Rhodium-Catalyzed [2+1+2+1] Cycloaddition of Benzoic Acids with Diynes through Decarboxylation and C≡C Triple Bond Cleavage

Article abstract of DOI:10.1002/chem.201901050

It has been established that an electron-deficient cyclopentadienyl rhodium(III) (Cp^ERh^III) complex catalyzes the oxidative and decarboxylative [2+1+2+1] cycloaddition of benzoic acids with diynes through C≡C triple bond cleavage, leading to fused naphthalenes. This cyclotrimerization is initiated by directed ortho C?H bond cleavage of a benzoic acid, and the subsequent regioselective alkyne insertion and decarboxylation produce a five-membered rhodacycle. The electron-deficient nature of the Cp^ERh^III complex promotes reductive elimination giving a cyclobutadiene–rhodium(I) complex rather than the second intermolecular alkyne insertion. The oxidative addition of the thus generated cyclobutadiene to rhodium(I) (formal C≡C triple bond cleavage) followed by the second intramolecular alkyne insertion and reductive elimination give the corresponding [2+1+2+1] cycloaddition product. The synthetic utility of the present [2+1+2+1] cycloaddition was demonstrated in the facile synthesis of a donor–acceptor [5]helicene and a hemi-hexabenzocoronene by a combination with the chemoselective Scholl reaction.

Full text of DOI:10.1002/chem.201901050

A Journal of
Accepted Article
Title: Rhodium-Catalyzed [2+1+2+1] Cycloaddition of Benzoic Acids
with Diynes via Decarboxylation and C≡C Triple Bond Cleavage
Authors: Yusaku Honjo, Yu Shibata, and Ken Tanaka
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To be cited as: Chem. Eur. J. 10.1002/chem.201901050
Link to VoR: http://dx.doi.org/10.1002/chem.201901050
Supported by

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Rhodium-Catalyzed [2+1+2+1] Cycloaddition of Benzoic Acids
with Diynes via Decarboxylation and C≡C Triple Bond Cleavage
Yusaku Honjo, Yu Shibata,*,[a] and Ken Tanaka*,[a]
Abstract: It has been established that an electron-deficient
cyclopentadienyl rhodium(III) (CpERhIII) complex catalyzes the
oxidative and decarboxylative [2+1+2+1] cycloaddition of benzoic
acids with diynes through C≡C triple bond cleavage, leading to fused
naphthalenes. This cyclotrimerization is initiated by directed ortho C–
with two alkynes is convenient and atom-economical, but the
directing group remains in the former method and the harsh
conditions require in the latter method (Scheme 1b, left). As a
traceless directing group, a carboxy group was successfully
employed in the pentamethylcyclopentadienyl (Cp*)-iridium(III)[14]
H
bond cleavage of
a
benzoic acid, and the subsequent
or cyclopentadienyl (Cp)-rhodium(III)[15]
complex-catalyzed
regioselective alkyne insertion and decarboxylation produce a five-
membered rhodacycle. The electron-deficient nature of the CpERhIII
complex promotes reductive elimination giving a cyclobutadiene-
rhodium(I) complex rather than the second intermolecular alkyne
insertion. The oxidative addition of the thus generated
cyclobutadiene to rhodium(I) (formal C≡C triple bond cleavage)
followed by the second intramolecular alkyne insertion and reductive
elimination give the corresponding [2+1+2+1] cycloaddition product.
