title: Entropy-driven hydrogen bonding: stereodynamics of a protonated, N,N-chiral "proton sponge".
abstract: 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.
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, UK. guy.lloyd-jones@bris.ac.uk
authors.forename: P
authors.initials: P
authors.lastname: Hodgson
authors.forename: G C
authors.initials: GC
authors.lastname: Lloyd-Jones
authors.forename: M
authors.initials: M
authors.lastname: Murray
authors.forename: T M
authors.initials: TM
authors.lastname: Peakman
authors.forename: R L
authors.initials: RL
authors.lastname: Woodward