Ligand-controlled remarkable regio- and stereodivergence in intermolecular hydrosilylation of internal alkynes: Experimental and theoretical studies

Article abstract of DOI:10.1021/ja405752w

The first highly efficient ligand-controlled regio- and stereodivergent intermolecular hydrosilylations of internal alkynes have been disclosed. Cationic ruthenium complexes [Cp*Ru(MeCN)₃]⁺ and [CpRu(MeCN)₃]⁺ have been demonstrated to catalyze intermolecular hydrosilylations of silyl alkynes to form a range of vinyldisilanes with excellent but opposite regio- and stereoselectivity, with the former being α anti addition and the latter β syn addition. The use of a silyl masking group not only provides sufficient steric bulk for high selectivity but also leads to versatile product derivatizations toward a variety of useful building blocks. DFT calculations suggest that the reactions proceed by a mechanism that involves oxidative hydrometalation, isomerization, and reductive silyl migration. The energetics of the transition states and intermediates varies dramatically with the catalyst ligand (Cp* and Cp). Theoretical studies combined with experimental evidence confirm that steric effect plays a critical role in governing the regio- and stereoselectivity, and the interplay between the substituent in the alkyne (e.g., silyl group) and the ligand ultimately determines the observed remarkable regio- and stereodivergence.

Full text of DOI:10.1021/ja405752w

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Article
Ligand-Controlled Remarkable Regio- and Stereodivergence in Intermolecular
Hydrosilylation of Internal Alkynes: Experimental and Theoretical Studies
Shengtao Ding, Li-Juan Song, Lung Wa Chung, Xinhao Zhang, Jianwei Sun, and Yun-Dong Wu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja405752w • Publication Date (Web): 23 Aug 2013
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Journal of the American Chemical Society
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Ligand-Controlled Remarkable Regio- and Stereodivergence in
Intermolecular Hydrosilylation of Internal Alkynes: Experi-
mental and Theoretical Studies
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‖
Shengtao Ding,§,† LiꢀJuan Song,§,‡ Lung Wa Chung,† Xinhao Zhang,‡ Jianwei Sun,*,† and YunꢀDong Wu*,‡,
†Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
SAR, China
‡Lab of Computational Chemistry and Drug Design, Laboratory of Chemical Genomics, Peking University Shenzhen Graduꢀ
ate School, Shenzhen 518055, China
‖College of Chemistry, Peking University, Beijing 100871, China
ABSTRACT: The first highly efficient ligandꢀcontrolled regioꢀ and stereodivergent intermolecular hydrosilylations of internal
alkynes have been disclosed. Cationic ruthenium complexes [Cp*Ru(MeCN)3]+ and [CpRu(MeCN)3]+ have been demonstrated to
catalyze intermolecular hydrosilylations of silyl alkynes to form a range of vinyldisilanes with excellent but opposite regioꢀ and
stereoselectivity, with the former being α anti addition and the latter β syn addition. The use of a silyl masking group not only proꢀ
vides sufficient steric bulk for high selectivity, but also leads to versatile product derivatizations towards a variety of useful buildꢀ
ing blocks. DFT calculations suggest that the reactions proceed by a mechanism that involves oxidative hydrometalation, isomeriꢀ
zation, and reductive silyl migration. The energetics of the transition states and intermediates varies dramatically with the catalyst
ligand (Cp* and Cp). Theoretical studies combined with experimental evidence confirm that steric effect plays a critical role in
governing the regioꢀ and stereoselectivity, and the interplay between the substituent in the alkyne (e.g., silyl group) and the ligand
that ultimately determines the observed remarkable regioꢀ and stereodivergence.
INTRODUCTION
with the R group for potential good regioꢀ and stereocontrol in
the hydrosilylation, and then can be easily converted to useful
functional groups in a stereodefined fashion, thereby addressꢀ
ing the above limitation in an indirect but attractive way.13 In
fact, stereodefined triꢀsubstituted vinyldisilanes, particularly
with two electronically distinct silyl groups, are extremely
versatile species towards various useful molecules by sequenꢀ
tial transformations of the two silyl groups.
