Organocatalyzed Amine-Free O-Phosphorylation of Alcohols with 4-Methylpyridine N-Oxide

Abstract

Amine-free phosphorylation of various alcohols was developed with 4-methylpyridine N-oxide in the presence of 4 Å molecular sieves at room temperature. This mild method gave various phosphorylated products in high yield and could be applied to acid- or base-sensitive substrates. Furthermore, this method was also effective for the chemoselective phosphorylation of diols or polyols.

Introduction

Phosphorus-containing compounds are involved in important processes and functions in biochemistry, biogeochemistry, ecology, and agriculture.^1–3) Pharmaceuticals containing phosphorus have also long attracted attention from the pharmaceutical industry, and many have been developed.^4–6) There are a wide range of phosphorus-containing functional groups, which are classified according to their structural characteristics and include phosphate esters and phosphate amides. Phosphate esters are important structures that are found in nucleotides, which are the building blocks of DNA and RNA. The functionalization of alcohols is the first choice for the chemical synthesis of phosphates. Due to the unique oxidation states of phosphorus, this can be accomplished either with phosphorous (III) reagents, such as phosphoramidites,⁷⁾ phosphites,⁸⁾ or phosphorochloridites,^9,10) followed by oxidation.^11–15) In particular, the phosphoramidite method has received much attention in the synthesis of oligonucleotides.¹⁶⁾ However, these methods are difficult to apply to substrates that are sensitive to oxidation due to the oxidation step. Other methods for the phosphorylation of alcohols involve Lewis acid or nucleophilic catalysts with phosphorous (V) reagents. For example, Jones et al. used Ti(Ot-Bu)₄, TiCl₄, or Cu(OTf)₂ as catalysts with phosphoryl chlorides or N-phosphoryl oxazolidinones,^17–19) and Sculimbrene and colleagues combined the Ti(Ot-Bu)₄ catalyst with tetrabenzyl pyrophosphate.²⁰⁾ Sculimbrene and Miller used N-methylimidazole as a nucleophilic catalyst in combination with diphenyl chlorophosphate (DPPCl).²¹⁾ Spivey and colleagues reported the phosphate esterification of alcohols using pyridine N-oxide as a catalyst.²²⁾ Although these reagents promote smooth phosphorylation, excess base, such as NEt₃, is often required to capture the protons generated in the reaction. Amines contribute to the deprotonation mechanism, but low-molecular-weight amines smell unpleasant, are toxic, and irritate the skin and eyes. The use of large amounts of transition metals should also be avoided for environmental reasons. In addition, these reaction systems tend to be strongly acidic or basic, which means that the reaction is difficult to use with acid- or base-sensitive substrates because of undesired side reactions. Thus, mild phosphorylation conditions for various alcohols are still required. Recently, we have developed methods for amine-free silylation²³⁾ or sulfonylation²⁴⁾ of primary and secondary alcohols with the combination of 4-methylpyridine N-oxide and 4 Å molecular sieves (MS) (Chart 1). In these studies, we demonstrated that MS could serve as an alternative to amines. In general, MS activated by heating can absorb water from various solvents appropriate to the MS pore size.

Chart 1. Our Previous Methods for Amine-Free Silylation and Sulfonylation of Primary and Secondary Alcohols

MS are crystalline aluminosilicates that have a three-dimensional interconnecting network of silica and alumina tetrahedra. Heating removes the natural water of hydration from this network to produce pores or cages that adsorb molecules of a specific size.²⁵⁾ Thus, MS act as a water scavenger to dry organic solvents and reagents in organic synthesis. We have reported that MS can also serve as an alternative to amines. In addition, we confirmed the presence of active species A or B, which consist of bulky silyl chloride or sulfonyl chloride with 4-methylpyridine N-oxide, by ¹H-NMR experiments in our previous reports.^23,24) Based on these studies, here we describe the mild amine-free phosphorylation of various alcohols using 4-methylpyridine N-oxide and 4 Å MS.

