Cyclopentadienyl-ruthenium(II) complexes as efficient catalysts for the reduction of carbonyl compounds

Article abstract of DOI:10.1016/j.tet.2017.01.067

This work reports the reduction of a large variety of aldehydes and ketones with the system PhSiH₃/[CpRu(PPh₃)₂Cl] in good to excellent yields and high chemoselectivity. The catalyst [CpRu(PPh₃)₂Cl] can be used in at least 12 catalytic cycles with excellent catalytic activity and several substrates were reduced under solvent free conditions.

Full text of DOI:10.1016/j.tet.2017.01.067

Accepted Manuscript
Cyclopentadienyl-ruthenium(II) complexes as efficient catalysts for the reduction of
carbonyl compounds
Ivânia R. Cabrita, Pedro R. Florindo, Ana C. Fernandes
PII:
S0040-4020(17)30112-6
10.1016/j.tet.2017.01.067
TET 28437
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 8 August 2016
Revised Date: 28 December 2016
Accepted Date: 27 January 2017
Please cite this article as: Cabrita IR, Florindo PR, Fernandes AC, Cyclopentadienyl-ruthenium(II)
complexes as efficient catalysts for the reduction of carbonyl compounds, Tetrahedron (2017), doi:
10.1016/j.tet.2017.01.067.
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ACCEPTED MANUSCRIPT
Graphical abstract

ACCEPTED MANUSCRIPT
Cyclopentadienyl-Ruthenium(II) Complexes as Efficient Catalysts for the
Reduction of Carbonyl Compounds
Ivânia R. Cabrita, Pedro R. Florindo and Ana C. Fernandes*
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av.
Rovisco Pais, 1049-001 Lisboa, Portugal, Tel: 351 218419264; Fax: 351 218464455,
anacristinafernandes@tecnico.ulisboa.pt
Abstract – This work reports the reduction of a large variety of aldehydes and ketones
with the system PhSiH3/[CpRu(PPh3)2Cl] in good to excellent yields and high
chemoselectivity. The catalyst [CpRu(PPh3)2Cl] can be used in at least 12 catalytic
cycles with excellent catalytic activity and several substrates were reduced under
solvent free conditions.
Keywords: reduction, carbonyl compounds, alcohols, CpRu(II) complexes, silanes.
1.Introduction
Alcohols are important building blocks for the synthesis of a number of
pharmaceuticals, agrochemicals and fine chemicals. The reduction of carbonyl
compounds to yield primary and secondary alcohols is a very desirable reaction, since
carbonyl compounds are among the most abundant starting materials for the synthetic
chemist. Therefore, the development of new methodologies for the chemoselective
reduction of carbonyl groups remains a challenge in organic synthesis.
Among the large variety of catalysts reported in the literature for the reduction of
carbonyl compounds, the use of ruthenium complexes has received significant attention.
For example, several ruthenium complexes have been employed as catalysts for the
hydrogenation of carbonyl compounds, providing an efficient access to the
corresponding alcohols.1-4
Transfer hydrogenation of carbonyl compounds catalyzed by ruthenium complexes
with 2-propanol is convenient in large scale synthesis since there is no need to employ a
high hydrogen pressure or to use hazardous reducing agents.5-8
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The hydrosilylation is also an important methodology for the reduction of carbonyl
compounds, producing initially a silyl ether which can easily be hydrolyzed to the
corresponding alcohol. Due to its safety and operational simplicity, the catalytic
hydrosilylation of carbonyl compounds has recently become an important alternative as
a reduction strategy. In contrast to the other reduction methodologies, ruthenium
complexes have not been widely used as catalysts in the hydrosilylation of carbonyl
compounds.9-12
Cyclopentadienyl-ruthenium(II) complexes have been the subject of investigation
by many research groups during past couple of decades not only because of their
potential as catalysts13-16 but also due to their strong anticancer activity.17-20
In continuation of our work about the hydrosilylation of carbonyl compounds
catalyzed by molybdenum21,22 and rhenium23 complexes, here we report a novel
methodology for the efficient reduction of aldehydes and ketones with silanes catalyzed
by cyclopentadienyl ruthenium (II) complexes.
