In-plane Aromaticity Research Articles

Theoretical predictions on In-Plane aromaticity in double hydrogen transfer reactions between ethane and ethylene

Aromaticity, traditionally understood as a cornerstone concept elucidating the stability, reactivity, and structure of unsaturated organic compounds, has evolved to encompass inorganic molecules, saturated systems with delocalized electrons, and, significantly, transition structures. Our research delves into the exploration of in-plane aromaticity within the transition state (TS) of the Double Hydrogen Transfer reaction (DHTR) between ethylene and ethane, marking a novel approach to understanding cyclic reaction mechanisms. Through the application of density functional theory and verification via the Gaussian09 program’s intrinsic reaction coordinate analysis, our investigation assesses parameters such as aromatic nucleus-independent chemical shift (NICS) and Mayer bond orders to characterize the TS. The findings reveal a distinctly synchronous in-plane aromatic TS, highlighting its aromatic features. This research paves the way for future studies aimed at unravelling the influence of aromaticity on the kinetics and pathways of similar reactions, enhancing our grasp of its pivotal role in chemical processes.

A dodecamethoxy[6]cycloparaphenylene consisting entirely of hydroquinone ethers: unveiling in-plane aromaticity through a rotaxane structure

[n]Cycloparaphenylenes ([n]CPPs, where n is the number of phenylene groups), consisting of 1,4-linked phenylene unit, have attracted much attention due to their cyclic π-conjugated structures and physical properties. However, functionalizing of the benzene rings of smaller [n]CPPs (n < 7) has been a challenge due to ring strain and steric hindrance of the substituents that hampers their synthesis. Here we show successful synthesis of a new [6]CPP derivative with twelve methoxy groups at the 2,5-positions of all benzene rings by utilizing our developed CPP synthesis method via a macrocyclic gold complex. This molecule exhibited a significantly higher oxidation potential caused by the electron-donating ability of the methoxy groups and the tubular molecular conformation, allowing facile oxidation to give dicationic species with in-plane aromaticity. Furthermore, this molecule successfully included with the guest molecules with a flexible alkyl chain in the cavity, enabling the creation of a CPP-based rotaxane, which exploited its mechanically interlocked molecular structure to the first experimental observation that the in-plane aromaticity in the center of the macrocycle.

Generation and Characterization of a Tetraradical Embedded in a Curved Cyclic Paraphenylene Unit.

Unique spin-spin (magnetic) interactions, ring-size effects on ground-state spin multiplicity, and in-plane aromaticity has been found in localized 1,3-diradicals embedded in curved benzene structures such as cycloparaphenylene (CPP). In this study, we characterized the magnetic interactions in a tetraradical consisting of two localized 1,3-diradical units connected by p-quaterphenyl within a curved CPP skeleton by electron paramagnetic resonance (EPR) spectroscopy and quantum chemical calculations. Persistent triplet species with zero-field splitting parameters similar to those of a triplet 1,3-diphenylcyclopentane-1,3-diyl diradical were observed by continuous wave (CW) or pulsed X-band EPR measurements. The quintet state derived from the ferromagnetic interaction between the two triplet diradical moieties was not detected at 20 K under glassy matrix conditions. At the B3LYP/6-31G(d) level of theory, the singlet state was lower in energy than the triplet and quintet states. These findings will aid in the development of open-shell species for material science application.

1,3-Diradicals Embedded in Curved Paraphenylene Units: Singlet versus Triplet State and In-Plane Aromaticity.

