The combination of relay and cooperative catalysis with a gold/palladium/brnsted acid ternary system for the cascade hydroamination/ allylic alkylation reaction

Article abstract of DOI:10.1002/adsc.201100922

The combination of relay and cooperative catalysis with a gold/palladium/Brnsted acid ternary system renders a cascade hydroamination/allylic alkylation reaction to provide an unprecedented entry to pyrrolidine derivatives in high yields. Copyright

Full text of DOI:10.1002/adsc.201100922

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DOI: 10.1002/adsc.201100922
The Combination of Relay and Cooperative Catalysis with
a Gold/Palladium/Brønsted Acid Ternary System for the Cascade
Hydroamination/Allylic Alkylation Reaction
Hua Wu,a Yu-Ping He,a and Liu-Zhu Gonga,*
a
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry, University of Science
and Technology of China, Hefei 230026, Peopleꢀs Republic of China
Fax : (+86)-551-360-6266; e-mail: gonglz@ustc.edu.cn
Received: November 27, 2011; Revised: January 15, 2012; Published online: April 4, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100922.
Abstract: The combination of relay and cooperative
catalysis with a gold/palladium/Brønsted acid terna-
ry system renders a cascade hydroamination/allylic
alkylation reaction to provide an unprecedented
entry to pyrrolidine derivatives in high yields.
structurally complex products from relatively simple
starting materials. In the last decades, homogeneous
catalysis has been dominantly dependent on single
catalyst species.[2] Although some successful attempts
have been performed on the development of organic/
metallic binary catalysis[3–5] or organogold dual metal
catalysis,[6,7] cascade reactions by heterometallic/or-
ganꢁc multiple catalyst systems, which represent the
mimic of variants occurring in nature by using disci-
plinarily diverse biocatalysts, remain elusive. As
a proof of concept, we herein report a gold/palladi-
um/Brønsted acid ternary system, which works in
relay and cooperative manner, offering an unprece-
dented access to pyrrolidine derivatives.
Keywords: allylic alkylation; Brønsted acids; coop-
erative catalysis; hydroamination; metal/organoca-
talyst ternary system; pyrrolidine derivatives; relay
catalysis
The capability of nature to produce structurally com-
Secondary amine-bridged enynes of type 1 are prin-
plex organic molecules from simple substrates relies cipally able to undergo a hydroamination to give an
highly on the cascade reaction by using disciplinarily N-allylic enamine intermediate II under the catalysis
diverse biocatalysts, which operate in cooperative or of gold complexes (Scheme 1).[8] Subsequently, the N-
relay manner.[1] In addition to these events occurred allylic enamine II might participate in an allylic alky-
in vivo, the conceptually similar strategy can princi- lation under the cooperative catalysis of a palladium
pally be realized in vitro upon reasonable tuning of complex and a Brønsted acid via an intermediate III
multiple catalyst systems. As such, unprecedented cas- as demonstrated by previous elegant work.[9] As such,
cade reactions would be created for the generation of this “relay-cooperative catalysis” would enable a cas-
Scheme 1. The proposed relay-cooperative catalysis.
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Hua Wu et al.
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Table 1. Recruiting the optimal catalyst system and conditions.[a]
Entry
Au
Pd
Ligand
B-H
Time [h]
Yield [%][b]
1
2
3
4
5
6
7
8
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuOTf
Ph3PAuMe
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Ph3PAuNTf2
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
N
–
3
3
3
3
3
3
3
3
3
9
25
63
79
50
77
PPh3
P
12
12
4
4
4
3
3
1
1
1
1
1
1
1
3
1
E
L1
L2
L3
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
78[c]
71[c]
50[d]
trace
17
9
3
3
10
11
12
13
14
15
16
17
PhCO2H
TFA
p-TSA
3
3
3
46
67
25[e]
61[f]
82 g]
11
none
[a]
Unless indicated otherwise, a mixture of 1 (0.1 mmol), a gold complex (5 mol%), palladium (5 mol%), phosphine ligand
(10 mol%), and Brønsted acid (5 mol%) in toluene (1.0 mL) was stirred at 708C under argon.
Isolated yield.
15 mol% L1 was used.
The Ph3PAuOTf was in situ generated from 5 mol% of Ph3PAuCl and 5 mol% of AgOTf.
5 mol% of L1 was used.
15 mol% of L1 was used.
7.5 mol% of Brønsted acid 3 was used.
[b]
[c]
[d]
[e]
[f]
[g]
cade hydroamination/allylic alkylation reaction, offer- efficiency. Thus, the presence of 10 mol% of triphen-
ing a new entry to dihydropyrroles 2. ylphosphine rendered the reaction to give a 25%
AHCTUNGTRENNUNG
The validation of the proposed strategy began with yield (entry 2). A further improved yield could be
a reaction of diethyl 2-(allylamino)-2-(4-phenylbut-3- achieved upon exploiting triphenyl phosphite as
ynyl)malonate (1a) in the presence of a combined cat- a ligand (entry 3, 63% yield). Among the Buchwald
alyst system of 5 mol% PdACHTNUTGRNEUNG
(PPh3)4, 5 mol% ligands[11] screened (entries 4–6), the electronically
Ph3PAuNTf2 and 5 mol% of binol-derived racemic rich and sterically bulky phosphine L1[11a] (Figure 1)
phosphoric acid 3 at 708C in toluene (Table 1, turned out to be best ligand for the reaction (entry 4).
entry 1). However, the desired product 2a was isolat- The comparison of other palladium sources with
[10]
ed in a poor yield (9%). Encouragingly, when Pd
ACHUTGTNERN(NUG dba)2 found that PdACHUTNGTRENN(UGN dba)2 was the precatalyst of
Pd(dba)2 was used as a precatalyst, the variation of choice (entries 4, 7, and 8). Although palladium ace-
ACHTUNGTRENNUNG
phosphine ligands was able to improve the reaction tate offered a comparable yield with PdACHTNUTGRNEUNG(dba)2, a rela-
Figure 1. The phosphine ligands and a phosphoric acid used in this study.
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Adv. Synth. Catal. 2012, 354, 975 – 980

