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Switch to Allylic Selectivity in Cobalt Catalyzed Dehydrogenative Heck Reaction with Unbiased Aliphatic Olefins Soham Maity, Rajesh Kancherla, Uttam Dhawa, Ehtasimul Hoque, Sandeep Pimparkar, and Debabrata Maiti ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01816 • Publication Date (Web): 11 Jul 2016 Downloaded from http://pubs.acs.org on July 11, 2016
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ACS Catalysis
Switch to Allylic Selectivity in Cobalt Catalyzed Dehydrogenative Heck Reaction with Unbiased Aliphatic Olefins Soham Maity, Rajesh Kancherla, Uttam Dhawa, Ehtasimul Hoque, Sandeep Pimparkar, and Debabrata Maiti* Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Supporting Information Placeholder ABSTRACT: A unique C–H allylation has been discovered with unbiased aliphatic olefins. An intimate M–L affiliation between a high-valent cobalt-catalyst and amino-quinoline derived benzamides, has been found to be crucial for this unprecedented selectivity. An exemplary set of aliphatic olefins, high yields coupled with excellent regio- and stereoselectivity, and wide functional group tolerances are noteworthy. In addition, a catalytically competent organometallic Co(III) species has been identified through X-ray crystallography. This study is expected to facilitate new synthetic designs towards unconventional allylic selectivity with aliphatic olefins.
Keywords: Allylic selectivity, Aliphatic Olefins, Organometallic Co(III) intermediate, Stereoselective, High yields
Introduction. The Mizoroki-Heck coupling and Fujiwara-Moritani reaction have contributed tremendously to synthetic organic chemistry. The most notable feature of these transformations is exclusive styrenyl selectivity.1-6 However, the orthogonal allylic selectivity, which potentially can be accessed from the same metal-alkyl intermediate, remained relatively unexplored till date (Scheme 1).3b, 7 In this work, we disclose our research effort for achieving exclusive allylic selectivity with unbiased aliphatic olefins.
the reactivity at various stages and stabilize the metal in such a way that it could distinguish between the subtle difference of reactivity of available C–H bonds. Unfortunately, superior stability of both styrenyl product and the incipient intermediate leading to its formation render the selectivity switch to allylation even more challenging. A plausible way out for countering this problem could be anticipated by designing a conformationally strained system, which could effectively inhibit β-H elimination leading to styrenyl analogue. This necessitated an intimate metal-ligand coordination and subsequent conformational alteration wherein the metal, its oxidation state and size/shape of the metalacycle would play a crucial role. We envisioned that these requisite parameters can be met in amino-quinoline derived amides of benzoic acids with an appropriate transition metal in higher oxidation state.9-10 The structural requirement of a relatively less flexible 6,7-fused bicyclic metalacycle intermediate could inhibit the energetically more feasible elimination of benzylic hydrogen; consequently, an unprecedented regioselective switch could be realized (Scheme 2).
Scheme 2. Proposed model for the selectivity switch
Results.
Scheme 1. Different aspects of regioselectivity with aliphatic olefins
Inspired by our earlier study with unactivated aliphatic olefin allowing a conventional styrenyl mode of olefination, we envisaged that the transition metal has to be put into a similar rigid yet reactive envelope to manipulate the allylic selectivity.8 Additional donor center in the ligand backbone would augment
The choice of metal was extremely crucial for the anticipated outcome of the reaction. Relevant recent studies indicated that a cobalt catalyst might be suitable for this purpose.11 The facile insertion into aliphatic olefins, β-H elimination, and higher oxidation state resulting in a stronger Co–N bond, could support our hypothesis.12,13 To examine the feasibility of our proposal, different acid derivatives containing bi-dentate directing groups were tested with 1-octene in presence of catalytic amount of Co(OAc)2.4H2O, base and oxidant. In most cases a mixture of regioisomers or the preferential formation of styrenyl isomer was observed. Several amines have also exhibited very similar reactivity. Nevertheless, in view of scarcity of reports of cobalt catalyzed Heck-type reactions with aliphatic olefins, these results seemed very encouraging. Specifically, entries 1b and 1f (Figure 1) suggested that the thermodynamically less favored allylic isomers could be accessed although some con-
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straint has to be placed in order to prevent the facile elimination of benzylic hydrogen. Intriguingly, we found that aminoquinoline derived benzamides, with certain steric presence at the vicinity of amide carbonyl, could be chosen as model substrates for allylic selectivity with aliphatic olefins.
