Gold nanoclusters as a catalyst for intramolecular addition of primary amines to unactivated alkenes under aerobic conditions

Article abstract of DOI:10.1246/cl.2010.46

Gold nanoclusters stabilized by the poly(N-vinyl-2-pyrrolidone) hydrophilic polymer (Au:PVP) catalyzed the intramolecular cycloaddition of primary amines to unactivated alkenes in the presence of formic acid derivatives. The reaction proceeded under neutral to slightly acidic conditions in air.

Full text of DOI:10.1246/cl.2010.46

doi:10.1246/cl.2010.46
46
Published on the web December 12, 2009
Gold Nanoclusters as a Catalyst for Intramolecular Addition of Primary Amines
to Unactivated Alkenes under Aerobic Conditions
Hiroaki Kitahara1 and Hidehiro Sakurai*1,2,3
1School of Physical Science, The Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8787
2Research Center for Molecular Scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki 444-8787
3PRESTO, Japan Science and Technology Agency, Tokyo 102-0075
(Received October 14, 2009; CL-090923; E-mail: hsakurai@ims.ac.jp)
Gold nanoclusters stabilized by the poly(N-vinyl-2-pyrroli-
(Table 1). As shown in eq 1 and Table 1, the cyclization with
Cs2CO3 did not proceed efficiently (Entry 1). When Cs2SO4 was
used, the rate of cyclization was sluggish at 50 °C, but increased
at elevated temperature (80 °C), to yield 2a with significant
amounts of the imines 3a/4a (Entry 2). Formate salts operate as
a reductant in the presence of Au:PVP;7 therefore, the imines
3a/4a were expected to be converted to 2a by reduction.8 The
yield of 2a was significantly increased when formate salts
such as HCO2Cs (Entry 3) or HCO2NH4 (Entry 4: condition A)
were added. The NH4OAc ammonium salt was not an effec-
tive additive, therefore a formate source is indispensable to
promote the cyclization (Entry 5). It was also found that the
reaction was dependent on the acidity of the solvent. When 1a
was treated with 400 mol % HCO2H in a pH 6.86 buffer/EtOH
mixed solvent (condition B), the yield of imines was slightly
decreased (Entry 6). Furthermore, when the reaction was carried
out in 1000 mol % aqueous HCO2H and 500 mol % NH3
solution/pH 4.01 buffer/EtOH mixed solvent (condition C),
2a was obtained in 94% yield (Entry 7). Thus, the yield of
the amine 2a can be maximized in weakly acidic to neutral
conditions.
Representative results of the hydroamination of primary
amines are summarized in Table 2. In the reaction with 1b and
1d, conditions B and A gave the best results, respectively,
affording the corresponding pyrrolidines 2b and 2c in quanti-
tative yields (Entries 4-9). The cis diastereomer was obtained as
a major product in both cases. On the other hand, no reaction
occurred with the nonsubstituted amine 1d (Entry 10). The
¤-methyl-substituted alkene 1e underwent cyclization in good
yield with the 3e imine as a minor product (Entries 11-13). 2f
was also obtained in moderate yield from 1f at 27 °C with the 4f
imine as a minor product (Entries 14-16).
done) hydrophilic polymer (Au:PVP) catalyzed the intramolec-
ular cycloaddition of primary amines to unactivated alkenes in
the presence of formic acid derivatives. The reaction proceeded
under neutral to slightly acidic conditions in air.
We have previously demonstrated that gold nanoclusters
(mean size: 1.3 nm) stabilized by poly(N-vinyl-2-pyrrolidone)
(Au:PVP), promote the intramolecular hydroamination of tolu-
enesulfonamide to unactivated alkenes1 in aqueous or polar
solvents under mild, basic, and aerobic conditions (eq 1). Among
the reported catalysts for the hydroamination of primary amines
such as organolanthanides,2 group IV metals,3 alkali metals,4 and
transition metals,5 there are still few moisture- and air-stable
catalyst systems.6 In addition, the reactivity of the catalysts is
highly dependent on the substituent located on nitrogen, and no
versatile catalyst for various types of amine derivatives is known.
