Direct alkylation of heteroaryls using potassium alkyl- and alkoxymethyltrifluoroborates

C–H functionalization of quinazolinones by transition metal catalysis

Quinazolinone and its derivatives are an important class of heterocyclic scaffolds in pharmaceuticals and natural products. This review provides the recent research advances in the transition metal catalyzed selective C–H bond functionalization of quinazolinone.

Computational Insight into the Mechanism of Ruthenium(II)-Catalyzed α-Alkylation of Arylmethyl Nitriles Using Alcohols

The ruthenium(II)-catalyzed α-alkylation reaction of arylmethyl nitriles (phenylacetonitrile) using alcohols (ethanol) in toluene has been extensively investigated by means of SMD-M06-2X/6-311G(d,p)-LANL2dz (LAnL2dz for Ru, 6-311G(d,p) for other atoms) calculations. Detailed mechanistic schemes have been proposed and discussed. The catalytically active Ru(II) complex was generated by the base-induced KCl elimination from the catalyst precursor [(PNP^Ph)RuHCl(CO)]. The overall Ru(II) catalytic cycle consists of three basic processes: (1) ethanol-to-aldehyde transformation catalyzed by the 16-electron unsaturated ruthenium pincer catalyst; (2) a 16-electron unsaturated ruthenium pincer catalyst catalyzed condensation reaction of arylmethyl nitrile with aldehyde, which leads to PhC(CN)=CHCH₃; (3) hydrogenation of PhC(CN)=CHCH₃, which leads to the formation of the α-alkylated arylmethyl nitrile product (PhCH(CH₂CH₃)CN). The DFT results revealed that the rate-determining barrier of the overall reaction was 23.9 kcal/mol of the H-transfer step in the third process. The reaction of PhC(CN)=CHCH₃ with the dihydride Ru complex, which is generated in the ethanol-to-aldehyde transformation process, is the more preferable hydrogenation mechanism than hydrogenation of vinyl nitrile-Ru complex by H₂. Using alcohol as the reactant not only fulfills the requirement of the borrowing-H strategy but also lowers the barriers of the H-migration steps.

Decarboxylative Alkylation of Heteroarenes Using N-Hydroxybenzimidoyl Chloride Esters

Functionalized N-heteroarenes are highly desired motifs in medicinal chemistry and pharmaceutical industry. Minisci-type reactions usually require a protonated N-heteroarene for the alkyl radical to attack. This work describes a leaving-group-assisted redox-active ester to enable direct coupling of an amino acid with N-heteroarenes. The efficient and sustainable photoredox strategy provides rapid access to an alkylated heterocyclic manifold.

Reaction of Papaverine with Baran DiversinatesTM

The reaction of papaverine with a series of Baran Diversinates^TM is reported. Although the yields were low, it was possible to synthesize a small biodiscovery library using this plant alkaloid as a scaffold for late-stage C–H functionalization. Ten papaverine analogues (2–11), including seven new compounds, were synthesized. An unexpected radical-induced exchange reaction is reported where the dimethoxybenzyl group of papaverine was replaced by an alkyl group. This side reaction enabled the synthesis of additional novel fragments based on the isoquinoline scaffold, which is present in numerous natural products. Possible reasons for the poor yields in the Diversinate^TM reactions with this particular scaffold are discussed.

Saturated oxygen and nitrogen heterocycles via oxidative coupling of alkyltrifluoroborates with alkenols, alkenoic acids and protected alkenylamines

The oxidative coupling of alkyltrifluoroborates with heteroatom-tethered vinylarenes leads to a broad range of saturated oxygen and nitrogen heterocycles.

Saturated heterocycles are important components of many bioactive compounds. The method disclosed herein enables a general route to a range of 5-, 6- and 7-membered oxygen and nitrogen heterocycles by coupling potassium alkyltrifluoroborates with heteroatom-tethered alkenes, predominantly styrenes, under copper-catalyzed conditions, in the presence of MnO₂. The method was applied to the synthesis of the core of the anti-depressant drug citalopram. The reaction scope and observed reactivity is consistent with a polar/radical mechanism involving intermolecular addition of the alkyl radical to the alkene followed by [Cu(iii)]-facilitated C–O (or C–N) bond forming reductive elimination.

