Iron(IV)-Corrole Catalyzed Stereoselective Olefination of Aldehydes with Ethyl Diazoacetate

Article abstract of DOI:10.1021/acs.organomet.5b00069

Iron(IV)-corrole complexes were first investigated as catalysts for olefination of aldehydes with ethyl diazoacetate in the presence of triphenylphosphine. Efficient olefination of aromatic aldehydes with high trans-selectivity was observed, showing iron corrole is a new kind of promising catalyst for olefination reaction. Transformation of the phosphazine to ylide by iron(IV) corrole was proved to be the key step in the present system.

Full text of DOI:10.1021/acs.organomet.5b00069

Article
pubs.acs.org/Organometallics
Iron(IV)-Corrole Catalyzed Stereoselective Olefination of Aldehydes
with Ethyl Diazoacetate
Huai-Bo Zou,† Hong Yang,† Ze-Yu Liu,† Mian HR Mahmood,† Guang-Quan Mei,‡ Hai-Yang Liu,*,†
and Chi-Kwong Chang*,§
†Department of Chemistry, South China University of Technology, Guangzhou 510640, China
‡Key Laboratory of Jiangxi University for Applied Chemistry & Chemical Biology, Yichun University, Yichun 336000, China
§Department of Chemistry, Michigan State University, E. Lansing, Michigan 48824, United States
S
* Supporting Information
ABSTRACT: Iron(IV)-corrole complexes were first inves-
tigated as catalysts for olefination of aldehydes with ethyl
diazoacetate in the presence of triphenylphosphine. Efficient
olefination of aromatic aldehydes with high trans-selectivity
was observed, showing iron corrole is a new kind of promising
catalyst for olefination reaction. Transformation of the
phosphazine to ylide by iron(IV) corrole was proved to be
the key step in the present system.
INTRODUCTION
■
Metallocorroles are efficient catalysts for many important
organic reactions such as epoxidation,1 asymmetric sulfoxida-
tion,2 hydroxylation,3 aziridination,4 and oxygen atom transfer
reactions.5 Iron corroles are especially proved to be effective
catalysts in oxidation,6 cyclopropanation of olefins,7 and the
insertion reactions involving −NH,7c−e −SH,7e and −CH7f
Figure 1. Structures of iron(IV)-corrole complexes 1−4.
groups by ethyl diazoacetate (EDA). Recent research revealed
that iron corroles can catalyze the copolymerization of epoxides
with carbon dioxide (CO2)8 and the [4 + 2] cycloaddition of
complexes. Among them, 2 and 3 are first reported here (for
dienes and aldehydes.9 These examples imply that iron corroles
characterization data, see Supporting Information).
may be potent catalysts for various other organic reactions.
RESULTS AND DISCUSSION
■
Construction of CC double bonds is one of the most
important organic reactions. Wittig reaction is a practical and
the most commonly used olefination method for this
purpose.10 An alternative approach is to use easily accessible
EDA to generate ylide in situ by using metal complexes as
catalysts.11 It was observed that iron porphyrin displayed good
catalytic activity for this olefination reaction in which metal-
carbene is involved as the intermediate.12 Corrole is a close
analogue of porphyrin tetrapyrrolic macrocycle,13 and we here
wish to report the first investigation of iron(IV)-corrole-
catalyzed olefination of aldehydes with EDA. The current
catalytic olefination reaction is efficient and can afford trans-
olefination products with high selectivity. Evidence showed that
the phosphazine to ylide transformation by iron(IV) corrole is
the key step for this catalytic system.
The catalytic olefination of benzaldehyde (1 equiv) with EDA
(2 equiv) by iron corroles (1 mol %) were conducted in
dichloromethane (DCM) at 40 °C for 12 h in the presence of
triphenylphosphine (PPh3, 1.1 equiv; Table 1). The olefination
product was obtained with yield 79−88% and good trans-
selectivity for iron corroles 1−4. The control experiment gave
no olefination product (entry 2), and no benzaldehyde
consumption was observed in the absence of PPh3 (entry 3),
demonstrating that PPh3 was necessary for this reaction. Of all
the investigated iron corroles, 1 exhibited the highest activity.