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olefins of different chain length (3a-3d) and other aliphatic olefins (3f, 3g and 3k) were indistinguishable in terms of regioselective outcome (exclusively allylic). The structural feature of 1-naphthoic acid itself was sufficient for the anticipated allylated products (3g-3i). Furthermore, compound 3g was unambiguously characterized through X-ray crystallography. A linear alkyl chain (3j), alkyl with carbonyl at distal position (3k), -CF3 (3n), -OMe (3p) and -iPr (3r) were well-suited with this new reactivity. Even much smaller -Cl substituent (3o) exhibited complete allylic selectivity with allylbenzene. Additionally, bulky -Me substituents (3f, 3k and 3r) and a fused cyclohexyl group (3l) at 5 position of the benzoic acid did not affect the yield or selectivity.
Figure 1. Reactivity with different directing group and substrate combinations After extensive optimization, Ag(I)-salts were found to be useful oxidants as Mn- and Cu-based oxidants remained unsuccessful. Of the solvents tested, relatively non-polar 1,2dichloroethane, chlorobenzene and trifluorotoluene seemed suitable for this reaction (Table 1). Presence of base was found to be essential and addition of 3 equiv Na2CO3 gave the desired product in 88% GC yield. Note that, conventional allylation reactions require a leaving group such as -OAc, -OPh, -Cl etc.1416
Table 1. Optimization of several reaction parameters
Entry
Catalyst
Oxidant
Solvent
Yield
1
Co(OAc)2.4H2O
AgOAc
DCE
20
2
Co(OAc)2.4H2O
Ag2CO3
DCE
41
3
Co(OAc)2.4H2O
Ag2SO4
DCE
47
4
Co(OAc)2.4H2O
Mn(OAc)2.4H2O
DCE
-
5
Co(OAc)2.4H2O
Mn(OAc)3.2H2O
DCE
17
6
Co(OAc)2.4H2O
Ag2CO3
PhCl
37
7
Co(OAc)2.4H2O
Ag2CO3
PhCF3
38
8
Co(OAc)2.4H2O
Ag2CO3
THF
33
9
Co(OAc)2.4H2O
Ag2SO4
DCE
88a
10
Co(acac)2
Ag2SO4
DCE
79a
11
CoI2
Ag2SO4
DCE
-
12
CoCl2
Ag2SO4
DCE
-
13
CoBr2
Ag2SO4
DCE
-
a
All reactions in 0.1 mmol scale. Reactions with 3 equiv Na2CO3.
To assess the generality of this new transformation, we tested several substituted benzoic acid derivatives with simple aliphatic olefins (Scheme 3). We found that relatively larger substituent such as -Me (3a, 82%) and -Ph (3e, 77%), which is supposed to have significant steric influence, exhibited excellent allylic selectivity with satisfactory yields. Interestingly, α-
Scheme 3. C–H allylation with unbiased aliphatic olefins;. ayield of gram-scale reaction in parenthesis.
In order to thoroughly outline the utility of this present method, we systematically examined a number of aliphatic olefins (Scheme 4). A wide variety of functional groups such as ester (4a, 78%), free-OH (4b, 75%), silyl ether (4c, 78%), -OTs (4d, 77%), -OBn (4e, 65%), protected diol (4f, 83%), epoxy (4g, 81%), -CN (4h, 72%) and amino acid (4i, 80%) containing olefins were amenable to the reaction condition. Cyclic and acyclic allyl olefins (4j, 4k and 4o), homoallyl benzene (4q) and even vinyl cycloalkanes (4l-4n) were found to be useful substrates. Presumably, the projection of tertiary C–H bonds at different angles in vinyl cycloalkanes was responsible for observed regioisomeric ratio (allylic: styrenyl) of varying extent.
