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Cobalt(III)-Catalyzed [5+1] Annulation for 2H-Chromenes Synthesis via Vinylic C-H Activation and Intramolecular Nucleophilic Addition Ramajayam Kuppusamy, Krishnamoorthy Muralirajan, and Chien-Hong Cheng ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00978 • Publication Date (Web): 16 May 2016 Downloaded from http://pubs.acs.org on May 17, 2016
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Cobalt(III)-Catalyzed [5+1] Annulation for 2H-Chromenes Synthesis via Vinylic C-H Activation and Intramolecular Nucleophilic Addition Ramajayam Kuppusamy, Krishnamoorthy Muralirajan, and Chien-Hong Cheng* Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan Supporting Information KEYWORDS: cobalt, 2-vinylphenols, allenes, C-H activation, 2H-chromenes
ABSTRACT: A new cobalt-catalyzed phenolic OH-assisted C–H functionalization of 2-vinylphenols with allenes to give various 2H-chromenes is described. It is the first time that allenes are used as the coupling partners in the cobaltcatalyzed C–H activation reactions. In most cases, cobalt-catalyzed oxidative annulation of arenes with alkenes or alkynes via C–H activation gave [4+2] or [3+2] cyclization products, but the present catalytic reaction afforded an oxidative [5+1] cyclization products with the allenes acting as a one carbon coupling partner. The catalytic reaction is proposed to proceed via the C–H activation of the vinyl group, allene insertion and an unusual intramolecular regioselective phenoxide addition.
The 2H-chromene structure is found as the central core in a wide range of natural products, pharmaceutically active intermediates and also in various biologically useful molecules (antiviral, antitumor, anti-inflammatory, antibacterial, fungicidal, and anti-HIV activity).1 The derivatives are also used as the photochromic materials, organic light emitting devices (OLEDs), fluorescence probes, and laser dyes.2 2H-Chromenes are known to be synthesized via Brønsted and Lewis acid/base catalysis, organocatalysis and transition metal-catalysis.3 In most cases, these reaction routes required multi-steps, pre-functionalized substrates and expensive noble metal complexes. Scheme 1. Cobalt-Catalyzed C–H Oxidative Annulation Reactions
Recently, Co-catalyzed C–H oxidative annulation of arenes has become a very attractive strategy for the synthesis of divergent N-heterocyclic compounds,4,5 because cobalt metal is earth abundant, less expensive and environmentally more friendly compared with noble metals. So far, only alkenes and alkynes are used as the πcomponent coupling partners for Co(II) or Co(III)catalyzed C–H oxidative annulation reactions (Scheme 1).4,5 A pioneer work involving Co(II) as the catalyst precursor via C–H oxidative annulation reactions for the synthesis of isoquinolone derivatives using N-(quinolin-8yl)benzamides and alkynes was demonstrated by Daugulis.4a We also reported a C–H activation reaction of N-
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(quinolin-8-yl)benzamides with alkenes to give [3+2] annulation products.4j Very recently, Co(III)Cp*-catalyzed C–H oxidative annulation reactions using alkynes as the coupling partners in the presence of internal and external oxidants were reported by Kanai5a-b, Ackermann5c, Sundararaju5d, Cheng5e-f and Glorius5g. Most of these oxidative annulation reactions resulted in [4+2] or [3+2] annulations after alkynes/alkenes insertion. Our continuous interest in cobalt catalyzed C–H activation reactions5e-f,6a-c and allene chemistry6d-g has motivated us to study the reaction of 2-vinylphenols and allenes using [CoCp*(CO)I2] as the catalyst. The reaction proceeds to give various substituted 2H-chromene