NH+–F Hydrogen Bonding in a Fluorinated “Proton Sponge” Derivative: Integration of Solution, Solid-State, Gas-Phase, and Computational Studies

Achieving Endo/Lysosomal Escape Using Smart Nanosystems for Efficient Cellular Delivery

The delivery of therapeutic agents faces significant hurdles posed by the endo-lysosomal pathway, a bottleneck that hampers clinical effectiveness. This comprehensive review addresses the urgent need to enhance cellular delivery mechanisms to overcome these obstacles. It focuses on the potential of smart nanomaterials, delving into their unique characteristics and mechanisms in detail. Special attention is given to their ability to strategically evade endosomal entrapment, thereby enhancing therapeutic efficacy. The manuscript thoroughly examines assays crucial for understanding endosomal escape and cellular uptake dynamics. By analyzing various assessment methods, we offer nuanced insights into these investigative approaches' multifaceted aspects. We meticulously analyze the use of smart nanocarriers, exploring diverse mechanisms such as pore formation, proton sponge effects, membrane destabilization, photochemical disruption, and the strategic use of endosomal escape agents. Each mechanism's effectiveness and potential application in mitigating endosomal entrapment are scrutinized. This paper provides a critical overview of the current landscape, emphasizing the need for advanced delivery systems to navigate the complexities of cellular uptake. Importantly, it underscores the transformative role of smart nanomaterials in revolutionizing cellular delivery strategies, leading to a paradigm shift towards improved therapeutic outcomes.

Cryogenic infrared spectroscopy reveals remarkably short NH+⋯F hydrogen bonds in fluorinated phenylalanines

In past decades, hydrogen bonds involving organic fluorine have been a highly disputed topic. Obtaining clear evidence for the presence of fluorine-specific interactions is generally difficult because of their weak nature. Today, the existence of hydrogen bonds with organic fluorine is widely accepted and supported by numerous studies. However, strong bonds with short H⋯F distances remain scarce and are primarily found in designed model compounds. Using a combination of cryogenic gas-phase infrared spectroscopy and density functional theory, we here analyze a series of conformationally unrestrained fluorinated phenylalanine compounds as protonated species. The results suggest proximal NH+⋯F hydrogen bonds with an exceptionally close H⋯F distance (1.79 Å) in protonated ortho-fluorophenylalanine.

Rational Computational Design of Systems Exhibiting Strong Halogen Bonding Involving Fluorine in Bicyclic Diamine Derivatives

Perhaps the most controversial and rare aspect of the halogen bonding interaction is the potential of fluorine in compounds to serve as a halogen bond donor. In this note, we provide clear and convincing examples of hypothetical molecules in which fluorine is strongly halogen bonding in a metastable state. Of particular note is a polycyclic system inspired by Selectfluor, which has been controversially proposed to engage in halogen bonding.

Ionic Liquid Clusters Generated from Electrospray Thrusters: Cold Ion Spectroscopic Signatures of Size-Dependent Acid-Base Interactions

We determine the intramolecular distortions at play in the 2-hydroxyethylhydrazinium nitrate (HEHN) ionic liquid (IL) propellant, which presents the interesting case that the HEH⁺ cation has multiple sites (i.e., hydroxy, primary amine, and secondary ammonium groups) available for H-bonding with the nitrate anion. These interactions are quantified by analyzing the vibrational band patterns displayed by cold cationic clusters, (HEH⁺)_n(NO₃^-)_n-1, n = 2-6, which are obtained using IR photodissociation of the cryogenically cooled, mass-selected ions. The strong interaction involving partial proton transfer of the acidic N-H proton in HEH⁺ cation to the nitrate anion is strongly enhanced in the ternary n = 2 cluster but is suppressed with increasing cluster size. The cluster spectra recover the bands displayed by the bulk liquid by n = 5, thus establishing the minimum domain required to capture this aspect of macroscopic behavior.

