Cobalt-catalyzed regioselective stereoconvergent Markovnikov 1,2-hydrosilylation of conjugated dienes

Article abstract of DOI:10.1039/c7sc04002d

We report the first stereoconvergent Markovnikov 1,2-hydrosilylation of conjugated dienes using catalysts generated from bench-stable Co(acac)₂ and phosphine ligands. A wide range of E/Z-dienes underwent this Markovnikov 1,2-hydrosilylation in a stereoconvergent manner, affording (E)-allylsilanes in high isolated yields with high stereoselectivities (E/Z = >99;:;1) and high regioselectivities (b/l up to > 99;:;1). Mechanistic studies revealed that this stereoconvergence stems from a σ-π-σ isomerization of an allylcobalt species generated by the 1,4-hydrometalation of Z-dienes. In addition, a cobalt catalyst that can only catalyze the hydrosilylation of the E-isomer of an (E/Z)-diene was identified, which allows the separation of the (Z)-isomer from an isomeric mixture of (E/Z)-dienes. Furthermore, asymmetric hydrosilylation of (E)-1-aryl-1,3-dienes was studied with Co(acac)₂/(R)-difluorphos and good enantioselectivities (er up to 90;:;10) were obtained.

Full text of DOI:10.1039/c7sc04002d

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DOI: 10.1039/C7SC04002D
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Cobalt-Catalyzed Regioselective Stereoconvergent Markovnikov
,2-Hydrosilylation of Dienes
1
Received 00th January 20xx,
Accepted 00th January 20xx
*
Hui Leng Sang, Songjie Yu, and Shaozhong Ge
DOI: 10.1039/x0xx00000x
We report the first stereoconvergent Markovnikov 1,2-hydrosilylation of conjugated dienes using catalysts generated from
bench-stable Co(acac) and phosphine ligands. A wide range of E/Z-dienes underwent this Markovnikov 1,2-hydrosilylation
2
www.rsc.org/
in a stereoconvergent manner, affording (E)-allylsilanes in high isolated yields with high stereoselectivities (E/Z = >99:1)
and high regioselectivities (b/l up to > 99:1). Mechanistic studies revealed that this stereoconvergence stems from a σ-π-σ
isomerization of an allylcobalt species generated by 1,4-hydrometalation of Z-dienes. In addition, a cobalt catalyst that can
only catalyze the hydrosilylation of the E-isomer of (E/Z)-diene was identified, which allows the separation of a (Z)-isomer
from an isomeric mixture of (E/Z)-dienes. Furthermore, asymmetric hydrosilylation of (E)-1-aryl-1,3-dienes was studied
2
with Co(acac) /(R)-difluorphos and good enantioselectivities (er up to 90:10) were obtained.
Markovnikov hydrosilylation (Scheme 1B). However, transition
Introduction
metal catalysts for Markovnikov 1,2-hydrosilylation of
conjugated dienes, especially for stereoconvergent
Markovnikov 1,2-hydrosilylation of E/Z-dienes, still remain
unknown. Driven by our research interest in developing base
metal catalysts for transformations of unsaturated organic
Allylsilanes are synthetically valuable building blocks due to
their non-toxicity, high stability and versatile applications in
1
organic synthesis and material science. Among the methods
2
for allylsilane synthesis,
transition metal-catalyzed
4
f,5f,10
molecules,
catalyzed stereoconvergent Markovnikov 1,2-hydrosilylation of
wide range of functionalized conjugated (E/Z)-dienes
herein we report a highly selective cobalt-
hydrosilylation of conjugated dienes is the most
straightforward approach to prepare these synthetically
3
4
valuable allylsilanes. Recently, hydrosilylation of alkenes and
a
5
alkynes has been extensively studied with cobalt catalysts,
6
(Scheme 1C). In addition, we also identified a cobalt catalyst
that selectively catalyzes the hydrosilylation of the (E)-isomer
of an (E/Z)-diene with the (Z)-isomer unreacted. This discovery
would represent a convenient protocol to purify (Z)-dienes
from (E/Z)-isomeric dienes, which are generally more
accessible than stereodefined (Z)- or (E)-dienes.
particularly due to their higher abundance and lower toxicity
7
compared to platinum catalysts for hydrosilylation reactions.