The catalytic activity gradually reduced with the enhancement
of electron-withdrawing property of iron corroles.
To explore the scope of this reaction, iron corrole 1 was
selected as catalyst for the olefination of various aryl aldehyde
substrates (Table 2). Most aldehydes underwent olefination
from moderate to high yields and with high trans-selectivity.
Except for 4-dimethylaminobenzaldehyde which gave a low
Iron-corrole complexes 1−4 used in this work are shown in
Figure 1. These iron-corrole complexes are peripherally
substituted with phenyl (−C6H5) and 2,3,4,5,6-pentafluoro-
phenyl (−C6F5) groups at meso-positions of the corrole ring,
which may impart different electronic features to the
Received: January 24, 2015
Published: May 28, 2015
© 2015 American Chemical Society
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Table 1. Iron-Corrole-Catalyzed Olefination of
Benzaldehyde with EDA
Table 2. Olefination of Aldehyde Substrates with EDA Using
Iron Corrole 1 Catalyst
a
a
b
b
entry catalyst catalyst loading (mol %) yield (%) trans/cis ratio
1
1
1.0
88 (86)
24:1
nd
c
e
2
nd
d
e
3
1
2
3
4
1.0
1.0
1.0
1.0
nr
nd
4
82
81
79
16:1
24:1
19:1
5
6
a
Reactions were carried out at 40 °C for 12 h in DCM under N2, with
1.0 equiv of benzaldehyde, 2.0 equiv of EDA, and 1.0 mol % of catalyst
(4.7 μmol) in the presence of 1.1 equiv of PPh3. Determined by GC.
Number in parentheses is isolated yield. Control reaction without
catalysts. Without PPh3. nd: not determined. nr: no reaction.
b
c
d
e
yield of 20% (entry 1), all other tested aryl aldehydes bearing
electron-donating groups (Table 2, entries 2−4) gave the
desired olefin in good to high yields (77−87%). Aryl aldehydes
bearing electron-withdrawing groups were also suitable
substrates for this transformation with moderate to high yields
(entries 5−11). It seems electron-deficient aryl aldehydes dis-
favor this reaction and give lower yields. 3,4-dichlorobenzalde-
hyde gave significantly lower yield (63%, entry 11) than 4-
chlorobenzaldehyde (88%, entry 9). And the steric hindrance at
ortho-position of aryl aldehyde obviously lowered the trans-
selectivity (entries 4 and 10). Interestingly, aromatic hetero-
cyclic furfural may give moderate yield of 56% (entry 12), while
2-formylpyridine gave very low yield (7%, entry 13). The sharp
drop in yield may be caused by the axial coordination of pyridyl
nitrogen atom and central iron atom of the catalyst, which
results in the lower catalytic activity by blocking the active
center. This was confirmed by the observation that the addition
of pyridine axial ligand to the reaction system will lower the
yield. The olefination yield of benzaldehyde drops from 86%
(Table 1, entry 1) to 60% (Table 2, entry 14) in the presence of
pyridine. The lowered olefination yield of 4-dimethylamino-
benzaldehyde (Table 2, entry 1) may also be rationalized by
axial coordination of amino group. PPh3 is a strong axial ligand
for iron corrole, and it was found a large excess of PPh3 would
hinder the reaction also (Figure S17, Supporting Information).
