Solid acids as cocatalysts in the chelation-assisted hydroacylation of alkenes and alkynes

Abstract

The use of homogeneous Brønsted acid cocatalysts (such as benzoic acid) in hydroacylation reactions via imine intermediates has been extensively studied. However, the use of heterogeneous cocatalysts has been limited to montmorillonite K10. Thus, we can use other solid acids to increase the efficiency of the reaction. In this study, we describe the effects of sulfated zirconia, Al-MCM-41 or superacid modified montmorillonite on the hydroacylation of alkenes and alkynes with aldehydes via imine intermediates and in the presence of the Wilkinson complex. Furthermore, we addressed the dual role of montmorillonite, a redox reagent in the presence of TEMPO and an acid solid, allowing the direct use of benzyl alcohols as substrates to generate saturated or α,β-unsaturated ketones.

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Introduction

The transformation of an aromatic or aliphatic aldehyde into a saturated or α,β-unsaturated ketone involves the hydroacylation of alkenes or alkynes.¹ This strategy of synthesis considers the aldehydic C–H bond as a functional group that can be activated to form a C–C bond in the presence of transition metal catalysis.² In the overall catalytic hydroacylation process, the principal drawback is the decarbonylation of aldehyde,³ which results in the formation of carbonyl-metal complexes and alkanes (Scheme 1a). The following strategies have been used to avoid this problem: (i) the use of a β-sulfide⁴ or β-OR⁵ group in the substrate² (Scheme 1b) and (ii) the use of reversible imine formation with 2-aminopyridines⁶ (Scheme 1c). Both of these approaches involve the assistance of chelating groups to stabilise the intermediate that results in the formation of a ketone. In the last protocol, the in situ formation of imines from aldehydes is performed in the presence of aniline and benzoic acid as cocatalysts.⁷ This imine reacts with alkenes in the presence of Wilkinson's catalyst to generate ketimines, which are hydrolysed to saturated ketones (Scheme 1c).⁸

Scheme 1 Hydroacylation reaction: (a) in the absence of chelating groups, (b) with the use of a β-sulfide or β-OR chelating group and (c) via an imine intermediate.

Alternatively, the use of montmorillonite K10 (ref. 9) as an efficient acidic solid and reusable cocatalyst to generate imines has been described. However, the study of other solid acids with different properties and structural characteristics has not been explored, even though the use of solid acids as cocatalysts in organic reactions is attractive due to the recyclability of solid materials.¹⁰

Thus, in this work, we chose to use three structurally different solid acids, namely, sulfated zirconia (SZ),¹¹ Al-MCM-41 (ref. 12) and montmorillonite¹³ modified with a superacid (CF₃SO₃H), to develop a cooperative system through which hydroiminoacylation of the corresponding aldimines generates ketones (Scheme 1c). Additionally, we considered extending this study to a process in which a simple alcohol is used as the aldehyde precursor in the presence of modified montmorillonite and TEMPO to generate the ketone. To the best of our knowledge, montmorillonite has not been used in similar dual processes (redox and acid catalysis).

Results and discussion

Materials

The SZ investigated in the present study was synthesised by the sol–gel technique and characterised by X-ray powder diffraction (XRD).¹⁴ Fig. 1a shows the diffractogram for SZ, in which the characteristic pattern of the crystalline tetragonal phase can be observed, given by the reflections at 2θ = 30.21°, 35.35°, 50.21°, 59.23°, 60.17°, 62.84°, 74.78° and 81.78°. These values are congruent with what is described in the literature¹⁴ for the crystalline tetragonal structure of a super acid material.

Fig. 1 XRD patterns of (a) SZ, (b) Al-MCM-41 and (c) montmorillonite modified with CF₃SO₃H.

The mesoporous material Al-MCM-41 was synthesised by an ultrasound-assisted technique.¹⁵ The structure of the mesoporous material was confirmed by XRD. Fig. 1b shows the diffractogram for Al-MCM-41, in which reflections can be observed at 2θ = 2.2° and 4.0°. These values are identical to those described in the literature for these kinds of materials.

