Synthesis and Characterization of Ibuprofen Impurities-A, F, Q and R

Received: 06-Aug-0026, Manuscript No. DPC-26-193375; Editor assigned: 10-Aug-2026, Pre QC No. DPC-26-193375 (PQ); Reviewed: 24-Aug-2026, QC No. DPC-26-193375; Revised: 15-Sep-2026, Manuscript No. DPC-26-193375 (R); Published: 22-Sep-2026, DOI: 10.4172/0975-413X.18.1.779-783

Introduction

Ibuprofen (1) a bioactive Nonsteroidal Anti-Inflammatory Drug (NSAID) and a propionic acid derivative helps alleviate symptoms related to arthritis, primary dysmenorrhea, and fever. It exhibits analgesic, anti-inflammatory, and antipyretic effects [1]. Development of synthetic chemical process can lead to generation of unwanted organic impurities. These usually include unreacted Starting Materials (SM’s), impurities originating from the SMs, unreacted Intermediates (IMs), reaction side and degradation products. Thus, the detection, identification, quantification, and control of such impurities originating in the manufacturing process have become an important element of drug development in order to ensure product quality and, ultimately, patient safety [2]. Ibuprofen can contain impurities that arise from manufacturing processes or storage. The common Ibuprofen Impurities A, F, Q and R (2-5) include Ibuprofen isomer impurities, related carboxylic acid impurities, organic impurities from starting materials or intermediates, inorganic impurities.

Figure 1: Ibuprofen and its impurities

In this work, we report the synthesis of ibuprofen impurities A, F, Q and R in a contemporary method during our regular research work.

Materials and Methods

The synthesis of IBU-impurity A was achieved by the hydrolysis of alkylated methyl ester 8 obtained by a series of alkylation of 6 with methyl iodide using LDA [3] followed by its coupling with isobutyl boronic acid using tetrakis.

Reagents and conditions: (a) LDA, THF, MeI, r.t., 3 h. (b) Isobutylboronic acid, toulene, K₂CO₃, tetrakis, 110°C, 20 h, 67.9%. (c) NaOH, methanol, r.t., 4 h, 40.5%.

Scheme 1: Synthesis of Ibuprofen impurity A (2)

The synthesis of IBU-impurity-F was achieved by LiOH [4] mediated hydrolysis of ester (12) obtained by C2-wittig olefination [5] of 4- isobutylbenzaldehyde (9).

Reagents and conditions: (a) t-BuO-K+, THF, 0°C-r.t, 39.1%. (b) CH3OH, 10% Pd/C, r.t, 4h, 92.8%. (c) LiOH, THF-H2O (2:1), r.t, 4 h, 70.2%.

Scheme 2: Synthesis of Ibuprofen impurity F (3)

In a similar way, the IBU-impurity-Q was synthesized by hydroboration [6] of alkene 15 obtained by C1-wittig olefination [7] of 4- isobutylbenzaldehyde (9) using methyl triphenyl phosphonium bromide (13).

Reagents and conditions: (a) n-BuLi, THF, 0°C-r.t, 3 h, 44.9%. (b) Borane-THF, NaOH, 0-5°C, 30 mins, 41%.

Scheme 3: Synthesis of Ibuprofen impurity Q (4)

The IBU-impurity-R (5) was achieved by reduction of alkene 18 which was deduced by alkyl boronic acid catalyzed alkylation followed by its C1 Wittig olefination.

Reagents and conditions: (a) Tetrakis, K₂CO₃, 1,4-dioxane, 100°C, 14 h, 60.5%. (b) 10, t-BuO-K+, THF, 0°C-r.t, 14 h, 41.3%. (c) CH₃OH, 10% Pd/C, r.t, 4 h, 31.2%.

Scheme 4: Synthesis of Ibuprofen impurity R (5)

All the synthesized products were characterized by their ¹H NMR, ¹³C NMR, IR and mass spectral analysis.

