Nickel/photoredox-catalyzed carbonylative transformations of α-phosphorus-, α-sulfur-, and α-boron-substituted alkyl halides

Abstract

Organophosphorus compounds are important motifs in living organisms, medicinal chemistry, agricultural chemistry, materials science, catalysts, ligands, etc. However, catalytic carbonylative transformation of α-phosphorus, α-sulfur or α-boron substituted alkyl halides remains a formidable challenge due to α-heteroatom effects. In this report, we describe a nickel/photoredox dual-catalytic strategy for the direct amino- and alkoxycarbonylation of α-phosphorus, α-sulfur, and α-boron substituted organohalides with an array of reaction partners under low CO gas pressure which furnished various high-value products in excellent yields. The utility of this process was also demonstrated by the development of a new α-phosphine amide ligand. Additionally, this synergistic protocol also facilitates a sequential four-component carbonylation in the presence of vinyl phosphonate.

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Introduction

Phosphorus is one of the essential elements for life and is closely related to living organisms. In addition, organophosphorus compounds are not only important structural motifs of genes, but also widely used in medicinal chemistry, agricultural chemistry, materials science, organic synthesis, and other fields.¹ In particular, β-phosphonyl acids and derivatives, an indispensable class of phosphorus skeletons, are widely utilized as ligands and key intermediates in organometallic species-mediated reactions due to their unique chemical properties.² Thus, developing efficient strategies to access β-phosphonyl acids from readily available starting materials remains an important task. One of the most attractive approaches is the use of broadly available carbon monoxide as the C1/carbonyl source toward organophosphorus molecules.

Carbonylation reactions have become indispensable tools for constructing carbonyl-containing compounds in organic and medicinal chemistry as they enable the efficient and robust union of molecular fragments and carbon monoxide.³ Over the last few decades, multiple generations of catalytic systems have been explored that have elevated the transition metal-catalyzed carbonylation of organohalides to an essential transformation.⁴ Compared with mature noble metal catalysts, cheap metal catalysts such as nickel have also been explored successfully by taking advantage of slow CO-releasing reagents and specialized ligand complexes to minimize the generation of highly toxic and low catalytic activity Ni(CO)_n.⁵ Concerning the substrates applied, aryl,⁶ benzylic,⁷ and alkyl halides⁸ have been relatively well studied, even with nickel catalysts (Fig. 1(a)); however, carbonylative transformations of α-heteroatom substituted organohalides to construct high-value α-heteroatom substituted amides or esters remain less developed, with some examples of α-phosphorus- and α-sulfur-substituted alkyl halides.⁹

Fig. 1 The background of α-heteroatom halide carbonylation.

α-Heteroatom functionalization is a key and challenging strategy in organic synthesis.⁹ However, because of the unique properties (electron cloud density, bond energy, resonance, etc.) of heteroatoms, the substituents containing heteroatoms could change the properties and reaction characteristics of the molecules, especially in adjacent positions.^9c,10 In addition, the coordination of π-acidic CO and heteroatoms with metal catalysts might decrease or even inhibit metal catalytic activity.¹¹ On the other hand, the rate of decarbonylation depends strongly on the nature of substituents, and the α-heteroatoms can effectively stabilize the adjacent carbon radicals, resulting in acyl radicals that tend to decarbonylate to form a stable radical species, especially at lower CO pressures or higher temperatures.^12,13 Additionally, owing to the relatively more polar carbon–halogen bonds, α-heteroatom substituted alkyl halides can readily undergo dehaloprotonation (Fig. 1(b)). To overcome these challenges, the development of novel and efficient strategies is highly in demand.

Considering the challenges discussed above, we questioned if photoredox catalysis may offer a unique pathway for the carbonylation of α-heteroatom substituted alkyl halides. Synergistic photoredox catalysis and metal catalysis offer a powerful catalytic platform for many challenging organic transformations. Several outstanding studies have shown that pairing a visible-light photocatalyst with a conventional nickel or palladium catalyst can accelerate challenging steps in reactions.¹⁴ Herein, we report the first example of nickel/photoredox dual-catalyzed carbonylation of α-heteroatom substituted alkyl halides with various nucleophiles under low CO pressure, furnishing a series of high-value compounds (Fig. 1(c)). Notably, this catalytic strategy also enables the multicomponent carbonylation of vinyl phosphonate and affords the target compounds in moderate yields and with high selectivity.

