Selective oxidation of terminal aryl and aliphatic alkenes to aldehydes catalyzed by iron(iii) porphyrins with triflate as a counter anion

Article abstract of DOI:10.1039/c1cc13574k

[Fe(Por)CF₃SO₃] (Por = porphyrin dianion) can efficiently catalyze selective oxidation of terminal aryl alkenes and aliphatic alkenes to aldehydes in good to high yields under mild conditions.

Full text of DOI:10.1039/c1cc13574k

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COMMUNICATION
Selective oxidation of terminal aryl and aliphatic alkenes to aldehydes
catalyzed by iron(III) porphyrins with triflate as a counter anionw
Guo-Qiang Chen,a Zhen-Jiang Xu,a Cong-Ying Zhoub and Chi-Ming Che*ab
Received 16th June 2011, Accepted 25th August 2011
DOI: 10.1039/c1cc13574k
[Fe(Por)CF3SO3] (Por = porphyrin dianion) can efficiently
catalyze selective oxidation of terminal aryl alkenes and aliphatic
alkenes to aldehydes in good to high yields under mild conditions.
[Fe(2,6-Cl2TPP)Cl] (2 mol%) in dichloromethane at room
temperature afforded styrene oxide without phenylacetaldehyde
being detected (Table 1, entry 1).
As iron(III) porphyrin with a weakly coordinating anion as
the axial ligand would have strong Lewis acidity that is needed
for promoting isomerization of epoxide to aldehyde,8 we tested
the activity of [Fe(2,6-Cl2TPP)CF3SO3] towards this tandem
E–I reaction. [Fe(2,6-Cl2TPP)CF3SO3] (generated in situ by
reacting [Fe(2,6-Cl2TPP)Cl] with AgSO3CF3) catalysed the
E–I reaction of styrene with PhIO to give phenylacetaldehyde
in 62% yield along with 5% yield of benzaldehyde, no epoxide
Aldehyde is frequently used in organic synthesis.1 Traditional
methods for the synthesis of aldehyde mainly rely on oxidation
of alcohol and reduction of ester.1,2 As many alkenes can be
obtained directly from petrochemicals, the conversion of
alkene to aldehyde by oxidation (Wacker-type reaction) and
hydroformylation is of great interest in both academia and
industry.3,4 Wacker oxidation is a well-known reaction that
converts alkenes to carbonyl compounds using palladium
catalysts.4 Its most significant industrial application is the
conversion of ethylene to acetaldehyde. However, when terminal
alkenes except ethylene are employed as substrates, methyl
ketones instead of aldehydes are obtained as the main products.
Although some modified Wacker oxidation reactions have been
developed, they either give a mixture of ketones and aldehydes,
or require a directing group such as heteroatom(s) installed in
substrates for chelation with palladium.5 Recently, we reported
that ruthenium porphyrin efficiently and selectively catalyzes
‘Wacker-type’ oxidation of terminal alkenes to aldehydes
through a tandem epoxidation–isomerization (E–I) pathway,
but the alkenes are confined to styrenes and cinnamenyl
alkenes, and aliphatic alkenes were found to be poor substrates
for the ruthenium catalysis.6 As part of our program to develop
sustainable catalysis, we herein describe a selective ‘Wacker-
type’ oxidation of terminal alkenes to aldehydes catalyzed by
iron(III) porphyrins. Importantly, both aryl and aliphatic alkenes
can be selectively oxidized to aldehydes when [Fe(Por)CF3SO3]
(Por = porphyrin dianion) is used as the catalyst.
À
was observed (Table 1, entry 2). ReplacingÀCF3SO3 by other
anions such as ClO4À, SbF6À, and PF6 afforded similar
Table 1 The iron(III) porphyrins catalyzed E–I reaction of styrene to
phenylacetaldehydea
Yieldb (%)
Entrya [Fe(Por)Cl]
AgX
—
Conversionb (%)
100
2a 2b 2c
1
2
3
4
5
6
7
8
9
[Fe(2,6-Cl2TPP)Cl]
—
62
60
43
41
37
3
60
—
—
—
—
—
—
30
—
5
5
3
[Fe(2,6-Cl2TPP)Cl] AgOTf 100
[Fe(2,6-Cl2TPP)Cl] AgClO4 100
[Fe(2,6-Cl2TPP)Cl] AgSbF6 100
8
[Fe(2,6-Cl2TPP)Cl] AgPF6
97
40
3
39
—
10
—
—
5
[Fe(F20-TPP)Cl]
[Fe(TTP)Cl]
[Fe(TPP)Cl]
—
AgOTf
AgOTf
AgOTf
AgOTf
3
—
—
a
0.2 mmol styrene, PhIO (0.3 mmol), catalyst (2 mol%) and silver
salts (2 mol%) were mixed in 2 mL DCM and stirred at room
b
temperature for 8 hours. Determined by GC with n-dodecane as
At the outset, we examined the oxidation of styrene with PhIO
using [Fe(2,6-Cl2TPP)Cl] [2,6-Cl2TPP = meso-tetrakis(2,6-di-
chlorophenyl)porphyrin] as the catalyst.7 Treatment of styrene
with PhIO (1.5 equiv.) in the presence of a catalytic amount of
the internal standard.
a Shanghai-Hong Kong Joint Laboratory in Chemical Synthesis,
Shanghai Institute of Organic Chemistry, 354 Feng Lin Road,
Shanghai, China. E-mail: cmche@hku.hk; Fax: +852 2857-1586;
Tel: +852 2859-2154
b Department of Chemistry, State Key Laboratory of Synthetic
Chemistry, and Open Laboratory of Chemical Biology of the
Institute of Molecular Technology for Drug Discovery and Synthesis,
The University of Hong Kong, Pokfulam Road, Hong Kong, China
w Electronic supplementary information (ESI) available: Experimental
procedures, compound characterization see DOI: 10.1039/c1cc13574k
c
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Chem. Commun., 2011, 47, 10963–10965 10963

