Zinc-catalyzed benzylic C–H bond oxidation

Tetrahedron Letters 53 (2012) 6123–6126

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Zinc-catalyzed benzylic C–H bond oxidation Xiao-Feng Wu ⇑ Department of Chemistry, Zhejiang Sci-Tech University, Xiasha Campus, Hangzhou, Zhejiang Province 310018, People’s Republic of China Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany

a r t i c l e

i n f o

Article history: Received 6 July 2012 Revised 29 August 2012 Accepted 31 August 2012 Available online 7 September 2012

a b s t r a c t A zinc-catalyzed oxidation of benzylic substrates has been developed. The corresponding carbonyl containing compounds have been produced in moderate to excellent yields. Both arenes and heteroarenes can be applied as substrates by using H2O2 as the oxidant. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Zinc catalyst Benzylic compounds Carbonyl compounds Oxidation H2O2

The development of environmental benign methodologies in organic synthesis is attracting more and more attentions during the last decades.1 As the requests of sustainable development, the direct activation of C–H bond is getting more and more interesting than the procedures needed pre-activation.2 Compounds containing carbonyl groups are an interesting and important class of compounds in organic synthesis.3 They are ready for further modification, while they themselves hold many important applications in advance materials, chemicals production, pharmaceuticals and so on. Chemically, carbonyl compounds can be synthesized by Friedel–Crafts acylation,4 oxidation of the corresponding benzylic alcohols,5 and also other methodologies.6 Additionally, palladium-catalyzed carbonylative Suzuki reaction offers another interesting procedure.7 But all of these mentioned methodologies generate significant amounts of wastes and hold the character of low atom efficiency. Meanwhile, the direct oxidations of benzylic compounds to the corresponding carbonyl containing products were also reported.8 A stoichiometric amount of metal oxidants, like Cr(VI), KMnO4, and Oxone, or a stoichiometric amount of hypervalent iodine was needed. The use of metal catalysts like Mn, Fe, Co, Ru, Rh, Bi, and Au together with an excess amount of oxidant was also reported. In addition, the use of organocatalysts has also been developed in the oxidation of benzylic compounds. But the reaction efficiency and generality are still problematic in most of the reported methods. Moreover, zinc salts have the characters as inexpensive, environmental benign, and low toxic.9 Even as non-redox metal, zinc ⇑ Tel.: +49 381 1281 343. E-mail address: [email protected] 0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2012.08.149

salts are still explored in redox reactions.10 More recently, some zinc-catalyzed oxidation reactions have been developed in our group.11 Such as the oxidative esterification of aldehydes,11a the oxidation of benzyl alcohols to aldehydes,11c as well as the oxidation of sulfides to sulfoxides.11b As our continual interest in this area, we wish to report an interesting zinc-catalyzed oxidation of benzylic substrates to the corresponding carbonyl compounds. H2O2, as a kind of green oxidant which generates water as the only by-product, was used as the oxidant in this methodology. Various ketones were prepared in good yields. Initially, the reaction was carried out with diphenylmethane in the presence of ZnBr2 (10 mol %), in 2 mL of methanol and in the presence of 0.1 mL of trifluoroacetic acid (TFA), at 70 °C, using 4 mmol of H2O2 as the terminal oxidant. Forty-eight percentage of benzophenone with 72% of conversion was observed after 16 h (Table 1, entry 1). Then several other zinc salts were tested, which all gave decreased conversions and yields (Table 1, entries 2–7). After finding the influence of zinc salts, we decided to use ZnBr2 as the pre-catalyst to test the effects of solvent (Table 1, entries 8–13). 22–28% of yield with 51–69% of conversion was obtained in alcoholic solvents (Table 1, entries 8 and 9). Similar results were detected in THF and MeCN as well. No desired product was formed in water with 5% of conversion (Table 1, entry 12). We believe the solubility of the substrate is responsible for the low conversion. The yield and conversion were improved to 66% and 79% respectively in dioxane (Table 1, entry 13). The results were further improved by increasing the reaction temperature to 100 °C (Table 1, entry 14). No better yields and conversions were succeeded by increasing the loading of H2O2 to 8 mmol (Table 1, entry 15). The best results were achieved in dioxane together with 0.2 mL of

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X.-F. Wu / Tetrahedron Letters 53 (2012) 6123–6126

Table 1 Zinc-catalyzed oxidation of diphenylmethane to benzophenonea

O [Zn]/H2O 2 1 mmol Entry

[Zn] (10 mol %)

Solvent (2 mL)

Conv.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

ZnBr2 ZnCl2 ZnI2 Zn(acac)2
xH2O Zn(ClO4)2
6H2O Zn(NO3)2
6H2O ZnSO4
7H2O ZnBr2 ZnBr2 ZnBr2 ZnBr2 ZnBr2 ZnBr2 ZnBr2 ZnBr2 ZnBr2 ZnBr2 CdBr2
4H2O

