English

• 1. Introduction

  Hydrogen atom transfer (HAT) is identified as an efficient and atom economic strategy that simultaneously transfers a proton and an electron from one species to another in a single step [1-6]. Over the past century, impressive advances have been made in the inert C–H bond functionalization via the HAT process in organic synthesis and materials science [7,8]. In the past decade, photocatalytic organic chemistry has gained enormous attention [9-18]. In this context, the photocatalytic HAT strategy is an environmentally friendly and energy-efficient platform to achieve C–H/Si–H functionalization by generating corresponding radical intermediates for diverse organic transformations [7,19-20]. Until now, a vast variety of HAT photocatalysts have been reported, such as aryl ketones [21], decatungstate [22-25], uranyl cations [26-27], neutral eosin Y [28], and so on.

  Halogen radicals (Cl^• or Br^•) are efficient HAT agents that could abstract hydrogen atoms from various C–H/Si–H/O–H bonds to give the corresponding C/Si/O-centered radical and H–X (X = Cl, Br). Among them, the bond dissociation energy (BDE) of the H–Cl bond (103 kcal/mol) is higher than H–Br (88 kcal/mol) [29,30]. Therefore, chlorine radical is generally a more reactive and electrophilic radical for the activation of inert aliphatic C–H bonds [31]. Chloride anion (Cl^–) should be an ideal source for the generation of Cl^• via single electron oxidation because it is innocuous and abundant in diverse salt forms.

  However, the single-electron oxidation of chloride anion into Cl^• is generally challenging due to the high redox potential (E_ox(Cl⁻/Cl^•) = +2.03 V versus SCE in MeCN) [32]. Significantly, in 1962 Kochi disclosed the generation of Cl^• and CuCl from the photolysis of CuCl₂ [33] via a successive ligand-to-metal charge transfer (LMCT) and Cu–Cl homolysis process. Typically, the photo-induced LMCT process involves an electronic transition from a filled orbital (n/π) of the ligand to an empty orbital (d) of the metal center, resulting in an excited LMCT state. The LMCT state undergoes a homolysis of the M−L to release an active radical L^•, which could be a useful radical initiator in various reactions (Scheme 1). Since then, the application of photogenerated Cl^• has emerged as a powerful and promising tool to initiate various organic transformations [34-42].

  Scheme 1

  

  Scheme 1.  Photo-inducted ligand-to-metal charge transfer (LMCT).

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  On the other hand, the functionalization of C–H bonds is highly attractive because C–H bonds are the most abundant moieties in organic molecules. However, the ubiquitous C–H bonds, particularly C(sp³)–H bonds are generally inert because of their relatively high BDEs [43,44]. Traditional methods of C–H activation rely on transition metal catalysts or stoichiometric amounts of oxidants, which usually require the pre-installation of suitable directing groups [45-47], or elevated reaction temperatures under harsh conditions [48,49]. Although elegant examples using traceless directing group strategy have been achieved [50-52], the direct functionalization of the C–H bond remains challenging in organic synthesis. Thus, the photogenerated Cl^•-mediated HAT process brings a useful and general method to achieve the functionalization of the C–H bond in organic synthesis.

  Iron is one of the most abundant minerals in the Earth's crust. Importantly, it is also an essential metal element for human health [53]. Over the past decades, iron catalysis has captured huge attention in oxidative coupling reactions, reductive reactions, and hydrometallation reactions [54-56], etc. Up to now, several reviews related to iron-catalyzed organic synthesis have been reported [8,57-59], they cover a wide variety of iron catalysts and many types of organic reactions. However, FeCl₃ as an inexpensive, environmentally friendly, and safe catalyst [60,61], has been rapidly developed in the field of organic synthesis in recent years. Particularly, FeCl₃ has emerged as an effective photocatalyst for the synthesis of complex molecules and post-modification of bioactive molecules in recent years [62-68]. Therefore, we herein summarize the recent advances in FeCl₃-photocatalyzed organic reactions for the formation of C–C, C–N, C–Si, C–S, C–B, and C-P bonds, which involve photogenerated chlorine radical-initiated HAT. Notably, the elegant examples of FeCl₃-catalyzed photooxidation of polystyrene are not included [69,70].

