Highly efficient copper-catalyzed hydroacylation reaction of aldehydes with azodicarboxylates

Tetrahedron Letters 52 (2011) 5880–5883

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Highly efficient copper-catalyzed hydroacylation reaction of aldehydes with azodicarboxylates Yuancheng Qin, Qiang Peng ⇑, Jinsheng Song, Dan Zhou School of Environmental and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, PR China

a r t i c l e

i n f o

Article history: Received 7 April 2011 Revised 27 August 2011 Accepted 30 August 2011 Available online 2 September 2011

a b s t r a c t The hydroacylation reaction of aldehydes with azodicarboxylates catalyzed by copper(II) acetate monohydrate has been reported. The reaction of various aldehydes gave the corresponding hydroacylation products in 60–98% yields under mild conditions. The method is simple, economical, and has practical advantages for the construction of the carbon–nitrogen bonds. Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.

Keywords: Hydroacylation Copper catalyst Azo compounds Aldehydes

Formation of C–N bond is considered to be an important strategy that finds wide applications in the synthesis of many substances such as drugs, materials, and natural products.1 During the last few decades, the enormous progress has been made in the application of polar, pericyclic, and radical reactions in such bond construction.1,2 Dialkyl azodicarboxylates, with a central azo functionality flanked by two carboalkoxy groups, are excellent electrophiles and can readily participate in the zwitterion formation. They are commercially available. So, over the last few decades, a very efficient reaction, which involves the use of azodicarboxylates as electrophiles, has been successfully used for such carbon–nitrogen bond formation. There are many types of reactions involving azodicarboxylates that have been studied extensively for the formation of C–N bond, such as the zwitterion intermediate reactions,3 a-amination of carbonyl compounds,4 the C–H activation at the a-position of amines and ethers,5 and the ene-type reaction with olefins.6 But the direct hydroacylation reaction, which involves aldehydes has not been studied extensively.7 Hydroacylation reaction with the direct functionalization of the aldehydic C–H to form a variety of hydrazine imides is a highly efficient synthetic methodology to form a carbon–nitrogen bond. But they often result in low yields and only a narrow scope of the substrates can be used under photolytic or thermal conditions.7 The transition-metal catalyst, rhodium acetate, has proven to be an effective catalyst for the hydroacylation reaction involving aliphatic saturated or unsaturated aldehydes with azodicarboxylates under ⇑ Corresponding author. Tel./fax: +86 07913953369. E-mail address: [email protected] (Q. Peng).

mild conditions to provide the hydrazine imides.8 However, relatively low yields were obtained when the aromatic aldehydes were used as the substrates for this method. In addition, very costly

Table 1 Optimization of the reaction condition for benzaldehyde with diisopropyl azodicarboxylatea

CHO+

O

N O

O N

O

Cu(OAc)2 H2O solvent, r.t., 48 h

O

N O

1

OO N H

O

Entry

Solvent

Cu (mol %)

Yieldb (%)

1 2 3 4 5 6 7 8 9 10 11

EtOAc MeCN MeOH THF CH2Cl2 EtOAc EtOAc EtOAc EtOAc EtOAc EtOAc

20 20 20 20 20 0 10 15 30 20 20

98 90 72 60 52 <5 84 87 98c 90d 82e

a Reaction conditions: diisopropyl azodicarboxylate (0.5 mmol), benzaldehyde (1 mmol), catalyst, solvent (5 mL), rt, 48 h. b Isolated by silica gel column chromatography and based on diisopropyl azodicarboxylate. c The reaction time was 36 h. d The ratio of benzaldehyde to diisopropyl azodicarboxylate was 1.5:1. e The ratio of benzaldehyde to diisopropyl azodicarboxylate was 1.2:1.

0040-4039/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.08.165

5881

Y. Qin et al. / Tetrahedron Letters 52 (2011) 5880–5883 Table 2 Hydroacylation of aldehydes with azobicarboxylatesa

R CHO+

Entry

Aldehyde

O O

NN

O O

Cu(OAc)2 H2O EtOAct, r.t.

Time (h)

R O

OO

NN O H

O Yieldb (%)

Product

CHO

O O

48

1

NN O H

O 98

O

1

H3CO

H3CO

CHO

O O

24

2

N O

O2N

N O

O

48

CHO

O

67

OO N H

82

O

4

N 96

5

O

N H

3

N O

N

86

Br

CHO

4

O

2

O O

96

Br

N H

O2N

CHO

3

O

O

N O

OO N H

60

O

5

S

S CHO 72

6

O

N O

OO N H

O

72

O

97

6

S

S CHO 16

7

OO

N

O

N H

O 7 n-C12H25

C12H25 S

S 8

CHO

48

O

N O

OO N H

8

O

CHO

O N

36

9

86

O

O

N H

O O

80

9 C11H23 10

n-C11H23-CHO

12

O

O

NN O H

O O

97

10

(continued on next page)

5882

Y. Qin et al. / Tetrahedron Letters 52 (2011) 5880–5883

Table 2 (continued) Entry

Aldehyde

Time (h)

Yieldb (%)

Product

CHO

O 20

11

O

N O

O

N H

O

96

O

99

O

98

11 O

CHO 8

12

O

N O

13

a b

n-C6H13-CHO

10

N H

12

C6H13 O

O N

O

O

N H

O

13

Reaction conditions: diisopropyl azodicarboxylate (0.5 mmol), aldehyde (1 mmol), copper(II) acetate monohydrate (0.1 mmol), EtOAc (5 mL), rt. Isolated by silica gel column chromatography and based on diisopropyl azodicarboxylate.

