Efficient microwave-assisted regioselective one

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Efficient microwave-assisted regioselective one pot direct ortho-formylation of phenol derivatives in the presence of nanocrystalline MgO as a solid base catalyst under solvent-free conditions† Hossein Naeimi

* and Elham Zakerzadeh

In this research, at first nanocrystalline MgO was prepared and then the solvent-free reactions of phenol Received 5th December 2017, Accepted 14th February 2018

derivatives with paraformaldehyde in the presence of the obtained nanocrystalline MgO as a new catalyst

DOI: 10.1039/c7nj04794k

yielded as products. This method seems to be comparable with other reported methods due to its high

under microwave irradiation were investigated. In this reaction, ortho-hydroxyaromatic aldehydes were yield and regioselectivity. The significant features of this method are short reaction times, high yields, and

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easy and quick isolation of the products.

Introduction Formylation of aromatic compounds is an important and classical reaction in organic chemistry for the synthesis of salicylaldehyde, and numerous methods are available.1 Salicylaldehydes are excellent precursors for the preparation of important classes of organic compounds such as salen derivatives.2 By directed ortho-metallation of an activated phenol, a formyl group can be selectively introduced, but this methodology requires the introduction and removal of the activating group for the synthesis of salicylaldehydes.3 On the other hand, salicylaldehydes are accessible from the corresponding phenols by several classical formylation reactions. However, for many of these reactions, the yields of salicylaldehydes are often only moderate and the lack of regioselectivity is problematic.4 Moreover, the reaction conditions are quite harsh and pose safety problems at a large scale, and environmentally disagreeable reagents are required. The regioselectivity is even more of a problem for 1,3-dihydroxylated phenols. Significant industrial demand exists for ortho-formylated phenols and their derivatives, for example as intermediates for the synthesis of various pharmaceuticals, agrochemicals, fragrance chemicals, mining chemical ligands and other products.5 Over the years, many formylation methods have been introduced, for example Duff,6,7 Reimer-Tiemann,8 Vilsmeier-Haack9 or Gattermann-Koch10 reactions. Unfortunately, these reactions are hampered by one or more disadvantages, for example the use of large amounts of Lewis acids, hazardous reagents, difficult separations of isomers, multistage synthesis and poor yields and/or poor regioselectivity. Department of Organic Chemistry, Faculty of Chemistry, University of Kashan, Kashan 87317, I.R. Iran. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj04794k

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Today, there is a general need for regioselective and more environmentally friendly synthetic methods. Solid strong bases are extremely desirable for developing environmentally benign processes to catalyze various reactions under mild conditions and to minimize the production of pollutants.11 Magnesium oxide has been extensively used in catalysis, toxic waste remediation, refractory materials, paint, superconductors and insulation applications.12 In the field of catalysis, MgO shows strongly basic property that can be applied to catalyze many organic reactions, such as alcohol dehydrogenation,13,14 aldol condensation,15 Meerwein–Ponndorf– Verley reaction,16 Wittig reaction,17,18 Friedel–Crafts benzylation reaction,19 isomerization of alkenes,20,21 cycloaddition of CO2 to epoxides22 and catalytic transfer hydrogenation.23,24 It can also be used as the catalyst support in the reforming reaction of methane with CO2 for anticoking.25 Thus, it is very desirable to develop methods to produce in a controllable manner highly porous MgO with high specific surface area.26 In this research, we hope to report regioselective orthoformylation of substituted phenols using a nano MgO base system and paraformaldehyde to afford salicylaldehyde derivatives in excellent yields. The salicylaldehydes obtained by this method are significant intermediates that can be applied for the preparation of various useful organic products.

Experimental Reagents and samples Chemicals were purchased from the Merck and Fluka Chemical Companies in high purity. All of the materials were of commercial reagent grade.

