A convenient and efficient rhenium-Catalyzed hydrosilylation of ketones and aldehydes

Article abstract of DOI:10.1002/adsc.200900246

The easily available rhenium(I) complex [Re(CH₃CN) ₃Br₂(NO)] catalyzes the homogeneous hydrosilylation of a great variety of organic carbonyl compounds (ketones and aldehydes). The reaction is quite sensitive to the solven

Full text of DOI:10.1002/adsc.200900246

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DOI: 10.1002/adsc.200900246
A Convenient and Efficient Rhenium-Catalyzed Hydrosilylation
of Ketones and Aldehydes
Hailin Donga and Heinz Berkea,*
a
Anorganisch-chemisches Institut, Universitꢀt Zꢁrich, Winterthurerstrasse 190, 8057 Zꢁrich, Switzerland
Fax : (+41)-1-63-568-02; phone: (+41)-1-63-546-81; e-mail: hberke@aci.uzh.ch
Received: April 9, 2009; Published online: July 27, 2009
Abstract: The easily available rhenium(I) complex
[Re(CH3CN)3Br2(NO)] catalyzes the homogeneous
drosilylation using high oxidation state rhenium oxo
complexes as catalysts.
ACHTUNGTRENNUNG
hydrosilylation of a great variety of organic carbon-
yl compounds (ketones and aldehydes). The reac-
tion is quite sensitive to the solvent applied.
We report here the efficient hydrosilylation of vari-
ous ketones and aryl aldehydes catalyzed by a phos-
phine-free rhenium(I) system. The reaction was car-
ried out at room temperature with relatively low cata-
lyst loading (0.2–1.0 mol%) giving good to excellent
yields. The TOF values of the reaction can be greatly
enhanced by elevating the reaction temperature to
858C.
Several rhenium complexes were examined in vari-
ous solvents at room temperature for the hydrosilyla-
tion of butanone with triethylsilane as a model reac-
tion (Table 1). In this reaction butanone was convert-
ed to the corresponding silyl ether. The yield of sec-
butoxytriethylsilane was dramatically affected by the
type of the rhenium complexes and solvents used in
ChloroACHTUNGTRENNUNGbenzene was found to be superior over all
the other solvents used. Various aliphatic and aro-
matic silanes were tested. Excellent yields were ach-
ieved at 858C in chlorobenzene using triethylsilane,
the reaction affording TOF values of up to 495 hÀ1.
A possible reaction mechanism for the hydrosilyla-
tion is presented.
Keywords: aldehydes; homogeneous catalysis; hy-
drosilylation; ketones; reaction mechanism; rheni-
um
the reaction. [Re
most reactive complex the reaction going to comple-
tion in 1.5 h (entry 1). [Re(THF)(CH3CN)2Br2(NO)]
ACHTUGNTRENUN(GN CH3CN)3Br2(NO)] proved to be the
A
ACHTUNGTRENNUNG
Transition metal-catalyzed hydrosilylations are useful gave a good yield, but the reaction time required was
alternatives to hydrogenations and transfer hydroge- 2.5 h (entry 9). Other rhenium complexes containing
nations of C=C and carbon-heteroatom multiple various phosphines gave only modest yields after 1–
bonds.[1] In recent years hydrosilylation of various or- 2 h (entries 11–14). Several water-soluble rhenium
ganic carbonyl compounds has made considerable complexes showed very little or no activity (en-
progress and became a major tool of synthetic organic tries 15–17). The rhenium complexes, such as
chemistry and organosilicon chemistry as well as pro- ([Re
viding efficient and versatile access to new com- not exhibit any activity at all (entries 10 and 18).
