Scope and mechanistic studies of catalytic hydrosilylation with a high-valent nitridoruthenium(VI)

Article abstract of DOI:10.1021/cs300848h

Hydrosilylation catalyzed by a high-valent nitridoruthenium(VI) compound, [RuN(saldach)(CH₃OH)]⁺[ClO₄]^- (1, where saldach is the dianion of racemic N,N'-cyclohexan-diyl- bis(salicylideneimine)) is described. Using phenylsilane as reductant, a variety of unsaturated organic substrates, including aldehydes, ketones, and imines, are effectively reduced to alcohols and amines, respectively, accompanied by the redistribution of PhSiH₃ at silicon. Mechanistic studies indicate that the catalysis proceeds via silane activation rather than carbonyl activation, and the silane is likely activated via multiple pathways, including a radical-based pathway.

Full text of DOI:10.1021/cs300848h

Research Article
pubs.acs.org/acscatalysis
Scope and Mechanistic Studies of Catalytic Hydrosilylation with a
High-Valent Nitridoruthenium(VI)
Srinivas Abbina, Shi Bian, Casey Oian, and Guodong Du*
Department of Chemistry, University of North Dakota, 151 Cornell Street Stop 9024, Grand Forks, North Dakota 58202, United
States
S
* Supporting Information
ABSTRACT: Hydrosilylation catalyzed by a high-valent nitridoruthenium(VI) compound,
[RuN(saldach)(CH3OH)]+[ClO4]− (1, where saldach is the dianion of racemic N,N′-
cyclohexan-diyl-bis(salicylideneimine)) is described. Using phenylsilane as reductant, a
variety of unsaturated organic substrates, including aldehydes, ketones, and imines, are
effectively reduced to alcohols and amines, respectively, accompanied by the redistribution
of PhSiH3 at silicon. Mechanistic studies indicate that the catalysis proceeds via silane
activation rather than carbonyl activation, and the silane is likely activated via multiple pathways, including a radical-based
pathway.
KEYWORDS: hydrosilylation, ruthenium nitrido, aldehydes and ketones, imines, catalytic reduction, silane activation,
radical mechanism
reaction to be carried out conveniently on the benchtop under
air. In addition to the synthetic utility of these reactions, the
high-valent transition metal catalysts also provide a new
paradigm in which important mechanistic questions can be
addressed as related to σ-bond activation.34,35
Several mechanisms have been proposed for the high-valent,
transition-metal-catalyzed hydrosilylation. For the cis-dioxo
ReO2I(PPh3)2 catalyst, the Si−H bond adds across one of the
two ReO bonds to afford a siloxy rhenium hydride, followed
by carbonyl insertion and silylether elimination (Scheme 1,
INTRODUCTION
■
Reduction of unsaturated organic compounds is an important
transformation in academic and industrial research. Particularly,
catalytic hydrosilylation of carbonyl compounds, the addition of
a Si−H bond across a CO double bond, has been extensively
studied.1 Because of the mild nature and ease of handling of
hydrosilanes, they are often used as a convenient alternative to
hydrogenation,2 especially in asymmetric synthesis.3 The field
has been traditionally dominated by catalysts based on low-
valent precious metals platinum, rhodium, and iridium.2,3
Oxidative addition of Si−H to low-oxidation-state, late
transition metals is believed to be a key step in the reaction.4
Therefore, it is noteworthy that a high-valent cis-dioxo
rhenium(V) compound, Re(O)2I(PPh3)2, was found to
effectively catalyze the hydrosilylation of aldehydes and
ketones.5 This represents a new reactivity mode for high-valent
transition metals in reduction catalysis6 because such complexes
are typically involved in catalytic oxidation and oxygen atom
transfer reactions.7−9 Significant progress has been made in the
use of high-valent complexes in catalytic reductions and the
understanding of their reaction mechanisms.10,11 A number of
catalysts have been identified, mostly high-valent rhenium (V,
VII)12,13 and molybdenum (IV, VI)14−16 complexes bearing
terminal oxo or imido groups. They have proved effective in a
variety of reduction reactions, such as hydrosilylation of
