Amnon Stanger

What is… aromaticity: a critique of the concept of aromaticity—can it really be defined?

Aromaticity has been a basic concept in chemistry for many years, yet it is poorly understood. This Feature Article introduces some questions regarding the concept, describes major paradigms in the field, and emphasizes the contradictions and paradoxes between these paradigms, and between different measures of aromaticity.

The NICS-XY-scan: identification of local and global ring currents in multi-ring systems.

Nucleus-independent chemical shift (NICS)-based methods are very popular for the determination of the induced magnetic field under an external magnetic field. These methods are used mostly (but not only) for the determination of the aromaticity and antiaromaticity of molecules and ions, both qualitatively and quantitatively. The ghost atom that serves as the NICS probe senses the induced magnetic field and reports it in the form of an NMR chemical shift. However, the source of the field cannot be determined by NICS. Thus, in a multi-ring system that may contain more than one induced current circuit (and therefore more than one source of the induced magnetic field) the NICS value may represent the sum of many induced magnetic fields. This may lead to wrong assignments of the aromaticity (and antiaromaticity) of the systems under study. In this paper, we present a NICS-based method for the determination of local and global ring currents in conjugated multi-ring systems. The method involves placing the NICS probes along the X axis, and if needed, along the Y axis, at a constant height above the system under study. Following the change in the induced field along these axes allows the identification of global and local induced currents. The best NICS type to use for these scans is NICSπZZ , but it is shown that at a height of 1.7 Å above the molecular plane, NICSZZ provides the same qualitative picture. This method, namely the NICS-XY-scan, gives information equivalent to that obtained through current density analysis methods, and in some cases, provides even more details.

Cyclopropenylcarbinol Derivatives as New Versatile Intermediates in Organic Synthesis: Application to the Formation of Enantiomerically Pure Alkylidenecyclopropane Derivatives

The copper-catalyzed carbomagnesiation (or hydrometalation) reaction of chiral cyclopropenylcarbinol derivatives, obtained by means of a kinetic resolution of secondary allylic alcohols, leads to an easy and straightforward preparation of enantiomerically pure alkylidenecyclopropane derivatives. The reaction mechanism is composed of a syn-carbometalation followed by a syn-elimination reaction. To gain further insight into the reaction mechanism of the carbometalation, the diastereoselective formation of cyclopropylcarbinol was also achieved and was found to be very sensitive to the nature of the organometallic species used for the addition reaction. Cyclopropylcarbinol could also be prepared through a diastereoselective reduction of cyclopropenylcarbinol derivatives. Finally, functionalization of enantiomerically enriched cyclopropenylcarbinols into the corresponding acetate or phosphinite derivatives leads, under mild conditions, to various enantiomerically pure heterosubstituted alkylidenecyclopropanes.

The Impact of Antiaromatic Subunits in [4n+2] π-Systems: Bispentalenes with [4n+2] π-Electron Perimeters and Antiaromatic Character

Three series of stable, neutral, π-extended bispentalene derivatives, with two pentalenes fused to a central benzene or naphthalene moiety, have been prepared through a modified double carbopalladation cascade reaction. While these chromophores feature skeletons with [4n+2] π-electron perimeters, the two 8 π-electron pentalene subunits strongly influence bonding and spectral properties. (1)H NMR spectra showed large upfield shifts of the protons in the pentalene moieties, comparable to antiaromatic monobenzopentalenes. Further investigations on magnetic ring currents through NICS-XY-scans suggest a global paratropic current and a local diatropic current at the central benzene ring in two of the series, while the third series, with a central naphthalene ring, showed more localized ring currents, with stronger paratropic ring currents on the pentalene moieties. X-ray diffraction analyses revealed planar bispentalene cores with large double- and single-bond alternation in the pentalene units, characteristic for antiaromaticity, and small alternation in the central aromatic rings. In agreement with TD-DFT calculations, both optical and electrochemical data showed much smaller HOMO-LUMO energy gaps compared to other neutral, acene-like hydrocarbons with the same number of fused rings. Both experimental and computational results suggest that the molecular properties of the presented bispentalenes are dominated by the antiaromatic pentalene-subunits despite the [4n+2] π-electron perimeter of the skeletons.

