Cobalt-Catalyzed Intramolecular Alkyne/Benzocyclobutenone Coupling: C-C Bond Cleavage via a Tetrahedral Dicobalt Intermediate

Article abstract of DOI:10.1021/acscatal.7b03852

A Co(0)-catalyzed intramolecular alkyne/benzocyclobutenone coupling through C-C cleavage of benzocyclobutenones is described. Co₂(CO)₈/P[3,5-(CF₃)₂C₆H₃]₃ was discovered to be an effective metal/ligand combination, which exhibits complementary catalytic activity to the previously established rhodium catalyst. In particular, the C8-substituted substrates failed in the Rh system, but succeeded with the Co catalysis. Experimental and computational studies show that the initially formed tetrahedral dicobalt-alkyne complex undergoes C1-C2 activation via oxidative addition with Co(0), followed by migratory insertion and reductive elimination to give the β-naphthol products.

Full text of DOI:10.1021/acscatal.7b03852

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Letter
Cobalt-Catalyzed Intramolecular Alkyne/Benzocyclobutenone
Coupling: C-C Bond Cleavage via a Tetrahedral Dicobalt Intermediate
Zixi Zhu, Xinghan Li, Sicong Chen, Penghao Chen, Brent Allen Billett, Zhongxing Huang, and Guangbin Dong
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03852 • Publication Date (Web): 22 Dec 2017
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CobaltꢀCatalyzed Intramolecular Alkyne/Benzocyclobutenone Couꢀ
pling: C−C Bond Cleavage via a Tetrahedral Dicobalt Intermediate
Zixi Zhu, † Xinghan Li, ‡ Sicong Chen, † Pengꢀhao Chen, ‡ Brent A. Billett, † Zhongxing Huang,†,‡ and
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Guangbin Dong*†,‡
†Department of Chemistry, University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637, United States
‡Department of Chemistry, University of Texas at Austin, Austin, Texas 78712, United States
Supporting Information Placeholder
ABSTRACT: A Co(0)ꢀcatalyzed intramolecular alkyne/benzocyclobutenone coupling through C−C cleavage of benzocyclobuꢀ
tenones is described. Co2(CO)8/P[3,5ꢀ(CF3)2C6H3]3 was discovered to be an effective metal/ligand combination, which exhibits
complementary catalytic activity to the previously established rhodium catalyst. In particular, the C8ꢀsubstituted substrates failed in
the Rh system, but succeeded with the Co catalysis. Experimental and computational studies show that the initially formed tetraheꢀ
dral dicobaltꢀalkyne complex undergoes C1−C2 activation via oxidative addition with Co(0), followed by migratory insertion and
reductive elimination to give the βꢀnaphthol products.
KEYWORDS: C−C activation, benzocyclobutenones, cobalt catalysis, cyclization, βꢀnaphthol.
Catalytic carbon−carbon bond (C−C) activation has emerged as
a useful tool for quickly building molecules with high complexiꢀ
ty.1 Numerous elegant C−C activation transformations have been
developed to date; however, the majority of them require the use
of noble transition metals (TMs), such as Rh, Ru, Pd and Ir.
Clearly, it would be highly desirable if the same transformations
can be catalyzed by earth abundant firstꢀrow TMs, which would
offer a more cost effective option. In addition, complementary
reactivity could be expected due to distinct properties of firstꢀrow
TMs from noble metals. Despite the recent advance of Ni catalyꢀ
sis in various C−C activations,1rꢀu using other firstꢀrow TMs as
catalysts has been rare.2 Our laboratory has been focusing on
developing catalytic methods based on activation of ketone αꢀ
C−C bonds, and our prior works exclusively relied on using Rh
catalysts.3 Herein, we disclose the first Co(0)ꢀcatalyzed C−C actiꢀ
vation of ketones,4 in which an intramolecular coupling between
benzocyclobutenones and alkynes was realized with an inexpenꢀ
sive cobalt precatalyst (Scheme 1). Preliminary mechanistic study
suggests that the reaction is promoted by a tetrahedral dicobaltꢀ
mediated C−C cleavage.
