MCl2(ampy)(dppf) (M = Ru, Os): Multitasking catalysts for carbonyl compound/alcohol interconversion reactions

Article abstract of DOI:10.1021/om201189r

The easily accessible complexes [MCl₂(dppf)(ampy)] (M = Ru (cis-1), Os (trans-2); dppf = 1,1′-bis(diphenylphosphino)ferrocene; ampy = 2-aminomethylpyridine) in the presence of base (NaOiPr, KOtBu) are efficient catalysts for several reactions involving carbonyl compounds and alcohols. The derivatives 1 and 2 catalyze the selective transfer hydrogenation of aldehydes and ketones to alcohols with 2-propanol using 0.1-0.005 mol % of catalyst at 82 °C with TOF values up to 3.0 × 10⁵ h^-1. These compounds (0.1-0.02 mol %) promote the hydrogenation of aldehydes and ketones in EtOH or a MeOH/EtOH mixture at 30-90 °C (5 atm of H₂) and the acceptorless dehydrogenation of alcohols to ketones in tBuOH at 130-145 °C (0.4 mol %). Complexes 1 and 2 (0.5 mol %) catalyze the racemization of chiral alcohols at 70 °C in 2-propanol and the isomerization of allylic alcohols to ketones in tBuOH at 70-120 °C (1 mol %). In addition, 1 and 2 (0.5 mol %) promote the α alkylation of α-tetralone with primary alcohols (EtOH, nPrOH, and nBuOH) at 120 °C in a tBuOH/toluene mixture (1/2 v/v). Complex 2 is easily obtained in 93% yield from [OsCl₂(PPh₃) ₃], dppf, and ampy in toluene.

Full text of DOI:10.1021/om201189r

Article
pubs.acs.org/Organometallics
MCl2(ampy)(dppf) (M = Ru, Os): Multitasking Catalysts for Carbonyl
Compound/Alcohol Interconversion Reactions
Elisabetta Putignano, Gianluca Bossi, Pierluigi Rigo, and Walter Baratta*
̀
Dipartimento di Chimica, Fisica e Ambiente, Universita di Udine, Via Cotonificio 108, I-33100 Udine, Italy
S
* Supporting Information
ABSTRACT: The easily accessible complexes [MCl2(dppf)-
(ampy)] (M = Ru (cis-1), Os (trans-2); dppf = 1,1′-
bis(diphenylphosphino)ferrocene; ampy = 2-aminomethylpyr-
idine) in the presence of base (NaOiPr, KOtBu) are efficient
catalysts for several reactions involving carbonyl compounds and
alcohols. The derivatives 1 and 2 catalyze the selective transfer
hydrogenation of aldehydes and ketones to alcohols with 2-
propanol using 0.1−0.005 mol % of catalyst at 82 °C with TOF
values up to 3.0 × 105 h−1. These compounds (0.1−0.02 mol %)
promote the hydrogenation of aldehydes and ketones in EtOH
or a MeOH/EtOH mixture at 30−90 °C (5 atm of H2) and the
acceptorless dehydrogenation of alcohols to ketones in tBuOH at
130−145 °C (0.4 mol %). Complexes 1 and 2 (0.5 mol %)
catalyze the racemization of chiral alcohols at 70 °C in 2-
propanol and the isomerization of allylic alcohols to ketones in tBuOH at 70−120 °C (1 mol %). In addition, 1 and 2 (0.5 mol
%) promote the α alkylation of α-tetralone with primary alcohols (EtOH, nPrOH, and nBuOH) at 120 °C in a tBuOH/toluene
mixture (1/2 v/v). Complex 2 is easily obtained in 93% yield from [OsCl2(PPh3)3], dppf, and ampy in toluene.
INTRODUCTION
■
The search for highly efficient transition-metal catalysts is a
current issue for applications in the synthesis of valuable
organic compounds. In this context ruthenium has become one
of the preferred metals because of its high performance and
versatility for a variety of organic catalyzed reactions. High
selectivity and productivity, which are crucial parameters in
catalysis, can be achieved through an appropriate design of the
ligands, resulting in the formation of structurally well-defined
catalysts. Nevertheless, there is also a great interest in
multitasking catalysts able to efficiently promote different
organic transformations by a careful switching of the reaction
parameters: namely temperature, solvent, and cocatalyst
concentration. Moreover, to make catalysts appealing for
industrial applications, a straightforward preparation of the
catalysts is highly desirable. As regards the reduction of
carbonyl compounds, outstanding results have been obtained
by Noyori in the enantioselective hydrogenation1 and transfer
hydrogenation2 of carbonyl compounds, leading to the isolation
of the systems trans-[RuCl2(PP)(1,2-diamine)]3 (A; PP =
diphosphine) and [(η6-arene)RuCl(Tsdpen)] (B),4 respec-
tively (Figure 1).
Figure 1. Ru and Os catalysts for TH and HY reactions.
for both transfer hydrogenation (TH) and hydrogenation (HY)
reactions. It is worth noting that the ampy ligand, having the
NH2 function and the pyridine moiety, leads to the acceleration
of both catalytic TH and HY reactions and allows reduction of
bulky substrates (e.g., tert-alkyl ketones), on account of the flat
pyridine ring that facilitates the approach of the ketone.