The Combination of Relay and Cooperative Catalysis with a Gold/Palladium/Brønsted Acid Ternary System
Table 2. Generality of the reaction.[a]
Scheme 2. The relay and cooperative catalysis with N-allyl-
5-phenylpent-4-yn-1-amine.
Entry 2 R1
R2
Time [h] Yield [%][b]
1
2
3
4
5
6
7
8
2b 4-FC6H4
H
H
H
H
H
H
H
H
3
3
3
3
3
3
3
3
63
75
80
72
60
81
76
53
71
77
79
with the substrates 1 bearing either electronically less
deficient, neutral or rich aryl substituents bonded to
the acetylene subunit (entries 2–4 and 6). The installa-
tion of a substituent at the olefin moiety was also tol-
erated, giving the desired products in high yields (en-
tries 9–11).
However, the extension of the optimal conditions
to N-allyl-5-phenylpent-4-yn-1-amine (1m), which has
no substiuents at the a-carbon to amine, gave the de-
sired product in a low yield.[13] Because the hydroami-
nation step proceeded smoothly under the catalysis of
Ph3PAuNTf2, we envisioned that the key problem
might exist in the allylic alkylation. Thus, a re-evalua-
tion of palladium catalysts[14] was performed and we
2c 4-ClC6H4
2d 4-PhC6H4
2e 4-t-BuC6H4
2f 4-CF3C6H4
2g 3-MeOC6H4
2h 2-naphthyl
2i 3,5-F2C6H3
2j Ph
9
10
11
Ph
4-MeC6H4
4-MeOC6H4 12
12
12
2k Ph
2l Ph
[a]
[b]
Unless indicated otherwise, a mixture of 1 (0.1 mmol),
Ph3PAuNTf2 (5 mol%), Pd(dba)2 (5 mol%), L1
(10 mol%), and 3 (7.5 mol%) in toluene (1.0 mL) was
stirred at 708C under argon.
AHCTUNGTRENNUNG
Isolated yield.
found that PdACHTNUGTRNEG(UN PPh3)4 was the best co-catalyst to
enable the whole sequential reaction to give the final
product in 80% yield (Scheme 2).
tively larger amount of phosphine ligands was re-
A series of control experiments was conducted to
quired, probably due to in situ generation of palladi- gain insight into the mechanism of the transformation.
um(0) species. The examination of gold(I) com- No reaction occurred when the acid 3 was used alone
plexes[10] indicated that the more cationic gold com- as the catalyst [Scheme 3, Eq. (1)]. In the absence of
plex provided a more complete reaction (entries 9 Ph3PAuNTf2 and 3, two compounds D1 and D2 were
and 10 vs. 4) and we found that Ph3PAuNTf2 turned afforded, but no desired product 2d was observed
out to be the best catalyst for the hydroamination [Eq. (2)]. An N-allylic enamine IId was isolated in
step. The evaluation of Brønsted acids led to a conclu- a moderate yield (55%) when 1d was treated with
sion that the acidity[9b,12] has a considerable effect on Ph3PAuNTf2 (5 mol%) in toluene at 708C for less
the reaction. Seemingly, the more acidic Brønsted than 10 min [Eq. (3)]. A subsequent treatment of the
acid delivered a cleaner reaction (entries 11–13) while intermediate IId with PdACTHNUTRGNE(UNG dba)2 (5 mol%), L1
the phosphoric acid gave the best results (entry 4). (10 mol%), and 3 (7.5 mol%) gave 2d in 62% yield at
The highest yield could be achieved when the ratio of 708C for 3 h [Eq. (4)].[15] On the contrary, without the
L1 to PdACHTUNGTRENNUNG(dba)2 was 2/1 and the loading of 3 was Brønsted acid 3 only a very low yield (11%) of 2a
tuned to 7.5 mol% (entries 4 and 14–16). However, in was obtained (Table 1, entry 17), which indicated that
the absence of the phosphoric acid, the reaction pro- the Brønsted acid 3 was necessary to facilitate the al-
ceeded incompletely to give the desired product in lylic alkylation step. These results revealed that the
a poor yield (entry 17), indicating that the palladium relay catalytic transformation really proceeded start-
