Volume 72, Issue 12

The 15 th International Conference on Physical Organic Chemistry (ICPOC 15) was held on the west coast of Sweden in the heart of Scandinavia in central Göteborg on 8–13 July 2000 in the middle of the summer. The conference was the first in the series to be held in the Nordic countries of Europe. IUPAC Conferences on Physical Organic Chemistry are for physical organic chemists working in all fields of science and its applications. ICPOC 15 covered progress in physical organic chemistry and emphasized its interaction with other sciences.In particular, those ways in which physical organic chemistry has crept into some "hot "fields were highlighted. The conference attracted 268 active participants and 51 accompanying persons. The program comprised 14 plenary lectures, 30 invited lectures, 75 oral contributions, 119 poster presentations, and 3 after-lunch concerts. The organizers are grateful for support from IUPAC, The Royal Swedish Academy of Sciences through its Nobel Institute for Chemistry, The Swedish National Committee for Chemistry, The Swedish Chemical Society, and Organic Chemistry at Göteborg University. Per Ahlberg Chairman, ICPOC 15

This review highlights, with a historical perspective, the development of the field, with focus on the evolution since its birth in 1987, the scope of applications in present-time research, and some future developments in femtochemistry and the sibling field, femtobiology.

Rotaxanes of the amide type have been accessible in preparative yields by a variety of reactions. Beneath S N 2- and S N 2 t -mechanisms we developed a synthesis of [2]rotaxanes that comes off a Michael addition. The motif of the attractive interactions between an axle-shaped and a macrocyclic wheel part to form rotaxanes consists of multiple hydrogen bonds in the nonionic strategy (threading), as well as in a new high yield anionic template synthesis (trapping). We introduce new synthetic routes for the preparation of [n]rotaxanes using nonionic as well as anionic templates. Furthermore, we report on the latest results of the statistical synthesis (slipping) by melting together axle and wheel to form rotaxanes. The chiroptical properties of a homologous series of cycloenantiomeric [1]rotaxanes as well as a cyclodiastereomeric [3]rotaxane have been described. The differences in the Cotton effects obtained show that small structural changes have an impact on the chiroptical properties of rotaxanes. The first X-ray structures obtained of cycloenantiomerically chiral amide-based [2]- and [1]rotaxanes as well as of the first topologically chiral amide-based knot compound were solved which show networks of H-bonds between the entities of the rotaxanes and the segments of the knot-shaped molecule. Our investigations in template effects based on hydrogen bonding for the synthesis of supramolecular structures open up a variety of strategies for the preparation of catenanes, rotaxanes andrecentlyeven molecular knots.

The great success in controlling chemical reactivity by spin manipulation was achieved in the last decade, and many remarkable spin and magnetic phenomena have been discovered. Among those discoveries, the most chemically important highlights are magnetic isotope effect, spin catalysis, and single-spin tunneling spectroscopy. This paper summarizes recent advances in these new and hot areas of modern chemistry.

The geometry about oxygen in the transition-state structures for oxygen transfers from a nitrone to phosphorous, from a percarboxylic acid to a carbon­carbon double bond, and from an N-sulfonyl oxaziridine to a carbon­carbon double bond have been evaluated by the endocyclic restriction test. The former can proceed at an oblique angle, while the latter two require a large angle between the entering and leaving groups on oxygen. This information is used to determine the mechanism of the aldehyde-dependent oxygen transfer from molecular oxygen to a carbon­carbon double bond.

Two different approaches are described for the creation of supramolecular systems potentially capable of recognition and catalysis. Using the design approach, we have been able to accelerate and influence two different Diels­Alder reactions within the cavities of porphyrin dimers and trimers; this is templating from the outside inwards. The selection approach is a synthetic chemical attempt to capture some of the key evolutionary features of biological systems: dynamic combinatorial chemistry is used to create equilibrating mixtures of potential receptors, and then a template is used to select and amplify the desired system. Five potential reactions for such dynamic chemistry are discussed: base-catalyzed transesterification, hydrazone exchange, disulfide exchange, alkene metathesis, and Pd-catalyzed allyl exchange, and preliminary templating results (inside outwards) are presented.

We report here the first systematic study of the effect of impurities and additives (e.g., water, chloride, and cosolvents) on the physical properties of room-temperature ionic liquids. Remarkably, it was discovered that the viscosity of mixtures was dependent mainly on the mole fraction of added molecular solvents and only to a lesser extent upon their identity, allowing viscosity changes during the course of a reaction to be entirely predictable. While the addition of such molecular solvents decreases the viscosity and density, chloride impurities, arising from the preparation of the ionic liquids, increase viscosity dramatically. The commonly used methods of preparation were validated with respect to chloride impurity.

