Iron corrole-catalyzed [4 + 2] cycloaddition of dienes and aldehydes

Article abstract of DOI:10.1246/cl.130672

We have developed cationic iron(IV) corrole-catalyzed formal hetero-Diels-Alder reaction of aldehydes with dienes, which opens the way for divergent synthesis of pyrans. The potential utility of the catalyst was also demonstrated by carrying out cycloaddition of α,β-unsaturated aldehyde with a diene to give pyran selectively. Furthermore, the catalytic reactivity of iron corrole complex on the cycloaddition was determined by comparing that of iron porphyrin complex.

Full text of DOI:10.1246/cl.130672

doi:10.1246/cl.130672
Published on the web October 5, 2013
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Iron Corrole-catalyzed [4 + 2] Cycloaddition of Dienes and Aldehydes
Toru Kuwano,1 Takuya Kurahashi,*1,2 and Seijiro Matsubara*1
1Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510
2JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012
(Received July 18, 2013; CL-130672; E-mail: kurahashi.takuya.2c@kyoto-u.ac.jp,
matsubara.seijiro.2e@kyoto-u.ac.jp)
We have developed cationic iron(IV) corrole-catalyzed
formal hetero-Diels-Alder reaction of aldehydes with dienes,
which opens the way for divergent synthesis of pyrans. The
potential utility of the catalyst was also demonstrated by
carrying out cycloaddition of ¡,¢-unsaturated aldehyde with
a diene to give pyran selectively. Furthermore, the catalytic
reactivity of iron corrole complex on the cycloaddition was
determined by comparing that of iron porphyrin complex.
yield as well as [Fe(tpc)]SbF6 catalyst. The use of other counter
anions such as TfO, ClO4, and Cl resulted in the formation 3aa
in trace amounts or lower yields.
With the optimized reaction conditions in hand, we
evaluated the cationic iron(IV) corrole-catalyzed [4 + 2] cyclo-
addition of various aldehydes 1 with diene 2a. Arylaldehydes
with electron-withdrawing substituents such as fluoride, chlo-
ride, and bromide afforded pyrans 3 in good yields (Table 1).
Arylaldehyde 1e, possessing an electron-donating methoxy
substituent, reacted with 2a to furnish 3ea in excellent yield
Metalloporphyrins perform important biochemical functions
in nature. Therefore, the related chemistry has also been a
research topic of great interest.1 Recently, we reported that
metalloporphyrins could behave as Lewis acid and catalyze
various reactions such as formal hetero-Diels-Alder reaction and
cycloisomerization.2,3 Herein, we report that iron corrole,4,5
which is an analogue of iron porphyrin with a tetravalent iron
center, can catalyze [4 + 2] cycloaddition of aldehydes and
unactivated dienes.6,7
[Fe(tpc)]X (5 mol%)
O
O
+
benzene, 80 °C, 12 h
Ph
H
Ph
Ph
Ph
1a
2a
3aa
yield
X
Ph
N
SbF6
BF4
>99%
>99%
84%
X–
N
Fe
N
TFPB
OTf
ClO4
Cl
Ph
Ph
N
30%
<1%
<1%
[Fe(tpc)]X
Initially, we prepared the cationic iron(IV) corrole catalyst
[Fe(tpc)]SbF6 (tpc: 5,10,15-triphenylcorrole). To a CH2Cl2
solution of pyrrole and benzaldehyde was added a catalytic
amount of trifluoroacetic acid (Scheme 1). After the reaction
mixture was stirred at r.t. for 2 h, chloranil was added and stirred
for 15 min, then free-base corrole (H3TPC) was obtained. The
free-base corrole (H3TPC) and FeCl2 were dissolved in DMF,
and the resulting mixture was heated to reflux. Purification of
the crude mixture with silica gel chromatography afforded
[Fe(tpc)]Cl. Cationic iron(IV) corrole complex [Fe(tpc)]SbF6
was prepared by treating [Fe(tpc)]Cl with AgSbF6 in CH2Cl2 at
ambient temperature.