ketones and aldehydes17,18 and reduction of imines,19 esters,20
amides,21 azides,22 nitriles,23 nitro compounds,24 and sulf-
oxides.25 Asymmetric reduction of ketones and imines has been
achieved with good to excellent enantioselectivity.26,27
Furthermore, other sigma bonds, such as H−H and B−H,
can be activated by high-valent transition metals, as hydro-
genation of alkynes,28,29 and reduction with boranes30−33 have
demonstrated. Because of their high oxidation state, these
complexes are usually moisture- and air-stable, allowing the
Scheme 1. Proposed Mechanism of Oxo and Imido Re- and
Mo-Catalyzed Hydrosilylation
path A).36,37 Depending on the substrates, the resting state of
the catalyst may vary. Related MoO2Cl2 catalyst likely follows a
similar mechanism.38,39 However, this unconventional pathway
seems to be unique to catalysts bearing cis-dioxo groups, as Si−
H addition to ReO is not observed for the monooxorhenium
Received: December 31, 2012
Revised: February 14, 2013
Published: February 21, 2013
© 2013 American Chemical Society
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catalysts, such as ReOCl3(PPh3)2.40 In these systems, silane
activation is believed to proceed via a η2-silane σ-adduct,
followed by heterolytic cleavage at the electrophilic rhenium
center (Scheme 1, path B). The rhenium hydride formation
step may not be needed if the reaction proceeds in a more
concerted manner,41,42 resembling the situation in catalytic
silane alcoholysis reactions.43 As in the ionic hydrogenation
mechanism, a carbonyl coordination step is not required.44
Indeed, a nonhydride ionic hydrogenation mechanism is
supported by a computational study.45 On the ground of a
stoichiometric labeling experiment, an alternative mechanism,
in which the metal center simply activates the carbonyls as a
Lewis acid, is suggested for imidomolybdenum catalysts, as well
as the rhenium catalysts mentioned above.46,47 In another
study, the expected Si−O elimination (the last step of path A in
Scheme 1) from the intermediate is not observed.48 The
disparity in mechanistic understanding is perhaps not too
surprising because hydrosilylation is often complicated and a
universal mechanism is not expected for different catalysts and
substrates. In any event, the roles of the multiply bonded
terminal ligands and hydrides in silane activation remain
unclear in catalysis.
selected results are summarized in Table 1. As noted before,
reduction of aldehydes was efficient, typically complete within 2
h, except for p-nitrobenzaldehyde, which is surprisingly sluggish
(Table 1, entry 3). Substituents such as halo, hydroxyl, and
nitro groups are tolerated, although there is evidence that the
nitro group may be reduced further after the carbonyl
reduction. The α,β-unsaturated cinnamaldehyde underwent
1,2-addition reaction of silane, leading to the corresponding
alcohol in 74% yield (entry 6), and 1,4 addition product is not
observed.
In comparison with aldehydes, the reduction of ketones was
relatively slow, taking ∼20 h to completion. In most of the
cases, the corresponding alcohols are successfully isolated by
column chromatography in good yields. Sometimes, deoxyge-
nation of carbonyl to corresponding alkyl compounds can be
observed. For example, 4-ethyl-anisole is isolated in 27% yield
along with the desired alcohol in the reduction of 4-
methoxyacetophenone (entry 8). On the other hand, α,β-
unsaturated enones seem to be challenging substrates in the
reaction. The acyclic trans-chalcone gave rise to a complex
mixture, from which only ∼16% expected 1,2-reduction
product is isolated (entry 12). Other isolated products include
16% 1,4 reduction product and the deoxygenation product
(18%). In the case of a cyclic enone, 3-methyl, 2-cyclo-
hexenone, no 1,4-addition product was observed, and the
desired unsaturated alcohol was identified by both GC/MS data
Given the utility of this novel type of catalysts in reductions,
we became interested in related transition metal compounds.
