Acceptorless and base-free dehydrogenation of alcohols and amines using ruthenium-hydride complexes

Article abstract of DOI:10.1002/adsc.201200532

An efficient, operatively simple, acceptorless, and base-free dehydrogenation of secondary alcohols and nitrogen-containing heterocyclic compounds was achieved by using readily available ruthenium hydride complexes as precatalysts. The complex RuH₂(CO)(PPh₃)₃ (1) and Shvo's complex (2) showed excellent activities for the dehydrogenation of secondary alcohols and nitrogen containing heterocycles. In addition to complexes 1 and 2, the complex RuH₂(PPh₃)₄ (3) also showed moderate to excellent activity for the acceptorless dehydrogenation of nitrogen-containing heterocyclic compounds. Kinetic studies on the oxidation reaction of 1-phenylethanol using complex 1 were carried out in the presence and the absence of external triphenylphosphine (PPh₃). External addition of PPh₃ had a negative influence on the rate of the reaction, which suggested that dissociation of PPh₃ occurred during the course of the reaction. Hydrogen was evolved from the oxidation reaction of 1-phenylethanol by using 1 mol% of 1 (88%) and 2 (92%), which demonstrated the possible usage of the catalytic systems in hydrogen generation. Copyright

Full text of DOI:10.1002/adsc.201200532

FULL PAPERS
DOI: 10.1002/adsc.201200532
Acceptorless and Base-Free Dehydrogenation of Alcohols and
Amines using Ruthenium-Hydride Complexes
b
a,
Senthilkumar Muthaiah and Soon Hyeok Hong *
a
Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul 151-747, Korea
Fax : (+82)-2-889-1568; phone: (+82)-2-880-6655; e-mail: soonhong@snu.ac.kr
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological
b
University, Singapore 637371, Singapore
Received: June 18, 2012; Revised: August 14, 2012; Published online: November 4, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200532.
Abstract: An efficient, operatively simple, acceptor- nylethanol using complex 1 were carried out in the
less, and base-free dehydrogenation of secondary al- presence and the absence of external triphenylphos-
cohols and nitrogen-containing heterocyclic com- phine (PPh ). External addition of PPh had a nega-
3
3
pounds was achieved by using readily available tive influence on the rate of the reaction, which sug-
ruthenium hydride complexes as precatalysts. The gested that dissociation of PPh occurred during the
3
complex RuH (CO) ACHTUNGNRTEUNG( PPh ) (1) and Shvoꢀs complex course of the reaction. Hydrogen was evolved from
2
3 3
(
2) showed excellent activities for the dehydrogena- the oxidation reaction of 1-phenylethanol by using
tion of secondary alcohols and nitrogen containing 1 mol% of 1 (88%) and 2 (92%), which demonstrat-
heterocycles. In addition to complexes 1 and 2, the ed the possible usage of the catalytic systems in hy-
complex RuH ACHTUNGTRENNUNG( PPh ) (3) also showed moderate to drogen generation.
2 3 4
excellent activity for the acceptorless dehydrogena-
tion of nitrogen-containing heterocyclic compounds. Keywords: alcohols; dehydrogenation; N-heterocy-
Kinetic studies on the oxidation reaction of 1-phe- cles; oxidation; ruthenium; Shvoꢀs catalyst
Introduction
that, similar to alcohols, nitrogen-containing heterocy-
cles can also serve as the organic hydrides which can
[
1]
[2]
[14]
Oxidation reactions of alcohols and amines are act as potential hydrogen storage systems. The in-
fundamentally important in chemistry with wide ap- clusion of nitrogen into the cyclic system facilitates
plications. Traditional methods for the oxidation of al- the dehydrogenation process by decreasing the endo-
[
14]
cohols involve stoichiometric amounts of oxidants thermicity of the reaction.
[
3]
[4]
such as hypochlorite, chromium salts, manganese
[7]
Notwithstanding their potential applications in en-
vironmentally friendly oxidation and hydrogen stor-
age, not so many catalytic systems have been devel-
[5]
[6]
salts, oxalyl chloride, and hypervalent iodines.
Several transition metal complexes, such as Ru,
[
8]
[
9]
[10]
[11]
Rh, Ir
and Au
complexes, in the presence of oped for the acceptorless and oxidant- or base-free
stoichiometric amounts of oxidizing agents have been dehydrogenation of alcohols and nitrogen-containing
[
10,13]
extensively used as catalysts in the oxidation reac- heterocycles.