Results and Discussion

All the air and moisture sensitive reactions were carried out under inert atmosphere (nitrogen or argon). Oven-dried glass apparatus were used to perform all the reactions. Freshly distilled anhydrous solvents were used for air and moisture sensitive reactions. Commercially available reagents were used as such. Purification of compounds was carried out via column chromatography by using silica gel (60-120 meah) packed in glass columns. ¹H NMR and ¹³C NMR were recorded in CDCl₃ on 400 MHz and 100 MHz spectrometer respectively, using TMS as an internal standard [8]. IR spectra were recorded on a Perkin-Elmer FT-RT 240-c Spectro photometer using Thin Film optics. Mass spectra were recorded on a Finnigan MAT 1020 mass spectrometer operating at 70 eV.

Methyl 2-(3-isobutylphenyl) propanoate (8)

To a stirred solution of methyl 2-(3-bromo phenyl acetate) (10 g, 0.043 mol) in dry THF (50 mL) was added LDA (2.0 M solution) drop wise at -70°C (26 ml, 0.052 mol) and stirred for 30 min at -70°C and added methyl iodide (6.1 g, 0.043 mol) slowly at same temperature. The reaction mixture was allowed to stirr for 3 h at room temperature. After the completion of the reaction (monitored by TLC), the reaction mixture was quenched with ammonium chloride solution and extracted with DCM (3 × 50 mL). The organic layer was rinsed with brine, dried over Na2SO4 and concentrated in vacuum to obtain crude compound 7 (6.5 g) [9].

Thus obtained crude compound 7 (6.5 g) was dissolved in dry toluene (100 mL), under inert atmosphere at room temperature was added potassium carbonate (9.1 g, 0.065 mol), isobutyl boronic acid (6.4 g, 0.043 mol) and tetrakis (1.9 g, 0.0017 mol). Then the reaction mixture was allowed to stir for 20 h at 110°C. After completion of the reaction (monitored by TLC) was diluted with water (30 ml). The reaction mixture was extracted with toluene (3 × 25 mL) the combined organic layers were dried over Na₂SO₄ and solvent was evaporated under reduced pressure to give crude compound, which was then purified by column chromatography by using silica gel (60-120 mesh) by eluting with EtOAc-Hexane (1:9) mixture to afford, compound 8 (4.0 g, 67.9%) as yellow oil over two steps [10].

MS-ESI-: m/z [M+H]⁺ peak at 221.30.

Ibuprofen impurity-A (2)

To a stirred solution of ester 8 (4 g, 1.11 mmol) in dry methanol (100 mL) was added a solution of NaOH (2.0 g, 0.05 mmol) in water (10 mL) at room temperature and the resulting mixture was stirred for 4h at 45oC. After completion of the reaction (TLC), the pH of the reaction mixture was adjusted to 2.0 with HCl. The aqueous layer was extracted with DCM (2 × 30 mL) and the combined organic layer was washed with brine, dried over Na₂SO₄, concentrated under reduced pressure to give crude compound, which was later purified by column chromatography by using silica gel (60-120 mesh) by eluting with EtOAc-Hexane (2:8) mixture to afford, compound 2 (1.50 g, 40.5%) as colorless oil.

IR (neat): υ 2954, 2868, 1704, 1462, 912 cm^-1

¹H NMR (400 MHz, CDCl₃): δ 12.23 (s, 1H), 7.22 (t, 1H), 7.09 (d, 1H), 7.05 (s, 2H), 7.02 (d, 1H), 3.60-3.65 (m, 1H), 2.42 (d, 1H), 1.75-1.85 (m, 1H), 1.34 (d, 1H), 0.85 (d, 6H).

¹³C NMR (100 MHz, CDCl₃): δ 17534, 141.2, 141.0, 128.1, 128.0, 127.3, 124.7, 44.6, 44.5, 22.6, 22.1, 18.5.

MS-ESI-: m/z [M-H]⁺ peak at 205.10.

