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Accepted Article Title: Visible-Light Promoted Distereodivergent Intramolecular Oxyamidation of Alkenes Authors: Xiang Ren, Qihang Guo, Jianhui Chen, Hujun Xie, Qing Xu, and Zhan Lu This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Chem. Eur. J. 10.1002/chem.201603977 Link to VoR: http://dx.doi.org/10.1002/chem.201603977
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10.1002/chem.201603977
Chemistry - A European Journal
COMMUNICATION Visible-Light Promoted Distereodivergent Intramolecular Oxyamidation of Alkenes Xiang Ren, [a]† Qihang Guo, [a]† Jianhui Chen, [a,c] Hujun Xie, *[b] Qing Xu* [c] and Zhan Lu*[a] Dedication ((optional))
The visible-light-promoted diastereodivergent intramolecular oxyamination of alkenes and amination of unactivated C(Sp3)-H bonds is described to construct oxazolindinones, pyrrolidinones and imidazolidones via mild generation of primary amidyl radicals from functionalized hydroxylamines. A unique phenomenon on highly diastereoselective ring-opening of aziridines controlled by electron sacrifices was observed. High diastereoselective amino alcohols derivatives were obtained efficiently through this protocol in gram scales. The mechanistic studies suggested the isolatable anti-aziridine intermediates were generated quickly from primary amidyl radicals and the diastereoselectivities were controlled by pKa values of electron sacrifices. Abstract:
1,2-Amino alcohols derivatives are highly-value, widely exist and well used in organic transformation for asymmetric catalysts, medicine, materials and so on. The directly and highly regio- and diastereoselective oxyamination of alkenes is one of the powerful methodologies to construct useful amino alcohols derivatives.[1] Since Bäckvall reported the first direct oxyaminations of alkenes using stoichiometric PdCl 2 in 1975[2] and Sharpless reported the first example of Os-catalyzed oxyamination of alkenes,[3] direct oxyamination of alkenes were developed to explore their utilities. Although highly chemo- and regioselective oxyamidations of alkenes were achieved through transition-metal catalysis, [4] hypervalent iodine,[5] acids,[6] NFSI,[7] as well as electrochemistry[8], distereoselectivity-controllable oxyamidation reaction of alkenes, particularly based on radicalinitiated reactions, is a remaining significant challenge. The diastereoselectivity-controllable oxyamination of alkenes used to be controlled by using Z or E-configuration alkenes.[9] Dauban and co-workers developed a diastereoselectivity-controllable
[a]
[b]
[c]
Mr. X. Ren, Mr. Q. Guo, Mr. J. Chen, Prof. Z. Lu Department of Chemistry, Zhejiang University Hangzhou, Zhejiang 310058, P. R. China E-mail: [email protected] Prof. H. Xie Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou, Zhejiang 310018, P. R. China E-mail: [email protected] J. Chen, Prof. Q. Xu College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, P. R. China E-mail: [email protected] † X. Ren and Q. Guo are contributed equally to this work Supporting information for this article is given via a link at the end of the document.
rhodium-catalyzed intermolecular oxyamination of indoles by using different oxygen nucleophiles.[3j] However, the substrates were limited in indoles. Xu and co-workers reported a diastereoselectively iron-catalyzed intramolecular oxyamination of olefins controlled by careful selection of counteranion/ligand combinations.[3w] However, in both cases, there were only one reversed example shown via amidyl-carbenes or amidyl-metal intermediates, and no direct evidences to explain the controllable selectivity. Visible light is a safe, inexpensive, abundant, and nonpolluting reagent, [10] meanwhile, could promote to generate amidyl radical mildly.[11] Very recently, Akita[12] and Leonori[13], respectively, reported the oxyamination of alkenes via visible light photocatalysis. However, the reactions of 1,2-disubstituted alkenes afforded products with poor diastereoselectivities. To the best of our knowledge, the use of photosensitizer to promote the diastereoselectivity-controllable oxyamination reaction of the same alkene has not previously been described.[14] Our group are particularly interested in one-electron reduction of nitrogencontaining electron-deficient groups.[15] Here, we described a mechanistic distinct diastereroselectivity-controllable intramolecular oxyamidation of alkenes with functionalized hydroxylamines in the presence of photocatalysts and electronsacrifices upon irradiation of visible light.
Scheme 1. Regio- and stereoselective oxyamination of alkenes.
We chose 1a as a model substrate. The transformation of 1a using transition metal catalysts has been studied to form aziridine or oxyamidation products. [4n,9c] In particular, the reaction of 1a gave the oxyamidation products in good yield with poor diastereoselectivity using iron catalysts. [3w] On the contrary, we proposed that the primary amidyl radical might be formed through the one-electron reductive process.[16] The reaction of 1a was conducted in the presence of 2 mol% of visible light
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10.1002/chem.201603977
Chemistry - A European Journal
COMMUNICATION Table 1. Optimization.
