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Photoredox-catalyzed Reductive Dimerization of Isatins and Isatin-derived Ketimines: Diastereoselective Construction of 3,3’-Disubstituted Bisoxindoles Chao-Ming Wang, Peng-Ju Xia, Jun-An Xiao, Jun Li, Hao-Yue Xiang, Xiao-Qing Chen, and Hua Yang J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b03056 • Publication Date (Web): 10 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017

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The Journal of Organic Chemistry

Photoredox-catalyzed Reductive Dimerization of Isatins and Isatin-derived Ketimines: Diastereoselective Construction of 3,3’-Disubstituted Bisoxindoles Chao-Ming Wang,§ Peng-Ju Xia,§ Jun-An Xiao, Jun Li, Hao-Yue Xiang,* Xiao-Qing Chen, Hua Yang* College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, P. R. China e-mail: [email protected]; [email protected]

ABSTRACT: Reductive dimerization of isatin and its derivatives can be regarded as a step-economical pathway to construct 3,3’-disubstituted bisoxindoles, which was unfortunately accompanied with severe direct reduction as well as low efficiency. A visible-light driven, photoredox-catalytic protocol was developed to readily furnish 3,3’-dihydroxy- (dl-, >20:1 dr) and 3,3’-diamino-bisoxindoles (meso-, 3.5:1 to 5:1 dr) in moderate to good yields, successfully circumventing the common problem. Two vicinal quaternary carbon centers were effectively assembled under the irradiation of visible light.

Oxindoles,

frequently found in a

variety of

natural products

and

pharmaceutically relevant compounds, are a class of extremely important bioactive motifs.1 Among them, 3,3’-disubstituted bisoxindoles bearing vicinal quaternary centers are particularly attractive due to their unique architectures and interesting biological activities.2 Moreover, 3,3’-disubstituted bisoxindoles served as key

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intermediates in total syntheses of many natural products, such as folicanthine, chimonanthine, and calycanthine etc..3 Not surprisingly, considerable efforts have been garnered toward the effective and facile assembly of these scaffolds. The synthetic challenge mainly arises from the construction of sterically demanding vicinal quaternary carbon centers. Obviously, direct C-C coupling at C3-position of oxindoles could be one of the most step/atom-economical and straightforward synthetic routes leading to 3,3′-bisoxindoles (Scheme 1).4 Back to 1994, the first C-C coupling through a dimerization of ethyl 2-(1-methyl-2-oxoindolin-3-yl)-acetate was achieved by Rodrigo et al. in good diastereoselectivity, but suffering from harsh reaction conditions and prolonged reaction time.3a Thereafter, the groups of Bisai employed the similar strategy to realize the direct oxidative dimerization of oxindoles.4a Noticeably, the synthetic interests in the dimerization of oxindoles were mainly focused on sp3-hybridized carbon coupling at C3-position of oxindole, while the reductive dimerization of isatin or sp2-hybridized carbon at C3-position of oxindole received surprisingly insufficient attention. Essentially, the corresponding reductive dimerization products were usually formed in low yields or as by-products since the direct reductions of substrates were extremely unavoidable.5 Obviously, an efficient pathway for the reductive dimerization of isatin and its derivatives with sp2-hybridized carbon at C3-position is in high demand, which would pave the way for the extensive biological studies on these classes of bisoxindoles. Scheme 1. Direct Dimerization of Oxindoles Assembling 3,3’-Disubstituted Bisoxindoles


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In recent years, photoredox catalysis has experienced a resurgence in interest as a powerful synthetic tool for organic transformations under mild and environmentally benign conditions.6 Up to now, a variety of elegant photoredox-catalyzed reactions for the construction of C−C bonds have been established.7 Encouraged by these pioneering works, we wondered whether the photoredox-catalyzed C-C dimerization could address the issue facing the sp2-hybridized carbon coupling of isatins to effectively install 3,3’-disubstituted bisoxindole scaffolds. Interestingly, Xiao et al. reported

a

photocatalytic

dimerization

of

3-ylideneoxindoles

to

construct

spirooxindoles with contiguous quaternary centers.8 We envisaged that the reactive ketyl radical derived from isatins via a photoredox process would be prone to dimerize,

overcoming

the

increasing

steric

barrier

in

the

formation

of

3,3’-disubstituted bisoxindoles and therefore suppressing the problematic straight reduction of C=O.9 Herein, we disclose a photoredox-catalyzed reductive dimerization of isatins and their imine derivatives to access 3,3’-bisoxindoles in good chemical yields and diastereoselectivities under mild reaction conditions. Initially, we used benzylisatin (1a) in the model reaction (Table 1). To our delight, irradiation of 1a with household light in the presence of Me3N (2 equiv) and Ir(ppy)3PF6 (1 mol %) in CH2Cl2 afforded the desired isatide 3a in 52% yield with excellent diastereoselectivity (>20:1) and the direct reduction of 1a was successfully suppressed under the reaction conditions (Table 1, entry 1). Concurrently, the