precious metals are used. Ni et al. have demonstrated that the hydroacylation reaction can be carried out successfully in the presence of the ionic liquid, 1-n-butyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide without the use of a catalyst9a and they also reported that the reaction can be carried out successfully in water without the use of a catalyst.9b However, the reaction gives relatively low yields and requires a long time when the aromatic aldehydes are used as the substrates. Therefore, it is highly desirable to develop a more efficient procedure while broadening the scope of the substrates without the use of an expensive catalyst. Copper stands out as a convenient alternative to costly precious metals, because copper salts are often cheap and environmentally benign. Copper catalysts have been proved to be active and versatile in many types of reactions.10 To the best of our knowledge, there has been no general study on the hydroacylation reaction between aldehydes and azodicarboxylates catalyzed by the copper catalysts. We report here that the copper-catalyzed hydroacylation between aldehydes and azodicarboxylates can provide the hydrazine imides in moderate to excellent yields under mild and ligand-free conditions. Initially, the hydroacylation reaction between benzaldehyde with diisopropyl azodicarboxylate was selected as a model reaction for optimizing the reaction conditions. Firstly, we studied the effect of various solvents for the hydroacylation reaction. As shown from the results summarized in Table 1, the reaction proceeded smoothly in conventional organic solvents, such as MeOH, CH3CN, and EtOAc. The isolated yields were significantly influenced by the solvent employed. Among the solvents tested, EtOAc was proven to be the best compared to the others (Table 1, entry 1). When MeCN was used as the solvent, the desired hydroacylation product was obtained in high yields, 90% (Table 1, entry 2). Methanol and THF could also provide relatively good yields of the desired hydroacylation products (Table 1, entries 3 and 4). Relatively low yields were obtained when dichloromethane was used as a solvent (Table 1, entry 5). The effect of catalyst loading on the cross-coupling reaction is shown in Table 1. Poor yields were obtained without the use of a catalyst (Table 1, entry 6).9 The amount of the copper(II) acetate monohydrate employed in the reaction is of importance. When the catalyst loading was less than 15 mol %, the yield of the corresponding product was relatively lower (Table 1, entries 7 and 8). A yield of 98% was obtained when 20 mol % Cu(OAc)2 was applied (Table 1, entry 1), and the yield was not apparently improved further when the catalyst loading was more than 30 mol %. Only different reaction rate was observed for the reaction (Table 1, entry

9). The effect of the ratio of benzaldehyde to diisopropyl azodicarboxylate was also investigated. Lower ratio of benzaldehyde to diisopropyl azodicarboxylate led to lower reaction yield (Table 1, entries 10 and 11). Under our optimized reaction conditions, the scope of this hydroacylation reaction was then explored. A variety of aldehydes were examined for the hydroacylation reactions. As shown in Table 2, most of the hydroacylation reactions afforded the hydroacylation products in good to excellent yields. Aromatic aldehydes were good substrates for this reaction and afforded the hydroacylation products in good to excellent yields. The hydroacylation reactions with benzaldehyde provided the desired products 1 in excellent yields (Table 2, entry 1). To our knowledge, this is the highest yields reported so far. The aromatic aldehydes, bearing either electron-withdrawing or electron-donating groups, gave the hydroacylation products 2–4 in good yields (Table 2, entries 2–4). These results are superior to that obtained by Lee8 and Ni,9 in which rhodium acetate catalyst and ionic liquid are used. To broaden the scope of the reaction, heterocyclic aldehydes were used in the reaction and also gave the desired products in good to excellent yields (72–97%, Table 2, entries 6–8). Especially the hydroacylation reaction with thiophene-3-carbaldehyde provided the desired product 7 in excellent yield (Table 2, entry 7). The trans-cinnamaldehyde also provided the desired product in good yield (Table 2, entry 9). The aliphatic saturated aldehydes, with either linear or branched substituents were treated with diisopropyl azodicarboxylated to give the desired products in excellent yields (96–99%, Table 2, entries 10–13). At present, the mechanism for the hydroacylation reaction is still not clear. It was described in previous studies that the hydroacylation of aldehydes with azodicarboxylate considered to proceed by a radical intermediate mechanism.7–10 In a radical mechanism, copper initiates the radical chain process by transferring its one electron in previous studies.11,12 Therefore, we assume that the hydroacylation reaction still proceed via a radical mechanism (Scheme 1). This radical mechanism is supported by a complete inhibition of the reaction when it was run with as low as 4 mol % of radical scavenger (hydroquinone). When 0.5, 1, 2, or 3 mol % radical scavenger was applied, the yields were 61%, 52%, 16%, and 10%. In conclusion, we have developed a highly efficient copper-catalyzed hydroacylation reaction of aldehydes with azobicarboxylates, the corresponding target products were obtained in good to excellent yields, and an array of functional groups are tolerated under the mild conditions with respect to the aldehydes. The protocol used inexpensive copper(II) acetate monohydrate as the catalyst, no additional ligand and additive were required, so the method

Y. Qin et al. / Tetrahedron Letters 52 (2011) 5880–5883

O R'

R R N NH O R'

O R'

References and notes H

O R'

R N N O R'

H

5883

R N N R

R

Scheme 1. Mechanism for the hydroacylation reaction.

is simple, economical, and has practical advantages for the construction of the carbon–nitrogen bonds. Acknowledgments The work was financially supported by the Natural Science Foundation of China (Grant No. 20802033), Program for New Century Excellent Talents in University (Grant No. NCET-10-0170), Yong Scientist of Jing Gang Zhi Xing Project of Jiangxi Province (Grant No. 2008DQ00700) and Natural Science Foundation of Jiangxi Province (Grant No. 2010GZH0110). Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tetlet.2011.08.165.

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