The synthetic utility of the present [2+1+2+1] cycloaddition was
demonstrated in the facile synthesis of a donor-acceptor [5]helicene
decarboxylative and oxidative [2+2+2] cycloaddition of benzenes
with two alkynes, which yields only water and carbon dioxide as
by-products. However, these reactions required high
temperatures (xylene, reflux to 160 °C) and stoichiometric
amounts of metal oxidants. Very recently, our research group
reported that the use of an electron-deficient Cp-rhodium(III)
complex (CpERhIII) significantly mildened the reaction conditions
(at RT–80 °C under air or O2).[16] The highly Lewis acidic nature
of the CpERhIII complex may accelerate the decarboxylation step
as well as the C-H bond cleavage step[17] as a result of the
strong π-rhodium interaction in their transition-states. Here, we
have established that the CpERhIII complex catalyzes the
oxidative and decarboxylative [2+1+2+1] cycloaddition of
benzoic acids with acyclic diynes (Scheme 1b, right). The highly
Lewis acidic nature of the CpERhIII complex may promote the
and
a hemi-hexabenzocoronene by a combination with the
chemoselective Scholl reaction.
The transition-metal-catalyzed alkyne cyclotrimerization is
one of the most useful and straightforward methods for the
synthesis of substituted benzenes. Particularly, the [2+2+2]
cycloaddition of three alkynes has been widely investigated by
using various transition-metal catalysts for the synthesis of
functional molecules such as polycyclic aromatic hydrocarbons
(Scheme 1a, left).[1] In rare cases, the [2+1+2+1] cycloaddition of
three alkynes that affords structurally isomeric benzenes through
C≡C triple bond cleavage also proceeds by using cyclic diynes
or electron-deficient and/or sterically demanding triynes as
substrates and cationic rhodium(I) complexes as catalysts
(Scheme 1a, right).[2,3] In this unusual cyclotrimerization, the
reductive elimination to form
a cyclobutadiene-rhodium(I)
intermediate. Furthermore, it is worthy of note that this report is
the first example of the [2+1+2+1] cycloaddition using the aryne
or its equivalent and acyclic diynes.
(a) construction of benzene ring
[2+2+2] (many examples)
[2+1+2+1] (triynes or cyclic diynes: few examples)
R3
transition-
metal
catalyst
R3
R3
•
•
R1
R2
RhI
catalyst
•
R1
R2
•
R3
R4
R4
R4
R1
R2
R4
R4
•
+
•
R4
R3
R3
reductive
elimination
of
a
cyclobutadiene
from
R3
•
[M]
•
R4
R1
R2
R3
rhodacyclopentadiene A, generated by the oxidative
cycloaddition of two alkynes and the cationic rhodium(I) complex,
is a crucial step to give regioisomeric rhodacyclopentadiene B
through the formal C≡C triple bond cleavage.
1 [M]
R1
R2
R
R3
R4
•
•
R4
•
•
R2
[M]
A
B
reductive elimination
(b) construction of naphthalene ring
On the other hand, replacing one alkyne with an aryne would
[2+2+2] (many examples)
[2+1+2+1] (no precedent)
afford
a substituted naphthalene. To date, various aryne
R3
NiII, PdII
(DG) R3
(DG) R3
(DG)
•
precursors and equivalents have been employed for the [2+2+2]
cycloaddition,[4–11] while their preparation and reactions require
harsh conditions. Alternatively, directed[12] or non-directed[13]
functionalization of the adjacent two C–H bonds of benzenes
RhIII
•
R3
R4
R4
R4
R4
R4
X
Y
•
•
catalyst
•
R1
R1
+
R1
•
R4
R3
R3
Scheme 1. Transition-metal-catalyzed cyclotrimerization to construct benzene
[a]
Y. Honjo, Dr. Y. Shibata, Prof. Dr. K. Tanaka
Department of Chemical Science and Engineering, Tokyo Institute
of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan
E-mail: yshibata@apc.titech.ac.jp
and naphthalene rings. [M] = transition-metal complex. DG = directing group.