We next examined the reaction of benzaldehyde (1a) with
4-methyl-2-phenyl-1,3-pentadiene (2a) in the presence of the
cationic iron(IV) corrole catalyst [Fe(tpc)]SbF6 (5 mol %) in
benzene at 80 °C for 12 h, affording pyran 3aa in 99% isolated
yield (Scheme 2). Upon optimization of the counter anion of the
cationic iron(IV) corrole complex, 3aa was obtained in excellent
yield. When [Fe(tpc)]BF4 was used, 3aa was obtained in 99%
Scheme 2. Effects of counter anion.
Table 1. Iron corrole-catalyzed [4 + 2] cycloaddition of alde-
hydes 1 and diene 2aa
O
[Fe(tpc)]BF4 (5 mol%)
O
+
R
H
benzene, 80 °C, 12 h
Ph
Ph
R
1
2a
3
Cycloadduct 3b
O
O
O
Ph
Ph
Ph
Ph
Ph
Ph
Ph
MeO
3ba 46%
3ca 98%
3da 77%
MeO
O
O
O
Ph
F
Cl
3ea 98%
3fa 99%
3ga 92%
O
O
O
Ph
Ph
H
O
TFA
chloranil
N
NH
N
+
Ph
Br
O2N
NC
Ph
Ph
H
NHHN
CH2Cl2, r.t., 2 h
CH2Cl2, r.t., 15 min
3ha 95%
3ia 95%
3ja 96%
H3TPC
O
O
O
BnO
Ph
Ph
Ph
Ph
N
–
Ph
N
Cl
SbF6
Ac
FeCl2
DMF, 170 °C, 0.5 h
AgSbF6
N
N
3ka 95%
3la 97%
3ma 92%
Fe
Fe
Ph
Ph
Ph
Ph
N
N
N
N
CH2Cl2, r.t., 2 h
aReactions were carried out using [Fe(tpc)]BF4 (5 mol %),
aldehyde 1 (0.5 mmol), and diene 2a (2.0 mmol, 4 equiv) in
2 mL of benzene at 80 °C for 12 h, unless otherwise noted.
bIsolated yields based on aldehyde 1.
[Fe(tpc)]Cl
[Fe(tpc)]SbF6
Scheme 1. Synthesis of [Fe(tpc)]SbF6.
Chem. Lett. 2013, 42, 1241-1243
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1242
Table 2. Iron corrole-catalyzed [4 + 2] cycloaddition of ben-
zaldehyde 1a and dienes 2a
O
O
[Fe(tpc)]SbF6 (5 mol%)
+
+
Ph
Ph
Ph
benzene, 80 °C, 12 h
Ph
H
R3
Ph
Ph
R4 R3
R2
R1
R2
R1
1n
2a
2a
3na 86%
O
R4
[Fe(tpc)]BF4 (5 mol%)
O
+
Ph
H
benzene, 80 °C, 12 h
Ph
1a
2
3
O
O
[Fe(tpc)]SbF6 (5 mol%)
Cycloadduct 3b
benzene, 80 °C, 12 h
O
O
O
1o
3oa 45%
Ph
Ph
Ph
Scheme 3. Iron corrole-catalyzed [4 + 2] cycloaddtion.
3ab 52%
3ac 64%
3ad 84%
(98%). Nitro and nitrile substituents on benzaldehyde were well
tolerated under the present reaction conditions and the corre-
sponding cycloaddition products 3ia and 3ja respectively, were
isolated in excellent yields. Furthermore, the reaction of p-
acetylbenzaldehyde (1k) with 2a gave 3ka in 95% yield. The
cationic iron(IV) corrole complex was also found to be effective
for the reaction of aliphatic aldehydes with 2a. Cyclohexane-
carbaldehyde (1l) reacted with 2a to afford the corresponding
substituted pyran 3la in 97% yield. Notably, cinnamaldehyde
(1n) has the potential of Diels-Alder reaction with alkenes and
dienes, however, it underwent cycloaddition with 2a to afford
pyran 3na in 86% yield (Scheme 3). Additionally, cyclohex-
anone (1o) in place of aldehydes reacted with 2a to furnish 3oa
in moderate yield (45%).8
O
O
O
Ph
Ph
Ph
Cl
Cl
OMe
OMe
3ae 57%
3af 81%
3ag 85%
O
O
O
S
Ph
Ph
Ph
3ah 99%
3ai 95%
3aj 92%
aReactions were carried out using [Fe(tpc)]BF4 (5 mol %),
benzaldehyde 1a (0.5 mmol), and diene 2 (2.0 mmol, 4 equiv)
in 2 mL of benzene at 80 °C for 12 h, unless otherwise noted.
bIsolated yields based on benzaldehyde 1.