On the other hand, natural montmorillonite was treated with concentrated CF₃SO₃H acid to generate the modified material. Fig. 1c shows the diffractogram with the characteristic reflections to a montmorillonite clay at 2θ = 7.0°, 20.0°, 35.0° and 61.8°.¹⁶

The textural properties of the synthesised materials were determined by nitrogen adsorption–desorption (BET) methods, allowing us to compare the surface area, pore volume and pore size of the catalysts. Compared with the SZ and Al-MCM-41 materials, the modified montmorillonite had a larger surface area and larger pore size (Table 1). This pore size makes this material go from being microporous to mesoporous according to IUPAC nomenclature.

Table 1 Textural properties of acid materials

	Materials
Properties	SZ	Al-MCM-41	Modified montmorillonite
BET area (m^2 g^−1)	90.35	1221.79	185.00
Pore volume (cm^3 g^−1)	0.12	0.85	0.60
Pore size (Å)	52.01	26.49	107.79

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Initially, we optimised the typical reaction parameters, including the catalytic reagents, solvent, temperature, and time, for the hydroacylation of 1-butene with benzaldehyde, as shown in Scheme 1c and Table S10 in the ESI.† Subsequently, and based on the results previously obtained, we focused on studying the reaction trend with different substrates and the following different solid acids: modified montmorillonite, SZ or Al-MCM-41. As shown in Table 2, the hydroacylation of alkenes in the presence of 2-amino-3-picoline, aniline, Wilkinson's complex, and toluene as the solvent at 80 °C afforded saturated ketones 3a–3e in very good yields (80–92%). Similar yields were observed when the reaction was carried out with alkynes to generate α,β-unsaturated ketones 3f–3j. In contrast, the yields of the reaction decrease considerably when the aldehyde is aliphatic (acetaldehyde), as demonstrated by the yields obtained for ketones 3k and 3l (Table 2). It is important to note that, in this catalytic system, 2-amino-3-picoline and aniline were used simultaneously to enhance the formation of the imine, as described by Castillón et al.⁹

Table 2 Hydroacylation reaction in the presence of solid acids^ab

a Reaction conditions: aldehyde 1 (1 mmol), alkene or alkyne 2 (1.5 mmol), RhCl(PPh₃)₃ (5% mmol), 2-amino-3-picoline (20% mmol), aniline (20%) solid acid (50 mg) and toluene (3 mL).b Yield of the isolated product after chromatographic purification.

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We think that the difference between the yields obtained with the three materials may be explained by their acidic properties and direct influence on the formation of picolyl imines. The SZ has H₀ ≤ −14,¹⁷ Al-MCM-41 H₀ ≤ 6.80 (ref. 18) and modified montmorillonite has H₀ ≤ −12.75.¹⁹ Therefore, materials with major acidity properties catalyse the formation of imines more efficiently.

The efficiency of the SZ and modified montmorillonite catalysts are compared with previously reported for the synthesis of 1-phenylheptan-1-one (3c). As shown in Table 3, similar conversions were obtained when the reaction was carried out in presence of montmorillonite K10.⁹ However, the main differences lie in the excess moles of the 1-hexene used. We use an excess of 0.5 mmol, in contrast to the 5 mmol excess required when using montmorillonite K10.⁹

Table 3 Comparison in the efficiency of different catalytic systems to obtain 3c

	Catalyst
	SZ	Mont/CF3SO3H	Mont/K10
a Yield of the isolated product after chromatographic purification.b 80% Conversion and 98% yield, which were determined by GC.
Aldehyde mmol	1	1	2.5
Alkene mmol	1.5	1.5	12.5
Catalyst loading (mg)	50	50	83
Temperature (°C)	80	80	110
Time (h)	1	1	2
Yield %	90^a	86^a	98^b

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Reuse of solid acids

We studied the reuse of solid acids in the hydroacylation of 1-butene with benzaldehyde to form 3a by reactivation of the materials at 100 °C in an oven under an O₂ atmosphere for 12 h. In the case of modified montmorillonite and SZ, the reaction yield did not decrease significantly after three reuses. After one run, Al-MCM-41 lost its catalytic activity (Fig. 2).

Fig. 2 Reuse of materials in the synthesis of 3a.

