Ferric hydrogensulfate catalyzed aerobic oxidative coupling of 2-naphthols in water or under solvent free conditions

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Chinese Chemical Letters 20 (2009) 663–667 www.elsevier.com/locate/cclet

Ferric hydrogensulfate catalyzed aerobic oxidative coupling of 2-naphthols in water or under solvent free conditions Hossein Eshghi *, Mehdi Bakavoli, Hassanali Moradi Department of Chemistry, Ferdowsi University of Mashhad, P.O. Box 91775-1436, Mashhad, Iran Received 29 October 2008

Abstract The symmetric oxidative coupling reactions of 2-naphthol derivatives with both ferric hydrogensulfate in water and silica ferric hydrogensulfate in solvent free conditions were carried out. The advantages of this green procedure are: inexpensive catalyst or cocatalyst, reusability of catalyst, organic solvent-free procedures and simple workup. # 2008 Hossein Eshghi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Silica ferric hydrogensulfate; Naphthol; BINOL; Solvent-free

1,10 -Bi-2-naphthol derivatives have successfully been utilized as chiral inducers for stereo- and enantioselective reactions because of their axial dissymmetry and molecular flexibility [1]. Because the enantiomerically pure binaphthols can be easily obtained from their racemates by number of methods including classical resolution via crystallization and diastereoisomeric derivatives [2], formation of inclusion crystals with chiral host molecules [3], deracemization of racemates with cupper complexes of chiral amines [4] or enzymatic hydrolysis of esters [5], it is essential to establish a simple and convenient method for preparation of racemic 1,10 -bi-2-naphthol derivatives. Therefore much attention has been paid to the development of efficient synthesis of 1,10 -binaphthalene-2,20 -diol (BINOL) derivatives, which are versatile source of the various 1,10 -binaphthalene skeletons. Several procedures have been developed for this purpose [6]. For example, the oxidative coupling of 2-naphthols is one of the most useful methods for the synthesis of BINOL derivatives, by using Fe3+, Cu2+, Mn3+ and Ti2+ as oxidants [7]. However, some of these methods suffer from one or more of disadvantages such as long reaction times, use of organic hazardous solvents, and difficult work up, use of stoichiometric excess amounts of the expensive reagents for successful oxidation. In continuation of our finding [8] on chemoselective reactions were carried out in the presence of Fe3+ salts, in this paper we describe an efficient, mild, and fast procedure for 1,10 -bi-2-naphthols (BINOLs) via oxidative coupling of 2naphthols by Fe(HSO4)3 (FHS) in water or Fe(HSO4)3 supported on silica gel (SFHS) under solvent free conditions. 1. Experimental All materials and solvents were purchased from Merck and Fluka. Melting points were determined in open capillary tubes in an Electrothermal IA 9700 melting point apparatus. 1H NMR spectra were recorded on a Bruker-100 MHz * Corresponding author. E-mail address: [email protected] (H. Eshghi). 1001-8417/$ – see front matter # 2008 Hossein Eshghi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2008.12.045

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instrument using tetramethylsilane (TMS) as an internal standard. IR spectra were recorded on a Shimadzu-IR 470 spectrophotometer. Elemental analyses were obtained on a Thermo Finnigan Flash EA microanalyzer. Ferric hydrogensulfate Fe(HSO4)3 was prepared according to previously reported procedure [8a]. Preparation of Fe(HSO4)3/SiO2 (10 mol%) (SFHS) Ferric hydrogensulfate (0.01 mol, 3.47 g) was ground in a mortar and the powder was added to a suspension of chromatographic grade SiO2 (0.1 mol, 6.01 g) in absolute ethanol (50 mL). The yellowish mixture was stirred for 10 h. Then the mixture was concentrated and the solid material was filtered and dried at 120 8C for 2 h. The homogeneous, free flowing and white powder catalyst (SFHS) was obtained in 99% yield (9.45 g). General procedures for oxidative coupling A In to a Pyrex-glass vial fitted with a reflux condenser were successively placed Fe(HSO4)3 (0.347 g, 1 mmol), naphthol (1 mmol), and water (10 mL). The reaction mixture was vigorously stirred at 100 8C under air atmosphere for appropriate time as indicated in Table 2. After the reaction was completed (TLC), the mixture were filtered off, followed washing with boiling water (2 20 mL). The crude product was then dried and crystallized from ethanol. B A finely mixed 2-naphthol (1 mmol), and (Fe(HSO4)3/SiO2 (10 mol%) (0.947 g, 1 mmol) was heated at 90 8C for an appropriate time as indicated in Table 2. After the reaction was completed (TLC), the reaction mixture was cooled, washed with CHCl3 (2 10 mL), and the catalyst was recovered. The solvent was evaporated and the crude product recrystallized from EtOH to afford the pure product. The catalyst was then washed with acetone, and dried in air. Recycling performance of SFSH was investigated in the oxidative coupling of 2-naphthol. The catalyst could be reused without significant loss of its catalytic activity until five runs. All the products are known compounds and were characterized by IR and NMR spectroscopic data and their melting points are compared with reported values [11–14]. 1