  2. Formation of C−C bond

  The Dowd–Beckwith rearrangement is a radical-mediated skeletal rearrangement involving a radical 1,2-rearrangement process. Rovis's group developed a FeCl₃-catalyzed C(sp³)–H alkylation reaction via a Dowd-Beckwith ring expansion procedure under the irradiation of 390 nm LEDs in 2021 (Scheme 2) [71]. In this reaction, chlorine radical is generated from FeCl₃ via an LMCT/homolysis procedure under irradiation. Then, the Cl^• abstracts a hydrogen atom of the C(sp³)–H bond of substrate 1 to give an unstable 1° carbon-centered radical 5. Then the addition of radical 5 to the olefin 2 produces the radical intermediate 8, followed by a single electron transfer (SET) and protonation to afford the unrearranged product 3. Alternatively, the unstable radical 5 could undergo an intramolecular skeletal rearrangement via cyclopropyl-containing radical intermediate 6 to generate a more stable radical intermediate 7, which is converted into the corresponding rearranged products 4.

  Scheme 2

  

  Scheme 2.  FeCl₃-photocatalyzed C(sp³)–H alkylation.

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  In this work, a mixture of unrearranged and rearranged products was obtained in moderate yields. The ratios of products generally can be tuned by changing the concentration and temperature of the reaction. It turns out that the ratio of product 3/4 is increased at a low temperature and high concentration (r.t., 0.3 mol/L), while it is decreased at a high temperature and low concentration (60 ℃, 0.1 mol/L). This protocol provides a simple method to accomplish the selective synthesis of the directly alkylated or the rearranged-alkylated products. However, the poor selectivity and the low yields in most cases limit the utilization of this method.

  As a class of important building blocks and fundamental structural motifs, alkynes are widely used in the synthesis of pharmaceuticals and electronic materials [72]. On the other hand, the desulfonylative transformations of organic sulfones via C−SO₂ bond cleavage have been recognized as a versatile tool for the construction of new bonds [73-75]. In this context, in 2021 Jin and Duan's group developed an efficient and practical desulfonylative coupling of light alkanes 10 and alkynyl aryl sulfones 11 for the construction of valuable internal alkynes 12, by using FeCl₃·6H₂O (10 mol%) as a direct HAT photocatalyst under the irradiation of 395 nm light (Scheme 3) [76]. This oxidant-free protocol proceeds smoothly under mild conditions, especially for the light alkanes, including liquid alkenes and gaseous alkanes such as methane, ethane, and propane. Notably, when isobutane was employed as an alkylation reagent, the 1° and 3° C(sp³)–H bond functionalization products (α/β) were observed with a ratio of 99/1. Additionally, a wide range of cycloalkanes such as cyclopentane, cyclohexane, cycloheptane, cyclooctane, and cyclododecane were also efficient alkylation reagents in this procedure to give the corresponding products in good to excellent yields. Similarly, the chlorine radical (Cl^•) generated from FeCl₃ via an LMCT/homolysis procedure under irradiation is a critical initiator to abstract the hydrogen atom of the C(sp³)–H bond of alkanes affording the corresponding alkyl radicals.

  Scheme 3

  

  Scheme 3.  FeCl₃-photocatalyzed alkynylation of light alkanes.

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  Selectively cleavage of C–C bond is a promising strategy to construct new compounds and complex molecules, which has received great attention in recent years [77-83]. However, selective cleavage of an inert C–C bond is always a tough task due to the large BDE [84]. Considering that alkoxy radicals can be transformed into C-centered radicals through an intramolecular HAT or the β-scission of the C–C bond [85-88], Hu's group in 2021 reported a visible-light-induced FeCl₃-catalyzed direct HAT strategy to generate alkoxy radicals from alcohols, leading to a C–C bond cleavage (Scheme 4) [89]. As can be seen in Scheme 4, under the irradiation of 430 nm blue light, Cl^• is generated from [FeCl₄⁻] via an LMCT procedure. Then, Cl^• abstracts a hydrogen atom from the O–H bond of the cyclic/linear alcohol 13 with the assistance of a base, which gives the alkoxy radical 17. Then, 17 undergoes an intramolecular β-scission to afford the carbon-centered radical 18. Subsequently, the HAT process between radical 18 and in situ formed ArSH generates the corresponding ketone product 19. As a result, a series of ketones and aldehydes were synthesized through this practical catalytic system.

