Photoswitchable electron-rich phosphines: using light to modulate the electron-donating ability of phosphines.

The synthesis and properties of photoswitchable electron-rich phosphines containing N-heterocyclic imines equipped with a photochromic dithienylethene unit are reported. Heteronuclear NMR spectroscopy and UV/vis studies reveal that the imine substituents undergo reversible electrocyclic ring-closing and ring-opening reactions upon exposure to UV and visible light, respectively. The photoisomerization alters the electron-donating ability of the phosphines by up to ΔTEP = 8 cm-1.

Pd-Catalyzed Activation of Carbon-Carbon Bonds in Hydroxymethylfurfural Derivatives.

Palladium-catalyzed activation of C-C bonds in organic molecules is a powerful tool for the synthesis of value-added compounds. 5-Hydroxymethylfurfural (HMF) derivatives are a promising class of biomass-derived chemicals that have received considerable attention due to their potential applications in the synthesis of biologically active molecules and materials. However, the selective activation of unstrained C-C bonds is a challenging task, mainly due to their relatively high bond dissociation energies. Herein, we report a palladium-catalyzed method for the efficient C-C bond activation of HMF derivatives, enabling their arylation with iodobenzenes. Mechanistic studies, including reaction-profile analysis, competition experiments and head-space IR spectroscopy suggest a decarboxylative mechanism.

2,2'-Biquinoline-Based Recyclable Electroauxiliaries for the Generation of Alkyl Radicals via C-C Bond Cleavage.

Alkyl radical precursors are essential for a wide variety of photocatalytic and 3d-metal-catalyzed C-C bond forming reactions. Neutral organic heterocycles as electroauxiliaries such as 4-alkyl Hantzsch esters have become reliable tools for alkyl radical formation. Here we show that 2,2'-biquinoline-derived alkyl-substituted dihydroquinolines act as competent radical precursors with the ability to form primary, secondary and tertiary alkyl radicals. Hydroalkylation of benzalmalononitriles and N-Boc protected diazenes has been achieved through copper catalysis under mild conditions of 50 °C with good to very good yields of up to 85 %. Furthermore, the dihydroquinolines' reactivity towards a denitrative alkylation of nitroolefins such as ß-nitrostyrene was discovered. Most importantly, the released biquinoline can be recycled, which greatly improves the overall atom-economy of these alkyl radical precursors in comparison to previous N-heterocyclic electroauxiliaries.

Photoswitchable Nitrogen Superbases: Using Light for Reversible Carbon Dioxide Capture.

Using light as an external stimulus to alter the reactivity of Lewis bases is an intriguing tool for controlling chemical reactions. Reversible photoreactions associated with pronounced reactivity changes are particularly valuable in this regard. We herein report the first photoswitchable nitrogen superbases based on guanidines equipped with a photochromic dithienylethene unit. The resulting N-heterocyclic imines (NHIs) undergo reversible, near quantitative electrocyclic isomerization upon successive exposure to UV and visible irradiation, as demonstrated over multiple cycles. Switching between the ring-opened and ring-closed states is accompanied by substantial pKa shifts of the NHIs by up to 8.7 units. Since only the ring-closed isomers are sufficiently basic to activate CO2 via the formation of zwitterionic Lewis base adducts, cycling between the two isomeric states enables the light-controlled capture and release of CO2 .

Ni-Catalyzed Carboxylation of Aziridines en Route to ß-Amino Acids.

A Ni-catalyzed reductive carboxylation of N-substituted aziridines with CO2 at atmospheric pressure is disclosed. The protocol is characterized by its mild conditions, experimental ease, and exquisite chemo- and regioselectivity pattern, thus unlocking a new catalytic blueprint to access ß-amino acids, important building blocks with considerable potential as peptidomimetics.

Remote sp2 C-H Carboxylation via Catalytic 1,4-Ni Migration with CO2.

A remote catalytic reductive sp2 C-H carboxylation of arenes with CO2 (1 bar) via 1,4-Ni migration is disclosed. This protocol constitutes the first catalytic 1,4-Ni migration reported to date, thus offering new vistas in the Ni-catalyzed reductive coupling arena while providing an unconventional new platform for incorporating electrophilic sites at remote sp2 C-H linkages.

Transition-Metal-Like Catalysis with a Main-Group Element: Bismuth-Catalyzed C-F Coupling of Aryl Boronic Esters.

The main-group age: The past decade has revealed main-group-element compounds that display transition-metal-like reactivity in stoichiometric and even catalytic transformations. Cornella and co-workers have now reported a bismuth complex that catalyzes fluoroarene formation from aryl boronate esters, a reaction that is almost unprecedented in transition-metal catalysis.

Cyclic (Aryl)(Amido)Carbenes: NHCs with Triplet-like Reactivity.

The synthesis and study of a library of cyclic (aryl)(amido)carbenes (CArAmCs), which represent a class of electrophilic NHCs that feature low calculated singlet-triplet gaps (ΔEST =19.9 kcal mol-1 ; B3LYP/def2-TZVP) and exhibit reactivity profiles expected from triplet carbenes, are described. The electrophilic properties of the CArAmCs were quantified by analyzing their respective selenium adducts, which exhibited the largest downfield 77 Se NMR chemical shifts (up to 1645 ppm) measured for any NHC derivative known to date, as well as their Ir carbonyl complexes, from which large Tolman electronic parameter (TEP) values (up to 2064 cm-1 ) were ascertained. The CArAmCs were found to engage in reactions that are typically observed with triplet carbenes, including C-H insertions, [2+1] cycloadditions with alkenes as well as alkynes, and spontaneous oxidation upon exposure to oxygen.