Among all the transition metal com-
[12]
tions. Examples of the oxidizing agents are perox- plexes employed in the dehydrogenation reactions,
ides, PhIO, N-methylmorpholine N-oxide, 2,6-dime- ruthenium complexes have played a vital role. How-
thoxybenzoquinone and K S O . However, from both ever, most of the reported ruthenium catalytic sys-
2
2
8
environmental and economic points of views, the use tems for the dehydrogenation reactions of alcohols
of stoichiometric amounts of oxidants is undesirable. and amines used more than stoichiometric amounts of
Significant efforts have been made to develop cata- reagents such as hydrogen acceptors, bases, and/or ox-
lytic oxidations with environmentally friendly oxi- idants, which are not atom-economical and environ-
dants. Transition metal-catalyzed, acceptorless dehy- mentally benign. A number of ruthenium complexes
[8a]
drogenation reactions are one of the environmentally in combination with oxidants, such as t-BuOOH, [17]di-
[
15]
[16]
benign solutions for the oxidations, providing a possi- oxygen, iodosylbenzene, and persulfate ions, or
[
13]
[18]
ble platform for storage and transportation of H2.
hydrogen acceptors such as 1,4-benzoquinone have
Recent experimental and theoretical studies showed
Adv. Synth. Catal. 2012, 354, 3045 – 3053
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3045

Senthilkumar Muthaiah and Soon Hyeok Hong
FULL PAPERS
shown good to excellent activities for the dehydrogen- alcohols have been oxidized to their corresponding
ation. ketones in moderate to excellent yields by using
For example, the reported ruthenium-based catalyt- RuH (CO)(PPh ) (1) and Shvo’s complex (2). Be-
AHCTUNGTRENNUNG
2
3 3
ic systems that showed excellent activities without any sides, the activities of complexes 1, 2, and RuH2
hydrogen acceptors, but needed a promoter or a basic (PPh ) (3) were tested for the dehydrogenation of ni-
condition are [Ru(OCOCF )(CO)(PPh ) ] with trogen-containing heterocyclic compounds. Further-
CF COOH as a promoter, base-promoted rutheni- more, to gain more insight into the mechanism involv-
AHCTUNGTRENNUNG
3
4
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG
3
3 2
[
19]
3
[
20]
um hydride-phosphine systems,
RuCl3 hydrate/ ing the dehydrogenation of alcohol using 1, we carried
phosphine and [RuCl (p-cymene)] /nitrogen-contain- out a series of kinetic studies in the presence of exter-
AHCTUNGTRENNUNG
2
2
[
21]
ing ligands in a basic medium, Grubbsꢀ catalyst and nally added PPh . The results supported a phosphine
3
[
22]
[
RuCl
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(p-cymene)] /PPh system with LiOH,
and ligand dissociative mechanism.
2
2
3
pincer diamine and diphosphane Ru complexes with
[
8f]
a catalytic amount of KO-t-Bu.
Hydrogen acceptorless and base- or additive-free Results and Discussion
oxidations have been reported with rather specially
designed catalytic systems such as heterogeneous Preliminary results on the oxidation of 1-phenyletha-
[
23]
Shvo-type ruthenium complex,
a recyclable Ru/ nol (7a) to acetophenone (8a) using a variety of
[
24]
AlO(OH) system, dinuclear ruthenium complexes ruthenium hydride complexes such as RuH (CO)-
bearing dicarboxylate and phosphine ligands,
zolidene ruthenium complexes,
PNN-type pincer ruthenium complexes. In addition, Ru (CO) (5) and Ru ACHUTGTNRENUNG( cod) ACTHGNUTRENNUNG( cot) (6) are shown in
2
[
25]
tria-
and PNP- and RuHCl(CO) ACHTUNGTRNEGNU( PPh ) (4) and Ru(0) complexes such as
3 3
ACHTUNGTNERNUNG( PPh ) (1), Shvoꢀs complex (2), RuH ACHTUNGTEURNN(G PPh ) (3),
3 3 2 3 4
[
26]
[
27]
3
12
Hartwig and co-workers have shown that some ruthe- Table 1. Complex 1 gave 8a in a low yield of 33%
nium complexes, including hydrido-phosphine, diphos- under toluene reflux conditions (entry 1). The yield
phine-diamine, and Shvoꢀs complex, are active for the was dramatically increased when NaH was added as
dehydrogenation of 1,4-butanediol to g-butyrolac- a base (entry 2). Matched with our hypothesis in
[
28]
tone.
