The application of a new aromaticity index to six-membered ring heterocycles

A previously described index of aromatic character has now been applied to six-membered ring heterocycles. Of particular interest is its ability to indicate the extent of aromatic character retained by the amide form of potential hydroxyheterocycles.     

Homoanomeric effects in six-membered heterocycles

Structural and energetic consequences of homoanomeric n(X) --> beta-sigma(C-Y) interactions in saturated six-membered heterocycles where X = O, N, S, Se and Y = H, Cl were studied computationally using a combination of density functional theory (B3LYP) and Natural Bond Orbital (NBO) analysis. Unlike the classic anomeric effect where the interacting donor and acceptor orbitals are parallel and overlap sidewise in a pi-fashion, orbital interactions responsible for homoanomeric effects can follow different patterns imposed by the geometric restraints of the respective cyclic moieties. For the equatorial beta-C-Y bonds in oxa-, thia- and selena-cyclohexanes, only the homoanomeric n(X)(ax) --> sigma(C-Y)(eq) interaction (the Plough effect) with the axial lone pair of X is important, whereas the n(X)(eq) --> sigma(C-Y)(eq) interaction (the W-effect) is negligible. On the other hand, the W-effect is noticeably larger than the n(X)(ax) --> sigma(C-Y)(eq) interaction in azacyclohexanes. Hyperconjugation is a controlling factor which determines relative trends in the equatorial beta-C-H bonds in heterocycloxanes. In contrast, all homoanomeric interactions are weak for the respective axial bonds where relative lengths are determined by intramolecular electron transfer through exchange interactions and polarization-induced rehybridization. Although the homoanomeric effects are considerably weaker than the classic vicinal anomeric n(X)(ax)-->alpha-sigma(C-Y)(ax) interactions, their importance increases significantly when the acceptor ability of sigmaorbitals increases as a result of bond stretching and/or polarization. Depending on the number of electrons and the topology of interactions, homoconjugation interactions can be cooperative (enhance each other) or anticooperative (compete with each other). Such effects reflect symmetry of the wave function and can be considered as weak manifestations of sigma homoaromaticity or homoantiaromaticity.     

Substituent effects on the energetics and aromaticity of aminomethylbenzoic acids

The standard (p0 = 0.1 MPa) molar enthalpies of combustion of six aminomethylbenzoic acids were measured at T = 298.15 K by static bomb calorimetry. With these values, the standard molar enthalpies of formation in the crystalline state were obtained. Combining these results with the standard molar enthalpies of sublimation, the standard molar enthalpies of formation in the gaseous phase were derived. For the 10 possible isomers, the obtained experimental results were compared to and correlated with the relative stability obtained by ab initio calculations at the B3LYP/6-311++G(d,p) level of theory. Seeking a better understanding of the aromatic behavior and energetics of aminomethylbenzoic acids in the gas phase, calculations of NICS values, HOMA indices, and dihedral angles between the aromatic carbon and the amino group, Phi(Ar-NHH), were also performed computationally. The significant differences observed in the energetics, as well as in the NICS values, HOMA indices, and Phi(Ar-NHH) dihedral angles for these 10 isomers suggest a strong dependency on the identity and relative position of the three substituents on the benzene ring. This study points out a marked tendency for a decrease of the ring aromaticity, accompanied by an increase in the respective system stability, as the conjugation between the substituents becomes more extensive.     

Criteria for aromaticity of mesoionic heterocycles

The quantum chemical calculations of the basic criteria for aromaticity (nucleus-independent chemical shift (NICS), aromatic stabilization energy (ASE), and parameters of harmonic oscillator model of aromaticity (HOMA), and geometric indices (I ₅)) of 54 mesoionic heterocycles in the 6–31G* split-valence basis set were performed in terms of the density functional theory (DFT) with the B3LYP exchange-correlation hybrid functional. The aromatic nature of the mesoionic heterocycles containing the pyridinium N atom was shown.     

Synthesis and characterisation of four- and six-membered P-Se heterocycles

The preparation, spectroscopic characterisation and crystal structures of [FcP(mu-Se)Se]2, [FcP(mu-Se2)Se]2 and [PhP(mu-Se2)Se]2 are reported. Crystallographic data reveal planar four-membered PSePSe and skewed six-membered P2Se4 rings, respectively, in all cases with trans arrangement of organic substituents and exo selenium atoms. Whilst stable at room temperature in solid state, NMR data suggest the six-membered rings of both the ferrocenyl and phenyl compounds decompose in the solution with loss of red selenium, forming PSe2PSe five-membered rings.     

The aromaticity of heterocycles containing the imine nitrogen

The method of predicting aromaticity based on simple Hückel calculations is applied to heterocyclic systems which contain the imine nitrogen. The necessary heteroatom integrals were determined from thermochemical data. Replacement of C by N in alternant systems produced no dramatic changes, but in nonalternants the effect can be large and unexpected as in the azapentalenes where interesting synthetic possibilities are suggested.     

