Stereoselective Synthesis of Tetrasubstituted Alkenes via a Cp*CoIII-Catalyzed C?H Alkenylation/Directing Group Migration Sequence

Article abstract of DOI:10.1002/anie.201703193

A highly atom economical and stereoselective synthesis of tetrasubstituted α,β-unsaturated amides was achieved by a Cp*Co^III-catalyzed C?H alkenylation/directing group migration sequence. A carbamoyl directing group, which is typically removed after C?H functionalization, worked as an internal acylating agent and migrated onto the alkene moiety of the product. The directing group migration was realized with the Cp*Co^III catalyst, while a related Cp*Rh^III catalyst did not promote the migration process. The product was further converted into two types of tricyclic compounds, one of which had fluorescent properties.

Full text of DOI:10.1002/anie.201703193

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201703193
German Edition: DOI: 10.1002/ange.201703193
À
C H Bond Activation
Hot Paper
Stereoselective Synthesis of Tetrasubstituted Alkenes via a
III
À
Cp*Co -Catalyzed C H Alkenylation/Directing Group Migration
Sequence
Hideya Ikemoto, Ryo Tanaka, Ken Sakata, Motomu Kanai, Tatsuhiko Yoshino,* and
Shigeki Matsunaga*
Abstract: A highly atom economical and stereoselective
synthesis of tetrasubstituted a,b-unsaturated amides was ach-
III
À
ieved by a Cp*Co -catalyzed C H alkenylation/directing
group migration sequence. A carbamoyl directing group,
À
which is typically removed after C H functionalization,
worked as an internal acylating agent and migrated onto the
alkene moiety of the product. The directing group migration
was realized with the Cp*CoIII catalyst, while a related Cp*RhIII
catalyst did not promote the migration process. The product
was further converted into two types of tricyclic compounds,
one of which had fluorescent properties.
T
etrasubstituted alkenes are found in many biologically
active molecules[1] and natural products.[2] They are also
important synthetic intermediates for the synthesis of highly
congested vicinal stereogenic carbon centers by various
difunctionalization reactions of tetrasubstituted alkenes.[3–5]
Stereoselective synthesis of all-carbon tetrasubstituted
alkenes, however, remains a great challenge because of their
congested nature and difficulties in controlling stereoselec-
tivity. General strategies for their construction involve
carbometalation of internal alkynes and successive cross-
coupling reactions or addition to electrophiles.[6] Although
remarkable advances have been made in these strategies, the
use of stoichiometric organometallic reagents is still inevi-
Scheme 1. a) Previous work: di- and trisubstituted alkene synthesis by
À
C H functionalization b) This work: synthesis of tetrasubstituted
À
olefins by C H alkenylation/donating group (DG) migration sequence.
table.[7] On the other hand, transition-metal-catalyzed C H
metallic reagents (Scheme 1a). In the latter reactions,
proto-demetalation or reductive elimination to form an
À
bond functionalizations[8] have emerged as atom-[9] and step-
economical[10] methods for synthesizing di- and trisubstituted
alkenes.[11] These reactions generally proceed by either
À
alkenyl H bond occurs after alkyne insertion, making the
À
formation of the fourth C C bond difficult.
a transition-metal-catalyzed C H/C H oxidative coupling
reaction with alkenes or a redox-neutral alkyne insertion at
À
We previously reported the Cp*CoIII-catalyzed[12–15] syn-
À
À
À
thesis of pyrroloindolones, in which C H alkenylation and
C H bonds without stoichiometric amounts of organo-
successive intramolecular nucleophilic addition to a carba-
moyl directing group of indole proceeded without proto-
demetalation (Scheme 1b, previous work).[15] During the
course of our further studies of this reaction, we found that
the tetrasubstituted alkene was formed as a kinetically
controlled product (Scheme 1b, this work). The obtained
tetrasubstituted alkene, which is difficult to access stereo-
selectively by other methods, is considered to be formed by
directing group migration.[16] Herein, we report the optimized
conditions for this atom economical directing group migration
process, in which the carbamoyl group works not only as
a directing group but also as an internal acylating agent.