More importantly, recent studies indicate that cobalt catalysts
5
can offer a precise control on regio- and stereoselectivities.
However, highly selective cobalt catalysts for hydrosilylation of
conjugated dienes still remain rare and are under
3
c,8
development.
Conjugated dienes can undergo 1,4- and 1,2-hydrosilylation
and the selectivity is dependent on the catalysts employed.
The majority of the catalysts based on Fe, Co, and Ni show high
3
selectivity towards 1,4-hydrosilylation (Scheme 1A). For
example, Hilt reported a highly selective cobalt catalyst for 1,4-
3
hydrosilylation of isoprene in the presence of P(n-Bu)3, and
c
Ritter reported a well-defined Fe(0) complex ligated by 2-
d
3
iminopyridines for 1,4-hydrosilylation of 1,3-dienes.
In
contrast, 1,2-hydrosilylation of conjugated dienes has been
barely studied, and two catalysts based on Pt and Co have
Scheme 1 Transition metal-catalyzed hydrosilylation of conjugated dienes.
8
been used to catalyze this 1,2-hydrosilylation. In addition,
,9
these two catalysts show high selectivity towards anti-
Results and Discussion
We chose the reaction of (E)-1-phenyl-1,3-butadiene with
Department of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore 117543 (Singapore). E-mail: chmgsh@nus.edu.sg
3
PhSiH to evaluate reaction conditions for this Co-catalyzed
†
DOI: 10.1039/x0xx00000x
Electronic
Supplementary
Information
(ESI)
available:
See
hydrosilylation of conjugated dienes. We tested this reaction
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2
with cobalt catalysts generated from Co(acac) and various desired allylsilane 1a in an increased yield with an excellent
nitrogen- and phosphine-based ligands. The selected examples Markovnikov selectivity (entry 9 in Table 1).
of these examples are summarized in Table 1. In general, these As (E/Z)-isomeric mixtures of dienes are synthetically more
reactions were conducted with 3 mol % of Co(acac)
ligand at 50 C for 3 h.
2
and 3 mol accessible than stereodefined (E)- or (Z)-dienes, we tested this
hydrosilylation with a mixture of (E/Z)-isomeric 1-phenyl-1,3-
butadiene to check the stereoconvergency for this reaction. To
our delight, both (Z)- and (E)-1-phenyl-1,3-dienes underwent
%
°
Table 1. Evaluation of Conditions for the Co-catalyzed Hydrosilylation of 1-
a
Phenyl-1,3-butadiene.
this cobalt-catalyzed Markovnikov 1,2-hydrosilylation and
produced branched allylsilane 1a in high yield with high
selectivity (entry 10 in Table 1). Different from the
hydrosilylation of (E)-1-phenyl-1,3-diene (entry 9 in Table 1),
the reaction with a mixture of (E/Z)-1-phenyl-1,3-butadienes
also generated
a detectable amount (4%) of branched
allylsilane 2a, a product resulted from 1,4-hydrosilylation of
diene.
Under the identified conditions (entry 9 in Table 1), we
studied the scope of conjugated trans-dienes for this reaction.
These results are summarized in Table 2. In general, a wide
range of conjugated trans-dienes reacted smoothly with
PhSiH in the presence of 1 mol % of Co(acac) and xantphos at
3
2
room temperature, affording the corresponding (E)-allylsilanes
1a 1n in Table 2) in high yields (70% 96%) with excellent
regioselectivities (b/l = >99:1). The scope of these trans-dienes
encompassed aryl-substituted 1a 1h in Table 2), alkyl-
substituted (1i 1k in Table 2), and multiply-substituted dienes
1a 1h in Table 2). The GC-MS analysis on the crude mixture of
(
−
−
(
−
−
(
−
these reactions revealed that organosilane products from
Table 2. Scope of trans-Dienes for this Co-Catalyzed Markovnikov 1,2-
a
Hydrosilylation
a
Conditions: (E)-diene (0.400 mmol), PhSiH
3
(0.500 mmol), Co(acac)
2
(12.0
mol), ligand (12.0 ꢀmol), THF (1 mL), 3 h; Conversion of diene, yield of
a, ratio of products 1a, 2a, and 3a were determined by GC analysis with
b
ꢀ
1
c
d
tridecane as the internal standard; Catalyst (1 mol %); A mixture of (E/Z)-
-phenyl-1,3-butadiene (E/Z = 45:55) was used.
