Indium Tribromide as a Highly Efficient and Versatile

LETTER

1727

Indium Tribromide as a Highly Efficient and Versatile Catalyst for Chemoselective Synthesis of Acylals from Aldehydes under Solvent-Free Conditions IndiumTribromideasaHighlyEficentandVersatileCat lyst Yin, Zhan-Hui Zhang, Yong-Mei Wang,* Mei-Li Pang Liang

Abstract: A mild and efficient method has been developed for the chemoselective preparation of acylals from aldehydes in the presence of catalytic amounts (0.01–1.0 mol%) of InBr3 under solventfree conditions in very good to excellent yields. Key words: acylals, aldehydes, chemoselective, indium tribromide, solvent-free conditions

Selective protection and deprotection is the heart and soul of multistep organic synthesis. Acylals (geminal diacetates) of aldehydes have already been recognized as important protecting group alternatives to acetals1 because of their stability in neutral and basic medium2 and their easy conversion into the parent aldehydes.3 In addition, acylals are useful starting materials and intermediates in organic synthesis4 and have been used as cross linking reagents for cellulose in cotton.5 Smonou and Angelis have reported the synthesis of optical aldehydes by lipase-catalyzed resolution of the corresponding acylals.6 As a result, a variety of methods have been examined for their formation. Generally, acylals are prepared from aldehydes and acetic anhydride using strong protonic acids, such as sulfuric, phosphoric, methanesulfonic or perchloric acids,7 and Lewis acids, such as zinc chloride, ferric chloride, ferrous sulfate, lithium bromide, phosphorus trichloride, tungsten hexachloride, indium chloride, ceric ammonium nitrate, lithium tetrafluoroborate, zinc tetrafluoroborate, Fe2(SO4)3·xH2O, zirconium chloride, copper(II) tetrafluoroborate, bismuth nitrate,8 heteropolyacid Well–Dawson (H6P2W18O62·24H2O) and aluminum dodecatungstophosphate (AlPW12O40),9 and heterogeneous catalysts like Nafion-H, expansive graphite, zeolites, clay and supported reagents.10 Other catalysts, such as iodine,11 N-bromosuccinimide,12 trimethylchlorosilane/sodium iodide,13 have also been used for this transformation. But these procedures are often accompanied by longer reaction times, low product yields, stringent conditions, high catalyst loading, corrosive reagents, high temperature and require the use of toxic metal ions and solvents. Although recently the triflates like Cu(OTf)2, Sc(OTf)3 and LiOTf have emerged as the most effective catalysts,14 the high cost and susceptibility to aqueous medium of the metal triflates SYNLETT 2004, No. 10, pp 1727–173009.08204 Advanced online publication: 15.07.2004 DOI: 10.1055/s-2004-829549; Art ID: U10604ST © Georg Thieme Verlag Stuttgart · New York

do not make triflates good contenders for use in large scale synthesis. Therefore, mild reaction conditions that can overcome the shortcomings of previous methods are necessary. Since Li and Chan initiated and applied indium-mediated reactions in aqueous media in 1991,15 indium complexes have emerged as mild and water-tolerant Lewis acid catalysts for organic synthesis with high chemo-, regio- and stereoselectivity.16 Particularly, indium tribromide has been found to be a more effective catalyst than conventional Lewis acids in promoting various transformations including thioacetalization, Biginelli reaction, sulfonation of indoles, stereoselective synthesis of alkynyl sugars, nucleophilic addition, intramolecular Michael addition, synthesis of 1,3-dioxane derivatives, Ferrier rearrangement, regioselective ring opening of epoxides, conversion of oxiranes to thiiranes, and conjugate addition.17 In this paper, we wish to report the results of the chemoselective conversion of aldehydes to acylals using a catalytic amount of InBr3 (0.01–1.0 mol%) under solvent-free conditions (Scheme 1). RCHO 1

Ac2O, InBr3 (0.01–1.0 mol%)

OAc R

neat, r.t.

OAc 2

Scheme 1

As shown in Table 1, a series of aliphatic and aromatic aldehydes reacted with acetic anhydride smoothly to afford the corresponding acylals in the presence of 0.1 mol% catalyst at ambient temperature in very good to excellent yields. In the solvent-free condition, the acetylations could be successfully performed with a ten-fold increase of benzaldehyde (1a) in the presence of lower catalyst loading (0.01 mol%) with only a marginal drop in reaction rate (entry 3). The presence of electron-donating and electron-withdrawing groups on the aromatic ring of aldehydes, irrespective of their positions in the ring, did not make any obvious difference in the acylation. Aliphatic (1b–f) as well as a,b-unsaturated (1g and 1t) aldehydes also gave the corresponding acylals in good yields under similar reaction conditions. 2,4-Dinitrobenzaldehyde (1y) and 4-(dimethylamino) benzaldehyde (1z), however, due to deactivation of the carbonyl group, remained unaffected even when the reaction mixtures were stirred at room

