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DOI: 10.1039/D4OB01244E (Paper) Org. Biomol. Chem., 2024, Advance Article

NaIO4/air-initiated phosphorylation of alcohols with H-phosphine oxides for the construction of P(O)–O bonds in water†

Huabin Wanga, Lianhua Xua, Xiongwei Liua, Yang Shia, Zhen Yaoa, Ying Zhou*a and Qiang Huang*bc
aCollege of Pharmacy, Guizhou University of Traditional Chinese Medicine, Guiyang 550025, P. R. China
bSchool of Pharmacy, Zunyi Medical University, Zunyi 563006, P. R. China. E-mail: huangqiang65@sina.com
cKey Laboratory of Basic Pharmacology of Ministry of Education and Joint International Research Laboratory of Ethnomedicine of Ministry of Education, Zunyi Medical University, Zunyi 563006, P. R. China

Received 28th July 2024 , Accepted 16th August 2024

First published on 17th August 2024

Abstract

A facile and efficient protocol for P(O)–O bond formation was discovered through NaIO4/air-initiated phosphorylation of alcohols with H-phosphine oxides in water. This reaction showed good functional group tolerance and a broad substrate scope, providing an alternative method for constructing P(O)–O bonds. Mechanistic studies suggested that a phosphoryl radical-involving process from H-phosphine oxides facilitated the phosphorylation of alcohols under air.

Introduction

The chemistry of organophosphorus compounds continuously attracts attention from organic and medicinal chemists because of their wide application in pharmaceuticals,1 agrochemicals,2 flame retardants,3 ligand scaffolds,4 and organic synthesis.5 The development of novel synthetic methodologies to construct various phosphorus substituted derivatives is significant. Recently, organophosphorus compounds bearing P(O)–O bonds have received much attention.6 Traditional methods for the construction of P(O)–O bonds primarily involve the direct esterification of alcohols with toxic and moisture-sensitive phosphoryl halides (Scheme 1a).6e,f In addition, the Atherton–Todd reaction is one of the most powerful and useful methods for the phosphorylation of alcohols (Scheme 1b).7 However, the use of highly toxic inert halogenating reagents (such as CCl4, CHCl3, etc.) limited the development of these methodologies.7a,b Recent strategies for the Atherton–Todd reaction with the employment of transition metal catalysts,8 H2O2/I2,9 Selectfluor,10 or electrosynthesis11 have emerged (Scheme 1c). Unfortunately, these oxidative phosphorylation reactions usually require metallic catalysts or additional oxidants, resulting in metal residues, damage the starting P(O)–H chemicals, and involve electrosynthesis accompanied by energy consumption and operational complexity. Meanwhile, direct esterification reactions of R2P(O)OH with alcohols or phenols in the presence of activation reagents or microwave irradiation have also been developed (Scheme 1d).12 These direct esterification syntheses provide a significant and alternative approach for obtaining various phosphoryl compounds but depend on transition metals or high temperatures. Therefore, more mild, green and efficient protocols should be further considered for the construction of P(O)–O bonds.
image file: d4ob01244e-s1.tif
Scheme 1 The methods for the construction of P(O)–O bonds.

In the past few years, phosphoryl radical-involved reactions have been widely investigated due to their high reactivities.13 Among them, phosphorylation of alcohols with P(O)H compounds via air/O2-initiated phosphoryl radical reactions represents a green and more environmentally friendly process to construct P(O)–O bonds.14 For example, Chen and co-workers found a phosphoryl radical-initiated Atherton–Todd reaction using air as a radical initiator and CHCl3 as a halogenating reagent for the phosphorylation of alcohols.14b However, these remarkable compositions mainly depend on the phosphoryl radical-initiated Atherton–Todd-type reaction using highly toxic chloroform or CCl4 as the halogenating reagent for the phosphorylation of alcohols, phenols, and amines. Very recently, electrochemical dehydrogenative cross-coupling phosphorylation of alcohols has been successfully carried out under mild conditions,15 which indicated that the generated phosphoryl radical might stably live in the aqueous phase. Based on our recent studies on the chemistry of organophosphorus compounds,16 we envisioned that the reactive phosphoryl radicals might also be generated in water by air or oxidant initiation without toxic halogenating reagents. Considering the mechanism of known oxidative dehydrogenation cross-coupling of alcohols with P(O)H,17 oxidants as additives might still be indispensable for constructing P(O)–O bonds. Herein, we wish to report our preliminary studies on the direct phosphorylation of alcohols with H-phosphine oxides by phosphoryl radical reactions in water, providing an alternative method to access various organophosphorus compounds.

