Efficient acceptorless dehydrogenation of secondary alcohols to ketones mediated by a PNN-Ru(II) catalyst

Article abstract of DOI:10.1039/c7cy00342k

Four types of ruthenium(II) complexes, [fac-PNN]RuH(PPh₃)(CO) (A), [fac-PN_HN]RuH(η¹-BH₄)(CO) (B), [fac-PN_HN]RuCl₂(PPh₃) (C) and [fac-PN_HN]RuH(η¹-BH₄)(PPh₃) (D) (where PN_HN and PNN are N-(2-(diphenylphosphino)ethyl)-5,6,7,8-tetrahydroquinoline-8-amine and its deprotonated derivative), have been synthesized and assessed as catalysts for the acceptorless dehydrogenation of secondary alcohols to afford ketones. It was found that C, in combination with t-BuOK, proved the most effective and versatile catalyst allowing aromatic-, aliphatic- and cycloalkyl-containing alcohols to be efficiently converted to their corresponding ketones with particularly high values of TON achievable. Furthermore, the mechanism for this PNN-Ru mediated process been proposed on the basis of a number of intermediates that have been characterized by EI-MS and NMR spectroscopy. These catalysts show great potential for applications in atom-economic synthesis as well as in the development of organic hydride-based hydrogen storage systems.

Full text of DOI:10.1039/c7cy00342k

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Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
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Ca Pt al el ya ss ies d So c ni eo nt ca ed j&u s Tt emc ah rng oi nl os gy
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DOI: 10.1039/C7CY00342K
Journal Name
ARTICLE
Efficient Acceptorless Dehydrogenation of Secondary Alcohols to
Ketones mediated by a PNN‐Ru(II) Catalyst
Received 00th January 20xx,
Accepted 00th January 20xx
a,b,c
b
,b
a
,a,d
a
Zheng Wang, Bing Pan, Qingbin Liu,* Erlin Yue, Gregory A. Solan,* Yanping Ma, and Wen‐
,
a,c
DOI: 10.1039/x0xx00000x
Hua Sun*
www.rsc.org/
1
Abstract: Four types of ruthenium(II) complexes, [fac‐PNN]RuH(PPh
3
)(CO) (A), [fac‐PN
H
N]RuH(η ‐BH
4
)(CO) (B), [fac‐
1
PN
H
N]RuCl
2 3
(PPh ) (C) and [fac‐PN
H
N]RuH(η ‐BH
4
)(PPh
3
) (D) (where PN
H
N and PNN are N‐(2‐(diphenylphosphino)ethyl)‐
5,6,7,8‐tetrahydroquinoline‐8‐amine and its deprotonated derivative), have been synthesized and assessed as catalysts for
the acceptorless dehydrogenation of secondary alcohols to afford ketones. It was found that C, in combination with t‐
BuOK, proved the most effective and versatile catalyst allowing aromatic‐, aliphatic‐ and cycloalkyl‐containing alcohols to
be efficiently converted to their corresponding ketones with particularly high values of TON achievable. Furthermore, the
mechanism for this PNN‐Ru mediated process been proposed on the basis of a number of intermediates that have been
characterized by EI‐MS and NMR spectroscopy. These catalysts show great potential for applications in atom‐economic
synthesis as well as in the development of organic hydride‐based hydrogen storage systems.