Another example is system B, originally developed for the
asymmetric TH of ketones, which was successfully applied for
Although no universal catalyst exists for different substrates
and for various organic transformations, the complexes [(η6-
arene)Ru(OTf)(Tsdpen)] (C)5 and [MCl2(PP)(ampy)] (D;
M = Ru,6 Os;7 ampy = 2-aminomethylpyridine) and the related
pincer [MCl(CNN)(PP)]8 (E; HCNN = 1-(6-arylpyridin-2-
yl)methanamine) were demonstrated to be efficient catalysts
Received: November 29, 2011
Published: January 26, 2012
© 2012 American Chemical Society
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Scheme 1. Catalytic Reactions Catalyzed by the Ruthenium and Osmium Complexes cis-1 and trans-2
enantioselective C−C bond formation reactions with a wide
ampy (2 h), affords the osmium derivative trans-[OsCl2(dppf)-
(ampy)] (2), which was isolated in 93% yield (eq 1).
scope.9
Despite the great interest in the asymmetric reduction of
ketones,10 the design of novel, highly productive, and easily
accessible nonchiral catalysts for the preparation of primary and
rac secondary alcohols from carbonyl compounds is a topic of
industrial relevance. In this regard, only few catalysts have been
reported to efficiently catalyze the chemoselective reduction of
aldehydes via TH.11 It is worth noting that the complexes
[MCl2(dppf)(en)] (M = Ru, Os; en = ethylenediamine),
containing 1,1′-bis(diphenylphosphino)ferrocene diphosphine
(dppf), have recently been found to be active in the
acceptorless dehydrogenation (DHY)12 of alcohols to
ketones.13 The presence of the flexible ferrocenyl diphosphine
allows the formation of catalytically active species which are
thermally more stable and productive with respect to the
analogous derivatives having diphosphines with an alkyl
backbone. Therefore, it is expected that these catalysts, which
can easily promote C−H activation, can enhance the alcohol
reactivity, giving access to new reactions: i.e., via hydrogen
borrowing.14
We report here that the complexes [MCl2(dppf)(ampy)] (M
= Ru (cis-1), Os (trans-2)) which are easily obtained in one-pot
reactions from commercially available compounds, are efficient
catalysts for a range of organic transformations. The complexes
1 and 2 in the presence of a base catalyze the fast and
productive TH and HY (5 atm of H2) of aldehydes and ketones
(TOF up to 3.0 × 105 h−1) with 0.1−0.0005 mol % loading, the
acceptorless DHY of alcohols, the racemization of secondary
alcohols, the isomerization of allylic alcohols, and ketone α-
alkylation with primary alcohols (Scheme 1). The catalytic
activities of the Ru and Os complexes are compared here.
The 31P{1H} NMR spectrum (CD2Cl2) of 2 displays two
doublets at δ −7.0 and −17.9 with 2J(P,P) = 19.7 Hz. In the 1H
NMR spectrum the two protons of the ampy CH2 group lead
to one signal at δ 4.39, whereas the resonance for the NH2
protons is at δ 3.59, which is consistent with a complex of trans
1
configuration. Notably, the H and 13C NMR data of 2 are
much the same as those of the corresponding trans-
[RuCl2(dppf)(ampy)], which is the kinetic product in the
formation of the cis-1 derivative.13 Attempts to prepare cis-
[OsCl2(dppf)(ampy)] by heating a mesitylene solution of 2 at
150 °C (3 h) results in the partial conversion of 2 and
formation of two uncharacterized Os dppf species, as inferred
from 31P NMR measurements.15
Transfer Hydrogenation of Aldehydes and Ketones.
The ruthenium and osmium complexes 1 and 2 are extremely
efficient catalysts for the TH of both aldehydes and ketones in
basic 2-propanol (eq 2).
RESULTS AND DISCUSSION
■
Synthesis of the Osmium Complex 2. The ruthenium
complex cis-[RuCl2(dppf)(ampy)] (1) was easily prepared in
high yield from [RuCl2(PPh3)3], dppf, and ampy, according to
the literature method.13 Treatment of [OsCl2(PPh3)3] with 1
equiv of dppf in toluene at 50 °C (2 h), followed by addition of
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Table 1. TH of Aldehydes and Ketones (0.1 M) with 1 and 2 with NaOiPr (2.0 mol %) in 2-Propanol at Reflux Temperature
a
The conversion and TOF (moles of aldehyde or ketone converted into alcohol per mole of catalyst per hour at 50% conversion) were determined
b
c
d
by GC analysis. NaOiPr 1.0 mol %. (+)-neomenthol/(−)-menthol =3.2. (+)-neomenthol/(−)-menthol =2.
Using a catalyst loading of 0.1−0.0005 mol % and in the
presence of NaOiPr (2 mol %), quantitative conversion of
several carbonyl compounds has been achieved in a short time,
achieving TOF values up to 3 × 105 h−1 at 82 °C (Table 1).
