Premium
Entropy‐Driven Hydrogen Bonding: Stereodynamics of a Protonated, N , N ‐Chiral “Proton Sponge”
Author(s) -
Hodgson Paul,
LloydJones Guy C.,
Murray Martin,
Peakman Torren M.,
Woodward Robert L.
Publication year - 2000
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20001215)6:24<4451::aid-chem4451>3.0.co;2-h
Subject(s) - protonation , hydrogen bond , sponge , proton , chemistry , stereochemistry , computational chemistry , crystallography , physics , organic chemistry , molecule , geology , quantum mechanics , paleontology , ion
The C 2 ‐symmetric (“[ DL ]”) and achiral (“[ meso ]”) diastereoisomers of the hydrogen iodide salt of 1,8‐bis‐( N ‐benzyl‐ N ‐methylamino)naphthalene ([ 2 H] + [I] − ) interconvert in solution. Direct interconversion of the diastereoisomers of [ 2 H] + must involve hydrogen bond fission (to give “[ non HB‐ 2 H + ]”) and rotation–inversion of the non‐protonated nitrogen centre. The global activation parameters (Δ H ≠ and Δ S ≠ ) for diastereoisomer interconversion in [D 7 ]DMF have been determined from rate data obtained by temperature‐drop and magnetisation‐transfer 13 C NMR spectroscopy over a temperature range of 170 °C. The process is found to have a high entropy of activation in both directions (Δ S ≠ =163(±4) and 169(±4) J K −1 mol −1 ) and this is suggested to arise through hydrogen bonding of the ammonium centre in [ non HB‐ 2 H + ] with the solvent ([D 7 ]DMF). Comparison of the enthalpy of activation (Δ H ≠ ) with that earlier found for diastereoisomer interconversion of the free‐base form 2 suggests that the intramolecular hydrogen bond in [ 2 H] + is roughly equal in enthalpic strength (Δ H ) with that made with the solvent ([D 7 ]DMF) in the non‐hydrogen‐bonded intermediate [ non HB‐ 2 H + ]. As such, the hydrogen bonding in [ 2 H] + may be considered as predominantly an entropically driven process, without any unusual enthalpic strength.