Results and Discussion

We chose (−)-menthol 1 as a standard substrate for catalyst screening for amine-free O-phosphorylation (Table 1). Treatment of 1 with 1.7 equivalent (equiv.) DPPCl and 350 wt % activated 4 Å MS powder in CH₂Cl₂ at room temperature for 4 h did not produce phosphorylated product 2a (entry 1). Therefore, 4 Å MS alone did not have phosphorylation activity toward the alcohol. Next, various pyridine N-oxide derivatives (20 mol %) were investigated (entries 2–8). Both 4-NO₂-pyridine N-oxide and 4-CN-pyridine N-oxide bearing a strong electron-withdrawing group were ineffective as catalysts for amine-free O-phosphorylation (entries 2, 3). The ¹H-NMR experiment showed that 4-NO₂-pyridine N-oxide did not form a complex with DPPCl (see Supplementary Materials). In contrast, pyridine N-oxide derivatives bearing H, Me, OMe, or N(Me)₂ at the 4-position of the pyridine ring were effective for O-phosphorylation (entries 5–8). In particular, 4-methylpyridine N-oxide (7) showed good activity for O-phosphorylation with a reaction rate faster than those of other pyridine N-oxide derivatives. When 20 mol % N,N-dimethyl-4-aminopyridine (DMAP) was used as the catalyst, the reaction was not completed even after 5 h, and 10% of the starting material was recovered (entry 9). This is because DMAP is more basic than pyridine N-oxide, acts as a better Brønsted base than 4 Å MS, and is deactivated by protonation. The conditions without 4 Å MS gave a low yield (entry 10). These results demonstrated that 4-methylpyridine N-oxide (7) worked with the 4 Å MS in this reaction.

Table 1. Catalyst Screening for the Phosphorylation of (−)-Menthol

Entry	Catalyst	Time	Yield of 2a (%)b)	Recovery of 1 (%)b)
1	None	4 h	0	approx. 100
2	4-NO2-Pyr-N-O (3)	4 h	0	approx. 100
3	4-CN-Pyr-N-O (4)	4 h	0	approx. 100
4	4-Ph-Pyr-N-O (5)	5 h	96	0
5	Pyr-N-O (6)	2 h	94	0
6	4-Me-Pyr-N-O (7)	1.5 h	96	0
7	4-OMe-Pyr-N-O (8)	4 h	91	0
8	DMAPO	4 h	98	0
9	DMAP	5 h	85	10
10a)	4-Me-Pyr-N-O (7)	24 h	18	71

a) Without 4 Å MS. b) Isolated yield.

Next, we optimized the conditions for amine-free O-phosphorylation using 7 (Table 2). When CH₂Cl₂ and CHCl₃ were used as the solvents, the reaction proceeded smoothly to give product 2 in excellent yield (entries 1, 2). Less polar solvents, such as CCl₄ and toluene, prolonged the reaction time (entries 3, 4). DMF gave desired product 2a in low yield with formylated product 9 due to the formation of the Vilsmeier reagent (entry 6). To determine the best pore size for the MS, 3 and 5 Å MS were also investigated (entries 7, 8), but they gave longer reaction times than the 4 Å MS. In particular, 5 Å MS, which have acidic properties, were not suitable.^26–30) The reaction time was prolonged when the amount of 4 Å MS was reduced to 200 and 300 wt %, respectively (entries 9, 10). The minimum amount of catalyst required to complete the phosphorylation in 24 h (entries 11, 12) was only 5 mol % 7, although the reaction time was longer than for 20 mol % 7. The amount of reagent was reduced from 1.7 to 1.3 equiv., but the reaction time did not change (entry 12).

Table 2. Optimization of Amine-Free Phosphorylation Using 4-Methylpyridine N-Oxide (7)

Entry	Additive	Solvent	X (mol %)	Time	Yield of 2a (%)e)	Recovery of 1 (%)e)
1	MS 4A	CH2Cl2	20	1 h	96	0
2	MS 4A	CHCl3	20	1 h	96	0
3	MS 4A	CCl4	20	24 h	74	11
4	MS 4A	Toluene	20	6 h	76	20
5	MS 4A	THF	20	24 h	72	10
6a)	MS 4A	DMF	20	24 h	8	19
7	MS 3A	CH2Cl2	20	24 h	88	8
8	MS 5A	CH2Cl2	20	24 h	33	52
9b)	MS 4A	CH2Cl2	20	24 h	78	11
10c)	MS 4A	CH2Cl2	20	22 h	96	0
11	MS 4A	CH2Cl2	5	24 h	94	0
12	MS 4A	CH2Cl2	10	6 h	96	0
13d)	MS 4A	CH2Cl2	20	1 h	95	0

a) Compound 9 was obtained in 21% yield. b) 200 wt % 4 Å MS was used. c) 300 wt % 4 Å MS was used. d) 1.3 equiv. DPPCl was used. e) Isolated yield.

Next, the scope of phosphoryl reagents in the catalyst system was investigated with various phosphoryl chloride reagents (Table 3). When dimethyl chlorophosphate was used, 2b was obtained in 88% yield (entry 2). The combination of 4-methylpyridine N-oxide (7) with diethyl chlorophosphate gave phosphorylated product 2c in excellent yield (entry 3). In contrast, 2-chloro-2-oxo-1,3,2-dioxaphospholane gave a complex mixture instead of desired product 2d due to hydrolysis.³¹⁾ These results show that the combination of 7 and 4 Å MS can also be applied to amine-free O-phosphorylation using acyclic dialkoxychlorophosphate.