We have been engaged in the systematic development of “cut
and sew” transformations1p for building bridged and fused rings,
which involves oxidative addition of a TM into a C−C bond, folꢀ
lowed by 2πꢀinsertion and reductive elimination. A suite of methꢀ
ods has been demonstrated for intramolecular “cut and sew” reacꢀ
tions with benzocyclobutenones,3a,b,d,m where insertion occurred
selectively at the C1ꢀC2 position. For example, the coupling with
alkynes generates fused βꢀnaphthols (Scheme 1A).3d The DFT
study by Liu and coworkers suggests that the reaction first
Scheme 1. “Cut and Sew” reaction between alkynes and benꢀ
zocyclobutenones.
cleaves the C1ꢀC8 bond, and then through a decarbonylaꢀ
tion/reinsertion sequence delivers Rh to the C2 position.5 Hence,
one major limitation with the Rh system is that substitution at the
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C8 position is not well tolerated due to the steric constrain in the
creased the yield, which is possibly caused by the steric hindrance
around the alkyne (2g). In general, less bulky and more electronꢀ
rich alkyne moieties gave higher yields. Both electronꢀdonating
(2t) and withdrawing (2s) substituents on the benzocyclobutenone
rings were well tolerated. Unfortunately, substrates with a longer
tether (1u) did not undergo the “cut and sew” transformation,
instead only giving alkyne trimerization products.8 Unsurprisingly,
the less reactive olefinꢀtethered substrate (1v) gave no reaction
under the catalytic conditions.3a
initial C−C cleavage step, which became a motivation to discover
alternative catalytic systems. During our total synthesis of cycloiꢀ
numakiol, a trace amount of the desired “cut and sew” product
was detected when stoichiometric Co2(CO)8 was used.6 While the
reaction was not catalytic, it inspired us to explore the use of
Co2(CO)8 as the precatalyst for the intramolecular alꢀ
kyne/benzocyclobutenone coupling (Scheme 1B).
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2
3
4
5
6
7
8
Table 1. Control Experiments.
Table 2. Substrate scopea
9
O
7.5 mol% Co2(CO)8
36 mol% P(3,5ꢀC6H3(CF3)2)3
20 mol% pyridine Nꢀoxide
O
R
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R
O
X
X
1,4ꢀdioxane, 15h, 110 o
C
OH
8
8
R'
R'
1a-t
2a-t
O
O
O
O
Me
Et
nPr
OH
OBn
OH
OH
OH
2a, 85%
with Rh: 93%
2b
2c
, 64%
, 96%
2d, 65%
with Rh: 67%
Me
O
O
O
Ph
Et
OTIPS
OH
OH
OH
2f, 73%
with Rh: 56%
2e, 77%
2g, 48%
O
Ar =
OMe
Cl
CH3
Ar
aUnless otherwise noted, yields were determined by 1H NMR. b
OH
2k, 72%
2h, 81%
2j, 73%
2i, 78%
o
Obtained by heating Co2(CO)8 alone in 1,4ꢀdioxane at 130 C for
with Rh: 86%
with Rh: 93%
6h. c18 mol% of dppb was used. dIsolated yield.
tBu
F
CF3
CO2Me
Benzocyclobutenone 1a with an alkyne moiety was employed
as the model substrate. After a careful survey of the reaction paꢀ
rameters (See Table S1, Supporting Information), the desired βꢀ
naphthol product (2a) was afforded in 88% yield using
Co2(CO)8/P[3,5ꢀ(CF3)2C6H3]3 as the metal/ligand combination
and pyridine Nꢀoxide as the additive. Control experiments were
carried out to understand the role of each reactant (Table 1). In the
absence of Co2(CO)8, no desired product was observed (entry 2).
The reaction without pyridine Nꢀoxide or the ligand still afforded
the desired product albeit in a lower yield (entries 3 and 4). Their
roles are proposed to create open coordination sites7 and assist
catalyst dissociation. Co(II) complexes, such as CoBr2 and salen
Co complex, are not reactive (entries 5 and 6). On the other hand,
cobalt nanoparticles, obtained from the decomposition of
Co2(CO)8, showed no catalytic activity (entry 7), suggesting that
the reaction is likely catalyzed by homogeneous Co species. The
use of other monoꢀdentate phosphine ligands, such as PPh3 or
SPhos, slightly lowered the yield (entries 9 and 10); in contrast,
bidentate ligands, such as dppb, significantly inhibited the reacꢀ
tion, probably through blocking reactive sites on Co (entry 8).
