Combining visible light catalysis and transition metal catalysis for the alkylation of secondary amines

Article abstract of DOI:10.1002/adsc.201300311

By combining the visible-light catalyst, Ru (bpy) ₃Cl ₂, and transition metal salts, Cu (OTf)₂, we are able to functionalize secondary amines to form C_sp3-C_sp3 bonds with b-keto esters efficiently by

Full text of DOI:10.1002/adsc.201300311

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DOI: 10.1002/adsc.201300311
Combining Visible Light Catalysis and Transition Metal Catalysis
for the Alkylation of Secondary Amines
Xue-Wang Gao,+a Qing-Yuan Meng,+a Ming Xiang,a Bin Chen,a Ke Feng,a
Chen-Ho Tung,a and Li-Zhu Wua,*
a
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry
and University of Chinese Academy of Sciences, the Chinese Academy of Sciences, Beijing 100190, Peopleꢀs Republic of
China
Fax : (+86)-10-8254-3580; e-mail: lzwu@mail.ipc.ac.cn
+
These two authors contributed equally to this work.
Received: April 15, 2013; Revised: June 8, 2013; Published online: August 9, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300311.
Abstract: By combining the visible-light catalyst,
Ru(bpy)3Cl2, and transition metal salts, Cu(OTf)2,
esters with high yields and enantioselectivity.[7] Never-
theless, all of the reactions need a stoichiometric
amount of oxidant (i.e., DDQ or t-BuOOH), and
some of them were undertaken at high temperature.
G
ACHTUNGTRENNUNG
we are able to functionalize secondary amines to
À
form Csp3 Csp3 bonds with b-keto esters efficiently
À
by an aerobic oxidative coupling reaction under
visible light irradiation at ambient conditions.
To activate the C H bond of secondary amines at
the a-position, no matter by which of the approaches
mentioned above, the following factors must be taken
into consideration: (i) the relatively high oxidation
potential that makes secondary amines difficult to be
oxidized; (ii) the hydrogen atom connected to the ni-
trogen atom of secondary amines has some acidity,
and therefore the cation radical intermediate generat-
ed from secondary amines is not as stable as that
À
Keywords: alkylation; C H activation; cross-cou-
pling reactions; visible light catalysis
The design of synthetic approaches to functionalize from tertiary amines, which may increase the possibil-
secondary amines for the generation of large arrays of ity of by-product formation.
diverse a-amino acid derivatives is always an impor-
Very recently, Li and co-workers established a mild
tant concern of synthetic chemists.[1] Although the method for the a-indolylation of a-amino carbonyl
À
direct oxidative coupling of a substituted group at the compounds to build a Csp3 Csp2 bond by using a visi-
a-position of tertiary amines has been extensively ble-light catalysis strategy at elevated temperatures,[8]
studied,[2,3] functionalization of secondary amines re- where molecular oxygen (O2) was employed as a ter-
mains far behind. Only four successful examples of minal oxidant. Rueping and co-workers, on the other
oxidative coupling reactions, to the best of our knowl- hand, achieved an oxidative alkynylation of N-substi-
edge, were reported for the synthesis of amino acid tuted tetrahydroisoquinolines by employing [Ru
derivatives.[4–7] In 2008, Li and co-workers developed
(dtbbpy)](PF6)2 and (MeCN)4CuPF6 as the photosen-
the first example of an oxidative coupling reaction of sitizer and metal catalyst, respectively.[9] Fu and co-
ACHTUNGTRENNUNG(bpy)2
A
ACHTUNGTRENNUNG
À
À
secondary amines by an oxidative C H/C H coupling workers developed the alkynylation of N-aryltetrahy-
process.[4,5] The functionality, such as aryl, vinyl, and droisoquinolines by merging Rose Bengal and CuI
alkynyl, had been specifically anchored at the a-posi- under visible light irradiation.[10] These appealing re-
tion of relatively unreactive glycine amines. Huang sults inspired us to initiate the study of visible-light
and Xie made use of a CuACTHNUTRGNEUNG
(OAc)2/pyrrolidine system catalysis for the alkylation of secondary amines,[11] es-
À
to catalyze the oxidative reaction, but they found that pecially for the formation of a Csp3 Csp3 bond of a-
the efficiency and selectivity of the reaction were amino acid esters, which is more challengiung than
poor.[6] Moreover, Wang and co-workers reported that that of a Csp3 Csp2 or a Csp3 Csp bond. Since tertiary
a CuACHTUNGTRENNUNG(OTf)2/chiral bisoxazoline ligand system was able amines can be oxidized by O2 under visible-light cat-
À
À
to realize the oxidative cross-coupling reaction of N- alysis for the subsequent aza-Henry cross-coupling re-
substituted glycine esters and a-substituted b-keto action,[12] we envisioned that secondary amino acid
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Combining Visible Light Catalysis and Transition Metal Catalysis for the Alkylation
Scheme 1. Combining visible-light catalysis and transition metal catalysis for the alkylation of secondary amines.
