Synthesis of amides through the Cannizzaro-type reaction catalyzed by lanthanide chlorides

Tetrahedron 65 (2009) 10022–10024

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Synthesis of amides through the Cannizzaro-type reaction catalyzed by lanthanide chlorides Lijun Zhang *, Shunpeng Su, Hongping Wu, Shaowu Wang * Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based Materials, Institute of Organic Chemistry, School of Chemistry and Materials Science, Anhui Normal University, Wuhu, Anhui 241000, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 May 2009 Received in revised form 22 September 2009 Accepted 25 September 2009 Available online 30 September 2009

Amidation of aldehydes with lithium amides through the LnCl3-catalyzed Cannizzaro-type reactions afforded a variety of amides in high yields. The electronic and steric effects on the reaction were investigated. The features of the economical catalysts, high yields, tolerance of a wide range of lithium amides and aromatic aldehydes make this methodology an easy and valid contribution to the direct synthesis of amides from aldehydes. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The amides are one of the most common functionalities in synthetic organic chemistry. Although different methods for preparations of amides from activated carboxylic acids or their derivatives are available,1,2 development of another facile, highefficiency, economical, simple methods by introduction of new starting materials or of new catalysts is still attracting a great deal of attention. Recently Susuma Saito’s group reported the synthesis of amides by the hydration of organonitriles under mild condition with Rh species as catalyst system.3 Aminocarbonylation of aryl halides by introducing CO to a pressurized continuous flow reactor was also described.4 In addition, the amides could be obtained by direct, endothermic amidation from alcohols and amines via aldehyde intermediates, using Ru PNN pincer catalysts by removal of H25a or catalyzed by [Ru(p-cymene)Cl2]2 with bis(diphenylphosphino)butane.5b Another attractive starting material is aldehyde. Although direct oxidative amination of aldehydes utilizing readily available aldehydes was considered an economically alternative route to amides, only a few examples have been reported to date. The routes to amides by direct oxidative amination of aldehydes catalyzed by the expensive late transition metals such as palladium6 and ruthenium catalyst7a or by the treatment with iodine and H2O27b or in the presence of the oxone are reported.7c Metal-free oxidative amination of aromatic aldehydes in the presence of TBHP provides a convenient access to amides.8 The N-heterocyclic carbene (NHC)

* Corresponding authors. Tel./fax: þ86 553 3883517. E-mail addresses: [email protected] (L. Zhang), [email protected]. edu.cn (S. Wang). 0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2009.09.101

catalysts were applied in the amidation of aldehydes.9 Ishihara and co-workers described the direct synthesis of amides from aldehydes through the Cannizzaro-type reactions catalyzed by lithium N,N-diisopropylamide (LDA).10 Our group reported that the corresponding amides could be obtained in good yields by treatment of aromatic aldehydes with a stoichiometric amount of lanthanide amides at ambient temperature or by treatment of aromatic aldehydes with lithium amide in the presence of lanthanide chlorides.11,12 In comparison with the catalyst of LDA,10 La[N(TMS)2]3,13a heterobimetallic lanthanide/sodium phenoxides,13b trisguanidinate lanthanide complexes,13c lanthanide chlorides are more readily available or economical.14 To date, the routes to amides directly from aldehydes through the Cannizzaro-type reactions initiated by the lanthanide chlorides were very limited. To explore the generality and scope of the reaction, we have investigated the Cannizzaro-type reaction of various aldehydes and lithium amides. Herein, we wish to report the results in this paper. 2. Results and discussion Preliminary optimization of the Cannizzaro reaction of the benzaldehyde with lithium benzamide in the presence of YCl3 was carried out under various conditions as shown in Table 1. To our great delight, the yield of N-phenylbenzamide could be isolated in as high as 99% by treatment of 2 equiv of the benzaldehyde with lithium benzamide in the presence of 5% YCl3 in toluene at room temperature for 2 days (Table 1, entry 6). The yield of the product was much higher than those through the conventional Cannizzaro methods catalyzed by other catalysts such as LDA10 and La[N(TMS)2]3.13 In all the reaction conditions examined, almost the same amount of the corresponding alcohol was isolated, which confirmed the Cannizzaro reaction mechanism.

L. Zhang et al. / Tetrahedron 65 (2009) 10022–10024 Table 1 Optimization of LnCl3-catalyzed amidation of aldehydes with lithium amides 2

O C NH

NHLi 1.YCl3 (1-10mol %)

CHO +

2. H2 O

CH 2OH

+

1a

2a

Entry

Ratioa

T ( C)

Solvent

Cat. (%)

T (h)

Yield of 1ab (%)

Yield of 2ab (%)

1 2 3 4 5 6 7 8 9 10

1:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1

rt rt rt rt rt rt rt rt 60 rt

Toluene Toluene THF Hexane Toluene Toluene Toluene Toluene Toluene Toluene

10 10 10 10 8 5 3 1 10 10

48 48 48 48 48 48 48 48 48 24

48 97 92 70 96 99 71 52 54 72

46 94 88 69 95 93 65 46 50 70

a b

Ratio of aldehyde/lithium amide. Isolated yield based on the lithium amide.

