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Cobalt(III)-Catalyzed Hydroarylation of Allenes via C-H Activation Sachiyo Nakanowatari, Ruhuai Mei, Milica Feldt, and Lutz Ackermann ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00207 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 2, 2017
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Cobalt(III)-Catalyzed Hydroarylation of Allenes via C−H Activation Sachiyo Nakanowatari, Ruhuai Mei, Milica Feldt and Lutz Ackermann* Institut für Organische und Biomolekulare Chemie, Georg-August-Universität, Tammannstraße 2, 37077 Göttingen, Germany ABSTRACT: The hydroarylation of allenes via C−H activation was achieved with a versatile cationic cobalt(III) catalyst. Stepand atom- economical access to alkenylated heteroarenes is thus provided in a highly chemo- and regioselective manner. Detailed mechanistic studies were conducted, including kinetic studies and Hammett analysis, suggesting the catalytic cycle being initiated with a reversible N-ligand dissociation followed by kinetically relevant arene C−H activation.
KEYWORDS: C–H activation, allenes, cobalt, heterocycles, alkenylation
Transition metal-catalysed hydroarylations1 of unsaturated C−C bonds via C−H activations2 offer direct access to alkenylated and alkylated arenes, forming a C−C bond with high levels of step- and atom-economy.3 Despite significant progress during recent years, catalysts based primarily on precious transition metals, such as rhodium, ruthenium, palladium and iridium, are typically required.4 Furthermore, these protocols are largely limited to alkynes and alkenes. In contrast hydroarylations of allenes continue to be scarce, despite their wide use in organic synthesis5-7 combined with the growing number of bioactive compounds and functional materials containing the allene moiety.7j,8,9 The first hydroarylation of allenes yielding allylated products was arguably disclosed by Krische, using a cationic iridium catalyst,10 while Ma identified powerful rhodium catalysis,11a which was subsequently exploited to asymmetric allylations by Cramer (Figure 1).11b Lately, notable efforts have been directed to replace noble transition metals by inexpensive, earth-abundant first-row 3d transition-metals as the catalysts.12 Particularly, high-valent cobalt catalysis13 has witnessed major progress,14 including successful hydroarylations with alkynes and alkenes, utilizing either low-valent15 or high-valent cobalt catalysts.16 In this context, Cheng, Maiti/Volla and Yu have very recently reported on the independent use of cobalt catalysis in C−H functionalisation of allenes for the assembly of heterocycles under oxidative conditions.17 Despite these indisputable advances, to the best of our knowledge, cobalt-catalyzed hydroarylations of allenes via C−H activation remain elusive. Within our program on sustainable C−H activation,18 we have developed the first cobalt-catalyzed hydroarylation of allenes, on which we report herein.
Figure 1. Catalytic hydroarylations via C−H activation.
We initiated our studies by probing various cobalt complexes in the hydroarylation of allene 2a with N-pyrimidylindole 1a (Table 1 and Table S1). The reaction proceeded smoothly in the presence of catalytic quantities of [Cp*CoI2(CO)], using AgSbF6 as the additive. Among a set of representative solvents, including 1,4-dioxane and 2,2,2-trifluoroethanol (TFE), 1,2-dichloroethane (DCE) was found to be optimal. The dimeric cobalt complex [Cp*CoI2]2 showed slightly higher catalytic activity, suggesting that the carbonyl ligand is not essential (entry 6-7). Control experiments revealed that the cobalt(III) complex was mandatory for C−H activation (entry 8), while the addition of catalytic amounts of metal acetates was found to inhibit the catalytic efficacy (entry 9).