To confirm the homogeneity of catalysts in the reuse reactions, the structures of materials were verified by XRD before use. Fig. 3 shows the diffractograms of the three materials after drying at 100 °C and before being their reused. For SZ, the characteristic pattern of the crystalline tetragonal phase observed in the original material was still present (Fig. 3a). The plane reflections at 2θ = 30.21°, 35.35°, 50.21°, 59.23°, 60.17°, 62.84°, 74.78° and 81.78° were observed again. In the case of Al-MCM-41, the original structure was lost (Fig. 3b), which helps explain the significant decrease in reaction yield. Fig. 3c shows the plane reflections for montmorillonite at 2θ = 7.0°, 20.0°, 35.0° and 61.8°, which are the same as those of the original catalyst.

Fig. 3 XRD patterns (a) SZ, (b) Al-MCM-41 and (c) montmorillonite modified before being reused.

Oxidation of benzylic alcohols and hydroacylation

In previous studies, we observed that montmorillonite modified with CF₃SO₃H can participate as a Brønsted or Lewis acid catalyst in organic reactions²⁰ or as a generator of free radical species in redox processes.²¹ From this double behaviour, we focus on studying the hydroacylation reaction of alkenes and alkynes with benzylic alcohols (Table 4).²² The process begins with the in situ oxidation of benzyl alcohol 4 to aldehyde 1 in the presence of modified montmorillonite and TEMPO²³ to subsequently catalyse the hydroacylation of alkene or alkyne 2 and generate corresponding ketone 3 (Tables 4 and S11, ESI†). This strategy allowed us to obtain saturated (3a–3e) and α,β-unsaturated (3f–3j) ketones in good yields (55–70%). In the case of aliphatic alcohols, the formation of the final product was not observed; only the corresponding carboxylic acid was obtained. This suggests that, under our reaction conditions, the overoxidation of the aliphatic alcohol occurs significantly faster that the hydroacylation reaction.

Table 4 Hydroacylation of benzylic alcohols^ab

a Reaction conditions: aldehyde 1 (1 mmol), alkene or alkyne 2 (1.5 mmol), TEMPO (10%), RhCl(PPh₃)₃ (5% mmol), 2-amino-3-picoline (20% mmol), aniline (20%) solid acid (50 mg) and toluene (3 mL).b Yield of the isolated product after chromatographic purification.

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To propose a pathway for this direct hydroacylation reaction from benzyl alcohol 4a, a series of control experiments were carried out. Fe³⁺ was detected in a sample of modified montmorillonite by EPR (150 mT and g = 4.36, Scheme 2); thus, the treatment with CF₃SO₃H may dissolve structural iron in the montmorillonite, allowing these free cations to move to the interlayer of the montmorillonite where the oxidation of benzyl alcohol to aldehyde is catalysed. In the absence of montmorillonite or its replacement by another free-Fe³⁺ solid acid (SZ or Al-MCM-41), ketone 3a was only detected at 10%. When the reaction was carried out in N₂ atmosphere, no final product formation was observed. In the absence of Wilkinson's complex and long reaction times (1 to 7 h), benzoic acid was generated in 65% yield. Moreover, in the absence of TEMPO, a complex mixture of benzyl alcohol oligomerization products was observed via NMR. Based on these results, a putative reaction mechanism is shown in Scheme 2. The Fe³⁺ present in the modified montmorillonite is responsible for the oxidation of TEMPO to generate TEMPO⁺, which oxidises alcohol 4a to aldehyde 1a.²⁴ Air acts as an oxidant in the first step of the process, promoting the oxidation of Fe²⁺ to Fe³⁺ from montmorillonite. On the other hand, protons and aluminium ions present in solid acids (Brønsted or Lewis acid sites) are responsible for catalysing the formation of imine 5a. Then, Wilkinson's complex participates in the second part of the process,²⁵ the hydroacylation of the imine to obtain 7a via the rhodium–aminoacyl complex 6a. Finally, the hydrolysis of 7a afforded saturated ketone 3a.

Scheme 2 Proposed mechanism for the hydroacylation of benzyl alcohols.