We expected that Au:PVP would function as a moisture-/air-
stable catalyst for the hydroamination of primary amines, but
disappointingly, the cyclized product was obtained in only 3%
yield under similar conditions to that used for the toluenesul-
fonamide reaction (eq 1). In this paper, we report that Au:PVP
can also be a good catalyst for the hydroamination of primary
amines in the presence of formic acid derivatives.
R
NHR
N
5 atom% Au:PVP
CH3
ð1Þ
Ph
Ph
300 mol% Cs2CO3
air, 50 °C
conditions
Ph
Ph
Conditions; R = Ts: EtOH, 1 h, >99%
R = H: H2O/EtOH, 4 h, 3%
2,2-Diphenyl-4-pentenyl-1-amine (1a) was treated with
5 atom % of Au:PVP in a H2O/EtOH mixed solvent in air
Table 1. Hydroamination of primary amines
H
NH2
N
N
N
CH3
CH3
CH3
5 atom% Au:PVP
Conditions, air
+
+
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
1a
2a
3a
4a
Yield/%b
Solventa
(H2O or buffer:EtOH = 2:1)
Entry
Additives
Temp/°C
Time/h
2a
3a
4a
1a
1
2
3
4
5
6
300 mol % Cs2CO3
300 mol % Cs2SO4
300 mol % HCO2Cs
1000 mol % HCO2NH4
300 mol % NH4OAc
400 mol % HCO2H
1000 mol % HCO2H
500 mol % NH3 aq
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
H2O/EtOH
50
50/80
50
50
50/80
50
4
4/6
4
4
4/6
4
3
25
64
83
18
80
®
2
5
4
trace
3
®
28
3
7
14
3
97
45
28
®
67
®
(Condition A)
pH 6.86 buffer/EtOH
(Condition B)
(Condition C)
7
pH 4.01 buffer/EtOH
50
4
94
2
4
®
aH2O or buffer/EtOH = 20 mL/10 mL, pH 9.18 buffer: 10 mM NaB4O7 solution, pH 6.18 buffer: 25 mM KH2PO4/24 mM Na2HPO4 solution, pH 4.01
buffer: 50 mM C6H4(COOH)(COOK) solution. b 1H NMR ratio (Entries 1-3 and 5) and isolated yield (Entries 4, 6, and 7).
Chem. Lett. 2010, 39, 46-48
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

47
Table 2. Au:PVP-catalyzed hydroamination9,10
Table 3. Hydroamination labeling experiments
H
H
NH2
N
N
N
CH3
R3
CH3
R3
NH2
R3
CH2D
(H)
CH2D
(H)
CH2D
(H)
N
N
N
5 atom% Au:PVP
CH3
5 atom% Au:PVP
+
Ph
+
+
+
Ph
Ph
R1
R2
Ph
1000 mol% additive
solvent, 50 °C, time
Ph
R1
R1
R1
condition
50 °C, air, time
Ph
Ph
Ph
R2
R2
R2
1a
2a
3a
4a
1
2
3
4
Yield/%a (%Db)
Time
/h
Entry
Alkene
NH2
Condition Time/h
Yield/%a
Entry Solvent
Additive
2a
3a 4a
H
N
CH3
R3
CH3
N
N
CH3
R3
1
2
3
4
D2O/EtOD
HCO2NH4
H2O/EtOH-d6 HCO2NH4
D2O/EtOH-d6 HCO2NH4
17
17
17
4
81 (0) 9 (0)
67 (9) 9 (5)
81 (11) 8 (4)
7 (0)
5 (2)
8 (1)
R3
R1
R1
R1
R1
R2
R2
R2
R2
1a–1e
2a–2e
3a–3e
4a–4e
H2O/EtOH
DCO2NH4
82 (89)
®
trace (82)
1a R1, R2 = Ph
3 = H
1
A
4
2a 83
3a 4
4a 7
R
aIsolated yield. Calcd by GCMS.
b
2
3
B
C
4
4
80
94
3
2
3
4
H
N
reductant of the imines. 3a and 4a were treated under condition
A to afford a quantitative recovery of the imines (eq 2).
