Iron-facilitated oxidative dehydrogenative C-O bond formation by propargylic Csp3-H functionalization

Article abstract of DOI:10.1002/anie.201205779

Hot couple: Propargyl azides were coupled with carboxylic acids by an iron-catalyzed dehydrogenative C-O bond formation (see scheme). This method enables propargylic C sp 3-H functionalization under mild reaction conditions and also may involve the applic

Full text of DOI:10.1002/anie.201205779

Angewandte
Chemie
DOI: 10.1002/anie.201205779
Dehydrogenative Coupling
À
Iron-Facilitated Oxidative Dehydrogenative C O Bond Formation by
Propargylic Csp3 H Functionalization**
À
Teng Wang, Wang Zhou, Hang Yin, Jun-An Ma, and Ning Jiao*
À
The formation of carbon–heteroatom bonds is one of the most
attractive and fundamental transformations in organic syn-
propargylic Csp3 H bond activation has not been reported
(Scheme 1c).
thesis.[1] For example, the construction of C O bonds plays an
We began our study by examining the oxidative rear-
rangement reaction of (3-azidoprop-1-ynyl)benzene (1a).[7]
Interestingly, the dehydrogenation product, 1-azido-3-phenyl-
prop-2-ynyl acetate (3aa), was obtained in 43% yield in the
presence of acetic acid 2a [Eq. (1); DCE = dichloroethane,
DDQ = 2,3-dichloro-5,6-dicyano-1,4-benzoquinone]. In con-
À
important role in producing alcohols, ethers, and esters in the
synthesis of drugs, materials, and natural products.[2] In
À
general, great progress has been made in C O bond
formation from carbon–halide bonds,[1,3] although prefunc-
tionalization of the substrates was often required and halide-
containing by-products were formed during the process
(Scheme 1a). Thus, the alternative strategy involving direct
trast, the reaction of 4, which has a similar structure to 1a but
does not contain an azido group, did not afford the
À
corresponding C O coupling product 5 [Eq. (2)]; this result
indicates that the azido group acted as an assisting group to
À
Scheme 1. The strategy for propargylic Csp3 H functionalization with
carboxylic acids. AG=assisting group.
À
À
C H/O H oxidative coupling has assumed increasing impor-
tance from the perspective of atom economy.[4] In this context,
some couplings have been achieved by employing carboxylic
acids as the O H donators.[5,6] Despite a significant number of
successful examples, most direct Csp3 H/O H oxidative
couplings are still restricted to allylic Csp3 H functionalization
facilitate this Csp3 H functionalization. Subsequently, the
À
À
À
À
yield of 3aa was improved to 49% in a one-pot procedure
when 3-chloro-1-phenyl-1-propyne (4a) was employed as
a precursor of 1a (Table 1, entry 1).
À
by palladium catalysis, as developed by the groups of ꢀker-
mark, Larsson, and White (Scheme 1b).[6] To the best of our
Recently, iron catalysts have attracted a lot of attention in
catalysis owing to their cheap, environmentally benign, and
À
À
knowledge, until now the direct C O bond formation through
insensitive characteristics.[8] Iron-catalyzed oxidative Csp3
H
À
functionalizations for carbon heteroatom bond formations
have been recently disclosed in a limited number of cases.[9]
To our delight, iron salts such as FeCl2 were very effective in
the oxidative coupling (Table 1, entries 2, and 4–6). Attempts
to use other oxidants and solvents were not successful (see the
Supporting Information for more details). Pivalic acid 2b,
instead of 2a, was used in the reaction and produced the
corresponding product 3ab in a similar moderate yield
(Table 1, entry 7). By increasing the amount of 2b used, 3ab
was obtained in 70% yield (Table 1, entry 8). The model
reaction in the presence of 2,2,6,6,-tetramethylpiperidine-N-
oxyl (TEMPO; 2 equiv) or O2 produced the desired 3ab with
low yields (see Table 1, entry 10, and the Supporting Infor-
mation). Attempts to use other catalysts such as manganese
salts (Table 1, entries 12 and 13) resulted in lower yields.
With the optimized conditions established, the substrate
scope was investigated. A diverse range of carboxylic acids
[*] T. Wang, W. Zhou, H. Yin, Dr. N. Jiao
State Key Laboratory of Natural and Biomimetic Drugs
Peking University, Xue Yuan Rd. 38, Beijing 100191 (China)
E-mail: jiaoning@bjmu.edu.cn
Homepage: http://sklnbd.bjmu.edu.cn/nj
T. Wang, Dr. J.-A. Ma
Department of Chemistry, Tianjin University, Tianjin 300072 (China)
Dr. N. Jiao
State Key Laboratory of Organometallic Chemistry
Chinese Academy of Sciences, Shanghai 200032 (China)
[**] Financial support from the National Basic Research Program of
China (973 Program 2009CB825300), the National Science Foun-
dation of China (No. 21172006), and Peking University are greatly
appreciated. We thank Zejun Xu in this group for reproducing the
results of 3ac, 3ib, and 6e.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205779.