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Instrumentation IR spectra were recorded as KBr pellets on a Perkin-Elmer 781 spectrophotometer and an Impact 400 Nicolet FT-IR spectrophotometer. 1H NMR and 13C NMR spectra were recorded in DMSO-d6/CDCl3 solvents on a Bruker DRX-400 spectrometer with tetramethylsilane as an internal reference. The elemental analyses (C, H, N) were obtained from a Carlo ERBA Model EA 1108 analyzer. Electron Ionization Mass (EI-MASS) spectra were recorded on an Agilent Technology (HP) 5973 instrument at an ionization potential of 70 eV. X-ray powder diffraction (XRD) measurements were performed using an X’pert diffractometer of Philips Company with monochromatized Cu Ka radiation. The crystallite sizes of selected samples were estimated using the Scherrer method. The samples were characterized using a scanning electron microscope (SEM) (Philips XL 30) with gold coating. The microwave irradiations were carried out in a microwave oven specially designed for organic synthesis (Milestone LAVIS 1000 Basic Microwave). Melting points obtained with a Yanagimoto micro melting point apparatus are uncorrected. The purity determination of the substrates and reaction monitoring were accomplished by TLC on silica-gel polygram SILG/UV 254 plates (from Merck Company). General procedure for the synthesis of magnesium oxide nanostructures To prepare the Mg(OH)2 precursor, a NaOH solution with a concentration of 0.1 M was added to a 0.1 M solution of Mg(NO3)2
6H2O in distillated H2O. Then the suspension was ultrasonically irradiated with a high-density ultrasonic probe immersed directly into the solution. To prepare MgO powders the obtained precipitate was calcinated at 400 1C in a furnace. General procedure for the synthesis of ortho-hydroxy benzaldehydes under microwave irradiation ortho-Hydroxy benzaldehyde derivatives were prepared according to the procedure as follows: 80 mmol of dry paraformaldehyde (2.4 g) was added to a mixture of 12 mmol of the desired phenol derivative and 2.5 mmol of MgO nanocrystalline (0.1 g). The resulting mixture was exposed under microwave irradiation with a power of 650 W. The progress of the reaction was monitored by thin layer chromatography (TLC) developed by n-hexane : ethyl acetate (8 : 2). After completion of the reaction, 100 mL of sulfuric acid (15% w/w) was added to the reaction mixture and heated at 50 1C for 15 min. Then, the reaction mixture was cooled to room temperature and the product was extracted by dichloromethane (2  50 mL). The organic layer was dried over anhydrous magnesium sulphate; the solution resulting from filtration was evaporated to obtain the crude product. In most cases the product was sufficiently pure for further use; the other products were purified by column chromatography using n-hexane : ethyl acetate (95 : 5 to 70 : 30). 2-Hydroxy-3-methyl benzaldehyde; yellow liquid; b.p. = 208 1C; IR (KBr)/n (cm 1): 3300–3500 (O–H, phenol), 2854, 2742 (C–H, aldehyde), 1641 (CQO, aldehyde), 1476 (CQC, Ar), 1201 (C–O); 1H NMR (400 MHz, DMSO)/d (ppm): 11.32