  Scheme 4

  

  Scheme 4.  FeCl₃-photocatalyzed C−C bond cleavage of cyclic alcohols.

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  This visible-light-induced reaction showed a wide range of substrate scope. For example, 1°, 2°, and 3° alcohols were all suitable for the transformation, and the yields of the ring-open products were up to 96%. Additionally, natural products and pharmaceutical molecules such as (-)-nopol, testosterone, and 7-methyltestosterone, showed good performance in this ring-open reaction, giving the corresponding products with moderate to good yields.

  After that, the same group developed another FeCl₃-photocatalyzed C–C bond cleavage alkylation of alcohols with electron-deficient alkenes via the direct HAT and β-scission of various alcohols (Scheme 5) [90]. A series of control experiments indicated that a β-scission of the alkoxy radical progress is involved in this transformation. In the plausible mechanism, alkoxy radical 24 is generated from the corresponding alcohol by the HAT process of Cl^•, which is generated from the photoexcited iron catalyst. Then, the alkoxy radical 24 undergoes a selective β-scission of the C–C bond to generate an alkyl radical. Subsequently, the addition of alkyl radical to electron-deficient olefins affords the radical intermediate 25, which is finally transformed into the corresponding product 26 after a single electron reduction and protonation. Although these two above-mentioned FeCl₃-photocatalyzed reactions were realized under the irradiation of visible light (450 nm), a relatively high light intensity (50 W × 2) and a stoichiometric amount of base are required to facilitate the reaction due to the high BDE of the O–H bond (~105 kcal/mol) [29].

  Scheme 5

  

  Scheme 5.  FeCl₃-photocatalyzed reaction of alcohols and electron-deficient alkenes.

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  Direct transformed gaseous alkanes (such as methane, ethane, and propane) into high-value-added chemicals in mild conditions without the need for functionalized reagents is a meaningful issue [91]. In 2022, Wang and Gong's group reported a FeCl₃/HCl-photocatalyzed procedure to achieve the diverse functionalization of the C(sp³)–H bonds under the irradiation of 390 nm LEDs (Scheme 6) [62], providing the alkylation, oxidation, chlorination, fluorination, amination, alkynylation, and sulfonylation products. Particularly, in the reaction of cyclohexane and 2-benzylidenemalononitrile, a turnover number (TON) of 9900 was observed. Light alkanes like ethane, propane, aldehyde and silane were successfully applied as substrates. In this catalytic system, the addition of HCl is found to be essential, which could promote FeCl₃ to generate more photoactive species [FeCl₄⁻]. Under the irradiation of 390 nm LEDs, [FeCl₄⁻] generates Cl^• via an LMCT procedure. Then, Cl^• abstracts a hydrogen atom from the C(sp³)–H bond substrates 27 to give the carbon-centered radical 31. Subsequently, 31 adds to olefin 30, forming the radical adduct 32. Finally, 32 undergoes a SET and protonation to afford the corresponding product 29.

  Scheme 6

  

  Scheme 6.  FeCl₃/HCl-photocatalyzed functionalization of C(sp³)–H bonds.

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  In the same year, Jin's group reported a similar FeCl₃-photocatalyzed protocol to achieve the transformation of gaseous alkanes via a Cl^•-mediated HAT process [92]. In this neutral system, methane (5 MPa), ethane (0.1 MPa), propane (0.1 MPa), and n-butane (0.1 MPa) reacted with electron-deficient alkenes smoothly, affording the alkylation products with moderate to excellent yields.

