title: Facile S(N)2 reaction in protic solvent: quantum chemical analysis.
abstract: We study the effects of protic solvent (water, methanol, ethanol, and tert-butyl alcohol) and cation (Na+, K+, Cs+) on the unsymmetrical SN2 reaction X- + RY --> RX + Y- (X = F, Br; R = CH3,C3H7;Y = Cl, OMs). We describe a series of calculations for the S(N)2 reaction mechanism under the influence of cation and protic solvent, presenting the structures of pre- and postreaction complexes and transition states and the magnitude of the activation barrier. An interesting mechanism is proposed, in which the protic solvent molecules that are shielded from the nucleophile by the intervening cation act as a Lewis base to reduce the unfavorable Coulombic influence of the cation on the nucleophile. We predict that the reaction barrier for the S(N)2 reaction is significantly lowered by the cooperative effects of cation and protic solvent. We show that the cation and protic solvent, each of which has been considered to retard the SN2 reactivity of the nucleophile, can accelerate the reaction tremendously when they interact with the fluoride ion in an intricate, combined fashion. This alternative S(N)2 mechanism is discussed in relation to the recently observed phenomenal efficiency of fluorination in tert-alcohol media [Kim, D. W.; et al. J. Am. Chem. Soc. 2006, 128, 16394].
authors: College of Environmental Science and Applied Chemistry (BK 21), Kyunghee University, Kyungki 449-701, Korea.
authors: Young-Ho
authors: YH
authors: Oh
authors: Doo-Sik
authors: DS
authors: Ahn
authors: Sang-Yoon
authors: SY
authors: Chung
authors: Jeong-Hwan
authors: JH
authors: Jeon
authors: Sung-Woo
authors: SW
authors: Park
authors: Seung Jun
authors: SJ
authors: Oh
authors: Dong Wook
authors: DW
authors: Kim
authors: Hee Seup
authors: HS
authors: Kil
authors: Dae Yoon
authors: DY
authors: Chi
authors: Sungyul
authors: S
authors: Lee
mesh_terms: D002412:Cations; D002586:Cesium / Q000737:chemistry; D002621:Chemistry / Q000379:methods; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D004364:Pharmaceutical Preparations / Q000737:chemistry; D011789:Quantum Theory; D012969:Sodium Fluoride / Q000737:chemistry; D012984:Software; D012997:Solvents / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry
chemical_list: D002412:Cations; D004364:Pharmaceutical Preparations; D012997:Solvents; D014867:Water; D002586:Cesium; D012969:Sodium Fluoride