Scheme 1, the corresponding ketone was produced in
During our study on Ru-catalyzed oxidative CÀN an excellent yield even in the absence of a base at an
bond formation reactions directly from alcohols and elevated temperature of 1658C (entry 3). This result
amines through hydrogen acceptor-free dehydrogena- suggested that the oxidative addition of an OÀH bond
[29]
tion of alcohols, we became interested in alcohol or itself in a neutral condition, without generation of an
amine oxidation with simpler ruthenium precursors alkoxide under a basic condition, requires higher tem-
without using any hydrogen acceptor and/or a base. perature conditions. The catalytic activity was main-
As dehydrogenation of alcohols can be facilitated by tained with a lowered catalyst loading of 1 mol%
hydrogen elimination from dihydride intermediates at (entry 5). However, further decreases in the catalyst
elevated temperatures and oxidative addition of hy- loading resulted in diminished activities (entries 3 to
droxy or amine NÀH to a transition metal center can 7). Then, we checked the activities of ruthenium-hy-
happen without any assistance of a base, we envi- dride complexes 2–4. The results clearly indicated
sioned that hydrogen acceptorless and base-free oxi- that all of the ruthenium-hydride complexes have the
dation of alcohol or amine could be realized with ability to oxidize the alcohol 7a into ketone 8a with
more readily available Ru hydride species moderate to excellent yields, even in the absence of
(
Scheme 1).
any base, additive, or hydrogen acceptor. Among the
In this report, we investigated a series of readily complexes, 1 and 2 showed the best activities (en-
available ruthenium hydride complexes for the ac- tries 5 and 9). Ruthenium(0) complexes such as 5 and
ceptorless and oxidant-free dehydrogenation of secon- 6 showed much less activity compared to ruthenium
dary alcohols and nitrogen-containing cyclic systems hydride complexes (entries 17 and 18). Shvoꢀs com-
under neutral reaction conditions. Several secondary plex (2) showed the highest turnover number of 2600
when a 0.01 mol% catalyst loading used (entry 12).
Having these optimized conditions in hand, we
then examined the scope of the reaction using com-
plexes 1 and 2 under a gentle flow of argon to assist
the removal of hydrogen generated during the reac-
tion (Table 2). Various secondary alcohols were suc-
cessfully oxidized to the corresponding ketones in
moderate to excellent yields. Initially, oxidation reac-
tions of a range of substituted 1-phenylethanols were
Scheme 1. Basic principle of hydrogen acceptor- and base- tried. All of the derivatives of 1-phenylethanol tested
free dehydrogenation of alcohol and amine.
gave their corresponding ketones in moderate to ex-
3046
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 3045 – 3053

Acceptorless and Base-Free Dehydrogenation of Alcohols and Amines
[a]
Table 1. Optimization of reaction conditions for dehydrogenation of 1-phenylethanol (7a).
[b]
Entry
[Ru] (mol%)
Base
Solvent
Yield of 8a [%] (TON)
1
2
3
4
5
6
7
8
9
RuH (CO)
A
T
N
T
E
N
N
(PPh ) 1 (5)
none[
NaH
none
none
none
none
none
none
none
none
none
none
none[
NaH
none
none
none
none
toluene
toluene
33
84
98
98
98 (98)
58 (580)
0
2
3
3
3
3
3
3
3
3
3
3
3
3
3
3
c]
RuH (CO)
A
H
U
G
R
N
U
G
2
RuH (CO)
A
H
U
G
R
N
U
G
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
toluene
2
RuH (CO)
A
H
U
G
R
N
U
G
2
RuH (CO)
A
H
U
G
R
N
U
G
2
RuH (CO)
A
H
U
G
R
N
U
G
2
RuH (CO)
A
H
U
G
R
N
U
G
2
Shvoꢀs complex 2 (5)
Shvoꢀs complex (1)
Shvoꢀs complex (0.5)
Shvoꢀs complex (0.1)
Shvoꢀs complex (0.01)
96
96 (96)
85 (170)
75 (750)
26 (2600)
34
67
49
95
36
10
11
12
13
14
15
16
17
18
RuH
RuH
RuH
ACHTUNGTRNENU(G PPh ) 3 (5)
2 3 4
c]
A
H
U
G
R
N
N
(PPh ) (5)
toluene
2
3 4
A
H
U
G
R
N
N
(PPh ) (5)
mesitylene
mesitylene
mesitylene
mesitylene
2
3 4
RuHCl(CO)
A
H
U
G
R
N
U
G
3
3
Ru (CO) 5 (1)
3
12
Ru
A
H
U
G
R
N
U
G
A
H
U
G
E
N
N
30
[
[
[
a]
[
Ru] complex (x mol%), alcohol (1.0 equiv.), reflux (1108C, toluene; 1658C, mesitylene), 24 h.
b]
c]
Determined by GC using dodecane as an internal standard, average of at least two runs.