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structure of isatide 3a was unambiguously confirmed by X-ray crystallographic analysis. Among the screened catalysts, Ru(bpy)3(PF6)2 was proved to be the best choice whereas eosin Y and rhodamine B were ineffective in the reaction (Table 1, entries 2-5). Various bases were then explored to facilitate the reaction, of which Me3N provided the highest yield (Table 1, entries 6-8). Screening of solvents revealed that CH2Cl2 was the optimal solvent (Table 1, entries 9-11). Furthermore, effects of H2O and atmosphere were also taken into consideration (Table 1, entries 12-14). The yield of 3a was drastically reduced in the absence of H2O, while an increase in the amount of H2O exerted negligible effect on the reaction (Table 1, entries 12-13). And the desired product 3a was unobtainable in the absence of oxygen (Table 1, entry 14). However, a significantly lowered yield for 3a (Table 1, entry 15, 9%) was obtained when the reaction was run under O2 atmosphere, which might be due to the severe oxidation caused by the presence of excessive O2. Obviously, O2 and H2O were playing important roles in this photoredox catalytic process. Furthermore, good yield was also achieved when the reaction was performed on 0.5 mmol scale under standard reaction conditions (Table 1, entry 16 vs 2).

Table 1. Optimization of Reaction Conditionsa

Entry Catalyst

Solvent

Base

Yield (%)

1

Ir(ppy)3PF6

CH2Cl2

Me3N

52

2

Ru(bpy)3(PF6)2

CH2Cl2

Me3N

80

3

Ir(ppy)(dtppy)2PF6 CH2Cl2

Me3N

45

4

Eosin Y

CH2Cl2

Me3N

NR

5

Rhodamine B

CH2Cl2

Me3N

NR

6

Ru(bpy)3(PF6)2

CH2Cl2

DIPEA 75


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7

Ru(bpy)3(PF6)2

CH2Cl2

i-Pr3N

76

8

Ru(bpy)3(PF6)2

CH2Cl2

NMP

23

9

Ru(bpy)3(PF6)2

CHCl3

Me3N

72

10

Ru(bpy)3(PF6)2

CH3CN

Me3N

75

11

Ru(bpy)3(PF6)2

DMF

Me3N

64

12b

Ru(bpy)3(PF6)2

CH2Cl2

Me3N

35

13c

Ru(bpy)3(PF6)2

CH2Cl2

Me3N

70

14d

Ru(bpy)3(PF6)2

CH2Cl2

Me3N

NR

15e

Ru(bpy)3(PF6)2

CH2Cl2

Me3N

9

16f

Ru(bpy)3(PF6)2

CH2Cl2

Me3N

81

a

Reaction conditions: 1 (0.2 mmol), base (2 equiv), catalyst (1 mol %),

irradiation with white LEDs (18 W), solvent (1 mL), room temperature, 6 h. bUsing anhydrous solvent. cAddition of 1 equiv of H2O. dPerformed under Ar atmosphere. ePerformed under O2 atmosphere. fPerformed on 0.5 mmol scale.

To further explore the versatility of this developed protocol, a series of substituted isatins 1 were then evaluated and the corresponding results are summarized in Scheme 2. Generally speaking, various substituents such as Me, F, Cl, and Br at C5 and C6 position of isatins were well tolerated, giving the corresponding products in good yields ranging from 66 to 82% (3b-3g). Pleasingly, 7-fluoroisatin also afforded the product 3h in 78% yield. However, introducing a strong electron-withdrawing group (NO2) at C5 position of isatin completely muted its reactivity and the desired product was unable to be obtained. Finally, different protecting groups on isatins were also examined, where MOMand methyl protected isatins proceeded smoothly to give the corresponding isatide 3i and 3j in good yields, respectively. However, t-butyloxy carbonyl (Boc) and benzoyl (Bz) protected isatin as well as straight isatin failed to give the desire product. Obviously, the protecting group has sizable effect on the photoredox reactivity of isatin. It should be noted that excellent diastereoselectivity (single dl- isomer) was persistently achieved in all the tested substrates.