[a]
Entry
Photocatalyst
Additives (equiv)
Yield [b] (%)
Dr [b] anti/syn
1
Ir(ppy)2(dtbbpy)PF6
Et3N (1)
65
>20:1
2
-
Et3N (1)
0
-
3
Ir(ppy)2(dtbbpy)PF6
-
0
-
4
Ir(ppy)2(dtbbpy)PF6
Et3N (1)
0
-
5
Ir(ppy)2(dtbbpy)PF6
PPh3 (1)
60
4:1
6
Ir(ppy)2(dtbbpy)PF6
PhSiH3 (1)
0
-
7
Ir(ppy)2(dtbbpy)PF6
Et3SiH (1)
0
-
8
Ir(ppy)2(dtbbpy)PF6
(EtO)3SiH (1)
21
1:3
9
Ir(ppy)2(dtbbpy)PF6
Ph3N (1)
50
1:10
10
Ir(ppy)2(dtbbpy)PF6
PhNMe2 (1)
68
1:6
11
Ir(ppy)2(dtbbpy)PF6
N-Me-morpholine (1)
68
[c]
1.4:1 [e]
[d]
12
Ir(ppy)2(dtbbpy)PF6
Et3N (1)
(69)
>20:1
13[d]
Ir(ppy)2(dtbbpy)PF6
Ph3N (1)
(74)[e]
1:12
[d]
14
Ir(ppy)2(dtbbpy)PF6
Et3N (0.1)
72
1:1.3
15[d]
Ir(ppy)2(dtbbpy)PF6
Et3N (0.01)
90
1:5
[d]
16
Tetrabromofluorescein
Et3N (1)
69
13:1
17[d]
Tetrabromofluorescein
Ph3N (1)
67
1:12
[d]
18
Rose bengal
Et3N (1)
70
13:1
19[d]
Rose bengal
Ph3N (1)
72
1:5
[a] The reaction was conducted using 2 mol% of photocatalyst, 1 equiv. of additive and 0.5 mol of substrate in a solution of MeCN (0.05 M) under the irradiation of blue LED (5 W). [b] Yields and diastereoselectivities were determined by H NMR using crude products and the trimethylsilylbenzene as an internal standard. [c] No light. [d] 50 oC. [e] In 1 hour, and isolated yields of the major isomer in the parenthesis.
photocatalyst Ir(ppy)2(dtbbpy)(PF6) and 1 equiv. of Et3N (pKa value is 10.75)[17] as an electron sacrificial donor in a solution of MaCN (0.05M) under the irradiation of 5 W of blue LED at room temperature to afford the oxyamidation product anti-2a, in 65% yield with >20:1 distereoselectivity (Table 1, entry 1). Control experiments suggested that photocatalysts, light, bases are all necessary (entries 2-4). Various e-sacrificial donors have been investigated. Triphenylphosphine is also a good electron sacrificial donor to afford the desired anti-product in 60% yield and 4:1 dr (entry 5). Silanes could not efficiently promote the process at room temperature, and diastereoselectivities was less ideal (entires 6-8). To our surprise, the reactions using Ph3N (pKa value is -3.04, entry 9) as the electron sacrificial donor afforded syn-product 3a as the major products with 1/10 dr.
PhNMe2 (pKa value is 5.2, entry 10) and N-Me-morpholine (pKa value is 7.38, entry 11) were selected as electron sacrificial donors to give the oxyamidation products with 1:6 and 1.4:1 dr, respectively. The reactions could be accelerated at 50 oC (entries12 and 13, as the two standard conditions). Using 10 mol% of Et3N, the reaction gave the oxyamidation products with 1:1.3 dr (entery 14). The reversed diastereoselectivity was observed using 1 mol% of Et3N (entery 15). This unique diastereoselectivity-controllable reactions were dependent on pKa value of electron sacrificial donors and pH value of reactions. The organic photosensitizers such as Tetrabromofluorescein and Rose Bengal could promote the same transiformations with slightly poor reactivities and diastereoselectivities (entries 16-19). With standard conditions in hand, we investigated the scope of substrates shown in Table 2. Using electron-deficient benzoyl group, the diastereoselectivities were improved dramatically under both Et3N and Ph3N conditions (2b-d and 3b-d). The reactions with ortho- and meta-methylphenyl alkenes gave antiselective products 2e-f with high diastereoselectivity under Et 3N conditions, and syn-selective products 3e-f with a slightly low diastereoselectivity under Ph3N conditions. Simple phenyl ring 1g, and phenyl ring with electron-donating groups 1h, phenyl group 1i and electron-withdrawing groups 1j could participate to give the anti-selective products 2g-j with high diastereoselectivity under Et3N conditions, and syn-selective products 3g-k with a slightly low diastereoselectivity. Both 1naphthyl and 2-naphthyl alkenes could be transferred to the desired anti- and syn-products with high diastereoselectivities (2l-m and 3l-m). Heterocycles such as 2-(N-tosyl)indolyl could be tolerated to afford the desired anti-2m in 52% yield with >20/1 dr. The syn-3m could also be obtained under Ph3N conditions in 76% yield with >20/1 dr. The alkyl alkene 1n could undergo oxyamidation reactions to afford the same major diastereoisomer under both conditions. Dimethyl alkenes and terminal alkenes afforded the oxyamidation product in 59% and 41% yield, respectively. The carbon-tethered substrate 4 could also participate to afford the pyrrolidinone 5 in 57% with 14/1 dr and 52% yield with 7/1 dr under conditions A and B, respectively. The configurations of major products were somehow the same. The nitrogen-tethered substrate 6 was converted to aziridine 7[18] in 66% yield under conditions A. The oxyamination product 8 was produced in 49% yield with 6/1 dr under conditions B. Notably, the reaction of 6 under conditions B with an excess equivalent of benzoic acid at 80 oC afforded the imidazolidone 8 in 62% yield with >20/1 dr. The relative configurations of oxyamination products were determined by X-ray crystallographic analysis of compounds 2e, 3e and 8. [19] The gram-scale reaction could be conducted smoothly to afford the anti- and syn-products in 77% and 93% isolated yield, respectively (Scheme 2). The amino alcohols could be obtained by hydrolysis of the oxyamidation products.
