Organophosphorus compounds (OPCs) are broadly defined as compounds that incorporate phosphorus-carbon (P—C) bonds, as well as phosphoric acid derivatives bearing organic substituents, such as phosphonates (with P— O—C linkages) and phosphonamides (containing P—N—C bonds).^[1] OPCs play crucial roles in organic synthesis,^[2-5] materials chemistry,^[6-7] pharmaceutical chemistry,^[8-11] and other fields. Compared to P-hetero bonds formation which could be conveniently accessed by nucleophilic substitution reactions of heteroatom nucleophile with halogenated OPCs,^[12] P—C bond construction seems to be more challenging, and methodologies for P—C bonds construction have garnered increasing attention.^[13-14] Traditional P—C bond formation relies on a multi-step sequence: the initial chlorination of white phosphorus (P₄) to generate PCl₃, subsequent conversion to dialkyl phosphites, and final P—C bond construction via hydrophosphination. Notable advancements have been achieved in the direct synthesis of OPCs from P₄,^[15-21] contemporary research on hydrophosphination predominantly focuses on pentavalent phosphorus oxides.^[22-32] The direct construction of P—C bonds using trivalent phosphine hydrides remains rarely reported.^[33-39] Transition metal catalysis has been proven powerful for OPCs synthesis especially for P—C(sp²) bond formation.^[40-49] However, for the construction of P—C(sp³) bond, radical approaches would be more efficient due to the easy generation of phosphorus radical and high reactivity of the open-shell organophosphorus radical species.^[50-59]

Hydrophosphination serves as an atom-economical approach to construct P—C bonds, allowing the preparation of OPCs via addition of P—H bonds to unsaturated substrates such as olefins or alkynes.^[60-64] Although several strategies have been developed for olefin hydrophosphination, most rely on the nucleophilic addition of trivalent H-phosphines to Michael acceptors or proceed through transition-metal complex intermediates.^[35-38] In contrast, the direct radical addition between H-phosphine molecules and olefins is a well-established and highly efficient route for forming P—C(sp³) bonds, which is believed to proceed via a phosphinyl radical intermediate.^[65-67] Recently, we reported a photocatalytic method for the straightforward synthesis of phosphonium salts through radical hydrophosphoniumylation of alkenes under acidic conditions. According to our density functional theory (DFT) calculations, a highly reactive phosphine radical cation (PRC) generated in situ adds to unactivated alkenes in a kinetically barrierless manner to form the P—C(sp³) bond, followed by a hydrogen atom transfer (HAT) step to furnish the phosphonium salt product. This reaction offers a notable alternative and complement to existing synthetic routes to phosphonium salts (Scheme 1a).^[68] We therefore questioned whether a H-substituted phosphine radical cation, once generated, could react with unsaturated systems through a similar mechanism to afford protonated trivalent phosphine intermediates. Subsequent deprotonation would then deliver the corresponding trivalent phosphine products (Scheme 1b).

We first selected diphenylphosphine (1a) and 1-dode- cene (2a) as model substrates to optimize the reaction conditions (Table 1). Using 1 mol% photocatalyst [Ir[dF(CF₃)- ppy]₂(dtbbpy)]PF₆, 2.0 equiv. of CF₃SO₃H as an additive, and 40 mol% PhS-SPh as the HAT reagent, the reaction in acetonitrile under blue LED irradiation at room temperature provided the desired hydrophosphination oxidation product 4a in 89% yield as determined by ³¹P NMR analysis (Entry 1). In contrast, when 2,4,6-trimethylphenyl thiol was employed as the HAT catalyst instead of PhS-SPh, the yield decreased significantly to 31% (Entry 2). Reducing the amount of CF₃SO₃H to 1.0 equiv. or replacing it with weaker acids such as acetic acid or trifluoroacetic acid, led to lower yields (Entries 3~5). Decreasing the loading of 2a from 3.0 equiv. to 1.5 equiv. similarly reduced the yield to 39% (Entry 6). Other photocatalysts, including Ir(ppy)₃, 4-CzIPN, and Eosin Y, proved less effective than [Ir[dF- (CF₃)ppy]₂(dtbbpy)]PF₆, resulting in diminished yields (Entries 7~9). Acetonitrile was identified as the optimal solvent, outperforming alternatives such as dichloroethane (DCE), N,N-dimethylformamide (DMF), and tetrahydrofuran (THF) (Entries 10~12). Finally, control experiments confirmed that both visible light and the photocatalyst are essential for the reaction to proceed efficiently (Entries 13, 14).

