Bonding properties of singlet and triplet cyclopentane-1,3-diyl diradicals (DRs) having C=O and C≡N groups (S-DR3 and T-DR3) were investigated by time-resolved IR spectroscopy. The C=O and C≡N wavenumber shifts observed upon the formation of S-DR3 and T-DR3 were both quite different from each other, indicating that the effects of the unpaired electrons to the C=O and C≡N bond properties in DR3 strongly depend on its spin multiplicity. The observed difference was discussed based on a resonance hybrid of DRs.

Abstract

Cyclopentane-1,3-diyl diradicals (DRs) provide excellent opportunities to study the properties of diradicals because their lifetimes can be significantly lengthened to up to milliseconds with the introduction of proper substituents. This study investigated the bonding characteristics of singlet and triplet DRs having C=O and p-cyanophenyl groups (S-DR3 and T-DR3) by monitoring the photo-induced formation of the diradicals from their precursor azo compounds using time-resolved IR (TR-IR) spectroscopy. Upon the formation of S-DR3, a C=O stretching wavenumber was upshifted by 22 cm⁻¹, whereas a C≡N stretching one was downshifted by 12 cm⁻¹. The observed shifts indicate that the unpaired electrons increase and decrease the C=O and C≡N bond orders, respectively. The effects of the unpaired electrons in S-DR3 were similar to those observed in our previous TR-IR studies on a singlet cyclopentane-1,3-diyl diradical having C=O but no C≡N groups (S-DR2) and on that having C≡N but no C=O groups (S-DR1), respectively. Contrastingly, upon the formation of T-DR3, the C=O wavenumber was downshifted by 16 cm⁻¹, indicating that the unpaired electrons decrease the C=O bond order. More notably, no detectable shifts were observed in the C≡N stretching wavenumber. These observations are not clearly explained by a model suggested in the previous studies on S-DRs. Here, we discuss and propose a more elaborated resonance hybrid of DRs that can explain the directions and relative magnitudes of the observed wavenumber shifts irrespective of spin multiplicities. We expect that the findings and suggestions presented here will stimulate research in both organic and theoretical chemistry.

Intrinsic conformational preferences (twisted Reichardt's vs. planar Brooker's zwitterions) through metameric induction (chemical perturbation) found to have strong impact on various tensorial and non-tensorial properties. Orbital energies, and because of this, absorption, and charge transfer properties of the metamers were found to be strongly affected. Reichardt's mode was found to be more efficient NLO-phore (large hyperpolarizability) than Brooker's mode. Brooker's mode was found to be effective in addressing the transparency trade-off problem than Reichardt's mode.

Abstract

This contribution reports influences of unusual conformational metamorphosis shown by Reichardt's and Brooker's metameric zwitterions by an earlier work, on various intrinsic electronic and optoelectronic properties. Detailed quantum mechanical investigations were carried out using HF, B3LYP, CAM-B3LYP, and ωB97xD methodologies. Observations suggest that whereas certain properties were directly and strongly influenced by the conformation preferences (twisted vs. planar), others were not strongly inclined to such conformational transformations. Interestingly, even with inherent conformational differences, observed properties were found to have only one major contributing component in each molecule and can be beneficial in one dimensional (1D) or pseudo-1D chromophore design strategies. Both coupled perturbed (CP) and finite field (FF) theories were used to compute dipole moments, polarizabilities, and hyperpolarizabilities, and so on, and excellent agreements (or exact matching results) were observed between the two theories. Reichardt's metamer was found to be more efficient in many aspects than Brooker's metamer. The direct and strong influences of metameric manipulations on structure–property correlations shown in this work can be adopted as a useful strategy for efficient chromophore design. Such a strategy is useful in the field of nonlinear optics, and may also find applications in various other areas of material sciences.

The exclusive mechanism of tandem reactions of fluorinated-oxadiazoles with several conjugated, unconjugated, cyclic, and acyclic dienes have been elucidated. The polarity of both inter- and intra-molecular cycloaddition steps can be influenced by two factors: the nature of the heteroatom present on the diene molecule and the size of the cyclic diene. These factors play a role in determining the reactivity and electron distribution within the diene, thereby impacting the overall polarity of the cycloaddition reactions.

