Bench-Stable Manganese NHC Complexes for the Selective Reduction of Esters to Alcohols with Silanes

Article abstract of DOI:10.1002/adsc.202000148

Selective reduction of esters to alcohols was accomplished through Mn(I)-mediated hydrosilylation reaction. The manganese tricarbonyl complex [Mn(bis-NHC)(CO)₃Br] resulted an active pre-catalyst for the reduction of a variety of esters using phenylsilane and the cheap and readily available polymethylhydrosiloxane. An in situ examination of the catalytic reaction using ⁵⁵Mn NMR spectroscopy allowed us to detect the formation of Mn(I) intermediate active species. (Figure presented.).

Full text of DOI:10.1002/adsc.202000148

Title: Bench-Stable Manganese NHC Complexes for the Selective
Reduction of Esters to Alcohols with Silanes
Authors: Sara C. A. Sousa, Sara Realista, and Beatriz Royo
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Advanced Synthesis & Catalysis
10.1002/adsc.202000148
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Bench-Stable Manganese NHC Complexes for the Selective
Reduction of Esters to Alcohols with Silanes
a†
a†
a
Sara C. A. Sousa, Sara Realista, and Beatriz Royo *
a
Instituto de Tecnologia Química e Biológica António Xavier, ITQB NOVA, Universidade Nova de Lisboa, Oeiras,
Portugal. E-mail: broyo@itqb.unl.pt
[
for corresponding author(s) please include phone and fax number(s) and e-mail address(es)]
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. Selective reduction of esters to alcohols was An in situ examination of the catalytic reaction using 55Mn
accomplished through Mn(I)-mediated hydrosilylation NMR spectroscopy allowed us to detect the formation of
reaction. The manganese tricarbonyl complex [Mn(bis- Mn(I) intermediate active species.
3
NHC)(CO) Br] resulted an active pre-catalyst for the
reduction of a variety of esters using phenylsilane and the Keywords: Manganese; N-heterocyclic carbenes; reduction
cheap and readily available polymethylhydrosiloxane.
esters; hydrosilylation; PMHS
Mn(0) N,N,N-pincer complex that catalyzed the
dihydrosilylation of esters to afford a mixture of
Introduction
[9]
alkoxysilane products.
The first Mn-mediated
The reduction of esters to alcohols is an important
process for both laboratory organic synthesis and the
chemical industry.[ Although traditional methods
involving the use of hazard and moisture-sensitive
selective hydrosilylation of esters to alcohols, was
disclosed three years later by Turculet and co-
workers using a (N-phosphinoamidinate)Mn complex
1]
[10]
and PhSiH
3
.
More recently, Leitner and co-workers
reducing agents such as LiAlH
4
and NaBH
4
are still
described a Mn(I) tricarbonyl complex capable to
efficiently reduce a variety of esters to a mixture of
applied,[ the use of catalytic methods is much more
2]
[3]
[11]
attractive from an environmental point of view. In
particular, hydrogenation constitutes very
the corresponding alcohols and ethers. Apart from
a
these studies, Darcel and Sortais group successfully
achieved the reduction of carboxylic acids with
convenient method from the point of view of atom-
economy.[ However, direct reaction with hydrogen
3]
silanes mediated by [Mn
(CO)10
]
under UV
2
[3]
[12]
generally requires high pressure and temperature.
irradiation. Despite these recent advances, further
improvements are still needed, e.g. use of cheaper
silanes, higher tolerance to functional groups, and
development of operationally simple methodologies.
In this respect, hydrosilylation represents an
interesting method, because allows milder conditions
and simpler and safer manipulation, avoiding the use
of hydrogen gas at high pressure.[4]
Metal-catalyzed systems for ester hydrosilylation
have mainly relied on the use of noble metals (eg. Pt,
[5]
Pd, Rh, Ir, Ru). The use of earth-abundant 3d metals
for the selective reduction of esters to alcohols
[6]
through hydrosilylation remains a challenge.