We have recently communicated the application of a cationic
nitride Ru(VI) complex, [Ru(VI)N(saldach)(MeOH)+]-
−
1
[ClO4 ] (1, where saldach is the dianion of racemic N,N′-
and H NMR spectroscopy, although the conversion is low
cyclohexan-diyl-bis(salicylideneimine), Figure 1),49 which is
(entry 13).
Reduction of Imines. Synthesis of amines from imines is
an important transformation in pharmaceutical and agricultural
chemistry that can be achieved with a stoichiometric reducing
agent or catalytic hydrogenation.52 Alternatively, this can be
done via catalytic hydrosilylation and a number of high-valent
Re- and Mo-based systems have been shown to effect the
reduction of imines. To expand the substrate scope of the
RuVIN-PhSiH3 system, we examined a number of imines under
the standard reaction conditions employed for carbonyl
compounds. The results are summarized in Table 2.
It is evident that the CN double bonds in imine react
readily under these conditions, leading to the formation of the
corresponding amines upon workup. The reactivity is
comparable with that of ketones and slower than aldehydes.
The imines derived from anilines are generally slower than
imines derived from alkyl amines (Table 2, entries 1−4 vs 5−
6). As expected, a similar level of functional group tolerance is
observed. The reduction of imine derived from isopropyl
amine, PhCHNiPr, seems to stop after ∼33% conversion,
and prolonged reaction time failed to improve the conversion
(entry 3). Similar reactivity was seen with an imine derived
from 2,6-dimethyl aniline (entry 6). Apparently, the steric bulk
near the imino nitrogen plays a significant role in the reduction
reaction.
Product Profile. Although the desired reduction products,
alcohols and amines, can generally be isolated in good yields, it
has been noted that the initial reaction products are rather
complex. For example, in the crude reaction mixture of an
imine and PhSiH3, various products can be detected by GC/
MS (Scheme 2). In the case of aldehydes and ketones, mono-,
di-, and trialkoxy silanes are observed among the products, with
the dialkoxysilane being dominant. The preference for
dialkoxysilane formation has been observed in other catalytic
hydrosilylations.15,53,54 This suggests that PhSiH2(OR) is more
active than PhSiH3 in the reaction. Further reduction to
Figure 1. RuN catalyst.
isoelectronic with rhenium (V) and Mo(IV) with a d2 electron
configuration, in catalytic hydrosilylation of aldehydes and
ketones.50 Herein, we describe a detailed study of the 1/PhSiH3
system in catalytic reduction of various carbonyl compounds
and imines. Furthermore, we present evidence that more than
one pathway, including a radical pathway, is at play for the
current hydrosilylation.
RESULTS AND DISCUSSION
■
As mentioned previously, high-valent transition metal catalysts
can effectively reduce a variety of unsaturated organic
substrates. We have shown that RuN(saldach)+ is an effective
catalyst for the reduction of a few ketones and aldehydes by
primary silane.50 To extend the scope of the reaction, we have
examined a number of different substrates, with particular
attention on the hydrosilylation of carbonyl and imine
compounds.
Reduction of Carbonyl Compounds. Using the standard
conditions (∼0.5 mmol substrate, 1.5 equiv of PhSiH3, 1 mol %
catalyst 1, ∼120 °C in benzene), we carried out the
hydrosilylation of a diverse set of representative aliphatic and
aryl carbonyl compounds, including acyclic, cyclic, aryl, acyclic
conjugated enone, and cyclic conjugated enone, etc.51 The
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a
Table 1. Hydrosilylation of Carbonyl Compounds Catalyzed by RuN
a
b
Reaction conditions: 0.3−0.8 mmol substrate, 1.5 equiv of PhSiH3, and catalyst 1 (1 mol %) in heated toluene or benzene (∼120 °C). Based on
c
d
e
NMR integration. Isolated yields. p-Ethylanisole was isolated in 27% yield. Other products isolated include 1, 3-diphenylpropan-1-ol (15%) and 1,
3-diphenylpropan-1-one (16%).