Scheme 2. Gram scale reactions.
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10.1002/chem.201603977
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COMMUNICATION Table 2. Substrate scope for diastereoselectivity-controllable oxyamidation of alkenes
[a] Conditions: Ir(ppy)2(dtbbpy)PF6 (2 mol%), base (1 equiv.), 1 (0.5 mol), and MeCN (0.05 M), blue LED (5 W), 50 oC. [b] 80 oC, with 1 equiv. of PhCOOH.
To elucidate the mechanism, a series of experiments were conducted. 1) The reaction did not occur in the presence of radical inhibitor tempo (Scheme 3). This suggested the reaction might undergo a radical initiated pathway. [20] 2) Time course studies were conducted to show that the reactions of 1a in the presence of Et3N or Ph3N afforded the same aziridine intermediate 9a which could be observed by analysis of crude mixtures, also isolated and identified by 1H and 13C NMR, NOE and HRMS. The control experiments were carried out under the irradiation by visible light in 15 min, then the mixtures were stirred in dark in 18 h to afford anti-2a or syn-3a, respectively (Scheme 4). In the presence of Et3N or Ph3N with benzolic acid, 9a could also be translated to the corresponding anti-2a in 61% yield and 16/1 dr or syn-3a in 74% yield and 10/1 dr, respectively (Scheme 4). These results suggested: one is that the aziridine might be the stable intermediate during the reaction pathways, another is that the controllable diastereoselectivity of the products should be somehow involved by the ring-open step of the aziridine intermediate.[21] 3) The reactions of Z-1a in the presence of Et3N or Ph3N afforded the anti-2a in 67% yield with 8/1 dr or syn-3a in 55% yield with 11/1 dr, respectively. These results were similar with ones using E-1a as the substrate. It would be unlikely to generate the nitrene intermediates which would react with alkenes with different configurations to afford stereo-specific products.[22] The nitrogen radical intermediates seemed more reasonable. The nitrene intermediates could not be absolutely ruled out from primary amidyl radicals.
Scheme 3. Radical trapped reactions.
Scheme 4. Time course studies and reactions of intermediates.
Scheme 5. Reactions of Z-1a.
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COMMUNICATION In order to elucidate the origin of the observed stereoselectivity, we carried out the density functional theory (DFT) calculations to investigate the reaction mechanisms for the nucleophilic attack step by the benzolic anion for the formation of products.[23] The corresponding Gibbs free energy profiles for this step in the presence of Ph3N (I) and Et3N (II) are illustrated in Figure 1. As shown in Figure 2(I), the Ph3N is not involved in the nucleophilic attack step due to the smaller proton affinity of Ph3N (218.8 kcal/mol) compared to Et3N (232.0 kcal/mol). This reaction can proceed via two possible pathways. The barrier difference (4.5 kcal/mol, between TS12 and TS13) for the formation of the two diastereoisomers can give a multiplicative difference of about 103 in the reaction rate if we assume an Arrhenius expression for the rate constant and similar preexponential factors, explaining the experimental observations that the reaction could afford syn3 as the major product in the presence of Ph3N. For the Et3N, the barrier difference (3.1 kcal/mol, between TS45 and TS46) for the formation of the two diastereoisomers can give a multiplicative difference of about 102 in the reaction rate. The calculation results agree with the experimentally observed selectivity that the reaction could afford anti-2 as the major product in the presence of Et3N.
Figure 1. Free energy profiles of the nucleophilic attack step by benzoyl group for the formation of different products in the presence of Ph3N (I) and Et3N (II). The solvation-corrected relative Gibbs free energies are given in kcal/mol.
aziridine and benzoic acid pair B might form a intermolecular D, then a SN1-reaction pathway might proceed to give syn-3 products.[24] This interesting phenomenon to control the diastereoselectivity will potentially provide a novel strategy for diastereoselective ring-opening of aziridines.
Scheme 6. Possible Mechanism.
In summary, we developed a first visible-light-promoted diastereodivergent intramolecular oxyamidation of alkenes with functionalized hydroxylamines. The unique diastereodivergent reactions were controlled by pKa value of electron sacrificial donors and pH value of reactions. This protocol afforded useful amino alcohols derivatives with high diastereoselectivities in gram scales. The mechanistic studies suggested that primary amidyl radicals were mildly generated from functionalized hydroxylamines in the presence of photocatalysts and amines upon irradiation of visible light, aziridine intermediates were formed quickly and the high diastereoselectivities were dependent on ring-opening of aziridine intermediates. Visiblelight-promoted selectivity-controllable intermolecular reactions and asymmetric transformations will be explored in our laboratory.