Despite the enormous interest raised in Mn catalysis,
hydrosilylation of esters mediated by Mn complexes
[7]
has been scarcely studied. To the best of our
knowledge, apart from the initial studies reported in
1
995 by Cutler with a manganese carbonyl acyl
complex, that catalyzed ester deoxygenation to yield
[8]
a mixture of siloxanes and their parent ethers, only
three additional reports can be found in the literature
(
Chart 1). In 2014, Trovitch´s group described a
1
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Advanced Synthesis & Catalysis
10.1002/adsc.202000148
6
h (Table 1, entry 2). Next, the influence of different
N
solvents was evaluated. As shown in Table 1,
quantitative yield was attained when the reaction was
carried out in THF, but negligible conversions were
obtained in chloroform, acetonitrile, or toluene
L(CO) MnC(O)CH3
PPh2
4
N
Mn
L = CO, PPh3
PPh2
(
<10%). A slight drop in the conversion was observed
Cutler, 1995
Product: ethers
and alkoxysilanes
N
when the amount of PhSiH
3
was reduced to 1
equivalent (95% yield, Table 2, entry 2).
Trovitch, 2014
Product: alkoxysilanes
i) 1 (1 mol%),
O
PhSiH (1.2 eqv.)
3
N
Neat/100
0
C
Ph
Br
OH + MeOH
CO
O
N
N
N
ii) NaOH/MeOH
N
Mn
CO
Mn
N
N(SiMe3)2
OC
P
P
98% yield
Ph
Ph
Scheme 1. Reduction of methyl benzoate with PhSiH
mediated by 1.
3
Leitner, 2019
Product: alcohols and ethers
Turculet, 2017
Product: alcohols
Table 1. Optimization of catalytic conditions using 1 with
[a]
PhSiH
3
Entry Solvent Catalyst Temp. Time Conv.[b]
N
N
N
loading
(mol%)
1
(ºC)
(h)
Br
N
Mn
OC
Br N
N
N
Mn
1
2
3
Neat
Neat
Neat
100
90
3
6
3
>99
57
CO
OC
CO
CO
1
CO
1
This work
Product: alcohols
2
0.75
100
<
5
4
5
Neat
0.75
1
100
100
6
95
<10
0
Chart 1. Hydrosilylation of esters mediated by Mn-based
catalysts.
CH
3
CN
16
As part of our interest in hydrosilylation of
functional groups with 3d metal compounds
6
7
CHCl
3
1
1
1
100
100
16
16
supported by N-heterocyclic carbene ligands
Toluene
THF
<10
98
(
NHC),[ we recently focused our attention on Mn-
13]
8
100
3
[14]
based catalysis. We report in this work an efficient
catalytic system for the selective reduction of esters
to alcohols using polymethylhydrosiloxane (PMHS)
or phenylsilane in the presence of [Mn(bis-
[a]
Reaction conditions: methyl benzoate (1 mmol), complex
, PhSiH (1.2 eqv.), solvent (0.4 mL).
Conversions determined by GC employing n-tetradecane
1
3
[b]
NHC)(CO) Br] (1). To the best of our knowledge,
3
as internal standard (Figures S6-S13).
this work represents the first Mn-mediated reduction
of esters using the cheap and readily available PMHS
as reducing agent. Notable, our catalytic system
operates under air atmosphere, without addition of
any auxiliary additives.
It is worth noting that in the absence of 1 or
using [Mn(CO) Br] as catalyst, no reaction took place.
5
When complex 2 (Chart 1) bearing a mixed NHC-
pyridine ligand, was applied as catalyst, longer
reaction time (6 h) was needed to afford high
conversion under similar reaction conditions (Table
S1). The courses of the hydrosilylation reaction using
complexes 1 and 2 were investigated by GC. The
catalytic reactions were performed in small reaction
vessels (5 mL); reagents were mixed in air and then
the vessels were closed and heated to 100 ºC. We
observed that when the vessels were opened to take
the aliquots for the monitoring of the reaction, the
catalytic reaction was altered, and the data obtained
was not reproducible. Therefore, to obtain
information about the profile of the reaction, several
vessels loaded with complex 1 (1 mol%), methyl
Results and Discussion
Initial experiments were performed using methyl
benzoate as a model substrate in the presence of
[
Mn(bis-NHC)(CO)
3
Br] (1) (1 mol%) as catalyst and
PhSiH (1.2 eqv.) as reducing agent, in neat
3
conditions at 100 ºC. Gratifyingly, after 3 h of
reaction, quantitative conversion of methyl benzoate
to benzyl alcohol was obtained (98% yield
determined by gas chromatography (GC), 83%
isolated yield, Scheme 1). When lower amount of 1
(
0.75 mol%) was used, longer reaction time (6 h) was
benzoate (1 mmol) and PhSiH
3
(1.2 mmol) in neat
o
needed to afford high conversion (Table 1, entries 3
and 4). Lowering the temperature to 90 ºC has also a
significant impact in the catalytic reaction; the
conversion of methyl benzoate dropped to 57 % after
conditions were heated at 100 C, and the reactions
were stopped at different times. As shown in Table
S1, during the first hours of reaction (2 h 45 min for 1
2
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Advanced Synthesis & Catalysis
10.1002/adsc.202000148
and 5 h 45 min for 2) no reaction occurred. After the
induction period, the reaction suddenly accelerated,
reaching full conversion of methyl benzoate in few
minutes. These findings indicate that complexes 1
of benzoates bearing electron donating and electron
withdrawing substituents was investigated. Benzoates
bearing groups such as CF
3
, Cl, and OMe were well
tolerated, and the corresponding alcohols were
and 2 are pre-catalysts for the hydrosilylation reaction. obtained in good to high yields (71-91%) (Table 3,
Complex 1 resulted less active than Turculet´s Mn entries 8-10). Notable limitations were detected in the
reduction of benzoates bearing the NO , NH2, and CN
2
pre-catalyst, [(k -P,N)Mn(N(SiMe
3
)
2
], which can
2
[10]
selectively reduce esters to alcohols at 25 ºC.
