Substitution reactions in the acenaphthene analog of quino[7,8-h]quinoline and an unusual synthesis of the corresponding acenaphthylenes by tele-elimination

The possibility of functionalization of dipyrido[3,2-e:2',3'-h]acenaphthene containing a quino[7,8-h]quinoline fragment and being a highly basic diazine analog of 1,8-bis(dimethylamino)naphthalene ("proton sponge") has been studied for the first time. In addition to the pronounced tendency of the title compound to form associates with an intramolecular hydrogen bond of the NHN type (new examples with the participation of pyridine rings, including self-associates are shown) and its inertness to amination reactions of the pyridine rings, the naphthalene core at positions 5(8) and the CH2CH2 bridge (dehydrogenation) undergo chemical modifications under mild conditions, giving the corresponding acenaphthylenes. The latter can also be obtained in an unusual way by tele-elimination from 5,8-dibromodipyridoacenaphthene by reaction with neutral or anionic bases.

Synthesis, structure, and properties of switchable cross-conjugated 1,4-diaryl-1,3-butadiynes based on 1,8-bis(dimethylamino)naphthalene

A set of novel 1,4-diaryl-1,3-butadiynes terminated by two 7-(arylethynyl)-1,8-bis(dimethylamino)naphthalene fragments was prepared via the Glaser-Hay oxidative dimerization of 2-ethynyl-7-(arylethynyl)-1,8-bis(dimethylamino)naphthalenes. The oligomers synthesized in this way are cross-conjugated systems, in which two conjugation pathways are possible: π-conjugation of 1,8-bis(dimethylamino)naphthalene (DMAN) fragments through a butadiyne linker and a donor-acceptor aryl-C≡C-DMAN conjugation path. The conjugation path can be "switched" simply by protonation of DMAN fragments. X-ray diffraction, UV-vis spectroscopy and cyclic voltammetry are applied to analyze the extent of π-conjugation and the efficiency of particular donor-acceptor conjugation path in these new compounds. X-ray structures and absorption spectra of doubly protonated tetrafluoroborate salts of the oligomers are also discussed.

Synthesis, conformational stability and molecular structure of 4-aryl- and 4,5-diaryl-1,8-bis(dimethylamino)naphthalenes

4-Bromo- and 4,5-dibromo-1,8-bis(dimethylamino)naphthalenes were arylated with arylboronic acids under Suzuki reaction conditions to provide 4-aryl- and 4,5-diaryl-1,8-bis(dimethylamino)naphthalenes, respectively. The interaction of 4,5-dibromo-1,8-bis(dimethylamino)naphthalene with pyridin-3-ylboronic acid was accompanied by heterocyclization leading...

A new family of 1,4-diaryl-1,3-butadiynes based on the “proton sponge”: synthesis, electronic and chemical properties

A new family of 1,4-diaryl-1,3-butadiynes containing two and four fragments of 1,8-bis(dimethylamino)naphthalene (“proton sponge”) were synthesized.

Combination of "Buttressing" and "Clothespin" Effects for Reaching the Shortest NHN Hydrogen Bond in Proton Sponge Cations

A series of previously unknown 2,4,5-tri- and 2,4,5,7-tetrasubstituted 1,8-bis(dimethylamino)naphthalenes and their salts with HBF₄ containing bulky spherically shaped substituents (Me, Br, and SiMe₃) in the naphthalene ring has been synthesized. Using XRD analysis of 11 samples, the influence of the so-called "buttressing" and "clothespin" effects on their molecular structure and the NHN hydrogen bond geometry in the solid cations were investigated. The combined action of both effects has been shown to significantly increase the compression of the hydrogen bond. As a result, the previous record of the hydrogen bond shortness (N···N = 2.524 Å) has been surpassed in favor of 2.502 Å found for the tetrafluoroborate of 2,4,5,7-tetramethyl-1,8-bis(dimethylamino)naphthalene. The molecular structure of the latter differs by perfect symmetry and practically barrier-free proton transfer in the [NHN]⁺ bond. On the basis of the results of quantum-chemical calculations, it is suggested that the value of 2.502 Å likely represents or lies very close to the theoretical limit for the short hydrogen bonds between the amine-type nitrogen atoms.

