Copper-Catalyzed 1,6-Hydrodifluoroacetylation ofpara-Quinone Methides at Ambient Temperature with Bis(pinacolato)diboron as Reductant

1,6-Nucleophilic Di- and Trifluoromethylation of para-Quinone Methides with Me3SiCF2H/Me3SiCF3 Facilitated by CsF/18-Crown-6

The direct 1,6-nucleophilic difluoromethylation, trifluoromethylation, and difluoroalkylation of para-quinone methides (p-QMs) with Me3SiRf (Rf = CF2H, CF3, CF2CF3, CF2COOEt, and CF2SPh) under mild conditions are described. Although Me3SiCF2H shows lower reactivity than Me3SiCF3, it can react with p-QMs promoted by CsF/18-Crown-6 to give structurally diverse difluoromethyl products in good yields. The products can then be further converted into fluoroalkylated para-quinone methides and α-fluoroalkylated diarylmethanes.

Photodriven Radical Perfluoroalkylation-Thiolation of Unactivated Alkenes Enabled by Electron Donor-Acceptor Complex

A clean and direct three-component radical 1,2-difunctionalization of various alkenes with perfluoroalkyl iodides and thiosulfonates enabled by the electron donor-acceptor complex has been developed under light illumination at room temperature. The approach offers a convenient and environmentally friendly route for the simultaneous incorporation of Csp3-Rf and Csp3-S bonds, affording valuable polyfunctionalized alkane derivatives containing fluorine and sulfur in satisfactory yields. Consequently, this methodology holds significant value and practicality in the field of organic synthesis.

DMSO-promoted direct δ-selective arylation of p-quinone methenylpiperidine bearinides to generate fuchsones under metal-free conditions by employing p-QMs themselves or substituted phenols as aryl sources

Fuchsones have wide applications in modern society. Present methods for generating fuchsones have many disadvantages and there are significant limitations for further exploration of fuchsone applications. Herein, we describe a DMSO-promoted direct δ-selective arylation of p-QMs to synthesize symmetrical and unsymmetrical fuchsones under metal-free conditions by employing p-QMs themselves or substituted phenols as aryl sources. As unprecedented methods, these novel strategies present a great advantage and significance for further exploration of fuchsones and the development of new applications.

BF3 ⋅Et2 O-Catalyzed Selective C-4 Alkylation of Isoquinolin-1(2H)-ones Employing p-Quinone Methides

The direct C-4 alkylation of isoquinolin-1(2H)-one moiety is a challenging transformation in organic synthesis. Here we present a practical and efficient synthesis of C-4 alkylated isoquinolin-1(2H)-ones through conjugate addition of isoquinolin-1(2H)-ones to p-quinone methides for the first time. The process is facilitated by Lewis acid catalysis and this operationally straightforward, mild, metal-free and one-pot transformation provides a wide range of C-4 alkylated isoquinolin-1(2H)-ones at ambient temperature in good to excellent yields.

Addition of sulfonylphthalides to para-quinone methides: Selective 1,6-additions and oxidative annulations

KO^tBu mediated addition of sulfonylphthalides to p-quinone methides led to the selective synthesis of isochroman-1,4-diones and addition products. Interestingly, isochroman-1,4-diones were obtained via an unprecedented oxidative annulation pathway. The present work highlights a wide variety of substrates, good yields, shorter reaction time and ambient reaction conditions. Furthermore, a few addition products were transformed into functionalized heterocyclic molecules. Additionally, the scale-up experiment suggests the practical feasibility of preparing isochroman-1,4-diones in higher-scale reactions.