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results (entries 3–5). The other iron porphyrins with triflate anion
generated in situ gave lower substrate conversions (entries 6–8).
Presumably this is due to the oxidative instability of [Fe(TPP)]+
and in the case of [Fe(F20-TPP)]+, the F substituents render the
more oxidizing ‘‘putative [Fe(F20-TPP)O]+’’ intermediate less
accessible. AgSO3CF3 alone failed to catalyze the E–I reaction
under the same conditions (Table 1, entry 9).
Table 2 The [Fe(2,6-Cl2TPP)OTf] catalyzed E–I reaction of aryl
alkenes to arylacetaldehydes
Entrya
Alkene
Product
Yieldb (%)
For easy operation, [Fe(2,6-Cl2TPP)CF3SO3] was prepared
according to the literature method.8b The effects of solvent,
temperature and substrate ratio were examined and the results
are depicted in Table SII (see ESIw). The reaction in dichloro-
methane at room temperature with substrate ratio of styrene :
PhIO = 3 : 1 gave best result and the byproduct benzaldehyde or
epoxide was minimized (83% yield, Table SIIw, entry 13). Excess
styrene was found to lead to a higher yield of aldehyde. With
other oxidants, such as tert-butylhydroperoxide, oxone or
pyridine oxide, the reaction yield significantly dropped (see ESIw).
With the optimized conditions, we examined the scope of
substrates for the [Fe(2,6-Cl2TPP) CF3SO3]-catalyzed E–I
reaction of terminal alkenes to aldehydes. As depicted in
Table 2, various aryl alkenes underwent the E–I reaction to
give corresponding aldehydes in good to high yields. For
examples, the reaction of both 4-methylstyrene 1b and 4-chloro-
styrene 1d with PhIO gave corresponding aldehydes in 93% and
87% yields respectively (Table 2, entries 1 and 3). para-Methoxy
substituents led to lower product yield (64% yield, entry 2).
Varying the halo substituent(s) on the phenyl ring was found to
have only a slight impact on the reaction with corres-
ponding aldehydes obtained in 86–94% yields (entries 3–7).
The 1,1-disubstituted alkene 1i was also reactive to undergo
the E–I reaction to give product 2i in high yield (entry 8).
When a trisubstituted alkene 1k was used, quaternary
aldehyde 2k was obtained in 68% yield, presumably via alkyl
migration in the isomerization step (entry 10).
1
2
3
93
64
87
4
5
93
94
6
7
86
92
8
9
85
57
68
With the success in selective oxidation of styrenes to arylacet-
aldehydes catalyzed by [Fe(2,6-Cl2TPP)CF3SO3], we extended this
protocol to aliphatic alkenes which are problematic substrates in
the ruthenium porphyrin-catalyzed E–I reaction and Pd-catalysed
Wacker oxidation. When dodec-1-ene 3a was treated with
excess PhIO in the presence of 2 mol% [Fe(2,6-Cl2TPP)
CF3SO3] in DCM at room temperature, only an epoxidation
product was obtained in 74% yield without aldehyde being
observed. Increasing temperature to 80 1C was found to effect
the E–I reaction of 3a to give aldehyde 4a in 40% yield. The
product yield was further improved to 70% using a stepwise
procedure in the DCE/dioxane system, details of which are
given in the ESI.w
With the optimized conditions, various aliphatic alkenes
were examined. Most aliphatic alkenes underwent the
Fe-catalyzed E–I reaction to give aldehydes in good yields
(Table 3). In this work, the aliphatic alkenes used did not bear
heteroatoms which are known to chelate with the palladium
ion, thereby facilitating the Wacker oxidation of terminal
alkenes to aldehydes. In the literature, this kind of aliphatic