2.Results and discussion
The reduction of the test substrate 4-(methylthio)benzaldehyde was investigated with
different CpRu(II) complexes, silanes and solvents. Initially, the reduction of
4-(methylthio)benzaldehyde was explored with different CpRu(II) complexes (1 mol%)
(Figure 1) with phenylsilane at 80 ºC under solvent free conditions. All catalysts
afforded the alcohol as the major product and the silyl ether as the minor one in 20-35%
yields. After hydrolysis of the silyl ether with tetrabutylammonium fluoride (TBAF),
4-(methylthio)benzyl alcohol was obtained in excellent yields. The complex
[CpRu(PPh3)2Cl] 1 (1 mol%) proved to be the best catalyst for this reduction, producing
the corresponding benzyl alcohol in 98% yield after 40 min (Table 1, entry 1). Using
only 0.5 mol% of [CpRu(PPh3)2Cl], the reaction also afforded the alcohol in excellent
yield, but required 24 h (Table 1, entry 2). At room temperature, the product was
isolated in 89% yield after 24 h (Table 1, entry 3). The ruthenium complex
[CpRu(dppe)Cl] 2 required 24 h to reduce the aldehyde in 95% yield (Table 1, entry 4).
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Ru
Ph3P
Ru
Ph2P
Cl
Cl
1
Ph3P
PPh2
2
[PF6]
[PF6]
[PF6]
N
Ru
N
Ru
Ru
Ph3P
Ph3P
Ph3P
Ph3P
N
OAc
OAc
N
O
O
N
Ph3P
N
N
O
AcO
O
3
H
4
5
O
OAc
Figure 1 – Structures of the CpRu (II)
In our previous works,17,18 we have demonstrated the excellent biological
activity of cyclopentadienyl ruthenium (II) complexes 3 and 4, containing fructose and
xylose carbohydrate derivative moieties, respectively. In this work we explored the
catalytic activity of these monocationic, N-bonded complexes in the reduction of
carbonyl compounds, to study the influence of these structural changes on their catalytic
activity when compared with the parent complex [CpRu(PPh3)2Cl].The reactions
catalyzed by complexes 3 and 4, were very fast (20 min-1h), producing the alcohol in
good to excellent yields (Table 1, entries 5-6). The complex [RuCp(PPh3)(2,2′-
bipy][PF6] 5 was also efficient in the reduction of the aldehyde with 84% yield after 3 h
(Table 1, entry 7). Finally, in absence of catalyst was observed the formation of the
alcohol in only 30% yield after 24 h (Table 1, entry 8). Complex [CpRu(PPh3)2Cl]
proved to be the most efficient catalyst for the conversion of 4-(methylthio)
benzaldehyde in 4-(methylthio)benzyl alcohol,within the CpRu(II) compounds tested.
Table 1
Reduction of 4-(methylthio)benzaldehyde catalyzed by different CpRu(II) complexesa
Catalyst
Catalyst
(mol%)
1.0
Temp. (ºC)
Time
Yield (%)b
Entry
1
2
3
[CpRu(PPh3)2Cl] 1
[CpRu(PPh3)2Cl] 1
[CpRu(PPh3)2Cl] 1
80
80
r.t.
40 min
24 h
98
93
89
0.5
1.0
24 h
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4
5
6
7
8
[CpRu(dppe)Cl] 2
1.0
80
80
80
80
80
24 h
1 h
95
98
72
84
30
[RuCp(PPh3)2(TFru)][PF6] 3
1.0
[RuCp(PPh3)2NCXylAc][PF6] 4 1.0
[RuCp(PPh3)(2,2’-bipy)][PF6] 5 1.0
20 min
3 h
Without catalyst
24 h
−
a
The reactions were carried out with 1.0 mmol of 4-(methylthio)benzaldehyde, 1.2 mmol of PhSiH3 and
1.0 mmol TBAF.
b Isolated yields.
