Visible light promoted synthesis of allenes

Jitender Singh† , Barakha Saxena† and Anuj Sharma *
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247667, India. E-mail: anuj.sharma@cy.iitr.ac.in; anujsharma.mcl@gmail.com; Tel: +91 1332 284751

First published on 17th July 2024

---

---

Jitender Singh

Jitender Singh obtained his B.Sc (2014) from Kurukshetra University, Kurukshetra, India, and M.Sc (2018) in Organic Chemistry from Central University of Punjab (CUP), Bathinda, India. He received the Junior Research Fellowship Award (CSIR-JRF) just after the completion of his Masters's. Recently, he received the Senior Research Fellowship Award (CSIR-SRF). Presently, he is a Ph.D. Scholar in the research group of Prof. Anuj Sharma in the department of chemistry, Indian Institute of Technology (IIT), Roorkee, India. His current research is focused on novel strategies for the synthesis of functionalized O- or N-containing aromatic heterocyclic compounds.

Barakha Saxena

Barakha Saxena obtained her B.Sc (2015) and M.Sc degree (2017) in Organic Chemistry from Mahatma Jyotiba Phule Rohilkhand University Bareilly, Uttar Pradesh, India. She received the Junior Research Fellowship Award (CSIR-JRF) just after completing her Master's. She joined as a Ph.D. scholar in the research group of Prof Anuj Sharma in the department of chemistry, Indian Institute of Technology (IIT), Roorkee, India. She received the Senior Research Fellowship Award (CSIR-SRF). Her current research is focused on novel strategies for C–C and C–X bond formation.

Anuj Sharma

Prof. Anuj Sharma earned his Ph.D. degree from the Institute of Himalayan Bioresource Technology (IHBT) in 2006. Afterward, he undertook two short postdoctoral assignments in UFSM, Santa Maria, Brazil in 2006 and KU Leuven, Belgium, in 2007 before finally moving to the University of Arizona as NIH postdoctoral fellow in Prof. Laurence Hurley's group. He is currently working as a professor in the department of chemistry, Indian Institute of Technology (IIT), Roorkee, India. His group focuses on the development of green organic synthetic methodologies including visible light utilization for C–H functionalization.

---

1. Introduction

Allenes are fascinating and essential organic molecules possessing two cumulated π-bonds, and show unique structural features and reactivities in many organic transformations.¹ Due to their immense applications in materials sciences, pharmaceutical industry, and organic synthesis, allenes have been recognized as crucially essential building blocks, and chemists have devoted great efforts for their synthesis (Fig. 1).² In this context, Burton and Pechmann first synthesized allenes in 1887, and later on, the first addition of the trifluoromethyl radical to allenes was achieved by Haszeldine in 1954.³ Later, classical methodologies for allene synthesis like sigmatropic rearrangement of propargylics, nucleophilic substitution, 1,2-elimination, 1,4-addition reactions, π-complexation-assisted deprotonation of propargylic C–H bonds, Crabbé homologation reactions of terminal alkynes, and other methods evolved.⁴ Besides, for the construction of multisubstituted allenes, radical-mediated 1,4-difunctionalization of 1,3-enynes was recognized as one of the most straightforward routes. Unfortunately, these aforementioned methodologies suffer from the use of costly metals and ligands, high temperature, and toxic radical initiators and thus, the exploration of simple and mild approaches for the synthesis of allenes is highly demanded.

Over the last decade, visible light photoredox catalysis has served as a powerful method for accomplishing synthetically valuable transformations, and has garnered significant attraction to access organic compounds under mild reaction conditions.⁵ In this context, the utilization of visible light photoredox catalysis to access allenes from 1,3-enynes has emerged as the most powerful tool for the organic synthetic community. The general mechanism of allene synthesis from 1,3-enynes is demonstrated in Scheme 1a. Under visible light irradiation, the photoexcited photocatalyst PC* undergoes either reductive or oxidative quenching with the radical precursor RX 2 to generate the respective radical species R˙ 1Avia a SET process. Therefore, the generated radical species R˙ 1A is trapped by the 1,3-enyne 1, followed by isomerization to give the allenyl radical 1D as a key intermediate, which upon a series of SET events leads to the target allene. Besides, reductive quenching of the propargyl ester 3 with photoexcited photocatalyst PC*, followed by C–O bond cleavage, gives the allenyl radical 1D′, leading to the desired allene (Scheme 1b).

There are a few review articles on the synthesis of allenes, albeit, majorly dealing with TM (transition metal)-based protocols.⁶ However, to the best of our knowledge, there has been no review article solely dedicated to visible light mediated synthesis of allenes. The current review article has presented visible light mediated synthesis of allenes from substrates like 1,3-enynes, propargylic carbonates, homopropargylic alcohols, propargylic oxalates, alkynyl diazo compounds, and terminal alkynyl aziridines. It is further categorized into the synthesis of allenes including sulfonylated, fluorinated, cyano, phosphonylated, silylated, arylated, alkylated, and acylated allenes from various substrates.

2. From 1,3-enynes

2.1. Sulfonylated allene synthesis

Sulfonyl-possessing compounds are essential in the fields of materials science and pharmaceuticals, and continuous efforts have been devoted to their synthesis.⁷ 1,3-Enynes are precious building blocks in the synthesis of allenes as they serve as acceptors for polar species as well as for radical species.⁸

In 2021, Wu and co-workers accomplished the synthesis of sulfonyl allenic alcohols 6 utilizing vinyl enynes 5 as substrates, sulfonyl chloride 4 as the sulfonyl radical precursor of 2A, and K₃PO₄ as a base under blue LEDs (Scheme 2).⁹ The protocol illustrated a broad functional group compatibility with respect to both vinyl enynes and sulfonyl chlorides. It is proposed that the photoexcited Ir(III)* reduces the sulfonyl chloride 4 to access the sulfonyl radical 2A. Thereafter, the Ir(IV) catalyst oxidizes the chloride anion to produce a Cl˙ radical, which upon reaction with H₂O furnishes the HO˙ radical (this step is promoted by the base). Subsequently, the generated HO˙ radical is trapped by the vinyl enyne 5 to produce the radical intermediate 2B, followed by coupling with the sulfonyl radical 2A to access the required allene 6.

Shortly thereafter, Oh and co-workers accomplished the visible light induced Eosin-Y-catalyzed 1,4-peroxidation-sulfonylation of enynones 7 using TBHP and sulfinic acids 8 under mild reaction conditions (Scheme 3).¹⁰ A variety of enynones as well as sulfinic acids containing electron-donating and electron withdrawing groups behaved well to afford the respective allenes with good regioselectivity. Under visible light illumination, TBHP oxidizes the photoexcited Eosin-Y* to generate the t-butoxy radical via a SET process, followed by the radical addition to the enynone 7, resulting in the α-keto radical species 3C. Meanwhile, deprotonation of sulfinic acid 8 by the hydroxyl anion gives sodium sulfinate 3A, followed by SET to [Eosin-Y]˙⁺, generating sulfonyl radical 3B. Finally, radical–radical coupling of the sulfonyl radical 3B and the α-keto radical 3C leads to the desired product 9.