  N—Heteroarenes are an omnipresent component of many natural products, bioactive compounds, and pharmaceuticals. Particularly, benzothiazoles and quinolines have captured the interest of chemists worldwide [93-96]. Recently, Jian and Tong's group developed an elegant Minisci reaction to achieve the alkylation of N-heteroarene compounds by using FeCl₃ as a photocatalyst and LiCl as an additive (Scheme 7) [64]. This procedure is compatible with a variety of N-heteroarene substrates, such as quinolines, p-hydroxyquinazoline, quinazolinone, and benzothiazole, leading to the corresponding alkylation products in moderate to good yields. Interestingly, brine or seawater can replace LiCl to promote the reaction, which corresponds to the concept of sustainable and green chemistry. In the proposed mechanism, Cl^• is generated from the FeCl₃ via an LMCT procedure under light irradiation. Then Cl^• abstracts a hydrogen atom from the unactivated C(sp³)–H bond of alkanes to give the alkyl radical 35. Subsequently, 35 adds to the protonated heteroarene 36, affording the intermediate 37. Finally, 37 undergoes deprotonation and oxidation to give the product 34. This photocatalytic reaction provides a convenient method for the installation of an alkyl group into N-heteroarene skeleton at room temperature. However, the excess amount of trifluoroacetic acid is necessary to assist the protonation procedure of N-heteroarenes.

  Scheme 7

  

  Scheme 7.  FeCl₃-photocatalyzed alkylation of heteroarenes.

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  Dearomatization is a constant challenge in both organic synthesis and pharmaceutical chemistry. Li's group developed a FeCl₃-photocatalyzed method to construct alkyl benzothiazolines through a dearomatizative addition of inert alkanes 38 and benzothiazoles 39 under mild conditions (Scheme 8) [97]. Under the irradiation of 390 nm LED, Cl^• was generated from FeCl₃ via an intramolecular LMCT procedure, which directly abstracted a hydrogen atom from 38 to generate an alkyl radical (^•R¹). Subsequently, the alkyl radical reacted with the protonated substrate 41 to produce intermediate 42. Finally, a SET between 42 and the in situ generated Fe(II) species provided the target product 40 and completed the catalytic cycle. This method can be conveniently scaled up, and most of the alkylated products can be purified without chromatography.

  Scheme 8

  

  Scheme 8.  FeCl₃-photocatalyzed dearomatizative alkylation of benzothiazoles.

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  Inexpensive and easily available aldehydes could be applied as useful acylation reagents because the BDE of the C(O)–H bond is relatively weak (~86.9 kcal/mol of benzaldehyde). Recently, Reiser's group disclosed a Cl^•-mediate acylation reaction by a FeCl₃/diphenylanthracene (DPA) dual photocatalytic system [98]. In this reaction, acyl radical was generated from aldehydes 43 in the presence of HCl (1 equiv.) and MgCl₂ (10 mol%) under the irradiation of 390 nm LEDs (Scheme 9). The in situ generated Cl^• interacts with 43 giving the acyl radical. Then the addition of acyl radical to the electron-deficient olefin 44 produces the radical adduct 46. Subsequently, the SET and protonation process between the radical 46 and the reduced DPA 48 generates the corresponding acylation product 45. As a result, a broad range of aldehydes are compatible with this method, ranging from aromatic to aliphatic aldehydes to give the corresponding products in moderate to excellent yields. It must be mentioned that this procedure is practical and scalable as 96% yield was obtained when the reaction was conducted on a 5.0 mmol scale.

  Scheme 9

  

  Scheme 9.  FeCl₃-photocatalyzed alkylation of aldehydes.

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  The construction of C(sp²)-C(sp³) bond from readily available starting materials benzenes and aliphatic hydrocarbons is generally a challenging task due to the intrinsic inertness of reactants. Recently, the coupling of benzenes with aliphatic hydrocarbons was realized by Gong's group with visible-light-induced iron salts (FeCl₃ or FeBr₃) catalytic system using air as a green oxidant (Scheme 10) [99]. This protocol provides an eco-friendly, cost-efficient approach to build C(sp²)-C(sp³) bond from a strong C(sp²)-H bond and a robust C(sp³)-H bond, affording a broad range of cross-coupling products with high yields and commendable chemo-, site-selectivity. It is worth mentioning besides the alkyl benzene derivates, the indole- and pyrrole-based heterocycles also showed good tolerance delivering alkylbenzene products in good regioselectivity and yields. Additionally, different C(sp³)-H compounds such as adamantane, 1-methyl adamantine, cyclopentane, cyclohexane, cycloheptane, and 1,1-dimethyl cyclohexane, were successfully converted into the coupling product in moderate yields. In this protocol, the in situ generated Cl^• initiates the C-centered radical 55 from the aliphatic hydrocarbon 52 via HAT. Subsequently, 55 was oxidized to carbocation 56 by Fe(III) species. Finally, the C(sp²)-C(sp³) cross-coupling of 56 and arene 53 was achieved through the electrophilic substitution process. Alternatively, the iron-catalyzed Friedel-Crafts-alkylation between arene and the in situ-produced alkyl halide pathway could not be ruled out.