Substituent effect on the aromaticity of the silolide anion

The effect of the substituents on the planarity and aromaticity of the silolide anion was studied computationally. It was revealed that π-electron acceptor groups (e.g., silyl or trimethylsilyl) at the α position of the ring reduce substantially the inversion barrier about the central silicon increasing the aromaticity according to isomeric stabilization energy (ISE) and NICS values. In the planar and highly aromatic silolide anions, the mesomeric structures with the largest weight exhibit a Si=C double bond and a negative charge which is located at the α or β carbons based on NRT calculations. 2,5-disilylsilacyclopentadienides coordinated by naked Li⁺ have planar minima, however, further coordination by THF molecules (as in a solution) reduces somewhat the flattening of the silicon pyramid. NMR calculations were carried out to understand the connection between the ²⁹Si chemical shift and the aromaticity of the ring. It was revealed that the commonly accepted charge transfer–chemical shift relationship is strongly influenced by the substituents and the counter cation. THF complexation of the Li counter cation has a small influence on the NMR shift.     

Synthesis of benzo-fused five- and six-membered heterocycles by palladium-catalyzed cyclocarbonylation

A novel and simple synthetic methodology, based on palladium-catalyzed cyclocarbonylation reaction, is presented for preparing five- and six-membered benzo-fused heterocycles. A mechanism for the process is also proposed and discussed.     

Local aromaticity of the six-membered rings in pyracylene. A difficult case for the NICS indicator of aromaticity

In this work, we have analyzed the local aromaticity of the six-membered rings (6-MRs) of planar and pyramidalized pyracylene species through the structurally based harmonic oscillator model of aromaticity (HOMA), the electronically based para-delocalization index (PDI), and the magnetic-based nucleus independent chemical shift (NICS) measurements, as well as with maps of ring current density. According to ring currents and PDI and HOMA indicators of aromaticity, there is a small reduction of local aromaticity in the 6-MRs of pyracylene with a bending of the molecule. In the case of NICS, the results depend on whether the NICS value is calculated at the center of the ring (NICS(0)) or at 1 A above (NICS(1)(out)) or below (NICS(1)(in)) the ring plane. While NICS(1)(out) values also indicate a slight decrease of aromaticity with bending, NICS(0) and NICS(1)(in) wrongly point out a large increase of aromaticity upon distortion. We have demonstrated that the NICS(0) reduction in the 6-MRs of pyracylene upon bending is due to (a) a strong reduction of the paratropic currents in 5-MRs and (b) the fact that, due to the distortion, the paratropic currents point their effects in other directions.     

Substituent effects on enthalpies of formation of nitrogen heterocycles: 2-substituted benzimidazoles and related compounds

The enthalpies of combustion, heat capacities, enthalpies of sublimation and enthalpies of formation of 2-tert-butylbenzimidazole (2tBuBIM) and 2-phenylimidazole (2PhIM) are reported and the results compared with those of benzene derivatives and a series of azoles (imidazoles, pyrazoles, benzimidazoles and indazoles). Theoretical estimates of the enthalpies of formation were obtained through the use of atom equivalent schemes. The necessary energies were obtained in single-point calculations at the B3LYP/6-311++G(d,p) on B3LYP/6-31G optimized geometries. The comparison of experimental and calculated values of all studied compounds bearing H (unsubstituted), methyl (Me) ethyl (Et), propyl (Pr), isopropyl (iPr), tert-butyl (tBu), benzyl (Bn) and phenyl (Ph) groups show remarkable homogeneity. The remarkable consistency of both the calculated and experimental results allows us to predict with reasonable certainty the missing experimental values. The crystal and molecular structure of the 2-benzylbenzimidazole (2BnBIM) has been determined by X-ray analysis. The observed molecular conformation permits the crystal being built up through N-H...N hydrogen bonds and van der Waals contacts between the molecules. An attempt has been made to relate the crystal structure to the enthalpies of sublimation.     

A new aromaticity index and its application to five-membered ring heterocycles

A new index of aromatic character has been devised based upon a statistical evaluation of the deviations in peripheral bond orders. The latter can be derived readily from experimentally determined bond lengths. The utility of this index is exemplified by its application to five-membered ring heterocycles and their mesoionic derivatives.     

Quantum-chemical interpretation of recyclization reactions. 10. Photoisomerization of six-membered heterocycles

Singlet photoisomerizations of a number of six-membered heterocycles have been described on the basis of the index approach in the framework of the coupled variant of perturbation theory for a one-electron transition density matrix in the Pariser-Parr-Pople method.For Communication 9, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 357–363, March, 1986.     

A synthesis of aromatic five- and six-membered B-N heterocycles via ring closing metathesis

The ring closing metathesis on appropriate vinyl or allyl aminoboranes (1 or 2) gives azaboracycloalkenes (3 or 4) which can be converted to azaborolides (5) or azaborines (6).