Regio- and Stereoselective Dehydrogenative Silylation and Hydrosilylation of Vinylarenes Catalyzed by Ruthenium Alkylidenes

Article abstract of DOI:10.1021/acs.orglett.6b02642

Development of regio- and stereoselective dehydrogenative silylation and hydrosilylation of vinylarenes with alkoxysilanes, catalyzed by ruthenium alkylidenes, is described. Varying L- and X-type ligands on ruthenium alkylidenes permits selective access to either (E)-vinylsilanes or β-alkylsilanes with high regio- and stereocontrol. cis,cis-1,5-Cyclooctadiene was identified as the most effective sacrificial hydrogen acceptor for the dehydrogenative silylation of vinylarenes, which allows use of a nearly equimolar ratio of alkenes and silanes.

Full text of DOI:10.1021/acs.orglett.6b02642

Letter
pubs.acs.org/OrgLett
Regio- and Stereoselective Dehydrogenative Silylation and
Hydrosilylation of Vinylarenes Catalyzed by Ruthenium Alkylidenes
Apparao Bokka and Junha Jeon*
Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019, United States
S
* Supporting Information
ABSTRACT: Development of regio- and stereoselective
dehydrogenative silylation and hydrosilylation of vinylarenes
with alkoxysilanes, catalyzed by ruthenium alkylidenes, is
described. Varying L- and X-type ligands on ruthenium
alkylidenes permits selective access to either (E)-vinylsilanes
or β-alkylsilanes with high regio- and stereocontrol. cis,cis-1,5-
Cyclooctadiene was identified as the most effective sacrificial
hydrogen acceptor for the dehydrogenative silylation of
vinylarenes, which allows use of a nearly equimolar ratio of
alkenes and silanes.
inylsilanes and alkylsilanes are important building blocks in
the synthesis of small molecules and polymers, based, in
spectroscopic and isotope-labeling experiments.11 However,
there are no examples of this type of Si−H activation by metal
alkylidenes (i.e., catalytic deprotonative silyl metalation) for
dehydrogenative silylation to afford vinylsilanes (Scheme 1).8,12
V
part, on their relatively high stability and virtually nontoxic
nature.1 These organosilanes have been extensively exploited as
useful synthetic intermediates whose silicon functional groups
can be directly converted to many other useful moieties through
further reactions.1c,2 Regio- and stereoselective dehydrogenative
silylation3−5 to provide vinylsilanes are challenging,6,7 owing to
either competitive hydrosilylation to afford alkylsilanes or
alternative β-hydride elimination to furnish allylsilanes.8 Because
alkenes are more readily accessible than alkynes and serve as one
of the most important starting materials, more direct silylation
methods to afford vinylsilanes are highly attractive. For example,
Falck4 and Hartwig5 recently reported Ir-catalyzed regio- and
stereoselective dehydrogenative silylation of terminal alkenes
with norbornene as a stoichiometric sacrificial hydrogen acceptor
(SHA). Watson demonstrated a Pd-catalyzed silyl Heck reaction
utilizing terminal alkenes and silyl triflates.9 Although there are a
number of developments in the dehydrogenative silylation to
afford vinylsilanes utilizing metal catalysts,3d,f,g−i,l,n−p such
methods generally require either excess alkene substrates or
silanes albeit employing excess SHA, air- and moisture-sensitive
catalysts, or more reactive alkylsilanes in lieu of more useful
alkoxysilanes for further manipulations. Chirik and co-workers
recently demonstrated highly selective Co-catalyzed dehydro-
genative silylation of alkenes for preparation of allylsilanes where,
for catalytic turnover, half of the alkenes served as sacrificial
hydrogen acceptors to furnish simple alkanes as byproducts.8
In a previous study, we first demonstrated that the preferential
Si−H activation over alkene activation utilizing Ru alkylidene
complexes was feasible to achieve intramolecular alkene
hydrosilylation. In contrast to a generally accepted Chauvin-
type silylation mechanism of addition of Si−H across the π-bond
of a Ru benzylidene,6b,d,10 a mechanism involving direct Si−H
activation by RuCl was proposed on the basis of a series of
Scheme 1. Ru Alkylidene Catalyzed Dehydrogenative
Silylation and Hydrosilylation of Vinylarenes
We now report regio- and stereoselective dehydrogenative
silylation to afford only (E)-vinylsilanes and hydrosilylation of
vinylarenes by altering the ruthenium alkylidene catalysts (L1 and
X ligand). Notably, preparation of both alkylsilanes and
vinylsilanes was achieved using a nearly equimolar ratio of
alkenes and silanes with a new sacrificial hydrogen acceptor.