  Scheme 10

  

  Scheme 10.  FeCl₃-photocatalyzed the cross-coupling of benzenes with aliphatic hydrocarbons.

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  3. Formation of C−N bonds

  C–N bonds are omnipresent in biologically active compounds, natural products, and materials [100,101]. Over the past decades, numerous efforts have been devoted to the development of efficient methods for the construction of C–N bonds. The unsaturated N=N bond of the di-tert-butyl azodicarboxylate (DBAD) makes it a particularly useful alkyl radical acceptor for the synthesis of nitrogen-contained compounds. Jin and Duan's group developed a FeCl₃·6H₂O-photocatalyzed protocol to furnish the direct amination of light alkanes, employing DBAD as the amination reagent (Scheme 11) [102]. This oxidant- and the acid-free procedure proceeds smoothly under the irradiation of 365 nm LEDs, giving the highest TON up to 8000. Similar to the above-mentioned systems, the in situ formed Cl^• initiates a HAT process generating alkyl radical (R^•) from the alkane substrate 57, which is further accepted by the N=N bond of DBAD (58) affording the radical intermediate 62. Subsequently, 62 undergoes a single electron reduction and protonation to give the corresponding products 60. This photocatalytic reaction provides a convenient method for the synthesis of various nitrogen-contained compounds under mild conditions. It is worth mentioning that in the purification stage, column chromatography is unnecessary in most cases.

  Scheme 11

  

  Scheme 11.  FeCl₃-photocatalyzed amination of alkanes.

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  In 2022, Ni and coworkers reported a FeCl₃-photocatalyzed N-alkylation of amides and heterocycles under the irradiation of blue LEDs (450–455 nm) (Scheme 12) [103]. Notably, an excess amount of di-tert-butyl peroxide (DTBP) was required. A myriad of amides and NH-containing heterocycles were applicable in this reaction, affording the corresponding products in moderate to good yields. Moreover, the alkylation product could be transformed into amino alcohol via a ring-opening reaction demonstrating the applicability of this protocol. In this protocol, alkyl radical was possibly generated from two pathways: (a) under the irradiation of visible light, L_nFe(III)-Cl* underwent an LMCT procedure to generate a Cl^•, which abstracted a hydrogen atom from 64 to produce the alkyl radical 66; (b) the homolytic cleavage of DTBP under visible light gave tert-butoxy radical (^tBuO^•), which then reacted with 64 to generate the alkyl radical 66 via a HAT procedure. On the other hand, substrate 63 coordinates with L_nFe(III)-Cl to generate the iron complex 67. Finally, the coupling of radical 66 and complex 67 generates product 65 along with the release of L_nFe(II).

  Scheme 12

  

  Scheme 12.  FeCl₃-photocatalyzed alkylation of amides and N-heterocycles.

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  Amides are important functional groups in organic chemistry [104], which are ubiquitous in biological systems, natural products, organic materials, and pharmaceutical drugs. In 2022, Zeng's group reported a green method to synthesize amides from readily available aldehydes and nitroarenes using FeCl₃ as the photocatalyst (Scheme 13) [105]. In this oxidant- and reductant-free protocol, aromatic and aliphatic aldehydes were smoothly converted to the corresponding amides in moderate to excellent yields. Moreover, alcohols could also be utilized as starting materials to react with nitrobenzene, affording the corresponding amides in moderate yields. A solvent-assisted nitro-reduction mechanism was proposed as shown in Scheme 13. Firstly, the alkyl radical 72 was generated from solvent DCE and Cl^• by a HAT process. On the other hand, the Cl^• can also abstract the hydrogen atom from 68 to generate the acyl radical 71, which reacts with 75 to produce 76 or couples with Cl^• to generate acyl chloride. The following Fe-cycle with the 76 would regenerate the iron(III) and 77. An alternative process might involve the direct reaction of the acyl chloride with 75 to produce 77. Finally, the reduction of 77 produced the amide product 70. This photocatalytic reaction provides a green method to construct amides from easily available material at room temperature. Notably, the excess amount of the reductant or oxidant is necessary for nitro-reduction in the previous reports [106-109]. However, the chlorine radical-mediated HAT process of 1,2-dichloroethane assists in the reduction of nitroarenes, avoiding to use of harsh reaction conditions.

