A rotationally hindered and thus stereogenic biaryl axis is the structurally and stereochemically decisive element of a steadily growing number of natural products, chiral auxiliaries, and catalysts. Thus, it is not surprising that significant advances have been made in the asymmetric synthesis of axially chiral biaryl compounds over the past decade. In addition to the classic approach (direct stereoselective aryl-aryl coupling), innovative concepts have been developed in which the asymmetric information is introduced into a preformed, but achiral-that is, symmetric or configurationally labile-biaryl compound, or in which an aryl--C single bond is stereoselectively transformed into an axis. This Review classifies these strategies according to their underlying concepts and critically evaluates their scope and limitations with reference to selected model reactions and applications. Furthermore, the preconditions required for the existence of axial chirality in biaryl compounds are discussed.

Atroposelective Synthesis of Axially Chiral Biaryl Compounds

Eine rotationsgehinderte und dadurch stereogene Biarylachse ist das strukturell und stereochemisch entscheidende Element einer standig wachsenden Zahl von Naturstoffen, chiralen Auxiliaren und Katalysatoren. Daher ist es nicht uberraschend, dass im letzten Jahrzehnt bedeutende Fortschritte in der asymmetrischen Synthese axial-chiraler Biaryle erzielt worden sind. Neben dem klassischen Zugangsweg, der direkten Aryl-Aryl-Kupplung, sind innovative Konzepte entwickelt worden, in denen die asymmetrische Information in ein schon vorhandenes, aber nicht optisch aktives – symmetrisches oder konfigurativ labiles – Biaryl eingefuhrt oder eine Aryl-C-Einfachbindung stereoselektiv in eine Achse umgewandelt wird. In diesem Aufsatz werden die Strategien nach den zugrunde liegenden Konzepten klassifiziert, und ihre Anwendungsbreite und ihre Beschrankungen werden anhand ausgewahlter Beispiele kritisch beurteilt. Ferner werden die Voraussetzungen fur das Auftreten von Axialchiralitat diskutiert.

Atropselektive Synthese axial‐chiraler Biaryle

Key approaches to the synthesis of merocyanine dyes are summarised. The dependence of the dye properties on their chemical structure, the solvent nature and the aggregation type is analysed. The main methods of studying the electronic structure of merocyanines are considered. Attention is focused on polyene–polymethine electron transitions. The applications of merocyanines in modern research, including the design of new materials and technologies (non-linear optics, photovoltaics, holography, etc.) are presented.

https://iopscience.iop.org/article/10.1070/RC2009v078n02ABEH003900/pdf

Merocyanine dyes: synthesis, structure, properties and applications

Enantioselective Synthesis of Axially Chiral Phthalides through Cationic [RhI(H8‐binap)]‐Catalyzed Cross Alkyne Cyclotrimerization

Synthesis, crystal growth and studies on non-linear optical property of new chalcones

A series of merocyamne dyes containing various donors and barbituric acid and cyclobutene-1,2-dione moieties as the acceptors have been synthesized and their first-order hyperpolarizabilities β were determined. The β values of barbituric acid derivatives increase as the strength of the donor is increased from 4-dimethylaminophenyl to trimethylindolinyl to benzothiazolinyl, apparently due to the gradual decrease in the bond length alternation (BLA) from a large positive value to an optimum one by a stronger donor. In contrast, the β values for the cyclobutene-1,2-dione derivatives decrease with the same variation of the donors even though the cyclobutene-1,2-dione is a poorer acceptor than the barbituric acid moiety. The results have been attributed to the electron-donating ability of the donors and the increased distortion of the chromophores from planarity.

Molecular hyperpolarizabilities of barbituric acid and cyclobutene-1,2-dione derivatives. electronic and steric effects

Measurement of first-order hyperpolarizability of several barbituric acid derivatives in solution by hyper-Rayleigh scattering

The steric effect on the first order hyperpolarizability of the β-nitrostyrene derivatives has been investigated by comparing the β values and the dihedral angles between the aryl and nitroalkenyl groups. In general the β value increased with the electron-donating ability of the substituent. The larger β value for 3,4-dimethoxy-β-nitrostyrene than that for p-methoxy-β-nitrostyrene has been attributed to the lower charge transfer energy for the former. The most striking substituent effect was observed in the β-methyl-β-nitrostyrene derivatives. Thus the β values for 3,4-dimethoxy- and p-methoxy derivatives of the latter decreased to near zero, probably because of the large distortion from planarity caused by the steric repulsion between the β-methyl and the aryl groups. The larger β value for p-dimethylamino-β-methyl-β-nitrostyrene has been interpreted with an increased electron-donating ability of the substituent and increased co-planarity.

Steric Effect on the Molecular Hyperpolarizabilities of β-Nitrostyrene Derivatives

Elimination reactions of (E)-O-pivaloylbenzaldoximes promoted by Et 3 N-MeCN, t-BuOK-t-BuOH, and t-BuOK-DMSO have been studied kinetically. The reactions produce benzonitrile quantitatively. The reactions are second-order and exhibit substantial values of ρ, β, and k H /k D and an E2 mechanism is evident. The rate of elimination from (E)-O-pivaloylbenzaldoxime increased with base-solvent system variation and gave relative rates of 1, 14.8, and 4.31×10 4 for Et 3 N-MeCN, t-BuOK-t-BuOH, t-BuOK-DMSO, respectively

Jong Tae Je

Papers

Molecular hyperpolarizabilities of barbituric acid and cyclobutene-1,2-dione derivatives. electronic and steric effects

Measurement of first-order hyperpolarizability of several barbituric acid derivatives in solution by hyper-Rayleigh scattering

Steric Effect on the Molecular Hyperpolarizabilities of β-Nitrostyrene Derivatives

Elimination Reactions of (E)-O-Pivaloylbenzaldoximes.

Reactions of (E)-O-arylbenzaldoximes with secondary amines in acetonitrile. Competition between E2 and SNAr reactions