We first investigated the optimal reaction parameters for the
dehydrogenative silylation depicted in Table 1. The results
revealed that a ratio of products (alkylsilane 2a, vinylsilane 3a,
stilbene 4a, and ethylbenzene 5a) was highly dependent upon
catalyst structure and silanes. The reaction of styrene 1a (1
equiv) and alkyl- or alkoxysilanes (1.1 equiv) with Ru-1,
constituting phosphine L-type ligand and dichloride X-type
ligands, afforded a mixture of products with low to moderate
conversion (entries 1−5). Gratifyingly, the use of HSiMe-
(OSiMe3)2 provided (E)-vinylsilane 3a as a major silylation
product with excellent product selectivity of dehydrogenative
̀
silylation vis-a-vis hydrosilylation as well as regio- and stereo-
selectivity (only E, of note, previously known metal-catalyzed
Received: September 2, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02642
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Letter
a
Table 1. Evaluation of Catalysts and Silanes
Scheme 2. Evaluation of Sacrificial Hydrogen Acceptors
b
c
entry
RuLn
solvent
HSiEt3
conversion (%)
2a:3a:4a:5a
1
Ru-1
Ru-1
Ru-1
Ru-1
Ru-1
Ru-1
Ru-2
Ru-3
Ru-4
Ru-5
Ru-6
Ru-7
Ru-8
77
94
1:6:0:4
1.5:1:0:1.5
nd
2
H2SiPh2
3
H2SiEt2
H2SiiPr2
32
4
25
nd
5
HSi(OEt)3
91
1:1:0:1
6
HSiMe(OSiMe3)2
HSiMe(OSiMe3)2
HSiMe(OSiMe3)2
HSiMe(OSiMe3)2
HSiMe(OSiMe3)2
HSiMe(OSiMe3)2
HSiMe(OSiMe3)2
HSiMe(OSiMe3)2
100
100
100
100
100
100
100
100
1:16:0:10
1:3:0:1
7
8
1:5:14:2
1:8:25:1
1:3:12:3
1:4:5:1
increasing ring strain14 the corresponding ratio as well as yield
(3a) proportionally increased.
Having established the optimized conditions, we explored the
scope of Ru alkylidene catalyzed dehydrogenative silylation of 1
with Ru-1 (X = Cl and L1 = PCy3) to afford (E)-3 (Scheme 3).
9
10
11
12
13
5:1:0:1
4:1:0:1
Scheme 3. Substrate Scope of Ruthenium Alkylidene (Ru-1)-
a,b
Catalyzed Dehydrogenative Silylation of Vinylarenes
a
Conditions: 1a (0.4 mmol), silane (0.44 mmol), THF (0.2 M).
b
c
Determined by GC/MS analysis. Determined by GC/MS analysis
and H NMR spectroscopy utilizing an internal standard (CH2Br2).
1
alkyne hydrosilylations or dehydrogenative silylations typically
afford Z-vinylsilanes as major) (entry 6). In contrast, Ru-7 and
Ru-8, best known for Z-selective olefin metathesis catalysts
containing an NHC L-type and bidentate nitrate X-type ligands,
as well as chelating adamantyl ligand,13 furnished alkylsilanes 2a
as a major product (entries 12 and 13). Interestingly, NHC/
dichloride-containing catalysts including Ru-3, Ru-4, Ru-5, and
Ru-6 produced olefin metathesis product stilbene 4a as a major
product, even in the presence of silane (entries 8−11). These
results are summarized in Table 1 (bottom), which comprises
three modes of Ru alkylidene reactivity toward dehydrogenative
silylation, olefin metathesis in the presence of silane, and
hydrosilylation.
