Volume 85, Issue 10

The IUPAC Conference on Physical Organic Chemistry (ICPOC) series of biennial conferences has a long history as the leading international meeting on physical organic chemistry. From its first installment in Crans sur Sierre (Switzerland) in 1972, ICPOC has acted as a focus point for the physical organic community worldwide, and the conference series enjoys a respected international reputation. With its focus on relating chemical behavior and properties to molecular structure through the development of (ideally quantitative) understanding of structure–property relationships, physical organic chemistry (POC) finds wide application in tackling current scientific challenges and has been undergoing something of a resurgence in recent years. ICPOC-21, held on 9-13 September 2012 at the University of Durham, UK, provided a forum for researchers (329 delegates from 38 countries) in academia and industry, and at all career stages, to present their results to the POC community and to exchange ideas, meet old friends, and make new contacts while enjoying spectacular views of the World Heritage castle and cathedral. In line with a developing interpretation of POC as a widely applicable approach to chemistry, the scientific program embraced three broad themes (physical underpinnings, mechanism and catalysis, and supramolecular chemistry) but often these strands were as inseparably interconnected as the three leaves held in unity in the minimal saddle trefoil adopted as the conference logo. The scope of the meeting is illustrated by the selection of contributions included in this issue of Pure and Applied Chemistry . In addition to the 24 plenary and keynote lectures, there were 120 contributed talks and 141 poster presentations. Donna Blackmond's paper on the interplay of thermodynamics and kinetics in dictating organocatalytic reactivity and selectivity demonstrates the use of kinetic modeling to provide mechanistic understanding leading to practical application. Martin Tanner's discussion of rearrangements catalyzed by indole alkaloid prenyltransferases shows how the experimental POC approach may shed light on enzyme mechanisms, while Dean Tantillo describes the application of quantum chemical dynamics calculations to mechanistic problems in the field of terpene biosynthesis, and a blend of experiment and computation is presented in Brian Ohta's account of the structure of halonium ions in superacidic solutions. Hiromitsu Maeda reviews research on various stimuli-responsive circularly polarized luminescence properties derived from π-conjugated molecules and related materials, and Jason Harper summarizes progress towards a predictive understanding of how ionic solvents affect and control organic reactivity. Finally, Izumi Iwakura reports directly observations of the Claisen rearrangement by timeresolved vibrational spectroscopy using a few-optical-cycle pulse laser. This selection of excellent work provides only a taste, but the conference as a whole showed that the international POC community is as vital and vibrant as it has ever been, promising exciting times ahead for this approach to chemistry. The next ICPOC will be held in Ottawa (Canada) 10-15 August 2014 under the chairmanship of Prof. Paul Mayer (University of Ottawa). Further information can be found at http://events.science.uottawa. ca/icpoc22/welcome.html. Ian H. Williams Conference Chair and Editor

Recent reports of the real-time identification of intermediates in organocatalytic reactions by NMR spectroscopy coupled with detailed kinetic studies highlight a potential role for stable intermediates reversibly formed downstream from what is generally considered to be the enantioselectivity-determining step. In this work, we employ kinetic modeling to explore these concepts further. We demonstrate that when an intermediate is common to multiple reaction pathways, the relative reactivity of these pathways dictates the ultimate outcome, regardless of the relative stability of other intermediates connected to these pathways. Kinetic modeling also illustrates important implications for enantioselectivity depending on whether such intermediates lie on or off the catalytic cycle.

An introduction to the application of quantum chemical dynamics calculations to mechanistic problems in the field of terpene biosynthesis is provided. A bare bones introduction to the fundamentals of chemical dynamics is followed by a brief account of previous applications to terpene-forming carbocation reactions, a discussion of questions in this field that dynamics calculations may help answer, and a description of current problems to which dynamics calculations are being applied.