Results and discussion

To establish this transformation and explore the optimal conditions, bromophosphate 1a and aniline 2a were selected as the model substrates. After a systematic evaluation of all the reaction parameters, the optimized reaction conditions were determined, as summarized in Table 1 (for more details, see the ESI†). Lower yields were obtained with other nickel catalysts than those with Ni(acac)₂ (Table 1, entries 2 and 3). The nature of the ligand played an essential role in improving the yield of this reaction, and the bidentate nitrogen ligand L2 resulted in decreased yield (Table 1, entry 4). Next, Acr-Mes + ClO₄⁻ made the reaction almost impossible to occur (Table 1, entry 5). When fac-Ir(ppy)₃ instead of 4-CzIPN was tested, a similar yet slightly diminished reactivity was observed (Table 1, entry 6). Diminished yield or no product was detected when using THF, DMAc, and DCE (the dehaloprotonation product was the main by-product), but PhCF₃ could give a good yield (Table 1, entries 7 and 8). Screening of the bases highlighted the significant promotion of this carbonylation by Cs₂CO₃ (Table 1, entry 9). Notably, a good yield was also obtained with only 1 bar CO (Table 1, entries 10 and 14). Further assessment on the amount of bromophosphate indicated that 1.5 equivalents performed the best (Table 1, entry 11). In addition, the best yield of the desired product was observed in the presence of 10 mol% KI (Table 1, entry 12). We hypothesize that the reaction begins with the nucleophilic substitution of bromophosphate 1a with KI to form the corresponding iodophosphate. The desired product could not be detected under 60 bar CO in the presence of nickel or a photocatalyst (Table 1, entry 14). Experiments without a photocatalyst, nickel catalyst, ligand, Cs₂CO₃, or light failed to produce the desired carbonylated product, implying that all these components are necessary for the reaction to proceed (Table 1, entry 15). It is worthy of mention that easily accessible chiral dioxazole ligands were tested with the idea of introducing a chiral center, but low or no yield of the desired product was obtained without enantioselectivity.

Table 1 Investigation of reaction conditions^a

Entry	Variation from standard conditions	3a (%)
a Reaction conditions: 1a (1.2 equiv.), 2a (0.2 mmol), Ni(acac)2 (5 mol%), L1 (6 mol%), 4-CzIPN (1 mol%), Cs2CO3 (1.5 equiv.), CO (10 bar), MeCN (2 mL), 30 W blue LEDs, 18–25 °C, 24 h, isolated yields. acac = acetylacetone; TMHD = 2,2,6,6-tetramethyl-3,5-heptanedione. b 60 bar CO.
1	None	68
2	NiBr2(DME) instead of Ni(acac)2	17
3	Ni(TMHD)2 instead of Ni(acac)2	36
4	L2 instead of L1	20
5	Acr-Mes^+ClO4^− instead of 4-CzIPN	2
6	fac-Ir(ppy)3 instead of 4-CzIPN	59
7	PhCF3 as solvent	62
8	THF, DMAc, DCE as solvent	0–10
9	DiPEA instead of Cs2CO3	0
10	1 bar CO instead of 10 bar CO	60
11	1.5 equiv. of 1a was used	75
12	With 10 mol% KI	93
13	1 bar CO, 1.5 equiv. of 1a, and 10 mol% KI	82
14^b	w/o Ni and PC	0
15	w/o PC or Ni or L1 or Cs2CO3 or light	0

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Encouraged by these results, we first investigated the scope of various nucleophiles under the optimized conditions (Fig. 2). Various aromatic amines substituted with an electron-donating group or an electron-withdrawing group were all suitable coupling partners. A variety of substituents including –F (3d), –Cl (3e), –Br (3f), and –CF₃ (3g) survived, providing possibilities for further derivatization. The sterically bulky amine 3h also reacted equally well. In addition, a wide range of strongly nucleophilic alkylamines could be used, including benzylamine (3m), butylamine (3n), and amantadine (3o). Gratifyingly, heterocycle-containing substrates that readily coordinate with metals could successfully participate in this transformation to deliver the corresponding products in good yields (3p, 3q, 3r, and 3s). Next, some alcohols were also tested, such as long chain alcohols (4a, 4b, and 4c), cycloalcohols (4d and 4e), 3-methoxy-1-propanol (4f), chiral alcohol (4g) and benzyl alcohol (4h), giving the target products in moderate to excellent yields. Only a trace amount of the carbonylated product was detected when using sterically bulky tert-butanol as the substrate. In addition, phenols, which readily quench radicals, were also suitable substrates for this transformation (4j, 4k, 4l, and 4m). However, tert-butanol led to only a trace amount of the desired product (4i). Subsequently, we turned our attention to challenging nucleophiles. Several alcohols with various sensitive functional groups including trimethylsilyl (–TMS) and halogen atoms (–Cl and –I) were converted into the corresponding products in moderate to good yields (4n, 4o, and 4p). CD₃OD was successfully converted into the corresponding D-containing product (4q) in 69% yield. Notably, the catalytic efficiency was unaffected when substrates with a carbon–carbon double bond (4r, 4s, 4t, and 4u) were used. Finally, we explored the reactivity of diols, and 1,6-hexanediol successfully delivered the diester product in good yield (4v). However, no target product could be obtained when thiophenol was used as the nucleophile.

