Copper-catalyzed asymmetric propargylic etherification of oximes promoted by chiral tridentate P,N,N-ligand

Tetrahedron Letters xxx (xxxx) xxx

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Copper-catalyzed asymmetric propargylic etherification of oximes promoted by chiral tridentate P,N,N-ligand De-Quan Wei a,b, Zhen-Ting Liu b, Xue-Ming Wang c, Chuan-Jin Hou a,⇑, Xiang-Ping Hu b,⇑ a

School of Light Industry and Chemical Engineering, Dalian Polytechnic University, Dalian 116034, China Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China c Tianjin Huijia New Material Technology LTC, Tianjing 300182, China b

a r t i c l e

i n f o

Article history: Received 6 September 2019 Revised 17 October 2019 Accepted 21 October 2019 Available online xxxx

a b s t r a c t A copper-catalyzed asymmetric etherification of propargylic acetates with oximes as O-nucleophiles has been developed. The employment of a chiral tridentate P,N,N-ligand proved to be crucial for the success of the reaction. The reaction features a broad scope on either reaction partners and led to a variety of etherification products in good yields and enantioselectivities. Ó 2019 Elsevier Ltd. All rights reserved.

Keywords: Asymmetric catalysis Copper Propargylic substitution Etherification

Since van Maarseveen [1] and Nishibayashi [2] independently reported the first copper-catalyzed asymmetric propargylic amination, the copper-catalyzed asymmetric propargylic substitution has witnessed significant progress in the past decade [3]. The reaction proceeds with copper-allenylidene complexes as the key intermediates and is suitable for a variety of nitrogen and carbon nucleophiles. In a comparison, oxygen nucleophiles are still less explored due to relatively poor nucleophilic ability of the oxygen nucleophiles. In 2015, Nishibayashi reported the first Cu-catalyzed asymmetric etherification of propargylic carbonates with both aliphatic alcohols and phenols as O-nucleophiles [4]. In this reaction, only aliphatic propargylic carbonates were suitable substrates, and the completion of reaction normally required a long time. Following this pioneering work, some propargylic etherification strategies have been developed to overcome these limitations [5]. By the employment of a chiral tridentate P,N,N-ligand and with Cs2CO3 as the additives, our group have realized a highly enantioselective Cu-catalyzed propargylic etherification of both aliphatic and aromatic propargylic esters with phenols as the O-nucleophiles [5a]. Niu and coworkers employed copper/borinic acid dual catalysis to enable the enantioselective propargylic etherification with aliphatic polyols as the O-nucleophiles [5b]. However, all these propargylic etherification reactions have been limited to alcohols or phenols as

⇑ Corresponding authors. E-mail addresses: [email protected] (C.-J. Hou), [email protected] (X.-P. Hu).

the O-nucleophiles. The search of other O-nucleophiles such as oximes for the copper-catalyzed enantioselective propargylic etherification is therefore highly desirable and remains a challenging task. Optically active oxime ethers are synthetically important compounds [6]. As oximes can be readily prepared from aldehydes and hydroxylamine by a simple condensation, the enantioselective etherification of oximes represents the most ideal and direct access to chiral oxime ethers. However, the enantioselective synthesis of these compounds via a catalytic way remains a huge challenge [7]. The present studies mostly focused on the transition metalcatalyzed allylic etherification of oximes, in which some Ir- and Pd-catalytic systems displayed the satisfactory catalytic performance [8]. As the demonstrated O-nucleophilic property of oxime in the catalytic allylic etherification, we envisioned that the oxime may also be a suitable O-nucleophile for the copper-catalyzed asymmetric propargylic substitution. As a result, herein we report the first copper-catalyzed asymmetric propargylic etherification of oximes by the employment of a chiral tridentate P,N,N-ligand developed within our group. The etherification displayed broad substrate scope, thus providing a variety of optically active oxime ethers in good to high enantioselectivity. Our optimization studies started with the reaction between 1phenylprop-2-yn-1-yl acetate 1a-1 and benzaldehyde oxime 2a as the benchmark substrates in the presence of iPr2NEt as the base in MeOH at 20 °C, evaluating various copper catalysts prepared in situ from copper salts and chiral tridentate P,N,N-ligand

https://doi.org/10.1016/j.tetlet.2019.151305 0040-4039/Ó 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: D.-Q. Wei, Z. T. Liu, X. M. Wang et al., Copper-catalyzed asymmetric propargylic etherification of oximes promoted by chiral tridentate P,N,N-ligand, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.151305

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D.-Q. Wei et al. / Tetrahedron Letters xxx (xxxx) xxx

Table 1 Optimization of reaction parameters.a

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

1a 1a-1 1a-1 1a-1 1a-1 1a-1 1a-1 1a-1 1a-1 1a-1 1a-1 1a-1 1a-1 1a-1 1a-1 1a-2 1a-3

[Cu] Cu(OAc)2
H2O CuF2
H2O Cu(OTf)2 CuBr2 CuBr Cu(CH3CN)4BF4 Cu(OTf)2 Cu(OTf)2 Cu(OTf)2 Cu(OTf)2 Cu(OTf)2 Cu(OTf)2 Cu(OTf)2 Cu(OTf)2 Cu(OTf)2 Cu(OTf)2