As mentioned above, benzoic acid (1a) reacted with two
internal monoynes in the presence of the CpERhIII complex to
give oxidative and decarboxylative [2+2+2] cycloaddition
E-mail: ktanaka@apc.titech.ac.jp
Homepage: http://www.apc.titech.ac.jp/~ktanaka/
Supporting information for this article is given via a link at the end
of the document
products
C
in good yields without forming [2+1+2+1]
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COMMUNICATION
cycloaddition products D (Scheme 2a).[16] On the contrary, the
reaction of 1a with 1,6-diyne 2a instead of two monoynes
afforded [2+1+2+1] cycloaddition product 3aa as a major product
along with a small amount of [2+2+2] cycloaddition product 4aa
(Scheme 2b).
and 1c) and withdrawing (1d) groups at the 4-position was
tolerable. Interestingly, in these reactions, regioisomeric
products 3’ were not generated at all. Sterically demanding 3,4-
dimethyl-substituted 1e also reacted with 2a to give the
corresponding naphthalene 3ea. Not only 1,6-diynes but
biphenyl-linked 1,7-diyne 2i also reacted with 1a at 80 °C to give
3ai in 36% yield along with a small amount of 4ai (7%). In this
reaction, lowering substrate concentration (0.01 M) improved the
yield to 42%. Concerning substituents at the alkyne termini, the
introduction of electron-donating (2j) and withdrawing (2k)
groups was tolerable, and the structure of 3aj was confirmed by
the X-ray crystallographic analysis.[21] The introduction of
electron-donating (2l) and withdrawing (2m) groups into the
biphenyl-linker was also tolerable, while the use of 2m afforded
(a) reactions of 1a with two monoynes
R3
cat. [CpERhCl2]2
cat. AgOAc
cat. Cu(OAc)2•H2O
R3
•
•
R3
•
•
R4
R3 (R4)
R4 (R3)
R4
R4
H
•
+
OH
PhCl or toluene
RT–80 °C
R4 (R3)
•
R4
R3 (R4)
C / good yields
[2+2+2]
O
1a
under air or O2
R3
D / not detected
[2+1+2+1]
Me
CO2Et
Me
EtO2C
Me
Cl
Rh
Cl
2
[CpERhCl2]2
Table 1. Screening of reaction conditions.[a]
(b) reaction of 1a with 1,6-diyne 2a
H
OH
NTs
5 mol % Rh or Ir complex
20 mol % additive
10 mol % Cu(OAc)2•H2O
O
1a
OH
•
•
NTs
•
5 mol % [CpERhCl2]2
20 mol % AgOAc
10 mol % Cu(OAc)2•H2O
•
•
O
1a
+
+
NTs
•
•
solvent, 60 °C, 16 h
under air
+
•
•
+
NTs
•
PhCl, 60 °C, 16 h
under air
NTs
•
•
NTs
3aa
4aa
2a
(1 equiv)
3aa / 46%
4aa / 3%
2a
(1 equiv)
[2+1+2+1]
[2+2+2]
Entry
Rh or Ir complex
Additive
Solvent
Yield [%][b]
Scheme 2. CpERhIII complex-catalyzed oxidative and decarboxylative [2+2+2]
3aa 4aa
and [2+1+2+1] cycloadditions of benzoic acid with alkynes.
1
[CpERhCl2]2
[CpERhCl2]2
[CpERhCl2]2
[CpERhCl2]2
[CpERhCl2]2
[CpERhCl2]2
[CpERhCl2]2
[CpERhCl2]2
[CpERhCl2]2
[IndERhCl2]2
[CpARhCl2]2
[Cp*RhCl2]2
[Cp*IrCl2]2
AgOAc
AgOAc
AgOAc
AgOAc
AgNTf2
AgSbF6
PhCl
46
25
57
38
0
3
2
toluene
2
To improve the yield of 3aa, we conducted further
investigations into the reaction conditions as shown in Table 1.
With respect to the solvent (entries 1–3), the use of toluene that
was a suitable solvent for our previously reported oxidative and
decarboxylative [2+2+2] cycloaddition markedly decreased the
yield (entry 2), but the use of (CH2Cl)2 improved the yield to 57%
(entry 3). Lowering the reaction temperature to 40 °C led to an
incomplete conversion of 2a even for 72 hours (entry 4).