The issue of simple reduction of the starting alkenes 1 was
addressed by employing a sacrificial hydrogen acceptor (SHA, 1
equiv) (Scheme 2). When well-known strained bicyclic alkenes
[e.g., norbornene (nbe), norbornadiene (nbd)] were tested, we
observed noticeable ring-opening metathesis polymerization
(ROMP) activity of Ru-1 in the presence of nbe or nbd. We
quickly discovered that moderately ROMP-active cycloalkenes
[including cyclopentene, cyclohexene, cycloheptene, cis-cyclo-
octene, and cis,cis-1,5-cyclooctadiene (cod)] not only afforded
good yields by diminishing the alkene reduction product 5a [use
of 2 equiv of SHA (i.e., cod) eventually further improved yields
(<5% of 5a)] but also exhibited excellent selectivity of
dehydrogenative silylation over hydrosilylation. The trend of
corresponding yield and the ratio of 2a and 3a were well
correlated with the ring strain of cycloalkenes, whereupon with
a
A ratio of 2 and 3 was determined by GC/MS and 1H NMR
b
spectroscopy. Reaction of 1b on 7 mmol (1.12 g) scale.
Electron-rich and -deficient styrenes afforded vinylsilanes (3a−r)
in moderate to good yields with excellent stereoselectivity (only
E) and a good to excellent ratio of dehydrogenative silylation and
hydrosilylation. The reaction of 1b on a 7 mmol (1.12 g) scale
provided 2b in 75% yield with good product selectivity (2b/3b =
7:93). Notably, carboxylic acid, ester, unprotected amine,
B
DOI: 10.1021/acs.orglett.6b02642
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Letter
protected o-amino group (potential chelation group to Ru),
indole, benzofuran, and boronate ester (3s-z) tolerated the
reaction conditions. Finally, structurally complex, C3-vinyl
estrone derivative afforded vinylsilane 3aa in 76% yield with
good product selectivity. Ru alkylidene catalytic systems
unfortunately did not effect the reaction of aryl-substituted
alkenes with an alkyl side chain.
We then continued to investigate the scope of Ru alkylidene-
catalyzed hydrosilylation of 1 and HSiMe(OTMS)2 with Ru-7
(Scheme 4). When Ru-7 (X = NO3 and L1 = NHC bearing
intermolecular KIE experiment using 1a-[2,2-D2] and 1a-[2,2-
H2] displayed minimal isotopic selectivity (kH/kD = 1.3),
suggesting that direct C−H activation/silylation to afford
vinylsilane is unlikely (Scheme 5b). Taken together, the observed
significant KIE in the parallel isotope experiments indicates that
the Si−H bond cleavage to generate the putative ruthenium silyl
complex and HCl is the turnover-limiting step, which is
consistent with observation of Ru alkylidene as the resting
state.6b,15
The catalytic mechanisms of these interesting processes
involving catalysts such as Ru-1 and Ru-7 have not been yet
fully elucidated. Based on discoveries from our current studies,
coupled with previous studies regarding Ru alkylidene catalyzed
intramolecular alkene hydrosilylation,11 we propose mechanisms
for the Ru alkylidene catalyzed dehydrogenative silylation and
hydrosilylation of alkenes (Scheme 6). First, dehydrogenative
Scheme 4. Substrate Scope of Ruthenium Alkylidene (Ru-7)-
a
Catalyzed Hydrosilylation of Vinylarenes
Scheme 6. Proposed Mechanisms
a
A ratio of 2 and 3 was determined by GC/MS and 1H NMR
silylation begins with the Si−H activation by Ru catalyst after
dissociation of phosphine ligand to afford putative Ru silyl
complex 6a and HCl. The resulting HCl can further react with
silane to give Si−Cl and H2, which were observed by GC−MS
spectroscopy.
adamantly moiety) was used, diverse mono- and disubstituted
styrenes provided alkylsilanes 2a−j with moderate to good yields
and a synthetically useful level of product selectivity. Again,
ester-, amino-, benzofuran-, and indole-containing styrenes
underwent hydrosilylation to provide 2k−n in good yields.