and Brønsted acid really operate cooperatively on the ing with the gold(I)-catalyzed hydroamination while
allylic alkyltion.[9]
Pd(0) and the Brønsted acid catalyzed the conversion
The generality of the protocol was next explored of N-allylic enamine II into the pyrrolidines 2 in a co-
(Table 2). A variety of secondary amine-bridged operative manner.
enynes 1 was probed as substrates. The introduction
In summary, we have developed a combination of
of a substituent to the phenylacetylene moiety of the relay and cooperative catalysis with a gold/palladi-
1 led to a smooth cascade reaction under the relay-co- um/Brønsted acid ternary system enabling the secon-
operative catalysis while the electronic feature exerts dary amine-bridged enynes to undergo cascade hydro-
some impact on the reaction performance. Basically, amination and allylic alkylation reactions, providing
the presence of a highly electron-withdrawing sub- an unprecedented entry to pyrrolidine derivatives in
stituent resulted in a moderate yield (entries 1, 5 and high yields. In this cascade reaction, the gold complex
8). In contrast, much higher yields were delivered serves a catalyst responsible for the hydroamination
Adv. Synth. Catal. 2012, 354, 975 – 980
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Hua Wu et al.
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Scheme 3. Experimental evidence for the proposed mechanism.
while Brønsted acid and palladium cooperatively cat- ether:ethyl acetate:Et3N=200:10:1) to yield pure product
2m.
alyze the allylic alkylation. This example might open
a window for the future development of heterometal-
lic/organo multiple catalyst systems. The investiga-
tions to reach this goal are now undergoing in our
group.
Supporting Information
Experimental details and characterization data of new com-
pounds are given in the Supporting Information.
Experimental Section
Acknowledgements
General Procedure for the Cascade Hydroamination/
Allylic Alkylation Reaction
We are grateful for financial support from NSFC (21172207),
MOST (973 program 2009CB825300), Ministry of Educa-
tion, and CAS.
Procedure 1: A solution of 1 (0.1 mmol) in toluene (1.0 mL)
was added to a mixture of Ph3PAuNTf2 (5 mol%), Pd
ACHTUNGERTN(NUNG dba)2
(5 mol%), L1 (10 mol%), and 3 (7.5 mol%) under argon.
The resultant reaction solution was stirred at 708C for 3 or References
12 h. Then, the reaction mixture was directly subjected to
a flash column chromatography on silica gel (eluent: petro-
leum ether:ethyl acetate=15:1 to 10:1) to yield pure prod-
ucts 2a–2l.
Procedure 2: A solution of 1m (0.1 mmol) in toluene
(1.0 mL) was added to a mixture of Ph3PAuNTf2 (5 mol%),
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Adv. Synth. Catal. 2012, 354, 975 – 980

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[13] A 32% yield was obtained after the reaction had pro-
ceeded for 3 h under the optimal conditions.
[14] Variation of the combined catalysts and conditions
revealed the following: (i) PPh3AuNTf2 (5 mol%),
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Hua Wu et al.
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Pd
ene (1.0 mL), 708C, 5 h, yield: 43%; (ii) PPh3AuNTf2
(5 mol%), Pd(CH3CN)2Cl2 (5 mol%), L1 (15 mol%), 3
(7.5 mol%), toluene (1.0 mL), 708C, 3 h, yield: 28%;
G
(15 mol%), 3 (5 mol%), toluene (1.0 mL), 708C , 6 h,
yield: 68%.
[15] The experiment details are described in the Supporting
Information.
(iii) PPh3AuNTf2 (5 mol%), PdACHTNUTRGNE(UNG OAc)2 (5 mol%), PPh3
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