Silicon-containing formal 4π-electron four-membered ring systems: antiaromatic, aromatic, or nonaromatic?

Density functional theory calculations (B3LYP) have been carried out to investigate the 4π-electron systems of 2,4-disila-1,3-diphosphacyclobutadiene (compound 1) and the tetrasilacyclobutadiene dication (compound 2). The calculated nucleus-independent chemical shift (NICS) values for these two compounds are negative, which indicates that the core rings of compounds 1 and 2 have a certain amount of aromaticity. However, deep electronic analysis reveals that neither of these two formal 4π-electron four-membered ring systems is aromatic. Compound 1 has very weak, almost negligible antiaromaticity, and the amidinate ligands attached to the Si atoms play an important role in stabilizing this conjugated 4π-electron system. The monoanionic bidentate ligand interacts with the conjugated π?system to cause π-orbital splitting. This ligand-induced π-orbital splitting effect provides an opportunity to manipulate the gap between occupied and unoccupied π?orbitals in conjugated systems. Conversely, compound 2 is nonaromatic because its core ring does not have a conjugated π?ring system and does not fulfill the requirements of a Hückel system.     

Stereoselective 6π-electron electrocyclic ring closures of 2-halo-amidotrienes via a remote 1,6-asymmetric induction

A diastereoselective 6π-electrocyclic ring closure employing halogen-substituted 3-amidotrienes via a 1,6-remote asymmetric induction is described. This new asymmetric manifold for pericyclic ring closure further underscores the significance of the allenamide chemistry.     

A versatile enantioselective synthesis of azabicyclic ring systems: a concise total synthesis of (+)-grandisine D and unnatural analogues

Closing in on azacines: We have developed a new six step approach for the rapid and enantioselective synthesis of indolizidine, pyrrolo[1,2-a]azepine, and pyrrolo[1,2-a]azocine azabicyclic systems and their respective lactam congeners, which are found in a host of natural products as well as pharmaceutical preparations. This protocol enables a concise enantioselective total synthesis of (+)-grandisine?D in 16.4?% overall yield from commercial materials (see scheme).     

A Simplified version of the Pariser-Parr-Pople treatment of π-electron systems

The semi-empirical configuration interaction treatment due to Pariser and Parr and to Pople has been modified by reducing the set of electronic interaction integrals to ₁₁ and ₁₂ and by limiting the number of configurations to 10. Hückel MOs are used as basis functions. The dependence of the resonance integral _v on the interatomic distance R _v is simulated by making _v a quadratic function of the Hückel bond orders P _v .

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Zusammenfassung Eine Modifikation des semiempirischen CI-Verfahrens von Pariser, Parr und Pople wurde zur Behandlung von -Systemen herangezogen. Als Vereinfachung wurde der Grad der CI-Matrix auf zehn beschränkt und von den Elektronenwechselwirkungsintegralen nur ₁₁ und ₁₂ berücksichtigt. Hückel-MOs wurden als Basisfunktionen verwendet. Die Abhängigkeit des Resonanzintegrals _v vom interatomaren Abstand R _v wurde durch den Ansatz von _v als quadratische Funktion der Hückel-Bindungsordnung P _v angenähert.

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Résumé Le procédé semi-empirique de l'interaction de configuration selon Pariser, Parr et Pople a été modifié en réduisant l'ensemble des intégrales d'interaction électronique à ₁₁ et limitant le nombre des configurations à 10. Les fonctions de base sont des orbitales moléculaires selon Hueckel. La dépendance entre l'intégrale de résonance _v et la distance interatomique R _v est simulée en posant que _v est une fonction quadratique des ordres de liaisons P _v selon Hückel.

     

On the aromatic character of some ten π-electron systems

The electronic structure of 1,4,5-triaza-cyclohepta-2,6-diene has been studied. The treatment accounts for the failure of detecting, by experiment, a cyclic conjugation in this and related seven membered ring systems.

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Zusammenfassung Die Elektronenstruktur des 1,4,5-triaza-cycloheptadien-2,6 wurde theoretisch untersucht. Das Ergebnis erklärt das Mißlingen von Versuchen, in diesem und verwandten Siebenringsystemen zyklische Konjugation aufzufinden.

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Résumé La structure électronique du 1,4,5-triaza-cycloheptadiène-2,6 a été étudiée. Le résultat explique pourquoi une conjugaison cyclique n'a pas été trouvée expérimentalement ni dans ce composé ni dans d'autres systèmes cycliques semblables à sept membres.

