Abstract
The mechanism of the rhodium‐catalyzed reductive coupling of 1,3‐diynes and vicinal dicarbonyl compounds employing H~2~ as reductant was investigated by density functional theory. Oxidative coupling through 1,4‐addition of the Rh^I^‐bound dicarbonyl to the conjugated diyne via a seven‐membered cyclic cumulene transition state leads to exclusive formation of linear adducts. Diyne 1,4‐addition is much faster than the 1,2‐addition to simple alkynes. The 1,2‐dicarbonyl compound is bound to rhodium in a bidentate fashion during the oxidative coupling event. The chemo‐, regio‐, and enantioselectivities of this reaction were investigated and are attributed to this unique 1,4‐addition pathway. The close proximity of the ligand and the alkyne substituent distal to the forming CC bond controls the regio‐ and enantioselectivity: coupling occurs at the sterically more demanding alkyne terminus, which minimizes nonbonded interaction with the ligand. A stereochemical model is proposed that accounts for preferential formation of the (R)‐configurated coupling product when (R)‐biaryl phosphine ligands are used.