reports of complexes that were capable of mediating this green
transformation were reported by Robinson in the 1970s and Cole‐
Introduction
Hamilton in the 1980’s, in which well‐defined ruthenium(II)
7
complexes of the type [Ru(OCOCF ) (CO)(PPh ) ]
and
3
2
3 2
The conversion of alcohols to carbonyl compounds can be regarded
as one of the most important fundamental reactions in organic
chemistry as evidenced by its extensive application in the synthesis
of fine chemicals and pharmaceutical intermediates. Indeed, a raft
of methods have been implemented over the years to accomplish
8
[
RuH (N )(PPh ) ] were employed. In the intervening years, the
2 2 3 3
9
concept of acceptorless alcohol dehydrogenation (AAD) has rapidly
grown in interest with not only ruthenium but iridium catalysts
now capable 2of the promoting the reaction. While both
1
1
10
1
6,10,11
1
heterogeneous and homogeneous
processes have been
this transformation. Traditionally, stoichiometric amounts of
common oxidants are used, but these reactants tend not only to be
developed, the catalytic efficiency of the homogeneous variant
remains insufficiently high to merit its industrial application and
hence needs to be improved. For example, the ruthenium‐ and
iridium‐based homogeneous catalysts reported so far tend to
require relatively high catalyst loadings in the 0.1 − 5 mol% range to
hazardous or toxic but can generate large quantities of noxious by‐
2
products. In recent years, the development of transition‐metal‐
catalyzed oxidation of alcohols using environmentally friendly
3
4
5
oxidants such as O2, H2O2 or acetone offers an improved
approach. However, from the viewpoint of atom economy and
reaction safety, the direct dehydrogenation of an alcohol to form a
carbonyl compound (e.g., a ketone or an aldehyde) without the
need for an oxidant altogether, is an even more desirable and
achieve
satisfactory
conversions.
Nevertheless,
recent
developments using pincer complexes that can operate using
metal−ligand cooperaꢀon (MLC) show great potenꢀal for improving
3
e,13,14,15
these catalytic performances.
6
In our previous work we have shown the ruthenium‐hydride [fac‐
sustainable route as the only by‐product is hydrogen. The first
PNN]RuH(PPh )(CO) (PNN = 8‐(2‐diphenylphosphinoethyl)amido‐
3
trihydroquinoline) (A) (Chart 1) to be an effective catalyst in the
coupling cyclizations of γ‐amino alcohols with secondary alcohols to
give pyridine or quinoline derivatives. Indeed, this system has
proved highly efficient and amenable to catalyst loadings of as low
as 0.025 mol% to achieve satisfactory results in this multistep
a.
Key Laboratory of Engineering Plastics and Beijing National Laboratory for
Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190, China
b.
c.
College of Chemistry and Material Science, Hebei Normal University, Shijiazhuang
050024, China
1
6
Beijing National Laboratory for Molecular Science, State Key Laboratory for dehydrogenative pathway. Furthermore, A and its derivative [fac‐
1
Structural Chemistry of Unstable and Stable Species, Institute of Chemistry,
PN N]RuH(η ‐BH )(CO) (B) (PN N = 8‐(2‐diphenylphosphinoethyl)‐
H
4
H
Chinese Academy of Sciences, Beijing 100190, China
Department of Chemistry, University of Leicester, University Road, Leicester LE1
d.
aminootrihydroquinoline) (Chart 1) can also be applied to the
catalytic hydrogenation of esters. Notably, when combined with 5
7RH, UK
†
Corresponding Authors: whsun@iccas.ac.cn; liuqingb@sina.com;
mol% of NaBH , A or B can deliver high efficiencies for the
4
Electronic Supplementary Information (ESI) available: Figures, tables, and giving
NMR spectra of the new compounds and CCDC 1533624 for C, See
DOI: 10.1039/x0xx00000x
hydrogenation of a wide range of esters under mild reaction
1
7
conditions.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1‐3 | 1
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Ca Pt al el ya ss ies d So c ni eo nt ca ed j&u s Tt emc ah rng oi nl os gy
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Journal Name
study could be grown by slow diffusion of n‐hexane into its solution
View Article Online
DOI: 10.1039/C7CY00342K
in dichloromethane. The structure consists of a distorted octahedral
H
geometry at ruthenium with the PN N ligand adopting a fac‐
configuration with the triphenylphosphine molecule trans to the
amine donor and the two chloride ligands mutually cis (Fig. 1, see
1
SI). The borohydride derivative of C, [fac‐PN N]RuH(η ‐BH )(PPh )
H
4
3
(
D), could be readily obtained by reacting complex C with NaBH in
4
o
a toluene/ethanol mixture at 65 C; the structure of D was
confirmed by multinuclear NMR spectroscopy, MS and elemental
analysis (see SI).
