(Z)-selective hydrosilylation of terminal alkynes with HSiMe(OSiMe3)2 catalyzed by a ruthenium complex containing an N-heterocyclic carbene

Article abstract of DOI:10.1021/acs.orglett.7b02477

The N-heterocyclic-carbene-ligated ruthenium complex [RuHCl(CO)(H₂IMes)(PCy₃)] exhibits high catalytic activity for the (Z)-selective hydrosilylation of various terminal alkynes with 1,1,1,3,5,5,5-heptamethyltrisiloxane (HSiMe-(OSiMe

Full text of DOI:10.1021/acs.orglett.7b02477

Letter
pubs.acs.org/OrgLett
(
Z)‑Selective Hydrosilylation of Terminal Alkynes with
HSiMe(OSiMe ) Catalyzed by a Ruthenium Complex Containing an
3
2
N‑Heterocyclic Carbene
Yuichiro Mutoh,* Yusei Mohara, and Shinichi Saito*
Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
*
S Supporting Information
ABSTRACT: The N-heterocyclic-carbene-ligated ruthenium
complex [RuHCl(CO)(H IMes)(PCy )] exhibits high catalytic
2
3
activity for the (Z)-selective hydrosilylation of various terminal
alkynes with 1,1,1,3,5,5,5-heptamethyltrisiloxane (HSiMe-
(
OSiMe ) ). The stereoretentive derivatization of the (Z)-
3 2
alkenylsiloxanes allows the synthesis of biologically active
compounds, e.g. potent antitumor agents and inhibitors for
induced-NO synthase.
lkenylsilanes have received substantial attention on
Scheme 1. Transition-Metal-Catalyzed β-(Z)-Selective
Hydrosilylation of Terminal Alkynes with HSiMe(OSiMe3)2
1
A
account of their utility in organic chemistry. The
hydrosilylation of alkynes using late-transition-metal catalysts
represents a reliable and straightforward synthetic route to
1
,2
alkenylsilanes.
As the silane source, the cost-effective
3
HSiMe(OSiMe3)2 is particularly attractive, given that
alkenylsiloxanes derived from HSiMe(OSiMe ) are resistant
3
2
toward hydrolysis due to the siloxane linkages, but still readily
4
undergo Hiyama cross-coupling reactions owing to the
presence of two electron-withdrawing (trimethylsilyl)oxy
3
c,f
groups.
During the hydrosilylation of terminal alkynes, several
1
,2
possible isomers can be produced, and the β-(E)-selective
reaction of terminal alkynes with HSiMe(OSiMe ) has been
3
2
3a,c
established using well-defined platinum complexes.
How-
ever, the β-(Z)-selective reaction with HSiMe(OSiMe3)2
remains challenging with respect to both catalytic efficiency
of the transition-metal catalysts, as well as the regio- and
stereoselectivity of the products. In fact, the ruthenium−N,S-
heterocyclic-carbene-catalyzed hydrosilylation of phenylacety-
lene with HSiMe(OSiMe ) affords a mixture of isomers in low
3
2
5
a
yield (Scheme 1, eq 1), while a latent (Z)-to-(E) isomer-
ization was observed when using an Ir−Cr bimetallic catalyst
5b
(
Scheme 1, eq 2). Alternative approaches to (E)- and (Z)-
carbene (NHC), should be a highly active catalyst for the
hydrosilylation of terminal alkynes (Scheme 1, eq 3). Herein,
we describe the [Ru(NHC)]-catalyzed β-(Z)-selective hydro-
alkenylsiloxanes are the dehydrogenative silylation of terminal
3f,i
alkenes with HSiMe(OSiMe ) , and the rhodium-catalyzed
3
2
β-(Z)-selective hydrosilylation of terminal alkynes with
silylation of terminal alkynes with HSiMe(OSiMe3)2.
HSiMe (OSiMe ) and HSiMe(OEt) , which generates the
2
3
2
6
Initially, we investigated the effect of the structure of NHC in
corresponding (Z)-alkenylsiloxanes.