L*

base

Solvent

Yield %b

Ee %c

L1a L1a L1a L1a L1a L1a L1b L1c L2 L3 L1a L1a L1a L1a L1a L1a

i

MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH MeOH Toluene CH3CN MeOH MeOH

65 60 78 54 55 30 – 60 – 68 66 50 45 21 80 –

85 82 85 83 85 80 – 85 – 65 84 80 86 72 89 –

Pr2NEt Pr2NEt Pr2NEt i Pr2NEt i Pr2NEt i Pr2NEt i Pr2NEt i Pr2NEt i Pr2NEt i Pr2NEt Et3N Cs2CO3 i Pr2NEt i Pr2NEt i Pr2NEt i Pr2NEt i i

a Reaction condition: 1a (0.3 mmol), 2a (0.45 mmol), [Cu] (0.01 mmol, 3.3 mol%), L* (0.011 mmol, 3.6 mol%), and base (0.36 mmol, 1.2 eqv.) in 3 mL of MeOH at 24 h. b Yield of isolated product. c Determined by HPLC using a chiral stationary phase.

(S)-L1a at 3.3 mol% of catalyst loadings. The results in Table 1 indicated that copper salts displayed less influence on the enantioselectivity but significantly affected the reactivity (entries 1–6). Among them, Cu(OTf)2 turned out to be the most efficient catalyst precursor in terms of yield and enantioselectivity, with which 78% yield with 85% ee was achieved (entry 3). Ligand structure showed a dramatic effect in the reaction outcome. Whereas tridentate ketimine ligands (S)-L1a and (S)-L1c exhibited good catalytic performance (entries 3 and 8), the corresponding aldimine ligand (S)-L1b displayed unexpectedly low reactivity (entry 7). Commercially available ligands (S)-BINAP (L2) and (S,S)-diMe-pybox (L3) were also examined in the reaction, but did not provide any improvement for the reaction outcome (entries 9 and 10). Screening the reaction using different bases did not show any benefits (entries 11 and 12). The nature of the solvent significantly affected the activity, and the protic solvent (MeOH) showed superiority over aprotic solvent such as toluene and CH3CN (entries 13 and 14). To further improve the reaction outcome, we then attempted two trialkylsilyl-protected substrates 1a-2 and 1a-3 for the reaction as we have recently demonstrated the efficiency of the desilylation-activated strategy in the catalytic asymmetric propargylic transformation [9]. As expected, an improvement of reaction out-

20 °C for

come was observed when 1-phenyl-3-(trimethylsilyl)prop-2-yn1-yl acetate (1a-2) was employed instead of 1a-1 (entry 15). However, 3-triethylsilylpropargylic acetate (1a-3) showed low reactivity (entry 16). Lower reactivity with 1a-3 may be ascribed to its higher stability toward copper-catalyzed C-Si bond cleavage as observed in the previous study [9a]. Having identified the optimal reaction conditions, we then investigated the scope of 3-trimethylsilylpropargylic acetates 1 for the catalytic propargylic etherification of benzaldehyde oxime 2a, and the results are summarized in Table 2. The substitution pattern on the phenyl ring showed some influence on the reactivity and enantioselectivity. The meta-substituted substrate 1c displayed better enantioselectivity but lower reactivity in comparison to its ortho- or para-analogues (1b or 1d) (entries 1–3). The results indicated that the substrate bearing an electron-donating group tended to give the inferior performance to those with the electron-withdrawing substituent (entries 4–8). 2-Naphthyl substrate 1i also worked, giving the corresponding oxime ether 3ia in 50% yield and with 90% ee (entry 9). Heteroaromatic substrate 1j was not so suitable for the reaction, leading to the etherification product 3ja in 41% yield and with 77% ee (entry 10). Aliphatic propargylic ester 1k was well tolerated, generating the oxime ether 3ka

Please cite this article as: D.-Q. Wei, Z. T. Liu, X. M. Wang et al., Copper-catalyzed asymmetric propargylic etherification of oximes promoted by chiral tridentate P,N,N-ligand, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.151305

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D.-Q. Wei et al. / Tetrahedron Letters xxx (xxxx) xxx Table 2 Scope of 3-trimethylsilylpropargylic acetates 1.a

Table 3 Scope of oximes 2.a

Entry

1 (R1)

3

Yield %b

ee %c

Entry

2 (R2)

3

Yield %b

ee %c

1 2 3 4 5 6 7 8 9 10 11

1a-2 (Ph) 1b (2-ClC6H4) 1c (3-ClC6H4) 1d (4-ClC6H4) 1e (4-FC6H4) 1f (4-BrC6H4) 1g (4-CF3C6H4) 1h (4-MeOC6H4) 1i (2-naphthyl) 1j (2-thienyl) 1k (cyclohexyl)