Molecular-sieve zeolites, Nafion membranes, low-density polyethylene films, and mixed surfactant vesicles have been used as microreactors to carry out organic photochemical reactions. The photo-cycloadditions of diaryl compounds with long flexible chains included in NaY zeolite or low-density polyethylene films yield intramolecular photocyclomers to the exclusion of intermolecular products. The photosensitized oxidation of alkenes included in pentasil zeolites or Nafion membranes or vesicles can be directed selectively toward either the singlet oxygen-mediated or the superoxide radical anion-mediated products by controlling the status and location of the substrate and sensitizer molecules in the reaction media. The photo-Fries rearrangement of phenyl phenylacetates included within NaY and pentasil zeolites or Nafion membranes gives either ortho -hydroxyphenones or decarbonylation products depending on the size/shape of the microreactors and the substrate molecules. All these results demonstrate the utility of microreactors to control the product selectivity in organic photochemical reactions.

Quinone methides were produced in aqueous solution by photochemical dehydration of o-hydroxybenzyl alcohols ( o -HOC 6 H 4 CHROH; R = H, C 6 H 5 , 4-CH 3 OC 6 H 4 ), and flash photolytic techniques were used to examine their rehydration back to starting substrate as well as their interaction with bromide and thiocyanate ions. These reactions are acid-catalyzed and show inverse isotope effects (k H+ /k D+ < 1), indicating that they occur through preequilibrium protonation of the quinone methide on its carbonyl carbon atom followed by rate-determining capture of the benzyl carbocations so formed by H 2 O, Br - , or SCN - . With some quinone methides (R = C 6 H 5 and 4-CH 3 OC 6 H 4 ) this acid catalysis could be saturated, and analysis of the data obtained in the region of saturation for the example with R = 4-CH 3 OC 6 H 4 produced both the equilibrium constant for the substrate protonation step and the rate constant for the rate-determining step. Energy relationships comparing the quinone methides with their benzyl alcohol precursors are derived.

Acid-catalyzed hydrocarbon reactions involve by far the largest amount of catalyst and the largest volume of transformation in oil refinery and chemical industry. However, despite the general agreement on the carbocationic nature of the reaction intermediates, the initial steps and the true nature of the cations on the surface are still open to debate. On these points our basic knowledge has gained enormously from landmark experiments in physical organic chemistry with spectroscopic observations using liquid superacids as solvents. As the range of superacidity covers over 11 logarithmic units of the Hammett acidity scale, the mechanistic behavior of small alkanes vary accordingly. This allows for comparison with strong solid acids such as zeolite or others used in industry. Isotopic labeling of the alkane and the acids is a powerful tool used to understand the nature of the intermediates, as well as the reaction mechanism in which carbenium and carbonium ions intervene.

Concepts for the activation of methane are derived from ion-molecule reactions of mass-selected, ground-state transition-metal cations M + . Elementary steps of industrially important processes are uncovered (e.g., oxygenation of methane or its coupling with carbon dioxide). In addition, the implications of the electronic structures of M(CH 2 ) + complexes for their reactions with nucleophiles are discussed, and the crucial role of contemporary quantum mechanical calculations in the elucidation of mechanistic details is emphasized.

Structures and reactions of a stable dialkylsilylene and various cyclic and acyclic disilenes, which we have recently synthesized, are discussed in detail. The stable silylene was found to be well protected sterically from dimerization but least perturbed electronically as evidenced by UV­vis and NMR spectroscopy. Persilylated acyclic and cyclic disilenes have shown interesting features in the Si=Si bond distances, electronic spectra, and reactions, due to the remarkable electronic and steric effects of trialkylsilyl substituents. Detailed theoretical calculations for the addition of water to disilene has shown that the intramolecular anti-addition pathway is rationalized using the orbital symmetry rule. The reactions of disilenes with haloalkanes were judged to proceed via the rate-determining halogen abstraction of disilenes from haloalkanes. The biradical character of disilenes may be related to the small π bond energy.

Dendrimers are macromolecular, nanoscale objects that are widely recognized as precise, mathematically defined, covalent core-shell assemblies. As such, they are composed of quantized numbers of atoms, monomers, and terminal functional groups relative to the respective shell levels (generations) surrounding their cores. Dendrimers have been referred to as molecular-level analogs of atoms. This perspective arises from their potential to function as precise macromolecular tectons (modules), suitable for the synthesis of structure-controlled complexity beyond dendrimers. We have termed this major new class of generic structures "megamers". Our group has now synthesized such "megamer complexity" in the form of both covalent and supra-macromolecular dendri-catenanes, dendri-macrocycles, dendri-clefts, and dendri-clusters. The covalent dendri-cluster subset of megamers has been coined "core-shell tecto(dendrimers)". New mathematically defined, covalent bonding rules for tecto(dendrimer) formation are consistent with sterically induced stoichiometry (SIS) predictions and have been verified experimentally.