Results and discussion

Initially, the reaction between 1-butanol (1a) and diphenylphosphine oxide (2a) was conducted by using PhI(OAc)2 as an oxidant at room temperature in water under air for 6 h. To our delight, the corresponding product 3aa was isolated in 76% yield (Table 1, entry 1), indicating that the high-valence iodine reagent could promote the phosphorylation of alcohols. Hence, different iodine reagents were examined including iodosobenzene, Togni's reagent, Dess–Martin periodinane, 2-iodoxybenzoic acid (IBX), NaIO4, I2 and (dichloroiodo)-benzene (entries 2–8). The less toxic NaIO4 was proved to be a more suitable oxidant, leading to the formation of 3aa in 85% yield (entry 6). Next, different organic solvents were compared instead of water. Although 76% of the yield could be retained in CH3CN, the use of organic solvents failed to improve the yield of 3aa (entries 9–14). Water was still considered the optimal reaction medium. The investigation into the reaction temperature demonstrated a gradual decline in performance with the enhancement of temperature (entries 15–18). In addition, when the reaction was carried out under an open O2 atmosphere or air without NaIO4, the formation of 3aa could not be observed (entries 19 and 20). Finally, the use of a specific amount of NaIO4 was explored (entry 21). Utilizing 1.5 equivalents of NaIO4 maintained the yield of 3aa at 86%. However, the use of a lower amount of NaIO4 resulted in a notable decline in the yield of 3aa, which indicated that an excess use of NaIO4 might be inevitable.
Table 1 Optimization of the reaction conditionsa,b

image file: d4ob01244e-u1.tif
Entry Oxidant Solvent T (°C) Yield (%)
a Reaction conditions: 1a (0.5 mL), 2a (0.5 mmol), oxidant (1.0 mmol), solvent (2.0 mL), open air, 25 °C, 6 h.b Isolated yields based on 2a.c 0.2 mmol NaIO4.d 0.5 mmol NaIO4.e 0.75 mmol NaIO4.
1 PhI(OAc)2 H2O 25 76
2 Iodosobenzene H2O 25 73
3 Togni's reagent H2O 25 66
4 Dess–Martin periodinane H2O 25 76
5 IBX H2O 25 62
6 NaIO4 H2O 25 85
7 I2 H2O 25 52
8 C6H5ICl2 H2O 25 69
9 NaIO4 CCl4 25 72
10 NaIO4 CHCl4 25 69
11 NaIO4 CH3CN 25 76
12 NaIO4 THF 25 73
13 NaIO4 DMF 25 68
14 NaIO4 DCM 25 69
15 NaIO4 H2O 45 78
16 NaIO4 H2O 60 72
17 NaIO4 H2O 80 53
18 NaIO4 H2O 100 38
19 O2 (1 atm) H2O 25 0
20 Air (1 atm) H2O 25 0
21 NaIO4 H2O 25 45c, 77d, 86e

With the optimized reaction conditions in hand, the scope of the various alcohols was studied, as shown in Table 2. Satisfactorily, most of the alcohols could easily react with 2a to afford the corresponding products in moderate to excellent yields. Generally, the chain lengths of saturated fatty alcohols have an obvious effect on phosphorylation. The degressive reactivity of alcohols was observed along with the extension of the carbon chain (3aa–3ai). When the longer chain representing 1-hexadecanol was used, no product 3ay was observed, which was attributed to the poor solubility of 1-hexadecanol. Taking into account the influence of solubility, we further conducted the reaction of 1-hexadecanol with 2a using THF and H2O as a mixed solvent, achieving 3ay in 81% yield.

