Lewis acid character of zero-valent gold nanoclusters under aerobic conditions: Intramolecular hydroalkoxylation of alkenes

Article abstract of DOI:10.1246/cl.2007.646

Gold nanoclusters stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP NCs, φ = 1.3 nm) behave as Lewis acid catalyst in aqueous media under aerobic conditions, to promote the intramolecular hydroalkoxylation of unactivated alkenes. Molecular oxygen generate

Full text of DOI:10.1246/cl.2007.646

6
46
Chemistry Letters Vol.36, No.5 (2007)
Lewis Acid Character of Zero-valent Gold Nanoclusters under Aerobic Conditions:
Intramolecular Hydroalkoxylation of Alkenes
1
1
1;2
ꢀ1
Ikuyo Kamiya, Hironori Tsunoyama, Tatsuya Tsukuda, and Hidehiro Sakurai
Research Center for Molecular-Scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki 444-8787
1
2
CREST, Japan Science and Technology Agency, Kawaguchi 332-0012
(Received February 19, 2007; CL-070190; E-mail: hsakurai@ims.ac.jp)
Gold nanoclusters stabilized by poly(N-vinyl-2-pyrrolidone)
aerobic oxidation (Entry 3).3,4,6 These results indicate that O2
does not act as an oxidant but plays an essential role in the
present reaction. The catalytic behavior of Au:PVP(1.3) appears
(
Au:PVP NCs, ꢀ ¼ 1:3 nm) behave as Lewis acid catalyst in
aqueous media under aerobic conditions, to promote the intra-
molecular hydroalkoxylation of unactivated alkenes. Molecular
oxygen generates a reaction center having the Lewis acidic
character on the surface of Au NCs in which constituent gold
atoms are formally in zero-valence state.
I
III
to be similar to that of Au /Au complexes, which are applied
as Lewis acid catalyst particularly for activation of C–C multiple
9
–11
bonds.
It is thought that O2 generates a reaction center with
Lewis acid character on the Au NC surface; the constituent
atoms of Au NCs are formally in a zero-valence state.
Representative results of hydroalkoxylation of alkenes cata-
lyzed by Au:PVP(1.3) are summarized in Table 2. The reactiv-
ity of tertiary alcohols is dependent to a remarkable extent on the
substituents at the ꢂ-position of the alcohols. Although the reac-
tion of 1a proceeds smoothly (Entry 1; also Table 1, Entry 1), no
reaction occurs with the phenyl-methyl derivative 1b (Entry 2).
In contrast, cyclization does occur with the ꢂ-naphthyl-methyl
derivative 1c to afford 2c in 62% yield (Entry 3). Although we
Oxidation reactions have been a central issue in the active
research area of gold nanocluster (Au NC) catalysis since the
landmark report by Haruta et al.1,2 In a series of publications,
we showed that Au NCs stabilized by poly(N-vinyl-2-pyrroli-
done) (Au:PVP NCs) exhibit high catalytic activity for aerobic
3
oxidation reactions such as oxidation of alcohols and generation
4
of hydrogen peroxide, but only when the diameter is reduced to
less than ca. 2 nm. It was proposed that a superoxo-like molecu-
lar oxygen species generated on the cluster surface plays a key
role in these oxidation reactions. We also observed that Au:PVP
Table 2. Hydroalkoxylation of alkenes catalyzed by Au:
PVP(1.3)a
Yield/%b
87
5
Entry
Alkene
OH
Product
O
NCs with an average diameter of 1.3 nm (Au:PVP(1.3)) cata-
lyzed homocoupling reactions of arylboron compounds under
Ph
Ph
CH3
CH3
1
6
Ph
Ph
aerobic conditions. This result hints at the possibility that an
Ph
1a
2a
electron-deficient site at the cluster surface, generated by adsorp-
tion of O2, behaves as a formal Lewis acid and provides a site
for transmetalation of aryl moieties. In order to test the idea that
Au NCs in aqueous media under aerobic conditions act as a
Lewis acid, we studied the activity of Au:PVP(1.3) in the intra-
molecular hydroalkoxylation of unactivated alkenes, which is
conventionally promoted by Lewis acid catalysts.
