cis-Selective Hydrogenation of Aryl Germanes: A Direct Approach to Access Saturated Carbo- and Heterocyclic Germanes

Photocatalytic Generation of Germyl Radicals from Digermanes Enabling the Hydro/Deuteriogermylation of Alkenes

We have developed a visible-light-induced method to photolyze digermanes through single-electron oxidation using a photocatalyst, in contrast to direct excitation, to generate germyl radicals and achieve the hydro/deuteriogermylation of alkenes. This protocol allows the previously elusive incorporation of the small trimethylgermyl group and deuterium, metabolically stable bioisosteres of tert-butyl and hydrogen, respectively, making this approach attractive in not only organic synthesis but also medicinal chemistry.

Visible-Light-Driven Germyl Radical Generation via EDA-Catalyzed ET-HAT Process

We have established a facile and efficient protocol for the generation of germyl radicals by employing photo-excited electron transfer (ET) in an electron donor-acceptor (EDA) complex to drive hydrogen-atom transfer (HAT) from germyl hydride (R3GeH). Using a catalytic amount of EDA complex of commercially available thiol and benzophenone derivatives, the ET-HAT cycle smoothly proceeds simply upon blue-light irradiation without any transition metal or photocatalyst. This protocol also affords silyl radical from silyl hydride.

Synthetic techniques for thermodynamically disfavoured substituted six-membered rings

Six-membered rings are ubiquitous structural motifs in bioactive compounds and multifunctional materials. Notably, their thermodynamically disfavoured isomers, like disubstituted cyclohexanes featuring one substituent in an equatorial position and the other in an axial position, often exhibit enhanced physical and biological activities in comparison with their opposite isomers. However, the synthesis of thermodynamically disfavoured isomers is, by its nature, challenging, with only a limited number of possible approaches. In this Review, we summarize and compare synthetic methodologies that produce substituted six-membered rings with thermodynamically disfavoured substitution patterns. We place particular emphasis on elucidating the crucial stereoinduction factors within each transformation. Our aim is to stimulate interest in the synthesis of these unique structures, while simultaneously providing synthetic chemists with a guide to approaching this synthetic challenge.

Site-Isolated Rhodium(II) Metalloradicals Catalyze Olefin Hydrofunctionalization

Rh(II) porphyrin complexes display pronounced metal-centered radical character and the ability to activate small molecules under mild conditions, but catalysis with Rh(II) porphyrins is extremely rare. In addition to facile dimerization, Rh(II) porphyrins readily engage in kinetically and thermodynamically facile reactions involving two Rh(II) centers to generate stable Rh(III)-X intermediates that obstruct turnover in thermal catalysis. Here we report site isolation of Rh(II) metalloradicals in a MOF host, which not only protects Rh(II) metalloradicals against dimerization, but also allows them to participate in thermal catalysis. Access to PCN-224 or PCN-222 in which the porphyrin linkers are fully metalated by Rh(II) in the absence of any accompanying Rh(0) nanoparticles was achieved via the first direct MOF synthesis with a linker containing a transition-metal alkyl moiety, followed by Rh(III)-C bond photolysis.

Arene Reduction by Rh/Pd or Rh/Pt under 1 atm Hydrogen Gas and Room Temperature

Current methods for arene hydrogenation generally need either harsh reaction conditions or complex catalyst preparation. Here we describe a mild and convenient protocol that only utilizes commercially available catalysts. Using [Rh(nbd)Cl]2 and Pd/C together as catalysts, arenes bearing various functional groups can be hydrogenated under 1 atm of H2 at room temperature. This arene hydrogenation can also be achieved using catalysts of [Rh(cod)Cl]2 and PtO2, thus avoiding glovebox manipulations and simplifying the reaction procedure.

Access to C(sp3) borylated and silylated cyclic molecules: hydrogenation of corresponding arenes and heteroarenes

This paper presents a simple and cost-effective hydrogenation method for synthesizing a myriad of cycloalkanes and saturated heterocycles bearing boryl or silyl substituents. The catalyst used are heterogeneous, readily available, bench stable, and recyclable. Also demonstrated is the application of the method to compounds that possess both boryl and silyl groups. When combined with Ir-catalyzed sp2 C-H borylation, such hydrogenations offer a two-step complementary alternative to direct sp3 C-H borylations that can suffer selectivity and reactivity issues. Of practical value to the community, complete stereochemical analyses of reported borylated compounds that were never fully characterized are reported herein.

Chemoselective Heterogeneous Hydrogenation of Sulfur Containing Quinolines under Mild Conditions

Sulfur, alongside oxygen and nitrogen, holds a prominent position as one of the key heteroatoms in nature and medicinal chemistry. Its significance stems from its ability to adopt different oxidation states, rendering it valuable as both a polarity handle and a hydrogen bond donor/acceptor. Nevertheless, the poisonous nature of its free electron pairs makes sulfur containing substrates inaccessible for many catalytic protocols. Strong and (at low temperatures) irreversible chemisorption to the catalyst's surface is in particular detrimental for heterogeneous catalysts, possessing only few catalytically active sites. Herein, we present a novel heterogeneous Ru-S catalyst that tolerates multiple sulfur functionalities, including thioethers, thiophenes, sulfoxides, sulfones, sulfonamides, and sulfoximines, in the hydrogenation of quinolines. The utility of the products was further demonstrated by subsequent diversifications of the sulfur functionalities.

Recent Developments with Organogermanes: their Preparation and Application in Synthesis and Catalysis

Within the sphere of traditional Pd0 /PdII cross coupling reactions, organogermanes have been historically outperformed both in terms of scope and reactivity by more conventional transmetalating reagents. Subsequently, this class of compounds has been largely underutilized as a coupling partner in bond-forming strategies. Most recent studies, however, have shown that alternative modes of activation of these notoriously robust building blocks transform organogermanes into the most reactive site of the molecule-capable of outcompeting other functional groups (such as boronic acids, esters and silanes) for both C-C and C-heteroatom bond formation. As a result, over the past few years, the literature has increasingly featured methodologies that explore the potential of organogermanes as chemoselective and orthogonal coupling partners. Herein we highlight some of these recent advances in the field of organogermane chemistry both with respect to their synthesis and applications in synthetic and catalytic transformations.

Hydrogenation of functionalised pyridines with a rhodium oxide catalyst under mild conditions

Piperidines are one of the most widely used building blocks in the synthesis of pharmaceutical and agrochemical compounds. The hydrogenation of pyridines is a convenient method to synthesise such compounds as it only requires reactant, catalyst, and a hydrogen source. However, this reaction still remains difficult for the reduction of functionalised and multi-substituted pyridines. Here we report the use of a stable, commercially available rhodium compound, Rh2O3, for the reduction of various unprotected pyridines. The reaction only requires mild conditions, and the substrate scope is broad, making it practically useful.

Visible-Light-Driven Silyl or Germyl Radical Generation via Si-C or Ge-C Bond Homolysis

We report a simple, rapid, and selective protocol for visible-light-driven generation of silyl radicals through photoredox-induced Si-C bond homolysis. Irradiating 3-silyl-1,4-cyclohexadienes with blue light in the presence of a commercially available photocatalyst smoothly generated silyl radicals bearing various substituents within 1 h, and these radicals were trapped by a broad range of alkenes to afford products in good yields. This process is also available for efficient generation of germyl radicals.