Table 3. Scope of Phosphoryl Reagents for Catalyst 7 with 4 Å MS

Entry	R	Time	Yield of 2a–2d (%)a)
1	PO(OPh)2	1 h	95 (2a)
2	PO(OMe)2	1.5 h	88 (2b)
3	PO(OEt)2	2.5 h	97 (2c)
4	PO(OCH2-)2	5 h	0 (2d)

a) Isolated yield.

Optimized reaction conditions were 20 mol % catalyst, 350 wt% 4 Å MS, and 1.3 equiv. chlorophosphate in CH₂Cl₂, and these conditions were applied to a series of alcohol substrates (Fig. 1). First, we explored the use of various primary alcohols (entries 1–3). Linear or branched alcohols, such as octanol, 3-methyl-1-butanol, and phenethyl alcohol derivatives, were treated under the optimized conditions to give O-phosphorylated products 10, 11, and 12 in 92, 92, and 93% yields, respectively (entries 1–3). Next, the primary alcohol with acetal, which is an acid-sensitive functional group, was investigated. The alcohol was converted to phosphate 13 in high yield without destruction of the functional group (entry 4). Treatment of 1,5-pentanediol with 2.6 equiv. DPPCl and 700 wt % activated 4 Å MS powder in the presence of 40 mol% catalyst afforded phosphate 14 in high yield (entry 5). In addition, the reaction proceeded smoothly when benzyl alcohol also was used as a substrate (entry 6). Various secondary alcohols were then investigated (entries 7–11). The reaction proceeded in good yields regardless of whether the substrates were cyclic or acyclic. When a secondary benzyl alcohol was treated under optimal conditions, the reaction appeared to proceed well on TLC, but purification of the crude product using silica gel column chromatography resulted in the degradation of phosphorylation product 18 (entry 10). The same result was reported by the Spivey group.²²⁾ However, smooth purification of compound 18 was achieved with an NH₂-coated silica gel column (entry 10). Furthermore, this condition was effective for a base-sensitive substrate, which can often undergo β-elimination with an amine base such as NEt₃ (entry 11). The exposure of the phenol to the optimized conditions afforded the phosphate ester 20, but the reaction was not completed even after 12 h (entry 12). Unfortunately, the optimized conditions were ineffective for the tertiary alcohol (entry 13).

Fig. 1. Phosphates from Various Alcohols^a

Chemoselective phosphorylation was investigated next (Fig. 2). The primary and phenolic hydroxy groups were distinguished under these conditions, with the primary alcohol selectively phosphorylated due to the weak basicity of 4 Å MS and the weak nucleophilicity of phenol (entries 1, 2). Next, we investigated the selective phosphorylation of substrates bearing both primary and secondary alcohols. The primary alcohol in 1,5-hexanediol was converted to phosphate 24 in good yield with good regioselectivity (entry 3). Furthermore, treatment of octyl-β-D-glucopyranoside under dilute conditions gave 6-O-phosphorylated product 25 in moderate yield with good regioselectivity (entry 4). In contrast, the chemoselective phosphorylation of 1,2-diols using DPPCl as the phosphorylation reagent was unsuccessful, probably due to cyclization of the product during the reaction or purification (entries 5, 6). In addition, phosphate esters with five-membered rings are unstable, so we could not isolate the cyclized products.³¹⁾

Fig. 2. Chemoselective Phosphorylation

The production of monophosphate with meso-cyclohexane 1,2-diol 28 as the substrate was suggested by the crude ¹H-NMR spectrum. However, the monophosphate 27 was degraded during purification by using column chromatography on silica gel or NH₂-coated silica gel. We tried the one-pot amine-free acetylation after monophosphorylation to prevent the cyclization during purification (Chart 2), and compound 29 was obtained in moderate yield over two steps. Furthermore, using diethyl chlorophosphate as the phosphoryl reagent for the monophosphorylation of meso-cyclohexane 1,2-diol 28 afforded monophosphate 30 in 79% yield without cyclization during purification (Chart 3). Thus, selective monophosphorylation could be achieved by choosing the appropriate phosphorylation reagent.

Chart 2. One Pot Synthesis of 29

Chart 3. Monophosphorylation Using Diethyl Chlorophosphate as the Phosphoryl Reagent

In summary, we developed a method for amine-free phosphorylation of various primary and secondary alcohols using 4-methylpyridine N-oxide and 4 Å MS powder. The reaction proceeds under mild conditions to provide various phosphorylated products in high yield and can be applied to acid- or base-sensitive substrates. Furthermore, this method was also effective for the chemoselective phosphorylation of diols or polyols, and selective monophosphate esterification of 1,2-diols could be achieved without a cyclization reaction by using the dialkyl chlorophosphate.