Unconventional Transformations of Difluorocarbene with Amines and Ethers

ConspectusFluorine-containing compounds are extensively involved in various fields originating from intriguing and unique characteristics of fluorine atom; notably, in pharmaceuticals, the involvement of a fluorine atom or a fluorine-containing group is a chief technique for improving the pesticide effect and developing new drugs. Difluorocarbene, one of the most important and powerful fluorine-containing reagents, is widely employed and studied in many areas mainly to assemble gem-difluoromethyl molecules, including but not limited to the abundant reactions between difluorocarbene with nucleophilic substrates, Wittig reaction with ketones or aldehydes, cascade reaction with both a nucleophile and an electrophile, or [2+1] cycloaddition with alkenes or alkynes. However, its unconventional and intriguing protocols beyond as a difluoromethyl synthon have rarely been studied, and thus, it is highly desired given its abundance, inexpensiveness and peculiar properties. In this Account, we mainly discuss our discovery with unconventional transformations of difluorocarbene, instead of as a sole difluoromethyl source (different from other dihalocarbene), actually can serve as an electron acceptor to activate C-X bonds (X = N and O) and thus promote a myriad of fascinating transformations for the assembly of versatile valuable products with various aza-compounds (primary/secondary/tertiary amines as well as NH₃ and NaNH₂ and so on) and aliphatic ethers in the absence of transition metals and expensive ligands. Inspired by the electron-deficient characteristics of difluorocarbene, we first found that the isocyanides could be readily formed in situ when the unoccupied orbital of difluorocarbene meets the lone-pair of primary amines; in basic condition, a cascade defluorination and cyclizations could afford plethora of valuable N-containing heterocycles. Meanwhile, we disclosed that cyano anion could be accessible in situ as well when difluorocarbene and NaNH₂ or NH₃ were mixed up in suitable basic conditions, and thus a series of aryl nitrile compounds were obtained in the presence of Pd catalysis and ArI. Interestingly, when difluorocarbene encountered secondary amines, formamides were rendered under mild reactions. Of note, concomitant functionalizations of C and N moieties via cleavage of the unstrained C(sp³)-N bond in the absence of metal and oxidant are sparce, which indeed significantly add versatility and diversity to products. Gratifyingly, by uitilizing difluorocarbene and cyclic tertiary amines, we achieved difluorocarbene-mediated deconstructive functionalizations for the first time, showing successive C(sp³)-N bond scission of amines and simultaneous functionalization of C and N atoms which would be introduced into the products in the absence of transition metals and oxidants. This method provides a brand-new while very universal synthetic pathway to selectively cleave inert unactivated Csp³-N bonds, in which halodifluoromethyl reagents act as both C1 synthon and halo (Cl, Br, I) sources. Fascinatingly, nitrogen ylides are generated in situ from difluorocarbene and tertiary amines, and an intriguing and universal approach for deaminative arylation or alkenylation of tertiary amines was disclosed for the first time in appropriate basic conditions, which represents an intriguing reaction mode to lead to a formal transition-metal free Suzuki cross coupling. Besides, we also disclosed that difluorocarbene could proceed novel atom recombination to render meaningful 2-fluoroindoles or 3-(2,2-difluoroethyl)-2-fluoroindoles from ortho-vinylanilines, 3-fluorined oxindoles from 2-aminoarylketones, in which difluorocarbene acts as a C1 synthon and F1 source simultaneously. Last but not the least, we recently found that the lone-pair-electron of oxygen could trap difluorocarbene as well to form oxonium ylide, which eventually leads to C-O bond cleavage with the formation of difluoromethyl ethers.

Metal-Free Reductive Coupling of para-Quinone Methides with 4-Cyanopyridines Enabled by Pyridine-Boryl Radicals: Access to Pyridylated Diarylmethanes with Anti-Cancer Activity

Reported herein is a streamlined protocol to produce pyridylated diarylmethanes through pyridine-boryl radical induced reductive coupling between para-quinone methides (p-QMs) and 4-cyanopyridines using bis(pinacolato)diboron (B₂ pin₂ ) as a templated reagent. The metal-free process is characterized by an operationally simple approach, excellent chemoselectivity (1,2- vs. 1,6-selectivity), and a broad substrate scope with good functional group compatibility. The mechanistic studies provided important insights into the reductive cross-coupling process between diarylmethyl radical and pyridine-boryl radical. Moreover, part of the obtained pyridylated diarylmethane products were screened against a panel of cancer cell lines, and 3 v was confirmed to significantly inhibit the proliferation of head and neck squamous cell carcinoma (HNSCC) cells. This method offers a platform for the preparation of new lead compounds with antitumor activity.