Fig. 2 Substrate scope. ^a Reaction conditions: 1a (1.2 equiv.), 2 (0.2 mmol), Ni(acac)₂ (5 mol%), L1 (6 mol%), 4-CzIPN (1 mol%), Cs₂CO₃ (1.5 equiv.), KI (10 mol%), CO (10 bar), MeCN (2 mL), 30 W blue LEDs, 18–25 °C, 24 h, isolated yields. ^b PhCF₃ (2 mL). ^c 1a (2.4 equiv.), 1,6-hexanediol (0.2 mmol), Ni(acac)₂ (10 mol%), L1 (12 mol%), 4-CzIPN (2 mol%), Cs₂CO₃ (3 equiv.), KI (20 mol%), CO (10 bar), PhCF₃ (3 mL).

The conditions shown in Table 1 were then used for the transformation of various α-heteroatom substituted halides (Fig. 3). A series of phosphates with different steric hindrance, including diethyl, diisopropyl (5a), and dibutyl (5b) phosphonates, could be tolerated in this reaction. Other α-alkyl substituents of bromophosphates, such as long chain alkanes (5d, 5e, 5g, and 5j), cycloalkane (5f), heteroatom substituted alkane (6h), and halogen atom substituted alkane (5i), could be used in this reaction. However, both internal alkene (5k) and terminal alkene (5l) were all well tolerated in the transformation. Subsequently, α-bromoalkyldiarylphosphine oxides were successfully employed in this reaction, affording the target products in good to excellent yields (5m, 5n, 5o, 5p, 5q, and 5r). To demonstrate the practicality of this transformation, several natural products and bioactive molecules were also tested (Fig. 3). Aminoglutethimide (6a), sulfalen (6b), and amino acid derivatives (6c and 6d) were all suitable substrates. Likewise, geraniol (6e), phytol (6f), cholesterol (6g), menthol (6h) diacetonefructose (6i), lanosterol (6j), and epiandrosterone (6k) also reacted smoothly.

Fig. 3 Substrate scope of α-phosphonates and complex molecules. Reaction conditions: 1a (1.2 equiv.), 2 (0.2 mmol), Ni(acac)₂ (5 mol%), L1 (6 mol%), 4-CzIPN (1 mol%), Cs₂CO₃ (1.5 equiv.), KI (10 mol%), CO (10 bar), MeCN (2 mL), 30 W blue LEDs, 18–25 °C, 24 h, isolated yields. ^a PhCF₃ (2 mL).

Next, we found that various sulfur-containing compounds were also suitable carbonylated substrates (Fig. 4). Good yields were obtained with different functionalized α-bromo sulfone compounds (7a, 7b, 7c, 7d, and 7e). Finally, a carbonylative homologation of halomethylorganoborons, which are also important C1 reagents,¹⁵ was tested in this transformation. However, only acetylation products (7f and 7g) were obtained in good yields, which formed through a base-promoted protodeboronation of the originally produced carbonylative α-acylboron products.¹⁶

Fig. 4 Testing of reaction diversity. Reaction conditions: 1a (1.2 equiv.), 2 (0.2 mmol), Ni(acac)₂ (5 mol%), L1 (6 mol%), 4-CzIPN (1 mol%), Cs₂CO₃ (1.5 equiv.), KI (10 mol%), CO (10 bar), MeCN (2 mL), 30 W blue LEDs, 18–25 °C, 24 h. ^a PhCF₃ (2 mL). ^b Yield reported post-deboronation. ^c R-Br (0.2 mmol), Cs₂CO₃ (3.5 equiv.), PhCF₃ (2 mL). ^d Vinyl phosphonate (1.5 equiv.), RF-I (2.5 equiv.), PhCF₃ (2 mL).