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Scheme 2. Photoredox-catalyzed Reductive Dimerization of Isatina,b

a

Reaction conditions: 1 (0.5 mmol, 1 equiv), Me3N (89 µL, 1 mmol, 2 equiv), Ru(bpy)3(PF6)2 (1

mol %), CH2Cl2 (2.5 mL), irradiation with white LEDs (18 W), room temperature. b>20:1 dr

It is noteworthy that the reductive dimerization of isatin-derived ketimine was scarcely described in the literature.10 Given the efficiency of this developed protocol, we questioned whether the photoredox-catalyzed pathway was also amenable to the reductive

dimerization

of

isatin-derived

ketimines.

As

a

consequence,

N-Boc-protected ketimine 4a (R1 = H, R2 = Bn) was subjected to the standard conditions. Not surprisingly, the desired diamine 5a was isolated in 49% yield with moderate diastereoselectivity (dr = 5.6:1). The structure of 5a was also confirmed by X-ray crystallographic analysis. Interestingly, meso-isomer was formed as the major isomer in this case. In this sense, the formation of dl- or meso-3,3’-disubstituted bisoxindole can be readily manipulated through a simple derivatization at the C3-position of isatins. The substrate scope of ketimine 4 was then explored and the results are shown in Scheme 3. Interestingly, methyl-protected ketimine 4b gave a


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relative higher yield (54% yield) but with a relatively lower dr. Taking into account the reproducibility and chemical yield, methyl group was thus chosen as the optimum protecting group for the following tests. Satisfyingly, excellent substituent tolerance was also observed as various substitution patterns consistently provided the desired products 5c-5i in moderate yields ranging from 52 to 62% with moderate to good dr (3.5:1 to 5:1). It was also found that the presence of a strong electron-withdrawing group (NO2) significantly eroded the reactivity of 4 and only trace amount of the desired product was formed in an elongated reaction time. Scheme 3. Photoredox-catalyzed Reductive Dimerization of Isatin-derived Ketiminesa

a

Reaction conditions: 5 (0.5 mmol, 1 equiv), Me3N (89 µL, 1 mmol, 2 equiv), Ru(bpy)3(PF6)2 (1

mol %), DCM (2.5 mL), irradiation with white LEDs (18 W), room temperature.

Scheme 4. Proposed Mechanism for the Visible-light Photoredox Catalyzed Reductive Dimerization



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Ru(bpy)32+ photocatalyst

O2 NBoc

visible light

*Ru(bpy)32+ oxidant

O

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N A

N Me 4

O2

+ N

N Ru(bpy)3+

B

reductant

O

O2

C HOO

secondary orbital interaction

O N Bn 1

OH O H2O

N R

O N Bn

N

OH

O

O dimerization N R

O

r elieved ster ic repulsion

Me N

O

O E

N Me

O H2O

O

O BocHN

O

NH HN

dimerization

O N R

O

N dl-3 Bn

D

NBoc

HO HO

Bn

O

N R

NHBoc O

N meso-5 Me

Based on the obtained results and previous reports,9a, 11 a plausible catalytic cycle for this reaction is proposed in Scheme 4. Initially, the oxidizing species *Ru(bpy)32+ was generated by the photoexcitation of Ru(bpy)32+. Subsequently, a single-electron oxidation of amine A proceeded to produce an amino radical cation B and *Ru(bpy)32+ was simultaneously reduced to give Ru(bpy)3+. Ru(bpy)3+ was then oxidized by O2 to regenerate the catalyst Ru(bpy)32+ and deliver O2•−, which could undergo the protonation to give HOO-.12 Afterwards, ketyl radical anion D or amine radical E was generated through the reduction of isatin 1 or imine 4 by O2•−, which underwent the following protonation by water and radical dimerization to afford the corresponding product 3 or 5. Noticeably, with regard to the case of 3-oxindole 1, the secondary orbital interaction between π-orbitals of two carbonyl groups would predominately direct the ketyl radicals D to approach each other side by side, providing dl-isatide 3 exclusively.13 On the other side, as for the amine radical E, the presence of sterically bulky Boc group would force two coupling partners to position


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face to face, where two Boc groups were thus oriented in the opposite directions to significantly relieve the steric repulsion. Ultimately, the dimerization of radicals proceeded to give meso-bisoxindole 5 as the major diastereomer. In summary, we described a facile method to construct 3,3’-disubstitued bisoxindoles bearing vicinal quaternary centers through the visible-light induced, photoredox-catalyzed reductive dimerization of isatins or isatin-derived ketimines. This developed protocol features mild reaction conditions, easy operation, and high atom-economy. This simple generation of ketyl and amine radicals from isatins and ketimines might pave the way for difficultly accessible transformations of isatins.

