Aerobic oxidative kinetic resolution of secondary alcohols with naphthoxide-bound iron(salan) complex

Article abstract of DOI:10.1021/ja204426s

The first general method for iron-catalyzed aerobic oxidative kinetic resolution of secondary alcohols was achieved with good to high enantiomeric differentiation (k_rel = 7-50). Although iron(salan) complex 1 does not catalyze alcohol oxidation, the naphthoxide-bound iron(salan) complex does.

Full text of DOI:10.1021/ja204426s

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Aerobic Oxidative Kinetic Resolution of Secondary Alcohols with
Naphthoxide-Bound Iron(salan) Complex
Takashi Kunisu, Takuya Oguma, and Tsutomu Katsuki*
Department of Chemistry, Faculty of Science, Graduate School, and International Institute for Carbon-Neutral, Energy Research,
Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
S Supporting Information
b
Scheme 1. Aerobic Oxidative Coupling of 2-Naphthols
Using Complex 1 as a Catalyst
ABSTRACT: The first general method for iron-catalyzed
aerobic oxidative kinetic resolution of secondary alcohols
was achieved with good to high enantiomeric differentiation
(
krel = 7À50). Although iron(salan) complex 1 does not
catalyze alcohol oxidation, the naphthoxide-bound iron-
(salan) complex does.
dvances in organic synthesis in the last several decades have
benefitted from the development of asymmetric catalysis by
A
Scheme 2. Proposed Mechanism for Asymmetric Aerobic
Oxidative Cross-Coupling of 2-Naphthols
transition metal complexes, especially with rare metals such as
Pd, Rh, Ru, etc. However, the reserves of rare metals are scarce,
and depletion is possible in the near future. Thus, much effort has
recently been directed to development of asymmetric catalysis
with abundant transition metals, i.e., iron, which is the second
1
most abundant metal in the earth’s crust. Another important
issue in organic synthesis is further development of ecologically
benign reactions.
In this context, we were interested in iron-catalyzed asym-
2
metric oxidation using molecular oxygen, a green oxidant. We
explored aerobic oxidative coupling of 2-naphthols using di-μ-
hydroxo iron(salan) complex 1 as the catalyst in air and achieved
a highly enantioselective homocoupling reaction of 3-substituted
3a
2
-naphthols (Scheme 1, eq 1). The subsequent kinetic and
X-ray studies indicated that the coupling reaction proceeds
through an iron(salan)(2-naphthoxo) complex A according to
a radical anion mechanism (Scheme 2) and leads to development
of an enantioselective oxidative cross-coupling reaction affording
C1-BINOLs (Scheme 1, eq 2). Intermediate A also catalyzes
the coupling of 2-naphthols with nearly identical enantioselec-
tivity to complex 1. Coordination of the naphthoxo ligand may
facilitate the single electron transfer.
Thus, we screened various 2-naphthols in search of an anti-
coupling additive. Addition of 1-methyl-2-naphthol 4 also promoted
oxidation but enantiomer differentiation decreased (entry 2).
Oxidation did not occur in the presence of 3-methoxycarbonyl-
3b
2
-naphthol 5 with a chelating property (entry 3). 6-Methoxy-
carbonyl-2-naphthol 6 was a poor additive (entry 4). Oxidation
was slightly accelerated by 4-tert-butylphenol 7, but little en-
antiomer differentiation was observed (entry 5). To our delight,
We expected that if the aerobic oxidation stops at the cationic
radical species (B) in the reaction cycle, we could use it as a
catalyst for alcohol oxidation. Thus, we examined aerobic
oxidative kinetic resolution (OKR) of secondary alcohols in
the presence of various aromatic alcohols, which are reluctant to
1
-naphthol 8 was found to enhance the oxidation rate and the
enantioselectivity (krel = 35) without undergoing undesired
coupling (entry 6). These results indicate that coordination of
an electron-donating and nondimerizable naphthol is essential
for this approach. Oxidation at 25 °C was slow and much less
selective (entry 7). Oxidation was best performed using 3 mol %
7
3b
undergo oxidative coupling. Although various metal complexes
2,4
have been used as catalysts for OKR, no general method of
5
of catalyst and 8 mol % of additive at 50 °C, after optimization
OKR using iron as a catalyst has been reported. Herein, we
(
(
entry 8). We also examined other iron(salan) complexes
9À11) in the presence of 8, but they did not show any catalysis
communicate an efficient iron-catalyzed OKR using air as an
oxidant.
of alcohol oxidation (entries 9À11). Of note, complex 1 did not
The expected OKR of (()-1-phenylethanol proceeded in the
6
presence of 2-naphthol with k value of 24 at 50 °C, albeit
rel
slowly (Table 1, entry 1). The oxidative homocoupling of
Received: May 14, 2011
Published: July 24, 2011
2
-naphthol also proceeded under the conditions.
r 2011 American Chemical Society
12937
dx.doi.org/10.1021/ja204426s
|J. Am. Chem. Soc. 2011, 133, 12937–12939