According to the literature, alkyl halides can be converted to the corresponding alcohols via oxyanions in the presence of bases.¹⁷ To our delight, we attempted to use alkyl halides instead of alcohols and the corresponding products were delivered in moderate yields (Fig. 4(c); 8a, 8b, 8c, and 8d). Catalytic carbonylative multicomponent reactions (CMCRs) represent a powerful and efficient strategy for the rapid construction of carbonyl-containing products in a single operation.¹⁸ Notably, a nickel/photoredox dual-catalyzed four-component α-heteroatom carbonylation reaction was also successfully achieved, and the target products were obtained in moderate yields (Fig. 4(d); 9a and 9b). In this way, the introduction of three useful fragments of phosphine, carbonyl, and fluorine into one molecule can be achieved.

To demonstrate the practicality and synthetic utility of this methodology, the carbonylation was performed on the 1 mmol scale and the target product 3a was delivered in 73% yield (Fig. 5(a)). Subsequently, we synthesized the Wittig–Horner reagent via a one-step reaction (Fig. 5(b)). Next, we successfully obtained the phosphine ligand 11 in 63% yield through the reduction reaction (Fig. 5(c)). The obtained phosphine ligand is a valuable and potential ligand in organic synthetic chemistry.¹⁹

Fig. 5 Synthetic applications.

Subsequently, to better understand the pathway of this transformation, several control experiments were performed, as presented in Fig. 6. First, radical inhibition experiments were carried out by adding radical scavengers, and the results indicated that the reaction possibly proceeded via a radical pathway (Fig. 6(a)). The captured radical intermediate was detected by GC-MS and HRMS. Next, competition experiments between primary and secondary alkyl bromides with aniline were performed, and the primary substrate gave a slightly better yield (Fig. 6(b)). In order to test the reactivity of 3-iodopropanol, we carried out the reaction in the presence of 1.5 equivalents and 3.5 equivalents of Cs₂CO₃, respectively, and the corresponding product 8e was not detected in both cases (Fig. 6(c)). Finally, control experiments were performed to compare the current catalytic conditions with the several previously reported reactions. The developed strategy (Fig. 6(d); entry 1) afforded the desired product in high yields, whereas the previously reported carbonylation protocols (Fig. 6(d); entries 2–5) could not yield the target product (for more details, see the ESI†).

Fig. 6 Control experiments.

Although elucidation of the detailed mechanism requires further studies, we proposed a possible catalytic pathway for this carbonylation reaction (Fig. 7), based on the above mechanistic studies and related literature.²⁰ First, blue light irradiation of the photoredox catalyst would generate the excited-state PC*, which will oxidize the NiI species via a SET process to generate Ni^II species. Subsequently, the α-phosphonate alkyl radical A was delivered from α-bromophosphate through a SET reduction process. Then, the α-phosphonate alkyl radical A was trapped by CO to give the carbamoyl radical B, which was then quickly intercepted by the Ni^II species generated above to generate intermediate D. Alternatively, the α-phosphonate alkyl radical A was intercepted by the Ni^II species to afford the alkyl Ni^III intermediate C, followed by migratory insertion with CO to deliver the same acyl species D. Next, the acyl complex D reacted with nucleophiles to afford the complex E. Finally, reductive elimination of the Ni^III complex E provided the corresponding product and regenerated the nickel(I) complex for the next catalytic cycle.

Fig. 7 Proposed mechanism.

Conclusions

In summary, we have identified a novel dual nickel/photoredox catalyzed direct amino- and alkoxycarbonylation of α-heteroatom substituted organohalides. The fundamental challenges posed by α-heteroatom effects, including decarbonylation, difficult oxidative addition, nucleophilic substitution, and reduction, can be circumvented by using the current nickel/photoredox catalyzed radical pathway. A variety of α-heteroatom substituted organohalides, including α-phosphorus, α-sulfur, and α-boron, reacted with amines, alcohols, phenols, and alkyl halides to deliver various α-heteroatom substituted amides and esters in excellent yields under mild conditions. In addition, a four-component carbonylation of vinyl phosphonate was also developed.

Author contributions

X.-F. W. conceived and directed the project. L.-C. W. performed all the experiments. L.-C. W. and X.-F. W. wrote and revised the manuscript and the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key R&D Program of China (2023YFA1507500) and the Dalian Institute of Chemical Physics DICP.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qo00167b

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