Fe-Catalyzed Pictet-Spengler-Type Cyclization via Selective Four-Electron Reductive Functionalization of CO2

Article abstract of DOI:10.1002/cjoc.202000521

Herein, we describe a novel catalytic Pictet-Spengler-type cyclization using CO₂ as a nontoxic and sustainable C1 feedstock with environmentally benign and non-precious-metal iron as catalyst. The reaction is achieved by selective four-electron reduction of CO₂ into methylene level intermediate through carefully tuning the reaction parameters. A variety of tetrahydro-β-carbolines and other nitrogen-containing heterocycles can be easily obtained under mild conditions. Mechanistic studies have shown that tetrahydro-β-carbolines are probably obtained via spiroindolenine intermediates.

Full text of DOI:10.1002/cjoc.202000521

Chinese Journal
of Chemistry
CJC
中 国 化 学 - An International Journal
Accepted Article
Title: Fe-Catalyzed Pictet-Spengler-type Cyclization via Selective Four-electron
Reductive Functionalization of CO2
Authors: Wen-Duo Li, Jie Chen, Dao-Yong Zhu* and Ji-Bao Xia*
This manuscript has been accepted and appears as an Accepted Article online.
This work may now be cited as: Chin. J. Chem. 2020, 38, 10.1002/cjoc.202000521.
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published online in Early View: http://dx.doi.org/10.1002/cjoc.202000521.
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Chinese Journal
of Chemistry
Carbon dioxide, Iron catalysis, Four-electron reduction, Cyclization
Report
CJC
中
国 化 学 - An International Journal
Fe-Catalyzed Pictet-Spengler-type Cyclization via Selective Four-electron
Reductive Functionalization of CO2
Wen-Duo Li,a,b Jie Chen,a Dao-Yong Zhu*a and Ji-Bao Xia* a,b
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Center for Excellence in Molecular Synthesis, Suzhou Research Institute of LICP, Lanzhou
Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou, 730000, China
b University of Chinese Academy of Sciences, Beijing 100049, China
Cite this paper: Chin. J. Chem. 2020, 38, XXX—XXX. DOI: 10.1002/cjoc.202000XXX
Summary of main observation and conclusion We herein describe a novel catalytic Pictet-Spengler-type cyclization using CO2 as a nontoxic and
sustainable C1 feedstock with environmentally benign and non-precious-metal iron as catalyst. The reaction is achieved by selective four-electron
reduction of CO2 into methylene level intermediate through careful tuning the reaction parameters. A variety of tetrahydro-β-carbolines and other
nitrogen-containing heterocycles have been produced under mild conditions. Mechanistic studies showed that tetrahydro-β-carbolines is probably
obtained via a spiroindolenine intermediate.
This article has been accepted for publication and undergone full peer review but has not been
through the copyediting, typesetting, pagination and proofreading process which may lead to
differences between this version and the Version of Record. Please cite this article as doi:
10.1002/cjoc.202000521
This article is protected by copyright. All rights reserved.

First author et al.
Report
Tetrahydro-β-carbolines are one type of biologically important
structural motifs widely distributed in natural products and
pharmaceutical agents.[10] The most general methods to
synthesize tetrahydro-β-carbolines is Pictet-Spengler reaction
from tryptamines and formaldehyde catalyzed or promoted by
acid.[11] In comparison with formaldehyde, CO2 is a more
abundant, nontoxic, and sustainable C1 feedstock. Recently, we
reported a selective N-methylation or N-formylation of amines
with CO2 using easily handled silanes as reducting agent under an
Fe/monophos catalytic system.[12] We hypothesized that a
Pictet-Spengler type cyclization with CO2 would be realized by
non-precious iron catalysis via selective four-electron reductive
functionalization of CO2 under suitable conditions. Herein, we
report a Fe-catalyzed Pictet-Spengler-type cyclization to afford
tetrahydro-β-carbolines and other N-heterocycles via selective
functionalization of CO2 under mild condtions (Scheme 1d).