OH
Ph
CH3
O
2
3
0
CH3
1b
2b
OH
α
-Nap
CH3
O
CH3
R3
62
(1.1:1)
α
-Nap
Ph
CH3
1c
2c
R3
OH
Ph
Ph
O
R2
Under aerobic oxidation conditions, ꢁ-hydroxyalkenes
generally undergo Wacker-type oxidation or oxygenation
R2
R1
7
Ph
R1
2
3
8
4
5
6
1d R1 = CH , R = H, R = H
3
2d
2e
2f
38
8
0
(
was treated with 10 atom % of Au:PVP(1.3) and 200 mol % of
Scheme 1). In contrast, when 1,1-diphenyl-4-penten-1-ol (1a)
1
2
3
1e R = H, R = CH , R = H
1f R1 = H, R2 = CH , R3 = CH3
3
3
ꢁ
DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) at 50 C for 16 h in
OH
O
CH3
CH3
CH3
7
93
H2O/DMF mixed solvent under air, 2a was obtained in 87%
yield without the formation of Wacker-type products 3, 4, or
oxygenation product 5 (Entry 1, Table 1 and Scheme 1).
Cyclization did not occur under carefully degassed conditions
Ph
Ph Ph
Ph
1g
2g
OH
O
CH3
Ph
8
93
(3.6:1)
CH3
Ph Ph
(Entry 2), or using a catalyst of greater cluster diameter
(Au:PVP(9.5)); the latter showed no catalytic activity for
Ph
1h
2h
OH
O
9
33
77
Table 1. The role of molecular oxygen in the cyclization of 1a
OH
Ph
Ph
O
1
2
0 atom % Au:PVP
00 mol % DBU
CH3
1
i
2i
Ph
Ph
1
OH
O
H2O/DMF, 50 °C, 16 h
a
2a
10
Entry
Catalyst
Conditions
under air
Yield/%
1
j
2j
1
2
3
Au:PVP(1.3)
87
0
aReaction conditions:12 Au:PVP(1.3) (10 atom %),13 alkene (0.05
Au:PVP(1.3) under degassed conditions
Au:PVP(9.5) under air
ꢁ
mmol), DBU (200 mol %), H2O (10 mL), DMF (5 mL), 50 C, 16–
b
0
2
4 h. Diastereomeric ratios are given in parentheses.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.5 (2007)
647
cannot provide a clear-cut explanation for such unprecedented
reactivity (Entries 1–3) at this stage, these results suggest that
the catalytic site is not a gold ionic species leached out from
the cluster but the surface of the cluster. The larger size of the
aromatic ring is expected to interact with the surface of the Au
NCs stronger.14 The efficiency of the hydroalkoxylation is also
Au
+
O2
O
-
1
1
O
R
O
CH3
δ
-
R
R'
DMF
DMF
O
R'
-
1
δ
+
2
Au
DMF
A
R
R'
O
O
O
O
O
sensitive to subsititution at the alkene moiety. The introduction
of other substituents at the internal and/or terminal alkenic car-
bon decreases the yields of the products (Entries 4–6), probably
due to steric hindrance. Next, the reactions of primary/second-
ary alcohols are examined. It is anticipated that cyclization
would compete with alcohol oxidation because the reaction is
conducted under conditions similar to those for aerobic oxida-
R
O
Au
Au
R'
D
B
O
O
O
R
Au
H2O R'
2
C
β
-elimination
O2
R
O
R
O
CH3
R
O
OH
3
R'
or R'
R'
tion. However, cyclization takes place predominantly for the
3
4
5
aliphatic alcohols, giving 2 (Entries 7–9). It is noteworthy that
substrates with aromatic rings gave better results in this type
of reaction. In contrast, aerobic oxidation is preferred in the
reaction of benzylic alcohols, giving the corresponding ketones
selectively (Entry 10).
0
Scheme 1. A possible mechanism for Au :PVP-catalyzed
hydroalkoxylation.
the present study represents the beginning of a new era of Au NC
use in areas other than oxidation catalysis.
To elucidate the source of the hydrogen at the terminal
carbon, the reaction was carried out in D2O and/or DMF-d7
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas ‘‘Chemistry of Coordination Space’’
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan, and the CREST program sponsored by JST.
(
Table 3).12 Compound 2a-D was obtained only when DMF-d7
was employed, which revealed that the hydrogen was introduced
via a radical process. It should be pointed out that the use of
either D2O or DMF-d7 diminished the reaction rate.
Table 3. Labeling experiment for hydroalkoxylation
References and Notes
M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 1989, 115,
301.
1
OH
Ph
Ph
O
1
0 atom % Au:PVP(1.3)
CH2D
(H)
Ph
For reviews, see: a) M. Haruta, M. Dat e´ , Appl. Catal. A 2001, 222,
427. b) M. Haruta, Gold Bull. 2004, 37, 27. c) A. S. K. Hashmi,
G. J. Hutchings, Angew. Chem., Int. Ed. 2006, 45, 7896.
a) H. Tsunoyama, H. Sakurai, Y. Negishi, T. Tsukuda, J. Am. Chem.