groups. Methyl 4-nitrobenzoate and methyl 3-
aminobenzoate were fully converted into a complex
product mixture. In the case of methyl 3-
aminobenzoate, the formation of several silylated
compounds was detected by Si NMR (Figure S2),
but after basic hydrolysis the target reduction product,
However, in contrast to Turculet´s catalyst, 1 do not
require inert atmosphere for its manipulation and can
operate under air and moisture conditions.
2
9
Next, we explored the reuse of 1 by adding new
charges of substrate, methyl benzoate, and PhSiH
3
under the optimized conditions (1 mol% of 1, 1.2 eqv. 3-aminobenzyl alcohol was not obtained. In addition,
of PhSiH
when the reaction vessels were opened and new
charges of substrate and PhSiH were added, the
3
, 100 °C, neat conditions). Interestingly,
reduction of conjugated systems such as the methyl
cinnamate gave a complex mixture of products.
Substrate limitations have also been encountered by
Turculet and co-workers using the manganese
3
reaction resumed. In this way, 5 cycles were
completed, reaching an overall TON number of 485
after several days (Figure S1).
Then, we explored the activity of less expensive
silanes, such as Ph
tetramethyldisiloxane (TMDS). Interestingly, the
readily available PMHS afforded 88% conversion of
methyl benzoate in 24 h, while Ph
resulted inactive (Table 2, entries 3-5).
2
[10]
complex [(k -P,N)Mn(N(SiMe
described with iron-based catalysts.
3
) ], and have been
2
[
6e,6g]
2
2
SiH , PMHS, and 1,1,3,3-
Table 3. Scope of the reduction of esters using 1 with
[
a]
PhSiH
3
2
SiH and TMDS
2
Table 2. Screening of various silanes in the reduction of
methyl benzoate with 1 [
a]
Entry Silane Amount Solvent Time Conversion
[
b]
silane
(h)
(
eqv.)
1.2
1
2
3
PhSiH
3
3
Neat
Neat
Neat
3
3
>99
95
PhSiH
1.0
3
2
Ph SiH
2
24
<8
88
4
5
PMHS
TMDS
3
3
THF
THF
24
24
<2
[
a]
Reaction conditions: methyl benzoate (1 mmol), complex
(1 mol%), silane, neat or THF (0.4 mL), at 100 ºC.
Conversions determined by GC employing n-tetradecane
1
[
b]
as internal standard (Figures S15-S19).
Having selected the optimised conditions (1
mol% of complex 1, 100 ºC), the applicability of this
catalytic system was assessed with PhSiH (under
3
neat conditions, Table 3) and PMHS (in THF, Table
). Ethyl benzoate and the acetates, methyl
4
phenylacetate and methyl 2-(naphthalen-2-yl)acetate
were quantitatively reduced to the corresponding
alcohols in high yields (82-89%) using PhSiH
3
(Table
3, entries 2-4). In addition, the reduction of the cyclic
aliphatic methyl cyclohexane carboxylate (entry 5)
and the linear aliphatic methyl octanoate (entry 6)
yielded the corresponding alcohols in high yields.
