Journal of Physical Organic Chemistry最新文献

Unreactive C-H bond activation is a new horizon for frustrated Lewis pairs (FLPs) chemistry. Although concerted mechanism (Science 2015, 349, 513) and stepwise carbene mechanism (Org. Lett. 2018, 20, 1102) have been proposed for the FLPs catalyzed C-H bond activation of 1-methylpyrrole, the influence of electronic properties of FLPs on the reaction mechanism is far away from well-understood. In this study, an assortment of P-B type FLPs with different electronic characteristic was employed to study the catalyzed C-H bond activation of 1-methylpyrrole by using density functional theory calculations. Detailed calculations demonstrated that the reactivity variation and the reaction mechanism binary of FLPs catalyzed C-H activation can be varied by tuning electronic effect of Lewis base center. On the one hand, the concerted C-H activation reactivity is mainly controlled by the electron donation of the lone pair of Lewis base center; thus, the FLPs with electron-donating substituents (FLP1, FLP2, and FLP3) catalyzed the C-H bond activation through concerted mechanism. On the other hand, the reactivity of stepwise carbene mechanism is mostly attributed to the vacant orbital of Lewis acid center; as a result, the FLP5 bearing -P(C₆F₅)₂ preferred to catalyzed the bond activation through concerted mechanism. In contrast, a metathesis mechanism through strained four-membered ring transition state is less feasible. These results should provide deeper insight and broader perspective to understand the structure and function of FLPs for rational design of FLPs catalyzed C-H bond activation.

Benzo[a]fluoranthene (4a) is one of non-alternate polycyclic aromatic hydrocarbons. A reaction of 4a in CF₃SO₃H yielded a dark yellow solution. Direct NMR observation indicated the exclusive formation of carbocation 4aH⁺ by protonation at the C(8) position. The most deshielded ¹H and ¹³C signals were observed at 8.99 ppm for H(12) and 8.29 ppm for H(1), 182.6 ppm for C(12b), 155.6 ppm for C(8a), and 149.4 ppm for C(7a). The signals for H(4) and H(5) were significantly more shielded than those of 4a. A reaction of 3-tert-butylbenzo[a]fluoranthene (4b) in CF₃SO₃H afforded carbocation 4bH⁺ by the protonation at the C(8) position. 4bH⁺ was gradually converted to 4aH⁺. The changes in ¹³C NMR chemical shifts (Δδ¹³C) suggested that positive charge was delocalized into mainly seven carbons in 4aH⁺ and 4bH⁺. The observed cations were found to be the most stable cations among the possible protonation cations by the DFT method. The NICS(1)_zz values for the five-membered rings were calculated to be 35.6 for 4aH⁺ and 34.4 for 4bH⁺ by GIAO-B3LYP/6-311+G(2d,p). The experimental NMR and the NICS(1)_zz data indicated that the five-membered rings in 4aH⁺ and 4bH⁺ exhibited anti-aromaticity.

Analysis of molecular electrostatic potential (MEP) on the surface of high-energy molecules is often used to predict detonation properties of these compounds since strong positive values of electrostatic potentials in the central molecular regions are related to the high sensitivity towards detonation. In this work, we combined bond dissociation energy (BDE) calculations with analysis of the MEPs to reveal the influence of the halogen substituents on the sensitivities towards detonation of a series of halogen-substituted dinitronaphthalenes. Obtained results showed that halogen substituents affect detonation properties of the studied molecules by tilting the neighboring NO₂ groups, which results in decreased stability of corresponding C–N bonds. In addition, halogen atoms affect the detonation properties of studied molecules by modifying the positive values of the electrostatic potentials in the central regions of the molecular surfaces.

Isothiazolones are important heterocyclics with pharmacological potency such as anti-inflammatory, anticancer, antimicrobial, and robust biocidal (used in agrochemicals). This study seeks to provide mechanistic insight into the chemo- and regio-selectivities of the [3 + 2] cycloaddition reaction of 5-benzoyl-3(2H)-isothiazolone (A1) with two stable nitrile oxides, that is, mesitonitrile oxide (A2) and dichlorobenzonitrile oxide (A3) using M06-2X hybrid density functional calculations coupled with the 6-311G (d, p) basis sets. Mesitonitrile oxide A2 chemo-selectively adds across the carbonyl of the benzyl group of A1 while dichloro benzonitrile oxide A3 preferentially adds across the ethylene center of A1. Derivatization of A1 with electron-donating groups lowers the activation barriers by a very minute margin ranging from 0.1 to 0.5 kcal/mol whereas electron-withdrawing groups significantly decrease the energetics of the reaction by a margin of 1.1 to 2.5 kcal/mol. Solvation with chloroform does not affect the selectivity of the reaction but tends to increase both activation and reaction energies of the various routes. Analysis of the Parr function on different reactive sites of A1 shows the addition of A2 via the atomic center with the largest Mullikan atomic spin densities. Substitution of the S-heteroatom with C, O, or N does not affect the regioselectivity of the reaction but lowers the activation energies in the reaction of A1 with A3. The global electron density transfer (GEDT) values predict a polar reaction between A1 and A2 whereas the reaction of A1 and A3 is non-polar.

