A simple route to 1,4-addition reactions by Co-catalyzed reductive coupling of organic tosylates and triflates with activated alkenes

Article abstract of DOI:10.1039/c7cc06881f

An efficient Co-catalyzed 1,4-addition reaction of alkyl/aryl triflates and tosylates with activated alkenes is described. In this reaction, an air-stable cobalt(ii) complex, a mild reducing agent Zn and a simple proton source (H₂O) are used. A radical mechanism for the addition of alkyl tosylates to activated alkenes is likely involved.

Full text of DOI:10.1039/c7cc06881f

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A Simple Route to 1,4-Addition Reactions by Co-
Catalyzed Reductive Coupling of Organic
Cite this: DOI: 10.1039/x0xx00000x
Tosylates and Triflates with Activated Alkenes
Jen-Chieh Hsieh,*a Yi-Hua Chu,b Krishnamoorthy Muralirajanb and Chien-
Hong Cheng*a
Received 00th ,
Accepted 00th
DOI: 10.1039/x0xx00000x
www.rsc.org/
has stimulated us to investigate the utilization of alkyl/aryl triflates
and tosylates for the 1,4-addition reactions. Herein, we report the
results of these studies. This new strategy complements nicely the
existing methods, because the triflate and tosylate substrates can be
readily obtained from the widely available aliphatic and aromatic
alcohols, greatly enlarging the scope of this 1,4-addition reaction. It
is noteworthy that there is no report on the Co-catalyzed 1,4-addition
reactions of aryl/alkyl triflates and tosylates with activated alkenes
known to date. Although some Mizoroki-Heck type reactions using
aryl triflates and tosylates with alkenes catalyzed by palladium and
ruthenium complexes have been reported (Scheme 1).8 The use of
organic triflates as coupling partners in the cobalt catalytic coupling
reactions has only been reported by Hayashi,9 and the tosylate has
never been used in the cobalt catalysis.10, 11
An efficient Coꢀcatalyzed 1,4ꢀaddition reaction of alkyl/aryl
triflates and tosylates with activated alkenes is described. In
this reaction, airꢀstable cobalt(II) complex, mild reducing
agent Zn and simple proton source (H2O) are used. A radical
mechanism for the addition of alkyl tosylates to activated
alkenes is likely involved.
Transition metal-catalyzed C-C bond formation through addition
reactions is one of the most prevailing methods in organic synthesis.1
Generally, organometallic coupling reagents (Si, Sn, B etc.), organic
halides or pseudohalides are used for these addition reactions.2 In
particular, C-C bond formation via 1,4-addition reaction using such
reagents with activated alkenes in the presence of metal complexes
have attracted considerable attention during the past decades. These
metal complexes mainly influence the reaction mechanism as well as
the reaction sequence to afford the desired products.3 In this fashion,
1,4-addition reactions using first row transition metal catalysts such
as Co, Ni and Cu are demonstrated. These reactions feature more
efficient, atom-economical and versatile processes for C-C bond
formation by using organometallic reagents. However, the reported
1,4-addition reactions generally require organometallic reagents,
halo compounds, nucleophiles or bases, which are mostly considered
as demerits and thus the development is still desired.4, 5 Hence, it is
highly necessary to develop a transition metal-catalyzed 1,4-addition
reaction with mild condition, absence of organometallic reagents and
compatible with a wide variety of substrates.
In this context, we have reported the mild Co-catalyzed 1,4-addition
reactions through oxidative addition or transmetalation using alkyl
halides or aryl boronic acids with activated alkenes.6 Our continuing
interest in the first row transition metal-catalyzed coupling reactions7
Scheme 1. Metal-catalyzed reactions using pseudohalides
We started to investigate the 1,4-addition reaction of phenyl
triflate 1a with n-butyl acrylate 2a by using Co(dppe)I2 (10 mol
aDepartment of Chemistry, Tamkang University, New Taipei City, 25137,
Taiwan (R.O.C.).