derivatives via vinylic C–H activation and [5+1] annulation with the allene acting as a one-carbon source for the ring formation. To the best of our knowledge, this report is the first to use allenes as the coupling partners in cobalt-catalyzed C–H activation. Previously, an example of Rh-catalyzed C–H activation of 2-vinylphenols and coupling with allenes to afford 2H-chromenes is known.7a We started to examine the Co-catalyzed [5+1] annulation reaction by using 2-vinylphenol (1a) and buta-2,3dien-1-ylbenzene (2a) as the substrates (see Table 1). The reaction of 1a (0.20 mmol) with 2a (0.30 mmol) in the presence of [CoCp*(CO)I2] (10 mol %) and Ag2CO3 (0.20 mmol) in DCE at 30 °C under N2 for 12 h gave 2Hchromene 3aa in 25% yield (Table 1, entry 1). To optimize the reaction conditions, we used different solvents, additives and oxidants (entries 2-21). The best result was achieved using PhCl as the solvent, NaOAc as the additive and Ag2CO3 as the oxidant affording single regioisomeric product 3aa in 93% yield (entry 3). The other solvents like DCE, toluene, o-xylene and benzene are also effective to afford 3aa in 30-60% yield (entries 2 and 4-6). The studies on different additives show that OAc- containing additives are important for the present [5+1] annulation reaction; the others were mostly ineffective. Finally, 10 mol % of NaOAc gave an excellent result, 3aa in 93% yield (entry 3). Similarly, Cu(OAc)2·H2O and Mn(OAc)3·2H2O are also active to give 3aa in 86 and 85% yields, respectively (entries 7 and 9). Other acetate additives such as Cu(OAc)2, Mn(OAc)2, Fe(OAc)2, KOAc, CsOAc, AgOAc and AcOH were slightly less effective providing 3aa in 55-81% yields (entries 8 and 10-15). The selection of oxidant is also important for the success of this transformation. The addition of 1.00 equiv of Ag2CO3 to the reaction mixture results excellent yield of 3aa. Other oxidants like Ag2O, AgOAc, Cu(OAc)2·H2O and Mn(OAc)3·2H2O were less effective (entries 16-17 and 20-21) and no desired product was observed when AgBF4 and AgOTf were used as oxidants (entries 18-19). We then tested the catalytic activity of various cobalt complexes. It appears that only [CoCp*(CO)I2] is active; the other cobalt complexes like CoI2, CoBr2, Co(OAc)2 and Co(acac)3 did not show any catalytic activity under the standard reaction conditions. Similarly, we also performed the reactions using highvalent metal catalysts such as [RhCp*Cl2]2 and [IrCp*Cl2]2, which affords 3aa in 53 and 12% yields, respectively (see Supporting Information).
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Table 1. Optimization of Cobalt-Catalyzed Oxidative Annulationa,b OH
1a
•
[CoCp*(CO)I2] (10 mol %), additive oxidant, solvent, 30 °C, 12 h
O
3aa
2a
Entry
Additive
Solvent
Oxidant
Yield (3aa, %)
1
---
DCE
Ag2CO3
25
2
NaOAc
DCE
Ag2CO3
55
3
NaOAc
PhCl
Ag2CO3
93(89)
4
NaOAc
toluene
Ag2CO3
60
5
NaOAc
o-xylene
Ag2CO3
35
6
NaOAc
benzene
Ag2CO3
30
7
Cu(OAc)2·H2O
PhCl
Ag2CO3
86
8
Cu(OAc)2
PhCl
Ag2CO3
65
9
Mn(OAc)3·2H2O
PhCl
Ag2CO3
85
10
Mn(OAc)2
PhCl
Ag2CO3
70
11
Fe(OAc)2
PhCl
Ag2CO3
59
12
KOAc
PhCl
Ag2CO3
80
13
CsOAc
PhCl
Ag2CO3
55
14
AgOAc
PhCl
Ag2CO3
81
15
AcOH
PhCl
Ag2CO3
74
16
NaOAc
PhCl
Ag2O
38
c
NaOAc
PhCl
AgOAc
28
17
c
NaOAc
PhCl
AgBF4
---
c
NaOAc
PhCl
AgOTf
---
20
c
Ag2CO3
PhCl
Cu(OAc)2·H2O
21
c
Ag2CO3
PhCl
Mn(OAc)3·2H2O
9
18
19 21 a
Unless otherwise mentioned, all reactions were carried out using 1a (0.20 mmol), 2a (0.30 mmol), [CoCp*(CO)I2] (10 mol %), additive (10 mol %), oxidant (0.20 mmol), solvent (1.5 b mL) at 30 °C for 12 h under N2. Yields were determined by 1 the H NMR integration method; the value in the parenthesis c indicates isolated yield. 0.40 mmol of Oxidant was used. PhCl = Chlorobenzene, DCE = 1,2-Dichloroethane.
