Chantu R. Saha-Möller

J‐Aggregates: From Serendipitous Discovery to Supramolecular Engineering of Functional Dye Materials

J-aggregates are of significant interest for organic materials conceived by supramolecular approaches. Their discovery in the 1930s represents one of the most important milestones in dye chemistry as well as the germination of supramolecular chemistry. The intriguing optical properties of J-aggregates (in particular, very narrow red-shifted absorption bands with respect to those of the monomer and their ability to delocalize and migrate excitons) as well as their prospect for applications have motivated scientists to become involved in this field, and numerous contributions have been published. This Review provides an overview on the J-aggregates of a broad variety of dyes (including cyanines, porphyrins, phthalocyanines, and perylene bisimides) created by using supramolecular construction principles, and discusses their optical and photophysical properties as well as their potential applications. Thus, this Review is intended to be of interest to the supramolecular, photochemistry, and materials science communities.

Biotransformations with Peroxidases

Enzymes are chiral catalysts and are able to produce optically active molecules from prochiral or racemic substrates by catalytic asymmetric induction. One of the major challenges in organic synthesis is the development of environmentally acceptable chemical processes for the preparation of enantiomerically pure compounds, which are of increasing importance as pharmaceuticals and agrochemicals. Enzymes meet this challenge! For example, a variety of peroxidases effectively catalyze numerous selective oxidations of electron-rich substrates, which include the hydroxylation of arenes, the oxyfunctionalizations of phenols and aromatic amines, the epoxidation and halogenation of olefins, the oxygenation of heteroatoms and the enantioselective reduction of racemic hydroperoxides. In this review, we summarize the important advances achieved in the last few years on peroxidase-catalyzed transformations, with major emphasis on preparative applications.

From the firefly bioluminescence to the dioxetane-based (AMPPD) chemiluminescence immunoassay: a retroanalysis

Chemiluminescent dioxetanes, which are enzymically triggerable, have proved to be a valuable tool in diagnostic clinical applications. In this context, the phenomenon of firefly bioluminescence (the most efficient light-producing system so far) has served as an excellent example to develop new chemiluminescent probes for bioanalysis, in particular chemiluminescence immunoassays. From the recognition of the molecular details of the firefly bioluminescence, a chemical equivalent has been conceived which possesses the essential features for efficient generation of light, i.e., thermal persistence through spiroadamantyl substitution, spontaneous chemically initiated electron exchange luminescence (CIEEL)-active decomposition after enzymic triggering, efficient light emission as a lasting glow and convenient synthesis by photooxygenation. This has led to a rationally designed dioxetane, namely the 3-(2′-spiroadamantyl)-4-methoxy-4-(3″-phosphoryloxy)-phenyl-1,2-dioxetane (AMPPD) derivative, which serves as a substrate for alkaline phosphatase and, in terms of sensitivity, has surpassed the hazardous radioactive immunoassay probes. This paper gives a historical account in the form of a retroanalysis of the success story behind the rational design of the first enzymically triggerable, CIEEL-active AMPPD chemiluminescent probe.

CHEMOSELECTIVE METHYLTRIOXORHENIUM(VII)-CATALYZED SULFOXIDATIONS WITH HYDROGEN PEROXIDE

Abstract The selective oxidation of sulfides to sulfoxides by hydrogen peroxide under methyltrioxorhenium(VII) catalysis was examined; a high selectivity of sulfoxide over sulfone was achieved, except in the presence of water, which enhanced sulfone formation.

Influence of hydroperoxides on the enantioselectivity of metal-catalyzed asymmetric Baeyer–Villiger oxidation and epoxidation with chiral ligands

Abstract Chiral hydroperoxides have a significant influence on the enantioselectivity of the metal-catalyzed asymmetric Baeyer–Villiger oxidation of cyclic ketones and the epoxidation of allylic alcohols, when chiral ligands are employed. If both the ligand and the hydroperoxide are enantiopure, the ligand determines the formation of the preferred product enantiomer in both reactions.

Oxidation of methoxybenzenes to p-benzoquinones catalyzed by methyltrioxorhenium(VII)

Abstract Methoxy-substituted benzenes 1 are oxidized with aqueous hydrogen peroxide catalyzed by methyltrioxorhenium (VII) in acetic acid to yield the p-benzoquinones 3 in moderate yields. An intermediary diperoxo rhenium(VII) complex rather than peracetic acid is the dominating oxidizing species, since oxidation also proceeds in ethanol under peracid-free conditions. Acid played an important role, especially in the oxidation of p-menzoquinones 2 to p-benzoquinones 3. An arene oxide mechanism is postulated for the formation of p-benzoquinones, which would account for the participation of the acid and also overoxidation by cleavage of the arene oxide ring with hydrogen peroxide.

Synthesis of optically active α-hydroxy ketones by enantioselective oxidation of silyl enol ethers with a fructose-derived dioxirane

Abstract Optically active α-hydroxy ketones 3 have been prepared in moderate to high enantioselectivities by the asymmetric oxidation of the silyl enol ethers 2 with in situ generated dioxirane from the fructose-derived ketone 1 . Best results (ee values up to 82%) for this novel non-transition metal mediated asymmetric α-hydroxylation may be obtained, when an excess of the fructose-derived ketone 1 is employed at pH ca. 8 and short reaction times. Valuable mechanistic information on the spiro versus planar transition states for the oxygen-transfer process has been aquired through the absolute configuration of the resulting α-hydroxy ketone 3 products.

Kinetic resolution of racemic α-hydroxy ketones by lipase-catalyzed irreversible transesterification

Abstract Asymmetric acetylation of racemic α-hydroxy ketones with isopropenyl acetate catalyzed by lipases afforded the optically active keto alcohols and acetates in high enantiomeric excess (up to 99%); an enzymatic kinetic resolution which may be performed on preparative scale.

Asymmetric epoxidation of olefins by chiral dioxiranes generated in situ from ketones of d-(−)-quinic acid

Abstract The in situ generated dioxiranes (Caroate as peroxide source) of the optically active ketones 4a and 4b , which may be conveniently prepared from d -(−)-quinic acid, serve as effective oxidants for the asymmetric epoxidation (ee values up to ca. 90% with 4a ) of prochiral olefins.

Control of enantioselectivity through a hydrogen-bonded template in the vanadium(V)-catalyzed epoxidation of allylic alcohols by optically active hydroperoxides

The vanadium(V)-catalyzed asymmetric epoxidation of primary allylic alcohols by the optically active TADDOL-derived hydroperoxide as the asymmetric controller provides the corresponding (R)-epoxides in up to 72% ee. From this mechanistic study we conclude that this novel enantioselective oxygen transfer takes place via a hydrogen-bonded template, held together by the vanadium metal.