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Scheme 5. Aliphatic olefins with additional tri-substituted double bonds. a a/s ratios from 1H NMR spectra. Scheme 4. Scope of aliphatic olefins. All the reactions were carried out in 0.25 mmol scale; aadditionally, 15% oxidized aldehyde product was obtained; byield of gram-scale reaction in parenthesis; c a/s ratios from 1H NMR spectra.
to classical styrene products. As a result, the other available hydrogen (Ha), was eliminated thus providing a unique way to achieve highly regio- and stereoselective C–H allylation starting from aliphatic olefins.
We further elaborated the scope of these reactions with several aliphatic olefins derived from natural products (Scheme 5). Specifically, tri-substituted olefin containing substrates, often considered problematic due to catalyst deactivation, were designed to evaluate the activity of cobalt catalyst. Gratifyingly, substituted olefin in tiglic acid (5a, 80%), electron rich olefin obtained from citronellol (5d, 77%), embedded olefin in cholesterol (5g, 62%) did not affect the efficacy of this new type of C– H allylation. Even substrates designed from linalool (5e) and nerol (5f), which contain additional spectator double bonds were found to be reactive under the optimal condition. Finally, 1, 7-octadiene with identical terminal olefins provided monoallylated product in 54% yield (5h).
Experimental Mechanistic Studies. In view of the unconventional selectivity observed in the present reaction, we tried to rationalize the origin of this regioselective switch. Interestingly, amides derived from unsubstituted as well as 3-bromo benzoic acid, gave a regioisomeric mixture consisting majorly of styrenyl isomer under identical reaction conditions (styrenyl: allylic = 4:1; Scheme 6, 6A). This observation clearly indicates that a common 7-membered insertion metalacycle (E, Scheme 13, vide infra) is a valid intermediate and the reaction is less likely to proceed through a prior abstraction of allylic C–H bond.17 We presumed that even subtle alteration in steric environment at the vicinity of amide carbonyl could induce a conformational flipping, thereby hindering the archetypal elimination of benzylic hydrogen leading
Scheme 6. Mechanistic investigations
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The nature of C–H elimination was probed with electronically diverse allyl alcohol derivatives.1d Intriguingly, electron rich allyl phenyl ether majorly provided allylic isomer (a/s = 4:1). On the other hand, with an electron withdrawing allyl acetate, allylic isomer was obtained only in slight excess. This observation likely suggested that the β-Ha elimination is hydridic in nature (Scheme 6, 6B). Labelling studies have been carried out with D+ source both in presence and absence of the coupling partner olefin. Analysis of the recovered starting material revealed that D could not be incorporated into the arene, thereby suggesting the C–H activation step to be irreversible in nature.
Kinetic Studies.
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Kinetic experiments were performed to determine the rate laws of the allylation reaction by measuring the GC yield of the product w.r.t. time. It was observed that the olefin exhibited a first order kinetics whereas amide is showing a partial (-ve) order (Scheme 7). Even more interestingly, the order w.r.t. catalyst was found to be 0.5 suggesting that a complex reaction pathway to be operative (see supporting information).
Radical Quenching Experiments. To gain some additional insight, a well-known radical quencher TEMPO was added to the reaction medium (Scheme 8). However, it was observed that the yield of the product remained unchanged, thus involvement of any radical species in the reaction may be ruled out.
Entry
Radical Quencher
Yield
1
-
73%
2
TEMPO (1 equiv)
75%
3
TEMPO (2 equiv)
69%
Scheme 8. Reactions with radical quenchers
Isolation of Reaction Intermediates. Although the control experiments and kinetic studies have helped us to shed some light into the plausible pathway, more efforts are still needed to understand the mechanism of this reaction. To this end, a 5-membered organometallic Co(III) metalacycle intermediate (C1) was identified which was characterized through X-ray crystallography, UV-Vis and ESI-MS analysis (Scheme 9, 9B and Scheme 10). Even more interestingly, the organometallic species (C1) was found to be catalytically competent; therefore the intermediacy of a related Co(III) system can be established in the present reaction.
