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Catalyst-Controlled Chemoselective and Enantioselective Reactions of Tryptophols with Isatin-Derived Imines Fei Jiang, Dan Zhao, Xue Yang, Fu-Ru Yuan, Guang-Jian Mei, and Feng Shi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02279 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017
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ACS Catalysis
Catalyst-Controlled Chemoselective and Enantioselective Reactions of Tryptophols with Isatin-Derived Imines Fei Jiang,† Dan Zhao,‡ Xue Yang,† Fu-Ru Yuan,† Guang-Jian Mei† and Feng Shi*,† †School of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou 221116, China;
‡Department of Chemistry and Environmental Engineering, Hebei University, Baoding, 071002, China. ABSTRACT: Catalyst-controlled chemoselective and enantioselective reactions of tryptophols with isatin-derived imines were demonstrated. Under catalysis with a chiral phosphoric acid, diastereo- and enantioselective dearomative cyclization occurred, while in the presence of a chiral squaramide-tertiary amine, enantioselective addition occurred. Both reactions afforded 3substituted 3-aminooxindoles in generally good yields (up to 99%) and excellent stereoselectivities (up to >95:5 dr, 99:1 er).
KEYWORDS: Asymmetric catalysis; Chemoselectivity; Dearomatization; Enantioselectivity; Organocatalysis; Tryptophol One of the ultimate goals in modern organic synthesis is to control the selectivity of reactions, which mainly includes chemoselectivity, regioselectivity and stereoselectivity (diastereo- and enantioselectivity).1 Among the different strategies, catalyst-controlled chemoselectivity has become an important tactic to achieve this goal.2-3 However, in asymmetric catalysis, simultaneously controlling the chemoselectivity and the enantioselectivity by tuning the chiral catalyst remains a great challenge.4 Therefore, it is highly desired to develop catalyst-controlled chemoselective and enantioselective reactions for synthesizing enantioenriched compounds with chemodiversity.
catalytic addition reactions of nucleophiles (Nu) to isatinderived imines (eq. 1). Nevertheless, catalyst-controlled chemoselective and enantioselective addition reactions of nucleophiles to isatin-derived imines have not yet been established, despite this approach having the ability to afford enantioenriched 3-substituted 3-aminooxindoles with diversified structures. Scheme 2. Design of catalyst-controlled chemoselective and enantioselective addition reactions of tryptophols to isatinderived imines
Scheme 1. Bioactive chiral 3-substituted 3-aminooxindoles and the method to construct this scaffold
The chiral 3-substituted 3-aminooxindole scaffold belongs to one of the most privileged heterocyclic structures, which exists in many natural alkaloids and bioactive compounds (Scheme 1).5 Thus, the enantioselective construction of this framework has gain much attention from organic chemists,6 and the most convenient method to construct this scaffold is
Tryptophols are a class of unique nucleophiles, which can attack electrophiles (E) using their C3-nucleophilicity to perform catalytic asymmetric dearomative (CADA) reactions.7 As a result, elegant achievements have been established in catalytic asymmetric dearomative cyclizations of tryptophols (eq. 2).8 To develop catalyst-controlled chemoselective and enantioselective addition reactions of nucleophiles to isatin-derived imines, and as a continuation of our efforts in constructing
