Mechanistic insights into grubbs-type ruthenium-complex-catalyzed intramolecular alkene hydrosilylation: Direct σ-bond metathesis in the initial stage of hydrosilylation

Article abstract of DOI:10.1021/acscatal.5b00431

Grubbs-type ruthenium-complex-mediated intramolecular alkene hydrosilylation of alkenylsilyl ethers has been developed to provide cyclic silyl ethers with high regioselectivity. This non-metathetical use of such ruthenium complexes for alkene hydrosilylation via preferential Si-H bond activation over alkene activation is notable, where the competing alkene metathesis dimerization was not detected. In addition to the synthesis of organosilicon heterocycles from readily available olefins, this study provides fundamental mechanistic insights into the non-metathetical function of Grubbs-type ruthenium catalysts. In the initial stage of hydrosilylation within a ruthenium coordination sphere, evidence for activation of a ruthenium complex by direct σ-bond metathesis between Si-H and Ru-Cl via a four-centered transition state is presented. This study counters the traditionally accepted Chauvin-type mechanism, specifically the addition of R₃Si-H across the π-bond of a Ru-benzylidene.

Full text of DOI:10.1021/acscatal.5b00431

Research Article
pubs.acs.org/acscatalysis
Mechanistic Insights into Grubbs-Type Ruthenium-Complex-
Catalyzed Intramolecular Alkene Hydrosilylation: Direct σ‑Bond
Metathesis in the Initial Stage of Hydrosilylation
Apparao Bokka, Yuanda Hua, Adam S. Berlin, and Junha Jeon*
Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019, United States
*
S Supporting Information
ABSTRACT: Grubbs-type ruthenium-complex-mediated intramolecular alkene hydrosilylation of alkenylsilyl ethers has been
developed to provide cyclic silyl ethers with high regioselectivity. This non-metathetical use of such ruthenium complexes for
alkene hydrosilylation via preferential Si−H bond activation over alkene activation is notable, where the competing alkene
metathesis dimerization was not detected. In addition to the synthesis of organosilicon heterocycles from readily available olefins,
this study provides fundamental mechanistic insights into the non-metathetical function of Grubbs-type ruthenium catalysts. In
the initial stage of hydrosilylation within a ruthenium coordination sphere, evidence for activation of a ruthenium complex by
direct σ-bond metathesis between Si−H and Ru−Cl via a four-centered transition state is presented. This study counters the
traditionally accepted Chauvin-type mechanism, specifically the addition of R Si−H across the π-bond of a Ru-benzylidene.
3
KEYWORDS: ruthenium, alkene hydrosilylation, metathesis, silane, σ-bond metathesis
lefin metathesis is a revolutionary technology for olefin
synthesis through a highly efficient C−C bond-forming
alkynes toward ruthenium catalysts, which are largely less
reactive toward hydrosilylation than other late transition metal
catalysts, and (ii) more importantly the potential cross-
metathesis of 1 via alkene activation to afford homocoupled
products 4 and 5. Due to the paucity of mechanistic studies
O
13
reaction. In particular, ruthenium-catalyzed olefin metathesis is
among the most powerful olefination processes due to the
stability of catalysts, high reactivity, and broad functional group
1
compatibility. New discoveries stemming from non-metathet-
(
e.g., isotope-labeling experiments, reaction kinetic studies, and
ical applications of Grubbs-type ruthenium complexes have led
spectroscopic observations), the origin of the reactivity and
selectivity of such processes is elusive. The regio- and
diastereoselectivity remain uncertain. Nonetheless, the outcome
2
to the development of new synthetic strategies in parallel. The
instructive non-metathetical use of Grubbs-type catalysts for
intermolecular alkyne hydrosilylation has been reported by the
of this process has merit by virtue of silicon functionality, which
3
4
5
Cox, Lee, and Cossy laboratories. However, Grubbs-type
ruthenium-complex-catalyzed alkene hydrosilylation to provide
cyclic silyl ethers has not been reported to date.
8,14,15
allows further elaboration.
Herein, we report Grubbs-type
ruthenium-complex-catalyzed regioselective intramolecular al-
kene hydrosilylation of alkenyl silyl ethers, unveiling mecha-
nistic insights into the process.
