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Enantioselective Synthesis of Indole-Based Biaryl Atropisomers via Palladium-Catalyzed Dynamic Kinetic Intramolecular C-H Cyclization Congfa He, Mengqing Hou, Zixi Zhu, and Zhenhua Gu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01855 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017
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ACS Catalysis
Enantioselective Synthesis of Indole-Based Biaryl Atropisomers via Palladium-Catalyzed Dynamic Kinetic Intramolecular C-H Cyclization Congfa He, Mengqing Hou, Zixi Zhu and Zhenhua Gu* Department of Chemistry and Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P. R. China ABSTRACT: We report here a palladium-catalyzed enantioselective synthesis of indole-based atropisomers via an intramolecular dynamic kinetic C-H cyclization. The TADDOL derived phosphoramidite ligand proves most efficient in this directing group-free transformation, delivering products with up to 96:4 er. The thermal stability of the axial chirality of the atropisomers has also been investigated.
KEYWORDS: atropisomer, homogeneous catalysis, palladium, C-H cyclization, indole
Atropisomerism is one of the most fundamental ways for molecules to manifest their stereochemical characters.1 Atropisomeric structures can often be seen in natural products [steganone (A) and galeon (B), Figure 1],2 synthetically useful catalysts and ligands (C and D).3 Among the various atropisomeric forms, such as atropisomeric amides and biaryl ethers, axially chiral biaryl compounds have received particular attention due to their extraordinary aptness as chiral ligands in asymmetric catalysis.
sigmatropic rearrangement of N,N’-binaphthyl hydrazines,9 and asymmetric Michael addition of 2naphthols to quinones.10 Scheme 1. Size-Increasing Strategy in Atropisomer Synthesis
Figure 1. Atropisomerism in Natural Products, Catalysts and Ligands Generally, for a non-cyclic biaryl atropisomer to be configurationally stable under ambient conditions, three or more ortho-substituents (or two very bulky orthosubstituents) are required. As a result it forms a sterically congested axis.4 It is a challenging task for the construction of sterically hindered aryl-aryl bonds meanwhile gaining high enantioselectivity. For these reasons, a series of outstanding ligands were developed and excellent enantioselectivity was achieved.5 To further gain axially chiral biaryl compounds with highly structural diversity, additional catalytically asymmetric methods have been established,6 including (dynamic) kinetic resolution,7 asymmetric [2+2+2]cycloaddition,8 [3,3]-
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It is known that biaryls with small ortho-substituents like hydrogen, would have low rotation barriers and are configurationally unstable. The chemical transformation that displace the small substituent with a larger group can increase the rotation barrier and thus create a stable atropisomer (Scheme 1a). To this end, C-H functionalization strategies have been developed, typically with the aid of a directing group. For instances, Murai and You groups reported the enantioselective construction of axially chiral biaryls through rhodium catalyzed asymmetric C-H alkylation/alkenylation of naphthyl-pyridine and isoquinoline derivatives.11 Sulfoxides, phosphates and imines were also used as directing groups (Scheme 1b).12 Alternatively, Miller and co-workers realized an asymmetric bromination of 3'-hydroxy-[1,1'-biphenyl]-2-carboxylic acid, where asymmetric induction hinged on the hydrogen bond interaction between amino acid catalyst with the substrates (Scheme 1b).13 Different from the aforementioned methods, we envisaged that the small substituent (hydrogen) could be increased to “large” groups via a palladiumcatalyzed asymmetric intramolecular cyclization reaction. As a result, no directing group is required (Scheme 1c). To fulfill this ambition, we must: 1) address the challenge issue of enantioselectivity due to the long distance between the axial chirality and the reactive site; 2) realize high chemoselectivity by differentiating the two different C-H bonds (Ha vs Hb in Scheme 1c) for good regioselectivity. Indoles are important subunits in bioactive natural products. The synthesis of indoles has caught significant attention of synthetic organic chemists. However, naturally occurred atropisomers bearing an indole subunit were unusually ignored. Interestingly, Murrastifoline F (1) was found as a mixture of enantiomers with low enantiomeric excess in nature, which was confirmed by the total synthesis (Figure 2).14 Neither the center chirality of sulfoxide nor the axial chirality of compound 2 was identified.15 No optical rotation data of Dendridine A (3) was given during its isolation.16 It is of curiosity and significance to investigate the synthesis indole-based atropisomers as well as study their physical and chemical properties. Recently, Kitagawa, Shi, Li and Tan groups reported the synthesis of atropisomers with indole or pyrrole skeletons, and the rotation barriers of naphthalenolindole atropisomers were also studied using the theoretical calculations.17