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enantioenriched heterocyclic frameworks,9 we decided to employ tryptophols as competent nucleophiles to attack isatinderived imines using different kinds of chiral organocatalysts (Scheme 2). In the presence of chiral Brønsted acids (B*-H), such as chiral phosphoric acids (CPA),10 tryptophols can use their C3-nucleophilicity to attack isatin-derived imines, thus performing stereoselective dearomative cyclizations to afford structurally complex 3-substituted 3-aminooxindoles bearing three adjacent stereogenic centers (eq. 3). Additionally, in the presence of chiral H-donor-amines (H-A*), such as squaramide-tertiary amines,11 tryptophols can utilize their Onucleophilicity to attack isatin-derived imines, leading to enantioselective additions to generate O-substituted 3aminooxindoles (eq. 4). Based on this design, we accomplished catalyst-controlled chemoselective and enantioselective addition reactions of tryptophols to isatin-derived imines, which afforded two series of enantioenriched 3-substituted 3aminooxindoles with structural diversity. Initially, the reaction of isatin-derived imine 1a and tryptophol 2a was employed as a model reaction to examine the possibility of our design (Schemes 3-4). As illustrated in Scheme 3, in the presence of chiral phosphoric acid (R)-4a, the dearomative cyclization reaction occurred to afford the desired product 3aa in a high yield of 85% and a moderate diastereoselectivity of 82:18 dr but with a low enantioselectivity of 38:62 er. To improve the enantioselectivity and the diastereoselectivity, a series of CPAs 4-5 were screened, and the reaction conditions were optimized (see the Supporting Information for details). Finally, the optimal reaction conditions were found, which delivered product 3aa in an acceptable yield of 62% and excellent stereoselectivity (>95:5 dr, 96:4 er). Scheme 3. Catalysts and model reaction employed to optimize the conditions of the catalytic asymmetric dearomative cyclization reaction
In contrast, in the presence of the chiral thiourea-tertiary amine catalyst 7a, the same isatin-derived imine 1a and tryptophol 2a substrates underwent the addition reaction to give the designed product 6aa in a moderate yield of 56% with a low enantioselectivity of 33:67 er (Scheme 4). Nevertheless, this result demonstrated that our designed catalyst-controlled chemoselective and enantioselective addition reactions of tryptophols to isatin-derived imines were feasible. Then, several chiral thiourea-tertiary amines and squaramide-tertiary amines 7 were screened, and various reaction conditions were evalu-
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ated, which finally provided the optimal conditions (see the Supporting Information for details). Under these conditions, the addition reaction proceeded in a good yield of 74% and an excellent enantioselectivity of 98:2 er. Scheme 4. Catalysts and model reaction employed to optimize the conditions of the catalytic asymmetric addition reaction
Table 1. Substrate scope of isatin-derived imines 1 in the dearomative cyclizationsa
Yield drc erd b (%) 1 Bn/Boc/H (1a) 62 >95:5 96:4 3aa 2e,f Me/Boc/H (1c) 73 83:17 95:5 3ca 3 Ph/Boc/H (1d) 65 >95:5 95:5 3da 4 Allyl/Boc/H (1e) 69 >95:5 95:5 3ea 5 Ac/Boc/H (1f) 57 90:10 95:5 3fa 6 Bn/BnCO2/H (1g) 73 >95:5 92:8 3ga 7 Bn/Boc/5-F (1h) 90 93:7 96:4 3ha 8 Bn/Boc/5-Cl (1i) 77 90:10 98:2 3ia 9 Bn/Boc/5-Br (1j) 85 83:17 95:5 3ja 10 Bn/Boc/5-Me (1k) 60 90:10 95:5 3ka 11 Bn/Boc/6-Br (1b) 75 96:4 97:3 3ba 12e,f 54 90:10 96:4 3m Bn/Boc/6-Me (1m) a 13 Bn/Boc/7-F (1n) 72 90:10 97:3 3na 14g Bn/Boc/7-Cl (1o) 66 >95:5 96:4 3oa 15g,h 3pa Bn/Boc/7-Br (1p) 49 90:10 96:4 16g,h 3qa Bn/Boc/7-Me (1q) 49 83:17 91:9 17 75 >95:5 98:2 3ra Bn/Boc/5,7-F2 (1r) 18 Bn/Boc/4-Me (1s) 80 >95:5 93:7 3sa a Unless indicated otherwise, the reaction was carried out at a 0.1 mmol scale and catalyzed by 10 mol % (S)-4f in EtOAc (1 mL) at 25 °C with 3 Å MS (100 mg) as additives for 5 h, and the molar ratio of 1:2a was 2:1. bIsolated yields. cThe dr value was determined by 1H NMR spectroscopy. dThe er value was determined by HPLC. eCatalyzed by 30 mol % (S)-4f. fPerformed at -10 °C. g Catalyzed by 20 mol % (S)-4f. hPerformed at 0 °C. Entry