pounds.[1p,q] Asymmetric hydrosilylations with high
When the reaction was carried out in coordinating
ACHTUTNGRENNU(G CH3CN)3Cl2(NO)], [ReACHTUNGTREN(NNGU TACN)(CO)2Br]), did
enantioselectivities are also been well documen- solvents such as THF and CH3CN, only modest yields
ted.[1b,d,m,n] In industry hydrosilylation has become also were obtained (entries 3 and 4) When aromatic hy-
an appropriate method to produce organosilicon com- drocarbons, such as benzene and toluene were used as
pounds, in particular with respect to the functionaliza- solvents, the reaction proceeded very slowly. In sol-
tion of polymers.[2]
vents of higher polarity, like DMF and DMSO, the
Exploring rhenium complexes for their use as cata- yields were also very low. Dichloromethane and diox-
lysts, our early work has led to the development of ane were therefore proper choices of reaction solvents
several efficient systems for transfer hydrogenation, (entries 1–2).
dehydrogenative silylation and hydrosilylation.[3] Re-
In order to understand the influence of the silane
cently, hydrosilylation reactions using various metals applied in the catalytic hydrosilylation, the reactions
such as zinc,[4 iron,[5] rhodium[6] and copper[7] have of a variety of silanes were tested with butanone in di-
been reported, including the work of other authors chloromethane at room temperature (Table 2). It ap-
with rhenium catalysts.[8] Toste et al.[8a–c] and Abu- peared that the choice of the silane had a significant
Omar et al.[8d–g] proposed new mechanisms for the hy- influence on the outcome of the reaction. Triethylsi-
Adv. Synth. Catal. 2009, 351, 1783 – 1788
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1783

Hailin Dong and Heinz Berke
COMMUNICATIONS
Table 1. Rhenium-catalyzed hydrosilylation of butanone with Table 2. Hydrosilylation of butanone catalyzed by the
triethylsilane.[a]
[ReACTHUNGETRNNUG
(CH3CN)3Br2(NO)] complex.[a]
Entry
Silane
t [h]
Yield [%][b]
1
2
3
4
5
6
7
8
HSiEt3
PhSiH3
Ph2MeSiH
PhMe2SiH
T
1.5
2
2
2
2
2
2
2
2
99
31
54
51
10
67
26
25
29
10
Entry Catalyst
Solvent
t
Yield
[h] [%][b]
1
2
3
4
5
6
7
8
9
10
[Re
[Re(CH3CN)3Br2(NO)]
[Re(CH3CN)3Br2(NO)]
[Re(CH3CN)3Br2(NO)]
[Re(CH3CN)3Br2(NO)]
[Re(CH3CN)3Br2(NO)]
[Re(CH3CN)3Br2(NO)]
[Re(CH3CN)3Br2(NO)]
[Re(THF)(CH3CN)2Br2(NO)]
(CH3CN)3Cl2(NO)]
G
CH2Cl2 1.5 99
Dioxane 1.5 90
N
THF
CH3CN 1.5 45
DMF 1.5 50
1.5 69
R
9
10
C
2
Benzene 1.5 75
DMSO 1.5 10
Toluene 1.5 8
CH2Cl2 2.0 75
CH2Cl2 10 0
Dioxane
CH2Cl2 10 0
Dioxane
CH2Cl2 18 5
CH2Cl2 18 37
CH2Cl2 18 40
CH2Cl2 10 1.3
[a]
Reaction conditions: 1.0 mol% of the complex
[Re(CH3CN)3Br2(NO)], 0.5 mmol of butanone and
AHCTUNGTRENNUNG
1.2 equiv. of appropriate silanes in dichloromethane
(0.8 mL) at room temperature for the given time, under
a nitrogen atmosphere.
[Re
[Re
[b]
[c]
Yield on the basis of the integration of the 1H NMR
spectrum.
11
A
N
Polymethylhydrosiloxane.
12
13
14
15
[Re
[Re
[Re
[Re
N
E
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
obtaining similar yields (entries 9–11). 2, 2-Dimethyl-
1-phenylpropan-1-one was converted to the silyl ether
with the yield of 58% after 8 h presumably due to the
hindrance of the bulky isopropyl group.
