Photocatalytic C–P bond formation based on the reaction of carbon-centered radicals with phosphides

Abstract

Organophosphorus compounds hold significant importance in the fields of materials science, medicinal chemistry, pesticides and organic synthesis. In particular, phosphonate esters can be used in sensors, antibacterial and anti-virus drugs, herbicides and organic phosphine ligands, etc. Therefore, the construction of C–P bonds has attracted tremendous attention from chemists. Although strategies for the photocatalytic synthesis of phosphonate esters have been reported, they are mainly limited to the reaction of phosphorus radicals with carbon-containing organic compounds. In recent years, with the rapid development of photocatalysis, organic chemists have made remarkable progress in the study of visible light-mediated carbon-centered radical phosphorylation, especially the construction of C(sp³)–P bonds based on alkyl radicals. This review focuses on the synthesis of C–P bonds using carbon-centered radicals with phosphorous compounds of different valence states (P^III, P^V, P₄) under photocatalysis.

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1. Introduction

Phosphorus is one of the most basic elements and is ubiquitous in nature and organisms. Among these, phosphonate compounds containing C–P bonds have been widely applied in the fields of material science, medicinal chemistry, pesticides and organic synthesis,^1–4 due to their unique electronic and structural properties. Examples include ACE-inhibitor (medicines), glyphosate (herbicides), bisphosphine ligand (asymmetric synthesis) and others (Fig. 1).⁵ Therefore, these have sparked significant interest among organic chemists for developing innovative and efficient synthesis methods for phosphonate esters.

Fig. 1 Examples of organophosphorus molecules with phosphoryl moieties and their applications in material science, medicinal chemistry, pesticides and organic synthesis.

In the past few decades, photochemical synthetic methods have received much attention due to being environmentally friendly and able to work in mild reaction conditions. Photochemical synthesis has become a crucial technique in organic synthesis for its energy transformation, range of light sources, ease of operation, and enhanced safety.⁶ Traditionally, the synthesis of phosphonate esters mainly relied on the classical Arbuzov reaction,⁷ electrophilic substitution reaction⁸ and transition-metal catalyzed coupling reaction.⁹ However, these methods may require the use of transition metals, large amounts of oxidants or higher temperatures, which is not in line with the development of green chemistry concepts today. Although strategies for the photocatalytic synthesis of phosphonate esters have been reported, especially direct synthesis from inexpensive and readily available starting materials, they are mainly limited to the reaction of phosphorus radicals with carbon-containing organic compounds (Fig. 2).¹⁰ For example, in 2013, Kobayashi and Yoo^10a used organic dye (rhodamine B) as the photocatalyst to achieve hydrophosphonation with secondary phosphine oxides (SPOs) and unactivated alkenes under visible light. Inspired by this, in 2017, Lakhdar et al.^10b achieved the hydrophosphinylation of unactivated alkenes using alkoxy phosphinate as a radical precursor. Based on these, organic chemists have increasingly ventured into related fields, leading to significant advancements in their research.

Fig. 2 Synthetic methods for the construction of C–P bonds.

In recent years, researchers have published several reviews on the construction of C–P bonds mediated by visible light. For example, Gao et al.^11a,b conducted a comprehensive study on the difunctionalization of unsaturated compounds using phosphorus radicals. A review on C–P bond formation reactions under visible light conditions was published by Cai et al.^11c A photo-mediated phosphorylation of C–H bonds was reviewed by Ung et al.^11d in 2021. These studies rarely involved the production of carbon-centered radicals. With the in-depth research of organic chemists in this field, recent research has made great progresses in the construction of C–P bonds using carbon-centered radicals, leading to the synthesis of various phosphonate esters under visible light. In this review, we summarize the recent advances on the construction of C–P bonds using carbon-centered radicals under photocatalysis, especially the construction of C(sp³)–P bonds based on alkyl radicals.