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Table 1: Optimization of reaction conditions for dehydrogenative cou-
pling of 3-chloro-1-phenyl-1-propyne 4a and acetic acid 2a.[a]
Entry
Cat.
RCO2H
Yield of 3 [%]
1
2
3
4
5
6
–
2a
2a
2a
2a
2a
2a
2b
2b
2b
2b
2b
2b
2b
49 (3aa)
64 (3aa)
43 (3aa)
55 (3aa)
60 (3aa)
61 (3aa)
63 (3ab)
70 (3ab)
59 (3ab)
57 (3ab)
50 (3ab)
30 (3ab)
trace (3ab)
FeCl2
CuBr2
FeCl3
FeBr2
FeF2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
MnBr2
MnCl2
7[c]
8[d]
9[d,e]
10[d,f]
11[d,g]
12[d]
13[d]
À
Scheme 2. Iron-catalyzed oxidative C O coupling of 3-chloro-1-phenyl-
[a] Reaction conditions: 4a (0.2 mmol), sodium azide (0.3 mmol), TBAB
(0.02 mmol) in anhydrous DCE (2.0 mL) at 308C under Ar for 24 h, then
acetic acid 2a (0.5 mmol), DDQ (0.4 mmol), and catalyst (10 mol%)
were added to the mixture, which was stirred at 308C for 16 h. [b] Yields
of the isolated products. [c] Pivalic acid (2b) was added instead of 2a.
[d] 2b (0.6 mmol) was used. [e] 20 mol% of FeCl2 was used. [f] This
reaction was carried out under O2. [g] In the second step the reaction
mixture was stirred at 508C for 6 h. Piv=trimethylacetyl, TBAB=tetra-
butylammonium bromide.
1-propyne (4a) with various carboxylic acids 2. Reaction conditions: 3-
Chloro-1-phenyl-propyne (4a; 0.2 mmol), sodium azide (0.3 mmol),
and TBAB (0.02 mmol) in anhydrous DCE (2.0 mL) at 308C under Ar
for 24 h, then carboxylic acid 2 (0.6 mmol), DDQ (0.4 mmol) and FeCl2
(10 mol%) were added to the mixture, which was stirred at 308C for
16 h. Yields are of the isolated products.
À
Table 2: Iron-catalyzed oxidative C O coupling of various 3-chloro-1-
aryl-1-propynes 1 with pivalic acid 2b.[a,b]
were employed in the oxidative coupling with 4a (Scheme 2).
Most aliphatic acids gave the desired propagyl carboxylic acid
esters in good yields (3aa–3al); substrates containing aryl
groups and double bonds were tolerated (see products 3af,
3ag, and 3ai–3al). Sterically hindered carboxylic acids also
performed well in the coupling. For instance, the reaction of
adamantane-1-carboxylic acid (2h) gave 3ah in 66% yield. A
few aromatic acids including heterocyclic acid 2p were
employed in this transformation to give the corresponding
products in moderate yield. Moreover, 3am was obtained
from cinnamic acid in 66% yield. Crystals of compound 3an
that were suitable for X-ray crystallographic analysis were
obtained, thus enabling the structure to be confirmed (see the
Supporting Information, Figure S1).
Entry
Ar
4
Yield of 3 [%]
1
2
3
4
5
6
7
8
2-MeC6H4
3-MeC6H4
4-MeC6H4
2-OMeC6H4
3-OMeC6H4
2,4-Me2C6H3
4-FC6H4
4-ClC6H4
4-BrC6H4
3-ClC6H4
1-naphthyl
2-thienyl
4b
4c
4d
4e
4 f
4g
4h
4i
67 (3bb)
80 (3cb)
74 (3 db)
83 (3eb)
68 (3 fb)
56 (3gb)
58 (3hb)
80 (3ib)
9
4j
4k
4l
56 (60)[c] (3jb)
74 (3kb)
45 (3lb)
10
11
12
4m
57 (3mb)
Subsequently, a range of different aryl propyne substrates
were used in the transformation with pivalic acid (Table 2). It
is noteworthy that the substrates containing electron-rich
arenes were well tolerated in this transformation (Table 2,
entries 2 and 4). Additionally, aromatic halides gave the
desired products in moderate yields under the standard
reaction conditions (Table 2, entries 7–10), thus providing the
possibility to perform other transformations in sequence.
Aromatic heterocyclic compounds were also tolerated in this
transformation (Table 2, entry 12).