E-mail: jchsieh@mail.tku.edu.tw; Tel: 886-2-26215656ext2545
bDepartment of Chemistry, National Tsing Hua University, Hsinchui,
30013, Taiwan (R.O.C.).
%) as a catalyst. The reaction proceeded nicely in the presence
of Zn (2.75 equiv) and H2O (1.0 equiv) in CH3CN (2.0 mL) at
80 ºC for 16 h, affording Michael-type addition product 3a in
75% NMR yield (Table 1, entry 1). To further understand the
effect of reaction, we surveyed catalysts and solvents for the
reaction and the results are summarized in Table 1. We first
evaluated the cobalt complexes and found the cobalt complex is
E-mail: chcheng@mx.nthu.edu.tw; Tel: 886-3-5715131ext33381
† Electronic Supplementary Information (ESI) available:
Experimental procedure, characterization data, spectral data and
copies of all compounds. See DOI: 10.1039/b000000x/
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DOI: 10.1039/C7CC06881F
which is better than the yield of its para analogue 3b. Different
quinoline derivatives (3i and 3j) were obtained in high yields. A
variety of activated alkenes were examined as well. As demonstrated,
different acrylates and acrylonitrile can participate in this reaction to
form the corresponding products (3k–3o) in 58 to 80% yields.
crucial to this reaction (entries 1–8). Among the complexes that
we examined, Co(dppe)I2 is the most effective catalyst, while
Co(dppe)X2 with other halo ligands such as Co(dppe)Cl2 and
Co(dppe)Br2 provided much lower yields of product 3a (entries
2, 3). Other bidentate phosphine cobalt complexes, Co(dppb)I2
and Co(dppm)I2, have no reactivity for this reaction (entries 4,
5). Additional amount of bidentate phosphine ligand is helpful
for this addition reaction, the combination of Co(dppe)I2 with
dppe provided significant improvement for the yield of product
3a (entry 6, 89%). Monodentate phosphine cobalt complexes
are less reactive for this reaction, additional amount of PPh3
also cannot show efficacy (entry 7, 8). The solvent is crucial for
this reaction as well. Apart from the CH3CN, reaction can only
proceed in THF with unsatisfied yield of 3a (entries 9–12). We
further tested some selected nickel complexes (entries 13–15),
and it was found that only Ni(dppb)Cl2 is active for this
reaction, but the product 3a could be obtained only in a low
yield (entry 15). Moreover, the blank experiments revealed that
in the absence of either cobalt or zinc, no desired product 3a
can be observed (entry 16, 17).
Scheme 2. Co-catalyzed 1,4-addition reaction of aryl triflates 1 with
activated alkenes 2.a, b
Table 1. Optimization of Reaction Conditionsa
aReaction condition:
1 (0.5 mmol, 1.0 equiv), 2 (1.0 mmol), Co(dppe)I2
(10 mol%), dppe (10 mol%), Zn (2.75 equiv), H2O (1.0 equiv) and
CH3CN (2.0 mL) at 80 °C for 16 h under N2. bIsolated yields.
Entry
1
2
3
4
5
6
7
8
Catalyst/Ligand
Co(dppe)I2
Co(dppe)Cl2
Co(dppe)Br2
Co(dppb)I2
Co(dppm)I2
Co(dppe)I2/dppe
Co(PPh3)2I2/PPh3
Co(PPh3)2Cl2/PPh3
Co(dppe)I2/dppe
Co(dppe)I2/dppe
Co(dppe)I2/dppe
Co(dppe)I2/dppe
Ni(dppe)Br2
Solvent
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
THF
Toluene
1,4-Dioxane
DCM
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Yield (%)b of 3a
To further investigate the scope of this 1,4-addition reaction,
we tried to extend the reactions to alkyl triflates 4. Therefore,
reactions for the long chain aliphatic triflates with various
activated alkenes were tested and shown in Scheme 3. As
75
21
53
0
trace
89 (85)c
indicated, the aliphatic triflates
4 worked well with n-butyl
17
trace
22
0
0
0
0
0
30
0
0
acrylate to form the products 5a and 5b in 87 and 88% yields,
respectively. In addition, the present addition reaction is also
compatible with vinyl ketones (5c and 5d) and vinyl sulfonyl
benzene (5e), the reaction yields significantly varied with the
activated alkenes.