  Scheme 13

  

  Scheme 13.  FeCl₃-photocatalyzed amidation with nitroarenes.

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  Subsequently, Xia and Yang's group developed an iron-catalyzed direct dehydrogenative amination reaction of aldehydes and amines under visible light irradiation [110]. This protocol has a good functional group tolerance and broad substrate scope toward both aliphatic and aromatic components. Secondary N-methyl benzylamine derivatives, alkyl amines, and primary alkyl amines were suitable substrates. Additionally, commercially available pharmaceutical molecules, such as haloperidol and donepezil, were also amenable to this transformation. A nucleophilic addition of amines process was proposed as shown in Scheme 14. Acyl halide 81 as an intermediate was generated from aldehyde 78 via a photoinduced HAT. Then 81 reacted with the substrate amines 79 giving the corresponding amides 80.

  Scheme 14

  

  Scheme 14.  FeCl₃-photocatalyzed dehydrogenative amination of aldehydes and amines.

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  4. Formation of C−Si bonds

  As a bioisostere of C–H, Si–H bond has a similar property, owing to both being group IV elements. Thus, organosilicon compounds were usually recognized as bio-isosteres of hydrocarbons. However, the difference in lipophilicity and electro-positivity between carbon and silicon makes silicone-containing compounds useful materials. In this context, the introduction of Si-containing functional groups into organic molecules has received huge attention in recent years [111].

  In 2022, Wang's group developed a FeCl₃/LiCl photocatalytic system for the reaction of triphenyl silane 82 and electron-deficient olefins 83 under the irradiation of visible light (440–445 nm, 25 W) (Scheme 15) [112]. With this 100% atom-efficient protocol, the hydrosilylation products 84 were obtained in moderate to good yields. Notably, this sustainable procedure was successfully used for the late-stage functionalization of naturally occurring compounds, such as L-menthol, vanillic acid, vitamin E. Based on the control experiments, a plausible reaction pathway was proposed. With the irradiation of visible light, Cl^• can be generated from FeCl₃ via an LMCT procedure, which abstracts a hydrogen atom from triphenyl silane 82 to give the silyl radical 85. Then 85 adds to the olefin 83 to deliver the intermediate 86, which undergoes a single electron reduction and protonation to give the product 84.

  Scheme 15

  

  Scheme 15.  FeCl₃-photocatalyzed hydrosilylation of electron-deficient alkenes.

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  The same FeCl₃/LiCl photocatalytic system was also applicable in the desulfonylation coupling of hydrosilanes 88 and alkynyl sulfones 89 (or vinyl sulfones 89′) for the synthesis of alkynyl silanes or vinyl silanes (Scheme 16) [113]. This method has good functional group compatibility. All the sulfones containing the electron-withdrawing group (F-, Cl-, Br-, Ac-, CN-) or the electron-donating group (Me-, Et-, tBu-, MeO-) work well in this procedure. When vinyl sulfones 89′ were applied as substrate, only E-vinylsilanes 91 were obtained, which might be a good tool for the regio- and stereoselective construction of vinylsilanes. Similarly, the desired silyl radical 92 is generated via HAT with the in situ generated Cl^•. The radical 92 adds to sulfone 89 giving the radical intermediate 93, which subsequently transformed into anion intermediate 94 via a SET procedure. Finally, the C–SO₂ bond cleavage of 94 affords the corresponding silylated product 90.

  Scheme 16

  

  Scheme 16.  FeCl₃-photocatalyzed alkynylation of hydrosilanes.

