H-transfers Coupled to Conformational Changes and Hydrogen Bond Pre-equilibria

2 H-transfers Coupled to Conformational Changes and Hydrogen Bond Pre-equilibria 

Replacing the CH unit of diarylamidines with imino nitrogens leads to diaryltri-azenes. As illustrated in Fig. 6.46, the aryl groups are found to be coplanar with the triazene unit. A consequence is that an intermolecular steric interaction between aromatic CH arises, which prevents the formation of cyclic dimers. Thus, diaryltriazenes are not able to exchange protons without the help of a catalyst, as has been shown recently [33]. In order to obtain more information about catalytic proton exchange, the base catalyzed transfer of l,3-bis(4-fluorophenyl)[l,3-i N2]triazene was studied in more detail using H and F NMR. As catalysts dimethylamine, trimethylamine and water were studied, using tetrahydrofuran-dg and methylethylether-dg as solvents. The latter is liquid down to 130 K. 

Surprisingly, both dimethylamine and trimethylamine were able to pick up the mobile proton of the triazene at one nitrogen atom and carry it to the other nitrogen atom, resulting in an intramolecular transfer process catalyzed each time by a different base molecule. Even more surprising is that the intramolecular transfer (Fig. 6.47(a)) catalyzed by dimethylamine is faster than the superimposed inter-molecular double proton transfer (Fig. 6.47(b)). 

The kinetic FI/D isotope effects are small, especially in the catalysis by trimethylamine, indicating a major heavy atom rearrangement and absence of tunneling. 

Al-Soufi et al. [83] have followed the kinetics of the intramolecular H- and D-transfer between the keto and the enol form of 2-(2 -hydroxy-4 -methylphenyl) benzoxazole (MeBO) dissolved in alkanes using optical methods. No dependence of the rate constants on the solvent viscosity could be found. The Arrhenius diagram obtained over a very wide temperature range is depicted in Fig. 6.48. At low temperatures, the very rare regime of temperature-independent rate constants is obtained, exhibiting a very large temperature independent kinetic H/D isotope effect of about 

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