Conclusions

In summary, we demonstrated that different solid acids (sulfated zirconia, Al-MCM-41 or montmorillonite superacid) can be used as cocatalysts in the hydroacylation of alkenes and alkynes via an imine intermediate and in the presence of Wilkinson's complex. Due to the stability of SZ, Al-MCM-41 and modified montmorillonite under reaction conditions, the catalysts can be reused with similar yields. Additionally, the catalytic process was carried out from benzylic alcohols with in situ oxidation of alcohol in the presence of modified montmorillonite and TEMPO to afford the corresponding aldehyde. In the latter case, we demonstrate the usefulness of the dual character of montmorillonite by utilizing it first in a redox process and then as an acid catalyst.

Experimental

General information

Trifluoromethanesulfonic acid, zirconium n-propoxide (70% n-propanol), 2-propanol, sulfuric acid, cetyltrimethylammonium bromide, triethylamine, tetramethylammonium hydroxide, TEOS, aluminium nitrate nonahydrate, benzaldehyde, chloridotris(triphenylphosphine)rhodium(I), 2-amino-3-picoline, aniline, 1-butene, 1-pentene, 1-hexene, 1-nonene, 1-dodecene, phenylacetylene, 1-ethynyl-4-nitrobenzene, 1-ethynyl-3-nitrobenzene, 1-chloro-2-ethynylbenzene, 1-bromo-4-ethynylbenzene, p-anisaldehyde, acetaldehyde, toluene, hexane, and ethyl acetate were purchased from Sigma-Aldrich. Microwave irradiation experiments to were performed using a Discover System (CEM Corporation) single-mode microwave with standard sealed microwave glass vials. The organic reactions were monitored by TLC carried out on 0.25 mm Merck silica gel plates. The developed TLC plates were visualised under a short-wave UV lamp or by heating after they were dipped in Ce(SO₄)₂. Flash column chromatography (FCC) was performed using silica gel (230–400) and employed a solvent polarity correlated with the TLC mobility. Yields refer to the chromatographically and spectroscopically (¹H and ¹³C) homogeneous materials. NMR experiments were conducted on a Varian 300 or Bruker 500 MHz instruments in CDCl₃ (99.9% D) and CD₃OD (99.8% D) as solvents; the chemical shifts (δ) were referenced to CHCl₃ (7.26 ppm ¹H, 77.00 ppm ¹³C), CH₃OH (4.87 ppm ¹H, 49.00 ppm ¹³C), or TMS (0.00 ppm). The chemical shifts are reported in parts per million (ppm) and coupling constants J are given in hertz (Hz). The elemental analyses were carried out in an Elemental Analyzer Thermo Scientific/Flash 2000 equipment. The measurements of EPR were made with a Jeol JES-TE300 X band fashions spectrometer with a cylindrical cavity in the mode TE011. The external calibration of the magnetic field was carried out with a precision gaussmeter, Jeol ES-FC5. Powder X-ray diffraction (XRD) was performed using a Stoe Stadi-P Cu diffractometer with Cu Kα1 (using 40 kV and 30 mA). The nitrogen adsorption–desorption analysis of the materials was obtained at −196 °C on Micromeritics ASAP 2020 equipment. The chemical compositions of natural montmorillonite and modified montmorillonite were determined by energy-dispersive X-ray spectroscopy (EDXS), using an electronic spectrometer microprobe EPMA, JXA8900-R, JEOL. FT-IR spectra were recorded on a Nicolet Magna 750 spectrometer and data collection was performed using DRIFT.

Synthesis of sulfated zirconia (SZ)

Zirconium n-propoxide (20 mL, 70% n-propanol) and 2-propanol (30 mL) were added to a 250 mL Erlenmeyer flask and stirred with a magnetic bar. Acid solution (1 mL 98% sulfuric acid in 3.2 mL distilled H₂O) was added dropwise to hydrolyse the zirconium n-propoxide to obtain a gel. The solid was filtered and dried at 80 °C until complete alcohol evaporation, then calcined in air at 600 °C for 6 h.

