Catalyzed Asymmetric Friedel–Crafts Reaction of Indole with

Letter Cite This: Org. Lett. 2018, 20, 7149−7153

pubs.acs.org/OrgLett

Revisiting the CuII-Catalyzed Asymmetric Friedel−Crafts Reaction of Indole with Trifluoropyruvate Thien Phuc Le,†,§ Kazuyuki Higashita,†,§ Shinji Tanaka,† Masahiro Yoshimura,‡ and Masato Kitamura*,† †

Org. Lett. 2018.20:7149-7153. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/16/18. For personal use only.

Graduate School of Pharmaceutical Sciences and Research Center for Materials Science, Nagoya University, Chikusa, Nagoya 464-8601, Japan ‡ Division of Liberal Arts and Sciences, Aichi Gakuin University, Iwasaki, Nisshin 470-0195, Japan S Supporting Information *

ABSTRACT: A CuII complex of bisamidine ligand LS catalyzes the Friedel−Crafts (FC) reaction of indole with trifluoropyruvate with high generality, yielding the highly enantiomerically enriched FC adduct. Electron-rich indoles have high reactivity due to a dual activation mechanism showing second-order kinetics for CuII, whereas indoles with a soft substituent at C(4) proceed at a rate that is three orders of magnitude lower via a coordination mechanism, which reverses the sense of enantioselectivity.

C

hiral organofluorine compounds constitute a core area of the development of pharmaceuticals and agrochemicals.1 For example, the chiral organofluorine efavirenz is widely used for AIDS treatment2 (Figure 1). In 2013, Zhou found that indole-substituted trifluoromethylethanol derivatives provide the same inhibition of the HIV-1 reverse transcriptase as efavirenz, attracting much attention from medicinal chemists as a new type of drug candidate for non-nucleoside reverse transcriptase inhibitors (NNRTIs).3 In view of the structural characteristics of these derivatives, the most direct synthetic approach is apparently enantioselective C−C bond formation between the nucleophilic C(3) of an indole and the electrophilic CF3(CO) carbonyl carbon via a catalytic asymmetric Friedel−Crafts (FC) reaction. Jørgensen reported the utility of a CuII complex of the bisoxazoline sp2N bidentate ligand (tBu-BOX in Figure 2) as a chiral Lewis acid in 2000,4,5a triggering an explosion in the number of reports on the FC reaction of indole with trifluoropyruvate.5−7 Although the reaction might seem to be established, there are still problems in terms of reactivity (5−10 mol %),5,6 substrate scope, and unambiguous determination of absolute configurations. Thus, the development of a highly reactive, selective, and productive catalyst aimed toward process chemistry in NNRTI research has remained at an early stage. With a clear focus on the frequently utilized CuII complex, in this work we investigated the utility of a CuII complex of our original bisamidine-type sp2N bidentate chiral ligand, (S,S)Naph-diPIM-dioxo-iPr (LS) (Figure 2).8 The Naph-diPIM system (naphthalene condensed with two dihydropyrroloimidazoles) looks similar to the privileged tBu-BOX ligand9 but is unique. Its strong metal ion capturing ability and enantiodiscrimination ability are realized through the characteristics of (i) the highly σ-electron-donating amidine sp2N atoms that are © 2018 American Chemical Society

Figure 1. Examples of NNRTIs.

Figure 2. tBu-BOX and (S,S)-Naph-diPIM-dioxo-iPr (LS).

fixed to the same side, similar to 1,10-phenanthroline; (ii) the ca. 90° bite angle that stabilizes various metal complexes; and (iii) the relatively large and clear C2 chiral reaction site, which is constructed by the 5,5,6,6,5,5 ring-fused rigid and planar structure with two 2,2-iPr-disubstituted dioxolane (dioxo-iPr) rings up and down at both ends of the plane. We investigated the catalytic performance of [CuLS](OTf)2 (CuLS) under the standard conditions of [indole (1a)] = [ethyl trifluoropyruvate (2)] = 100 mM, [Cu(OTf)2] = [LS] = 0.5 mM (substrate/catalyst (S/C) = 200; 0.5 mol % for 1a). Received: September 26, 2018 Published: October 31, 2018 7149