Besides sulfonyl chlorides and sulfinic acids, Wang and co-workers published the photoredox catalyzed reaction of 1,3-enynes 1 and dithiosulfonates 10, and obtained dithiosulfonylated allenes 11 in moderate yields (Scheme 4).¹¹ The dithiosulfonates bearing secondary or tertiary alkyl groups (R) exhibited good reaction efficiency. Interestingly, the strategy showed good regioselectivity under blue LEDs at room temperature. In the putative mechanism, the dithiosulfonate 10 is reduced by the photoexcited Ir(III)* to produce the sulfonyl radical 4A, which upon incorporation with the enyne delivers the allenyl radical 4B. Next, Ir(IV) oxidizes the radical intermediate 4B to carbocation species 4C, followed by a reaction with intermediate 4D, affording the desired allene 11.

Aryl halides are essential chemical feedstocks and gained substantial recognition as aryl radical precursors in organic synthesis.^12a In 1,4-radical addition to 1,3-enynes, the merging of photoredox/TM (Cu, Ni)-catalysis got remarkable attention. In this regard, the synthesis of α-allenyl sulfones 14 from sodium sulfinates 12 and 1,3-enynes 1 by employing dual nickel/photoredox catalysis was achieved by Hu, Li and co-workers (Scheme 5a).^12b The reaction demonstrated a good substrate scope with a broad functional group tolerance with respect to aryl halides, 1,3-enynes, and sulfinate salts. Furthermore, the strategy also afforded drug 14a with good yield. Experimental results favored the generation of sulfonyl radicals 5A in the reaction system. As illustrated in Scheme 5a, the photoexcited Ru(II)* oxidizes the sodium sulfinate 12 to access the sulfonyl radical 5A, which upon interaction with the 1,3-enyne 1 forms the allenyl radical intermediate 5C. Next, this resulting allenyl radical 5C gets coupled with the Ni(0)-complex to furnish the allenyl-Ni(I) complex 5D. Therefore, oxidative addition of the aryl halide 13 to the intermediate 5D leads to the Ar–Ni(III)-allenyl intermediate 5E. Lastly, reductive elimination of intermediate 5E accesses the required allenes 14 with concomitant release of the Ni(I)-complex. The Ni(I)-complex reduces Ru(I), resulting in the accomplishment of the photocatalytic cycle with the regeneration of the active Ni(0)-complex.

Surprisingly, a similar strategy was accomplished by Ke, Wang, and co-workers via 1,4-arylsulfonylation of 1,3-enyne 1 utilizing dual NiCl₂ and Ru(phen)₃Cl₂·6H₂O (photocatalyst) under blue LEDs (Scheme 5b).¹¹ This methodology displayed a wide substrate scope with good catalytic activity, employed low catalytic loading, and afforded α-allenyl sulfones 15 in nice yields.

In previous strategies (Scheme 5a and b), metal-photocatalysts were utilized which were expensive, and to overcome this barrier, a visible light induced organophotocatalyzed (4CzIPN as a photocatalyst) synthesis of α-allenyl sulfones 16 was disclosed by Lu and co-workers (Scheme 5c).¹² The reaction parameters implied that dual 4CzIPN/NiCl₂·glyme catalysis in the presence of blue LEDs gave the best results to afford allenes 16 with good regio-, chemo-, and stereoselectivity.

Besides photocatalyst-assisted strategies, Li, Wang, and co-workers demonstrated a photocatalyst-free Ni-catalyzed three-component arylsulfonylation of 1,3-enynes 1 to access sulfone-containing allenes 17 with excellent functional group tolerance with respect to 1,3-enynes, aryl halides (X = I, Br), and aryl sulfinates under purple LEDs (Scheme 6).¹³ The reaction system revealed that the Ni-catalyst, ligand, and light were crucial for the reaction to occur. Various aryl sulfinates like aryl (heterocyclic) and alkyl sulfinates as well as aryl halides possessing electron-withdrawing or electron-rich substituents worked well in this transformation; however, ortho-substituted iodobenzene was found inert towards the reaction system probably owing to steric hindrance. Notably, ortho-substituted enynes gave lower product yields which might be due to steric repulsion. The mechanistic studies revealed that the reaction might be proceeded via an LMCT (ligand-to-metal charge transfer) process.

N-Heterocyclic carbene (NHC)-catalyzed radical reactions have paved a new avenue for acyl radical cross-coupling chemistry.¹⁴ In this context, a dual Ir-photoredox/NHC-catalyzed 1,4-sulfonylation of 1,3-enyne 1 for the synthesis of tetrasubstituted allenyl ketones 19 was accomplished by Zheng, Zhang and co-workers by the reaction between 1,3-enyne 1, aroyl fluoride 18, and sodium sulfinate 12 (Scheme 7).¹⁵ Furthermore, drugs 19a, 19b, and 19c were also achieved with satisfactory yields. Mechanistic studies reveal that the NHC-catalyst reacts with the aroyl fluoride 18 to generate the acylazolium intermediate 7E, which gets reduced by the photoexcited Ir(III)*, resulting in the ketyl radical 7F. On the other hand, a SET process between Ir(IV) and sodium sulfinate 12 produces a sulfonyl radical 7A, followed by addition to the 1,3-enyne 1, giving allenyl radical 7C. Finally, the intermediate 7C upon coupling with the ketyl radical 7F gives an NHC-bound intermediate 7D, followed by disintegration leading to the desired allene 19 with regeneration of the NHC-catalyst.

2.2. Fluorinated allene synthesis

The introduction of fluorine-containing functional groups to organic molecules enhances the biological- and physicochemical properties due to their fascinating lipophilicity, bioactivities, and binding affinity, and these compounds play a crucial role in medicine, materials sciences, agrochemicals, and pharmaceuticals.¹⁶

Recently, Zheng, Zhang, and co-workers extended their previous strategy (Scheme 7) for the synthesis of mono/difluoromethylated allenes via 1,4-mono/di-fluoromethylative acylation of 1,3-enynes 1 by employing acyl fluorides 20 and R_fSO₂Na 21 under mild reaction conditions (Scheme 8).¹⁷ In this protocol, R_fSO₂Na (R_f = CH₂F, CF₂H) worked as monofluoromethyl or difluoromethyl radical precursors under dual photoredox/NHC-catalysis. The methodology demonstrated a wide substrate scope with high functional group tolerance with respect to both 1,3-enynes and acyl fluorides. Moreover, monofluoromethylated allene 22c could be achieved with excellent yield. Furthermore, the strategy could be applied for the synthesis of drugs 22a and 22b with good yields.

Synthesis of bis(trifluoromethylated) allenes 24 from 1,3-enynes 1 under visible light triggered copper-catalyzed conditions was achieved by Hu and co-workers (Scheme 9).¹⁸ The methodology did not require any photocatalysts, and oxygen played a crucial role in facilitating CF₃ group transfer to enhance the reaction efficiency. The 1,3-enyne having electron-donating substituents on the phenyl ring afforded higher yields in comparison to electron-withdrawing ones. Notably, cyclic alkenyl 1,3-enyne worked smoothly to furnish the respective allene with good diastereoselectivity. Moreover, the methodology also accessed drug 24a with excellent yield. Furthermore, the reaction could be applied on a gram-scale with satisfactory yield. Under blue LEDs, (bpy)Cu(CF₃)₃23 upon photoexcitation releases trifluoromethyl radical 9I, followed by trapping with the 1,3-enyne 1 to generate the allenyl radical 9B. Thereafter, the resulting allenyl radical 9B coupled with the intermediate 9A, followed by reductive elimination of the intermediate 9C, leads to the formation of the required allene 24. Alternatively, complex 23 upon oxidation by oxygen gives Cu(II) species 9D, followed by dimerization to afford intermediate 9E, which undergoes isomerization to produce another intermediate 9F. Finally, the generated intermediate 9F captures the radical species 9B, followed by reductive elimination of 9G to furnish the desired allene 24.

