Dynamic Liquid State NMR and IR Study of Tautomerism and Conformations of Tetraphenyloxalamidine, a Novel Small Intramolecular Double Hydrogen Transfer System

Abstract

Using Dynamic NMR Spectroscopy we have detected a novel intramolecular double hydrogen transfer in the oxalamidine (OA) system. OA is, therefore, so far the smallest known molecular unit capable of such a transfer. Because of solubility problems, experiments were performed on tetraphenyloxalamidine (TPOA, Ia) and the isotopically labeled compound TPOA‐15N4‐d20 (Ib) dissolved in CD2CI2. At low temperatures TPOA exists in a number of different conformations, of which only one conformation with two intramolecular hydrogen bonds is capable of an intramolecular hydrogen transfer which shows up in the 1H‐15N signal of this conformer. At low temperatures a 1H‐15N doublet (88 Hz coupling constant) is observed which changes into a triplet (44 Hz coupling constant) as the temperature is raised. The position of this signal is independent of temperature and concentration. Two further conformations were detected, of which one formed strong intermolecular hydrogen bonds and underwent fast intermolecular proton exchange. Above room temperature all NH signals coalesce into an exchange‐broadened singlet. 13C experiments prove that this effect is due to a fast internal conversion of all conformers. IR experiments on TPOA in CCl4 did not reveal the complicated conformational behavior of TPOA. By lineshape analysis we obtained the rate constants of the HH migration, which are given by kHH = 1010.8 exp(−43 kJ mol−1/RT). An upper limit for the HD exchange rates were found for a sample deuterated to 90% in the labile proton sites: kHD = exp(−53 kJ mol −1/RT), i.e. kHH/kHD = 2.1 at 298 K. The data are compared to previous results on the azophenine rearrangement.

Heavy atom motions and tunneling in hydrogen transfer reactions: the importance of the pre‐tunneling state

“…The intramolecular degenerate double proton transfers of azophenine (AP)95 and of oxalamidine (OA) derivatives (Scheme )96–99 represent interesting cases which demonstrate the importance of the molecular structure on how hydrogen bond compression leading to the pre‐tunneling state assists the hydrogen transfer. These systems have been studied by dynamic NMR spectroscopy of the 15 N labeled compounds.…”

“… Intramolecular double proton transfers in azophenine (AP)95, 100 and oxalamidine (OA) derivatives96–99 TPOA: tetraphenyloxalamidine; BIM: bisimidazolyl; OA5: 2,2′‐bis(4,5‐dihydro‐l,3‐diazole); OA6: 2,2′‐bis‐(3,4,5,6‐tetrahydro‐1,3‐diazixine). OA7: 2,2′‐bis‐(4,5,6,7‐tetrahydro‐1,3‐diazixine) …”

NMR Stratagems for the Study of Multiple Kinetic Hydrogen/Deuterium Isotope Effectsof Proton Exchange. Example: Di-p-fluorophenylformamidine/THF

“…Unauthenticated Download Date | 5/10/18 3: 29 PM still remains because the vibrational frequencies of transition states cannot be observed spectroscopically and because of proton tunneling, which also affects kinetic hydrogen/deuterium isotope effects [1], In order to check the validity of the RGM we have, for a number of years, been studying kinetic isotope effects of well characterized multiple symmetric pro ton transfer reactions by dynamic NMR spectroscopy in non aqueous liquids [17,[20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and, recently, also in the solid state [35][36][37][38][39][40][41][42]. Large deviations from (2) were observed for the intramolecular HH migrations in meso-tetraphenylporphine [17] and azophenine [17,32,33] as well as for proton exchange between acetic acid and methanol in tetrahydrofuran (THF) [17,21], as replacement of the first H atom by D resulted in a larger decrease in the rate constants than replacement of the second H atom by D. These results have been useful in elucidating kinetic results of enzyme reaction mechanisms [ 10,11 ].…”

“…In order to measure the above mentioned multiple kinetic isotope effects, NMR proton inventory tech niques [17,21,29,31,32] had to be designed that involve dynamic NMR measurements as a function of the deuterium fraction D in the mobile proton sites. When the chemical shifts of the jumping protons or deuterons are altered during the exchange, kinetic constants can be derived by a combination of !H and 2H NMR spectroscopy [17,21].…”

“…The use of the modulation of a scalar spin-spin interaction between the mobile hydrogen isotopes L and "re mote" spins in the residual group A of a molecule AL is only a partial solution to the problem because of the following circumstance. Although kinetic HH/HD isotope effects can be obtained from the analysis of A -H splitting patterns by *H NMR experiments [17,28,29,31,33,34], scalar A -D coupling is not strong enough to obtain kinetic HD/DD isotope effects by 2H NMR spectroscopy because of the smaller gryomagnetic ratio of the 2H nucleus. Therefore, kinetic HH/DD isotope effects of several intramolecular dou ble proton transfer reactions were obtained by per forming NMR experiments on remote spins such as 13C [17,33].…”