a
Table 2. Hydrosilylation of Imines Catalyzed by RuN
a
b
Reaction conditions: 0.4−0.8 mmol imine, 1.5 equiv of PhSiH3, and catalyst (1 mol %) in heated toluene or benzene. Isolated yields
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activation pathway;46 rather, a silane activation pathway is more
likely, although it should be noted that the observation of
Ru(VI) reduction by silane is not necessarily related to a silane
activation pathway, and the possibility of Lewis acid catalysis
cannot be completely ruled out.
a
Scheme 2. Products in Hydrosilylation of an Imine
The question is then how silane is activated in the reaction,
as diverse pathways have been proposed for high-valent
catalysts.10,11 Compared with isoelectronic Re(V) and Mo(IV),
Ru(VI) is certainly more oxidizing, because complex 1 can
abstract hydrogen from relatively weak C−H bonds via a
hydrogen atom transfer mechanism.59 It is thus conceivable
that RuN abstracts hydrogen from silane in the initial step,
generating a silyl radical, PhH2Si•. This could explain the
formation of disilane via combination of two silyl radicals. The
radical pathway in catalytic hydrosilylation has been indicated
for a titanium(IV)/silane system via a single electron transfer
process.60 In a high-valent-Mo-catalyzed hydrosilylation, radical
mechanism could provide a feasible pathway on the basis of the
computational studies.61
To investigate the possibility of radical involvement, the
catalytic reduction of PhCHO (1 equiv) by PhSiH3 (1.5 equiv)
was carried out in the presence of 1 equiv of a silyl radical
scavenger, 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO).62
The hydrosilylation reaction slowed down, as shown in Figure
2, but still was able to finish. At the end of the reaction, 2,2,6,6-
a
The top two species are the major products.
deoxygenation products is typically noted for ketones when
longer reaction time is needed for complete conversion. After
acidic workup, however, the majority of the species isolated are
the corresponding alcohols or amines. In comparison, the
ReVO-catalyzed hydrosilylation of acetophenone was accom-
panied mostly by the formation of ethylbenzene and dl, meso-
di(1-phenyl ethyl)ethers.13 The different product profiles
suggest that these catalysts may have different features in the
reaction.
Redistribution at Silicon. Very notable is the presence of
silane redistribution products, mostly Ph2SiH2, with a small
amount of Ph3SiH (Scheme 3). SiH4, the other possible
Scheme 3. Redistribution of PhSiH3
redistribution product, has not been detected, supposedly
because of its high volatility and reactivity. Sometimes,
hydrosilylation products derived from Ph2SiH2 can be detected
(see Scheme 2). In the literature, silane redistribution reactions
have been observed in the presence of low-valent, late transition
metal complexes, such as Ru, Rh,55 Ir,56 or others.57 However,
in high-valent transition-metal-catalyzed hydrosilylations with
PhSiH3, silane redistribution has been rarely reported.14,15,21 To
further probe this reactivity with the present ruthenium catalyst,
reaction in the absence of carbonyl substrate was carried out
under similar conditions with a catalytic amount of 1. The
conversion of PhSiH3 reached a plateau of ∼25% after 24 h.
Among the products, Ph2SiH2 can be easily identified by both
1H NMR (5.1 ppm) and GC/MS (m/z 184; tR = 13.55 min). A
small amount of Ph3SiH can be detected by GC/MS (m/z 260)
in the crude mixture, along with silane dehydrocoupling
product PhH2SiSiH2Ph (m/z 214). It is unclear how the
redistribution occurs. One obvious choice is catalysis by low-
valent Ru resulting from reduction by PhSiH3; however, the
observation that the redistribution stops before completion
seems to suggest that high-valent Ru is important in the present
redistribution.