1
0 mol% of NaH was used.
cellent yields (entries 1 to 8). Comparable yields were used, complexes 1 and 2 gave 46% and 73% of H , re-
2
obtained when methoxy group was present either in spectively.
the meta- or para-positions of the phenyl ring (en-
tries 3 and 4). Functional groups such as ether of nitrogen-containing heterocycles were tested.
entry 4), halogen (entries 5 and 6), amide (entry 7), Recent theoretical and experimental studies have
Encouraged by the above results, dehydrogenations
(
and amino groups (entry 8) can be tolerated in our shown that similar to alcohols, nitrogen-containing
catalytic systems. The presence of amide and amino heterocycles can also act as potential hydrogen stor-
[
14a]
groups did not have a significant influence on the ac- age systems.
tivity of 2, whereas considerably diminished yields of tested ruthenium complexes 1–4 for the dehydrogena-
g and 8h were observed with 1 (entries 7 and 8). An tion of 1,2,3,4-tetrahydroisoquinoline (11a) under
aliphatic cyclic secondary alcohol such as cyclooctanol base- and oxidant-free reaction conditions (Table 3).
As a beginning of this study, we
8
(
(
7i) gave an excellent yield of cyclooctanone (8i) When 1 mol% of 1 was used, both the fully and the
entry 9). In the case of bulky secondary alcohols partially dehydrogenated products isoquinoline (12a)
such as 1-cyclohexylethanol (7j) and menthol (7k), and 3,4-dihydroisoquinoline (13) were formed in low
complex 2 showed better activity than complex 1. yields (entry 1). Increase of the catalyst loading to
When olefinic secondary alcohols such as 7l and 7m 2.5 mol% resulted in 12a as the major product in an
were reacted, they gave the double bond reduced ke- improved yield of 53% (entry 2). When the catalyst
tones, 8l and 8m, in excellent yields (entries 12 and loading was increased to 5 mol%, only the product
13). The oxidation of primary alcohol such as 2-phe- 12a was formed in 73% yield and there was no obser-
nylethanol (9) did not give the corresponding alde- vation of 13 (entry 3). Other reported systems such as
[
8b]
[9]
hyde, instead, it resulted in the formation of an ester Ru AHCTNUGTRENNUN(G OAc) Cl/O , Rh AHCNTURGTENNNUG( cap) /t-BuOOH, and RuCl2
2 4 2 2 4
[
32]
10 (entry 15). The formation of the ester from pri-
ACHTUNGTNERNU(GN PPh ) /PhIO, exclusively resulted in the formation
3 3
mary alcohols using ruthenium hydride complexes has of partially dehydrogenated product 13 with little or
[
30]
been well reported previously.
The results prompted us to measure the amount of 65% of compound 12a was reported while using
hydrogen evolved out of the reaction. The hydrogen RuCl (PPh ) as catalyst along with large excess of
no formation of compound 12a. A maximum yield of
AHCTUNGTRENNUNG
2
3 3
t
[8a]
evolution was measured using a gas burette apparatus Bu OOH (5 equiv.) as an oxidant. Notably, our oxi-
in the dehydrogenation reaction of 7a using 1 mol% dant-free catalytic conditions produced 12a as the
[
31]
of precatalysts 1 and 2.
Complexes 1 and 2 pro- major product. Consistent with the results of the alco-
duced hydrogen with 88% and 92% yields, respective- hol dehydrogenations, an excellent yield of 99% was
ly. When a lowered catalyst loading of 0.1 mol% was obtained when 2 was used (entry 5). Complexes 3 and
Adv. Synth. Catal. 2012, 354, 3045 – 3053
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3047

Senthilkumar Muthaiah and Soon Hyeok Hong
FULL PAPERS
[a]
Table 2. Oxidation of alcohols using ruthenium-hydride
complexes
Table 3. Catalyst screening for the dehydrogenation of 9.
[a]
[b]
Entry Substrate
Product
Yield [%]
with catalyst
1
2
Entry
[Ru] (mol%)
RuH (CO)(PPh ) 1 (1)
Yield of
Yield of
[b]
[b]
1
2a [%]
13 [%]
1
2
3
4
5
6
7
A
H
U
G
R
N
U
G
28
53
73
82
99
78
64
30
5
0
0
0
1
2
3
4
5
6
7
8
R=H (7a)
8a
8b
8c
8d
8e
8f
98
99
93
96 (87)
99 (88)
100 (94)
2
3 3
RuH (CO) ACHTUNGNRTEUNG( PPh ) (2.5)
R=4-Me (7b)
R=4-OMe (7c)
R=3-OMe (7d)
R=3-Cl (7e)
R=4-Cl (7f)
2
3 3
RuH (CO)
A
H
U
G
R
N
N
(PPh ) (5)
2
3 3
Shvoꢀs complex 2 (1)
Shvoꢀs complex (2.5)
100 99 (91)
100 100 (92)
85
64
12
RuH
A
H
U
G
R
N
N
(PPh ) 3 (5)
0
3
94 (88)
86 (73)
86 (77)
2
3 4
RuHCl(CO)
A
H
U
G
R
N
U
G
R=3-NH(CO)CH (7g) 8g
3 3
3
R=4-NH (7h)
8h
2
[a]
Ru complex (x mol%), amine (1.0 equiv), mesitylene,
[b]
1
658C, 24 h. Determined by GC using dodecane as an
internal standard and average of at least two runs.
