LG

A series of gold(I) complexes bearing bulky tri-(ortho-biaryl)phosphine ligands were synthesized. They were shown to exhibit remarkable catalytic activities thus demonstrating their value as synthetic tools in organic chemistry.

Abstract

The confinement of a catalytic site is an efficient strategy to control a reaction and modulate its selectivity. In the present work, a new class of structurally simple and easily accessible bulky tri-(ortho-biaryl)phosphine ligands were accessed, and their gold(I) complexes synthesized. Their X-ray diffraction analysis and the comparative evaluation of their V _Bur% and G steric parameters against a series of gold complexes commonly employed in catalysis demonstrated their confined nature. Despite their notable steric congestion, these complexes exhibited remarkable catalytic activities and unusual selectivities, both in nature and level, that make them unique in the field of synthetic homogeneous gold catalysis.

Nature Communications, Published online: 04 May 2022; doi:10.1038/s41467-022-30176-z

Abstract

The application of proton coupled electron transfer (PCET) processes in organic synthesis has opened the door to new radical intermediates for synthesis such as alkyl radicals in remote positions to a ketone. Herein, we present the addition of these remote alkyl radicals to electron deficient double bonds under photoorganocatalyzed and very mild conditions. The method is not only applicable to diactivated double bonds, but monoactivated ones are also accessible using more stabilized alkyl radicals, and alkyl chains of any length can be introduced. The final products can be easily converted into more complex structures via a one-pot process, and the activating functional groups were transformed in the more versatile methyl esters. Mechanistic investigations support a mechanistic proposal based on a PCET process.

A versatile electron donor–acceptor (EDA) complex thioetherification was developed using simple organic compounds (thianthrenium salts and thiols) and visible-light irradiation under open-to-air conditions. This protocol allows the retention of the C−X bond, as well as the late-stage thioetherification of biomolecules.

Abstract

The synthesis of sulfides has been widely studied because this functional subunit is prevalent in biomolecules and pharmaceuticals, as well as being a useful synthetic platform for further elaboration. Thus, various methods to build C−S bonds have been developed, but typically they require the use of precious metals or harsh conditions. Electron donor–acceptor (EDA) complex photoactivation strategies have emerged as versatile and sustainable ways to achieve C−S bond formation, avoiding challenges associated with previous methods. This work describes an open-to-air, photoinduced, site-selective C−H thioetherification from readily available reagents via EDA complex formation that tolerates a wide range of different functional groups. Moreover, C(sp²)−halogen bonds remain intact using this protocol, allowing late-stage installation of the sulfide motif in various bioactive scaffolds, while allowing yet further modification through more traditional C−X bond cleavage protocols. Additionally, various mechanistic investigations support the envisioned EDA complex scenario.

Under aprotic conditions, Breslow intermediates can be generated from N-heterocyclic carbenes such as imidazolidin-2-ylidenes ((hyphenation: “imidazol-idin-2-ylidenes”)) and aldehydes. But how is the intermediary “primary adduct” (PA) converted to the diaminoenol (BI)? Our kinetic study shows that the highly unfavorable 1,2-C-to-O H-shift is autocatalyzed ((hyphenation: “au-to-cata-lyzed”)) by the Breslow intermediate, and that a hemiacetal plays a crucial role in the presence of excess aldehyde.

Abstract

Under aprotic conditions, the stoichiometric reaction of N-heterocyclic carbenes (NHCs) such as imidazolidin-2-ylidenes with aldehydes affords Breslow Intermediates (BIs), involving a formal 1,2-C-to-O proton shift. We herein report kinetic studies (NMR), complemented by DFT calculations, on the mechanism of this kinetically disfavored H-translocation. Variable time normalization analysis (VTNA) revealed that the kinetic orders of the reactants vary for different NHC-to-aldehyde ratios, indicating different and ratio-dependent mechanistic regimes. We propose that for high NHC-to-aldehyde ratios, the H-shift takes place in the primary, zwitterionic NHC-aldehyde adduct. With excess aldehyde, the zwitterion is in equilibrium with a hemiacetal, in which the H-shift occurs. In both regimes, the critical H-shift is auto-catalyzed by the BI. Kinetic isotope effects observed for R-CDO are in line with our proposal. Furthermore, we detected an H-bonded complex of the BI with excess NHC (NMR).

Dark BODIPY dyes are able to fully recover their fluorescence properties when linked via a flexible ethylene tether. Various excited state deactivation channels are shown to become drastically mitigated by formation of stable excitons within conformationally unstable, discrete superstructures. This study might open doors to a novel assay for detection and visualization of spatial, molecular encounters.

Abstract

Herein we present a systematic study demonstrating to which extent exciton formation can amplify fluorescence based on a series of ethylene-bridged oligo-BODIPYs. A set of non- and weakly fluorescent BODIPY motifs was selected and transformed into discrete, chain-like oligomers by linkage via a flexible ethylene tether. The prepared superstructures constitute excitonically active entities with non-conjugated, Coulomb-coupled oscillators. The non-radiative deactivation channels of Internal Conversion (IC), also combined with an upstream reductive Photoelectron Transfer (rPET) and Intersystem Crossing (ISC) were addressed at the monomeric state and the evolution of fluorescence and (non-)radiative decay rates studied along the oligomeric series. We demonstrate that a “masked” fluorescence can be fully reactivated irrespective of the imposed conformational rigidity. This work challenges the paradigm that a collective fluorescence enhancement is limited to sterically induced motional restrictions.

Synergistic enamine, photoredox and cobalt triple catalysis enables a desaturative approach for the preparation of aromatic aldehydes.

Abstract

Aromatic aldehydes are fundamental intermediates that are widely utilised for the synthesis of important materials across the broad spectrum of chemical industries. Accessing highly substituted derivatives can often be difficult as their functionalizations are generally performed via electrophilic aromatic substitution, S_EAr. Here we provide an alternative and mechanistically distinct approach whereby aromatic aldehydes are assembled from saturated precursors via a desaturative process. This novel strategy harnesses the high-fidelity of Diels–Alder cycloadditions to quickly construct multi-substituted cyclohexenecarbaldehyde cores which undergo desaturation via the synergistic interplay of enamine, photoredox and cobalt triple catalysis.

Aliphatic and aromatic carboxylic acids are rapidly converted into potassium acyltrifluoroborates (KATs) by copper-catalyzed borylation of their mixed anhydrides. This scalable method is compatible with a broad range of functional groups and allows for the facile preparation of aliphatic and aromatic KATs, including KATs derived from Fmoc- and Boc-protected amino acids.

Abstract

We report the preparation of potassium acyltrifluoroborates (KATs) from widely available carboxylic acids. Mixed anhydrides of carboxylic acids were prepared using isobutyl chloroformate and transformed to the corresponding KATs using a commercial copper catalyst, B₂(pin)₂, and aqueous KHF₂. This method allows for the facile preparation of aliphatic, aromatic, and amino acid-derived KATs and is compatible with a variety of functional groups including alkenes, esters, halides, nitriles, and protected amines.