A cationic high-valent Cp*CoIII complex for the catalytic generation of nucleophilic organometallic species: Directed C-H bond activation

Article abstract of DOI:10.1002/anie.201209226

Active without activation: In an inexpensive and atom-economical approach to C-H bond functionalization, a cationic Co^III complex (see scheme) was used to generate nucleophilic organometallic species in situ without additional activating reagen

Full text of DOI:10.1002/anie.201209226

Angewandte
Chemie
DOI: 10.1002/anie.201209226
À
C H Activation
ACationic High-Valent Cp*CoIII Complex for the Catalytic Generation
À
of Nucleophilic Organometallic Species: Directed C H Bond
Activation**
Tatsuhiko Yoshino, Hideya Ikemoto, Shigeki Matsunaga,* and Motomu Kanai*
The nucleophilic addition of organometallic reagents to polar
electrophiles, such as aldehydes, imines, and Michael accept-
tionship of various Cp*CoIII complexes (Scheme 1) for the
catalytic generation of nucleophilic organometallic species.
We found that the [Cp*CoIII(arene)](PF6)2 complex 1a (5–
10 mol%) promoted the addition of 2-aryl pyridines to
imines, enones, and a,b-unsaturated N-acyl pyrroles as ester
and amide surrogates.
À
ors, is a fundamental C C bond-forming reaction in organic
synthesis. The generation of nucleophilic organometallic
reagents, however, generally requires stoichiometric amounts
of strong bases and/or reducing metals, such as Mg and Li, and
stoichiometric salt waste is therefore inevitably produced.
Thus, the development of atom-economical processes[1]
involving the catalytic generation of nucleophilic organo-
metallic species and their addition to polar electrophiles
without additional activating reagents is highly desirable.
À
Transition-metal-catalyzed C H bond functionalization could
be an attractive method for addressing these issues,[2] but
À
À
addition reactions of C H bonds to polar C X multiple bonds
(X = N, O, or C) have been investigated much less[3] than
related reactions with nonpolar alkenes and alkynes.[2] Quite
recently, it was disclosed that Cp*RhIII complexes (Cp* =
pentamethylcyclopentadienyl)[4] catalyze addition reactions
of arene C H bonds to imines, aldehydes,[6] Michael
[5]
À
acceptors,[7] and other polar electrophiles.[8] Although
Cp*RhIII-catalyzed processes are useful and versatile, the
need for expensive and precious rhodium sources is econom-
ically and environmentally disadvantageous. Therefore, stud-
ies are needed for the development of an inexpensive base
metal catalyst as an alternative to the cationic Cp*RhIII
complexes.[9] Herein, we describe the utility of a cationic
high-valent cobalt complex and the structure–activity rela-
Scheme 1. Cationic high-valent cobalt(III) complexes synthesized and
investigated in this study.
We selected cobalt,[10] which is homologous with rhodium
but a more abundant first-row transition metal,[11] and
investigated the ability of high-valent cobalt catalysts to
À
promote C H bond functionalization. Optimization studies
[*] T. Yoshino, H. Ikemoto, Dr. S. Matsunaga, Prof. Dr. M. Kanai
Graduate School of Pharmaceutical Sciences
The University of Tokyo
with 2-phenylpyridine (2a) and the sulfonyl imine 3a[12–14] as
model substrates are summarized in Table 1. Several com-
mercially available CoII and CoIII salts showed no catalytic
activity at 1008C in 1,2-dichloroethane (Table 1, entries 1 and
3–5). Even the addition of a silver salt to CoCl2 to afford
a cationic Co species resulted in no reaction (Table 1, entry 2).
By analogy with Cp*RhIII catalysis, we expected that the
cyclopentadienyl–cobaltIII structure would be an appropriate
catalyst core for the present reaction. The use of a dimeric
[{Cp*CoCl2}2] complex, however, resulted in no reaction
(Table 1, entry 6). The addition of AgPF6 was effective, and
a cationic Cp*CoIII complex generated in situ afforded 4aa in
48% yield (Table 1, entry 7).
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
E-mail: smatsuna@mol.f.u-tokyo.ac.jp
kanai@mol.f.u-tokyo.ac.jp
Homepage: http://www.f.u-tokyo.ac.jp/~kanai/e_index.html
Prof. Dr. M. Kanai
Japan Science and Technology Agency, ERATO
Kanai Life Science Catalysis Project
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Dr. S. Matsunaga
Japan Science and Technology Agency, ACT-C
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
[**] This research was supported in part by the ERATO program of JST
(M.K.), a Grant-in-Aid for Scientific Research on Innovative Areas
(“Molecular Activation Directed toward Straightforward Synthesis”)
from MEXT (S.M.), the ACT-C program of JST (S.M.), and the Naito
Foundation. T.Y. thanks the JSPS for fellowships. Cp*=penta-
methylcyclopentadienyl.
