Sterically congested molecules, 14. Imine protonation in toluene solution: A problem akin to the N ‐protonation in rhodopsin

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.