Vinylsilanes are valuable building blocks in organic synthesis1
not only because of their versatility in a wide range of useful
organic transformations, such as Pdꢀcatalyzed crossꢀcoupling
reactions2 and TamaoꢀFleming oxidation,3 etc.,4 but also due
to their amenable features, including ease of handling, low
cost, low toxicity, and stability relative to other vinylꢀmetal
species. Among the various synthetic methods towards viꢀ
nylsilanes, hydrosilylation of alkynes represents the most
straightforward and atomꢀeconomical approach.5 In the past
few decades, there has been significant progress in the develꢀ
opment of metalꢀcatalyzed stereoselective alkyne hydrosilylaꢀ
tion processes.6 However, the majority of these reactions deal
with terminal alkynes or symmetrical internal alkynes.5ꢀ7 In
contrast, the use of unsymmetrical internal alkynes has only
met with limited success due to the increased difficulty in reꢀ
gioꢀ and stereocontrol.8 Indeed, such examples with general
and excellent control in both regioꢀ and stereoselectivity have
been only limited to alkynes with a highly polarized triple
bond (e.g., alkynones and alkynoates),9 or alkynes bearing an
internal directing group,10 or intramolecular hydrosilylations.11
Scheme 1. Design of Intermolecular Hydrosilylation of
Silyl Alkynes
While the stereoselectivity, i.e., syn or anti addition, for inꢀ
termolecular hydrosilylation of internal triple bonds has been
demonstrated to be wellꢀcontrollable by the proper choice of
catalytic systems,12 however, the regioselectivity, i.e., α or β
addition, is largely influenced by the two substituents (R and
R’, Scheme 1) on the triple bond in terms of their electronic
and steric difference. We were intrigued by the possibility of
overcoming this intrinsic substrate limitation by an alternative
approach. We hypothesized that a masking group, such as a
silyl group, can be employed for temporary differentiation
Herein we report highly efficient and general Ruꢀcatalyzed
intermolecular hydrosilylation reactions of internal silyl alꢀ
kynes with excellent regioꢀ and stereoselectivity. More intriꢀ
guingly, with subtle variation of the ligand, we have observed
unprecedentedly remarkable switch of regioꢀ and stereoselecꢀ
tivity. Extensive density functional theory (DFT) calculations
combined with experimental evidence provide insight into the
reaction mechanism and the origin of the ligandꢀcontrolled
regioꢀ and stereodivergence.
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Page 2 of 9
catalyst ligand. These results suggest that steric effect plays an
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3
4
5
6
7
8
important role in the hydrosilylation processes. Finally, 1,7ꢀ
diynes with the silyl groups at either terminal or internal posiꢀ
tions are also effective substrates. The corresponding viꢀ
nyldisilane products 3nꢀo and 4nꢀo from bis(hydrosilylation)
were obtained with excellent efficiency and selectivity, and no
cyclic products from the hypothetical hydrosilylationꢀ
cyclization cascade across the two triple bonds were observed.