Considering mechanistic studies on the β-(Z)-selective
7
[
RuHCl(CO)(NHC)(PCy )] on the hydrosilylation of phenyl-
3
acetylene (1a) with HSiMe(OSiMe3)2 (Table 1). When
hydrosilylation of alkynes with HSiEt catalyzed by [RuHCl-
3
8
,9
(
CO)(PCy ) ] and the Grubbs second generation catalyst
system in olefin metathesis, we envisioned that [RuHCl-
CO)(NHC)(PCy )], which carries an N-heterocyclic
3 2
10
Received: August 10, 2017
1
1
(
3
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02477
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Scope of the β-(Z)-Selective Hydrosilylation of 1a
Scheme 2. Scope of the [RuHCl(CO)(H IMes)(PCy )]-
2 3
a
with HSiMe(OSiMe ) Regarding Ruthenium Complexes
Catalyzed β-(Z)-Selective Hydrosilylation with Respect to
3
2
a
Terminal Alkynes 1
time
(h)
yield
b
β-(Z)/
c
entry
[RuHCl(CO)(L)(PCy )]
(%)
β-(E)
3
1
2
3
4
5
[RuHCl(CO)(PCy ) ]
1
34
95
89
78
80
97:3
3
2
[RuHCl(CO)(H IMes)(PCy )]
0.5
1
>99:1
>99:1
>99:1
>99:1
2
3
[RuHCl(CO)(H IPr)(PCy )]
2
3
[RuHCl(CO)(IMes)(PCy )]
1
3
[RuHCl(CO)(IPr)(PCy )]
1
3
a
Reaction conditions: [RuHCl(CO)(L)(PCy )] (1 mol %), 1a (1.1
3
equiv), HSiMe(OSiMe ) (1.0 equiv), CH Cl (0.1 M). In all cases,
3
2
2
2
1
the α-isomer was not detected by GC-MS or H NMR spectroscopy.
Isolated yields. The β-(Z)/β-(E) ratio was estimated by H NMR
b
c
1
analysis of the crude product.
bis(phosphine) complex [RuHCl(CO)(PCy ) ] was used as
3
2
the catalyst, and the reaction was conducted at room
temperature for 1 h, styrylsiloxane 2a was obtained in low
yield as a mixture of isomers (Table 1, entry 1). The
corresponding H IMes-ligated complex [RuHCl(CO)-
2
(
9
H IMes)(PCy )] was more active and furnished β-(Z)-2a in
5% yield as a single stereoisomer after 0.5 h (Table 1, entry 2).
2 3
A ruthenium complex with a bulkier H IPr ligand also exhibited
2
good performance (Table 1, entry 3). Although decreased
catalytic activity was observed for ruthenium complexes
containing unsaturated NHC ligands, the stereoselectivity of
the products was nearly perfect (Table 1, entries 4 and 5). In
a
General reaction conditions: [RuHCl(CO)(H IMes)(PCy )] (1 mol
2
3
general, complexes of the type [RuHCl(CO)(NHC)(PCy )]
3
%), 1 (1.1 equiv), HSiMe(OSiMe ) (1.0 equiv), CH Cl (0.1 M).
Unless otherwise specified, the Z/E ratio (>99:1) was based on the H
3 2 2 2
1
are efficient precatalysts for the (Z)-selective hydrosilylation of
terminal alkynes with HSiMe(OSiMe ) .
NMR analysis of the crude product. Isolated yields are shown in
parentheses. The reaction was conducted at 70 °C in toluene.
3
2
b
Considering the mechanistic studies on the hydrosilylation of
7
terminal alkynes with HSiEt , the NHCs should affect the rate-
3
limiting dissociation and recoordination of PCy3 for all
intermediates, e.g. [RuCl{SiMe(OSiMe ) }(CO)(NHC)-
(Z)-styrylsiloxanes 2b and 2c were obtained in high yields.
Trifluoromethyl-substituted phenylacetylene (1d) exhibited a
comparatively low reactivity, albeit 2d was formed in 96% yield
when the reaction was performed at 70 °C. This decreased
reactivity may be rationalized in terms of a weaker coordinating
ability of the alkyne to the ruthenium center. Notably, the
bromo functionality is tolerated in this hydrosilylation, and the
corresponding products (2e−g) were obtained regardless of the
position of the bromine atom on the benzene ring. The
relatively low reactivity in the reaction leading to 2g is probably
due to steric congestion.