Scheme 7. Rate and order of the reaction
Scheme 9. Synthesis, characterization, X-ray structure and catalytic competence of organometallic Co(III) species
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ACS Catalysis The relatively lower basicity of -OAc was not sufficient for the C–H activation step and additional base was required (entries 3 vs. 4, Scheme 11B). On the other hand Co(acac)2 could generate the organometallic species even in absence of a base (-acac being more basic than -OAc). These observations likely suggested that a base-assisted concerted metallation-deprotonation pathway (CMD) was responsible for the C–H activation step. Interestingly, a M–L Co(III) complex could be isolated from the same reaction mixture, which has been characterized through ESI-MS analysis and UV-Vis study (Scheme 12).
Scheme 10. UV-Vis spectra of complex C1
The cobalt complex C1 was obtained from a stoichiometric reaction in trifluoroethanol (TFE) solvent in presence of a base (NaOPiv) and an oxidant (K2S2O8). To look into further, we performed stepwise control experiments with two different Co-salts, which are most relevant in the present transformation: Co(OAc)2.4H2O and Co(acac)2. It was observed that the complex formation is dependent on the basicity of the counteranion (-OAc or -acac) or the presence of an exogenous base (Scheme 11, 11B).
Scheme 12. UV-Vis spectra of complex C3
Prolongled exposure in the reaction medium failed to completely covert complex C3 to complex C1. However, addition of equivalent amount of NaOPiv in the reaction mixture gave C1 as the sole complex. Therefore, some valuable informations could be gained about the oxidation states of the active catalyst in the catalytic cycle.
Proposed Mechanism. 11B. Control experiments
Entry
Co-source
K2S2O8
NaOPiv
Result
1
Co(OAc)2.4H2O
2
Co(OAc)2.4H2O
×
×
3
Co(OAc)2.4H2O
×
4
Co(OAc)2.4H2O
×
×
×
5
Co(acac)2
6
Co(acac)2
×
7
Co(acac)2
×
8
Co(acac)2
×
×
Scheme 11. Control studies and oxidation states of cobalt in the proposed catalytic cycle
In accordance with the experimental mechanistic studies and current literature, a plausible mechanism has been depicted in Scheme 13.
Scheme 13. Plausible mechanism for C–H allylation
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The reaction is initiated with a Co(III) species generated in-situ in the reaction medium, probably through a disproportionation step involving two Co(II) species. The complexometric study under stoichiometric conditions and the order w.r.t. catalyst (0.5) also support this hypothesis. Control experiments strongly suggest that this Co(III) species is responsible for the C–H activation step (C). A cyclometalated complex (analogous to C1) with an additional amide unit as bis-chelating ligand might also remain in equilibrium with the more competent species C which can account for the partial negative order of the reaction w.r.t. starting amide. Olefin co-ordination and subsequent 1,2migratory insertion resulting in a conformationally strained setup is likely responsible for the observed allylic selectivity. Reductively eliminated Co(I) is oxidized back to Co(III) by Ag(I) thus sustaining the continuity of the conceived catalytic cycle.
Conclusions. To summarize, we have developed a unique C–H allylation starting from aliphatic olefins. The highly unusual allylic selectivity has been observed with a diverse range of substrate combinations. Operational simplicity, high yields and excellent stereoselectivity are some of the most attractive features of this method. Specifically, this study demonstrates the power of appropriate M–L affiliation, and provides useful insights for future ligand design. A complete evaluation of this method, mechanistic understanding in detail and application of the current knowledge in other related set-up is presently ongoing in our laboratory. ASSOCIATED CONTENT Supporting Information Additional experimental procedures, X-ray crystallographic data (CIF file for compound 3g and complex C2) and spectroscopic data for synthesized compounds. This material is available free of charge via the Internet at http://pubs.acs.org. The authors thank Mr. Sujoy Rana and Mr. Arun Maji for X-ray crystallographic analysis and kinetic studies, respectively.
AUTHOR INFORMATION Corresponding Author
[email protected] Present Addresses Department of Chemistry, Indian Institute of Technology Bombay Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This activity is supported by SERB, India (EMR/2015/000164). Financial assistance provided by CSIR-New Delhi (fellowship to S. M.) and DST under the Fast Track Scheme (R.K.) is gratefully acknowledged.
