New anionic rhodium complexes as catalysts for the reduction of acetophenone and its derivatives

Article abstract of DOI:10.1039/C8RA08954J

New anionic rhodium(iii) complexes, obtained by a simple reaction of RhCl₃ with organic chlorides (derivatives of imidazole and pyridine), have been employed as catalysts for hydrosilylation (reduction) of acetophenone derivatives. The reaction

Full text of DOI:10.1039/C8RA08954J

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New anionic rhodium complexes as catalysts for
the reduction of acetophenone and its derivatives
Cite this: RSC Adv., 2019, 9, 711
a
a
Olga Bartlewicz,
Magdalena Jankowska-Wajda
ab
*
and Hieronim Maciejewski
New anionic rhodium(III) complexes, obtained by a simple reaction of RhCl3 with organic chlorides
(derivatives of imidazole and pyridine), have been employed as catalysts for hydrosilylation (reduction) of
acetophenone derivatives. The reactions, in which 1,1,1,3,5,5,5-heptamethyltrisiloxane was a reducing
agent, proceeded in a biphasic system because the above complexes are insoluble in the reaction
medium. Thereby easy isolation of the complexes from post-reaction mixtures was possible after
reaction completion. This is the first example of the application of rhodium complexes of this type as
catalysts for ketone reduction. The complexes have shown high activity and enabled obtaining the
hydrosilylation product in a very short time and in the range of low concentrations (0.1 mol%). By using
FT-IR in situ analysis that enables measuring product concentrations in real time, a comparison has been
made of the catalytic activity for hydrosilylation of acetophenone and methoxyacetophenone isomers
shown by four rhodium complexes ([C+][RhCl4ꢀ]) differing in cations and the most effective catalyst for
this process has been distinguished.
Received 29th October 2018
Accepted 16th December 2018
DOI: 10.1039/c8ra08954j
rsc.li/rsc-advances
a plethora of transition metal catalysts have been exploited for
the hydrosilylation of ketones.5 Efficient catalysts include
mainly transition metal complexes5 but also main group metal
complexes.6 Furthermore, metal-free methods have also been
developed using acids,7 bases8 and alkali uorides9 to effect
these transformations, although such methods have mostly
required harsh reaction conditions.
Introduction
Catalytic reduction of C]O is one of the most fundamental
transformations in organic chemistry.1 It can be performed
using a number of reducing agents with dihydrogen being the
most frequently used.2 This transformation can also be ach-
ieved by using stoichiometric metal hydrides, although this is
concomitant with issues of low product selectivity and poor
atom economy. One appealing alternative to these methods is
the catalytic hydrosilylation of a carbonyl group. Formally,
hydrosilylation of ketones produces (in a single step) silyl
ethers, which are protected forms of secondary alcohols,
frequently used in organic synthesis. Among them, of particular
importance, are chiral secondary alcohols which are of great
signicance in agrochemicals and pharmaceuticals as well as in
ne chemicals.3 Alcohol synthesis requires an additional
hydrolysis (deprotection) step, which is realized by one of the
standard techniques depending on the specicity of the reac-
tion system. The discovery by Ojima of the high catalytic activity
of Wilkinson's catalyst [RhCl(PPh3)3] in hydrosilylation of
carbonyl compounds stimulated great development in hydro-
silylation of C]O bonds, which makes catalytic hydrosilylation
one of the most important and convenient reduction methods
for obtaining secondary alcohols.4 Up to the present day,
Within transition metal complexes the most efficient cata-
lysts are those based on Zn,10–13 Cu,14–18 Fe,19–23 Re,24–26 Ti27 and
Rh,4,28–33 and Mo,34 of which the rhodium ones have been most
comprehensively studied. Since the discovery of the activity of
Wilkinson's catalyst in hydrosilylation of ketones, rhodium
complexes are intensively studied and widely used in hydro-
silylation of carbonyl compounds.28 Over the last few years
a number of rhodium N-heterocyclic carbene complexes have
been developed (including the chiral ones) and proved active
catalysts for the reduction of carbonyl compounds.28,29 Exam-
ples could be NHC complexes with ferrocenyl substituents
which were employed as catalysts for hydrosilylation of aceto-
phenone derivatives and resulted in good selectivity and good
yield of the reaction.35 Carbene complexes with alkosilyl groups
(that make easier the immobilization on silica surface) have
been used in hydrosilylation of acetophenone with 1,1,1,3,5,5,5-
heptamethyltrisiloxane and very good catalytic results were
obtained.36 Unfortunately, homogeneous N-heterocyclic car-
bene complexes, although show satisfactory conversion of
reactants, not always are characterized by satisfactory selec-
tivity. Moreover, they are very susceptible to process conditions,
therefore the process carried out in their presence requires the
a
´
Faculty of Chemistry, Adam Mickiewicz University in Poznan, Umultowska 89B,
´
61-614 Poznan, Poland. E-mail: maciejm@amu.edu.pl
b
´
˙
Poznan Science and Technology Park, A. Mickiewicz University Foundation, Rubiez 46,
´
61-612 Poznan, Poland
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application of special conditions (inert atmosphere, Schlenk
technique).31,37 Yigit et al. have synthesized new carbene
complexes in the reaction of rhodium dimer [(Rh(OMe)COD)]2
with 1,3-dialkylimidazolium chloride in tetrahydrofuran and
used them in the reaction of acetophenone hydrosilylation with
triethylsilane. They reported obtaining good catalytic yield.38 It
is worth of mentioning that 1,3-dialkylimidazolium chloride is
a representative of a large group of compounds that are called
ionic liquids which recently arouse a large interest not only as
“green solvents”39,40 but also as immobilizing agents for metal
complexes. At present, over 100 types of chemical reactions
realized in the medium of ionic liquids and catalyzed by tran-
sition metal complexes are known.41,42 We have also developed
a number of effective catalytic systems based on diverse plat-
inum and rhodium complexes immobilized in the most
important kinds of ionic liquids.43–46 However, in addition to the
application of ionic liquids as immobilizing agents, more and
more frequently attempts are made at obtaining metal ion-
containing ionic liquids with catalytic properties. One of
examples are halometallate ionic liquids which can be dened
as ionic liquids formed by the reaction of metal halide with an
organic halide salt. The rst representatives of this group of
compounds were chloroaluminate ionic liquids,47 however, at
present, examples are known of compounds that contain tran-
sition metals such as cobalt, nickel, iridium, gold, palladium as
well as platinum.48,49 All the above compounds are basically
classied into the group of halometallate ionic liquids,
however, their melting points which exceed 150 ꢁC make it
difficult to classify them among ionic liquids, whose melting
points should be below 100 ꢁC. This is why it would be better to
call them anionic metal complexes. Very recently our research
group synthesized durable and air-stable anionic rhodium
complexes as a result of a direct reaction between rhodium(III)
chloride and derivatives of imidazole and pyridine (Scheme 1).50
The above complexes were applied as catalysts for hydro-
silylation processes. In order to compare the catalytic activity of
the obtained new rhodium complexes, acetophenone and its
methoxy group-containing derivatives were used. These
compounds are present in many fruits and plants and they
exhibit a number of interesting chemical and biological prop-
erties, due to which they are very attractive for researchers. The
other reagent was 1,1,1,3,5,5,5-heptamethyltrisiloxane that is
readily available, stable and, rst and foremost, cheap
compound compared to arylsilanes which were most frequently
employed as hydride donors in reduction reactions catalyzed by
rhodium complexes. To the best of our knowledge, this study on
the above anionic rhodium complexes is the rst example of the
employment of such complexes in hydrosilylation processes,
particularly for reduction of ketones.