2. The construction of C–P bonds

Photons can be used to “activate” molecules so that new synthesis strategies can be developed.¹² Organic compounds can produce various active intermediates excited by irradiation, especially carbon-centered radicals (Fig. 3A). This has paved the way for the development of innovative methods in organic chemistry to form new bonds using visible-mediated photoredox catalysis. By employing photosensitizers and photocatalysts that become excited under specific wavelengths, electron transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) (Fig. 3B) can occur, giving rise to strong redox properties in the photocatalyst. This enables the photocatalyst to drive other molecules to undergo a single electron transfer (SET) process (Fig. 3C),¹³ ultimately facilitating the formation of new bonds. In recent years, organic chemists have developed different ways to obtain carbon-centered radicals under visible light and construct C–P bonds with phosphorous compounds of different valence states (P^III, P^V, P₄). Phosphorus atoms with three single bonds form a kind of P^III organic compound, such as phosphinites and phosphonites. These two kinds of phosphorus compounds have different reactivity due to the diverse types of substituents (phenyl or alkyl substitution is more reactive). This kind of compound has lone pair electrons on the P atom, so it can react with electron deficient molecules or intermediates to construct C–P bonds. Finally, the alkyl group of alkoxy can be easily replaced to synthesize P^V through a transesterification-type reaction. Additionally, the P–H bond in P^V can be converted into a relatively stable phosphine radical, which can then form the C–P bond through radical coupling with an alkyl radical. And P₄ (white phosphorus) as a “free radical trap” due to its tetrahedral structure's ability to capture two alkyl radicals, leading to the formation of alkyl phosphine through oxidation cleavage (Fig. 3D). Currently, the majority of products synthesized by organic chemists are pentavalent phosphorus compounds. Due to the high electron cloud density on the P=O bond in these compounds, the bond length between P and O in the double bond is shorter compared to the P–O bond in the single bond. As a result, the resulting phosphate ester compound (P^V) exhibits greater stability.

Fig. 3 (A) Partial active intermediates produced by visible light excitation; (B) electron transition under visible light excitation; (C) production of alkyl radicals under visible light excitation; (D) phosphorus sources with different valence states.

2.1 P^III involved in the construction of C–P bonds

In the past, there has been great progress on the phosphonation of carbon radicals centered on C(sp²). These studies¹⁴ mainly developed the methods of phosphonation of aryl radicals (Scheme 1). Utilizing halogenated aromatic hydrocarbons and other aromatic compounds with leaving groups as radical precursors, aryl radicals are obtained via the single electron transfer (SET) process under visible light. The aryl radicals react with the phosphite ester to form the phosphorus radical intermediates, which produce the target compound when the alkyl radical leaves. Since the previous reviews have summarized this field to some extent, here we mainly elaborate on construction of C(sp³)–P bonds based on alkyl radicals.

Scheme 1 Construction of C(sp²)–P bonds using aryl radicals.

In 2018, Lei's group¹⁵ achieved C(sp³)–H phosphorylation of aniline derivatives under visible light catalysis (Scheme 2). Although this method did not involve the generation of alkyl radicals, this reaction mechanism is of great significance for the subsequent synthesis of alkyl phosphonates. It also promoted the development of visible light-induced radical reactions.

Scheme 2 P^III participation in previous work on the synthesis of alkyl phosphonates.

Visible light-induced catalysis is commonly used to convert carboxylic acids and their derivatives into alkyl radical intermediates.¹⁶ However, the application of this method to the synthesis of α-aminophosphonate esters is challenging due to the limitation of the substrate range and excessive oxidant usage.¹⁷ In order to overcome these problems, Aggarwal's group¹⁸ first developed an efficient method for the direct synthesis of α-amino phosphonate esters from activated derivatives of α-amino acids through a radical–polar crossover (RPC) process¹⁹ in 2022 (Scheme 3). This method combines photocatalytic radical–polar crossover with Arbuzov-type phosphorylation for synthesizing α-amino phosphonate esters. The method is compatible with a wide range of functional groups, and it can also be successfully transformed into non-typical branched chains with amino and carboxyl groups. A reaction pathway to the formation of C(sp³)–P bonds from phosphite ester is proposed: initially, PC is excited to the excited state (PC*) under blue light irradiation, leading to the protonation and single-electron reduction of 1 by PC* to generate alkyl radical 4 with the elimination of CO₂ and HPhth. Subsequently, radical 4 is oxidized by PC˙⁺ to obtain acyl imide cationic intermediate 5, which is attacked by phosphite to yield intermediate 7. The intermediate 7 is demethylated by the trifluoro-acetic acid anion to obtain the target product 2. At the same time, 5 and HPhth could form by-product 6, which is reversible under TFA, to again obtain 5.

