Copper-mediated oxidative homocoupling and rearrangement of N-alkoxyamides: an efficient method for the preparation of aromatic esters

Tetrahedron Letters 56 (2015) 4634–4637

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Copper-mediated oxidative homocoupling and rearrangement of N-alkoxyamides: an efficient method for the preparation of aromatic esters Xiyan Duan ⇑, Kun Yang, Shuang Tian, Junying Ma ⇑, Yaning Li, Jiao Zou, Dongliang Zhang, Huanqing Cui School of Chemical Engineering & Pharmaceutics, Henan University of Science and Technology, No. 263, Kaiyuan Road, Luolong District, Luoyang 471023, PR China

a r t i c l e

i n f o

Article history: Received 13 May 2015 Revised 5 June 2015 Accepted 9 June 2015 Available online 14 June 2015 Keywords: N-Alkoxyamides m-CPBA Heron rearrangement Copper-catalyzed oxidative homocoupling Esters

a b s t r a c t N-Alkoxyamides are successfully converted into their corresponding esters in a moderate to satisfactory yields via copper-catalyzed oxidative homocoupling and Heron rearrangement. The process tolerates a wide variety of functional groups and allows the synthesis of sterically hindered ester products not readily accessible by traditional acylation chemistry. A radical-mediated pathway has been tentatively proposed for the oxidative process. Ó 2015 Elsevier Ltd. All rights reserved.

The ester unit is an important structural moiety in pharmaceuticals, natural products, polymers, and biologically relevant compounds.1 A very large number of methods available for formation of esters are well known, including the most direct synthetic routes in which alcohol reacts with activated carboxylic acid derivative or transesterification.2 Although those approaches are widely used, it is not without limitations. The synthesis of sterically hindered esters can be cumbersome due to a bulky group on either the carboxylic acid or the alcohol significantly hampers esterification.3 To address the formation of these important ester products, special reagents and procedures are recently reported in the literature.4 However, the high utility of esters calls for the continuous development toward their synthesis.4a In recent years, N-alkoxyamides have recently attracted attention in the field of organic chemistry and medicinal chemistry because of their easy conversion into esters, amides, and nitriles, as well as their applications in the synthesis of heterocyles.5–7 Notably, the additional heavy atom between the acyl and alkoxy side chains reduces the influence of steric effects,5,8 so a direct conversion of N-alkoxyamides to carboxylic esters is an efficient strategy to the formation of highly hindered esters. Existing methods for the formation of esters from corresponding N-alkoxyamides ⇑ Corresponding authors. Tel./fax: +86 379 64240991. E-mail addresses: [email protected] (X. Duan), [email protected] (J. Ma). http://dx.doi.org/10.1016/j.tetlet.2015.06.023 0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

can be summarized as three general approaches: (1) Oxidation of various N-alkoxyamides with ceric ammonium nitrate (CAN)9 or nickel peroxide (NiO2H2O)9 silver oxide (Ag2O)10 or lead(IV) acetate (Pb(OAc)4)11,12 leads to the corresponding esters (Scheme 1,

path a O R

1

N H

OR2

CAN, Ag 2O, Pb(OAc) 4 or NiO2 H2 O

O R

1

OR 2

path b O R

1

N H

OR2

O

(1) t-BuOCl R1

(2) NaN 3

OR 2

path c O R

1

N H

OR2

O

NBS R

1

OR 2

path d O R1

N H

OR2

CuBr2 , m-CPBA 1,4-dioxane rt-90 o C

O R

1

OR 2

this work Scheme 1. Methods for the synthesis of esters from N-alkoxyamides.