Scheme 1 Synthesis of anionic rhodium(III) complexes.
heptamethyltrisiloxane, n-decane, toluene, Karstedt's catalyst
[Pt2{(CH2]CHSi(CH3)2)O}3] and Wilkinson's catalyst
[RhCl(PPh3)3], chloro(1,5-cyclooctadiene) rhodium(I) dimer
[{Rh(m-Cl)COD}2] and chlorobis(cyclooctene) rhodium(I) dimer
[{Rh(m-Cl)C8H14}2] were purchased from Sigma Aldrich and
used as received. The (1,5-cyclooctadiene)-m-(trimethylsiloxy)
rhodium(I) dimer complex [{Rh(m-OSiMe3)COD}2] was obtained
in accordance to the preparation procedure described by Mar-
ciniec et al.,51 whereas anionic platinum complexes [BMPy]2[-
PtCl4] and [BMIM]2[PtCl4] were prepared following the
procedure described by Zhong et al.52
The rhodium(III) chloride, RhCl3 (Pressure Chemicals Co.) was
used as the metal precursor for the obtained complexes, while the
ligand precursors were ionic liquids: 1-butyl-3-methylimidazolium
chloride, 1-butyl-2,3-dimethylimidazolium chloride, 1-butyl-4-
methylpyridinium chloride and 1-decyl-3-methylimidazolium
chloride, obtained from Iolitec. The yield of a product of a given
reaction was determined using a Clarus 680 gas chromatograph
(Perkin Elmer) equipped with a 30 m capillary column Agilent VF-
5ms and TCD detector, using the temperature program: 60 ꢁC
(3 min.), 10 ꢁC minꢀ1, 290 ꢁC (5 min.). FT-IR in situ measurements
were performed using a Mettler Toledo ReactIR 15 instrument. For
selected samples, spectra were recorded with 256 scans for 1 or 2
hours at 30 second intervals with a resolution of 1 cmꢀ1. For the
products obtained, NMR spectra were made with Bruker BioSpin
(400 MHz) spectrometer using acetonitrile-d3 as a solvent; chem-
ical shis are given in ppm.
The GC-MS analysis was conducted using
a Varian
3300 chromatograph equipped with a 30 m DB-1 capillary
column connected to the Finnigan Mat 700 mass detector.
ICP-MS analysis of post-reaction samples was carried out on
a
PerkinElmer Nexion 300D inductively coupled mass
spectrometer.
Synthesis of catalysts
The synthesis of anionic rhodium complexes was performed
according to the procedure described in the patent applica-
tion.50 Anionic rhodium complexes were obtained by placing
1 mmol of ionic liquid and 1 mmol of RhCl3$xH2O in hot
Experimental
All reagents used in the catalytic studies, i.e. acetophenone, 2- CH3CN (2 ml) in a Schlenk tube equipped with a magnetic
methoxyacetophenone, 3-methoxyacetophenone, 4-methox- stirring bar. The mixture was stirred for 3 h under reux. The
yacetophenone, 4-nitroacetophenone, 4-acetylbenzonitrile, 4- obtained solution was cooled to room temperature and the
iodoacetophenone,
acetophenone,
4-aminoacetophenone,
benzophenone,
4-methyl- solvent was evaporated. The product was washed with diethyl
1,1,1,3,5,5,5- ether (3 ꢂ 5 ml) and dried under vacuum.
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Spectroscopic characterization of the obtained rhodium
complexes:
(1-Butyl-3-methylimidazolium)tetrachlororhodiate(III). 1H
1-Phenyl-1-(3-siloxy-1,1,1,3,5,5,5-heptamethyltrisiloxane)
ethylene (product of dehydrogenative silylation). GC-MS: m/z
339.70 [M+], 73.10 [SiMe3]+, 147.30 [SiMe3OSiMeO]+, 221.50
[Me3OSiMeOSiMe3]+, 267.54 [PhC]CH2OSiMeOSiMe3O]+.
3-(1-(2-Methoxyphenyl)ethoxy)-1,1,1,3,5,5,5-heptamethyltri-
NMR (DMSO-d6): 8.40 (s, 1H, –N–CH]N–), 7.32 (s, 1H –CH]
CH–), 7.31 (s, 1H –CH]CH), 4.20 (t, 2H N–CH2–), 3.9 (s, 3H –N–
CH3), 1.75 (m, 2H, –CH2–), 1.23 (m, 2H, J ¼ 7.5, –CH2–), 1.0 (t,
3H, J ¼ 7.5, –CH3).
1
siloxane. H NMR (CD3CN): 7.48 (m, J ¼ 7.6 Hz, 1H, C]C–C),
7.24 (m, J ¼ 8.0 Hz, 1H, C–C]C), 6.96 (m, 2H, C]C–C), 5.40 (q,
J ¼ 6.3 Hz, 1H, CH), 3.83 (s, 3H, O–CH3), 1.37 (d, J ¼ 6.3 Hz, 3H,
C–CH3), 0.07–0.01 (m, 21H, Si–CH3).