Optimization studies using N-morpholinocarbamoyl
indole 2a and alkyne 3a under Cp*CoIII/KOAc catalysis are
summarized in Table 1. The best reaction conditions for the
synthesis of pyrroloindolone 5 using Cp*CoIII-arene catalyst
[*] H. Ikemoto, Prof. Dr. M. Kanai
Graduate School of Pharmaceutical Sciences, The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
R. Tanaka, Dr. T. Yoshino, Prof. Dr. S. Matsunaga
Faculty of Pharmaceutical Sciences, Hokkaido University
Kita-ku, Sapporo 060-0812 (Japan)
E-mail: tyoshino@pharm.hokudai.ac.jp
smatsuna@pharm.hokudai.ac.jp
Dr. K. Sakata
Faculty of Pharmaceutical Sciences, Hoshi University
Ebara, Shinagawa-ku, Tokyo 142-8501 (Japan)
Dr. K. Sakata, Dr. T. Yoshino, Prof. Dr. S. Matsunaga
ACT-C Japan Science and Technology Agency (Japan)
Supporting information for this article can be found under:
https://doi.org/10.1002/anie.201703193.
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Table 1: Optimization studies and control experiments.[a]
The optimal reaction conditions were then applied to
À
various alkynes, as summarized in Table 2. The C H
alkenylation/donating group (DG) migration sequence of
indole 2a proceeded well with various aryl/alkyl alkynes that
À
Table 2: Substrate scope of C H alkenylation/DG migration with
alkynes.[a]
Entry
Cat.[b]
X [m]
T [8C]
Yield [%][c]
4aa
5
6
1
2
3
4
5
6
1a
1a
1a
0.10
0.10
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
130
100
100
100
100
100
100
100
100
100
100
0
82
23
<1
5
4
7
5
0
0
0
0
10
22
10
6
6
5
6
0
0
3
37
40
50
50
69
Entry
R1
R2
3
4
Yield[b] [%]
1b-BF4
1b-PF6
1b-SbF6
1b-SbF6
1b-SbF6
1c (Rh)
1c (Rh)
1c (Rh)
1
2
Me
Me
Me
Me
Me
Me
Me
Et
Ph
3a 4aa
3b 4ab
74
55
77
58
74
71
69
76
69
87
85
71
99
76
70
54
0
4-Me-C6H4
4-Cl-C6H4
4-Br-C6H4
4-CO2Et-C6H4
2-naphthyl
6-MeO-naphth-2-yl 3g 4ag
Ph
Ph
Ph
3[c]
4[c]
5[c]
6
3c
4ac
7[d]
8[f]
9[d]
10[g]
11[h]
80 (74)[e]
3d 4ad
3e 4ae
10
0
0
3 f
4af
7
8
9
0
8
3h 4ah
[a] Reaction conditions: 2a (0.40 mmol) and 3a (0.20 mmol).
Pr
Ph
3i
3j
4ai
4aj
10[d]
11[d]
12[d]
13[d]
14
15
16
17
18
[b] 1a=[Cp*Co(C6H6)](PF6)2, 1b-X=[Cp*Co(CH3CN)3]X2, 1c=[Cp*Rh-
(CH3CN)3](SbF6)2. [c] Determined by 1H NMR analysis with an internal
standard. [d] KOAc (5 mol%) was used. [e] Isolated yield of 4aa after
purification by silica gel column chromatography. [f] Reaction without
KOAc. [g] CsOAc (5 mol%) was used instead of KOAc. [h] CsOPiv
(5 mol%) was used instead of KOAc.