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X. Duan et al. / Tetrahedron Letters 56 (2015) 4634–4637 Table 1 Optimization of the reaction conditionsa

O MeO

O N H

OMe

MeO

oxidant solvent

1a Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 a b c d

Oxidant (equiv) DDQ(1.0) MnO2(1.0) FeCl3(1.0) H2O2(1.0) TEMPO(1.0) m-CPBA(1.0) m-CPBA(1.0) m-CPBA(1.0) m-CPBA(1.0) m-CPBA(1.0) m-CPBA(1.0) m-CPBA(1.0) m-CPBA(1.0) m-CPBA(1.0) m-CPBA(1.0) m-CPBA(1.0)

Catalyst (equiv) c

— —c —c —c —c —c CuBr2 (0.1) CuBr2 (0.1) CuCl(0.1) Cu(acac)2 (0.1) Cu(OAc)2 (0.1) CuBr (0.1) CuBr2 (0.1) CuBr2 (0.1) CuBr2 (0.1) CuBr2 (0.1)

MeO

OMe

MeO 2a

Solvent

Temp (°C)

Time (h)

Yieldb (%)

CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 THF 1,4-dioxane DCE CH3CN

rt rt rt rt rt rt rt 70 °C 70 °C 70 °C 70 °C 70 °C 70 °C 70 °C 70 °C 70 °C

3h 3h 3h 3h 3h 1d 1d 3h 3h 3h 3h 3h 3h 3h 3h 3h

n.r.d n.r.d n.r.d n.r.d 0 45 60 68 40 34 39 65 65 88 49 67

Reaction conditions: 1a (1 mmol), oxidant (1 mmol), catalyst (0.1 mmol) in solvent(10 mL) unless otherwise stated. Isolated yield. No catalyst. n.r. = no reaction.

path a). (2) Heron rearrangement of N-alkoxyamides substrates by SN2 azidation of N-alkoxy-N-chloroamides (Scheme 1, path b).8 (3) Direct conversion of N-Alkoxyamides to carboxylic esters involving NBS-mediated oxidative procedure for homocoupling and thermal denitrogenation (Scheme 1, path c).5 Nevertheless, some of these methods involve toxic reagents, harsh reaction conditions, multistep operations or generation of wastes that not only reduce process efficiency but also pose environmental problems.8–12 Compared with the late transition metal catalysts, the first-row transition metals, especially copper13 have received more and more attention recently, owing to their availability, low cost, low toxicity, and ease of use. Herein, we report a method for the synthesis of esters starting from N-alkoxyamides, via Heron rearrangement processes in the presence of CuBr2 and m-CPBA in 1,4-dioxane (Scheme 1, path d). Initially we utilized N-methoxybenzamide 1a as a model substrate to investigate different reaction conditions. Selected data are listed in Table 1. Firstly, various oxidants were screened for the reaction in CH2Cl2 at room temperature. Unfortunately, with the use of oxidants such as DDQ, MnO2, or FeCl3, no desired product was detected (Table 1, entries 1–3). Therefore, other oxidants were tested (Table 1, entries 4–5). When m-CPBA was used, the desired product 2a was obtained in 45% yield (Table 1, entry 6). Interestingly, we found that the introduction of CuBr2 greatly improved the product yield at room temperature in 1 day (Table 1, entry 7). The reaction temperature is crucial for this reaction. A change in the reaction temperature from room temperature to 70 °C increased the reaction yield and rate. Probably the increase of reaction temperature will accelerate the coupling reaction and rearrangement occurs (Table 1, entry 8). An examination of other copper catalysts revealed that CuCl, Cu(acac)2, Cu(OAc)2, and CuBr gave decreased yields of 2a (Table 1, entries 9–12). Screening of a series of other solvents, including THF, 1,4-dioxane, DCE, and MeCN (Table 1, entries 13–16), showed that the reaction in 1,4-dioxane afforded the best yield in 81% (Table 1, entry 14). With the optimized reaction conditions, we proceeded to explore the substrate scope of this transformation (Table 2). The