Methyl acetates bearing a heteroaromatic substituent
such as methyl nicotinate (Table 3, entry 7), afforded
a complex mixture of products which could not be
identified. To evaluate the tolerance of the catalytic
system to functional groups, the reduction of a variety
3
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Advanced Synthesis & Catalysis
10.1002/adsc.202000148
[
a]
Reaction conditions: substrate (1 mmol), catalyst (1
mol%), PhSiH
3
(1.2 mmol), n-tetradecane (0.5 mmol), 100
ºC, neat conditions.
[
b]
Conversions and yields determined by GC (Figures S20-
S27). Isolated yields in parenthesis.
Reaction performed in THF (0.5 mL).
[
c]
Next, we explored the scope of the catalytic
reaction using PMHS as a reducing agent. A variety
of esters were efficiently reduced with 1 using PMHS
(
3 eqv.) in THF at 100 °C affording the
corresponding alcohols in high yields (66-88% yield,
Table 4, entries 1-9). Notably, ethyl benzoate, methyl
phenylacetate and methyl 2-(naphthalen-2-yl)acetate
were
quantitatively
reduced
affording
the
corresponding alcohols in high yields (82-88%). Both
the aliphatic linear and cyclic esters, methyl
octanoate and methyl cyclohexane carboxylate, were
also successfully reduced (Table 4, entries 5 and 6).
Finally, para-substituted methyl benzoates containing
CF
to the corresponding alcohols in good yields (Table 4,
entries 7-9), while NO and NH groups were not
tolerated (Table 4, entries 10 and 11).
3
, Cl, and MeO functional groups were converted
2
2
To the best of our knowledge, this work
represents the first Mn-catalyzed reduction of esters
through hydrosilylation using PMHS. Hydrosilylation
of esters using 3d metals as catalysts and PMHS as
[
6d,f,g,13d]
reducing agent is rare.
examples reported in the literature was described by
Adolfsson and co-workers using ZnEt in the
One of the few
2
[6f]
presence of LiCl. This catalytic system displayed
high functional group tolerance and provided a facile
access to a wide range of alcohols. Broad substrate
scope was also exhibited by the iron-based catalytic
system, Fe(stearate)
2
/NCH
2
CH
2
NH
2
and PMHS,
[
6d]
described by Beller. Recently, Findlater and co-
workers reported an iron-based catalytic system
capable to convert esters to alcohols using PMHS in
the presence of n-BuLi (30 mol%), although in this
[6g]
case substrate limitations were encountered.
In
contrast to these reports, no auxiliary reagents (LiCl,
n-BuLi) were required using complex 1.
To demonstrate the utility of our catalytic system,
we performed the scale up of the hydrosilylation
reaction; 0.7 g of methyl benzoate were reacted with
PMHS in the presence of 1 (5 mol%) and THF (1
mL) at 100 ºC. After 24 h of reaction, benzyl alcohol
was obtained in 90 % yield.
Table 4. Scope of the reduction of esters using 1 with
[
a]
PMHS
4
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Advanced Synthesis & Catalysis
10.1002/adsc.202000148
catalytic reaction using 55Mn NMR spectroscopy
allow us to detect the formation of Mn(I)
intermediate active species. First, we recorded the
5
5
Mn NMR spectrum of 1 in THF-d , which showed a
8
resonance at -1684 ppm (Fig. S3a), shifted to lower
frequency than those observed for tricarbonyl Mn
complexes
bearing
bidentate
phosphines
[
Mn(CO)
3
Br(P-P)] (-890 to -1254).[
15a]
This
observation is consistent with the stronger donating
character of the NHC ligand versus phosphines. Then,
three J- Young valve NMR tubes loaded with methyl
benzoate (0.75 mmol), complex 1 (0.75 mol%),
PhSiH
3
(0.9 mmol) and THF-d (0.4 mL) were heated
8
to 100 C for 1, 3 and 6 h, respectively, and their
5
5
55
Mn NMR spectra recorded at 25 C. The Mn
NMR spectrum of the sample that was heated for 1 h
showed the loss of the peak at -1684 ppm and the
formation of one new resonance at -2185 ppm
(
Figure S3b). After 3 h of heating, the resonance at -
185 ppm remained in the spectrum as the major
peak, and the appearance of a new signal at -2230
2
5
5
ppm was observed (Figure S3c). The Mn NMR
spectrum recorded after 6 h of heating displayed a
new resonance at -2124 ppm along with the peaks at -
2185 and -2230 ppm. These three signals remained in
the spectrum until completion of the reaction. It must
be noted that the catalytic reactions performed in J-
Young valve NMR tubes (without stirring) took
longer reaction times than those performed in 5 mL
vessels.