13C NMR (DMSO-d6): 138 (N–CH]N); 123 (–CH]CH–), 121
(–CH]CH–); 50 (N–CH2), 37 (N–CH3); 32, 31 (–CH2–); 13 (–CH3).
1H
13C NMR (CD3CN): 154.91 (C–OCH3), 134.50 (C–C]C),
128.26, 120.35 (C]C–C), 116.67 (C–CN), 64.26 (C–O), 54.85
(-OCH3), 24.74 (C–CH3), 0.90 (Si–(CH3)3), ꢀ3.95 (O–Si–CH3).
29Si NMR (CD3CN): 8.07 (OSi(CH3)3), ꢀ58.14 (O–Si–O–).
GC-MS: 356.9 [PhOMeCHOSiMe(OSiMe3)2]+, 221.5 [Me3-
OSiMeOSiMe3]+, 151.1 [PhOCH3CHOCH3]+, 135.2 [PhOCH3-
CHCH3]+, 105.1 [PhCHCH3]+, 79.2 [C6H7]+, 78.2 [C6H6]+, 77.2
[C6H5]+, 73.2 [SiMe3]+.
(1-Butyl-4-methylpyridinium)tetrachlororhodiate(III).
NMR (DMSO-d6): 9.25 (d, 4H, J ¼ 7.21 Hz, Py–H), 8.9 (d, 4H, J ¼
7.21 Hz, Py–H), 4.92 (t, 2H, J ¼ 7.36 Hz, –N–CH2–), 2.65 (m, 2H, J
¼ 7.24, –CH2–), 2.41 (m, 2H, J ¼ 7.14, –CH2–), 1.86 (s, 3H, –CH3),
0.99 (t, 3H, J ¼ 7.15, –CH3).
13C NMR: 145 (CAr), 129 (CAr), 62 (–N–CH2), 33 (Ar–CH3), 19,
18 (–CH2–), 14 (–CH3).
(1-Butyl-2,3-dimethylimidazolium)tetrachlororhodiate(III).
1H NMR (DMSO-d6): 9.58 (s, 1H, –N–CH]N–), 7.32 (s, 1H,
–CH]CH–), 7.31 (s, 1H, –CH]CH), 4.51 (t, 2H,–N–CH2), 4.05
(s, 3H, –CH3), 3.93 (s, 3H, N–CH3), 2.25 (m, 2H, J ¼ 7.5, –CH2–),
1.75 (m, 4H,–CH2–), 1.27 (m, 2H, J ¼ 7.5, –CH2–), 0.9 (t, 3H, J ¼
7.5, –CH3).
1-(2-Methoxyphenyl)-1-(3-siloxy-1,1,1,3,5,5,5-heptamethyltri-
siloxane)ethylene (the product of dehydrogenative silylation).
GC-MS: 356.8 [PhOMeCHOSiMe(OSiMe3)2]+, 281.0 [PhOMeC]
CH2OSiMeOSiMe3]+, 267.0 [PhC]CH2OSiMeOSiMe3O]+, 221.5
[Me3OSiMeOSiMe3]+, 149.1 [PhOMeC]CH2O]+.
13C NMR (DMSO-d6): 138 (C–CH3); 123 (CH]CH), 117 (CH]
CH); 45 (N–CH2), 32 (N–CH3); 29.20 (–CH2–); 25.14 (–CH3).
(1-Decyl-3-methylimidazolium)tetrachlororhodiate(III). 1H
NMR (DMSO-d6): 9.50 (s, 1H, N–CH]N), 7.5 (s, 1H, CH]CH),
7.5 (s, 1H, CH]CH), 5.35 (t, 2H, –N–CH2), 4.1 (s, 3H N–CH3),
2.5–1.2 (m, 14H, –CH2–), 0.8 (t, 3H, J ¼ 7.15, –CH3).
13C NMR (DMSO-d6): 138 (N–CH]N); 123 (–CH]CH–), 121
(CH]CH); 47.5(N–CH3), 37 (–N–CH2–); 32–23 (–CH2–), 13 (–CH3).
3-(1-(3-Methoxyphenyl)ethoxy)-1,1,1,3,5,5,5-heptamethyl-
trisiloxane. 1H NMR (CD3CN): 7.34 (m, J ¼ 7.6 Hz, 1H, C]C–C),
7.0 (m, 2H, C–C]C), 6.83 (m, 1H, C]C–C), 5.06 (q, J ¼ 6.4 Hz,
1H, CH), 3.80 (s, 3H, O–CH3), 1.44 (d, J ¼ 6.3 Hz, 3H, C–CH3),
0.19–0.05 (m, 21H, Si–CH3).
13C NMR (CD3CN): 159.67 (C–OCH3), 148.06 (C–CHCH3),
129.15, 117.59 (C]C–C), 117.24 (C–CN), 112.15, 110.89 (C–C]C),
69.85 (C–O), 54.78 (–OCH3), 26.10 (C–CH3), 0.99 (Si–(CH3)3), ꢀ3.87
(O–Si–CH3).
29Si NMR (CD3CN): 9.08 (OSi(CH3)3), ꢀ57.83 (O–Si–O–).
GC-MS: 356.9 [PhOMeCHOSiMe(OSiMe3)2]+, 221.5 [Me3-
OSiMeOSiMe3]+, 151.1 [PhOCH3CHOCH3]+, 135.2 [PhOCH3-
CHCH3]+, 105.1 [PhCHCH3]+, 79.2 [C6H7]+, 78.2 [C6H6]+, 77.2
[C6H5]+, 73.2 [SiMe3]+.
1-(3-Methoxyphenyl)-1-(3-siloxy-1,1,1,3,5,5,5-heptamethyltri-
siloxane)ethylene (the product of dehydrogenative silylation).
GC-MS: 356.8 [PhOMeCHOSiMe(OSiMe3)2]+, 281.0 [PhOMeC]
CH2OSiMeOSiMe3]+, 267.0 [PhC]CH2OSiMeOSiMe3O]+, 221.5
[Me3OSiMeOSiMe3]+, 149.1 [PhOMeC]CH2O]+.
3-(1-(4-Methoxyphenyl)ethoxy)-1,1,1,3,5,5,5-heptamethyltri-
siloxane. 1H NMR (CD3CN): 7.30 (m, 2H, C]C–C), 6.90
(m, 2H, C–C]C), 5.02 (q, J ¼ 6.4 Hz, 1H, CH), 3.79 (s, 3H, O–
CH3), 1.41 (d, J ¼ 6.45 Hz, 3H, C–CH3), 0.10–0.01 (m, 21H, Si–
CH3).