Curved π-conjugated molecules and open-shell structures have attracted much attention from the perspective of fundamental chemistry, as well as materials science. In this study, the chemistry of 1,3-diradicals (DRs) embedded in curved cycloparaphenylene (CPPs) structures, DR-(n+3)CPPs (n = 0-5), was investigated to understand the effects of the curvature and system size on the spin-spin interactions and singlet versus triplet state, as well as their unique characteristics such as in-plane aromaticity. A triplet ground state was predicted for the larger 1,3-diradicals, such as the seven- and eight-paraphenylene-unit-containing diradicals DR-7CPP (n = 4) and DR-8CPP (n = 5), by quantum chemical calculations. The smaller-sized diradicals DR-(n+3)CPPs (n = 0-3) were found to possess singlet ground states. Thus, the ground-state spin multiplicity is controlled by the size of the paraphenylene cycle. The size effect on the ground-state spin multiplicity was confirmed by the experimental generation of DR-6CPP in the photochemical denitrogenation of its azo-containing precursor (AZ-6CPP). Intriguingly, a unique type of in-plane aromaticity emerged in the smaller-sized singlet states such as S-DR-4CPP (n = 1), as proven by nucleus-independent chemical shift calculations (NICS) and an analysis of the anisotropy of the induced current density (ACID), which demonstrate that homoconjugation between the 1,3-diradical moiety arises because of the curved and distorted bonding system.

Near-Infrared Fluorescence from In-Plane-Aromatic Cycloparaphenylene Dications.

Cycloparaphenylenes (CPPs) are hoop-shaped conjugated hydrocarbons corresponding to partial structures of fullerenes or armchair carbon nanotubes. Here, we examined the fluorescence properties of a series of [ n]cycloparaphenylene dications ([ n]CPP2+, n = 5-9), which have unique in-plane aromaticity. The fluorescence peak positions of the [ n]CPP2+s shifted to the longer-wavelength region with increasing ring size, reaching the near-infrared region for those with n > 5. The fluorescence quantum yield of [6]CPP2+ was the highest among the [ n]CPP2+s examined in this study, and the value was on the same order as that of carbon nanotubes. The Stokes shifts of [ n]CPP2+s were smaller than those of neutral [ n]CPPs, which do not have in-plane aromaticity. Theoretical calculations indicate that [ n]CPP2+s undergo smaller structural changes upon S0-S1 transition than [ n]CPPs do, and this is responsible for the difference of the Stokes shift. Furthermore, molecular orbital analysis reveals that the S0-S1 transition of smaller [ n]CPP2+s has an electric-dipole-forbidden character due to HOMO → LUMO/HOMO → LUMO+1 mixing. The relatively high fluorescence quantum yield of [6]CPP2+ is considered to arise from the balance between relatively allowed character and the dominant effect of energy gap.

Interplay between aromaticity and strain in double group transfer reactions to 1,2-benzyne

Density Functional Theory calculations are used to explore the double hydrogen atom transfer from different alkanes to 1,2-benzyne. State-of-the-art calculations including the Activation Strain Model of reactivity, Energy Decomposition Analysis, and Valence Bond methods, reveal the origins of the relatively low activation barriers computed for these processes compared to the analogous reaction involving acetylene. In addition, the interplay between the in-plane aromaticity of the corresponding transition states and the variation of the π-aromaticity associated with the benzyne moiety as well as their influence on the barrier heights of the transformations are analyzed in detail.

Synthesis and Characterization of [n]CPP (n = 5, 6, 8, 10, and 12) Radical Cation and Dications: Size-Dependent Absorption, Spin, and Charge Delocalization.

Radical cations and dications of [n]cyclo-p-phenylenes ([n]CPPs, n = 5, 6, 10, and 12), which are the models of those of linear oligo-p-phenylenes without a terminus, were synthesized as hexafluoroantimonate salts by the one- and two-electron chemical oxidation of CPP by NOSbF6 or SbF5. The radical cations, [n]CPP(•+), and dications, [n]CPP(2+), exhibited remarkable bathochromic shifts in their UV-vis-NIR absorption bands, suggesting that [n]CPP(•+) and larger [n]CPP(2+) exhibit longer polyene character than the shorter analogues. The larger bathochromic shift was consistent with the narrower HOMO-SOMO and HOMO-LUMO gaps in larger [n]CPP(•+) and [n]CPP(2+), respectively. In [n]CPP(•+), the spins and charges were equally and fully delocalized over the p-phenylene rings of the CPPs, as noted by ESR. (1)H NMR revealed that the hydrogen of [n]CPP(2+) shifted to a high magnetic field from the neutral compounds due to the diamagnetic ring current derived from the in-plane aromaticity of [n]CPP(2+). The single resonances observed in all [n]CPP(2+) strongly suggest the complete delocalization of the charges over the CPPs. Furthermore, the contribution of biradical character was clarified for [10]- and [12]CPP by VT-NMR experiment and theoretical calculation.