The reactivity of different silanes was also evaluated in the reduction of the
4-(methylthio)benzaldehyde catalyzed by [CpRu(PPh3)2Cl] (1 mol%). The best result
was obtained with PhSiH3, affording the alcohol in 96% yield after 40 min (Table 2,
entry 1). The silanes PhMe2SiH, Ph3SiH, Pr3SiH, Et3SiH and polymethylhydrosiloxane
(PMHS) also produced good yields of 4-(methylthio)benzyl alcohol, but these reactions
required 24 h (Table 2, entries 2-6). In absence of silane we did not observe the
reduction of the aldehyde (Table 2, entry 7).
Table 2
Reduction of 4-(methylthio)benzaldehyde catalyzed by [CpRu(PPh3)2Cl] using different
silanesa
Entry
1
Silane
Time
(Yield)b
96
PhSiH3
40 min
2
3
4
5
6
7
Me2PhSiH
Ph3SiH
24 h
24 h
24 h
24 h
24 h
24 h
87
83
PMHS
75
Pr3SiH
70
Et3SiH
66
Without silane
No reaction
a The reactions were carried out with 1.0 mmol of 4-(methylthio)benzaldehyde,
1.2 mmol of silane and TBAF (1.0 mmol).
b Isolated yields.
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The reduction of 4-(methylthio)benzaldehyde catalyzed by [CpRu(PPh3)2Cl] (1 mol%)
with PhSiH3 was also investigated in different solvents (Table 3). The best result was
obtained under solvent free conditions at 80 ºC, producing the benzyl alcohol in 98%
yield after 40 min (Table 3, entry 1). The reduction performed at reflux of toluene gave
the alcohol in 96% (Table 3, entry 2). At room temperature, this reaction afforded the
alcohol in 89% yield, but the reaction took 24 h (Table 3, entry 3). THF was also a good
solvent for this reaction, giving the benzyl alcohol in 92% yield after 2 h (Table 3, entry
4). Excellent yields of the product were also isolated in benzene, dichloromethane and
chloroform (Table 3, entries 5-7). Finally, the reduction performed in acetonitrile
produced the alcohol in only 21% yield (Table 3, entry 8).
Table 3
Reduction of 4-(methylthio)benzaldehyde catalyzed by [CpRu(PPh3)2Cl] in different
solventsa
Entry
1
Solvent
Neat
Temp. (ºC)
80
Time
Yield (%)b
98%
40 min
2
3
4
5
6
7
8
Toluene
Toluene
THF
Reflux
r. t.
40 min
24 h
2 h
96%
89%
92%
97%
95%
94%
21%
Reflux
Reflux
Reflux
Reflux
Reflux
Benzene
CH2Cl2
CHCl3
24 h
24 h
24 h
24 h
CH3CN
a The reactions were carried out with 1.0 mmol of 4-(methylthio)benzaldehyde,
1.2 mmol of PhSiH3 and TBAF (1.0 mmol).
b Isolated yields.
To evaluate the applicability of the system PhSiH3/[CpRu(PPh3)2Cl], we tested it in the
reduction of aldehydes and ketones. The reactions of liquid aldehydes were carried out
under solvent free conditions at 80 ºC and the reactions of solid aldehydes were
performed in toluene at reflux temperature (Table 4). The results obtained demonstrated
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that this system was very efficient for the reduction of a large variety of aldehydes in
good to excellent yields with high chemoselectivity, tolerating several of functional
groups, such as -F, -Br, -CF3, -OMe, -CN, -CO2Me, heterocyclic ring and double bond.
Other interesting results were also obtained in the reduction of hydrocinnamaldehyde in
85% (Table 4, entry 7) and in the reduction of ferrocenecarboxaldehyde in 68% yield
(Table 4, entry 11).