Under the optimized conditions (Table 1, entry 6), the scope and generality of the lanthanide chlorides-catalyzed amidation of various aromatic aldehydes with different lithium amides R1R2NLi were explored. In general, the reactions, in most cases, proceeded efficiently at room temperature to provide the amides in good to excellent yields (Table 2). As shown in Table 2, the electronic effects and the steric effects of the substituents of the amines or aldehydes play a significant role in the yields of products. Table 2 Synthesis of other amides through the Cannizzaro-type reaction

2 R

CHO + Li N

R1 R2

O R1 C N R2

1.YCl3 (5 mol %) 2.H2O

R

+

1a-w

CH2 OH R 2

Entry

Aldehydes

Lithium amines

Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

C6H5CHO C6H5CHO C6H5CHO C6H5CHO C6H5CHO C6H5CHO C6H5CHO C6H5CHO C6H5CHO C6H5CHO C6H5CHO C6H5CHO C6H5CHO C6H5CHO C6H5CHO 4-CH3C6H4CHO 4-CH3OC6H4CHO 4-Me2NC6H4CHO 4-ClC6H4CHO 4-NO2C6H4CHO 2-CH3OC6H4CHO 2,4-(Cl)2C6H4CHO 2,4,6-Me3C6H4CHO (CH3)3CCHO

C6H5NHLi 4-CH3C6H4NHLi 4-ClC6H4NHLi 4-NO2C6H4NHLi 2-CH3C6H4NHLi 2-NH2C6H4NHLi 2,6-Me2C6H3NHLi 2,6-iPr2C6H3NHLi a-C10H7–NHLi C6H5CH2NHLi i PrNHLi t BuNHLi Ph2NLi Et2NLi

99 (1a) 98 (1b) 71 (1c) 27 (1d) 94 (1e) 57 (1f) 79 (1g) 66 (1h) 91 (1i) 83 (1j) 66 (1k) 29 (1l) 83 (1m) 89 (1n) 81 (1o) 88 (1p) 67 (1q) 85 (1r) 97 (1s) 99 (1t) 58 (1u) 97 (1v) 66 (1w) Trace

NLi

C6H5NHLi C6H5NHLi C6H5NHLi C6H5NHLi C6H5NHLi C6H5NHLi C6H5NHLi C6H5NHLi C6H5NHLi

Isolated yield based on the lithium amide. Reaction conditions: toluene as solvent, room temperature, 2d.

It was found that the electron-donating groups such as CH3 on the amines facilitated the reactions. For example, reaction of benzaldehyde with lithium 4-methylanilinide or lithium 2methylanilinide gave the amides in nearly quantitative yields

10023

(Table 2, entries 2 and 5). However, only 27% yield of product was obtained when benzaldehyde was treated with lithium 4nitroanilinide (Table 2, entry 4). As the result of the steric effects, lower yields were observed with the introduction of the substituents in the 2- or 6-position substituted of anilinides (Table 2, entries 6–8). Reactions of benzaldehyde with lithium 2,6-dimethylanilinide or lithium 2,6-diisopropylanilinide afforded the corresponding amides only in 79% and 66% yields, respectively (Table 2, entries 7 and 8). Besides lithium anilinide, other lithium primary amides such as lithium napthanamide or lithium alkyl amides could also achieve the good results (66– 91%) (Table 2, entries 9–11). The reason for low yield of the reaction of lithium tert-butyl amide with benzaldehyde might lie in the steric effects (Table 2, entry 12). Good to moderate yields of the amides can also be obtained when the secondary aromatic or alkyl lithium amides such as Ph2NLi, Et2NLi, and lithium piperidine-1-ide were treated with benzaldehyde at room temperature (Table 2, entries 13–15). The results were much better than those reported previously.13a,b A variety of substituted aromatic aldehydes could be successfully converted to the corresponding amides in good to excellent yields by treatment with lithium anilinide under the optimized conditions (Table 2). It was found that the electron-withdrawing groups (such as O2N–, X–) on the aromatic aldehydes favor the reactions producing the products in high yields (Table 2, entries 19, 20, 22). While treatment of the aromatic aldehydes having electron-donating groups such as CH3O–, CH3–, (CH3)2N– (Table 2, entries 16–18, 21, 23) with lithium anilinide produced the corresponding amides in good to moderate yields (Table 2, entries 16–18, 21, 23), but relatively low than those of the aldehydes incorporating electron-withdrawing groups. These results suggested that electronic effects of aromatic aldehydes have influence on the addition of the corresponding amido group to the carbonyl group. As shown from Table 3, almost all the lanthanide chlorides exhibited good catalytic activities on the amidation reaction of benzaldehyde with lithium anilinide The results were in consistent with the Lewis acidity of the Ln3þ cations.15 The higher the Lewis acidity of the Ln3þ is, the higher yield of product could be obtained. Only 45% yield of amide could be obtained in the absence of the catalyst of lanthanide chlorides, which confirmed the enhancement of the catalysts of lanthanide chlorides.

