Dehydrogenative coupling of alcohols and carboxylic acids with hydrosilanes catalyzed by a salen-Mn(v) complex

Article abstract of DOI:10.1039/c5cy01912e

A Mn(v)-salen complex was found to be an effective catalyst for the dehydrogenative coupling of hydroxyl groups with hydrosilane. The reaction conditions were optimized with different silanes and efficient dehydrogenative coupling was achieved by using triethoxysilane and diphenylsilane. Various alcohols and phenols and a limited number of carboxylic acids were converted into the corresponding silyl ethers and silyl esters. A range of functional groups such as chloro, nitro, methoxy, carbonyl and carbon-carbon multiple bonds are tolerated in the reaction.

Full text of DOI:10.1039/c5cy01912e

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Dehydrogenative coupling of alcohols and
carboxylic acids with hydrosilanes catalyzed by a
Cite this: DOI: 10.1039/c5cy01912e
salen–MnIJV) complex†
*
Srikanth Vijjamarri, Vamshi K. Chidara, Jana Rousova and Guodong Du
A MnIJV)–salen complex was found to be an effective catalyst for the dehydrogenative coupling of hydroxyl
groups with hydrosilane. The reaction conditions were optimized with different silanes and efficient de-
hydrogenative coupling was achieved by using triethoxysilane and diphenylsilane. Various alcohols and
phenols and a limited number of carboxylic acids were converted into the corresponding silyl ethers and
silyl esters. A range of functional groups such as chloro, nitro, methoxy, carbonyl and carbon–carbon multi-
ple bonds are tolerated in the reaction.
Received 6th November 2015,
Accepted 23rd December 2015
DOI: 10.1039/c5cy01912e
www.rsc.org/catalysis
hydrogen gas as the only by-product. Due to these attractive
features, a number of catalysts have been reported for the de-
Introduction
Si–O bond-containing compounds such as silyl ethers and
silyl esters are important chemicals with an assortment of ap-
plications. For example, silyl ethers are commonly employed
for protecting hydroxyl groups in multi-step syntheses,1 and
serve as precursors to silicone polymers.2 They are also valu-
able for surface modifications, constructing organic–inorganic
hybrids, functionalizing silica materials and tethering organo-
metallic catalysts.3 Silyl esters are useful for protecting carbox-
ylic acid groups1 and polyIJsilyl ester)s are an interesting class
of degradable materials with tunable degradation profiles.4
The standard method for the synthesis of silyl ethers and silyl
esters is treating alcohol and carboxylic acid substrates with
chlorosilanes, for which the HCl by-product is neutralized
with a stoichiometric amount of bases such as imidazole and
diisopropylethylamine.5 Moreover, the corrosiveness and hy-
drolytic sensitivity of chlorosilanes make them inconvenient
for handling and storage. For tertiary and hindered alcohols,
the use of more expensive silyl triflate reagents is required.6
Hexamethyldisilazane (HMDS) has also been utilized for silyl
ether synthesis, but it usually takes longer reaction times and
the ammonia by-product needs to be removed continuously.7
Dehydrogenative coupling of alcohols and carboxylic acids
with hydrosilanes has emerged as an alternative for the syn-
thesis of silyl ethers and silyl esters. Compared to chloro-
silanes, hydrosilanes are generally less prone to hydrolysis
and easier to handle. The dehydrogenative coupling reaction
is a single-step process with high atom economy, generating
hydrogenative coupling of alcohols and hydrosilanes. They
are typically complexes of various transition metals,8 while
metal-free systems such as N-heterocyclic carbenes9 and
Lewis acid BIJC6F5)3 (ref. 10) have also been developed for the
process. In comparison, only a few catalytic systems are avail-
able for the dehydrogenative coupling of carboxylic acids and
hydrosilanes.11
Recent concerns about sustainability have led to signifi-
cant efforts towards the development of catalytic systems de-
rived from inexpensive, biocompatible and abundant
metals.12,13 Taking this into consideration, manganese is of
particular interest, since its global reserve is one of the larg-
est among transition metals and it is an essential element for
biological systems. Traditionally, manganese complexes have
been extensively investigated in oxidation reactions, and their
applications in other types of transformations have only been
explored more recently.14 During the course of our study on
high-valent transition metal catalysts for hydrosilylations,15
we observed that hydroxyl groups reacted with hydrosilanes
in the presence of a MnIJV) complex, [MnNIJsalen-3,5-tBu2)] (1),
that is readily available and can be easily prepared.16 Herein,
we report our study on the dehydrogenative coupling of alco-
hols and carboxylic acids with hydrosilanes catalyzed by the
Mn complex 1.