Chart 1. PNN‐ruthenium(II) complexes (A – D) to be explored for the
AAD reactions
Herein we explore the use of A and B, along with two new
examples of this class of ruthenium(II) complex, [fac‐
1
PN N]RuCl (PPh ) (C) and [fac‐PN N]RuH(η ‐BH )(PPh ) (D) (Chart 1),
H
2
3
H
4
3
for the acceptorless dehydrogenation of alcohols. In particular, C is
shown to exhibit unprecedented efficiency in the acceptorless omitted for clarity. Bond lengths [Å] and angles [deg]: Ru(1)‐N(2)
dehydrogenation of secondary alcohols to afford the corresponding
ketones, with very low catalytic loadings required to achieve high
conversions. We view these complexes as showing great potential
Fig. 1. ORTEP representation of C. All hydrogen atoms have been
2
2
.089(5), Ru(1)‐N(1) 2.162(5), Ru(1)‐P(1) 2.2622(17), Ru(1)‐P(2)
.3249(17), Ru(1)‐Cl(1) 2.4275(17), Ru(1)‐Cl(2) 2.5013(16), P(1)‐Ru(1)‐
Cl(2) 167.48(6), P(2)‐Ru(1)‐Cl(2) 89.40(6), Cl(1)‐Ru(1)‐Cl(2) 84.44(6),
N(2)‐Ru(1)‐N(1) 78.61(19), N(2)‐Ru(1)‐P(1) 90.62(14), N(1)‐Ru(1)‐P(1)
for applications in atom‐economic synthesis and the development 84.06(14).
of organic hydride hydrogen storage systems. Furthermore, we
propose
a
mechanism for the catalyzed acceptorless
Catalytic Studies
dehydrogenation reactions that is based on various intermediates
Firstly, we screened the catalytic activity of all four PNN‐ruthenium
complexes, A – D, for the test AAD reaction of cycloheptanol to give
cycloheptanone; the results are listed in Table 1.
that have been characterized by EI‐MS and NMR spectroscopy.
Results and discussion
Synthesis and characterization of PNN‐ruthenium complexes
Table 1. Acceptorless dehydrogenation of cycloheptanol by
complexes A ‐ D.
a
Entry
Cat.
Base
Conv.
b
(
%)
Scheme 1. Synthesis of PNN‐ruthenium complexes C and D
1
2
3
4
5
6
7
8
A
B
B
C
t‐BuOK
none
50
26
52
76
35
74
12
16
Reaction of RuCl (PPh ) with 8‐(2‐diphenylphosphinoethyl)‐
2
3 3
1
6
t‐BuOK
t‐BuOK
none
aminotrihydroquinoline (PN N) in toluene at 100 ºC for 3 hours
H
gave on work‐up, [fac‐PN N]RuCl (PPh ) (C), in good yield (Scheme
H
2
3
1). Complex C displays peaks in its ESI mass spectrum corresponding
D
D
to a protonated molecular ion and a fragmentation peak
attributable to a loss of chloride from the molecular ion.
Unexpectedly, on standing in deuterated chloroform for four hours,
C undergoes partial isomerization resulting in a mixture consisting
t‐BuOK
t‐BuOK
t‐BuOK
c
1
d
2
3
1
1
of C and C' in a 72:28 ratio. The P{ H} NMR spectrum of C exhibits
two mutually coupled doublets at δ 55.68 and δ 45.61 with a
coupling constant of ca. 28 Hz corresponding to a cis‐arrangement
of the phosphine donors. In C' a cis‐arrangement of the phosphine
a
‐3
Reaction conditions: cycloheptanol (5 mmol), A ‐ D (1.25 × 10 mmol),
t‐BuOK (5 mmol) in p‐xylene (5 mL) at 160 ºC (oil bath temperature) for
24 hours.
The conversion was determined by GC analysis using dodecane as an
internal standard .
is RuH(CO)(PPh
is RuCl
b
2
donors is again apparent with the two doublets (each ca. J(PP) = 28
Hz) in this case appearing at δ 48.12 and δ 43.16 (see SI). Crystals of
the major isomer C suitable for a single‐crystal X‐ray diffraction
c
‐3
1
2
3
)
3
(1.25 × 10 mmol)as catalyst.
d
‐3
2
(PPh
3
)
3
(1.25 × 10 mmol) as catalyst.