Abstract

The tandem reactions of 2,5-bis(trifluoromethyl)-1,3,4-oxadiazole with conjugated, unconjugated, acyclic, and cyclic dienes have been studied at the M06-2X/6-311++G(d,p) level of theory. The rate-determining step is the initial [4+2] cycloaddition in the tandem reaction of 2,5-bis(trifluoromethyl)-1,3,4-oxadiazole with acyclic, cyclic, conjugated, and unconjugated dienes, whereas the stereochemistry of the tandem adduct is determined by the [3+2] step. The exo-coupling is kinetically favored over the endo-coupling in the initial [4+2] reaction of 2,5-(bis-trifluoromethyl)-1,3,4-oxadiazole with all the considered dienes. In the retro [3+2] step (N₂ extrusion), higher activation energy is required to furnish the carbonyl ylide in the reaction of 2,5-(bis-trifluoromethyl)-1,3,4-oxadiazole with conjugated and unconjugated cyclic dienes as compared with the reaction with unconjugated acyclic dienes. At the stereochemistry [3+2] step, the intermolecular addition is kinetically favored over the intramolecular addition in the [3+2] reaction of 2,5-(bis-trifluoromethyl)-1,3,4-oxadiazole with both conjugated or unconjugated cyclic dienes and unconjugated acyclic dienes. The reaction proceeds with low activation energies when conjugated and unconjugated cyclic dienes are participating, compared with those of unconjugated acyclic dienes. Overall, the tandem process proceeds via an asynchronous one-step mechanism [4+2] coupling in an exo- or endo-cycloaddition fashion, followed by a retro [3+2], which extrudes the N₂, and then the stereo-determining intermolecular or intramolecular [3+2] cycloaddition step, which leads to the tandem products. The polarity of both inter- and intra-molecular cycloaddition steps can be influenced by two factors: the nature of the heteroatom present on the diene molecule and the size of the cyclic diene. These factors play a role in determining the reactivity and electron distribution within the diene, thereby impacting the overall polarity of the cycloaddition reactions.

The influence of the ethylene substitution and the tether in the intramolecular [3+2] cycloaddition reactions of cyclic nitrones have been studied within the Molecular Electron Density Theory. The increase in the polar character of the reaction decreases the activation Gibbs free energies, while the highly polar reactions are disfavored. The preferred regioselectivity in low polar reactions having three methylene units is reversed to that in nitrones separated with four methylene units, in conformity with the experimental outcome.

Abstract

The influence of ethylene substitution and the tether length between the two reacting counterparts on the selectivity and reactivity of the intramolecular [3 + 2] cycloaddition (IM32CA) reactions of cyclic nitrones leading to tricyclic isoxazolidines have been studied within the Molecular Electron Density theory at the MPWB1K/6-311G(d,p) computational level. These zw-type IM32CA reactions follow one-step mechanism, and the activation barrier decreases with the introduction of electron withdrawing (EW) substituent at the alkene moiety in both the intramolecular and intermolecular versions. The IM32CA reactions involving unsubstituted alkene have non-polar character with minimal electron density flux classified as null electron density flux type, while that involving the EW nitro substituted ethylene is more facile with a strong electron density flux from the nitrone to the ethylene moiety, classified as forward electron density flux type. The increase in the polar character of the IM32CA reaction decreases the activation Gibbs free energies associated with these intramolecular processes, while the highly polar IM32CA reactions are disfavored with respect to the intermolecular ones. Interestingly, the preferred regioselectivity observed in low polar IM32CA reactions having three methylene units between the nitrone and ethylene frameworks is reversed to that in nitrones separated with four methylene units, in conformity with the experimental outcome. Finally, electron localization function and quantum theory of atoms-in-molecules studies reveal that, in general, these IM32CA reactions involve early transition state structures in which the formation of new C-C and C-O single bonds have not yet started.

The IQA@IRC protocol enables us to see, in a much deeper detail, how the energy of each atom or group is varying along the IRC. It acts like a magnifying glass, allowing us to see things that are unreachable from the standard IRC analysis.

Abstract

A fruitful debate took place recently in literature, discussing the enhanced Diels–Alder reactivity of tropone derivatives for which the carbonyl polarity was reversed by means of umpolung. Karas et al. sustained that the umpolung increases the antiaromatic character of the ring, affecting the highest occupied molecular orbital (HOMO)/least unoccupied molecular orbital (LUMO) energies, speeding up the reaction. Tiekink et al. challenged this interpretation by sustaining that the asynchronicity of the reaction mechanisms, rather than orbital energy perturbation, was the main responsible for the smaller reaction barriers. We shed light on this dispute by computing full interaction quantum atom (IQA) and quantum theory of atoms in molecules (QTAIM) analyses over complete intrinsic reaction coordinate (IRC) paths for the Diels–Alder reaction of tropone and its umpolung derivatives, using the same systems studied by Karas et al. and Tiekink et al. Our results confirm that the asynchronicity is indeed very high for those reactions with smaller reaction barriers and offer an atom-by-atom and bond-by-bond analysis of the entire IRC pathways. Even though asynchronicity and lower reactions barriers seem to be related, antiaromaticity and lower barriers are related as well, but discussing both these interpretations does not necessarily require arguments on HOMO/LUMO energies to be invoked.