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Department of Chemistry, the State Key Laboratory of Functional Polymer Materials for Absorption and Separation, Nankai University, Tianjin 300071, P. R. China Fax +86(22)23502654; E-mail: [email protected] Received 14 April 2004

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Entry

InBr3-Catalyzed Conversion of Aldehydes to Acylals R

Catalyst load

Time (min)

Yield (%)a

(mol%)

Mp (°C) or bp (°C)/Torr Found

Reported

44–45

44–4510g

1

Ph (1a)

1.0

5

98

2

Ph (1a)

0.1

10

96

3

Ph (1a)

0.01

30

95b

4

Ph (1a)

0.1

60

91c

5

CH3(1b)

0.1

20

84

87–88/30

73–75/158f

6

CCl3 (1c)

0.1

40

76

111–113/10

85–87/510a

7

CH3(CH2)3 (1d)

0.1

30

89

128–130/10

145/2010f

8

CH3(CH2)4 (1e)

0.1

30

90

128–130/2

127–129/28h

9

CH3(CH2)8 (1f)

0.1

30

92

141–143/10

10

CH3-CH=CH (1g)

0.1

35

83

81–83/10

11

2-MeC6H4 (1h)

0.1

6

97

58–59

12

4-MeC6H4 (1i)

0.1

8

98

82–83

80.5–81.510g

13

2-MeOC6H4 (1j)

0.1

6

94

75–76

74–757c

14

4-MeOC6H4 (1k)

0.1

20

96

64–65

64–6510g

15

2-ClC6H4 (1l)

0.1

8

97

57–58

56–577c

16

3-ClC6H4 (1m)

0.1

6

96

65–66

65–6610g

17

4-ClC6H4 (1n)

0.1

15

97

80–81

81–8210g

18

3,4-(OCH2O)C6H3 (1o)

0.1

3

98

78–79

78–7910g

19

2-NO2C6H4 (1p)

0.1

10

98d

86–87

85.5–86.57c

20

3-NO2C6H4 (1q)

0.1

12

96d

66–67

64–6510g

21

4-NO2C6H4 (1r)

0.1

15

99d

127–128

125.5–126.510g

22

4-FC6H4 (1s)

0.1

20

98

50–51

50.5–51.57c

23

(E)-PhCH=CH (1t)

0.1

20

96

84–85

83.5–84510g

24

Furfural (1u)

0.1

30

88

50–51

50.5–51.510g

25

4-OHC6H4 (1v)

0.1

12 h

96d,e

89–91

89–907c

26

2-OH-4-BrC6H3 (1w)

0.1

14 h

95d,e

90–91

27

3-MeO-4-OHC6H3(1x)

0.1

10 h

94d,e

90–91

28

2,4-(NO2)2C6H3 (1y)

0.1

24 h

No reaction

29

4-Me2NC6H4 (1z)

1.0

24 h

No reaction

a

Isolated yield of the corresponding acylals. Ten-fold benzaldehyde was used. c InCl3 as catalyst. d 6 Equiv of acetic anhydride (with respect to substrate) was used. e Yield refers to pure triacetate. b

Synlett 2004, No. 10, 1727–1730

© Thieme Stuttgart · New York

89–90/158f

90–9110g


Table 1

LETTER

L. Yin et al.

Indium Tribromide as a Highly Efficient and Versatile Catalyst

temperature for one day, and the starting materials were quantitatively recovered. The reaction conditions are mild enough not to induce any isomerization for conjugated aldehydes (1g and 1t) or damage to moieties such as methoxy (1j and 1k) and methylenedioxy (1o), which often undergo cleavage in strongly acidic reaction media. Side product formation was not observed in the reactions we studied. It is worth noting that hydroxy groups in 4-hydroxy-, 2hydroxy-4-bromo-benzaldehydes (1v and 1w) and vaniline (1x) were also acetylated to afford the corresponding triacetates under similar conditions, but the acetylation of phenolic aldehydes required a longer reaction period. The selectivity of the present method is demonstrated by a competition experiment for the acylation of aldehydes in the presence of ketones under solvent-free conditions at room temperature. When a 1:1 mixture of benzaldehyde and acetophenone was allowed to react with acetic anhydride in the presence of 0.1 mol% InBr3 for 15 minutes, TLC analysis of reaction mixture indicated complete disappearance of benzaldehyde, while acetophenone was still intact, even when the mixture continued to stir for 24 hours, due to the steric effects of methyl group in acetophenone. In comparison with other catalysts such as InCl3, CAN, LiOTf, Cu(OTf)2, Sc(OTf)3, ZrCl4, AlPW12O40, LiBF4 and Cu(BF4)2·xH2O, which were recently reported in the acylation of 4-nitrobenzylaldehyde, InBr3 employed here shows more effective catalytic reactivity than InCl3, CAN, LiOTf, Cu(OTf)2, ZrCl4 and LiBF4 in terms of the amount of catalyst, yields and reaction times (Table 2). It should be pointed out that acylation of aldehydes in the Table 2 Comparison of the Effect of Catalysts in the Acylation of 4Nitrobenzylaldehyde Catalyst

