Efficient aerobic oxidation of alcohols to aldehydes and ketones using a ruthenium carbonyl complex of a tert-butyl-substituted tetramethylcyclopentadienyl ligand as catalyst

Article abstract of DOI:10.1007/s11243-018-0251-3

Tert-butyl-substituted tetramethylcyclopentadiene [C₅HMe₄^tBu] was reacted with Ru₃(CO)₁₂ to prepare [(η⁵-C₅Me₄^tBu)Ru(CO)(μ-CO)]₂. The complex was characterized by IR, ¹H NMR, ¹³C NMR, elemental analysis, and single-crystal X-ray diffraction. The complex was investigated as a catalyst in the aerobic oxidation of alcohols to the corresponding aldehydes and ketones in the presence of 2,2’,6,6’-tetramethylpiperidine N-oxide (TEMPO) as co-oxidant. The combination of [(η⁵-C₅Me₄^tBu)Ru(CO)(μ-CO)]₂ and TEMPO afforded an efficient catalytic system for the aerobic oxidation of a variety of primary and secondary alcohols, giving the corresponding carbonyl compounds in good-to-excellent yields.

Full text of DOI:10.1007/s11243-018-0251-3

Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-0251-3
Efcient aerobic oxidation oꢀ alcohols to aldehydes and ketones
using a ruthenium carbonyl complex oꢀ a tert‑butyl‑substituted
tetramethylcyclopentadienyl ligand as catalyst
1
1
1
1
2
3
1
Zhiqiang Hao · Xinlong Yan · Zhanwei Li · Ruitao Wu · Zhihong Ma · Suzhen Li · Zhangang Han ·
1
1
Xuezhong Zheng · Jin Lin
Received: 4 April 2018 / Accepted: 29 May 2018
Springer International Publishing AG, part of Springer Nature 2018
©
Abstract
t
5
t
Tert-butyl-substituted tetramethylcyclopentadiene [C HMe Bu] was reacted with Ru (CO) to prepare [(η -C Me Bu)
5
4
3
12
5
4
1
13
Ru(CO)(μ-CO)] . The complex was characterized by IR, H NMR, C NMR, elemental analysis, and single-crystal X-ray
2
difraction. The complex was investigated as a catalyst in the aerobic oxidation oꢀ alcohols to the corresponding aldehydes
5
and ketones in the presence oꢀ 2,2’,6,6’-tetramethylpiperidine N-oxide (TEMPO) as co-oxidant. The combination oꢀ [(η -
t
C Me Bu)Ru(CO)(μ-CO)] and TEMPO aforded an eꢁcient catalytic system ꢀor the aerobic oxidation oꢀ a variety oꢀ primary
5
4
2
and secondary alcohols, giving the corresponding carbonyl compounds in good-to-excellent yields.
Introduction
ꢀrom the reaction mixture is oꢀten costly and diꢁcult. Only
a ꢀew examples oꢀ catalytic oxidations using cheap oxida-
tion agents such as oxygen or hydrogen peroxide have been
reported so ꢀar. The use oꢀ molecular oxygen as an oxidant is
clean, economic, and attractive ꢀrom both an economic and
environmental viewpoint. For this reason, several studies
have ꢀocused on the use oꢀ molecular oxygen as an oxidant
in aerobic oxidation reactions [13]. Noble metal particles
or complexes have been reported as good homogeneous
oxidation catalysts [14]. Most examples oꢀ alcohol oxida-
tions, both homogeneous and heterogeneous, involve the
use oꢀ group 8 metal complexes as catalysts. In particular,
ruthenium compounds, which are widely used as catalysts
in organic synthesis [15], have been thoroughly investigated.
However, the number oꢀ ruthenium-catalyzed alcohol oxida-
tions, employing oxygen (or air) as the sole oxidant, is lim-
The oxidation oꢀ primary and secondary alcohols to their
corresponding aldehydes and ketones is oꢀ importance in
organic synthetic chemistry, both at laboratory and at indus-
trial scale [1]. In recent years, much efort has been devoted
to the development oꢀ alcohol oxidation [2–10]. Stoichio-
metric amounts oꢀ oxidants have been utilized in oxidation
reactions, with quite high activity [11, 12]. However, most
such reactions require stoichiometric amounts oꢀ oꢀten toxic
oxidation reagents. The removal oꢀ traces oꢀ those reagents
Zhiqiang Hao, Xinlong Yan and Zhanwei Li contributed equally to
this work.
Electronic supplementary material The online version oꢀ this
article (https://doi.org/10.1007/s11243-018-0251-3) contains
supplementary material, which is available to authorized users.
ited [16]. For example, RuCl (PPh ) , [17] hydrated RuO
2
2
3 3
[
18], and Ru-hydrotalcite [19] have been shown to catalyze
*
Zhihong Ma
the oxidation oꢀ alcohols using oxygen as the primary oxi-
dant, but in these cases, the scope was generally limited
to activated alcohols, i.e., allylic and benzylic alcohols. In
addition to metal complexes, stable nitroxyl radicals, such
mazhihong-1973@163.com
*
Jin Lin
linjin64@126.com
′
′
1
2
3
as 2,2 ,6,6 -tetramethylpiperidine N-oxide (TEMPO), have
been used as co-oxidants ꢀor the mild and selective oxidation
oꢀ alcohols to aldehydes and ketones [20]. In this work, we
The College oꢀ Chemistry and Material Science, Hebei
Normal University, Shijiazhuang 050024, China
School oꢀ Pharmacy, Hebei Medical University,
Shijiazhuang 050017, China
5
t
have investigated the combination oꢀ [(η -C Me Bu)Ru(CO)
5
4
(μ-CO)] and TEMPO as an eꢁcient catalytic system ꢀor
Hebei College oꢀ Industry and Technology,
Shijiazhuang 050091, China
2
the aerobic oxidation oꢀ a variety oꢀ primary and secondary
Vol.:(0
1
123456789)
3