General procedure for catalytic tests
The catalytic activity of the obtained anionic rhodium
complexes was determined in the reactions of hydrosilylation of
acetophenone and its derivatives with 1,1,1,3,5,5,5-heptame-
thyltrisiloxane (HMTS). For this purpose, 0.001 mmol of cata-
lyst, 1 mmol of ketone, 1.25 mmol of HMTS and 1 mmol of n-
decane as an internal standard were used. In addition, for the
systems with methoxyacetophenone, due to the fact that the
substrates were insoluble, it was necessary to add toluene in the
amount of 1.88 mmol for 2-methoxyacetophenone, 3.76 mmol
for 3-methoxyacetophenone and 5.64 mmol for 4-methox-
yacetophenone. The reaction was carried out in a reaction vessel
in the presence of air at 110 ꢁC with vigorous stirring for 1 h. The
reaction mixture was then cooled down and subjected to GC
analysis to determine the reaction yield. The product was iso-
lated and subjected to NMR and GC-MS analyses.
13C NMR (CD3CN): 158.89 (C–OCH3), 138.84 (C–CHCH3),
126.56 (C]C–C), 117.23 (C–CN), 113.39 (C–C]C), 69.58 (C–O),
54.85 (–OCH3), 26.04 (C–CH3), 0.97 (Si–(CH3)3), ꢀ3.81 (O–Si–CH3).
29Si NMR (CD3CN): 8.58 (OSi(CH3)3), ꢀ58.65 (O–Si–O–).
GC-MS: 356.9 [PhOMeCHOSiMe(OSiMe3)2]+, 221.50 [Me3-
OSiMeOSiMe3]+, 151.1 [PhOCH3CHOCH3]+, 135.2 [PhOCH3-
CHCH3]+, 105.1 [PhCHCH3]+, 79.2 [C6H7]+, 78.2 [C6H6]+, 77.2
[C6H5]+, 73.2 [SiMe3]+.
3-(1-Phenylethoxy)-1,1,1,3,5,5,5-heptamethyltrisiloxane. 1H
NMR (CD3CN): 7.28–7.10 (m; 5H; C]C), 4.98 (q; 1H; CH), 1.36
(d; 3H; CH3), 0.07–0.01 (m, 21H, Si–CH3).
13C NMR (CD3CN): 146.49 (C–C), 127.93 (C–C]C), 126.58,
124.64 (C]C–C), 116.67 (C–CN), 69.69 (C–O), 26.29 (C–CH3),
1.16 (Si–CH3), ꢀ3.84 (O–Si–CH3).
29Si NMR (CD3CN): 8.58 (OSi(CH3)3), ꢀ58.01 (O–Si–O–).
GC-MS: 327.30 [PhCHOSiMe3OSiMeOSiMe3]+, 221.50 [Me3-
OSiMeOSiMe3]+, 105.20 [PhCHCH3]+.
1-(4-Methoxyphenyl)-1-(3-siloxy-1,1,1,3,5,5,5-heptamethyltri-
siloxane)ethylene (the product of dehydrogenative silylation).
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GC-MS: 356.8 [PhOMeCHOSiMe(OSiMe3)2]+, 281.0 [PhOMeC]
CH2OSiMeOSiMe3]+, 267.0 [PhC]CH2OSiMeOSiMe3O]+, 221.5
[Me3OSiMeOSiMe3]+, 149.1 [PhOMeC]CH2O]+.
Hydrolysis of the hydrosilylation product
Scheme
2
Hydrosilylation of acetophenone with 1,1,1,3,5,5,5-
The product (1 ml, 2.75 mmol) of hydrosilylation of acetophe-
none and 1,1,1,3,5,5,5-heptamethyltrisiloxane and 3 ml of
0.4 mol HCl solution in acetone were placed in a round bottom
ask equipped with a magnetic stirrer. The reaction of hydro-
lysis was carried out for 4 h at room temperature. Then acetone
was evaporated and the mixture was allowed to stand in the
fridge for about 1 hour. Aer this time, the separation and GC
analysis of phases were possible. The obtained chromatograms
showed that the upper phase contained mainly by-products of
the silyl ether hydrolysis, while the desired product was in the
lower phase. Aer extracting the upper phase, the obtained
heptamethyltrisiloxane.
of post-reaction mixtures of all the systems studied has shown
that in some cases the product of dehydrogenative silylation
was formed, indeed. However, no other products were found in
addition to the main product of hydrosilylation as can be seen
in chromatograms of post-reaction mixtures. Exemplary chro-
matograms of the reaction mixtures before and aer the reac-
tion are shown in Fig. 2.
The employed anionic rhodium complexes were insoluble in
the reaction medium which enabled easy separation of post-
reaction mixture from catalyst and analysis of the mixtures
composition. The yields of products of the reaction of aceto-
phenone hydrosilylation catalyzed by the studied anionic
complexes are shown in Table 1. In all cases one can notice high
catalytic activity of the studied complexes in the presence of
which the hydrosilylation product (A) was mainly formed in
such a short time as 1 hour. Additionally, it should be stressed
1
sample was analyzed by H and 13C NMR to uniquely identify
the product. 1 ml of hydrosilylation reaction product (product
A) gave about 0.25 ml (2 mmol) of 1-phenylethanol (the yield of
about 73%).
1
1-Phenylethanol. H NMR (CD3CN): 7.54–7.13 (m, 5H), 4.84
(q, J ¼ 6.5 Hz, 1H, C–H), 3.19 (s, 1H, C–OH), 1.42 (d, J ¼ 6.5 Hz,
3H, –CH3); 13C NMR (CD3CN): 146.96 (C–C–OH), 128.21, (C–C),
126.85 (–C]C–), 125.34 (–C]C–C–), 117.36 (C–CN), 69.15 (C–
OH), 25.05 (C–CH3); GC-MS: 122.1 [PhCHOHCH3]+, 107.1
[PhCHOH]+, 105.1 [PhCHCH3]+, 77.2 [C6H5]+, 78.2 [C6H6]+, 79.2
[C6H7]+, 51.2 [C4H3]+.