esters could be oxidized to generate iminium ions in keto esters (Scheme 1). As will be demonstrated later,
the same way and, on the other hand, the aza-Henry the expectation was found to be indeed the case. The
reaction would be accelerated when transition metal functionalization of secondary glycine esters has been
salts are added to increase the nucleophilicity of b- achieved with enhanced efficiency and selectivity by
Table 1. Optimization of the reaction conditions.[a]
2+
Entry
Ru
(bpy)3
Cu
–
E
Solvent
Time [h]
dr[b]
Yield [%][c]
1
2
3
4
5
6
7
8
1 mol%
–
CH2Cl2
CH2Cl2
CH2Cl2
THF
CHCl3
toluene
DMSO
DMF
24
24
24
5
–
–
<5
<5
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
10 mol%
5 mol%
20 mol%
10 mol%
10 mol%
10 mol%
–
1 mol%
1 mol%
1 mol%
1 mol%
1 mol%
1 mol%
1 mol%
1 mol%
1 mol%
2 mol%
3 mol%
1 mol%
1 mol%
–
1:1
2:1
3:2
2:1
1:1
1:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
30 (20)
27 (18)
42 (31)
50 (37)
32 (23)
30 (22)
47 (35)
43 (34)
44 (32)
51 (37)
50 (37)
93 (86)[d]
30 (23)[d]
47 (36)[d]
NR[e]
6
12
24
24
24
12
12
12
12
12
12
12
12
9
DCE
10
11
12
13
14
15
16
17
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
10 mol%
10 mol%
1 mol%
[a]
Unless otherwise specified, the reaction was carried out with 1a (0.2 mmol) and 2a (0.1 mmol) in the presence of
Ru(bpy)3Cl2 and Cu(OTf)2 in the solvent used (3 mL) under irradiation by blue LEDs at room temperature.
G
ACHTUNGTRENNUNG
[b]
[c]
1
The diastereomeric ratio (dr) was determined by H NMR spectroscopy.
Yields were measured by 1H NMR spectroscopy using an internal standard, the isolated product yields are given in paren-
theses.
[d]
[e]
The reaction was carried out with 1a (0.1 mmol) and 2a (0.2 mmol) and the yields is based on the consumption of 1a.
The reaction was carried out under an argon (Ar) atmosphere. DCE=1,2-dichloroethane, PMP=para-methoxyphenyl,
Tf=trifluoromethanesulfonyl and bpy=2,2’-bipyridine, NR=no reaction.