With 0.005 mol % of 1 benzaldehyde and p-anisaldehyde are
efficiently reduced to the corresponding primary alcohols in 2
and 1 h, respectively (TOF up to 5.4 × 104 h−1). The latter
substrate is also reduced (91%, 6 h) using a remarkably low
amount of 1 (0.0005 mol %). Interestingly, the osmium
complex 2 (0.005 mol %) displays a higher activity for these
aromatic aldehydes, leading to quantitative conversion in 5 min
with TOF values up to 3.0 × 105 h−1. Aliphatic aldehydes such
as hexanal and 2-methylbutanal are also promptly converted
into alcohols using 1 and 2 (0.05−0.005 mol % TOF up to 1.5
× 105 h−1). Under these catalytic conditions, the unsaturated
aldehydes citronellal and trans-cinnamaldehyde have selectively
been reduced at the CO bond using 1 and 2, without
hydrogenation or isomerization of the CC bond (TOF =
103−104 h−1). These results indicate that the Os complex
efficiently catalyzes the reduction of aldehydes, with rates
comparable to or even higher than those of the Ru complex. To
show the catalytic potential of these complexes, p-anisyl alcohol
(1.74 g) has been isolated in 90% yield, starting from p-
anisaldehyde and using 0.05 mol % of 1 (30 min at 82 °C),
whereas citronellol (1.92 g) has been prepared in 88% yield
with 2 (2 h). The ketones acetophenone, 4-methoxyacetophe-
none, and the bulkier 2,2-dimethylpropiophenone have also
been reduced to secondary alcohols with 1 and 2 at 0.1−0.002
mol %, affording TOF = 103−105 h−1. It is worth noting that
employment of the isomer trans-[RuCl2(dppf)ampy] (0.002
mol %) leads to TH of acetophenone with only 43%
conversion after 20 h, whereas 1 gives 92%, indicating that
the cis derivative is more active than the trans analogue, in
accordance with the literature data.6a The diaryl ketone
benzophenone has also quantitatively reduced to benzhydrol
with both 1 and 2 (0.005 mol %), the Os complex being more
efficient. Also, the cyclic derivative (−)-menthone was rapidly
reduced with 1 and 2 (0.05 mol %) in 10 min. With 1 the ratio
(+)-neomenthol/(−)-menthol was 3.2, whereas with 2 the ratio
was 2. Finally, complexes 1 and 2 (0.05 mol %) catalyze the
selective TH of the unsaturated ketone 5-hexen-2-one with the
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Table 2. Influence of the Base on the TH of Aldehydes (0.1 M) with Complex 1 in 2-Propanol at Reflux Temperature
a
Conversion into the alcohol was determined by GC analysis.
formation of 5-hexen-2-ol, without reduction at the CC
bond, 2 displaying a higher rate with respect to 1.
alcohol and p-anisyl alcohol at 50 °C, using 0.02 mol % of 1 in
10 and 5 min, respectively (TOF = 7.5 × 104 and 5.0 × 104 h−1)
(Table 3).
The effect of the base in the TH of aldehydes has also been
investigated. With NaOiPr (2 mol %) and in the absence of 1,
the aromatic p-anisaldehyde undergoes reduction (96%
conversion) after 20 h, whereas with K2CO3 only 47%
conversion is obtained (Table 2).
With the osmium complex 2 (0.1 mol %) quantitative
conversion to alcohols has been achieved at 90 °C in 10 and 30
min (TOF up to 1.0 × 104 h−1). Phenylacetaldehyde is
efficiently converted into 2-phenylethanol (98% in 10 min)
using 1 (0.1 mol %), whereas with 2 at 90 °C, 91% conversion
is achieved after 8 h. The substrate trans-cinnamaldehyde is
reduced to cinnamyl alcohol, using 1 (0.02 mol %) at 50 °C in
60 min (TOF = 2.8 × 104 h−1) and with 2 (0.1 mol %) at 90
°C, with no hydrogenation at the CC bond. In addition, 2-
phenylethanol (1.04 g) was isolated in 85% yield using 0.02
mol % of 2 at 90 °C under 5 atm of H2 (8 h).
Addition of 1 (0.005 mol %), in the presence of NaOiPr,
increases the rate of the TH with production of the p-anisyl
alcohol in 1 h. As regards the aliphatic aldehydes, hexanal in the
presence of NaOiPr and without 1 is poorly converted to
alcohol (3%) in 20 h. In the presence of 1 (0.005 mol %) the
conversions were 12 and 81% (20 h) with 0.5 and 1 mol % of
NaOiPr, respectively. The use of K2CO3, which is moderately
soluble in 2-propanol, leads to a lower conversion of alcohol
(9%). With 1, 2-methylbutyraldehyde is rapidly reduced to the
alcohol in the presence of NaOiPr (99%) in 10 min, whereas
with K2CO3 92% conversion was obtained in 20 h. These
results indicate that while aromatic aldehydes can be reduced in
basic 2-propanol, the addition of 1 dramatically increases the
rate of conversion. In contrast, aliphatic aldehydes necessitate
the presence of 1 with NaOiPr, the base K2CO3 being less
efficient. It is likely that the TH of carbonyl compounds
catalyzed by 1 and 2 involves the formation of the
[MHX(dppf)(ampy)] (M = Ru, Os; X = H, OR′) species, via
a β-hydrogen elimination reaction,16 similarly to the Ru pincer
complexes [RuX(CNN)(PP)], for which a reversible β-
hydrogen elimination has been observed.17 On account of the
easy cleavage of the alcohol α-C−H bond with 1 and 2, a
broadening of the alcohol reactivity is expected with these
complexes (see below).
