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Selective Synthesis of Indoles by Cobalt(III)Catalyzed C-H/N-O Functionalization with Nitrones Hui Wang, Marc Moselage, María González, and Lutz Ackermann ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02937 • Publication Date (Web): 29 Feb 2016 Downloaded from http://pubs.acs.org on February 29, 2016
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ACS Catalysis
Selective Synthesis of Indoles by Cobalt(III)-Catalyzed C−H/N−O Functionalization with Nitrones Hui Wang, Marc Moselage, María J. González, and Lutz Ackermann* Institut für Organische und Biomolekulare Chemie, Georg-August-Universität, Tammannstraße 2, 37077 Göttingen, Germany Supporting Information Placeholder ABSTRACT: The redox-neutral annulation of alkynes by differently decorated nitrones set the stage for a step-economical access to indoles with ample substrate scope. The redox-neutral C–H/N–O functionalization process proceeded through kinetically relevant C–H activation by carboxylate assistance, and displayed an excellent siteand regio-selectivity with unsymmetrical nitrones and alkynes.
KEYWORDS: C–H activation, carboxylates, cobalt, indole, mechanism Substituted indoles are important structural motifs found in Notable features of our findings include (i) unprecedented compounds of relevance to medicinal chemistry, crop protecrobust cobalt(III)-catalyzed indole synthesis by C–H activation tion, and drug discovery, among others.1 As a consequence, (ii) isohypsic, i.e. redox-neutral, C–H/N–O functionalization there is a continued strong demand for the development of with easily-accessible nitrones 1 that avoid external oxidants, generally applicable de-novo syntheses of these heteroarenes.2 and (iii) – in contrast to a very recently reported rhodium(III)In this context, the recent years have witnessed the emergence catalyzed transformation16 – significantly improved selectivity of C–H functionalizations as increasingly powerful tools for patterns in C–H functionalizations with unsymmetrical althe assembly of heteroarenes,3 thereby avoiding the use of kynes 2 (Figure 1). prefunctionalized substrates.4 Specifically, versatile transition At the outset of our studies, we tested various reaction conmetal catalysts set the stage for the direct synthesis of indoles.5 ditions for the envisioned C–H/N–O functionalization with Despite undisputable advances, these protocols largely renitrone 1a and alkyne 2a (Table 1, and Tables S-1–S-3 in the quired precious second-row transition metals.6 In the past few Supporting Information). Preliminary experiments highlighted years, naturally-abundant base metal catalysts proved instruCp*CoI2(CO) to be the catalyst of choice, and indicated that mental for efficient C–H activation reactions,7 with major HFIP and TFE were most suitable among a variety of sol8 advances being accomplished by high-valent cobalt catalysts vents.17,18 While K2CO3 and PivOH as additives gave rather 9 10 11 as reported by Matsunaga/Kanai, Ackermann, Glorius, unsatisfactory results (entries 1, and 2), acetates proved to be Daugulis,12 and Chang,13 among others.14 In spite of this sigsuperior, with NaOAc being optimal (entries 3–5). The synnificant progress, a cobalt(III)-catalyzed de-novo synthesis of thesis of indole 3aa occurred in the absence of silver(I) salts indoles by C–H activation technology has thus far proven (entry 6), but was more effective when employing co-catalytic elusive. Within our program on base metal-catalyzed C–H amounts of AgSbF6 (entries 5–10). The isohypsic alkyne annufunctionalizations,15 we became attracted to devise protocols lation proceeded at lower reaction temperatures, yet best for cobalt(III)-catalyzed indole syntheses, on which we report yields were accomplished at 100 °C (entries 11–14). In recent herein. years, N-acyl amino acid ligands have been identified as powerful ligands in metal-catalyzed C–H functionalizations, as reported by Yu19 and our group.20 Thus, we conducted a comparative study with a series of amino acid ligands for the cobalt-catalyzed C–H/N–O functionalization of nitrone 1a (entries 16–27). Notably, Piv-Leu-OH emerged as a powerful ligand, efficiently delivering the desired indole 3aa (entry 27). Interestingly, the rhodium(III) complex [Cp*RhCl2]2 failed to Figure 1. Cobalt-Catalyzed Indole Synthesis provide the desired product 3aa in comparable yields (entry 28–30). Moreover, the key importance of the cobalt catalyst and the carboxylate additive for the C–H activation process was verified (entries 31 and 32).