DOI: 10.1021/acs.orglett.8b03086 Org. Lett. 2018, 20, 7149−7153

Letter

Organic Letters The CuLS complex was prepared by mixing Cu(OTf)2 and LS in a 1:1 ratio in cyclopentyl methyl ether (CPME) at 25 °C for 3 h. The resulting clear solution of the CuLS complex was added to a CPME solution of 1a at 0 °C, and then 2 was introduced.10 Table 1 summarizes the results. The standard

Table 2. Substrate Scope for the (S,S)-Naph-diPIM-dioxoiPr/CuII-Catalyzed Friedel−Crafts Reactiona

Table 1. Catalytic Asymmetric Friedel−Crafts Reaction of Indole (1a) with CF3COCOOC2H5 (2) Using CuLS

conc. (mM) entry 1 2c 3 4d 5d 6e

a

[1a]0, [2]0

[CuLS]0

S/C

mol %

S:Rb

100 100 100 200 200 500

0.5 0.5 0.1 0.04 0.02 0.025

200 200 1000 5000 10000 20000

0.5 0.5 0.1 0.02 0.01 0.005

98:2 2:98 98:2 98:2 98:2 90:10

a

In all cases, Friedel−Crafts adduct 3a was quantitatively obtained.10 Determined by Flack parameter analysis and chiral HPLC analysis.10 c LR was used as the ligand. d2 h. e−20 °C, 48 h. b

conditions quantitatively afforded (S)-3a with a 98:2 enantiomeric ratio (er), and the enantiomeric catalyst gave (R)-3a (entries 1 and 2). The reaction was complete within 1 min.10 The S/C ratio could be increased from 200 to 1000, 5000, or 10000 without loss of enantioselectivity (entries 3− 5). Even with an S/C ratio of 20 000, the reaction reached completion to give 3a, albeit with a 90:10 er (entry 6). CPME was found to be the best solvent, and other CuII and metal precursors were not as effective as Cu(OTf)2.10 With tBu-BOX under the standard conditions, the reactivity declined by 1−2 orders of magnitude, and the enantioselectivity was reversed (93:7 S/R er from (S,S)-tBu-BOX with an up/down mode opposite to LS).10 Table 2 summarizes the generality of the catalyst. First, the reliability of the present asymmetric reaction was confirmed on a 20 g scale using 1a with S/C = 2000 (entry 1). The reactivity and enantioselectivity were sensitive to the properties and position of the substituent on indole 1. With C(4)-substituted indoles 1b−g, the catalyst performance deteriorated (entries 2−7), and the extent of deterioration was significant for 4CH3S- and 4-I-indole (1d and 1e), yielding 3b−g with a range of 52:48 to 12:88 er. The sense of enantioface selection of C(4)-sulfur- and -halogen-substituted substrates 1d−g was reversed from that of 1a, furnishing (R)-3d−g (see below). Both electron-donating and -withdrawing C(5) substituents could be introduced (entries 8−15). In particular, 5-CH3- and -CH3O-indole (1h and 1i) resulted in rapid full conversion to yield enantiomerically pure (S)-3h and -3i (entries 8 and 9). Introduction of I, Br, Cl, F, and CO 2 CH 3 at C(5) quantitatively afforded (S)-3j−n with 95:5 to 97:3 er (entries 10−14). Highly electron-withdrawing CF3 and NO2 groups decreased the efficiency (entries 15 and 16). Introduction of a CH3, Br, or CH3O substituent at C(6) or C(7) was also acceptable, giving (S)-3q, -3s, -3t, and -3u in high yields with 95:5 to 99:1 er (entries 17 and 19−21). With C(6)-CH3O and C(7)-Br-substituted indoles, the enantioselectivity was lowered

a

Conditions unless otherwise specified: [1] = [2] = 100 mM; [Cu(OTf)2] = [LS] = 0.5 mM; CPME; 0 °C; 1 h. bIsolated yields. c Determined by HPLC analysis. dAll of the absolute configurations were determined by Flack parameter analyses of the FC products and the derivatives, by derivatization of the FC products to the stereochemistry-determined compounds, or by VCD analysis.10 e20 g scale of 1a, [1a] = [2] = 400 mM; [Cu(OTf)2] = [LS] = 0.20 mM, 1 h. f16 h. g3 h.

