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Letter
Cp*Co(III) Catalyzed C(sp3)-H Bond Activation: A Highly Stereoand Regioselective Alkenylation of 8-Methylquinoline with Alkynes Malay Sen, Balakumar Emayavaramban, Nagaraju Barsu, J. Richard Premkumar, and Basker Sundararaju ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00612 • Publication Date (Web): 25 Mar 2016 Downloaded from http://pubs.acs.org on March 26, 2016
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Cp*Co(III)-Catalyzed C(sp3)-H Bond Activation: A Highly Stereoand Regioselective Alkenylation of 8-Methylquinoline with Alkynes Malay Sen,† Balakumar Emayavaramban,† Nagaraju Barsu,† J. Richard Premkumar,‡ Basker Sundararaju†,* †
Fine Chemical Laboratory, Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur - 208016, Uttar Pradesh, India. ‡ Center for Molecular Modelling, Indian Institute of Chemical Technology, Hyderabad – 500 067, India. ABSTRACT: Efficient, atom economical, highly regioselective C(sp3)-H bond alkenylation of 8-methylquinoline catalyzed by (Cp*)Co(III) is reported. A well defined, air stable, molecular cobalt catalyst Cp*Co(III) is employed for the first time in C(sp3)-H bond activation. The developed methodology is broadly applicable, tolerN ates variety of functional groups under mild conditions. Experimental Co R1 R1 R2 + N and DFT results suggest that the initial cyclometallation was occurred H R2 H via external base assisted concerted metallation deprotonation pathway.
KEYWORDS: sp3 C-H bond activation • Cobalt • Alkenylation • Quinoline • Trisubstituted olefins The Transition metal catalyzed functionalization of C-H bonds is considered as an atom and step economical route compared to traditional cross coupling that requires prefunctionalization in organic synthesis.1 Since the pioneering work of catalytic C-H bond functionalization reported by Fujiwara/Moritani, Murai, Murahashi, Jones and others, much time was devoted by chemists for the activation/functionalization of C(sp2)-H bonds.2-3 Only recently, there is growing interest to develop molecular catalyst that have the ability to activate C(sp3)-H bonds.4 In particular, catalytic C-H bond alkenylation is considered as an atom economical transformation as all atoms in reactants are retained in the product.5 Such a reaction was mostly employed for C(sp2)-H bonds with alkynes using precious metals such as Pd, Rh, Ru, Ir and Re.6 But earth abundant first row transition metals were seldom utilized for the above-mentioned transformation.7 Due to poor reactivity and selectivity, alkenylation of C(sp3)-H bond is rarely found in the literature,8 as the reactivity of C(sp3)-H bond is determined by strong coordination ability of incoming groups. Activation of C(sp3)-H bonds catalyzed by first row transition metals are at its infancy stage especially with Cobalt. Ceinin and Zhang reported functionalization of C(sp3)-H bonds intramolecularly proceed through outer sphere mechanism using Co(I) catalyst.9 Brookhart reported unusual intramolecular hydride transfer of cyclic amine via activation of C(sp3)-H bond catalyzed by Cp*Co(I).10 Very recently, Ge et al and Zhang co-workers independently reported the functionalization of unactivated C(sp3)-H bond under oxidative conditions using 8-aminoquinoline as a bidentate chelating group for amidation and cyclization with terminal alkynes at 150°C.11 Lately, Cp*Co(III) catalyst emerged as a promising first row
molecular catalyst for various C(sp2)-H bond functionalization (Scheme 1a).12
Scheme 1. Overview of Cp*Co(III) catalyzed C-H bond functionalization: a) C(sp2)-H bond activation with Co(III) - reported (b) C(sp3)-H bond activation with Co(III). Cyclometallated cobaltacycle has its unique nucleophilic character, high reactivity, and excellent selectivity. Further, our continued interest in developing C-H bond functionalization using first row late transition metal catalysts 13 envisage us to probe the possibility of activation of C(sp3)-H bonds using Cp*Co(III) as a catalyst.14 Recently, Wang et. al. reported C(sp3)-H bond alkenylation of 8-methylquinoline with alkynes under Rh(III) catalysis.15a However, the reaction requires substoichiometric amount of Copper(II) acetate (although the role of copper was not known) to achieve high yield.15b We herein report the first, highly regio- and stereo-selective, functional
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group tolerant, carboxylate assisted Cp*Co(III) catalyzed C(sp3)-H bond alkenylation of 8-methylquinoline with alkynes under mild conditions corresponds to cis-addition of C(sp3)-H bond to the triple bond (Scheme 1b). Table 1. Optimization studies and control experimentsa
Entry
Catalyst
[Ag]
Additive
Yield (%)
1
Cp*Co(CO)I2
AgSbF6
NaOPiv
57
2
Cp*Co(CO)I2
AgSbF6
NaOAc
39
3
Cp*Co(CO)I2
AgSbF6
Cu(OAc)2
28
4
Cp*Co(CO)I2
AgSbF6
PivOH
54
5
Cp*Co(CO)I2
AgSbF6
AgOPiv
64
6
Cp*Co(CO)I2
AgSbF6
AdCO2Na
69
7
Cp*Co(CO)I2
AgSbF6
AdCO2H
61
8
Cp*Co(CO)I2
AgSbF6
AdCO2H
68b
9
Cp*Co(CO)I2
AgSbF6
AdCO2H
84c
10
Cp*Co(CO)I2
AgOTf
AdCO2H
95
11
Cp*Co(CO)I2
AgPF6
AdCO2H
76
12
[Cp*CoI2]2
AgOTf
AdCO2H
89
13
Cp*Co(CO)I2
AgOTf
AdCO2H
77d
14
[Cp*RhCl2]2
AgOTf
AdCO2H
62
15
[Cp*IrCl2]2
AgOTf
AdCO2H
5
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20h where as PF6- gave only 76% yield (Table 1, entry 10-11). Slightly lower yield of 3a was obtained when we employed [Cp*CoI2]2 as dimer. We further demonstrated that the alkenylation product 3a could be obtained in good yield using 2 mol% of Cp*Co(III) catalyst along with 4 mol% of silver salt and 4 mol% of admantyl carboxylic acid as additive at 80° C in 20h (entry 13). Under our reaction conditions Rh(III) gave 62% and Ir(III) did not give any satisfactory result (entry 14-15).