Background and Originality Content
As an abundant, nontoxic, and sustainable carbon feedstock,
carbon dioxide (CO2) has been widely used as a C1 building block
for the synthesis of valuable chemical compounds.[1] Bearing the
highest oxidation state of carbon, CO2 can be reduced into many
intermediates by single- or multi-electron reduction using various
types of reducing agents such as dihydrogen, hydrosilanes, and
hydroboranes.[2] In between, four-electron reduction of CO2
generating formaldehyde remains a challenge, because over
reduction to methanol is a much easier process. As a consequence
of this limitation, several types of reactions have been achieved
affording formaldehyde oxidation level compounds via
four-electron reductive functionalization of CO2. First, selective
reduction of CO2 into bis(silyl)acetal species has been widely
investigated with easily handled hydrosilanes (Scheme 1a).[3]
Under hydroboration conditions, bis(boryl)acetals have been
accessed[4] and transient formaldehyde have been successfully
detected with well-defined iron catalyst by Bontemps,
SaboEtienne and co-workers.[4c] Next, four-electron reduction of
CO2 via deoxygenation has also been achieved affording
symmetrical molecules via formation of two identical chemical
bonds (Scheme 1b). Catalytic reduction of CO2 generating aminals
with amine and silanes was achieved by Cantat first and then He
and Han respectively.[5] Furthermore, Klankermayer and others
reported that Ru- or Co-catalyzed reductive functionalization of
CO2 selectively generated acetals with alcohols using H2 as
reductant.[6] Recently, Xi and co-workers reported reduction of
CO2 into dithioacetals with thiols under NaBH4/I2 system.[7] In
Scheme 1 Selective four-electron reductive functionalization of CO2
or
Hydrosilylation hydroboration
or
of CO2 with silanes boranes
a)
b)
or
cat.
promoter
or
O
O
O
O
or
CO2
SiR3
BR2
R
R2B
3Si
R
R2BH
3SiH
or
of CO2 to aminals acetals
Reductive
Deoxygenation
or
R
cat.: TBD
betaine
glycine
3SiH
NR2
OR
SR
R2N
RO
RS
R2NH
or
cat.
[Ru] [Co]
ROH
RSH
CO2
H2
I2
NaBH4
of 2-substituted
with CO
2
c) Spirocyclization
tryptamines
2017, we reported
a
organocatalytic spirocyclization of
NHPh
Ph
N
2-substituted tryptamine derivatives via four-elcetron reductive
functionalization of CO2 (Scheme 1c).[8] Later, a metal-free
deoxygenative olefination of phosphorus ylides with CO2 was also
reported under reductive conditions.[9] However, research in this
area still has its limitations, such as limited reaction types and
harsh reaction conditions including high pressure of CO2 and high
reaction temperature. Therefore, the development of novel and
efficient methods for selective four-electron reductive
functionalization of CO2 under mild conditions with
environmentally benign catalyst is of great significance and
rewarding.
20 mol% TBD
+
+
PhSiH3
CO2
°
CH3CN, 100
C
N
Me
Me
reaction with
CO2
Pictet-Spengler-type
H
N
work:
This
Fe-catalyzed reductive
d)
3 mol%
15 mol% P( -Bu)3
[CpFe(CO)2]2
NHR
n
+
+
PhSiH3
CO2
°
H
H
THF/CH
60
3CN,
C
N
R
N
mild conditions
simple ligand
 selective 4-electron reduction  C C & C
formation



iron catalysis
 low
of CO
pressure
Non-precious
2
or FeCl3 as catalyst (Table 1, entrie 5). A variety of MonoPhos
ligands were screened. Moderate yield of 2a was obtained with
triphenylphosphine (PPh3) as ligand, and the yield of 4a was
increased significantly (Table 1, entrie 6). Using other
trialkylphosphine ligands, such as PMe3, PCy3 and P(t-Bu)3,
60–81% yields of 2a were obtained, (Table 1, entries 7–9).
