Journal of Physical Organic Chemistry最新文献

The Curtin–Hammett principle, widely recognized in thermal reactions, has been extended to photosensitization processes in this study, providing new insights into the reactivity of photogenerated singlet oxygen (¹O₂) with phenol and phenolate anion species. Here, we explore mechanistic and Curtin–Hammett studies of the equilibrium between the phenol and phenolate anion forms of a prenylated natural product, prenylphloroglucinol. This study uses density functional theory (DFT) to examine phenol and phenolate anion-quenching pathways of ¹O₂ showing distinct pathways for each form. In the phenolate anion, ¹O₂ is quenched to form a peroxy anion. In contrast, in the phenol form, ¹O₂ leads to a potent epoxidizing agent in a seemingly pro-oxidant path. An iso-hydroperoxyhydrofuran intermediate is proposed to be key in the epoxidation. Meanwhile, the phenolate anion cyclizes and protonates forming a comparatively benign hydroperoxyhydrofuran species. The phloroglucinol is next to the C-prenyated group directs the reaction pathway towards the formation of a dihydrobenzofuran, deviating from the conventional ¹O₂ “ene” reaction mechanism and the production of allylic hydroperoxides typically observed in trisubstituted alkenes.

Nonclassical hydrogen bond (NCHB) as a weak interaction force not only plays an important role in organic conversion and catalysis but also has significant implications for understanding and predicting the properties of compounds. At present, the influence of homoring competition effect of substituents on the intramolecular properties of NCHB has not been studied. In this work, 51 diphenylnitroketone XArCH=N(O)ArY (XPNY) compounds containing different substituents were chosen as model molecules, the bond energy (E_HB) in CH⋯O and aromaticity of quasi-aromatic ring (PDI_Q) were computed, and the chemical shifts δ_H(CH⋯O) of ¹H NMR in CH⋯O were collected or measured. This work focuses on analyzing the NCHB CH⋯O hydrogen bond in compounds XPNY and explored the influence of homoring competition effect of substituents on the energy E_HB of CH⋯O and chemical shift value δ_H(CH⋯O). The results show that (1) the chemical shift values δ_H(CH⋯O) of CH⋯O in XPNY are abnormally large, and the values of E_HB are 14.15–15.51 kJ/mol, lower than 16.74 kJ/mol, which is consistent with the characteristics of intramolecular NCHB. Therefore, it can be considered that the CH⋯O hydrogen bonds are NCHB in XPNY. (2) The substituent X in compound XPNY exhibits the homoring competition effect. (3) The aromaticity of quasi-aromatic ring Q is the main factor affecting the E_HB. The substituent X affects the E_HB and δ_H(CH⋯O) along different paths. This study can help understanding the impact of substituent effects on NCHB in depth.

Although the energies of intermolecular hydrogen bonds, E_HB, can be ascertained by a variety of approaches, there is not a general method to accurately determine the energies for intramolecular hydrogen bonds, E_IMHB. Structures for compounds X(CH₂)_nOH {X = F, OH, NH₂, Cl, Br, SH; n = 4–5} are calculated and then “clipped” to form complexes CH₃CH₂X•CH₃CH₂OH such that the critical geometric, spectroscopic, and electron density features are preserved. The E_IMHB of the parent molecule is assumed to equal the E_HB of the complex. Of the previous methods of determining E_IMHB, the molecular tailoring approach (MTA) comes closest to the values from this work with the differences due to incomplete cancellation of conformational effects in the MTA. In general, parametric methods fare poorly, only being effective for groups of similar molecules. The cis–trans and isodesmic approaches are of limited value for longer carbon chains due to conformational strain.

The NHC/photoredox cocatalysis proposes an alternative to conventional methods for acylation of the unsaturated bonds. But it is still a great challenge to effectively control the stereoselectivities stemming from couplings of the ketyl radicals and the alkyl radicals. In this work, we have selected the photo-/NHC cocatalyzed fluoroaroylation of benzofurans as the computational model for density functional theory (DFT) studies, intending to unveil mechanisms of the fluorine-induced diastereoselectivities and thus to provide guidance to future rational design of promising catalytic reactions with high stereoselectivities. The computational results reveal that a fluorine-bearing zwitterion intermediate is yielded after nucleophilic addition of the NHC catalyst to the acyl fluorides, because fluorine anion is a poor leaving group. The cross-coupling of the ketyl radical and the tertiary carbon radical is the diastereoselectivity-determining step, and the configuration with the bulky NHC-stabilized ketyl radical bonded trans to the α-fluorine substitution is energetically favor over that giving the cis configuration. The distortion–interaction energy calculations combined with the geometry analysis indicate that the repulsion of the α-fluorine atom on the neighboring carbon results in significant distortions of the two coupling partners in the cis-configuration transition state and therefore leads to high diastereoselectivities. In addition, it is unveiled that the nucleophilicity of the carbene atom could be substantially influenced by electron delocalization. Moreover, the steric hindrance arose from the N(2)-phenyl group warrants attention as it may reduce remarkable geometry distortions with approach of the acyl fluoride compound.