The effect of steric hindrances in tertiary amine and solvent polarity on the regioselectivity and kinetics of the reaction of epichlorohydrin with acetic acid were investigated. Correlations between the reaction rate and the parameters of nucleophilicity and structure of amines as well as the polarity of the solvent were established. The regioselectivity of the reaction was studied by ¹H NMR spectroscopy by the ratio of regioisomeric reaction products.

Abstract

The asymmetric oxirane ring-opening reaction leading to the formation of regioisomeric chlorohydrin esters was studied in the reaction series “acetic acid–epichlorohydrin–tetrahydrofuran (nitrobenzene)–trialkylamine” by kinetic methods and FT-IR spectroscopy. The effect of solvent polarity and the structure of tertiary amines on the regioselectivity and reaction rate were studied. Tertiary amines with comparable basicity but different nucleophilicity and spatial structure were chosen as catalysts. It was shown that in solvents of different polarity, the components of the initial reaction system are present both as hydrogen-bonded complexes and as individual substances. The reaction orders with respect to acid and amine in solvents of different polarity were established. Correlations between the reaction rate and the parameters of nucleophilicity and structure of amines as well as the polarity of the solvent were established. The regioselectivity of the reaction was studied by ¹H NMR spectroscopy using the ratio of regioisomeric reaction products. It was shown that the regioselectivity and rate of catalytic acetolysis of epichlorohydrin are effectively controlled by the structure of tertiary amines and the polarity of the solvent. The scheme of reaction regio-flows was detailed.

Photoexcitation of SmI₂ and related Sm^II-based reagents generate a potent single electron transfer reagent capable of reductions which are otherwise impossible in the ground state. This review article illustrates the role of light in synthetic reactions mediated by SmI₂. Advances made in understanding mechanistic aspects of these reactions are also described.

Abstract

SmI₂ is a versatile reagent in single electron transfer-mediated reductive transformations. Photoexcitation of SmI₂ generates a reactive excited state capable of transferring an electron to substrates that are recalcitrant towards accepting electrons. Synthetic results unequivocally indicate light as a green and sustainable promoter of SmI₂-mediated chemistry, with the potential to replace the suspected carcinogen hexamethylphosphoramide (HMPA). Rate constants of photoinduced electron transfer from SmI₂ are in the range of 10⁷–10⁹ M⁻¹ s⁻¹, which are an order of magnitude higher in comparison with the ground state process. Recent advancement in Eu^II- and Ce^III-based photo-redox catalysis rejuvenated the area of photo-catalyzed reactions of low-valent lanthanides. This review article aims to illustrate the role of photoexcitation on SmI₂-mediated reductive transformations.

Reaction of 1-chloro-2,4-dinitronaphthalene with amino acids and their methyl esters.

Abstract

A variety of novel naphthimindazol-N-oxides and naphththiazol-N-oxide have been prepared in a simple two-step process. The first step involves the reaction of 1-chloro-2,4-dinitronaphthalene with glycine, alanine, glycolic acid, thioglycolic acid, and their methyl esters affording substitution products, the subsequent treatment of which with base furnishes naphthimindazol-N-oxide and naphththiazol-N-oxide derivatives. Stepwise reaction mechanisms via carbanions, nitrogen anions, and spiro Meisenheimer intermediates are proposed. The action of 10% NaOH in dioxane on the substitution products was measured spectrophotochemically, and the kinetic studies suggested that the N-naphthyl glycine and N-naphthyl alanine follow a second-order rate law while S-naphthyl thioglycolic acid is accurately first-order kinetics.

The cover image is based on the Research Article Solvent effects on the sodium borohydride reduction of 2-halocyclohexanones by Daniela Rodrigues Silva, Lucas A. Zeoly, Pascal Vermeeren, Rodrigo A. Cormanich, Trevor A. Hamlin, Célia Fonseca Guerra and Matheus P. Freitas, https://doi.org/10.1002/poc.4556.

This article uses two theories, DFT and TD-DFT, to study a series of Ir complexes through theoretical calculations. By analyzing their luminescence color, molecular frontier orbitals, absorption and spin orbit coupling coefficients in CH2Cl2 solution, and other data, their performance is evaluated. Finally, complexes with high spin orbit coupling coefficients are obtained.

Abstract

In this paper, several Ir (III) complexes with transition metal as the central atom formed by the corresponding combination of two main ligands and three auxiliary ligands have been studied theoretically. The electronic structure, frontier molecular orbital, and spin orbit coupling data are used to analyze its application value in light emitting devices. The density functional theory is used to study (tbi)₂Ir(bpp), (tbi-c)₂Ir(bpp), (tbi)₂Ir(dbm), (tbi-c)₂Ir(dbm), (tbi)₂Ir(pic), and (tbi-c)₂Ir(pic). bpp = (2Z)-3-hydroxy-13-diphenylprop-2-en-1-one; dbm = 1,3-di-phenyl-1, 3-propanedione; pic = picolinate.