Correction: Privileged chiral N-heterocyclic carbene ligands for asymmetric transition-metal catalysis.

Correction for 'Privileged chiral N-heterocyclic carbene ligands for asymmetric transition-metal catalysis' by Daniel Janssen-Müller et al., Chem. Soc. Rev., 2017, DOI: .

Privileged chiral N-heterocyclic carbene ligands for asymmetric transition-metal catalysis.

Chiral ligands play a central role in enantioselective transition-metal catalysis. The success of achiral N-heterocyclic carbenes (NHCs) as stable electron-rich neutral ligands in homogeneous catalysis led to the development of a manifold of chiral NHCs as stereodirecting ancillary ligands for various enantioselective transformations. Due to the modular design of NHCs and the ease of access to their azolium salt precursors, tailor-made NHCs are readily available. Many chiral NHC scaffolds have been synthesised and tested in catalysis. Herein, we highlight only those NHC structures which have enabled high degrees of enantioselectivity in transition-metal catalysis. Following a brief introduction to the field of chiral NHCs, this tutorial review introduces different categories of chiral NHCs and provides a guide to the structural fine-tuning of ligand requirements and stereochemical models.

Mechanistic Studies on a Cooperative NHC Organocatalysis/Palladium Catalysis System: Uncovering Significant Lessons for Mixed Chiral Pd(NHC)(PR3) Catalyst Design.

A comprehensive investigation of the mechanism of the highly enantioselective Pd(PPh3)4/NHC-catalyzed annulation of vinyl benzoxazinanones and enals has been conducted. A study of reaction orders supports the postulated cooperative catalysis. Interestingly, a detailed investigation of the catalytically active palladium species pointed toward a dual role of the NHC acting as an organocatalyst and forming a novel mixed ligand Pd/NHC/phosphine complex. The catalytically active Pd/NHC/phosphine complex represents a new class of chiral palladium catalyst. Remarkably, phosphine plays a crucial role in this transformation. These complexes could be characterized by X-ray crystallographic analysis and employed as catalysts for the enantioselective [4 + 1] annulation reaction of vinyl benzoxazinones and sulfur ylides in good yields and good enantioselectivities.

NHC-Organocatalyzed CAr -O Bond Cleavage: Mild Access to 2-Hydroxybenzophenones.

A Truce-Smiles rearrangement of acyl-anion equivalents generated by N-heterocyclic carbene (NHC) catalysis has been achieved. The developed method includes CAr -O, CAr -S, or CAr -N bond cleavage for the formation of a CAr -C bond and enables access to 2-hydroxybenzophenones, an important structural motif that is present in several bioactive natural products. By utilizing this procedure, the alkaloid taxilamine was synthesized in three steps. DFT calculations and control experiments support a classical SN Ar mechanism with a catalyst-bound Meisenheimer-type intermediate. The method features mild reaction conditions, excellent functional-group tolerance, and a broad substrate scope, including various classes of (hetero)arenes.

Annulation of o-Quinodimethanes through N-Heterocyclic Carbene Catalysis for the Synthesis of 1-Isochromanones.

The activation of 2-(bromomethyl)benzaldehydes using N-heterocyclic carbenes represents a novel approach to the generation of o-quinodimethane (o-QDM) intermediates. Coupling with ketones such as phenylglyoxylates, isatins, or trifluoromethyl ketones via [4 + 2] annulation gives access to functionalized 1-isochromanones.

Cooperative N-Heterocyclic Carbene/Palladium-Catalyzed Enantioselective Umpolung Annulations.

A combination of NHC organocatalysis and transition-metal catalysis gives rise to fundamentally new cooperative reactivity and enables the regio- and enantioselective annulation reaction between enals and vinyl benzoxazinanones. The cooperative umpolung annulation eliminates mutual deactivation and leads to a diverse set of benzazepine derivatives in good yields with excellent enantioselectivities (up to 99% ee). The development of such a cooperative catalytic system dramatically expands the scope of NHC organocatalysis by opening up new metal-catalyzed reaction pathways for homoenolate intermediates.

Switchable selectivity in an NHC-catalysed dearomatizing annulation reaction.

The development of general catalytic methods for the regio- and stereoselective construction of chiral N-heterocycles in a diversity-oriented fashion remains a formidable challenge in organic synthesis. N-heterocyclic carbene (NHC) catalysis has been shown to produce a variety of outcomes, but control of the reactivity has rarely been demonstrated. Here we report a switchable catalytic activation of enals with aromatic azomethine imines that provides high selectivity using NHC organocatalysts. The original selectivity corresponds to the acidity of the base used in the reaction. The catalytically generated chiral homoenolate or enol intermediate undergoes enantioselective annulation with electrophiles such as N-iminoquinolinium ylides, N-iminoisoquinolinium ylides and ß-N-iminocarboline ylides. The good-to-high overall yields, high regioselectivities and excellent enantioselectivities observed are controlled by the catalyst and reaction conditions.

Enantioselective intramolecular hydroacylation of unactivated alkenes: an NHC-catalyzed robust and versatile formation of cyclic chiral ketones.

A highly enantioselective intramolecular N-heterocyclic carbene (NHC)-catalyzed hydroacylation reaction gives access to a range of cyclic ketones from unactivated olefin-substituted aldehydes (up to 99 % ee). Remarkably, aliphatic aldehydes were also transformed efficiently in an NHC-catalyzed hydroacylation reaction for the first time.