The efficiency of the system PhSiH3/[CpRu(PPh3)2Cl] was tested in the reduction of
several ketones containing electron-donating and electron-withdrawing groups,
producing the alcohols in good yields (Table 4). These reductions were carried out at
reflux temperature of toluene and required a higher amount of [CpRu(PPh3)2Cl] (5
mol%). Curiously, this system was also appropriated for the reduction of the natural
product flavanone, producing the corresponding alcohol in 77% yield after 4h 30min
(Table 4, entry 15).
The reduction of 4-(methylthio)benzaldehyde was also carried out in the
presence
of
4′-(trifluoromethyl)acetophenone
with
the
system
PhSiH3/[CpRu(PPh3)2Cl], affording 4-(methylthio)benzyl alcohol in 97% yield along
with 5% yield of 1-(4-(trifluoromethyl)phenyl)ethanol after 4 h at 80ºC, demonstrating
the selective reduction of aldehydes in the presence of ketones.
The catalytic system PhSiH3/[CpRu(PPh3)2Cl] proved to be very efficient for the
reduction of aldehydes and ketones with excellent chemoselectivity, however this
method required higher reaction temperature than other methodologies reported in the
literature.9-12
Table 4
Reduction of carbonyl compounds with the system PhSiH3/[CpRu(PPh3)2Cl] a
O
OH
R2
1) [CpRu(PPh3)2Cl] (1-5 mol%), PhSiH3
2) TBAF
R1
R2
R1
Aldehyde or Ketone
Entry Substrate
Alcohol
Solvent
Neat
Temp. (ºC) Time
Yield (%)b
98
1
80
40 min
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2
Toluene
Reflux
2 h 30min
97
3
4
5
Toluene
Neat
Reflux
80
20 h
96
93
92
1 h
Toluene
Reflux
30 min
6
Neat
80
1 h 30 min 88
7
8
Neat
Neat
80
80
7 h
85
82
40 min
9
Toluene
Reflux
2 h 30 min 74
1 h 30 min 71
10
11
Neat
80
Toluene
Reflux
30 min
68
63
OH
HO
12
13
Toluene
Toluene
Reflux
Reflux
24 h
4 h
OCH3
78
14
15
Toluene
Toluene
Reflux
Reflux
3 h
77
77
4 h 30 min
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16
17
Toluene
Toluene
Toluene
Reflux
Reflux
Reflux
2 h
75
71
61
3 h 30 min
2 h 30 min
18
a
The reactions were carried out with 1.0 mmol of carbonyl compound, 1.2 mmol of PhSiH3, TBAF (1.0
mmol) using 1 mol% of catalyst in the reaction of aldehydes and 5 mol% of catalyst in reduction of
ketones.
b Isolated yields.
In order to study the possible use of the complex [CpRu(PPh3)2Cl] (1 mol%) as catalyst
in multiple cycles, we carried out successive reactions by sequential addition of fresh
substrate 4-(methylthio)benzaldehyde and phenylsilane to the reaction mixture. The
results obtained showed that [CpRu(PPh3)2Cl] can be used in at least 12 catalytic cycles
with excellent yields (Graphic 1).
Graphic 1. Use of the catalyst [CpRu(PPh3)2Cl] in multiple cyclesa
aThe reactions were carried out by successive addition of 0.5 mmol
of 4-(methylthio)benzaldehyde and 0.6 mmol of PhSiH3.
bYields determined by 1H NMR using mesitylene as internal standard.
We have also performed the reduction of 4-(methylthio)benzaldehyde with deuterated
1
dimethylphenylsilane (Me2PhSiD) catalyzed by [CpRu(PPh3)2Cl] (Eq. 1). In the H
NMR spectrum of the deuterated alcohol 2, the signal at δ 4.60 ppm integrates only one
8

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proton, which is consistent with the reduction of the carbonyl to the corresponding
methylene group and the incorporation of one deuterium atom in alcohol 2.