The scope of the [4 + 2] cycloaddition was explored by
using various dienes 2 with benzaldehyde (1a), and the results
are summarized in Table 2. The reaction of 2,3-dimethyl-1,3-
butadiene (2b) with 1a provided 3ab in 52% yield. Cyclo-
addition of conformationally s-cis fixed diene such as 1,2-
dimethylidenecyclohexane (2c) with 1a also proceeded success-
fully to give hexahydro-1H-isochromene derivative 3ac in 64%
yield. 2-Aryl-1,3-butadienes having an electron-withdrawing or
-donating substituent on the phenyl moiety reacted with 1a to
afford the corresponding substituted pyrans regioselectively in
good yield (3ad, 3ae, and 3af). Trisubstituted 1,3-butadienes
also participated in the reaction with 1a to provide correspond-
ingly substituted pyrans in good yields (3ag, 3ah, and 3ai). 1,3-
Butadiene possessing a thiophene moiety reacted with 1a to give
3aj in 92% yield.
O
O
Fe catalyst (5 mol%)
+
Ph
Ph
benzene, 80 °C, 12 h
Ph
H
Ph
1a
2k
3ak (dr)
[Fe(tpc)]BF4
[Fe(tpp)]BF4
99% (87/13)
99% (92/8)
O
O
Fe catalyst (5 mol%)
+
Ph
Ph
benzene, 80 °C, 12 h
Ph
H
Ph
1a
2l
3ak (dr)
[Fe(tpc)]BF4
[Fe(tpp)]BF4
89% (54/46)
84% (55/45)
tBu
tBu
O
O
Fe catalyst (5 mol%)
+
Ph
Ph
benzene, 80 °C, 12 h
Ph
H
Ph
1a
2m
3am (dr)
We next examined the differences between cationic iron(IV)
corrole catalyst and cationic iron(III) porphyrin catalyst in terms
of reactivity and selectivity, and the results are summarized in
Scheme 4. It was found that (E)-2-phenyl-1,3-pentadiene (2k)
reacted with 1a in the presence of cationic iron(IV) corrole
catalyst to afford cis-3ak in 99% yield in a regioselective
manner, with a stereoisomer ratio of 87/13, while the reaction
with cationic iron(III) porphyrin catalyst [Fe(tpp)]BF4 (tpp:
5,10,15,20-tetraphenylporphyrin) gave cis-3ak with slightly
better stereoselectivity. However, the [4 + 2] cycloaddition of
1a with (Z)-2-phenyl-1,3-pentadiene (2l) afforded pyran 3ak as a
mixture of stereoisomers regardless of the catalysts employed.
The iron corrole-catalyzed reaction of 1a with 1,3-butadiene 2m
possessing sterically hindered substituent afforded 3am in 85%
yield with a stereoisomer ratio of 69/31, while the reaction with
iron porphyrin catalyst gave 3am in 70% yield with a stereo-
isomer ratio of 82/18. These results suggest that the iron corrole
complex with tetravalent iron is more Lewis acidic than iron
porophyrin complex with trivalent iron which may cause
reduced stereoselectivity.
[Fe(tpc)]BF4
[Fe(tpp)]BF4
85% (69/31)
70% (82/18)
Scheme 4. Iron corrole-catalyzed [4 + 2] cycloaddtion.
In conclusion, we have developed cationic iron(IV) corrole-
catalyzed [4 + 2] reaction of aldehydes with dienes to afford
pyrans. It was also demonstrated that cationic iron(IV) corrole
complex has potential catalytic ability as well as iron(III)
porphyrin complex. Efforts for detailed studies to elucidate the
mechanism underlying the unique reactivity of cationic iron(IV)
corrole are underway.9
This work was supported by JST, ACT-C and Grants-in-Aid
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan. T.K. acknowledges the Asahi Glass Foun-
dation, the Uehara Memorial Foundation, Tokuyama Science
Foundation, and Kurata Memorial Hitachi Science and Tech-
nology Foundation.
Chem. Lett. 2013, 42, 1241-1243
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1243
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© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/