EXPERIMENTAL SECTION General Experimental Methods. Unless otherwise noted, all the reagents were purchased from commercial suppliers and used without further purification. 1H NMR spectra was recorded at 400 MHz. The chemical shifts were recorded in ppm relative to tetramethylsilane and with the solvent resonance as the internal standard. Data were reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, br = broad), coupling constants (Hz), integration. 13C NMR data were collected at 100 MHz with complete proton decoupling. Chemical shifts were reported in ppm from the tetramethylsilane with the solvent resonance as internal standard. Infrared spectra (IR) were measured by FT-IR apparatus. High resolution mass spectroscopy (HRMS) was recorded on TOF MS ES+ mass spectrometer and acetonitrile was used to dissolve the sample. Column chromatography was carried out on silica gel (200-300 mesh). Substrates 1 and 4 were prepared according to the reported procedures.14 General Procedure for Visible-light Driven Photoredox Reductive Dimerization of 1 and 4. A mixture solution of substrate 1 or 4 (0.5 mmol, 1 equiv), Ru(ppy)3(PF6)2 (4.3 mg, 1 mol %), and trimethylamine (89 µL, 1 mmol, 2 equiv) in CH2Cl2 (2.5 mL) was irradiated with an 18 W white LED light bulb (distance app. 2.0 cm from the bulb) at room temperature. After completion of the reaction (confirmed by TLC analysis), the solvent was removed in vacuo. The resulting residue was purified via flash chromatography (EtOAc/PE = 1/9-3/7) to yield the corresponding product. Characterization Data for 3a-3j and 5a-5i.



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1,1'-Dibenzyl-3,3'-dihydroxy-[3,3'-biindoline]-2,2'-dione 3a White crystals (94.6 mg, 0.203 mmol, yield 81%, > 20:1 dr); m.p. 135-136 °C; IR (KBr) ν 3417, 3057, 1706, 1609, 1462, 1368, 1171, 1114 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 7.26-7.30 (m, 2H), 7.19-7.20 (m, 6H), 6.94-7.09 (m, 4H), 6.84-6.94 (m, 2H), 6.72 (d, J = 8.0 Hz, 2H), 6.50 (s, 2H), 4.85 (d, J = 16.0 Hz, 2H), 4.67 (d, J = 16.0 Hz, 2H) ;

13

C NMR (DMSO-d6, 100 MHz) δ

175.4, 143.7, 136.2, 130.6, 128.9, 127.6, 127.3, 127.1, 126.0, 122.7, 109.7, 77.8, 43.2; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C30H24N2O4Na 499.1634, found 499.1665. 1,1'-Dibenzyl-3,3'-dihydroxy-5,5'-dimethyl-[3,3'-biindoline]-2,2'-dione 3b White crystals (90.8 mg, 0.180 mmol, yield 72%, >20:1 dr); m.p. 150-151 °C; IR (KBr) ν 3442, 2919, 1714, 1602, 1496, 1371, 1147, 990 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 7.18-7.19 (m, 6H), 7.08 (d, J = 7.6 Hz, 2H), 6.99-7.06 (m, 4H), 6.60 (d, J = 8.0 Hz, 2H), 6.42 (br s, 2H), 4.84 (d, J = 16.0 Hz, 2H), 4.62 (d, J = 15.2 Hz, 2H), 2.06 (s, 6H) ;