Scheme 3 Photocatalytic decarboxylative phosphorylation via a radical–polar crossover.

In 2023, Wu's group²⁰ reported a method to construct C(sp³)–P bonds via intramolecular hydrogen atom transfer (HAT) (Scheme 4). The method can control intramolecular phosphonylation using the radical translocating group to trigger a kinetically favoured intramolecular HAT, which effectively avoids the intermolecular phosphonylation. Therefore, it resolves the incompatibility issues between phosphite reagents and HAT agents for the first time.²¹ The authors utilized the inexpensive and easily removed 2-iodobenzoyl group as an intramolecular HAT reagent, which promoted the generation of an α-amino radical and lead to the oxidative quenching pathway of PC required for the radical–polar crossover (RPC) process. A plausible mechanism was proposed: first, under light irradiation, PC transforms to an excited state (PC*) and undergoes single electron reduction with 8, producing PC˙⁺ and a transient aryl radical following the loss of an iodide ion. The facile 1,5-HAT process translocates the radical to the α-amino position, resulting in the key intermediate alkyl radical 10. Finally, radical 10 undergoes single electron oxidation with PC˙⁺ to undergo the RPC process. Iminium 11 is readily captured by trialkyl phosphite through nucleophilic attack to form 12, which is then dealkylated by iodide through an Arbuzov-type process to yield the target molecule 9. Notably, the reaction is acid-free and base-free, redox neutral, and shows broad substrate applicability for amines and phosphonate esters.

Scheme 4 Photocatalytic HAT-induced C(sp³)–H phosphonylation.

In addition to the aforementioned reaction mechanism of “intramolecular hydrogen atom transfer (HAT)”, Li's group²² introduced a novel protocol for C(sp³) phosphinylation in 2024, namely the radical oxidation mode (Scheme 5). By utilizing 4CzIPN as the photocatalyst, the reaction of NHPI esters with phosphonites leads to the production of decarboxylative phosphinylation products under visible light at room temperature. Compared with phosphites, this method requires the use of more stable phosphonite, which avoids the use of copper catalyst. It also showcases a broad range of substrates and compatibility with different functional groups, enabling the modification of intricate molecules and the swift creation of bioactive phosphinic acids. A proposed mechanism involves a radical–polar crossover initiated by aryl radicals and concluded with nucleophilic dealkylation. The rational mechanism of response is as follows: blue light irradiation of 4CzIPN generates the triplet excited state [4CzIPN]* that is oxidatively quenched by 13 to produce the [4CzIPN]˙⁺, alkyl radical 14, CO₂ and phthalimide anion. Subsequently, the alkyl radical 14 adds to dimethyl phenylphosphonite to form phosphoranyl radical 15 that then undergoes a single-electron reduction with the [4CzIPN]˙⁺ to provide phosphonium cation 16 and the ground state 4CzIPN. The following Arbuzov-type demethylation of 16 by the phthalimide anion leads to the product formation and the generation of N-methylphthalimide. In this method, the author notes that the target product can still be obtained using PhCO₂Li instead of 4CzIPN. Therefore, we speculate that the N–O bond is destroyed by the alkali under light irradiation, and the energy transfer process may occur, thus obtaining the target product. The use of 4CzIPN can promote the production of alkyl radical and complete the reaction cycle, and improve the utilization of the molecule.

Scheme 5 Phosphinylation via addition–oxidation.

In 2023, Aggarwal's group²³ developed a new kind of phosphonylation reagent “BecaP”, offering a new approach to synthesise alkyl phosphonate esters (Scheme 6). The reagent contains an aryl o-diol ligand which can stabilize the phosphorus center radical and carry a leaving group for oxidation or reduction. Based on this particular structure, “BecaP” enables reaction with both nucleophilic and electrophilic alkyl radical precursors. The possible reaction mechanism is as follows: upon light irradiation, PC transitions to an excited state (PC*). Alkyl borate 17 and NHPI ester 18 are promoted by PC* to produce alkyl radical 21 through the reduction or oxidation quenching pathway, respectively. Then 21 is combined with “BecaP” to obtain a phosphine radical 22, which can split to gain a diphenyl methyl radical 23 and intermediate 24via β-scission. Subsequently, 24 is ring-opened in the presence of MeOH to form a more stable alkyl phosphonate ester 19. Diphenyl methyl radical 23 can be further reduced to negative ion 26 or oxidized to positive ion 25 for catalytic cycling. This method demonstrates excellent functional group tolerance, and different alcohols (equivalent addition) are used, the corresponding products can be obtained with good yield. It can also be applied to the synthesis of the bioactive compound Phaclofen through a concise four-step procedure starting from Baclofen.

