Intramolecular Hydrogen-Bond Activation: Strategies, Benefits, and Influence in Catalysis

Dependence of Intramolecular Hydrogen Bond on Conformational Flexibility in Linear Aminoalcohols

Intramolecular hydrogen bonds (H-bonds) are abundant in physicochemical and biological processes. The strength of such interaction is governed by a subtle balance between conformational flexibility and steric effect that are often hard to predict. Herein, using linear aminoalcohols NH2(CH2)nOH (n = 2-5) as a model system, we demonstrated the dependence of intramolecular H-bond on the backbone chain length. With sensitive photoacoustic Raman spectroscopy (PARS), the gas-phase Raman spectra of aminoalcohols were measured in both N-H and O-H stretching regions at 298 and 338 K and explained with the aid of quantum chemistry calculations. For n = 2-4, two conformers corresponding to the O-H···N intramolecular H-bond and free OH were identified, whereas for n = 5, only the free-OH conformer was identified. Compared to free OH, a striking spectral dependence was observed for the intramolecular H-bonded conformer. According to the red shift of the OH-bonded band, the strongest intramolecular H-bond yields in n = 4, but the favorable chain length to form an intramolecular hydrogen bond at room temperature was observed in n = 3, which corresponds to a six-membered-ring in 3-aminopropanol. This is in good agreement with statistical analysis from the Cambridge Structural Database (CSD) that the intramolecular hydrogen bond is preferred when the six-membered ring is formed. Furthermore, combined with the calculated thermodynamic data at the MP2/aug-cc-pVTZ//M062X/6-311++G(d,p) level, the origin of decrease in intramolecular hydrogen-bond formation was ascribed to an unfavorable negative entropy contribution when the backbone chain is further getting longer, which results in the calculated Gibbs free energy optimum changing with increasing temperature from n = 4 (0-200 K) to n = 3 (200-400 K) and to n = 2 (above 400 K). These results will provide new insight into the nature of intramolecular hydrogen bonds at the molecular level and the application of intramolecular hydrogen bonds in rational drug design and supramolecular assembly.

Site-selective post-modification of short α/γ hybrid foldamers: a powerful approach for molecular diversification towards biomedical applications

The extensive research work in the exhilarating area of foldamers (artificial oligomers possessing well-defined conformation in solution) has shown them to be promising candidates in biomedical research and materials science. The post-modification approach is successful in peptides, proteins, and polymers to modulate their functions. To the best of our knowledge, site-selective post-modification of a foldamer affording molecules with different pendant functional groups within a molecular scaffold has not yet been reported. We demonstrate for the first time that late-stage site-selective functionalization of short hybrid oligomers is an efficient approach to afford molecules with diverse functional groups. In this article, we report the design and synthesis of hybrid peptides with repeating units of leucine (Leu) and 5-amino salicylic acid (ASA), regioselective post-modification, conformational analyses (based on solution-state NMR, circular dichroism and computational studies) and morphological studies of the peptide nanostructures. As a proof-of-concept, we demonstrate the applications of differently modified peptides as drug delivery agents, imaging probes, and anticancer agents. The novel feature of the work is that the difference in reactivity of two phenolic OH groups in short biomimetic peptides was utilized to achieve site-selective post-modification. It is challenging to apply the same approach to short α-peptides having a poor folding tendency, and their post-functionalization may considerably affect their conformation.

The Role of H-Bonds in the Excited-State Properties of Multichromophoric Systems: Static and Dynamic Aspects

Given their importance, hydrogen bonds (H-bonds) have been the subject of intense investigation since their discovery. Indeed, H-bonds play a fundamental role in determining the structure, the electronic properties, and the dynamics of complex systems, including biologically relevant materials such as DNA and proteins. While H-bonds have been largely investigated for systems in their electronic ground state, fewer studies have focused on how the presence of H-bonds could affect the static and dynamic properties of electronic excited states. This review presents an overview of the more relevant progress in studying the role of H-bond interactions in modulating excited-state features in multichromophoric biomimetic complex systems. The most promising spectroscopic techniques that can be used for investigating the H-bond effects in excited states and for characterizing the ultrafast processes associated with their dynamics are briefly summarized. Then, experimental insights into the modulation of the electronic properties resulting from the presence of H-bond interactions are provided, and the role of the H-bond in tuning the excited-state dynamics and the related photophysical processes is discussed.

Dynamic Construction and Maintenance of Confined Nanoregions via Hydrogen-Bond Networks between Acetylene Reactants and a Polyoxometalate-Based Metal-Organic Framework

The nanoconfinement effect in catalysis has attracted much attention because it provides a novel means of regulating the molecular properties and related reactions. Confined nanoregions composed of both reactants and catalysts through weak interactions are expected to improve the catalytic performance and promote the mass transport of relevant molecules simultaneously. However, at reaction temperatures, the structural variation of such confined spaces constructed via weak interactions remains unclear. Herein, through density functional theory calculations combined with ab initio molecular dynamics simulations, we have systematically investigated the dynamic structural evolution of the confined space constructed by acetylene reactants and a polyoxometalate-based metal-organic framework (POMOF) via hydrogen-bond networks. It is found that, at the reaction temperature of acetylene semihydrogenation, the hydrogen-bond networks and generated confined nanoregions are not rigid but are constantly changing and dynamically maintained. The steering role played by the O atoms at the surfaces of the polyoxometalate clusters is essential for generation of the hydrogen-bond networks and maintenance of the nanoregions. Upon confinement, the acetylene reactants can be better activated than those in an unconstrained atmosphere, which is reflected by the different dynamic distributions of the ∠CHC bending magnitude. Moreover, from a comparison of the distinct interaction characteristics between acetylene/ethylene and POMOF, the different manifestations in the adsorption energy variations of the confined molecules can be interpreted. This work helps to elucidate the underlying mechanisms of confined catalysis and may promote its application in practical catalytic processes.

Isothiourea-catalyzed formal enantioselective conjugate addition of benzophenone imines to β-fluorinated α,β-unsaturated esters

The isothiourea-catalyzed formal enantioselective conjugate addition of 2-hydroxybenzophenone imine derivatives to α,β-unsaturated para-nitrophenyl esters has been developed. Investigations of the scope and limitations of this procedure showed that β-electron withdrawing substituents within the α,β-unsaturated ester component are required for good product yield, giving rise to a range of β-imino ester and amide derivatives in moderate to good isolated yields with excellent enantioselectivity (20 examples, up to 81% yield and 97 : 3 er).