N-Methylated 1,8-Diaminonaphthalenes as Bifunctional Nucleo­philes in Reactions with α,ω-Dihalogenoalkanes: A Facile Route to Heterocyclic and Double Proton Sponges

The reaction of 1-dimethylamino-8-(methylamino)naphthalene with 1,3-dibromopropane chemoselectively leads to the product of N,N′-heterocyclization, while in the case of 1,4-dibromobutane and 1,2-bis(bromomethyl)benzene the process results in heterocyclization onto the same nitrogen atom with the formation of previously unknown 1-dimethylamino-8-pyrrolidino- and 1-dimethylamino-8-isoindolino-naphthalenes. The same reactions conducted without adding any auxiliary base lead to the formation of N,N′-linked double proton sponges as a new type of polynitrogen organic receptor. Proceeding as a sequence of quaternization–demethylation–cyclization steps, this heterocyclization process can also be used to construct six-membered rings (piperidino, morpholino), albeit in lower yields. The ability of 1,2-dibromoethane to brominate N-alkylated 1,8-diaminonaphthalenes is also described. It is shown for the first time that a commercially available 1,8-bis(dimethylamino)naphthalene (DMAN) can be used as a starting material in a heterocyclization reaction, which via a one-pot approach and in a short time can be converted into 1,5-dimethylnaphtho[1,8-bc]-1,5-diazacyclooctane or 1-dimethylamino-8-(pyrrolidin-1-yl)naphthalene.

Perimidines: a unique π-amphoteric heteroaromatic system

Data on the physicochemical characteristics, theoretical calculations, reactivity and synthetic methods for perimidines are summarized. Although perimidine and some of its simple 2-substituted derivatives were obtained by Sachs back in 1909, their chemistry and key physical properties remained unknown until the early 1970s. Subsequent studies revealed many fundamental features of the perimidine system, previously not encountered in the heterocyclic series. The first comprehensive review on perimidines was published forty years ago. The period that has passed since 1980 led to the emergence of new directions and trends. Several hundred new publications have appeared, the generalization of which has become the main purpose of this article. This primarily concerns the obtaining of highly nucleophilic and stable perimidine carbenes, new methods of electrophilic substitution and oxidation, establishment of a close relationship between perimidines and proton sponges, and modern theoretical calculations. Based on perimidines, many different polycondensed systems have been obtained. Applied research has developed especially rapidly in recent years. Many new compounds based on perimidines related to chemosensors, analytical reagents, dyes, metal catalysts, electronic devices, nanotechnology, and medical chemistry have been proposed. Some information under review is presented as Supplementary Materials. It contains six tables, which include data on the basicity constants of perimidines, details of some synthetic methods for perimidines and fused analogs and also a list of biological activities of perimidines.

The bibliography includes 387 references.

How Strong is Hydrogen Bonding to Amide Nitrogen?

The protonation of the carboxamide nitrogen atom is an essential part of in vivo and in vitro processes (cis-trans isomerization, amides hydrolysis etc). This phenomenon is well studied in geometrically strongly distorted amides, although there is little data concerning the protonation of undistorted amides. In the latter case, the participation of amide nitrogen in hydrogen bonding (which can be regarded as the incipient state of a proton transfer process) is less well-studied. Thus, it would be a worthy goal to investigate the enthalpy of this interaction. We prepared and investigated a set of peri-substituted naphthalenes containing the protonated dimethylamino group next to the amide nitrogen atom ("amide proton sponges"), which could serve as models for the study of an intramolecular hydrogen bond with the amide nitrogen atom. X-Ray analysis, NMR spectra, basicity values as well as quantum chemical calculations revealed the existence of a hydrogen bond with the amide nitrogen, that should be attributed to the borderline between moderate and weak intramolecular hydrogen bonds (2-7 kcal ⋅ mol^-1 ).

Proton-induced fluorescence in modified quino[7,8-h]quinolines: dual sensing for protons and π-donors

The synthesis, as well as spectral, structural and photoluminescence properties of dipyrido[3,2-e:2',3'-h]acenaphthene 5 and quinazolino[7,8-h]quinazolines 6 as representatives of the bidentate -N[double bond, length as m-dash]/-N[double bond, length as m-dash] superbases, are reported. These nitrogen bases being more rigid (5) or π-extended (6) analogs of optically-mute quino[7,8-h]quinoline are both active in terms of fluorescence with quantum yields up to φ = 0.71-0.77. At the same time, their luminescence behavior is opposite to that of peri-NMe₂/NMe₂ naphthalene proton sponges and their hybrid NMe₂/-N[double bond, length as m-dash] analogs. Although 5 and 6 exhibit visible region emission upon protonation, for the hybrid systems the fluorescence is manifested only for bases. The most remarkable observation is that the fluorescence of compound 5 can be switched on not only by means of organic or inorganic acids, but also through the formation of chelate complexes with such weak H-donors as water and primary alcohols. It was disclosed that water is present in the complex as a cluster comprising 8 interconnected H₂O molecules. Overall, the studied compounds demonstrate a previously unobserved type of dual mode optical response, H-sensing (emission enhancement in 5 and 6 on protonation) and π-sensing (emission quenching in 5H⁺ and 6H⁺ on coordination with π-donors). This work seems to be an important contribution to areas such as chemosensorics, the creation of new ligands, hydrogen transfer and some other phenomena representing different types of supramolecular interactions.