Scheme 6 Radical phosphonylation with “BecaP” reagent.

Almost simultaneously, Li's group²⁴ developed a new method to synthesize phosphonate esters with another different phosphation reagent (Scheme 7). Compared to Aggarwal's work, the process involves a photoredox-catalyzed reaction between various alkyl bromides or iodides and radical phosphonylation reagents. This versatile method demonstrates remarkable functional group compatibility and enables the efficient synthesis of pharmaceutical compounds like fosarilate and phosphonoalaine. Based on mechanism experiments, the following mechanism is proposed: (a) the visible-light-induced excitation of photocatalyst 4DPAIPN generates the triplet-excited state [4DPAIPN]*, that is reductively quenched by DIPEA to form the [4DPAIPN]˙⁻. [4DPAIPN]˙⁻ is further excited to the triplet-excited state that engages in the single-electron reduction with 27. Alkyl radical 33 is thus produced by the consecutive photoexcitation process along with the regeneration of the photocatalyst. Then 33 reacts with 29, which undergoes β-cleavage to produce 31 and radical 34. 31 is transesterified to obtain the product 32, while 34 is generated from the DIPEA radical cation to generate fluorene. (b) Photo-excitation of [Ir] generates the triplet-excited state [Ir]*, and is oxidatively quenched by 28 to give an iridium radical cation and a radical anion. The latter undergoes C–I bond cleavage to provide 33 and an iodide anion. Then 33 reacts with 30, which undergoes β-cleavage to produce 31 and the radical 23. 31 is transesterified to obtain the product 32. [Ir]˙⁺ oxidizes 23 to carbon cation 25 to return to the ground state [Ir] and completes the catalytic cycle. The 25 is trapped by a zinc acetate to deliver diphenylmethyl acetate. Unlike alkyl iodides, alkyl bromides are inert under the conditions of method b. The advantage of the phosphine reagent used in the method is that the large substituent groups on phosphine are easy to form stable carbon cations, thus realizing rapid removal. Secondly, the stable and synthetically more valuable dimethyl alkylphosphonate is obtained by the second step. The corresponding phosphonic acid compounds can be obtained by hydrolysis under acidic conditions.

Scheme 7 Visible light-induced dehalogenation of various alkyl bromides and iodides.

In recent years, many dual catalytic systems of photoredox and transition metals have been developed to construct C–X bonds.²⁵ In 2020, Li's group^25d reported a Cu/photoredox dual catalytic approach to build C–SO₂ bonds starting from NHPI esters and sodium sulfinates. On this basis, Li’s group²⁶ went on to report the dual catalytic approach to construct C(sp³)–P bonds in 2023 (Scheme 8). In this catalytic cycle of transition metals, there is an oxidative addition/reduction elimination path between the copper catalyst and phosphite ester. Based on control experiments, the possible mechanism of this reaction is as follows: the visible-light-induced excitation of 4DPAIPN generates the triplet-excited state [4DPAIPN]* that is oxidatively quenched by the NHPI ester 35 to give the [4DPAIN]˙⁺ and 37. With the N–O bond cleavage of 37 and elimination of CO₂, 37 generates the corresponding alkyl radical 38. Simultaneously, triethyl phosphite forms a complex with Cu(I) and undergoes the single electron transfer (SET) process with [4DPAIN]˙⁺ to form 4DPAIN and the Cu(II) complex. The Cu(II) complex then interacts with 38 to produce 39, which further reacts with the benzoic acid anion to give 36, regenerating Cu(I). The protocol shows a broad range of compatible functional groups and substrate scope, enabling modifications of intricate molecules during later stages. For example, the NHPI ester of anti-inflammatory medication fenbufen was effectively changed into a phosphonate ester with a yield of 76%. Likewise, oleic acid was converted to a phosphonate ester with a 73% yield. The technique also could successfully transform steroids into the corresponding phosphonate esters such as dehydrolithocholic acid and dehydrocholic acid.