In the HY of ketones the ruthenium 1 is catalytically active at
30−50 °C, leading to high conversion of acetophenone, 4-
methoxyacetophenone, and 4-chlorobenzophenone in 1−5 h
(93−98%). It is worth noting that 1 leads to 1-phenylethanol
(93%) in 60 min, whereas the analogous trans-[RuCl2(dppf)-
(ampy)] displays a lower activity, with 94% conversion in 20 h,
under the same catalytic conditions. With osmium complex 2,
quantitative HY of these ketones has been achieved in 10−30
min at 90 °C (TOF up to 9.0 × 103 h−1). As regards cyclic
ketones, cyclohexanone was reduced to cyclohexanol with 1 at
50 °C in 1 h, whereas with 2 at 90 °C the reaction was
complete after 15 min. The HY of (−)-menthone with 1 at 50
°C leads to poor conversion (35%, 3 h), whereas with 2 at 90
°C, 90% conversion is attained in 2 h, leading to (+)-neo-
menthol/(−)-menthol = 2. These data suggest that osmium is a
valid complement to ruthenium for both HY and TH reactions,
taking into account that osmium is more robust and generally
requires a higher temperature to become catalytically active.
Dehydrogenation of Alcohols. Catalytic alcohol dehy-
drogenation represents a straightforward route to prepare
carbonyl compounds through the direct formation of H2
without the need for oxidizing agents.12 Several ruthenium
complexes were described to be efficient catalysts for both
oxidation of alcohols to ketones18 and production of hydro-
gen.19 The derivatives 1 and 2 in the presence of KOtBu display
high catalytic activity in the dehydrogenation of secondary
alcohols to ketones in tert-butyl alcohol or a tert-butyl alcohol/
toluene mixture at reflux and in an open system (eq 4).
Hydrogenation of Aldehydes and Ketones. The
complexes 1 and 2 are catalytically active in the HY of
aldehydes and ketones at low hydrogen pressure (5 atm) using
0.1−0.02 mol % loading, affording TOF values up to 7.5 × 104
h−1 (eq 3).
With 1 (0.1−0.02 mol %) a rapid HY occurs in ethanol at
30−50 °C in the presence of NaOEt (2 mol %), whereas with 2
(0.1 mol %) good performances are achieved in a methanol/
ethanol mixture (3/1, v/v), using KOtBu (2 mol %) at 90 °C.
Benzaldehyde and p-anisaldehyde are quickly reduced to benzyl
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Table 3. HY of Carbonylic Compounds (0.5 M) with the Ru and Os Catalysts 1 and 2 (0.1 mol %) under 5 atm of H2
a
b
c
NaOEt (2 mol %) in ethanol. KOtBu (2 mol %) in methanol/ethanol (3/1, v/v). The conversion and TOF (moles of aldehyde or ketone
d
e
converted into alcohol per mole of catalyst per hour at 50% conversion) were determined by GC analysis. Catalyst 0.02 mol %. Reaction with
trans-[RuCl2(dppf)(ampy)]. (+)-neomenthol/(−)-menthol =2.
f
Table 4. DH of Alcohols with the Ru and Os Catalysts 1 and 2 (0.4 mol %) with KOtBu (2 mol %) in tert-Butyl Alcohol
a
The conversion and TOF (moles of alcohol converted into ketone per mole of catalyst per hour at 50% conversion) were determined by GC
b
analysis. The conversion and TOF were determined by NMR analysis of cholesterol (0.21 M) with 0.8 mol % of catalyst and KOtBu (4 mol %) in
tert-butyl alcohol/toluene (2/1, v/v).