1
Cobalt catalysts generated from the combination of
Ph TF
Co(acac) and nitrogen-based ligands, such as PDI, ADPI or
2
PyBox, did catalyze the 1,2-hydrosilylation of (E)-1-phenyl-1,3-
butadiene to full conversions, but these reactions produced a
mixture of products 1a and 3a with low selectivities (entries
8
in Table 1). The reactions conducted with the
1−
3
combination of Co(acac)2 and bisphosphine ligands, such as
dppm, dppe, or dppbz, proceeded to low to modest
conversions, but with complete selectivities (>99%) to
Markovnikov 1,2-hydrosilylation (entries 4−6 in Table 1). In
particular, the reactions catalyzed by the combination of
Co(acac)2 and binap or xantphos proceeded to high or full
conversions with excellent selectivities to branched allylsilane
a
Conditions: (E)-diene (0.400 mmol), PhSiH
3
(0.500 mmol), Co(acac)
2
(12.0
mol), ligand (12.0 ꢀmol), THF (1 mL), 3 h; Conversion of diene, yield of
1a, ratio of products 1a, 2a, and 3a were determined by GC analysis with
1a
(enties 7 and 8 in Table 1). As dienes are thermally less
b
ꢀ
stable and can undergo polymerization, we tested this
hydrosilylation at lower temperatures. The reaction conducted
c
d
tridecane as the internal standard; Catalyst (1 mol %); A mixture of (E/Z)-
-phenyl-1,3-butadiene (E/Z = 45:55) was used.
1
with 1 mol % of Co(acac) and 1 mol % of xantphos at room
2
temperature proceeded to full conversion and afforded the
2
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either
1,4-hydrosilylation
or
anti-Markovnikov
1,2- hydrosilylation are listed in Table 3 and abbreviated as 1,2/1,4
hydrosilylation of these trans-dienes were not formed during ratios.
this Co-catalyzed Markovnikov 1,2-hydrosilylation. In addition,
The data in Table 3 indicated that the electronic properties
we also tested this hydrosilylation with secondary hydrosilanes of the aryl substituents do not have a significant influence on
(
Ph
smoothly to afford tertiary cinnamylsilanes (1a’ and 1a” in hydrosilylation reactions (e.g. 1a
Table 2) in high isolated yields. However, this Co-catalyzed Co-catalyzed transformation can tolerate various reactive
hydrosilylation did not occur with dialkylsilane (Et SiH ) or groups, such as trifluoromethyl ether (1r), halogens (1f 1s, and
tertiary hydrosilanes, such as (EtO) SiH and (EtO) MeSiH. 1t), triflate (1u), ester (1v and 1w), acetal (1x), unprotected
2
SiH
2
and PhMeSiH
2
), and these reactions proceeded the stereoconvergence and regioselectivity of these
,
1e, and 1o in Table 3). This
2
2
,
3
2
Subsequently, we studied the scope of conjugated dienes primary aniline (1y), and pinacolboronic ester (1z). In addition,
containing a mixture of (E/Z)-isomeric 1,3-dienes for this ((1E/Z,3E)-hexa-1,3-dien-1-yl)benzene, an internal 1,3-diene,
hydrosilylation reaction, and the results are summarized in also underwent this stereoconvergent hydrosilylation to afford
Table 3. Generally, a wide range of (E/Z)-dienes, with E/Z ratios allylsilane 1aa in high isolated yield with high stereoselectivity
given in the brackets in Table 3, underwent this Markovnikov (E/Z = >99:1).