Palladium-catalyzed direct C2-arylation of azoles with aromatic triazenes

A highly efficient palladium-catalyzed arylation of azoles at the C2-position using 1-aryltriazenes as aryl reagents was developed. Azoles including oxazoles, thiazoles, imidazoles, 1,3,4-oxadiazoles, and oxazolines could react with 1-aryltriazenes smoothly to generate the corresponding products in good to excellent yields, and various substitution patterns were tolerated toward the reaction.

Visible Light-Promoted Aliphatic C-H Arylation Using Selectfluor as a Hydrogen Atom Transfer Reagent

A mild, practical method for direct arylation of unactivated C(sp³)-H bonds with heteroarenes has been achieved via photochemistry. Selectfluor is used as a hydrogen atom transfer reagent under visible light irradiation. A diverse range of chemical feedstocks, such as alkanes, ketones, esters, and ethers, and complex molecules readily undergo intermolecular C(sp³)-C(sp²) bond formation. Moreover, a broad array of heteroarenes, including pharmaceutically useful scaffolds, can be alkylated effectively by the protocol presented here.

Ligand-Accelerated Iron Photocatalysis Enabling Decarboxylative Alkylation of Heteroarenes

A mild, practical protocol for the decarboxylative alkylation of heteroarenes has been accomplished via iron photocatalysis. A diverse range of carboxylic acids readily undergo oxidative decarboxylation and then couple with a broad array of heteroarenes in this transformation. The photoexcited state lifetimes of iron complexes are typically much shorter than those of iridium and ruthenium complexes. Here we describe our effort on iron photocatalysis by utilizing the intramolecular charge transfer pathway of iron-carboxylate complexes.

Desulfonative photoredox alkylation of N-heteroaryl sulfones - an acid-free approach for substituted heteroarene synthesis

Minisci-type alkylation of electron-deficient heteroarenes has been a pivotal technique for medicinal chemists in the synthesis of drug-like molecules. However, such transformations usually require harsh conditions (e.g., strong acids, stoichiometric amount of oxidants, elevated temperatures, etc.). Herein, by utilizing photoredox catalysis, a highly-selective alkylation method using heteroaryl sulfones has been developed that can be carried out under acid-free and redox-neutral conditions. Because of these mild conditions, challenging yet privileged structures, such as monosaccharides and unprotected secondary amines, can be installed.

Electrochemical cobalt-catalyzed C-H or N-H oxidation: a facile route to synthesis of substituted oxindoles

Two comparable protocols for the electrochemical cobalt-catalyzed C-H/N-H oxidation have been exploited for the synthesis of substituted oxindoles via radical pathways. The electrochemical cobalt-catalyzed system was demonstrated to be efficient and eco-friendly and avoided the use of stoichiometric oxidants to afford the arylation or alkylation products in good yields at room temperature.

Advancements in Visible-Light-Enabled Radical C(sp)2-H Alkylation of (Hetero)arenes

The Minisci reaction, which encompasses the radical C-H alkylation of heteroarenes, has undergone revolutionary development in recent years. The application of photoredox catalysis for alkyl radical generation has given rise to a multitude of methods that feature enhanced functional group tolerance, generality, and operational simplicity. The intent of this short review is to bring readers up to date on this rapidly expanding field. Specifically, we will highlight key examples of visible light-driven Minisci alkylation strategies that represent key advancements in this area of research. The scope and limitation of these transformations will be discussed, with a focus on examining the underlying pathways for alkyl radical generation. Our goal is to make this short review a stepping stone for further synthetic research development.