protocol proved to be compatible with a broad range of N-alkoxyamides and tolerated a wide variety of functional groups. On the whole, the reaction was benefited by electron donating groups and hindered by electron withdrawing groups attached to the aromatic ring. Aromatic N-alkoxyamides bearing electron donating groups reacted smoothly under the optimized conditions to form the corresponding esters in good yields (2a–2c). With electron withdrawing groups, the yield of the product decreased (2d–2f). For example, when 4-fluoro-N-methoxybenzamide was used as substrate only 46% esterification product 2f was obtained at higher temperature even for a longer reaction time of 12 h. Using of heteroaromatic N-alkoxyamide, such as N-methoxythiophene-2carboxamide as substrate, resulted in product 2g in a moderate yield of 65%. Besides the aromatic N-alkoxyamides, we found that N-benzylcinnamamides and N-methoxycinnamamide, also worked efficiently (2h and 2i). Furthermore, this catalytic reaction worked well with aliphatic carboxylic amides. It resulted in esters 2j and 2k in 50% and 55% yields, respectively. Finally, when the R2 group was changed as benzyl, the desired product was also obtained in 87% yield. Encouraged by these promising results, we further applied the optimized reaction conditions to examine the substrate scope of N-tert-butoxy-amides. Aromatic N-tert-butoxy-amides bearing electron donating groups, such as methoxy, afforded the desired products in good yields (2m and 2n). Also this catalytic reaction is nicely tolerant of halo substituents on the aromatic ring of the N-tert-butoxy benzamides (2o and 2p). Replacing the phenyl ring with a naphthalene ring in the substrate does not hamper the reaction, and the desired product 2q was obtained in 68% yield. It is important to note that the substrate with bulky R1 groups, could also afford the desired product 2r in 45% yield. From the above results, we believe that the reaction proceeds via oxidative homocoupling and Heron rearrangement process by a radical mechanism. Our hypothesized mechanism for this transformation is shown in Scheme 2. Firstly, alkoxyamide 1 was converted to radical intermediate A via radical oxidation in the presence of Cu/m-CPBA.14 Next, the N-centered radical

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X. Duan et al. / Tetrahedron Letters 56 (2015) 4634–4637 Table 2 Investigation of the scope of the procedurea,b

O R1

O

m-CPBA, CuBr 2

OR2

N H

R1

1,4-dioxane rt-90 oC

1 O

O MeO

OR 2 2

O

O

OMe

OMe

OMe

OMe

MeO

MeO

Br

2a

2b

2c

2d

90%

81%

70%

75%

O

O

O

O

OMe

S

OMe

Cl

OBn

OMe

F 2e

2f c

67%

46% O

F

2g

2h

65%

68% O

O 2N

O

O

OBn

OMe OMe

OMe

2i

2j

2k

65%

50%

55%

O MeO

O

2l

O

O

O

O

O

O

MeO

87%

MeO

Br

Cl

2m

2n

2o

2p

88%

78%

67%

51%

O O

O

O 2r 45%

2q 68%

a General conditions: 1 (1.0 mmol), m-CPBA (1 mmol), and CuBr2 (0.1 mmol) in dioxane (10 mL) at 70 °C for 3 h unless otherwise stated. b Isolated yield. c Reaction conditions: 1 (1.0 mmol), m-CPBA (1 mmol), and CuBr2 (0.1 mmol) in dioxane (10 mL) at 90 °C for 12 h.

Cu

m-CPBA

O O

O 2 R1

N 1H

Cl

Cu m-CPBA OR2 -2H

O O 2 R1

R1 N

OR2 N N R2 O R1 O B

OR2

A

O R1

OR 2 2

O R1

N N

-N 2 OR 2

R1

R 2O

2

C

unstable,5 it decomposed to generate ester 2 and intermediate 1,1-disubstituted diazene C by the Heron process. Finally, intermediate 1,1-disubstituted diazene C decomposed to afford another molecule of ester with liberation of nitrogen via Heron rearrangement.8 In summary, the efficient and direct synthesis of ester from alkoxyamide via copper-catalyzed oxidative homocoupling and Heron rearrangement has been developed. This methodology provides an avenue to the synthesis of various carboxylic esters from diverse alkoxyamides. Moreover, the generality of this method has been demonstrated by the efficient synthesis of some sterically hindered esters, which are not easy to obtain by other methods. Further investigations to explore the mechanistic details and the synthetic applications of the reaction are currently ongoing.

O

Scheme 2. Proposed mechanistic pathways.

species A was dimerized to form the symmetrical N,N0 -diacylN,N0 -dialkoxyhydrazine B. As intermediate B is thermally

Acknowledgments X. Duan acknowledges the Scientific Research Foundation of Henan University of Science and Technology (2014QN022, 09001680). Supported by students research training program of the Henan University of Science and Technology.

X. Duan et al. / Tetrahedron Letters 56 (2015) 4634–4637

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