Different silanes were also evaluated as reducing agents.
Diphenylsilane (Ph2SiH2) gave 2a in 62% yield, whereas no
reaction occurred with Ph3SiH, Et3SiH, or PMHS (Table 1, entries
10–11). Reducing the amount of PhSiH3 to 1.5 equivalent led to a
decreased yield of 2a (Table 1, entry 12). Switch the co-solvent to
THF or CH3CN resulted in diminished yield of the product and
recovery of 1a (Table 1, entries 13-14). When reducing the
reaction temperature to 30 °C, 2a was obtained in 33% yield,
along with N-formylation product 3a in 46% yield. (Table 1, entry
15).
Results and Discussion
We started our research by studying Fe-catalyzed
Pictet-Spengler-type cyclization of tryptamine derivative 1a with
atmospheric pressure of CO2 (1 atm) under reductive conditions.
After extensive investigations, we found that the desired
cyclization product 2a was obtained in 90% yield with
[CpFe(CO)2)]2 as catalyst, P(n-Bu)3 as ligand, and phenylsilane
(PhSiH3) as the reducing reagent in mixed THF and CH3CN at 60 °C
(Table 1, entry 1). The trace amount of N-formylation or
N-methylation product 3a and 4a was detected. Control
experiments indicated that the iron catalyst and the phosphine
ligand are both necessary (Table 1, entry 2). The iron pre-catalyst
has a significant effect on the reaction and product distribution.
Lower yield of 2a was obtained with Fe(acac)2 or Fe(acac)3 as
catalyst (Table 1, entrie 3 and 4). No reaction occurred with FeCl2
With the optimal reaction conditions in hand, we next
Chin. J. Chem. 2020, template
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Running title
Chin. J. Chem.
investigated the substrate scope of this Fe-catalyzed
Pictet-Spengler-type cyclization with 1 atm of CO2 (Table 2). The
substituent effect on the amine was first explored. Moderate to
excellent yields were obtained with aryl group bearing both
electron-donating and electron-withdrawing groups on the amine
(2b–2f). Furthermore, substrates bearing a heteroaryl group
(2-pyridyl) or an alkyl group (benzyl) on the amine also delivered
the corresponding product in good yield (2g and 2h). The pyridyl
and benzyl group can be removed through routine method,
providing the opportunities for further manipulation of these
useful products. Next, different substituents on the indole core of
tryptamine derivatives were tested. The reaction tolerates
electron-donating (MeO, BnO) and electron-withdrawing (F, Cl)
groups on the different positions of indole ring, affording the
corresponding products in 71–94% yields (2i–2o). It should be
noted that cyclization of free hydroxyl containing substrate also
delivered the desired product 2p in 51% yield. In addition,
moderate yield of 2q was obtained with N-methyl indole substrate.