H NMR data

2a: 1H NMR (100 MHz, CDCl3, dppm): 7.9 (d, 2H, J = 8.1 Hz), 7.85 (d, 2H, J = 8.0 Hz), 7.55–7.05 (m, 8H), 5.06 (s, 2H). 2b: 1H NMR (100 MHz, acetone-d6, dppm): 8.5 (s, 2H), 8.0 (s, 2H), 7.66 (d, 2H, J = 8.2 Hz), 7.3–6.95 (m, 8H). 2c: 1H NMR (100 MHz, acetone-d6, dppm): 8.5 (s, 1H), 8.3 (s, 1H), 7.75 (d, 2H, J = 8.3 Hz), 7.70 (d, 2H, J = 8.3 Hz), 7.1 (d, 2H, J = 8.4 Hz), 6.90 (dd, 2H, J = 9.1 and 1.7 Hz), 6.36 (d, 2H, J = 2.4 Hz). 2d: 1H NMR (100 MHz, acetoned6, dppm): 8.25 (s, 2H), 8.10 (d, 2H, J = 2.5 Hz), 7.40 (d, 2H, J = 4.8 Hz), 7.35 (dd, 2H, J = 9.1 and 2.4 Hz), 6.97 (d, 2H, J = 8.6 Hz). 2e: 1H NMR (100 MHz, acetone-d6, dppm): 10.95 (br, 2H), 8.81 (s, 2H), 8.06 (m, 2H), 7.40 (m, 4H), 7.12 (m, 2H), 5.0 (br, 2H).

Scheme 1.

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2. Results and discussion To optimize the reaction conditions, the oxidative coupling with FHS and SFHS reagent systems were examined using 2-naphthol (1a) as substrate (Scheme 1), under air atmosphere in different solvents. The reaction did not proceed at all without catalyst. The use of non-polar solvents such as 1,2-dichloroethane and n-hexane gave the BINOL (2a) in low to moderate yields, and in polar solvents conversion increased to good. The oxidative coupling of (1a) was found to be very slow at low temperatures, but it gave the best conversion yields at temperature of about 100 8C in water as an effective solvent system. Furthermore the results showed that reaction under solvent free conditions using SFHS was carried out at Table 1 Ferric hydrogensulfate catalyzed the oxidative coupling of 2-naphthols in water and solvent-free conditions. Entry

Substrate

Product

Fe(HSO4)3, H2O, reflux Time (h)

1

2c 3c 4c 5c

1a 1a 1a 1a

2a 2a 2a 2a

6

7d

1b

2b

8

conv/yielda,b (%)

216–218

216–218 [11]

– – – –

– – – –

1 1 1 1

95/91 95/90 94/90 93/88

216–218 216–218 216–218 216–218

216–218 216–218 216–218 216–218

0.25

100/92

0.2

100/94

274(dec)

275 [11]

0.2

100/95

–

–

274(dec)

275 [11]

0.6

98/95

0.3

97/92

134–139

131–132 [12]

4

99/97

206–208

198–199 [12]

72

n. r

b c d

Lit [ref]

95/91

10

2e

Found

1

50/30

1e

conv/yielda,b (%)

98/92

10

a

Time (h)

Mp (8C)

0.3

9

11 d

Fe(HSO4)3/SiO2 (10%), grinding, 90 8C

6.5

90/84

24

–

n. r

–

Conversion yields according to the TLC or NMR. Isolated yield. Refer to the recycling of catalyst in new subsequent runs. Reaction was carried out in the presence of 3 equivalents of DMSO at the same condition.

