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

Abstract

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 + BF 4 – –6H + BF 4 – 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 + BF 4 – –6H + BF 4 – 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 + BF 4 – , 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.

How Strong is Hydrogen Bonding to Amide Nitrogen?

“…This is the reason why indirect methods of E HB evaluation are the only possibility for intramolecular HBs. The so‐called “rotational” method, previously applied for proton sponge derivatives, was used to estimate E HB for 5aH + , 5bH + , 5hH + and for 4H + (Table , see Table S4 for expanded list). The main idea of this method is as follows: the HB energy is determined by the difference between electronic energies of “ in ” and “ out ” forms of molecule (Figure S20).…”

“…The potential curves demonstrate a single‐well potential, i. e. there is no second energetic minima for protonated amide nitrogen (Figure S21). Similarly, for proton sponge analogue 16H + (Scheme ) with a weak HB, the single‐well potential curve is also reported . In contrast, there are double‐well potential curves for most proton sponges, or even a barrier‐free proton transfer with one minimum …”

“…This is a result inherent drawbacks of this method (column “Gas phase, A”). In order to obtain more precise E HB values, we considered (a) the solvent effect (PCM model, acetonitrile solution, column “Acetonitrile, A”), (b) a combination of the solvent effect and the presence of BF 4 − counterion (column “Acetonitrile, B”), (c) a combination of the solvent effect and the repulsion between peri ‐substituents (for description of the method see SI, p. S12, column “Acetonitrile, C”). Thus, the more corrections are considered, the more E HB values are reduced.…”

Nucleophilic Substitution of Hydrogen Atom in Initially Inactivated Pyrrole Ring

“…Primarily, we aimed to clarify whether the pyrrole nitrogen atom can serve as a proton acceptor ( n -donor) in the formation of the NHN hydrogen bond. This has indeed been confirmed for a number of protonated 9-(dimethylamino)­benzo­[ g ]­indoles 1 . However, when we used 1-(dimethylamino)-8-(pyrrolyl-1)­naphthalene ( 2 ), the situation developed completely differently.…”

A Spectroscopic Overview of Intramolecular Hydrogen Bonds of NH…O,S,N Type

“…The left hand molecule with X=N(CH 3 ) 2 is the well known DMANH + proton sponge. With X=pyrrole in Figure4hydrogen bonding to the π-electron system is found,[7] whereas with X=N(CH 3 )C=OCH 3 hydrogen bonding to nitrogen have been tested[8]. The right hand molecule, N,N ,N"-tris(p-tolyl)azacalix[3](2,6)pyridine (TAPH), shows an extremely low field NH proton chemical shift of 22.1 ppm[9,10].…”