Ethyl (E)-3-(4-isobutylphenyl) acrylate (11)

To a stirred solution of 4-isobutyl benzaldehyde (9) (12 g, 0.077 mol) in dry THF (60 ml) was added with C₂ ylide (2-methoxy-2-oxoethyl) triphenylphosphonium bromide (10) (30 g, 0.085 mol) at 30^oC, stirred for 1 h, then reaction mass was added potassium tert-butoxide at 0°C and allowed to stirr at room temperature for 14h After the completion of the reaction (monitored by TLC), the reaction mixture was quenched with ice cooled water (20 ml) and extracted with ethylacetate (2 × 100 ml). The combined organic layer was washed with brine dried over Na₂SO₄, concentrated under reduced pressure to give crude compound. The crude was purified by column chromatography by using silica gel (60-120 mesh) by eluting with EtOAc-Hexane (1: 4) mixture to afford compound 11 (7.0 g, 39.1%) as yellow oil.

MS-ESI: m/z [M+H]⁺ peak at 233.42.

Ethyl 3-(4-isobutylphenyl) propanoate (12)

A solution of methyl ester 11 (7.0 g, 0.60 mmol) in dry methanol (50 mL) was stirred for 10 min at 30^oC and then added (10% w/w) Pd/carbon (2.5 g) and allowed to stir for 14 h under hydrogen atmosphere at room temperature. After the completion of reaction (confirmed by TLC), the reaction mixture was filtered with methanol [11]. The combined organic layers were concentrated under reduced pressure to obtain the crude product which was further used without any purification 12 (6.5 g, 92.8%) as yellow oil.

MS-ESI: m/z [M+H]⁺ peak at 235.33.

Ibuprofen impurity-F (3)

To a stirred solution of 12 (6.5 g, 38.5 mmol) in THF: H₂O (2:1) (60 mL) was added lithium hydroxide (0.17 g, 24.06 mmol) at room temperature and stirred for 4 h. After completion of the reaction (TLC), the reaction mixture was diluted with EtOAc (50 mL), pH was adjusted to 2.0 with HCl solution and extracted with ethylacetate (2 × 30 ml). The combined organic layer was washed with brine dried over Na₂SO₄, concentrated under reduced pressure to give the compound (4 g, 70.2%) as white solid.

IR (neat): υ 2950, 2862, 1695, 1421, 934 cm^-1.

¹H NMR (400 MHz, CDCl3): δ 12.10 (s, 1H), 7.12 (d, 2H, J = 8.0 Hz), 7.04 (d, 2H, J = 8.0 Hz), 2.78 (t, 2H), 2.51 (t, 2H), 2.39 (d, 2H), 1.76-1.80 (m, 1H), 0.84 (d, 6H).

¹³C NMR (100 MHz, CDCl3): δ 173.8, 138.6, 138.0, 128.8, 127.9, 44.2, 35.3, 29.9, 29.6, 22.1.

MS-ESI: m/z [M-H]⁺ peak at 205.25.

1-isobutyl-4-vinylbenzene (14)

To the stirred solution of methyl triphenyl phosponium bromide (13) (11 g, 0.03 mol) in dry THF (50 mL), was added n-butyl lithium at 0oC and stirred at room temperature. After 1 h, 4-isobutyl benzaldehyde (9) (5 g, 0.03 mol) in dry THF was added slowly at 0oC and allowed to stirr at room temperature for 3 h. After completion of the reaction (monitored by TLC), the reaction mixture was quenched with sat. ammonium chloride solution and the aqueous layer was extracted with methyl tert-butyl ether (2 × 30 mL). The combined organic layer was washed with brine, dried over Na₂SO₄ and the solvent was evaporated under reduced pressure. The crude product was purified by column chromatography using silica gel (60-120 mesh) by eluting with EtOAc-hexane (1;1) mixture to give 14 (2.2 g, 44.9%) as colorless solid.

MS-ESI: m/z [M+H]⁺ peak at 161.30.

IBU-Impurity-Q (4)

To stirred solution of alkene in dry THF was added Boron. THF solution (30 mL, 0.03 mol) and sat. sodium hydroxide solution (10 ml) at 0^oC and stirred for 30 min at same temperature. After 30 min, H₂O₂ (30%) was added slowly at room temperature and allowed to stirr for 4 h. After completion of the reaction (monitored by TLC), the reaction mixture was quenched with sat. ammonium chloride solution and the aqueous layer was extracted with methyl tert-butyl ether (2 × 30 mL). The combined organic layer was washed with brine, dried over Na₂SO₄ and the solvent was evaporated under reduced pressure. The crude product was purified by column chromatography using silica gel (60-120 mesh) by eluting with EtOAc-hexane (1;1) mixture to give IBU-Impurity-Q (1 g, 41%) as colorless solid.