Journal of the American Chemical Society
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Table 1. Kinetic Resolution of 1-Phenylethanol Using
Iron(salan) Catalysts
catalyze aerobic oxidation of 1-phenylethanol 3a in the absence
of 1-naphthol.
a
Under the optimized conditions, we examined OKR of other
secondary carbinols (Table 2).
Regardless of the electronic nature of aryl substituents
methoxy, cyano, chloro, and trifluoromethyl groups), a series
(
of secondary benzylic alcohols were enantiomerically differen-
tiated with good selectivity (krel = 19À39). It is noteworthy
that the basic dimethylamino group did not disturb oxidation
(
(
entry 2). 1-(2-Naphthyl)ethanol was an excellent substrate
krel = 50) (entry 6). 1-Phenylpropanol was also oxidized, but
b
b
c
entry cat. additive time (h) conv. (%)
ee (%)
config.
k
rel
the enantioselectivity was moderate (entry 7). Cyclic carbinols
were also oxidized, although enantioselectivity was dependent on
ring size (entries 8À10). The reactions of alkenyl carbinols were
also highly enantioselective (entry 11). It should be noted that
nonactivated alcohols 3m and 3n were also available as substrates
(entries 12 and 13).
In summary, we demonstrated that the catalytic activity of
iron(salan) complexes can be enhanced by coordination of a
naphthoxide ligand and this modified complex catalyzes aerobic
kinetic resolution of secondary alcohols. Further investigation of
iron-catalyzed oxidation is in progress.
d
H
1
2
3
4
5
6
7
8
9
1
1
1
2
4
5
6
7
8
8
8
8
8
8
48
48
48
48
48
12
48
24
12
12
12
50
26
nr
26
9
81
22
nd
24
2
(R)
(R)
nd
24
5
1
1
nd
7
1
(R)
(R)
(R)
(R)
(R)
nd
1
2
1
54
11
95
6
35
3
e
f
1
g
1
55 (41)
nr
98
nd
nd
nd
41
nd
nd
nd
9
0
1
10
11
trace
nr
nd
’
ASSOCIATED CONTENT
nd
a
Reactions were run in toluene (0.1 M) with iron catalyst (5 mol %) and
S
Supporting Information. Experimental procedures; GC
b
additive (20 mol %) on a 0.1 mmol scale under air, unless otherwise
mentioned. Determined by GC analysis on a chiral capillary column using
bicyclohexyl as an internal standard. See SI. Determined by comparison of
the optical rotation with the literature value. 2 = 2-naphthol. Run at
2
1
and HPLC conditions. This material is available free of charge via
the Internet at http://pubs.acs.org.
b
c
d
e
H
f
’
AUTHOR INFORMATION
5 °C. Run on a 0.5 mmol scale (0.5 M) in the presence of 3 mol % of
g
and 8 mol % of 10. The number in parentheses refers to recovered 3a.
Corresponding Author
katsuscc@chem.kyushu-univ.jp
Table 2. Aerobic OKR of Secondary Alcohols Using 1 as a
a
Catalyst in the Presence of 1-Naphthol
’
ACKNOWLEDGMENT
Financial support from Nissan Chemical Industries, Ltd. and
the Global COE Program, ‘Science for Future Molecular Sys-
tems’ from MEXT, Japan is gratefully acknowledged. T.O. is
grateful for the JSPS Research Fellowships for Young Scientists.
time conv.
ee
yield
1
2
b
b,c
d
entry
R
R
(h) (%)
(%)
(%)
k
rel
’
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23
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36
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24
60 97 (R) 37 (3b) 19
54 96 (R) 46 (3c) 39
56 92 (R) 42 (3d) 20
56 91 (R) 43 (3e) 19
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62 99 (R) 35 (3n) 20
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1
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Journal of the American Chemical Society
COMMUNICATION
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(
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(
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(
1
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dx.doi.org/10.1021/ja204426s |J. Am. Chem. Soc. 2011, 133, 12937–12939