Screening of additives revealed that AgOAc was the best one
(entries 5–9), although the use of NaOAc instead of AgOAc still
promote the desired reaction (entry 8). The use of moderately
electron-deficient IndERhIII[18] and CpARhIII[19] complexes lowered
the yields of 3aa (entries 10 and 11). Importantly, the use of
electron-rich Cp*RhIII and Cp*IrIII complexes failed to give 3aa
(entries 12 and 13), and no reaction was observed in the
absence of the rhodium/iridium complex (entry 14).
3
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
4
2
0
0
4
3
0
1
4
0
2
0
4[c]
5
6
0
[d]
7
Ag2CO3
NaOAc[e]
none
49
35
0
8
9
10
11
12
13
14
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
16
3
0
0
none
0
With the optimized conditions in hand, we investigated the
substrate scope of the CpERhIII complex-catalyzed [2+1+2+1]
cycloaddition (Scheme 3).[20] The investigation of the diyne
termini revealed that not only non-substituted phenyl (2a) but
also electron-withdrawing (2b) and donating (2c) phenyl groups
are tolerable to afford 3ab and 3ac, although the product yields
decreased. Unfortunately, the use of diyne 2d possessing alkyl
groups at the alkyne termini significantly decreased the product
yields. Concerning tethers of the 1,6-diynes, oxygen-linked diyne
2e reacted with 1a to give 3ae as a sole product, although its
yield was moderate. Carbon-linked diynes 2f and 2g showed low
reactivity even at 80 °C, especially, the use of malonate-linked
diyne 2g led to an incomplete conversion (ca. 40%). Importantly,
the reaction of naphthalene-linked 1,6-diyne 2h with 1a afforded
[2+2+2] cycloaddition product 4ah as a predominant product.
Concerning benzoic acids, introduction of electron-donating (1b
[a] [CpERhCl2]2 (0.0050 mmol), AgOAc (0.020 mmol), Cu(OAc)2•H2O (0.010
mmol), 1a (0.100 mmol), 2a (0.100 mmol), and solvent (2.0 mL) were used. [b]
Determined by 1H NMR of the crude reaction mixture using dimethyl sulfone
as an internal standard. [c] At 40 °C for 72 h. [d] Ag2CO3 (10 mol %) was used.
[e] NaOAc (40 mol %) was used.
Ph
Me
H
N
Me
Me
Me
Me
Cl
Ph
O
EtO2C
Cl
Me
Ph
O
Rh
Cl
Rh
M
Cl
Cl
Cl
2
2
2
[IndERhCl2]2
[CpARhCl2]2
[Cp*RhCl2]2 (M = Rh)
[Cp*IrCl2]2 (M = Ir)
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5 mol % [CpERhCl2]2
20 mol % AgOAc
10 mol % Cu(OAc)2•H2O
R3
R3
R1
R1
R1
R1
R3
R3
R3
H
+
R2
(CH2Cl)2, under air
R2
R3
R2
R2
CO2H
R3
R3
2
1
3
4
3' (not detected)
(1 equiv)
NTs
NTs
NTs
O
NTs
Me
CF3
OMe
Me
CF3
OMe
3aa+4aa / 61% (94:6)[a]
3ab / 34%
3ac+4ac / 41% (97:3)[a]
3ad+4ad / 17% (95:5)[a]
3ae / 33%
3af+4af / 26% (71:29)[a]
(60 °C, 16 h)
(60 °C, 24 h)
(60 °C, 24 h)
(60 °C, 23 h)
(60 °C, 12 h)
(80 °C, 72 h)
MeO2C
CO2Me
NTs
NTs
NTs
NTs
Me
MeO
Cl
Me
Me
3ag / 5%
3ah / 8%[b] (4ah / 55%)
3ba+4ba / 61% (91:9)[a]
3ca / 38%
3da+4da / 55% (96:4)[a]
3ea+4ea / 52% (77:23)[a]
(80 °C, 72 h)
(60 °C, 48 h)
(60 °C, 24 h)
(60 °C, 24 h)
(60 °C, 24 h)
(60 °C, 24 h)
OMe
CF3
OMe
OMe
CF3
Me
