Catalysed anti-Markovnikov oxidation of terminal aryl alkenes to aldehydes and transformation of methyl aryl tertiary amines to formamides with H2O2 as a terminal oxidant

Article abstract of DOI:10.1039/c4cc05972g

Anti-Markovnikov oxidation of terminal aryl alkenes to aldehydes and transformation of N-methyl aryl tertiary amines to formamides with H₂O₂ as a terminal oxidant under mild conditions have been achieved with moderate to good product yields using [Fe^III(TF₄DMAP)OTf] as catalyst. This journal is

Full text of DOI:10.1039/c4cc05972g

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[FeIII(TF4DMAP)OTf] catalysed anti-Markovnikov
oxidation of terminal aryl alkenes to aldehydes
and transformation of methyl aryl tertiary amines
to formamides with H2O2 as a terminal oxidant†
Cite this: Chem. Commun., 2014,
50, 12669
Received 31st July 2014,
Accepted 22nd August 2014
Yi-Dan Du,a Chun-Wai Tse,b Zhen-Jiang Xu,*a Yungen Liub and Chi-Ming Che*ab
DOI: 10.1039/c4cc05972g
www.rsc.org/chemcomm
Anti-Markovnikov oxidation of terminal aryl alkenes to aldehydes and intermediate.7 As iron is earth abundant and biocompatible,3,8
transformation of N-methyl aryl tertiary amines to formamides with H2O2 we studied an iron porphyrin catalyzed anti-Markovnikov oxidation
as a terminal oxidant under mild conditions have been achieved with of both terminal aryl and aliphatic alkenes to aldehydes. High
moderate to good product yields using [FeIII(TF4DMAP)OTf] as catalyst.
product yields were obtained but with a shortcoming of using
PhIO as oxidant.9 Our attempts to replace PhIO by H2O2 for
Metal catalysed selective oxidation of CQC and C–H bonds is a [Fe(2,6-Cl2TPP)OTf] or [FeIII(F20TPP)OTf]10 (2,6-Cl2TPPH2
=
useful tool for organic synthesis and fine chemical industry but meso-tetrakis(2,6-dichlorophenyl)porphyrin), H2F20TPP = meso-
remains to be accomplished with high selectivity and product yields (tetrakis(pentafluorophenyl)porphyrin) catalysed E–I reaction
using environmentally benign oxidants such as O2/air or H2O2.1 We using styrene as substrate afforded phenylacetaldehyde with
are attracted to the use of high valent iron–oxo complexes in organic 30% yield and with low selectivity. We envisioned that the
+
synthesis as iron–oxo complexes with oxidation states IV and V are highly oxidizing [(Porꢀ )(FeIVQO)]+ intermediate,11 once gener-
well documented strong oxidants capable of performing oxidative ated, underwent CQC bond epoxidation as well as over oxidation.
functionalization of alkenes and alkanes.2 We are particularly [FeIII(TF4DMAP)]+ (H2TF4DMAP
=
meso-tetrakis(o,o,m,m-
interested in developing synthetic applications of oxidation reactions tetrafluoro-p-(dimethylamino)phenyl)porphyrin) is an analogue
that proceed via [Fe(Por)O]+ (Por = porphyrinato dianion) reaction of [FeIII(F20TPP)]+ obtained by replacing the para-F substituent of
intermediates, as their methods of generation and reactivity have meso-C6F5 groups with the electron-donating dimethylamino
+
already been subjected to extensive studies over the past several (NMe2) moiety. It is envisioned that the [(TF4DMAPꢀ )(FeIVQO)]+
decades.2,3 In the literature, examples of iron catalyzed selective intermediate generated by the oxidation of [FeIII(TF4DMAP)]+
organic oxidation reactions using O2/air or H2O2 as a terminal with oxygen atom donors should be less oxidizing and more
oxidant that can be used in organic synthesis are sparse but have stable; thus, the accompanying side over oxidation reactions in
been increasing over the past several years.3,4 An organic oxidation the course of E–I oxidation may be minimized. Herein, we report
reaction that has a profound impact in both industry and academia [FeIII(TF4DMAP)OTf] as an effective catalyst for anti-Markovnikov oxida-
is the anti-Markovnikov oxidation of terminal alkenes to aldehydes tion of terminal aryl alkenes to aldehydes with good product
+
without cleavage of CQC bonds.5 Very recently, Grubbs reported yields. The synthetic application of the putative [(TF4DMAPꢀ )-
PdCl2(MeCN)2 catalyzed selective oxidation of styrenes to phenyl (FeIVQO)]+ intermediate has also been revealed by the examples of
acetaldehydes with p-benzoquinone or O2 as oxidant.6 We have [FeIII(TF4DMAP)]+ catalysed selective oxidation of a panel of N-methyl
previously reported that selective oxidation of terminal alkenes to aryl tertiary amines to formamides using H2O2 as a terminal oxidant.