The indole prenyltransferases are a family of metal-independent enzymes that catalyze the transfer of a prenyl group from dimethylallyl diphosphate (DMAPP) onto the indole ring of a tryptophan residue. These enzymes are remarkable in their ability to direct the prenyl group in either a “normal” or “reverse” fashion to positions with markedly different nucleophilicity. The enzyme 4-dimethylallyltryptophan synthase (4-DMATS) prenylates the non-nucleophilic C-4 position of the indole ring in free tryptophan. Evidence is presented in support of a mechanism that involves initial ion pair formation followed by a reverse prenylation at the nucleophilic C-3 position. A Cope rearrangement then generates the C-4 normal prenylated intermediate and deprotonation rearomatizes the indole ring. The enzyme tryprostatin B synthase (FtmPT1) catalyzes the normal C-2 prenylation of the indole ring in brevianamide F ( cyclo -L-Trp-L-Pro). It shares high structural homology with 4-DMATS, and evidence is presented in favor of an initial C-3 prenylation (either normal or reverse) followed by carbocation rearrangements to give product. The concept of a common intermediate that partitions to different products via rearrangements can help to explain how these evolutionarily related enzymes can prenylate different positions on the indole ring.

Isotopic perturbation of equilibrium was applied to 1,2-bridged halonium ions to determine whether they exist as single symmetric structures or as a rapid equilibrium of asymmetric structures. The observed deuterium isotope shifts are qualitatively and quantitatively consistent with the presence of intrinsic and equilibrium isotope shifts. The presence of equilibrium shifts suggests that these ions exist as a rapid equilibrium of asymmetric structures. Though the asymmetric structures were initially ascribed to β-halocarbenium ions, subsequent computational data suggest that 1,2-bridged halonium ions react with sulfur dioxide (SO 2 ), the experimental solvent. Our current hypothesis is that the equilibrium isotope shifts result from rapid labile addition of SO 2 to the halonium ions. Other hypotheses have been invoked to explain the results and are considered in the context of the available data.

This short review article summarizes recent progress in research on various stimuli-responsive circularly polarized luminescence (CPL) properties derived from π-conjugated molecules and related materials. As representative examples, assembled structures of chiral π-conjugated polymers and molecules showed tunable CPL resulting from the enhancement of chirality induction by aggregation. Fascinating CPL-active species, pyrrole-based anion-responsive π-conjugated molecules exhibiting CPL induced by anion binding and ion pairing, are also discussed.

Ionic liquids are frequently touted as alternatives to traditional molecular solvents but are limited in their applicability as the outcome of reactions may be altered on moving from a molecular to an ionic solvent. This manuscript summarizes our progress towards a predictive framework through understanding how ionic solvents affect organic processes, with an emphasis on how these findings might be applied. Particularly, we will consider the importance of the mole fraction of the ionic liquid used, including some hitherto undisclosed results, as well as the importance of understanding the key interactions of the solvent with the components along the reaction coordinate.

Time-resolved vibration spectroscopy is the only known way to directly observe reaction processes. In this work, we measure time-resolved vibration spectra of the Claisen rearrangement triggered and observed by few-optical-cycle pulses. Changes in molecular structure during the reaction, including its transition states (TSs), are elucidated by observing the transient changes of molecular vibration wavenumbers. We pump samples with visible ultrashort pulses of shorter duration than the molecular vibration period, and with photon energies much lower than the minimum excitation energy of the sample. The results indicate that the “nonthermal Claisen rearrangement” can be triggered by visible few-optical-cycle pulses exciting molecular vibrations in the electronic ground state of the sample, which replaces the typical thermal Claisen rearrangement.

Procedures for the determination of photoluminescence quantum yields with optical methods are addressed, and challenges associated with these measurements are discussed. Special emphasis is dedicated to relative measurements of fluorescent (i.e., short excited-state lifetime), transparent, dilute dye solutions in conventional cuvettes in a 0°/90° measurement geometry. Recommendations on the selection of suitable quantum yield standards are presented, and requirements for the documentation of photoluminescence quantum yields are derived.