With 1 (0.4 mol %) at 130 °C, α-tetralol (1.25 M) in tert-
butyl alcohol was efficiently converted into α-tetralone (3 h)
with TOF = 600 h−1 (Table 4). It is worth noting that the
analogous diamine derivative [RuCl2(dppf)(en)] afforded
dehydrogenation of α-tetralone with a lower rate (TOF =
200 h−1).13
The osmium catalyst 2 shows a lower activity with respect to
1, leading to 87% conversion in 20 h. The substrate 1-(4-
methoxyphenyl)ethanol alcohol is oxidized to 4-methoxyace-
tophenone in 4 h, whereas with 2 a longer reaction time is
required (20 h). The oxidation of sterols is a significant reaction
for the synthesis of 4-en-3-one steroidal compounds. Several
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routes were reported, including the use of Cr(VI) and Mn(IV)
oxidants,20 Oppenauer oxidation,21 and hydrogen transfer with
the Shvo catalyst using acetone.22 The acceptorless dehydro-
genation of cholest-5-en-3β-ol with 1 at 0.8 mol % at 145 °C in
a tert-butyl alcohol/toluene mixture (2/1, v/v), leads to cholest-
4-en-3-one quantitatively in 3 h. A faster reaction has been
observed with the osmium catalyst 2, for which the formation
of the steroid compound has been achieved in 1 h, affording a
remarkably high rate (TOF = 210 h−1). These data show that
the ampy complexes 1 and 2 display a significantly higher
activity for dehydrogenation, with respect to the analogous
diamine compounds [MCl2(dppf)(en)] (M = Ru, Os) (TOF =
15 h−1 for cholest-5-en-3β-ol) and the CNN pincer complexes
[MCl(CNN)(PP)], with an increase of the reaction rate of up
to 1 order of magnitude.13
through a reversible hydrogen transfer process. In the
racemization, no formation of side products such as ketones
was observed, since the reaction was carried out in 2-propanol,
which acts as a hydrogen donor.
Isomerization of Allylic Alcohols. The catalytic isomer-
ization of allylic alcohols to ketones is an interesting route for
the preparation of carbonyl compounds. Among the several
transition-metal catalysts which have been developed for this
transformation,25 particular attention has been devoted to
ruthenium, which led to the most active systems.26 Recently,
cyclopentadienyl osmium complexes with pendant amine
ligands were found to be active in the alcohol allylic
isomerization.27 Complexes 1 and 2 (1 mol %) with KOtBu
(2 mol %) in tert-butyl alcohol catalyzes the isomerization of
monosubstituted aliphatic allylic alcohols (eq 6).
Racemization of Secondary Alcohols. Chiral alcohols
can be converted to racemates by several transition-metal
complexes. This catalytic transformation is particularly relevant
for dynamic kinetic resolution in which a lipase is combined
with a ruthenium catalyst for the preparation of chiral
alcohols.23 Recently, the pincer complexes [MCl(CNN)(PP)]
(M = Ru, Os) were found to be active in the racemization of
chiral alcohols.24 Since 1 and 2 are efficient catalysts for both
TH and DHY reactions, these complexes were investigated for
the racemization of chiral alcohols (eq 5).
Thus, with 1 3-buten-2-ol (0.33 M) gave 2-butanone in 64,
90, and 94% conversion after 1, 5, and 10 min at 70 °C,
respectively, affording TOF = 6000 h−1 (Table 6). The
substrates 1-penten-3-ol and 1-hepten-3-ol are isomerized to
3-pentanone and 3-heptanone in 30 min with TOF values of
480 and 440 h−1, respectively.
The substrate 1-phenyl-2-propen-1-ol is isomerized to
propiophenone in 2 h at 120 °C, while at 70 °C the reaction
is sluggish. With the osmium complex 2, the substrates 3-buten-
2-ol, 1-penten-3-ol, 1-hepten-3-ol, and 1-phenyl-2-propen-1-ol
were converted into ketones at 120 °C with TOF values up to
460 h−1. These data clearly indicate that osmium complex 2
displays catalytic activity at higher temperature with respect to
the ruthenium catalyst 1. The GC and NMR data for these
reactions showed that the yield is much the same as the
conversion and no other products were detected, indicating
that the isomerization occurs with high selectivity.
Reaction of (R)-1-phenylethanol (0.33 M) in 2-propanol at
70 °C with 1 (0.5 mol %) and in the presence of NaOiPr (2.0
mol %), leads to 18% ee in 5 min, whereas complete
racemization is achieved after 10 min (Table 5).
Table 5. Racemization of Chiral Alcohols (0.33 M) with the
Ru and Os Catalysts 1 and 2 (0.5 mol %) with NaOiPr (2.0
mol %) in 2-Propanol at 70 °C
α-Alkylation of α-Tetralone. The ruthenium complexes
[RuCl2(PPh3)3]28 and [Ru(DMSO)4Cl2]29 and the iridium
30
system [Ir(COD)Cl]2/PPh3 have been described to catalyze
the α-alkylation of ketones with primary alcohols. This reaction,
which occurs through a step-economical synthesis, is an
attractive route which avoids the use of toxic alkylating (e.g.,
organohalides) reagents. The transformation can be envisaged
as a sequence of oxidation of alcohol/aldol condensation/
reduction of the unsaturated ketone (Scheme 2).12a
Complexes 1 and 2 (0.5 mol %) efficiently catalyze the
alkylation of α-tetralone with several primary alcohols at 120
°C and in the presence of KOtBu (30 mol %) (eq 7). Reaction
a
The conversion was determined by chiral GC analysis.