This study explores the potential mechanisms (Paths 1 and 2) involved in the regioselective dipolar cycloaddition of thioamides, selenoamides, and amides with propargyl alcohol using density functional theory (DFT). Our calculations reveal that the initial step involves the formation of a cation with catalyst. Subsequently, isomerization occurs between cations I and II via 1,3-hydride transfer in the second step. We analyzed the global reactivity index and frontier molecular orbital (FMO) theory to gain insights into the mechanism. In the third step, chalcoamides attack cations I and II, forming an intermediate. The formation of a five-member ring intermediate constitutes the fourth step, followed by hydrogen transfer to produce stable five-member heterazole compounds in the final step. We demonstrated the influence of substituents in the electrophile by employing various electron-withdrawing and donating groups. Additionally, we examined the effect of the dielectric medium on the reaction barrier using polarizable continuum model. Thus, this study provides valuable insights for the rational design of more efficient 1,3-dipolar cycloaddition reactions yielding regioselective products.

Paddlanes are tricyclic molecules with four bridging chains that share the same two bridgehead carbon atoms. The smallest known paddlane containing only carbon atoms in the main framework has a total of 18 atoms. We utilized computational chemistry (MP2/cc-pVTZ) to locate the smallest possible all-carbon paddlanes that meet the criterion suggested by Hoffmann, Schleyer, and Schaefer as being “fleeting,” meaning frequency calculations suggest the structures are energy minima on their respective potential energy surfaces (PES). Our results suggest that paddlane compounds with a total of 10 or fewer atoms can be considered to be non-viable either because they are not energy minima on the PES or due to strain in the system that manifests as especially long carbon–carbon bonds. Some paddlanes with a total of 11 carbon atoms proved to be energy minima but still exhibited longer carbon–carbon bonds than normal. Finally, several paddlanes with a total of 12 carbons appear to be the least strained since carbon–carbon bond lengths remain reasonable.

The Latin American Conference on Physical Organic Chemistry (CLAFQO) is recognized as the most important meeting of Physical Organic Chemistry in Latin America. CLAFQO-15 took place in Santa Catarina, Brazil, from November 13 to 18, 2022. The Conference was attended by 208 researchers. This special issue of the JPOC includes a total of 7 contributions based on works presented at the CLAFQO-15, representing a significant sample of the high quality of the work presented during the Conference.

The condensation of 1,3-diketones with hydrazine to access 4H-pyrazoles is a well-established synthetic route that travels through a 4H-pyrazol-1-ium intermediate. In the route to a 3,5-diphenyl-4H-pyrazole containing a cyclobutane spirocycle, density functional theory calculations predict, and experiments show that the protonated intermediate undergoes a rapid 1,5-sigmatropic shift to form a tetrahydrocyclopenta[c]pyrazole. Replacing the 3,5-diphenyl groups with 2-furanyl groups decreases the calculated rate of the 1,5-sigmatropic shift by 6.2 × 10⁵-fold and enables the isolation of new spirocyclic 4H-pyrazoles for click chemistry.

Molecular dynamics simulations were carried out to study the structural and diffusion properties of a cyanobiphenyl monolayer composed of pentyl cyano biphenyl (5CB) and pentyloxy cyano biphenyl (5OCB) molecules on a graphene surface coated with alkane and alcohol molecules. To investigate the diffusion properties of 5CB and 5OCB molecules on the graphene surface, the molecules of pentane ($�C_5$) and dodecane ($�C_{12}$) from alkanes and dodecanol ($�C_{12}�\mathrm{OH}$) from alcohols with a polar hydroxyl ($�-�\mathrm{OH}$) terminal functional group were chosen as orienting surfactant molecules. It was found that the diffusion ability of cyanobiphenyl molecules in a graphene substrate coated with alkane and alcohol molecules depends on the chain length of these molecules and on the presence of polar hydroxyl –OH terminal groups. With an increase in the length of alkane molecules from $�C_5$ to $�C_{12}$, the value of the diffusion coefficient $�D_z�\left(�5�\mathrm{CB}�\right)$ (or $�D_z�\left(�5�\mathrm{OCB}�\right)$), it decreases slightly, while the presence of the polar hydroxyl $�-�\mathrm{OH}$ terminal group leads to a significant decrease in this diffusion coefficient of cyanobiphenyl molecules.

We devised a quantum chemical simulation protocol that can rapidly and accurately predict ground and excited state reduction potentials (E⁰ and E*, respectively) of acridinium photoredox catalysts (PCs). To bypass time-consuming excited state geometry optimizations, we used ground state equilibrium geometries to compute the electronic energy of excited states, which provides reasonable E⁰ and E* estimates. The contribution of Hartree-Fock exchange (HFX) in density functionals was systematically varied to estimate E⁰ and E*. In addition to reproducing experimental results, physically sensible models, such as correct descriptions of exciton behavior, are highly necessary. Based on the exciton correlation values, the appropriate amount of HFX in the B3LYP functional was determined to be 30 % to yield physically sensible bound electron-hole pairs for small organic molecules. We also investigated the impact of basis sets on the predictability of E⁰ and E*. Geometry optimizations with B3LYP-D2(HFX 20 %)/6-31G and single point energy refinement with (TD-)B3LYP-D2(HFX 30%)/6-311++G(d,p) yielded the best results. The transferability of the suggested protocol was confirmed with a test set consisting of eight acridinium derivatives. This study can provide reasonably accurate results with relatively small amounts of computational resources, and it is therefore expected to greatly contribute to the development of new acridinium PCs.