TY - JOUR
T1 - Free-energy surfaces for liquid-phase reactions and their use to study the border between concerted and nonconcerted α, β-elimination reactions of esters and thioesters
AU - Kim, Yongho
AU - Mohrig, Jerry R.
AU - Truhlar, Donald G.
PY - 2010/8/18
Y1 - 2010/8/18
N2 - Distinguishing between the concerted second-order mechanism for I-eliminations and nonconcerted mechanisms with discrete carbanion intermediates is very difficult experimentally, but the ability of quantum chemistry to find stationary points of the free-energy surface in liquid-phase solutions, even for complex reagents, provides a new tool for elucidating such mechanisms. Here we use liquid-phase density functional theory calculations to find transition states and intermediates on the free-energy surfaces of four base-initiated α, β-eliminations of acetoxy and mesyloxy esters and their analogous thioesters. The geometries, free energies, and charge distributions of these structures support a stepwise irreversible first-order elimination from a conjugate base (E1cBI) mechanism with acetoxy ester 3, acetoxy thioester 4, and mesyloxy thioester 6. However, mesyloxy ester 5, which has an excellent nucleofuge and a less-acidic proton, follows a concerted but asynchronous E2 mechanism with an E1cB-like transition state. The anti transition state is more favorable than the syn one, even for the poorer nucleofuge and more-acidic thioesters. The article includes a general scheme for describing liquid-phase reactions in terms of free-energy surfaces.
AB - Distinguishing between the concerted second-order mechanism for I-eliminations and nonconcerted mechanisms with discrete carbanion intermediates is very difficult experimentally, but the ability of quantum chemistry to find stationary points of the free-energy surface in liquid-phase solutions, even for complex reagents, provides a new tool for elucidating such mechanisms. Here we use liquid-phase density functional theory calculations to find transition states and intermediates on the free-energy surfaces of four base-initiated α, β-eliminations of acetoxy and mesyloxy esters and their analogous thioesters. The geometries, free energies, and charge distributions of these structures support a stepwise irreversible first-order elimination from a conjugate base (E1cBI) mechanism with acetoxy ester 3, acetoxy thioester 4, and mesyloxy thioester 6. However, mesyloxy ester 5, which has an excellent nucleofuge and a less-acidic proton, follows a concerted but asynchronous E2 mechanism with an E1cB-like transition state. The anti transition state is more favorable than the syn one, even for the poorer nucleofuge and more-acidic thioesters. The article includes a general scheme for describing liquid-phase reactions in terms of free-energy surfaces.
UR - http://www.scopus.com/inward/record.url?scp=77955573850&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=77955573850&partnerID=8YFLogxK
U2 - 10.1021/ja101104q
DO - 10.1021/ja101104q
M3 - Article
C2 - 20698673
AN - SCOPUS:77955573850
SN - 0002-7863
VL - 132
SP - 11071
EP - 11082
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 32
ER -