In-plane aromaticity in cycloparaphenylene dications: a magnetic circular dichroism and theoretical study.

The electronic structures of [8]cycloparaphenylene dication ([8]CPP(2+)) and radical cation ([8]CPP(•+)) have been investigated by magnetic circular dichroism (MCD) spectroscopy, which enabled unambiguous discrimination between previously conflicting assignments of the UV-vis-NIR absorption spectral bands. Molecular orbital and nucleus-independent chemical shift (NICS) analysis revealed that [8]CPP(2+) shows in-plane aromaticity with a (4n + 2) π-electron system (n = 7). This aromaticity appears to be the origin of the unusual stability of the dication. Theoretical calculations further suggested that not only [8]CPP(2+) but also all [n]CPP (n = 5-10) dications and dianions exhibit in-plane aromaticity.

Noyori Hydrogenation: Aromaticity, Synchronicity, and Activation Strain Analysis

By means of density functional theory calculations, we have computationally explored the intimacies of the crucial step of Noyori hydrogrogenation reactions of multiple bonds. This process can be considered analogous to the so-called double group transfer reactions. Both kinds of transformations proceed concertedly via the simultaneous migration of two hydrogen atoms/groups in a pericyclic [σ2s + σ2s + π2s] reaction through six-membered transition structures. Despite the structural resemblances of both types of saddle points, significant differences are found in terms of synchronicity and in-plane aromaticity. In addition, the activation strain model has been used to get quantitative insight into the factors which control the corresponding barrier heights. It is found that the presence of a heteroatom in the acceptor moiety is responsible for a remarkable increase of the interaction energy between the reactants which can compensate the destabilizing effect of the strain energy associated with the deformation of the initial reagents leading to low reaction barriers.

Highly Strained Heterometallacycles of Group 4 Metallocenes with Bis(diphenylphosphino)methanide Ligands

A study of the coordination chemistry of different bis(diphenylphosphino)methanide ligands [Ph2PC(X)PPh2] (X = H, SiMe3) with Group 4 metallocenes is presented. The paramagnetic complexes [Cp2Ti{κ(2)-P,P-Ph2PC(X)PPh2}] (X = H (3 a), X = SiMe3 (3 b)) have been prepared by the reactions of [(Cp2TiCl)2] with [Li{C(X)PPh2}2(thf)3]. Complex 3 b could also be synthesized by reaction of the known titanocene alkyne complex [Cp2Ti(η(2)-Me3SiC2SiMe3)] with Ph2PC(H)(SiMe3)PPh2 (2 b). The heterometallacyclic complex [Cp2Zr(H){κ(2)-P,P-Ph2PC(H)PPh2}] (4 aH) has been prepared by reaction of the Schwartz reagent with [Li{C(H)PPh2}2(thf)3]. Reactions of [Cp2HfCl2] with [Li{C(X)PPh2}2(thf)3] gave the highly strained corresponding metallacycles [Cp2M(Cl){κ(2)-P,P-Ph2PC(X)PPh2}] (5 aCl and 5 bCl) in very good yields. Complexes 3 a, 4 aH, and 5 aCl have been characterized by X-ray crystallography. Complex 3 a has also been characterized by EPR spectroscopy. The structure and bonding of the complexes has been investigated by DFT analysis. Reactions of complexes 4 aH, 5 aCl, and 5 bCl did not give the corresponding more unsaturated heterometallacyclobuta-2,3-dienes.