When using pyrrole-containing amine or aniline as substrate, the
a
Table 1 Fe-catalyzed Pictet-Spengler-type cyclization with CO2
O
3 mol%
15 mol% P( -Bu)3
[CpFe(CO)2]2
n
HN
N
N
C
Ph
H
3
N
+
+
+
+
Ph
CO2
atm)
PhSiH3
equiv)
(2.0
Ph
Ph
H
THF/CH3 1)
(1/
60 °C, 36 h
CN
N
N
N
N
H
(1
H
H
H
"Standard Conditions"
1a
2a
3a
4a
Yield (%)
Entry
Variations from the ''Standard Conditions''
1a
2a
3a
5
4a
none
<5
>95
<5
9
90 (87)b
0
<5
0
1
2
without Fe catalyst or ligand
Fe(acac)2 instead of [CpFe(CO)2]2
Fe(acac)3 instead of [CpFe(CO)2]2
FeCl2 or FeCl3 instead of [CpFe(CO)2]2
PPh3 instead of P(n-Bu)3
0
52
26
0
27
30
0
9
3
24
0
4
>95
16
<5
8
5
50
81
60
78
62
0
5
23
6
6
PMe3 instead of P(n-Bu)3
6
7
PCy3 instead of P(n-Bu)3
8
11
5
8
P(t-Bu)3 instead of P(n-Bu)3
3.0 equiv Ph2SiH2 instead of PhSiH3
<5
14
>95
27
31
12
7
6
9
7
12
0
10
11
12
13
14
15
6.0 equiv Ph3SiH, Et3SiH, or PMHS instead of PhSiH3
1.5 equiv PhSiH3 instead of 2.0
with THF as solvent
0
50
48
65
33
8
<5
8
7
with CH3CN as solvent
9
5
at 30 oC
46
<5
a
1
All reactions were carried out with 0.2 mmol of 1a in THF/CH3CN (2 mL/2 mL) unless otherwise noted. The yield was determined by H NMR with
1,1,2,2,-tetrachloroethane as an internal standard. b Isolated yield in the parenthesis.
a
Table 2 Scope of Fe-catalyzed Pictet-Spengler-type cyclization with CO2
Chin. J. Chem. 2020, 38, XXX－XXX
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First author et al.
Report
3 mol%
15 mol% P( -Bu)3
H
[CpFe(CO)2]2
CO
Fe
CO
CO
n
N
Fe
CO
R
+
+
Ar
CO2
atm)
PhSiH3
equiv)
(2.0
Ar
N
THF/CH CN 1)
(1/
3
°C,
R
60
N
36 h
(1
1
2
[CpFe(CO)2]2
=
heteroaryl,
H)
alkyl,
aryl,
N
(R
Me
N
N
N
Me
OMe
F
N
N
N
H
N
N
H
H
H
H
2a
2b
2c
2d
2e
,
87%
N
70%
N
55%
90%
84%
,
,
,
,
,
F
MeO
BnO
N
Ph
N
N
N
N
N
N
N
Cl
H
N
N
H
H
F
H
70%b
2f
2g
2i
2j
62%
87%
86%
89%
,
,
,
2h
,
BnO
N
N
N
N
MeO
MeO
F
Cl
N
F
N
N
N
N
H
F
N
H
H
H
H
2l
2m
2n
2o
71%
,
91%
94%
92%
,
,
,
2k
,
94%
CH3
HO
MeO
MeO
N
N
Ph
HN
N
N
N
N
N
NH
70%
Me
H
R
H
=
=
2p
2q
2r
2s
,
R
R
4t
51%
50%
96%
90%
91%
,
,
,
,
,
OMe 4u
aReaction conditions: 1 (0.2 mmol), CO2 (1 atm), PhSiH3 (2 equiv), [CpFe(CO)2)]2 (3 mol%), P(n-Bu)3 (15 mol%), THF/CH3CN (2 mL/2 mL), 60 °C, 36 h, and the
isolated yield was provided. b At 100 oC.
Scheme 2 Gram-scale reaction and application for the synthesis of Tryptoline
H
3 mol%
15 mol% P( -Bu)3
1.1
4Å
MeOTf
equiv
MS, DCM,
[CpFe(CO)2]2
1)
2)
N
n
rt, 12 h
+
+
CO2
atm)
PhSiH3
equiv)
(2.0
N
NH
N
1.5
MeOH,
NaBH
equiv
rt, 1 h
THF/CH
3CN,
°C,
60 36 h
4
N
N
N
N
H
(1
H
H
1g
2g
84%
70%
g)
Tryptoline
(1.42
reaction also occurred smoothly generating the corresponding
N-heterocycles in good to excellent yields (2r and 2s). It is
noteworthy that cyclization of N,N-unsubstituted primary aniline
1s gave the desired product 2s in 70% yield. Finally, this
Fe-catalyzed Pictet-Spengler-type cyclization was tested using
substituted β-phenethylamines as substrates. Unfortunately, no
desired cyclization product was observed but the corresponding
N-methylation products were obtained in excellent yield (4t and
4u). This result indicates that the intramolecular strong
nucleophile (indole or pyrrole) is crucial for the
Pictet-Spengler-type cyclization reaction.
