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Visible Light-Induced Radical Rearrangement to Construct C-C Bonds via Intramolecular Aryl Migration/Desulfonylation Process Yuyuan Li, Bei Hu, Wuheng Dong, Xiaomin Xie, Jun Wan, and Zhaoguo Zhang J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b00735 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on June 28, 2016
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The Journal of Organic Chemistry
Visible Light-Induced Radical Rearrangement to Construct C-C Bonds via Intramolecular Aryl Migration/Desulfonylation Process Yuyuan Li,† Bei Hu,‡ Wuheng Dong,‡ Xiaomin Xie,‡ Jun Wan*† and Zhaoguo Zhang*‡ †
Key Laboratory of Sensor Analysis of Tumor Marker, Ministry of Education, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China, Fax: (+86)-532-8402-2617; phone: (+86)-532-8402-2617; E-mail:
[email protected] ‡
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, China Fax: (+86)-21-5474-8925; phone: (+86)-21-5474-8925; E-mail:
[email protected] ABSTRACT A highly efficient intramolecular selective aryl migration/desulfonylation of 2-bromo-N-aryl-N(arenesulfonyl)amide via visible light-induced photoredox catalysis has been accomplished. This approach allows for the construction of a variety of multi-substituted N,2-diarylacetamide under mild reaction conditions.
INTRODUCTION Aryl migration reaction was considered as an efficient method to reconstruct complex compounds and had been widely applied to the synthesis of natural products.1 Among them, the free radical aryl migrations can occur between carbon and carbon radicals, carbon and heteroatom-centered radicals to reconstruct the bond of C-X (X = C, Si, N). While in the early reports, poisonous radical initiator was usually used in the aryl migration reactions,2-3 and a tremendous effort has been made to seek more efficient methods for this kind of reactions. Recently, Zhu’s group reported an aryl migration reaction using 3 equivalents of dicumyl peroxide as radical initiator under the temperature of 140 °C to give desired products in good yields.4 In addition, transition metalmediated reactions have also been applied to the field of aryl migration reactions. For example, Nevado’s group has reported a copper-catalyzed aryl migration reaction by using Togni’s reagent5 to generate trifluoromethyl radical species.6 Although some progress has been made, these transformations still suffered the common drawbacks of harsh reaction conditions including high temperature, high catalyst loading, narrow range of substrates and the use of poisonous or environment-unfriendly radical initiators.7 Therefore, it is
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profound and significant to develop mild and effective methodologies for aryl migration reactions. In recent years, visible light photoredox catalysis has received widespread attention as a powerful tool in the field of synthetic organic chemistry owing to its attractive features including mild reaction conditions, excellent functional group tolerance and high reactivity.8-9 To the best of our knowledge, several groups have reported intermolecular radical addition/1,2-aryl migration reaction via visible light-induced conditions.10 However，few examples of intramolecular aryl migration reactions under the visible light-mediated condition have been reported until some works have been introduced by Xia’s group and Stephenson’s group. 11 Xia’s group has reported a intermolecular radical addition/ 1,4-aryl shift/desulfonylation cascade reaction with a trifluoromethyl radical involved as the key radical source (scheme 1a). On the other hand, Stephenson’s group provided an efficient approach to achieve the benzylic difluoroethanol motif via an intramolecular radical addition/1,4-aryl shift/desulfonylation process (scheme 1b). Besides that, the activation of C-X (X = Cl, Br, I) by visible-light participation has been widely reported in recent years
12
and an efficient intramolecular
dearomative cyclization reaction occurred via C-Br bond fragmentation has been reported by our group.13 Therefore, We envisioned that
2-bromo-N-phenyl-N-(phenylsulfonyl)acetamide could generate α-carbonyl
radical under visible light catalysis, which may then undergo intramolecular ipso addition/elimination and hydrogen abstraction followed by polar extrusion of SO2 leading to the desired product. Herein, we report a visible light-induced intramolecular aryl migration/desulfonylation process for the synthesis of N,2diarylacetamide products under a mild reaction condition (scheme 1c).
Scheme 1. Reported the aryl migration reactions and our strategy
RESULTS AND DISCUSSION We
initiated
our
investigation
by
examining
the
reaction
of
2-bromo-2-methyl-N-phenyl-N-
(phenylsulfonyl)propanamide (1a) with fac-Ir(ppy)3 (2 mol%) in DMF under a 23W household fluorescent
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The Journal of Organic Chemistry
lamp irradiation (Table 1, entry 1). Fortunately, the reaction occurred at room temperature, giving the desired product (2a) with 26% yield. However, other photocatalysts, such as Ir(ppy)2(bpy)PF6, Ru(ppy)3Cl2 and Eosin Y do not work. Next, our attentions were paid on screening the additives to improve the efficiency of the reaction.14 When Li2CO3, Na2CO3 or NaHCO3 was added, the reaction gave much higher yields (Table 1, entries 2-4), however, the byproducts (see supporting information) were also obtained. So we turned to use a weaker base like KH2PO4, a lower yield was observed without any byproducts (Table 1, entry 5). On the other hand, water was introduced as an additive because it could serve as a hydrogen donor in the reaction.15 To our delight, the yield could be increased to 85% when 5 equivalents of water was used as the additive (Table 1, entry 6). By increasing the amount of water to 20 equivalents, a significantly increase of yield was observed and an 88% isolated yield was obtained (Table 1, entry 7). There is no influence on the yield when the amount of water was further increased (Table 1, entry 8). We then screened the solvents in the presence of water as the additive. Among solvents tested, DMF was proved to be the best choice for this reaction (Table 1, entries 9-12). The control experiments showed that both photocatalyst and visible light were essential for this reaction. When the reactions were conducted without catalyst or visible light, no product was formed (Table 1, entries 13 and 14). Table 1: Optimization of reaction conditionsa
Entry
Cat. (2% mol)
Add. (eq)
Solvent
Conv.(%)
Yieldb(%)
1
fac-Ir(ppy)3
-----
DMF
36
26
2
fac-Ir(ppy)3
Li2CO3(1)
DMF
88
50
3
fac-Ir(ppy)3
Na2CO3(1)
DMF
100
63
4
fac-Ir(ppy)3
NaHCO3(1)
DMF
88
40
5
fac-Ir(ppy)3
KH2PO4(1)
DMF
53
43
6
fac-Ir(ppy)3
H2O(5)
DMF
100
85
7
fac-Ir(ppy)3
H2O(20)
DMF
100
93(88) c
8
fac-Ir(ppy)3
H2O(50)
DMF
100
93
9
fac-Ir(ppy)3
H2O(20)
DMA
85
72
10
fac-Ir(ppy)3
H2O(20)
CH3CN
57
52
11
fac-Ir(ppy)3
H2O(20)
DCM
50
43
12
fac-Ir(ppy)3
H2O(20)
MeOH