Dioxane
(CH3CN)][Br]2 CH2Cl2 10 0
Dioxane
0.8
16
17
18
[ReACHTUNGTRENNUNG(PTAH)2Br2(NO)ACTHUNGTRENNNUG
[Re
[Br]
[Re
(TACN)[d](CO)2Br]
ACHTUNGTRENNUNG(PTAH)ACHTUNGTRENNUNG(PTA)Br2(NO)ACTHUNTGRENNNGU
Trying to extend the scope of the rhenium-cata-
lyzed hydrosilylation, a screening of various aldehyde
substrates was initiated bearing a variety of substitu-
ents at varying aromatic positions (Table 4). Benzal-
dehyde, 2-phenylpropanal and 4-methylbenzaldehyde
gave high yields of 95%, 99% and 98%, respectively
(entries 1–3). The reaction proceeded smoothly inde-
pendent of the position of the aryl substitution (ortho,
meta or para), which did not affect the reaction rate
to a great extent. Substrates with methyl or chlorine
substitution required relatively longer reaction times
to render satisfactory yields. The catalyst showed
great tolerance for electron-withdrawing and elec-
tron-donating substitution on the aryl rings (entries 7–
A
CH2Cl2 10 0
DMSO
[a]
All reactions were carried out with butanone (0.5 mmol),
Et3SiH (0.6 mmol), and the rhenium complex (1.0 mol%) in
the appropriate solvent (0.8 mL) at room temperature under
a nitrogen atmosphere. Progress of the reaction was moni-
tored by 1H NMR.
Yield on the basis of the integration of the H NMR spec-
trum.
PTA=1,3,5-triaza-7-phosphaadamantane.
TACN =1,4,7-triazacyclononane.
[b]
1
[c]
[d]
lane proved to be the most suitable silane source for 12). The aliphatic ketones worked better than aromat-
the reaction with yields of 99% in 1.5 h (entry 1). In ic ones presumably because of their higher electron-
general, aliphatic silanes led to better yields in com- donating abilities and consequently their better ligand
parison with arylsilanes. An exception was i-Pr3SiH, properties.
which gave a low yield, presumably due to its bulky
To improve the catalytic activity (TOFs) and for
substituents, as also did HSi(SiMe3)3 (entries 5 appropriate practical purposes, higher temperatures
AHCTUNGTRENNUNG
and10). Secondary silanes, alkoxysilanes and poly- and lower catalyst loadings were tested. Representa-
silanes also did not provide satisfactory yields (en- tive yields for the hydrosilylation of various organic
tries 7–9).
A variety of ketones were reduced to the corre- ized in Table 5. Low catalyst loadings (0.2 mol%) and
sponding silyl ethers catalyzed by the relatively shorter reaction times (1–2 h) were success-
[Re(CH3CN)3Br2(NO)] complex (Table 3). In most ful when the reactions were carried out in chloroben-
cases the reactions were complete within 2 h or less zene at 858C.
carbonyl compounds with triethylsilane are summar-
ACHTUNGTRENNUNG
with excellent yields (entries 1–8). Substrates contain-
A possible mechanism for the hydrosilylation of ke-
ing bulky groups, such as 3, 4-dihydro-1(2H)-naphtha- tones catalyzed by the [ReACHTNUGTRENNUNG
ACHTUNGTRENNUNGlenone, 9-fluorenone and 2-decanone, required 3 h for plex is proposed in Figure 1. [ReACTHUNGTRENNUNG
1784
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A Convenient and Efficient Rhenium-Catalyzed Hydrosilylation of Ketones
COMMUNICATIONS
Table 3. [Re
(CH3CN)3Br2(NO)]-catalyzed hydrosilylation of
Table 4. [Re
aldehydes with HSiEt3
ACHTNUGRETN(NUGN CH3CN)3Br2(NO)]-catalyzed hydrosilylation of
[a]
ketones with triethylsilane.[a]
Entry
Substrate
t [h]
1.5
Yield [%][b]
98[c]
Entry
Substrate
t [h]
1.5
2
Yield [%][b]
1
2
1
2
3
4
95
99
98
94
1.5
99[c]
3
1.5
95[c]
2
4
5
6
1.5
1.5
1.5
99
2
98[c]
93
5
6
2
2
93
91
7
8
9
1.5
2
99[c]
96
7
8
4
4
89
87
5
90
9
6
99
10
11
3
95
10
11
8
8
8
95
95
90
3
8
92
58
12
12
[a]
Reaction conditions: 1.0 mol% of the complex
[Re(CH3CN)3Br2(NO)], 0.5 mmol substrate, and
[a]
Reaction conditions: 1.0 mol% of the complex
[Re(CH3CN)3Br2(NO)], 0.5 mmol of ketone and
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
1.2 equiv. of HSiEt3 in dichloromethane (0.8 mL) at
room temperature, under nitrogen atmosphere. Progress
1.2 equiv. of HSiEt3 in dichloromethane (0.8 mL) at
room temperature for given time, under nitrogen atmos-
phere.