9
93
67
89 (84)
94 (91)
4
showed moderate activities with major production
of compound 12a (entries 6 and 7).
After optimization of the reaction conditions, we
extended the substrate scope using complexes 1, 2
and 3 (Table 4). Compared to 11a, 1,2,3,4-tetrahydro-
quinoline (11b) required a longer reaction time. An
excellent yield (98%) of 12b was obtained in 48 h
1
0
1
[c]
1
36
81 (69)
Table 4. Dehydrogenation of nitrogen-containing heterocy-
[a]
clic compounds.
1
2
3
100 98 (92)
[b]
Entry Substrate
Product
Yield with catalyst
1
2
3
1
2
73 99 (91)
78
1
1
1
83
45
77
100 (96)
58 (42)
98
[c]
9
8
[
[c]
[c]
45
32
c]
(
93)
4
5
3
4
87 100 (96) 74
95 98 (93)
89
5
97 100 (96) 73
[
[
a]
Ru complex (1 mol%), alcohol (1.0 equiv.), mesitylene,
658C, 24 h.
[a]
Ru complex (5 mol% of [Ru], that is, 2.5 mol% for 2),
amine (1.0 equiv.), mesitylene, 1658C, 24 h unless other-
wise noted.
1
b]
Determined by GC using dodecane as the internal stan-
dard and average of at least two runs. Isolated yield in
parenthesis.
c]
[b]
Determined by GC using dodecane as an internal stan-
dard and average of at least two runs. Isolated yield in
parenthesis.
c]
[
36 h.
[
4
8 h
3048
asc.wiley-vch.de
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 3045 – 3053

Acceptorless and Base-Free Dehydrogenation of Alcohols and Amines
when 2 was used. Other ruthenium complexes 1 and 3
gave 12b in moderate yields after 48 h (entry 2). 6-
Methoxy-1,2,3,4-tetrahydroisoquinoline 11c gave good
to excellent yields of the product 12c (entry 3). Dehy-
drogenations of indolines were highly efficient with
all three ruthenium complexes (entries 4 and 5).
Morton and Cole-Hamilton reported an RuN H
2
2
ACHTUNGTRENNUNG( PPh ) -catalyzed dehydrogenation of alcohols in
3 3
[20]
a basic medium.
They proposed a mechanism, in
which the first step involves the displacement of dini-
trogen ligand by an alkoxide moiety. Then, the result-
ing alkoxide complex undergoes b-hydride transfer to
form the oxidation product, aldehyde or ketone. Fur-
ther, an anionic ruthenium trihydride complex [RuH
3
À
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(PPh ) ] was suggested to be the resting state of the
3
3
catalyst. It was argued that release of dihydrogen
would be rate-limiting and the three PPh ligands
3
Figure 1. Effect of concentration of externally added PPh3
on the rate of dehydrogenation of 1-phenylethanol
could remain coordinated to the metal center during
[20a]
the process.
However, Shinoda and co-workers
showed that externally added phosphines in the reac-
tion mixture retard the reaction, which indicated that
the reaction mechanism involves PPh ligand dissocia- proposed a mechanism involving a ligand dissociative
3
[33]
tion.
workers suggested that the dehydrogenation reaction ation of one of the phosphine ligands coordinated to
of methanol using [RuN H (PPh ) ] complex prefer- the ruthenium to give a coordinatively unsaturated
A recent theoretical study by Bolm and co- pathway (Scheme 2). The first step can be the dissoci-
AHCTUNGTRENNUNG
2
2
3 3
entially proceeds through dissociation of one of the complex A. Complex A in turn can react with an al-
[
34]
phosphine ligands. The mechanism also involves in- cohol to form ruthenium-alkoxide-trihydride complex
termolecular proton transfer from methanol to the B, which further undergoes b-hydride transfer fol-
catalyst followed by the release of dihydrogen. The lowed by reductive elimination to give compound C
rate-limiting b-hydride elimination from the resulting and the product. Release of dihydrogen from complex
methoxide species was proposed to regenerate the C will reproduce the unsaturated complex A and
resting state of the catalyst and complete the catalytic complete the catalytic cycle.
cycle.