Visible-light-mediated Minisci C-H alkylation of heteroarenes with unactivated alkyl halides using O2 as an oxidant

Herein, we report a protocol for direct visible-light-mediated Minisci C-H alkylation of heteroarenes with unactivated alkyl halides using molecular oxygen as an oxidant at room temperature. This mild protocol is compatible with a wide array of sensitive functional groups and has a broad substrate scope. Notably, functionalization of (iso)quinolines, pyridines, phenanthrolines, quinazoline, and other heterocyclic compounds with unactivated primary, secondary, and tertiary alkyl halides proceeds smoothly under the standard conditions. The robustness of this protocol is further demonstrated by the late-stage functionalization of complex nitrogen-containing natural products and drugs.

Photoredox-mediated remote C(sp3)-H heteroarylation of free alcohols

We report an efficient and economical method for remote δ C(sp3)-H heteroarylation of free aliphatic alcohols using a hypervalent iodine PFBI-OH oxidant under photoredox catalysis. The reaction sequence involves in situ alcoholysis of PFBI-OH with alcohol, generation of an alkoxy radical intermediate by SET reduction, 1,5-HAT, and Minisci-type C-C bond formation. This method uses a slight excess of alcohols, can facilitate reaction at δ methyl and methylene positions, and has been successfully applied to modification of complex drug molecules.

Photoinitiated Three-Component α-Perfluoroalkyl-β-heteroarylation of Unactivated Alkenes via Electron Catalysis

A visible-light-initiated α-perfluoroalkyl-β-heteroarylation of various alkenes with perfluoroalkyl iodides and quinoxalin-2(1 H)-ones is presented. This three-component radical cascade reaction allows an efficient synthesis of a range of perfluoroalkyl containing quinoxalin-2(1 H)-one derivatives in moderate to excellent yields under mild conditions. Reactions proceed via acidic aminyl radicals that are readily deprotonated to give the corresponding radical anions able to sustain the radical chain as single electron transfer reducing reagents. Hence, the overall cascade classifies as an electron-catalyzed process.

Direct C-3 alkylation of coumarins via decarboxylative coupling with carboxylic acids

A metal-free direct alkylation of coumarins using carboxylic acids in aqueous media with a broad substrate scope is devised.

Green oxidant H2O2 as a hydrogen atom transfer reagent for visible light-mediated Minisci reaction

A metal-free protocol for oxidative coupling of heteroarenes and aliphatic C–H components has been achieved via a radical pathway.

Visible-light induced decarboxylative C2-alkylation of benzothiazoles with carboxylic acids under metal-free conditions

A photoredox catalyzed C2-alkylation of benzothiazoles with carboxylic acids was developed by using an acridinium salt as a photocatalyst and air as an oxidant.

Metal-, photocatalyst-, and light-free late-stage C–H alkylation of N-heteroarenes with organotrimethylsilanes using persulfate as a stoichiometric oxidant

Minisci C–H alkylation of N-heteroarenes with readily available benzylsilanes and heteroatom substituted silanes was developed.

Visible-light-induced deboronative alkylarylation of acrylamides with organoboronic acids

A visible-light-induced deboronative alkylarylation of acrylamides with boronic acids was developed via a tandem reaction process.

A Predictive Tool for Electrophilic Aromatic Substitutions Using Machine Learning

At the early stages of the drug development process, thousands of compounds are synthesized in order to attain the best possible potency and pharmacokinetic properties. Once successful scaffolds are identified, large libraries of analogues are made, which is a challenging and time-consuming task. Recently, late stage functionalization (LSF) has become increasingly prominent since these reactions selectively functionalize C-H bonds, allowing to quickly produce analogues. Classical electrophilic aromatic halogenations are a powerful type of reaction in the LSF toolkit. However, the introduction of an electrophile in a regioselective manner on a drug-like molecule is a challenging task. Herein we present a machine learning model able to predict the reactive site of an electrophilic aromatic substitution with an accuracy of 93% (internal validation set). The model takes as input a SMILES of a compound and uses six quantum mechanics descriptors to identify its reactive site(s). On an external validation set, 90% of all molecules were correctly predicted.