Results and discussion
The reduction of ketones using rhodium catalysts is a well-
known and widely used reaction. This is why the above
process has been chosen as a test for determining catalytic
activity of four anionic rhodium complexes, the formulae of
which are as follows on Fig. 1.
The above complexes were applied as catalysts for the
reduction (hydrosilylation) of acetophenone with heptamethyl-
trisiloxane, which is presented in Scheme 2.
According to literature data, hydrosilylation of ketones can
be accompanied by dehydrogenative silylation which results in
the formation of a respective silyl enol ether (B) in addition to
the hydrosilylation product (A).53 Gas chromatographic analysis
Fig.
2 Exemplary chromatograms of hydrosilylation reaction
mixtures: (a) before the reaction, (b) after the reaction resulting in one
product only, and (c) after the reaction yielding two products (A and B).
Fig. 1 Anionic rhodium complexes studied.
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Table
1 Yields of the products of acetophenone hydrosilylation
data, were characterized by high effectiveness in hydrosilylation
processes, namely RhCl3, [RhCl(PPh3)3], [{Rh(m-OSiMe3)COD}2],
[{Rh(m-Cl)(C8H14)}2] and [{Rh(m-Cl)COD}2]. To this end, we have
carried out the reaction of acetophenone hydrosilylation in
conditions analogous to those used for the complexes synthe-
sized by us. The yields of products of the hydrosilylation reac-
tion catalyzed by the above compounds are presented in
Table 2.
catalyzed by anionic rhodium complexesa
Total yield Yield of product Yield of product
Catalyst
[%]
A [%]
B [%]
[BMPy]+[RhCl4]ꢀ
[BMIM]+[RhCl4]ꢀ
98
91
98
80
83
79
0
11
6
[BMMIM]+[RhCl4]ꢀ 89
[C10MIM]+[RhCl4]ꢀ 83
4
The performed measurements have shown that none of the
commercially available catalysts was able to give the product
with the yield greater than 37%, although they were highly
active in other reactions of hydrosilylation. All the above
rhodium complexes, except the simple salt RhCl3, had similar
activity, therefore one can be assume that such a low yield was
caused by conditions at which the reaction was conducted,
namely too low catalyst concentration and too short reaction
time. The above assumption was veried by a considerable
increase in the yield when the reaction was performed again
with tenfold greater concentration of the Wilkinson's catalyst
[RhCl(PPh3)3] and twice as long time. Thereby, higher activity of
anionic rhodium complexes was conrmed. Real-time moni-
toring of the reaction catalyzed by the latter complexes using in
situ FT-IR probe enabled to determine particular reaction
proles and to show differences in their activity. Fig. 3 presents
the change in hydrosilylation product yield as a function of time
as observed for particular reactions catalyzed by anionic
rhodium complexes (at the concentration of 0.1 mol%), con-
a
Reaction conditions: Rh-catalyst (0.001 mmol), acetophenone (1
mmol), HMTS (1.25 mmol), n-decane (1 mmol), T ¼ 110 ꢁC, t ¼ 1 h.
that the applied concentration of the catalyst was at least 10
times lower than commonly applied concentrations of catalysts
for this type of reactions (according to literature data, the
concentration is 1–5 mol%). Moreover, frequently a large excess
of the reducing agent (Si–H) was used, whereas in our case we
have applied the reactant ratio [Si–H] : [C]O] ¼ 1.25. The
highest yield and 100% selectivity was obtained for the reaction
catalyzed by the complex [BMPy][RhCl4]. In other cases, yields
were somewhat lower (91–83%) and the product of dehydro-
genative silylation was also present, albeit its amount was rather
small (4–11%). Thereby, one can conclude that complexes
containing imidazolium derivatives show somewhat poorer
selectivity than pyridinium cation-containing complexes. A
comparison of catalytic performances of complexes containing
imidazolium derivatives indicate that their activity is quite
similar, however, the complex [BMIM][RhCl4] shows poorer
selectivity.
It is known from literature data that rhodium complexes
dominate in hydrosilylation of ketones, although in general, the
most popular and commonly used catalysts for olen hydro-
silylation are platinum complexes. This is why we have
checked catalytic activity of two anionic platinum complexes
[BMPy]2[PtCl4] and [BMIM]2[PtCl4] (analogous to the studied
rhodium complexes) in the same conditions of the test reaction.
To our surprise, the effectiveness of these catalysts was very low
and product yields ranged between 3–10%. By comparison, the
activity of Karstedt's catalyst (which was also studied in the test
reaction) was higher and 30% yield was achieved. Nevertheless,
all platinum complexes (anionic ones in particular) showed
a lower activity for hydrosilylation of ketones compared to that
of anionic rhodium complexes. For this reason we also
compared the activity of the studied complexes with that of
commercially available rhodium complexes. We have chosen
several rhodium compounds which, according to literature
ꢁ
ducted at 110 C.
All the complexes make it possible to achieve maximum yield
in a very short time. The complex [BMPy][RhCl4] enabled to
obtain almost 100% yield already in the third minute of the
reaction, whereas the slowest catalyst [BMIM][RhCl4] reached
its maximum yield (91%) in the eighth minute. As it has been
already mentioned, the complex [BMPy][RhCl4] is characterized
not only by the highest yield but also by the fastest reaction rate.
Of course, the reaction course is also inuenced by the process
temperature. The mentioned FT-IR probe was also employed in
the studies of reactions catalyzed by [BMPy][RhCl4], but carried
out at different temperatures and the obtained results show
signicant differences in their course, as can be seen in Fig. 4
and Table 3.
The results proved that the catalyst was also active at a lower
temperature, however, the reaction time has been signicantly
extended. At the temperature of 80 ꢁC, the catalyst activation
time is about one hour. The reaction time is shortened as the
temperature rises, as shown by the activation times at 90 ꢁC to
Table 2 Yields of the products of acetophenone hydrosilylation catalyzed by commercially available rhodium complexesa
Catalyst
Total yield [%]
Yield of product A [%]
Yield of product B [%]
[RhCl(PPh3)3]
32
37
22
3
29
36
21
3
3
1
1
0
2
[{Rh(m-Cl)(COD)}2]
[{Rh(m-Cl)(C8H14)}2]
RhCl3
[{Rh(m-OSiMe3)COD}2]
33
31
a
Reaction conditions: Rh-catalyst (0.001 mmol), acetophenone (1 mmol), HMTS (1.25 mmol), n-decane (1 mmol), T ¼ 110 ꢁC, t ¼ 1 h.