Scheme 8 Visible light-induced dehalogenation of various alkyl bromides and iodides.

In 2024, Li and co-workers²⁷ published a study on the remote C(sp³)–H phosphonylation of amides via amidyl radical-mediated 1,5-HAT (Scheme 9). Amidyl radical-mediated 1,5-HAT has proven to be a powerful tool for selective C–H functionalization.²⁸ In a previous article (Scheme 4), Wu's work²⁰ mainly uses the conversion of phenyl radicals to alkyl radicals. But for this work, transfer from the N-centered radicals to the C-centered radicals using the 1,5-HAT process is used to achieve the phosphonylation of unactivated C(sp³)–H bonds. The method demonstrates high tolerance to a wide array of functional groups. The wide range of functional group compatibility allows for the direct alteration of complex molecules or derivatives from natural products. For example, successful synthesis has been achieved with both oleic acid and ibuprofen derivatives. Notably, this method maintains the integrity of allylic or benzylic C(sp³)–H bonds, showcasing the impressive selectivity in chemical transformations. Control experiments suggest a mechanism involving the addition of alkyl radical 40 to phosphite ester followed by β-scission. Radical 40 is likely generated through amidyl radical-mediated 1,5-HAT. One possible explanation involving the three-state excitation of the one-electron reduction process between 4DPAIPN and N-(acyl) amide is feasible. But the reducing agents needed to complete the catalytic cycle are still unknown.

Scheme 9 Phosphonylation of unactivated C(sp³)–H bonds.

Alcohols are cheap and easy to obtain, and are an important starting material for the synthesis of various complex molecules. Due to the large energy of the C–O bond (about 95 kcal mol⁻¹) and high REDOX potential of alcohols, it is difficult to directly produce alkyl radicals in the reaction. So, for alcohols, alkyl radicals can be produced by pre-esterification and decarboxylation, such as the oxalic ester, thiol ester and NHPI ester.²⁹ In 2024, MacMillan's group³⁰ introduced a strategy for the direct activation of alcohols to build C(sp³)–P bonds by NHC under photocatalysis (Scheme 10). The “plug-and-play” approach presented in this study represents a novel and practical method for enhancing molecular synthesis. By harnessing the in situ activation of alcohols and O=P(R₂)H motifs, researchers can expedite the development of intricate molecular structures with greater efficiency and ease. The proposed mechanism involves the rapid condensation of alkyl radical precursor alcohol 41 with NHC to form activated alcohol adduct 43. Simultaneously, in a separate reaction system, 47 is prepared using a phosphating reagent and diphenylmethyl derivative, which is then added to the system containing 43. Under blue light irradiation, [Ir]^III is converted to *[Ir]^III, initiating a single-electron oxidation process with 43, leading to the formation of heterocyclic radical 44 and [Ir]^II. Subsequent steps involve the generation of alkyl radical 45 and the formation of heavy aromatized by-product NHC, through β-cleavage. This is followed by the reversible addition of 45 to 47, resulting in the formation of 46. An irreversible β-cleavage driven by a weak C–O bond yields product 42 and 19. Finally, 19 undergoes a single-electron reduction process with [Ir]^II to reduce to 22, regenerating [Ir]^III to complete the reaction cycle. In this reaction, different phosphine hydrogen compounds and corresponding thiocarbonyl derivatives are compatible. It is worth noting that some sugars, nucleosides and drug molecules can obtain corresponding products with yields of 37%–89% in this reaction.

Scheme 10 Deoxyphosphonylation of alcohols.

2.2 P^V involved in the construction of C–P bonds

In 2011, Rueping's group³¹ achieved the first C–H phosphorylation at the benzyl position using the trapping agent (P^V) (Scheme 11). The approach involves the generation of an imide ion via nitrogen cation radicals, which are subsequently trapped by P^V. This methodology serves as a valuable reference for future investigations on C(sp³)–P bond formation using alkyl radicals and P^V.