Synthesis and Basicity Studies of Quinolino[7,8-h]quinoline Derivatives

Quinolino[7,8-h]quinoline is a superbasic compound, with a pK_aH in acetonitrile greater than that of 1,8-bis(dimethylaminonaphthalene) (DMAN), although its synthesis and the synthesis of its derivatives can be problematic. The use of halogen derivatives 4,9-dichloroquinolino[7,8-h]quinoline (16) and 4,9-dibromoquinolino[7,8-h]quinoline (17) as precursors has granted the formation of a range of substituted quinolinoquinolines. The basicity and other properties of quinolinoquinolines can be modified by the inclusion of suitable functionalities. The experimentally obtained pK_aH values of quinolino[7,8-h]quinoline derivatives show that N⁴,N⁴,N⁹,N⁹-tetraethylquinolino[7,8-h]quinoline-4,9-diamine (26) is more superbasic than quinolino[7,8-h]quinoline. Computationally derived pK_aH values of quinolinoquinolines functionalized with dimethylamino (NMe₂), 1,1,3,3-tetramethylguanidino (N═C(NMe₂)₂) or N,N,N',N',N″,N″-hexamethylphosphorimidic triamido (N═P(NMe₂)₃) groups are significantly greater than those of quinolino[7,8-h]quinoline. Overall, electron-donating functionalities are observed to increase the basicity of the quinolinoquinoline moiety, while the substitution of electron-withdrawing groups lowers the basicity.

Close Amide NH···F Hydrogen Bonding Interactions in 1,8-Disubstituted Naphthalenes

In this note, we present a series of N-(8-fluoronaphthalen-1-yl)benzamide derivatives designed to maximize amide-NH···F hydrogen bond interactions therein. A combination of IR and NMR spectroscopy indicates a linear correlation between the high energy shift in NH stretching frequency and the electron withdrawing nature of the substituent, consistent with the trend predicted by DFT calculations. Additionally, a limiting case of hydrogen bonding is observed when the benzamide derivatives are replaced with trifluoroacetamide, causing an additional red shift of 44 cm^-1 in the NH stretching frequency. Most importantly, ¹H-¹⁹F coupling constants in this series are among the largest measured for amide-NH···F interactions. X-ray crystallography reveals face-to-face alignment of naphthalene rings in these derivatives resulting in part from the NH···F hydrogen bonds. This motif also dictates the formation of sheets composed of stacked naphthalene rings in the crystal structure as opposed to unfluorinated analogues wherein NH···OC hydrogen-bonding interactions force benzamide and naphthalene rings to engage in T-shaped π-π interactions instead. Additionally, the NH proton in the trifluoroacetamide derivative engages in extended H-bond interactions in its crystal structure.

Mechanism and Origin of the Unexpected Chemoselectivity in Fluorocyclization of o-Styryl Benzamides with a Hypervalent Fluoroiodane Reagent

The mechanism and origin of the unexpected chemoselectivity in fluorocyclization of o-styryl benzamide with a cyclic hypervalent fluoroiodane reagent were explored with DFT calculations. The calculations suggested an alternative mechanism that is broadly similar to, but also critically different from, the previously proposed mechanism for the formation of an unexpected structurally novel seven-membered 4-fluoro-1,3-benzoxazepine. The amide group of o-styryl benzamide was revealed to be crucial for activating the fluoroiodane reagent and facilitating C-F bond formation. In contrast to the popular electrophilic N-F reagent Selectfluor, the F atom in the fluoroiodane reagent is nucleophilic, and the I(III) atom is the most electrophilic site, thus inducing a completely different reactivity pattern. The insights reported here will be valuable for the further development of new reactions based on the hypervalent fluoroiodane reagent.

A Au(I)-Precatalyst with a Cyclopropenium Counterion: An Unusual Ion Pair

The synthesis and X-ray crystal structure of a novel Au(I)-precatalyst applied to intermolecular alkyne hydroamination is reported. Density functional theory (DFT) calculations revealed the cyclopropenium counterion of this Au(I)-precatalyst imparts stability through H-bonding and other noncovalent interactions.