Acknowledgements The possible mechanism was shown in Scheme 6. The visible light photocatalyst (PC) could absorb the visible light to generate the excited state PC* which could be reduced by electron sacrifices to produce the low valent photocatalyst PC-. The substrate could be reduced by PC species to regenerate PC and afford the radical ion intermediate which might undergo O-N bond cleavage to form a distonic radical ion A. The primary amidyl radical could undergo the intramolecular radical cyclization to afford a benzylic radical which might lose an electron to form a stable aziridine intermediate [21] with the benzoic acid. Although the possibility of the nitrene obtained through deprotonation of A is small, we can not absolutely exclude this pathway. In the presence of Et3N, the benzoic acid could react with Et3N to form +NHEt3 and benzoic ion which might be favor to attack the aziridine intermediate undergo SN2reaction pathway to give anti-2. Due to the weak acidic property, Ph3N could not react with the benzoic acid efficiently. The
We thank the NSFC (21472162). Keywords: visible light photocatalysis • diastereodivergent • oxyamidation • aziridines • alkenes [1] [2] [3]
[4]
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10.1002/chem.201603977
Chemistry - A European Journal
COMMUNICATION
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Weinheim, 2001; Vol. 2, pp 407-424; f) S. Z. Zard, Chem. Soc. Rev. 2008, 37, 1603. For selected examples on primary amidyl radicals, see: g) M. Newcomb, M. T. Burchill, J. Am. Chem. Soc. 1988, 110, 6528; h) Y. Guindon, B. Guérin, S. R. Landry, Org. Lett. 2001, 3, 2293; i) Y. Tang, C. Li, Org. Lett. 2004, 6, 3229; j) P. Gaudreault, C. Drouin, J. Lessard, Can. J. Chem. 2005, 83, 543; k) J. Kemper, A. Studer, Angew. Chem. Int. Ed. 2005, 44, 4914; l) A. Martin, I. Pérez-Martín, E. Suárez, Org. Lett. 2005, 7, 2027. m) T. Hu, M. Shen, Q. Chen, C. Li, Org. Lett. 2006, 8, 2647; n) J. Guin, C. Mück-Lichtenfeld, S. Grimme, A. Studer, J. Am. Chem. Soc. 2007, 129, 4498. K. Miyazawa, T. Koike, M. Akita, Chem. Eur. J. 2015, 21, 11677. J. Davies, S. G. Booth, S. Essafi, R. A. W. Dryfe, D. Leonori, Angew. Chem. Int. Ed. 2015, 54, 14017. During revising this manuscript, a photocatalytic oxyamination of alkenes was reported by Xiao group, see: X.-Q. Hu, J. Chen, J.-R. Chen, D.-M. Yan, W.-J. Xiao, Chem. Eur. J. 2016, 22, 14141. C. Wang, X. Ren, H. Xie, Z. Lu, Chem. Eur. J. 2015, 21, 9676. a) K. Okada, K. Okamoto, N. Morita, K. Okubo, M. Oda, J. Am. Chem. Soc. 1991, 113, 9401; b) M. J. Schnermann, L. E. Overman, Angew. Chem. Int. Ed. 2012, 51, 9576; c) G. Cecere, C. M. Konig, J. L. Alleva, D. W. C. MacMillan, J. Am. Chem. Soc. 2013, 135, 11521; d) D. Kalyani, K. B. McMurtrey, S. R. Neufeldt, M. S. Sanford, J. Am. Chem. Soc. 2014, 136, 5607; e) Q. Qin, S. Yu, Org. Lett. 2014, 16, 3504. All of the pKa data from Evan’s pKa table. For examples on visible-light-promoted aziridinations of olefins, see: a) S. O. Scholz, E. P. Farney, S. Kim, D. M. Bates, T. P. Yoon, Angew. Chem. Int. Ed. 2016, 55, 2239; b) K. Matsuzawa, Y. Nagasawa, E. Yamaguchi, N. Tada, A. Itoh, Synthesis 2016, DOI:10.1055/s-00351561635. During revising this manuscript, a cobalt-catalyzed aziridination of alkenes was reported by Zhang group, see: H. Jiang, K. Lang, H. Lu, L. Wojtas, X. P. Zhang, Angew. Chem. Int. Ed. 2016, 55, 11604. CCDC 1443389, 1443391 and 1496350. The energy transfer mechanism could not be absolutely ruled out. For selected examples, see: a) Z. Lu, T. P. Yoon, Angew. Chem. Int. Ed. 2012, 51, 10329; b) E. P. Farney, T. P. Yoon, Angew. Chem. Int. Ed. 2014, 53, 793; c) R. Alonso, T. Bach, Angew. Chem. Int. Ed. 2014, 53, 4368; d) E. Arceo, E. Montroni, P. Melchiorre, Angew. Chem. Int. Ed. 2014, 53, 12064. For a acidity and polarity controlled divergent aziridine opening process, see: a) Berti, C.; Camici, G.; Macchia, B.; Macchia, F.; Monti, L. Tetrahedron Lett 1972, 25, 2591; for a regiodivergent aziridine opening under Schiff base catalysis, see: b) Xu, Y.; Kaneko, K.; Kanai, M.; Shibasaki, M.; Matsunaga, S. J. Am. Chem. Soc. 2014, 136, 9190; for a two-step examples on the stereodivergent synthesis of vic-amino alcohols from aziridines, see: c) B. Olofsson, U. Khamrai, P. Somfai, Org. Lett. 2000, 2, 4087; d) B. Olofsson, P. Somfai, J. Org. Chem. 2002, 67, 8574. For selected reviews on nitrenes and aziridines, see: a) I. D. G. Watson, L. Yu, A. K. Yudin, Acc. Chem. Res. 2006, 39, 194; b) G. S. Singh, M. D’hooghe, N. De Kimpe, Chem. Rev. 2007, 107, 2080; c) H. Ohno, Chem. Rev. 2014, 114, 7784; d) L. Degennaro, P. Trinchera, R. Luisi, Chem. Rev. 2014, 114, 7881. The computational details can be found in supporting information. D. Ferraris, W. J. Drury III, C. Cox , T. Lectka, J. Org. Chem. 1998, 63, 4568.