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i.e. we deal with a heterogeneous catalytic reaction in the
presence of a solid catalyst. Such a reaction is a complex process
which consists of several steps, the nature of which is physical
(diffusion of substrates and products in catalyst pores) or
chemical (chemisorption of at least one substrate on the cata-
lyst surface, surface reaction between chemisorbed species and
product desorption). With a rise in temperature, both the rate of
diffusion and the rate of chemisorption increase. Another factor
that seems to affect the activation of the catalysts is their
hydrophobicity. As results from Fig. 3, the fastest activation
occurs in the case of the complexes [BMPy][RhCl4] and
[C10MIM][RhCl4], whose cations are more hydrophobic (due to
the presence of a long alkyl substituent or aromatic ring) and
the slowest activation proceeds in the case of the complex
[BMIM][RhCl4], whose cation is less hydrophobic. This permits
to conclude that a greater hydrophobicity and higher tempera-
ture favor the reaction studied. Since at 110 ꢁC the reaction
starts immediately and no undesirable products are formed, the
above temperature can be regarded as optimal for the reaction.
All the above studies concerned hydrosilylation of aceto-
phenone, but at the next stage of our research we determined
catalytic performance of anionic rhodium complexes in hydro-
silylation of methoxyacetophenone. The effect of the position of
the substituent attached to the benzene ring on the catalytic
activity of the obtained catalysts was also investigated. For this
purpose, acetophenone with the methoxy group in the ortho,
meta and para positions was used. The yields achieved in the
particular reactions are presented in Table 4.
Fig. 3 The change in the hydrosilylation product yield as a function of
time for the reactions catalyzed by anionic rhodium complexes.
30 min. and 100 ꢁC to 9 min. At 110 ꢁC, the reaction starts
almost immediately aer the addition of the catalyst and no by-
product formation is observed in this case.
In other cases, lower temperatures of the system, except
longer reaction times, also affected the formation of the unde-
sirable by-product. Unquestionably, all the studied catalysts are
very active as evidenced by almost immediate conversion of
substrates (a nearly vertical course of the curves in Fig. 2 and 3
at the moment of the reaction initiation). As concerns the effect
of temperature on the reaction initiation time, one can notice
that the studied catalysts are insoluble in the reaction medium,
The obtained results show that catalytic activities of the
studied complexes are even more diversied. The complex
[BMPy][RhCl4] is one of the most active and selective catalysts,
but also the complex [C10MIM][RhCl4] proved to be a very effi-
cient catalyst which, as the only one, made it possible to obtain
the product yield of 89% already aer 1 hour of the reaction
with 2-methoxyacetophenone. Other complexes required longer
reaction times, albeit aer 2 hours only the complex [BMPy]
[RhCl4] enabled the achieve the yield of 96% (in Table 4, the
values in parentheses present the yields aer 2 hours of reac-
tion). Certainly the steric hindrance that makes the reaction
progress difficult is the location of the methoxy substituent at
the ortho position. In hydrosilylation of the two remaining
derivatives having the methoxy substituent in meta and para
positions, the greatest effectiveness was shown by [BMPy]
[RhCl4], which made it possible to obtain aer 1 hour of the
reaction the yields of 100 and 85%, respectively. Also the
complex [C10MIM][RhCl4] enabled to achieve high yields of 67
and 82%, respectively. On the other hand, the complex [BMIM]
[RhCl4] proved to be denitely the poorest catalyst in all the
reactions conducted. The two aforementioned complexes
contain imidazole derivatives and differ only in the length of
alkyl substituent attached to imidazole ring. Taking into
consideration that the alkyl chain length inuences hydro-
phobic properties, one can conclude that the highest hydro-
phobic properties of the catalytic system the easier the reaction
proceeds. Since all the above complexes are insoluble in the
reaction medium and catalysis is a surface phenomenon, then
surface properties are crucial for the course of catalysis.
Fig. 4 The change in the hydrosilylation product yield as a function of
time as observed for the reaction catalyzed by [BMPy][RhCl4], carried
out at different temperatures.
Table
3 Yields of the products of acetophenone hydrosilylation
catalyzed by [BMPy][RhCl4], carried out at different temperaturesa
Temperature
[ꢁC]
Total yield
[%]
Yield of product
A [%]
Yield of product
B [%]
80
90
100
110
89
93
96
98
86
90
96
98
3
3
0
0
a
Reaction conditions: Rh-catalyst (0.001 mmol), acetophenone (1
mmol), HMTS (1.25 mmol), n-decene (1 mmol).
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Table 4 Yields of the products of hydrosilylation of acetophenone derivatives catalyzed by anionic rhodium complexesa
Ketone
Catalyst
Total yield [%]
Yields of products A/B [%]
[BMPy]+[RhCl4]ꢀ
[BMIM]+[RhCl4]ꢀ
[BMMIM]+[RhCl4]ꢀ
[C10MIM]+[RhCl4]ꢀ
16 (96)
11 (16)
0 (33)
16/0 (95/1)
11/0 (16/0)
0/0 (33/0)
89/0 (96/4)
89 (100)
[BMPy]+[RhCl4]ꢀ
[BMIM]+[RhCl4]ꢀ
[BMMIM]+[RhCl4]ꢀ
[C10MIM]+[RhCl4]ꢀ
100
36
52
100/0
36/0
52/0
67/0
67
[BMPy]+[RhCl4]ꢀ
[BMIM]+[RhCl4]ꢀ
[BMMIM]+[RhCl4]ꢀ
[C10MIM]+[RhCl4]ꢀ
85
24
72
82
81/4
16/8
71/1
80/2
a
Reaction conditions: Rh-catalyst (0.001 mmol), acetophenone (1 mmol), HMTS (1.25 mmol), T ¼ 110 ꢁC, t ¼ 1 h. Yields of the reaction carried out
for 2 hours were given in parentheses.
In the case of hydrosilylation of other acetophenone deriv- participation of 4-methylacetophenone (Table 5) which
atives, a signicant effect was observed of substituents in ace- contains an electron-donating substituent, just as 4-
tophenone on the catalytic activity of the studied complexes. methoxyacetophenone.