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  5. Formation of C−S and C–B bonds

  Sulfur-containing compounds are important chemicals in pharmaceuticals, agrochemicals, and functional materials [114]. Among the commercially available drugs approved by the FDA, over 20% of the molecules contain the sulfur element, which is the third largest heteroatom after oxygen and nitrogen. Particularly, amido-N,S-acetal is one of the most important moieties in natural products and antibacterials, such as penicillin, penicillin derivatives, and fusaperazine A. In 2022, Laulhéa and co-workers developed a FeCl₃-photocatalyzed protocol for the synthesis amido-N,S-acetal derivatives from diaryl disulfides/diselenides 95 and N-methyl amides 96 (Scheme 17) [115]. In this work, the absorbance of FeCl₃ in different solvents was studied by UV–vis spectrum. When CH₃CN was employed as the solvent, FeCl₃ showed the strongest absorbance in the visible region suggesting that acetonitrile could act as a ligand in this system. Under 390 nm LEDs irradiation, the acetonitrile-coordinated FeCl₃ generates Cl^• via the LMCT procedure. Subsequently, Cl^• selectively abstracts a hydrogen atom from the C–H bond of substrate 96 to deliver amidoalkyl radical 98, which reacts with diaryl disulfides 95 to give the desired product 97 and radical 99. Subsequently, 99 undergoes a SET process and oxidation to reproduce the substrate 95.

  Scheme 17

  

  Scheme 17.  FeCl₃-photocatalyzed thiolation of N-methyl amides.

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  Very recently, Guo and Xia's group reported an undirected iron-catalyzed C(sp³)–H borylation, thiolation, and sulfinylation strategy that showed unconventional regioselectivity, compared with previous reports in which preference for activated and thermodynamically favored bonds. However, in this work, all reactions were preferentially at the distal methyl position (Scheme 18) [65]. The site selectivity is not only relevant to the HAT species but also largely affected by the nature of radical acceptors. For instance, in the C–H thiolation reaction the selectivity was controlled by the steric effect furnishing the distal methyl functionalized products. While the borylation reaction preferentially happened on the β-methylene rather than the terminal methyl group, giving borylation products of β-C(sp³)-H and γ-C(sp³)-H bond almost in 1:1 ratio proportion, which is probably due to an effect of boron–carbonyl oxygen interaction.

  Scheme 18

  

  Scheme 18.  FeCl₃-photocatalyzed borylation, thiolation, and sulfinylation of C(sp³)–H.

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  A broad range of ketones, nitriles, aliphatic esters, ethers, amides, and sulfonamides were all found to be efficiently transformed into the corresponding boronate esters in moderate to good yields. It is worth pointing out that this transformation preferentially occurred at β- or γ-position of the functionalized alkanes, and no borylation of α-C(sp³)–H bond was observed. In addition, this strategy enabled C(sp³)–H borylation of simply silanes, germane, and stannane, generating the corresponding borylation products in acceptable yields. Moreover, this iron photocatalyzed protocol can also be used for thiolation and sulfinylation as shown in Scheme 18. These reactions exhibit a remarkably broad scope (>150 examples), excellent substrate applicability, and functional group compatibility.

  6. Formation of C-P bonds

  Organophosphorus compounds play a vital role in organic synthesis, catalysis, materials chemistry, medicinal chemistry, and coordination chemistry. In this context, tertiary phosphines are a class of important compounds widely used as valuable reactants, organocatalysts, and ligands in organic chemistry. However, its synthesis usually requires harsh conditions, sensitive organometallic reagents, and prefunctionalized substrates. Therefore, the development of new synthetic methods to create tertiary phosphine species is of great importance. In 2023, Hu's group developed a novel C(sp³)–H phosphorylation protocol to construct a series of tertiary phosphines species from industrial phosphine(III) sources using FeCl₃ as a photocatalyst (Scheme 19) [116]. With the irradiation of 370 nm LEDs, Cl^• can be generated from FeCl₃ via an LMCT procedure, then reacted with hydrocarbon 105 to give an alkyl radical via the HAT process. On the other hand, diphenylphosphinyl radical 108 was generated from phosphine(III) sources 106 and Fe(II)-species through a SET step with the generation of the Fe(III) catalyst for the next catalytic cycle. Subsequently, the corresponding product 107 was generated from alkyl radical and radical 108. Moreover, the homo-coupling intermediate 109 should be a possible precursor for the final product.

  Scheme 19

  

  Scheme 19.  FeCl₃-photocatalyzed phosphorylation of C(sp³)–H.

