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Sterically congested molecules, 14. Imine protonation in toluene solution: A problem akin to the N ‐protonation in rhodopsin
Author(s) -
Knorr Rudolf,
Ferchland Kathrin
Publication year - 1995
Publication title -
liebigs annalen
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 0947-3440
DOI - 10.1002/jlac.199519950251
Subject(s) - chemistry , protonation , steric effects , imine , photochemistry , hydrogen bond , medicinal chemistry , toluene , trifluoroacetic acid , pyridine , solvation , solvent , organic chemistry , molecule , ion , catalysis
The protonation state of N ‐(1,1,3,3‐tetramethyl‐2‐indanylidene)methylamine (5/6) is assessed on the basis of fully assigned 1 H‐ and 13 C‐NMR spectra in [D 8 ]toluene and in CDCl 3 . In either solvent the basicity of 5 remains intermediate between those of pyridine (weaker) and triethylamine (stronger). Nearly complete proton transfer to 5 in toluene occurs with trifluoroacetic and dichloroacetic acids, whereas partial protonation is achieved by chloroacetic acid or acetic acid and even by biphenyl‐2,2′‐diol (7). As a possible model for imine protonation in the hydrophobic region of proteins like the rhodopsins, the sterically shielded iminium cation 6 (or 2 ) does not require solvation by a polar solvent; rather it can be stabilized in toluene solution by a counteranion whose proton affinity is diminished by an assisting hydrogen bond.