RESULTS AND DISCUSSION
Experimental Results
We began our investigation with 1ꢀtrimethylsilylꢀ1ꢀhexyne
(1a) as the model substrate and triethoxysilane as the silylation
reagent. As mentioned above, the two silyl groups in the reacꢀ
tants are electronically distinct and the vinyldisilane products
are expected to be particularly useful for subsequent chemoseꢀ
lective derivatizations. After brief survey of some potential
catalytic systems (see the SI for details), we were pleased to
identify that, with [Cp*Ru(MeCN)3]PF6 (5) as the catalyst14
and DCM as the solvent, the reaction proceeded smoothly at
room temperature to furnish vinyldisilane 3a in essentially
quantitative yield (Scheme 2). Notably, the reaction displays
not only high chemical efficiency, but also excellent regioꢀ
and stereoselectivity for the Si‒H bond addition, with excluꢀ
sive α anti addition (α/β >50:1, Z/E >50:1). More intriguingly
and surprisingly, replacement of the catalyst ligand Cp* by Cp
(i.e., 6)14 resulted in complete switching of both regioꢀ and
stereoselectivity. Thus, under otherwise identical conditions,
the reaction afforded β syn addition product 4a as a single
isomer in quantitative yield. It is worth noting that such a reꢀ
markable ligandꢀcontrolled divergence in both regioꢀ and steꢀ
reoselectivity is unprecedented.15
Next, we examined the silane scope of the Ruꢀcatalyzed hyꢀ
drosilylation processes (Table 2). We were pleased to find that,
with the smaller catalyst 6, various silanes including trialꢀ
kylsilanes and chlorosilanes are suitable reaction partners,
providing the desired β syn addition products 4pꢀv all with
high efficiency and selectivity. However, these silanes exhibit
different reactivity when the bulkier catalyst 5 was used. For
example, alkoxysilanes 2bꢀd and chlorosilane 2h can particiꢀ
pate in the hydrosilylation process with good chemical effiꢀ
ciency and excellent regioꢀ and stereoselectivity, but the bulky
tris(trimethylsiloxy)silane 2e and the relatively electronꢀrich
trialkylsilanes 2fꢀg are unreactive (entries 4ꢀ6, conditions A).
The high reactivity of alkoxysilanes and chlorosilanes relative
to trialkylsilanes indicates that the reaction favors electronꢀ
deficient silanes. Unfortunately, arylꢀsubstituted alkynes are
not suitable substrates under both conditions (entries 16ꢀ17).
Finally, it is noteworthy that, although the chemical efficiency
varies with silanes and catalysts, the same remarkable regioꢀ
and stereodivergence with high levels of selectivity was alꢀ
ways observed as long as these reactions proceed.
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Scheme 2. Ligand Control in Regioꢀ and Stereodivergent
Hydrosilylations
In order to gain insight into the reaction mechanism, particꢀ
ularly the origin of the intriguing regioꢀ and stereodivergence
controlled by ligands, we have carried out a series of theoretiꢀ
cal and experimental mechanistic studies. We expected that
these catalytic systems might have similarity with the unusual
regioꢀ and stereoselectivity of the intermolecular reactions
with terminal alkynes (Markovnikov and anti addition) and the
intramolecular reactions (endoꢀdig and anti addition) Trost
and Ball developed.12dꢀf
Theoretical Results
Two of us, together with Trost and Ball, previously reported
a DFT study and proposed a new mechanism that involves a
concerted oxidative addition/hydrometalation step followed by
the stereoselective Cα‒Cβ rotation to give a metallacycloproꢀ
peneꢀlike intermediate and then a novel reductive silyl migraꢀ
tion.17,18 The proposed mechanism successfully explained all
the observed unusual regioꢀ and stereochemistry Trost and
Ball reported. We postulated that the present intermolecular
reactions of silyl alkynes may proceed by a similar mechanism,
but the energetics of the transition states and intermediates
may vary due to the incorporation of the silyl groups in the
substrates and thus result in the observed unusual regioꢀ and
stereoselectivity. Therefore, we followed the previous compuꢀ
tational methods (B3LYP, Lanl2dz effective core potential and
basis set augmented with f functions for Ru; 6ꢀ31G* basis sets
for the other atoms)17,19ꢀ21 to study the present hydrosilylation
reactions.