1) [CpRu(PPh3)2Cl] (1 mol%)
O
D
H
PhMe2Si-D, Toluene, reflux, 24 h
OH
H
(Eq. 1)
2) TBAF, r.t., 30 min, 60%
H3CS
H3CS
2
1
This result also suggests that the mechanism for the reduction of carbonyl compounds
with silanes catalyzed by [CpRu(PPh3)2Cl] should involve the activation of the silane by
the catalyst, with formation of a hydride that reduces the carbonyl group.
3. Conclusion
We report a novel procedure for the reduction of carbonyl compounds catalyzed
by CpRu(II) complexes using a silane as reducing agent. The system
PhSiH3/[CpRu(PPh3)2Cl] proved to be very efficient for the reduction of a large variety
of aldehydes and ketones in good to excellent yields with high chemoselectivity,
tolerating a large range of functional groups such as -CO2R, -CN, -CF3, -F, -Br, -OMe,
-SMe, heterocyclic ring and double bond. Furthermore, this procedure has the
advantages that many of the substrates can be reduced under solvent free conditions and
the catalyst can be used in at least 12 catalytic cycles with excellent catalytic activity.
This work demonstrates that cyclopentadienyl-ruthenium(II) complexes activate the
silanes and they are good catalysts for the reduction of carbonyl compounds.
Further studies to explore the catalytic activity of other CpRu(II) complexes as
well to extend this methodology to the reduction of other substrates are now under
investigation in our group.
4. Experimental section
4.1. General information
All the reactions were carried out under air atmosphere and without any dry solvents.
Carbonyl compounds and silanes were obtained from commercial suppliers and were
used without further purification. The catalysts [CpRu(PPh3)2Cl],24 [CpRu(dppe)Cl],24
[RuCp(PPh3)(2,2′-bipy][PF6],25
[RuCp(PPh3)2(TFru)][PF6],18
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[RuCp(PPh3)2NCXylAc][PF6],17 and PhMe2SiD26 were prepared according to literature
procedures. Flash chromatography was performed on MN Kieselgel 60M 230-400
mesh. 1H NMR and 13C NMR spectra were measured on a Bruker Avance II+ 400 MHz
and 300 MHz spectrometers. Chemical shifts are reported in parts per million (ppm)
downfield from an internal standard.
4.2 General procedure for the reduction of liquid aldehydes with the system
PhSiH3/[CpRu(PPh3)2Cl]
To a solution of [CpRu(PPh3)2Cl] (1 mol%) and liquid aldehyde (1.0 mmol) was added
PhSiH3 (1.2 mmol). The reaction mixture was stirred at 80 ºC under an air atmosphere
(the reaction times are indicated in Table 4). Then, TBAF (1.0 mmol) was added and the
reaction mixture was stirred at room temperature during 30 minutes. After evaporation,
the reaction mixture was purified by silica gel column chromatography with ethyl
acetate:n-hexane (1:3) to afford the corresponding alcohols.
4.3 General procedure for the reduction of solid aldehydes with the system
PhSiH3/[CpRu(PPh3)2Cl]
To a solution of [CpRu(PPh3)2Cl] (1 mol%) and solid aldehyde (1.0 mmol) in toluene (3
ml) was added PhSiH3 (1.2 mmol). The reaction mixture was stirred at reflux
temperature under an air atmosphere (the reaction times are indicated in Table 4).
Then, TBAF (1.0 mmol) was added and the reaction mixture was stirred at room
temperature during 30 minutes. After evaporation, the reaction mixture was purified by
silica gel column chromatography with ethyl acetate:n-hexane (1:3) to afford the
corresponding alcohols.
4.4 General procedure for the reduction of ketones with the system
PhSiH3/[CpRu(PPh3)2Cl]
To a solution of [CpRu(PPh3)2Cl] (5 mol%) and ketones (1.0 mmol) in toluene (3 ml)
was added PhSiH3 (1.2 mmol). The reaction mixture was stirred at reflux temperature
under an air atmosphere (the reaction times are indicated in Table 4). Then, TBAF (1.0
mmol) was added and the reaction mixture was stirred at room temperature during 30
minutes. After evaporation, the reaction mixture was purified by silica gel column
chromatography with ethyl acetate:n-hexane (1:3) to afford the corresponding alcohols.