aldehydes without CQC bond cleavage can be readily accomplished
with high product yields via a tandem epoxidation–isomerization
(E–I) pathway, which involves a reactive Ru–oxo reaction
a Shanghai-Hong Kong Joint Laboratory in Chemical Synthesis,
Shanghai Institute of Organic Chemistry, 354 Feng Lin Road, Shanghai, China.
E-mail: xuzhenjiang@mail.sioc.ac.cn
b Department of Chemistry and State Key Laboratory of Synthetic Chemistry,
The University of Hong Kong, Pokfulam Road, Hong Kong, China.
E-mail: cmche@hku.hk; Fax: +852-2857-1586; Tel: +852-2859-2154
Initially, the oxidation of styrene by H2O2 was used for the
optimization of the reaction conditions. As the ligand/counter-anion
† Electronic supplementary information (ESI) available: Experimental procedures,
detailed characterizations, additional tables and figures. See DOI: 10.1039/c4cc05972g
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Table 1 [FeIII(TF4DMAP)X] catalysed E–I reactiona
Table 2 [Fe(TF4DMAP)OTf] catalysed E–I reactiona
Conversionb Yieldb
Yieldc (%)
Entry Substrate
Product
(%)
(%)
Entry Additive Timeb (h) Conversionc (%)
a
b
c
d
1
2
3
4
5
6
7
8
1b: R1 = p-Me; R2 = H
1c: R1 = p-OMe; R2 = H 2c
1d: R1 = p-ClCH2; R2 = H 2d
2b
100
100
100
100
100
90
94
80
90
73
55
64
76
1
2
3
4
5
6
7
8
AgOTf
NaBArF
NaBArF
AgSbF6
AgPF6
AgPF6
AgBF4
AgBF4
5 + 5
5 + 5
5 + 21
5.5 + 5
5 + 5
5 + 22
5 + 5
B100
B100
B100
94
77
29 65
57 21 20
10 79
11 71
24 51 13
41 25
46
0
15
5
7
1
2
5
2
2
1
2
1e: R1 = p-F; R2 = H
1f: R1 = p-Br; R2 = H
1g: R1 = p-CF3; R2 = H
1h: R1 = m-Me; R2 = H
1i: R1 = m-NO2; R2 = H
1j: R1 = m-F; R2 = H
1k: R1 = m-Cl; R2 = H
1l: R1 = m-Br; R2 = H
1m: R1 = o-F; R2 = H
2e
2f
2g
2h
2i
2j
2k
2l
72
58c
87
0
3
87
91
75
75
67d,e
75d
70d
80d,e
60
8
9
5 + 22
5
20
10
11
12
100
100
92
a
Styrene (0.2 mmol), catalyst (2 mol%) and additive were mixed in
2m
dioxane (1.5 mL) at room temperature, and then H2O2 (0.4 mmol,
diluted in dioxane (0.5 mL)) was added via syringe pump. The first
b
13
86
60
number was the addition time of H2O2 and the second number was the
c
further stirring time. Determined by GC.