Aromatic Transition States in Nonpericyclic Reactions: Anionic 5-Endo Cyclizations Are Aborted Sigmatropic Shifts

The transition states (TSs) of 5-endo-dig and 5-endo-trig anionic ring closures are the first unambiguous examples of nonpericyclic reactions with TSs stabilized by aromaticity. Their five-center, six-electron in-plane aromaticity is revealed by the diatropic dissected nucleus-independent chemical shifts, -24.1 and -13.7 ppm, respectively, resulting from the delocalization of the lone pair at the nucleophilic center, a σ CC bond, and an in-plane alkyne (or alkene) π bond. Other seemingly analogous exo and endo cyclization TSs do not have these features. A symmetry-enhanced combination of through-space and through-bond interactions explains the anomalous geometric, energetic, and electronic features of the 5-endo ring closure transition state. Anionic 5-endo cyclizations can be considered to be "aborted" [2,3]-sigmatropic shifts. The connection between anionic cyclizations and sigmatropic shifts offers new possibilities for the design and electronic control of anionic isomerizations.

In-Plane Aromaticity in Double Group Transfer Reactions

The main features of double group transfer reactions have been studied under the density-functional theory framework. It is found that a wide range of structure-types and processes including type II-dyotropic reactions and the Meerwein-Ponndorf-Verley reduction take place via highly synchronous in-plane aromatic transition structures. Actually, the orbital topology of these saddle points is equivalent to that which corresponds to a D2h-symmetric aromatic molecule such as pyrazine.

In-Plane Aromaticity in 1,3-Dipolar Cycloadditions. Solvent Effects, Selectivity, and Nucleus-Independent Chemical Shifts

The aromaticity and the regiochemistry of several 1,3-dipolar cycloadditions have been studied computationally. It is found that all the transition structures associated with concerted supra-supra processes are in-plane aromatic, and this aromaticity is compatible with a ring current circulating along the molecular plane. Solvent effects enhance the activation barrier of the reaction and diminish its synchronicity, with little impact on aromaticity. According to our calculations, aromaticity is important but does not determine the regiochemistry of the reaction. Other phenomena such as electrostatic interactions and solvent effects can modify the regiochemical and the stereochemical outcome.

A Simple Ring Current Model for Describing In-Plane Aromaticity in Pericyclic Reactions.

The ring current model allows the characterization of the in-plane aromaticity present in the transition structures of several pericyclic reactions. The differences between in-plane and pi aromaticity can be readily characterized by means of this model. In the presence of a magnetic field, in-plane aromaticity is manifested by a ring current moving along the molecular plane, thus yielding a maximum diamagnetic shielding at the ring point of the electron density. This shielding decays monotonically above and below the molecular plane. In contrast, pi aromaticity can be characterized by two ring currents moving at a certain distance above and below the molecular plane. This distance corresponds to the covalent radius of the atoms involved. Therefore, the diamagnetic shielding is not maximum at the ring point of the electron density.

Biphenylene Dimer. Molecular Fragment of a Two-Dimensional Carbon Net and Double-Stranded Polymer

Biphenylene dimers, which may be considered as “macrocyclic” 2 × 2 fragments of net 1 and dimeric fragments of a double-stranded polymer, 5, are prepared. X-ray crystallography on two polymorphs of the dimer 9 reveals that its bond alternation is analogous to that in biphenylene; the tetraphenylene part of 9 has approximately C2h symmetry with a small dihedral angle (<12°) between the benzene rings in the adjacent biphenylenes. Cyclic voltammetry of 9 gives two reversible waves corresponding to a radical anion and a dianion; the lithium and potassium salts of these anions, 9•-,M+ (M = Li, K) and 92-,2M+ (M = Li, K), were generated and characterized by ESR, NMR, and UV−vis spectroscopies. The spectral data for the dianions are compatible with classical structures; no evidence for “in-plane aromaticity” is found. For polymer 5, a localized band and large density of states near the Fermi level are found, using extended Huckel theory calculations.

In-plane aromaticity and trishomoaromaticity: a computational evaluation

In-plane aromaticity and trishomoaromaticity: a computational evaluation