13

C NMR (DMSO-d6, 100 MHz) δ

175.4, 141.3, 136.2, 131.5, 130.6, 128.9, 127.6, 127.3, 127.0, 126.8, 109.3, 77.8, 43.2, 21.4; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C32H28N2O4Na 527.1941, found 527.1946. 1,1'-Dibenzyl-5,5'-difluoro-3,3'-dihydroxy-[3,3'-biindoline]-2,2'-dione 3c White crystals (98.7 mg, 0.193 mmol, yield 77%, >20:1 dr); m.p. 147-148 °C; IR (KBr) ν 3415, 1722, 1701, 1489, 1456, 1347, 1177, 1148 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 7.22-7.81 (m, 6H), 7.13-7.18 (m, 2H), 6.99-7.12 (m, 4H), 6.74-6.78 (m, 4H), 6.60 (br s, 2H), 4.89 (d, J = 16.0 Hz, 2H), 4.67 (d, J = 16.0 Hz, 2H) ; 13C NMR (DMSO-d6, 100 MHz) δ 174.7, 158.4 (d, 1JC−F = 237 Hz), 139.8, 136.0, 128.9, 128.7 (d, 3JC−F = 8.0 Hz), 127.7, 127.4, 116.9 (d, 2JC−F = 23 Hz), 113.7 (d, 2JC−F = 24 Hz), 110.6 (d, 3JC−F = 7.0 Hz), 78.0, 43.4; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C30H22N2O4F2Na 535.1445, found 535.1420. 1,1'-Dibenzyl-5,5'-dichloro-3,3'-dihydroxy-[3,3'-biindoline]-2,2'-dione 3d White crystals (55.6 mg, 0.160 mmol, yield 80%, >20:1 dr); m.p. 137-138 °C; IR (KBr) ν 2937, 1816, 1710, 1656, 1450, 1320, 961, 880 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 7.38 (dd, J = 8.4, 2.0 Hz, 2H), 7.21-7.23 (m, 6H), 6.95-7.12 (m, 4H), 6.76-6.78 (m, 4H), 4.89 (d, J = 16.0 Hz, 2H), 4.67 (d, J = 16.0 Hz, 2H); 13C NMR (DMSO-d6, 100 MHz) δ 174.4, 142.5, 135.6, 130.4, 129.1, 129.0, 127.7, 127.3, 126.7, 126.0, 111.1, 78.0, 43.4; HRMS (TOF-ES+) m/z: [M+H]+ calcd for C30H23N2O4Cl2 545.1035, found 545.1009. 1,1'-Dibenzyl-5,5'-dibromo-3,3'-dihydroxy-[3,3'-biindoline]-2,2'-dione 3e


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White crystals (130.0 mg, 0.205 mmol, yield 82%, >20:1 dr); m.p. 137-138 °C; IR (KBr) ν 3453, 3376, 1731, 1709, 1480, 1342, 1167, 813 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 7.49 (dd, J = 8.0, 1.6 Hz, 2H), 7.21-7.23 (m, 6H), 7.00-7.10 (m, 4H), 6.93 (br s, 2H,), 6.77 (s, 2H), 6.71 (d, J = 8.4 Hz, 2H), 4.88 (d, J = 16.0 Hz, 2H), 4.66 (d, J = 16.0 Hz, 2H) ; 13C NMR (DMSO-d6, 100 MHz) δ 174.3, 142.9, 135.8, 133.2, 129.4, 129.0, 128.8, 127.7, 127.3, 114.4, 111.6, 77.9, 43.4; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C30H22N2O4Br2Na 654.9844, found 654.9825. 1,1'-Dibenzyl-6,6'-dichloro-3,3'-dihydroxy-[3,3'-biindoline]-2,2'-dione 3f White crystals (108.0 mg, 0.198 mmol, yield 79%); m.p. 161-162 °C; IR (KBr) ν 3467, 3367, 1708, 1612, 1484, 1437, 1344, 1168 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 7.22-7.23 (m, 6H), 7.01-7.13 (m, 4H), 6.97 (d, J = 7.2 Hz, 2H), 6.87 (d, J = 1.6 Hz, 2H), 6.67 (s, 2H), 4.90 (d, J = 16.0 Hz, 2H), 4.69 (d, J = 16.0 Hz, 2H); 13C NMR (DMSO-d6, 100 MHz) δ 175.0, 145.1, 135.9, 135.0, 128.8, 127.8, 127.5, 126.1, 122.5, 110.0, 77.5, 43.2; HRMS (TOF-ES+) m/z: [M+H]+ calcd for C30H23N2O4Cl2+ 545.1035, found 545.1017. 1,1'-Dibenzyl-6,6'-dibromo-3,3'-dihydroxy-[3,3'-biindoline]-2,2'-dione 3g White crystals (104.7 mg, 0.165 mmol, yield 66%, > 20:1 dr); m.p. 120-121 °C; IR (KBr) ν 3422, 3061, 1706, 1606, 1487, 1428, 1372, 1175 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 7.23-7.24 (m, 6H), 7.12 (d, J = 7.6 Hz, 2H), 7.02-7.10 (m, 4H), 6.99 (d, J = 1.6 Hz, 2H), 6.82 (br s, 2H), 6.68 (br, 2H,), 4.90 (d, J = 16 Hz, 2H), 4.70 (d, J = 16 Hz, 2H) ; 13C NMR (DMSO-d6, 100 MHz) δ 175.0, 145.2, 135.9, 135.1, 128.9, 127.8, 127.7, 127.4, 126.5, 123.5, 112.6, 77.5, 55.4, 43.2; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C30H22N2O4Br2Na 654.9844, found 654.9821. 1,1'-Dibenzyl-7,7'-difluoro-3,3'-dihydroxy-[3,3'-biindoline]-2,2'-dione 3h White crystals (100.0 mg, 0.195 mmol, yield 78%, > 20:1 dr); m.p. 112-113 °C; IR (KBr) ν 3445, 3056, 1708, 1627, 1472, 1350, 1158, 1118 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 7.18-7.23 (m, 8H), 6.95-6.98 (m, 6H), 6.77 (s, 2H), 4.86 (d, J = 16.0 Hz, 2H), 4.80 (d, J = 16.0 Hz, 2H) ; 13C NMR (DMSO-d6, 100 MHz) δ 174.7, 147.3 (d, 1JC−F = 242 Hz), 137.1, 130.2, 130.1(d, 3JC−F = 9.0 Hz), 128.9 (d, 2JC−F = 12.0 Hz), 128.8, 127.5, 126.7, 123.8 (d, 3JC−F = 6.0 Hz), 122.3, 118.7 (d, 2 C−F