4-Me-C6H4
4-Br-C6H4
4-TMS-C6H4
4-Me-C6H4
4-Br-C6H4
4-TMS-C6H4
3k 4ak
3l 4al
3m 4am
3n 4an
3o 4ao
3p 4ap
3q 4aq
TBDPSO(CH2)3- Ph
Bu
TMS
Pr
2-thienyl
Ph
Pr
À
1a are shown in entry 1, in which C H alkenylation,
H
Ph
3r
4ar
0
successive intramolecular nucleophilic addition to the carba-
moyl directing group, and elimination of an amine, proceed
with high selectivity. When the reaction temperature was
decreased to 1008C, the yield of 5 dropped to 23% along with
the alkenylation product 6 (22%). After careful analysis of
the reaction mixture, we identified 4aa as a major product
(37% yield, entry 2). To decelerate the undesired proto-
demetalation leading to 6, the concentration of 3a was
decreased to 0.05m and the desired alkene 4aa was obtained
in 40% yield with moderate selectivity. Although further
minor modification of the reaction conditions was unfruitful,
screening of Cp*CoIII catalysts led to improvement (entries 4–
6). For our desired reaction, the [Cp*CoIII(CH3CN)3]X2
catalysts[17] 1b-X had higher reactivity and selectivity than
the Cp*CoIII-arene catalyst 1a, and the best catalyst 1b-SbF6
afforded 69% yield (entry 6). We screened the reaction
conditions again using 1b-SbF6 and determined that 5 mol%
KOAc was optimal (entry 7) to give 4aa in 80% yield (74%
isolated yield). The reactivity dramatically decreased without
a catalytic amount of KOAc (entry 8), indicating that KOAc
[a] Reaction conditions: 1b-SbF6 (5 mol%), KOAc (5 mol%), 2a
(0.40 mmol), 3 (0.20 mmol), 1,2-dichloroethane (4.0 mL), Ar atmos-
phere at 1008C, unless otherwise noted. [b] Isolated yields of 4 (single
regioisomer) after purification by silica gel column chromatography.
[c] 1,2-dichloroethane (6.0 mL, 0.033m) [d] 908C.
have electron-donating and electron-withdrawing substitu-
ents on the aromatic rings, and products were obtained in 55–
77% yield (entries 1–7). The indicated products were
obtained as single regioisomers in all cases. Good reactivities
and high selectivities were also observed with other alkyl
substituents, such as ethyl or propyl on alkynes (entries 8, 9).
Diaryl-substituted alkynes were also compatible to give the
desired products in 71–99% yield (entries 10–13). A func-
tionalized alkyne 3n bearing a silyl ether unit also gave 4an in
76% yield without deprotection (entry 14). An alkyne 3o
with a thiophene ring gave tetrasubstituted alkene 4ao,
containing two different heteroaromatic rings, in 70% yield
(entry 15). While alkyne 3p protected with trimethylsilyl
(TMS) group resulted in 4ap in a moderate yield (entry 16);
dialkylalkyne 3q afforded the alkenylation product 6aq
exclusively (entry 17). No reaction proceeded with a terminal
alkyne 4r (entry 18).
À
had an important role in efficient C H bond cleavage by
a concerted metalation–deprotonation mechanism.[18] We also
confirmed that Cp*RhIII catalyst 1c[19,20] did not promote the
desired directing group migration, and only a small amount of
the alkenylated product 6 was obtained, even after screening
of the carboxylate additives (entries 9–11). These results
The scope of indoles is summarized in Table 3. Both
electron-donating and electron-withdrawing substituents
were compatible, and various 4- or 5-substituted indoles
resulted in 60–96% yield (4bm–4hm). Indoles with other
À
indicate that high nucleophilicity of the C Co bond is
essential.[12a,15]
2
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À
Table 3: Substrate scope of C H alkenylation/DG migration with indoles
wavelength of 501 nm. Our synthetic method is expected to
and pyrroles.[a]
provide easy access to various substituted analogues of this
fluorescent molecule.
A plausible catalytic cycle involving the Cp*CoIII complex
in the presence of KOAc is shown in Figure 1. As the initial
step, three acetonitrile ligands dissociate from [Cp*CoIII-
(CH3CN)3](SbF6)2, and ligand exchange with acetate
[a] Reaction conditions: 1b-SbF6 (5 mol%), KOAc (5 mol%), 2
(0.40 mmol), 3 (0.20 mmol), 1,2-dichloroethane (4.0 mL), Ar atmos-
phere at 908C, unless otherwise noted. Isolated yields of 4 (single
regioisomer) after purification by silica gel column chromatography are
listed. [b] Reaction conditions: 808C, 12 h. [c] 1008C [d] Reaction
conditions: KOAc (10 mol%), 1,2-dichloroethane (2.0 mL), 1008C.
carbamoyl directing groups also gave the corresponding
tetrasubstituted alkenes 4im–4lm although the reactivity
was significantly affected by the structure of the directing
group. Moreover, N-carbamoyl pyrrole 4m also underwent
the desired alkenylation/DG migration reaction to give
pyrrole-substituted tetrasubstituted alkenes 4mf and 4mm.