To demonstrate the feasibility of Grubbs-type ruthenium-
complex-catalyzed alkene hydrosilylation and relieve our
concern regarding the competing alkene metathesis dimeriza-
tion, several ruthenium catalysts were examined (Table 1). At
ambient temperature, the reaction using Ru-1 (first-generation
Grubbs catalyst) did not proceed, and homoallyl silyl ether 1a
6
Metal-catalyzed alkene hydrosilylation, an addition reaction
of silicon−metal hydride across a carbon−carbon double bond,
is an important homogeneous catalytic process used to produce
7
−9
not only versatile silicon-containing synthetic intermediates
10
but also functionalized materials and medicinally useful
1
1
molecules from readily available hydrocarbon alkene feed-
stock. We wondered if Grubbs-type ruthenium complexes can
effectively promote intramolecular alkene hydrosilylation of
12
alkenylsilyl ethers 1 via preferential Si−H bond activation,
over alkene activation, to selectively afford cyclic silyl ethers
such as 2 or 3 (Scheme 1). The challenge of such processes
would be associated with (i) inferior reactivity of alkenes to
Received: February 27, 2015
Revised: April 10, 2015
©
XXXX American Chemical Society
3189
DOI: 10.1021/acscatal.5b00431
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ACS Catalysis
Scheme 1. Intramolecular Alkene Hydrosilylation vis-a-
Homo-crossed Alkene Metathesis Dimerization Using a
Grubbs-Type Ruthenium Complex
Research Article
̀
vis
Table 1. Evaluation and Optimization of Grubbs-Type
Ruthenium-Catalyzed Intramolecular Alkene
Hydrosilylation
a
b
c
entry
RuLn
solvent
2a/3a/4a
yield (%)
1
2
3
4
5
6
7
8
9
Ru-1 (2 mol %)
Ru-2 (2 mol %)
Ru-2 (2 mol %)
Ru-2 (2 mol %)
Ru-2 (2 mol %)
Ru-2 (0.5 mol %)
Ru-2 (0.1 mol %)
Ru-3 (2 mol %)
Ru-4 (2 mol %)
Ru-5 (2 mol %)
Ru-6 (2 mol %)
Ru-7 (2 mol %)
PhMe
PhMe
100:0:0
100:0:0
100:0:0
100:0:0
100:0:0
100:0:0
100:0:0
100:0:0
100:0:0
100:0:0
100:0:0
100:0:0
27
75
50
52
85
85
73
67
50
28
15
13
CH Cl2
2
PhH
THF
THF
THF
PhMe
PhMe
PhMe
PhMe
PhMe
1
1
1
0
1
2
a
b
Conditions: silane 1a (0.1 mmol), solvent (0.2 M). Determined by
c
1
GC/MS analysis. Determined by H NMR spectroscopy utilizing an
internal standard (CH Br ).
2
2
to detect homo-crossed dimers such as 4a or the styrene
byproduct released by the initial metathesis event in the GC/
1
19
MS and H NMR analyses. These results established that the
regioselective synthesis of oxasilacyclopentanes 2 is feasible via
hydrosilylative cyclization, exploiting Grubbs-type ruthenium
catalysts.
The effect of substituents on silicon was also examined
Table 2). Dimethyl and diphenyl substituents cleanly effected
(
Table 2. Silyl Substituent Effect on the Intramolecular
Alkene Hydrosilylation Catalyzed by Ru-2
was cleanly recovered. However, we were able to observe the
formation of oxasilacyclopentane 2a via a 5-exo-trig hydro-
silylative cyclization by exploiting Ru-2 (first-generation
a
16
Hoveyda−Grubbs catalyst), which indicates the viability of
Si−H bond activation (ca. 20% conversion for 7 days). Among
the seven ruthenium catalysts examined, Ru-2 was identified as
the most effective catalyst, permitting intramolecular alkene
hydrosilylation of 1a to afford 2a (73−85% in tetrahydrofuran
b
c
entry
R
2a/3a
yield (%)
(
%
THF), entries 5−7). A low catalyst loading of Ru-2 (0.1 mol
) gave slightly lower yield (73%, entry 7). Catalysts bearing
1
2
3
4
Me
Ph
100:0
100:0
100:0
100:0
63
85
20
0
bistricyclohexylphosphine ligands (i.e., Ru-1, entry 1), a mono-
ortho-substituted N-heterocyclic carbene ligand developed for
olefin metathesis of hindered alkenes (i.e., Ru-5 and Ru-6,
i-Pr
t-Bu
a
b
Conditions: silane 1a (0.1 mmol), THF (0.2 M). Determined by
17
entries 10 and 11), and a bidentate nitrate and adamantyl
c
1
GC/MS analysis. Determined by H NMR spectroscopy utilizing an
ligands developed for Z-selective olefin metathesis (i.e., Ru-7,
internal standard (CH Br ).
