title: Theoretical studies on the mechanism and stereoselectivity of Rh(Phebox)-catalyzed asymmetric reductive aldol reaction.
abstract: Density functional theory calculations (B3LYP) have been carried out to understand the mechanism and stereochemistry of an asymmetric reductive aldol reaction of benzaldehyde and tert-butyl acrylate with hydrosilanes catalyzed by Rh(Phebox-ip)(OAc)(2)(OH(2)). According to the calculations, the reaction proceeds via five steps: (1) oxidative addition of hydrosilane, (2) hydride migration to carbon-carbon double bond of tert-butyl acrylate, which determines the chirality at C2, (3) tautomerization from rhodium bound C-enolate to rhodium bound O-enolate, (4) intramolecular aldol reaction, which determines the chirality at C3 and consequently the anti/syn-selectivity, and (5) reductive elimination to release aldol product. The hydride migration is the rate-determining step with a calculated activation energy of 23.3 kcal mol(-1). In good agreement with experimental results, the formation of anti-aldolates is found to be the most favorable pathway. The observed Si-facial selectivity in both hydride migration and aldol reaction are well-rationalized by analyzing crucial transition structures. The Re-facial attack transition state is disfavored because of steric hindrance between the isopropyl group of the catalyst and the tert-butyl acrylate.
authors: School of Chemical Biology and Biotechnology, Shenzhen Graduate School of Peking University, Shenzhen, China.
authors: Yun-Fang
authors: YF
authors: Yang
authors: Ting
authors: T
authors: Shi
authors: Xin-Hao
authors: XH
authors: Zhang
authors: Zong-Xun
authors: ZX
authors: Tang
authors: Zhen-Yi
authors: ZY
authors: Wen
authors: Jun-Min
authors: JM
authors: Quan
authors: Yun-Dong
authors: YD
authors: Wu
mesh_terms: D000179:Acrylates / Q000737:chemistry*; D000447:Aldehydes / Q000737:chemistry; D001547:Benzaldehydes / Q000737:chemistry*; D002384:Catalysis; D008958:Models, Molecular; D010084:Oxidation-Reduction; D011789:Quantum Theory; D012238:Rhodium / Q000737:chemistry; D012821:Silanes / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D000179:Acrylates; D000447:Aldehydes; D001547:Benzaldehydes; D012821:Silanes; C116609:3-hydroxybutanal; C522411:tert-butyl acrylate; D012238:Rhodium; C032175:benzaldehyde