OMe
CF3
OMe
Me
OMe
CF3
OMe
OMe
3ai / 36%,[b] 42%[b,c]
(80 °C, 16 h)
3aj / 38%[c]
(80 °C, 24 h)
3ak / 36%[b,c]
(80 °C, 24 h)
3al / 27%[c]
(80 °C, 24 h)
3am / 18% (4am / 36%)[c]
(80 °C, 24 h)
Me
MeO
Cl
Me
Me
Me
OMe
OMe
OMe
OMe
OMe
OMe
3bj / 32%[c]
OMe
3cj / 28%[c]
OMe
3dj / 43%[c]
OMe
3ej / 32%[c]
OMe
3fj / 33%[c]
(80 °C, 24 h)
(80 °C, 24 h)
(80 °C, 24 h)
(80 °C, 24 h)
(80 °C, 24 h)
Scheme 3. Substrate scope. [CpERhCl2]2 (0.0050 mmol), AgOAc (0.020 mmol), Cu(OAc)2•H2O (0.010 mmol), 1 (0.100 mmol), 2 (0.100 mmol), and (CH2Cl)2 (2.0
mL) were used. The cited yields are of the isolated products. [a] A mixture of 3 and 4 was isolated. The ratios were determined by 1H NMR. [b] The yields were
determined by 1H NMR of the crude reaction mixture using dimethyl sulfone as an internal standard. Analytically pure compounds were isolated by sequentiual
silica gel chromatography and/or gel permeation chromatography. See the Supporting Information for details. [c] (CH2Cl)2 (10 mL) was used.
[2+2+2] cycloaddition product 4am as a major product. As with
1,6-diyne 2a, 1,7-diyne 2j reacted with a series of substituted
benzoic acids 1b–f to furnish the corresponding [2+1+2+1]
cycloaddition products 3bj–fj.
proceed. Thus, reductive elimination furnishes cyclobutadiene-
rhodium(I) complex J, and the subsequent oxidative addition of
tautomeric cyclobutadiene J’ (dearomatized form) to rhodium(I)
generates rhodacycle K. Subsequent alkyne insertion and
reductive elimination generate [2+1+2+1] cycloaddition product 3
and rhodium(I) complex I. Finally, oxidation of I by the copper(II)
co-catalyst regenerates rhodium(III) complex E. This mechanism
is consistent with the observed complete regioselectivity
(absence of regioisomers 3’ in Scheme 3) in the reactions of
substituted benzoic acids. According to the above mechanism,
Based on the previous reports concerning the oxidative and
decarboxylative [2+2+2] cycloaddition of benzoic acids with two
alkynes,[14–16] and the [2+1+2+1] cycloaddition of three
alkynes,[2,3] mechanisms for the formation of 3 and 4 are
proposed as shown in Scheme 4a. Rhodium(III) acetate E,
generated by the reaction of [CpERhCl2]2 with AgOAc, reacts
with 1 to give rhodacycle F via directed ortho C-H bond cleavage.
Subsequently, insertion of one alkyne unit of diyne 2 affords
the chemoselectivity between
3 and 4 depends on the
regioselectivity of the first alkyne insertion step giving G or G’.
The phenyl group of 2a rather than the tosylamide group may
prefer to locate on the rhodium side (F-2a) giving G’ as a result
of the coordination of the phenyl group to rhodium. In the
reaction of biphenyl-linked 1,7-diynes 2j, the phenyl group rather
than the biphenyl group may prefer to locate on the rhodium side
(F-2j) giving G’ as a result of the steric repulsion between the
biphenyl group and the ligand.[22,23] On the contrary, the
regioisomeric seven-membered rhodacycle
G
or G’.