A proposed mechanism for the N-heterocycle dehy-
As the working mechanism of Shvoꢀs catalystꢀs for drogenation by 2, based on the reports on Shvoꢀs
[
35]
[8c]
alcohol dehydrogenation has been well studied, we complex-catalyzed amine dehydrogenation
and
were keen to study the mechanism involved in Wangꢀs mechanistic proposal on the dehydrogenation
[
36]
RuH (CO) ACHTUNGTRENNUNG( PPh ) (1)-catalyzed oxidation of 1-phe- of N-heterocycles, is illustrated in Scheme 3.
3 3
2
nylethanol (7). To investigate whether hydrogen, CO,
or PPh dissociates to generate an active catalytic spe-
3
cies, we have carried out a series of kinetic studies
with varied externally added PPh concentrations. The
3
kinetic studies were performed using 0, 5, 7.5 and
1
0 equiv. of external PPh with respect to 0.00545M
3
of 1. When the reciprocal of the rate constant was
plotted as a function of concentration of added PPh3,
a linear dependence was observed (Figure 1). The
rate dependence on [PPh ] is in good agreement with
3
[
33]
the other previous reports,
suggesting that the
mechanism involves the phosphine ligand dissociative
[
33]
[34]
pathway similarly to Shinodaꢀs and Bolmꢀs stud-
ies.
In addition, through NMR spectroscopic investiga-
tion on the oxidation of 1-phenylethanol in mesity-
lene-d , we found that free PPh was observed in
12
3
31
P NMR spectra within 5 min without any significant
production of the oxidation product 8a. Based on our Scheme 2. Proposed catalytic cycle for the dehydrogenation
experimental evidence and the literature reports, we of alcohols using RuH (CO)(PPh )
AHCTUNGTRENNUNG
2
3 3.
Adv. Synth. Catal. 2012, 354, 3045 – 3053
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3049

Senthilkumar Muthaiah and Soon Hyeok Hong
FULL PAPERS
Scheme 3. Proposed catalytic cycle for the dehydrogenation of an N-heterocycle using Shvoꢀs complex.
Conclusions
comparison with other reported systems. Hydrogen
evolution measurements showed that the reported cat-
In summary, we have reported an acceptorless, base- alytic processes could provide a possible platform for
and oxidant-free dehydrogenation of secondary alco- the storage and transformation of H . The kinetic and
2
hols and nitrogen-containing heterocyclic compounds NMR studies suggested that the mechanism involves
using commercially available RuH (CO) ACTHUNGTRENN(NGU PPh ) , the PPh ligand dissociation pathway when RuH (CO)-
2
3
3
3
2
Shvoꢀs complex, and RuH
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG( PPh ) was used as a precatalyst.
3 3
2
3
4
tion conditions. Among the ruthenium-hydride com-
plexes tested, Shvoꢀs complex showed the best activity
for the oxidation of both secondary alcohols and nitro-
gen-containing heterocyclic compounds. Notably, the
dehydrogenation of 1,2,3,4-tetrahydroisoquinoline ex-
clusively resulted in the formation of the completely
Experimental Section
General Considerations
dehydrogenated product, isoquinoline, which shows All reactions were carried out using standard Schlenk tech-
the stronger oxidizing ability of our catalytic systems in nique under inert argon atmosphere. Toluene and THF
3050
asc.wiley-vch.de
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 3045 – 3053

Acceptorless and Base-Free Dehydrogenation of Alcohols and Amines
[37]
1
were dried over a solvent purification system. Anhydrous
mesitylene was purchased from Aldrich and used without
further purification. GC analyses were carried out using do-
decane as an internal standard. Shvoꢀs complex (2) was pur-
chased from Strem and used without further purification.
white solid; isolated yield: 73% (using complex 2). H NMR
(400 MHz, CDCl ): d=2.21 (s, 3H), 2.59 (s, 3H), 7.38–7.42
3
(m, 1H), 7.65–7.67 (m, 1H), 7.92–7.94 (m, 1H), 8.01 (s, 1H),
8.07 (bs, 1H).
[45]
1-(4-Aminophenyl)ethanone (8h): Purified by silica gel
[38]
[39]
RuH (CO)
RuHCl(CO)
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(PPh )
(1),
RuH2
A
H
U
G
R
N
U
(PPh )
(3),
and
column chromatography (hexane:EA 3:2) to give a white
2
3
3
3 4
[
38]
1
A( PPh ) (4)
C
H
T
U
N
G
T
R
E
N
N
U
N
G
were prepared according to the
solid; isolated yield: 77% (using complex 2). H NMR
3
3
reported procedures.
