Chiral tridentate P,N,N ligands for highly enantioselective copper-catalyzed propargylic amination with both primary and secondary amines as nucleophiles

Article abstract of DOI:10.1002/adsc.201200589

Chiral tridentate P,N,N ligands have been demonstrated to be highly efficient for the coppercatalyzed enantioselective propargylic amination of propargylic acetates with both primary and secondary amines as nucleophiles, affording the corresponding propar

Full text of DOI:10.1002/adsc.201200589

UPDATES
DOI: 10.1002/adsc.201200589
Chiral Tridentate P,N,N Ligands for Highly Enantioselective
Copper-Catalyzed Propargylic Amination with both Primary and
Secondary Amines as Nucleophiles
Cheng Zhang,a,b Ya-Hui Wang,a Xin-Hu Hu,a Zhuo Zheng,a Jie Xu,a
and Xiang-Ping Hua,*
a
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, Peopleꢀs
Republic of China
Fax : (+86)-411-8468-4746; e-mail: xiangping@dicp.ac.cn
Graduate University of Chinese Academy of Sciences, Beijing 100039, Peopleꢀs Republic of China
b
Received: July 6, 2012; Published online: September 28, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200589.
Abstract: Chiral tridentate P,N,N ligands have been
demonstrated to be highly efficient for the copper-
catalyzed enantioselective propargylic amination of
propargylic acetates with both primary and secon-
dary amines as nucleophiles, affording the corre-
sponding propargylic amines in high yields and with
excellent enantioselectivities (up to 97% ee for sec-
ondary amines, and up to 96% ee for primary
amines). Furthermore, the present catalytic system
was also effective for the more challenging aliphatic
propargylic acetate substrates.
chiral pyridine-2,6-bisoxazoline ligand (pybox) 1 and
only primary amines served as efficient nucleophiles.[6]
More recent studies on the propargylic amination of
aliphatic propargylic esters showed the same result.[7]
A comparison of van Maarseveenꢀs and Nishibaya-
shiꢀs protocols disclosed that the major difference be-
tween them is the structure of the chiral ligands, a tri-
dentate N ligand (pybox, 1) and a bidentate P ligand
(biphep, 2) respectively (Figure 1). We therefore
Keywords: asymmetric catalysis; copper; primary
amines; propargylic amination; secondary amines
Figure 1. Structures of diPh-pybox
biphep 2.
1 and (R)-Cl-MeO-
Propargylic amines are structural units of widespread
chemical significance, having been heavily utilized as
building blocks for organic synthesis.[1] Among the
methods usually employed to generate such scaf- speculated that the different donor atoms of the li-
folds,[2] catalytic propargylic amination presents an at- gands used in these two methods may be responsible
tractive strategy.[3] A seminal report by Murahashi for their distinct reactivity and enantioselectivity in
and co-workers sets the stage for an enantioselective the Cu-catalyzed propargylic amination with primary
Cu-catalyzed propargylic amination.[4] While still in and secondary amines as nucleophiles. On the basis of
the racemic series at that stage, it was shown that Cu this speculation, we imagine that a chiral ligand bear-
sources promote the amination of propargylic esters ing the structural feature of both pybox and biphep,
smoothly under mild reaction conditions. In 2008, van such as a tridentate P,N,N ligand, should be efficient
Maarseveen and Nishibayashi independently reported for this transformation with both primary and secon-
the asymmetric version of the copper-catalyzed prop- dary amines as nucleophiles.