Lowering the reaction temperature to 90°C slightly decreased the
yield (entry 11). Surprisingly, raising the reaction temperature to
130°C gave a much lower yield (entry 12), likely due to the accelꢀ
erated decomposition of the Co catalyst or competitive alkyne
trimerization reaction.8 In the previous Rh system, adding ZnCl2
was found to be beneficial to the reactivity;3d however, it was
detrimental to the Co catalyst, though the exact reason remains
unclear (entry 13).
2l, 77%
2o, 69%
2n, 51%
2p, 49%
2m, 93%
with Rh: 77%
O
O
O
Et
OH
Et
Et
MeO2C
OH
Me
OH
Me
2q, 48%
with Rh: 23%
2t, 82%
with Rh: 83%
2s, 58%
with Rh: 78%
Et
O
O
O
Me O
no
alkyne
trimerization
reactivity
1u
1v
a All yields are isolated yields. The yields with Rh are from ref.
3d. Others are all new substrates.
Compared to the Rh system, the Co catalysis gave comparable
yields but at a lower reaction temperature (110 vs 130 C). Howꢀ
o
ever, one most significant difference is in the case of C8ꢀ
substituted benzocyclobutenones. Using [Rh(cod)Cl2]2ꢀdppp, 8ꢀ
methylꢀsubstituted substrate 1q exhibited low reactivity and only
afforded 23% yield; in contrast, the Co system gave a more than
doubled yield of product 2q. In addition, 8,8ꢀdimethly substituted
substrate 1r was completely unreactive with the Rh system owing
to the bulkiness of the C1−C8 bond; however, the Co catalysis
successfully provided the corresponding benzocyclohexenone
product (2r) in 52% yield (Scheme 2). The enhanced reactivity of
the Co catalyst with 8ꢀsubstituted benzocyclobutenones suggests
that the reaction with Co is unlikely initiated by cleavage of the
C1−C8 bond; therefore, it may operate through a different mechꢀ
anistic pathway from the Rh system.
With the optimized conditions in hand, the scope of this reacꢀ
tion was examined (Table 2). Similar to the [Rh(cod)Cl]2ꢀdppp
system,3d substrates containing alkyl, aryl, or alkenyl alkynes
were converted into the corresponding βꢀnaphthols in good to
excellent yields. Numerous functional groups, such as aryl chloꢀ
ride (2k), ethers, esters (2o), olefins (2f), benzylꢀprotected alcoꢀ
hols (2d), and triisopropylsilyl (TIPS) ethers (2e) were well tolerꢀ
ated. Introducing a methyl group at the propargylic position deꢀ
2
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Scheme 2. Unique reactivity with the 8,8ꢀdimethly substituted
substrate
The unique reactivity of the Co system motivated us to explore
the reaction mechanism. Consequently, a number of experiments
were carried out. Analogous to the Pauson−Khand reaction,9 mixꢀ
ing of Co2(CO)8 and the substrate (1a) at room temperature rapidꢀ
ly generated a tetrahedral dicobaltꢀalkyne adduct (S2) (Scheme
3a). This species was found to be catalytically active for the inꢀ
tramolecular benzocyclobutenone alkyne coupling, giving compaꢀ
rable yields as those with Co2(CO)8 (Scheme 3b). Considering the
established high stability of the dicobaltꢀalkyne adduct,10 it is
reasonable to assume that this intermediate may be involved in
the catalytic cycle. On the other hand, running the reaction under
a
13CO atmosphere yielded the desired product without much 13C
incorporation. Given that the exchange between free CO and the
coordinated CO on Co2(CO)8 is known to be facile,11 if the reacꢀ
tion proceeded through cleavage of the C1−C8 bond first, the
subsequent CO desertion and reꢀinsertion would have introduced
substantial 13C into the product. The absence of significant 13C
incorporation disfavored the pathway involving cleavage of the
C1−C8 bond.
Figure 1. Plausible catalytic cycle.