When using terephthalaldehyde (entry 15) as substrate, the
mono-olefinated product was isolated with a yield of 83%, and
diolefinated product could not be isolated. This is probably due
to its low yield in the current catalytic conditions. α,β-
unsaturated cinnamaldehyde (entry 17) can obtain the desired
product with high yield and trans-stereoselectivity. Polycyclic 1-
naphthalenecarboxaldehyde was also a good substrate (83%,
entry 16). Aliphatic phenylacetaldehyde gave 33% yield and
almost 100% trans-selectivity (entry 18) in the present catalytic
system. As compared to iron porphyrin catalyzed olefintion of
aldehyde with EDA,12a,c,f iron(IV) corrole although gives
comparable yields, but the trans-selectivity are remarkably
higher for a lot of substrates (Table 2, entries 11, 12, 14, 15, 17,
18).
a
b
c
Reaction conditions, see Table 1. Isolated yield. Determined by 1H
d
NMR. 23 μL of pyridine (0.6 equiv relative to benzaldehyde) was
added into the reaction. Mono-olefinated product.
e
catalyzed aldehyde olefination reactions proceed via path 1 or
path 2; only one example was reported via the path 3
mechanism.14a In the current catalytic system, we found the 1H
NMR spectrum of iron corrole 1 was exactly the same in the
presence and absence of EDA. This implies that iron corrole
could not react with EDA to yield iron-carbene species or the
formed carbene species is too reactive to be detected by H
NMR.
In the absence of catalyst 1 (e.g., Table 1, entry 2), no olefin
could be detected, but the reaction could still go on to give
azine PhCHN−NCHCO2Et (I) and OPPh3 as
previously observed.15 We also prepared azine I (for its
1
Three possible mechanisms have been suggested for the
olefination of aldehydes with EDA catalyzed by transition metal
complexes (Scheme 1).14 Most transition metal complexes
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Scheme 1. Three Mechanisms for the Olefination Catalyzed
by Transition Metal Complexes
Figure 2. 31P NMR changes of the reaction mixture of PPh3, EDA, and
iron corrole 1 in CDCl3 at room temperature: (A) phosphazine II (δ =
22.80 ppm); (B) phosphorus ylide (δ = 17.65 ppm); (C) OPPh3 (δ
= 29.08 ppm).
characterization, see the Supporting Information) purposely
and found it did not react with iron-corrole catalyst 1 to yield
olefin as detected by GC-MS. This suggests that azine I is not
the intermediate of the present olefination reaction. It is known
that PPh3 can react rapidly with EDA to form phosphazine
II14a,c,16 (Scheme 1), and the formation of phosphazine can be
easily monitored by the 31P NMR signal at 22.39 ppm (Figure
S18, Supporting Information). The direct reaction of
phophazine II with aldehydes produces azine I.14a
Figure 3. 31P NMR changes of the reaction mixture of phosphazine II
and iron corrole 1 in CDCl3 at room temperature: (A) phosphazine II
(δ = 22.85 ppm); (B) phosphorus ylide (δ = 17.60 ppm); (C) O
PPh3 (δ = 29.01 ppm).
Phosphazine II was detected as the major product in the
mixture of PPh3, EDA, and catalyst 1 (0.5 mol % relative to
EDA) after 0.5 h reaction time, and PPh3 was almost
consumed. At the same time, a small amount of phosphorus
ylide Ph3PCHCO2Et could also be detected in the 31P NMR
spectrum (Figure 2, signal B). After some time, the amount of
phosphazine II (signal A) decreased gradually and the ylide
concentration increased. This indicates that iron corrole 1 can
catalyze the transformation of phosphazine II to phosphorus
ylide. Although the aldehyde substrate is absent in the mixture,
phosphine oxide (signal C) was also observable; this may result
from the oxidation of ylide by oxygen. Further evidence comes
from the direct reaction of prepared phosphazine II17,18 and
iron corrole 1 (1 mol % relative to II). Figure 3 shows the 31P
NMR changes of the reaction mixture of II and 1 in CDCl3.
Phosphazine II (signal A) was gradually transformed into the
phosphorus ylide (signal B). The peak shape and chemical shift
of signal B are identical to those of the commercial ylide sample
(Figure S19, Supporting Information). The ylide then reacts
with aryl aldehyde to give olefin via Wittig reaction.