argylic amination of propargylic acetates. In Nishi-
Very recently, we have developed a series of chiral
bayashiꢀs method, a combination of copper(I) triflate P,N,N ligands for highly diastereo- and enantioselec-
with (R)-Cl-MeO-biphep 2 was used as the catalyst tive Cu-catalyzed [3+3]cycloaddition of propargylic
and only secondary amines worked as suitable nucleo- esters with cyclic enamines, which is believed to pro-
philes,[5] whereas van Maarseveenꢀs method employed ceed via the propargylic substitution firstly.[8] We
a catalyst formed in situ from copper(I) iodide and therefore envisioned that these P,N,N ligands should
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Adv. Synth. Catal. 2012, 354, 2854 – 2858

Chiral Tridentate P,N,N Ligands for Highly Enantioselective Copper-Catalyzed Propargylic Amination
and a catalytic amount of CuACHTUNRGTNEUNG(OTf)2 (5 mol%). To our
delight, planar-chiral ferrocenyl ligand (Sc,Rp)-3 dis-
played excellent enantioselectivity (95% ee) with
good yield (87%) for the secondary amine, N-methyl-
aniline 6a (entry 1), and good enantioselectivity (80%
ee) with moderate yield (71%) for the primary amine,
o-anisidine 7a (entry 2). In comparison with (Sc,Rp)-3,
its phenyl analogue (Rc)-4 showed lower enantioselec-
tivities with both N-methylaniline 6a and o-anisidine
Figure 2. Structures of chiral P,N,N ligands (Sc,Rp)-3 and
(Rc)-4.
be also efficient to the Cu-catalyzed propargylic ami- 7a as nucleophiles (entries 3 and 4). Since the result
nation of propargylic esters. As a result, herein we for the amination of 5a with primary amine 7a was
present our studies on the application of chiral triden- not so satisfactory with the CuACHTNUTRGENNG(U OTf)2/ACHTUNTGRENN(GUN Sc,Rp)-3 catalyt-
tate P,N,N ligands (Sc,Rp)-3 and (Rc)-4 (Figure 2) for ic system, the search for an appropriate Cu source
the copper-catalyzed propargylic amination of various was then performed. After a careful survey of copper
aliphatic and aromatic propargylic acetates with both salts in the propargylic amination of 5a with o-anisi-
primary and secondary amines as nucleophiles, in dine 7a (entries 5-10), CuCl was identified as the best
which excellent performance was achieved.
Cu source, affording the corresponding propargylic
To test our speculation, chiral P,N,N ligands (Sc,Rp)- amine 9a in 88% yield and with 93% ee (entry10).
3 and (Rc)-4 were then subjected to the Cu-catalyzed CuCl is also highly effective for the propargylic ami-
propargylic amination of 1-(4-chlorophenyl)-2-pro- nation of 5a with N-methylaniline 6a, giving the sub-
pynyl acetate 5a with both N-methylaniline 6a and o- stituted product 8a in 89% yield and with 95% ee,
anisidine 7a, and some representative results are sum- comparable to that obtained with Cu
ACHTUNGRTEN(NUNG OTf)2 (entry 11
marized in Table 1. vs. entry 1). These results demonstrate that P,N,N
The reaction was performed in methanol at 08C for ligand (Sc,Rp)-3 is a promising ligand scaffold for the
12 h in the presence of N,N-diisopropylethylamine Cu-catalyzed amination of propargylic esters with
both primary and secondary amines as nucleophiles.