Scheme 11 P^V participates in the synthesis of alkyl phosphonates.

Amino acids are organic compounds found in nature that are vital for life.³² While the decarboxylation of α-amino acids has been extensively researched in the past, achieving photocatalytic decarboxylation of α-amino acids without the use of metal and base has been a challenge.³³ In 2024, Yang and coworkers³⁴ introduced a practical method for decarboxylative phosphorylation of N-aryl glycines to obtain α-amino phosphine oxides using visible-light photoredox catalysis (Scheme 12). The method involves a coupling reaction between the generated phosphine radical and the carbon radical to synthesize alkyl phosphonate. This differs from MacMillan's method³⁰ which involves pre-activation of P^V. This method is compatible with hydroxyl and ester groups, but not with N-alkyl glycine. This may be due to the high reactivity of carbon radicals formed during decarboxylation and the need for stabilization of aromatic rings. Furthermore, this protocol can be easily scaled up to the gram scale, all avoiding the use of metal and base. The possible mechanism obtained according to the control experiments is as follows: initially, MB⁺ is excited by visible light to generate the excited state *MB⁺, which then undergoes single-electron oxidate decarboxylation to form 48 (or its ionic form) along with the formation of 50 and MB˙. The reduced MB˙ is oxidized by O₂ to complete the catalytic cycle and release the O₂˙⁻. Simultaneously, the resulting O₂˙⁻ combines with phosphine reagent to produce 51 and HOO˙ via the HAT process. Finally, the desired product 49 is obtained via a cross-coupling of 51 and 50. According to a previous summary,³⁵ it was found that the pK_a of different phosphate esters (P^V) is not the same. The pK_a of diphenylphosphine oxide is 20.6, and the pK_a of dimethyl phosphite is 18.4, which can explain why this method can react better with diphenylphosphine oxide but cannot get a good result with dimethyl phosphite.

Scheme 12 Photocatalytic decarboxylative α-phosphorylation of N-aryl glycines.

2.3 P₄ involved in the construction of C–P bonds

White phosphorus (P₄) is commonly used as the primary source of phosphorus atoms in the synthesis of valuable organophosphorus compounds.³⁶ The conventional stepwise processes for producing alkylated phosphines from P₄ are associated with environmental, safety, and selectivity challenges.³⁷ In 2022, Tang and coworkers³⁸ developed a novel photoinduced selective alkylation method for P₄, utilizing cost-effective carboxylic acid derivatives as alkylating agents to yield dialkylphosphines and trialkylphosphines (Scheme 13). The innovative phosphorylation process demonstrates one-pot functionality, exceptional product selectivity, and the ability to work with various alkyl NHPI esters, even those originating from intricate natural compounds and medications. The effectiveness of this research approach relies on the subtle non-covalent bonding between NaI, HE, and NHPI esters, generating carbon radical 52 under visible-light irradiation in the presence of EDA complex. Then 52 undergoes the HAT process with P₄ to produce dialkylphosphines and trialkylphosphines, which are further oxidized to obtain alkyl phosphine oxides. This efficient approach enables the direct installation of diverse alkyl substituents with a wide range of functional groups at phosphorus, using P₄ as the P-atom source, thereby avoiding the traditional chlorination stage.³⁹ Therefore, we expect that the various organophosphorus compounds obtained by this method can be used as common phosphorylation reagents to promote the synthesis of complex natural products and drug molecules.

Scheme 13 Visible light-induced synthesis of alkyl phosphine oxides from P₄.

In 2023, Tang and coworkers⁴⁰ proposed a strategy for the direct functionalization of white phosphorus (P₄) catalyzed by visible light (Scheme 14). Dialkylphosphines with various structures were synthesized using alkyl iodide as the starting material using 4CzIPN as the catalyst and HE as the reducing agent. But unfortunately, compared with the last work,³⁸ this method is not able to produce trialkylphosphines. The reaction mechanism of this visible light promoting selective alkylation of P₄ is as follows: under the irradiation of blue light, 4CzIPN produces the corresponding excited state 4CzIPN*, which is then quenched by HE to produce HE˙⁺ and 4CzIPN*⁻. HE˙⁺ reacts with alkyl iodide to produce 53 and HE⁺. Then, the tetrahedral structure of P₄ is destroyed by 53, which produces unstable 54, and 55 is produced by the single electron transfer (SET) process. 55 reacts with alkyl iodide to produce phosphine hydride, which is oxidized to a dialkylphosphine in air. The method has better conversion of phosphorus and good functional group compatibility.

