Highly efficient redox isomerization of allylic alcohols and transfer hydrogenation of ketones and aldehydes catalyzed by ruthenium complexes

Article abstract of DOI:10.1002/adsc.201000667

A dicationic dichloro-bipyridine-ruthenium complex shows very high catalytic activity in redox isomerization of allylic alcohols but a relatively low one in transfer hydrogenation of ketones; surprisingly, the analogous dimethyl-bipyridine-ruthenium complex shows reverse catalytic activities in the two reactions.

Full text of DOI:10.1002/adsc.201000667

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DOI: 10.1002/adsc.201000667
Highly Efficient Redox Isomerization of Allylic Alcohols and
Transfer Hydrogenation of Ketones and Aldehydes Catalyzed by
Ruthenium Complexes
Pei Nian Liu,a,* Kun Dong Ju,a and Chak Po Laub,*
a
Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130
Meilong Road, Shanghai, 200237, Peopleꢀs Republic of China
Fax : (+86)-21-6425-2758; phone: (+86)-21-6425-0552; e-mail: liupn@ecust.edu.cn
Department of Applied Biology & Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon,
b
Hong Kong
Fax : (+852)-2364-9932; phone: (+852)-3400-8717; e-mail: bccplau@polyu.edu.hk
Received: August 27, 2010; Revised: December 3, 2010; Published online: February 8, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000667.
Abstract: A dicationic dichloro-bipyridine-rutheni-
um complex shows very high catalytic activity in
redox isomerization of allylic alcohols but a rela-
tively low one in transfer hydrogenation of ketones;
surprisingly, the analogous dimethyl-bipyridine-
ruthenium complex shows reverse catalytic activi-
ties in the two reactions.
The ruthenium-mediated transfer hydrogenation
(TH) of carbonyl compounds under basic conditions
employing i-PrOH or other hydrogen donor solvents
has attracted increasing interest in organometallic
chemistry as well as organic synthesis methodology,[7]
due to its practical simplicity and potential use at am-
bient pressure. Although a range of structurally di-
verse ruthenium complexes have been reported as
À
Keywords: aldehydes; isomerization; ketones; re-
duction; ruthenium
catalysts for TH, the precatalysts featuring a Ru NH
linkage offer high levels of activity and selectivity.[7,8]
To date, exploration of highly effective catalysts with-
[9]
À
out ancillary N H functionality is still in demand.
Reported herein is our study on the highly efficient
redox isomerization of allylic alcohols and the TH of
The catalytic redox isomerization of allylic alcohols ketones and aldehydes catalyzed by the dicationic
into the corresponding saturated carbonyl compounds bipy-ruthenium(II) complexes.
represents an atom-economic and elegant shortcut to
valuable carbonyl compounds. Conventionally, the pyridine (bipy) ligands such as cis-[Ru(6,6’-Cl2bipy)2
2A
The dicationic ruthenium complexes containing bi-
transformation involves a two-step sequential oxida-
ACHTUNGTRENU(NG H2O)2] (OTf)2 (1) and cis-[Ru(6,6’-Me2bipy) CHTUGNTRENN(NUG H2O)2]
tion and reduction process.[1] In the last few decades, (OTf)2 (2) (Figure 1) have been known for decades,
various transition metal complexes of Rh, Co, Ni, Mo, but their applications as catalysts for organic reactions
Ir, Fe, Os, Pd and Pt have been explored as catalysts are still very limited.[10] We first studied the catalytic
for this isomerization[1] and, in particular, ruthenium isomerization of 1-butene-3-ol (3a) with the dichloro-
complexes have been described as the most efficient
catalysts.[2–6] However, most of these catalytic systems
have limited substrate scopes, and harsh reaction con-
ditions and high catalytic loadings are usually re-
quired for the catalytic processes. Recently, bisACTHNUGRTNEUNG(allyl)-
ruthenium(IV) complexes have been disclosed by
Gimeno et al as the most efficient catalysts for the
redox isomerization of allylic alcohols.[2,3] The redox
isomerization catalyzed by the Ru(IV) complexes
demonstrated turnover frequency (TOF) values in the
range of 1–3000 hÀ1 for most substrates when
0.2 mol% catalyst was used.[3]
Figure 1. Structures of complexes 1 and 2.