In 2022, Kong, Wang, and co-workers synthesized tetrasubstituted CF₃-allenes 28 by utilizing the combination of decatungstate photo-hydrogen atom transfer (HAT) and nickel catalysis (Scheme 10).¹⁹ A variety of aryl halides 27 like aryl bromides, alkenyl bromides, alkynyl bromides, and acyl chlorides exhibited good compatibility under the reaction system. An important feature of this protocol was that the abundant hydrocarbons 26 were utilized as feedstocks without the necessity of any pre-activated radical surrogate, and biologically active compounds (28a, 28b, 28c, 28e) could be achieved with good yields. The mechanistic studies revealed the involvement of 1,3-nickel rearrangement in the reaction mechanism. Under 390 nm irradiation, the photoexcited *[W₁₀O₃₂]⁴⁻ upon HAT (from alkyl 26) generates the alkyl radical 10A, followed by radical addition to the allene 25 which gives the radical intermediate 10B. Subsequently, the resulting intermediate reacts with LnNi(0) and 1,3-nickel rearrangement produces the intermediate 10C. Eventually, the generated intermediate 10C undergoes an oxidative addition with R³Br 27 to produce 10D, followed by reductive elimination, leading to the final product 28.

2.3. Allenyl nitrile synthesis

Nitrile-containing compounds are an essential class of organic compounds and common structural motifs in natural products.²⁰ A visible light induced dual Ir-photoredox/copper-catalyzed 1,4-carbocyanation of 1,3-enynes 1 using alkyl N-hydroxyphthalimide esters (NHP) 29 and TMSCN 30 was reported by Lu and co-workers (Scheme 11).²¹ An array of 1,3-enynes and alkyl N-hydroxyphthalimide esters (primary, secondary, tertiary, and allylic) were found compatible, leading to the respective allenes in good yields. Moreover, the methodology also furnished drugs (31a, 31b, and 31c) with good yields. The authors proposed that the photoexcited Ir(III)* reduces the NHP 29 to produce the alkyl radical 11A, followed by a reaction with the 1,3-enyne 1, leading to the allenyl radical 11B. Subsequently, in the presence of TMSCN 30, the allenyl radical 11B gets coupled with the LnCu(II)-complex to access the Cu(III)-complex 11C. Finally, the targeted product 31 is obtained via reductive elimination of 11C, with the regeneration of the LnCu(I)-catalyst.

Besides, Wang, Shang, and co-workers demonstrated a three-component 1,4-alkylcyanation of 1,3-enynes to construct the functionalized allenes 33 by employing 1,3-enynes 1, TMSCN 30, and 2-cyclobutoxyisoindoline-1,3-dione 32 under dual photoredox/copper catalysis (Scheme 12).²² The protocol proceeded smoothly with a wide substrate scope, high functional group tolerance, and good product yields with respect to both 1,3-enynes and cyclic alcohol derivatives. Very interestingly, the cyclopentyl alcohol derivative, bicyclo[4.2.0]octa-1,3,5-trien-7-ol, and spirocyclic substrate reacted well to afford respective products 33d, 33e, and 33f in good yields. Unfortunately, cyclohexanol derivatives are found unreactive to the reaction system. Furthermore, the strategy also afforded drugs 33a, 33b, and 33c with good yields. Under blue LEDs, the photoexcited Ir(III)* reduces the 2-cyclobutoxyisoindoline-1,3-dione 32, followed by N–O bond cleavage and β-scission of 12A to access the aldehyde-functionalized primary radical 12B. Thereafter, the resulting radical 12B gets trapped by 1,3-enyne 1 to give radical intermediate 12C, followed by isomerization to generate the allenyl radical 12D. Next, the generated allenyl radical 12D is captured by Cu(II)–CN to produce the allenyl-Cu(III) species 12E, which upon reductive elimination furnishes the desired product 33.

Recently, Wang and co-workers presented a dual photoredox/copper catalyzed synthesis of allenenitriles 35 from 1,4-difunctionalization of 1,3-enynes 1via a 1,5-HAT under blue LEDs (Scheme 13).²³ A library of aryl- and alkyl-functionalized 1,3-enynes reacted smoothly to give satisfactory yields. Meanwhile, N-alkoxyphthalimides 34 were found suitable for 1,3-enynes (R¹ = aryl, R² = alkyl), while replacing with N-alkoxypyridinium salts 36 gave difunctionalized allenes 37 without any necessity of deprotection (copper catalyst and ligand loading increased to 20% and 30% respectively) (Scheme 14). Furthermore, the strategy also furnished cholesterol bearing allene 35a with 60% yield. Based upon previous literature reports and experimental results it is proposed that the photoexcited Ir(III)* reduces the substrate 34 to generate the alkoxyl radical 13Avia a SET process. This is followed by 1,5-HAT to give the carbon radical intermediate 13B. Therefore, the resulting Ir(IV) oxidizes the LnCu(I) catalyst in the presence of TMSCN 30 to furnish LnCu(II)CN with concurrent regeneration of Ir(III) to accomplish the photocatalytic cycle. Finally, the radical species 13B addition to the 1,3-enyne 1, followed by trapping of allenyl radical 13C by LnCu(II)CN, delivers the target allene 35.

Synthesis of α-amido allenyl nitriles 39via dual photoredox/copper-catalyzed 1,4-amidocyanation of 1,3-enynes 1 with N-amidopyridin-1-ium salts 38 was reported by Li and co-workers (Scheme 15).²⁴ The strategy presented an excellent substrate scope with high functional group compatibility, good regioselectivity, and gram-scale synthesis. 1,3-Enynes possessing electron-rich, electron-deficient, and bulky substituents were readily engaged in the 1,4-sulfonamidocyanation to afford the required allenes in good yields. Likewise, N-amidopyridin-1-ium salts having substituents like, Me, Ph, F, Cl, Br, and CF₃ on the aryl ring behaved well, while N-methylsulfonamidopyridin-1-ium salt was found inert towards the reaction probably owing to the instability of the methylamido radical.

Dual photoredox/copper-catalyzed 1,4-perfluoroalkylcyanation of 1,3-enynes 1 using fluoroalkyl halides 40 (X = Br, I), 1,3-enynes 1, and TMSCN 30 under blue LEDs was accomplished by Deng and co-workers (Scheme 16).²⁵ 1,3-Enynes containing electron-donating, electron-withdrawing groups, and heterocycles like pyridine, thiophene, and 1,3-benzodioxole worked smoothly to give the corresponding products in good yields. Likewise, perfluoroalkyl halides including perfluoropropyl, ethyl iododifluoroacetate, ethyl bromodifluoroacetate, perfluoroisopropyl, and tetrachloromethane were proven to be compatible to furnish the respective allenes in acceptable yields. Notably, 1,2-dibromotetrafluoroethane gave poor yields under the reaction system. Moreover, the reaction could be carried out at the gram-scale with moderate yields.

