Ag-Catalyzed Remote Unactivated C(sp3)-H Heteroarylation of Free Alcohols in Water

Minisci Reaction by an MW-Boost: A Simple Protocol for Site-Selective Dehydrogenative Cross-Coupling Between Nitrogen-Aromatics and C(sp3)─H Bonds

The Minisci-type reaction between C(sp3)─H bonds and nitrogen-containing aromatics was carried out using a simple protocol based on the persulfate anion (PS) under microwave (MW) irradiation. The MW/PS protocol is applicable to a wide variety of dehydrogenative coupling reactions of N-heteroaromatics and C(sp3)─H bonds, leading to the formation of alkylated heteroaromatics. Site-selectivity is ensured by hydrogen atom transfer (HAT) with the sulfate radical affecting the polar/steric effects in the transition state. The MW/PS protocol was applicable to direct intramolecular cyclization to give five and six-membered rings fused by pyridine and lutidine, which, in previous methods, it was necessary to use tedious derivatization to imino-oxyacetic acids as HAT precursors.

Ag-Catalyzed cross-dehydrogenative-coupling for the synthesis of 1,4-dioxan-2-yl substituted quinazoline hybrids in an aqueous medium

We herein developed an effective approach for the construction of 2- or 4-(1,4-dioxan-2-yl) substituted quinazolines under mild conditions. A silver-K2S2O8 catalyzed direct CDC reaction between quinazolines and 1,4-dioxane for the synthesis of a series of 2- or 4-(1,4-dioxan-2-yl) substituted quinazoline hybrids is reported. The reaction proceeded well in water under mild conditions and showed a broad substrate scope and good functional group compatibility.

Unified ionic and radical C-4 alkylation and arylation of pyridines

C-H Functionalization of pyridines is an efficient strategy to access pyridine derivatives occurring in pharmaceuticals, agrochemicals, and materials. Nucleophilic additions to pyridiniums via both ionic and radical species have proven particularly useful. However, these reactions suffer from poor regioselectivity. By identifying an enzyme-mimic pocket-type urea activation reagent, we report a general platform for pyridine C-4 functionalization. Both ionic and radical nucleophiles can be incorporated to achieve the alkylation and arylation. Notably, the highly regioselective C-4 radical arylation is disclosed for the first time. The broad scope of nucleophiles and pyridines renders this platform applicable to the late-stage functionalization of drug-like molecules and the preparation of complex biologically important molecules.

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Regioselective synthesis of isoquinolinonediones through remote unactivated C(sp3)-H bonds

Herein, a general strategy for the remote-site-selective cascade addition/cyclization of unactivated C(sp3)-H bonds in free alcohols and sulfonamides to build isoquinolinonedione skeletons is developed. The site selectivity occurs predominantly via a 1,5-hydrogen atom transfer (HAT) process, triggered by heteroatom-centred radicals generated directly under silver catalysis. A broad substrate scope and excellent regio-/chemo-selective control are demonstrated in this method.

Radical-mediated photoredox hydroarylation with thiosulfonate

Herein, we report a novel visible light-induced photocatalytic system that enables intramolecular hydroarylation of unactivated alkenes. Thiosulfonate compounds were found to be the key radical precursor that mediates the Minisci-type intramolecular cyclization reaction. Under the optimal reaction conditions, a wide range of pyridyquinazolinone and pyrroloquinazolinone products were obtained in moderate to good yields.

Phototriggered Self-Catalyzed Three-Component Minisci Reaction: A Route to β-C(sp3) Heteroarylated Alcohols/Ethers

Herein, a visible-light-triggered photocatalyst-free radical cascade Minisci reaction of heteroarenes, alkenes, and water/alcohols to obtain diverse β-C(sp³) heteroarylated alcohols/ethers has been developed. Achieved under mild and simple conditions, this protocol is scalable and features broad substrate scope and functional group tolerance. Mechanistic studies demonstrate that the heteroarene can be served as a photocatalyst to engage single-electron transfer with persulfate.

Visible Light-Induced Unactivated δ-C(sp3 )-H Amination of Alcohols Catalyzed by Iron

An iron-catalyzed remote C(sp³ )-H amination of alcohols through 1,5-hydrogen atom transfer is developed. This protocol provides a method to generate δ-C(sp³ )-N bonds from primary, secondary, and tertiary alcohols under mild conditions. A wide substrate scope and a good functional group tolerance are presented. Mechanistic studies show that a LMCT course of an Fe-OR species and a chlorine radical-induced hydrogen abstraction of an alcohol are possible to generate the alkoxy radical intermediate.

Bioinspired Synthesis of Pinoxaden Metabolites Using a Site-Selective C-H Oxidation Strategy

A bioinspired synthesis of Pinoxaden metabolites 2-5 is described herein. A site-selective C-H oxidation strategy validated by density functional theory (DFT) calculations was devised for preparing metabolites 2-4. Oxidation of the benzylic C-H bond in tertiary alcohol 7 using K₂S₂O₈ and catalytic AgNO₃ formed the desired metabolite 2 that enabled access to metabolites 3 and 4 in a single step. Unlike most metal/persulfate-catalyzed transformations reported for the C-C and C-O bond formation reactions wherein the metal acts as a catalyst, we propose that Ag(I)/K₂S₂O₈ plays the role of an initiator in the oxidation of intermediate 7 to 2. Metabolite 2 was subjected to a ruthenium tetroxide-mediated C-H oxidation to form metabolites 3 and 4 as a mixture that were purified to isolate pure standards of these metabolites. Metabolite 5 was synthesized from readily available advanced intermediate 9 via a House-Meinwald-type rearrangement in one step using a base.

Tandem vinyl radical Minisci-type annulation on pyridines: one-pot expeditious access to azaindenones

A new regiospecific alkylative/alkenylative cascade annulation of pyridines has been achieved whilst the corresponding classic Minisci alkylative annulation failed. This protocol provides a novel and expeditious access to azaindenones and related compounds via cross-dehydrogenative coupling with the long-standing problem of C2/C4 regioselectivity of pyridines being well addressed.

Visible-light-promoted photocatalyst-free alkylation and acylation of benzothiazoles

Herein we report a protocol for the visible-light-mediated alkylation/acylation reaction of benzothiazoles. Alkyl/acyl substituted Hantzsch esters are easily prepared and rationally used as radical precursors. In the presence of BF3·Et2O and Na2S2O8, various benzothiazole derivatives were readily obtained in good yields. Our user-friendly protocol can proceed by simple irradiation with blue LEDs (λ = 465 nm) and without the assistance of external photocatalysts. The reaction is also characterized by mild conditions and scalability, thus offering an alternative and efficient tool for the synthesis of 2-functionalized benzothiazoles.