Derivatives containing electron-donating and electron-
Also in this case one can notice a similar tendency of activity
withdrawing substituents were employed in the study and the changes because the best catalysts proved to be the complexes
obtained results showed that in the case of compounds con- [BMPy]{RhCl4] and [C10MIM][RhCl4]. However, the yields are
taining electron-withdrawing substituents, namely 4-nitro- lower than those obtained with methoxyacetophenone, despite
acetophenone, 4-acetylbenzonitrile, 4-iodoacetophenone and the extension of the reaction time to two hours. A comparison of
pentauoroacetophenone, no conversion of the substrates was the two acetophenone derivatives shows that methoxy substit-
observed in the presence of the investigated complexes. No uents have stronger electron-donating properties than methyl
reaction also occurred in the case of benzophenone, most likely groups, therefore, one can state that anionic rhodium
due to a steric hindrance effect. On the other hand, a better complexes catalyze reactions of acetophenone derivatives con-
result was obtained in the reaction conducted with the taining electron-donating substituents and the stronger the
Table 5 Yields of the products of hydrosilylation of 4-methylacetophenone catalyzed by anionic rhodium complexesa
Ketone
Catalyst
Total yield [%]
Yields of products A/B [%]
[BMPy]+[RhCl4]ꢀ
[BMIM]+[RhCl4]ꢀ
[BMMIM]+[RhCl4]ꢀ
[C10MIM]+[RhCl4]ꢀ
56
14
25
51
54/2
12/2
22/3
51/0
[BMPy]+[RhCl4]ꢀ
[BMIM]+[RhCl4]ꢀ
[BMMIM]+[RhCl4]ꢀ
[C10MIM]+[RhCl4]ꢀ
71
23
45
67
71/0
19/4
40/5
65/2
a
Reaction conditions: Rh-catalyst (0.001 mmol), methylacetophenone (1 mmol), HMTS (1.25 mmol), T ¼ 110 ꢁC, t ¼ 2 h.
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Table
7
Yields of the products of acetophenone hydrosilylation
electron-donating effect, the easier the reaction proceeds.
Admittedly, we have also conducted hydrosilylation of 4-ami-
noacetophenone (amino substituent is classied into strong
electron donors), but the achieved yields were low. However, in
this case the strong electron-donating effect of amino group
probably caused catalyst poisoning, which is frequently
observed in hydrosilylation of amino derivatives.5
catalyzed by anionic rhodium complexes, obtained in two subsequent
catalytic reaction cycles using the same catalyst portion without its
isolation
Yields of products (A/B) [%]
Catalyst
Catalytic cycle 1 h
2 h
3 h
[BMPy]+[RhCl4]ꢀ
[C10MIM]+[RhCl4]ꢀ
1
2
1
2
98 (98/0)
82 (80/2) 84 (82/2) 87 (83/4)
84 (80/4)
As was already mentioned, all the studied catalysts are
insoluble in the reaction medium, which makes it possible to
separate them from post-reaction mixtures. This is why we have
made an attempt at their reusing in subsequent catalytic cycles.
Table 6 presents the yields acetophenone hydrosilylation
products obtained in subsequent reaction runs. Aer each
cycle, the post-reaction mixture was decanted from the solid
catalyst and subjected to chromatographic analysis, while the
catalyst was reused in a subsequent reaction run with a new
portion of substrates.
The catalytic tests showed that all the studied complexes
were catalytically active in the second cycle, albeit their activity
was lower than in the rst cycle. Subsequent cycles required
a longer reaction time, but aer the third cycle the yield, even at
extended time, was only on the level of a dozen percent. It is
worth mentioning that the employed catalysts were in the form
of a ne-crystalline powder and, despite being insoluble despite
being insoluble in the reagents, they remained dispersed in
post-reaction mixtures for quite a long time. Although the post-
reaction mixtures were allowed to stand for a long time, placed
62 (60/2) 65 (62/3) 70 (67/3)
a possibility of conducting the reaction at even lower catalyst
concentration which is an additional economic asset. By using
coarse–crystalline complexes or applying efficient sedimenta-
tion methods a more effective isolation of catalysts should be
achieved, thus creating a possibility of their multiple use.
Further research on a better isolation of catalysts as well as
on the mechanism of the reactions catalyzed by anionic
rhodium complexes is underway but the studies carried out
hitherto permit to designate two complexes [BMPy][RhCl4] and
[C10MIM][RhCl4] (particularly the former) as very effective
catalysts for hydrosilylation of ketones.
Conclusions
in a refrigerator and attempts were made at centrifuging, it is Synthesis of anionic rhodium complexes is very simple and
possible that a part of the catalyst suspension was removed enables obtaining durable air-stable complexes, due to which
together with reaction products and this can be the reason for one can deal with them without any protection measures. The
the yield reduction caused by a decrease in the concentration of above compounds proved to be catalytically active for the
catalyst. This is why we conducted for the two most active process of ketone reduction. This is the rst example of the
catalysts a subsequent reaction cycle without isolation, i.e. aer application of complexes of this type to hydrosilylation of
the reaction completion a new portion of substrates was added. ketones. Additionally, the reactions of hydrosilylation were
The obtained yields are presented in Table 7.
conducted using heptamethyltrisiloxane which is generally
The performed experiment proved that the studied available and cheap compound compared to arylsilanes that
complexes do not lose their activity and that there is were most frequently applied as hydride donors in the reactions
catalyzed by rhodium complexes. Anionic rhodium complexes
are insoluble in reactions medium, due to which they enable
easy separation of catalyst from post-reaction mixtures. The
performed tests proved that the catalysts are active in the
acetophenone-HMTS system (1 mmol : 1.25 mmol) even with
a slight excess of siloxane. It has also been shown that the above
catalytic complexes are active in the reaction of acetophenone
hydrosilylation at much lower concentration (0.1 mol%) than
commonly used in numerous studies reported in the literature.