2.4. Phosphonylated and silylated allene synthesis

A visible light mediated photoredox catalyzed regioselective radical hydrophosphinylation of 1,3-enynes 1 by employing diaryl phosphine oxides 42 as phosphinoyl radical precursors was achieved by Ding and co-workers (Scheme 17).²⁶ The methodology demonstrated a wide substrate scope with high functional group tolerance with respect to both 1,3-enynes and diaryl phosphine oxides. Moreover, the protocol afforded chiral allene (norethindrone 43a) with 77% yield. Furthermore, drugs like estrone 43b and (±)-ibuprofen 43c could be accomplished with 82% and 40% yields respectively. Unfortunately, dicyclohexylphosphine oxide, ethyl phenylphosphinate, and diethyl phosphite were found inert towards the given reaction system. Mechanistically, the photoexcited Ir(III)* oxidatively quenched by oxygen to produce Ir(IV), which oxidizes the diaryl phosphine oxide 42 to generate the phosphinoyl radical 17Avia PCET (proton-coupled electron transfer). Next, the resulting radical 17A is trapped by the 1,3-enyne 1 to give the propargyl radical 17B, followed by SET (by photoexcited Ir(III)*) leading to the anionic intermediate 17C. Eventually, protonation of the allenyl carbanion 17C delivers the target allene 43.

The incorporation of silicon substituents to organic molecules enhanced the lipophilicity, which is highly desirable for the development of new drugs.²⁷ Recently, Chen and co-workers reported a dual photoredox/nickel catalyzed regioselective hydro(deutero)silylation of 1,3-enynes 1 utilizing K₂HPO₄ as a base and silanecarboxylic acids 44 as silyl radical precursors under blue LEDs (Scheme 18).²⁸ Various 1,3-enynes and silanecarboxylic acids showed good compatibility under the given reaction conditions. Notably, switching the solvent to MeOD resulted in the deuterosilylation of 1,3-enynes with good yields. Furthermore, the protocol could be applied to gram-scale synthesis with moderate yields. Based upon previous reports and control experiments, it is concluded that the photoexcited 4CzIPN* oxidizes the silanecarboxylic acid 44 to furnish the carboxyl radical 18A, followed by decarboxylation leading to the silyl radical 18B. Subsequently, trapping of silyl radical 18B by the 1,3-enyne 1 provides the allenyl radical 18C, which reacts with NiⁿL to access the allenyl nickel complex 18D. Lastly, protonation and SET of the intermediate 18D gives the required allene 45 with regeneration of the nickel catalyst and 4CzIPN.

2.5. Arylated and alkylated allene synthesis

Aryl boronic acids are easily accessible and highly stable which makes them convenient surrogates for building molecules in organic synthesis.²⁹ Owing to various advantages of aryl boronic acids, Qiu, Guo, and co-workers reported a dual copper/photoredox catalyzed three-component reaction for 1,4-difunctionalization of 1,3-enynes 1 with high chemo- and regioselectivity using cyclobutanone oxime esters 46 and aryl boronic acids 47 in batch and continuous flow (Scheme 19).³⁰ The substrate scope revealed that aryl boronic acids having groups like Cl, Br, OBn, and Me at the ortho-, meta-, and para-positions of the phenyl ring were found suitable for this transformation. Meanwhile, 3- or 4-monosubstituted cyclobutanone O-benzoyl oximes containing Et, Me, Bn, Ph, CO₂t-Bu, and OBn substituents reacted smoothly to give products in moderate yields. Furthermore, the protocol could be achieved on a gram-scale synthesis with 69% yield. Mechanistically, SET from the photoexcited Ir(III)* to the oxime ester 46, followed by ring opening via homolytic C–C bond scission, gives cyanoalkyl radical 19A. Next, the resulting radical 19A is trapped by the 1,3-enyne 1, followed by a radical shift to afford the allenyl radical 19B. On the other hand, the LnCu(II) species reacts with the aryl boronic acid 47 to form LnCu(II)–Ar 19D, which gets oxidized by the allenyl radical 19B to produce the Cu(III) intermediate 19C. Lastly, reductive elimination of the intermediate 19C provides the desired allene 48.

Apart from it, Deng, Chen, and co-workers reported a three component 1,4-alkylarylation of 1,3-enynes 1 by merging nickel and photoredox catalysis using TMEDA as a reductant, and obtained a library of tetrasubstituted allenes 50 with high chemo- and regioselectivity (Scheme 20).³¹ 1,3-Enynes as well as aryl bromides with electron-donating, electron-withdrawing, and electron-neutral groups were amenable to the reaction system. Likewise, tertiary alkyl bromides bearing aryl, naphthalene, furan, and thiophene also worked well to afford the required allenes 51 in good yields. Interestingly, the protocol also delivered drug 50d with good yield. Mechanistically, the photoexcited 4CzIPN* oxidizes TMEDA to produce 4CzIPN˙⁻, which reduces the alkyl bromide 49, generating the alkyl radical 20A. Therefore, addition of alkyl radical 20A to the 1,3-enyne 1 and isomerization gives allenyl radical 20C, which gets trapped by LNi(0) to access allenyl-Ni(I)L 20D. Subsequently, oxidative addition of aryl bromide 13 to the intermediate 20D affords Ni(III) species 20E, followed by reductive elimination which delivers the target allene 50.

In continuum, Zhu and co-workers reported a visible light triggered photoredox catalyzed regioselective 1,4-hydroalkylation of 1,3-enynes 1, and accomplished di- and trisubstituted allenes 53 in moderate to excellent yields (Scheme 21).³² 1,3-Enynes bearing different groups including F, Cl, alkyl, amino, alkoxy, phenyl, pyridine, thiophene, and cyclopropyl showed good compatibility under the optimized reaction conditions. Likewise, dimethyl malonates containing methyl, iso-butyl, ethyl, benzyl, n-hexyl, and cyclohexylmethyl proved to be tolerable to furnish the respective products in moderate yields. Very interestingly, the strategy afforded L-menthol 53a, isoborneol 53b, (−)-α-terpineol 53c, and (+)-cedrol 53d derived allenes in moderate yields. Mechanistically, the enol 21A of malonate 52 gets oxidized by the photoexcited Ir(III)*, leading to the radical species 21B. This is followed by radical addition to the 1,3-enyne 1 to produce the propargyl radical 21C, which upon tautomerization gives the allenyl radical intermediate 21D. Finally, SET from Ir(II) to the radical intermediate 21D resulted in allene anion 21E, followed by protonation to furnish the desired allene 53.

A dual photoredox/nickel catalyzed 1,4-dicarbofunctionalization of 1,3-enynes 1 with tertiary N-methylamines 54 and aryl/alkyl bromides 55 to access tetrasubstituted allenes was accomplished by Li, Wu, and co-workers (Scheme 22).³³ The protocol displayed excellent functional group tolerance and a wide substrate scope with excellent site-selectivity. Drugs (56a and 56b) could also be accomplished with moderate yields. Upon visible light illumination, the photoexcited 4CzIPN* oxidizes the tertiary N-methylamine to radical cation 22A, followed by deprotonation by the base and coordination with LnNi(0) to give the intermediate 22B. In path b, the resulting intermediate 22B gets trapped by the 1,3-enyne 1 to form the intermediate 22D, followed by Ni migration to afford the allenyl Ni intermediate 22E. Eventually, oxidative insertion of the intermediate 22E with R⁵X 55 gives allenyl C–Ni(R⁵)X 22F, which undergoes reductive elimination, leading to the target allene 56. Alternatively (path a), intermediate 22F could be generated via a reaction of allenyl radical 22C with the intermediate 22G (path a).