–

300(dec)

[11] [11] [11] [11]

–

300 [14]

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Table 2 Comparison of our results with results obtained by other groups. Entry

1 2 3 4

Product

2a 2b 2c 2d

FeCl3/MgBr CH3NO2, rt [11]

CuSO4/Al2O3, PhCl, 140 8C [12]

FeCl3, H2O/MeOH, 70 8C [13]

Fe(HSO4)3, H2O, 100 8C [present work]

Fe(HSO4)3/SiO2, 90 8C [present work]

Time (h)

Yield (%)

Time (h)

Yield (%)

Time (h)

Yield (%)

Time (h)

Yield (%)

Time (h)

Yield (%)

5 5 – –

77 28 – –

8 – 15 2

97 – 93 98

2 – – 17

60 – – 91

0.3 0.25 0.6 10

92 92 95 30

1 0.20 0.3 4

91 94 92 97

90 8C under air atmosphere gave the best conversion yield of (2a). At the best reaction conditions, no byproducts such as perylens, quinines and polymers could be detected by 1H NMR analysis of the crude reaction mixtures. The present procedures were further applied to the oxidative coupling of substituted naphthols (Scheme 1) and the results are presented in Table 1. The oxidative coupling of substituted 2-naphthols (1a–1e) efficiently proceeded to afford the corresponding BINOL derivatives (2a–2e) in moderate to high yields (entries 1, 6–11). All the substrates studied were converted smoothly and selectively to their corresponding ortho–ortho coupled products in good to excellent yields. The SFHS catalyst was easily recovered and reused for the next set of oxidative coupling of 2naphthol, without significant decrease in activity even after five runs (entries 1–5). These results clearly indicate that substrates with an electron donating group react better and have higher yields at both standard reaction conditions (entries 6–8). This may be because of either lower oxidation potentials (Eox) attributed to higher HOMO orbitals and thus higher ability of generation of oxidized state of substrate naphthol to achieve better results in formation of self coupling product or better ability of reacting as better naphthol nucleophiles with other oxidized species [9]. However, the reaction rate of substrates with electron withdrawing groups is slow and the yield of them is low except of the excellent improvement in selectivity, yield and the reaction time for 6-bromo-2naphthol (1d) in solvent free conditions (entry 9). 3-Hydroxy-2-naphthoic acid does not react under both reaction conditions (entry 10). Furthermore we showed that by use of appropriate amounts of DMSO as co-oxidant under above reaction conditions the rate and yield of the reaction increased predominately (entry 11). To highlight the action of DMSO as a co-oxidant or base in the oxidative coupling reaction, we examined the reaction of (1b, 1e) by different amounts of FHS under heterogeneous system. It was demonstrated that the little acceleration for more easily oxidizable substrate (1b) was achieved, whereas the unreactive substrate (1e) reacts easily in the presence of DMSO to afford (2e) in 84% yield. Thus we can point out the important effect of DMSO on increasing the initiating oxidized species. This may be due to either the fact that polar DMSO is an efficient base to accelerate formation of radical from cation radical to react better with neutral naphthols or radicals created at the surface of crystals, or DMSO can participate in transfer of singlet oxygen with special mechanism to Fe(III) to form Fe(IV) or Fe(V) as powerful oxidants for generation of radical species [10]. Table 2 compares our results (time, yield and reaction conditions) with some other results obtained by other researchers [11–13]. As can be seen, our procedure is the best in the reaction conditions. 3. Conclusion In conclusion, we can develop the efficient aerobic oxidative biaryl coupling system by novel Fe(HSO4)3 and Fe(HSO4)3/SiO2 (10 mol%) catalysts with high yield and selectivity. The present systems has the following significant advantages: (i) the use of easily prepared and inexpensive Fe(HSO4)3 and Fe(HSO4)3/SiO2 (10 mol%) catalysts, (ii) the use of molecular oxygen as a sole oxidant (Fe3+ regenerated by air drying of the separated catalyst under the solvent-free procedure), (iii) the use of water as a solvent, or solvent free system are important from the standpoint of green chemistry and technology, (iv) simple workup in both heterogeneous and solvent free procedures, and (v) reusability of catalyst in solvent-free procedure. References [1] (a) P. Kocovsky, S. Vyskocil, M. Smrcina, Chem. Rev. 103 (2003) 3213; (b) Y. Chen, S. Yekta, A.K. Yudin, Chem. Rev. 103 (2003) 3155.

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