J

= 18 Hz), 78.1, 45.0; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C30H22N2O4F2Na 535.1445,

found 535.1472. 3,3'-Dihydroxy-1,1'-bis(methoxymethyl)-[3,3'-biindoline]-2,2'-dione 3i White crystals (72.1 mg, 0.188 mmol, yield 75%, > 20:1 dr); m.p. 150-151 °C; IR (KBr) ν 3350,



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2878, 2932, 1725, 1613, 1492, 1346, 1249 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 7.33 (t, J = 7.6 Hz, 2H), 6.96-6.99 (m, 4H), 6.84 (br s, 2H), 6.48 (s, 2H), 4.90 (s, 4H), 2.91 (s, 6H);

13

C NMR

(DMSO-d6, 100 MHz) δ 175.6, 143.0, 130.7, 126.8, 126.0, 123.0, 109.9, 78.2, 71.1, 55.8; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C20H20N2O6Na 407.1219, found 407.1223. 3,3'-Dihydroxy-1,1'-dimethyl-[3,3'-biindoline]-2,2'-dione 3j White crystals (68.9mg, 0.213 mmol, yield 85%, >20:1 dr); m.p. 147-148 °C; IR (KBr) ν 3535, 3380, 1712, 1613, 1471, 1352, 1211, 1091 cm-1; 1H NMR (DMSO-d6, 400 MHz) δ 7.32-7.36 (m, 2H), 6.90-6.94 (m, 4H), 6.76 (br s, 2H), 6.32 (s, 2H), 2.89 (s, 6H);

13

C NMR (DMSO-d6, 100

MHz) δ 175.0, 144.4, 130.6, 126.8, 125.6, 122.4, 108.9, 78.0, 26.1; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C18H16N2O4Na 347.1008, found 347.1016. di-tert-Butyl (1,1'-dibenzyl-2,2'-dioxo-[3,3'-biindoline]-3,3'-diyl)dicarbamate 5a White crystals (82.7 mg, 0.123 mmol, yield 49%, 5.6:1 dr); m.p. 212-215 °C; IR (KBr) ν 3477, 1706, 1609, 1490, 1436, 1369, 1172, 1114 cm-1; 1H NMR (CDCl3, 400 MHz, 5.6:1 dr) δ 7.40-7.41 (m, 5.2H), 7.27-7.34 (m, 6.3H), 7.20 (s, 1.7H), 7.04-7.09 (m, 0.9H), 6.89-6.92 (m, 4.0H), 6.62 (t, J = 7.2 Hz, 2.3H), 6.33 (d, J = 7.6 Hz, 2.0H), 6.20-6.27 (m, 0.28H), 5.13-5.17 (m, 1.7H), 5.00-5.05 (m, 0.58H), 4.62-4.66 (m, 2.0H), 4.47-4.51 (m, 0.35H), 1.36 (s, 18H), 0.96 (s, 3.2H);

13

C NMR

(CDCl3, 100 MHz, major diastereomer) δ 174.2, 153.9, 143.2, 135.2, 129.5, 128.7, 127.8, 127.6, 125.1, 122.8, 122.1, 108.8, 80.4, 62.3, 44.5, 28.3; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C40H42N4O6Na 697.3002, found 697.3004. di-tert-Butyl (1,1'-dimethyl-2,2'-dioxo-[3,3'-biindoline]-3,3'-diyl)dicarbamate 5b White crystals (70.6 mg, 0.135 mmol, yield 54%, 3.5:1 dr); m.p. 189-190 °C; IR (KBr) ν 3379, 2983, 1706, 1614, 1498, 1373, 1256, 1086 cm-1; 1H NMR (CDCl3, 400 MHz, 3.5:1 dr) δ 7.12 (s, 1.4H), 7.02-7.09 (m, 3.7H), 6.97-6.98 (m, 1.9H), 6.80-6.91 (m, 3.2H), 6.41-6.43 (m, 2.5H), 3.18 (s, 5.9H), 3.10 (s, 1.6H), 1.34 (s, 18.0H), 1.03 (s, 5.1H);

