Dynamic NMR study of the tautomerism of bicyclic oxalamidines: Kinetic HH/HD/DD isotope and solvent effects

Article abstract of DOI:10.1021/ja00083a009

The tautomerism of oxalamidine, which consists of an intramolecular degenerate double proton transfer between nitrogen atoms, has been studied by dynamic NMR spectroscopy. Experiments were performed on two bicyclic oxalamidines, 2,2′-bis(3,4,5,6-tetrahydro-1,3-diazine) (OA6) and 2,2′-bis(4,5,6,7-tetrahydro-1,3-diazepine) (OA7) dissolved in organic solvents. For this purpose, both compounds had to be labeled with ¹⁵N and partly with ²H. The tautomerism of OA6 was too slow to be detectable by NMR, in contrast to the case of OA7. Rate constants of the tautomerism of OA7 dissolved in methylcyclohexane-d₁₄ (MCY) and in acetonitrile d₃ (AN) could be obtained, including the full kinetic HH/HD/DD isotope effects. The data for MCY can be represented by the following: k_MCY^HH = 10^11.2±0.1 exp(-57.6 ± 06 kJ mol^-1/RT) s^-1, 303 K ≤ T≤ 415 K, k_298K^HH = 14 s^-1; k_MCY^HD = 10^11.1±0.1 exp(-59.6 ± 1.0 kJ mol^-1/RT) s^-1, 340 K ≤ T ≤ 415 K, k_298K^HD = 4.4 s^-1; k_MCY^DD = 10^10.9±0.1 exp(-59.5 ± 0.7 kJ mol^-1/RT) s^-1, 303 K ≤ T ≤ 415 K, k_298K^DD= 2.9s^-1. From these data, we determine that k_MCY^HH/k^MCYHD ≈ 3.1, k_MCY^HH/k_MCY^HH/k_MCY ^DD ≈4.8, and k_MCY^HD/k_MCY^DD ≈ 1.5 at 298 K. The data for AN are as follows: k_AN^HH = 10^11.7±0.1 exp(-56.2 ± 0.9 kJ mol^-1/RT) s^-1, 300 K ≤ T ≤ 382 K, k_298K^HH = 75s^-1; k_AN^HD = 10^11.5±0.1 exp(-57.6 ± 1.4 kJ mol^-1/RT) s^-1, 312 K ≤ T ≤ 374 K, k_298K^HD = 23 s^-1; and k_AN^DD = 10^11.1±0.2 exp(-56.7 ± 1.2 kJ mol^-1/RT) s^-1, 300 K ≤ T ≤ 393 K, k_298K^DD = 15 s^-1. From these data, we determine that k_AN^HH/k_AN^HD = 3.2, k_AN^HH/k_AN^DD = 5.2, k_AN^HD/k_AN^DD = 1.6, and k_AN^HH/k_MCY^HH = 5.4 at 298 K. No dependence of chemical shifts nor of rate constants on concentration was observed, which indicates that OA7 is not subject to intermolecular hydrogen bonding. Therefore, the observed increase of the rate constants with solvent polarity indicates the formation of a highly polar transition state, as expected for a stepwise proton transfer. This interpretation is independently supported by the observed multiple kinetic solvent isotope effects. The size of the kinetic isotope effects indicates a substantial heavy atom reorganization preceding each single proton-transfer step. The differing dynamic behavior of OA6 and OA7 indicates that this phenomenon consists not only of solvent reorganization but mainly of a compression of the hydrogen bond in which the proton transfer takes place. In the bicyclic oxalamidines, this compression is more or less coupled to an elongation of the other hydrogen bond and/or to a reorganization of the methylene bridges.