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Table 1. Isomerization of 3-butene-2-ol (3a) catalyzed by
substituted bipy-Ru(II) complex 1. It can be seen
from Table 1 that with a substrate/catalyst (S/C) ratio
of 1000, quantitative conversions were obtained when
the reactions were carried out neat (808C) and in re-
fluxing THF and DCE solutions (entries 1–5). The re-
action also proceeds, but at a much slower rate, in
aqueous solution (entry 5). However, with a higher
S/C ratio (5000), THF stands out as the solvent of
choice (entries 6–8). It should be pointed out that
base is required for the catalytic isomerization reac-
tions; it seems that the weak base K2CO3 is inferior to
the stronger ones, KOBu-t and KOH (entries 6, 9 and
10). Interestingly, the complex containing the 6,6’-di-
bipy-ruthenium(II) complexes.[a]
Entry Catalyst S/C
Solvent Time Temp.
[oC]
Conv.
[%][b]
1
2
3
1
1
1
1
1
1
1
1
1
1
1
2
1000 Neat
1000 THF
1000 DCE
10 min 80
10 min 72
10 min 50
100
100
100
91
4
1000 acetone 10 min reflux
5[c]
6
1000 H2O
5000 THF
5000 Neat
5000 DCE
5000 THF
5000 THF
10000 THF
1000 THF
4 h
90
72
10 min 72
10 min 80
10 min 50
10 min 72
10 min 72
20 min 72
20 min 72
100
<5
16
98
25
7
8
9[c]
10[d]
11
12
methylbipyridine ligand cis-[Ru(6,6’-Me2bipy)2ACHTNURGTENUNG(H2O)2]
69
38
(OTf)2 (2) exhibits much lower activity (TOF=
1200 hÀ1) in comparison to complex 1 (entry 12).
Subsequently, the catalytic redox isomerization of
allylic alcohols in THF was examined for more sub-
strates at 728C in the presence of 1/KOBu-t, and the
results are listed in Table 2. Monosubstituted secon-
dary aliphatic allylic alcohols [RCH(OH)CH=CH2]
3a–d exhibit high reactivity (TOF=12,000–49,000 hÀ1)
[a]
Reaction conditions: 3a (2 mmol), catalyst:KOBu-t=1:2,
solvent (1 mL), unless otherwise noted.
Determined by H NMR spectroscopy.
Using KOH as the base.
Using K2CO3 as the base.
[b]
[c]
[d]
1
Table 2. Redox isomerization of allylic alcohols catalyzed by complex 1.[a]
Entry
1
Substrate
Product
S/C
Time
Conversion [%][b]
TOF [hÀ1][c]
5000
10 min
>99
49,000 (5 min)
2
3
4
5
6
7
8
5000
5000
1000
1000
20
10 min
20 min
10 min
60 min
30 min
2 h
>99
>99
>99
>99
78
30,000 (10 min)
39,000 (10 min)
12,000 (5 min)
1,200 (30 min)
31 (30 min)
20
>99
72
19 (30 min)
20
23 h
13 (0.5 h)
9
20
48 h
74
0.5 (24 h)
[a]
Reaction conditions: substrate (2 mmol), 1:KOBu-t=1:2, THF (1 mL), 728C.
Determined by H NMR spectroscopy.
TOF values were determined according to the conversions at the time noted in parentheses.