2
| J. Name., 2012, 00, 1‐3
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Typically, the catalytic screen was performed using an equimolar
ratio of cycloheptanol to the t‐BuOK base (5 mmol), using 0.025 mol%
of the corresponding ruthenium complex in p‐xylene at reflux for 24
hours. To our delight, all the ruthenium species were active with
the conversion to cycloheptanone being 50% for A, 52% for B, 76%
for C and 74% for D (Table 1, entries 1, 3, 4 and 6). In the absence of
t‐BuOK, the conversion dropped to 35% using D, while for B it
lowered to 26% (Table 1, entries 2 and 5). In order to rule out any
catalyst precursor effects on performance, RuH(CO)(PPh3)3 and
RuCl (PPh ) in the presence of 5 mmol t‐BuOK, were independently
Table 2. Substrate scope in the acceptorless alcohol
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a
dehydrogenation by C.
DOI: 10.1039/C7CY00342K
b
Entry
Alcohol
Product
t (h)
24
Conv. (%)
TON
920
1
23
OH
2
3 3
2
24
43
1720
evaluated under the same conditions; the conversion to
cycloheptanone in these cases was 12 and 16%, respectively (Table
OH
24
36
76
94
3040
3760
3
4
5
1, entries 7, 8). Overall, the best results were obtained using the
PNN‐ruthenium(II) complexes C and D. However, due to D being
synthesized from C, coupled with the fact that D showed some
instablity in solution, we choose C as our catalyst for subsequent
studies.
24
100
4000
24
100
4000
6
24
98
3920
7
8
24
24
100
91
4000
3640
9
1
24
24
100
97
4000
3880
0
1
1
1
1
2
3
24
24
24
24
24
10
24
24
24
96
90
94
95
68
97
27
30
21
3840
3600
3760
3800
2720
3880
1080
1200
840
Fig. 2 The acceptorless dehydrogenation of cycloheptanol by complex C
a
under different bases
a
Reaction conditions: cycloheptanol (5 mmol), base (5 mmol), C (1.25 ×
0 mmol) in toluene (5 mL) at reflux for 24 hours.
The conversion was determined by GC using dodecane as an internal
‐3
1
b
14
15
standard.
c
3
Base: CsCO (2.5 mmol).
d
Reaction conditions: cycloheptanol (5 mmol), t‐BuOK (5 mmol), C (1.25
μmol), at 160 ºC (oil‐bath temperature) in p‐xylene (5 mL) in 36 hours.
16
Secondly, with a view to establishing the most compatible base,
the acceptorless dehydrogenation of cycloheptanol to
cycloheptanone with C as catalyst was screened with four different
types of bases, NaOH, K CO , CsCO and t‐BuOK (Fig. 2, see SI, Table
17
1
8
9
2
3
3
OH
F
O
F
S1), in toluene at reflux. It was found that the type of base
introduced had significant effect on the conversion of
cycloheptanol to cycloheptanone with t‐BuOK the standout
performer. When equivalent molar ratios of NaOH or K CO were
1
a
20
21
10
40
48
48
48
100
100
29
4000
8000
1160
1080
c
2
3
employed (5 mmol), the conversion observed after 24 hours is
markedly less (23 and 39%, respectively) than that seen with t‐BuOK
22
23
24
27
(
74%). With 2.5 mmol of CsCO (relatively expensive and toxic), 69%
3
d
41
55
1640
2200
780
of cycloheptanone was produced after 24 hours. On increasing the
temperature to reflux in p‐xylene, the conversion using C/t‐BuOK
increased to 76% in 24 hours and 94% in 36 hours (see SI, Table S1).
Notably in the absence of base and under the same reaction
conditions using C as catalyst, only 5% of cycloheptanone was
produced after 24 hours. Overall, carrying out the AAD in p‐xylene
at reflux using t‐BuOK as the base proved the optimal operating
conditions to deliver high conversions to cycloheptanone.