Many studies have revealed that derivatives of 4,5-
disubstituted-1,2,3-triazoles are chemically and biologically
active.[10] However, few methods have been developed that
provide an efficient entry to these aforementioned tria-
zoles.[11] The copper-catalyzed azide-alkyne cycloaddition
[a] Reaction conditions: 3-chloro-1-aryl-propyne 4 (0.2 mmol), sodium
azide (0.3 mmol), and TBAB (0.02 mmol) in dry DCE (2.0 mL) at 308C
under Ar for 24 h, then pivalic acid 2b (0.6 mmol), DDQ (0.4 mmol) and
FeCl2 (10 mol%) were added to the mixture, which was stirred at 308C
for 16 h. [b] Yields are of the isolated product. [c] Yield for a larger scale
(5 mmol) reaction, see the Supporting Information for details.
reaction (CuAAC)[12] often gives N-substituted products.[11c]
It has been reported that propargyl azides can isomerize to
the allenyl azides by the shift of azide group, followed by
rapid cyclization to give 4,5-disubstituted-1,2,3-triazoles.[13]
Therefore, an advantage of the present protocol is that the
products could potentially undergo click reactions in an
efficient one-pot procedure. A brief investigation of the
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conditions for the rearrangement of propargyl azide 3ab,
showed methanol to be the optimal solvent. After a solution
of 3ab in methanol was heated to reflux for 36 hours in the
absence of any other additives, 4-(dimethoxymethyl)-5-
phenyl-1,2,3-triazole 6a was obtained in 65% yield
(Scheme 3).
Furthermore, the present novel method for propargylic
À
Csp3 H functionalization can be combined with many trans-
À À
formations. For example, compounds containing Bt C O
functional groups have been proven to be efficient building
blocks in organic sythesis.[16] These compounds can be
smoothly prepared from our multifunctionalized products
by
a simple click reaction with benzyne precursors
[Eq. (3);TMS = trimethylsilyl, Ts = p-toluenesulfonyl]. For
example, when 3jb was treated with 2-(trimethylsilyl)-
phenyl-4-methylbenzenesulfonate benzotriazole
obtained in 60% yield.
8
was
To obtain further understanding of the oxidative coupling
of carboxylic acids with propargyl azides, a propargyl ether 9
was treated to the standard reaction conditions. However, the
desired coupling product, which would be formed via an
oxonium intermediate,[17] was not observed and instead
phenylpropiolaldehyde 10 was obtained in 40% yield
[Eq. (4)]; this result indicates that azide groups are the
appropriate assisting groups in this coupling.
Scheme 3. Formation of 4-(dimethoxymethyl)-5-aryl-1,2,3-triazoles 6 in
methanol. Reaction conditions: 1-azido-3-arylprop-2-ynyl carboxylic
ester 3 (0.2 mmol) in methanol (2.0 mL) at reflux under Ar for 36 h.
Yields are of the isolated products.
The scope of the formation of triazoles 6 was examined
with a series of aryl propargyl azides 3 (Scheme 3). The
substrates with electron-rich substituents at a variety of
positions on the aryl ring gave the corresponding products
(6b–6d) in good yields (51–74%). Aryl propargyl azides with
chloro- and bromo- substituents at the para-position gave the
corresponding triazoles (6e, and 6 f) in moderate yields (54
and 56%, respectively). Triazole 6g, containing an naptha-
lene group, was obtained in 47% yield. The structure of the
triazoles was confirmed by X-ray crystallographic analysis of
6 f (see the Supporting Information, Figure S2).
Interestingly, in the presence of inorganic bases such as
NaOH 3-alkoxyenal analogues (E)-7, instead of 6, were
stereoselectively obtained[14] as the sole products (Scheme 4).
Walizei and Breitmaier have shown that these 3-alkoxyenal
analogues are useful synthons for a wide range of synthetic
targets.[15] As an example, 2-substituted pyrroles have been
produced by the reaction of 3-alkoxypropenals with glycine
esters.[15]
On the basis of the above results, a plausible mechanism is
proposed in Scheme 5. Initially, substrates 1 undergo hydro-
gen abstraction through an iron-facilitated single-electron
oxidation with DDQ[18] to form the radical species A, which
may be stabilized by the azido group.[19] Subsequently, radical
species A was further oxidized to give the aryl propargyl
cation B. Then, nucleophilic attack of cation B by carboxylic
acid 2 gave the desired product 3 with the regeneration of the
catalyst.
Scheme 4. Base-facilitated formation of (E)-3-methoxy-3-arylacryl-
aldehyde 7.
Scheme 5. Proposed mechanism for this oxidative dehydrogenative
À
C O bond formation.
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In conclusion, we have demonstrated a novel and practical
À
protocol for iron-promoted Csp3 H functionalization of aryl
propargyl azides. This method offers an opportunity to
À
achieve propargylic Csp3 H functionalization under mild
reaction conditions and also may involve the application of
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À
azido group as an assisting group in C H activation. The
products of this oxidative dehydrogenative coupling are
useful synthons for a wide range of synthetic targets such as
4,5-disubstituted-1,2,3-triazoles, 3-alkoxyenals, and benzo-
triazoles. Further studies on the mechanism and applications
of this transformation are in progress in our laboratory.
Received: July 20, 2012
Revised: August 7, 2012
Published online: October 9, 2012
À
À
Keywords: carboxylic acids · cations · C H activation · C
.
O coupling · iron
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