato, Michael J. Krische
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.