The results in entries 6 and 7 of Table 1 prompted us to
examine cationic CoIII complexes with thermally labile
ligands. Although the synthesis, structure, and electrochem-
ical properties of some cationic cyclopentadienyl–cobaltIII
complexes with arene ligands have been reported,[15] the
application of these complexes for synthetic organic trans-
formations has not been investigated. Therefore, several
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209226.
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Table 1: Optimization of the reaction conditions for the arylation of
imine 3a.
Scheme 2. Gram-scale synthesis of 1a.
Entry
Catalyst (mol%)
t [h]
Yield [%][a]
1
2
3
4
5
6
7
8
9
CoCl2 (10)
CoCl2 (10) + AgPF6 (20)
Co(OTf)2
12
12
13
12
13
13
12
12
20
20
20
20
0
0
0
0
0
Table 2: Substrate generality in the arylation of imines 3.[a]
[Co(acac)3]
[Co(NH3)6Cl3]
[{Cp*CoCl2}2] (5)
[{Cp*CoCl2}2] (5) + AgPF6 (20)
1a (10)
1b (10)
1c (10)
0
48
80[b]
39
11
trace
22
10
11
12
Entry
R: imine 3
Product
Yield [%][b]
1d (10)
1e (10)
1
2
3
4
5
6
7
8
9
Ph
3a
3b
3c
3d
3e
3 f
3g
3h
3i
4aa
4ab
4ac
4ad
4ae
4af
4ag
4ah
4ai
80
79
83
72
64
76
76
57
71
69
66
2-naphthyl
p-Cl-C6H4
p-Br-C6H4
p-CF3-C6H4
m-Cl-C6H4
p-Me-C6H4
p-MeO-C6H4
o-Me-C6H4
2-thienyl
[a] The yield was determined by 1H NMR spectroscopic analysis of the
crude reaction mixture with dibenzyl ether as an internal standard.
[b] Yield of the isolated product after purification by column chroma-
tography on silica gel. acac=acetylacetonate, Tf=trifluoromethane-
sulfonyl.
cationic [CpCoIII(arene)] complexes with different Cp-type
ligands were synthesized to evaluate the catalyst structure–
activity relationship in detail (Scheme 1). The [Cp*Co-
(benzene)] complex (1a)[15] showed higher reactivity than
the cationic catalyst generated in situ, and the yield of 4aa was
improved to 80% (Table 1, entry 8). This improved catalytic
activity is probably due to the more efficient generation of an
active catalytic species through thermal arene dissociation
than through chloride abstraction with a silver salt. The
sterically more hindered CoIII complexes 1b–1d[16] were much
less active than 1a (Table 1, entries 9–11). Attempts to
improve the catalytic activity with the sterically less crowded
complex 1e failed (Table 1, entry 12), because complex 1e
seemed to be unstable under the reaction conditions. The
[Cp*CoIII(arene)](PF6)2 complex 1a showed the best balance
between reactivity and stability. In contrast to the related
10
11
3j
3k
4aj
4ak
2-furyl
[a] Reaction conditions: 1a (0.04 mmol), 2a (0.60 mmol), 3
(0.40 mmol), 1,2-dichloroethane (0.2 mL), Ar atmosphere. [b] Yield of
the isolated product after purification by column chromatography on
silica gel.
withdrawing substituent at the para or meta position showed
good reactivity (Table 2, entries 3–7), whereas an imine with
a strongly electron donating methoxy substituent showed
slightly lower reactivity (Table 2, entry 8). The reaction also
proceeded with the sterically hindered ortho-substituted
imine 3i (Table 2, entry 9) and other heteroaryl imines
(Table 2, entries 10 and 11).[18]
THF was the best solvent for conjugate addition to a,b-
unsaturated carbonyl compounds, and the catalyst loading
was successfully reduced to 5 mol% in most cases. As shown
in Scheme 3, a broad range of enone substrates were
applicable. Chalcone and its derivatives containing either
electron-donating or electron-withdrawing substituents were
converted into the desired products in 67–80% yield. Other
acyclic alkyl-substituted enones and cyclic enones were
transformed into products 6af–6aj in 76–90% yield. The
reaction was also successfully carried out on a preparative
scale (5.0 mmol) to give 6ai without significant loss of yield.