1
,2-hydrosilylation in
conversions, affording the corresponding (E)-allylsilanes reaction are bench-stable, we tested the hydrosilylation of 1-
1a 1z in Table 3) in high isolated yields with high regio- and (buta-1,3-dien-1-yl)-4-methoxybenzene with PhSiH3 on a 10
a
stereoconvergent manner in full
As both Co(acac) and xantphos used for this hydrosilylation
2
(
−
stereoselectivities (b/l = >99:1; E/Z = >99:1). The GC-MS mmol scale with 1 mol % of Co(acac) /xantphos weighed on
2
analysis on the crude reaction mixtures indicated that these the benchtop without using a dry box. This reaction proceeded
reactions also produced small amounts of 1,4-hydrosilylation to full conversion of the diene substrate and afforded 1o in
products, and the ratios of these products from 1,2- and 1,4- 87% isolated yield (eq 1).
a
Table 3. Scope of (E/Z)-Dienes for Stereoconvergent Hydrosilylation Reactions
To understand the stereoconvergence of this hydrosilylation
of (E/Z)-dienes, we analyzed the reaction of (E/Z)-1-phenyl-1,3-
butadiene and found that the (E)-isomer was consumed at a
1
1
significantly higher rate than (Z)-isomer. In addition, the
hydrosilylation of (Z)-1-phenyl-1,3-butadiene was studied, and
this reaction afforded 1,2-hydrosilylation product 1a together
with a significant amount of 1,4-hydrosilylation product 2a
(
1a:2a = 74:26, Scheme 2A). The results of this reaction and
the reactions of the E-isomer (entry 9 in Table 1) and the
mixture of the (E/Z)-isomers (entry 10 in Table 1) suggest that
allylsilane 2a was formed by 1,4-hydrosilylation of the (Z)-
a
Conditions: (E/Z)-Diene (0.400 mmol), PhSiH
mol), xantphos (5.0 ꢀmol), THF (1 mL), rt, 6 h; yield of isolated product;
,2/1,4 ratios refer to 1,2-/1,4-hydrosilylation, ratios in brackets are E/Z
3 2
(0.500 mmol), Co(acac) (5.0
ꢀ
1
b
ratios of conjugated butadiene reagents. Reactions were conducted at 5
c
2
Scheme 2 Hydrosilylation of stereodefined diene by Co(acac) /xantphos
°
C for 24 h. 3 mol % catalyst, rt, and 48 h.
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isomer. To provide insight into the isomerization of the
After developing this stereoconvergent hydrosilylation
internal Z-alkene in the diene to the E-alkene in product 1a, we reaction, we rationalized that the separation of a (Z)-diene
subsequently conducted a deuterium-labelling experiment from Z/E-diene mixtures could be achieved if we could identify
using the (E/Z)-isomers and PhSiD
deuterium was solely incorporated onto the methyl group of We tested various cobalt catalysts generated from the
and 2a. This lack of deuterium incorporation into the combination of Co(acac) and bisphosphine ligands for this
internal vinylic carbons suggests that this E/Z-isomerization purpose. To our delight, we found that the cobalt complex
through migratory insertion of Z-alkene into a Co-H species from Co(acac) /binap was active for Markovnikov 1,2-
-H elimination, as indicated in Scheme 2C, is hydrosilylation of (E)-1-phenyl-1,3-diene (Scheme 4A) but did
unlikely. Furthermore, we also tested the reaction of (E)- not catalyze the hydrosilylation or the isomerization of (Z)-1-
isomer with PhSiD and the same deuterium incorporation was phenyl-1,3-diene (Scheme 4B). Then we conducted this
observed (Scheme 2D). hydrosilylation reaction with a E/Z-mixture of 1-phenyl-1,3-
Based on the results of experiments in Scheme 2 and the diene (Scheme 4C). As expected, this reaction afforded (E)-
3
(Scheme 2B) and found that a cobalt catalyst that can convert only the (E)-isomer of dienes.
1a
2
2
1
2
followed by
β
3
4
f,6
precedent of Co-catalyzed hydrosilylation of alkenes,
we allylsilane 1a in 58% isolated yield and (Z)-1-phenyl-1,3-diene
proposed a hydrometalation pathway with a Co(I)-H was recovered in 45% isolated yield with Z/E ratio of 98:2.
intermediate for this Co-catalyzed Markovnikov hydrosilylation
of conjugated dienes (Scheme 3). 2,1-Migratory insertion of
(
E)-diene into a Co-H species forms an allylcobalt intermediate
, which turns over with PhSiH3 to release the allylsilane
product and to regenerate the Co-H species.