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(s, 1H, O–H), 9.86 (s, 1H, aldehyde), 7.42 (d, 1H, J = 2.41 Hz), 6.69 (s, 1H, Ar), 6.45 (d, 1H, J = 2.41 Hz), 2.32 (s, 3H); 13 C NMR/(100 MHz, CDCl3)/d (ppm): 189.3, 156.3, 135.9, 131.1, 123.6, 120.4, 118.4, 58.9. 2-Hydroxy-4-methyl benzaldehyde; yellow solid; m.p. = 60–62 1C; IR (KBr)/n (cm 1): 3420–3500 (O–H, phenol), 2980 (C–H aldehyde), 1690 (CQO, aldehyde), 1570, 1480 (CQC, Ar), 1270 (C–O); 1H NMR (400 MHz, CDCl3)/d (ppm): 9.95 (s, 1H, O–H), 8.9 (s, 1H, aldehyde), 7.17 (d, 1H, J = 2.41 Hz), 7.01 (d, 1H, J = 8.24 Hz), 6.80 (d, 1H, J = 2.41 Hz), 2.60 (s, 3H); 13C NMR/ (100 MHz, CDCl3)/d (ppm): 196.7, 162.6, 149.3, 132.8, 124.9, 119.5, 117.2, 21.3. 2-Hydroxy-5-methyl benzaldehyde; yellow solid; m.p. = 55–57 1C; IR (KBr)/n (cm 1): 3200–3500 (O–H, phenol), 2862, 2736 (C–H aldehyde), 1651 (CQO, aldehyde), 1590, 1502 (CQC, Ar), 1215 (C–O); 1H NMR (400 MHz, DMSO)/d (ppm): 9.40 (s, 1H, O–H), 9.05 (s, 1H, aldehyde), 6.77 (s, 1H), 6.68 (dd, 1H, J = 8.24 Hz, J = 2.24 Hz), 6.59 (d, 1H, J = 2.41 Hz), 2.11 (s, 3H); 13C NMR/(100 MHz, CDCl3)/d (ppm): 198.2, 160.5, 135.2, 133.1, 130.3, 119.9, 117.0, 20.1. 2-Hydroxy-5-ethyl benzaldehyde; yellow solid; m.p. = 55–57 1C; IR (KBr)/n (cm 1): 3200–3400 (O–H, phenol), 2869 (C–H, aldehyde), 1651 (CQO, aldehyde), 1610, 1510 (CQC, Ar), 1226 (C–O); 1H NMR (400 MHz, CDCl3)/d (ppm): 11.19 (s, 1H, O–H), 9.90 (s, 1H, aldehyde), 7.61 (s, 1H), 7.34 (d, J = 6.2 Hz, 1H), 7.17 (d, J = 6.2 Hz, 1H), 2.62 (q, 2H, J = 8.1 Hz), 1.25 (t, 3H, J = 8.1 Hz); 13C NMR/(100 MHz, CDCl3)/d (ppm): 198.2, 163.0, 135.8, 133.4, 132.0, 120.8, 118.4, 28.2, 15.7. 2-Hydroxy-5-isopropyl benzaldehyde; yellow solid; m.p. = 63–65 1C; IR (KBr)/n (cm 1): 3200–3400 (O–H, phenol), 2871 (C–H, aldehyde), 1654 (CQO, aldehyde), 1590, 1501 (CQC, Ar), 1240 (C–O); 1H NMR (400 MHz, DMSO)/d (ppm): 10.98 (s, 1H, O–H), 9.93 (s, 1H, aldehyde), 7.45 (s, 1H), 7.25 (s, 1H), 7.08 (d, 1H, J = 2.0 Hz), 4.01 (m, 1H), 2.9 (d, 6H, J = 7.7 Hz); 13 C NMR/(100 MHz, CDCl3)/d (ppm): 198.2, 161.4, 142.3, 132.8, 127.9, 119.8, 116.8, 34.55, 24.2. 2-Hydroxy-5-tertbuthyl benzaldehyde; yellow solid; m.p. = 51–53 1C; IR (KBr)/n (cm 1): 3200–3400 (O–H, phenol), 2862 (C–H, aldehyde), 1640 (CQO, aldehyde), 1583, 1502 (CQC, Ar), 1276 (C–O); 1H NMR (400 MHz, DMSO)/d (ppm): 11.13 (s, 1H, O–H), 9.83 (s, 1H, aldehyde), 7.26 (s, 1H), 7.19 (s, 1H, Ar), 7.03 (s, 1H, Ar), 2.34 (s, 9H); 13C NMR/(100 MHz, CDCl3)/d (ppm): 198.1, 160.3, 144.9, 131.1, 128.7, 120.4, 117.4, 31.5, 25.0. 2-Hydroxy-3,5-dimethnyl benzaldehyde; yellow liquid; b.p. = 223–224 1C; IR (KBr)/n (cm 1): 3300–3500 (O–H, phenol), 2860, 2736 (C–H, aldehyde), 1646 (CQO, aldehyde), 1508, 1481 (CQC, Ar), 1263 (C–O); 1H NMR (400 MHz, DMSO)/d (ppm): 11.25 (s, 1H, O–H), 9.79 (s, 1H, aldehyde), 6.98 (d, 1H, J = 8.0 Hz), 6.80 (d, 1H, J = 7.6 Hz), 2.38 (s, 6H); 13C NMR/ (100 MHz, CDCl3)/d (ppm): 190.7, 155.9, 135.1, 131.3, 127.9, 125.0, 120.8, 21.0, 15.9. 2-Hydroxy-3,5-ditertbuthyl benzaldehyde; yellow solid; m.p. = 53–55 1C; IR (KBr)/n (cm 1): 3200–3500 (O–H, phenol), 2868, 2741 (C–H, aldehyde), 1652 (CQO, aldehyde), 1607, 1506 (CQC, Ar), 1213 (C–O); 1H NMR (400 MHz, CDCl3)/d (ppm): 11.65 (s, 1H, OH), 9.88 (s, 1H, aldehyde), 7.60 (s, 1H), 7.35