Soc. 2005, 127, 9374. b) H. Tsunoyama, H. Sakurai, T. Tsukuda,
Chem. Phys. Lett. 2006, 429, 528. c) H. Tsunoyama, T. Tsukuda, H.
Sakurai, Chem. Lett. 2007, 36, 212.
2
5
00 mol % DBU, air
0 °C, 16 h
2
Ph
1
a
2a
Yield/%
3
Entry
Solvent
1a
2a-H
2a-D
1
2
3
4
H2O/DMF
0
3
87
80
0
0
0
D2O/DMF
4
5
6
H. Sakurai, H. Tsunoyama, T. Tsukuda, Trans. Mater. Res. Soc. Jpn.
2006, 31, 521.
H2O/DMF-d7
D2O/DMF-d7
32
50
63
42
In the present paper, the catalyst is described with respect to the
average diameter given in parentheses (Refs. 3b and 4).
a) H. Tsunoyama, H. Sakurai, N. Ichikuni, Y. Negishi, T. Tsukuda,
Langmuir 2004, 20, 11293. b) H. Sakurai, H. Tsunoyama, T. Tsukuda,
J. Organomet. Chem. 2007, 692, 368.
0
A possible mechanism is shown in Scheme 1. As proposed
in previous studies,3 the reaction is initiated by the formation
of key intermediate A, which possesses an electron-deficient site
generated by adsorption of O2 onto the surface of the Au NCs.
A acts as a Lewis acid, activating both the alkoxide and alkene
by adsorption onto the surface (B), and giving C by the insertion
of an alkene into the O–Au bond. From C, neither ꢃ-elimination,
O2 insertion, nor protonation proceeds; only homolytic dissoci-
ation takes place, generating the radical intermediate D, which
afforded 2 via hydrogen abstraction from DMF accompanied
by the regeneration of free Au NCs. Judging from the decrease
in the reaction rate in DMF-d7, as shown in Table 3, all the steps
between A and D may be in equilibrium.
,4,6
7
a) J. Muzart, Tetrahedron 2005, 61, 5955. b) R. C. Larock, W. W.
Leong, in Comprehensive Organic Synthesis, ed. by B. M. Trost, I.
Fleming, Pergamon Press, New York, 1991, Vol. 4. c) T. Hosokawa,
S. Murahashi, Acc. Chem. Res. 1990, 23, 49.
8
9
S. Inoki, T. Mukaiyama, Chem. Lett. 1990, 67.
For reviews, see: a) A. Hoffmann-R o¨ der, N. Krause, Org. Biomol.
Chem. 2005, 3, 387. b) A. S. K. Hashmi, Gold Bull. 2004, 37, 51.
c) A. S. K. Hashmi, Gold Bull. 2003, 36, 3. d) G. Dyker, Angew.
Chem., Int. Ed. 2000, 39, 4237.
I
1
0 For Au /AuIII-catalyzed hydroalkoxylation of alkenes, see: a) A.
Hoffmann-R o¨ der, N. Krause, Org. Lett. 2001, 3, 2537. b) C.-G. Yang,
C. He, J. Am. Chem. Soc. 2005, 127, 6966. c) Z. Zhang, C. Liu, R. E.
Kinder, X. Han, H. Qian, R. A. Widenhoefer, J. Am. Chem. Soc. 2006,
128, 9066.
In summary, we showed that Au:PVP(1.3) possesses a
unique character as a formal Lewis acid catalyst, emerging only
under aerobic conditions in basic aqueous media. Of special note
is the unique character of the C–Au intermediate (C). That is, the
C–Au bond is resistant to ꢃ-hydrogen elimination, insertion of
O2, and even protonation, but does undergo homolytic cleavage.
On the basis of these phenomena, we consider that Au NCs con-
taining organic ligands have potential as an efficient source of
long-lived radicals; this has also been postulated based on a trap-
ping reaction of organic radicals using Au NCs.15 We hope that
1
1 For heterogeneous Au catalyst as a Lewis acid, see: S. Carrettin, M. C.
Blanco, A. Corma, A. S. K. Hashmi, Adv. Synth. Catal. 2006, 348,
1
283.
1
2 Supporting Information is also available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
3 Due to the significant growth of cluster size during the reaction,
1
1
0 atom % of Au:PVP(1.3) is required.
4 T. Nagata, J. Organomet. Chem. 2007, 692, 225.
5 a) F. Mirkhalaf, J. Paprotny, D. J. Schiffrin, J. Am. Chem. Soc. 2006,
1
1
1
28, 7400. b) C. Aprile, M. Boronat, B. Ferrer, A. Corma, H. Garc ´ı a,
J. Am. Chem. Soc. 2006, 128, 8388.
Published on the web (Advance View) April 14, 2007; doi:10.1246/cl.2007.646