With the optimal CuCl/ACTHNUTRGNE(UNG Sc,Rp)-3 catalytic system,
Table 1. Influence of ligand and Cu precursor on the reacti-
ACHTUNGTRENNUNG
on.[a]
we first examined the substrate scope of the propar-
gylic amination of propargylic esters 5 with secondary
amines 6. As shown in Table 2, a wide range of 1-aryl-
Table 2. Cu-Catalyzed asymmetric propargylic amination
with N-methylaniline (6a): scope of aromatic propargylic
acetates (5).[a]
Entry Nu [Cu]
L* Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
8
6a Cu
U
3
3
4
4
3
3
3
3
3
3
3
87
71
41
83
81
58
87
31
91
88
89
95
80
85
72
84
91
92
79
89
93
95
Entry Substrate (5: Ar) Product (8) Yield [%][b] ee [%][c]
7a Cu
6a Cu
7a Cu
7a Cu
7a CuF2·2H2O
7a Cu(OTf)·(C6H6)0.5
7a CuI
7a Cu(CH3CN)4ClO4
1
2
3
4
5
6
7
8
5a: 4-ClC6H4
5b: 3-ClC6H4
5c: 2-ClC6H4
5d: Ph
5e: 4-FC6H4
5f: 4-BrC6H4
5g: 4-CF3C6H4
5h: 4-MeC6H4
5i: 2-naphthyl
5j: 2-furyl
8a
8b
8c
8d
8e
8f
8g
8h
8i
89
86
87
86
90
90
88
84
86
88
95
97
96
95
97
96
96
92
94
91
R
9
10
11
7a CuCl
6a CuCl
9
10
8j
[a]
Reaction conditions: Cu salts (0.015 mmol), ligand
(0.03 mmol), 5a (0.3 mmol), N-methylaniline (6a) or o-
anisidine (7a) (0.36 mmol), and (i-Pr)2NEt (0.36 mmol)
were stirred in 2 mL of methanol at 08C for 12 h (for 6a)
or 24 h (for 7a).
[a]
Reaction conditions: CuCl (0.015 mmol), (Sc,Rp)-3
(0.03 mmol), (0.3 mmol), N-methylaniline 6a
5
(0.36 mmol), and (i-Pr)2NEt (0.36 mmol) were stirred in
2 mL of methanol at 08C for 12 h.
Isolated yield.
[b]
[c]
[b]
[c]
Isolated yield after column chromatography.
Determined by HPLC on a chiral column.
Determined by HPLC on a chiral column.
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Cheng Zhang et al.
UPDATES
2-propynyl acetates with varying electronic demands matic amines except that a longer reaction time (24 h)
were successfully reacted with N-methylaniline 6a, was required, affording the corresponding propargylic
giving the corresponding proparylamines 8a–j in good amine 8r in high yield (87%) and with excellent enan-
yields (84–90%) and with excellent enantioselectivi- tioselectivity (95% ee) (entry 9). However, when
ties (91–97% ee) (entries 1–10). The reaction had cyclic amines 6j and 6k were used as nucleophiles, the
a high tolerance to the substitution pattern on the results were very poor. By replacing CuCl with
phenyl ring of the substrates. Thus, all of chloro-sub- CuOTf·ACHTUNRTGNE(UNG C6H6)0.5, propargylic amines 8s and 8t were
stituted substrates 5a–c gave similar results (entries 1– formed in good yields and with excellent ee values
3). The electronic property of the substituent in the (entries 10 and 11).
para-position of the phenyl ring had little effect in the
reactivity (84–90% yield) and enantioselectivity (91– the present CuCl/
97% ee) (entries 4–8). tigated the catalytic asymmetric propargylic amina-
To further demonstrate the scope and flexibility of
AHCTUNGTRENNUNG
We also examined the effect of the substituent on tion using aromatic primary amines as nucleophiles,