The unusual effect of the catalyst ligand prompted us to
evaluate the generality of the phenomenon. As shown in Table
1, a wide range of different silyl alkynes and silanes can parꢀ
ticipate smoothly in the intermolecular hydrosilylation proꢀ
cesses. More importantly, the ligandꢀcontrolled regioꢀ and
stereodivergence is indeed general, i.e., with the same pair of
substrates, α anti addition products 3 were selectively formed
with catalyst 5, but β syn addition products 4 were formed
with catalyst 6. It is worth noting that Ruꢀcatalyzed syn selecꢀ
tive alkyne hydrosilylations are very scarce.16 The mild reacꢀ
tion conditions can tolerate a diverse set of functional groups,
such as esters, mesylates, acetals, silylꢀprotected alcohols,
Bocꢀprotected amines, ethers, etc. In addition to the TMS
group, we also evaluated other silyl groups in the alkyne subꢀ
strates (entries 10ꢀ13). The desired β syn addition products 4jꢀ
m were all formed with excellent efficiency and selectivity
when catalyst 6 was used. In contrast, with the bulkier catalyst
5, the corresponding reactions proceeded with much lower
conversions (3jꢀm, entries 10ꢀ13), although the levels of reꢀ
gioꢀ and stereoselectivity remain high. The observed low conꢀ
versions as well as slow reaction rates are presumably due to
the increased steric bulk in both the alkyne silyl group and the
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Journal of the American Chemical Society
Table 1. Scope of Alkynes
1
2
3
4
5
Si
Si
R
H
R
5
6
(2 mol%)
(2 mol%)
(1)
Si
R
H
+
Si(OEt)3
(EtO)3Si
DCM, r.t., 24 h
DCM, r.t., 24 h
(EtO)3SiH
2a
)
(
3
4
"Conditions A"
"Conditions B"
6
7
8
1
Conditions A
Conditions B
entry
product yieldb
β/αc
E/Zc
product
yieldb
α/βc
Z/Ec
R
Si
9
3a
4a
1
2
3
4
5
6
7
8
nBu
Me
TMS
TMS
TMS
TMS
96%
89%
99%
93%
90%
90%
95%
96%
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
>50:1
96%
88%
97%
94%
94%
85%
91%
75%
>50:1
>50:1
>50:1
15:1
>30:1
>30:1
>30:1
5:1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
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45
46
47
48
49
50
51
52
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57
58
59
60
3b
3c
4b
4c
nOct
CH2OTBS
3d
3e
3f
4d
4e
4f
(CH2)2OTBS TMS
>50:1
>50:1
12:1
25:1
(CH2)2OAc
(CH2)2OMs
TMS
TMS
25:1
3g
3h
3i
4g
4h
4i
25:1
(CH2)2OTHP TMS
C2H4NHBoc TMS
>50:1
>30:1
9
84%
28%
25%
<5%e
<5%e
>50:1
>50:1
>50:1
−
>50:1
>50:1
>50:1
−
77%
87%
93%
81%
92%
>50:1
>50:1
15:1
>50:1
>50:1
>30:1
>50:1
>30:1
3j
4j
10
11
12
13
nBu
nBu
nBu
nBu
Et3Si
TBS
3k
3l
4k
4l
PhMe2Si
BnMe2Si
>50:1
>50:1
3m
4m
−
−
nBu
H
H
Si(OEt)3
(EtO)3Si
Me
Me Si
Me
Si
Me
Me
nBu
nBu
nBu
nBu
Me Si
14d
3n
4n
95%
90%
>50:1
>50:1
>50:1
>50:1
84%
>50:1
>50:1
>50:1
>50:1
Si
Me
Me
Me
Si
Si
H
Me
Me
(EtO)3Si
Me
nBu
Si(OEt)3
H
H
H
Si(OEt)3
TMS
TMS
TMS
TMS
TMS
O
O
(EtO)3Si
(EtO)3Si
O
15d
83%
3o
4o
H
H
Si(OEt)3
<5%e
<5%e
TMS
16
17
Ph
TMS
−
<5%e
<5%e
−
−
−
−
−
−
−
−
−
−
3-thiophenyl TMS
−
a Standard conditions: A mixture of 1 (0.40 mmol), 2a (0.80 mmol), [Ru]ꢀcatalyst (2 mol %) in DCM (3 mL) was stirred under N2 at
b
c
d
e
room temperature for 24 h. Isolated yields. Determined by 1H NMR; 4 equivalents of silane were used. Low conversion with
the starting alkyne as the mass balance.