IR (neat): υ 3327, 3009, 2868, 1418, 835 cm^-1.

¹H NMR (400 MHz, CDCl3): δ 7.10 (d, 2H, J=8.3 Hz), 7.03 (d, 2H, J=7.6 Hz), 4.59 (t, 1H), 3.55-3.59 (m, 2H), 2.67 (t, 2H), 2.39 (d, 1H), 1.73-1.84 (m, 1H), 0.84 (d, 6H).

¹³C NMR (100 MHz, CDCl3): δ 138.4, 136.6, 128.7, 128.5, 62.2, 44.2, 38.6, 29.6, 22.1.

MS-ESI: m/z [M+H]+ peak at 195.10.

Bis(4-isobutylphenyl) methanone (17)

To the stirred solution of 4, 4-dibromo acetophenone (15) (5 g, 0.015 mol) in dry 1,4-dioxane (50 mL) was added Isobutyl boronic acid (16) (4.5 g, 0.044 mmol) and potassium carbonate (7.1 g, 0.051 mol) slowly at room temperature and allowed to stirr for 10 min. The reaction mixture was added with tetrakis (0.3 g, 0.003 mol) allowed to stir for 14 h at 100oC. After completion of the reaction (monitored by TLC), the reaction mixture was extracted with EtOAc (2 × 30 mL). The combined organic layer was washed with brine, dried over Na₂SO₄ and the solvent was evaporated under reduced pressure to obtain crude product 17 (2.5 g, 60.5%) as yellow solid.

MS-ESI: m/z [M-H]⁺ peak at 293.20.

4,4'-(ethene-1,1-diyl) bis (isobutylbenzene) (18)

To stirred solution of compound 17 in dry THF (25 mL) was added methyl triphenyl phosponium bromide (13) (6.3 g, 0.176 mol) and Potassium tert-butoxide (2.5 g, 0.022 mol) at 0oC and stirred for 14 h at room temperature. After completion of the reaction (monitored by TLC), the reaction mixture was quenched with ice chilled water and the aqueous layer was extracted with ethyl acetate (2 × 30 mL). The combined organic layer was washed with brine, dried over Na₂SO₄ and the solvent was evaporated under reduced pressure. The crude product was purified by column chromatography using silica gel (60-120 mesh) by eluting with EtOAc-hexane (1;1) mixture to give compound 18 (1 g, 40.3%) as colorless solid.

MS-ESI: m/z [M+H]⁺ peak at 293.20.

IBU-Impurity-R (5)

A solution of 18 (1.5 g, 0.60 mmol) in dry methanol (15 mL) was stirred for 10 min at room temperature and then added 10% w/w Pd/carbon (0.5 g,) and allowed to stir for 14 h under hydrogen atmosphere at room temperature. After the completion of reaction (confirmed by TLC), the reaction mixture was filtered with methanol under vacuum. The combined organic layers were concentrated under reduced pressure to obtain the product 5 (0.5 g, 31.2%) as colorless solid.

IR (neat): υ 2969, 2871, 1415, 929 cm^-1.

¹H NMR (400 MHz, CDCl₃): δ 7.14 (d, 4H, J=8.0 Hz), 7.04 (d, 4H, J=8.0 Hz), 4.06 (q, 1H), 2.37 (d, 4H), 1.72–1.82 (m, 2H), 1.53 (d, 3H), 0.83 (d, 12H).

¹³C NMR (100 MHz, CDCl₃): δ 143.7, 138.5, 128.8, 126.7, 44.2, 43.4, 29.6, 22.1, 21.6.

MS-ESI: m/z [M-H]⁺ peak at 293.20.

Conclusion

In summary, we report the synthesis of ibuprofen impurities-A, F, R and Q in a simple and convergent method involving the alkylation, wittig olefination and hydroboration as key reactions.

Acknowledgement

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