9
10
11
12
13
14
15
16
17d
Ni(dppe)Cl2
Ni(dppb)Cl2
Scheme 3. Co-catalyzed 1,4-addition reaction of alkyl triflates 4
with activated alkenes 2.a, b
-
Co(dppe)I2/dppe
aUnless otherwise mentioned, all reactions were carried out using 1a (0.5
mmol, 1.0 equiv), 2a (1.0 mmol), catalyst (10 mol %), ligand [PPh3 (30 mol
%), dppe (10 mol %)], Zn (2.75 equiv), H2O (1.0 equiv) and solvent (2.0
mL) at 80 °C for 16 h under N2. bYields were measured from the crude
products by 1H NMR integration method using mesitylene as an internal
standard. cIsolated yield. dWithout Zn.
The scope of aryl triflates 1 and activated alkenes 2 were then
investigated under the optimized reaction condition. As indicated in
Scheme 2, reactions performed smoothly to provide corresponding
Michael-type addition products, and the reaction yields significantly
depend on electron density of aryl triflates 1. Various substituents on
the para position of aryl triflates, including electron-donating and
electron-withdrawing groups, are tolerated to give the corresponding
products (3b–3g). Products with an electron-withdrawing group
were provided in very high yields (3e–3g), while products with an
electron-donating group were provided in moderate yields (3b–3d).
Product with a meta methoxy group (3h) was provided in 70% yield,
aReaction condition: 4 (0.5 mmol, 1.0 equiv), 2 (1.0 mmol), Co(dppe)I2 (10
mol%), dppe (10 mol%), Zn (2.75 equiv), H2O (1.0 equiv) and CH3CN (2.0
mL) at 80 °C for 10 h under N2. bIsolated yield.
After carrying out the Co-catalyzed 1,4-addition reactions of
aryl/alkyl triflates with activated alkenes, we then tried to
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7CC06881F
O
Co(PPh3)2I2 (10 mol%)
PPh3 (30 mol%)/Zn (2.5 equiv)
O
extend this reaction to include diversely aliphatic tosylates 6.
Ph
OTs
OR
O
Ph
4
Further optimization has been conducted, and we found slight
modification of the reaction condition is able to tolerate a wide
variety of aliphatic tosylates and alkenes. Thus, 10 mol% of
Co(PPh3)2I2, 30 mol% of PPh3, 2.5 equiv of Zn and 1.0 equiv of
H2O in CH3CN at 80 °C under N2 for 24 h was performed as
standard condition of this protocol, the scope of reaction was
investigated according to this reaction condition (Scheme 4).
As indicated, various acrylates can participate in this reaction to
H2O (1.0 equiv), TEMPO (2.0 equiv)
CH3CN, 80 °C, 24 h, < 5%
6a
2e: R = p-methylbenzyl
5i
O
Co(PPh3)2I2 (10 mol%)
PPh3 (30 mol%)/Zn (2.5 equiv)
O
OTs
OR
2e
O
3
H2O (1.0 equiv), CH3CN
80 °C, 24 h, 52%
6f
7
Scheme 5. Control experiments.
Although reactions for the aryl triflates may have alternative
pathway, a tentative mechanism for the alkyl tosylates (or triflates)
can be proposed according to above results and previous reports. As
shown in Scheme 6, the catalytic reaction pass through generation of
alkyl radical from alkyl tosylates by the SET (single electron transfer)
process, followed by the alkene insertion and the protonation.12 This
afford their corresponding products (5f–5m) in good yields.