[CpRu(MeCN)3]PF6ꢀCatalyzed β Syn Addition. Similar to
the proposed mechanism,17 the precursor complexes A3 (A3α
and A3β, with different alkyne orientation but same stability)
can be generated after ligand displacement of the ruthenium
catalyst A1 by the alkyne and then the silane (Figure 1). Subꢀ
sequently, H‒Si addition across the Ru‒C bond of A3 can
occur,22 with hydrogen migration to the substrate more favoraꢀ
ble than silyl migration (Figure S1).23 Regarding the regioꢀ
chemistry of the favorable oxidative hydrometalation step
(oxidative addition of H‒Si concerted with hydrometalation),
the βꢀselective pathway leading to A6β (via A5ꢀTSβ) is comꢀ
puted to be kinetically and thermodynamically more favorable,
because of the substantial steric repulsion between the Cp ring
and the TMS group in A5ꢀTSα (Figure S2).23 In contrast to the
previous calculations,17 planar σꢀvinyl intermediates A6α/β
can be first formed when the internal alkyne and trimethoxꢀ
ysilane are used in this study. Agostic interactions between the
newly formed Cβ‒H bond and the Ru center stabilizes A6α/β.
Interestingly, a reductive elimination transition state from
A6α/β could not be located. Instead, subsequent isomerization
of A6α/β, which involves rotation of the Cα=Cβ bond, could
lead to four possible ruthenacyclopropene isomers A8.
In general, isomerization via counterclockwise rotation (e.g.,
A7ꢀTSαꢀanti and A7ꢀTSβꢀanti) requires a lower barrier than
that via clockwise rotation (e.g., A7ꢀTSαꢀsyn and A7ꢀTSβꢀ
syn), because a relatively large group (e.g., TMS in A6β and
Me in A6α) moves towards the bulky Cp moiety in the latter
case (Figure S3).23 Finally, reductive silyl migration delivers
the corresponding complexes A10, leading to the hydrosilylaꢀ
tion product and the active catalytic species A1 for the next
catalytic cycle. The bulky silyl group is more favored to miꢀ
grate when the small hydrogen atom at Cβ is placed on the
same side of the metallacyclopropene plane (Figure S4): A9ꢀ
TSαꢀsyn
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Table 2. Scope of Silanes
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
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39
40
41
42
43
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45
46
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48
49
50
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52
53
54
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58
59
60
a Standard conditions: A mixture of 1a (0.40 mmol), 2a (0.80 mmol), [Ru]ꢀcatalyst (2 mol %) in DCM (3 mL) was stirred under N2
at r.t. for 24 h. b Isolated yield. c Determined by 1H NMR. d The reaction was quenched by Et3N (3.0 equiv) and MeOH (3.0 equiv),
and the corresponding methyl silyl ether derivative was obtained.
Figure 1. Free energy profile of the [CpRu(MeCN)3]PF6ꢀcatalyzed hydrosilylation. The favorable pathway is shown with solid blue
line.
(24.6 kcal/mol) and A9ꢀTSβꢀsyn (21.7 kcal/mol) are lower in
free energy than A9ꢀTSαꢀanti (27.2 kcal/mol) and A9ꢀTSβꢀ
anti (28.9 kcal/mol).23
[Cp*Ru(MeCN)3]PF6ꢀCatalyzed α Anti Addition. The
above computational findings indicate that hydrosilylation of
internal silyl alkynes, in contrast to terminal alkynes,17 can be
significantly influenced by steric effect. As a result, the Cp*
ligand is expected to have more pronounced steric effect. Inꢀ
deed, as shown in Figure 2, in the oxidative hydrometalation
Overall, the most favorable pathway leading to the observed
β syn addition product (A10βꢀsyn) occurs through A9ꢀTSβꢀ
syn, because the migrating silyl group approaches the less
step, the energy gap between
α addition (B5ꢀTSα) and β addiꢀ
bulky carbon. As shown in Figure 1, the
β regioselectivity and
tion (B5ꢀTSβ) is larger (2.4 kcal/mol) with the Cp* ligand
than that with the Cp ligand (1.5 kcal/mol), supporting a
stronger repulsion between the Cp* ligand and the TMS subꢀ
stituent.
syn stereoselectivity are primarily determined in the initial
oxidative hydrometalation step and the final silyl migration
step, respectively. The computational results are fully conꢀ
sistent with the observed experimental outcomes with
[CpRu(MeCN)3]PF6 as the catalyst.
