N-Heterocyclische Carbene in der Übergangsmetall- und Organokatalyse

Design und Verwendung neuer N-heterocyclischer Carbene als Liganden in der Übergangsmetallkatalyse (Suzuki-Kupplung). Verwendung verschiedener N-heterocyclischer Carbene in der konjugierten Umpolung von alpha,beta-ungesättigten Aldehyden

Visible-Light-Mediated Deaminative Three-Component Dicarbofunctionalization of Styrenes with Benzylic Radicals

The visible-light-mediated three-component dicarbofunctionalization of styrenes using simple benzylic radicals is described. Notably, this work describes a rare example of undirected dicarbofunctionalization using unsubstituted benzyl radicals. Key to the success of this strategy was the rational design and use of benzylic pyridinium salts as radical precursors. Using this approach, abundant styrenes, electron-rich heterocycles, and benzylic amines were combined to rapidly afford a number of densely functionalized 1,1-diarylalkanes. A dipeptide-derived pyridinium salt was applied in this transformation, which resembles a visible-light-mediated deaminative generation of radicals from peptides

Visible-Light-Mediated Deaminative Three-Component Dicarbofunctionalization of Styrenes with Benzylic Radicals

The visible-light-mediated three-component dicarbofunctionalization of styrenes using simple benzylic radicals is described. Notably, this work describes a rare example of undirected dicarbofunctionalization using unsubstituted benzyl radicals. Key to the success of this strategy was the rational design and use of benzylic pyridinium salts as radical precursors. Using this approach, abundant styrenes, electron-rich heterocycles, and benzylic amines were combined to rapidly afford a number of densely functionalized 1,1-diarylalkanes. A dipeptide-derived pyridinium salt was applied in this transformation, which resembles a visible-light-mediated deaminative generation of radicals from peptides

Substituted Azolium Disposition: Examining the Effects of Alkyl Placement on Thermal Properties

We describe the thermal phase characteristics of a series of 4,5-bis(n-alkyl)azolium salts that were studied using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), polarized-light optical microscopy (POM), and synchrotron-based small- to wide-angle X-ray scattering (SWAXS) measurements. Key results were obtained for 1,3-dimethyl-4,5-bis(n-undecyl)imidazolium iodide (1-11), 1,3-dimethyl-4,5-bis(n-pentadecyl)imidazolium iodide (1-15), and 1,2,3-trimethyl-4,5-bis(n-pentadecyl)imidazolium iodide (2), which were found to adopt enantiotropic smectic A mesophases. Liquid-crystalline mesophases were not observed for 1,3-dimethyl-4,5-bis(n-heptyl)imidazolium iodide (1-7), 3-methyl-4,5-bis(n-pentadecyl)thiazolium iodide (3), and 2-amino-4,5-bis(n-pentadecyl)imidazolium chloride (4). Installing substituents in the 4- and 5-positions of the imidazolium salts appears to increase melting points while lowering clearing points when compared to data reported for 1,3-disubstituted analogues

Visible-Light-Mediated Charge Transfer Enables C−C Bond Formation with Traceless Acceptor Groups

The development and application of traceless acceptor groups in photochemical C−C bond formation is described. This strategy was enabled by the photoexcitation of electron donor–acceptor (EDA) complexes with visible light. The traceless acceptors, which were readily prepared from amino acid and peptide feedstocks, could be used to alkylate a wide range of heteroarene and enamine donors under metal- and peroxide-free conditions. The crucial role of the EDA complexes in the mechanism of these reactions was explored through combined experimental and computational studies

Photosensitized intermolecular carboimination of alkenes through the persistent radical effect

An intermolecular, two‐component vicinal carboimination of alkenes has been accomplished by energy transfer catalysis. Oxime esters of alkyl carboxylic acids were used as bifunctional reagents to generate both alkyl and iminyl radicals. Subsequently, addition of the alkyl radical to an alkene generates a transient radical for selective radical–radical cross‐coupling with the persistent iminyl radical. Furthermore, this process provides direct access to aliphatic primary amines and α‐amino acids by simple hydrolysis

Influence of N-Substituents on the Adsorption Geometry of OH-Functionalized Chiral N-Heterocyclic Carbenes

Adsorption of chiral molecules on heterogeneous catalysts is a simple approach for inducing an asymmetric environment to enable enantioselective reactivity. Although the concept of chiral induction is straightforward, its practical utilization is far from simple, and only a few examples toward the successful chiral induction by surface anchoring of asymmetric modifiers have been demonstrated so far. Elucidating the factors that lead to successful chiral induction is therefore a crucial step for understanding the mechanism by which chirality is transferred. Herein, we identify the adsorption geometry of OH-functionalized N-heterocyclic carbenes (NHCs), which are chemical analogues to chiral modifiers that successfully promoted α-arylation reactions once anchored on Pd nanoparticles. Polarized near-edge X-ray absorption fine structure (NEXAFS) measurements on Pd(111) revealed that NHCs that were associated with low enantioselectivity were characterized with a well-ordered structure, in which the imidazole ring was vertically positioned and the OH-functionalized side arms were flat-lying. OH-functionalized NHCs that were associated with high enantioselectivity revealed a disordered/flexible adsorption geometry, which potentially enabled better interaction between the OH group and the prochiral reactant

Energy transfer catalysis mediated by visible light : principles, applications, directions

Harnessing visible light to access excited (triplet) states of organic compounds can enable impressive reactivity modes. This tutorial review covers the photophysical fundamentals and most significant advances in the field of visible-light-mediated energy transfer catalysis within the last decade. Methods to determine excited triplet state energies and to characterize the underlying Dexter energy transfer are discussed. Synthetic applications of this field, divided into four main categories (cyclization reactions, double bond isomerizations, bond dissociations and sensitization of metal complexes), are also examined

N-heterocyclic carbene modified palladium catalysts for the direct synthesis of hydrogen peroxide

Heterogeneous palladium catalysts modified by N-heterocyclic carbenes (NHCs) are shown to be highly effective toward the direct synthesis of hydrogen peroxide (H2O2), in the absence of the promoters which are typically required to enhance both activity and selectivity. Catalytic evaluation in a batch regime demonstrated that through careful selection of the N-substituent of the NHC it is possible to greatly enhance catalytic performance when compared to the unmodified analogue and reach concentrations of H2O2 rivaling that obtained by state-of-the-art catalysts. The enhanced performance of the modified catalyst, which is retained upon reuse, is attributed to the ability of the NHC to electronically modify Pd speciation