1,8-Bis(dimethylamino)naphthyl-2-ketimines: Inside vs outside protonation

Abstract

The structure and protonation behaviour of four ortho-arylketimines of 1,8-bis(dimethylamonio)naphthalene with a different number of methoxy groups in an aromatic substituent were investigated in solution by NMR (acetone, DMSO, MeCN), in solid state by X-ray analysis and in the gas phase by DFT calculations. Both mono- and diprotonated species were considered. It has been shown that E-isomers of neutral imines can be stabilised by an intramolecular C=N−H···OMe hydrogen bond with a neighbouring methoxy group. Electron-donating OMe groups dramatically increase the basicity of the imino nitrogen, forcing the latter to abstract a proton from the proton sponge moiety in monoprotonated forms. The participation of the out-inverted and protonated 1-NMe2 group in the Me2N−H···NH=C hydrogen bond is experimentally demonstrated. It was shown that the number and position of OMe groups in the aromatic substituents strongly affects the rate of the internal hindered rotation of the NH2 + fragment in dications.

Actual Symmetry of Symmetric Molecular Adducts in the Gas Phase, Solution and in the Solid State

“…Due to the strength of the intramolecular H-bond and a slow intermolecular proton exchange in solution, proton sponges are very popular model systems for benchmark studies of spectral manifestations of H-bonding [80][81][82][83][84][85][86]. Alternatively, the H-bond symmetry can be purposefully lowered to investigate competing interactions [87][88][89][90].…”

Transition‐Metal‐Free Synthesis of 2‐Substituted Benzo[cd]Indoles via the Reaction of 1‐Halo‐8‐lithionaphthalenes with Nitriles

“…Instead, the matching signal of an acidic proton appears at ≈11 ppm (10.84 ppm for 11 g ⋅ HBF 4 ) which is very similar to the corresponding chemical shift of 7 ⋅ HBF 4 (12.60 for 7 g ⋅ HBF 4 , see SI). This is rather surprising, since the protonation of “proton sponges” containing a second basic center generally results in the formation of an equilibrating mixture of protonated species with a proton preferably localized in the internitrogen space [48–50] . We believe that the selectivity of the protonation of 11 originates from the abovementioned strong conjugation of the 5‐NMe 2 group with an aromatic core: the stabilization of form 11’ boosts the basicity of the indole nitrogen on the one hand, and prevents the protonation of the non‐conjugated NMe 2 group by inducing a positive charge in close proximity, on the other hand (Scheme 5).…”

“…This is rather surprising, since the protonation of "proton sponges" containing a second basic center generally results in the formation of an equilibrating mixture of protonated species with a proton preferably localized in the internitrogen space. [48][49][50] We believe that the selectivity of the protonation of 11 originates from the abovementioned strong conjugation of the 5-NMe 2 group with an aromatic core: the stabilization of form 11' boosts the basicity of the indole nitrogen on the one hand, and prevents the protonation of the non-conjugated NMe 2 group by inducing a positive charge in close proximity, on the other hand (Scheme 5). A similar effect was previously observed in the case of cations 14, where the conjugation of the 1-NMe 2 group with the aromatic system also prevented the protonation of the molecule.…”

peri-Interactions in 1,8-bis(dimethylamino)naphthalene ortho-ketimine cations facilitate [1,5]-hydride shift: selective synthesis of 1,2,3,4-tetrahydrobenzo[h]quinazolines

“…S16–S28†) revealed a correlation between the yield of the reaction and the ratio of the forms 7a protonated in various positions (Scheme 4). Actually, when adding of 1 equivalent of the acid, two sets of signals appeared in the 1 H NMR spectrum of the resulting monocations 7a H + in the region of the dimethylamino groups: 30,32 the first one in the higher field corresponds to the isomer 7a H + (A) protonated at the CNH group and the second one in the lower field can be attributed to 7a H + (B) protonated at peri -NMe 2 nitrogen atoms. Both forms of monocation 7a H + upon addition of more than one equivalent of the acid start to generate dication 7a 2H + (B); the designation B indicates that the NMe 2 group 1 H NMR signals of 7a 2H + (B) merge with the 7a H + (B) ones.…”