Photocatalytic Synthesis of Diarylmethyl Silanes via 1,6-Conjugate Addition of Silyl Radicals to p-Quinone Methides

A novel photocatalytic method for the preparation of diarylmethyl silanes was reported through silyl radicals addition strategy to p-QMs (p-quinone methides). This protocol could tolerate a variety of functional groups affording the corresponding silylation products with moderate to excellent yields. The resulting silylation products could be easily converted into a series of bioactive GPR40 agonists and useful p-QMs precursors for the synthesis of compounds possessing both quaternary carbon centers and silicon substituents through simple operation. A plausible mechanism of silyl radicals to p-QMs was proposed on the basis of experimental results and previous literature.

1,6-Addition of 1,2,3-NH triazoles to para-quinone methides: Facile access to highly selective N1 and N2 substituted triazoles

The regioselective syntheses of N¹ and N² substituted triazoles through a 1,6-addition reaction of 1,2,3-NH triazoles to p-quinone methide were achieved under mild reaction conditions. The present reactions showed superior results in terms of selectivity, mild reaction conditions, short reaction time and broad substrate scope with good functional-group compatibility. Considering the high synthetic value of N¹- and N²-substituted compounds and p-QM related research, the present strategy will greatly benefit researchers in various fields.

Photoinduced decarboxylative 1,6-addition of para-quinone methides with α-keto acids: an eco-friendly approach to α,α′-diarylated ketones

The photoinduced decarboxylative 1,6-addition of para-quinone methides with α-keto acids in an eco-friendly approach to α,α′-diarylated ketones is developed.

Copper-Catalyzed Difluoroalkylation of Alkene/Nitrile Insertion/Cyclization Tandem Sequences: Construction of Difluorinated Bicyclic Amidines

A copper-catalyzed difluoroalkylation of an alkene/nitrile insertion/cyclization tandem sequence of N-cyanamide alkene was described, which provided a convenient synthetic approach for accessing difluorinated bicyclic amidines bearing imine moieties in a sustainable fashion. This protocol is characterized by high yields, a broad substrate scope, and good functional group compatibility. In addition, the desired product can be readily converted into other valuable functionalized fluorinated aza-heterocycles.

Iron-Catalyzed Ring Opening of Cyclopropanols and Their 1,6-Conjugate Addition to p-Quinone Methides

A novel iron-catalyzed ring opening of cyclopropanols and their 1,6-conjugate addition to p-quinone methides for accessing substituted phenols is disclosed. In this protocol, various cyclopropanols are converted to alkyl radicals and undergo 1,6-conjugate addition to p-quinone methides toward C-C bond formation. The salient features of this methodology include operationally simple and mild reaction conditions, environmentally benign protocol, high efficiency, inexpensive catalyst, good to excellent yield, and a wide range of substrate scope.

Electrochemical Radical δ-H Sulfonylation Reaction for the Synthesis of 4-((Aryl,Arylsul fonyl)methylene)-2,5-Cyclohexadiene Derivatives

A novel and efficient electrochemical radical δ-H sulfonylation reaction of para-quinone methides (p-QMs) and sodium sulfinates has been achieved under common laboratory conditions. In this strategy, a new C(sp²)-S bond was constructed for the synthesis of 4-((aryl,arylsulfonyl)methylene)-2,5-cyclohexadiene derivatives with a broad substrate scope, good functional group tolerance, and mild conditions. Further studies showed that the reaction had an excellent regional selectivity.

Uranyl-catalyzed hydrosilylation of para-quinone methides: access to diarylmethane derivatives

An efficient and convenient uranyl-catalyzed reductive hydrosilylation reaction of para-quinone methides (p-QMs) was developed by employing silane as the reductant. The hydrosilylation procedure using the UO2(NO3)2·6H2O/Et3SiH catalytic system proceeded smoothly and provided an expedient method for the construction of various diarylmethane derivatives in one step with good to excellent yields.