MeOH MeOH MeOH MeOH MeOH MeOH MeOH t BuOH EtOH THF MeCN H2O dioxane dioxane dioxane dioxane dioxane dioxane

72 22 11 27 16 31 33 69 51 47 63 5 79 94 94 92 96 97

b

(%)

Yieldb (%) 48 13 3 14 16 1 13 38 28 22 19 0 66 74c 75d 78e 85f 88f

a Diphenylmethane (1 mmol), zinc catalyst (10 mol %), solvent (2 mL), trifluoroacetic acid (0.1 mL), H2O2 (4 mmol; 30% in water), 70 °C, 16 h. b Conversion and yield were determined by GC using hexadecane as the internal standard, based on diphenylmethane. c 100 °C. d 100 °C, H2O2 (8 mmol). e 100 °C, TFA (0.15 mL). f 100 °C, TFA (0.2 mL).

TFA, using 4 mmol of H2O2, at 100 °C (Table 1, entry 17). Here, TFA possibly has two roles in this method but we do not have direct evidence currently: (1) to activate the formed ZnO2; (2) and to react with H2O2 to form 2,2,2-trifluoroethaneperoxoic acid as the real oxidant. And it is important to mention that the same reaction was also tested using Cu2O, CuBr2, FeCl3, FeCl3
6H2O, Fe3O4, Fe2O3, CuO, CuCl, and PdCl2 as the pre-catalyst under our best conditions (Table 1, entry 17), but only traces of product were formed. No product was detected in the absence of TFA. In contrast, CdBr2 succeeded to give comparable results in the oxidation of diphenylmethane (Table 1, entry 18). With the best reaction conditions in our hand, we started to test the generality and efficiency of this methodology (Table 2).12 To our delight, methyl-, chloride, bromide, iodide, and fluoride substituted diphenylmethanes all gave good to excellent yields of the corresponding benzophenones with excellent selectivities (Table 2, entries 2–6). To our surprise, the methyl group was unaffected and no over oxidation was observed. Pyridine can also be tolerated, but the coordination of nitrogen to the zinc catalyst may inhibit its activity and may be responsible for the decreased conversion (Table 2, entry 7). 93% of 9H-xanthen-9-one was produced from the corresponding 9H-xanthene (Table 2, entry 8). In the case of 9,10-dihydroanthracene, 78% of anthracene-9,10-dione was formed after oxidation on both sides (Table 2, entry 9). When 2,3-dihydro-1H-indene was tested under our standard conditions, only 26% of 2,3-dihydro-1H-inden-1-one was detected (Table 2, entry 10). But to our surprise, 1,3-dihydroisobenzofuran was totally oxidized and gave 68% of isobenzofuran-1(3H)-one (Table 2, entry 11). Isobenzofuran-1,3-dione, phthalaldehyde, and 2-formylbenzoic acid were also formed as the over oxidized products.

Table 2 Zinc-catalyzed oxidation of benzylic substratesa

O R ZnBr2, H2O2, TFA, dioxane

Entry

R

100oC, 16h

R'

R'

Product

Product

1

2

Cl

5

O

6

96

85 80c

91

80

97

89 88c

92

88

96

89

98

90 89c

63

52

100

93

F

I

N

Yieldb (%)

Br

3

4

Conv.b (%)

O

7

8

F

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X.-F. Wu / Tetrahedron Letters 53 (2012) 6123–6126 Table 2 (continued) Entry

Product

Product

O

O

O

O

Conv.b (%)

Yieldb (%)

100

78

37

26

100

68

100

88

50

35

61

49

70

35

45

18

9

10

Br

Cl O

O

11

F

I

O

O 12

N

O O

O 13

O O 14

O

O

O

O 15

CHO

CHO

16

F a b c

Substrate (1 mmol), ZnBr2 (10 mol %), dioxane (2 mL), trifluoroacetic acid (0.2 mL), H2O2 (4 mmol; 30% in water), 100 °C, 16 h. Conversion and yield were determined by GC using hexadecane as the internal standard, based on benzylic substrate. Isolated yield.

9H-Fluorene can also be used as the substrate in this reaction, and gave 88% of the corresponding product with total conversion of the starting material (Table 2, entry 12). In addition to the aromatic substituted substrates, alkyl substituted arenes can also be applied as substrates (Table 2, entries 13–16). Moderate yields of acetophenone and propiophenone were produced with good selectivities (Table 2, entries 13 and 14). Moreover, benzaldehydes can also be prepared from the corresponding toluenes (Table 2, entries 15 and 16). The corresponding benzoic acids were also produced as the main by-products. Concerning the reaction mechanism, it is under investigation in our group. In conclusion, the first zinc-catalyzed oxidation of benzylic substrates has been developed. Various carbonyl containing compounds were produced in moderate to excellent yields. Both arenes and heteroarenes are tolerable under our reaction conditions. H2O2 was used as the terminal oxidant, which generates water as the only by-product. Acknowledgments The financial support from the state of Mecklenburg-Vorpommern and the Bundesministerium für Bildung und Forschung (BMBF)

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