In the last few years, radical-polar crossover reactions have emerged as a powerful methodology to bridge the gap between radical and conventional polar chemistry.^34a,b Meanwhile, 1,2-allenyl ketones are highly important in pharmaceuticals and organic synthesis, and therefore, substantial strategies dealing with their synthesis have been achieved.³⁵ In this context, Li and co-workers synthesized 1,2-allenyl ketones 59 by employing alkyl silicates 58 as alkyl radical precursors under a dual Ir-photoredox/Cu-catalyzed system (Scheme 23).^34c Mechanistic studies revealed that the reaction proceeded via a reductive radical-polar crossover pathway. However, the role of the copper catalyst was not clearly illustrated. Under the given reaction conditions, the photoexcited Ir(III)* is reductively quenched by the substrate to produce the alkyl radical 23A, which upon trapping by the 1,3-enyne 57 and SET reduction (by Ir(II)) gave the anionic intermediate 23C. Finally, the intermediate 23C reacts with Cu to furnish the allenyl copper species 23D, followed by protonation to access the target allene 59. Alternatively, the intermediate upon isomerization and protonation of allenyl carbanion 23E affords the desired product 59.

In 2022, Wang and co-workers demonstrated a dual photoredox/chromium-catalyzed asymmetric 1,4-functionalization of 1,3-enynes 1 to chiral α-allenols 62 by employing DHP esters 60 as alkyl radical precursors under mild reaction conditions (Scheme 24).³⁶ The reaction offered a broad substrate scope, good functional group tolerance, and high regioselectivity. An array of aldehydes like aromatic, aliphatic, and heteroaromatic aldehydes reacted well to afford the respective allenols in acceptable yields. Likewise, DHP esters (primary, secondary, and tertiary alkyl) and 1,3-enynes worked well under the reaction system. Notably, the protocol also furnished drug 62c with excellent yield. Besides DHP esters, other alkyl radical precursors such as alkyl trifluoroborates 60′ and NHPI esters 60′′ also engaged well to deliver the respective products with good regio-, diastereo-, and enantioselectivity. Under visible light irradiation, the photoexcited 4CzIPN* gets reductively quenched by the DHP ester 60 to generate the alkyl radical 24B. Then, the resulting radical species 24B is added to the 1,3-enyne 1 to give radical intermediate 24C, which gets captured by L1Cr(III) to furnish the intermediate 24D. Thereafter, the generated intermediate 24D reacts with the aldehyde 61 to deliver the intermediate 24E, which upon dissociation of a C–O bond by pyridinium 24A′ affords the desired chiral allenol 62.

2.6. Acylated allene synthesis

Apart from dual photoredox-/TM-catalysis, Hong and co-workers reported the visible light promoted photocatalyst-free NHC-catalysed synthesis of 1,4-difunctionalized allenes 65 from 1,3-enynes 1 under blue LEDs (Scheme 25).³⁷ The substrate scope implied that 2-aryl substituted 1,3-enynes possessing cyclopropyl, long linear alkyl groups, 2,4-dialkyl substituted enynes, and 2-alkyl-4-aryl substituted 1,3-enynes were proved to be efficient substrates to provide the respective products in good yields. Mechanistically, the in situ generated xanthate 25B upon photoexcitation reduces the acyl azolium 25A to produce the xanthate radical 25C and the NHC-derived ketyl radical 25D. Next, trapping of the xanthate radical 25C by phosphine (PCy₃) gives the phosphoranyl radical 25E, which upon β-scission delivers the transient alkyl radical 25F. Finally, the alkyl radical 25F reacts with the 1,3-enyne 1 to access the allenyl radical 25G, followed by radical–radical coupling with the ketyl radical 25D to furnish the required acylated allene 65.

In 2023, Shi and co-workers presented a photoredox/Cu dual catalyzed acyl trifluoromethylthionation of 1,3-enynes 1 using K₂CO₃ and THF as a base and solvent respectively, and obtained δ-trifluoromethylthiolated β-allenyl ketones 67 in moderate to good yields (Scheme 26).³⁸ The substrate scope revealed that the 2-phenyl-1,3-enynes having F, Br, Me, and OMe substituents at the para position of the phenyl ring behaved smoothly. Meanwhile, thioesters 66 possessing electron-withdrawing groups reacted better in comparison to the electron-rich thioesters. Furthermore, the mechanistic studies implied the generation of acyl radical 26A, which upon radical addition to the 1,3-enyne 1, generates the allenyl radical 26B. This is followed by a reaction with LnCu(II)SCF₃ to give the intermediate 26C, which undergoes reductive elimination to provide the required allene 67.

3. From propargylic propargyl esters

3.1. Alkylated allene synthesis

In 2021, Liang et al. demonstrated a dual photoredox/nickel catalyzed highly regioselective synthesis of trisubstituted allenes 70 by employing propargylic carbonates 68 and 1,4-dihydropyridine derivatives 69 under blue LEDs at room temperature (Scheme 27).³⁹ The most important features of the protocol were that it afforded alkylated allenes without any alkyl organometallic reagents, and offered a wide substrate scope with respect to propargylic carbonates and 1,4-dihydropyridine derivatives. Based on previous literature surveys and experimental results it is proposed that SET from the photoexcited Ir(III)* to the 1,4-dihydropyridine 69 generates the alkyl radical 27A. Meanwhile, decarboxylation of propargylic carbonate 68 promoted by the Ni(0) catalyst gives the allenyl nickel intermediate 27C. This is followed by the trapping of the intermediate 27C by the alkyl radical 27A to accesses the Ni(III) intermediate 27D, which upon reductive elimination affords the desired allene 70.

On the other hand, Xiao and co-workers published a photoinduced regioselective alkylation of propargylic carbonates 71 with alkyl N-hydroxyphthalimide ester (NHP) 72via a nickel-promoted cross-electrophilic coupling process (Scheme 28).⁴⁰ The protocol constitutes features like a wide substrate scope, excellent regioselectivity, and involvement of the primary alkyl radical from a bench-stable alkyl radical reagent. Under visible light illumination, NHP ester, HE, and TBAI generated an EDA-complex, leading to the formation of the alkyl radical 28B. Meanwhile, the photoexcited HE* reduces Ni(II) to Ni(0), which reacts with the substrate 71 to produce the hybrid allenyl-Ni(I) intermediate 28A, followed by trapping of alkyl radical 28B, leading to the required allene 73.

3.2. Allenyl nitrile, chloroallene, and bromoallene synthesis

During the same period, Xiao, Lu, and co-workers presented synthesis of allenylnitriles 75via C–O bond scission of propargyl esters 74 under dual photoredox and copper catalysis (Scheme 29).⁴¹ The reaction features a good substrate scope, high functional group tolerance, and nice reaction efficiency. Notably, propargyl esters having linear, branched, and cyclic aliphatic substituents served well in this transformation with excellent yields. Very interestingly, propargyl esters with internal alkynes also engaged well to give the desired allene 75a with good yield. Furthermore, a gram-scale synthesis could be achieved with good yields. In the putative reaction mechanism, the photoexcited Ph-PTZ* reduces the propargyl ester 74, followed by C–O bond cleavage that gives the propargyl radical 29A and the carboxylic anion AxO⁻29D. Next, the resulting carboxylic anion 29D reacts with TMSCN 30 to generate the cyanide anion slowly, which gets coordinated with the LnCu(I)(CN) catalyst to produce the LnCu(I)(CN)₂ species. Thereafter, the generated LnCu(I)(CN)₂ species is oxidized by Ph-PTZ˙⁺ to LnCu(II)(CN)₂, which captures the allenyl radical 18B to form the intermediate 29C. Lastly, reductive elimination of the intermediate 29C leads to the formation of the required allene 75.