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COMMUNICATION Entry for the Table of Contents (Please choose one layout) Layout 2:
COMMUNICATION Author(s), Corresponding Author(s)* Page No. – Page No. Visible-Light Promoted Distereodivergent Intramolecular Oxyamidation of Alkenes
The visible-light-promoted diastereodivergent intramolecular oxyamination of alkenes and amination of unactivated C(Sp3)-H bonds is described to construct oxazolindinones, pyrrolidinones and imidazolidones via mild generation of primary amidyl radicals from functionalized hydroxylamines. A unique phenomenon on highly diastereoselective ringopening
of
aziridines
controlled
by
electron
sacrifices
was
observed.
High
diastereoselective amino alcohols derivatives were obtained efficiently through this protocol in gram scales. The mechanistic studies suggested the isolatable anti-aziridine intermediates were generated quickly from amidyl radicals and the diastereoselectivity were controlled by pKa values of electron sacrifices.
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Accepted Article Title: Visible-Light Promoted Distereodivergent Intramolecular Oxyamidation of Alkenes Authors: Xiang Ren, Qihang Guo, Jianhui Chen, Hujun Xie, Qing Xu, and Zhan Lu This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Chem. Eur. J. 10.1002/chem.201603977 Link to VoR: http://dx.doi.org/10.1002/chem.201603977
Supported by
10.1002/chem.201603977
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COMMUNICATION Visible-Light Promoted Distereodivergent Intramolecular Oxyamidation of Alkenes Xiang Ren, [a]† Qihang Guo, [a]† Jianhui Chen, [a,c] Hujun Xie, *[b] Qing Xu* [c] and Zhan Lu*[a] Dedication ((optional))
The visible-light-promoted diastereodivergent intramolecular oxyamination of alkenes and amination of unactivated C(Sp3)-H bonds is described to construct oxazolindinones, pyrrolidinones and imidazolidones via mild generation of primary amidyl radicals from functionalized hydroxylamines. A unique phenomenon on highly diastereoselective ring-opening of aziridines controlled by electron sacrifices was observed. High diastereoselective amino alcohols derivatives were obtained efficiently through this protocol in gram scales. The mechanistic studies suggested the isolatable anti-aziridine intermediates were generated quickly from primary amidyl radicals and the diastereoselectivities were controlled by pKa values of electron sacrifices. Abstract:
1,2-Amino alcohols derivatives are highly-value, widely exist and well used in organic transformation for asymmetric catalysts, medicine, materials and so on. The directly and highly regio- and diastereoselective oxyamination of alkenes is one of the powerful methodologies to construct useful amino alcohols derivatives.[1] Since Bäckvall reported the first direct oxyaminations of alkenes using stoichiometric PdCl 2 in 1975[2] and Sharpless reported the first example of Os-catalyzed oxyamination of alkenes,[3] direct oxyamination of alkenes were developed to explore their utilities. Although highly chemo- and regioselective oxyamidations of alkenes were achieved through transition-metal catalysis, [4] hypervalent iodine,[5] acids,[6] NFSI,[7] as well as electrochemistry[8], distereoselectivity-controllable oxyamidation reaction of alkenes, particularly based on radicalinitiated reactions, is a remaining significant challenge. The diastereoselectivity-controllable oxyamination of alkenes used to be controlled by using Z or E-configuration alkenes.[9] Dauban and co-workers developed a diastereoselectivity-controllable
[a]
[b]
[c]
Mr. X. Ren, Mr. Q. Guo, Mr. J. Chen, Prof. Z. Lu Department of Chemistry, Zhejiang University Hangzhou, Zhejiang 310058, P. R. China E-mail: [email protected] Prof. H. Xie Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou, Zhejiang 310018, P. R. China E-mail: [email protected] J. Chen, Prof. Q. Xu College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou 325035, P. R. China E-mail: [email protected] † X. Ren and Q. Guo are contributed equally to this work Supporting information for this article is given via a link at the end of the document.