À
Figure 1. Plausible catalytic cycle of C H alkenylation/DG migration
sequence with indole and alkyne.
generates a catalytically active monocationic species I. After
coordination of the carbamoyl group of indole 2 (II),
regioselective C H metalation at the C2 position occurs by
À
The DG migration during the first C H functionalization
À
À
inhibits the second functionalization of another C H bond of
the pyrrole ring.
a concerted metalation–deprotonation (CMD) mechanism[18]
to afford the indolyl–Co species III. The result without KOAc
(Table 1, entry 8) indicates that an electrophilic aromatic
substitution (SEAr) pathway and/or a CMD pathway with
external base other than acetate competes, but an acetate-
assisted CMD pathway is dominant under the optimal
The product 4aj was further converted to two different
tricyclic compounds, 8 and 9 (Scheme 2). After N-methylation
(7), treatment with trifluoromethanesulfonic anhydride
(Tf2O) and 4-dimethylaminopyridine (DMAP)[21] promoted
electrophilic cyclization at the C3 position to afford 8. On the
other hand, the spirocyclic compound 9 was obtained by
oxidation using pyridinium chlorochromate (PCC)[22] or
3-chloroperbenzoic acid (mCPBA)/Co(ClO4)2.[23] Spirocycle
9 exhibited green fluorescence with a maximum emission
À
conditions. Insertion of alkyne 3 into the C Co bond
generates the key alkenyl–Co intermediate (IV). Nucleo-
philic addition of the C Co bond to the carbamoyl directing
À
group proceeds to afford the intermediate V. A low concen-
tration was essential to avoid undesired proto-demetalation
of IV, leading to 6. Elimination of the indole results in DG
migration, giving tetrasubstituted alkene 4. Our previous
report revealed that pyrroloindolone 5 is a major product at
1308C,[15] and we confirmed the formation of 5 from 4 at
1308C in the presence of 1a/KOAc or KOAc (see Supporting
Information). These data suggest that 4 is a kinetically
favored primary product that undergoes cyclization, leading
to a thermodynamically favored product 5; although direct
formation of 5 from V at 1308C is also possible to some
extent.
In conclusion, the Cp*CoIII-catalyzed directing group
migration reaction of carbamoyl-protected indoles/pyrrole
and alkynes afforded tetrasubstituted alkenes 4 in 17–99%
yield with complete stereoselectivity and high atom economy
Scheme 2. Transformation of 4aj into tricyclic compounds.
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Acknowledgements
This work was supported in part by JST ACT-C Grant
Number JPMJCR12Z6, Japan, JSPS KAKENHI Grant
Number JP15H05802 in Precisely Designed Catalysts with
Customized Scaffolding, JSPS KAKENHI Grant Number
JP15H05993, Toray Science Foundation, and the Naito
Foundation (S.M.). H.I. thanks JSPS for fellowships. We
acknowlege Professors Masako Kato and Masaki Yoshida in
Hokkaido University for fluorescence spectroscopy.
[9] B. M. Trost, Science 1991, 254, 1471.
[10] P. A. Wender, B. L. Miller, Nature 2009, 460, 197.
Conflict of interest
À
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The authors declare no conflict of interest.
À
Keywords: C H activation · cobalt · first row transition metals ·
homogeneous catalysis · indoles
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À
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Manuscript received: March 28, 2017
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
À
C H Bond Activation
Tetrasubstituted alkenes were stereo-
selectively synthesized by a Cp*CoIII-cat-
À
H. Ikemoto, R. Tanaka, K. Sakata,
M. Kanai, T. Yoshino,*
alyzed C H alkenylation/directing group
migration sequence, within which a car-
bamoyl functional group operates as
a directing group and an internal acylat-
ing agent. A fluorescent spirocyclic mol-
ecule was generated from the resulting
alkene.
S. Matsunaga*
&&&& — &&&&
Stereoselective Synthesis of
Tetrasubstituted Alkenes via a Cp*CoIII-
À
Catalyzed C H Alkenylation/Directing
Group Migration Sequence
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!