In 2022, Ma and co-workers presented a visible light mediated dual photoredox/copper-catalyzed synthesis of allenenitriles 77 from propargylic oxalates 76 and TMSCN 30 with excellent regioselectivities (Scheme 30).⁴² Both terminal and internal propargylic oxalates worked efficiently under the given reaction conditions. Very interestingly, the strategy also afforded products possessing pentoxifylline 77a, raspberry ketone tetra-O-acetyl-β-D-glucopyranoside 77b, mestranol 77c, and Boc-protected L-proline 77e with acceptable yields, illustrating the practicality of the methodology. Notably, the protocol could be further extended to the construction of chloroallenes 78 and bromoallenes 79 using MgCl₂ or MgBr₂·6H₂O as a nucleophile (Scheme 31). Based upon experimental results, it is believed that the reaction might proceed via reductive quenching involving a SET process to generate the allenyl radical 30C by utilizing TMSCN 30 as a nucleophile.

4. From homopropargylic alcohols

Besides 1,3-enynes, Hu, Jiang, and co-workers synthesized α-allene aldehydes/ketones 81 by utilizing homopropargylic alcohols 80, K₂S₂O₈, Bu₄NCl, [Ir(dF(CF₃)(ppy)₂(dtbbpy))]PF₆, and MeCN as a substrate, oxidant, additive, photocatalyst, and solvent respectively (Scheme 32).⁴³ Various homopropargylic alcohols bearing substituents like Me, OMe, CF₃, CN, Br or fused rings such as 1-naphthyl furan, thiophene, and quinoline smoothly afforded the respective products in good yields. Besides, tertiary homopropargylic alcohols 82 also demonstrated good tolerance under the reaction conditions. Upon blue LED irradiation, K₂S₂O₈ oxidizes the photoexcited Ir(III)* to generate Ir(IV), which reduces the substrate 80 to produce the alkoxy radical 32Avia a PCET (proton-coupled electron transfer) process. Next, a remote HAT of the radical intermediate 32A gives allenyl radical 32B, followed by distal migration of the aryl (heteroaryl) group leading to the formation of the desired allene 81.

5. From alkynyl/aryl diazo compounds

Alkynyl diazo compounds are valuable surrogates of alkynyl carbenes and application of these compounds in organic synthesis is highly desirable.⁴⁴ In 2022, Zhou and co-workers reported a visible light induced photocatalyst-free synthesis of selenoallenic sulfones 86via radical 1,3-difunctionalization of alkynyl diazo compounds 84 under blue LEDs (Scheme 33).⁴⁵ The protocol offered a wide substrate scope and good functional group tolerance with good to excellent product yields. Besides selenosulfonates, thiosulfonates and sulfonyl iodides were also found suitable to furnish the respective products in good yields. Under visible light irradiation, R²SO₂X 85 generates the radical species 33A, followed by radical addition to the substrate 84 to afford the radical intermediate 33B. Next, the resulting vinyl radical intermediate 33B upon N₂ extrusion and 1,2-radical shift generates the allenyl radical 33C, followed by reaction with R²SO₂X 85 to deliver the desired allene 86.

In 2020, Gryko and co-workers reported a visible light mediated Doyle-Kirmse reaction of propargyl sulphides 87 with aryl diazocarbonyl compounds 88 to synthesize allenes 89 under blue LEDs (Scheme 34).⁴⁶ A library of aryl diazo compounds showed good reactivities regardless of electronic effects and positions of groups on the phenyl ring, delivering the target products in satisfactory yields. Besides, diazoacetates possessing unsubstituted aryl- and naphthyl rings as well as propargyl sulphides containing alkyl and heteroaryl (pyridine, tetrazole, quinoline) rings were also found efficient to furnish the respective products in moderate to good yields. Based upon mechanistic studies it is proposed that the photoexcited aryl diazoacetate* 34A upon nitrogen extrusion generates a singlet carbene (can undergo intersystem crossing to produce the triplet state) 34B, which reacts with the propargyl sulphide 87 to furnish the sulfonium ylide 34C. Thereafter, the resulting sulfonium ylide 34C undergoes [2,3]-sigmatropic rearrangement to afford the required allene 89.

6. From alkynyl aziridines

Alkynyl aziridines could be easily accessed from the reaction of propargylic sulfur ylides and aldimines.⁴⁷ In this regard, Xu, Lan, and co-workers achieved a photoredox-catalyzed synthesis of fluorinated allenes 92 from terminal alkynyl aziridines 90 by employing Na-ascorbate as a reductant under visible light irradiation (Scheme 35).⁴⁸ A library of fluoroalkyl iodides 91 reacted smoothly with aziridines 90 to afford the corresponding fluorinated allenes 92 with good yields. Interestingly, sterically encumbered fluoroalkyl iodides such as perfluoroisopropyl, trifluoromethyl, and ethyl idodifluoroacetate also successfully gave the respective desired products in good yields. Notably, replacement of Ru(bpy)Cl₂ with fac-Ir(ppy)₃ gave the best results, when R_fBr 93 was employed instead of R_fI 24 under the reaction system. It is noteworthy that various difluoromethylene bromides behaved well under the given reaction conditions, and corresponding difluoromethylene allenes 94 were obtained in good yields. Furthermore, the reaction could be calibrated on a gram-scale synthesis. The mechanistic studies revealed that the photoexcited Ru(II)* is reductively quenched by the reductant sodium ascorbate, which reduces perfluoroalkyl iodide 91 to generate the perfluoroalkyl radical 35A. Next, the resulting fluoroalkyl radical 35A reacts with the alkynyl aziridine 90 to give alkenyl radical 35B, followed by C–N bond scission to furnish the radical intermediate 35C. Eventually, the generated radical intermediate 35C reacts with ascorbate to afford the desired allene 92. Furthermore, DFT calculations revealed that the formation of the observed product was kinetically favorable.

7. Conclusions

In the recent decade, literature reports for the synthesis of allenes have witnessed an upsurge in visible light induced methodologies owing to their mild reaction conditions, becoming one of the most appealing approaches. This review has showcased visible light mediated synthesis of allenes from substrates like 1,3-enynes, propargylic carbonates, homopropargylic alcohols, propargylic oxalates, alkynyl diazo compounds, and terminal alkynyl aziridines. Despite impressive accomplishments and breakthroughs, further exploration in access of allenes is listed here: 1) in TM- and photoredox dual catalyzed reactions, Mn and Fe have not received attention in the synthesis of allenes. 2) Photocatalyst-free strategies, including EDA (electron-donor-acceptor) complexes, are also unexplored. 3) To access allenes, propargylic carbonates, homopropargylic alcohols, propargylic oxalates, alkynyl diazo compounds, and terminal alkynyl aziridines need more exploration. 4) Enantiomerically enriched allenes using dual TM-/photoredox catalysis are highly desirable. 5) Utilization of heterogeneous photocatalysts is an important avenue for the synthesis of allenes due to their reusability, and needs to be explored. Overall, the present review will stimulate organic synthetic chemists to explore the aforementioned strategies to access allenes, and new accomplishments are expected to appear for the synthesis of allenes in the near future.

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Conflicts of interest

The authors have no conflicts of interest.

Acknowledgements

Financial support from UCOST (UCS & T/R & D-35/20-21). Govt. of Uttarakhand, and SERB (CRG/2022/002691), India is gratefully acknowledged. JS and BS thank CSIR for the SRF Fellowship respectively.