Results and discussion
Dehydrogenative coupling of alcohols and silanes: screening
Department of Chemistry, University of North Dakota, 151 Cornell Street Stop
9024, Grand Forks, North Dakota 58202, USA. E-mail: Guodong.du@und.edu;
Tel: +1 701 777 2241
In the initial experiments, we examined the dehydrogenative
coupling reaction by taking benzyl alcohol as a template sub-
strate. Various hydrosilanes were examined under different
conditions in the presence of 0.5 mol% (vs. alcohol) MnN
† Electronic supplementary information (ESI) available: Detailed experimental
procedures and characterization data of products. See DOI: 10.1039/c5cy01912e
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Table 1 Dehydrogenative coupling of benzyl alcohol with hydrosilanesa
Entry
Silane
BnOH : silane
Products
T (°C)
t (h)
Conversionb (%)
1c
2
Ph2SiH2
Ph2SiH2
Ph2SiH2
PhMe2SiH
tBuMe2SiH
Et3SiH
Ph3SiH
PhSiH3
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
(EtO)3SiH
2 : 1
2 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
(PhCH2O)2-SiPh2
(PhCH2O)2-SiPh2
PhCH2O-SiHPh2
N.R.
N.R.
N.R.
PhCH2OSiPh3
(PhCH2O)2-SiHPhe
PhCH2O-SiIJOEt)3
PhCH2O-SiIJOEt)3
PhCH2O-SiIJOEt)3
N.R.
80
80
80
80
80
80
80
80
80
80
RT
80
2.5
3
1
72
24
24
72
1.5
89
85
90
—
—
—
57
90
98
94
90
—
3c
4
d
5
6
7
8
9c
10
11
12f
0.67
1
7
12
a
Reaction conditions: substrate (0.7–0.8 mmol), silanes (one or two equivalents), PhSiMe3 (10 mol%, as internal standard) and MnN catalyst 1
(0.5 mol%). Determined by H NMR on the basis of consumption of Ph2CH2OH or silane. c Reaction was performed under N2. Dialkoxy silyl
b
1
d
ether was observed as a minor product. e Mono and trialkoxy silyl ethers were observed as minor products. f Catalyst was not used.
complex 1, and the selected results are presented in Table 1.
Primary PhSiH3, secondary Ph2SiH2 and tertiary (EtO)3SiH
hydrosilanes reacted readily with PhCH2OH to produce the
corresponding silyl ethers, with (EtO)3SiH being the fastest
among them (entries 3 vs. 9, 8 vs. 10). According to the prod-
uct analysis of the crude reaction mixture, monoalkoxysilane
Ph2SiHIJOCH2Ph) was produced when one equivalent of
Ph2SiH2 was employed, with a small amount of dialkoxysilane
Ph2SiIJOCH2Ph)2 (entry 3). When the ratio of alcohol : Ph2SiH2
was 2 : 1, dialkoxysilane Ph2SiIJOCH2Ph)2 was produced as the
predominant product (entry 2). In this case, the time profile
of the reaction monitored by 1H NMR spectroscopy shows
rapid consumption of Ph2SiH2 and buildup of
Ph2SiHIJOCH2Ph), followed by slower disappearance of
Ph2SiHIJOCH2Ph) (Fig. 1). Similar to the Mn-catalyzed
hydrosilylation of carbonyl compounds,15a the coupling reac-
tion could be carried out at room temperature with extended
reaction time (entry 11). Running the reaction under inert ni-
trogen atmosphere could shorten the reaction time (entries 1
vs. 2, 9 vs. 10), though the effect seems to be less substantial
than that in hydrosilylation. Thus, subsequent reactions were
performed without extrusion of air for convenience. On the
t
other hand, tertiary silanes such as PhMe2SiH, BuMe2SiH
and Et3SiH exhibited no reactivity, while Ph3SiH showed rea-
sonable conversion with prolonged reaction time (72 h), likely
due to their electronic and steric characters (entries 4–7). In
a control experiment, no reaction between PhCH2OH and
(EtO)3SiH was observed after 12 h without adding the catalyst
(entry 12).