Complex 2 is also catalytically active, affording 54% ee in 10
min and 0% ee in 30 min. The aliphatic alcohol (S)-2-butanol
with 1 gives 46 and 22% ee in 5 and 10 min, respectively, and
complete racemization in 30 min. With 2 the reaction is slower
(64 and 21% ee in 10 and 30 min), leading to the racemate in 1
h. It is worth noting that the analogous chiral derivatives cis-
[RuCl2(Josiphos)(R-ampy)]6b were found to be extremely
active catalysts for the asymmetric TH of methyl aryl ketones,
giving alcohols with up to 99% ee. These data suggest that the
alcohol racemization as well as the enantioselective ketone
reduction, catalyzed by the ampy Ru and Os complexes, occur
of α-tetralone with EtOH (3 equiv) in the presence of 1 leads
to 96% of the alkylated ketone in 4 h (Table 7).
With nPrOH and nBuOH the conversions were 87 and 92%
in 2 and 3 h, respectively, with TOF values up to 420 h−1. The
alkylated products were characterized by NMR analysis, which
revealed that the alkylation occurs selectively, and the data of
the compounds were compared with those reported in the
literature.28b,31Apparently, no example of alkylation of ketones
with simple alcohols (EtOH and nPrOH) was previously
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Table 6. Isomerization of Allylic Alcohols (0.33 M) with the Ru and Os Catalysts 1 and 2 (1 mol %) with KOtBu (2 mol %) in
tert-Butyl Alcohol
a
The conversion and TOF (moles of allylic alcohol converted into ketone per mole of catalyst per hour at 50% conversion) were determined by GC
analysis.
Scheme 2. α-Alkylation of Ketones Using Primary Alcohols
double bond or via reduction of the CO bond, followed by
isomerization of the allylic alcohol.
CONCLUDING REMARKS
■
We have found that the Ru and Os complexes [MCl2(dppf)-
(ampy)] (M = Ru (cis-1), Os (trans-2)), bearing the 2-
aminomethylpyridine (ampy) ligand, efficiently catalyze a
variety of organic transformations involving ketones and
alcohols, namely (a) fast and productive reduction of aldehydes
and ketones to alcohols via both transfer hydrogenation with 2-
propanol and hydrogenation with H2, (b) dehydrogenation of
alcohols (cholesterol) to ketones, (c) racemization of chiral
alcohols, (d) isomerization of allylic alcohols to ketones, and
(e) α-alkylation of α-tetralone with primary alcohols. The fast
rate of the transfer hydrogenation with 2-propanol (TOF up to
105 h−1) indicates that complexes 1 and 2 efficiently promote
the cleavage of the α-C−H bond of the alcohols, leading to a
broad alcohol reactivity. The comparison of the activity of the
Ru and Os complexes 1 and 2 shows that 2 usually requires a
higher temperature to catalyze the reactions and can lead to
better performance with respect to 1. The straightforward
synthesis of 1 and 2 makes these efficient catalysts attractive for
applications in several organic transformations.
described; most of the reactions involved the use of nBuOH or
PhCH2OH.12a The relatively low loading of catalyst and the
high rate indicates that 1 is an efficient system for this reaction.
Surprisingly, the osmium derivative 2 displays a higher activity
compared to 1. Thus, the alkylation of α-tetralone with EtOH,
nPrOH, and nBuOH is attained in 1 h, 30 min, and 10 min,
respectively, with TOF values up to 2800 h−1. No, example of
the use of Os in the α-alkylation of ketones has been reported.
It is worth noting that the reduction of the unsaturated ketone
intermediate may occur through hydrogenation at the CC
Table 7. α-Alkylation of α-Tetralone (0.33 M) with Primary Alcohols (3 equiv) using the Ru and Os Catalysts 1 and 2 (0.5 mol
%) with KOtBu (30 mol %) in tert-Butyl Alcohol/Toluene (1/2, v/v) at 120 °C
a
The conversion and TOF (moles of ketone converted into α-alkylated ketone per mole of catalyst per hour at 50% conversion) were determined by
NMR analysis.
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Article
filtered over a short silica pad, and the conversion was determined by
GC analysis (Ru 0.02 mol %, NaOEt 2 mol %, substrate 0.5 M).
Typical Procedure for Catalytic Hydrogenation of Alde-
hydes and Ketones with 2. Complex 2 (2.0 mg, 2.2 μmol) was
dissolved in methanol (2 mL). The substrate (2.0 mmol), KOtBu (4.5
mg, 0.04 mmol), and 1.82 mL of the catalyst solution (2.0 μmol) were
added to a methanol (1.2 mL)/ethanol (1 mL) mixture (final volume
of the solution 4 mL; MeOH/EtOH 3/1, v/v). The solution was
transferred into a thermostated reactor, and the reduction was
performed by introducing H2 at 5 atm. The reaction was sampled by
removing an aliquot of the reaction mixture, and diethyl ether was
added (1/1, v/v). The solution was filtered over a short silica pad, and
the conversion was determined by GC analysis (Os 0.1 mol %, KOtBu
2 mol %, substrate 0.5 M).
Preparation of 2-Phenylethanol by Catalytic Hydrogenation
with 2. Complex 2 (18.5 mg, 0.02 mmol) was dissolved in 10 mL of a
MeOH/EtOH mixture (3/1, v/v). After addition of phenyl-
acetaldehyde (1.20 g, 10 mmol) and KOtBu (22.4 mg, 0.2 mmol)
the solution was transferred into a thermostated reactor at 90 °C for 8
h and the reduction was performed by introducing H2 at 5 atm.
