title: Mechanistic Insights into Carbonyl-Directed Rhodium-Catalyzed Hydroboration: ab Initio Study of a Cyclic γ,δ-Unsaturated Amide.
abstract: A two-point binding mechanism for the cationic rhodium(I)-catalyzed carbonyl-directed catalytic asymmetric hydroboration of a cyclic γ,δ-unsaturated amide is investigated using density functional theory. Geometry optimizations and harmonic frequency calculations for the model reaction are carried out using the basis set 6-31+G** for C, O, P, B, N, and H and LANL2DZ for Rh atoms. The Gibbs free energy of each species in THF solvent is obtained based on the single-point energy computed using the PCM model at the ECP28MWB/6-311+G(d,p) level plus the thermal correction to Gibbs free energy by deducting translational entropy contribution. The Rh-catalyzed reaction cycle involves the following sequence of events: (1) chelation of the cyclic γ,δ-unsaturated amide via alkene and carbonyl complexation in a model active catalytic species, [Rh(L2)2S2]+, (2) oxidative addition of pinacol borane (pinBH), (3) migratory insertion of the alkene double bond into Rh-H (preferred pathway) or Rh-B bond, (4) isomerization of the resulting intermediate, and finally, (5) reductive elimination to form the B-C or H-C bond with regeneration of the catalyst. Free energy profiles for potential pathways leading to the major γ-borylated product are computed and discussed in detail. The potential pathways considered include (1) pathways proceeding via migratory insertion into the Rh-H bond (pathwaysI,I-1, andI-2), (2) a potential pathway proceeding via migratory insertion into the Rh-B bond (pathwayII), and two potential competing routes to a β-borylated byproduct (pathwayIII). The results find that the Rh-H migratory insertion pathwayI-2, followed in sequence by an unanticipated isomerization via amide rotation and reductive elimination, is the most favorable reaction pathway. A secondary consequence of amide rotation is access to a competing β-hydride elimination pathway. The pathways computed in this study are supported by and help explain related experimental results.
authors.affiliation: Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology , Harbin 150040, People's Republic of China ; Department of Chemistry, University of Nebraska-Lincoln , Lincoln, Nebraska 68588, United States.
authors.forename: Zhao-Di
authors.initials: ZD
authors.lastname: Yang
authors.affiliation: Department of Chemistry, Yale University , New Haven, Connecticut 06518, United States.
authors.forename: Rhitankar
authors.initials: R
authors.lastname: Pal
authors.affiliation: Department of Chemistry, University of Nebraska-Lincoln , Lincoln, Nebraska 68588, United States.
authors.forename: Gia L
authors.initials: GL
authors.lastname: Hoang
authors.affiliation: Department of Chemistry, University of Nebraska-Lincoln , Lincoln, Nebraska 68588, United States.
authors.forename: Xiao Cheng
authors.initials: XC
authors.lastname: Zeng
authors.affiliation: Department of Chemistry, University of Nebraska-Lincoln , Lincoln, Nebraska 68588, United States.
authors.forename: James M
authors.initials: JM
authors.lastname: Takacs
keywords: Density functional theory (DFT); catalysis mechanism; catalytic asymmetric hydroboration; rhodium-catalyzed; two-point binding mechanism