25
24
e
24
78
a
‐3
Reaction conditions: alcohol (5 mmol), C (1.25 × 10 mmol) and t‐BuOK
5 mmol) in p‐xylene (5 mL) at reflux.
(
b
The conversion was determined by GC using dodecane as an internal
standard.
c
‐3
Under the same reaction conditions but with C (0.625 × 10 mmol).
The reaction temperature at 50 C.
Reaction conditions: Benzyl alcohol (10 mmol), C (0.1 mmol) and t‐
d
e
o
To explore the versatility of C, a broad range of secondary
BuOK (1 mmol) in toluene (5 mL) at 117 ºC (oil‐bath temperature).
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6
alcohols were selected for study in the AAD using the optimal C to M‐1. Subsequently, the hydroxyl group in the secondary
View Article Online
conditions established (viz., alcohol: t‐BuOK = 1:1, 0.025 mol% C in alcohol adds across the Ru=N bond in M‐1, tDoOfoI:r1m0.1i0n3te9/rCm7eCdYi0a0te34M2K‐
p‐xylene at reflux); the results are compiled in Table 2.
2. The CHR R O hydrogen atom belonging to the coordinated
1 2
It was found that for the cyclic alcohols, the smaller ring sizes (n ≤ 6) alkoxide in M‐2 transfers to the Ru center to generate hydride M‐3
led to very low conversions, e.g., only 23% and 34% of and ketone. In the last step elimination of hydrogen gas from M‐3
cyclohexanone and cyclopentanone were obtained after 24 hours, occurs reforming M‐1.
respectively (Table 2, entries 1‐2). By contrast, the larger cyclic
alcohols (n ≥ 7) gave excellent conversions to the corresponding
ketones, with cycloheptanone and cyclooctanone being obtained in
t-BuOK
N
N
Cl
KCl
t-BuOH
OH
94% in 36 hours and 100% in 24 hours, respectively (entries 3‐4).
H
N
Cl
N
Ru
Cl
Ru
R1
3
Ph P
R2
With regard to the aromatic‐containing secondary alcohols, the
conversions to the corresponding ketones were usually high, for
example 1‐phenylethanol, diphenylmethanol, 4‐methylphenyl‐
Ph
3
P
P
H
P
2
Ph Ph
Ph Ph
M-1
C
(phenyl)methanol, 1,2,3,4‐tetrahydro‐1‐naphthalenol, 1‐indanol and
1
0, 11‐dihydro‐5H‐dibenzo [a, d] cyclohepten‐5‐ol, were amenable
N
N
Cl
Cl
to 100%, 98%, 100%, 91%, 100% and 97% conversions in 24 hours,
respectively (Table 2, entries 5‐10). With regard to secondary
alcohols with electron‐rich groups (Me or MeO) in the aromatic ring,
they also gave very high conversions (Table 2, entries 11‐13). Even
the aromatic‐containing secondary alcohols with sterically hindered
groups gave moderate to high conversions (Table 2, entries 14‐15).
N
N
H
Ph
Ph
Ph
3
P
Ru
Ru
H
3
Ph P
P
O
H
Ph
P
R2
Ph
O
R
1
M-2
M-3
R1
R2
Conjugative and electronic‐rich effects due to the presence of the Scheme 2. Proposed mechanism for the acceptorless dehydrogenation of
aromatic ring are likely responsible for this good performance. On secondary alcohols catalyzed by C
the other hand, the alcohols containing electron‐withdrawing
groups in the aromatic ring gave very poor conversions: percentage
Table 3. Species detected using ESI mass spectrometry under
conversions
for
4‐chlorophenyl(phenyl)‐methanol,
1‐(4‐
catalytic conditions using catalyst C
fluorophenyl)‐ethanol, 1‐(2‐fluorophenyl)ethan‐1‐ol being only 27%, Intermediat ESI MS:
m/z
Structural
assignment
30% and 21% (Table 2, entries 17‐19). These findings imply that
e
detected
species
catalyst C is very sensitive to the structure of the secondary alcohol
employed. Alkyl‐containing alcohols can also be effectively
dehydrogenated particularly those incorporating a C=C double bond
+
C
[C+1]
795.4
759.1
(Table 2, entries 16, 20), e.g., 1‐octen‐3‐ol could be quantitatively
converted to 1‐octen‐3‐one in only 10 hours (Table 2, entry 20).