Table 2 Substrate scope of various alcoholsa,b,c
a Reaction conditions: 1 (0.5 mL), 2a (0.5 mmol), NaIO4 (0.75 mmol), H2O (2.0 mL), open air, 25 °C, 6 h.b Isolated yields based on 2a.c 1 mmol of 1 was added; THF and H2O were used as a mixed solvent, THF[thin space (1/6-em)]:[thin space (1/6-em)]H2O = 4[thin space (1/6-em)]:[thin space (1/6-em)]1 (volume ratio, 2 mL).
image file: d4ob01244e-u2.tif

Next, different cycloalkanols, branched alcohols, substituted alcohols and unsaturated alcohols were also explored, but just moderate yields were realized (3aj–3ar and 3av and 3aw). In contrast, more by-products were observed by TLC analysis when alkenyl alcohols such as 3-buten-1-ol and citronellol were employed, which might be relative to the stability of alcohols under oxidative conditions. Here, 2-(dimethylamino)ethan-1-ol, 3-methoxypropan-1-ol and 2-ethoxyethan-1-ol could also react with 2a in the mixed solvent, affording products 3aq, 3ar and 3av in moderate to good yields. Unfortunately, employment of ethanethiol, tertiary alcohols like tetrahydrolinalool, and heterocycles like 2-hydroxypridine and phenol failed to obtain the corresponding products, even if a mixed solvent of THF and H2O was used in the reaction.

Furthermore, additional examples of alcohols were further examined, including aromatic alcohols, protected carbohydrates and steroids such as cholesterol and dexamethasone. However, the use of aromatic alcohols only gave 3bc–3bk in moderate yields. Sadly, despite the reaction between cholesterol and 2a occurring in the mixed solvent, 1,2,3,4-tetra-O-acetyl-β-D-glucopyranose and dexamethasone did not react with 2a.

Finally, a variety of H-phosphine oxides were reacted with n-butanol or ethanol in water. The results can be seen in Table 3. Obviously, different H-phosphine oxides could react well with alcohols in this system, affording the desired products 3ca–3ci in good yields. The electronic effect and steric hindrance of substituents have no significant impact on the performance of the reaction. Furthermore, a gram-scale reaction was performed under the standard reaction conditions and the corresponding product 3aa was obtained in 78% yield (see the Experimental section), indicating the practical applicability of this protocol in organic synthesis.

Table 3 Substrate scope of various phosphine oxidesa,b
a Reaction conditions: 1 (0.5 mL), 2 (0.5 mmol), NaIO4 (0.75 mmol), H2O (2.0 mL), open air, 25 °C, 3 h.b Isolated yields based on 2.
image file: d4ob01244e-u3.tif

To explore whether the NaIO4/air-initiated phosphoryl radical process was involved, a series of control experiments were carried out to identify the possible pathway in the phosphorylation of alcohols with H-phosphine oxides (Scheme 2). Firstly, when the radical scavengers TEMPO and BHT were employed under the standard conditions, the yield of 3aa was obviously decreased, indicating that a free radical might be involved in the reaction. To capture the adducts, an LC-HRMS analysis was conducted after the reaction occurred for 2 h. To our delight, the free-radical adduct A was detected (calcd for C21H28NNaO2P [M + Na]+, 380.1749; found, 380.1744). In addition, when the reaction was carried out under an O2 atmosphere, a higher conversion efficiency for the formation of 3aa was observed. In contrast, the generation of product 3aa was seriously inhibited under an argon atmosphere. As acknowledged by the examination of reaction conditions, the phosphorylation of alcohols could not occur using O2 as the only oxidant without NaIO4. These results fully indicated that NaIO4/air initiated the phosphoryl radical reactions.


image file: d4ob01244e-s2.tif
Scheme 2 Control experiments.