3
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R1/R2/R3 (1)
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After establishing the optimal conditions for the catalystcontrolled chemoselective and enantioselective addition reactions of tryptophols to isatin-derived imines, we investigated the substrate scope of the two reactions. First, the substrate scope of isatin-derived imines 1 in the dearomative cyclizations was studied. As shown in Table 1, this reaction was applicable to a wide range of isatin-derived imines 1 bearing different R1/R2/R3 groups, which offered the dearomative cyclization products 3 in generally good yields, high diastereoselectivities and excellent enantioselectivities. It seemed that the alteration of the R1/R2/R3 groups did not impose evident effects on the enantioselectivity because all the products were generated in good-to-excellent enantioselectivities (91:9 to 98:2 er). Then, the substrate scope of tryptophols 2 in the dearomative cyclizations was examined by reactions with isatinderived imine 1b. As listed in Table 2, a series of Nunprotected tryptophols 2 bearing electronically distinct substituents (R2) at different positions of the indole moiety (C4C7) smoothly participated in the dearomative cyclizations to give products 3 in moderate-to-good yields and excellent diastereo- and enantioselectivities (entries 1-7). In addition, Nmethyl protected substrate 2j took part in the catalytic asymmetric dearomative cyclization, achieving a moderate yield, excellent diastereoselectivity and high enantioselectivity (entry 8). Table 2. Substrate scope of tryptophols 2 in the dearomative cyclizationsa
Entry
3
R1/R2 (2)
Yield (%)b
drc
erd
1 H/4-Br (2b) 63 >95:5 97:3 3bb 2e,f H/5-Br (2d) 66 >95:5 91:9 3bd 3 68 90:10 95:5 3be H/5-OMe (2e) H/6-Cl (2f) 68 83:17 94:6 4e,f 3bf 5 H/6-Me (2g) 82 >95:5 96:4 3bg 6 H/7-F (2h) 91 >95:5 94:6 3bh 7 H/7-Me (2i) 51 >95:5 94:6 3bi 8 Me/H (2j) 50 >95:5 93:7 3bj a Unless indicated otherwise, the reaction was carried out at a 0.1 mmol scale and catalyzed by 10 mol % (S)-4f in EtOAc (1 mL) at 25 °C with 3 Å MS (100 mg) as additives for 5 h, and the molar ratio of 1b:2 was 2:1. bIsolated yields. cThe dr value was determined by 1H NMR spectroscopy. dThe er value was determined by HPLC. eCatalyzed by 30 mol % (S)-4f. fPerformed at -10 °C.
Next, we investigated the generality of the catalytic asymmetric addition. As illustrated in Table 3, this reaction was amenable to a wide scope of isatin-derived imines 1, which gave rise to 3-substituted 3-aminooxindoles 6 in uniformly excellent enantioselectivities (96:4 to 99:1 er). Similarly, to the dearomative cyclization, the electronic nature and the position of the R1/R2/R3 substituents seemed to have no obvious influence on the enantioselectivity of the addition reaction; however, the yields varied remarkably in some cases.
Table 3. Substrate scope of isatin-derived imines 1 in the addition reactionsa
Entry R1/R2/R3 (1) Yield (%)b erc 6 1 Bn/Boc/H (1a) 82 99:1 6aa 2 Me/Boc/H (1c) 91 99:1 6ca Ph/Boc/H (1d) 71 97:3 3 6da 4 Allyl/Boc/H (1e) 90 97:3 6ea 5 Bn/BnCO2/H (1g) 99 96:4 6ga 6 Bn/Boc/5-F (1h) 90 99:1 6ha 7 Bn/Boc/5-Br (1j) 82 99:1 6ja 8 Bn/Boc/5-Me (1k) 58 99:1 6ka 9 Bn/Boc/6-F (1l) 73 97:3 6la 10 Bn/Boc/6-Br (1b) 74 98:2 6ba 11 60 99:1 6ma Bn/Boc/6-Me (1m) 12 Bn/Boc/7-F (1n) 70 98:2 6na 13 Bn/Boc/7-Cl (1o) 48 96:4 6oa 14 Bn/Boc/7-Br (1p) 45 97:3 6pa 15 Bn/Boc/7-Me (1q) 94 96:4 6qa 16 Bn/Boc/4-Me (1s) 51 98:2 6sa a Unless indicated otherwise, the reaction was carried out at a 0.1 mmol scale and catalyzed by 10 mol% 7g in toluene (1 mL) at 25 °C with 3 Å MS (100 mg) as additives for 12 h, and the molar ratio of 1:2a was 2:1. bIsolated yields. cThe er value was determined by HPLC.