Transition Metal Chemistry
alcohols to the corresponding aldehydes and ketones, in
good-to-excellent yields.
atoms. Crystallographic data and experimental details ꢀor the
structure determination are given in Table 1. These data can
be obtained ꢀree oꢀ charge via http://www.ccdc.cam.ac.uk,
or ꢀrom the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; e-mail: deposit@
ccdc.cam.ac.uk.
Experimental
General considerations
General procedure for aerobic oxidation reactions
All procedures were perꢀormed under a dry, oxygen-ꢀree
argon atmosphere using standard Schlenk techniques. All
solvents were distilled ꢀrom appropriate drying agents under
argon prior to use. Methylene chloride was distilled over
P O under argon. Column chromatography was carried
The catalytic activity oꢀ the complex ꢀor the aerobic oxi-
dation oꢀ various substrates was tested in the presence oꢀ
TEMPO as a co-oxidant. In a typical reaction, a solution
5
t
oꢀ [(η -C Me Bu)Ru(CO)(μ-CO)] in toluene (2 mL) was
2
5
5
4
2
1
13
out on an alumina column. H and C NMR spectra were
added to a solution oꢀ substrate (1 mmol) and TEMPO
(0.4 mmol) with constant stirring under air. The mixture
was reꢂuxed ꢀor 6 h, and the solvent was then evaporated
under reduced pressure. The residue was puriꢃed by Al O
recorded on a Bruker AV 500 spectrometer with CDCl as
3
1
13
solvent. Chemical shiꢀts oꢀ H and C NMR spectra are
reported in ppm. IR spectra were recorded as KBr disks
on an FTIR 8900 spectrometer. Elemental analysis was
2
3
perꢀormed on a VARIO EL III analyzer. The proligand
5
t
Table 1 Crystal data and structure reꢃnement ꢀor [(η -C Me Bu)
5
4
t
[
[
C HMe Bu] was prepared as described in the literature
Ru(CO)(μ-CO)]
2
5
4
21, 22].
Empirical ꢀormula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C H O Ru
30 42 4 2
668.78
298(2)
0.71073
Triclinic
P–1
8.4650 (8)
9.6001 (9)
9.8089 (9)
104.602 (3)
105.520 (3)
101.358 (2)
712.74 (12)
1
Preparation of the complex
t
Under an argon atmosphere, a solution oꢀ C HMe Bu
5
4
(
0.37 g, 2.1 mmol) and Ru (CO) (0.3 g, 0.47 mmol) in
3 12
xylene (20 mL) was boiled under reꢂux ꢀor 10 h. The mixture
was then cooled to room temperature. Aꢀter removal oꢀ sol-
vent the residue was loaded onto an alumina column. Upon
elution with petroleum ether/CH Cl (10:1), an orange band
2
2
was collected. Removal oꢀ solvent ꢀrom the eluate ꢀollowed
by crystallization ꢀrom a mixture oꢀ dichloromethane and
hexane gave 0.147 g (31.2% yield) oꢀ orange crystals. M.P.:
γ (°)
Volume (Å )
3
2
12.9–213.4 °C; Anal. Calcd ꢀor C H O Ru : C, 53.88;
3
0
42
4
2
Z
1
H, 6.33. Found (%): C, 54.09; H, 6.35; H NMR (CDCl ,
3
Dcalc (g cm−3)
1.558
1.092
5
00 MHz): δ 1.38 (s, 18H, CH ), 1.88 (s, 12H, C Me ), 2.06
3 5 2
−1
μ (mm
)
1
3
(
s, 12H, C Me ); C NMR (CDCl , 125 MHz):δ 9.7, 13.6,
5 2 3
F (000)
Crystal size(mm)
θ range (°)
Reꢂections collected
R (int)
342
−
1
3
1
2.4, 38.0, 101.6, 104.3, 110.7, 198.9. IR(υ , KBr, cm ):
CO
0.28×0.25×0.11
2.28–25.01
3655/2481
0.0535
925(s), 1745(s).
Crystallographic studies
Completeness to θ
Absorption correction
Max. and min. transmission
Reꢃnement method
Data/restraints/parameters
98.5%
Single crystals oꢀ the complex suitable ꢀor X-ray difrac-
tion were obtained ꢀrom the slow evaporation oꢀ a hex-
ane–dichloromethane solution. X-ray crystallographic data
were collected on a Bruker AXS SMART 1000 CCD diꢀ-
Semiempirical ꢀrom equivalents
0.8893/0.7497
Full-matrix least-squares on F2
2481/0/170
ꢀ
ractometer, using graphite monochromated Mo Kα radiation
φ/ω scan, λ=0.71073 Å). Semiempirical absorption correc-
2
Goodness-oꢀ-ꢃt on F
1.052
(
R , ωR [I>2σ(I)]
0.0676, 0.1912
0.0715, 0.1989
1.815
− 1.417
1559101
1
2
tions were applied. The structure was solved by direct meth-
ods and reꢃned by ꢀull-matrix least-squares. All calculations
were carried out using the SHELXL-97 program system.
Hydrogen atoms were included in calculated positions and
reꢃned with ꢃxed thermal parameters riding on the parent
R , ωR (all data)
1
2
−3
Max. peak (e Ǻ
)
)
Mini. Peak (e Ǻ−
CCDC
3
1
3