CH3
1b R1 = Ph, R2 = CH3
4
A
2
—
—
R3 = H
Ph
H
2b 84b (1.2:1)c
N
N
Me
CH3
CH3
N
5 atom% Au:PVP
CH3
or
ð2Þ
B
C
>99b (1.2:1)c
—
—
—
—
5
6
3
2
Ph
Ph
Ph
1000 mol% HCO2NH4
H2O/EtOH, air
50 °C, 4 h
Ph
Ph
Ph
3a
4a
2a
72b (1.2:1)c
H
Due to the character of Au:PVP as an aerobic oxidation
N
CH3
1c R1 = Ph, R2 = H
A
7
2
—
—
catalyst,12 the imines 3a and 4a could be formed by the
secondary oxidation of 2a.13 Since the aerobic oxidation is
performed under basic conditions, it is expected that the pathway
for the secondary oxidation of 2a might be more efficient in a
higher pH reaction mixture caused by the consumption of formic
acid as a hydrogen source. Therefore, 2a was treated with
5 atom % Au:PVP in various pH (pH 9.18, 6.86, and 4.01
buffer)/EtOH solutions at 50 °C. Although the oxidation to 3a or
4a did not proceed sufficiently in every case, the reaction rate
was slightly higher under basic conditions (eq 3). The imines 3a
and 4a were not detected by 1H NMR from the pH 6.86 and 4.01
buffer/EtOH solutions after 4 h, which indicates that secondary
oxidation is promoted under basic conditions.
R3 = H
Ph
2c 99b (1.5:1)c
H
88b (1.2:1)c
—
—
—
—
8
9
B
C
A
10
4
11b (1.1:1)c,e
1d R1, R2, R3 = H
1e R1, R2 = Ph
—
10
11
no reaction
H
CH3
CH3
CH3
N
N
4
—
A
CH3
3 = CH3
Ph
Ph
R
Ph
4
Ph
2e 83
3e 9
—
—
12
13
B
C
13
4
77
55e
—
H
N
Ph
Ph
24d
Ph
Ph
NH2
—
14 1f
A
N
CH3
CH3
2f 63
Ph
H
N
N
4f 26
Ph
N
CH3
CH3
5 atom% Au:PVP
CH3
13d
+
ð3Þ
15
16
B
C
56
29
—
—
38
26
Ph
Ph
Ph
air, 50 °C, 4 h
Ph
Ph
24d,e
2a
3a
4a
Ph
b
c
aIsolated yield. GC yield. Diastereomer ratio was determined as the
1000 mol% HCO2NH4
H2O/EtOH,
6%
7%
d
e
corresponding tosyl amide. 27 °C. 1 was recovered (Entry 10: 42%;
Entry 13: 35%; Entry 16: 29%).
pH 9.18 Buffer/EtOH
pH 6.86 Buffer/EtOH
pH 4.01 Buffer/EtOH
5%
3%
-
-
-
-
To elucidate the hydrogen source for the product and the role
of the formic acid derivatives, 1a was treated with 1000 mol % of
HCO2NH4 or DCO2NH4 in D2O and/or EtOH-d6 (condition A),
and the results are listed in Table 3. For the cyclization of
alcohol11 and toluenesulfonamide,1 the hydrogen at the terminal
methyl group of the products was introduced from the formyl
group of DMF and the ethyl group of EtOH, both of which were
used as a solvent, respectively. In contrast, EtOH was not a major
hydrogen source in the case of the primary amines, as shown in
Entries 1-3. 2a-D was obtained effectively when DCO2NH4 was
employed, which revealed that hydrogen was introduced from
the formyl group of HCO2NH4 (Entry 4).