As listed in Table 4, a series of tryptophols 2 bearing either electron-withdrawing or electron-donating groups at the C4C7 positions of the indole ring were successfully applied to the catalytic asymmetric addition reactions with isatin-derived imines 1a (entry 1-8), affording products 6 in moderate-togood yields (48%-99%) and high enantioselectivities (97:3 to 99:1 er). Obviously, the electronic nature and the position of the R2 substituents did not impose an evident effect on the enantioselectivity of the reaction. Notably, N-methyl substituted tryptophol 2j smoothly underwent the addition reaction, achieving a considerable yield of 62% and an excellent enantioselectivity of 99:1 er (entry 9). Table 4. Substrate scope of tryptophols 2 in the addition reactionsa
Entry
6
R1/R2 (2)
Yield (%)b
erc
1 H/4-Br (2b) 48 98:2 6ab 2 H/4-Me (2c) 77 98:2 6ac 3 H/5-Br (2d) 65 98:2 6ad 4 H/5-OMe (2e) 77 99:1 6ae 5 H/6-Cl (2f) 50 97:3 6af 6 H/6-Me (2g) 90 99:1 6ag 7 H/7-F (2h) 99 98:2 6ah 8 H/7-Me (2i) 74 98:2 6ai 9 Me/H (2j) 62 99:1 6aj a Unless indicated otherwise, the reaction was carried out at a 0.1 mmol scale and catalyzed by 10 mol% 7g in toluene (1 mL) at 25 °C with 3 Å MS (100 mg) as additives for 12 h, and the molar
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Scheme 6. Possible reaction pathways and activation modes a) Chiral phosphoric acid-catalyzed dearomative cyclization:
2,
-(i 4,6
H2 C6 r) 3 -P
Br H
O
(S)-4f
H
O ,6(i -
R
BocHN
O
O
O
S N S H
HO
N H 2a
Pr) 3C
Bn
N
Bn
N BocHN
O 2,4
Br Br
1j
Boc
P
N Bn O
N
O
(S)-BINOL
It should be mentioned that the two chemodivergent reactions occurred in exclusive chemoselectivities. Specifically, in the presence of catalyst 4f, only dearomative cyclization products 3 were generated, and no addition products 6 were produced, while in the presence of catalyst 7g, only addition products 6 were afforded, and no dearomative cyclization products 3 were formed. In some cases, the yields of products 3 or 6 were less than 50% because isatin-derived imines 1 decomposed into isatins during the reaction. The structures of products 3ja and 6af were confirmed by single-crystal X-ray diffraction analysis.12 In addition, the absolute configuration of product 3ja (99:1 er after recrystallization) was unambiguously determined to be (R, S, S) by single-crystal X-ray diffraction analysis.12 The absolute configuration of product 6ab was determined to be (R) by comparing experimental and calculated electronic circular dichroism data (see the Supporting Information for details).13 Therefore, the absolute configurations of the other products 3 and 6 were analogously assigned. To investigate the role of the N-H group in tryptophols 2 and to get insight on the possible activation mode of catalysts 4f and 7g to tryptophols 2, we performed two control experiments using N-methyl protected tryptophol 2j as a substrate in the chemodivergent reactions (Scheme 5). The two reactions occurred smoothly to afford products 3bj and 6aj in moderate yields and high stereoselectivities. These results indicated that the N-H group of tryptophols 2 had no evident effect on controlling the reactivity and the stereoselectivity. Therefore, it was deduced that there might not be obvious interactions between the catalysts and the N-H group of substrates 2.