Interestingly, the 55Mn NMR spectrum of the
reaction of complex 1 with PhSiH (in 1:100 ratio)
3
recorded after 3 h of heating at 100 C (in the absence
of substrate) showed a single peak at -2185 ppm
(
Figure S4).
These findings indicate that the
resonance observed at -2185 ppm corresponds to a
Mn species (A) formed upon reaction of 1 with
PhSiH , while the resonances at -2124 and -2230 ppm,
3
might be formed by interaction of A with the
substrate (methyl benzoate), affording two new Mn
intermediate species, B (at -2230 ppm) and C (at -
5
5
2
124 ppm). The Mn chemical shifts of A, B and C
species lie in the range of those resonances observed
for other Mn(I) complexes reported in the
[
15]
55
literature. The shifting of the Mn signals to lower
frequencies reflects the shielding of the manganese
nucleus, which is in accord with the formation of Mn-
2
H or Mn( -H-SiH
2
Ph) species that have been
proposed by us based on stochiometric reactions of 1
[14b]
with PhSiH
valve NMR tube was opened, the three resonances at
2124, -2185 and -2230 ppm rapidly disappeared
3
.
Interestingly, when the J- Young
-
[
a]
Reaction conditions: substrate (1 mmol), catalyst (1
from the spectrum (Figure S5b), but if a new charge
mol%), PMHS (3 mmol), n-tetradecane (0.5 mmol), 100 ºC, of PhSiH was added, the three resonances appeared
3
in THF (0.4 mL), 24 h.
again (Figure S5c). These results indicate that the Mn
active species are regenerated by addition of new
charges of silane.
We speculate that the reaction occurs through an
outer-sphere mechanism.
[
b]
Conversions and yields determined by GC (Figures S28-
S36). Isolated yields in parenthesis.
[
c]
3
mol% of catalyst 1 and 5 mmol of PMHS.
[16,6a]
This hypothesis was
supported by our in situ IR spectroscopy study of the
stochiometric reaction of 1 with PhSiH in which no
CO dissociate Mn intermediate was observed.
3
In order to gain information on the mechanism
of the catalytic reaction, an in situ examination of the
[14b]
The
5
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Advanced Synthesis & Catalysis
10.1002/adsc.202000148
We are grateful to Fundação da Ciência e a Tecnologia, FCT, for
outer-sphere mechanism has also been proposed for 1
in the selective N-alkylation of anilines and -
alkylation of ketones with alcohols.[17]
Projects
PTDC/QUI-QIN/28151/2017,
LISBOA-01-0145-
FEDER-007660 (Microbiologia Molecular, Estrutural
e
Celular) funded by FEDER funds through COMPETE2020,
POCI, and FCT, and Green-it “Bioresources for Sustainability”
(
UID/Multi/04551/2013). The NMR spectrometers at CERMAX
through project 022162. S.C.A.S
thanks FCT for grant
Conclusion
PTDC/QUI-QIN/28151/2017. We acknowledge Helena Matias
5
5
for her support in Mn NMR experiments.
In summary, we have described the first manganese-
catalyzed reduction of esters to alcohols using the
cheap and readily available PMHS as reducing agent. †S. C. A. Sousa and S. Realista contributed equally to this work.
The reduction of a variety of esters with PhSiH or
3
PMHS afforded the corresponding alcohols in good to
excellent yields. Further investigation of the reaction References
mechanism employing computational methods are
currently ongoing in our laboratory. Future research in [1] a) J. Magano, J. R. Dunetz, Org. Process Res. Dev.
our group aims to develop new Mn-NHC catalysts
with improved activities.
2012, 16, 1156-1184; b) P. G. Andersson, I. J.
Munslow in Modern Reduction Methods, John Wiley
and Sons, 2008; c) P. J. Dunn, K. K. Hii, M. J. Krische,
M. T. Williams, Sustainable Catalysis: Challenges and
practices for the Pharmaceutical and Fine Chemical
Industries, Wiley, 2013.
Experimental Section
General Procedure for the Reduction of Esters with
Phenysilane Catalyzed by 1: A 5 mL sealed cap flask
with a stirring bar was loaded with complex 1 (1 mol%,
[
[
2] a) A. Patra, S. Batra, A. P. Bhaduri, Synlett 2003,
1
2
611–1614; b) D. Das, S. Roy, P. K. Das, Org. Lett.
