The reaction of the inside protonated form of the tricyclic amine 1,4,8,12-tetraazatricyclo[188.8.131.52(4.12)]nonadecane (1) with iron(III) affords the inside monoprotonated form of the corresponding imine 4,8,12-triaza-1-azoniatricyclo[6.6,3.2(4.12)]nonadec-1 (15)-ene (2), which was isolated as the tetrabromozincate salt (2a) in a yield of 78%. The crystal structure of 2a has been solved by X-ray diffraction at T= 120 K. In the imine cation the acidic hydrogen atom and the lone pairs of the nitrogen atoms are oriented toward the inside of the cavity. The acidic hydrogen atom is bound to a nitrogen atom belonging to the triazacyclononane entity. The imine double bond is situated between the N-atom of the triazacyclononane entity and the C-atom belonging to one of the three trimethylene bridges. The imine 2 is stable in acidic solution and the inside coordinated proton is very robust in acidic solution. In basic solution the imine reacts fast to give a quantitative formation of the inside protonated form of the hemiaminal 1,4,8,12-tetraazatricyclo[6.6.3,2(4.12)]nonadecan-5-ol (3). The equilibrium constant K-im = [H+]/ was determined at three different temperatures from potentiometric measurements, which gave K-im = 1.57(1) x 10(-5) M at 25 degreesC, DeltaSdegrees = -83(1) J mol(-1) K-1 and DeltaHdegrees = 2,6(3) kJ mol(-1) at 1 = 1.0 M (NaCl). The inside coordinated proton in 3 is labile in basic solution and the rate for NH/ND exchange was determined by H-1 NMR at three different temperatures. The reaction followed the expression k(obs) = k(ex)[OD-] with k(ex) = 0.0978(30) dm(3) mol (1) s(-1) at 25 degreesC, DeltaSdegrees = 87(4) J mol(-1) K-1, and DeltaHdegrees = 104.9(11) kJ mol(-1) at I = 1.0 M (NaCl). The exchange rate is more than 5 x 106 times faster than that of the parent saturated cage 1. This extreme enhancement of reactivity is explained by an intramolecular proton transfer reaction mediated by hydroxy and oxy groups flipping in and out of the cavity, which mechanistically has resemblance to the transport of ions in a biological system.
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 2002|