the control experiments in Scheme 5, indicated that catalyst (S)-4f likely formed a hydrogen bond with the O-H group of tryptophol 2a rather than the N-H group. Second, we investigated the interaction between chiral squaramide-tertiary amine 7g and substrates 1a and 2a using a 1H NMR control experiment. In a mixture of catalyst 7g and isatin-derived imine 1a, the peak for the Boc group shifted from 1.59 to 1.60 ppm, and the peak for the aromatic protons adjacent to the CF3 groups in catalyst 7g shifted from 8.10 to 8.12 ppm. These phenomena implied that there should be an interaction between catalyst 7g and substrate 1a. On the other hand, in a mixture of catalyst 7g and tryptophol 2a, the peak for the O-H group in tryptophol 2a shifted from 3.57 to 3.71 ppm, which indicated that the tertiary amine group of catalyst 7g might act as a base to form a hydrogen bond with the O-H group of tryptophol 2a and finally deprotonated the O-H proton.
N
O
3ja
6H 2
b) Chiral squaramide-tertiary amine-catalyzed substitution: CF3 F3C
Scheme 5. Control experiments to investigate the role of the N-H group in the tryptophols
1a N Bn
Boc N N
O
O
H
Br
Bn N O R
O H N
2b H
O
N H
NH
O BocHN
Ph
N
Br
Ph
6ab
7g
c) Catalyst-controlled chemodivergence:
R2
GP HN O
*
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ratio of 1a:2 was 2:1. bIsolated yields. cThe er value was determined by HPLC.
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O
chiral H-donor basic catalyst O-nucleophilicity
R
To further elucidate the possible activation mode of the catalysts to the substrates, we performed some 1H NMR control experiments (see the Supporting Information for details). First, we studied the interaction between chiral phosphoric acid (S)4f and substrates 1a and 2a. In a mixture of catalyst (S)-4f and isatin-derived imine 1a, the peak for the Boc group shifted from 1.59 to 1.40 ppm. This result implied that there might be a hydrogen-bonding interaction between the hydroxyl group of catalyst (S)-4f and the imine functionality of substrate 1a. While in a mixture of catalyst (S)-4f and tryptophol 2a, the peak for the O-H group (3.57 ppm) in tryptophol 2a disappeared. In contrast, the peak for the N-H group in tryptophol 2a displayed almost no shift. These phenomena, along with
R2
N R3
N R1
OH 3
R N R1 + substrates 1
N
chiral H-donor acidic catalyst C3-nucleophilicity
GP
N H
R3 O
*
R
* * O
N R1
Based on the experimental results and previous reports,6-7 we suggest possible reaction pathways and activation modes for the catalyst-controlled chemoselective and enantioselective reactions. As illustrated in Scheme 6a, chiral phosphoric acid (S)-4f simultaneously formed two hydrogen bonds with the imine group of substrate 1j and the hydroxyl group of tryptophol 2a, thus facilitating an enantioselective nucleophilic addition of the indole C3-position to the imine group and a subsequent intramolecular oxa-Mannich reaction to give the dearomative cyclization product 3ja with an (R, S, S)configuration. When using the chiral squaramide-tertiary amine 7g as the catalyst (Scheme 6b), the squaramide moiety
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generated a hydrogen-bonding interaction with substrate 1a, and the tertiary amine group acted as a base to deprotonate the proton of the hydroxyl group in tryptophol 2b, thus promoting an enantioselective nucleophilic addition of the oxygen anion to the imine group to afford the addition product 6ab with an (R)-configuration. The chemodivergence of the reaction arises from the different properties of the two catalysts (Scheme 6c). In detail, chiral phosphoric acid 4f is type of chiral Brønsted acid, which serves as a H-donor acidic catalyst. Under suitable acidic conditions, the C3-position of the indole moiety in tryptophols 2 will possess strong nucleophilicity, thus performing dearomative cyclizations with isatin-derived imines 1. In contrast, squaramide-tertiary amine 7g is a type of chiral bifunctional catalyst, which serves as a H-donor basic catalyst due to