To verify the applicability of this Fe-catalyzed
Pictet-Spengler-type cyclization with CO2, a gram-scale reaction
with 1g was carried out (Scheme 2). The product 2g was obtained
in 84% yield under the standard conditions. Then, removal of
2-pyridyl protecting group of 2g was realized by sequential
methylation of pyridine with methyl triflate and reduction with
NaBH4. The natural alkaloid tryptoline was obtained in 70% yield
by two steps in this one-pot reaction.[13]
To provide some insights into the reaction mechanism, a
series of control experiments were carried out. When the
reaction was performed at a lower temperature (30 °C), both
Pictet-Spengler-type cyclization product 2a and N-formylation
product 3a were obtained (Scheme 3a). And the ratio of 2a and
3a was influence a lot by the amount of silane. At a high
temperature (100 oC), both the cyclization product 2a and
N-methylation product 4a were obtained (Scheme 3b). These two
reactions illustrates that 2a, 3a, and 4a may come from the same
intermediate. Next, 3a or 4a was used as substrate in the
Fe-catalyzed Pictet-Spengler-type cyclization. However, no
cyclization product 2a was obtained (Scheme 3c and 3d). It
demontrates that 3a or 4a was not the reaction intermediate of
this Pictet-Spengler-type cyclization. Then, reaction with 2-methyl
tryptamine 5 was carried out under the standard conditions,
spirocyclization reaction occurred smoothly generating 6 in 75%
yield (Scheme 3e). This result indicates that the Fe-catalyzed
Pictet-Spengler-type cyclization with CO2 might take place via
rearrangement from a spiroindolenine intermediate. In order to
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Running title
Chin. J. Chem.
capture the reductive intermediate form CO2, a stoichiometric
reaction was carried out with [CpFe(CO)2)]2, P(n-Bu)3, PhSiH3 and
CO2 (Scheme 3f, for details see Supporting Information). The
P(n-Bu)3 coordinated iron catalyst Cat-I was isolated in 44% yield.
Unfortunately, we didn’t detect the reductive intermediate from
CO2, such as silyl formate or bis(silyl)acetal, by LC-MS analysis.
Notabley, the Pictet-Spengler-type cyclization ran smoothly with
Cat-I as the catalyst affording 2a in excellent yield, indicating that
this P(n-Bu)3 coordinated Fe complex Cat-I as is an active catalyst
(Scheme 3g). Finally, a stepwise reaction was conducted. With
Scheme 3 Control experiments
Ph
3 mol%
15 mol% P( -Bu)3
3 mol%
15 mol% P( -Bu)3
[CpFe(CO)2]2
[CpFe(CO)2 2
]
N
n
HN
HN
Me
Ph
n
Ph
+
+
+
+
2a
3a
CO2
atm)
CO2
atm)
(a)
(e)
2.0
PhSiH
3
equiv
3CN,
30
°C,
36 h