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(s, 1H), 1.44 (s, 9H), 1.34 (s, 9H); 13C NMR/(100 MHz, CDCl3)/ d (ppm): 190.8, 156.0, 135.1, 131.3, 127.9, 125.0, 120.8, 21.0, 15.9. 2-Hydroxy naphtaldehyde; dusty solid; m.p. = 83–85 1C; IR (KBr)/n (cm 1): 3300–3500 (O–H, phenol), 2888, 2766 (C–H, aldehyde), 1642 (CQO, aldehyde), 1592, 1464 (CQC, Ar), 1247 (C–O);1H NMR (400 MHz, CDCl3)/d (ppm): 13.18 (s, 1H, O–H), 10.83(s, 1H, aldehyde), 8.36 (s, 1H), 8.0 (s, 1H), 7.81 (s, 1H), 7.63 (s, 1H), 7.45 (s, 1H), 7.16 (s, 1H); 13C NMR (100 MHz, CDCl3)/ d (ppm): 193.2, 164.9, 139.1, 132.8, 129.5, 129.1, 127.8, 124.5, 119.1, 118.6, 111.2. 2,3-Dihydroxy benzaldehyde; gray solid; m.p. = 106–108 1C; IR (KBr)/n (cm 1): 3200–3400 (O–H, phenol), 2875, 2723 (C–H, aldehyde), 1653 (CQO, aldehyde), 1585, 1484 (CQC, Ar), 1233 (C–O); 1H NMR (400 MHz, CDCl3)/d (ppm): 11.10 (s, 1H, O–H), 9.91 (s, 1H, aldehyde), 7.19 (dd, 2H, J = 15.6 Hz, J = 7.6 Hz), 6.96 (d, 1H, J = 7.2 Hz), 5.69 (s, 1H, O–H); 13C NMR (100 MHz, DMSO)/ d (ppm): 193.50, 150.16, 146.56, 123.08, 122.06, 120.31, 119.78. 2,4-Dihydroxy benzaldehyde; pink solid; m.p. = 136–138 1C; IR (KBr)/n (cm 1): 3100–3300 (O–H, phenol), 2785, 2733 (C–H, aldehyde), 1629 (CQO, aldehyde), 1581, 1499 (CQC, Ar), 1229 (C–O); 1H NMR (400 MHz, CDCl3)/d (ppm): 11.43 (s, 1H, O–H), 9.71 (s, 1H, aldehyde), 7.44 (d, 1H, J = 7.2 Hz), 6.50 (d, 1H, J = 6.0 Hz), 6.40 (s, 1H), 6.06 (s, 1H, O–H); 13C NMR (100 MHz, DMSO)/d (ppm): 191.82, 165.66, 163.74, 133.66, 115.56, 109.11, 102.65. 2,5-Dihydroxy benzaldehyde; green solid; m.p. = 98–100 1C; IR (KBr)/n (cm 1): 3200–3400 (O–H, phenol), 2877, 2730 (C–H, aldehyde), 1650 (CQO, aldehyde), 1577, 1487 (CQC, Ar), 1284 (C–O); 1H NMR (400 MHz, CDCl3)/d (ppm): 10.62 (s, 1H, O–H), 9.82 (s, 1H), 7.10 (dd, 1H, J = 8.8 Hz, J = 2.8 Hz), 7.02 (d, 1H, J = 2.8 Hz), 6.92 (d, 1H, J = 8.8 Hz), 5.15 (s, 1H, O–H); 13C NMR (100 MHz, DMSO)/d (ppm): 192.17, 154.39, 150.46, 125.02, 122.62, 118.67, 113.56.