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ACS Catalysis Table 1. Optimization of Alkyne Annulation a
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
entry 1
additive 1 AgSbF6
additive 2 K2CO3
T (°C) 120
3aa (%) 10
2
AgSbF6
HOPiv
120
66
3
AgSbF6
KOAc
120
86
4
AgSbF6
CsOAc
120
88
5
AgSbF6
NaOAc
120
90
6
---
NaOAc
120
62
7
AgPF6
NaOAc
120
82
8
AgBF4
NaOAc
120
80
9
AgOTf
NaOAc
120
82
10
AgOTs
NaOAc
120
86
11
AgSbF6
NaOAc
23
9
12
AgSbF6
NaOAc
50
46
13
AgSbF6
NaOAc
80
77
14
AgSbF6
NaOAc
100
92
15
AgSbF6
NaOAc
100
67b
16
AgSbF6
Piv-Ile-ONa
100
73
17
AgSbF6
Ac-Ile-ONa
100
83
18
AgSbF6
Ac-Leu-ONa
100
80
19
AgSbF6
Ac-IIe-OH
100
77
20
AgSbF6
Piv-IIe-OH
100
78
21
AgSbF6
Piv-IIe-ONa
100
80
22
AgSbF6
Fmoc-Met-OH
100
6
23
AgSbF6
Piv-Pro-OH
100
74
24
AgSbF6
Piv-Ala-OH
100
72
25
AgSbF6
Piv-Asn-OH
100
73 71
26
AgSbF6
Piv-Phe-OH
100
27
AgSbF6
Piv-Leu-OH
100
88
28
AgSbF6
NaOAc
100
13c
29
AgSbF6
HOPiv
100
31c
30
AgSbF6
CsOAc
100
40c
31
AgSbF6
NaOAc
120
--- d
AgSbF6
---
120
7
32 a
Reaction conditions: 1a (0.50 mmol), 2a (0.75 equiv), Cp*CoI2(CO) (5.0 mol %), additive 1 (10 mol %), additive 2 (20 mol %), HFIP (2.0 mL), T, 16 h. b 3 h. c Using [Cp*RhCl2]2 (2.5 mol %). d Without Cp*CoI2(CO). PMP = para-methoxyphenyl.
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With optimized reaction conditions being identified, we probed the scope of the cobalt(III) catalysis in the C–H/N–O functionalization with various nitrones 1 (Scheme 2). The most user-friendly cobalt catalyst showed a considerable chemo-selectivity, by fully tolerating important electrophilic functional groups, such as fluoro, chloro, bromo, ester or thiophene substituents. The intramolecular competition experiments with meta-substituted nitrones 1h and 1j bearing two inequivalent ortho-C–H bonds site-selectively delivered indoles 3ha and 3ja,9a,10b,g respectively, as the sole products.
R N
Cp*CoI 2(CO) (5.0 mol %) AgSbF 6 (20 mol %) additive (20 mol %)
Ph
H
Ph
+
O
Ph
R N H
HFIP, 100 °C, 16 h under air
Ph PMP
1
2
3
Ph
Ph
Ph F
Me Ph
Ph
Ph
N H
N H
N H
3aa NaOAc: 92% gram-scale: 82% Piv-Leu-OH: 88%
3ea NaOAc: 70% Piv-Leu-OH: 67%
3da NaOAc: 76% Piv-Leu-OH: 68%
Ph
Ph
Ph
Cl
Br Ph
Ph
Ph
N H
N H
3fa NaOAc: 70% Piv-Leu-OH: 76% O
Me H
3ha NaOAc: 78% Piv-Leu-OH: 89%
3ga NaOAc: 91% Piv-Leu-OH: 80%
Ph
Ph
EtO
N H
Ph N H 3ia NaOAc: 67% Piv-Leu-OH: 61%
S
Ph H
N H
3ja NaOAc: 82% Piv-Leu-OH: 77%
Scheme 2. C–H/N–O Functionalization with Nitrones 1 The versatile cobalt(III) catalysts also enabled the annulation of substituted tolane derivatives 2b–2f by the C–H/N–O functionalization process (Scheme 3). Thereby, a variety of 2,3-diaryl indoles 3 was accessed in a step-economical fashion.