I
2
Scheme 4 Hydrosilylation of E/Z-mixture of dienes by Co(acac) /binap
We subsequently studied cobalt catalysts generated from
Co(acac) and chiral bisphosphine ligands in order to develop
2
an asymmetric Markovnikov 1,2-hydrosilylation of conjugated
1
4
dienes. After evaluating various chiral phosphine ligands (see
ESI for the details), we found that the hydrosilylation of trans-
diene with PhSiH3 conducted with Co(acac)2 and (R)-
difluorphos proceeded smoothly to afford the corresponding
allylsilanes in good yield and good er (eq 2).
Scheme
3
Stereoconvergent Markovnikov Hydrosilylation of Dienes
Proposed Catalytic Pathways for this Co-Catalyzed
For the hydrosilylation of (Z)-dienes, 2,1-migratory insert of Conclusions
Z)-dienes occurs to generate an allylcobalt species II, which
(
In summary, we have developed the first Co-catalyzed
Markovnikov 1,2-hydrosilylation of conjugated dienes with the
undergoes
σ
intermediate III
-
π
.
-
σ
isomerization to form the allylcobalt
1
3
This allylcobalt species reacts with PhSiH to
3
2
catalyst generated from Co(acac) and xantphos. A broad
give a 1,4-hydrosilylation product. In addition, the allylcobalt
scope of trans-dienes underwent this Markovnikov
hydrosilylation to afford (E)-allylsilanes in high isolated yields
and excellent regioselectivities (b/l = >99:1). In addition, (E/Z)-
isomeric 1,3-dienes reacted in a stereoconvergent manner to
form (E)-allkylsilanes with good to excellent regioselectivity
species III also undergoes
σ
I. and this explains the observed
-
π
-
σ
isomerization to generate the
allylcobalt intermediate
stereoconvergency of (Z/E)-diene hydrosilylation. Both
allylcobalt species and III can react with PhSiH to generate
I
3
allylsilane products and the difference in the sterics around the
Co-C bonds in these two allylcobalt specis may account for the
product ratio observed for the reaction listed in Scheme 2A.
(
ratio of 1,2/1,4-hydrosilylation up to 99:1). This
stereoconvergence was resulted from a isomerization of
the allkylcobalt intermediate. In particular, we also identified a
cobalt catalyst, Co(acac) /binap, for selectively converting the
σ-π-σ
2
4
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E)-isomer of a mixture of (E/Z)-isomers, and this allows the
ARTICLE
(
Chem., Int. Ed. 2016, 55, 10839; (e) A. Rivera-Hernández, B. J.
Fallon, S. Ventre, C. Simon, M. -H. Tremblay, G. Gontard, E.
Derat, M. Amatore, C. Aubert and M. Petit, Org. Lett. 2016,
separation of (Z)-dienes from a mixture of (E/Z)-dienes.
18, 4242; (f) W. J. Teo, C. Wang, Y. W. Tan and S. Ge, Angew.
Chem., Int. Ed. 2017, 56, 4328; (g) J. Guo, X. Shen and Z. Lu,
Angew. Chem., Int. Ed. 2017, 56, 615.
Experimental Details
6
7
For recent reviews: (a) J. Sun and L. Deng, ACS Catal. 2016,
90; (b) X. Du and Z. Huang, ACS Catal. 2017, , 1227.
(a) B. D. Karstedt, Platinum Complexes of Unsaturated
Siloxanes and Platinum Containing Organopolysiloxanes. U.S.
Patent 3775452A, Nov. 27, 1973; (b) L. N. Lewis, J. Stein, Y.
Gao, R. E. Colborn and G. Hutchins, Platinum. Met. Rev.
6,
General Procedures for Stereoconvergent Hydrosilylation of (E/Z)-
2
7
Dienes
2
In an Ar-filled glovebox, a mixture of Co(acac) (5.0 µmol) and
xantphos (5.0 µmol) in THF (1 mL) was added into a 4 mL
screw-capped vial containing a magnetic stirring bar. The
resulting mixture was stirred for 2 mins prior adding
phenylsilane (0.500 mmol) and (E/Z)-1,3-dienes (0.400 mmol)
successively. The vial was removed from the glove box, and
the mixture was stirred at room temperature for 6 hours. After
that, the crude reaction mixture was concentrated under
vacuum and the residue was purified by flash column
chromatography using a mixture of ethyl acetate and hexane
as eluent. The conditions for flash chromatography and data
for characterization of the products are listed in the ESI.
