Ruthenium catalyzed selective hydrosilylation of aldehydes

Article abstract of DOI:10.1039/c3cc47593j

A chemoselective hydrosilylation method for aldehydes is developed using a ruthenium catalyst [(Ru(p-cymene)Cl₂)₂] and triethylsilane; a mono hydride bridged dinuclear complex [{(η⁶-p- cymene)RuCl}₂(μ-H-μ-Cl)] a

Full text of DOI:10.1039/c3cc47593j

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Ruthenium catalyzed selective hydrosilylation
of aldehydes†
Cite this: Chem. Commun., 2014,
50, 888
Basujit Chatterjee and Chidambaram Gunanathan*
Received 3rd October 2013,
Accepted 15th November 2013
DOI: 10.1039/c3cc47593j
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A chemoselective hydrosilylation method for aldehydes is developed can be carried out under neat conditions or in toluene and
using a ruthenium catalyst [(Ru( p-cymene)Cl2)2] and triethylsilane; benzyl(triethylsilyl)ether could be obtained in 95% yield from
a mono hydride bridged dinuclear complex [{(g6-p-cymene)RuCl}2- benzaldehyde using 1 mol% of 1 (Table 1, entry 1, see also
(l-H-l-Cl)] and a Ru(IV) mononuclear dihydride complex [(g6-p- Tables S1–S3, ESI†). The substrate scope of hydrosilylation of an
cymene)Ru(H)2(SiEt3)2] are identified as potential intermediates in assortment of aldehydes using [Ru( p-cymene)Cl2]2 1 is investi-
the reaction and the proposed catalytic cycle involves a 1,3-hydride gated and summarized in Table 1. Silyl ether products are
migration.
obtained in good yields using a range of substituents on alde-
hydes. Notably, aldehydes containing electron-releasing groups
Hydrosilylation of carbonyl compounds is a valuable transformation (entries 2–4) and electron withdrawing groups (entries 6–9), and
in chemical synthesis as a single step operation serves on both both (entries 10–12) are tolerated and underwent facile hydro-
reduction of carbonyl motifs and protection of resulting alcohols silylation reactions. The heteroaromatic aldehydes such as furfur-
featuring high atom-economy. Catalytic hydrosilylation, which can aldehyde and 2-thenaldehyde underwent hydrosilylation reactions
be carried out under mild experimental conditions, is often used as a under neat conditions (entries 13 and 14). The dinuclear
convenient alternative to hydrogenation reaction and is advanta- [Ru( p-cymene)Cl2]2 1 also catalyzed the hydrosilylation of aliphatic
geous over those performed using the traditional and harsh reducing aldehydes at room temperature and under neat conditions provid-
agents.1 Alkoxysilanes function as useful synthetic intermediates, ing silyl ether products in good yields (entries 16–19).
used for the synthesis of silicon-containing polymers, ceramic
Furthermore, reaction of benzaldehyde with triethylsilane cata-
materials and thus produced in both small as well as large scales.2 lyzed by 1 was carried out in the presence of a stoichiometrically
A number of transition metal complexes were reported to equivalent amount of acetophenone and diphenyl carbonate
catalyze the hydrosilylation of carbonyl motifs.3–11 However, (Scheme 1a). In both competition experiments hydrosilylation of
known catalysts for hydrosilylation reactions are highly reactive benzaldehyde was alone observed and benzyl triethylsilyl ether was
and they invariably reduce a range of different functionalities. isolated in 92% and 94% yields, respectively. Both acetophenone
Thus, a synthetic method that can differentiate and catalyze a and diphenylcarbonate were quantitatively recovered. Similar
selective hydrosilylation of aldehydes using a simple, inexpen- chemoselectivity is observed in hydrosilylation of substrates
sive catalyst would be valuable and have potential synthetic containing aldehydes and a range of other functional groups
applications.12,13 Here, we report a chemoselective hydrosilyla- (Scheme 1b).14 No hydrosilylation of ester or amide function-
tion of aldehydes under mild and neutral reaction conditions alities was observed under these conditions.11
catalyzed by commercially available [(Ru( p-cymene)Cl2)2] 1.
To understand the reaction mechanism of this fundamental
A detailed investigation and optimization studies using the com- and interesting catalytic transformation, reaction of benzaldehyde
mon ruthenium complexes and silanes revealed [Ru( p-cymene)Cl2]2 1 with triethylsilane catalysed by 1 was monitored by 1H NMR, which
and triethylsilane as suitable catalyst and silane, respectively, indicated a first order kinetics (see, Fig. S1, ESI†) and formation of
for hydrosilylation of aldehydes.14 Interestingly, the reaction metal hydride intermediates during the catalysis. A sharp singlet
was observed at d À10.18 ppm within 5 min of heating the reaction
School of Chemical Sciences, National Institute of Science Education and Research
mixture. Over the time, the intensity of this signal increased and
then began to decrease after 30 min upon appearance of another
(NISER), Bhubaneswar 751005, India. E-mail: gunanathan@niser.ac.in
† Electronic supplementary information (ESI) available: Experimental procedure,
singlet at d À13.53 ppm (see, Fig. S2, ESI†). Both of these singlet