Synthesis of Al-MCM-41

Cetyltrimethylammonium bromide (7.3 g) was mixed with deionized H₂O (200 mL) at 30 °C, then triethylamine (3.3 mL) was added. After stirring for 15 minutes, TMAOH (18 mL, 10 wt%), TEOS (22.4 mL) and Al(NO₃)₃·9H₂O (18.75 g) were added, and the solution was stirred for 1 hour. The resulting gel, with molar composition 1TEOS : 0.2CTMABr : 0.6EA : 0.2TMAOH : 0.05Al(NO₃)₃·9H₂O : 150H₂O, was placed in glass bottles and sonicated for 4 h. The precipitated solid was recovered by filtration and washed with deionized water, dried at 80 °C overnight and calcinated at 540 °C for 6 h under air flow.

Modification of montmorillonite

50 g of natural montmorillonite was ground in a mortar and suspended in 1000 mL of deionized H₂O. The mixture was stirred for 24 h and the suspended montmorillonite was separated by centrifugation (600 rpm for 15 min). This process was repeated three times and then the montmorillonite was dried at 100 °C under vacuum for 72 h to obtain a white solid. 10 g of this solid was suspended in 300 mL of deionized H₂O and stirred for 72 h. Subsequently, a solution of CF₃SO₃H (100 mL, 0.18 M) was added, and the mixture was stirred for 24 h. The solid was recovered by vacuum filtration and washed with acetone (50 mL). Finally, the montmorillonite was dried at 100 °C under vacuum for 72 h to obtain a white solid.

Standard reaction procedure to hydroacylation reaction from aldehydes

In a 10 mL tube of microwave with stir bar, 50 mg of solid acid, 1 mmol of aldehyde 1, 1.5 mmol of alkene or alkyne 2, 5% mmol of RhCl(PPh₃)₃, 20% mmol of 2-amino-3-picoline, 20% mmol of aniline and 3 mL of toluene were added. It was irradiated for 1 h in the microwave at 80 °C. The mixture was dried in a rotavapor and purified for chromatography column.

Standard reaction procedure to oxidation of benzylic alcohols and hydroacylation

In a 10 mL tube of microwave with stir bar, 50 mg of solid acid, 1 mmol of alcohol 4, 1.5 mmol of alkene or alkyne 2, 10% TEMPO, 5% mmol of RhCl(PPh₃)₃, 20% mmol of 2-amino-3-picoline, 20% mmol of aniline and 3 mL of toluene were added. It was irradiated for 2 h in the microwave at 80 °C. The mixture was dried in a rotavapor and purified for chromatography column.

Characterization data

1-Phenylpentan-1-one 3a. ¹H NMR (CDCl₃, 300 MHz): δ 7.96 (ddd, J = 8.4, 2.4, 1.2 Hz, 2H), 7.54 (tt, J = 7.5, 2.4 Hz, 1H), 7.44 (m, 2H), 2.96 (t, J = 7.2 Hz, 2H), 1.71 (quint, J = 7.2 Hz, 2H), 1.40 (sext, J = 7.5 Hz, 2H), 0.95 (t, J = 7.5 Hz, 3H). ¹³C NMR (CDCl₃, 75 MHz): δ 200.6, 137.2, 132.9, 128.6, 128.1 38.3, 26.5, 22.5, 14.0. Anal. calcd for C₁₂H₁₆O: C, 81.44; H, 8.70. Found: C, 81.39; H, 8.65.

1-Phenylhexan-1-one 3b. ¹H NMR (CDCl₃, 300 MHz): δ 7.96 (m, 2H), 7.52 (m, 1H), 7.43 (m, 2H), 2.93 (t, J = 7.5 Hz, 2H), 1.72 (quint, J = 7.5 Hz, 2H), 1.36 (m, 4H), 0.89 (t, J = 6.3 Hz, 3H). ¹³C NMR (CDCl₃, 75 MHz): δ 200.5, 137.2, 132.8, 128.5, 128.0, 38.6, 31.6, 24.1, 22.5, 13.9. Anal. calcd for C₁₂H₁₆O: C, 81.77; H, 9.15. Found: C, 81.72; H, 9.09.