[b]
[c]
1
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Highly Efficient Redox Isomerization of Allylic Alcohols
with 0.02–0.1 mol% catalyst and the conversions in the well-known transfer hydrogenation (TH) of ke-
(>99%) are quantitative within 10–20 min (entries 1– tones and aldehydes is the 1,2-addition of hydride to
4). The results previously reported for the isomeriza- the carbonyl carbon,[13] we are therefore interested to
tion of 3a–3d were 100% conversion after 15–20 min study and compare the catalytic activities of 1 and 2
(TOF=1,000–2000 hÀ1) when 0.2 mol% of Ru(IV) toward the TH reactions.
catalyst was used.[3] The isomerization of 3e to ketone
Interestingly, it was found that 1 exhibits low cata-
4e was also effective and >99% conversion was lytic activity in the TH of acetophenone while the di-
achieved after 1 hour with a high TOF (1220 hÀ1, methyl analogue 2 is highly active for the reduction
entry 5). For this reaction, TOF values of 600– reaction (TOF: 130 vs. 150,000 hÀ1, entries 1 and 2,
1200 hÀ1 have been reported using ruthenium com- Table 3). Acetophenones, the analogous acetonaph-
plexes,[5c,11] and even 2400 hÀ1 was achieved when a thone, and benzophenone show very high reactivities
rhodium complex was used.[12] When allylic alcohols (entries 2–6, 9). Steric hindrance seems to play a role
with internal C=C bonds (3f–h) were used as sub- in reducing the reactivity of the substrate (entries 7
strates, a higher catalyst loading (5 mol%) was and 8). In the TH of the cyclic ketone 5i, excellent
needed for high conversions, and the TOF values de- yields and TOF are observed with complex 2 as the
creased to 13–31 hÀ1 (entries 6–8). The strong depend- catalyst; and complexes 1 also demonstrates low cata-
ence of catalytic reactivity upon the substitution of lytic activity (TOF: 140,000 vs. 510 hÀ1, entries 10 and
the C=C bonds of the allylic alcohols has been previ- 11). The TH of cyclohexanone provides good yield al-
ously observed with other catalytic systems.[3,5b] When though the TOF is much lower (entry 12). Aliphatic
the trisubstituted substrate 3i was used as the sub- ketones are reduced with high yields, however, much
strate, 74% conversion was also obtained after 48 h longer reaction times are required (entries 13 and 14).
(entry 9). If the trisubstituted allylic alcohol 4-methyl- The catalytic reduction of aldehydes to primary alco-
3-penten-2-ol was applied in this isomerization, no hols,[14] which is still uncommon by use of the TH re-
isomerized product could be detected after 48 h, action, is found to be successful with complex 2 as the
which indicated the disubstitution at the teminal catalyst. For aromatic benzaldehyde, the reaction was
carbon of the C=C bond prohibited the isomerization completed after 1 min and very high reactivity
due to the steric hindrance.
In the mechanism investigations for the ruthenium- reduction of aliphatic octanal, quantitative yield was
catalyzed redox isomerization of allylic alcohols,[3,5]
obtained in 20 seconds, although higher catalyst load-
(TOF=92,000 hÀ1) was observed (entry 15). For the
a
mechanism for a half-sandwich ruthenium complex- ing was required (2 mol%, entry 16). Note that all the
catalyzed redox isomerization of allylic alcohols was transfer hydrogenation reactions were quenched at
suggested and substantiated by Bꢂckvall and co-work- the indicated reaction time by adding 1M aqueous
ers.[6] It was proposed that the catalytic reaction in- HCl to the reaction, which then could not proceed
volves the intermediacy of a ketohydride species, gen- any more under reflux conditions.
erated via b-elimination of the alkoxide species. The
In summary, we have found that the electrophilic
hydride can be added to the carbonyl group (1,2-addi- dichloro-bipy ruthenium complex 1 is highly efficient
tion) to reform the alkoxide or alternatively to the for the redox isomerization of allylic alcohols but
double bond of the keto ligand (1,4-addition) to form shows low activity for TH of ketones. It is, however,
the metal enolate; the overall rate of the isomeriza- very interesting to learn that, with respect to 1, com-
tion reaction is dependent on the relative rates of the plex 2 shows reverse catalytic activities in the redox
two addition steps. The isomerization of [D]-3g cata- isomerization and TH reactions. A preliminary mech-
lyzed by the complex 1 was performed and [D]-4g anistic study on the 1-catalyzed isomerization of allyl-
was obtained (Scheme 1), which confirmed a Bꢂck- ic alcohol indicates that the reaction probably pro-
vall-type reaction pathway in the complex 1-catalyzed ceeds via the intermediacy of a ketohydride species.