This work aims to explore and characterize citrate as an anion of nontoxic and biocompatible origin, which is a crucial step to developing a sustainable CO₂ capture process through ionic liquids (ILs). Citrate ILs have recently been synthesized and utilized as solvents and catalysts for various synthetic purposes in the industry. These are found to be easily recycled, nonpolluting, less corrosive, and easy to synthesize. In this work, citrate–CO₂ and citrate–bmim (1-butyl-3-methylimidazolium) ion pair (IP)–CO₂ interactions have been theoretically explored via carboxylation reactions and various electronic structure calculations. The results indicate favorable citrate–CO₂ interactions in the gas as well as the aqueous phase resulting in monocarboxylates, dicarboxylates, and tricarboxylates of citrates owing to the availability of its three carboxylate O atoms. Even as citrate is paired with bmim, it shows the possibility of multiple site CO₂ absorptions. This system should thus serve as a pathway for enhanced CO₂ capture and better desorption by reducing the formation of carbene–CO₂ complex (reduced basicity of the anion and enhanced steric hindrance of the cation). The study reveals that in the IP, at least one of the citrate O atoms can form a covalent carboxylate (chemisorption) with CO₂ while other available O sites may weakly bind CO₂ (physisorption).

Using cyclic voltammetry, the electrochemical properties and electrocatalytic activity in the reaction of molecular hydrogen formation in the presence of 2,2′-bipyridinium-1,1′-diium and 2,2′-bipyridinium-1-ium perchlorates were studied. The electrochemical reduction product of 2,2′-bipyridinium-1,1′-diium perchlorate decomposes to form molecular hydrogen, but in the case of 2,2′-bipyridinium-1-ium perchlorate, a two-electron reduction product is formed −(E)-1H,1′H-2,2′-bipyridinylidene. It has been shown that the hydrogen evolution reaction (HER) in the presence of 2,2′-bipyridinium-1,1′-diium and 2,2′-bipyridinium-1-ium perchlorates occurs at the same potentials but through different mechanisms. In both cases, the potential is −0.85 V according to the CECE mechanism, but at a potential of −1.25 V according to the ECEC mechanism. In both cases, the key intermediate, through which both mechanisms are realized, is (E)-1H,1′H-2,2′-bipyridinylidene.

Bridgehead radicals are increasingly commonly-encountered intermediates in organic chemistry. These carbon-centered radicals, however, may experience unique strain energies due to pyramidalization at the bridgehead carbon as enforced by the bicyclic frameworks. In this study, a series of related bicyclic bridgehead radicals were analyzed using the G3MP2B3 computational protocol. The adamantyl and cubyl radicals were also included in the study. The energies of the radicals were analyzed with reference to the similarly-substituted tert-butyl radical via a hyperhomodesmotic equation. While the relative energies were found to weakly correlate with the change in pyramidalization at the bridgehead carbons upon radical formation, a much stronger correlation was observed with the difference in strain energies between the saturated bicyclic compounds and the corresponding bridgehead radicals.

Fluoride ion possessing substantial biological importance demands widespread applications on the basis of physiological and detrimental effects on human body and environmental aspects. Therefore, attractive attention has been paid by the researchers to reach the utmost stage of development of versatile colorimetric and fluorometric fluoride ion sensors. For this purpose, proton transfer phenomenon is an excellent pathway to trigger the recognition of fluoride ion using various kinds of sensor molecules. A strong mode of binding interaction between the chemosensor and F⁻ ion was streamlined in terms of colorimetry, fluorometry and NMR- and DFT-based theoretical studies. This minireview article summarizes the recent development of various fluoride sensors based upon the proton transfer–mediated signalling mechanisms, namely, hydrogen bonding–promoted proton transfer, excited state intermolecular proton transfer (ESPT) and excited state intramolecular proton transfer (ESIPT).