14
15
16
1o: R1 = H; R2 = Me
1p: R1 = H; R2 = Ph
1q: R1 = H; R2 = BrCH2 2q
2o
2p
100
100
100
64
70
73
X may affect the reactivity of [FeIII(Por)X],9,12 the catalytic activities Àof
a
0.2 mmol substrate, catalyst (0.5 mol%) in dioxane (1.5 mL), H2O2
a panel of [Fe(TF4DMAP)X] (X = OTfÀ, BArFÀ, SbF6À, PF6À and BF4
,
(0.4 mmol, diluted in dioxane (0.5 mL)) was added via syringe pump
generated in situ by reacting [FeIII(TF4DMAP)Cl] with AgX, see ESI†)
catalysts were evaluated (Table 1). [FeIII(TF4DMAP)OTf] catalysed
the oxidation of styrene with H2O2 to give phenylacetaldehyde
with 77% yield along with 1-phenylethane-1,2-diol and benzaldehyde
over 5 h, r.t., 8–9 h. Based on 1H NMR with PhTMS as the internal
b
c
d
standard. Epoxide was obtained. After addition of H2O2, it was
heated to 80 1C. 2 mol% catalyst was used.
e
(15% and 7% yields, respectively) and no epoxide was observeÀd 1-phenylethane-1,2-diol. A trace amount of phenylacetaldehyde
(Table 1, entry 1). The use of other AgX additives (X = SbF6À, PF6
was obtained with o2% substrate conversion when the oxidant
t t
and BF4À) and NaBArF resulted in lower substrate conversion and/or was changed to BuOOBut or BuOOH (Table S3, ESI†). Hence,
lower aldehyde selectivity (Table 1). The recrystallization of the the protocol with 0.5 mol% iron catalyst and 2.0 equivalents
product (obtained by the reaction of [FeIII(TF4DMAP)Cl] and NaBArF H2O2 was used in the subsequent experiments.
Using this protocol, we examined the substrate scope of the
[FeIII(TF4DMAP)OTf] catalysed E–I reaction. As listed in Table 2,
a panel of aryl alkenes gave the corresponding aldehydes with
in THF) in CH2Cl2/hexane (1 : 9) gave [Fe(TF4DMAP)2]O (see ESI†).
Examination of solvent effect revealed dioxane to be the
solvent of choice (Table S1, ESI† entry 1). Low product yields were
obtained when THF, toluene, CH3CN, diethyl ether, methyl tert- good to high yields. Styrenes 1b–1f bearing para-substituents
butyl ether (MTBE), or MeOH/CH2Cl2 (3/1) was used (Table S1, afforded the corresponding phenylacetaldehydes 2b–2f with
ESI† entries 2–7). Phenylacetaldehyde was obtained with 47% 55%–76% yields (entries 1–5), while 58% p-trifluoromethyl styrene
yield when 1,2-dimethoxyethane (DME) was employed as solvent oxide was obtained from 1g (entry 6). Compounds 1i–1l, bearing
(Table S1, ESI† entry 8). 2-tert-Butoxy-2-phenylethanol via the ring electron-withdrawing meta-substituents, afforded 2i–2l with 67%–
80% yields (entries 8–11); however, a higher reaction temperature or
higher loading of catalyst was needed in these cases. a-Substituted
styrenes 1o–1q could also be converted to corresponding aldehydes
opening reaction of styrene oxide was obtained with 70% yield
when tert-butanol was used as a solvent (Table S1, ESI† entry 9).
Products were obtained in acetal forms when methanol or acetone
was used as a solvent (Table S1, ESI† entries 10 and 11). Even with with 64%–73% yields (entries 14–16).