rhodium-catalyzed intermolecular oxyamination of indoles by using different oxygen nucleophiles.[3j] However, the substrates were limited in indoles. Xu and co-workers reported a diastereoselectively iron-catalyzed intramolecular oxyamination of olefins controlled by careful selection of counteranion/ligand combinations.[3w] However, in both cases, there were only one reversed example shown via amidyl-carbenes or amidyl-metal intermediates, and no direct evidences to explain the controllable selectivity. Visible light is a safe, inexpensive, abundant, and nonpolluting reagent, [10] meanwhile, could promote to generate amidyl radical mildly.[11] Very recently, Akita[12] and Leonori[13], respectively, reported the oxyamination of alkenes via visible light photocatalysis. However, the reactions of 1,2-disubstituted alkenes afforded products with poor diastereoselectivities. To the best of our knowledge, the use of photosensitizer to promote the diastereoselectivity-controllable oxyamination reaction of the same alkene has not previously been described.[14] Our group are particularly interested in one-electron reduction of nitrogencontaining electron-deficient groups.[15] Here, we described a mechanistic distinct diastereroselectivity-controllable intramolecular oxyamidation of alkenes with functionalized hydroxylamines in the presence of photocatalysts and electronsacrifices upon irradiation of visible light.
Scheme 1. Regio- and stereoselective oxyamination of alkenes.
We chose 1a as a model substrate. The transformation of 1a using transition metal catalysts has been studied to form aziridine or oxyamidation products. [4n,9c] In particular, the reaction of 1a gave the oxyamidation products in good yield with poor diastereoselectivity using iron catalysts. [3w] On the contrary, we proposed that the primary amidyl radical might be formed through the one-electron reductive process.[16] The reaction of 1a was conducted in the presence of 2 mol% of visible light
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COMMUNICATION Table 1. Optimization.
[a]
Entry
Photocatalyst
Additives (equiv)
Yield [b] (%)
Dr [b] anti/syn
1
Ir(ppy)2(dtbbpy)PF6
Et3N (1)
65
>20:1
2
-
Et3N (1)
0
-
3
Ir(ppy)2(dtbbpy)PF6
-
0
-
4
Ir(ppy)2(dtbbpy)PF6
Et3N (1)
0
-
5
Ir(ppy)2(dtbbpy)PF6
PPh3 (1)
60
4:1
6
Ir(ppy)2(dtbbpy)PF6
PhSiH3 (1)
0
-
7
Ir(ppy)2(dtbbpy)PF6
Et3SiH (1)
0
-
8
Ir(ppy)2(dtbbpy)PF6
(EtO)3SiH (1)
21
1:3
9
Ir(ppy)2(dtbbpy)PF6
Ph3N (1)
50
1:10
10
Ir(ppy)2(dtbbpy)PF6
PhNMe2 (1)
68
1:6
11
Ir(ppy)2(dtbbpy)PF6
N-Me-morpholine (1)
68
[c]
1.4:1 [e]
[d]
12
Ir(ppy)2(dtbbpy)PF6
Et3N (1)
(69)
>20:1
13[d]
Ir(ppy)2(dtbbpy)PF6
Ph3N (1)
(74)[e]
1:12
[d]
14
Ir(ppy)2(dtbbpy)PF6
Et3N (0.1)
72
1:1.3
15[d]
Ir(ppy)2(dtbbpy)PF6
Et3N (0.01)
90
1:5
[d]
16
Tetrabromofluorescein
Et3N (1)
69
13:1
17[d]
Tetrabromofluorescein
Ph3N (1)
67
1:12
[d]
18
Rose bengal
Et3N (1)
70
13:1
19[d]
Rose bengal
Ph3N (1)
72
1:5
[a] The reaction was conducted using 2 mol% of photocatalyst, 1 equiv. of additive and 0.5 mol of substrate in a solution of MeCN (0.05 M) under the irradiation of blue LED (5 W). [b] Yields and diastereoselectivities were determined by H NMR using crude products and the trimethylsilylbenzene as an internal standard. [c] No light. [d] 50 oC. [e] In 1 hour, and isolated yields of the major isomer in the parenthesis.
photocatalyst Ir(ppy)2(dtbbpy)(PF6) and 1 equiv. of Et3N (pKa value is 10.75)[17] as an electron sacrificial donor in a solution of MaCN (0.05M) under the irradiation of 5 W of blue LED at room temperature to afford the oxyamidation product anti-2a, in 65% yield with >20:1 distereoselectivity (Table 1, entry 1). Control experiments suggested that photocatalysts, light, bases are all necessary (entries 2-4). Various e-sacrificial donors have been investigated. Triphenylphosphine is also a good electron sacrificial donor to afford the desired anti-product in 60% yield and 4:1 dr (entry 5). Silanes could not efficiently promote the process at room temperature, and diastereoselectivities was less ideal (entires 6-8). To our surprise, the reactions using Ph3N (pKa value is -3.04, entry 9) as the electron sacrificial donor afforded syn-product 3a as the major products with 1/10 dr.