1
of the reaction was monitored by H NMR.
[b]
Yield on the basis of the integration of the 1H NMR
spectrum.
[b]
[c]
Yield on the basis of the integration of the 1H NMR
spectrum.
Yield measured by GC-MS, based on the ketone.
takes the role of a pre-catalyst via primary dissocia- a classical ReACHTGNUTRENNUNG
1
tion of one of the CH3CN ligands. In the H NMR
ACHTUNGTRENNUNG
spectrum this dissociation became evident by a singlet dition a long range C,H-correlation experiment was
at 1.98 ppm assigned to the CH3 group of free carried out in support of the simultaneous presence of
CH3CN. Then a silane molecule coordinates to the a classical rhenium hydride and acetonitrile in the
rhenium center of complex 2, forming the h2-R3SiH ligand sphere. However, the NMR response was nega-
1
À
complex 3. The H-NOE experiment provides some tive, since scalar coupling between HRe and NC CH3
evidence for the presence of a side-on rhenium was not observed. This latter experiment does not
bonded Si H unit, since saturation of the dissociated allow the conclusion that oxidative addition would
acetonitrile methyl group at around 2 ppm gives rise not take place. We take two alternative pathways into
À
to a very broad (>100 Hz) NOE signal, which could consideration, which are direct inter-ligand transfer of
À
be assigned to an Re attached Si H group. For the al- a hydride from the silane to the Cketone atom or indeed
ternative structure of an oxidatively added silane with oxidative addition of the silane and then b-hydride
Adv. Synth. Catal. 2009, 351, 1783 – 1788
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Hailin Dong and Heinz Berke
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Table 5. [Re
ACHTUNGTRENNUNG(CH3CN)3Br2(NO)]-catalyzed hydrosilylation of
organic carbonyl compounds.[a]
Entry
Substrate
t [h]
Yield [%][b]
TOF[c]
475
1
2
1
1
95
99
495
3
1
71
355
4
5
1.5
1.5
94
95
313
317
6
7
1
1
89
90
445
450
8
1.5
1.5
90
92
300
307
Figure 1. Proposed pathway for the hydrosilylation of a
ketone, R’R’’C=O, catalyzed by the [ReACHTUNGTRNEUNG(CH3CN)3Br2(NO)]
complex
9
10
1.5
2
85
88
283
220
or 858C and with reasonable activities of the catalyst.
A plausible mechanism for this hydrosilylation has
been put forward, which follows a course with pri-
mary addition of the silane to the rhenium center and
consecutive H-shift and silyl transfer, the latter step
via reductive elimination to generate the silyl ethers.
This catalysis is suggested to function so well, because
11
[a]
Reaction conditions: 0.2 mol% of the complex
[Re(CH3CN)3Br2(NO)], 2.5 mmol of substrate, and
1.5 equiv. of HSiEt3 in chlorobenzene (2.0 mL) at 858C
for the given time, under nitrogen atmosphere.
Yield on the basis of the integration of the 1H NMR
spectrum.