(400 MHz, CDCl ): d=2.48 (s, 3H), 4.09 (bs, 2H), 6.61 (m,
3
2
H), 7.77 (m, 2H).
Cyclooctanone (8i): Purified by silica gel column chro-
[46]
General Procedure for Dehydrogenation
matography (hexane:EA 9:1) to give a colourless oil; isolat-
ed yield: 84% (using complex 2). H NMR (400 MHz,
1
Ruthenium complex (1 mol% for alcohol oxidation and
CDCl ): d=1.31–1.37 (m, 2H), 1.48–1.55 (m, 4H), 1.81–1.88
3
5
mol% for amine oxidation) and mesitylene (0.6 mL) were
(
m, 4H), 2.36–2.40 (m, 4H).
-Cyclohexylethanone (8j):
placed in an oven-dried Schlenk tube inside the glove box.
The Schlenk tube was taken out and starting material
[47]
1
Purified by silica gel
column chromatography (hexane:EA 9:1) to give a colour-
(
1.0 equiv.) was added under an argon atmosphere. The re-
1
less oil; isolated yield: 91% (using complex 2). H NMR
action mixture was heated at 1658C for 24 h in an oil bath
with a cooling reflux condenser under a gentle flow of argon
to facilitate removal of hydrogen. After completion of the
reaction, the flask was cooled to room temperature, and do-
decane (1.0 equiv.) was added as an internal standard. The
reaction mixture was filtered through neutral alumina using
dichloromethane as the solvent and injected into GC for
analysis. GC yields were measured and calculated consider-
ing the detector response factor of the product versus dodec-
ane. For isolation of the products, the flask was cooled to
room temperature after completion of the reaction. All vol-
atiles were removed under vacuum, and the residue was pu-
rified by silica gel flash chromatography to afford the pure
dehydrogenated product. All the dehydrogenated products
were identified by spectral comparison with literature data.
(
5
400 MHz, CDCl ): d=1.16–1.35 (m, 5H), 1.64–1.86 (m,
3
H), 2.10 (s, 3H), 2.30–2.32 (m, 1H).
[48]
Menthone (8k): Purified by silica gel column chroma-
tography (hexane:ether 1:1) to give a white solid; isolated
yield: 69% (using complex 2). H NMR (400 MHz, CDCl ):
d=0.85 (d, 3H), 0.77 (d, 3H), 1.01 (d, 3H), 1.19–1.30 (m,
2
1
3
H), 1.78–2.26 (m, 5H), 2.30–2.31 (m, 1H)
[42]
1
-Phenylbutan-1-one (8l): Purified by silica gel column
chromatography (hexane:EA 4:1) to give a pale yellow
liquid; isolated yield: 92% (using complex 2). H NMR
1
(400 MHz, CDCl ): d=1.00 (t, 3H), 1.74- 1.80 (m, 2H), 2.95
3
(
t, 2H), 7.42–7.54 (m, 3H), 7.95 (m, 2H).
[49]
1
-(4-Methylphenyl)butan-1-one (8m): Purified by silica
gel column chromatography (hexane:EA 4:1) to give a pale
[40]
yellow liquid; isolated yield: 96% (using complex 2).
Acetophenone (8a): Purified by silica gel column chro-
1
H NMR (400 MHz, CDCl ): d=0.53 (t, 3H), 1.30 (m, 2H),
3
matography (hexane:EA 9:1) to give a colourless oil; isolat-
1
1.93 (s, 3H), 2.45 (t, 2H), 6.7–6.79 (d, 2H), 7.39 (m, 2H).
ed yield: 87% (using complex 2). H NMR (400 MHz,
[50]
4
-Benzoylbenzonitrile (8n):
Purified by silica gel
CDCl ): d=2.6 (s, 3H), 7.2–7.5 (m, 3H), 7.9 (m, 2H).
3
[40]
column chromatography (CH Cl :hexane 1:1) to give
1
-p-Tolylethanone (8b):
Purified by silica gel column
2
2
a white solid; isolated yield: 42% (using complex 2).
chromatography (hexane:EA 9:1) to give a colourless oil;
1
1
H NMR (400 MHz, CDCl ): d=7.48–7.52 (m, 2H), 7.62–
isolated yield: 88% (using complex 2). H NMR (400 MHz,
3
7
.76 (m, 1H), 7.77- 7.82 (m, 4H), 7.82–7.88 (m, 2H).
CDCl ): d=2.39 (s, 3H), 2.58 (s, 3H), 7.25 (m, 2H), 7.84 (m,
3
[51]
Isoquinoline (12a): Purified by silica gel column chro-
2
H).