     

π-Relaxation of the ring strain: design of polycyclic unsaturated silicon molecules

Introduction of a double bond into cyclic silanes lowers the ring strain by the cyclic delocalization of π-electrons through the hyperconjugation with the σ bonds, which is favored by the high π-orbital energy of the SiSi bond and the low σ*-orbital energy of the Si-H bonds. The π-relaxation of strains significantly occurs in the small rings. Unsaturated small silicon ring molecules are less strained than the saturated ones and the unsaturated carbon congeners. We calculated a series of polycyclic silicon molecules to confirm the π-relaxation and suggested that some unknown molecules could be prepared due to the low strain.     

Annulations of isoquinoline and β-carboline ring systems: synthesis of 8-oxoprotoberberine derivatives

Annulation processes of isoquinoline and β-carboline compounds have been investigated leading to synthetic routes for the preparation of 8-oxoprotoberberine derivatives. The key steps combined a diene formation/Diels-Alder cycloaddition reaction to afford the targeted polycyclic skeletons. Further oxidative transformations of the cycloadducts produced the 8-oxoprotoberberine type products. The alkaloids of this class are important natural products with a wide range of biological activity and the synthethic methodology described in this paper could prove to be useful for the preparation of the D-ring functionalised derivatives.     

Planarity of ethylene/linear polyene analogues focused on π-electron holding ability of the components

Ethylene/polyene analogues composed of heavier group 14 elements, such as silicon and germanium, do not prefer a planar structure. In the repulsion dominant (RD) model of our previous study mainly focusing on the planarity of hexasilabenzene, it was demonstrated that electron repulsion promotes nonplanarization of heavy benzene analogues. In this study, we have investigated a correlation between intramolecular π-electron transfers (polarization effect) and planarity in various linear unsaturated compounds in order to deepen the RD model. Herein, it was revealed that the ability to hold π-electrons in the planar molecular structure is characteristic of each element. For example, carbon can hold more than one π-electron, whereas silicon and germanium cannot tolerate even one π-electron to keep the planar structure. Thus, π-accepting substituents on the heavy atom were found to make the heavy ethylenes and linear polyenes planar by controlling the number of π-electrons on each skeletal atom.     

Anionic polymerization of ϵ-caprolactam-lix effect of the ratio of reacting components,of the medium and of the ring size on the initial stage of the anionic polymerization of lactams

Effects of the concentrations and ratio of reacting components and of temperature on the kinetics of reaction of N-propionyllactam (I) with potassium salt of lactam (KL) were studied for derivatives of ϵ-caprolactam, 8-octanelactam and 12-dodecanelactam. For ϵ-caprolactam, the initial rates of propagation, acylation of open-chain amide groups and condensation of growth centres were estimated. At the ratio of the starting components [I]₀/[KL]₀ = 0.5-3, the participation of the polymerization reaction is constant, amounting to ca 45% of the overall consumption of 1. The condensation reaction is 14–36% of the total consumption of I; its initial rate passes through a maximum at [I]₀/[KL]₀ = 2. With increasing permittivity of the medium, the total rate of consumption of I increases; in two media with the same bulk permittivity, however, the rates may differ by as much as one order of magnitude.     

A tandem Diels–Alder/Mannich approach to the synthesis of AE and ABE ring analogues of Delphinium alkaloids

The one-pot TiCl₄ catalysed Diels–Alder/Mannich reaction of α-cyanoaminoacrylates with 2-silyloxy-1,4-butadienes gives 6-keto-3-azabicyclo[3.3.1]nonane-1-carboxylates. Reduction of the ketone and alkylation of the resultant alcohol gives 6-alkoxy-3-azabicyclo[3.3.1]nonanes mimicking the AE rings of a number of Delphinium and Aconitum alkaloids, with the same stereochemistry as the natural products.     

Synthesis and properties of conjugated oligoyne-centered π-extended tetrathiafulvalene analogues and related macromolecular systems