Scheme 14 Visible-light-induced phosphorylation of unactivated alkyl iodides with P₄.

In 2023, Zhang's group⁴¹ reported a direct benzylation method of P₄ under visible light (Scheme 15). P₄ was directly functionalized via the metallaphotoredox catalysis strategy, which obtained organophosphorus compounds. According to the control experiments, the possible mechanism is as follows: at first, 4CzIPN is excited to 4CzIPN* under irradiation of blue light, then 4CzIPN* is quenched by HE or DIPEA to produce 4CzIPN*⁻, which reacts with Ti^IV to produce Ti^III. Then benzyl bromide reacts with Ti^III to form the key benzyl radical 56 through the halogen atom transfer (XAT) process. Finally, the benzyl radical 56 is directly combined with P₄ to form tetrabenzylphosphonium salts. The method can directly synthesize the Witting reagent, which can react with aldehydes or ketones to prepare a series of styrene derivatives.

Scheme 15 The metallaphotoredoxcatalysis strategy of P₄.

Although the above studies are of great significance, it is still challenging to develop more methods to selectively synthesize organophosphorates from white phosphorus (P₄). In 2024, Zhang's group⁴² synthesized phosphoryltriacetates using α-bromo esters and P₄ as raw materials via a one-step process (Scheme 16). The mechanism study shows that Ir^III is converted to *Ir^III under light irradiation, and the single electron transfer (SET) process occurs with α-bromo ester to produce 57 and Ir^IV. The alkyl radical 57 reacts with P₄ to produce phosphine bromide, which eliminates HBr under the action of sodium acetate, resulting in 58. 58 forms a quaternary ring 59, which is accompanied by the elimination of acetylene ether to obtain the product. In the system, bromine ions and acetate ions are oxidized by Ir^IV to produce different kinds of radicals and Ir^III. These radicals are coupled to produce different kinds of by-products.

Scheme 16 Visible light-induced synthesis of phosphoryltriacetates from P₄.

3. Conclusions

In recent years, organic chemists have made significant progress by utilizing various strategies in photocatalysis to form C–P bonds involving alkyl radicals. These strategies include β-cleavage, Cu/photoredox dual catalysis, radical–polar crossing, radical-oxidation, and intramolecular HAT. Utilizing various alkyl radicals with phosphine-containing compounds of different valences (P^III, P^V, P₄), researchers have successfully obtained alkyl phosphonate esters. Some research groups have also introduced new phosphonylation reagents to participate in the synthesis of alkyl phosphonate esters. These methods offer mild reaction conditions, simple procedures, high functional group tolerance, and the ability to complex with natural molecules and drug intermediates. Despite the significant advancements made by organic chemists in this field, several challenges lie ahead. Firstly, the variety and origins of carbon-centered (sp³, sp², sp) radicals remain constrained, prompting further exploration. Secondly, the potential for developing additional phosphorus-containing reagents for phosphonate synthesis warrants investigation. Lastly, the feasibility of achieving direct catalytic cracking through visible light without the addition of a photocatalyst, is a key area of interest. In conclusion, there are numerous challenges in this research area, providing ample opportunities for exploration by organic chemists. We hope that this review will illustrate the further development of C(sp³, sp², sp)–P bond construction methods.

Data availability

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

Author contributions

Shi-Yi Zhao, Yi-Yun Huang and Shi-Hao Deng wrote the initial draft. Zhi-Bing Dong and Zhi-Peng Guan revised the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the Natural Science Foundation of Hubei Province (2024AFB315), Open Research Fund of School of Chemistry and Chemical Engineering, Henan Normal University (2020ZD02), Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province (CSPC202306), Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (BM2012110), Innovation and Entrepreneurship Training Program Funded by Wuhan Institute of Technology (202310490008), Wuhan Science and Technology Bureau and the Open and Innovation Fund of Hubei Three Gorges Laboratory (SC240004) is greatly appreciated.

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Footnote

† These authors contributed equally to this work.

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