Mechanistic Consideration. To probe the mode of
activation with the RuN catalyst, stoichiometric reactions of 1
and substrates were studied. Treatment of 1 with a carbonyl
substrate, PhCHO, showed no observable change in the NMR
or UV−vis spectra. On the other hand, reaction between 1 and
PhSiH3 in CH3CN is indicated by the facile color change from
reddish brown to green. Although the reaction product was not
isolated, ESI-MS analysis revealed predominantly a peak at m/z
422, in agreement with a RuIII(saldach)+ species.58 These
observations do not support a Lewis acid-catalyzed carbonyl
Figure 2. Time profile for the catalytic hydrosilylation of PhCHO in
the absence (circle) and presence (square) of 1 equiv of TEMPO.
tetramethyl-1-piperidine, the reduction product derived from
TEMPO, was detected in ∼0.5 equiv (relative to PhCHO).
Control experiment shows that TEMPO reacts with PhSiH3
under similar conditions, but takes much longer (>48 h). These
observations are suggestive of the involvement of a silyl radical
in the catalytic reaction.63
In addition, cyclopropyl phenyl ketone was employed in
catalytic hydrosilylation as a mechanistic probe; formation of
cyclopropyl ring-opening products indicates a radical mecha-
nism.64 Analysis of the crude reaction products showed again a
complex mixture, but both direct hydrosilylation and ring-
opening products were detected (Scheme 4). After hydrolysis,
mono- and dialkoxy silanes (E and F) disappeared, and the
major components are identified in a ratio of A/B/C/D =
2.4:1.0:1.7:5.2. The reappearance of the starting ketone (such
as A) upon workup has been noted previously,50 presumably
through a silyl enol ether intermediate. The presence of
cyclopropyl ring-opening products B (and C) clearly suggests
the involvement of a radical pathway, but perhaps not as a
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Scheme 4. Hydrosilylation Products Derived from
Cyclopropyl Phenyl Ketone
Scheme 5. Catalytic Reduction of Some Unsaturated
Substrates
principal contributor.58 As to the major pathway or pathways, a
heterolytic cleavage of the SiH bond activated by coordination
to the highly electrophilic RuN unit49 can be surmised, similar
to that of monooxo ReV-based systems in Scheme 1B.
However, a more detailed conclusion cannot be reached at
this point.
The effect of electronic factors was further studied with a
series of substituted benzaldehydes in competition reactions
with PhCHO. Both electron-donating (4-OMe) and electron-
withdrawing groups (4-Cl, 4-NO2) seem to accelerate the
reaction, lending further support for a radical contribution.65
However, a linear free energy relationship between relative
rates and σ• or other Hammet constants (σ and σ+) could not
be established, which may be a reflection of the presence of
multiple pathways. Curiously, at competition conditions,
reduction of p-nitro benzaldehyde is much faster, even faster
than benzaldehyde.
conversions are low (10−35%). It is also noted that the
consumption of PhSiH3 is considerably larger than the
unsaturated substrates, although the products were not always
tractable. In addition to Ph2SiH2, which is easily detected by
both NMR and GC/MS, we suspect that phenylsilane
oligomers/polymers were formed in these reactions, as
indicated by the presence of broad but feartureless signals
62
1
between 4 and 6 ppm in H NMR.
CONCLUSION
■
In summary, we have demonstrated that a high-valent
nitridoruthenium(VI) compound, [RuN(saldach)-
(CH3OH)]+[ClO4]− (1), is an effective catalyst for hydro-
silylation of unsaturated organic substrates, particularly
aldehydes, ketones, and imines. The reaction mixture contains
various species, including the redistribution products of PhSiH3.
This and other mechanistic studies indicate that the catalysis
likely proceeds by silane activation via several pathways; in
particular, evidence for a radical pathway is presented. Efforts
are underway to improve the performance of this type of RuN-
based catalyst and gain more insights into the mechanistic
aspects.