[41]
matography (hexane:EA 7:3) to give a yellow liquid; isolat-
ed yield: 91% (using complex 2). H NMR (400 MHz,
CDCl ): d=7.58–7.70 (m, 3H), 7.80–7.82 (m, 1H), 7.95–7.97
1-(4-Methoxyphenyl)ethanone (8c):
Purified by silica
1
gel column chromatography (hexane:EA 4:1) to give a col-
1
ourless oil; isolated yield: 94% (using complex 2). H NMR
3
(
m, 1H), 8.51 (s, 1H), 9.24 (s, 1H).
(
400 MHz, CDCl ): d=2.54 (s, 3H), 3.86 (s, 3H), 6.91–6.93
3
[51]
Quinoline (12b): Purified by silica gel column chroma-
(
m, 2H), 7.91–7.94 (m, 2H).
[42]
tography (hexane:EA 4:1) to give a pale yellow liquid; iso-
lated yield: 93% (using complex 2). H NMR (400 MHz,
CDCl ): d=7.36–7.40 (m, 1H), 7.51–7.56 (m, 1H), 7.68–7.73
1-(3-Methoxyphenyl)ethanone (8d):
Purified by silica
1
gel column chromatography (hexane:EA 4:1) to give a col-
1
ourless oil; isolated yield: 91% (using complex 2). H NMR
3
(
m, 1H), 7.79–7.82 (m, 1H), 8.09–8.16 (m, 2H), 8.91
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(s, 1H).
(
(
400 MHz, CDCl ): d=2.5 (s, 3H), 3.8 (s, 3H), 7.10 À7.12
3
[52]
m, 1H), 7.35–7.38 (m, 1H), 7.48–7.54 (m, 2H).
6-Methoxyisoquinoline (12c):
Purified by silica gel
[43]
1
-(3-Chlorophenyl)ethanone (8e): Purified by silica gel
column chromatography (hexane:EA 3:2) to give a yellow
oil, isolated yield: 96% (using complex 2). H NMR
(400 MHz, CDCl
(m, 2H), 8.00–8.06 (m, 2H), 8.74 (d, 1H).
1
column chromatography (hexane:EA 7:3) to give a colour-
1
less oil; isolated yield: 92% (using complex 2). H NMR
): d=3.91 (s, 3H), 7.09 (d, 1H), 7.33–7.38
3
(
400 MHz, CDCl ): d=2.54 (s, 3H), 7.25–7.41 (m, 1H),
3
[53]
7
.51–7.54 (m, 1H), 7.80–7.84 (m, 1H), 7.91–7.92 (d, 1H).
Indole (12d): Purified by silica gel column chromatog-
[40]
1-(4-Chlorophenyl)ethanone (8f): Purified by silica gel
raphy (hexane:EA 3:2) to give a white solid; isolated yield:
93% (using complex 2). H NMR (400 MHz, CDCl
6.59 (s, 1H), 7.15–7.25 (m, 3H), 7.38–7.40 (m, 1H), 7.68–
1
column chromatography (hexane:EA 7:3) to give a colour-
): d=
3
1
less oil; isolated yield: 88% (using complex 2). H NMR
(
400 MHz, CDCl ): d=2.57 (s, 3H), 7.42 (d, 2H), 7.88 (d,
7.70 (m, 1H), 8.05 (bs, 1H).
2-Methylindole (12e):
chromatography (hexane:EA 3:2) to give a white solid. Iso-
lated yield: 96% (using complex 2). H NMR (400 MHz,
3
[53]
2
H).
N-(3-Acetylphenyl)acetamide (8g):
gel column chromatography (hexane:EA 3:2) to give a off
Purified by silica gel column
[44]
Purified by silica
1
Adv. Synth. Catal. 2012, 354, 3045 – 3053
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3051

Senthilkumar Muthaiah and Soon Hyeok Hong
FULL PAPERS
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Gas Burette Measurements
The volume of H was quantitatively measured using a gas
2
[31]
burette as reported in the literature. In a typical proce-
dure, RuH (CO)(PPh ) or Shvoꢀs complex (0.01, 0.1 and
ACHTUNGTRENNUNG
2
3 3
1
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arm. To the flask, 1-phenylethanol (0.1 mL, 0.8 mmol) in
mesitylene (1 mL) was added under an argon atmosphere.
The Schlenk flask was connected to the gas burette via
tubing and was heated in a preheated oil bath (1658C).
After a few seconds the side arm was opened up to the bu-
rette, and the reaction was continued for 24 h before taking
the measurement of the increased volume.
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Acknowledgements
This research was supported by Basic Science Research Pro-
gram through the National Research Foundation of Korea
(
NRF) funded by the Ministry of Education, Science and
Technology (grant number 2012R1A1A1004077). S.M.
thanks Prof. Drasko Vidovic for his generous support of this
work.
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