PhNMe2 (pKa value is 5.2, entry 10) and N-Me-morpholine (pKa value is 7.38, entry 11) were selected as electron sacrificial donors to give the oxyamidation products with 1:6 and 1.4:1 dr, respectively. The reactions could be accelerated at 50 oC (entries12 and 13, as the two standard conditions). Using 10 mol% of Et3N, the reaction gave the oxyamidation products with 1:1.3 dr (entery 14). The reversed diastereoselectivity was observed using 1 mol% of Et3N (entery 15). This unique diastereoselectivity-controllable reactions were dependent on pKa value of electron sacrificial donors and pH value of reactions. The organic photosensitizers such as Tetrabromofluorescein and Rose Bengal could promote the same transiformations with slightly poor reactivities and diastereoselectivities (entries 16-19). With standard conditions in hand, we investigated the scope of substrates shown in Table 2. Using electron-deficient benzoyl group, the diastereoselectivities were improved dramatically under both Et3N and Ph3N conditions (2b-d and 3b-d). The reactions with ortho- and meta-methylphenyl alkenes gave antiselective products 2e-f with high diastereoselectivity under Et 3N conditions, and syn-selective products 3e-f with a slightly low diastereoselectivity under Ph3N conditions. Simple phenyl ring 1g, and phenyl ring with electron-donating groups 1h, phenyl group 1i and electron-withdrawing groups 1j could participate to give the anti-selective products 2g-j with high diastereoselectivity under Et3N conditions, and syn-selective products 3g-k with a slightly low diastereoselectivity. Both 1naphthyl and 2-naphthyl alkenes could be transferred to the desired anti- and syn-products with high diastereoselectivities (2l-m and 3l-m). Heterocycles such as 2-(N-tosyl)indolyl could be tolerated to afford the desired anti-2m in 52% yield with >20/1 dr. The syn-3m could also be obtained under Ph3N conditions in 76% yield with >20/1 dr. The alkyl alkene 1n could undergo oxyamidation reactions to afford the same major diastereoisomer under both conditions. Dimethyl alkenes and terminal alkenes afforded the oxyamidation product in 59% and 41% yield, respectively. The carbon-tethered substrate 4 could also participate to afford the pyrrolidinone 5 in 57% with 14/1 dr and 52% yield with 7/1 dr under conditions A and B, respectively. The configurations of major products were somehow the same. The nitrogen-tethered substrate 6 was converted to aziridine 7[18] in 66% yield under conditions A. The oxyamination product 8 was produced in 49% yield with 6/1 dr under conditions B. Notably, the reaction of 6 under conditions B with an excess equivalent of benzoic acid at 80 oC afforded the imidazolidone 8 in 62% yield with >20/1 dr. The relative configurations of oxyamination products were determined by X-ray crystallographic analysis of compounds 2e, 3e and 8. [19] The gram-scale reaction could be conducted smoothly to afford the anti- and syn-products in 77% and 93% isolated yield, respectively (Scheme 2). The amino alcohols could be obtained by hydrolysis of the oxyamidation products.
Scheme 2. Gram scale reactions.
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COMMUNICATION Table 2. Substrate scope for diastereoselectivity-controllable oxyamidation of alkenes
[a] Conditions: Ir(ppy)2(dtbbpy)PF6 (2 mol%), base (1 equiv.), 1 (0.5 mol), and MeCN (0.05 M), blue LED (5 W), 50 oC. [b] 80 oC, with 1 equiv. of PhCOOH.
To elucidate the mechanism, a series of experiments were conducted. 1) The reaction did not occur in the presence of radical inhibitor tempo (Scheme 3). This suggested the reaction might undergo a radical initiated pathway. [20] 2) Time course studies were conducted to show that the reactions of 1a in the presence of Et3N or Ph3N afforded the same aziridine intermediate 9a which could be observed by analysis of crude mixtures, also isolated and identified by 1H and 13C NMR, NOE and HRMS. The control experiments were carried out under the irradiation by visible light in 15 min, then the mixtures were stirred in dark in 18 h to afford anti-2a or syn-3a, respectively (Scheme 4). In the presence of Et3N or Ph3N with benzolic acid, 9a could also be translated to the corresponding anti-2a in 61% yield and 16/1 dr or syn-3a in 74% yield and 10/1 dr, respectively (Scheme 4). These results suggested: one is that the aziridine might be the stable intermediate during the reaction pathways, another is that the controllable diastereoselectivity of the products should be somehow involved by the ring-open step of the aziridine intermediate.[21] 3) The reactions of Z-1a in the presence of Et3N or Ph3N afforded the anti-2a in 67% yield with 8/1 dr or syn-3a in 55% yield with 11/1 dr, respectively. These results were similar with ones using E-1a as the substrate. It would be unlikely to generate the nitrene intermediates which would react with alkenes with different configurations to afford stereo-specific products.[22] The nitrogen radical intermediates seemed more reasonable. The nitrene intermediates could not be absolutely ruled out from primary amidyl radicals.
Scheme 3. Radical trapped reactions.
Scheme 4. Time course studies and reactions of intermediates.
Scheme 5. Reactions of Z-1a.
