title: Mechanism and origins of regio- and enantioselectivities in RhI-catalyzed hydrogenative couplings of 1,3-diynes and activated carbonyl partners: intervention of a cumulene intermediate.
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 C-C 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.
authors.affiliation: Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, CA 90095-1569, USA.
authors.forename: Peng
authors.initials: P
authors.lastname: Liu
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Krische
authors.forename: Kendall N
authors.initials: KN
authors.lastname: Houk
mesh_terms: D002384:Catalysis; D053280:Diynes / Q000737:chemistry*; D006865:Hydrogenation; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D011090:Polyenes / Q000737:chemistry*; D011789:Quantum Theory; D012238:Rhodium / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D053280:Diynes; D011090:Polyenes; D012238:Rhodium