Experimental

General Information: Melting point (mp) was measured with a melting point apparatus (MP-500D, Yanaco, Tokyo, Japan) and was uncorrected. Optical rotations were measured on a polarimeter (P-2200, JASCO, Tokyo, Japan). IR spectra of samples in CHCl₃ were measured with a spectrometer (FT/IR-4100, JASCO). ¹H-NMR and ¹³C-NMR spectra were recorded with a spectrometer (Avance III 600, Bruker, Billerica, MA, U.S.A.) at 600 MHz (150 MHz for ¹³C-NMR) and 25 °C with tetramethylsilane (δ = 0.0 ppm) as an internal standard. ³¹P-NMR spectra were recorded with 85% H₃PO₄ (δ = 0.0 ppm) as an external standard. The data are reported as follows: chemical shift in ppm (δ), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet), integration, and coupling constant (Hz). High-resolution mass spectra (HR-MS) were measured with a mass spectrometer (Exactive Plus Orbitrap, Thermo Scientific, Waltham, MA, U.S.A.). Analytical TLC was performed on silica gel (grade 60 F254, MERCK, Burlington, MA, U.S.A.). The spots and bands were detected under UV light (254 nm) and/or by staining with 5% phosphomolybdic acid followed by heating. Column chromatography to isolate the products was performed on silica gel 60 (230–400 mesh, KANTO) and Wakosil 50NH₂(HC) (FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan). Organic extracts were dried over anhydrous Na₂SO₄. Molecular sieves were activated by heating under reduced pressure. All dried solvents (CH₂Cl₂, CHCl₃, CCl₄, toluene, THF and DMF) were purchased from KANTO CHEMICAL CO., INC. (Tokyo, Japan) and used without further purification. Phosphoryl reagents were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan) and used without further purification. Pyridine N-oxide derivatives were purchased from Tokyo Chemical Industry Co., Ltd. and used after azeotrope with toluene.

Procedure for the Phosphorylation of 1 by Using DPPCl

To a stirred suspension of (−)-menthol (1) (31.5 mg, 0.202 mmol), 7 (4.4 mg, 4.0 × 10⁻² mmol, 20 mol %) and MS4A (110 mg, 350 wt % of 1) in CH₂Cl₂ (0.25 mL) was added DPPCl (70.5 mg, 0.263 mmol). After being stirred at room temperature for 1 h, the mixture was quenched with 1 M aqueous HCl (10 mL) and extracted with Et₂O (10 mL × 3). The combined extracts were washed with water (30 mL × 2), saturated aq. NaHCO₃ (30 mL) and saturated brine (30 mL), dried and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (CH₂Cl₂/hexane, 1 : 1) to provide 74.4 mg (95% yield) of 2a as colorless solids. mp. 39–41 °C; IR (CHCl₃): 1018, 1192, 1282, 1490, 1592, 2873, 2929, 2960, 3010 cm⁻¹; ¹H-NMR (600 MHz, CDCl₃): δ 0.73 (d, J = 7.2 Hz, 3H), 0.84 (d, J = 7.2 Hz, 3H), 0.85 (m, 1H), 0.89 (d, J = 7.2 Hz, 3H), 1.00 (m, 1H), 1.18 (q, J = 12.0 Hz, 1H), 1.37–1.49 (m, 2H), 1.62–1.68 (m, 2H), 2.02 (m, 1H), 2.22 (d, J = 12.0 Hz, 1H), 4.42 (m, 1H), 7.16 (t, J = 7.2 Hz, 1H), 7.17 (t, J = 7.2 Hz, 1H), 7.22–7.24 (m, 4H), 7.30–7.34 (m, 4H); ¹³C-NMR (150 MHz, CDCl₃) δ: 15.5, 20.8, 21.8, 22.7, 25.4, 31.5, 33.8, 42.4, 48.3 (d, J_CP = 7.5 Hz), 81.5 (d, J_CP = 7.5 Hz), 120.0 (d, J_CP = 4.5 Hz, 2C), 120.1 (d, J_CP = 4.5 Hz, 2C), 125.0, 125.1, 129.6 (4C), 150.70 (d, J_CP = 7.5 Hz), 150.72 (d, J_CP = 7.5 Hz); ³¹P-NMR (243 MHz, CDCl₃): δ −11.9; HR-MS (DART) Calcd for C₂₂H₃₀O₄P (M + H)⁺: m/z 389.1876. Found 389.1877.

Acknowledgments

This work was supported by JSPS KAKENHI Grant Number: JP23K06038.

Conflict of Interest

The authors declare no conflict of interest.

Supplementary Materials

This article contains supplementary materials.

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