the squaramide functionality acting as a H-donor and the tertiary amine group acting as a Brønsted base. Under suitable basic conditions, the O-H group of tryptophols 2 will be deprotonated and exhibit strong nucleophilicity, thus leading to addition reactions with isatin-derived imines 1. Finally, two representative reactions were performed on a large scale to demonstrate the utility of this protocol (Scheme 7). Compared to the small-scale reactions (Table 1, entry 11 and Table 3, entry 7), the two reactions afforded products 3ba and 6ja in higher yields and maintained enantioselectivities, which implied that the reactions could be scaled up. Scheme 7. Large-scale syntheses
In summary, we established catalyst-controlled chemoselective and enantioselective reactions of tryptophols with isatinderived imines. Under catalysis with a chiral phosphoric acid, diastereo- and enantioselective dearomative cyclization occurred, which afforded structurally complex 3-substituted 3aminooxindoles bearing three adjacent stereogenic centers in generally good yields (up to 91%) and excellent stereoselectivities (up to >95:5 dr, 98:2 er). In the presence of a chiral squaramide-tertiary amine, enantioselective addition occurred, providing 3-substituted 3-aminooxindoles with diversified structures in overall high yields (up to 99%) and excellent enantioselectivities (99:1 er). This approach not only serves as a successful example of catalyst-controlled chemoselective and enantioselective reactions but also provides a powerful method for synthesizing enantioenriched 3-substituted 3aminooxindoles with structural diversity.
ASSOCIATED CONTENT Supporting Information. Screening of catalysts and condition optimization, experimental procedures, characterization data, 1H
NMR control experiments, NMR and HPLC spectra for products 3 and 6, crystallographic data (CIF) for compounds 3ja and 6af, and determination of the absolute configuration of product 6ab. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *
[email protected] Author Contributions All authors have given approval to the final version of the manuscript.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT We are grateful for financial supports from NSFC (21372002 and 21232007), PAPD and Natural Science Foundation of Jiangsu Province (BK20160003).
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Zhuo, C.-X.; Zheng, C.; You, S.-L. Acc. Chem. Res. 2014, 47, 25582573; (c) Liang, X.-W.; Zheng, C.; You, S.-L. Chem. Eur. J. 2016, 22, 11918-11933; (d) Sun, W.-S.; Li, G.-F.; Hong, L.; Wang, R. Org. Biomol. Chem. 2016, 14, 2164-2176. (8) For some prominent examples: (a) Trost, B. M.; Quancard, J. J. Am. Chem. Soc. 2006, 128, 6314-6315; (b) Lozano, O.; Blessley, G.; Martinez del Campo, T.; Thompson, A. L.; Giuffredi, G. T.; Bettati, M.; Walker, M.; Borman, R.; Gouverneur, V. Angew. Chem. Int. Ed. 2011, 50, 8105-8109; (c) Han, L.; Liu, C.; Zhang, W.; Shi, X.-X.; You, S.-L. Chem. Commun. 2014, 50, 1231-1233; (d) Zhang, X.; Han, L.; You, S.-L. Chem. Sci. 2014, 5, 1059-1063; (e) Liu, H.; Jiang, G.D.; Pan, X.-X.; Wan, X.-L.; Lai, Y.-S.; Ma, D.W.; Xie, W.-Q. Org. Lett. 2014, 16, 1908-1911; (f) Shao, W.; Li, H.; Liu, C.; Liu, C.-J.; You, S.-L. Angew. Chem. Int. Ed. 2015, 54, 7684-7687; (g) Zhang, X.; Liu, W.-B.; Tu, H.-F.; You, S.-L. Chem. Sci. 2015, 6, 4525-4529. (9) (a) Zhang, Y.-C.; Zhao, J.-J.; Jiang, F.; Sun, S.-B.; Shi, F. Angew. Chem. Int. Ed. 2014, 53, 13912-13915; (b) Zhao, J.-J.; Sun, S.B.; He, S.-H.; Wu, Q.; Shi, F. Angew. Chem. Int. Ed. 2015, 54, 54605464; (c) Zhang, H.-H.; Wang, C.-S.; Li, C.; Mei, G.-J.; Li, Y.-X.; Shi, F. Angew. Chem. Int. Ed. 2017, 56, 116-121. (10) For some reviews: (a) Akiyama, T. Chem. Rev. 2007, 107, 5744-5758; (b) Terada, M.; Chem. Commun. 2008, 35, 4097-4112; (c) Terada, M. Synthesis 2010, 1929-1982; (d) Yu, J.; Shi, F.; Gong, L.Z. Acc. Chem. Res. 2011, 44, 1156-1171; (e) Parmar, D.; Sugiono, E.; Raja, S.; Rueping, M. Chem. Rev. 2014, 114, 9047-9153; (f) Wu, H.; He, Y.-P.; Shi, F. Synthesis 2015, 47, 1990-2016.