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1 A similar difference in the reaction rates for (E/Z)-isomers of
Acknowledgement
dienes was observed in
conjugated dienes, see: Y. N. Timsina, S. Biswas and T. V.
RajanBabu, J. Am. Chem. Soc. 2014, 136, 6215.
12 For examples of Co-catalyzed Z/E-isomerization of alkenyl
compounds, see: (a) C. Chen,T. R. Dugan, W. W. Brennessel,
a Co-catalyzed vinylation of
This work was supported by the National University of
Singapore (R-143-000-614-133) and the grant from NUS Young
Investigator Award (R-143-000-630-133).
D. J. Weix and P. J. Holland, J. Am. Chem. Soc. 2014, 136,
45; (b) S. Fu, N. -Y. Chen, X. Liu, Z. Shao, S. -P. Luo and Q.
9
Liu, J. Am. Chem. Soc. 2016, 138, 8588.
Notes and references
1
(a) Hydrosilylation: A Comprehensive Review on Recent
Advances (Ed: B. Marciniec), Springer, Berlin, 2009. (b) Y.
1
1
3 (a) J. Kwiatek and J. K. Seyler, J. Organomet. Chem. 1965, 3
4
,
21. (b) M. G. Burnett, P. J. Connolly and C. Kemball, J. Chem.
Nakajima and S. Shimada, RSC Adv. 2015,
For recent examples of allylsilane synthesis, see: (a) R.
Moser, T. Nishikata and B. H. Lipshutz, Org. Lett. 2010, 12
8; (b) N. Selander, J. R. Paasch and K. Szabó, J. Am. Chem.
5, 20603.
Soc. A 1968, 991. (c) I. Ojima and M. Kumagai, J. Organomet.
Chem. 1978, 157, 359. (d) R. K. Sharma and and T. V.
RajanBabu, J. Am. Chem. Soc. 2010, 132, 3295.
4 Asymmetric Markovnikov 1,2-hydrosilylation of conjugated
dienes was not reported preciously. For examples of
asymmetric 1,4-hydrosilylation of conjugated dienes, see: (a)
T. Hayashi, Y. Matsumoto,I. Morikawa and Y. Ito,
2
,
2
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Please Cꢀ dh oe ꢀmn oi c taꢀ al Sd cj ui e snt cꢀ me arginsꢀ
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DOI: 10.1039/C7SC04002D
JournalꢀNameꢀ
ꢀ
ARTICLEꢀ
Cobalt-CatalyzedꢀRegioselectiveꢀStereoconvergentꢀMarkovnikovꢀ
,2-HydrosilylationꢀofꢀDienesꢀꢀ
1
*
ꢀ
HuiꢀLengꢀSang,ꢀSongjieꢀYuꢀandꢀShaozhongꢀGe
Receivedꢀ00thꢀJanuaryꢀ20xx,ꢀ
Acceptedꢀ00thꢀJanuaryꢀ20xxꢀ
Theꢀ firstꢀ transitionꢀ metal-catalyzedꢀ stereoconvergentꢀ Markovnikovꢀ 1,2-hydrosilylaitonꢀ ofꢀ (E/Z)-dienesꢀ wasꢀ effectivelyꢀ
achievedꢀwithꢀexcellentꢀE-selectivitiesꢀusingꢀaꢀcobaltꢀcatalyst.ꢀꢀ
DOI:ꢀ10.1039/x0xx00000xꢀ
www.rsc.org/ꢀꢀ
ꢀ
R1
R1 SiH Ph
2
Co(acac) /L
R3
2
R3
ꢀ
R
+ PhSiH3
Ar
R
L = xantphos
ꢀ
R2
R2
b/l > 99:1
,2-selectivity up to 99:1
(
E/Z)-dienes
SiH Ph
2
1
Me
L = (R)-difluorphos
er up to 90:10
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
J.ꢀName.,ꢀ2013,ꢀ00,ꢀ1-3ꢀ|ꢀ1ꢀꢀ
Pleaseꢀdoꢀnotꢀadjustꢀmarginsꢀ