Alcohol substrate scope
Having established the activity of complex 1 in the dehydro-
genative coupling reactions, we moved on to a variety of alco-
hol substrates including primary, secondary, tertiary, cyclic
and phenolic alcohols (Table 2). Under specified reaction
conditions, most of the alcohols yielded the corresponding
silyl ethers with >90% conversions and slightly lower isolated
yields. Unlike primary alcohols, the dehydrogenative coupling
of secondary and tertiary alcohols with Ph2SiH2 afforded
monoalkoxysilanes as the primary product, even when excess
alcohols were used (entries 2–4). This observation indicates
that it was more challenging to couple the last Si–H bond
with a bulky alcohol because of the steric crowding. For ex-
ample, when 2 equivalents of a tertiary alcohol (tBuOH) were
allowed to react with Ph2SiH2, only ∼50% of the alcohol was
consumed, even after prolonged reaction time (6 h). Addition
of another equivalent of Ph2SiH2 led to the complete conver-
t
sion of BuOH within 2.5 h and the monoalkoxysilane was
the predominant product detected (Table 2, entry 3). Simi-
larly, reaction of a secondary alcohol, cyclohexanol, with
Ph2SiH2 resulted in the formation of the monoalkoxysilane
Fig. 1 Time profiles for the reaction of benzyl alcohol with Ph2SiH2
(one equiv.).
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Table 2 MnN-catalyzed dehydrogenative coupling of various alcohols with silanesa
Entry
1
Silaneb
Products
t (h)
Conv.c (%)
95 (81)
Ph2SiH2 (0.5)
7.5
2
Ph2SiH2 (1.0)
Ph2SiH2 (1.0)
Ph2SiH2 (1.0)
Ph2SiH2 (0.5)
Ph2SiH2 (0.5)
(EtO)3SiH (1.0)
1.5
8.5
1
96 (67)
97 (79)
95 (63)
85 (50)
95 (80)
90 (65)
3
4
5
3
6d
2.7
1
7
8
9
(EtO)3SiH (1.2)
(EtO)3SiH (1.2)
1.5
2.5
95 (81)
95 (73)
10
(EtO)3SiH (1.0)
3
98 (64)
11
12
(EtO)3SiH (1.0)
(EtO)3SiH (1.4)
4
5
98 (71)
96 (65)
13
(EtO)3SiH (1.3)
2.5
90 (64)
14
15e
16
17
18
19
20
(EtO)3SiH (1.0)
(EtO)3SiH (1.0)
(EtO)3SiH (1.0)
(EtO)3SiH (1.3)
(EtO)3SiH (1.2)
(EtO)3SiH (1.2)
(EtO)3SiH (1.3)
2.8
0.67
3
90 (72)
99
98 (81)
90 (78)
95 (73)
100 (90)
95 (57)
1.5
2
4
1
a
b
Reaction conditions: substrate (0.7–0.8 mmol), silane and MnN catalyst (0.5 mol%) at 80 °C. The equivalent amounts of silanes used (vs.