1-Phenylheptan-1-one 3c. ¹H NMR (CDCl₃, 300 MHz): δ 7.95 (ddd, J = 8.1, 2.4, 1.2 Hz, 2H), 7.52 (tt, J = 7.2, 2.4 Hz, 1H), 7.42 (m, 2H), 2.94 (t, J = 7.5 Hz, 2H), 1.72 (quint, J = 7.5 Hz, 2H), 1.33 (m, 6H), 0.88 (t, J = 6.3 Hz, 3H). ¹³C NMR (CDCl₃, 75 MHz): δ 200.4, 137.2, 132.8, 128.5, 128.0, 38.6, 31.7, 29.0, 24.3, 22.5, 14.0. Anal. calcd for C₁₃H₁₈O: C, 82.06; H, 9.53. Found: C, 81.97; H, 9.46.

1-Phenyldecan-1-one 3d. ¹H NMR (CDCl₃, 300 MHz): δ 7.96 (ddd, J = 8.1, 2.4, 1.5 Hz, 2H), 7.54 (tt, J = 7.5, 2.4 Hz, 1H), 7.44 (m, 2H), 2.95 (t, J = 7.8 Hz, 2H), 1.73 (quint, J = 7.8 Hz, 2H), 1.30 (m, 12H), 0.87 (t, J = 6.9 Hz, 3H). ¹³C NMR (CDCl₃, 75 MHz): δ 200.5, 137.0, 132.7, 128.4, 128.0, 38.5, 31.8, 29.4, 29.3, 29.2, 24.3, 22.6, 14.0. Anal. calcd for C₁₆H₂₄O: C, 82.70; H, 10.41. Found: C, 82.63; H, 10.34.

1-Phenyldodecan-1-one 3e. ¹H NMR (CDCl₃, 300 MHz): δ 7.96 (ddd, J = 7.2, 2.4, 1.2 Hz, 2H), 7.54 (tt, J = 7.2, 2.4 Hz, 1H), 7.44 (m, 2H), 2.95 (t, J = 7.8 Hz, 2H), 1.73 (quint, J = 7.8 Hz, 2H), 1.34 (m, 2H), 1.26 (m, 14H), 0.87 (t, J = 6.9 Hz, 3H). ¹³C NMR (CDCl₃, 75 MHz): δ 200.7, 137.2, 132.9, 128.6, 128.1, 38.7, 32.0, 29.7, 29.6, 29.5, 29.4, 24.5, 22.8, 14.2. Anal. calcd for C₁₈H₂₈O: C, 83.02; H, 10.84. Found: C, 82.97; H, 10.77.

(E)-Chalcone 3f. ¹H NMR (CDCl₃, 300 MHz): δ 8.02 (dd, J = 8.1, 2.7, 1.2 Hz, 2H), 7.81 (d, J = 15.6 Hz, 1H), 7.63 (m, 2H), 7.53 (m, 5H), 7.42 (m, 2H). ¹³C NMR (CDCl₃, 75 MHz): δ 190.6, 144.9, 138.3, 135.0, 132.9, 130.6, 129.0, 128.7, 128.6, 128.5. Anal. calcd for C₁₆H₁₃BrO: C, 86.51; H, 5.81. Found: C, 86.47; H, 5.76.

(E)-3-(4-Nitrophenyl)-1-phenylprop-2-en-1-one 3g. ¹H NMR (CDCl₃, 300 MHz): δ 8.26–7.53 (m, 11H). ¹³C NMR (CDCl₃, 75 MHz): δ 189.6, 148.5, 141.4, 141.0, 137.5, 133.3, 128.9, 128.8, 128.5, 125.7, 124.1. Anal. calcd for C₁₅H₁₁NO₃: C, 71.14; H, 4.38; N, 5.53. Found: C, 71.09; H, 4.31; N, 5.45.

(E)-3-(3-Nitrophenyl)-1-(p-tolyl)prop-2-en-1-one 3h. ¹H NMR (CDCl₃, 300 MHz): δ 8.49 (dd, J = 2.1, 1.8 Hz, 1H), 8.24 (ddd, J = 8.1, 2.1, 0.9 Hz, 1H), 7.96 (AA′BB′, d, J = 8.4 Hz, 2H), 7.91 (d, J = 7.8 Hz, 1H), 7.81 (d, J = 15.9 Hz, 1H), 7.65 (d, J = 15.9 Hz, 1H), 7.62 (dd, J = 8.1, 7.8 Hz, 1H), 7.33 (AA′BB′, d, J = 8.4 Hz, 2H), 2.44 (s, 3H). ¹³C NMR (CDCl₃, 75 MHz): δ 189.1, 148.8, 144.3, 141.2, 136.8, 135.1, 134.3, 130.1, 129.6, 128.8, 124.7, 124.6, 122.4, 21.8. Anal. calcd for C₁₆H₁₃NO₃: C, 71.9; H, 4.9; N, 5.24. Found: C, 71.81; H, 4.83; N, 5.18.