To further explore the reaction mechanism, preliminary DFT
study was conducted (without considering pyridine Nꢀoxide and
the phosphine ligand).12 The computation started from the active
tetrahedral dicobalt complex, and the energy diagram is depicted
in Figure 2 with 3D structures of key transition states TS1 and
TS2 included. The computational study shows that oxidative adꢀ
dition into the distal C1−C8 bond exhibits extremely high activaꢀ
tion barrier (TS1’, 50.5 kcal/mol) due to a highly strained transiꢀ
tion state. The cyclometalation pathway (path a) also shows a
high activation energy (TS1’’, 56.7 kcal/mol), which is attributed
to a nonꢀcoplanar structure of Co−C−C−O fourꢀmembered ring
and consequently less efficient orbital overlap. In contrast, the
direct cleavage of the C1−C2 bond has a much lower energy barꢀ
rier (TS1, 29.9 kcal/mol), supporting path b to be a more favoraꢀ
ble pathway.13 The migratory insertion step was found to exhibit
the highest overall activation energy (TS2, 36.6 kcal/mol) to give
Int3. The following reductive elimination readily takes place to
form the second C−C bond with a lower activation barrier. The
two cobalt centers in the resulting intermediate (Int4, ꢀ28.7
kcal/mol) are stabilized by both C=C and C=O πꢀbond to form a
dicobalt/enone complex via a µ2ꢀcarbonyl bridge.14 As a potential
competitive pathway, decarbonylation from Int3 would lead to a
sixꢀmembered cobalt cycle, the activation energy of which (TS3’,
8.2 kcal/mol) is 11.5 kcal/mol higher than the one of the reductive
elimination step; in addition, the free energy of the subsequent
intermediate (Int4’, ꢀ1.6 kcal/mol) is 27.1 kcal/mol higher than
Int4. Thus, the CO deꢀinsertion pathway is neither thermodynamꢀ
ically nor kinetically favorable. This observation is consistent
with the result of the 13Cꢀlabelling study (vide supra), in which
CO exchange was not significant. Computational methods and
detailed information regarding each intermediates and transition
states are available in the Supporting Information.
Scheme 3. Preliminary mechanistic study
Having ruled out the pathway involving C1−C8 bond cleavage,
a plausible catalytic cycle is proposed for the alkyne insertion
(Figure 1). The reaction begins with decarbonylation of dicobaltꢀ
alkyne complex A, which is accelerated by pyridine Nꢀoxide7, and
followed by coordination of the ketone carbonyl to form complex
B. At this stage, two pathways are possible. Path a involves cyꢀ
clometalation between the alkyne moiety and the carbonyl to form
intermediate C that can undergo subsequent βꢀcarbon elimination,
reductive elimination and tautomerization to give the βꢀnaphthol
product and reꢀgenerates the catalyst. Path b involves direct oxiꢀ
dative addition into the benzocyclobutenone C1−C2 bond with
one cobalt center. The resulting cobaltacycle D then undergoes
migratory insertion into the alkyne to afford intermediate E,
which upon reductive elimination and tautomerization forms the
βꢀnaphthol product and reꢀgenerates the catalyst.
In conclusion, we discovered a simple Co system that can cataꢀ
lyze βꢀnaphthol synthesis via an intramolecular alꢀ
kyne/benzocyclobutenone coupling. To the best of our knowledge,
this reaction represents the first example of Coꢀcatalyzed C−C3
activation of ketones. Compared to the Rh system, the Coꢀ
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catalyzed reaction not only operates at a lower temperature with
similar yields, but also effectively tolerates C8ꢀsubstituted subꢀ
strates. Its unique mechanistic feature disclosed here should have
broad implications for developing more efficient and economical
C−C activation methods in the future.
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oxidative
cyclization
OC
CO
Co
G in kcal/mol
M06/6ꢀ311+G(d,p)
OC
CO
C1-C8 cleavage
56.7
O
Co
O
CO
CO
OC
CO
OC Co
OC
CO
8
Co
50.5
O
CO
CO
CO
O
9
Co
O
Co
O
CO
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36.6
OC
CO
OC Co
Co CO
CO
O
29.9
O
TS2
G‡ = 36.6 kcal molꢀ1
23.4
OC
22.3
CO
CO
Co
OC
OC
CO
CO
Co
O
CO
carbonyl
exchange
OC
CO
CO
Co
O
Co
Co
13.3
OC
CO
O
O
CO
CO
CO
O
Co
O
CO
O
Co(CO)3
8.2
OC Co
C1-C2
cleavage
CO
CO
CO
Co
Co
O
O
CO
OC
ꢀ1.6
0.0
ꢀ3.3
OC
ꢀ4.6
OC
CO
Co
CO
OC Co
CO
OC
OC
O
CO
CO
CO
Co
CO
O
Co
CO
O
Co
O
CO
O
O
CO
Co
ꢀ28.7
CO
O
OC CO
O
Co
OH
TS1
G‡ = 29.9kcal molꢀ1
O
ꢀ46.2
CO
Co
O
CO
Figure 2. DFT calculated pathways for the intramolecular alkyne insertion.