a
All reactions were carried out under argon atmosphere unless otherwise stated using 1a/2a/[Co]/[Ag]/Additive in 0.26/0.2/0.01 /0.02/0.02 mmol in Trifluoroethanol (1.5 mL) at 80° C for 24h. b Reaction carried out for 36 h. cReaction carried out for 48 h. d2 mol% of [Co] and 4 mol% of each AgOTf, AdCO2H was used. Cp* =1,2,3,4,5-pentametheylcyclopentadienyl.
Optimization conditions for C(sp3)-H bond alkenylation of 8methylquinoline are summarized in Table 1. We started our investigation by operating 8-methylquinoline as a model substrate and 5-decyne as coupling partner, Cp*Co(CO)I2 as a catalyst (5 mol%), AgSbF6 as an additive (10 mol%) along with sodium pivalate (10 mol%) in trifluoroethanol at 80°C for 24h gave 3a as colourless oil in 57% yield (Table1, entry 1). Change of solvent did not improve the efficiency of the reaction (ESI, Table S1, entry 2-3). Among the additives we screened, bulkier carboxylate gave good yield (Table 1, entry 2-6). Both carboxylic acid and their sodium salt gave the similar yield; hence, we continued our optimization with adamantyl carboxylic acid as an additive (entry 7). Control experiments showed that all the three components such as [Co] catalyst, [Ag] salt and Carboxylic acids are necessary to obtain good yield (ESI, Table 1, entry 10-12). High yields were obtained when the reaction was carried out for longer reaction time (Table 1, entry 8-9). Finally, our persistent effort led us to find the best condition by altering counter ion from SbF6- to OTf- to facilitate the alkenylation product 3a in 95% yield in
Scheme 2. Scope of 8-methylquinolinea a
Yields are under the same reaction conditions.
With the optimized conditions in hand, we then investigated the scope of 8-methylquinoline using 5-decyne as alkyne source (Scheme 2). Due to the attractive biological properties of quinoline,16 we examined various substituted 8methylquinoline that are proceeded smoothly under our optimized conditions in good yield (3aa-3oa). Sensitive functional groups such as Br-, NO2- at various positions of 1 are tolerable and gave alkenylation product in moderate to good yield (3ba-3ca, 3na). Key functional groups, for example alkenes, alkynes, electron rich and electron poor arenes and naphthalene at C-5 position of 1a are amenable under our reactions conditions without compromise the yield of 3 (3da3ja). Substitution at various positions of quinoline (5-, 6-, -7) did not alter the reactivity (3ka-3oa). Further, we examined the steric parameter by employing 8-benzylquinoline as substrate but resulted in no product formation, which is in line with our results for selective mono alkenylation without formation of bis C(sp3)-H bond alkenylation. The alkenylation reaction can further be extended to other symmetrical and unsymmetrical internal alkynes using 1a as a coupling partner (Scheme 3). Alkyl substituted symmetrical
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alkynes gave very high yield (3ab) whereas biaryl substituted alkynes provided moderate yield (3ac-3ae).
+
N
R1
80oC,
nPr
Ph
Q
H
H
3ab; 85%
Ph
Ph
H
Q
Ph
Ph Q
H
H
3aj; 53% (9:1) b
Q
3al; 88% 3ak; 43%c (6.8:1) b,c with Rh(III); 71%
HO
R H
H
Q
N
+
R
H
R N
(3ka/3ca)
nBu
nBu
2a
Cp*Co(CO)I 2 - 5 mol% AdCO2H/Na - 10 mol% AgOTf - 10 mol%
N
DCE or TFE, 80o C, 24 h Cp*Co(CO)I 2 - 5 mol% AdCO2H/Na - 10 mol% AgOTf - 10 mol% CD3OD - 5 equiv. TFE, 80o C, 24 h
H/D