2
2
1
8
entry 12) gave yields lower than those of other catalysts (i.e.,
Ru-2 to Ru-5). Nonetheless, all of the Grubbs-type ruthenium
catalysts that we tested exhibited a complete regioselectivity,
and regioisomer 3a was consistently absent upon completion of
the reaction. Notably, metathesis activity was not detected in
any of these cases; no change to the resonance of a benzylidene
proton in H NMR was observed nor was the formation of
corresponding Ru-alkylidene/methylidene intermediates de-
tected in any of the reactions. Additionally, we were not able
the hydrosilylation. However, a substrate possessing diphenyl
substituents clearly exhibited a superior yield and conversion
over those containing alkyl substituents, suggesting that the silyl
substituent plays a critical role in the success of this process.
With the reaction conditions optimized, two other substrates
were examined, which successfully produced 2b (62%) and 2c
(65%), depicted in Scheme 2. Our preliminary results establish
1
3
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Research Article
Scheme 2. Selective Synthesis of Either
Oxasilacyclopentanes and Oxasilacyclohexanes via Grubbs-
Type Ruthenium-Catalyzed Hydrosilylative Cyclization
Scheme 3. Insightful Observations of the Formation of
Vinylsilanes (7a and 7c) and Chlorosilane (8a)
19
metathesis, the metathesis of 9a and silane 1 may furnish
either 9g via 9e (metal-out: unproductivethe silicon never
goes to the ruthenium metal center) or 9d via 9f (metal-in:
productive). It could be an analogous situation where the
outcome of competitive cross-metathesis (CM) [i.e., productive
CM: an ethylene-producing process (cf., 9f to 9d) and
unproductive CM: a degenerate metathesis (cf., 9e to 9g)] is
substantially dependent upon the steric hindrance of olefins, as
that the regioselective synthesis of either oxasilacyclopentanes
(e.g., 2a or 2b) via a 5-exo-trig (for homoallylic silyl ethers) or a
5
-endo-trig (for allylic silyl ethers) hydrosilylative cyclization or
oxasilacyclohexanes (e.g., 2c) via a 6-exo-trig cyclization is
feasible.
Several speculative mechanisms of Grubbs-type ruthenium-
complex-catalyzed alkyne hydrosilylation have been proposed
by Cox and Cossy. Cox proposed either (i) a sequence of an
28
well as the ligand set of the ruthenium catalyst.
To sort out these two mechanistic hypotheses for the initial
stage of the catalysis, we first performed a control experiment
(Scheme 5A); we speculated that bulkier substituents such as a
t-Bu group at silicon (i.e., 1a-t-Bu) could be favored for
unproductive σ-bond metathesis to yield chlorosilyl ether 8a-t-
initial addition of R Si−H across the π-bond of a Ru-
3
2
0
benzylidene (Chauvin mechanism), silylruthenation, α-
21
elimination (to metal alkylidene/hydride), and reductive
elimination or (ii) the traditional organometallic route
oxidative addition, migratory insertion, and reductive elimi-
nation. Cossy conjectured either hydroruthenation or
silylruthenation. However, neither study provided full
̀
Bu (via 10a vis-a-vis 10b). However, the formation of 2a-t-Bu
3
via the sequential addition of R Si−H across RuCHAr and
3
5
reductive elimination is unlikely because an addition of di-tert-
butylsilane 1a-t-Bu to Ru-2 is greatly hindered, as seen in a
experimental details regarding any such elemental processes.
During our initial study, we made an observation that addressed
the initial stage of Grubbs-type ruthenium-complex-catalyzed
hydrosilylation within a ruthenium coordination sphere. In
detail, upon treatment with Ru-2, silane 1a produced
29
Grubbs’ classification of general reactivity patterns of olefins.
When 1a-t-Bu was subjected to the reaction conditions
employing 100 mol % of Ru-2, only 8a-t-Bu (1a-t-Bu/8a-t-Bu
= 19:81) was observed without any notable cyclization,
corroborating our mechanistic hypothesis for the σ-bond
metathesis. In an effort to support this hypothesis, a
deuterium-labeling experiment was carried out using deuter-
iosilane 11-D and Ru-4 (Scheme 5B). The benzylidene proton
within the resulting putative ruthenium complex 12 remained
intact; we did not detect deuterium incorporation at this
22
vinylsilane 7a (ca. 0.5%) and chlorosilane 8a (ca. 1%),
which were detected and characterized by GC/MS analysis
(
(
Scheme 3A). We were able to isolate cyclic vinylsilane 7c
20% isolated yield) from hydrosilylation of 1c (Scheme 3B).