Even when the catalyst loading was reduced to 0.0125 mol%, 100%
conversion of 1‐octen‐3‐ol could be achieved in 40 hours leading to
a remarkable TON of 8000 (100% conversion) (Table 2, entry 21). For
alkyl‐containing alcohols without an alkene unit in the chain such as
nonan‐3‐ol and nonan‐2‐ol, the conversions were lower at 29% and
+
+
M‐1
[(M‐1)+1]
[(M‐2)+1]
M‐2
831.8
867.8
27% in 48 hours, again highlighting the importance of conjugative
effects in the transformation (Table 2, entries 22 and 23). As for
benzyl alcohol as the primary alcohol, the conversion to
benzaldehyde was lower at 41% after 48 hours at a relatively low
M‐2"
[(M‐2")+1]
o
temperature (50 C) (Table 2, entry 24). However, at higher
o
temperatures (>117 C), benzyl benzoate was produced instead of
the corresponding benzaldehyde (Table 2, entry 25). A few other
examples of primary alcohols were also converted using C to the
corresponding esters (see SI, Table S2). These findings are consistent
+
[
[
(M‐3)+1]
761.3
783.9
M‐3
+
(M‐3)+Na]
1
8
with the work reported by Gusev.
Mechanistic and Characterization aspects
In order shed some light on this mechanism, we monitored the
reaction of an equimolar ratio of C, t‐BuOK and benzyl alcohol in
A proposed catalytic cycle for these AAD reactions that makes
1
1
3j,k,19
CDCl at 45 ºC for 24 hours, by ESI‐MS and H NMR spectroscopy. In
3
use of bifunctional metal‐ligand cooperativity,
is shown in
+
‐
the ESI‐MS peaks corresponding to C along with four different
Scheme 2. Firstly, C undergoes the loss of H and Cl , under the
action of the strong base t‐BuOK, forming amide M‐1 along with KCl
and t‐BuOH. Crabtree has previously noted that the NH of a pincer
ligand needs to be deprotonated to form the active catalyst and we
similarly propose a related step occurring during the conversion of
intermediates have been identified: M‐1 [(PNN)RuCl(PPh )], M‐2
3
[
(PN N)RuCl(O‐t‐Bu)(PPh )], M‐2" [(PN N)RuCl(OCH Ph)(PPh )] and
H 3 H 2 3
1
8a
M‐3 [(PN N)RuCl(H)(PPh )] (Table 3 and SI). At the same time, the
H
3
1
H NMR spectrum was recorded after 1, 6 and 24 hours (see Fig. 3).
Close examination of the spectra reveals the signal corresponding to
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t al el y
a
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i
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o
c
n
ie
o
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a
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&
us
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t
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a
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i nl o
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the benzyl alcohol CH
with the time: at t = 0 h, peak area = 3.76; at t = 1 h, peak area = distillation). All H NMR (500 MHz), C NMR (125 MHz) and
.57; at t = 6 h, peak area = 3.49; at t = 24 h, peak area = 3.14. NMR spectra were recorded on a Bruker AV‐III (500 MHz)
2
group at 4.72 ppm decreases in intensity re‐purified according to standard procedures (e.g., vacuum
View Article Online
31
1
13
DOI: 10.1039/C7CY00342K
P
3
Furthermore, after one hour a new peak at 5.32 ppm forms with a spectrometer. GC analyses were carried out on an Agilent 6820
peak area of 0.25 which can be assigned to the benzyl alkoxide instrument using an OV‐1701 column. GC conditions: Injector Temp:
intermediate M‐2''. After 6 hours, this peak area increases in 250 ºC; Detector Temp: 250 ºC; column temperature 150 ºC. ESI‐MS
intensity to 0.35 while a new peak at 10.5 ppm becomes visible analysis was performed on a 3200 QTRAP 1200 infinity series
which can be attributed to the formation of benzaldehyde. After 24 instrument using a column C18, acetonitrile: water = 70:30, flow
hours, the peak area at 5.32 ppm for M‐2'' integrates to 0.31 which rate = 1 mL / min, electronic energy = 50 eV, Q1MS scan range =
is similar in intensity to that observed after 6 hours. This would 100~1000.