004, 6, 4133–4136; c) B. M. Trost, I. Fleming in
0
1
.01 mmol) and ester (1 mmol). Then, PhSiH
.2 mmol) and the internal standard (n-tetradecane, 0.5
3
(1.2 eqv. ,
Comprehensive Organic Synthesis, Pergamon, Oxford,
991.
1
mmol) were added. The mixture was stirred at 100 ºC for
3
-24 h. Then, the reaction mixture was diluted with 4 mL
3] a) S. C. Berk, K. A. Kreutzer, S. L. Buchwald, J. Am.
Chem. Soc. 1991, 113, 5093–5095; b) J. G. De Vries, C.
J. Elservier in Handbook of Homogeneous
Hydrogenation, Wiley-VCH, Weinheim, Germany,
of chloroform and quenched with 0.3 mL of 25% NaOH in
MeOH at room temperature. An aliquot (1 mL) was taken,
filtered through celite and subjected to GC-FID analysis.
To obtain the isolated products, all the volatiles were
evaporated after the quenching. The crude residue was
dissolved in ethyl acetate and washed with water 3 x 20
2
007; c) S. Elangovan, M. Garbe, H. Jiao, A.
Spannenberg, K. Junge, M. Beller, Angew. Chem. Int.
Ed. 2016, 55, 15364-15368. d) S. Wermeister, K. Junge,
M. Beller, Org. Process. Res. Dev. 2014, 18, 289-302.
2 4
mL, dried with Na SO , filtered and evaporated. The crude
was purified by using silica gel column chromatography
with the appropriate mixture of n-hexane and ethyl acetate
to afford the alcohol.
[4] a) B. MarciniecSpringer in Hydrosilylation: A
Comprehensive Review on Recent Advances,
Netherlands, 2009; b) D. Addis, S. Das, K. Junge, M.
Beller, Angew. Chem. Int. Ed. 2011, 50, 6004-6011.
General Procedure for the Reduction of Esters with
PMHS Catalyzed by 1: A 5 mL sealed cap flask with a
stirring bar was loaded with complex 1 (1-3 mol%, 0.01 –
[
5] Selected examples: a) T. Ohta, M. Kamiya, K. Kusui, T.
Michibata, M. Nobutomo, I. Furukawa, Tetrahedron
Lett. 1999, 40, 6963-6966; b) K. Matsubara, T. Lura, T.
Maki, H. Nagashima, J. Org. Chem. 2002, 67, 4985-
4988; c) A. C. Fernandes, C. C. Romão, J. Mol. Catal.
A 2006, 253, 96-98; d) J. Nakanishi, H. Tatamidani, Y.
Fukumoto, N. Chatani, Synlett 2006, 869-872.
0
.03 mmol), ester (1 mmol) and THF (0.4 mL). Then,
PMHS (3-5 eqv , 3 - 5 mmol) and the internal standard (n-
tetradecane, 0.5 mmol) were added. The mixture was
stirred at 100 ºC for 24 h. Then, the reaction mixture was
diluted with 4 mL of chloroform and quenched with 0.3
mL of 25% NaOH in MeOH at room temperature. An
aliquot (1 mL) was taken, filtered through celite and
subjected to GC-FID analysis.
To obtain the isolated products, all the volatiles were
evaporated after the quenching. The crude residue was
stirred with diethyl ether for 1 h at room temperature. Then,
the organic phase was washed with water (3 x 20 mL),
[
6] a) B. Royo, in Adv. Organomet. Chem., Academic
Press Inc., 2019, pp. 59–102; c) R. J. Trovitch, Synlett
2
014, 25, 1638-1642; d) K. Junge, B. Wendt, S. Zhou,
M. Beller, Eur. J. Org. Chem. 2013, 2061-2065; e) D.
Bézier, G. T. Venkanna, L. C. M. Castro, J. Zheng, T.
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dried over anhydrous Na SO , filtered and evaporated. The
crude was purified by using silica gel column
chromatography with the appropriate mixture of n-hexane
and ethyl acetate to afford the alcohol.
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Bench-Stable Manganese NHC Complexes for the
Selective Reduction of Esters to Alcohols with
Silanes
Adv. Synth. Catal. Year, Volume, Page – Page
Sara C. A. Sousa, Sara Realista, Beatriz Royo*
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