Catalyst load (mol%) Time

Yield (%)

InCl3

10

4h

888h

CAN

10

24 h

968i

LiOTf

20

15 h

9414c

1729

presence of InCl3 required longer reaction times and higher catalyst loading and led to poorer yields of products than those obtained with InBr3. This result clearly shows the strong catalytic activity of InBr3 in comparison with InCl3. In conclusion, InBr3 was found to be a novel and highly efficient Lewis acid catalyst for the conversion of aldehydes into acylals under mild conditions. The advantages include the low loading of catalyst, operation at room temperature, high yields, excellent chemoselectivity, and applicability to large-scale reactions.

General Procedure for the InBr3-Catalyzed Conversion of Aldehydes into Acylals: A mixture of aldehydes (10 mmol), freshly distilled Ac2O (30 mmol) and InBr3 (3.6 mg, 0.01 mmol, 0.1 mol%) was stirred at r.t. The progress of the reaction was monitored by TLC or GC. After completion of the reaction, EtOAc (100 mL) was added and the mixture was washed successively with 1 M NaOH solution (2 × 20 mL), brine (10 mL), and H2O (10 mL). The organic layer was separated and dried (Na2SO4). Evaporation of the solvent under reduced pressure gave almost pure product. Further purification was achieved by column chromatography on silica gel or recrystallization from EtOAc–hexane to give pure product in good to excellent yield (Table 1). The products were characterized by IR and 1H NMR and by comparison of the spectral data with those of authentic samples. Selected spectroscopic data: 2c: IR (neat): 797, 1029, 1219, 1375, 1431, 178, 2941, 2985 cm–1. H NMR (300 MHz, CDCl3): d = 2.21 (s, 6 H), 7.25 (s, 1 H) ppm.

1

2f: IR (neat): 1004, 1201, 1240, 1371, 1760, 2860, 2921 cm–1. 1H NMR (300 MHz, CDCl3): d = 0.88 (t, J = 6.6 Hz, 3 H), 1.26–1.35 (m, 14 H), 1.72–1.77 (m, 2 H), 2.08 (s, 6 H), 6.77 (t, J = 5.4 Hz, 1 H) ppm. 2h: IR (KBr): 683, 746, 840, 944, 105, 1120, 1220, 1373, 1492, 1583, 1604, 1767, 3014, 3063, 3106 cm–1. 1H NMR (300 MHz, CDCl3): d = 2.13 (s, 6 H), 2.46 (s, 3 H), 7.19–7.31 (m, 3 H), 7.51 (dd, J = 7.8, 2 Hz, 1 H), 7.83 (s, 1 H) ppm. 2w: IR (KBr): 731, 768, 848, 928, 969, 1011, 1045, 1213, 1247, 1374, 1468, 1606, 1751, 2936, 2965, 3034 cm–1. 1H NMR (300 MHz, CDCl3): d = 2.12 (s, 6 H), 2.34 (s, 3 H), 7.02 (d, J = 8.7 Hz, 1 H), 7.54 (dd, J = 7.8, 2.7 Hz, 1 H), 7.77 (d, J = 2.7 Hz, 1 H), 7.85 (s, 1 H) ppm.

Cu(OTf)2

2.5

4h

9414a

Acknowledgment

Sc(OTf)3

2

10 min

9914b

ZrCl4

5

30 min

928m

We thank Tianjin Natural Science Foundation (0236127711), the State Key Laboratory of Functional Polymer Materials for Absorption and Separation, the State Key Laboratory of Elemento-Organic Chemistry and Nankai University for financial support.

AlPW12O40

0.1

45 min

899b

7.5 h

808j

LiBF4

100

Cu(BF4)2·xH2O

1

3 min

928n

InBr3

0.1

15 min

99

References (1) Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic Synthesis, 3rd ed.; John Wiley and Sons: New York, 1999, 306. (2) Gregory, M. J. J. Chem. Soc. B 1970, 1201.

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(17)

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