the phenyl ring of aromatic secondary amines with and the results are summarized in Table 4. The results
regard to the reactivity and enantioselectivity, and the
results are summarized in Table 3. The results indicat-
ed that the substitution pattern and electronic proper-
ty had little effect on the reactivity and enantioselec-
tivity. All of N-methyl aromatic amines 6a–h reacted
with 1-phenyl-2-propynyl acetate 5d in excellent
enantioselectivities and yields (entries 1–8). The prop-
argylic amination of 1-phenyl-2-propynyl acetate 5d
with secondary dialkylamine 6i was also investigated
under the same reaction conditions as those with aro-
Table 4. Cu-Catalyzed asymmetric propargylic amination of
aromatic propargylic acetates (5) with primary amines (7).[a]
Entry Substrate (5:
Amine (7:
R)
9
Yield
[%][b]
ee
Ar)
[%][c]
1
5a: 4-ClC6H4
5b: 3-ClC6H4
5c: 2-ClC6H4
5d: Ph
5e: 4-FC6H4
5f: 4-BrC6H4
5g: 4-CF3C6H4 7a: 2-OMe 9g 85
5h: 4-CH3C6H4 7a: 2-OMe 9h 84
5i: 2-naphthyl 7a: 2-OMe 9i 86
7a: 2-OMe 9a 88
7a: 2-OMe 9b 86
7a: 2-OMe 9c 76
7a: 2-OMe 9d 89
7a: 2-OMe 9e 88
7a: 2-OMe 9f 90
93
96
93
93
93
94
96
90
91
89
92
90
90
Table 3. Cu-Catalyzed asymmetric propargylic amination of
1-phenyl-2-propynyl acetate (5d): scope of secondary amines
(6).[a]
2
3[d]
4
5
6
7
8
9
10[d] 5j: 2-furyl
11[d] 5d: Ph
12[d] 5d: Ph
13[d] 5d: Ph
7a: 2-OMe 9j 92
7b: 3-OMe 9k 86
7c: 4-OMe 9l 80
Entry Amine (6: R1, R2) Product
(8)
Yield
[%][b]
ee
[%][c]
1
2
6a: Ph, CH3
6b: 2-MeOC6H4,
CH3
8d
8k
86
82
95
93
7d: H
9m 85
[a]
Reaction conditions: CuCl (0.015 mmol), (Sc,Rp)-3
(0.03 mmol), 5 (0.3 mmol), primary amine 7 (0.36 mmol),
and (i-Pr)2NEt (0.36 mmol) were stirred in 2 mL of meth-
anol at 08C for 24 h.
Isolated yield after column chromatography.
Determined by HPLC on a chiral column.
3
4
6c: 3-MeOC6H4,
CH3
6d: 4-MeOC6H4,
CH3
8l
86
91
94
95
8m
8n
[b]
[c]
[d]
5
6
7
6e: 4-FC6H4, CH3
89
92
91
84
87
77
67
95
93
94
92
95
91
87
The reaction was performed at À208C using
5
6f: 4-ClC6H4, CH3 8o
6g: 4-BrC6H4, CH3 8p
6h: 4-MeC6H4, CH3 8q
(0.3 mmol) and primary amine 7 (0.6 mmol) and (i-
Pr)2NEt (1.2 mmol) for 48 h.
8
9[d]
6i: Et, Et
8r
8s
8t
10[e] 6j: piperidine
11[e] 6k: morpholine
disclosed that the substitution pattern of the substitu-
ent on the phenyl ring displayed some influence on
the enantioselectivity (entries 1–3). Thus, 4-Cl and 3-
Cl substituted substrates 5a and 5b proceeded
smoothly at 08C to give the corresponding amines in
high enantioselectivities (entries 1 and 2), whereas the
reaction of 1-(2-chlorophenyl)-1-propynyl acetate 5c
with o-anisidine 7a should be performed at lower
temperature (À208C) to obtain a satisfactory enantio-
[a]
Reaction conditions: CuCl (0.015 mmol), (Sc,Rp)-3
(0.03 mmol), (0.3 mmol), secondary amine
5
6
(0.36 mmol), and (i-Pr)2NEt (0.36 mmol) were stirred in
2 mL of methanol at 08C for 12 h.
Isolated yield.
Determined by HPLC on a chiral column.
Reaction time: 24 h.
[b]
[c]
[d]
[e]
CuOTf·ACHTUNGTRENNUNG(C6H6)0.5 was used instead of CuCl.