The reaction yields were not significantly influenced by the
substituents on the OR moiety of acrylates. Acrylonitrile and
vinyl ketone are also able to proceed this reaction and generate
the corresponding products 5n and 5o in 67 and 45% yields,
respectively. Ether-type tosylate is a good coupling partner; the reaction mechanism includes the following steps: (1) initiation of the
catalytic reaction by the reduction of CoI2(L)2 to Co(I) A by using
zinc powder,5c-h (2) generation of alkyl radical C from alkyl tosylate
via the SET of cobalt complex, (3) addition of the free alkyl radical
to the activated alkene to give D,12 (4) trapping of intermediate D by
Co(II) complex B to afford cobalt(III) intermediate E and E′, (5)
protonation of alkyl–cobalt(III) intermediate E by water and (6)
subsequent reduction of Co(III) complex F by zinc powder to
regenerate the active Co(I) catalyst.
corresponding product 5p was obtained in 85% yield. Reactions
of the secondary tosylates with acrylate, acrylonitrile and vinyl
ketone (5q–5s) proceeded smoothly, and the reaction tendency
is the same with reactions of the primary tosylates. Products 5a
and 5e could be also provided in slightly lower yields by using
tosylates as substrates. It is very important to mention that the
Michael-type 1,4-addition reaction of aliphatic tosylates with
activated alkenes has never been reported.10, 11
Scheme 4. Co-catalyzed 1,4-addition reaction of alkyl tosylates 6
with activated alkenes 2.a, b
Scheme 6. Proposed Mechanism
In conclusion, we have successfully demonstrated a cobalt-
catalyzed 1,4-addition reactions of organic triflates and tosylates to
activated alkenes. The reactions are compatible with diverse types of
triflates and tosylates. In addition, because of mild conditions, high
yields and relatively low cost of cobalt catalyst, this reaction shows
high potential to be a prevailing method for C–C bond formation.
Finally, the result seems to be an encouraging area with scope for
future development.
aReaction condition: 4 (0.5 mmol, 1.0 equiv), 2 (2.0 mmol), Co(PPh3)2I2 (10
mol%), PPh3 (30 mol%), Zn (2.5 equiv), H2O (1.0 equiv) and CH3CN (2.0
mL) at 80 °C for 24 h under N2. bIsolated yield.
To understand the mechanism, some control experiments were
conducted to clarify the reaction pathway (Scheme 5). First, we
added stoichiometric amount of TEMPO as a radical scavenger and
found the 1,4-addition reaction was significantly suppressed. Second,
we introduced 5-hexenyl tosylate 6f as a substrate and found the
intramolecular cyclization proceeds first then the cobalt catalytic
1,4-addition reaction. No olefin containing product has been detected.
These behaviors have been considered as strong evidence for the
presence of alkyl radicals during the catalytic reaction.
Acknowledgment
We thank the Ministry of Science and Technology of Republic
of China (MOST-105-2113-M-032-005-MY2) for financial
support of this research.
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DOI: 10.1039/C7CC06881F
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Graphical Abstract
A Simple Route to 1,4-Addition Reactions by Co-Catalyzed Reductive
Coupling of Organic Tosylates and Triflates with Activated Alkenes
Jen-Chieh Hsieh,* Yi-Hua Chu, Krishnamoorthy Muralirajan and Chien-Hong Cheng*
Abstract:
[CoL2I2]/Zn, H2O
R1
X
EWG
EWG
R1
(L = PPh3 or L2 = dppe)
(X = OTf and OTs)
(R1 = Alkyl or Aryl)
An efficient Co-catalyzed 1,4-addition reaction of alkyl/aryl triflates and tosylates with activated alkenes is described. In this reaction,
air-stable cobalt(II) complex, mild reducing agent Zn and simple proton source (H2O) are used. A radical mechanism for the addition of
alkyl tosylates to activated alkenes is likely involved.