alcohol). Determined by 1H NMR on the basis of consumption of alcohol and/or silane; isolated yields in parenthesis. Reaction under N2.
c
d
e
1 mol% catalyst used.
as the major product with 95% conversion in 1 h (Table 2,
entry 4).
hydrogenation of double and triple bonds with silanes. It is
noted that saturated primary alcohols seem to be more reac-
tive towards dehydrogenative coupling with triethoxysilane
than unsaturated ones, as it took less reaction time (entries
8 vs. 11 & 12). The time profile of their reactions under iden-
tical conditions with 1 equivalent of (EtO)3SiH confirms that
butan-1-ol has the fastest reaction time among them, though
the difference is not large (Fig. 2). Nevertheless, this suggests
that even though the double and triple bonds are not reactive
To examine the chemoselectivity of the dehydrogenative
coupling process, we further worked with alcohol substrates
having different functional groups such as CC and CC
multiple bonds (Table 2, entries 11–13 and 15), since such
groups may react with silanes under catalytic conditions.17
All these alcohols were converted to silyl ethers with excellent
conversions. There were no side products of hydrosilylation/
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Fig. 3 Reactions of substituted phenols p-X-C6H4OH (X = H, OMe,
NO2, Cl, tBu) with (EtO)3SiH (1 : 1).
was obtained as the major product (entry 2). Similarly, treat-
ment of pinacol with one equivalent of Ph2SiH2 or PhMeSiH2
afforded five-membered cyclic dioxasilacycles as the major
products (entries 3 and 4).8s
Fig. 2 Comparison of reactions of three primary aliphatic alcohols
with (EtO)3SiH (1 : 1).
In another set of experiments, we repeated the reaction
between benzyl alcohol and triethoxysilane to test the reus-
ability of the catalyst. After the reaction was nearly complete
(∼60 min), second equivalents of alcohol and silane (1 : 1 mo-
lar ratio) were added to the reaction mixture, and with vigor-
ous bubbling, >90% of the substrates were consumed within
30 min. A third addition of the 1 : 1 mixture of PhCH2OH and
(EtO)3SiH led to a similar outcome (Fig. 4). These results
suggested that the catalyst can be effectively reused without
much loss in reactivity.
towards hydrosilanes under these conditions, they may still
inhibit the coupling, presumably by coordinating to the ac-
tive metal site. Since the Mn complex is known to catalyze
the hydrosilylation of ketones,15a a multifunctional substrate,
5-hydroxy-2-pentanone was tested for the coupling reaction
(entry 14). Only the hydroxyl group reacted with silane under
these conditions, and the ketone moiety was preserved in the
isolated product. The preference for hydroxyl groups over car-
bonyls has been observed in other catalytic systems.8j,l,r
To further probe the selectivity and electronic effect of dif-
ferent functional groups, we performed dehydrogenative cou-
pling reactions of a series of para-substituted phenolic sub-
strates and (EtO)3SiH (Table 2, entries 16–20). Chloro, nitro
and methoxy groups were tolerated in the reaction and de-
hydrogenative coupling proceeded with high conversion and
decent yields within a few hours. When the reactions of 1 : 1
(EtO)3SiH : phenols under otherwise identical conditions were
monitored by 1H NMR, the silane conversion profile indi-
cated that both electron-withdrawing and electron-donating
groups lead to faster reactions (Fig. 3), with the exception of
p-nitrophenol, for which the reaction was considerably slower
and featured an induction period. The peculiarity of para-
NO2-substituted benzaldehyde and acetophenone has been
noticed previously in hydrosilylation reactions,15a though the
reason behind it is still unclear.