alkenes usually give methyl ketones as the main products
under Wacker oxidation conditions.5b,5e,9
10
a
b
0.2 mmol PhIO; 0.6 mmol alkene; 2 mL DCM, rt, 6 h. Yield based
1
on H NMR with PhTMS as the internal standard.
catalyst results in isomerization giving aldehyde as the final
product. In this work, epoxide was observed in the course of
the catalysis. When compared to the previous work on the
ruthenium porphyrin catalyzed E–I reaction,6 the Àcationic
iron(III) porphyrin with the weakly ligating CF3SO3 ligand
is a strong Lewis acid, and this is needed to promote the E–I
reaction of aliphatic alkenes.
a,b-Unsaturated esters are versatile compounds with many
applications in organic synthesis.10 As both E–I reaction and
olefination of aldehyde11 could be catalysed by iron(III)
porphyrin, a one pot protocol to synthesize a,b-unsaturated
esters from alkenes was developed. As shown in Scheme 2,
when styrene was treated with PhIO in the presence of [Fe(2,6-
Cl2TPP) CF3SO3] at room temperature for 4 hours, followed
by reaction with EDA and Ph3P, a,b-unsaturated ester 5 was
obtained in 84% isolated yield (Scheme 2, eqn 1). This iron
catalysis could be easily modified to give 1,2-diols which are
useful compounds in organic synthesis.12 Treatment of styrene
A tentative mechanism is proposed as depicted in Scheme 1.
Iron(III) porphyrin catalyzes oxidation of alkene to epoxide.
Subsequent activation of epoxide by the iron(III) porphyrin
c
10964 Chem. Commun., 2011, 47, 10963–10965
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Table 3 The [Fe(2,6-Cl2TPP) CF3SO3] catalyzed E–I reaction of
aliphatic alkenes to aldehydes
Funding Scheme for Joint Laboratory, and the Areas of
Excellence Scheme established under the University Grants
Committee of the Hong Kong SAR, China (AoE/P-10/01).
G.-Q. Chen thanks the Croucher Foundation of Hong Kong
for the postgraduate studentship.
Entrya
Alkene
Product
Yieldb (%)
Notes and references
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Wiley-VCH, Weinheim, 1999.
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´
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Elsevier, Oxford, 1995, vol. 3.
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70
68
80
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5
a
b
0.2 mmol alkene; 0.22 mmol PhIO. Yield based on 1H NMR with
PhTMS as the internal standard.
Scheme 1
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Scheme 2
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¨
¨
¨
In conclusion, we have developed an efficient and selective
‘Wacker-type’ oxidation of terminal aryl- and aliphatic
alkenes to aldehydes in high yields catalyzed by cationic
iron(III) porphyrins with triflate as the counter anion. A one
pot method for the synthesis of a,b-unsaturated esters from
alkenes has also been developed by combining the iron
porphyrin catalyzed E–I reaction and olefination of aldehyde.
By ring-opening of the epoxide intermediate, a dihydroxylation
of alkenes was achieved.
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12 For an analogous ruthenium porphyrin catalysis, see: W.-X. Hu,
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2010, 352, 3190.
We are thankful for the financial support of Hong Kong
Research Grant Council (HKU 7052/07P, CityU 2/06C and
HKU1/CRF/08), CAS-GJHZ200816 and CAS-Croucher
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 10963–10965 10965