THF/CH
N
H
N
Me
(1
H
(1
THF/CH
3CN,
60
°C,
36 h
N
with 2.0
with 1.0
PhSiH
PhSiH
equiv
equiv
33%
14%
46%
71%
3
3
1a
1a
5
75%
6
n
4.0
4.0
P( -Bu)
equiv
equiv
3
3 mol%
15 mol% P( -Bu)3
[CpFe(CO)2]2
CO
Fe
CO
CO
PhSiH
HN
Ph
n
3
+
+
CO2
atm)
+
+
Fe
CO
Fe
2a
4a
CO2
atm)
(b)
(c)
n
P( -Bu)3
OC
(f)
2.0
THF/CH
PhSiH
THF/CH
60
3CN,
44%
°C,
36 h
equiv
3CN,
3
N
H
n
P( -Bu)3
(1
(1
28%
100
°C,
36 h 64%
Cat-I
equiv)
(1.0
3 mol%
15 mol% P( -Bu)3
[CpFe(CO)2]2
n
HN
Ph
N
CHO
Cat-I
6 mol%
2.0
N
N
Ph
Ph
(g)
CO2
atm)
CO2
atm)
Ph
PhSiH
equiv
THF/CH
2.0
PhSiH
equiv
THF/CH
3
N
N
H
N
N
3
(1
H
H
(1
3CN,
60
°C,
36 h
H
3CN,
60
°C,
36 h
84%
no reaction
1a
2a
3a
2a
2a
3 mol%
15 mol% P( -Bu)3
[CpFe(CO)2]2
CO2
-I
°C,
Cat
30
N
atm)
2 (1
6 mol%
THF/CH
n
N
Me
N
atm)
+
+
Ph
(1
(d)
CO2
atm)
3CN,
24 h
CO2
1a
1.0
PhSiH
,
Ph
equiv
3
+
+
3a
1a
2a
2.0
PhSiH
equiv
THF/CH
N
H
N
H
(h)
3
then
solvents
(1
THF/CH
60
3CN,
°C,
36 h
PhSiH3
equiv)
3CN,
60
°C,
36 h
36% 42% 6%
remove
and
no reaction
(1
4a
Cat-I as catalyst, reduction of CO2 (1 atm) with PhSiH3 (1.0
equiv)was perfomred in mixed solvent THF/CH3CN at 30 C for 24
Scheme 4 Plausible reaction pathways
o
h (Scheme 3h). Then the solvents and CO2 was removed under
reduced pressure. To the resulting residue was added mixed
solvent THF/CH3CN, 1a, and PhSiH3 (1.0 equiv) and the reaction
Pictet-Spengler-type cyclization
[CpFe(CO)2]2
H
H
N
O
R
N
+
-
=
PR3
CO
-
L
L
CO
R
+
o
N
H
N
H
mixture was stirred at 60 C for 36 h under N2 atmosphere. This
Si
stepwise reaction produced 2a in 42% yield, 3a in 6% yield, and
recovered 1a in 36% yield (Scheme 3h). Together with reaction of
Scheme 3a, this result indicates that phenylsilyl formate
V
VI
I
2
CpFe(CO)L2 ( )
H
N
Si
H
R
Si
L
H
Si
H
o
N
intermediate might be formed at 30 C, which can react with
H
L
1
o
o
R
amine generating formamide at 30 C. While at 60 C, phenylsilyl
formate would be easily reduced to bis(silyl)acetal species, which
can react with 1a producing 2a as the product.
N
Si
Fe L
O
O
methylation
amine
100 o
CH3
N
OC
H
N
H
VII
Si
H
C
Fe L
4
OC
IV
Si
H
×
II
H
Si
formylation
amine
H
O
Fe L
O
Si
×
O
H
C
N
N
R
CO2
OC
30 o
C
N
H
III
3
2
Based on the mechanistic studies and previous reports on the
reductive functionalization of CO2,[14] a possible reaction pathway
of this Fe-catalyzed Pictet-Spengler-type cyclization has been
proposed as shown in Scheme 4. First, η2-silane iron species II
could be generated from precatalyst [CpFe(CO)2]2 via ligand
coordination and disassociation.[15] Next, insertion of Fe−hydride
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First author et al.
