We report the first hydrogen-mediated C−C bond formations catalyzed by iridium. Hydrogenation of commercially available alkyl-substituted alkynes, 3-hexyne 1a and 1-phenylpropyne 1b, in the presence of α-ketoesters 2a − 2g using cationic iridium precatalysts results in reductive C−C coupling to afford β,γ-unsaturated α-hydroxyesters 3a − 3f and 4a − 4e and 4g, which are formed in excellent yield, with complete control of olefin geometry and, in most cases, with excellent regiocontrol. Similar levels of selectivity are observed across a range of alkynes 1c − 1h . Reductive coupling of 3-hexyne 1a and α-ketoester 2a under an atmosphere of elemental deuterium provides the monodeuterated product deuterio- 3a, consistent with a catalytic mechanism involving alkyne−carbonyl oxidative coupling followed by hydrogenolysis of the resulting oxametallacycle.
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Ngai et al. (2006) studied this question.