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18
19
20
21
22
23
Figure 2. Free energy profile of the [Cp*Ru(MeCN)3]PF6ꢀcatalyzed hydrosilylation. The favorable pathway is shown with solid blue
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line.
In the isomerization step, the counterclockwise rotation is
again found to have a much lower barrier than the clockwise
rotation (Figure S5).23 Importantly, for α additions, more seꢀ
vere steric repulsion results in a larger energy difference beꢀ
tween the counterclockwise and clockwise rotation transition
states with Cp* (6.2 kcal/mol, calculated from B7ꢀTSαꢀsyn
and B7ꢀTSαꢀanti) than that with Cp (2.2 kcal/mol, calculated
from A7ꢀTSαꢀsyn and A7ꢀTSαꢀanti, Figure 1).
onic Ruꢀcatalyzed hydrosilylations. Instead, our calculations
demonstrated that the current mechanism is similar to that
proposed by Wu and Trost.17 The major difference is that the
16ꢀelectron planar σꢀvinylruthenium complexes here are obꢀ
tained to be intermediates, rather than transition states in the
previous mechanism. The formation of these intermediates and
their subsequent isomerization affect the regioꢀ and stereoseꢀ
lectivity. Additionally, due to significant steric effect, the rateꢀ
determining step also depends on the isomerization and reducꢀ
tive silyl migration steps. In contrast, electronic effect is domꢀ
inant in the previously proposed mechanism of the hydrosiꢀ
lylation of terminal alkynes.17c
Although the oxidative hydrometalation (via B5ꢀTSβ) and
isomerization steps (via B7ꢀTSβꢀanti) for
β additions are enꢀ
ergetically more favorable, the reaction barrier of the final
silyl migration step via B9ꢀTSβꢀanti is too high (36.1
kcal/mol) to be accessed,24 because of the steric repulsion beꢀ
tween TMS and the Cp* ligand (HꢀꢀH: 2.08 Å, Figure S6).23
Also, the rateꢀdetermining isomerization step via B7ꢀTSβꢀsyn',
which lacks stabilization by either formation of threeꢀ
membered ring or an agostic interaction, has a very high barriꢀ
We have summarized the two catalytic cycles that lead to
the selective formation of α anti and β syn products with cataꢀ
lysts 5 and 6, respectively (Scheme 3). For both catalytic cyꢀ
cles, the reaction starts with replacement of the two MeCN
ligands by the alkyne and the silane. Next, oxidative hydroꢀ
metalation forms the planar σꢀvinyl intermediate (e.g., A6β
and B6α), which can rotate to form a metallacyclopropeneꢀlike
intermediate (e.g., A8βꢀsyn and B8αꢀanti). The more favoraꢀ
ble rotation leads the βꢀsubstituent to be opposite to Cp (or
Cp*) to avoid steric repulsion (i.e., syn configuration). The
final step is silyl migration to the carbene carbon center. In
both catalytic cycles, the reductive silyl migration is principalꢀ
ly the rateꢀdetermining step. In the case of Cp, the rateꢀ
determining step (via A9ꢀTSβꢀsyn) is most favorable among
the four possible pathways, thereby leading to the formation of
β syn addition products. The mechanistic pathway is more
complicated for the case of Cp*. Although the β pathway is
more favorable in the oxidative hydrometalation step, it is
suppressed by a very high barrier in the isomerization step
(B7ꢀTSβꢀsyn') or the silyl migration step (B9ꢀTSβꢀanti). Inꢀ
stead, the α anti pathway has the overall lowest activation
barrier (B9ꢀTSαꢀanti), thus leading to the observed α anti
addition product.