Alkynyl-substituted phenyldithiafulvenes have been found to act as versatile building blocks for the construction of π-conjugated molecular rods, shape-persistent macrocycles (SPMs), and conducting polymers. Through Cu(I)-catalyzed alkynyl homocoupling, a series of linear-shaped π-extended tetrathiafulvalene analogues (exTTFs) carrying conjugated oligoynes (ranging from diyne to hexayne) as the central π-bridge were readily prepared. The solid-state properties and reactivities of diyne- and tetrayne-centered exTTFs were characterized by X-ray crystallography and differential scanning calorimetry (DSC), while the electronic properties of the oligoyne-exTTFs were elucidated by UV-vis absorption spectroscopy and density functional theory (DFT) calculations. Cyclic voltammetric analysis showed that the terminal phenyldithiafulvene groups of the oligyne-exTTFs could undergo oxidative coupling to form tetrathiafulvalene vinylogue (TTFV)-linked polymer wires. Through a different synthetic route involving oxidative dimerization and Pd/Cu-catalyzed alkynyl homocoupling, the acetylenic phenyldithiafulvene precursors led to shape-persistent macrocycles where the formation of trimeric macrocycles was particularly favored due to the small ring strain incurred. Finally, spectroelectrochemical studies on these oligoyne and TTF hybrid materials disclosed electrochromic and molecular redox-controlled switching properties applicable to molecular electronic and optoelectronic devices.     

Synthesis and characterization of a stable cyclic gem-bis(phosphaylide): a 4π-electron three-membered heterocycle

The mixed P,S-bisylide 1 reacts with phosphenium salts, affording phosphoniophosphinocarbenes, with elimination of diphenyl sulfide. Using a more compact cyclic phosphenium cation, the reaction leads to the formation of an original three-membered heterocycle featuring a gem-bisylide function. This molecule represents the first isolable analogue of the phosphirenyl anion derivative.     

Normal-coordinate analysis of the TiO6−5 ion

Normal-coordinate analysis for the TiO⁶⁻₅ ion in M₂TiO₅ (M = rare earths) has been made on the basis of C_4ν-symmetry and using the Wilson's GF matrix method. The observed frequencies have been quantitatively assigned according to the obtained values of the potential-energy distributions. Force constants for the different bonds and angles have been calculated. The TiO′ (axial) and TiO (equatorial) stretching force constants were calculated to be 4.15 and 2.48 mdyn Å⁻¹, respectively.     

Energetics of n-π* and π-π* charge-transfer complexes formed between amine donors and chloranil as π-electron acceptor

The heats of formation ofn- ^* and- ^* charge-transfer interactions have been computed from the charge-transfer spectra of molecular complexes formed in the pyridinechloranil and aniline-chloranil systems.

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Zusammenfassung Anhand der Charge-Transfer-Spektren von Molekülkomplexen aus Pyridin-Chloranil und Anilin-Chloranil wurde die Wärme für die Bildung vonn- ^* und -^* Charge-Transfer-Wechselwirkungen berechnet.

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Total synthesis of benzo[c]phenanthridine alkaloids based on a microwave-assisted electrocyclic reaction of the aza 6π-electron system and structural revision of broussonpapyrine

Total syntheses of the des-N-methyl (nor) type of benzo[c]phenanthridine alkaloids 1a-f and 19 and benzo[c]phenanthridine alkaloids, chelerythrine (2d), and broussonpapyrine (2f) were achieved. The key step was the construction of tetracyclic 10,11-dihydrobenzo[c]phenanthridines using a microwave-assisted electrocyclic reaction of the 2-cycloalkenylbenzaldoxime methyl ether 4 as an aza 6π-electron system, which was derived in two steps from a Suzuki-Miyaura cross-coupling reaction of 2-bromobenzaldehyde 6 with 2-(3,4-dihydro-6,7-methylenedioxynaphthyl)boronic acid pinacol ester 7. In addition, the exact structure of broussonpapyrine (2f) (2,3,9,10-tetraoxygenated type) was determined to be chelerythrine (2d).     

Studies on the formation and properties of polypyrrole doped with ionised β-cyclodextrins: influence of the anionic pendants

Journal of Solid State Electrochemistry - Polypyrrole films doped with sulfonated β-cyclodextrin (PPy/sβ-CD) and carboxymethyl β-cyclodextrin (PPy/CMβ-CD) were formed on...     

6π/10π-Electrocyclization of ketene-iminium salts for the synthesis of substituted naphthylamines

An intramolecular 6π/10π-electrocyclization from ketene-iminium salts was developed for the preparation of naphthylamines. Various substituents on the nitrogen, on the aromatic ring, and on the olefin were studied. Tricyclic skeletons were obtained in few steps and good overall yields. The electrocyclization of ketene-iminium salts has been computationally explored by means of DFT calculations and their activation barriers were compared to the parent triene as well as the corresponding dienyl allenes and dienyl ketenes. Electrocyclizations for ketene-iminium salts were shown to be highly exergonic and have much smaller barriers to activation.