Decarboxylation from G furnishes five-membered rhodacycle H,
and intramolecular alkyne insertion and reductive elimination
generate [2+2+2] cycloaddition product
4 and rhodium(I)
complex I.[14–16] On the other hand, decarboxylation from G’
affords H’, in which intramolecular alkyne insertion cannot
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10.1002/chem.201901050
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COMMUNICATION
naphthalene moiety of 2h may prefer to locate to the rhodium
side (F-2h) giving G presumably due to the stronger interaction
between the rhodium center and the π-electron of naphthalene
than benzene,[10b,24] and thus the [2+2+2] cycloaddition product
4ah was generated as a major product. Consistent with the
above mechanism, the reaction of 1a with monoyne 5 under the
standard conditions furnished 2,3-diarylnaphthalene 6 as a
major regioisomer (Scheme 4b).[25] The electron-deficient nature
of the CpERhIII complex may accelerate the reductive elimination
to form cyclobutadiene-rhodium(I) complex J.[26] Additionally, the
intramolecular coordination of the arylacetylene moiety to
rhodium in intermediate H’ may deter the intermolecular reaction.
dehydrogenation to give hemi-hexabenzocoronene
8
in
quantitative yield. Whereas, the Scholl reaction of none-fused
tetraarylnaphthalene 9[27] under the same conditions furnished
partially fused tetraarylnaphthalene 10, in which only the
electron-rich aryl groups were dehydrogenated.
CF3
CF3
FeCl3 (5 equiv)
CF3
CH2Cl2-MeNO2
28 °C, 52 h
OMe
CF3
(a)
Ar
•
Ar
•
•
OMe
7 / 82%
Ar
Z
R
R
•
•
R
Z
Z
•
CF3
[Rh]
[Rh]
O
–CO2
decarboxylation
OMe
Ar
Ar
O
Z
4
Ar
CF3
OMe
R
•
H
G
FeCl3 (20 equiv)
3am
•
Ar
[2+2+2] pathway
2
CH2Cl2-MeNO2
40 °C, 30 min
Ar
3
[Rh]
R
OMe
Cu(OAc)2
1
O
[Rh]
[Rh]
I
AcO
OAc
–2AcOH
OMe
8 / >99%
O
E
Z
C-H cleavage
F
R
•
[Rh]
CF3
CF3
2
[2+1+2+1] pathway
•
Ar
Ar
CF3
CF3
Z
[Rh]
K
Z
Z
R
Ar
•
[Rh]
FeCl3 (20 equiv)
•
•
•
R
R
•
CH2Cl2-MeNO2
40 °C, 1 h
Ar
•
Ar
Ar
–CO2
Ar
O
[Rh]
OMe
OMe
J'
Ar
Z
O
H'
G'
reductive
elimination
decarboxylation
OMe
OMe
[Rh]
R
9
10 / 78%
•
•
Ar
Scheme 5. Synthetic application.
J
Ar
In conclusion, we have established that an electron-deficient
CpERhIII complex catalyzes the oxidative and decarboxylative
[2+1+2+1] cycloaddition of benzoic acids with 1,6- and 1,7-
diynes through C≡C triple bond cleavage, which produces fused
unsymmetric naphthalenes. In this cyclotrimerization, the
electron-deficient nature of the CpERhIII complex may facilitate
TsN
[Rh] CpE
[Rh] CpE
[Rh] CpE
O
O
O
O
F-2a
O
F-2j
O
F-2h
reductive elimination giving
a
cyclobutadiene-rhodium(I)
(b)
Ts
N
5 mol % [CpERhCl2]2
20 mol % AgOAc
10 mol % Cu(OAc)2•H2O
complex that is
a
key intermediate of this [2+1+2+1]
Me
Ph
Ph
cycloaddition. The present CpERhIII-catalyzed [2+1+2+1]
cycloaddition of benzoic acid with a biphenyl-linked donor-
acceptor 1,7-diyne was successfully applied to the facile
+
OH
NTs
Me
(CH2Cl)2, 60 °C, 16 h
Ph
O
5
Me
N
Ts
1a
(2 equiv)
synthesis of
a
donor-acceptor [5]helicene and
a
hemi-
6 / 71%
hexabenzocoronene by a combination with the chemoselective
Scholl reaction using iron(III) chloride.