Nucleophilic Substitution of Hydrogen Atom in Initially Inactivated Pyrrole Ring

It has been found that 1-dialkylamino-8-(pyrrolyl-1)naphthalenes 1 and 6, upon treatment with an equimolar amount of HBF₄ under ambient conditions, produce 1-dialkylammonium salts which are transformed into 7,7-dialkyl-7 H-pyrrolo[1,2- a]perimidine-7-ium tetrafluoroborates 5 and 7, respectively. The reaction proceeds in a highly selective manner and represents the first case of nucleophilic substitution of hydrogen in the initially inactivated pyrrole ring. The scope and limitations of the transformation, apparently operating due to the joint action of the "proximity effect" and proton catalysis, are outlined.

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.

Positively and negatively charged NHN hydrogen bonds in one molecule: synergistic strengthening effect, superbasicity and acetonitrile capture

The first full nitrogen zwitterions were synthesized. Both [NHN]⁻ and [NHN]⁺ bridges strongly influence each other, resulting in superbasicity and receptor properties.

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

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.

The first proton sponge-based amino acids: synthesis, acid–base properties and some reactivity

The first hybrid bases constructed from 1,8-bis(dimethylamino)naphthalene and glycine or alanine residues were synthesised and structurally characterised and unusual channels of their reactivity revealed.

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.

Out-basicity of 1,8-bis(dimethylamino)naphthalene: the experimental and theoretical challenge

A possibility of non-conventional two-step protonation of 1,8-bis(dimethylamino)naphthalene (proton sponge) is discussed. Unlike the generally accepted mechanism, involving relatively slow direct penetration of a proton into the cleft between the peri-NMe2 groups, it consists of the rapid addition of a proton to the out-inverted NMe2 group with the subsequent slower rotational transfer of the proton into the inter-nitrogen space to produce a stable chelated cation. The following approaches were employed during the work: (1) competitive hydrogen bond formation in a specially designed alcohol in which the OH group might chelate either the proton sponge 1-NMe2 group or another basic center (N,N-dimethylaniline) of known basicity; (2) measuring the basicity of naphtho[1,8-b,c]diazabicyclo[3.3.3]undecane considered to be a close analogue of the proton sponge capable exclusively of out-protonation; (3) X-ray, spectral and DFT studies of structure, energy and stereodynamics of compounds obtained, including their conformers. For the first time, the pKa value of an organic base with a perfectly flat nitrogen atom is reported. The final conclusion is made that both pathways of proton sponge protonation, traditional and non-conventional, contribute in parallel with a still undefined ratio. The estimated out-basicity of the proton sponge is at least 5.5 orders of magnitude lower than the directly measured in-basicity.

1,8,1',8'-Tetrakis(dimethylamino)-2,2'-dinaphthylmethanols: double in/out proton sponges with low-barrier hydrogen-bond switching

Previously unknown bis[1,8-bis(dimethylamino)naphth-2-yl]phenylmethanol (5) and bis[1,8-bis(dimethylamino)naphth-2-yl]methanol (6) have been obtained and studied by combination of X-ray, NMR, and IR techniques at variable temperature. It has been established that both proton sponge units in the solid tertiary alcohol 5 exist in nonconventional in/out form, one of which is fixed by intramolecular O-H...N hydrogen bonding. In solution, a fast interconversion of two isoenergetic hydrogen chelates occurs which can be frozen below 183 K. Unlike this, the secondary alcohol 6 in the solid at 100 K adopts the in/out-in/in conformation and at 293 K demonstrates a kind of dynamic behavior which can be described as temperature-driven dimer-induced rechelation. In solution under ambient conditions 6 exists as an equilibrating mixture of chelated and unchelated monomeric forms in a approximately 1:1.8 molar ratio.