Base-promoted transformation of 2-C(O)R-1,8-bis(dimethylamino)naphthalenes into benzo[g]indole derivatives

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

A simple and effective organolithium approach to the synthesis of 2-substituted benzo[cd]indoles from peri-dihalonaphthalenes and nitriles has been developed. The reaction proceeds via a surprisingly easy intramolecular aromatic nucleophilic substitution facilitated by the "clothespin effect". The discovered transformation provides good isolated yields, allows usage of an extensive range of nitriles, and demonstrates a good substituents tolerance. UV-absorption and NMR spectra of the obtained benzo[cd]indoles and their protonated forms demonstrated exclusive protonation to the indole nitrogen atom even in the presence of two NMe2 groups in positions 5 and 6 (i. e. "proton sponge" moiety).

Sterically Facilitated Intramolecular Nucleophilic NMe2 Group Substitution in the Synthesis of Fused Isoxazoles: Theoretical Study

The influence of steric repulsion between the NMe₂ group and a second ortho-(peri-)substituent in the series of 1-dimethylaminonaphthalene and N,N-dimethylanilene ortho-oximes on the ease of the NMe₂ group’s intramolecular nucleophilic substitution is studied. Possible reaction intermediates for three mechanisms are calculated (ωB97xd/def-2-TZVP), and their free Gibbs energies are compared to model reaction profiles. Supporting experiments have proved the absence of studied reactivity in the case of simple 2-dimethylaminobenzaldoxime, which allowed us to establish reactivity limits. The significant facilitation of NMe₂ group displacement in the presence of bulky substituents is demonstrated. The possibility of fused isoxazoles synthesis via the intramolecular nucleophilic substitution of a protonated NMe₂ group in the aniline and naphthalene series is predicted.

Neutral Pyrrole Nitrogen Atom as a π- and Mixed n,π-Donor in Hydrogen Bonding

9-Dimethylaminobenzo[ g]indoles 3-6 and 1-dimethylamino-8-(pyrrolyl-1)naphthalene 7 were examined as possible models for establishing the ability of the pyrrole nitrogen atom to participate in [NHN]⁺ hydrogen bonding as a proton acceptor. Indoles 3-5 (to a lesser extent 6) form rather stable tetrafluoroborates, with the proton mostly located on the NMe₂ group but simultaneously engaged in the formation of a charged intramolecular [NHN]⁺ hydrogen bond (IHB) with the pyrrole N atom. The theoretically estimated energies of IHB in salts 3H⁺BF₄^--6H⁺BF₄^- vary between 7.0-10.7 and 6.2-7.0 kcal mol^-1 in vapor and MeCN, respectively. The pyrrole N atom undergoes a perceptible pyramidalization but still remains involved in the 6π-electron aromatic system, suggesting that the hydrogen bonding in salts 3H⁺BF₄^--6H⁺BF₄^- represents a previously unknown mixed NH···N(n,π) interaction. Despite the favorable orientation of the N-H bond and the pyrrole ring in salt 7H⁺BF₄^-, no signs of NH···N(n) bonding in it were noticed, and the existing interaction was classified as pure NH···N(π). The results obtained may be useful in studies of secondary protein structures, especially those α-helix sections which contain tryptophan residues.

Ring lithiation of 1,8-bis(dimethylamino)naphthalene: another side of the ‘proton sponge coin’

A set of X-ray, multinuclear NMR and DFT approaches was used to address a question of selectivity, relative stability and enhanced reactivity of ring lithiated 1,8-bis(dimethylamino)naphthalenes.