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Adv. Synth. Catal. 2012, 354, 2854 – 2858

Chiral Tridentate P,N,N Ligands for Highly Enantioselective Copper-Catalyzed Propargylic Amination
selectivity (entry 3). As was observed in the reaction (95% ee) (entry 1). However, the reaction of 5k with
with N-methylaniline, the electronic property of the primary amine 7a gave very low conversion under the
substituent on the phenyl ring of the substrates had same condition (entry 2). Replacing CuCl with
little effect on the reactivity and enantioselectivity. Cu
All of 4-substituted 1-phenyl-2-propynyl acetates 5a rate, leading to 84% yield and 87% ee (entry 3). With
and 5e–h reacted smoothly with o-anisidine 7a to give this Cu(OAc)2·H2O/(Sc,Rp)-3 catalytic system, n-Pr-
ACHTUNGTRNEN(UNG OAc)2·H2O significantly promoted the reaction
A
ACHTUNGTRENNUNG
the corresponding amines in high yields (84–90%) substituted substrate 5l reacted with primary amine
and with high enantioselectivities (90–96% ee) (en- 7a in 82% yield and 79% ee (entry 4). For the reac-
tries 1 and 5–8). Similar results to those with o-anisi- tion of n-Pr-substituted substrate 5l with secondary
dine 7a were obtained when m-anisidine 7b, p-anisi- amine 6a, however, a combination of Cu
dine 7c and aniline 7d were used as nucleophiles (en- and (Rc)-4 proved to be the suitable catalyst, giving
tries 11–13). the propargylamine 8v in 91% ee (entry 5). The Cu-
The success of the CuCl/ (OAc)2·H2O/(Rc)-4 catalytic system showed good
ACHTUNGTERN(NUNG OAc)2·H2O
A
ACHTUNGTRENNUNG
in the propargylic amination of aromatic propargylic enantioselectivities for the reaction of cyclohexyl- and
acetates prompted us to investigate its application in benzyl-substituted substrates 5m and 5n with both pri-
more challenging substrates, aliphatic propargylic ac- mary and secondary amines as nucleophiles (en-
tetates (Table 5). Under
a catalyst loadings of tries 6–9). The results suggested that the reactions of
10 mol% and performing the reaction at room tem- aliphatic propargylic acetates with secondary amine
perature for 60 h, the reaction of the simplest aliphat- 6a usually afforded higher yield and better enantiose-
ic propargyl acetate, but-3-yn-2-yl acetate 5k with sec- lectivity than those with primary amine 7a.
ondary amine 6a proceeded smoothly, providing the
Based on the crystal structure of the CuCl/ACHTGUNTERN(NUG Rc,Sp)-3
corresponding propargylic amine 8u in high yield complex[8] and the observed absolute stereochemistry
(84% yield) and with excellent enantioselectivity of the major enantiomer, we proposed a preliminary
model for the enantioinduction (Scheme 1). An edge-
to-face aromatic interaction makes a phenyl group of
Table 5. Cu-Catalyzed asymmetric propargylic amination of
the substrate close to a phenyl group of the ligand in
the copper acetylide complex.[7b,c] As a result, the
attack of the g-carbon atom by a nucleophile amine
happened favorably from the Si face to form (R)-
product while the Re face was hampered due to the
steric hindrance of the ligand.
aliphatic propargylic acetates.[a]
In conclusion, we have demonstrated that chiral tri-
dentate P,N,N ligands, (Sc,Rp)-3 and (Rc)-4, were
highly efficient for the Cu-catalyzed asymmetric prop-
argylic amination of propargylic acetates with various
amines. The unique feature of the present CuCl/
(Sc,Rp)-3 catalytic system lies in its significant activity
ACHTUNG-
TERNNUG
and selectivity in the use of both primary and secon-
dary amines as nucleophiles for the catalytic propar-
gylic amination. With secondary amines as nucleo-
philes, a series of 1-aryl-2-propynyl acetates was con-
verted into the corresponding amines with excellent
enantioselectivities (91–97% ee). Primary aromatic
Entry Substrate (5: R) Nu Product Yield [%][b] ee [%][c]
1[d]
2[d]
3[e]
4[e]
5
5k: Me
5k: Me
5k: Me
5l: n-Pr
5l: n-Pr
5m: Cy
5m: Cy
5n: benzyl
5n: benzyl
6a 8u
7a 9n
7a 9n
7a 9o
6a 8v
7a 9p
6a 8w
7a 9q
6a 8x
84
–
95
–
84
82
76
52
69
45
76
87
79
91
81
93
78
86
6
7
8
9[f]
[a]
Reaction conditions: CuACHTUNGTRENUNG(OAc)2·H2O (0.03 mmol), (Rc)-4
(0.036 mmol), 5k–n (0.3 mmol), amine (0.6 mmol), and
(i-Pr)2NEt (1.2 mmol) were stirred in 2 mL of methanol
at 08C for 60 h, unless otherwise specified.
