Entropy-driven hydrogen bonding: stereodynamics of a protonated, N,N-chiral "proton sponge"

The C2-symmetric ("DL]") and achiral ("meso]") diastereoisomers of the hydrogen iodide salt of 1,8-bis-(N-benzyl-N-methylamino)naphthalene (2H]-I] ) interconvert in solution. Direct interconversion of the diastereoisomers of 2H]+ must involve hydrogen bond fission (to give "nonHB-2H+]") and rotation-inversion of the non-protonated nitrogen centre. The global activation parameters (deltaH++ and deltaS++) for diastereoisomer interconversion in D7]DMF have been determined from rate data obtained by temperature-drop and magnetisation-transfer 13C NMR spectroscopy over a temperature range of 170 degrees C. The process is found to have a high entropy of activation in both directions (deltaS++=163(+/-4) and 169(+/-4) JK(-1)mol(-1)) and this is suggested to arise through hydrogen bonding of the ammonium centre in nonHB-2H+] with the solvent (D7]DMF). Comparison of the enthalpy of activation (deltaH++) with that earlier found for diastereoisomer interconversion of the free-base form 2 suggests that the intramolecular hydrogen bond in 2H]+ is roughly equal in enthalpic strength (deltaH) with that made with the solvent (D7]DMF) in the non-hydrogen-bonded intermediate nonHB-2H+]. As such, the hydrogen bonding in 2H]+ may be considered as predominantly an entropically driven process, without any unusual enthalpic strength.