Moreover, there is a possibility of catalyst reuse in the next
reaction cycle (both with isolation and without isolation of the
catalyst). This fact allows to reduce production costs (e.g.
secondary alcohols) and also affects the purity of the isolated
product. Compared to standard homogeneous rhodium cata-
lysts used in hydrosilylation reactions, this is an important
advantage. It was also shown that the hydrosilylation reaction in
the presence of anionic rhodium complexes in the
acetophenone-HMTS system proceeds very quickly and with
very good selectivity. The most effective catalyst for acetophe-
none reduction proved to be the complex containing the
Table
6 Yields of the products of acetophenone hydrosilylation
catalyzed by anionic rhodium complexes, in subsequent catalytic
reaction cycles using the same portion of catalysta
Yields of products (A/B) [%]
Catalyst
Catalytic cycle 1 h
2 h
3 h
[BMPy]+[RhCl4]ꢀ
1
2
3
1
2
3
1
2
3
1
2
3
98 (98/0)
27 (26/1)
6 (6/0)
91 (80/11)
23 (22/1)
6 (5/1)
89 (83/6)
30 (27/3)
7 (5/2)
38 (37/1) 46 (45/1)
12 (10/2) 15 (13/2)
[BMIM]+[RhCl4]ꢀ
[BMMIM]+[RhCl4]ꢀ
[C10MIM]+[RhCl4]ꢀ
41 (40/1) 53 (52/1)
11 (10/1) 13 (12/1)
37 (34/3) 43 (40/3)
14 (11/3) 17 (14/3)
83 (79/4)
62 (59/3)
8 (8/0)
66 (63/3) 67 (64/3)
10 (10/0) 16 (16/0)
a
Reaction conditions: Rh-catalyst (0.001 mmol), acetophenone (1
mmol), HMTS (1.25 mmol), T ¼ 110 ꢁC.
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pyridine ligand [BMPy]+[RhCl4]ꢀ. In the case of hydrosilylation
of methoxyacetophenone isomers visible is the effect of the
presence and position of methoxy group on the reaction course
and product yields. Hydrosilylation of 2-methoxyacetophenone
is more difficult because of a steric hindrance which made
necessary to extend the reaction time to obtain yields compa-
rable to those achieved in the reaction of acetophenone.
Nevertheless, the obtained yields were similar to those obtained
6 N. J. Lawrence and S. M. Bushell, Tetrahedron Lett., 2000, 41,
4507–4512.
7 N. Asao, T. Ohishi, K. Sato and Y. Yamamoto, J. Am. Chem.
Soc., 2001, 123, 6931–6932.
8 F. Le Bideau, C. T. D. Gourier, J. Hanique and E. Samuel,
Tetrahedron Lett., 2000, 41, 5215–5218.
9 Y. Kawanami, H. Yuasa, F. Toriyama, S. Yoshida and T. Baba,
Catal. Commun., 2003, 4, 455–459.
¨
with unsubstituted acetophenone only in the case of two most 10 S. Enthaler, K. Schroder, S. Inoue, B. Eckhardt, K. Junge,
effective complexes, namely [BMPy][RhCl4] and [C10MIM]
M. Beller and M. Drieß, Eur. J. Org. Chem., 2010, 25, 4893–
[RhCl4]. Reactions with other methoxyacetophenone isomers
4901.
having the CH3O group in the position meta or para proceed 11 T. Inagaki, Y. Yamada, L. Thanh Phong, A. Furuta, J. Ito and
easier, however, also in this case the signicant superiority of H. Nishiyama, Synlett, 2009, 2, 253–256.
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also the most selective) is visible. A similar effect is observed for Chem. Commun., 2003, 332–333.
hydrosilylation of ortho and para isomers of methyl- 13 W. Sattler, S. Ruccolo, M. R. Chaijan, T. N. Allah and
acetophenone, however, in this case the activity of the studied G. Parkin, Organometallics, 2015, 34, 4717–4731.
complexes is lower because of a weaker electron-donating effect 14 S. Rendler and M. Oestreich, Angew. Chem., Int. Ed., 2007, 46,
of methyl group, compared to methoxy group. On the other 498–504.
hand, it has been unequivocally established that anionic 15 S. Diez-Gonzales and S. P. Nolan, Acc. Chem. Res., 2008, 41,
rhodium complexes effectively catalyze reactions of acetophe- 349–358.
none derivatives with electron-donating substituents, whereas 16 B. H. Lipshutz in Modern Organocopper Chemistry, ed.
are not active for reactions of the derivatives with electron- N.Krause, Wiley-VCH, Weinheim, 2002, pp. 167–187.
withdrawing substituents. Summing up, the above complexes 17 F. Lazreg and C. S. J. Cazin in N-Heterocyclic Carbenes, ed. S.
can be a very good alternative to hitherto applied ketone P.Nolan, Wiley-VCH, Weinheim, 2014, pp. 199–242.
reduction catalysts due to their easy synthesis, stability under 18 C. Deutsch, N. Krause and B. H. Lipshutz, Chem. Rev., 2008,
atmospheric conditions, high effectiveness in the range of low 108, 2916–2927.
concentrations as well as easy isolation from post-reaction 19 B. Blom, S. Enthaler, S. Inoue, R. Irran and M. Driess, J. Am.
mixtures resulting from their insolubility in reaction medium. Chem. Soc., 2013, 135, 6703–6713.
Moreover, the complexes catalyze reactions with the use of 20 Z. Zuo, L. Zhang, X. Leng and Z. Huang, Chem. Commun.,
´
simple and cheap heptamethyltrisiloxane as a hydride donor.
2015, 51, 5073–5076.
21 F. S. Wekesa, R. Arias-Ugarte, L. Kong, Z. Summer,
G. P. McGovern and M. Findlater, Organometallics, 2015,
34, 5051–5056.
Conflicts of interest
22 A. Raya-Baron, M. A. Ortuna, P. Ona-Burgos, A. Rodriguez-
Dieguez, R. Langer, C. J. Cramer, I. Kuzu and I. Fernandez,
Organometallics, 2016, 35, 4083–4089.
There are no conicts of interest to declare.
Acknowledgements
23 K. Junge, K. Schroeder and M. Beller, Chem. Commun., 2011,
47, 4849–4859.
Financial support from the National Science Center (Poland),
24 J. J. Kennedy-Smith, K. A. Nolin, H. P. Gunterman and
F. D. Toste, J. Am. Chem. Soc., 2003, 125, 4056–4057.
25 K. A. Nolin, J. R. Krumper, M. D. Pluth, R. G. Bergman and
F. D. Toste, J. Am. Chem. Soc., 2007, 129, 14684–14696.
26 H. Dong and H. Berke, Adv. Synth. Catal., 2009, 351, 1783–
1788.
grant
OPUS
UMO-2014/15/B/ST5/04257,
is
gratefully
acknowledged.
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