Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena, California 91125, United States.
J Org Chem. 2013 Mar 1;78(5):2005-11. doi: 10.1021/jo302049z. Epub 2013 Jan 14.
The conformational populations of cis-1,3-cyclopentanedicarboxylic acid (1) and its mono- and dianion were established in DMSO solution by comparing the vicinal proton–proton coupling constants (3J(HH)) obtained in solution to their theoretical counterparts. Geometries used for 3J(HH) theoretical estimation (using Karplus-type equations) were obtained from optimized structures at the B3LYP/6-31G(2d,2p) level. The diacid (1) adopted many conformations, whereas the ionized species (1A mono- and 1B dianion) assumed single conformations. A downfield chemical shift of 19.45 ppm (Δδ(H) = 7.43 ppm) observed at −60 °C was indicative of intramolecular hydrogen bonding in 1A, which was later corroborated by determining the ratio of the first (K1) to the second (K2) ionization constants. K1/K2 in DMSO (1.3 × 10(7)) was significantly larger than the value in water (2 × 10). In addition, K1/K(E) = 200 (where K(E) is the acidity constant of the monomethylester of 1) was greater than the intramolecular hydrogen bonding threshold value of 2. The calculated intramolecular hydrogen bond strength of 1A was ~3.1 kcal mol(–1), which is ~2.7 kcal mol(–1) more stable than the values for cis-1,3-cyclohexanedicarboxylic acid (2A). Thus, the relative energies of intramolecular hydrogen bonding in the monoanions 1A and 2A suggests that 1,3-diaxial conformers are more favored for cyclopentane than for cyclohexane rings.
顺式-1,3-环戊烷二甲酸（1）及其单阴离子和二阴离子在 DMSO 溶液中的构象分布通过比较溶液中获得的相邻质子-质子偶合常数（3J（HH））与其理论值来确定。用于 3J（HH）理论估算（使用 Karplus 型方程）的几何形状是从 B3LYP/6-31G（2d，2p）水平优化结构获得的。二酸（1）采用了多种构象，而离子化物种（1A 单阴离子和 1B 二阴离子）则采用了单一构象。在-60°C 下观察到 19.45ppm 的高场化学位移（Δδ（H）=7.43ppm）表明 1A 中存在分子内氢键，后来通过确定第一（K1）和第二（K2）电离常数的比值得到了证实。DMSO 中的 K1/K2（1.3×10(7)）明显大于水（2×10）中的值。此外，K1/K(E) = 200（其中 K(E)是 1 的单甲酯的酸度常数）大于 2，这是分子内氢键的阈值。1A 的计算分子内氢键强度约为 3.1kcal mol(-1)，比顺式-1,3-环己烷二甲酸（2A）的值稳定约 2.7kcal mol(-1)。因此，单阴离子 1A 和 2A 中分子内氢键的相对能量表明，对于环戊烷而言，1,3-双轴向构象比环己烷环更有利。