Figure 2. Axially Chiral Natural Products Bearing Indoles
We started our initial trials with compound 4a. It was anticipated that palladium-catalyzed intramolecular C-H arylation/cyclization would favour the formation of a five-
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membered N-heterocycle 5a rather than a six-membered product 6a. Pleasingly, in the presence of palladium acetate with various phosphine ligands, the expected product 5a was formed exclusively. However, enantioselectivity control proved extremely arduous. With all the screened chiral ligands, such as BINAP, MOP, phosphoramidite and DIOP etc. the reaction gave very low ee values (Scheme 2). Scheme 2. Screening of Various Chiral Ligands
TADDOL-based phosphoramidites are a class of useful ligands that can be easily modified starting from cheap and readily available tartrates. Thus, our primary modification focused on the variation of amine moiety and ligands L1-4 were synthesized and tested for the cyclization reaction (Table 1). Disappointingly, no satisfactory results were achieved for substrate 4b, whose hydroxyl moiety was masked with methoxymethyl group, a readily removable protecting group (entries 1-4). Next, we varied the aryl groups in the TADDOL skeleton (L5-9) and found that electron-withdrawing substituents on aryl groups imparted negative effects on the enantioselectivity (entries 5-9). When the aryl group was 2-thiophenyl (L8), only trace amount of 5b was formed (entry 8). Introducing a biphenyl group into the ligand (L9) improved the selectivity to 72:28 er (entry 9). We were pleased to find that 1-phenylpiperazine-based phosphoramidite L10 gave 5b with 86:14 er at 90 oC, and lowering the reaction temperature to 65 oC boosted the enantioselectivity to 91:9 er (entries 10 and 11). Monitoring the ee values at different reaction time revealed that the enantiomeric excesses of 5b at the early reaction stage would slightly decrease (12%) as time went by (see SI for details). The ligand loading could be lowered to 6 mol% (Pd:ligand = 1:1.2) without affecting the selectivity, and in all the subsequent studies 6 mol% of ligands were used (entries 11-15). Further modifications of the piperazine structure proved fruitless (entries 12-14). Combination of PdCl2 and pivalic acid could give a reliable/reproducible yield (entry 15). Fortunately, a single crystal structure of compound 5b was obtained and the absolute configuration was determined to be R with the assistance of heavy atom chlorine of CH2Cl2, which was co-crystalized with compound 5b (Figure 3).
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With the optimized conditions in hand, we next examined the substrate scope of this transformation. Excellent yields and good enantioselectivity could be achieved regardless the substituents at naphthalenol or indole moiety (Table 2). Different substituents at the 7- and 6position of naphthalenol moiety, such as aryl and methyl groups were compatible, and up to 95.5:4.5 er was achieved (5c-f). Both chloro and fluoro-substituted indoles smoothly underwent the cyclization to give the desired atropisomers (5g and 5h). Both electronwithdrawing and electron-donating groups in the indole ring were compatible. These substituents did not show significant effects on the enantioselectivity albeit the yields insignificantly declined for those bearing electrondonating groups (5i-o). Further investigations indicated that para-substituents on the iodobenzene rings affected neither the yields nor the enantioselectivities (5q and 5r). There was no change of the yield and er value by switching the O-MOM group to O-ethoxymethyl group (5s). However, both the reactivity and selectivity were slightly diminished with the use of a benzyl protected naphthalenol derivative as the substrate (5t). Table 1. Reaction Condition Optimizationa
OMOM
Pd(OAc)2 , Ligand Cs2 CO3, solvent, Temp.
OMOM
Br N
N 5b
4b
Ph
Ph Ph
Ph O P R O
O O Ph
R=
O
O
N
L1
N
L2 L3
O P N O
O O
Ar
Figure 3. Structure of (R)-5b·CH2Cl2
The palladium-catalyzed cyclization reaction could be scaled up to gram scale. Pleasingly, after routine extraction and filtration, product 5b was conveniently precipitated with 96:4 er and 86% yield upon the treatment of hexanes, obviating the necessity of column chromatography (Scheme 3, top). Compound (R)-5b was rather acid sensitive, and the deprotection of MOM group with aqueous HCl led to its decomposition. In the presence of catalytic Co(acac)2 and TBHP, compound 5b was oxidized to oxo-isoindoline derivative 6b, which underwent facile deprotection to deliver 7 with high enantioselectivity (Scheme 3, bottom). Table 2. Substrate Scopea
Ar O P N O
O
Ar
Ph
N
Ar
a
The reactions were performed with 0.10 mmol of 4a, Pd(OAc)2 (5 mol%), ligand (15 mol%), Cs2CO3 in the indicatb c ed solvent (2.0 mL). The amine moiety is morpholine. Aryl d iodide was used. Cyclopentanecarboxylic acid (30 mol%) e was added; Pd:ligand = 1:1.2. PdCl2 (5 mol%) and Pivalic acid (30 mol%) were added; Pd:ligand = 1:1.2.