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  This sustainable procedure provides a valuable solution toward tertiary phosphines(III) synthesis, avoiding the prefunctionalization of substrates and sensitive organometallic reagents in traditional approaches. Under mild photocatalytic conditions, a wide range of hydrocarbons were suitable for this transformation, including inert alkanes, cyclic ethers, dimethyl ether, and toluene derivatives. Additionally, this catalytic system can be applied for the polymerization of electron-deficient alkenes demonstrating the synthetic potential of this reaction in polymer material chemistry.

  7. Conclusions

  Photocatalytic HAT has been recognized as a concise and efficient tool to generate active radical intermediates for various organic reactions under mild conditions. From the viewpoint of green chemistry, the earth-abundant transition metal salt FeCl₃ is an inexpensive, nontoxic, and easily available green catalyst. Under light irradiation, FeCl₃ could produce the chlorine radical (Cl^•) via an LMCT process, which is an initiator for the photocatalytic HAT processes. In this minireview, the recent advances in the photocatalytic organic transformations enabled by FeCl₃ were summarized. These catalytic systems are generally simple and practical by using a catalytic amount of FeCl₃ as a photocatalyst. The aid of halide compounds, such as LiCl, TBACl, and TBABr is also important to enhance the HAT process in some cases. Overall, the direct functionalization of C–H, O–H, and Si–H bonds could be achieved under mild conditions for the construction of C–C, C–N, C–Si, C–S, C–B and C–P bonds. It should be pointed out that in most of the above-mentioned examples, UV light with a wavelength of 365–395 nm is necessary to excite the FeCl₃ photocatalyst, which might limit the application of such a useful strategy. However, in some cases, the transformation could be conducted under irradiation of 440–455 nm blue light, probably due to the different additives and substrates. Therefore, it should be possible to realize FeCl₃-photocatalyzed reactions under the irradiation of visible light by using appropriate additives or solvent systems. This minireview should be helpful for the researchers interested in FeCl₃-promoted photocatalytic organic transformations. We believe the FeCl₃ photocatalysis strategy could be applied in more organic reactions even under the irradiation of visible light.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  We acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21971224, 22171249), Program for Science & Technology Innovation Talents in Universities of Henan Province (No. 23HASTIT003).

  
  
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• 

  Scheme 1  Photo-inducted ligand-to-metal charge transfer (LMCT).

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  Scheme 2  FeCl₃-photocatalyzed C(sp³)–H alkylation.

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  Scheme 3  FeCl₃-photocatalyzed alkynylation of light alkanes.

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  Scheme 4  FeCl₃-photocatalyzed C−C bond cleavage of cyclic alcohols.

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  Scheme 5  FeCl₃-photocatalyzed reaction of alcohols and electron-deficient alkenes.

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  Scheme 6  FeCl₃/HCl-photocatalyzed functionalization of C(sp³)–H bonds.

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  Scheme 7  FeCl₃-photocatalyzed alkylation of heteroarenes.

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  Scheme 8  FeCl₃-photocatalyzed dearomatizative alkylation of benzothiazoles.

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  Scheme 9  FeCl₃-photocatalyzed alkylation of aldehydes.

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  Scheme 10  FeCl₃-photocatalyzed the cross-coupling of benzenes with aliphatic hydrocarbons.

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  Scheme 11  FeCl₃-photocatalyzed amination of alkanes.

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  Scheme 12  FeCl₃-photocatalyzed alkylation of amides and N-heterocycles.

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  Scheme 13  FeCl₃-photocatalyzed amidation with nitroarenes.

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  Scheme 14  FeCl₃-photocatalyzed dehydrogenative amination of aldehydes and amines.

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  Scheme 15  FeCl₃-photocatalyzed hydrosilylation of electron-deficient alkenes.

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  Scheme 16  FeCl₃-photocatalyzed alkynylation of hydrosilanes.

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  Scheme 17  FeCl₃-photocatalyzed thiolation of N-methyl amides.

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  Scheme 18  FeCl₃-photocatalyzed borylation, thiolation, and sulfinylation of C(sp³)–H.

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  Scheme 19  FeCl₃-photocatalyzed phosphorylation of C(sp³)–H.

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•