Scheme 4. (a) Proposed mechanisms for formation of 3 and 4. [Rh] = CpERh
complex. (b) Intermolecular reaction of 1a with 5.
Acknowledgements
A combination of the present [2+1+2+1] cycloaddition and
the chemoselective Scholl reaction enabled the facile synthesis
of a donor-acceptor [5]helicene and a hemi-hexabenzocoronene
as shown in Scheme 5. The Scholl reaction of partially fused
tetraarylnaphthalene 3am with iron(III) chloride at room
temperature promoted the selective dehydrogenation between
the electron-rich aryl groups to give donor-acceptor [5]helicene 7
in good yield. The use of a large excess amount of iron(III)
chloride at the elevated temperature (40 °C) enabled complete
This work was supported partly by ACT-C (No.
JPMJCR1122YR) from JST (Japan), and Grants-in-Aid for
Young Scientists (No. 17K14481) and Scientific Research on
Innovative Areas “π-System Figuration: Control of Electron and
Structural Dynamism for Innovative
Functions” (No.
JP26102004) from JSPS (Japan). We thank Umicore for
generous support in supplying RhCl3•nH2O.
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10.1002/chem.201901050
Chemistry - A European Journal
COMMUNICATION
Keywords: C≡C Triple Bond Cleavage • C-H Bond
Functionalization • Cyclopentadienyl Rhodium Complexes •
Decarboxylation • [2+1+2+1] Cycloaddition
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were detected in the crude reaction mixtures as minor products (trace
to ca. 25% yield) unless otherwise noted.
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carbons shown below.
[3]
[4]
[5]
CF3
!–
C
MeO
MeO
!+
C
CF3
Rh CpE
O
C
O
F-2m
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A small amount (ca. 15%) of regioisomer 6’ was also detected,
[6]
[7]
[8]
[9]
although this compound could not be isolated in a pure form.
Ph
For a recent review using halobenzenes, see: J. Le Bras, J. Muzart,
Synthesis 2014, 46, 1555.
Me
N
Ts
For benzoyl chlorides, see: T. Yasukawa, T. Satoh, M. Miura, M.
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Ph
Me
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6'
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9
was prepared by the cross-[2+2+2]
cycloaddition of 1a with two different alkynes. See the Supporting
Information for details.
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10.1002/chem.201901050
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Me
Yusaku Honjo, Yu Shibata,* Ken
Tanaka*
CO2Et electron-deficient
CpERh accerelates...
EtO2C
Me
Cl
cat.
H
CpERhIII
Ar
Ar
•
•
•
Me
+
R
- C-H bond cleavage
- decarboxylation
Rh
Cl
R
CO2H
•
Ar
Page No. – Page No.
2
- reductive elimination
Ar
[2+1+2+1] cycloaddition
[CpERhCl2]2
Rhodium-Catalyzed [2+1+2+1]
Cycloaddition of Benzoic Acids with
Diynes via Decarboxylation and C≡C
Triple Bond Cleavage
It has been established that an electron-deficient CpERhIII complex catalyzes the
oxidative and decarboxylative [2+1+2+1] cycloaddition of benzoic acids with diynes
through C≡C triple bond cleavage. The electron-deficient nature of the CpERhIII
complex may facilitate reductive elimination giving the key cyclobutadiene-
rhodium(I) intermediate. The combination of the present [2+1+2+1] cycloaddition
and the chemoselective Scholl reaction enabled the facile synthesis of a donor-
acceptor [5]helicene and a hemi-hexabenzocoronene.
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