Results and discussion Characterization of nanocrystalline MgO Magnesium oxide is considered as one of the most important compounds of the magnesium industry. It is used in many important applications, such as catalysis, refractory materials, pharmaceuticals, toxic waste remediation, paint, superconductors and the glass industry.27 MgO has a simple sodium chloride structure and thermal stability. It has been reported that nanoscale MgO exhibits different reactivities from conventionally prepared MgO.28,29 The previously reported studies have shown that nanocrystalline MgO exhibits remarkable reactivity and rates of adsorption primarily due to some main properties, such as huge surface area, high concentration of low coordinated sites, structural defects on their surface and usage as an antibacterial agent.30–33 Various MgO structures, such as nanocrystals, nanoparticles, nanotubes, nanowires, and nanosheets have been fabricated successfully. Commercial MgO powders, which are usually produced by thermal-decomposition of magnesium hydroxide or carbonate, exhibit small specific surface areas, in homogeneous morphologies, and varied grain sizes, all of which are disadvantageous for their application.34 Therefore, many researchers have

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Scheme 1

The formation of MgO nanocrystals.

endeavoured to synthesize MgO with a higher specific surface area and/or a uniform porous structure. The reaction between magnesium nitrate and sodium hydroxide was performed under ultrasonic irradiation to prepare magnesium hydroxide, then, this was calcined at 400 1C to afford nanocrystalline magnesium oxide (Scheme 1). Fig. 1a shows the XRD pattern of Mg(OH)2 prepared by the sonochemical process in distillated H2O and Fig. 1b shows the XRD pattern of the above sample after heating at 400 1C. The obtained XRD patterns are matched with the standard patterns of Mg(OH)2 and MgO. The crystalline phases of Mg(OH)2 and MgO are hexagonal and cubic, with space groups P3% m1 and Fm3m and the lattice parameters a = 3.1250 Å, c = 4.7500 Å, z = 1 and a = 4.2130 Å, z = 4 for Mg(OH)2 and MgO, respectively, which are close to the reported values (JCPDS cards number 07-0239 and 04-0829). The sharp diffraction peaks of the sample indicated that well-crystallized MgO crystals can be easily obtained under the current synthetic conditions. Estimated from the Scherrer formula, D = 0.891l/b cos y, where D is the average crystallite size, l is the X-ray wavelength (0.15405 nm), and y and b are the diffraction angle and full-width at a half maximum of an observed peak, respectively,35 the average size of the crystals of sample number 1 (Mg(OH)2 particles) was 21 nm and for the above sample after heating at 400 1C (MgO particles) it was 15 nm. The surface morphology, structure, dispersity and size of the catalyst were investigated by using a Scanning Electron Microscopy (SEM) technique (Fig. 2). The SEM analysis displayed that the MgO was well-dispersed nanosheets. Also, a TEM image of MgO is shown in Fig. 3. As can be seen, the sample has a nanocrystalline structure with a plate-like shape. In order to select the optimum conditions for formylation of phenolic derivatives, the method was evaluated for various parameters such as the type of base, molar ratio of the base,

Fig. 1

The XRD pattern of (a) Mg(OH)2 and (b) MgO nanoparticles.

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Table 1

Entry

Base

Yield (%)

1 2 3 4

Calcium oxide Magnesium oxide Nano magnesium oxide Caesium carbonate

— 60 90 50

Table 2

Fig. 2

SEM images of MgO nanoparticles (the scale bar is 200 nm).