3aa 3ba 3ca 3da 3ea 3fa 3ga 3ha 3ia 3ja 3ka

80 80 61 78 81 82 68 56 50 41 51

89 86 91 85 84 84 84 78 90 77 95

1 2 3 4 5 6 7 8 9 10 11 12 13

2a (Ph) 2b (2-MeC6H4) 2c (3-MeC6H4) 2d (4-MeC6H4) 2e (4-FC6H4) 2f (4-BrC6H4) 2g (4-ClC6H4) 2h (4-NO2C6H4) 2i (4-tBuC6H4) 2j (1-naphthyl) 2k (2-naphthyl) 2l (2-thienyl) 2m (iPr)

3aa 3ab 3ac 3ad 3ae 3af 3ag 3ah 3ai 3aj 3ak 3al 3am

80 80 70 89 78 87 80 75 60 86 78 25 –

89 88 87 85 86 86 86 88 86 89 86 92 –

a Reaction condition: 1 (0.3 mmol), 2a (0.45 mmol), Cu(OTf)2 (0.01 mmol, 3.3 mol %), (S)-L1a (0.011 mmol, 3.6 mol%), and iPr2NEt (0.36 mmol, 1.2 eqv.) in 3 mL of MeOH at 20 °C for 24 h. b Yield of isolated product. c Determined by HPLC using a chiral stationary phase.

in moderate yield (51%) and with excellent enantioselectivity (95% ee) (entry 11). An attempt to directly use N-phenylhydroxylamine 4 for the etherification proved unsuccessful, leading to the amination product 5 instead of the etherification one. A variety of aromatic oximes 2 were next assessed, and the results are listed in Table 3. The substitution pattern of the functionality on the phenyl ring showed some influence on the reactivity but less effect in the enantioselectivity. Thus, 3-Me substrate 2c gave lower yield and similarly good enantioselectivity in comparison to its 2-Me or 4-Me analogues (2b or 2d) (entries 2–4). The electronic property of the substituent at the para-position of the phenyl ring was well tolerated, and all these substrates gave similar levels of enantioselectivity (entries 4–9). The reaction with 1naphthaldehyde oxime 2j proceeded smoothly, resulting in the corresponding oxime ether 3aj in 86% yield and with 89% ee (entry 10). In a comparison, 2-naphthaldehyde oxime 2k led to a slightly decreased reaction performance (entry 11). However, the present catalytic system was less efficient to heteroaromatic thiophene2-carbaldehyde oxime 2l in term of reactivity although excellent enantioselectivity of 92% ee was observed (entry 12). Furthermore, aliphatic oximes were not tolerated due to its easy decomposition under the reaction condition (entry 13). To explore the potential synthetic application of this methodology, the synthesis of chiral oxime ether 3af on a gram scale was carried out as shown in Scheme 1. Under standard conditions, the reaction afforded the desired compound 3af in 72% yield and with 86% ee. The imino-moiety of chiral oxime ether 3af could be reduced with NaBH3(CN) in a mixture of EtOH/THF to give (S)-N-(4-bromobenzyl)-O-(1-phenylprop-2-yn-1-yl)hydroxylamine 6af in moderate yield. The absolute stereochemistry of the

a Reaction condition: 1a (0.3 mmol), 2 (0.45 mmol), Cu(OTf)2 (0.01 mmol, 3.3 mol %), (S)-L1a (0.011 mmol, 3.6 mol%), and iPr2NEt (0.36 mmol, 1.2 eqv.) in 3 mL of MeOH at 20 °C for 24 h. b Yield of isolated product. c Determined by HPLC using a chiral stationary phase.

Scheme 1. Gram-scale synthesis, derivation and determination of absolute configuration.

resulting chiral oxime ethers was determined to be R by the comparison of the optical rotation with the reported value of 7ea in the literature [8d], which was obtained via the hydrogenation of 3ea with Pd/BaSO4 under 1 atm of H2 pressure. In conclusion, we have realized the first copper-catalyzed asymmetric propargylic etherification of propargylic acetates with oximes as the O-nucleophiles. The success of the present reaction was ascribed to the employment of a chiral tridentate P,N,N-ligand. The etherification displayed broad substrate scope, moderate to

Please cite this article as: D.-Q. Wei, Z. T. Liu, X. M. Wang et al., Copper-catalyzed asymmetric propargylic etherification of oximes promoted by chiral tridentate P,N,N-ligand, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.151305

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D.-Q. Wei et al. / Tetrahedron Letters xxx (xxxx) xxx

high yields and good to excellent enantioselectivities, thus providing a facile access to a variety of optically active oxime ethers. A further investigation on the copper-catalyzed asymmetric propargylic substitution with chiral tridentate P,N,N-ligands is ongoing in our laboratory.

[4] [5]

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments

[6]

Financial supports from the National Natural Science Foundation of China (21572226 and 21772196), Dalian Science and Technology Innovation Project (2018J12GX054) and Natural Science Foundation of Liaoning Province of China (2019-MS-022) are gratefully acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.tetlet.2019.151305.

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Please cite this article as: D.-Q. Wei, Z. T. Liu, X. M. Wang et al., Copper-catalyzed asymmetric propargylic etherification of oximes promoted by chiral tridentate P,N,N-ligand, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.151305