With 2-cyclopenten-1-one as the electrophile, we also inves-
tigated the scope of the reaction with respect to the donor
(Scheme 3). The reaction of donors with either an electron-
withdrawing or an electron-donating group at the para
position to the directing 2-pyridyl group afforded the desired
products 6bi–6di in good yield. Notably, the present
À
catalysis of low-valent Co for the addition of C H bonds to
imines,[3e] the present system requires no additional reagents;
a catalytic amount of the high-valent Co complex 1a alone
promoted the desired reaction in an atom-economical
manner.
The optimal complex 1a was readily synthesized from
CoCl2 in gram quantities without the use of any expensive
reagents (Scheme 2).[15c,17] The reaction with Cp*Li, oxida-
tion, and counterion exchange gave 1a as an air- and
moisture-stable solid in 24% overall yield after purification
by reprecipitation.
With the optimized catalyst 1a in hand, we investigated
the scope of the reaction with various polar electrophiles,
including imines (Table 2), enones (Scheme 3), and a,b-
unsaturated N-acyl pyrroles as ester surrogates (Scheme 4).
Aryl imines with either an electron-donating or an electron-
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Scheme 4. Addition to a,b-unsaturated N-acyl pyrroles and transforma-
tion of the products into esters and an amide. DBU=1,8-diazabicyclo-
[5.4.0]undec-7-ene.
a C H bond with Cp*RhIII complexes, the results in Scheme 4
À
are synthetically useful.
A plausible catalytic cycle based on related RhIII cata-
lysis[5c,d] is shown in Scheme 5. Initially, the coordinating
benzene ring in 1a would dissociate upon heating to generate
À
the 2-phenylpyridine complex I. The process of C H activa-
tion is assumed to proceed through electrophilic aromatic
substitution or a concerted metalation–deprotonation mech-
anism to form the cyclometalated intermediate II. After
Scheme 3. Scope of the arylation of a,b-unsaturated ketones. Reaction
conditions: 1a (0.02 mmol, 5 mol%), 2 (0.48 mmol), 5 (0.40 mmol),
THF (0.2 mL), Ar atmosphere. The yield of 6 after purification by
column chromatography on silica gel is shown for each reaction.
[a] The reaction was carried out on a 5.0 mmol scale (quantity of 5).
[b] The reaction was carried out with 10 mol% of 1a.
[Cp*CoIII(arene)](PF6)2 complex showed good functional-
group compatibility: an acetyl group (in product 6ei), a free
phenolic hydroxy group (in 6 fi), and a tertiary amino group
(in 6gi) did not interfere with the desired reactions. Further-
more, meta-substituted donors showed modest reactivity with
the predominant formation of products 6hi–6ji.
In contrast to enones, a,b-unsaturated esters and amides
did not afford the desired Michael adducts, possibly owing to
their low electrophilicity. Therefore, a,b-unsaturated N-acyl
pyrroles 7 were used as ester and amide surrogates[19,20] to
expand the utility of the present system. As shown in
Scheme 4, the desired products 8aa and 8ab were obtained
from the corresponding b-aryl and b-alkyl a,b-unsaturated N-
acyl pyrroles in 78 and 91% yield, respectively. The N-acyl
pyrrole unit was readily converted into an ester group (9aa,
9ab: 91%) by treatment with NaOEt at room temperature or
into an amide (10aa: 94%) with an amine and DBU. Because
b-substituted a,b-unsaturated esters and amides have not
been used successfully in the corresponding direct addition of
Scheme 5. Proposed catalytic cycle for the arylation of imines.
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In summary, we have demonstrated the utility of a cation-
ic, high-valent Co complex for atom-economical directed
À
addition reactions of aryl C H bonds to polar electrophiles.
Studies of the catalyst structure–activity/stability relationship
revealed that the [Cp*CoIII(arene)](PF6)2 complex 1a was the
most suitable catalyst. The Cp*CoIII complex 1a catalytically
generated the nucleophilic species in situ without any addi-
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À
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À
cost- and atom-efficient C H bond-functionalization process-
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À
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Received: November 17, 2012
Published online: January 16, 2013
À
Keywords: atom economy · C H activation · cobalt ·
homogeneous catalysis · nucleophilic addition
.
À
À
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[14] The p-toluenesulfonylimine derived from benzaldehyde was also
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À
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