13

C NMR (CDCl3, 100 MHz, major

diastereomer) δ 173.6, 154.0, 143.4, 129.7, 125.0, 121.9, 107.8, 80.4, 62.5, 28.2, 27.7, 26.1, 25.8; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C28H34N4O6Na 545.2376, found 545.2384. di-tert-Butyl (1,1',5,5'-tetramethyl-2,2'-dioxo-[3,3'-biindoline]-3,3'-diyl)dicarbamate 5c White crystals (71.6 mg, 0.130 mmol, yield 52%, 3.5:1 dr); m.p. 184-186 °C; IR (KBr) ν 3378, 2976, 1703, 1496, 1362, 1166, 1065, 809 cm-1; 1H NMR (CDCl3, 400 MHz, 3.5:1 dr) δ 7.09 (s, 1.4H), 7.00 (s, 0.5H), 6.81-6.88 (m, 5.7H), 6.30 (d, J = 7.6 Hz, 2.6H), 3.15 (s, 5.8H), 3.08 (s,


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1.7H), 2.19-2.21 (m, 7.8H), 1.35 (s, 18.0H), 1.04 (s, 5.1H); 13C NMR (CDCl3, 100 MHz, major diastereomer) δ 173.5, 154.0, 141.0, 131.6, 129.9, 125.0, 122.8, 107.4, 80.3, 62.6, 28.3, 26.1, 21.0; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C30H38N4O6Na 573.2689, found 573.2666. di-tert-Butyl

(5,5'-difluoro-1,1'-dimethyl-2,2'-dioxo-[3,3'-biindoline]-3,3'-diyl)dicarbamate

5d White crystals (78.2 mg, 0.140 mmol, yield 56%, 5:1 dr); m.p. 172-173 °C; IR (KBr) ν 3390, 2982, 1713, 1366, 1276, 1166, 1022, 805 cm-1; 1H NMR (CDCl3, 400 MHz, 5:1 dr) δ 7.07 (s, 1.5H), 6.97 (s, 0.4H), 6.47-6.68 (m, 5.3H), 6.43 (q, J = 3.2 Hz, 2.4H), 3.21 (s, 6.0H), 3.14 (s, 1.2H), 1.35 (s, 18.0H), 1.08(s, 3.6H); 1 C−F

J

13

C NMR (CDCl3, 100 MHz, major diastereomer) δ 173.2, 159.0 (d,

= 240.0 Hz), 154.0, 139.3, 126.5 (d, 3JC−F = 8.0 Hz), 116.2 (d, 2JC−F = 23 Hz), 110.3 (d, 2JC−F

= 26 Hz), 108.6 (d, 3JC−F = 8.0 Hz), 80.9, 62.5, 28.2, 26.4; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C28H32N4O6F2Na 581.2188, found 581.2168. di-tert-Butyl

(5,5'-dichloro-1,1'-dimethyl-2,2'-dioxo-[3,3'-biindoline]-3,3'-diyl)dicarbamate

5e White crystals (88.7 mg, 0.150 mmol, yield 60%, 5:1 dr); m.p. 195-197 °C; IR (KBr) ν 3380, 2976, 2362, 1722, 1490, 1362, 1256, 1165 cm-1; 1H NMR (CDCl3, 400 MHz, 5:1 dr) δ 7.01-7.40 (m, 2.9H), 6.96-6.98 (m, 3.5H), 6.78-6.90 (m, 0.8H), 6.41 (d, J = 8.4 Hz, 2.4H), 3.21 (s, 6.0H), 3.14 (s, 1.2H), 1.36 (s, 18.0H), 1.08 (s, 3.6H); 13C NMR (CDCl3, 100 MHz, major diastereomer) δ 172.9, 153.9, 141.9, 129.8, 127.8, 126.4, 122.6, 109.0, 81.0, 62.5, 28.2, 26.3; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C28H32N4O6Cl2Na 613.1597, found 613.1572. di-tert-Butyl