Dehydrogenative coupling of carboxylic acids and silanes
Based on the results of the alcohols, we next extended this
process to the dehydrogenative coupling of carboxylic acids
and silanes to synthesize silyl esters. Several reactions were
performed in several solvents at different temperatures to op-
timize the reaction conditions, and selected results are listed
in Table 4. Among the solvents tested, CDCl3 has generally
given rise to better conversions. Compared with the dehydro-
genative reactions of alcohols, reactions of carboxylic acids
are much slower, even with increased catalyst loading. Both
aromatic (entry 1) and aliphatic carboxylic acids (entries 2 &
3) undergo dehydrogenative coupling reactions with (EtO)3-
SiH to yield the corresponding silyl esters.
Mechanistic consideration
To extend the scope of dehydrogenative coupling reac-
tions, several diols were also examined under the same con-
ditions and the results are summarized in Table 3. Diol sub-
strates tend to be more demanding for the coupling because
of the competing formation of cyclic and polymeric species.
When 1,4-butanediol was allowed to react with two equiva-
lents of (EtO)3SiH, bissilylated (EtO)3SiOIJCH2)4OSiIJOEt)3 was
the only product as expected (entry 1). When a slight excess of
Ph2SiH2 was used, the seven-membered cyclic dioxasilacycle
During the reactions, one of the most notable features was
the conspicuous change in color of the starting MnN complex
from green to reddish brown, then to dark yellow, and finally
to pale yellow, similar to the observations in hydrosilylation
reactions.15a The color change can be attributed to the reduc-
tion of MnIJV) to lower oxidation states such as MnIJIII) and
MnIJII),18 and appears to be correlated with the progress of
the reaction. The observation that the reactions upon second
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Table 3 MnN-catalyzed dehydrogenative coupling of diols with silanesa
Entry
1
Silane
Diol : silane
1 : 2
Product(s)
t (h)
Conv.b (%)
90 (56)
(EtO)3SiH
3
2
3
Ph2SiH2
Ph2SiH2
1 : 1.2
1 : 1
3.5
5.5
70 (65)
95
4
PhMeSiH2
1 : 1
5
98
a
b
Substrate (0.7–0.8 mmol), silane and MnN catalyst (0.5 mol%) at 80 °C. Determined by 1H NMR on the basis of consumption of diols and/
or silane; isolated yields in parenthesis.
hypothesis that MnIJV) is not the actual active species and it
needs to be reduced first to act as the active catalyst. Con-
versely, when there was no reaction between the hydroxyl
groups and tertiary hydrosilanes such as PhMe2SiH, Et3SiH
t
and BuMe2SiH, the reaction mixture retained the same green
color. In these cases, the less active tertiary hydrosilanes were
not able to reduce manganeseIJV), and as a result, catalytic de-
hydrogenative coupling did not occur. Hence, we believe that
the activation of manganese by reduction is a crucial step for
the reactivity.
Conclusion
In conclusion, we have reported a Mn–salen complex for the
effective synthesis of silyl ethers and silyl esters via dehydro-
genative coupling of alcohols, phenols and carboxylic acids
with hydrosilanes. The coupling reaction is compatible with
Fig. 4 Reusability of the catalyst for dehydrogenative coupling.
a variety of functional groups, and seems to involve a reduced
Mn active species. Our future studies will be focusing on the
mechanistic understanding of the current dehydrogenative
and third additions of the reactants took place even faster, as
discussed earlier (see Fig. 4), is in agreement with the
coupling reactions and further extension of the methodology
to other applications.
Table 4 MnN-catalyzed dehydrogenative coupling of carboxylic acids with (EtO)3SiHa
Entry
1
Substrate
Acid : silane
1 : 1
Solvent
CDCl3
Products
t (h)
Conversionb (%)
73
32
2
3
1 : 1.3
1 : 1
CDCl3
C6D6
29
28
77
52
a
b
Reaction conditions: substrate (0.4–0.6 mmol), (EtO)3SiH and 1 (1 mol%); 80 °C. Determined by 1H NMR on the basis of consumption of
acids and/or silane. Repeated attempts at isolation of silyl esters by column chromatography on silica led to decomposition, and isolated yields
were not obtained.
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