Diethyl ether was added to the solution (1/1, v/v), the mixture was
filtered through a silica pad, and the solvent was evaporated to afford
the product (1.04 g, 85% yield).
EXPERIMENTAL SECTION
■
All reactions were carried out under an argon atmosphere using
standard Schlenk techniques. The solvents were carefully dried by
standard methods and distilled under argon before use. The
diphosphine dppf and all other chemicals were purchased from
Aldrich and used without further purification. The compounds cis-
[RuCl2(dppf)(ampy)] (1)13 and [OsCl2(PPh3)3]32 were prepared
according to the literature procedure. NMR measurements were
recorded on a Bruker AC 200 spectrometer, and the chemical shifts, in
1
ppm, are relative to TMS for H and 13C{1H} and 85% H3PO4 for
31P{1H}. Elemental analysis (C, H, N) was carried out with a Carlo
Erba 1106 elemental analyzer, whereas the GC analyses were
performed with a Varian GP-3380 gas chromatograph equipped with
a MEGADEX-ETTBDMS-β chiral column.
Synthesis of trans-[OsCl2(dppf)(ampy)] (2). [OsCl2(PPh3)3]
(100 mg, 0.095 mmol) and dppf (58 mg, 0.105 mmol) were treated
with toluene (1.5 mL), and the suspension was stirred for 2 h at 50 °C.
After addition of ampy (11 μL, 0.107 mmol) the suspension was
stirred for 2 h at 50 °C and then concentrated to about 0.5 mL.
Addition of heptane (2 mL) afforded a yellow precipitate, which was
washed with heptane (3 × 1 mL) and diethyl ether (3 × 1 mL) and
1
dried under reduced pressure. Yield: 82 mg (93%). H NMR (200.1
MHz, CD2Cl2, 20 °C): δ 8.54 (m, 1H; o-C5H4N), 8.08 (pseudo t,
J(H,H) = 8.7 Hz, 4H; aromatic protons), 7.60−7.13 (m, 18H;
aromatic protons), 6.58 (pseudo t, J(H,H) = 6.6 Hz, 1H; aromatic
proton), 4.77 (m, 2H; C5H4), 4.39 (m, 2H; CH2N), 4.22 (m, 2H;
C5H4), 4.18 (m, 2H; C5H4), 4.05 (m, 2H; C5H4), 3.59 (m, 2H; NH2).
13C{1H} NMR (50.3 MHz, CD2Cl2, 20 °C): δ 152.7 (d, J(C,P) = 3.0
Hz; NCH), 138.1−117.3 (m, aromatic carbons), 74.1 (d; J(C,P) = 8.2
Hz; C5H4), 72.0 (d; J(C,P) = 7.4 Hz; C5H4), 68.3 (d; J(C,P) = 6.1 Hz;
C5H4), 65.4 (d; J(C,P) = 6.3 Hz; C5H4), 46.9 (s, CH2). 31P{1H} NMR
(81.0 MHz, CD2Cl2, 20 °C): δ −7.0 (d, J(P,P) = 19.7 Hz), −17.9 (d,
J(P,P) = 19.7 Hz). Anal. Calcd for C40H36Cl2FeN2OsP2: C, 52.02; H,
3.93; N, 3.03. Found: C, 51.36; H, 3.88; N, 2.90.
Typical Procedure for the Catalytic Transfer Hydrogenation
of Aldehydes and Ketones. Complex 1 or 2 (1.2 μmol) was
dissolved in 3 mL of 2-propanol. The aldehyde or ketone (1 mmol)
was dissolved in 9 mL of 2-propanol, and the solution was refluxed
(100 °C bath temperature) under argon. After addition of 200 μL of
NaOiPr in 2-propanol (0.1 M; 0.02 mmol), 125 μL of the catalyst
solution (0.05 μmol), and 2-propanol (final volume of the solution 10
mL), the reduction of the aldehyde or the ketone started immediately.
The reaction was sampled by removing an aliquot of the reaction
mixture, and diethyl ether was added (1/1, v/v). The solution was
filtered over a short silica pad, and the conversion was determined by
GC analysis (Ru or Os 0.005 mol %, NaOiPr 2 mol %, aldehyde or
ketone 0.1 M).
Preparation of p-Anisyl Alcohol by Catalytic Transfer
Hydrogenation with 1. Complex 1 (5.8 mg, 7.0 μmol) was
dissolved in 50 mL of 2-propanol. After addition of p-anisaldehyde
(1.7 mL, 14.0 mmol) and NaOiPr (0.1 M, 2.8 mL, 0.28 mmol) the
solution was refluxed under argon for 30 min. Diethyl ether was added
to the solution (1/1, v/v), the mixture was filtered through a silica pad
and the solvent was evaporated to afford the product (1.74 g, 90%
yield).