dioxane as a solvent, the substrate conversion decreased significantly
Oxidative cross coupling of tertiary amines with nucleophiles is
from 100% to 40% when the amount of H2O2 was decreased a useful strategy for the synthesis of amino compounds.13 In this
from 2.0 equivalents to 1.2 equivalents. 2-Phenylacetic acid (the study, oxidative cross coupling reaction of tertiary amines and
over-oxidized product from phenylacetaldehyde) was obtained TMSCF3 with [FeIII(TF4DMAP)OTf] as catalyst and H2O2 as a
terminal oxidant gave formamides as the major product. Adding
1 equivalent (based on amine) of acetic acid increased both the
substrate conversion and product yield (Table S4, ESI†). Further
with 28% yield when the amount of H2O2 was increased to
5.0 equivalents (Table S2, ESI†). Pre-treatment of commercially
available styrene (from Aldrichs) by filtration through Al2O3 and
adding 1% BHT (BHT = 2,6-di-tert-butyl-4-methylphenol) did not optimization of the reaction conditions led to the use of 0.3 mol%
significantly improve the substrate conversion, product yield and of Fe catalyst with acetic acid (1 equivalent) as the additive, MeOH
selectivity. Using 2.0 equivalents of H2O2, dioxane as solvent and as the solvent, and H2O2 (2.5 equivalent) as terminal oxidant at
a lower loading of [FeIII(TF4DMAP)OTf] (0.5 mol%), phenyl- room temperature. As shown in Table 3, a panel of N,N-dimethyl
acetaldehyde was obtained with the highest yield of 86% with anilines having electron-donating or electron-withdrawing substi-
tuents were oxidized to corresponding N-methyl formamides with
minimal amounts of by-products, i.e. benzaldehyde (6%) and
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Table 3 [Fe(TF4DMAP)OTf] catalysed formamide formation reactiona
Entry Substrate
1
Product
Conversionb (%) Yieldc (%)
91
99
100 (82)
81 (65)
2
3
4
5
70
100
87
70
95 (80)
94 (75)
Fig. 1 Time-course plot for [Fe(TF4DMAP)OTf] catalysed E–I reaction.
the maximum after 3 h, and then started to decrease afterwards
and completely vanished after 7.5 h. The desired product, pheny-
lacetaldehyde, together with two other side products, 1-phenyl-1,2-
diol and benzaldehyde present in minor amounts, increased
gradually as the reaction proceeded. Controlled experiments with
styrene oxide as substrate revealed that phenylacetaldehyde was
obtained with 78% yield with 1-phenylethane-1,2-diol and benzal-
dehyde with 13% and 9% yields, respectively. Upon replacing H2O2
by H218O in the aforementioned controlled experiment of ring-
opening reaction of styrene oxide, no 18O labelled diol was observed
(a mixture of partially 18O labelled phenylacetaldehyde, benzaldehyde
and styrene oxide was detected). Thus, phenylacetaldehyde
was formed from isomerization of styrene oxide via a tandem
epoxidation–isomerization (E–I) pathway (see ESI†).
6
100
48
7
8
100
82
94 (74)
54
9
24
21
High resolution ESI-MS experiments were performed to gain
insight into the reaction mechanism. Analysis of a mixture of
[FeIII(TF4DMAP)OTf] and H2O2 (5 equiv.) in acetonitrile revealed two
new species at m/z 1144.1793 and 1160.1798 (Fig. S1, ESI†) attribu-
table to [Fe(TF4DMAP)O]+ (Fig. S2, ESI†) and [Fe(TF4DMAP)O2]+
(Fig. S3, ESI†), respectively. Collision-induced dissociation of
both species could produce a daughter ion peak at m/z 1128.1
corresponding to [Fe(TF4DMAP)]+ (Fig. S4 and S5, ESI†). In the
a
[FeIII(TF4DMAP)OTf] (3 mmol), substrate (1.0 mmol), and the additive
(acetic acid 1.0 mmol) were added successively to MeOH (1.2 mL), and it
was stirred under argon at room temperature. H2O2 (2.5 mmol) were
added via syringe pump over
1
h. b Analysed by GC and GC-
MS. c Determined by GC and GC-MS based on conversions, and the isolated
yields are shown in brackets.