Cu-Catalyzed radical-triggered spirotricyclization of enediynes and enyne-nitriles for the synthesis of pentacyclic spiroindenes

A new copper-catalyzed radical-triggered fluoromethylation-spirotricyclization of enediyne- and enyne-nitrile-containing para-quinone methides (p-QMs) was reported for the first time, and used to produce a series of hitherto unreported pentacyclic spiroindenes.

Copper-catalyzed 1,6-conjugate addition of para-quinone methides with diborylmethane

Presented herein is the first 1,6-conjugate addition of diborylmethane. This reaction features high yields, mild reaction conditions, and broad functional group compatibilities.

Photocatalytic intramolecular radical cyclization involved synergistic SET and HAT: synthesis of 3,3-difluoro-γ-lactams

A mild and metal-free protocol for visible-light induced intramolecular radical cyclization of N-allyl(propargyl)-2-bromo-2,2-difluoro-N-arylacetamide has been developed.

Visible-Light-Enabled C-H Functionalization by a Direct Hydrogen Atom Transfer Uranyl Photocatalyst

The development of the uranyl cation as a powerful photocatalyst is seriously delayed in comparison with the advances in its fundamental and structural chemistry. However, its characteristic high oxidative capability in the excited state ([UO₂ ]²⁺ * (+2.6 V vs. SHE; SHE=standard hydrogen electrode) combined with blue-light absorption (hv=380-500 nm) and a long-lived fluorescence lifetime up to microseconds have reveals that the uranyl cation approaches an ideal photocatalyst for visible-light-driven organic transformations. Described herein is the successful use of uranyl nitrate as a photocatalyst to enable C(sp³ )-H activation and C-C bond formation through hydrogen atom transfer (HAT) under blue-light irradiation. In particular, this operationally simple strategy provides an appropriate approach to the synthesis of diverse and valuable diarylmethane motifs. Mechanistic studies and DFT calculations have provided insights into the detailed mechanism of the photoinduced HAT pathway. This research suggests a general platform that could popularize promising uranyl photocatalytic performance.

Copper-Catalyzed Bromodifluoroacetylative Cyclization of Enynes

A copper-catalyzed bromodifluoroacetylative cyclization reaction is described. The treatment of bromodifluoroacete derivatives by CuI and B₂Pin₂ enables difluoroalkyl radical generation and triggers the radical addition/cyclization/bromination sequences. Bromodifluoroacetyl-derived ester, amide, and ketone were compatible and gave various vinyl C-Br bonds containing functionalized heterocycles in good yields.

Iridium(III)-Catalyzed Difluoroalkylation–Bicyclization of 1,7-Enynes toward Benzo[a]fluoren-5-ones under Visible-Light Photoredox Conditions

A new visible-light-induced Ir(III)-catalyzed difluoroalkylation–bicyclization of 1,7-enynes with ethyl 2-bromo-2,2-difluoroacetate (BrCF2CO2Et) is described, furnishing a wide range of difluoromethyl-containing benzo[a]fluoren-5-ones in good to excellent yields. The reaction is operationally simple, proceeds with high efficiency under mild conditions, and shows excellent functional group compatibility.

Recent progress on selective deconstructive modes of halodifluoromethyl and trifluoromethyl-containing reagents

Halodifluoromethyl and trifluoromethyl-containing compounds are widely employed in organic chemistry, pharmaceuticals and materials science. Therefore, their applications and transformations have received significant attention during the past few decades. The single, double, triple and quadruple cleavage of halodifluoromethyl compounds and various deconstructive modes of trifluoromethyl-containing compounds could generate a variety of synthons to prepare more valuable products. Herein, we summarize the most significant achievements in this field with an intriguing focus on results from the last decade.

Deconstructive Functionalizations of Unstrained Carbon-Nitrogen Cleavage Enabled by Difluorocarbene

Transition-metal- or oxidant-promoted deconstructive functionalizations of noncyclic carbon-nitrogen bonds are well established, usually only leaving one moiety functionalized toward the final product. In contrast, concomitant C- and N-functionalizations via the unstrained C(sp3)-N bond under metal- and oxidant-free conditions are very rare, which would favorably confer versatility and product diversity. Disclosed herein is the first difluorocarbene-induced deconstructive functionalizations embodying successive C(sp3)-N bond cleavage of cyclic amines and synchronous functionalization of both constituent atoms which would be preserved in the eventual molecular outputs under transition-metal-free and oxidant-free conditions. Correspondent access to deuterated formamides with ample isotopic incorporation was demonstrated by a switch to heavy water which is conceivably useful in pharmaceutical sciences. The current strategy remarkably administers a very convenient, operationally simple and novel method toward molecular diversity from readily available starting materials. Therefore, we project that these findings would be of broad interest to research endeavors encompassing fluorine chemistry, carbene chemistry, C-N bond activation, as well as medicinal chemistry.