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4.5 NMR characterization of products
1
Table 4, entry 1: H NMR (CDCl3, 400.1 MHz) δ: 7.31-7.25 (m, 4H), 4.65 (s, 2H),
2.49 (s, 3H), 1.61 (brs, 1H) ppm. 13C NMR (CDCl3, 100.6 MHz) δ: 138.0, 137.9, 127.8,
127.0, 65.1, 16.1 ppm.
Table 4, entry 2: 1H NMR (CDCl3, 400.1 MHz) δ: 7.58 (d, J = 8.0 Hz, 2H), 7.42 (d, J =
8.0 Hz, 2H), 4.70 (s, 2H), 2.57 (brs, 1H) ppm. 13C NMR (CDCl3, 100.6 MHz) δ: 144.9,
132.4, 130.8, 130.0, 129.7, 128.9, 127.0, 126.9, 125.6, 125.5, 125.4, 125.3, 122.9,
116.2, 64.4 ppm.
Table 4, entry 3: 1H NMR (CDCl3, 400.1 MHz) δ: 7.59 (d, J = 8.0 Hz, 2H), 7.43 (d, J =
8.0 Hz, 2H), 4.71 (s, 2H), 2.88 (brs, 1H) ppm. 13C NMR (CDCl3, 100.6 MHz) δ: 146.7,
132.2, 127.0, 119.0, 110.8, 63.9 ppm.
1
Table 4, entry 4: H NMR (CDCl3, 400.1 MHz) δ: 7.33-7.30 (m, 2H), 7.03 (t, J = 8.8
Hz, 2H), 4.63 (s, 2H), 1.94 (brs, 1H) ppm. 13C NMR (CDCl3, 100.6 MHz) δ: 163.6,
161.2, 136.7, 136.7, 128.9, 128.8, 115.6, 115.4, 64.7 ppm.
Table 4, entry 5: 1H NMR (CDCl3, 400.1 MHz) δ: 7.49 (d, J = 8.0 Hz, 2H), 7.25 (d, J =
8.0 Hz, 2H), 4.66 (d, J = 4.8 Hz, 2H), 1.67 (brs, 1H) ppm. 13C NMR (CDCl3, 100.6
MHz) δ: 139.9, 131.8, 128.7, 121.6, 64.8 ppm.
Table 4, entry 6: 1H NMR (CDCl3, 400.1 MHz) δ: 7.29 (d, J = 8.4 Hz, 2H), 6.90 (d, J =
13
8.8 Hz, 2H), 4.61 (s, 2H), 3,82 (s, 3H), 2.08 (brs, 1H) ppm. C NMR (CDCl3, 100.6
MHz) δ: 159.3, 133.3, 1128.7, 114.0, 65.0, 55.4 ppm.
1
Table 4, entry 7: H NMR (CDCl3, 400.1 MHz) δ: 7.31-7.17 (m, 5H), 3.68 (t, J = 6.4
13
Hz, 2H), 2,71 (t, J = 7.2 Hz, 8.0 Hz, 2H), 1,93-1.87 (m, 2H), 1.43 (brs, 1H) ppm. C
NMR (CDCl3, 100.6 MHz) δ: 141.9, 128.5, 128.4, 125.9, 62.2, 34.3, 32.1 ppm.
Table 4, entry 8: 1H NMR (CDCl3, 400.1 MHz) δ: 7.38 (d, J = 7.2 Hz, 2H), 7.31 (t, J =
7.2 Hz, 7.6 Hz, 2H), 7.26-7.24 (m, 1H), 6.60 (d, J = 16 Hz, 1H), 6.39-6.32 (m, 1H),
4.31 (dd, J = 1.2 Hz, 4.4 Hz, 2H), 1.78 (brs, 1H) ppm. 13C NMR (CDCl3, 100.6 MHz) δ:
136.8, 131.2, 128.7, 128.6, 128.5, 128.4, 127.8, 126.6, 63.8 ppm.