Table 3 The influence of the lanthanide metal on the reaction 2

NHLi

CHO +

O C NH

1. LnCl3 (5 mol %) 2. H2 O

CH 2OH

+

1a

Ln

None

Y

Pr

Nd

Sm

Eu

Dy

Yb

Yield of 1a (%)

45

99

81

87

91

89

89

98

Reaction conditions: toluene as solvent, room temperature, 2d. Isolated yield based on the lithium amide.

The reaction mechanism is proposed as shown in Scheme 1. Reaction of lithium amide with lanthanide chloride afforded the lanthanide amide.16 Coordination of the oxygen atom of aldehydes with metal center followed by addition of the amido group to the carbonyl group of the aldehyde generated the intermediate A, which transferred a hydride to the second molecule of the aldehyde to give the corresponding product of amide B and the lanthanide alkoxide C. Reaction of C with the lithium amide led to the regeneration of the lanthanide amide and furnishing a catalytic cycle.

10024

L. Zhang et al. / Tetrahedron 65 (2009) 10022–10024

LnCl3 RCH 2OH

LiNR 1R 2

H 2O Ln N

RCH 2OLi

R1 R2

R

RCH2 O Ln C O R

B

R O Ln

NR1 R 2

Ln O

R2 R 1N

R

O

H

LiNR 1R 2

O H R

H

R H

Ln O O

H R1 N R2

NR1 R2

R

H A

another 12 h. And the solvents were extracted. Then toluene (10 mL), YCl3 (0.010 g, 0.05 mmol) were charged. To the mixture was added benzaldehyde (0.212 g, 2.0 mmol). After stirring the reaction for 2 days at room temperature, diluted hydrochloric acid (0.1 M, 1 mL) was then added and the mixture was extracted with ethyl acetate, dried over anhydrous MgSO4, and filtered. After the solvents were evaporated, the residue was purified by silica gel column chromatography by using a mixture of n-hexane and ethyl acetate (1:1, v/v) as an eluent to give N-phenylbenzamide (1a) as a white solid in 99% yield (0.195 g). Following similar procedures, the amides 1b–1w were obtained. Acknowledgements The work was supported by the National Natural Science Foundation of China (20702001, 20832001), and a grant from the Anhui Education Department (TD200707, KJ2007A011). We are grateful to Prof. Jiping Hu and Prof. Baohui Du for their assistance in running NMR and IR spectra.

Scheme 1. Proposed mechanism for the reaction catalyzed by LnCl3.

Supplementary data 3. Conclusion In summary, a variety of amides were obtained by treatment of aldehydes with lithium amides in the presence of catalytic amount of LnCl3. The results indicated that electronic deficient groups on the anilinide disfavored the reaction, in contrast, the electronic deficient groups on the aromatic aldehydes favor the reactions. The features of the economical catalysts, high yields, tolerance of a wide range of amides and aromatic aldehydes make this methodology an easy and valid contribution to the direct synthesis of amides from aldehydes. 4. Experimental 4.1. General Melting points were determined using a Gallenkamp melting point apparatus and are uncorrected. 1H NMR spectra were recorded on a Bruker Avance 300 instrument in CDCl3 solutions using TMS as internal standard. Chemical shifts (d) are reported in parts per million. IR spectra were obtained with a UV-4100 FT-IR spectrometer. Mass spectra were performed on a Micromass GCT-MS CA064. All aldehydes, amines, and solvents were pre-dried, redistilled or recrystallized before use. The lanthanide chlorides were prepared according to references.17 4.2. General procedure for the synthesis of amides from the reaction of aldehydes with lithium amides catalyzed by lanthanide chlorides Under dried argon, to a THF solution PhNH2 (0.093 g, 1.0 mmol) at 78  C in a 30-mL Schlenk tube was slowly added an n-hexane solution of n-BuLi (0.8 mL, 1 mmol). The temperature of the reaction mixture was then gradually raised to room temperature after the addition. The mixture was stirred at room temperature for

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