moderate to good yields (entries 1–8). Changing N,N-dimethyl aniline presence of styrene (50 equiv.), the two oxygenated species could
to N,N-diethyl aniline decreased both the substrate conversion and also be detected but their signal intensities were weakened by
product yield dramatically. The corresponding N-ethyl acetamide 40% and 50% respectively (relative to [Fe(TF4DMAP)]+ at m/z
was obtained with 21% yield and 24% substrate conversion (entry 9). 1128.1, Fig. S6, ESI†). In the presence of H218O (500 equiv.), the
Direct formation of formamides from N-methyl amines is an signal at m/z 1144.2 showed 85% 18O incorporation (Fig. 2),
important synthetic strategy in organic chemistry. In the literature, whereas the signal at m/z 1160.2 showed 35% 18O incorporation
+
l-oxo-2,2,6,6-tetramethylpiperidinium ion, benzyltriethylammonium (Fig. S7, ESI†). [(Porꢀ )FeIVQO]+ is known to exchange its oxo–
permanganate (Et3N+CH2Ph)MnO4À, pyridinium chlorochromate ligand with H218O and is reactive towards alkenes.16 Based on
(PCC) and pyridinium dichromate (PDC) were reported as stoichio- the aforementioned findings, we assigned the m/z 1144.2 signal
+
metric oxidants for this type of reaction.14 To the best our knowledge, to be predominantly [(TF4DMAPꢀ )FeIVQO]+. The doubly-oxygenated
these are the first examples of Fe-catalysed formation of formamides signal at m/z 1160.2 might be a mixture of species, the nature of
from N-methyl amines using H2O2 as an oxidant.15
which remains to be studied. We also made attempts to generate
The time course for the [FeIII(TF4DMAP)OTf] catalysed E–I analogous species by changing the porphyrin ligand from TF4DMAP
reaction is depicted in Fig. 1. Styrene was completely consumed to F20TPP. Under similar conditions, reaction of [Fe(F20TPP)OTf]
within 5.5 hours, whereas the amount of styrene oxide reached with H2O2 produced a new signal at m/z 1043.9742, which can be
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Laboratory of Synthetic Chemistry, the External Cooperation
Program of Chinese Academy of Sciences (CAS-GJHZ200816),
NSFC (21272197 and 21102162) and the CAS-Croucher Funding
Scheme for Joint Laboratories.
Notes and references
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Fig. 2 ESI-MS spectrum of [Fe(TF4DMAP)O]+ in the absence (top) and
presence of 18O–H2O (500 equiv., bottom).
3 C. Bolm, J. Legros, J. Le Paih and L. Zani, Chem. Rev., 2004,
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Fig. 3 Plausible mechanism for the E–I reaction.
+
formulated as [(F20TPPꢀ )FeIVQO]+ (Fig. S8, ESI†). Its signal
˜
intensity was largely diminished in the presence of 50 equiv.
styrene (Fig. S9, ESI†) and it showed 80% 18O incorporation with
500 equiv. H218O (Fig. S10, ESI†). These results are similar to
those observed with the TF4DMAP system, revealing the likely
8 Reviews of iron-catalysed reactions: A. Correa, O. G. Mancheno and
C. Bolm, Chem. Soc. Rev., 2008, 37, 1108.
9 G.-Q. Chen, Z.-J. Xu, C.-Y. Zhou and C.-M. Che, Chem. Commun.,
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+
involvement of [(Porꢀ )FeIVQO]+ as reaction intermediate. Treatment
of [FeIII(TF4DMAP)OTf] with excess H2O2 in CH2Cl2 for 15 min led to
broadening and red-shift of the lowest energy absorption peak
maximum to 665 nm, which is similar to the absorption of
porphyrin p-cation radical (Fig. S11, ESI†).11
11 (a) J. T. Groves and Y. Watanabe, J. Am. Chem. Soc., 1986, 108, 507;
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(E–I) pathway is proposed (Fig. 3). Firstly, [Fe(Por)]+ is converted to
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N-methyl aryl tertiary amines to formamides with H2O2 as a
terminal oxidant under mild conditions has been achieved with
moderate to good product yields.
(d) J. Danielsson and P. Somfai, Org. Lett., 2014, 16, 784.
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We acknowledge the financial support from the Innovation
and Technology Commission (HKSAR, China) to the State Key 16 W. Nam and J. S. Valentine, J. Am. Chem. Soc., 1993, 115, 1772.
12672 | Chem. Commun., 2014, 50, 12669--12672
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