isomerization of allylic alcohols.[6]
Our work reported here shows that a subtle modifica-
It can be seen from Table 1 that the 6,6’-dimethyl- tion of the electronic properties of the metal centers
bipy-ruthenium complex 2 is a much less active cata- in similar complexes could lead to very different cata-
lyst than its dichloro analogue 1 for the isomerization lytic properties. It is definitely worth spending much
reaction (entry 12, Table 1); and since the crucial step effort, in terms of experimental and theoretical stud-
ies, to gain more understandings and to provide ex-
planations for this phenomenon. Such mechanism in-
vestigations are in progress in our laboratories.
Scheme 1. The isomerization of [D]-3g catalyzed by complex
1.
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Table 3. TH of ketones and aldehydes catalyzed by ruthenium complex 2.[a]
Entry
Substrate
Product
Time
Conversion [%][b]
40
TOF [hÀ1][c]
130 (3 h)
1[d]
3 h
2
3
4
5
1 min
1 min
1 min
1 min
93
96
84
68
150,000 (20 s)
170,000 (20 s)
100,000 (20 s)
83,000 (20 s)
6
7
8
1 min
20 min
18 h
95
94
94
130,000 (20 s)
13,000 (3 min)
300 (1.5 h)
9
1 min
92
110,000 (20 s)
10
1 min
3 h
94
89
140,000 (20 s)
510 (1 h)
11[d]
12
20 min
99
8,800 (3 min)
13
14
14 h
14 h
92
94
800 (0.5 h)
840 (0.5 h)
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Highly Efficient Redox Isomerization of Allylic Alcohols
Table 3. (Continued)
Entry
Substrate
Product
Time
1 min
20 s
Conversion [%][b]
TOF [hÀ1][c]
92,000 (20 s)
8,900 (20 s)
15
98
99
16[e]
[a]
Reaction conditions: substrate (1.5 mmol), substrate:2:KOBu-t=1000:1:25, 0.5M solution of substrate in i-PrOH, 858C,
unless otherwise noted.
Determined by GC-FID.
TOF values were determined according to the conversions at the time noted in parentheses.
Complex 1 used as catalyst.
Substrate:2:KOBu-t=50:1:25 was used.
[b]
[c]
[d]
[e]
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Typical Procedure for the Isomerization of Allylic
Alcohols Catalyzed by Complex 1
A THF (1 mL) solution of 1 (0.0004 mmol), allylic alcohol
(2.0 mmol), and KOBu-t (0.0008 mmol) was stirred in a test-
tube sealed with a rubber septum at 728C under N2 for the
designated time. The course of the reaction was monitored
by regular sampling and the conversion was determined by
1H NMR spectroscopy.
Typical Procedure for the Transfer Hydrogenation of
Ketones and Aldehydes Catalyzed by Complex 2
A solution of 2 (0.0015 mmol) and KOBu-t (0.038 mmol) in
i-PrOH (3.0 mL) in a test-tube sealed with a rubber septum
was heated at 858C under N2 for 30 min. The substrate
(1.5 mmol) was added and the resulting solution was heated
in a sealed tube at 858C for the required period of time.
The reaction was then quenched by adding 1M aqueous
HCl (80 mL) to the reaction mixture. The course of the reac-
tion was monitored by regular sampling and the conversion
was determined by GC-FID. Before GC analysis, the sample
was passed through a short collum throgh silica gel (eluent:
diethyl ether) to remove the ruthenium complex.
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Acknowledgements
This work was supported financially by NSFC (Project No.
20902020), the Shanghai Pujiang Talent Program (Project
No. 09J1403500), the Fundamental Research Funds for the
Central Universities, and the Hong Kong Research Grant
Council (Project No PolyU 5006/07P).
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