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the dual catalysis, visible-light catalysis and transition amino acid esters. When substrates 1a and 2a were ir-
À
metal (copper) catalysis, giving rise to Csp3 Csp3 cross- radiated by blue LEDs (l=450 nm) at ambient condi-
coupling products with moderate to excellent yields tions for 12 h, the cross-coupling product 3a could be
under ambient condition.
achieved in 50% yield (Table 1, entry 6). Considering
Initially, ethyl 2-oxocyclopentanecarboxylate (1a) that the rate of the oxidative coupling reaction might
and N-(4-methoxy)phenylglycine methyl ester (2a) depend on the amount of iminium ion intermediate,
were selected as the typical substrates for the photo- we modulated the ratio of 1a and 2a to increase the
chemical reaction. When either Ru
ACHTUNGTREN(UNGN bpy)3Cl2 or concentration of the iminium ion intermediate. As
Cu(OTf)2 was used, respectively, as the photocatalyst shown in Table 1, the yield of the coupling reaction
ACHTUNGTRENNUNG
or the transition metal catalyst in CH2Cl2, only a trace was enhanced from 50% to 93% (entries 6 and 14)
of the desired cross-coupling product 3a was obtained when the mole ratio of 1a/2a was changed from 0.2/
upon irradiation of the reaction mixture by blue 0.1 to 0.1/0.2, which further confirmed that the
LEDs (l=450 nm) for 24 h (Table 1, entries 1 and 2). amount of iminium ion generated from amino acid
However, the combination of Ru
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
product 3a in a yield of 30% under the same condi- process because no photoproduct could be detected
tions (Table 1, entry 3). The evaluation of solvents when the reaction was operated in an atmosphere of
suggested that toluene was the best solvent for the argon (Table 1, entry 17). RecentÀstudy has revealed
cross-coupling reaction, where a 50% yield of 3a was that superoxide radical anion (O2 C) is the real active
obtained (Table 1, entries 3–9). Furthermore, the species in the oxidative cross-coupling reaction of ter-
amounts of Ru
investigated and the results are summarized in lar oxygen in the oxidative reaction of glycine esters,
Table 1. It is clear that 10 mol% of Cu(OTf)2 showed 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and
the best result, more or less Cu(OTf)2 resulted in de- 2,2,6,6-tetramethyl-1-piperidine (TEMP) were em-
(OTf)2 were carefully tiary amines.[3a–d] To figure out the function of molecu-
ACHTUNGTRENNUNG(bpy)3Cl2 and CuCAHUTNGTRENNGUN
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
À
1
creases of the reaction yield and efficiency (Table, en- ployed to capture active species, O2 C and O2, respec-
tries 6, 10 and 11). On the other hand, increasing the tively, by electron spin resonance (ESR) spectrosco-
concentration of Ru
3 mol% gave rise to almost the same yield of product 2a and Ru
3a, indicating that a small amount of Ru(bpy)3Cl2 was irradiated with blue LED at 450 nm, a single radi-
(bpy)3Cl2 from 1 mol% to py.[13] It is evident that when the solution of DMPO,
ACHTUNGTRENNUNG
AHCTUNTGREN(GNUN bpy)3Cl2 in air-saturated toluene solution
AHCTUNGTRENNUNG
(1 mol%) is sufficient for this reaction (Table 1, en- cal was trapped (Figure 1a), the spectrum and hyper-
tries 12 and 13). Taken all together, we were delighted fine coupling constants of which are in line with the
À
to see that combination of the visible light photocata- reported values for the adduct of O2 C with DMPO.
1
lyst Ru
Cu
(bpy)3Cl2 and the transition metal catalyst By contrast, when TEMP, an O2 scavenger, was used
(OTf)2 is feasible to functionalize the secondary instead of DMPO in the same air-saturated solution,
no signal was deÀtected (Figure 1b). From these results,
we infer that O2 C generated from molecular oxygen is
the active species participating in the reaction of sec-
ondary amines.