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Table 1. Optimization of cobalt(III)-catalyzed C−H alkenylationa
H +
R1
R2 1
R3
[Cp*Col 2(CO)] (5.0 mol %) AgSbF6 (10-15 mol%) 1,4-Dioxane or 1,2-DCE 100-120 °C
R2
Het
R3
R1
2
3 Me
t-Bu
R
t-Bu
t-Bu
R n-Bu
n-Bu
N N
N
N
R = H (3aa): 92% R = F (3ba): 84% R = Br (3ca): 86%
Entry
[Co] (x mol %)
Solvent
3aa (%)
1
CoI2 (5.0)
1,4-dioxane
---b
2
[Cp*CoI2(CO)] (5.0)
1,4-dioxane
26c
3
[Cp*CoI2(CO)] (5.0)
1,4-dioxane
79
R
4
[Cp*CoI2(CO)] (5.0)
TFE
75
5
[Cp*CoI2(CO)] (5.0)
1,2-DCE
90
R = H (3ea): R = Me (3fa): R = t-Bu (3ga): R = OMe (3ha): R = CO2Me (3ia): R = F (3ja):
6
[Cp*CoI2]2 (2.5)
1,2-DCE
92
[Cp*CoI2]2 (1.25)
1,2-DCE
79
8
---
1,2-DCE
---b
[Cp*CoI2(CO)] (5.0)
9
1,2-DCE
---
N
N
N
R = H (3ab): 70% R = Me (3db): 82%
R
N
n-Bu
64% 54% 52% 58% 55% 61%
N
N
3da: 94%
R
N
N
N
N n-Pent
n-Bu
t-Bu
7
n-Pent
N
t-Bu
t-Bu R = R= R= R=
Me (3ka): 64% OMe (3la): 56% (83:17) CF3 (3ma): 61% Cl (3na) : 41%
R = OMe (3lb): 54% (85:15) R = F (3ob): 52%
Me
d
a
Reaction conditions: 1a (0.50 mmol), 2a (0.72 mmol), [Co] (5.0 mol %), AgSbF6 (10 mol %), solvent (1.0 mL), 100 °C, 20 h, under nitrogen. Yield of isolated products given. b GC conversion using n-dodecane as the internal standard. c Without AgSbF6. d With NaOAc (10 mol %).
N R
R = H (3pa): R = OMe (3qa):
54% 53%
N n-Pent
n-Bu
t-Bu
t-Bu
t-Bu 3ra:
57%
3sb: 43%
N N N
N
N
t-Bu
N
R1
n-Bu
3ta: 65%
With these optimized reaction conditions in hand, the versatility of the cobalt(III)-catalyzed hydroarylation with different allenes 2 was tested. To our delight, the cobalt(III) metal catalyst was widely applicable to include variously decorated arenes and indole derivatives 1, yielding the alkenylated products 3 with excellent levels of positional selectivity control (Scheme 1). Substituents, such as fluoro, chloro, bromo, or ester, were well tolerated by the robust catalyst. High siteselectivity was observed with meta-substituted arenes (1k–1o, 1s),19 giving the desired alkenylated products at the sterically less hindered position. Allenes featuring bulky alkyl groups gave the alkenylated products (3eb and 3ec), while di-n-alkyl substituted allene provided the allylated product (3ed). In contrast, mono-substituted allenes, 1,3-di- and 1,1,3-trisubstituted allenes, as well as aryl allenes thus far gave unsatisfactory results.
F
N n-Bu
R
2
R1 = n-Pent, R2 = t-Bu (3eb): 64% R1 = n-Bu, R2 = c-Hep (3ec): 54%
n-Bu n-Bu 3ed: 66%
Scheme 1. Cobalt(III)-catalyzed hydroarylation of allenes
To gain insights into the reaction mechanism, a series of experiments was carried out (Scheme 2). To this end, subjecting substrate 1a to the optimized reaction conditions in the presence of CD3OD demonstrated a significant H/D scrambling at the C-2 and C-3 positions, after reisolation of the starting material. In contrast, no H/D exchange was observed in the presence of CD3OH, indicating an organometallic - and not a radical - mode of C–H activation. Analogous experiment using substrate 1e showed no scrambling of the hydrogen atoms on the phenyl ring. Instead, significant H/D exchange was observed at the somewhat more acidic C-4 site of the pyrazole ring through an electrophilic mode of action. C−H activations with the isotopically labeled substrates [D]1-1a and [D]5-1e afforded the alkenylation products 3aa and 3ea with both arene and allene H/D exchange. A kinetic isotope effect (KIE) value of kH/kD = 2.2 was determined by initial rates for independent reactions with the substrates 1e or [D]5-1e, which can be rationalized with a inetically relevant C–H activation likely proceeding via a ligand-to-ligand hydrogen transfer.20 An independently prepared allylated indole21 4a did not isomerize under the reaction condition, implying that isomerization is not occurring subsequent to a potential C–H allylation. These observations suggest that the catalyzed C–H alkenylation directly occurs after C–C bond formation.