The anomeric effect at silicon

The anomeric effect at silicon has been studied by ab initio calculations. The geometries and energies of H2Si(XH)2, X = O, S were optimized with the basis sets 3-21G, 3-21G(*), and 6-31G *. Single point MP3/6-31G * //6-31G * calculations were also carried out. H2Si(XCH3)2, X = O, S, were studied with the 3-21G basis set. All compounds are most stable in the gauche, gauche conformation, pointing to the operation of an anomeric effect. The total anomeric interactions given by the energies of the equations: H2Si(XH)2 + SiH4 → 2H3SiXH are (MP3/6-31G *): 8.6 and 2.2 kcal/mol for X = O and X = S, respectively. Rotation barriers on going from the (g,g), to the (g,a) and to the (a,a) conformations are (MP3/6-31G *, kcal/mol): 2.5 and 3.8 in H2Si(OH)2 and 2.1 and 3.2 in H2Si(SH)2. Thus, the anomeric effect in H2Si(OH)2 is large, although smaller than in H2C(OH)2. In H2Si(SH)2 and H2C(SH)2 the anomeric effects are comparable, and both relatively small. The anomeric effect is predicted to be important in determining the conformations of compounds with silicon bonded to 2 oxygens such as R2Si(OR′)2, disilaoxiranes, and related molecules.

Enantio- and diastereoselective tandem zn-promoted brook rearrangement/ene-allene carbocyclization reaction.

The zinc-catalyzed addition of various alkynes to acylsilanes followed by a Zn-Brook rearrangement and either the Zn-ene-allene or Zn-yne-allene cyclization led to the enantio- and diastereoselective formation of carbocycles in a single-pot operation.

An Excursion from Normal to Inverted C-C Bonds Shows a Clear Demarcation between Covalent and Charge-Shift C-C Bonds

What is the nature of the C-C bond? Valence bond and electron density computations of 16 C-C bonds show two families of bonds that flesh out as a phase diagram. One family, involving ethane, cyclopropane and so forth, is typified by covalent C-C bonding wherein covalent spin-pairing accounts for most of the bond energy. The second family includes the inverted bridgehead bonds of small propellanes, where the bond is neither covalent nor ionic, but owes its existence to the resonance stabilization between the respective structures; hence a charge-shift (CS) bond. The dual family also emerges from calculated and experimental electron density properties. Covalent C-C bonds are characterized by negative Laplacians of the density, whereas CS-bonds display small or positive Laplacians. The positive Laplacian defines a region suffering from neighbouring repulsive interactions, which is precisely the case in the inverted bonding region. Such regions are rich in kinetic energy, and indeed the energy-density analysis reveals that CS-bonds are richer in kinetic energy than the covalent C-C bonds. The large covalent-ionic resonance energy is precisely the mechanism that lowers the kinetic energy in the bonding region and restores equilibrium bonding. Thus, different degrees of repulsive strain create two bonding families of the same chemical bond made from a single atomic constituent. It is further shown that the idea of repulsive strain is portable and can predict the properties of propellanes of various sizes and different wing substituents. Experimentally (M. Messerschmidt, S. Scheins, L. Bruberth, M. Patzel, G. Szeimies, C. Paulman, P. Luger, Angew. Chem. 2005, 117, 3993-3997; Angew. Chem. Int. Ed. 2005, 44, 3925-3928), the C-C bond families are beautifully represented in [1.1.1]propellane, where the inverted C-C is a CS-bond, while the wings are made from covalent C-C bonds. What other manifestations can we expect from CS-bonds? Answers from experiment have the potential of recharting the mental map of chemical bonding.

Strain induced bond localization in strained aromatic compounds with extended ? systems

The influence of small rings annulated to aromatic rings is usually additive. However, in some systems the behavior is nonadditive, for example, in tricarbonyliron‐cyclobutadienobenzene versus tris(tricarbonylironcyclobutadieno)benzene. A study of dimethylenecyclobutabenzene, 1,3‐bis(dimethylenecyclobuta)benzene, tris(dimethylenecyclobuta)benzene, and their bora, imino, and oxo derivatives is presented. The behavior of these systems ranges from an almost perfect additivity (in the bora derivatives) to nonadditive behavior (in the oxo derivatives) where the single ring annulated system shows a negative difference between the bonds endocyclic and exocyclic to the annulated small ring (negative ΔR; anti‐Mills–Nixon distortion), the doubly annulated system shows ΔR=0, and the triply annulated system shows positive ΔR (Mills–Nixon distortion). The geometrical properties of the systems are analyzed in terms of the strain, bond curvature, and electronegativity of the exocyclic substituent. These atomic and σ properties are found to be responsible for the structural properties whereas π factors are of minor or no importance.

The Different Aromatic Characters of Some Localized Benzene Derivatives

Localized benzene derivatives can be separated into two classes, one that retains large diamagnetic ring currents and a second that loses the diamagnetic ring current. Energetic criteria and NICS scan are used to evaluate the nature of the two classes. Hückel-type treatment, MO analysis, and comparison to model compounds suggest that there is no connection between geometric localization and the loss of aromaticity.

The crystal structures of (R3P)2Ni-anthracene (R = Et, Bu)

Abstract The title complexes have solid state structures in which the Ni atom is bound η 2 to the anthracene. Crystal packing forces and interactions between an alkyl group of one molecule and the anthracene ring system of a second molecule may favour the η 2 structure over the η 4 structure observed in solution.