It is of particular significance that upon irradiation
of 2a and RuACTHNUGTRNEUNG(bpy)3Cl2 in an air-saturated solution of
toluene, the iminium intermediate 4a could be isolat-
ed (Scheme 2). Strikingly, the obtained iminium inter-
mediate 4a can directly react with 1a assisted by
CuACTHNUTRGEN(UGN OTf)2 to afford the desired product 3a in an iso-
lated yield of 75% within 2 h at room temperature
(see the Supporting Information). These findings sug-
gest that aerobic visible-light catalysis is responsible
for the formation of an important iminium intermedi-
ate from glycine ester 2a, and copper transition metal
catalysis is helpful for the enhancement of the nucleo-
philicity of b-keto esters 1a and the stabilization of
the iminium intermediate 4a as well.[14] As a result,
secondary glycine ester 2a and b-keto ester 1a under-
go the aerobic cross-coupling reaction quickly with
suppression of competition from side reactions.
Figure 1. ESR spectrum of air-saturated toluene solution of
a) 2a (3.0ꢂ10À5 molLÀ1), Ru(bpy)3Cl2 (0.3ꢂ10À6 molLÀ1),
and DMPO upon irradiation by blue LEDs for 30 s; b) 2a
(0.3ꢂ10À4 molLÀ1), Ru(bpy)3Cl2·(0.3ꢂ10À6 molLÀ1), and
TEMP upon irradiation for 30 s.
AHCTUNGTRENNUNG
With this initial success, several copper salts were
screened and CuCAHTUNGTRENNUNG
AHCTUNGTRENNUNG
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Combining Visible Light Catalysis and Transition Metal Catalysis for the Alkylation
on the phenyl ring showed better yields. For the reac-
tions with N-(4-methoxy)phenyl-glycine ethyl ester
(2b), b-keto esters also have a wide range of applica-
bility. Substituted methyl, sterically bulky isopropyl,
tert-butyl or benzyl groups showed good to excellent
yields of 73–91% (Table 3, entries 6–9). Similar results
were also observed for the reaction with 2a (Table 3,
entries 10 and 11), even for less reactive acyclic b-
keto esters (Table 3, entries 12–14).
Scheme 2. The oxidation of N-PMP glycine ester (2a) by the
dual catalysis.
On the basis of the above results, we could specu-
late a plausible mechanism for the reaction as follows
(Scheme 3): upon irradiation of [Ru
cient transition metal catalyst for the alkylation of N- ble light, a single electron transfer from substrate 2 to
substituted gylcine ester (Table 2). As tabulated in the excited state of [Ru
(bpy)3]2+ affords an aminyl
Table 3, various b-keto esters and N-substituted gly- cation radical 5 and reduced [Ru
(bpy)3]+, the latter is
(bpy)3]2+ by visi-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
cine esters were studied under the optimized condi- regenerated by molecular oxygen, and at the same
À
tions, i.e., 1 (0.1 mmol) and 2 (0.2 mmol) in the pres- time to form an active species O2 C (supported by
ence of RuACHTUNGTRENNUNG(bpy)3Cl2 (0.001 mol) and CuAHCUTTGNREN(NUGN OTf)2 ESR experiments). Then aminyl cation radical 5 is
(0.01 mol) in air-saturated toluene solution (3 mL). further transformed to iminium ion 6 by interacting
À
For the reaction with ethyl 2-oxocyclopentanecarbox- with the superoxide radical anion O2 C. Subsequently,
ylate (1a), a series of N-substituted glycine esters, the HOOÀ ion generated in situ abstracts another hy-
bearing electron-donating and electron-withdrawing drogen atom from iminium ion 6 to produce iminium
groups on the 4-position of phenyl ring of 2, was ex- intermediate 4, as shown in Scheme 2. On the other
amined. All of them could be successfully trans- hand, transition copper salts can coordinate with b-
formed to their desired products 3 in the moderate to keto esters to increase their nucleophilicity.[15] And
excellent yields (58–93%, Table 3, entries 1–5). Note hence the generated strong nucleophilic species 7 at-
that substrates 2 containing electron-donating groups tacks iminium intermediate 4 quickly to yield the
À
Csp3 Csp3 cross-coupling product 3 and to release the
copper salt for the next catalytic cycle.