Preparation of ( )-Citronellol by Catalytic Transfer Hydro-
genation with 2. ( )-β-Citronellol was obtained following the
procedure used for p-anisyl alcohol, by employment of ( )-citronellal
in place of p-anisaldehyde, complex 2 in place of 1, and reflux of the
solution for 2 h (1.92 g, 88% yield).
Typical Procedure for Catalytic Hydrogenation of Alde-
hydes and Ketones with 1. Complex 1 (2.0 mg, 2.4 μmol) was
dissolved in ethanol (2 mL). The substrate (2.0 mmol), 0.16 mL of
NaOEt in ethanol (0.25 M, 0.04 mmol), and 0.33 mL of the catalyst
solution (0.4 μmol) were added to ethanol (final volume of the
solution 4 mL). The solution was transferred into a thermostated
reactor, and the reduction was performed by introducing H2 at 5 atm.
The reaction was sampled by removing an aliquot of the reaction
mixture, and diethyl ether was added (1/1, v/v). The solution was
Typical Procedure for Catalytic Dehydrogenation of
Alcohols. The complex 1 or 2 (0.01 mmol) was dissolved in 2 mL
of tBuOH. After addition of the alcohol substrate (2.5 mmol) and
KOtBu (0.05 mmol) the solution was heated at 130 °C (bath
temperature) under argon in an open system. The reaction was
sampled by removing an aliquot of the reaction mixture, and diethyl
ether was added (1/1, v/v). The solution was filtered over a short
silica pad, and the conversion was determined by GC analysis (Ru or
Os 0.4 mol %, KOtBu 2 mol %, alcohol 1.25 M).
Typical Procedure for Catalytic Dehydrogenation of
Cholesterol. The complex 1 or 2 (5.0 μmol), cholesterol (242 mg;
0.626 mmol), and KOtBu (25.0 μmol) were dissolved in 2 mL of
tBuOH and 1 mL of toluene. The solution was heated at 145 °C (bath
temperature) under argon in an open system. The reaction was
sampled by removing an aliquot of the reaction mixture, and diethyl
ether was added (1/1, v/v). The solution was filtered over a short
silica pad, the solvent was evaporated, and the conversion was
determined by NMR analysis (Ru or Os 0.8 mol %, KOtBu 4 mol %,
sterol 0.21 M).
Typical Procedure for the Racemization of Chiral Alcohols.
The complex 1 or 2 (1.6 μmol), NaOiPr (66 μL, 0.1 M solution in 2-
propanol, 6.6 μmol), and the chiral alcohol (0.33 mmol) were
dissolved in 2-propanol (1 mL) under argon. The solution was stirred
at 70 °C. The reaction was sampled by removing an aliquot of the
reaction mixture, and diethyl ether was added (1/1 in volume). The
solution was filtered over a short silica pad, and the conversion was
determined by GC analysis (Ru or Os mol 0.5 mol %, NaOiPr 2 mol
%, chiral alcohol 0.33 M).
Typical Procedure for the Isomerization of Allylic Alcohols.
The complex 1 or 2 (0.01 mmol), the allylic alcohol (1 mmol), and
KOtBu (2.2 mg, 0.02 mmol) were dissolved in 3 mL of tBuOH. The
solution was heated at 70 °C (Ru) or 120 °C (Os, bath temperature)
under argon. The reaction was sampled by removing an aliquot of the
reaction mixture, and diethyl ether was added (1/1, v/v). The solution
was filtered over a short silica pad, and the conversion was determined
by GC analysis (Ru or Os 1 mol %, KOtBu 2 mol %, allylic alcohol
0.33 M).
Typical Procedure for the α-Alkylation of α-Tetralone with
Primary Alcohols. The complex 1 or 2 (5.0 μmol), α-tetralone
(146.2 mg, 1.0 mmol), the primary alcohol (3.0 mmol), and KOtBu
(34 mg, 0.30 mmol) were dissolved in 1 mL of tBuOH and 2 mL of
toluene. The solution was heated at 120 °C (bath temperature) under
argon. The reaction was sampled by removing an aliquot of the
reaction mixture, and diethyl ether was added (1/1, v/v). The solution
was filtered over a short silica pad, the solvent was evaporated, and the
conversion was determined by NMR analysis (Ru or Os 0.5 mol %,
KOtBu 30 mol %, α-tetralone 0.33 M).
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ASSOCIATED CONTENT
* Supporting Information
Text giving characterization data of the isolated alcohols and α-
alkylated ketones. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: walter.baratta@uniud.it.
■
ACKNOWLEDGMENTS
This work was supported by the Ministero dell’Universita
della Ricerca (MIUR) and the Regione Friuli Venezia Giulia.
We thank Johnson-Matthey/Alfa Aesar for a generous loan of
ruthenium and Mr. P. Polese for carrying out the elemental
analysis.
■
̀
e
(14) Nixon, T. D.; Whittlesey, M. K.; Williams, J. M. J. Dalton Trans.
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2
(15) 31P{1H} NMR: doublets at δ −3.3 and −11.2 with J(P,P) =
2
20.2 Hz and δ −5.1 and −9.8 with J(P,P) = 20.0 Hz.
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