Diffuse Vibrational Signature of a Single Proton Embedded in the Oxalate Scaffold, HO2CCO2(-)

To understand how the D2d oxalate scaffold (C2O4)(2-) distorts upon capture of a proton, we report the vibrational spectra of the cryogenically cooled HO2CCO2(-) anion and its deuterated isotopologue DO2CCO2(-). The transitions associated with the skeletal vibrations and OH bending modes are sharp and are well described by inclusion of cubic terms in the normal mode expansion of the potential surface through an extended Fermi resonance analysis. The ground state structure features a five-membered ring with an asymmetric intramolecular proton bond. The spectral signatures of the hydrogen stretches, on the contrary, are surprisingly diffuse, and this behavior is not anticipated by the extended Fermi scheme. We trace the diffuse bands to very strong couplings between the high-frequency OH-stretch and the low-frequency COH bends as well as heavy particle skeletal deformations. A simple vibrationally adiabatic model recovers this breadth of oscillator strength as a 0 K analogue of the motional broadening commonly used to explain the diffuse spectra of H-bonded systems at elevated temperatures, but where these displacements arise from the configurations present at the vibrational zero-point level.

Synthesis and characterization of a novel N-F reagent derived from the ethano-Tröger's base: (1)J(FN) coupling constants as a signature for the N-F bond

Methylation of 2,8-dimethyl-6H,12H-5,11-ethanodibenzo[b,f][1,5]-diazocine (ethano-Tröger's base) with methyl iodide followed by ion metathesis and fluorination with N-fluoro-2,3,4,5,6-pentachloropyridinium triflate affords a new electrophilic N-F reagent, that is more reactive than Selectfluor. 2D (19)F-(15)N HMQC experiments provide (1)JNF coupling constants which are diagnostic for the N-F functional group.

Friedel-Crafts reaction of benzyl fluorides: selective activation of C-F bonds as enabled by hydrogen bonding

A Friedel-Crafts benzylation of arenes with benzyl fluorides has been developed. The reaction produces 1,1-diaryl alkanes in good yield under mild conditions without the need for a transition metal or a strong Lewis acid. A mechanism involving activation of the C-F bond through hydrogen bonding is proposed. This mode of activation enables the selective reaction of benzylic C-F bonds in the presence of other benzylic leaving groups.

Pyrazole-5-carboxamides, novel inhibitors of receptor for advanced glycation end products (RAGE)

In an effort to develop novel inhibitors of receptor for advanced glycation end products (RAGE) for the treatment of Alzheimer's disease, a series of pyrazole-5-carboxamides were designed, synthesized and biologically evaluated. Analyses of the extensive structure-activity relationship (SAR) led us to identify a 4-fluorophenoxy analog (40) that exhibited improved in vitro RAGE inhibitory activity and more favorable aqueous solubility than the parent 2-aminopyrimidine, 1. Surface plasmon resonance (SPR) and molecular docking study strongly supported the RAGE inhibitory activity of pyrazole-5-carboxamides. The brain Aβ-lowering effect of 40 is also described.

Triol-promoted activation of C-F bonds: Amination of benzylic fluorides under highly concentrated conditions mediated by 1,1,1-tris(hydroxymethyl)propane

Activation of the C-F bond of benzylic fluorides was achieved using 1,1,1-tris(hydroxymethyl)propane (2) as a hydrogen bond-donating agent. Investigations demonstrated that hydrogen bond-donating solvents are promoting the activation and hydrogen bond-accepting ones are hindering it. However, the reaction is best run under highly concentrated conditions, where solvents cannot interfere with the interaction between the organofluorine compound and the triol. Various benzylic fluorides react with secondary amines or anilines to form benzylic amines in good yields.

Enabling nucleophilic substitution reactions of activated alkyl fluorides through hydrogen bonding

It was discovered that the presence of water as a cosolvent enables the reaction of activated alkyl fluorides for bimolecular nucleophilic substitution reactions. DFT calculations show that activation proceeds through stabilization of the transition structure by a stronger F···H2O interaction and diminishing C-F bond elongation, and not simple transition state electrostatic stabilization. Overall, the findings put forward a distinct strategy for C-F bond activation through H-bonding.