Table 2. Selection of copper catalyst.[a]
In summary, we have developed a dual catalytic
À
system for the a-C H functionalization of secondary
amines. Taking advantage of visible-light catalysis and
Entry
[Cu]
Cu(OTf)2
CuBr
CuCl
CuI
Solvent
dr[b]
Yield [%][c]
1
2
3
4
5
6
G
toluene
toluene
toluene
toluene
toluene
toluene
2:1
2:1
3:2
3:1
2:1
2:1
93 (86)
62 (50)
70 (61)
48 (37)
80 (71)
69 (62)
CuACHTUNGTRENNUNG(OTf)
Cu(OAc)2·H2O
ACHTUNGTRENNUNG
[a]
Unless otherwise specified, the reaction was carried out
with 1a (0.1 mmol) and 2a (0.2 mmol) in the presence of
RuACHTUNGTRENNUNG(bpy)3Cl2 (0.001 mol) and CuACHTUNGTRNEN(GUN OTf)2 (0.01 mol) in the
solvent (3 mL) under bule LEDs irradiation at room
temperature.
[b]
[c]
The diastereomeric ratio (dr) was determined by
1H NMR spectroscopy.
Yields were measured by H NMR spectroscopy using an
1
internal standard, the isolated product yields are given in
parentheses.
Scheme 3. The proposed reaction mechanism.
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Table 3. Investigation of the scope of the procedure.[a]
transition metal catalysis, the efficiency of the oxida- avoiding usage of stoichiometric amounts of oxidant
tive coupling reaction was improved greatly, i.e., from (e.g., DDQ or t-BuOOH). It is anticipated that this
30% for RuACHTUNGTRENNUNG(bpy)3Cl2 and 47% for CuACHTUNGTRNEN(NGU OTf)2, respec- research line would provide an effective way to func-
tively, to 93% for a combination of the two catalysts tionalize secondary amino acid esters under mild con-
under the optimal conditions. The reaction proceeds dition, which is actively under study in our group.
efficiently in the presence of molecular oxygen, thus
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Combining Visible Light Catalysis and Transition Metal Catalysis for the Alkylation
Table 3. (Continued)
[a]
Unless otherwise specified, the reaction was carried out with 1 (0.1 mmol) and 2 (0.2 mmol) in the presence of
RuACHTUNGTRENNUNG(bpy)3Cl2 (0.001 mol) and CuACHTUNGTREN(NUNG OTf)2 (0.01 mol) in anhydrous toluene (3 mL) under blue LEDs irradiation at room
temperature.
[b]
[c]
1
The diastereomeric ratio (dr) was determined by H NMR spectroscopy.
Yields were measured by 1H NMR spectroscopy using an internal standard, the isolated product yields are given in paren-
theses.
Experimental Section
Experimental Procedure for the Alkylation of
Secondary Amines
For detailed experimental procedures, spectral data and
characterization see the Supporting Information.
A 10-mL Pyrex tube equipped with a rubber septum and
magnetic stir bar was charged with b-keto ester
1 (0.1 mmol), N-arylglycine ester 2 (0.2 mmol), Ru
ACHTUNGTRNE(NUNG bpy)3Cl2
(0.001 mmol), Cu(OTf)2 (0.01 mmol) and anhydrous toluene
AHCTUNGTRENNUNG
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Xue-Wang Gao et al.
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(3 mL). The resulting mixture was irradiated by blue LEDs
for 12 h. After the reaction was completed (monitored by
TLC), the reaction mixture was evaporated under reduced
pressure. The residue was purified by flash chromatography
on silica gel using petroleum ether/ethyl acetate as eluent to
afford the desired product 3.
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We are grateful to the Ministry of Science and Technology of
China (2009CB220008, 2013CB834505 and 2013CB834804),
the National Natural Science Foundation of China (21090343
and 91027041), and the Knowledge Innovation Program of
the Chinese Academy of Sciences for financial support of this
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