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COMMUNICATION In order to elucidate the origin of the observed stereoselectivity, we carried out the density functional theory (DFT) calculations to investigate the reaction mechanisms for the nucleophilic attack step by the benzolic anion for the formation of products.[23] The corresponding Gibbs free energy profiles for this step in the presence of Ph3N (I) and Et3N (II) are illustrated in Figure 1. As shown in Figure 2(I), the Ph3N is not involved in the nucleophilic attack step due to the smaller proton affinity of Ph3N (218.8 kcal/mol) compared to Et3N (232.0 kcal/mol). This reaction can proceed via two possible pathways. The barrier difference (4.5 kcal/mol, between TS12 and TS13) for the formation of the two diastereoisomers can give a multiplicative difference of about 103 in the reaction rate if we assume an Arrhenius expression for the rate constant and similar preexponential factors, explaining the experimental observations that the reaction could afford syn3 as the major product in the presence of Ph3N. For the Et3N, the barrier difference (3.1 kcal/mol, between TS45 and TS46) for the formation of the two diastereoisomers can give a multiplicative difference of about 102 in the reaction rate. The calculation results agree with the experimentally observed selectivity that the reaction could afford anti-2 as the major product in the presence of Et3N.
Figure 1. Free energy profiles of the nucleophilic attack step by benzoyl group for the formation of different products in the presence of Ph3N (I) and Et3N (II). The solvation-corrected relative Gibbs free energies are given in kcal/mol.
aziridine and benzoic acid pair B might form a intermolecular D, then a SN1-reaction pathway might proceed to give syn-3 products.[24] This interesting phenomenon to control the diastereoselectivity will potentially provide a novel strategy for diastereoselective ring-opening of aziridines.
Scheme 6. Possible Mechanism.
In summary, we developed a first visible-light-promoted diastereodivergent intramolecular oxyamidation of alkenes with functionalized hydroxylamines. The unique diastereodivergent reactions were controlled by pKa value of electron sacrificial donors and pH value of reactions. This protocol afforded useful amino alcohols derivatives with high diastereoselectivities in gram scales. The mechanistic studies suggested that primary amidyl radicals were mildly generated from functionalized hydroxylamines in the presence of photocatalysts and amines upon irradiation of visible light, aziridine intermediates were formed quickly and the high diastereoselectivities were dependent on ring-opening of aziridine intermediates. Visiblelight-promoted selectivity-controllable intermolecular reactions and asymmetric transformations will be explored in our laboratory.
Acknowledgements The possible mechanism was shown in Scheme 6. The visible light photocatalyst (PC) could absorb the visible light to generate the excited state PC* which could be reduced by electron sacrifices to produce the low valent photocatalyst PC-. The substrate could be reduced by PC species to regenerate PC and afford the radical ion intermediate which might undergo O-N bond cleavage to form a distonic radical ion A. The primary amidyl radical could undergo the intramolecular radical cyclization to afford a benzylic radical which might lose an electron to form a stable aziridine intermediate [21] with the benzoic acid. Although the possibility of the nitrene obtained through deprotonation of A is small, we can not absolutely exclude this pathway. In the presence of Et3N, the benzoic acid could react with Et3N to form +NHEt3 and benzoic ion which might be favor to attack the aziridine intermediate undergo SN2reaction pathway to give anti-2. Due to the weak acidic property, Ph3N could not react with the benzoic acid efficiently. The
We thank the NSFC (21472162). Keywords: visible light photocatalysis • diastereodivergent • oxyamidation • aziridines • alkenes [1] [2] [3]
[4]
T. J. Donohoe, C. K. A. Callens, A. Flores, A. R. Lacy, A. H. Rathi, Chem. Eur. J. 2011, 17, 58. J. E. Bäckvall, Tetrahedron Lett. 1975, 16, 2225. a) K. B. Sharpless, A. O. Chong, K. Oshima, J. Org. Chem. 1976, 41, 177; b) G. Li, H.-T. Chang, K. B. Sharpless, Angew. Chem. Int. Ed. 1996, 35, 451. For selected examples on transition-metal catalyzed oxyamination of alkenes, see: [Pd]: a) E. J. Alexanian, C. Lee, E. J. Sorensen, J. Am. Chem. Soc. 2005, 127, 7690; b) P. Szolcsanyi, T. Gracza, Chem. Commun. 2005, 3948; c) G. Liu, S. S. Stahl, J. Am. Chem. Soc. 2006, 128, 7179; d) L. V. Desai, M. S. Sanford, Angew. Chem. Int. Ed. 2007, 46, 5737; e) H. Zhu, P. Chen, G. Liu, J. Am. Chem. Soc. 2014, 136,
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COMMUNICATION
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10.1002/chem.201603977
Chemistry - A European Journal
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COMMUNICATION Author(s), Corresponding Author(s)* Page No. – Page No. Visible-Light Promoted Distereodivergent Intramolecular Oxyamidation of Alkenes
The visible-light-promoted diastereodivergent intramolecular oxyamination of alkenes and amination of unactivated C(Sp3)-H bonds is described to construct oxazolindinones, pyrrolidinones and imidazolidones via mild generation of primary amidyl radicals from functionalized hydroxylamines. A unique phenomenon on highly diastereoselective ringopening
of
aziridines
controlled
by
electron
sacrifices
was
observed.
High
diastereoselective amino alcohols derivatives were obtained efficiently through this protocol in gram scales. The mechanistic studies suggested the isolatable anti-aziridine intermediates were generated quickly from amidyl radicals and the diastereoselectivity were controlled by pKa values of electron sacrifices.
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