title: Theoretical investigations on the mechanism of benzoin condensation catalyzed by pyrido[1,2-a]-2-ethyl[1,2,4]triazol-3-ylidene.
abstract: A new annulated N-heterocyclic carbene (NHC), pyrido[1,2-a]-2-ethyl[1,2,4]triazol-3-ylidene, has been synthesized and its good catalytic activity for benzoin condensation has been experimentally determined by You and co-workers recently [ Ma , Y. J. , Wei , S. P. , Lan , J. B. , Wang , J. Z. , Xie , R. G. , and You , J. S. J. Org. Chem. 2008 , 73 , 8256 ]. In this work, the mechanism of the title reaction has been intensively studied computationally by employing the density functional theory (B3LYP) method in conjunction with 6-31+G(d) and 6-311+G(2d,p) basis sets. Our results indicate that path A (in which a sequence of intermolecular proton transfers between two carbene/benzaldehyde coupling intermediates affords enamine) and path B (in which a t-BuOH assisted hydrogen transfer generates enamine) proposed on the basis of the Breslow mechanism are competitive for their similar barriers. In path A, the first intermolecular proton transfer between two N-heterocyclic carbene/benzaldehyde coupled intermediates to form tertiary alcohol and enolate anion is theoretically the rate-determining step with corresponding barrier (30.93 kcal/mol), while the t-BuOH assisted hydrogen transfer generating Breslow enamine is the rate-determining step with corresponding barrier (28.84 kcal/mol) in path B. The coupling of carbene and benzaldehyde, and the coupling of enamine and another benzaldehyde to form a C-C bond are partially rate-determining for their relatively significant barriers (24.06 and 26.95 kcal/mol, respectively), being the same in both paths A and B. Our results are in nice agreement with the experimental result in a kinetic investigation of thiazolium ion-catalyzed benzoin condensation performed by White and Leeper in 2001.
authors.affiliation: College of Chemistry, Sichuan University , Chengdu 610064, People's Republic of China.
authors.forename: Yunqing
authors.initials: Y
authors.lastname: He
authors.forename: Ying
authors.initials: Y
authors.lastname: Xue
mesh_terms: D001547:Benzaldehydes / Q000737:chemistry; D001573:Benzoin / Q000737:chemistry*; D002384:Catalysis; D005663:Furans / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry; D008697:Methane / Q000031:analogs & derivatives / Q000737:chemistry; D008956:Models, Chemical; D015394:Molecular Structure; D011522:Protons; D011725:Pyridines / Q000737:chemistry*; D011789:Quantum Theory; D014230:Triazoles / Q000737:chemistry*; D020002:tert-Butyl Alcohol / Q000737:chemistry
chemical_list: D001547:Benzaldehydes; D005663:Furans; D011522:Protons; D011725:Pyridines; D014230:Triazoles; C000592837:pyrido(1,2-a)-2-ethyl(1,2,4)triazol-3-ylidene; C030011:carbene; C018674:tetrahydrofuran; D006859:Hydrogen; D001573:Benzoin; D020002:tert-Butyl Alcohol; D008697:Methane; C032175:benzaldehyde