11

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Table 4, entry 9: 1H NMR (CDCl3, 400.1 MHz) δ: 7.98 (d, J = 8.0 Hz, 2H), 7.39 (d, J =
13
7.6 Hz, 2H), 4.73 (s, 2H), 3.89 (s, 3H), 2.46 (brs, 1H) ppm. C NMR (CDCl3, 100.6
MHz) δ: 167.2, 146.2, 129.9, 129.3, 126.6, 64.7, 52.2 ppm.
1
Table 4, entry 10: H NMR (CDCl
3
, 400 MHz) δ: 7.39 (t, J = 0.8 Hz, 1H), 6.32-6.33
13
(m, 1H), 6.26 (d, J = 3.2 Hz, 1H), 4.56 (s, 2H), 2.61 (s, 1H) ppm. C NMR (CDCl3,
100.6 MHz) δ: 154.2, 142.5, 110.4, 107.6, 56.9 ppm.
Table 4, entry 11: 1H NMR (CDCl3, 400.1 MHz) δ: 4.33 (s, 2H), 4.24 (s, 2H), 4.18 (s,
13
7H), 1,55 (brs, 1H) ppm. C NMR (CDCl3, 100.6 MHz) δ: 88.6, 68.5, 68.4, 68.0, 60.9
ppm.
Table 4, entry 12: 1H NMR (CDCl3, 400.1 MHz) δ: 6.92-6.85 (m, 3H), 5.62 (brs, 1H),
13
4.60 (s, 2H), 3.90 (s, 3H) ppm. C NMR (CDCl3, 100.6 MHz) δ: 146.8, 145.3, 133.1,
120.4, 114.4, 110.1, 65.6, 56.1 ppm.
Table 4, entry 13: 1H NMR (CDCl3, 400.1 MHz) δ: 7.55 (d, J = 8.0 Hz, 2H), 7.40 (d, J
= 8.0 Hz, 2H), 4.83 (q, J = 8.0 Hz, 1H), 3.11 (brs, 1H), 1.42 (d, J = 8.0Hz, 3H) ppm. 13C
NMR (CDCl3, 100.6 MHz) δ: 149.8, 125.8, 125.6, 125.5, 125.4, 125.3, 69.8, 25.3 ppm.
Table 4, entry 14: 1H NMR (CDCl3, 400.1 MHz) δ: 7.41 (d, J = 8.0 Hz, 2H), 7.16 (d, J
= 8.0 Hz, 2H), 4.74 (q, J = 8.0 Hz, 1H), 2,79 (brs, 1H), 1.38 (d, J = 8.0 Hz, 3H) ppm.
13C NMR (CDCl3, 100.6 MHz) δ: 144.8, 131.5, 127.2, 121.1, 69.6, 25.2 ppm.
Table 4, entry 15: 1H NMR (CDCl3, 400.1 MHz) δ: 7.50 (d, J = 8.0 Hz, 1H), 7.45-7.36
(m, 4H), 7.20 (t, J = 8.0 Hz, 1H), 6.98 (t, J = 8.0 Hz, 1H) , 6.89 (d, J = 6.0 Hz, 1H),
5.19-5.10 (m, 2H), 2.52 (dd, J = 4.0 Hz, 1H), 2.15 (q, J = 12.0 Hz, 1H), 1.72 (t, J = 8.0
Hz, 1H) ppm. 13C NMR (CDCl3, 100.6 MHz) δ: 154.5, 140.5, 129.2, 128.7, 128.2,
127.0, 126.1, 125.7, 121.0, 116.7, 76.8, 65.8, 40.0 ppm.