Organocatalytic Enantioselective Addition of α-Aminoalkyl Radicals to Isoquinolines

With a dual organocatalytic system involving a chiral phosphoric acid and a dicyanopyrazine-derived chromophore (DPZ) photosensitizer and under the irradiation with visible light, an enantioselective Minisci-type addition of α-amino acid-derived redox-active esters (RAEs) to isoquinolines has been developed. A variety of prochiral α-aminoalkyl radicals generated from RAEs were successfully introduced on isoquinolines, providing a range of valuable α-isoquinoline-substituted chiral secondary amines in high yields with good to excellent enantioselectivities.

Metal-Free and Redox-Neutral Conversion of Organotrifluoroborates into Radicals Enabled by Visible Light

Converting organoboron compounds into the corresponding radicals has broad synthetic applications in organic chemistry. To achieve these transformations, various strong oxidants such as Mn(OAc)₃ , AgNO₃ /K₂ S₂ O₈ , and Cu(OAc)₂ , in stoichiometric amounts are required, proceeding by a single-electron transfer mechanism. Established herein is a distinct strategy for generating both aryl and alkyl radicals from organotrifluoroborates through an S_H 2 process. This strategy is enabled by using water as the solvent, visible light as the energy input, and diacetyl as the promoter in the absence of any metal catalyst or redox reagent, thereby eliminating metal waste. To demonstrate its synthetic utility, an efficient acetylation to prepare valuable aryl (alkyl) methyl ketones is described and applications to construct C-C, C-I, C-Br, and C-S bonds are also feasible. Experimental evidence suggests that triplet diacetyl serves as the key intermediate in this process.

DMSO as a Switchable Alkylating Agent in Heteroarene C-H Functionalization

Herein, we report a novel strategy for the activation of DMSO to act as a versatile alkylating agent in heteroarene C-H functionalization. This direct, simple, and mild switch between methylation/trideuteromethylation and methylthiomethylation of heteroarenes was achieved under reagent-controlled photoredox catalysis conditions. The proposed mechanism is supported by both experimental and computational studies.

Xanthate-Mediated Incorporation of Quaternary Centers into Heteroarenes

The xanthate-mediated addition of tertiary alkyl radicals to heteroarenes enabled the easy functionalization of heteroaromatic rings as well as more decorated structures, such as marketed drugs or agrochemicals. This work provides a synthetic tool for efficiently exploring the chemical space by allowing late-stage diversification with a high tolerance toward functional groups.

Visible Light-Mediated Decarboxylative Alkylation of Pharmaceutically Relevant Heterocycles

A net redox-neutral method for the decarboxylative alkylation of heteroarenes using photoredox catalysis is reported. Additionally, this method features the use of simple, commercially available carboxylic acid derivatives as alkylating agents, enabling the facile alkylation of a variety of biologically relevant heterocyclic scaffolds under mild conditions.

Highly Discriminative and Chemoselective Deprotection/Transformations of Acetals with the Combination of Trialkylsilyl Triflate/2,4,6-Collidine

Acetals are the most useful protecting groups for carbonyl functional groups. In addition to the role of protection, they can also be used as synthons of carbonyl functions. Previously, we developed a chemoselective deprotection and nucleophilic substitution of acetals from aldehydes in the presence of ketals. This article describes the highly discriminative and chemoselective transformations of acetals bearing different substitution patterns, different types of acetals, as well as mixed acetals. These reactions can achieve the transformations that cannot be attained by conventional methods, and their results strongly suggest the combination of R₃SiOTf/2,4,6-collidine to promote such unprecedented phenomena.

The synergistic effect of self-assembly and visible-light induced the oxidative C-H acylation of N-heterocyclic aromatic compounds with aldehydes

Constructing carbon-carbon bonds through oxidative cross-coupling between two hydrocarbon compounds is regarded as a foundational issue in green chemistry. High atom-economy and mild conditions are long term pursued goals in this field. Herein, we developed a visible-light mediated direct cross-coupling between N-heterocycles and aldehydes without the requirement of a photocatalyst. Several N-heterocycles afforded the acylation products with aromatic or aliphatic aldehydes in good yields. The reaction can be conducted under room temperature and easily scaled up.