Report
into CO2 leads to intermediate III.[16] Reaction of III with amine at
lower temperature (30 °C) in the presence of less phenylsilane
would generate formamide as the product. In the presence of
excess amount of phenylsilane at a higher temperature (60 °C),
the intermediate III is prone to be reduced into bis(silyl)acetal
species IV. Then, attack of intermediate IV with tryptamine 1 by a
substitution reaction affords hemiaminal V via C−N bond
formation and generates silanol. Subsequent release of another
silanol gives the iminium intermediate VI. The following
intramolecular nucleophilic addition of indole to the iminium
affording the protonated spiroindolenine intermediate VII. Finally,
tetrahydro-β-carboline 2 is easily accessed by rearrangement of
VII.[17] If the reaction temperature is further increased to 100 °C,
methylation of amine would be the observed, which may be
realized by further reduction of the iminium intermediate VI. In
this Fe-catalyzed reductive Pictet-Spengler-type cyclization of
tryptamines with CO2, formamide 3a or methylamine 4a has been
detected as by-product. According to our previous result, 4a is
easily obtained with less reductant (1 equiv PhSiH3) at lower
temperature (30 °C), whereas 3a is easily accessed with more
reductant (3 equiv PhSiH3) at higher temperature (100 °C).[13] The
successful Pictet-Spengler-type cyclization is attributed to careful
selection of solvent, temperature, and amount of reductant.
Acknowledgement
We are grateful to the financial support from NSFC (21772208,
21702212, 21602230) and Key Research Program of Frontier
Sciences of CAS (QYZDJSSW-SLH051).
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In summary, we have developed a novel Fe-catalyzed
Pictet-Spengler-type cyclization with heteroaryl ethylamines using
CO2 as a sustainable C1 feedstock. Tetrahydro-β-carbolines and
other nitrogen-containing heterocycles have been easily accessed
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Experimental
General Procedure for Fe-Catalyzed Pictet-Spengler-type
cyclization with CO2: An oven-dried Schlenk tube (25 mL) was
charged with the substrate 1 (0.20 mmol, 1.0 equiv) and
[CpFe(CO)2]2 (2.2 mg, 0.006 mmol, 0.03 equiv). The tube was then
evacuated and back-filled with CO2 for 3 times. Then, P(n-Bu)3
(7.5 μL, 0.03 mmol, 0.15 equiv), PhSiH3 (50 μL, 0.4 mmol, 2.0
equiv) and anhydrous CH3CN/THF (2.0 mL/2.0 mL) were added via
syringe under CO2. Then the Schlenk tube was sealed at
atmospheric pressure of CO2 (1 atm) and the resulting mixture
was stirred for 36 h at 60 °C. The mixture was cooled to room
temperature and concentrated under reduced pressure. The
residue was purified by column chromatography (petroleum
ether/EtOAc 10/1~1/1) to give the pure desired product 2.
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2020xxxxx.
www.cjc.wiley-vch.de
© 2020 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2020, 38, XXX－XXX
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Running title
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First author et al.
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(The following will be filled in by the editorial staff)
Manuscript received: XXXX, 2020
Manuscript revised: XXXX, 2020
Manuscript accepted: XXXX, 2020
Accepted manuscript online: XXXX, 2020
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the
Pyridine
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Running title
Chin. J. Chem.
Entry for the Table of Contents
Page No.
Fe-Catalyzed Pictet-Spengler-type Cyclization
via
Selective
Four-electron
Reductive
2
PhSiH
equiv
3 mol%
3
Functionalization of CO2
NHR
[CpFe(CO)2]2
+
CO2
H
H
N
R
n
15 mol% P( -Bu)3
THF/CH
°
3CN,
60
C
A catalytic Pictet-Spengler-type cyclization has been developed using CO2 as a nontoxic and
sustainable C1 building block with base-metal iron as catalyst. The reaction is achieved by selective
four-electron reduction of CO2 into methylene level intermediate through careful tuning the
reaction parameters.
Wen-Duo Li,a,b Jie Chen,a Dao-Yong Zhu*a and
Ji-Bao Xia*a,b
Chin. J. Chem. 2020, 38, XXX－XXX
© 2020 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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This article is protected by copyright. All rights reserved.