The formation of the vinylsilane suggests that Grubbs-type
ruthenium-complex-catalyzed alkene hydrosilylation likely
proceeds through a modified Chalk−Harrod mechanism (i.e.,
2
position by H NMR spectroscopy. This result suggests that the
1
2b
silylruthenation)
rather than the Chalk−Harrod (i.e.,
R Si−H addition across the RuCHAr and HCl elimination
3
1
2a,23
hydroruthenation) pathway.
cascade is improbable.
The formation of the chlorosilane 8a offers indirect
information for the initial stage of the hydrosilylation. The
result suggests two plausible mechanisms (Scheme 4): (i) A
The experiment performed to directly detect a ruthenium
silane complex is shown in Figure 1. In this prototype, the use
of an essentially equimolar ratio of Ru-4 and silane 11-H, which
does not bear an alkene moiety, resulted in full conversion to a
putative ruthenium silane complex (e.g., 9d in Scheme 4 or 12
in Scheme 5). Over time, the Si−H bond disappeared, yet
two-step sequence, namely, an addition of R Si−H across the π-
3
3
bond of a Ru-benzylidene to give 9b, followed by HCl
2
4
elimination
(
to form a putative Ru−Si complex 9d
2
5
7
productive), could be responsible for alkene hydrosilylation
benzylidene proton (H ) and other protons within the catalyst
7
11
to furnish 2 or reductive elimination to afford the chlorosilane
byproduct 8 (e.g., 8a) and 9c (unproductive) (path A). (ii)
Direct σ-bond metathesis between Si−H and Ru−Cl via a four-
centered transition state could be involved in the activation of
Ru-4 (H to H ) remained intact; an isopropoxy group was
12
still anchored to the ruthenium center (H , 4.74 ppm).
Interestingly, all protons in substrate 11-H were shifted
downfield, particularly, those at the ortho-position of
26,27
1
the ruthenium complex by silanes (path B).
Based primarily
diphenylsilyl substituents (H , shifted downfield by 0.12
3
upon a bottom-face olefin coordination mechanism for olefin
ppm) and methylene protons (H , shifted downfield by 0.063
3
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Scheme 4. Activation of Grubbs-Type Ruthenium Catalysts by Silanes
Scheme 5. Stoichiometric Reactions of Alkenylsilyl Ethers
(10a-t-Bu and 11-D) and Ru-2
Figure 1. Monitoring the stoichiometric reaction of hydrosilyl ether
1
1
1-H and Ru-4 by H NMR spectra (500 MHz, C D ) over time.
6
6
Proton resonances colored in red emanate from a newly generated
putative Si−Ru complex (for the stoichiometric reaction of 11-H and
Ru-2 monitored by 1H NMR spectra over time, see Supporting
Information).
ppm) within 11-H. These protons were drawn closer to the
ruthenium center. No additional benzylidene or alkylidene
resonances were observed during this series of experiments.
Further insights into the reaction mechanism of the
hydrosilylation were garnered by examining the impact of the
X-type halide ligands within ruthenium catalysts (Scheme 6).
The catalytic activity of the hydrosilylation was generally
increased by having an electron-withdrawing and smaller halide
group from iodide to chloride. This trend is similar to the
observed olefin metathesis reactivity of the ruthenium
dibromide and diiodide (Ru-2-Br and Ru-2-I, respectively).
The reasons behind the reactivity difference are unclear at this
moment, but a sterically less demanding and electron-
withdrawing X-type chloride ligand perhaps favors ruthenium
binding to Si−H, dictating a facile σ-bond metathesis.
1
7a
catalyst.
Particularly, the reaction with dichloride catalyst
We carried out an additional set of deuterium-labeling studies
to further understand the nature of this cyclization (Scheme 7).
Ru-2-Cl was significantly faster than catalysts containing
3
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Research Article
Scheme 6. Catalytic Activities Varying Halides on
Ruthenium Catalysts
Scheme 8. Addition of a Proton Scavenger
reactions with and without base were essentially identical (t1/2
=
ca. 40 min), albeit resulting in slightly diminished yields of 2a.
This result indicates that the dissociated HCl did not affect the
overall reaction efficiency. In addition, a potential mechanism
involving heterolysis of a Si−H bond by a ruthenium catalyst
24,30
can be eliminated.
The plausible overall mechanism, based upon our mecha-
nistic studies and observations, is depicted in Scheme 9. The
Scheme 9. Proposed Mechanism
Scheme 7. Intramolecular and Intermolecular Deuterium-
Labeling Studies
initial productive σ-bond metathesis between a ruthenium
catalyst and silyl ether 1 affords ruthenium−silyl complex 9d
(
see Scheme 4, path B). Alkene coordination to ruthenium
within 9h followed by silylruthenation affords 9i. At this stage,
either β-hydride elimination (to 8) or σ-bond metathesis (to
product 2) via 9j takes place and regenerates 9h. Alternatively,
protonation by HCl would afford 2 and ruthenium catalyst
RuLn.