Involvement of RuIII. Low-valent Ru species have been
studied extensively as hydrosilylation/hydrogenation cata-
lysts.66 In the present system, RuVIN can be easily reduced in
the presence of PhSiH3. RuVIN complexes could also undergo
N−N coupling reactions to afford RuIII.49 A recent work has
called attention to low-valent rhenium, which may be
responsible for hydrosilylation with oxorhenium(V) catalysts.67
It is also worth mentioning that the Brookhart’s [(POCOP)-
IrIIIH]+ system has been proposed to catalyze hydrosilylation of
carbonyl compounds through an ionic mechanism featuring an
η1-silane intermediate.68 Thus, we investigated the possibility of
Ru(III) as the actual catalyst. An independently prepared
EXPERIMENTAL SECTION
■
General. The ruthenium catalyst was prepared according to
the literature.49 Imine substrates were obtained by condensa-
tion of the corresponding carbonyl compounds and amines. All
other reagents and reactants were obtained commercially and
69
−
+
Ru(III)-saldach complex, [Ru(saldach)(H2O)2 ][PF6 ], did
not exhibit much reactivity at similar conditions, but in situ
generation for Ru(III) from Ru(VI)N and PhSiH3 in C6D6
seemed to lead to decomposition. In addition, under catalytic
conditions, the reaction mixture maintained an orange-brown
color throughout, not the green color, characteristic of RuIII,
observed in the absence of carbonyl substrates.58 Furthermore,
ESI-MS analysis of the reaction mixture under catalytic
conditions reveals the absence of mononuclear [RuVIN]+ or
[RuIII]+; instead, the majority of the Ru-containing species is
observed at m/z 858 and 892, with correct isotopic patterns for
a dinuclear form, possibly [(saldach)Ru]2N, although its nature
remains uncertain. These observations do not support RuIII as
the primary active catalyst, but it may still be involved in a
minor pathway.
1
were used as received. All H and 13C NMR spectra were
recorded on a Bruker Avance-500 NMR spectrometer and
referenced to the residue peaks in CDCl3 (7.26) or C6D6
(7.16). UV−vis measurement was performed on a PerkinElmer
Lamda 35 spectrophotometer. GC/MS analyses were per-
formed on an HP 5890 GC/HP 5971/B MSD system with
electron impact ionization (70 eV) and a DB5 column (30m ×
0.53 mm i.d., 0.25 μm thick; initial temperature, 50 °C; initial
time, 1 min; ramp rate, 10 °C/min; final temperature, 310 °C;
final time, 5 min). High-resolution mass spectrometry (HRMS)
was performed using high-resolution time-of-flight G1969A
instrumentation (Agilent, Santa Clara, CA).
Catalytic Hydrosilylation. In a typical procedure, RuN
catalyst 1 (3−5 mg, 1 mol %), substrate (0.5−0.8 mmol, 1.0
equiv), C6D6 (0.30−0.35 mL), and PhSiH3 (1.5 equiv) were
charged into a J Young NMR tube, usually in that order.
Trimethylphenyl silane or hexamethyl benzene was used as
internal standard (5−20%). This was then mixed and heated in
an oil bath at ∼120 °C. The reaction progress was monitored
Reduction of Other Unsaturated Substrates. A wide
variety of unsaturated substrates has been subjected to high-
valent rhenium- and molybdenum-catalyzed reductions. To
further examine the scope of the present RuVIN system, we
tested a few less-active, unsaturated groups, including ester,
amide, nitro, and alkyne. In these reactions, the desired
reduction products can be observed (Scheme 5), but the
1
by periodically taking H NMR. The reduction for ketones is
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HCl or tetrabutylammonium fluoride (TBAF). Before and after
the hydrolysis, a small sample was taken for GC/MS analysis.
After hydrolysis, the organic layer was extracted with ether and
then subjected to column chromatography on silica with an
appropriate mixture of hexane/EtOAc as eluent. The reduction
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1
products were identified by H NMR and GC/MS analysis in
comparison with literature data or authentic samples.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details and characterization data. UV−vis spectra
of 1 and related species. ESI-MS analysis of catalytic reactions.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Phone: 1-701-777-2241. Fax: 1-701-777-2331. E-mail: gdu@
chem.und.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported in part by the ND EPSCoR through
NSF Grant no. EPS-0814442 and the University of North
Dakota. We thank Dr. A. Kubatova and R. Cochran for
assistance with ESI-MS.
REFERENCES
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