3
2
(
PCy )]. The lower efficacy of the complexes bearing
3
unsaturated NHCs in the hydrosilylation is consistent with
the results obtained from Grubbs second generation catalyst
system in olefin metathesis, which involves a rate-determining
12
dissociation of PCy3. These results suggest that saturated
NHCs favor the liberation of PCy from a resting-state 16-
3
electron complex to generate the active 14-electron species
such as [RuCl{SiMe(OSiMe ) }(CO)(H IMes)] during the
3
2
2
hydrosilylation.
The observed high stereoselectivity of the hydrosilylation
could be attributed to the steric bulk of the NHC-ligated
The compatibility of heteroaryl-substituted terminal acety-
lenes such as ethynylthiophenes (1h and 1i) and 2-ethynyl-
benzofurane (1j) was also evaluated, as the reactivity of
ethynylheteroaromatics in the hydrosilylation reaction has not
yet been documented. The reactions were generally sluggish,
but 2h−j were obtained diastereoselectively in high yields
without the need to increase the catalyst loading. The reaction
of 4-ethynylcoumarin (1k), a bulkier cyclic ester with an alkyne
moiety, reached completion after 60 h at 70 °C to afford (Z)-2k
as the major product. Longer reaction times and/or higher
temperatures were required for the reactions of 1h−k, given
that these alkynes contain sulfur and oxygen atoms, which are
able to coordinate to the ruthenium center. The oxygen atom
ruthenium complex. Even when the less bulky HSiMe Ph was
2
employed in the [RuHCl(CO)(H IMes)(PCy )]-catalyzed
2
3
hydrosilylation of phenylacetylene, the corresponding product
was also obtained in a Z/E ratio of 99:1. Use of the less bulky
ruthenium catalysts such as [RuHCl(CO)(Pi-Pr ) ] and
3
2
[
RuHCl(CO)(PCy ) ] in the hydrosilylation with HSiMe Ph
3 2 2
8
afforded Z/E product ratios of 97:3 and 96:4, respectively.
To explore the scope of this reaction with respect to the
substrates, various terminal alkynes were used to afford the
corresponding alkenylsilanes with excellent (Z)-selectivity
(Scheme 2). Under the established conditions, 4-substituted
B
DOI: 10.1021/acs.orglett.7b02477
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
in 1j and 1k may act as a directing group that leads to the
formation of α-isomers.
stereospecific access to biologically relevant (Z)-stilbene
derivatives.
9b,13
In addition to aryl-substituted alkynes, alkyl-substituted
alkynes also underwent this stereospecific hydrosilylation. The
reaction of 1-hexyne (1l) at room temperature yielded 2l in
ASSOCIATED CONTENT
Supporting Information
■
*
S
90% after 24 h. The hydrosilylation of an alkyne derived from
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b02477.
dimethyl malonate furnished the corresponding product (2m)
in 60% yield after 14 h at 70 °C. Again, the relatively low yield
of this reaction should be attributed to the steric demand of the
substrate and the coordination ability of the functional group.
Finally, the synthetic utility of 2 was explored by synthesizing
biologically active compounds. We examined the fluoride-free
Experimental procedures (PDF)
X-ray crystallographic data for [RuHCl(CO)(H IMes)-
2
(
PCy )] (CCDC 1543773) (CIF)
3
3
f,14
Hiyama cross-coupling
of 2 with 3,4,5-trimethoxy-
[
RuHCl(CO)(IPr)(PCy )] (CCDC 1543774) (CIF)
3
iodobenzene (Scheme 3, eq 1). The reaction proceeded in
3b (CCDC 1566459) (CIF)
Scheme 3. Functionalization of the Obtained
Hydrosilylation Products
a
AUTHOR INFORMATION
■
Corresponding Authors
*
*
E-mail: ymutoh@rs.tus.ac.jp (Y.M.).
E-mail: ssaito@rs.kagu.tus.ac.jp (S.S.).
ORCID
Yuichiro Mutoh: 0000-0002-5254-9383
Shinichi Saito: 0000-0001-8520-1116
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Mr. Yuta Inoue, Dr. Shintaro Kodama, and
Prof. Youichi Ishii for elemental analysis measurements (Chuo
University).
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(
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(
6
(
H IMes)(PCy )] serves as an active precatalyst for the β-(Z)-
2 3
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D
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