Ligand electronic fine-tuning and its repercussion on the photocatalytic activity and mechanistic pathways of the copper-photocatalysed aza-Henry reaction

A family of six structurally related heteroleptic copper(i) complexes of the form of [Cu(N^N)(P^P)]+ bearing a 2,9-dimethyl-1,10-phenanthroline diimine (N^N) ligand and a series of electronically tunable xantphos (P^P) ligands have been synthesized and their optoelectronic properties characterized. The reactivity of these complexes in the copper-photocatalyzed aza-Henry reaction of N-phenyltetrahydroisoquinoline was evaluated, while the related excited state kinetics were comprehensively studied. By subtlety changing the electron-donating properties of the P^P ligands with negligible structural differences, we could tailor the photoredox properties and relate them to the reactivity. Moreover, depending on the exited-state redox potential of the catalysts, the preferred mechanism can shift between reductive quenching, energy transfer and oxidative quenching pathways. A combined study of the structural modulation of copper(i) photocatalysts, optoelectronic properties and photocatalytic reactivity resulted in a clearer understanding of both the rational design of the photocatalyst and the complexity of competing photoinduced electron and energy transfer mechanisms. © The Royal Society of Chemistry

P-stereogenic wide bite angle diphosphine ligands

This work was supported by an EASTCHEM fellowship. We would like to acknowledge the European Union for additional funding through a Marie Curie Excellence Grant MEXT-2004-014320 and COST action CM0802 PhoSciNet.Two modular synthetic approaches for the preparation of novel wide bite angle diphosphine ligands containing stereogenic P-atoms have been developed, leading to compounds (S,S)-2,2′-bis(methylphenylphosphino)diphenyl ether (L1) and (S,S)-2,2′-bis(ferrocenylphenylphosphino)diphenyl ether (L2) in very good diastereomeric ratios. Both protocols involve diphenyl ether as backbone and (2RP,4SC,5RC)-(+)-3,4-dimethyl-2,5-diphenyl-1,3,2-oxazaphospholidine borane (RP)-5 as initial auxiliary to induce chirality at phosphorus. The absolute configuration of intermediates (S,S)-9-(BH3)2 and (R,R)-10-(BH3)2 as well as the ligands (S,S)-L1-BH3 and (S,S)-L2 was determined by X-ray crystallographic analysis.PostprintPeer reviewe

Preparation of a Series of Supported Nonsymmetrical PNP-Pincer Ligands and the Application in Ester Hydrogenation

In contrast to their symmetrical analogues, nonsymmetrical PNP-type ligand motifs have been less investigated despite the modular pincer structure. However, the introduction of mixed phosphorus donor moieties provides access to a larger variety of PNP ligands. Herein, a facile solid-phase synthesis approach towards a diverse PNP-pincer ligand library of 14 members is reported. Contrary to often challenging workup procedures in solution-phase, only simple workup steps are required. The corresponding supported ruthenium-PNP catalysts are screened in ester hydrogenation. Usually, industrially applied heterogeneous catalysts require harsh conditions in this reaction (250–350 °C at 100–200 bar) often leading to reduced selectivities. Heterogenized reusable Ru-PNP catalysts are capable of reducing esters and lactones selectively under mild conditions. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

An Introduction to Model Compounds of Lignin Linking Motifs; Synthesis and Selection Considerations for Reactivity Studies

The development of fundamentally new valorization strategies for lignin plays a vital role in unlocking the true potential of lignocellulosic biomass as sustainable and economically compatible renewable carbon feedstock. In particular, new catalytic modification and depolymerization strategies are required. Progress in this field, past and future, relies for a large part on the application of synthetic model compounds that reduce the complexity of working with the lignin biopolymer. This aids the development of catalytic methodologies and in-depth mechanistic studies and guides structural characterization studies in the lignin field. However, due to the volume of literature and the piecemeal publication of methodology, the choice of suitable lignin model compounds is far from straight forward, especially for those outside the field and lacking a background in organic synthesis. For example, in catalytic depolymerization studies, a balance between synthetic effort and fidelity compared to the actual lignin of interest needs to be found. In this Review, we provide a broad overview of the model compounds available to study the chemistry of the main native linking motifs typically found in lignins from woody biomass, the synthetic routes and effort required to access them, and discuss to what extent these represent actual lignin structures. This overview can aid researchers in their selection of the most suitable lignin model systems for the development of emerging lignin modification and depolymerization technologies, maximizing their chances of successfully developing novel lignin valorization strategies. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Facile synthesis of supported Ru-Triphos catalysts for continuous flow application in selective nitrile reduction

The selective catalytic hydrogenation of nitriles represents an important but challenging transformation for many homogeneous and heterogeneous catalysts. Herein, we report the efficient and modular solid-phase synthesis of immobilized Triphos-type ligands in very high yields, involving only minimal work-up procedures. The corresponding supported ruthenium-Triphos catalysts are tested in the hydrogenation of various nitriles. Under mild conditions and without the requirement of additives, the tunable supported catalyst library provides selective access to both primary amines and secondary imines. Moreover, the first application of a Triphos-type catalyst in a continuous flow process is presented demonstrating high catalyst life-time over at least 195 hours without significant activity loss. © 2019 The Royal Society of Chemistry