Transition Metal Chemistry
5
Scheme 1 Synthesis oꢀ [(η -
t
C Me Bu)Ru(CO)(μ-CO)]
5
4
2
O
C
CO
xylene
reflux,10 h
Ru Ru
+
Ru (CO)
3 12
OC
C
O
column chromatography. Elution with petroleum ether/ethyl
acetate (1:10, V/V) gave the desired product.
Results and discussion
Preparation and crystal structure of the complex
t
Reꢀluxing a mixture oꢀ Ru (CO) and [C HMe Bu] in
3
12
5
4
xylene ꢀor 10 h resulted in ꢀormation oꢀ the Ru–Ru single-
5
t
bonded dinuclear complex [(η -C Me Bu)Ru(CO)(μ-CO)]
2
5
4
as orange crystals (Scheme 1). The complex is stable to air
and moisture, and it is soluable in common solvents, such as
dichloromethane, ethyl acetate, tetrahydroꢀuran, and toluene.
The IR spectrum oꢀ the complex showed two CO absorp-
Fig. 1 A thermal ellipsoid plot with atom labeling (displacement
5
t
ellipsoids at 30% probability) oꢀ [(η -C Me Bu)Ru(CO)(μ-CO)] .
−
1
5
4
2
tion bands at 1925 cm in the terminal υ(CO) region and
Hydrogen atoms have been omitted ꢀor clarity
−
1
one at 1745 cm in the bridging υ(CO) region, indicating
that there are two terminal CO ligands which have the same
chemical environment and two bridging CO ligands which
Catalytic oxidation
1
also have the same chemical environment. The H NMR
spectrum oꢀ the complex showed two groups oꢀ singlets ꢀor
the ꢀour methyl protons and a singlet ꢀor the t-butyl protons,
indicating the symmetrical structure in solution (Scheme 1).
The crystal structure oꢀ the complex was determined
by X-ray difraction. The structure is illustrated in Fig. 1.
Selected bond parameters are presented in Table 2. The com-
Our investigation began with the aerobic oxidation oꢀ
1-phenylethanol as a test substrate, under the conditions
that 1.0 mmol 1-phenylethanol was stirred with 10 mol%
oꢀ TEMPO in 2 mL toluene at reꢂux. As shown in Table 3,
the reaction gave only the acetophenone product in 11%
yield in the absence oꢀ catalyst (entry 1; Scheme 2). The
reaction proved to be highly dependent on the amount oꢀ
plex has a trans conꢀormation and C symmetry. It is a nor-
i
5
t
mal trans diruthenium complex, in which two tetramethylcy-
[(η -C Me Bu)Ru(CO)(μ-CO)] . Thus, addition oꢀ 1.0 mol%
5
4
2
5
t
clopentadienyl ligands are coordinated with two ruthenium
[(η -C Me Bu)Ru(CO)(μ-CO)] gave the desired product
5
4
2
5
atoms through their cyclopentadienyl rings in η -bonding.
in 51% yield (entry 2). The yield oꢀ the product increased
with increasing ruthenium carbonyl complex loadings up
to 3.0 mol% and TEMPO loadings up to 40.0 mol%, but
decreased as the loading amounts increased ꢀurther (entries
The complex has two types oꢀ carbonyl ligand, speciꢃcally
two terminal and two bridging. The two cyclopentadienyl
ring planes are parallel. The Ru–CO (bridged) distances are
5
t
2
.051(6) Å and 2.046(6) Å, while the Ru–CO (terminal)
3–7), suggesting that [(η -C Me Bu)Ru(CO)(μ-CO)]
5 4 2
distances are both 1.866(7) Å. The Ru–Ru bond distance
3.0 mol% was the optimal loading amount. The efect oꢀ
diꢀꢀerent solvents on catalytic eꢀꢀicacy was also studied
(Table 3, entries 6 and 9–12). Toluene proved to be the most
efective solvent, while the reactions using xylene, THF,
CH CN, or CH Cl all gave lower yields.
[
2.7677(9) Å] is slightly longer than those ꢀound in the anal-
5
ogous complexes trans-[η -CpRu(CO)(μ-CO)] [2.735(2)
2
5
i
Å] [23], trans-[η -CpMe PrRuCO)(μ-CO)] [2.7510(10)
4
2
5
i
Å] [24], and [(η -C Me Bu)Ru(CO)(μ-CO)] [2.7519(8) Å]
5
4
2
3
2
2
5
[
25], but slightly shorter than those in trans-[(η -C Me Ph)
With the optimized conditions in hand, we set out to
evaluate the scope oꢀ this aerobic oxidation protocol.
Generally, the reaction was compatible with a range oꢀ
5
4
5
Ru(CO)(μ-CO)] [2.7769(4) Å] and trans-[(η -C Me Ph-
2
5
4
OMe)Ru(CO)(μ-CO)] [2.7701(6) Å] [26].
2
1
3