On the basis of the above observations and previous reports,14a,18 a tentative mechanism for the phosphorylation of alcohols with H-phosphine oxides was proposed and is shown in Scheme 3. Firstly, the diphenyl phosphine oxide cation radical B was initiated by O2 under air. Simultaneously, alcohol 1a reacted with NaIO4 to generate an unstable intermediate D and release NaOH. Next, with the help of in situ-generated NaOH, the H+ of B was grabbed to release the phosphoryl radical C, which could be trapped, leading to the free-radical adducts A by using TEMPO. Finally, the phosphoryl radical C reacted with D to afford the corresponding product 3aa and released iodine radical E that could combine with NaOO˙ to accomplish the cycle of catalysis under an air atmosphere.


image file: d4ob01244e-s3.tif
Scheme 3 The tentative reaction mechanism for phosphorylation of alcohols.

Conclusions

In summary, we developed a novel NaIO4/air-initiated phosphoryl radical reaction to construct P(O)–O bonds. Importantly, the phosphorylation of alcohols with H-phosphine oxides can occur in water via a NaIO4/air-initiated P-radical pathway. This protocol shows good functional group tolerance and a broad substrate scope without an additive of a metal or base, providing a mild, green, efficient, highly atom-economical, and gram-scale access to various valuable phosphonates in moderate to excellent yields. Further studies concerning the potential utility of P-centered radicals in organophosphorus chemistry are ongoing in our laboratory.

Experimental section

General procedure for the synthesis of compound 3

To a solution of alcohols 1 (0.5 ml) and diarylphosphine oxide 2 (101 mg, 0.5 mmol) in water (2 mL) was added NaIO4 (215 mg, 1 mmol) under air, and the reaction mixture was stirred at 25 °C for 6 h. After completion of the reaction, a solution of 20% NaHCO3 was added into the mixture, and then the solution was extracted three times with EA (3 × 20 mL). The combined organic phase was concentrated and purified by flash chromatography on silica gel (petroleum ether/ethyl acetate = 10[thin space (1/6-em)]:[thin space (1/6-em)]1) to afford diarylphosphinate 3. The structures of the isolated products were identified by NMR and HRMS.

Large-scale synthesis of 3aa

To a solution of alcohol 1a (5 ml) and diarylphosphine oxide 2a (1.01 g, 5 mmol) in water (20 mL) was added NaIO4 (2.15 g, 10 mmol) under air, and the reaction mixture was stirred at 25 °C for 6 h. After completion of the reaction, a solution of 20% NaHCO3 was added into the mixture, and then the solution was extracted three times with EA (3 × 50 mL). The combined organic phase was concentrated and purified by flash chromatography on silica gel (petroleum ether/ethyl acetate = 10[thin space (1/6-em)]:[thin space (1/6-em)]1) to afford diarylphosphinate 3aa with a yield of 78%.

Author contributions

Huabin Wang and Ying Zhou drafted most parts of the manuscript and conducted some of the experiments of this research. Lianhua Xu mainly performed the experiments. Qiang Huang designed the study, coordinated the work, and revised the manuscript. Xiongwei Liu, Yang Shi, and Zhen Yao contributed to the analysis of results. All authors agreed to the final version of the manuscript.

Data availability

The data supporting this article have been included as part of the ESI,† including experimental details and compound characterization, such as 1H, 13C and 31P of NMR, and the data of HRMS.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for financial support from the National Natural Science Foundation of China (82360679), the Guizhou Provincial Natural Science Foundation (QKHJC-ZK[2022]592), and the Guizhou Province Administration of the Traditional Chinese Medicine Project (no. QZYY-2021-008).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ob01244e
This journal is © The Royal Society of Chemistry 2024