Microwave-Assisted Copper Catalysis of α-Difluorinated gem-Diol toward Difluoroalkyl Radical for Hydrodifluoroalkylation of para-Quinone Methides

Reported herein is a unified strategy to generate difluoroalkyl radicals from readily prepared α-difluorinated gem-diols by single electron oxidation. Under microwave irradiation, a catalytic amount of oxidant Cu(OAc)₂ succeeds in the formation of transient difluoroalkyl radicals in situ, for the first time. The reaction features a simple protocol, short reaction time, scalability, and high yield. The synthetic utility of this new methodology was also explored for the synthesis of difluoroalkylated spiro-cyclohexadienones, which is an important core structure in natural products and pharmaceuticals.

Asymmetric synthesis of dihydrocoumarins via catalytic sequential 1,6-addition/transesterification of α-isocyanoacetates with para-quinone methides

Dihydroquinidine-derived aminophosphine/AgNO₃ catalyzed asymmetric tandem 1,6-addition/transesterification of α-isocyanoacetates with o-hydroxyphenyl-substituted p-QMs was developed.

Organocatalytic asymmetric cascade 1,6-addition/hemiketalization/retro-Henry reaction of ortho-hydroxyphenyl-substituted p-QMs with α-nitroketones

Novel organocatalytic asymmetric cascade 1,6-addition/hemiketalization/retro-Henry reaction is developed to give new chiral 2-(1-(4-hydroxyphenyl)ethyl)phenols with good yields and excellent enantioselectivities under mild reaction conditions.

Recent developments in 1,6-addition reactions of para-quinone methides (p-QMs)

In this review, we provide a comprehensive overview of recent progress in this rapidly growing field by summarizing the 1,6-conjugate addition and annulation reactions of p-QMs with consideration of their mechanisms and applications.

δ-Cyano substituted para-quinone methides enable access to unsymmetric tri- and tetraarylmethanes containing all-carbon quaternary stereocenters

Placing an electron-withdrawing cyano group into the δ-position of para-quinone methides enables facile access to unsymmetrical tri- and tetraarylmethanes bearing all-carbon quaternary stereocenters.

Visible-light-promoted radical cross-coupling of para-quinone methides with N-substituted anilines: an efficient approach to 2,2-diarylethylamines

A series of 2,2-diarylethylamines were accessed via visible-light-promoted radical cross-coupling of p-QMs with N-alkyl anilines.

tert-Butyl Nitrite Mediated Synthesis of Fluorinated O-Alkyloxime Ether Derivatives

A tert-butyl nitrite (TBN)-mediated synthesis of fluorinated O-alkyloxime ether derivatives with bromodifluoroalkyl reagents as the fluorine sources has been developed. A variety of halodifluorinated compounds were found compatible, delivering the desired products in moderate to excellent yields. This transformation features a simple operation, can be done in air, and found to involve radicals. This protocol represents a straightforward approach to access various fluorinated O-alkyloxime ether derivatives.

Catalyst- and reagent-free 1,6-hydrophosphonylation of p-quinone methides: a practical approach for the synthesis of diarylmethyl phosphine oxides

We developed a catalyst-, reagent-, and additive-free protocol with 100% atom economy for the synthesis of diarylmethyl phosphine oxides via 1,6-hydrophosphonylation of p-quinone methides using water as a green solvent. The reaction showed broad scope with excellent functional group tolerance. The practicability of this method was demonstrated by carrying out the reaction on the gram scale whereby product was obtained in high yield by the filtration technique avoiding chromatographic purification.