References

1. (a) D. R. Taylor, The Chemistry of Allenes, Chem. Rev., 1967, 67, 317–359 CrossRef CAS ; (b) N. Krause and A. S. K. Hashmi, Modern Allene Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2004, vol. 1–2 CrossRef ; (c) H. E. Schuster and G. M. Coppola, Allenes in Organic Synthesis, John Woley and Sons, New York, 1984 Search PubMed ; (d) B. Alcaide and P. Almendros, Chem. Soc. Rev., 2014, 43, 2886 RSC ; (e) J. M. Alonso and P. Almendros, Chem. Rev., 2020, 121, 4193 CrossRef PubMed ; (f) S. Yu and S. Ma, Angew. Chem., Int. Ed., 2012, 51, 3074 CrossRef CAS PubMed ; (g) J. Singh, A. Sharma and A. Sharma, Org. Chem. Front., 2021, 8, 5651 RSC .
2. (a) A. Hoffmann-Röder and N. Krause, Angew. Chem., Int. Ed., 2004, 43, 1196 CrossRef PubMed ; (b) S. Ma, Acc. Chem. Res., 2009, 42, 1679 CrossRef CAS PubMed ; (c) S. Yu and S. Ma, Angew. Chem., Int. Ed., 2012, 51, 3074 CrossRef CAS PubMed ; (d) P. Rivera-Fuentes and F. Diederich, Angew. Chem., Int. Ed., 2012, 51, 2818 CrossRef CAS PubMed ; (e) C.-Y. He, Y.-X. Tan, X. Wang, R. Ding, Y.-F. Wang, F. Wang, D. Gao, P. Tian and G.-Q. Lin, Nat. Commun., 2020, 11, 4293 CrossRef CAS PubMed .
3. (a) B. S. Burton and H. V. Pechmann, Ueber die Einwirkung von Chlorphosphor auf Acetondicarbonsäureäther, Ber. Dtsch. Chem. Ges., 1887, 20, 145 CrossRef ; (b) K. Leedham and R. N. Haszeldine, J. Chem. Soc., 1954, 2040–2042 Search PubMed .
4. (a) S. Ma, Chem. Rev., 2005, 105, 2829–2872 CrossRef PubMed ; (b) K. M. Brummond and J. E. DeForrest, Synthesis, 2007, 2007, 795 CrossRef ; (c) S. Ma, Acc. Chem. Res., 2009, 42, 1679 CrossRef CAS PubMed ; (d) C. Aubert, L. Fensterbank, P. Garcia, M. Malacria and A. Simonneau, Chem. Rev., 2011, 111, 1954 CrossRef CAS PubMed ; (e) D. Tejedor, G. Mendez-Abt, L. Cotos and F. Garcia-Tellado, Chem. Soc. Rev., 2013, 42, 458 RSC ; (f) J. Ye and S. Ma, Org. Chem. Front., 2014, 1, 1210 RSC ; (g) R. K. Neff and D. E. Frantz, ACS Catal., 2014, 4, 519 CrossRef CAS ; (h) W. Li, R. Wu, H. Ruan, B. Xiao, X. Gao, H. Jiang, K. Chen, T.-Y. Sun and S. Zhu, Angew. Chem., Int. Ed., 2024, e202409332,  DOI:10.1002/anie.202409332 ; (i) J. Zhu, H. Xiang, H. Chang, J. C. Corcoran, R. Ding, Y. Xia, P. Liu and Y.-M. Wang, Angew. Chem., Int. Ed., 2024, 63, e202318040,  DOI:10.1002/anie.202318040 ; (j) X. Huang and S. Ma, Acc. Chem. Res., 2019, 57, 1301 CrossRef PubMed ; (k) X. Zeng, J. Yang, W. Deng, X.-T. Feng, H.-Y. Zhao, L. Wei, X.-S. Xue and X. Zhang, J. Am. Chem. Soc., 2024, 146, 16902 CrossRef CAS PubMed ; (l) S. Zhang, Y. Liang, K. Liu, X. Zhan, W. Fan, M.-B. Li and M. Findlater, J. Am. Chem. Soc., 2023, 145, 14143 CrossRef CAS PubMed ; (m) X. Tang, C. Zhu, T. Cao, J. Kuang, W. Lin, S. Ni, J. Zhang and S. Ma, Nat. Commun., 2013, 4, 2450 CrossRef PubMed .
5. (a) D. A. Nicewicz and D. W. C. MacMillan, Science, 2008, 322, 77 CrossRef CAS PubMed ; (b) M. A. Ischay, M. E. Anzovino, J. Du and T. P. Yoon, J. Am. Chem. Soc., 2008, 130, 12886 CrossRef CAS PubMed ; (c) Visible Light Photocatalysis in Organic Chemistry, ed. C. R. J. Stephenson, T. R. Yoon and D. W. C. MacMillan, John Wiley & Sons, 2018 Search PubMed ; (d) L. Marzo, S. K. Pagire, O. Reiser and B. König, Angew. Chem., Int. Ed., 2018, 57, 10034–10072 CrossRef CAS PubMed ; (e) J. Xuan and W.-J. Xiao, Angew. Chem., Int. Ed., 2012, 51, 6828 CrossRef CAS PubMed ; (f) Q. Liu and L.-Z. Wu, Natl. Sci. Rev., 2017, 4, 359–380 CrossRef CAS ; (g) L. Buzzetti, G. E. M. Crisenza and P. Melchiorre, Angew. Chem., Int. Ed., 2018, 58, 3730 CrossRef PubMed ; (h) J. Singh, S. Sharma and A. Sharma, J. Org. Chem., 2021, 86, 24 CrossRef CAS PubMed ; (i) J. Singh and A. Sharma, Adv. Synth. Catal., 2021, 363, 3411 CrossRef CAS ; (j) J. Singh and A. Sharma, Adv. Synth. Catal., 2021, 363, 4284 CrossRef CAS ; (k) J. Singh, R. I. Patel and A. Sharma, Adv. Synth. Catal., 2022, 364, 2289 CrossRef CAS .
6. (a) S. Yu and S. Ma, Chem. Commun., 2011, 47, 5384 RSC ; (b) W.-D. Chu, Y. Zhang and J. Wang, Catal. Sci. Technol., 2017, 7, 4570 RSC ; (c) Y. Li and H. Bao, Chem. Sci., 2022, 13, 8491 RSC .
7. (a) S. Ye, M. Yang and J. Wu, Chem. Commun., 2020, 56, 4145 RSC ; (b) R. J. Reddy and A. H. Kumari, RSC Adv., 2021, 11, 9130 RSC ; (c) S. Liang, K. Hofman, M. Friedrich and G. Manolikakes, Eur. J. Org. Chem., 2020, 2020, 4664 CrossRef CAS ; (d) B. Saxena, R. I. Patel, S. Sharma and A. Sharma, Green Chem., 2024, 26, 2721 RSC .
8. (a) L. Fu, S. Greßies, P. Chen and G. Liu, Chin. J. Chem., 2019, 38, 91 CrossRef ; (b) L. Chen, C. Lin, S. Zhang, X. Zhang, J. Zhang, L. Xing, Y. Guo, J. Feng, J. Gao and D. Du, ACS Catal., 2021, 11, 13363 CrossRef CAS ; (c) Y. Song, C. Fu and S. Ma, ACS Catal., 2021, 11, 10007 CrossRef CAS .