Interestingly, the reaction of benzaldehyde (1 equiv) and
phosphazine II (1 equiv) in the presence of catalyst 1 (1 mol %
relative to benzaldehyde) under the catalytic conditions gives
the same GC olefination yield (88%) and trans-selectivity
(96%) as entry 1 of Table 1 (Figure S20, Supporting
Information). Thus, the current catalytic reaction is suggested
to proceed via a phosphazine II to ylide route (path 3, Scheme
1); a similar mechanism had been suggested by Lu in the
molybdenum complex mediated olefination of aldehyde.14a
On the basis of these observations, a proposed reaction
mechanism for the olefination of aldehyde catalyzed by
iron(IV) corrole is depicted in Scheme 2. First, PPh3 reacts
with EDA to form the phosphazine II. It then coordinates with
iron corrole 1 to form intermediate III. The electronegative
carbon atom attacks the electropositive phosphorus atom to
form four-membered-ring intermediate IV, which gives an ylide
via intramolecular rearrangement and releases an iron corrole
catalyst. The ylide then reacts with aldehydes to give olefins.
The trans-selectivity might be explained by the Wittig reaction,
in which stabilized ylide Ph3PCHCO2Et reacts with aldehyde
to give exclusively trans double bonds.19 Interestingly, in a
similar olefination reaction catalyzed by iron(II or III)
porphyrin,12 iron carbenes were suggested to be the active
intermediates. The different catalytic behavior of iron(IV)
corrole might be related to its higher oxidation state of the
central iron atom.
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EtOAc, the desired target product was obtained upon evaporation of
the solvent.
Scheme 2. Proposed Mechanism for Olefination of Aldehyde
with EDA Catalyzed by Iron(IV) Corrole
Procedure for the Synthesis of Azine.14a A mixture of
benzaldehyde (100 mg, 0.94 mmol), EDA (150 mg, 1.3 mmol), and
PPh3 (300 mg, 1.1 mmol) in benzene (5 mL) was refluxed for 5 h.
After removal of the solvent, the residue was separated by column
chromatography (SiO2, 1.5 cm × 25 cm) using petroleum ether/
EtOAc (4:1, v/v), giving a yellow oil (154 mg, 80% yield).
Procedure for the Synthesis of Phosphazine.17 PPh3 (1311
mg, 5.0 mmol) was dissolved in 20 mL of n-pentane. EDA (57 mg, 0.5
mmol) was added to the clear mixture, and then the solution was
stirred for 8 h. The resulting solid was filtered and washed three times
with cold n-pentane, giving a white powder (1562 mg, 83% yield).
ASSOCIATED CONTENT
■
S
* Supporting Information
Characterization data and copies of 1H, 13C, 19F, and 31P NMR
and UV−vis spectra and HR-MS (ESI). The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/acs.organomet.5b00069.
CONCLUSION
■
In summary, we have presented the first and efficient iron-
corrole-catalyzed olefination of aldehydes with EDA. The iron-
corrole-catalyzed transformation of phosphazine to ylide is a
plausible key step in the present system.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: chhyliu@scut.edu.cn.
*E-mail: changc@msu.edu.
Notes
EXPERIMENTAL SECTION
■
The authors declare no competing financial interest.
General Considerations. Unless otherwise noted, all solvents and
chemicals were reagent grade, purchased commercially, and used
without further purification. The iron(IV) corroles 1−4 were readily
prepared according to the reported procedure,8 although they included
some inseparable impurity such as hydrocarbon, and they are used for
the olefination without further purification. Column chromatography
was performed using 200−300 mesh silica gel. Melting points were
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the National
Natural Science Foundation of China (NSFC 21171057,
21371059, 21261024).
obtained on a Buchi melting point B-545 apparatus and are
̈
1
uncorrected. Yields are based on the pure products isolated. All H,
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1
(18) H NMR (400 MHz, CDCl3): δ 1.28 (t, 3H, −CH3), 4.21 (q,
2H, −CH2CH3), 7.46−7.69 (m, 15H, −phenyl), 7.77 (d, 1H, −N
CH). 13C NMR (100 MHz, CDCl3): δ 165.49, 138.14, 133.55, 132.56,
128.77, 127.65, 59.77, 14.43. 31P NMR (162 MHz, CDCl3): δ 22.89.
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