therefore suggest that an equilibrium is established between M‐2" Synthesis of complex C
and benzyl alcohol. In summary, all this in‐situ determined data
firmly support the steps shown in the catalytic cycle in Scheme 2.
2 3 3
RuCl (PPh ) (2.00 g, 2.087 mmol) and N‐(2‐(diphenylphosphino)‐
ethyl)5,6,7,8‐tetrahydro‐quinolin‐8‐amine (0.75 g, 2.087 mmol)
were dissolved in toluene (100 mL) and stirred at 100 ºC for 3 h.
After being cooled to room temperature, the resulting precipitate
was filtered and washed with diethyl ether (3 × 10 mL). The title
1
complex was obtained as pale yellow solid (1.08 g, 65%). H NMR
(
500 MHz, CDCl ) δ 7.67 (q, J = 7.8, 7.2 Hz, 6H), 7.43 (t, J = 8.7 Hz,
3
2
6
2
1
1
1
1
1
1
H), 7.39 – 7.18 (m, 10H), 7.17‐7.06 (m, 8H), 6.97 (d, J = 7.8 Hz, 1H),
.88 (t, J = 7.6 Hz, 1H), 5.35 (s, H, N‐H), 3.54 (q, J = 48.6, 38.2 Hz, 1H),
.86 – 2.57 (m, 4H), 2.39 – 2.29 (m, 2H), 2.17‐1.98 (m, 3H), 1.78 –
1
3
.62 (m, 1H). C NMR (126 MHz, CDCl ) δ 161.21, 154.69, 138.71,
3
37.89, 137.17, 136.87, 136.77, 135.23, 135.18, 135.10, 135.07,
35.02, 134.90, 134.78, 134.71, 134.00, 133.93, 133.57, 131.42,
31.35, 129.07, 128.84, 128.78, 128.62, 128.26, 127.83, 127.76,
27.61, 127.54, 127.34, 127.29, 127.27, 127.21, 127.14, 127.08,
27.01, 125.34, 122.59, 61.22, 45.65, 44.78, 37.98 (d, J = 27.2 Hz),
1
28.96, 27.97, 21.50, 20.73 (CH
3
‐Toluene)
) δ 55.68 (d, J = 27.6 Hz), 45.61 (d, J = 27.8
Hz).ESI‐MS (m/z) Calcd for [C H Cl N RuP ], 795.1; found: 795.4
Fig. 3. H NMR (500 MHz, CDCl
3
) spectra of an equimolar ratio of C and
3
1
P NMR (202 MHz, CDCl
3
benzyl alcohol recorded: 1. after sample dissolution, 2. with t‐BuOK (1 eq.) at
5 ºC after 1 hour, 3. with t‐BuOK (1 eq.) at 45 ºC after 6 hours, 4. with t‐
4
1
41
2
2
2
4
+
+
[
C+1] ; Calcd for[C H Cl N RuP ], 759.1; found: 759.6 [C‐Cl] .
41 40 2 2 2
BuOK (1 eq.) at 45 ºC after 24 hours.
Anal. Calcd for C H Cl N RuP : C, 61.96; H, 5.073; N, 3.52. Found:
4
1
40
2
2
2
C, 61.77; H, 5,159; N, 3.34.
On standing in CDCl for 4 h, complex C was obtained as a mixture
3
Conclusions
of two isomers, C/C' 72:28. No free PPh was detected after this
3
Four types of PNN‐Ru(II) complexes, A – D, have been evaluated as
catalysts in the acceptorless dehydrogenation of secondary alcohols
to give ketones. Complex [fac‐PN N]RuCl (PPh ) (C), in the presence
time (NMR sample: 20 mg of C in 0.8 mL CDCl3).