It should be noted that the D/H ratios in the imines 3a/4a
were slightly smaller than that in 2a in every case, which
indicates that a minor pathway without hydrogenation by a
formate could occur. It is also noteworthy that D was selectively
introduced at the terminal methyl position and no other
positions, such as the methine position, as determined from
2H NMR. This indicates that the formate does not act as a
As proposed in previous studies,1,7,11,12 the reaction is
initiated by the adsorption of O2 onto the surface of the Au
nanoclusters as a key intermediate (A) that possesses electron-
deficient site (Scheme 1). The most important difference from
the previous reactions, such as cyclization of alcohols and
toluenesulfonamides, is that the reaction should be carried out
under neutral to slightly acidic conditions. In the reaction with
alcohols or toluenesulfonamides, basic conditions are necessary
to assist the adsorption of less nucleophilic substrates. In
contrast, primary amines can adsorb onto the gold surface, even
under neutral/slightly acidic conditions (B).14 Although the
reason why the cyclization does not proceed under basic
conditions is not yet clear, the formal Lewis acidic character
of Au clusters might be more efficient under neutral/acidic
conditions. After insertion of a N-H bond into the olefin
followed by the formation of a Au-C intermediate (C), selective
hydrogenation from HCO2H might occur due to insufficient
Chem. Lett. 2010, 39, 46-48
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

48
NH2
128, 1828. f) K. Li, P. N. Horton, M. B. Hursthouse, K. K. Hii,
J. Organomet. Chem. 2003, 665, 250.
H. Sakurai, H. Tsunoyama, T. Tsukuda, Trans. Mater. Res. Soc. Jpn.
2006, 31, 521.
δ-
O
H
N
O
δ+
O2
CH3
Ph
Au
Au
Ph
7
8
Ph
1
A
2
Ph
H2CO2 + HOOH
a) B. Basu, S. Jha, M. M. H. Bhuiyan, P. Das, Synlett 2003, 555. b) B.
Basu, M. M. H. Bhuiyan, P. Das, I. Hossain, Tetrahedron Lett. 2003,
44, 8931. c) V. Berdini, M. C. Cesta, R. Curti, G. D’Anniballe, N. D.
Bello, G. Nano, L. Nicolini, A. Topai, M. Allegretti, Tetrahedron
2002, 58, 5669.
General procedure for the hydroamination reaction of primary amine is
as follows: a test tube (¤ = 30 mm) was placed with 1 (0.1 mmol), and
dried Au:PVP (38.1 mg, 5 atom %). Water (20 mL), EtOH (10 mL), and
HCO2NH4 (63.1 mg, 0.1 mol) were added. The reaction mixture was
stirred vigorously (1300 rpm) at 50 °C for the time specified. The
reaction mixture was quenched by saturated NaHCO3 solution (10 mL)
and extracted with ethyl acetate (3 © 20 mL), and then the combined
organic layers were washed with water and brine, dried over Na2SO4,
and concentrated in vacuo. Purification of the product was carried out
by PTLC.
δ-
H
N
N
O
O
CH3
NH2
OH
O
Au
Au
δ+
Ph
Ph
Ph
H
Ph
3
Ph
Ph
O
H
B
Ph
HO
D
OH
N
CH3
4
H
N
9
OH
Ph
Au
O
H
Ph
Ph
N
HCO2H
H2O
3 + 4
C
Ph
Ph
Scheme 1. Proposed mechanism.
neutral/slightly acidic conditions (D), in contrast with the
hydroamination of toluenesulfonamide under basic conditions.
The formyl group hydrogen is abstracted, affording amine 2.
The formate should be transformed to carbon dioxide, which
may act as a reductant of oxygen, yielding hydrogen peroxide.
The formation of imines 3 and 4 might occur through both the
secondary oxidation of amine 2 as a major pathway and through
isomerization after ¢-hydrogen elimination from the Au-C
intermediate C as a minor pathway. The latter is supported by
the smaller D/H ratio in 3/4 than that in 2. The former oxidation
pathway can be reduced using the buffer as a co-solvent, because
the pH of the reaction mixture gradually increases by the
consumption of formic acid as a hydrogen source.