spectral data for intermediate complexes 2, 3 and silyl ethers and X-ray data for 2
signals remained in the spectra recorded further throughout the
reaction. Stoichiometric reactions were performed in order to
and 3. CCDC 932852 (2) and 971350 (3). For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c3cc47593j
888 | Chem. Commun., 2014, 50, 888--890
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a
Table 1 Hydrosilylation of aldehydes catalyzed by [Ru( p-cymene)Cl2]2
Entry
1c
Silyl ethers
Time (h)
3.5
Yieldb (%)
95
89
84
2c
4.5
3
4.5
Scheme 1 Chemoselective hydrosilylation of aldehydes catalyzed by
[Ru( p-cymene)Cl2]2 1.
4
12
74
5
6
90
74
6c
3.5
Scheme 2 Synthesis and X-ray structure of [{(Z6-p-cymene)RuCl}2-
(m-H-m-Cl)] 2. ORTEP diagram of 2 is drawn with 30% probability.
7c
8
4.5
9
93
54
86
identify these metal hydride intermediates involved in the hydro-
silylation of aldehydes. When [Ru( p-cymene)Cl2]2 1 was reacted
with triethylsilane in benzene-d6 at room temperature, a mono-
hydrido bridged complex [{(Z6-p-cymene)RuCl}2(m-H-m-Cl)] 2 was
obtained in quantitative yield (Scheme 2). The ‘bridged hydride’ in
9
9
1
complex 2 displayed a singlet in the H NMR spectrum, which is
10
11
12
7
75
85
74
attributable to the metal hydride signal observed at d À10.18 ppm
during the catalysis. Suitable crystals were obtained from a toluene
solution of 2 and the structure was unequivocally corroborated by
single crystal X-ray analysis (Scheme 2).
4.5
12
When a toluene solution of benzaldehyde, triethylsilane and 2
(1 mol%) was heated for 3 h the corresponding benzyl triethylsilyl
ether was obtained in 95% yield, confirming the potential inter-
mediacy of 2 in the reactions.15 Further, the metal hydride inter-
mediate 3 that resonates a singlet at d À13.53 ppm was also
independently prepared from both 1 and 2 and characterized as a
mononuclear Ru(IV) dihydride complex [(Z6-p-cymene)Ru(H)2(SiEt3)2]
(Scheme 3).14,16 The structure of 3 was also further confirmed by
X-ray analysis of the single crystals obtained from a hexane solution
of 3. Isolated complex 3 also exhibited catalysis, indicating it’s
possible intermediacy in the catalytic cycle.17
13c
14c
4.5
15
80
53
15
15
77
16d
12
12
70
63
17c,d
On the basis of these observed data, a catalytic cycle for the
hydrosilylation of aldehydes with silanes is proposed (Scheme 3).
Either direct oxidative addition of triethylsilane18 on dinuclear Ru(II)
2 or reductive elimination of triethylsilane from intermediate 3
in the presence of aldehydes could produce the transient inter-
mediate I, which undergoes 1,3-hydride migration from the metal
centre to the carbonyl carbon leading to the reduction and con-
comitant formation of Ru(II) alkoxo intermediate II. Further oxidative
addition of triethylsilane generates the Ru(IV) intermediate III.
Coupling of silyl and alkoxo ligands on III by reductive elimination
creates O–Si bonds and liberates silyl ethers providing Ru(II)
18c,d
19c,d
a
9
70
82
12
Catalyst [Ru( p-cymene)Cl2]2 (1 mol%), aldehyde (1 mmol), triethyl-
silane (1.3 mmol) and toluene (2 mL) were heated at 50 1C under an
b
argon atmosphere. Yields of isolated products obtained after column
c
chromatography. Reactions carried out under neat conditions.
d
Reactions carried out at 25 1C. Analyses of reaction mixtures
(1H NMR, TLC) indicated the presence of unreacted aldehydes and
products and no other side product is observed.
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and closes the catalytic cycle.
In conclusion, we have demonstrated a facile transformation
for hydrosilylation of aldehydes using a commercially available
dinuclear ruthenium complex [Ru( p-cymene)Cl2]2 1 as a catalyst.
Using this novel catalytic method chemoselective hydrosilylation of
aldehydes in the presence of other functionalities is also achieved.
Potential intermediates [{(Z6-p-cymene)RuCl}2(m-H-m-Cl)] 2 and
[(Z6-p-cymene)Ru(H)2(SiEt3)2]
3 involved in the reaction are
identified and independently synthesized and characterized. The
structures of 2 and 3 are solved by single crystal X-ray analyses. The
proposed mechanism involves Ru(II)–Ru(IV) cycles and a 1,3-hydride
migration that reduce the aldehyde at the metal centre.
We thank the Department of Science and Technology,
New Delhi, and NISER for financial support. B.C. is grateful
to UGC for a research fellowship. C.G. is a Ramanujan Fellow.
Notes and references
1 For general reviews on hydrosilylation, see: (a) Hydrosilylation: A
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¨
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Synthesis, 2004, 2943–2958.
2 The Chemistry of Organic Silicon Compounds, ed. Z. Rappoport and
Y. Apeloig, Wiley, New York, 2001, vol. 3.
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triphenylsilane was also observed in situ by 1H NMR. The mono-
nuclear metal dihydride with triphenylsilane ligands resonated a
characteristic hydride signal at À11.71 ppm.
¨
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890 | Chem. Commun., 2014, 50, 888--890
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