Under the optimized reaction conditions, the substrate scope of the alkene hydrophosphination reaction was next investigated (Table 2). Variations at the alkene C(2) position demonstrated that both primary and secondary alkyl substituents were compatible, furnishing the corresponding products 4b and 4c in 91% and 75% yields. Phenyloxy group was also tolerated under the photo catalytic condition, delivering product 4d in 81% yield, indicating that electron-rich substituent does not impede the radical addition process. Olefins bearing aromatic substituents (4e) reacted smoothly in 83% yield, highlighting the compatibility of π-extended systems. Likewise, substrates containing ester groups (4f) and carboxyl groups (4g), afforded the corresponding tertiary phosphine products in good to excellent yields (73%, 95%). These results underscore the method’s robustness to electron-withdrawing carbonyl functionalities. In addition, olefins bearing sulfone substituents provided product 4h in 83% yields. Notably, sensitive functional group like cyano (4i) was also compatible under the photochemical conditions, delivering the desired products in 65% yields. These examples further highlight the mild conditions of the reaction and its applicability to late-stage functionalization. Heterocyclic substituted olefins also participated successfully in this transformation. Nitrogen-containing heterocycles afforded products 4j in 51% yields. Furthermore, strained bicyclic olefins such as norbornene were suitable substrates, providing product 4k in 55% yield. This result illustrates the capacity of the reaction to engage rigid and sterically constrained frameworks. Additionally, the reaction employing dicyclohexylphosphine proceeded efficiently, affording product 4l in 75% yields.

Subsequently, gram-scale reactions employing 9 mmol of 1a and 3 mmol of 2a were performed to verify the synthetic practicality of this photo-induced hydrophosphination strategy. Gratifyingly, the target product 4a could be isolated in a satisfactory yield of 76%, highlighting the scalability of this protocol (Scheme 2a). Moreover, the synthetic transformations of the protonated trivalent phosphine were investigated via a one-pot two-step procedure. As illustrated in Scheme 2b, the corresponding borane adduct and sulfide products 6 and 7 can be readily prepared. Additionally, 3a can react with benzyl bromide to furnish the corresponding quaternary phosphonium salts 8. These results clearly demonstrate the promising synthetic application potential of the developed photo-induced hydrophosphination strategy.

On the basis of our previous work^[68] and relevant litera-ture reports,^[69-70] a plausible reaction mechanism was proposed involving the addition of phosphine radical cations to carbon-carbon double bonds as the key step (Scheme 3). To validate this hypothesis, the reaction process was monitored by ³¹P NMR spectroscopy, which confirmed the formation of protonated trivalent phosphine intermediate 3a. Meanwhile, the characteristic signals of the deprotonated trivalent phosphorus species 5a were also observed.

In conclusion, a novel visible-light-promoted hydropho- sphination of unactivated alkenes was developed, providing efficient access to a series of tertiary phosphine deriva-tives with moderate to excellent yields (48%~95%). The reaction proceeds under mild conditions and tolerates a wide range of functional groups, demonstrating its synthetic utility in organophosphorus chemistry. Based on prior mechanistic studies, a radical pathway is proposed, in which the key step is the addition of a phosphine radical cation (PRC), generated via single-electron oxidation of the trivalent phosphine under the photo catalytic condition, across the carbon-carbon double bond.