10 1f: Pale yellow oil; IR (neat): 3374, 3314, 3060, 3023, 2931, 1639,
¹1
1492, 1445, 911 cm
;
1H NMR (CDCl3): ¤ 1.66 (br, 2H), 1.90-1.96
(m, 2H), 2.25-2.29 (m, 2H), 4.90 (dd, J = 10.3, 1.7 Hz, 1H), 4.96 (dd,
J = 17.1, 1.7 Hz, 1H), 5.79 (dddd, J = 17.1, 10.3, 6.6, 6.6 Hz, 1H),
7.15-7.19 (m, 2H), 7.23-7.38 (m, 8H); 13C NMR: ¤ 148.56, 138.65,
128.05, 126.49, 126.26, 114.38, 60.92, 41.56, 28.61; Anal. Calcd for
C17H19N: C, 86.03; H, 8.07; N, 5.90%. Found: C, 85.96; H, 8.18; N,
5.85%. HRFAB m/z Calcd for C17H20N [M + H]+: 238.1596. Found:
238.1595. 2f: Pale yellow solid; mp 77-78 °C; IR (KBr): 3434, 2963,
1
1486, 1448 cm¹1; H NMR (CDCl3): ¤ 1.22 (d, J = 6.4 Hz, 3H), 1.46
(dddd, J = 12.4, 8.8, 6.6, 5.5 Hz, 1H), 1.91 (dddd, J = 12.4, 7.6, 7.5,
7.1 Hz, 1H), 2.37 (ddd, J = 12.5, 8.8, 7.6 Hz, 1H), 2.61 (ddd, J = 12.5,
7.5, 5.5 Hz, 1H), 3.34 (ddq, J = 7.1, 6.6, 6.4 Hz, 1H), 7.14-7.18 (m,
2H), 7.24-7.29 (m, 4H), 7.43-7.48 (m, 4H); 13C NMR: ¤ 148.83,
147.98, 128.12, 128.08, 126.40, 126.33, 126.23, 125.98, 71.89, 53.26,
39.16, 33.33, 22.24; Anal. Calcd for C17H19N: C, 86.03; H, 8.07; N,
5.90%. Found: C, 85.86; H, 8.02; N, 5.86%. HRMS m/z Calcd for
C17H19N: 237.1517. Found: 237.1522. 3a: Colorless oil; IR (neat):
3058, 2968, 1620 cm¹1; 1H NMR (CDCl3): ¤ 1.40 (d, J = 7.2 Hz, 3H),
2.01 (dd, J = 13.1, 9.2 Hz, 1H), 2.84 (dd, J = 13.1, 6.7 Hz, 1H), 4.16
(ddq, J = 9.2, 7.2, 6.7 Hz, 1H), 7.15-7.35 (m, 10H), 7.84 (s, 1H);
13C NMR: ¤ 168.72, 145.65, 143.38, 128.59, 128.59, 127.28, 127.04,
126.76, 126.59, 67.72, 66.83, 46.32, 21.61; HRMS m/z Calcd for
C17H17N: 235.1361. Found: 235.1353. 3e: Yellow oil; IR (neat): 3380,
2967, 2926, 1631, 1493, 1446 cm¹1; 1H NMR (CDCl3): ¤ 1.28 (s, 6H),
2.51 (s, 2H), 7.16-7.18 (m, 4H), 7.20-7.24 (m, 2H), 7.29-7.33 (m,
4H), 7.74 (s, 1H); 13C NMR: ¤ 166.01, 145.50, 128.58, 127.24, 126.54,
74.06, 67.23, 50.57, 29.46; HRMS m/z Calcd for C18H19N: 249.1517.
Found: 249.1512. 4f: White solid; mp 80-81 °C; IR (KBr): 3432,
3020, 2948, 1644, 1488, 1446 cm¹1; 1H NMR (CDCl3): ¤ 2.16 (s, 3H),
2.59 (br, 4H), 7.14-7.37 (m, 10H); 13C NMR: ¤ 173.98, 147.67,
128.07, 126.55, 126.28, 83.86, 40.57, 37.71, 19.99; Anal. Calcd for
C17H17N: C, 86.77; H, 7.28; N, 5.95%. Found: C, 86.65; H, 7.46; N,
5.91%. HRMS m/z Calcd for C17H17N: 235.1361. Found: 235.1365.
11 I. Kamiya, H. Tsunoyama, T. Tsukuda, H. Sakurai, Chem. Lett. 2007,
36, 646.
As described above, Au:PVP can be used to catalyze both
cycloaddition from toluenesulfonamides and primary amines by
changing the reaction conditions. The Au:PVP catalyst is expect-
ed to be a versatile and easy-to-handle catalyst for cycloami-
nation reactions, due to its moisture-/air-stable characteristics.
This work was supported by PRESTO-JST (Search for
Nanomanufacturing Technology and Its Development), MEXT,
and NEDO. We also thank Ms. Noriko Kai for preparation of the
Au:PVP catalyst.
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© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/