to 71:29 to 82:18 er, and (S)-3r and -3v were obtained in >95% isolated yield (entries 18 and 22). Among the asymmetric FC reactions reported to date,5 the absolute configuration has been definitively determined only for (S)-3g, a product of 4-Cl-indole (1g) and 2.5a On the basis of those results, the absolute configurations of the FC products 3 in previous studies have been estimated by comparing either retention times in chiral HPLC analyses or optical rotations, leading to confusion and errors.10 Here, therefore, the absolute configurations of all of the FC products 3a−v were determined by X-ray analyses of the FC products and the derivatives, by derivatization of the FC products to the stereochemistrydetermined compounds, or by VCD analysis.10 As shown in Table 3, the kinetic behaviors of indole substrates 1a, 1l, 1o, 1p, and 1g changed in accordance with the electronic and steric properties of 1. The reaction with 1a was second-order for CuLS,11 negative-first-order for 1a, and first-order for 2 (entry 1). The same behavior was observed for indole 1l with an electron-donating 5-Cl substituent (entry 2), whereas the introduction of an electron-withdrawing sub7150

DOI: 10.1021/acs.orglett.8b03086 Org. Lett. 2018, 20, 7149−7153

Letter

Organic Letters Table 3. Reaction Orders and Rates in the Reactions of C(4)- or C(5)-Substituted Indoles 1 with CF3COCOOC2H5 (2)a nucleophilic substrate entry

1

C(4)

1 2

1a 1l

H H

3 4

1o 1p

5

1g

C(5)

reaction orders time for [2]0 full conv. (min)

absolute config. of 3

[CuLS]0

[1]0

H Cl

2 2

−1 −1

1 1

14 18

S S

H H

CF3 NO2

1 1

1 1

1 1

485 548

S S

Cl

H

1

1

1

1272b

R

a

Conditions: [1]0 = [2]0 = 200 mM; [CuLS]0 = 0.05 mM (0.025 mol %); CPME; 0 °C. b[CuLS]0 = 0.5 mM (0.25 mol %).

stituent (CF3 or NO2) at C(5) altered the reaction orders to first, first, and first (entries 3 and 4). Unlike 5-Cl-indole, the reaction of 4-Cl-indole (1g) showed first-, first-, and first-order kinetics (entry 5). The reactivity was also significantly changed: the indole substrates 1a and 1l required only 14− 18 min for full conversion to 3a and 3l in the presence of 0.025 mol % CuLS, whereas the reactivity of 1o and 1p was ca. 20fold lower; furthermore, 1g required >1200 min for reaction completion, even when 10-fold more CuLS (0.25 mol %) was used, and led to reversed enantioselectivity. Figure 3 presents a potential explanation for the differences in kinetic behavior and reactivity. A reaction with second-order (CuLS),11 negative-first-order (1a or 1l), and first-order (2) kinetics and high reactivity is thought to proceed via the dual activation of 1 and 2 by CuLS. A cation−π interaction between CuLS and 1 increases the acidity of the indole NH, enhancing the nucleophilicity of C(3) by facilitating deprotonation by the Lewis basic TfO−;12 in addition, the electrophilicity of the CF3(CO) carbonyl carbon of 2 is enhanced by coordination of 2 to the Lewis acidic CuLS. The synergistic effect is thought to be the reason for the high reactivity. The rate law of the kinetic scheme in Figure 3a is expressed by eq 1.10 For consistency with the observed rate orders (second, negativefirst, first), K1[1] ≫ 1 + K2[2] must be established (eq 2), indicating that the CuLS(1) complex is more easily generated than CuLS(2). Indoles 1o and 1p, which have an electronwithdrawing substituent, would not form the cation−π complex; therefore, generally considered simple kinetic behavior (first, first, first) would be observed. The rate law shown in eq 3 (Figure 3b) can be approximated as eq 4 (first, first, first) when the inequality of 1 ≫ K2[2] is established, consistent with the case shown in Figure 3a. Figure 3c shows the kinetic scheme for the reaction of 4-Cl-indole (1g). The nucleophilic C(3) carbon of 1g is located at the γ-position of the Cl atom at C(4), making it difficult to approach CuLS(2) and decreasing the reactivity by at least three orders of magnitued compared with 1a. The CuLS(1)(2) complex generated from CuLS(2) and 1 with K3 is thought to be involved in this case, establishing the rate law shown in eq 5. The 1 ≫ K2[2] + K2K3[1][2] inequality is satisfied, which explains the first-, first-, first-order kinetics. In the molecular structure of CuCl2LS in the crystal (Figure 4), the Cl1−Cu−Cl2 plane is distorted by 28° from the N1− Cu−N2 coordination plane in order to minimize steric repulsion between Cl and the dioxolane methine proton.