In order to gain insight into the reaction mechanism of the Ru
alkylidene catalyzed dehydrogenative silylation, we conducted
two KIE experiments (Scheme 5). In the first experiment, parallel
KIE experiments with HSiMe(OTMS)2 and DSiMe(OTMS)2
were carried out (Scheme 5.a). Analysis of the products
established a significant KIE (kH/kD = 3.2). Second, the
1
and H NMR spectroscopy. A similar type of a bond-exchange
reaction has been seen in Noyori’s asymmetric hydrogenation,
where early activation of H2, by Ru(II)Cl2, provided HRu(II)Cl
and HCl.16 An alkene coordination and olefin migratory
insertion then give rise to 6c via 6b. When the catalyst contains
phosphine/dichloride ligands (e.g., Ru-1), 6d, produced by
coordination of moderately ROMP-active and bulky cycloalkene
cod (Scheme 2) to Ru metal, could dictate the product selectivity
by facilitating the C−C single-bond rotation (to 6e) and
subsequent β-hydride elimination to give a thermodynamic
product (E)-vinylsilane 3 and Ru−H (6f). The catalytically
responsible Ru−Si (6a) is regenerated by a sequence of olefin
migratory insertion of cod into ruthenium hydride (to 6g) and
Scheme 5. Preliminary Mechanistic Studies
1
reaction with silane by releasing cyclooctene (observed in H
NMR spectroscopy and GC−MS spectrometry).
Activation of the Si−H bond with hydrosilylation catalyst Ru-
7, which includes NHC, nitrate, and adamantyl ligands, furnishes
the putative Ru silyl complex 7a. The resulting monobound
nitrate can quickly react with additional silane to afford bis-silyl
Ru complex 7b. Because of this dehydrogenative Si−O coupling,
as shown in Scheme 4, the hydrosilylation process necessitates
the use of a slight excess of silane. Otherwise, diminished product
selectivity was generally observed. Side-bound olefin in 7c then
undergoes olefin migratory insertion to provide 7d. Finally,
reaction of 7d with silane releases alkylsilanes 2 and produces the
C
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2005, 127, 13094−13095. (d) Parrott, M. C.; Finniss, M.; Luft, J. C.;
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is nearly four times slower compared to dehydrogenative
silylation (t1/2 = 30 min for dehydrogenative silylation of 1d
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dehydrogenative silylation and hydrosilylation of vinylarenes and
alkoxysilanes by exploiting ruthenium alkylidene catalysts to
access vinylsilanes and alkylsilanes. Notably, variation of catalyst
structure, specifically both L- and X-type ligands at ruthenium,
greatly altered the reaction pathways to dehydrogenative
silylation and hydrosilylation. The readily accessible catalysts,
with a cis,cis-1,5-cyclooctadiene hydrogen acceptor for the
dehydrogenative silylation, exhibited relatively broad functional
group tolerance and high regio- and stereoselectivity. Although a
variety of nonmetathetical synthetic applications of Grubbs-type
ruthenium alkylidenes are known, including silylation reactions, a
mechanistic understanding of nonmetathetical catalytic function
of such catalysts is still limited. Our preliminary studies on
dehydrogenative silylation showed that the turnover-determin-
ing step is the Si−H cleavage by Ru alkylidene. The origin of such
ligand-controlled selectivity regarding dehydrogenative silylation
and hydrosilylation as well as their detailed mechanism are
currently under investigation.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02642.
(8) Atienza, C. C. H.; Diao, T.; Weller, K. J.; Nye, S. A.; Lewis, K. M.;
Delis, J. G. P.; Boyer, J. L.; Roy, A. K.; Chirik, P. J. J. Am. Chem. Soc. 2014,
136, 12108−12118.
Experimental details and spectroscopic characterization
data for all compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: jjeon@uta.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the American Chemical Society Petroleum Research
Fund (PRF No. 54831-DNI1) for support of our program. We
acknowledge Materia, Inc., for the generous donation of Ru
alkylidene catalysts.
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