ACHTUNGTRENNUNG
[b]
[c]
of a facile Re(I) and ReACTHNUTRGNE(NUG III) redox interplay in the
Defined as mol product per mol of catalyst per hour.
given ligand environment. Further studies regarding
the detailed mechanism and the extension of this rhe-
nium-mediated catalysis to the hydrosilylation of ole-
transfer to form in either case the organyloxy com- fins are in progress.
plex 5. The catalytic cycle “closes” with reductive
elimination of the siloxy product 6 and re-formation
of complex 2. Attempts to isolate the h2-R3SiH com-
Experimental Section
plex 3 were not successful, presumably due to its in-
stability. The solution turns black after removal of the
solvent under vacuum. Further detailed studies are
currently underway to achieve full understanding of
the mechanism.
In summary, we have successfully explored a quite
generally applicable phosphine-free rhenium(I) hy-
drosilylation catalysis utilizing the easily available
General Information
All manipulations were carried out using standard vacuum
line, Schlenk and cannula techniques or in a dry box (M.
Braun 150B-G-II) containing an atmosphere of purified ni-
trogen. Solvents were initially distilled under N2 atmosphere
using standard procedures and were degassed by freeze-
thaw cycles prior to use. The following rhenium
[Re
silyl ethers derived from ketones and aryl aldehydes
were accessed in excellent yields at room temperature [Re(NO)
ACHTUNGTRENNUNG(CH3CN)3Br2(NO)] complex. A great variety of
complexes:
(PCy3)2A
(CH3CN)Br2],[9]
(CH3CN)3Br2],[10] [Re
[Re(NO)
(PPh3)
[Re(NO)
(THF)ACHTNUGTRENNUNG
R
[Re(NO)-
A
U
G
CHTUNGTRENNUNG
N
U
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A Convenient and Efficient Rhenium-Catalyzed Hydrosilylation of Ketones
COMMUNICATIONS
[Re
[Re
N
E
[Re
E
N
[ReACTHNGUTERNN(UG PTAH)-
A
E
G
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ACHTUNGTRENNUNG
known procedures. Except for the rhenium compounds, all
the chemicals were purchased from Aldrich Chemical Co. or
Fluka. Unless otherwise noted all commercial materials (si-
lanes, ketones and aldehydes) were used without purifica-
tion. H, 11B, 31P and 13C{1H} NMR spectra were recorded on
1
a Varian Gemini 300 (299.78 MHz), or on a Varian Mercury
200 (199.78 MHz) spectrometer. GC-MS analyses were car-
ried out on a CP-3800 Saturn 2000 MS/MS spectrometer
(CP-Sil8CB low bleed/MS 30 m, ID 0.25 mm, OD 0.39 mm,
Film thickness 0.25 mm from Chrompack).
General Procedure for the Catalytic Hydrosilylations
according to Tables 1–4
A solution of the appropriate substrates (0.5 mmol) and the
silanes (0.6 mmol) and the given catalytic amount of the
rhenium complex in the given solvent (0.8 mL) was stirred
for an appropriate period of time at room temperature
under a nitrogen atmosphere. Upon completion, the reac-
tion the mixture was filtered over celite. The resulting solu-
tion was analyzed by NMR spectroscopy or GC-MS. The
obtained NMR data of the silyl ethers were identical to
those of the literatures.[3d,8a,f,g] The yields are listed in
Table 1, Table 2, Table 3 and Table 4. The GC-MS analysis
furnished the retention time listed in the following (tR =re-
tention time, CH2Cl2 was used as the sovlent): triethyl(iso-
propoxy)silane: m/z=174.1, tR =2.73 min; sec-butoxytri-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
General Procedure for the Catalytic Hydrosilylations
of Table 5
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Schlenk tube according to the procedures of the hydrosilyla-
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858C and stirred for the required reaction time. Upon com-
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Acknowledgements
We gratefully acknowledge financial support from the Swiss
National Science Foundation and the funds of the University
of Zurich.
Adv. Synth. Catal. 2009, 351, 1783 – 1788
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
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Hailin Dong and Heinz Berke
COMMUNICATIONS
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