N R
Ar = Ph, L4 Ar = 4-tBuC6 H4 , L5 Ar = 3,5-(CF3 )2 C6 H 3, L6 Ar = 3,5-(Me)2 C 6H 3, L7 Ar = 2-thiophenyl, L8 b Ar = 4-PhC 6H 4 , L9
R= R= R= R=
Ph Ph, L10 4-MeC 6H 4, L11 2-MeC 6H 4, L12 4-MeOC 6H 4, L13
I N
R
Ligand
Solvent
T/ C
Yield/ %
er/%
1
L1
PhMe
110
99
66:34
2
L2
PhMe
110
99
65:35
3
L3
PhMe
110
99
70:30
4
L4
PhMe
110
99
58:42
5
L5
PhMe
110
96
68:32
6
L6
PhMe
110
99
55:45
7
L7
PhMe
110
97
66:34
8
L8
PhMe
110
trace
-
9
L9
PhMe
110
91
72:28
c
L10
DCE
90
99
86:14
c,d
L10
DCE
65
99
91:9
cd
L11
DCE
65
86
90:10
L12
DCE
65
99
86:14
cd
L13
DCE
65
99
85:15
c,e
L10
DCE
65
99
92:8
OMOM
11
12
cd
13
14 15
O Ar
N Ph
Ar
Ar N
R
Ar =
Ph L10
Ph
6
6
7
Ph
OMOM
N 5c, 97%, er 90:10 (36 h)
p-CF3C 6H 4
OMOM
OMOM
N
N
5d, 91%, er 91.5:8.5 (12 h)
5e, 99%, 95.5:4.5 (10 h)
6
OMOM
OMOM
10
O P N O
O
R'
Ar
Me
entry
o
PdCl2, L10 pivalic acid Cs 2CO3, DCE, 65 oC
OMOM
Ph
Ar
Ar R'
X
R
N
N
5f, 96%, er 94.5:5.5 (36 h)
OMOM
5'
5g, X = Cl, 87%, er 90.5:9.5 (24 h) 5h, X = F, 93%, er 91.5:8.5 (12 h)
OMOM
5'
N
5i, R = NO2, 95%, er 92.5:7.5 (5 h) 5j, R = CO2Me, 92%, er 91:9 (36 h)
OMOM
OMOM
2'
R
5'
N
N
N
1'
5k, R = MeO, 81%, er 92.5:7.5 (10 h) 5l, R = Me, 83%, er 93.5:6.5 (18 h)b 5m, R = Ph, 97%, er 90:10 (12 h)c
Me
7'
6'
Me
5n, 99%, er 91:9 (12 h)b
OMOM
OMOM
N
10'
5o, 86%, er 92.5:7.5 (24 h)
Me
5p, 99%, er 91.5:8.5 (22 h)
a
N
10'
OR X
5q, X = Cl, 99%, er 91.5:8.5 (8 h) 5r, X = F, 99%, er 92:8 (12 h)
N 5s, R = CH2 OEt 97%, er 92:8 (12 h) 5t, R = Bn, 75%, er 91:9 (36 h)
The reactions were performed with 0.10 mmol of 4, palladium (5 mol%), L10 (6 mol%), Cs2CO3 (2.0 equiv), pivalic acid o (30 mol%) in dichloroethane (2.0 mL) at 65 C. The absolute
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configuration of 5 was tentatively assigned as R by the comb parison with compound (R)-5b. 1Methylcyclohexanecarboxylic acid (30 mol%) was used inc o stead of PivOH. 80 C.
Corresponding Author
Scheme 3. Gram-Scale Reaction and Applications
The authors declare no competing financial interests.
I
4b (1.00 g, 1.93 mmol)
OMOM
OMOM HCl(c), MeOH 50 oC, 3 h N
N (R)-5b, er 96:4
N 5b, 86%, er 96:4
Co(acac)2 (10 mol%) TBHP (5.0 equiv) acetone, rt, 40 h
O 6b, 82%, er 96:4
OH
N O 7, 99%, er 96:4
Analysis of the crystal structure of (R)-5b revealed that the angles of C16-C7-C6 and C16-C7-C8 were 126.7o and 127.8o, respectively, which are larger than the classic 120o for six-membered aromatic rings. Therefore, the indolebased atropisomers are expected to have less steric congestion and relatively lower rotation barriers than the corresponding benzene derivatives. Experimentally, heating the ortho tetra-substituted biaryl compound (R)-6b indicated that the half-life of (R)-6b was around 67 h at Ɵ of (R)90 oC. The energy barrier for racemization ∆ G -1 o 6b was determined to be 31.3 kcal mol (90 C) by the analyzing the rate of thermal racemization at three different temperatures (see SI for details) (Figure 4).
Figure 4. Plot of Racemization Rate of (R)-6b at Three Different Temperatures
In summary, we have developed a palladium-catalyzed intramolecular C-H cyclization reaction for the enantioselective construction of indole-based atropisomers. The modified TADDOL-phosphoramidite L10 was shown to display good to excellent enantioselectivities in this dynamic kinetic resolution process. This strategy also features the obviation of using strongly coordinative directing group.
ASSOCIATED CONTENT Supporting Information. Experimental procedure, spectro1 13 scopic data, and the H, C NMR spectra of the products (PDF), the single crystal structure of compound 5b (CIF). This material is available free of charge via the internet at http://pubs.acs.org.
AUTHOR INFORMATION
Notes
This work was partly supported by the '973' project from the MOST of China (2015CB856600), NSFC (21472179, 21622206), and Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).
OMOM
precipitated with hexanes no column chromatography
N
*
[email protected] ACKNOWLEDGMENT
PdCl2 (5.0 mol%), L10 (6.0 mol%) Cs2 CO3, DCE, 65 oC
OMOM
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