Seiser, T.; Saget, T.; Tran, D. N.; Cramer, N. Angew. Chem. Int. Ed. 2011,
50, 7740ꢀ7752. (k) Murakami, M.; Matsuda, T.; Chem. Commun. 2011,
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2835ꢀ2840. (m) Cramer, N.; Seiser, T. Synlett. 2011, 449ꢀ460; (n) Aïssa, C.
Synthesis 2011, 3389ꢀ3407. (o) Murakami, M.; Ishida, N. J. Am. Chem.
Soc. 2016, 138, 13759ꢀ13769. (p) Chen, P.ꢀH.; Billett, B. A.; Tsukamoto,
T.; Dong, G. ACS Catal. 2017, 7, 1340ꢀ1360. (q) Dermenci, A.; Coe, J. W.;
Dong, G. Org. Chem. Front. 2014, 1, 567ꢀ581. (r) Souillart, L., Cramer, N.;
Chem. Rev. 2015, 115, 9410ꢀ9464. (s) CꢀC Bond Activation; Dong,
G., Ed.; In Topics in Current Chemistry. Springer: Berlin, 2014, Vol. 346.
(t) Cleavage of carbonꢀcarbon single bonds by transition metals; Muraꢀ
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AUTHOR INFORMATION
Corresponding Author
*Email: gbdong@uchicago.edu
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
Supporting Information is available free of charge via the Interꢀ
net at http://pubs.acs.org.
Detailed experimental procedures, characterization of products,
(2) (a) Nakazawa, H.; Itazaki, M.; Kamata, K.; Ueda, K. Chem.ꢀAsian J.
2007, 2, 882ꢀ888. (b) Sherry, B. D.; Fu
rstner, A. Chem. Commun. 2009,
and computational discussion and data.
7116ꢀ7118. (c) Dieskau, A. P.; Holzwarth, M. S.; Plietker, B. J. Am. Chem.
Soc. 2012, 134, 5048ꢀ5051. (d) Perthuisot, C.; Edelbach, B. L.; Zubris, D.
L.; Jones, W. D. Organometallics 1997, 16, 2016ꢀ2023. (e) Kurahashi, T.;
de Meijere, A. Synlett. 2005, 17, 2619ꢀ2622. (f) Kurahashi, T.; de Meijere,
A. Angew. Chem. Int. Ed. 2005, 44, 7881ꢀ7884. (g) Zell, D.; Bu, Q.; Feldt,
M.; Ackermann, L. Angew. Chem. Int. Ed. 2016, 55, 7408ꢀ7412.
(3) (a) Xu, T.; Dong, G. Angew. Chem. Int. Ed. 2012, 51, 7567ꢀ7571.
(b) Xu, T.; Ko, H.ꢀM.; Savage, N.ꢀA.; Dong, G. J. Am. Chem. Soc. 2012,
134, 20005ꢀ20008. (c) Dermenci, A.; Whittaker, R. E.; Dong, G. Org. Lett.
2013, 15, 2242ꢀ2245. (d) Chen, P.ꢀH.; Xu, T.; Dong, G. Angew. Chem. Int.
Ed. 2014, 53, 1674ꢀ1678. (e) Ko, H. M.; Dong, G. Nat. Chem. 2014, 6,
739–744. (f) Chen, P.ꢀH.; Sieber, J.; Senanayake, C.ꢀH.; Dong, G. Chem.
Sci. 2015, 6, 5440ꢀ5445. (g) Zhou, X.; Dong, G. J. Am. Chem. Soc. 2015,
137, 13715–13721. (h) Zeng, R.; Dong, G. J. Am. Chem. Soc. 2015, 137,
1408–1411. (i) Whittaker, R. E.; Dong, G. Org. Lett. 2015, 17, 5504ꢀ5507.
(j) Dermenci, A.; Whittaker, R. E.; Gao, Y.; Cruz, F.; Yu, Z.; Dong, G.
Chem. Sci. 2015, 6, 3201ꢀ3210. (k) Zeng, R.; Chen, P.ꢀH.; Dong, G. ACS
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ACKNOWLEDGEMNTS
This project was supported by NIGMS (R01GM109054). USTC is
acknowledged for fellowship to Z.Z. and X.L. THU is acknowlꢀ
edged for fellowship to S.C. We thank Prof. Peng Liu from the
University of Pittsburgh for DFT advice.
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ACS Catalysis
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