Scheme 2. The Scope of 2-Vinylphenolsa,b
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a
a
Unless otherwise mentioned, all reactions were carried out using 1 (0.20 mmol), 2a (0.30 mmol), [CoCp*(CO)I2] (10 mol %), NaOAc (10 mol %), Ag2CO3 (0.20 mmol) and PhCl (1.5 b mL) at 30 °C for 12 h. Isolated yield.
To know the scope of the present cobalt-catalyzed [5+1] annulation reactions, various substituted 2-vinylphenols and benzylallene (2a) were tested under the optimized reaction conditions (Scheme 2). Thus, the reactions of 4methyl- and 4-t-butyl-2-vinylphenols with 2a afforded the expected products 3ba and 3ca in 70 and 60%, respectively. Halo substituents on the aryl group of the 2vinylphenols worked very well with 2a under the standard reaction conditions to give 3da–ga in excellent yields. Interestingly, the reaction of 4-nitro substituted 2vinylphenol (1h) with 2a also delivered the expected product 3ha in 73% yield. However, an electron-donating MeO group at ortho or meta position of 2-vinylphenols drastically reduces the product yields (3ja and 3ma). A close comparison of the results in Scheme 2 reveals that meta and ortho substituted 2-vinylphenols gave lower yield of the expected products than the corresponding para one (3ia–na). It is noteworthy that the presence of an alkyl and aryl group at the internal carbon of the vinyl group of 2-vinylphenols (1o–r) did not significantly affect the reaction and gave excellent product yields up to 90%. However, the substituent at terminal carbon of the vinyl group totally inhibits the reaction.
Unless otherwise mentioned, all reactions were carried out using 1a (0.20 mmol), 2 (0.30 mmol), [CoCp*(CO)I2] (10 mol %), NaOAc (10 mol %), Ag2CO3 (0.20 mmol) and PhCl (1.5 b c mL) at 30 °C for 12 h. Isolated yield. Toluene.
We then probed the reaction with various allenes under the optimized reaction conditions and the results are presented in Scheme 3. The reactions of 2-Me-, 3-Me-, 4-Me and 4-Br-substituted benzylallenes (2b−e) with 1a provided [5+1] annulation products 3ab−ae in 68−81% yields. Similarly, 5-phenyl-1,2-pentadiene (2f) and 1-naphthylsubstituted allene (2g) also furnished the desired products 3af and 3ag in 64 and 38% yields, respectively. However, the reactions of long chain alkyl/aryl allenes (2h and 2i) with 1a provided 3ah and 3ai in 66 and 45% yields, respectively. The unsymmetrical allenes 2j and 2k were also tested at 30 °C and 80 °C to afford regio isomeric mixtures of 3aj+3aj′ and 3ak+3ak′ in reasonable yields. Finally, we have also tested allenes having tertiary and quaternary carbon at the α-position under the standard reaction conditions. The reaction of penta-3,4-dien-2ylbenzene (2l) with 2-vinylphenol (1a) afforded corresponding product 3al with less than 5% yield. This is because of the presence of a methyl group at α-carbon, which increases steric hindrance and thus reduces the chance of β-hydrogen elimination. Surprisingly, the reaction of 4,4-dimethylpenta-1,2-diene (2m) with 1a did not provided any cyclized or non-cyclized product. Scheme 4. Competition Experiments
Scheme 3. The Scope of Allenesa,b
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To further understand the nature and the mechanism of the present catalytic reaction, we performed two intermolecular competition experiments (Scheme 4). The result of the reaction of 1a with benzylallene 2a and aliphatic allene 2h suggest that electron rich allene 2h is less effective to undergo an intramolecular nucleophilic attack of the oxygen−cobalt bond to π-allylic intermediate. Similarly, the competition reaction of 2a with vinylic arenes 1c and 1h demonstrates that the reaction favors electron deficient vinylic arene 1h. The result indicates that the C−H metalation step is faster with more acidic protons of the vinylic C−H bond.8