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Gandeepan, P.; Rajamalli, P.; Cheng, C.-H. Angew. Chem. Int. Ed., 2016, 55, 4308. (13) For a recent review on Co-catalysis, see: Moselage, M.; Li, J.; Ackermann, L. ACS Catal. 2016, 6, 498. (14) For early refernces of conventional allylation, see: (a) Tsuji, J.; Takahashi, H.; Morikawa, M. Tetrahedron Lett. 1965, 6, 4387. (b) Heck, R. F. J. Am. Chem. Soc. 1968, 90, 5531. (c) Trost, B. M.; Fullerton, T. J. J. Am. Chem. Soc. 1973, 95, 292. (15) For selected recent references of conventional allylation reactions, see: (a) Oi, S.; Tanaka, Y.; Inoue, Y. Organometallics 2006, 25, 4773. (b) Fan, S.; Chen, F.; Zhang, X. Angew. Chem. Int. Ed. 2011, 50, 5918. (c) Yao, T.; Hirano, K.; Satoh, T.; Miura, M. Angew. Chem. Int. Ed. 2011, 50, 2990. (d) Makida, Y.; Ohmiya, H.; Sawamura, M. Angew. Chem. Int. Ed. 2012, 51, 4122. (e) Asako, S.; Ilies, L.; Nakamura, E. J. Am. Chem. Soc. 2013, 135, 17755. (f) Wang, H.; Schroeder, N.; Glorius, F. Angew. Chem. Int. Ed. 2013, 52, 5386. (g) Kim, M.; Sharma, S.; Mishra, N. K.; Han, S.; Park, J.; Kim, M.; Shin, Y.; Kwak, J. H.; Han, S. H.; Kim, I. S. Chem. Commun. 2014, 50, 11303. (h) Cong, X.; Li, Y.; Wei, Y.; Zeng, X. Org. Lett. 2014, 16, 3926. (i) Yu, D.-G.; Gensch, T.; de Azambuja, F.; Vásquez-Céspedes, S.; Glorius, F. J. Am. Chem. Soc. 2014, 136, 17722. (j) Suzuki, Y.; Sun, B.; Sakata, K.; Yoshino, T.; Matsunaga, S.; Kanai, M. Angew. Chem. Int. Ed. 2015, 54, 9944. (k) Gensch, T.; VasquezCespedes, S.; Yu, D.-G.; Glorius, F. Org. Lett. 2015, 17, 3714. (l) Deng, H.-P.; Wang, D.; Szabó, K. J. J. Org. Chem. 2015, 80, 3343. (m) Feng,
C.; Feng, D.; Loh, T.-P. Chem. Commun. 2015, 51, 342. (n) Moselage, M.; Sauermann, N.; Koeller, J.; Liu, W.; Gelman, D.; Ackermann, L. Synlett 2015, 26, 1596 (o) Cera, G.; Haven, T.; Ackermann, L. Angew. Chem. Int. Ed. 2016, 55, 1484. (16) For allylation with allene, see: (a) Zhang, Y. J.; Skucas, E.; Krische, M. J. Org. Lett. 2009, 11, 4248. (b) Zeng, R.; Fu, C.; Ma, S. J. Am. Chem. Soc. 2012, 134, 9597(c) Ye, B.; Cramer, N. J. Am. Chem. Soc. 2013, 135, 636. (17) (a) Luzung, M. R.; Lewis, C. A.; Baran, P. S. Angew. Chem. Int. Ed. 2009, 48, 7025. (b) Reed, S. A.; Mazzotti, A. R.; White, M. C. J. Am. Chem. Soc. 2009, 131, 11701. (c) Howell, J. M.; Liu, W.; Young, A. J.; White, M. C. J. Am. Chem. Soc. 2014, 136, 5750. (d) Pattillo, C. C.; Strambeanu, I. I.; Calleja, P.; Vermeulen, N. A.; Mizuno, T.; White, M. C. J. Am. Chem. Soc. 2016, 138, 1265. (e) Zhang, H.; Hu, R.-B.; Liu, N.; Li, S.-X.; Yang, S.-D. Org. Lett. 2016, 18, 28. f) Jiang, H.; Yang, W.; Chen, H.; Li, J.; Wu, W. Chem. Commun. 2014, 50, 7202.
TOC graphic.
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