Isolated yield after column chromatography.
Determined by HPLC on a chiral column.
[b]
[c]
[d]
The reaction was performed at room temperature using
CuCl/ACHTUNGTRENNUNG
[e]
[f]
A
Scheme 1. Proposed model for enantioinduction. A CpFe
group in the ligand has been omitted for clarity.
Adv. Synth. Catal. 2012, 354, 2854 – 2858
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Cheng Zhang et al.
UPDATES
Wei, Z.-G. Li, C.-J. Li, Synlett 2004, 1472–1483; c) L.
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amines were also suitable nucleophiles, leading to the
corresponding propargylic amines in high yields and
excellent enantioselectivities (up to 96% ee). Aliphat-
ic propargylic acetates turned out to serve well as the
substrate for this process, providing good to excellent
enantioselectivities (up to 95% ee). To the best of our
knowledge, the present Cu/P,N,N ligand catalytic
system represents the first example with which both
primary and secondary amines can be used as effi-
cient nucleophiles for the highly enantioselective cata-
lytic propargylic amination of both aliphatic and aro-
matic propargylic acetates.
Experimental Section
General Experimental Procedure
CuCl (1.5 mg, 0.015 mmol) and (Sc,Rp)-3 (15.1 mg,
0.03 mmol) were stirred in 1 mL of anhydrous methanol
under a nitrogen atmosphere for 1 h. The mixture was
cooled to 08C, and then a solution of propargylic acetate 5
(0.3 mmol), amine 6 or 7 (0.36 mmol) and N,N-diisopropyl-
[3] For reviews, see: a) N. Ljungdahl, N. Kann, Angew.
Chem. 2009, 121, 652–654; Angew. Chem. Int. Ed. 2009,
48, 642–644; b) R. J. Detz, H. Hiemstra, J. H. Maar-
seveen, Eur. J. Org. Chem. 2009, 6263–6276; c) Y.
Miyake, S. Uemura, Y. Nishibayashi, ChemCatChem
2009, 1, 342–356.
ACHTUNGTRENNUNGethylamine (0.36 mmol) in 1 mL of anhydrous MeOH was
added. The reaction mixture was kept at 08C for 12 h. The
solvent was removed under reduced pressure, and the resi-
due was purified by the silica gel column chromatography
with petroleum and ethyl acetate as eluent. The enantiomer-
ic excess was determined by HPLC on a chiral column.
[4] Y. Imada, M. Yuasa, I. Nakamura, S.-I. Murahashi, J.
Org. Chem. 1994, 59, 2282–2284.
[5] G. Hattori, H. Matsuzawa, Y. Miyake, Y. Nishibayashi,
Angew. Chem. 2008, 120, 3841–3843; Angew. Chem. Int.
Ed. 2008, 47, 3781–3783.
[6] R. J. Detz, M. M. E. Delville, H. Hiemstra, J. H. Maar-
seveen, Angew. Chem. 2008, 120, 3837–3840; Angew.
Chem. Int. Ed. 2008, 47, 3777–3780.
[7] a) G. Hattori, A. Yoshida, Y. Miyake, Y. Nishibayashi, J.
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Acknowledgements
Support for this research from Dalian Institute of Chemical
Physics (CAS) is gratefully acknowledged.
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Adv. Synth. Catal. 2012, 354, 2854 – 2858