Fig. 3

TEM image of nanocrystalline MgO.

microwave power, phenolic derivatives and paraformaldehyde. In all cases, resorcinol was selected as representative for phenolic derivatives and the product yields were recorded as a response. Firstly, the type of base was studied. For this purpose, various bases such as calcium oxide, magnesium oxide, nanosized magnesium oxide and caesium carbonate were investigated in this reaction. The results are given in Table 1. As can be seen, a high yield was achieved in the case of nanosized magnesium oxide. Moreover, the effect of microwave power on the product yield was also investigated in the range of

Yield of formylation of resorcinol with various bases

Yield of the formylation reaction with various microwave powers

Entry

Microwave power (W)

Yield (%)

1 2 3 4 5

100 300 450 650 900

— 30 60 90 70

100–900 W and 650 W was selected as the best power for further studies (Table 2, entry 4). Finally, the effect of all reagent amounts on the yield of product was studied. It was found that the best ratio of phenol : nanoscale MgO : paraformaldehyde was 1 : 2 : 6.7. The obtained results are summarized in Table 3. Under the optimum conditions, the formylation of phenolic derivatives was carried out from their reaction with 12 mmol of phenol with 2.5 mmol of nanoscale MgO and 80 mmol paraformaldehyde. The results are shown in Table 4. As seen in this table, the corresponding products were obtained in excellent yields and appropriate reaction times. Scheme 2 shows the reaction between the phenolic derivatives and paraformaldehyde in the presence of nanoscale magnesium oxide under microwave irradiation. The structures of the products have been assigned by spectroscopic data. In the IR spectra, the stretching frequency of CQO is formed in the region between n = 1630 and 1690 cm 1 as a strong signal band. The OH stretching frequency is found at n = 3000 cm 1 with a particular width. The stretching frequency of aromatic CQC is observed in the region between n = 1480 and 1610 cm 1. The stretching vibration of C–H in the formyl groups appears in the region between n = 2730 and 2890 cm 1. In the 1H NMR spectra, the broad signals at d = 9.40–13.18 ppm are assigned to the protons of the hydroxyl groups. The signal

Table 3 The formylation reaction with various amounts of nano MgO and paraformaldehyde

Entry

Nano MgO (g)

Paraformaldehyde (g)

Microwave power (W)

Yield (%)

1 2 3 4 5 6 7 8 9

0.02 0.04 0.06 0.08 0.10 0.12 0.10 0.10 0.10

0.25 0.25 0.25 0.25 0.25 0.25 0.15 0.20 0.30

650 650 650 650 650 650 650 650 650

35 55 70 78 90 90 10 75 90

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Table 4 Products and yields of formylation of various phenol derivatives with nano MgO

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Entry Substrate

Product

Time (min) Yielda (%)

1

12

80

2

16

85

Table 4

Entry Substrate

10

90

4

17

80

5

17

75

Product

Time (min) Yielda (%)

11

15

90

12

15

80

a

3

(continued)

Isolated yield.

Scheme 2

The formation of ortho-hydroxy benzaldehyde derivatives.

around d = 9.05–10.83 ppm is assigned to the proton of CHQO. The signals around d = 6.40–8.36 ppm are assigned to the protons of CHQCH of aromatic rings. The signal around d = 5.15–6.06 ppm is assigned to the proton of a hydroxyl group of the compounds that contain two hydroxyl groups.

Conclusions 6

20

70

7

15

75

In the present work, a series of ortho-hydroxy benzaldehyde derivatives were obtained by the reaction of phenol derivatives and paraformaldehyde in the presence of nanoscale magnesium oxide under microwave irradiation and solvent-free conditions. All products were obtained in excellent yields, short reaction times, and high purity through an efficient and convenient method under mild conditions.

Conflicts of interest There are no conflicts to declare. 8

20

70

Acknowledgements The authors are grateful to the University of Kashan for supporting this work by Grant No. 159148/89. 9

25

65

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