9. C.-Y. Zhang, J. Zhu, S.-H. Cui, X.-Y. Xie, X.-D. Wang and L. Wu, Org. Lett., 2021, 23, 3530 CrossRef CAS PubMed .
10. D. Bhatt, K. Miyake, S. Nakamura, H. Y. Kim and K. Oh, Org. Lett., 2024, 26, 2955 CrossRef CAS PubMed .
11. X. Ren, Q. Ke, Y. Zhou, J. Jiao, G. Li, S. Cao, X. Wang, Q. Gao and X. Wang, Angew. Chem., Int. Ed., 2023, 62, e202302199 CrossRef CAS PubMed .
12. (a) J. Singh, N. Singh and A. Sharma, Adv. Synth. Catal., 2024, 366, 1719–1737 CrossRef CAS ; (b) T. Xu, S. Wu, Q.-N. Zhang, Y. Wu, M. Hu and J.-H. Li, Org. Lett., 2021, 23, 8455 CrossRef CAS PubMed ; (c) Y. Chen, K. Zhu, Q. Huang and Y. Lu, Chem. Sci., 2021, 12, 13564–13571 RSC .
13. D.-D. Hu, Q. Gao, J.-C. Dai, R. Cui, Y.-B. Li, Y.-M. Li, X.-G. Zhou, K.-J. Bian, B.-B. Wu, K.-F. Zhang, X.-S. Wang and Y. Li, Sci. China Chem., 2022, 65, 753 CrossRef CAS .
14. X. Wang, S. Wu, R. Yang, H. Song, Y. Liu and Q. Wang, Chem. Sci., 2023, 14, 13367 RSC .
15. L. Wang, R. Ma, J. Sun, G. Zheng and Q. Zhang, Chem. Sci., 2022, 13, 3169 RSC .
16. H. Xiao, Z. Zhang, Y. Fang, L. Zhu and C. Li, Chem. Soc. Rev., 2021, 50, 6308 RSC .
17. J. Xia, R. Ma, L. Wang, J. Sun, G. Zheng and Q. Zhang, Org. Chem. Front., 2024, 11, 3089 RSC .
18. Y.-R. Zhao, Y. Zhou and M. Hu, J. Fluorine Chem., 2023, 267, 110105 CrossRef CAS .
19. W. Liu, C. Liu, M. Wang and W. Kong, ACS Catal., 2022, 12, 10207 CrossRef CAS .
20. (a) A. Rakshit, H. N. Dhara, A. K. Sahoo and B. K. Patel, Chem. Asian J., 2022, 17, e202200972 CrossRef PubMed ; (b) J. Singh and A. Sharma, New J. Chem., 2022, 46, 16220 RSC .
21. Y. Chen, J. Wang and Y. Lu, Chem. Sci., 2021, 12, 11316–11321 RSC .
22. Y. Pu, S. Ding, H. Zhao, Q. Xue, H. Zhang, X. Xie, Y. Shang and J. Wang, Org. Lett., 2024, 26, 1834–1839 CrossRef CAS PubMed .
23. Y.-B. Li, Y.-X. Bi, W.-C. Ke, C.-Y. Wang, D.-D. Hu, Y. Li and X.-S. Wang, Asian J. Org. Chem., 2023, e202300234 CrossRef CAS .
24. L. Yang, L. Zeng, Y. Zhu, Q. Wang and J. Li, Chin. Chem. Lett., 2024, 109685,  DOI:10.1016/j.cclet.2024.109685 .
25. Y. Chen, Q. Wang, K. Peng, L. Cheng, Z. She and G.-J. Deng, Chin. J. Chem., 2024, 42, 2023 Search PubMed .
26. Q. Cao, M.-M. Li, X. Mao, Q.-Q. Zhou and W. Ding, Org. Lett., 2024, 26, 4678–4683 CrossRef CAS PubMed .
27. C. Rücker and K. Kümmerer, Chem. Rev., 2023, 115, 466 CrossRef PubMed .
28. G. Zhang, W. Tan, D. Zhang, K. Wang, P. Gao, S. Wang, S.-L. Liu and F. Chen, Org. Lett., 2024, 26, 536 CrossRef CAS PubMed .
29. D. G. Hall, Chem. Soc. Rev., 2019, 48, 3475 RSC .
30. Q. Sun, X.-P. Zhang, X. Duan, L.-Z. Qin, X. Yuan, M.-Y. Wu, J. Liu, S.-S. Zhu, J.-K. Qiu and K. Guo, Chin. J. Chem., 2022, 40, 1537 CrossRef CAS .
31. Q. Wang, Y. Chen, K. Peng, Y. Li, L. Cheng and G.-J. Deng, Org. Lett., 2023, 25, 8889 CrossRef CAS PubMed .
32. H. Hou, Y. Pan, Y. Sun, Y. Han, C. Yan, Y. Shi and S. Zhu, Chem. – Eur. J., 2023, 29, e202301633 CrossRef CAS PubMed .
33. C. Cheng, G.-F. Lv, S. Wu, Y. Li and J.-H. Li, Org. Lett., 2023, 25, 4236 CrossRef CAS PubMed .
34. (a) L. Pitzer, J. L. Schwarz and F. Glorious, Chem. Sci., 2019, 10, 8285 RSC ; (b) S. Sharma, J. Singh and A. Sharma, Adv. Synth. Catal., 2021, 363, 3146 CrossRef CAS ; (c) W. Lei, Y. Liu, Y. Fang, Y. Li, C. Du and J. Fang, Org. Biomol. Chem., 2021, 19, 8502 RSC .
35. J. M. Alonso and P. Almendros, Adv. Synth. Catal., 2023, 365, 1332 CrossRef CAS .
36. F.-H. Zhang, X. Guo, X. Zeng and Z. Wang, Nat. Commun., 2022, 13, 5036 CrossRef CAS PubMed .
37. C.-Y. Tan, M. Kim and S. Hong, Angew. Chem., Int. Ed., 2023, 62, e202306191 CrossRef CAS PubMed .
38. Z. Zhang, Y. Tian, X. Li, Z. Wang, R. Liu, P. Chen, X. Li, J. Dai and D. Shi, Green Chem., 2023, 25, 2723 RSC .
39. Z.-Z. Zhou, X.-R. Song, S. Du, K.-J. Xia, W.-F. Tian, Q. Xiao and Y.-M. Liang, Chem. Commun., 2021, 57, 9390 RSC .
40. Z.-Z. Zhou, X.-F. Zhai, S.-L. Zhang, K.-J. Xia, H. Ding, X.-R. Song, W.-F. Tian, Y.-M. Liang and Q. Xiao, Org. Chem. Front., 2023, 10, 298 RSC .
41. F.-D. Lu, Z.-C. Shu, G.-F. He, J.-C. Bai, L.-Q. Lu and W.-J. Xiao, Org. Chem. Front., 2022, 9, 5259 RSC .
42. Q. Liu, J. Zheng, X. Zhang and S. Ma, Nat. Commun., 2022, 13, 3302 CrossRef CAS PubMed .
43. D.-F. Jiang, X.-X. Zeng, D.-F. Li, S.-M. Wen and M.-H. Hu, Org. Biomol. Chem., 2023, 21, 5240 RSC .
44. M. M. Q. Simões, J. A. S. Cavaleiro and V. F. Ferreira, Molecules, 2023, 28, 6683 CrossRef PubMed .
45. W. Li and L. Zhou, Org. Lett., 2022, 24, 3976 CrossRef CAS PubMed .
46. K. Orłowska, K. Rybicka-Jasinska, P. Krajewski and D. Gryko, Org. Lett., 2020, 22, 1018 CrossRef PubMed .
47. C. Dank and L. Lelo, Org. Biomol. Chem., 2023, 21, 4553 RSC .
48. T. Song, L. Zhu, H. Li, C.-H. Tung, Y. Lan and Z. Xu, Org. Lett., 2020, 22, 2419 CrossRef CAS PubMed .

---

Footnote

† These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2024