Transition Metal Chemistry
5
t
Table 2 Selected bond distances (Å) and angles (°) ꢀor [(η -C Me Bu)Ru(CO)(μ-CO)]
5
4
2
Bond distances
Ru(1)–Ru(1i)
Ru(1)–C(5)
Ru(1)–C(4)
Ru(1)–C(1)
Ru(1)–C(3)
Ru(1)–C(2)
Ru(1)–C(15)
2.7677 (9)
2.258 (7)
2.277 (6)
2.288 (6)
2.303 (6)
2.315 (6)
1.866 (7)
Ru(1)–C(14i)
Ru(1)–C(14)
O(1)–C(14)
O(2)–C(15)
C(1)–C(6)
C(6)–C(7)
Ru(1)–CEN
2.046 (6)
2.051 (6)
1.167 (8)
1.147 (9)
1.535 (9)
1.546 (11)
1.9323
Bond angles
Ru(1i)–C(14)–Ru(1)
O(2)–C(15)–Ru(1)
O(1)–C(14)–Ru(1i)
O(1)–C(14)–Ru(1)
C(15)–Ru(1)–C(14i)
C(15)–Ru(1)–C(14)
85.0 (2)
173.4 (7)
137.3 (5)
137.7 (5)
92.7 (3)
93.1 (3)
C(14i)–Ru(1)–C(14)
C(15)–Ru(1)–Ru(1i)
C(14i)–Ru(1)–Ru(1i)
C(14)–Ru(1)–Ru(1i)
C(1)–C(2)–C(3)
95.0 (2)
94.3 (2)
47.58 (17)
47.44 (18)
108.0 (5)
109.6 (6)
C(1)–C(6)–C(7)
Symmetry code: (i): −x, −y, −z
5
Table 3 Aerobic oxidation oꢀ 1-phenylethanol catalyzed by [(η -
1-(4-Bromophenyl)ethanol gave the product in 85% yield,
and a similar result was obtained in the case oꢀ 1-(3-bromo-
phenyl)ethanol, suggesting that steric hindrance was only a
moderate inꢂuence on the reaction. When 1-phenylethanols
with electron-withdrawing substituents were employed as
substrates, all the oxidation reactions proceeded smoothly
with good-to-excellent yields (Table 4, entries 3–8). Thus,
t
C Me Bu)Ru(CO)(μ-CO)]
5
4
2
a
Entry
Catalyst
mol%)
TEMPO
(mol%)
Solvent
Yield (%)
(
1
2
3
4
5
6
7
8
9
–
10
10
20
30
40
40
50
–
40
40
40
40
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Xylene
THF
11
51
56
72
73
83
75
42
78
61
56
39
1.0
2.0
3.0
4.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
5
t
the [(η -C Me Bu)Ru(CO)(μ-CO)] catalyst can catalyze
5
4
2
the oxidation oꢀ secondary 1-phenylethanols. Aryl alkyl
alcohols and diaryl alcohols gave the corresponding ketone
products in good-to-excellent yields.
The benzyl alcohols were all good substrates (Table 4,
entries 10–12). Reaction oꢀ a benzyl alcohol with an elec-
tron-rich methoxy substituent gave the product in 86% yield,
while the reaction oꢀ a benzyl alcohol with an electron-with-
drawing chloro substituent gave the product in 74% yield
1
1
1
0
1
2
CH CN
3
CH Cl
2
2
(
Table 4, entries 11 and 12), revealing that electronic efects
5
t
Reaction conditions: 1 mmol 1-phenylethanol, [(η -C Me Bu)Ru(CO)
have only a modest inꢂuence on the reaction. Naphthylalco-
hols also proved to be excellent substrates (Table 4, entries
5
4
(
μ-CO)] , 2 mL solvent, 6 h, open air
2
Isolated yield was determined by column chromatography
a
9
, 13).
Conclusions
In summary, we have synthesized a new ruthenium car-
bonyl complex oꢀ a tert-butyl-substituted tetramethylcyclo-
Scheme 2 The oxidation oꢀ 1-phenylethanol
5
pentadienyl ligand and explored its use in an eꢁcient [(η -
t
C Me Bu)Ru(CO)(μ-CO)] /TEMPO-catalyzed oxidation
5
4
2
substituents, speciꢃcally alkoxy, chloro, bromo, and triꢂuo-
romethyl groups (Table 4, entries 2–7). 4-Methoxypheny-
lethanol proved to be the best substrate (Table 4, entry 2).
system ꢀor a variety oꢀ alcohols. Using this system, both
primary and secondary alcohols gave the corresponding
aldehydes and ketones in good-to-excellent yields.
1
3