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(11) For an early example: (a) Okino, T.; Hoashi, Y.; Takemoto, Y. J. Am. Chem. Soc. 2003, 125, 12672-12673; For some recent reviews: (b) Chauhan, P.; Mahajan, S.; Kaya, U.; Hack, D.; Enders, D. Adv. Synth. Catal. 2015, 357, 253-281; (c) Held, F. E.; Tsogoeva, S. B. Catal. Sci. Technol. 2016, 6, 645-667; For selected recent examples: (d) Liu, Y.-L.; Wang, X.; Zhao, Y.-L.; Zhu, F.; Zeng, X.-P.; Chen, L.; Wang, C.-H.; Zhao, X.-L.; Zhou, J. Angew. Chem. Int. Ed. 2013, 52, 13735-13739; (e) Yu, J.-S.; Liao, F.-M.; Gao, W.-M.; Liao, K.; Zuo, R.-L.; Zhou, J. Angew. Chem. Int. Ed. 2015‚ 54, 7381-7385; (f) Yao, W.; Dou, X.; Lu, Y. J. Am. Chem. Soc. 2015, 137, 54-57; (g) Wang, T.; Yu, Z.; Hoon, D. L.; Phee, C. Y.; Lan, Y.; Lu, Y. J. Am. Chem. Soc. 2016, 138, 265-271. (12) CCDC 1553884 for 3ja and CCDC 1553885 for 6af, see SI for details. (13) (a) Zhu, H.-J. In Organic Stereochemistry---Experimental and Theoretical Methods, 1st ed.; Zhu, H.-J. Eds.; John Wiley & Sons, 2015; Vol. 2, p 24; (b) Zhu, H.-J. In Current Organic Stereochemistry, 1st ed.; Zhu, H.-J. Eds.; Science Presses of China: Beijing, 2009; Vol. 1, p 8; (c) Zhu, H.-J.; Ren, J.; Jr, C. U. P. Tetrahedron, 2007, 63, 2292-2314; (d) Ren, J.; Jiang, J.-X.; Li, L.-B.; Liao, T.-G.; Tian, R.R.; Chen, X.-L.; Jiang, S.-P.; Pittman, C. U.; Zhu, H.-J. Eur. J. Org. Chem. 2009, 3987-3991; (e) Zhao, S.-D.; Shen, L.; Luo, D.-Q.; Zhu, H.-J. Cur. Org. Chem. 2011, 15, 1843-1862; (f) Ren, J.; Li, G.-L.; Shen, L.; Zhang, G.-L.; Nafie, L.; Zhu, H.-J. Tetrahedron, 2013, 69, 10351-10356; (g) Zhu, H.-J.; Li, W.-X.; Hu, D.-B.; Wen, M.-L. Tetrahedron, 2014, 70, 8236-8243; (h) He, P.; Wang, X.; Guo, X.; Zhou, C.; Shen, S.; Hu, D.; Yang, X.; Luo, D.; Dukor, R.; Zhu, H.-J.; Tetrahedron Lett. 2014, 55, 2965-2968.
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