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addition10 of the oxygen−cobalt bond to π-allylic intermediate, followed by protonation by AcOH forms intermediate VII. Subsequent β-hydride elimination of intermediate VII affords the expected 2H-chromene 3aa and Co(I). Finally, oxidation of Co(I) by Ag2CO3 to regenerate the active Co(III) complex for next catalytic cycle. It should be noted that two pathways have been proposed to account for the formation of intermediate IV from II. One is formed via a concerted metalation deprotonation (CMD) method (pathway A), while the other is an intramolecular attack of the alkene π-bond to Co(III) to give intermediate III followed by rearomatization (pathway B).7a,b However, the results of KIE experiments appear to favor the C−H metalation forms via a CMD pathway. Notably, cobalt-assisted nucleophilic addition to π-allyllic carbon to form a C−O bond and also nucleophilic addition to the center carbon of the allyl moiety are rare. The allene substrate acts as a one carbon source during the formation of the six-membered chromene ring in the reaction. Another pathway for the formation of product 3aa from VI via triene intermediate IX also cannot be excluded and is shown in Scheme 6. Scheme 6. A Plausible Reaction Mechanism
Scheme 5. Kinetic Isotope Experiments
Additionally, we performed deuterium-labeled kinetic experiments (Scheme 5). The parallel reactions of 1q and [D2]-1q with 2a gave kinetic isotope effect (kH/kD) value of 2.2. Also, the competition reaction of 1q, [D2]-1q and 2a provided kH/kD value of 2.7. Both experimental KIE values suggest that the C–H bond cleavage is probably involved in the rate-determining step.5 We also conducted the reaction in the presence of D2O under the standard reaction conditions affording 3aa-D1 in 12% yield of product with 27% (by 1H NMR) deuterium at one of the methyl protons. On the basis of the obtained results and previous cobalt(III)5 and rhodium(III)9 chemistry, a possible cobalt catalyzed [5+1] annulation mechanism for the formation of 3aa is shown in Scheme 6. Generation of an active cobalt(III) complex I from [Cp*Co(CO)I2], Ag2CO3 and NaOAc is followed by cyclocobaltation of substrate 1a to provide the six-membered cobaltacycle IV. Allene (2a) coordination and regioselective insertion give π-allylic cobaltacycle VI.6d-e,9h Then, intramolecular nucleophilic
To demonstrate the synthetic application of this [5+1] annulation products (Scheme 7), 3aa was treated with Pd/C as the catalyst under H2 balloon pressure to give the corresponding chromane derivative in 86% yield. The chromane structure is also found as an important core for many bioactive compounds such as tocopherols and tocotrienols. The cobalt-catalyzed [5+1] annulation offers a new effective route to synthesize different functional group containing six-membered chromene and chromane heterocycles in good yields. Scheme 7. Synthesis of Chromane from 2HChromene
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In summary, this study demonstrates a new cobaltcatalyzed [5+1] annulation reactions between 2vinylphenols and allenes to give a wide range of functional groups containing a chromene core in high yields. It is the first time using allenes as the coupling partners in cobalt catalyzed C−H annulation reactions and the proposed mechanism involves an intramolecular regioselective nucleophilic attack of the oxygen−cobalt bond at the center carbon of the π-allylic group to furnish a six-membered chromene heterocycles under mild reaction conditions. The asymmetric version of the current reaction is in progress.
ASSOCIATED CONTENT Supporting Information General experimental procedures, characterization details 1 13 and Hand C NMR spectra of new compounds. This material is available free of charge on ACS publication website at http://pubs.acs.org
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] ACKNOWLEDGMENT We thank the Ministry of Science and Technology of the Republic of China (MOST-104-2633-M-007-001) for support of this research.
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