Transition Metal Chemistry
Table 4 Aerobic oxidation oꢀ
Producta
O
b
Entry
1
Substrate
Isolated yield [%]
various substrates catalyzed by
5
t
[
(η -C Me Bu)Ru(CO)(μ-CO)]
2
OH
5
4
83
92
82
2
3
4
84
85
5
6
7
87
74
8
83
9
88
1
1
1
1
0
1
2
3
78
86
74
83
5
t
Reaction conditions: 1 mmol substrate, 0.03 mmol [(η -C Me Bu)Ru(CO)(μ-CO)] , 0.4 mmol TEMPO,
5
4
2
2
mL toluene, reꢂux temperature, 6 h, open air
a
The product was characterized by H and 13C NMR
1
b
Determined by column chromatography
1
3

Transition Metal Chemistry
1
1
1
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Arends IWCE, Sheldon RA (2016) ChemCatChem 8:2704
Supplementary information
1. Xie A, Zhou X, Feng L, Hu X, Dong W (2014) Tetrahedron
The characterization data ꢀor related catalytic products.
7
0:3514
2. Gunasekaran N, Jerome P, Ng SW, Tierink ERT, Karvembu R
(2012) J Mol Catal A Chem 353:156
Acknowledgements The authors grateꢀully acknowledge the ꢃnan-
cial support ꢀrom the National Natural Science Foundation oꢀ China
13. Allen SE, Walvoord RR, Padilla-Salinas R, Kozlowski MC (2013)
Chem Rev 113:6234
(
No. 21372061), the Hebei Natural Science Foundation oꢀ China (No.
B2017205006), and the Key Research Fund oꢀ Hebei Normal Univer-
sity (No. L2017Z02).
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1
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