In light of these new mechanistic insights, we investigated the
substrate scope and regio- and stereoselectivity of Grubbs-type
ruthenium-complex-catalyzed intramolecular alkene hydrosily-
lation (Table 3). Homoallylic silyl ethers with 3° alkoxy carbon
(2d−i) showed good conversions and yields. To understand
the impact of relative stereochemistry, substrates 1f−i were
subjected to the reaction conditions. We found that substrates
containing a silyl ether and a syn-substituent (Ph or t-Bu) at the
4-position on a cyclohexyl moiety gave products (2f and 2h)
with higher yields when compared with their counterparts (2g
Under the same reaction conditions, the deuterium of 1a-D was
mainly transferred to two positions of 2a-D, supporting the
modified Chalk−Harrod mechanism (Scheme 7A). Further-
more, the crossover experiment established that the proton
transfer occurs intermolecularly (Scheme 7B). The hydrogen
and deuterium scrambling, shown as 2a-H/D and 2e-H/D,
reinforces our mechanistic hypothesis involving the σ-bond
metathesis.
Lastly, we examined the reaction of 1a employing a
stoichiometric amount of base [2,6-di-tert-butyl-4-methylpyr-
idine (DTBMP) or NaHCO ] (Scheme 8). The rates of two
3
3
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Research Article
a
Table 3. Substrate Scope of Grubbs-Type Ruthenium Catalysts for Intramolecular Alkene Hydrosilylation
a
b
1
c
Conditions: silane 5 (0.3 mmol), THF (0.2 M). Determined by H NMR spectroscopy utilizing an internal standard (CH Br ). Isolated yield of
2
2
d
1
oxidation/acetylation or benzoylation products 6. Diastereomeric ratio was determined by GC/MS analysis and H NMR spectroscopy.
and 2i). Substrates with 1° silyl ether (1j) afforded 2j in modest
yield. Substrates with 2° silyl ether (1k) yielded 2k with a 3:1
ratio (cis/trans) of diastereomers. We also studied allylic sillyl
ether 1l, which provided 2l. 2-Allylphenoxydiphenylsilanes
Chalk−Harrod mechanism. Further efforts toward synthetic
applications of Grubbs-type ruthenium-catalyzed hydrosilyla-
tion are currently underway.
ASSOCIATED CONTENT
Supporting Information
The following file is available free of charge on the ACS
1m−p afforded oxasilacyclohexanes 2m−p with respective
■
yields, regardless of electronic nature of the substituents at the
para-position to the phenoxy group. The resulting organo-
silicon heterocylces 2 were subjected to oxidation and acylation
conditions, which provided diacetates (6d and 6m−p), hydroxy
acetates (6e−i), and dibenzoate (6j).
*
S
Publications website at DOI: 10.1021/acscatal.5b00431.
Experimental details and spectroscopic characterization
data for all compounds (PDF)
In summary, we have developed a Grubbs-type ruthenium-
complex-catalyzed intramolecular alkene hydrosilylation of
alkenylsilyl ethers to provide cyclic silyl ethers. Preferential
Si−H bond activation over alkene activation was observed,
where alkene metathesis was effectively suppressed. This study
expands our understanding of fundamental mechanistic aspects
of non-metathetical function of Grubbs-type ruthenium
catalysts for alkene hydrosilylation, with potential implications
for other associated transformations such as dehydrogenative
AUTHOR INFORMATION
Corresponding Author
E-mail: jjeon@uta.edu.
■
*
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
31
32
condensation between alcohols and silanes, direct arylation,
The authors dedicate this article to Professor Thomas R. Hoye
(University of Minnesota) on the occasion of his 65th birthday.
We thank UT Arlington Research Enhancement Program and
the American Chemical Society Petroleum Research Fund
(PRF# 54831-DNI1) for support of our program. The NSF
3
3
and hydrogenation. Notably, the initial stage of the
hydrosilylation involving the σ-bond metathesis between Si−
H and Ru−Cl is proposed. The Grubbs-type ruthenium-
complex-catalyzed alkene hydrosilylation follows the modified
3
194
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ACS Catalysis
CHE-0234811 and CHE-0840509) is acknowledged for partial
funding of the purchases of the NMR spectrometers used in
this work. We acknowledge Mr. Naphtali Reyna and Mr.
William Scaggs for their preliminary studies. We also
acknowledge at Materia, Inc. for generous donation of
ruthenium complexes.
Research Article
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DOI: 10.1021/acscatal.5b00431
ACS Catal. 2015, 5, 3189−3195