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(Figure 2c). These data indicates that the reaction is zero-order in the allene and provides support for a rate-determining C−H metalation. (a) log(∆[3ea]/∆t) (mM/sec)
1.00 0.70 0.40 0.10 n = 1.01 ± 0.05 R² = 0.99
-0.20 -0.50 0.20
0.50
0.80
1.10
1.40
1.70
log([Co]) (mM)
log(∆[3ea]/∆t) (mM/sec)
(b) -2.30 -2.60
n = −1.22 ± 0.11 R² = 0.98
-2.90 -3.20 -3.50 -3.80 -1.30
-1.00
-0.70
-0.40
-0.10
log([1e]) (M)
(c) log(∆[3ea]/∆t) (mM/sec)
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0.80 0.60 0.40 0.20 0.00 -1.10 -0.90 -0.70 -0.50 -0.30 -0.10 log([2a]) (M)
Figure 2. Kinetic analysis of the cobalt(III)-catalyzed C−H alkenylation.
Scheme 2. Summary of key mechanistic findings.
To further probe the turnover-limiting step of the C−H alkenylation, the kinetic order of each reaction component was established by studying the initial rates (Figure 2). The linear plot of log(kobs) versus the logarithm of the concentration of the cobalt catalyst demonstrated the reaction to be the first order in the cobalt catalyst (Figure 2a). The order of the reaction in substrate 1e was found to be inverse first-order (Figure 2b), implying that reversible dissociation of heteroarene substrates occurs prior to the slow cleavage of the C−H bond. The initial rate was independent of the concentration of the allene 2a over a wide range of concentrations
Next, to gain insights into the electronic effects on these transformations, a set of experiments with electronically differentiated arenes 1 were carried out. A Hammett-plot analysis22 using substituted phenylpyrazoles 1e–1m showed a linear fit with a positive slope of ρ = +0.66, being indicative of increasing reaction rate with more electron-deficient arenes 1 (Figure 3).
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product 3 proceeds through ligand exchange with a further molecule of arene 1.
0.4 0.3
y = 0.6643x - 0.0974 R² = 0.9185 4-CO2Me
0.2 log(kX/kH)
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0.1 0 -0.1 -0.2 -0.3 -0.4 -0.3
-0.1
0.1
0.3
0.5
0.7
σ Figure 3. Hammett-plot analysis for substituted phenylpyrazoles 1e–1m.
To complement our experimental studies, density functional theory (DFT) calculations were conducted for substrate 1a23 (Figure 4, Figure S1 and S2). The insertion of the terminal allene double bond into the Co−C bond via transition state TS1-2 is followed by the formation of cobalt intermediate IM4, which finally transfers the Hα to the γ carbon via the adjacent β carbon.
Scheme 3. Plausible catalytic cycle
In conclusion, we have reported on the first cobalt-catalysed hydroarylation of allenes via C−H activation. The use of sustainable cobalt(III) catalysis set the stage for a novel method of C–H alkenylation with high atom economy. The catalytic system showed broad functional group tolerance, along with excellent elements of chemo and positional selectivities. Detailed mechanistic studies were performed, which provided valuable insights into the Cp*Co(III)-catalyzed hydroarylation by kinetically relevant C–H activation.
AUTHOR INFORMATION Corresponding Author * Institut für Organische und Biomolekulare Chemie, GeorgAugust-Universität, Tammannstraße 2, 37077 Göttingen, Germany. Email:
[email protected]. Figure 4. Calculated energy profile in DCE with energies given in kcal mol-1.[23]
Notes The authors declare no competing financial interest.
ASSOCIATED CONTENT Based on our detailed mechanistic studies and literature precedence, a putative mechanism for the C−H alkenylation was proposed in Scheme 3. First, [Cp*Co(CO)I2] will generate the cationic Cp*Co(III) species 5 by the action of the AgSbF6 additive and coordination of two molecules of pyrazole 1. The plausible catalytic cycle is initiated by reversible dissociation of substrate 1, followed by irreversible C−H cobaltation proceeding by means of ligand-to-ligand hydrogen transfer, delivering the five-membered metallacycle 6. Thereafter, subsequent coordination of allene 2 occurs, and the intermediate 7 then undergoes a migratory insertion at the terminal position of the allene to generate intermediate 8, which is followed by double bond isomerization and protonation (9) in the coordination sphere of the cobalt catalyst. A final decobaltation step to deliver the alkenylated
Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Experimental procedures, characterization data, 1H and 13C NMR spectra for new compounds and additional information on mechanistic experiments and DFT calculations (PDF)
ACKNOWLEDGMENT Generous support by the European Research Council under the European Community’s Seventh Framework Program (FP7 2007– 2013)/ERC Grant agreement no. 307535, the JASSO (fellowship to S. N.) and the CSC (fellowship to R. M.) is gratefully acknowledged. We further thank Marcus Thater, Dr. Michael John and Dr. Alan James Reay for experimental support and helpful discussion.
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