Synthesis of Chiral Tetrahydrofurans and Pyrrolidines by Visible-Light-Mediated Deoxygenation

Total Synthesis of the Diterpenes (+)-Randainin D and (+)-Barekoxide via Photoredox-Catalyzed Deoxygenative Allylation

We report the first enantioselective total synthesis of diterpenoid randainin D, which possesses a hydroazulenone core with a β-substituted butenolide moiety on the cycloheptane ring. The trans-5/7 ring system was formed via a highly challenging ring-closing metathesis delivering the tetrasubstituted cycloheptenone. The butenolide moiety was installed via a novel deoxygenative allylation under Ir-photoredox catalysis, employing methyl oxalate as a red/ox tag. Moreover, the developed allylation was successfully utilized in the 7-step total synthesis of (+)-barekoxide. This study suggests that this deoxygenative allylation method is a promising strategy for the formation of Cq-C(sp3) bonds (Cq = quaternary center) in the context of natural product synthesis.

Dehydroxylative Alkylation of α-Hydroxy Carboxylic Acid Derivatives via a Spin-Center Shift

A strategically distinct dehydroxylative alkylation reaction of α-hydroxy carboxylic acid derivatives with alkenes is developed. The reaction starts with the attack of a 4-dimethylaminopyridine (DMAP)-boryl radical to the carbonyl oxygen atom, followed by a spin-center shift (SCS) to trigger the C-O bond scission. The resulting α-carbonyl radicals couple with a wide range of alkenes to furnish various alkylated products. This strategy allows for the efficient conversion of a wide array of α-hydroxy amides and esters derived from several biomass molecules and natural products to value-added compounds. Experimental and computational studies verified the reaction mechanism.

Silver-catalysed double decarboxylative addition-cyclisation-elimination cascade sequence for the synthesis of quinolin-2-ones

An atom-efficient silver-catalysed double carboxylative strategy for the one-step synthesis of quinolin-2-ones via an addition-cyclisation-elimination cascade sequence of oxamic acids to acrylic acids, mediated either thermally or photochemically, is reported. The reaction was applicable to the synthesis of a broad range of quinolin-2-ones and featured a double-disconnection approach that constructed the quinolin-2-one core via the formal and direct addition of a C(sp²)-H/C(sp²)-H olefin moiety to a phenylformamide precursor.

Iron-Catalyzed Reductive Vinylation of Tertiary Alkyl Oxalates with Activated Vinyl Halides

We present herein a rare and efficient method for the creation of vinylated all carbon quaternary centers via Fe-catalyzed cross-electrophile coupling of vinyl halides with tertiary alkyl methyl oxalates. The reaction displays excellent functional group tolerance and broad substrate scope, which allows cascade radical cyclization and vinylation to afford complex bicyclic and spiral structural motifs. The reaction proceeds via tertiary alkyl radicals, and the putative vinyl-Br/Fe complexation appears to be crucial for activating the alkene and enabling a possibly concerted radical addition/C-Fe forming process.

Formation of allylated quaternary carbon centers via C-O/C-O bond fragmentation of oxalates and allyl carbonates

Disclosed herein emphasizes Fe-promoted cross-electrophile allylation of tertiary alkyl oxalates with allyl carbonates that generates all C(sp3)-quaternary centers. The reaction involves fragmentation of tertiary alkyl oxalate C-O bonds to give tertiary alkyl radical intermediates, addition of the radicals to less hindered alkene terminals, and subsequent cleavage of the allyl C-O bonds. Allylation with 2-aryl substituted allyl carbonates was mediated by Zn/MgCl2, and Fe is used to promote the radical addition efficiency. By introduction of activated alkenes, a three-component radical cascade reaction took place.

New avenues for C-B bond formation via radical intermediates

This perspective gives an overview on recent findings in the emerging area of C-radical borylation using diborons as radical trapping reagents. Aryl, vinyl and alkyl boronic esters can be accessed via such an approach under mild conditions. These processes are complementary to established transition metal catalysed cross coupling reactions. Radical borylations can be conducted in the absence of a transition metal but some processes require transition metals as catalysts. It will be shown that various readily available C-radical precursors can be used to run these borylations. For a better understanding of the chemistry, mechanistic discussions are also presented and an outlook on this topic will be provided at the end of the article.

Deoxygenative Borylation of Secondary and Tertiary Alcohols

Two different approaches for the deoxygenative radical borylation of secondary and tertiary alcohols are presented. These transformations either proceed through a metal‐free silyl‐radical‐mediated pathway or utilize visible‐light photoredox catalysis. Readily available xanthates or methyl oxalates are used as radical precursors. The reactions show broad substrate scope and high functional‐group tolerance, and are conducted under mild and practical conditions.

Zn-Mediated Fragmentation of Tertiary Alkyl Oxalates Enabling Formation of Alkylated and Arylated Quaternary Carbon Centers

Zn-mediated reduction of readily accessible dialkyl oxalates derived from tertiary alcohols provides an efficient approach to C-O bond fragmentation and alkyl radical formation. With MgCl₂ as the indispensable additive and Ni as the promoter, trapping the radical with activated alkenes and aryl-Ni intermediates allows for the generation of alkylated and arylated all-carbon quaternary centers.

Photoredox Catalysis with Metal Complexes Made from Earth-Abundant Elements

Photoredox chemistry with metal complexes as sensitizers and catalysts frequently relies on precious elements such as ruthenium or iridium. Over the past 5 years, important progress towards the use of complexes made from earth-abundant elements in photoredox catalysis has been made. This review summarizes the advances made with photoactive Cr^III , Fe^II , Cu^I , Zn^II , Zr^IV , Mo⁰ , and U^VI complexes in the context of synthetic organic photoredox chemistry using visible light as an energy input. Mechanistic considerations are combined with discussions of reaction types and scopes. Perspectives for the future of the field are discussed against the background of recent significant developments of new photoactive metal complexes made from earth-abundant elements.

Selective photocatalysis of lignin-inspired chemicals by integrating hybrid nanocatalysis in microfluidic reactors

In an ideal symbiosis of nanophotocatalysis and microflow chemistry, this tutorial review delineates a micro-highway decorated with semiconductor-based photocatalytic thin layers for the selective transformations of lignin-based chemicals to high-value organic products.