Table 4, entry 16: 1H NMR (CDCl3, 400 MHz) δ: 7.23 (d, J = 8.0 Hz, 2H), 7.13 (d, J =
8.0 Hz, 2H), 4.82 (q, J = 8.0 Hz, 1H), 2.33 (s, 3H), 2.07 (brs, 1H), 1.45 (d, J = 8.0 Hz,
13
3H) ppm. C NMR (100 MHz, CDCl3) δ: 143.0, 137.2,129.2, 125.5, 70.3, 25.2, 21.2
ppm.
Table 4, entry 17: 1H NMR (CDCl3, 400 MHz) δ: 7.25 (d, J = 8.0 Hz, 2H), 6.85 (d, J =
8.0 Hz, 2H), 4.79 (q, J = 4.0 Hz, 1H), 3.76 (s, 3H), 2.30 (brs, 1H), 1.43 (d, J = 8.0 Hz,
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13
3H) ppm. C NMR (100 MHz, CDCl3) δ: 159.0, 138.1, 126.7, 113.9, 69.9, 55.3, 25.1
ppm.
Table 4, entry 18: 1H NMR (CDCl3, 400 MHz) δ: 7.62 (d, J = 8.0 Hz, 2H), 7.05 (d, J =
8.0 Hz, 2H), 4.75 (q, J = 4.0 Hz, 1H), 2.60 (brs, 1H), 1.39 (d, J = 4.0 Hz, 3H) ppm. 13C
NMR (100 MHz, CDCl3) δ: 145.3, 137.5, 127.5, 125.4, 69.7, 25.3 ppm.
4.6 Use of the catalyst [CpRu(PPh3)2Cl] in several catalytic cycles
To a solution of [CpRu(PPh3)2Cl] (1 mol%) and 4-(methylthio)benzaldehyde (0.5
mmol) was added PhSiH3 (0.6 mmol). The reaction mixture was stirred at 80 ºC under
an air atmosphere. The reaction mixture was cooled and the yield was determined by 1H
NMR spectroscopy using mesitylene (0.5 mmol) as the internal standard. In the next
catalytic cycles, the aldehyde (0.5 mmol), phenylsilane (0.6 mmol) and mesitylene (0.5
mmol) were added to the reaction mixture and stirred at 80 ºC. The yields were
determined by 1H NMR spectroscopy.
4.7 Synthesis of deuterated alcohol 2
To a solution of [CpRu(PPh3)2Cl] (1 mol%) and 4-(methylthio)benzaldehyde (1.0
mmol) was added PhMe2SiD (1.2 mmol). The reaction mixture was stirred at 80 ºC
during 24 h under an air atmosphere. Then, TBAF (1.0 mmol) was added and the
reaction mixture was stirred at room temperature during 30 minutes. After evaporation,
the reaction mixture was purified by silica gel column chromatography with ethyl
acetate:n-hexane (1:3), giving the deuterated alcohol 2 in 60% yield). 1H NMR (CDCl3,
400 MHz) δ: 7.28-7.23 (m, 4H), 4.60 (s, 1H), 2.47 (s, 3H), 1.89 (brs, 1H) ppm. 13C
NMR (100 MHz, CDCl3) δ: 137.9, 137.8, 127.8, 127.0, 64.8, 16.1 ppm.
Acknowledgments
This research was supported by Fundação para a Ciência e Tecnologia (FCT) through
project PTDC/QUI-QUI/110080/2009. Ivânia Cabrita (SFRH/BD/74280/2010) and
Pedro Florindo thank FCT for grants. ACF (IF/00849/2012) acknowledges FCT for the
“Investigador FCT” Program. Authors thank the project UID/QUI/00100/2013 and the
Portuguese NMR Network (IST-UTL Center) for providing access to the NMR
facilities.
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Highlights
CpRu(PPh3)2Cl is highly efficient catalyst for the reduction of carbonyl compounds
The reductions can be performed under solvent free conditions
The catalyst CpRu(PPh3)2Cl can be use in 12 catalytic cycles with excellent activity