(E)-3-(2-Chlorophenyl)-1-phenylprop-2-en-1-one 3i. ¹H NMR (CDCl₃, 300 MHz): δ 8.17 (d, J = 15.9 Hz, 1H), 8.01 (d, J = 7.5 Hz, 2H), 7.74 (m, 1H), 7.51 (m, 5H), 7.32 (m, 2H). ¹³C NMR (CDCl₃, 75 MHz): δ 190.5, 140.7, 138.0, 135.5, 133.3, 133.0, 131.2, 130.4, 128.8, 128.7, 127.9, 127.2, 124.9. Anal. calcd for C₁₅H₁₁ClO: C, 74.23; H, 4.57. Found: C, 74.17; H, 4.51.

(E)-3-(4-bromophenyl)-1-(p-tolyl)prop-2-en-1-one 3j. ¹H NMR (CDCl₃, 300 MHz): δ 7.87 (AA′BB′, m, 2H), 7.78 (d, J = 15.1 Hz, 1H), 7.62 (AA′BB′, m, 2H), 7.59 (AA′BB′, m, 2H), 7.34 (d, J = 15.1 Hz, 1H), 6.93 (AA′BB′, m, 2H), 3.85 (s, 3H). ¹³C NMR (CDCl₃, 75 MHz): δ 189.2, 161.7, 145.1, 137.1, 131.7, 130.2, 129.8, 127.4, 127.3, 119.0, 114.3, 55.2. Anal. calcd for C₁₆H₁₃BrO: C, 63.81; H, 4.35. Found: C, 63.76; H, 4.29.

4-Phenylbutan-2-one 3k. ¹H NMR (CDCl₃, 300 MHz): δ 7.34 (m, 2H), 7.25 (m, 3H), 2.96 (t, J = 8.0 Hz, 2H), 2.79 (t, J = 8.0 Hz, 2H), 2.17 (s, 3H). ¹³C NMR (CDCl₃, 75 MHz): δ 207.4, 140.8, 128.2, 128.0, 125.9, 44.79, 29.48. Anal. calcd for C₁₀H₁₂O: C, 81.04; H, 8.16. Found: C, 81.35; H, 8.46.

(E)-4-Phenylbut-3-en-2-one 3l. ¹H NMR (CDCl₃, 300 MHz): δ 7.54 (m, 2H), 7.52 (d, J = 12.3 Hz, 2H), 7.40 (m, 3H), 6.72 (d, J = 12.3 Hz, 2H), 2.38 (s, 3H). ¹³C NMR (CDCl₃, 75 MHz): δ 198.5, 143.6, 134.6, 130.6, 129.0, 128.5, 127.3, 27.5. Anal. calcd for C₁₀H₁₀O: C, 82.16; H, 6.90. Found: C, 82.29; H, 6.98.

Data availability

The datasets supporting this article have been uploaded as part of the ESI.†

Author contributions

J. A. M. S. and R. G. conceived the project and acquired the funds. B. I. V. A., E. G. R., R. G., J. C. and J. A. M. S designed the experiments. B. I. V. A., E. G. R., A. M. G., E. A. J. A. and A. L. T. conducted the experimental work. J. C., R. G. and J. A. M. S. coordinated the whole project. B. I. V. A., E. G. R., R. G., J. C. and J. A. M. S wrote the manuscript. All the authors contributed to the discussions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

J. A. M. S is grateful to the Royal Society (UK) for the International Alumni Grant (AL\180003 and AL\191055).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: General experimental procedures, full characterization of materials, analytical data of individual compounds and ¹H, ¹³C NMR spectra of the synthesised compounds. See DOI: https://doi.org/10.1039/d4ra05791k

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