2.3. [4 + 4] Cycloadditions 1058 2.4. Photocycloadditions of Aromatic Compounds 1058 2.4.1. Benzene Derivatives 1058 2.4.2. Condensed Aromatic Compounds 1060 3. Photochemical Rearrangements 1061 4. Cyclizations 1064 4.1. Pericyclizations 1064 4.2. Norrish−Yang Reaction 1066 5. Photochemical Extrusion of Small Molecules 1067 6. Photochemical Electron Transfer 1068 6.1. Photochemical Electron-Transfer Reactions with a Catalytic Sensitizer 1068

Photochemical Reactions as Key Steps in Organic Synthesis

Ball-, Bowl-, and Belt-Shaped Conjugated Systems and Their Complexing Abilities: Exploration of the Concave−Convex π−π Interaction

In 1747, Johann Sebastian Bach wrote a series of canons and fugues on the occasion of his visit to King Frederick the Great of Prussia in Potsdam. His music of homage, the “Musikalisches Opfer” (Musical Offering, BWV1079), includes a canon that is probably one of the oldest man-made examples of an object with Mobius topology.1 The canon cancricans (crab canon) is written for two violins playing the same score in two different directions. If you cut the score, paste it in such way that it forms a band with the notes of the first violin on top and those of the second violin below, fold it lengthwise between the two notes, give one end a half twist, and join both ends, you end up with a endless Mobius-shaped score (Figure 1). After every turn the two violins change their parts. Karl Amadeus Hartmann, Nicolas Slonimsky,2 and others several centuries later wrote further examples of Mobius music. Note that there are two different ways to form the Mobius score, by twisting the strip either clockwise or anticlockwise. The two Mobius strips are enantiomers (the music, unfortunately, is not) and exhibit the highest symmetry a Mobius object can attain: C2. More than 100 years later, August Ferdinand Mobius and Johann Benedict Listing (Figure 2), both students of Carl Friedrich Gauss in Gottingen, published their seminal work on projective planes and one-sided surfaces. Nowadays, the credit goes almost exclusively to Mobius3 after whom the famous band is named, even though Listing published 4 years earlier.4 Both mathematicians mentioned the “Mobius” band in unpublished papers for the first time in 1858; however, again Listing’s unpublished note (July) predates the one of Mobius (September) by 2 months. It would probably be fair to name the one-sided band a “Listing band”. There are good reasons to assume that Listing was not adequately credited because of social rather than scientific reasons.5 Most of the objects we are dealing with in our everyday lives are two-sided. They have an inside and an outside surface like a sphere, a cube, or a torus. This is also true for most two-dimensional mathematical objects. According to the mathematical definition, a normal vector on the surface of a two-sided object cannot be shifted to any other point on the surface without crossing a border or interpreted in anthropomorphic terms: walking upright on the outside surface you cannot enter the inside and vice versa. The Mobius band is probably the simplest and best-known exception (Figure 3). Any closed band with an odd number of 180° twists is one-sided, and those with an even number of 180° twists are two-sided. Unfortunately, the sidedness is not an intrinsic property, because it requires that the surface is embedded in a † In memory of Professor Edgar Heilbronner. * To whom correspondence should be addressed. Telephone: ++49 431 88

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Topology in chemistry: designing Möbius molecules.

This review describes the current state of magnetic criteria of aromaticity. The introduction contains the fundamentals of ring currents in aromatic and antiaromatic systems, followed by a brief description of experimental and computational tools: NMR, diamagnetic susceptibility exaltation, current density analyses (CDA) and nucleus independent chemical shifts (NICS). This is followed by more comprehensive chapters: NMR – focusing on the work of R. Mitchell – NICS and CDA – describing the progress and development of the methods to their current state and presenting some examples of representative work.

Magnetic criteria of aromaticity

Benzo[a]pyrene is a known carcinogen, which derives from fossil fuel combustion, cigarette smoke, and generic biomass combustion including traffic emissions. This potent carcinogen has a well-known mechanism of action, leading to the formation of adducts with the DNA, primarily at guanosine positions. The reactivity and chemistry of this notorious compound are, however, dependent on the electronic configuration of the biologically activated metabolite, the benzo[a]pyrene diol epoxide. The activated metabolite exists mainly as four isomers, which have particular chemical reactivities toward guanosine sites on the DNA. These isomers exert also a different carcinogenicity compared to one another, which is a feature that is conventionally attributed to their geometry. However, the reactivity and properties of the isomers are not fully defined, and a determination of these properties by wavefunction behavior is required. This study reports the electronic properties of the benzo[a]pyrene diol epoxide enantiomers, along with a detailed analysis of the energy landscape, geometry, and electronic configuration of the epoxide ring. The results show that the epoxide ring, the core of the reactivity, bears different properties at the level of wavefunction for each isomer. Each of the isomers has a distinct profile on the epoxide ring, in terms of hydrogen bonds and in terms of the non-covalent interaction between the diol groups and the epoxide. These profiles generate differential reactivities of epoxide group, which can be attributed to its local bond lengths, the electron localization function, and polarized bonds. Most interestingly, the quantum chemical calculations showed also that the epoxide ring is inclined more perpendicularly toward the angular ring plane for the more carcinogenic isomers, a feature which suggests a potential geometrical relationship between the inclination of the epoxide group and its interaction with the guanosine group upon adduct formation. Our results introduce novel and crucial information, which assist in understanding the mechanism of toxic potential of this known molecule, and display the strength and level of detail of applying quantum chemical methods to reveal the reactivity, energy properties, and electronic properties of a mutagen.

Wavefunction and reactivity study of benzo[a]pyrene diol epoxide and its enantiomeric forms

Which NICS aromaticity index for planar pi rings is best

The first experimental example of a [4n]annulene derivative with one Mobius twist, 1, was synthesized recently [Ajami, D.; Oeckler, O.; Simon, A.; Herges, R. Nature 2003, 426, 819] and was purported to possess aromatic character. However, critical analysis of the published crystallographic data indicates that the Mobius [16]annulene core of 1 shows large bond alternation (Deltar up to 0.157 A). Delocalization in this core is inhibited by large dihedral angles, which hinders effective pi overlap. This conclusion is supported by computational results (B3LYP/6-311+G) on 1 and several less benzannelated derivatives, based on geometric (Deltar, Deltar(m), Julg A, HOMA) and magnetic (NICS, magnetic susceptibility exaltation) criteria of aromaticity. That benzannelation results in bond localization in the [16]annulene core is shown by additional computations on benzannelated derivatives of other Mobius aromatic species. Additionally, the aromatic stabilization energy (ASE) of 1 has been reinvestigated using two different procedures. Evaluation of uncorrected ISE(II) values of just the polyene bridge portion of 1 and its Huckel counterpart suggests that stabilization of 1 relative to its Huckel isomer is confined to the polyene bridge and is not due to a delocalized pi circuit. Furthermore, application of s-cis/s-trans corrections lowers the ISE(II) value of 1 from 4.0 kcal/mol to 0.6 kcal/mol, suggesting that 1 is nonaromatic.

Chaitanya S Wannere

Papers

Which NICS aromaticity index for planar pi rings is best

Investigation of a putative mobius aromatic hydrocarbon. The effect of benzannelation on mobius [4n]annulene aromaticity.