C and C'
13
H
2
3
3
C NMR (126 MHz, CDCl ) δ 162.74 (C'), 161.21 (C), 157.58 (C'),
of t‐BuOK, proved the most suitable and was able to operate
efficiently with a catalyst loading of just 0.025 mol%. Indeed, using
C/t‐BuOK as catalyst, seventeen different kinds of secondary
alcohols could be dehydrogenated to give their corresponding
ketones with yields in the range 21‐100%; structural variations in
the substrate greatly affect the catalyst performance. In addition, a
mechanism for the PNN‐Ru mediated dehydrogenation has been
proposed that is supported by various intermediates that have been
characterized by EI‐MS and NMR spectroscopy.
1
1
1
1
1
6
2
2
54.69 (C), 138.71, 137.89, 137.17, 136.87, 136.77, 135.23, 135.18,
35.10, 135.07, 135.02, 134.90, 134.78, 134.71, 134.00, 133.93,
33.57, 131.42, 131.35, 129.07, 128.84, 128.78, 128.62, 128.26,
27.83, 127.76, 127.61, 127.54, 127.34, 127.29, 127.27, 127.21,
27.14, 127.08, 127.01, 125.34, 122.59 (C), 120.98 (C'), 61.93 (C'),
1.22 (C), 45.65 (C), 44.78 (C'), 37.98 (d, J = 27.2 Hz, C), 35.11 (d, J =
7.6 Hz, C'), 28.96 (C), 28.27 (C') 27.97 (C), 27.21 (C'), 21.50 (C),
1.21 (C'), 20.73 (CH ‐Toluene).
3
31
1
P{ H} NMR (202 MHz, CDCl
3
) C: δ 55.68 (d, J = 27.6 Hz), 45.61 (d, J
=
27.8 Hz); C': δ 48.12 (d, J = 27.7 Hz), 43.16 (d, J = 29.3 Hz).
Catalytic study details.
Experimental section
Under an atmosphere of argon, a Schlenk vessel equipped with a
‐
3
stir bar, was loaded with the ruthenium complex (A ‐ D) (1.25 × 10
mmol) to be investigated, the corresponding alcohol (5 mmol) and
the desired amount of base (NaOH, K CO , CsCO t‐BuOK) (1 ‐ 5
mmol) in p‐xylene (5 mL) (or toluene). The reaction was then stirred
and heated to the desired oil‐bath temperature (130 C or 160 C)
General information.
All experiments with metal complexes and phosphine ligands were
carried out under an atmosphere of nitrogen. All solvents were
reagent grade or better and were used after being distilled under
nitrogen. Most of the chemicals used in the catalytic reactions were
2
3
3,
o
o
with the reaction vessel open to the bubbler. After the specified
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1
1c,e
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We acknowledge support from the National Natural Science
Foundation of China (21476060 and U1362204) and the Nature
Science Foundation of Hebei Province (B2014205049). G.A.S. thanks
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Journal Name
ARTICLE
Graphical for Table of Contents
Efficient Acceptorless Dehydrogenation of Secondary Alcohols to Ketones mediated
by a PNN‐Ru(II) Catalyst
a,b,c
b
,b
a
,a,d
a
,a,c
Zheng Wang, Bing Pan, Qingbin Liu,* Erlin Yue, Gregory A. Solan,* Yanping Ma, and Wen‐Hua Sun*
a
Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, China. Email: whsun@iccas.ac.cn
b
College of Chemistry and Material Science, Hebei Normal University, Shijiazhuang 050024, China. Email: liuqingb@sina.com
University of Chinese Academy of Sciences, Beijing 100049, China
Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, UK. E‐mail: gas8@leicester.ac.uk
c
d
H 2 3
The ruthenium(II) complex, [fac‐PN N]RuCl (PPh ) (C), in combination with t‐BuOK proved an effective and versatile catalyst allowing
aromatic‐, aliphatic‐ and cycloalkyl‐containing alcohols to be efficiently converted to their corresponding ketones with particularly high
values of TON achievable.
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