Next, we explored the dependence of the catalytic efficacy on the C-substitution pattern of the nitrones 1 (Scheme 1). In stark contrast to a recently reported rhodium-catalyzed transformation,16 the PMP substituent proved to be superior to the bulky, and less atom-economical mesityl substitution pattern that proved to be mandatory for the rhodium(III) catalysis.16
Scheme 1. Influence of the C-Substitution Pattern
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ACS Catalysis Interestingly, the dialkyl-substituted alkyne 2l delivered 3,3disubstituted 3H-indole 4al (Scheme 5), which contrasts Chang’s22a report on rhodium(III)-catalyzed syntheses of indolines,16,22a while being in agreement with a more recent finding.22b
Scheme 5. Formation of 3H-Indole 4al The user-friendly nature of the C–H activation strategy was reflected by the use of a cationic single-component catalyst, thus avoiding the use of any additional silver salts (Scheme 6).
Scheme 3. Cobalt(III)-Catalyzed Annulation of Tolanes 2 In contrast to a recent rhodium-catalyzed indole synthesis,16 the expedient cobalt catalysis proved particularly effective for the challenging annulation of unsymmetrically substituted alkynes 2 in a regio-selective fashion (Scheme 4).21 Hence, the C–H/N–O functionalization process furnished products 3ag– 3hj, generally placing the aryl moiety proximal to the nitrogen heteroatom. It is particularly noteworthy that the unsymmetrical diaryl-substituted alkyne 2k delivered indole 3ak with excellent levels of regio control, while a rhodium(III) catalyst previously furnished the two regioisomers without a significant element of selectivity.16
Scheme 6. Single-Component Catalyst In consideration of the unique selectivity features of the cobalt(III)-catalyzed C–H/N–O functionalization, we became intrigued by unravelling the catalysts mode of action.17 To this end, intermolecular competition experiments revealed electron-rich nitrones 1 to be inherently more reactive. This finding can be rationalized in terms of a base (acetate)-assisted intramolecular electrophilic substitution-type (BIES)10a,23 mechanism being operative within the C–H cobaltation via carboxylate24 assistance (Scheme 7a). Electron-rich alkynes 2 were found to be preferentially converted (Scheme 7b), which is suggestive of a kinetically relevant alkyne coordination. In contrast to a rhodium-catalyzed alkyne annulation,16 we did not observe any H/D scrambling when using isotopically labeled co-solvents (Scheme 7c),17 while independent reactions with substrates 1a and [D]5-1a revealed a kinetic isotope effect (KIE) of kH/kD ≈ 2.7 (Scheme 7d). These observations provide strong support for a rate-determining C–H activation.
Scheme 4. Annulation of Unsymmetrical Alkynes 2
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Scheme 8. Plausible Catalytic Cycle In summary, we have described the first cobalt-catalyzed de-novo indole synthesis by C–H activation. The versatile cationic cobalt(III) catalyst proved to be broadly applicable in the alkyne annulation by easily accessible nitrones via C– H/N–O functionalization. The robust cobalt-catalyzed indole synthesis was characterized by an excellent functional group tolerance and a unique regio-selectivity in the annulation of unsymmetrical alkynes. Mechanistic studies provided strong support for a kinetically relevant C–H cobaltation through carboxylate assistance.
Scheme 7. Key Mechanistic Findings Based on these mechanistic studies we propose the catalytic cycle for the C–H/N–O functionalization to commence by a rate-determining C–H cobaltation of cationic cobalt(III)carboxylate complex 5 that generates metallacyle 6 (Scheme 8). The subsequent insertion of alkyne 2 delivers the intermediate 7, which thereafter undergoes N–O cleavage, along with C–O formation. The thus-formed cobalt(III) enolate 8 regenerates the catalytically active cationic complex 5 by proto-demetalation with HOAc. Thereby, protected orthoamino ketone 9 is formed, which upon hydrolysis and subsequent intramolecular condensation furnishes the desired indole 3.
ASSOCIATED CONTENT Supporting Information Experimental procedures, characterization data, and 1H and 13C NMR spectra for new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT Support by the European Research Council under the European Community’s Seventh Framework Program (FP7 2007– 2013)/ERC Grant agreement no. 307535, the CSC (fellowship to H.W.), and the DAAD (fellowship to M.J.G.) is gratefully acknowledged.
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