(5,5'-dibromo-1,1'-dimethyl-2,2'-dioxo-[3,3'-biindoline]-3,3'-diyl)dicarbamate

5f White crystals (105.5 mg, 0.155 mmol, yield 62%, 5:1 dr); m.p. 199-200 °C; IR (KBr) ν 3383, 2977, 1724, 1609, 1488, 1364, 1255, 1166 cm-1; 1H NMR (CDCl3, 400 MHz, 5:1 dr) δ 7.16-7.25 (m, 2.8H), 7.093-7.097 (m, 1.7H), 6.96 (s, 1.6H), 6.76-6.87 (m, 0.9H), 6.37 (d, J = 8.4 Hz, 2.4H), 3.21 (s, 6.0H), 3.14 (s, 1.2H), 1.36 (s, 18.0H), 1.08 (s, 3.6H); 13C NMR (CDCl3, 100 MHz, major diastereomer) δ 172.8, 156.2, 153.9, 142.3, 132.7, 126.7, 125.3, 114.8, 109.5, 81.0, 62.5, 28.2, 26.3; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C28H32N4O6Br2Na 701.0586, found 701.0604. di-tert-Butyl

(6,6'-dichloro-1,1'-dimethyl-2,2'-dioxo-[3,3'-biindoline]-3,3'-diyl)dicarbamate

5g



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

White crystals (74.6 mg, 0.153 mmol, yield 61%, 5:1 dr); m.p. 187-188 °C; IR (KBr) ν 3400, 2982, 2364, 1735, 1701, 1497, 1370, 1161 cm-1; 1H NMR (CDCl3, 400 MHz, 5:1 dr) δ 7.01 (s, 1.5H), 6.95-6.97(s, 0.6H), 6.80-6.90 (m, 5.1H), 6.51 (s, 2.3H), 3.17 (s, 6H), 3.10 (s, 1.2H), 1.34 (s, 18.0H), 1.07 (s, 3.8H); 13C NMR (CDCl3, 100 MHz, major diastereomer) δ 173.7, 153.9, 144.8, 135.8, 123.3, 123.0, 122.2, 109.1, 80.8, 62.0, 28.2, 26.3; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C28H32N4O6Cl2Na 613.1597, found 613.1594. di-tert-Butyl

(6,6'-dibromo-1,1'-dimethyl-2,2'-dioxo-[3,3'-biindoline]-3,3'-diyl)dicarbamate

5h White crystals (115.7 mg, 0.170 mmol, yield 58%, 5:1 dr); m.p. 192-193 °C; IR (KBr) ν 3401, 2980, 1734, 1702, 1606, 1495, 1369, 1163 cm-1; 1H NMR (CDCl3, 400 MHz, 5:1 dr) δ 6.99-7.06 (m, 4.0H), 6.90-6.91 (m, 0.8H), 6.82-6.84 (m, 2.3H), 6.67 (s, 2.4H), 3.17 (s, 6.0H), 3.10 (s, 1.2H), 1.33 (s, 18.0H), 1.07 (s, 3.6H); 13C NMR (CDCl3, 100 MHz, major diastereomer) δ 173.6, 153.9, 144.8, 125.2, 123.81, 123.75, 123.3, 111.9, 80.9, 62.0, 28.2, 26.3; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C28H32N4O6Br2Na 701.0586, found 701.0577. di-tert-Butyl (7,7'-difluoro-1,1'-dimethyl-2,2'-dioxo-[3,3'-biindoline]-3,3'-diyl)dicarbamate 5i White crystals (82.4 mg, 0.148 mmol, yield 59%, 4.8:1 dr); m.p. 172-173 °C; IR (KBr) ν 3349, 2977, 1723, 1486, 1371, 1281, 1167, 861 cm-1; 1H NMR (CDCl3, 400 MHz, 4.8:1 dr) δ 7.08 (s, 1.5H), 7.00 (s, 0.4H), 6.78-6.87 (m, 7.8H), 3.38-3.39 (m, 5.9H), 3.32 (s, 1.2H), 1.35 (s, 18.0H), 1.10 (s, 3.7H); 13C NMR (CDCl3, 100 MHz, major diastereomer) δ 173.2, 154.0, 147.2 (d, 1JC−F = 243 Hz), 130.1 (d, 3JC−F = 9.0 Hz), 127.8, 122.8 (d, 3JC−F = 6.0 Hz), 118.1 (d, 2JC−F = 19 Hz), 80.8, 62.5, 28.2; HRMS (TOF-ES+) m/z: [M+Na]+ calcd for C28H32N4O6F2Na 581.2188, found 581.2175.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.xxxxxxx 1

H NMR and 13C NMR spectra for new compounds 3a-3j and 5a-5i; X-ray structures of 3a, 3i, 5a

and 5f.


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AUTHOR INFORMATION Corresponding author *Tel.: +86-731-88830833; e-mail: [email protected]; [email protected].

Author Contributions §

These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We gratefully acknowledge the financial support from National Natural Science Foundation of China (Nos. 21576296 and 21676302) and Central South University.

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