Figure 3. Presumed catalytic cycles and rate laws.

The Cl1−H distance of 2.73−3.07 Å, which is shorter than the sum of the van der Waals radii of the Cl and H atoms (3.34 Å),13 suggests that there is a balance between the inherently preferred square-planar (SP) geometry of CuCl2LS and the attractive H···Cl interaction, leading to distortion of the Cl1− Cu−Cl2 plane. Such a distorted SP geometry is also thought to occur in CuLS(2) to enable generation of 4 in Figure 5, where nucleophilic attack of 1 is impossible from either the top or bottom side because of the dioxo-iPr unit. By contrast, the tetrahedral-type intermediate 5, proposed by Evans14a and

Figure 4. Molecular structure of the CuCl2LS complex in the crystal. The crystals were obtained using LR and CuCl2.10 7151

DOI: 10.1021/acs.orglett.8b03086 Org. Lett. 2018, 20, 7149−7153

Organic Letters

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Jørgensen14b in a Diels−Alder reaction, would enable the nucleophilic attack of CuLS(1a) and other CuII-free 1 from the bottom-right area of the Re face, explaining the observed enantioselectivity. Complex 5 may adopt an SH, TMP, SMP, or TBP geometry.15,16 The intermolecular pathway via 5 is difficult for the sterically disadvantaged 1g; therefore, the distorted SMP-type catalyst−substrate complex 6 (X = Cl; CuLS(1)(2) in Figure 5) is thought to be involved.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03086. Experimental procedures for asymmetric FC reaction, Xray crystallographic analysis of Cu complexes, NMR spectral data of products, rate deduction, and kinetic experimental data (PDF) Accession Codes

CCDC 1867039−1867057 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by e-mailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Masato Kitamura: 0000-0002-0543-8795 Author Contributions §

T.P.L. and K.H. contributed equally.

Notes

Figure 5. Enantioface selection in the CuLS-catalyzed Friedel−Crafts reaction.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was aided by an Advanced Catalytic Transformation Program for Carbon Utilization (ACT-C; Grant JPMJCR12YC) from the Japan Science and Technology Agency (JST) and the Platform Project for Supporting Drug Discovery and Life Science Research funded by the Japan Agency for Medical Research and Development (AMED; Grant JP18am0101099). The authors thank Professor Eiji Yashima and Mr. Daisuke Taura at Nagoya University and Mr. Quan Vuong at the University of Tennessee for determination of the absolute configuration of 3o.

Coordination of the soft chlorine atom to the central CuII metal brings the nucleophilic C(3) atom closer to the CF3(CO) carbonyl carbon, providing an opportunity for the inherently less reactive 1g to react. This intramolecular nucleophilic attack of the C(3) carbon on the Si face of CF3(CO) gives (R)-3g, explaining the reversal of the enantioselectivity (Table 2 (gray-shaded) and Table 3). The indole substrates 1d−f, which have other soft atoms such as S, I, and Br, would also react with 2 via this route. In the Rucatalyzed hydrogenation of functionalized ketones, the halo group is known to act as a good directing group.17 In summary, we have developed a high-performance CuII catalyst for the FC reaction of various indoles with ethyl trifluoropyruvate, attaining a 2−3 orders of magnitude improvement in the reactivity. Our mechanistic study of the catalytic reaction revealed the following four points. First, the high reactivity is ascribed to the second-order kinetics for CuII that is expressed by dual activation of both indole and trifluoropyruvate, leading to an enhancement of the nucleophilicity and electrophilicity via a CuII cation−π interaction and coordination to another CuII, respectively. Second, the reaction of an electron-deficient indole that makes no cation−π interaction proceeds via generally considered first-order kinetics. Third, a C(4)-substituted indole is inherently less reactive because of the proximity of the substituent to the reacting C(3) carbon. Fourth, the indoles with a soft substituent (S, I, Br, or Cl) at C(4) proceed via a coordination mechanism, reversing the sense of enantioselectivity. The present study makes a significant conceptual advancement to our knowledge of the mechanism underlying the conventional and novel CuII-catalyzed FC reaction and will provide a firm basis for facilitating NNRTI research.

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DOI: 10.1021/acs.orglett.8b03086 Org. Lett. 2018, 20, 7149−7153