er (30.6 kcal/mol). Therefore, the
β addition paths are strongly
destabilized with the bulky Cp* ligand. In this respect, the
transition states for α anti and α syn additions have much lowꢀ
er barriers (26.4 and 23.4 kcal/mol for B9ꢀTSαꢀanti and B9ꢀ
TSαꢀsyn, respectively), since relatively small group (Me or H)
is on the same side of the silyl migration. However, a high
isomerization barrier (30.3 kcal/mol) via B7ꢀTSαꢀsyn (HꢀꢀH:
2.22 Å, Figure S6)23 prevents α syn addition. Therefore, the
energetically most favorable pathway for hydrosilylation cataꢀ
lyzed by 6 is α anti addition via intermediates B6α and B8αꢀ
anti as well as transition states B5ꢀTSα, B7ꢀTSαꢀanti, and
irreversible B9ꢀTSαꢀanti. The computed regioꢀ and stereoseꢀ
lectivity are also consistent with the experimental results.
Proposed Mechanisms and Theoretical Prediction for tꢀ
Butylꢀsubstituted Alkynes. The above computational results
have clearly explained the experimentally observed regioꢀ and
stereodivergence. Traditional ChalkꢀHarrod mechanism and its
variant,25 which typically refer to the Pdꢀcatalyzed hydrosiꢀ
lylations of terminal alkynes, proved inapplicable to the catiꢀ
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Scheme 3. Proposed Catalytic Cycles
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The calculations also indicate that the activation barrier for
the α anti addition with catalyst 5 (Cp*) is higher (∆G≠ = 26.4
kcal/mol) than that for the β syn addition with catalyst 6 (Cp,
∆G≠ = 21.7 kcal/mol). Thus, we evaluated the reaction of 1a
and 2a in the presence of an equal amount of catalysts 5 and 6
(eq. 2). A mixture of 3a (α anti product) and 4a (β syn product)
was obtained in good yield, with the latter being the major
(3a/4a = 1:3). The results are in qualitative agreement with the
theoretical studies.
5
6
(1 mol %)
(1 mol %)
(2)
3a 4a
+
1a 2a
+
DCM, N2, r.t.
80% yield
3a 4a
= 1:3)
(
/
Product Derivatizations. In order to demonstrate the utility
of our efficient regioꢀ and stereoselective hydrosilylation proꢀ
cesses, we have done derivatization reactions of the viꢀ
nyldisilane products 3h and 4h, representing the α anti and β
syn addition products. For example, they can undergo Hiyama
crossꢀcoupling reactions with pꢀtolyl iodide to form products
10 and 11, respectively (eq. 3). The triethoxysilyl unit, rather
than the TMS group, was selectively reacted, and the reactions
are stereospecific in terms of alkene configuration. Alternaꢀ
tively, under electrophilic iodination conditions, the TMS
group can be selectively converted, with the triethoxysilyl unit
untouched (eq. 4).
On the basis of the above calculation results, we conclude
that the dominant factor for the highly selective reactions is
steric effect, rather than electronic effect, which explains that
the steric bulk of the catalyst ligand and silyl group in the alꢀ
kyne substrates are critically important. It is the interplay beꢀ
tween the bulky silyl group and the ligand that ultimately deꢀ
termines the observed complete regioꢀ and stereodivergence.
Thus, we further hypothesized that other bulky substituents,
such as tꢀbutyl group, may also result in similar selectivity
t
divergence. We carried out additional calculations with BuC
≡CMe as the substrate, and the results show that similar ligꢀ
andꢀcontrolled regioꢀ and stereodivergence can also be obꢀ
served (Figure S7).23
Some experiments were carried out to confirm the theoretiꢀ
cal results. Firstly, alkyne 7 was subjected to the standard reꢀ
action conditions. As shown in eq. 1, the reactions proceed
selectively to afford the α anti addition product 8 with catalyst
5, but the β syn addition product 9 with catalyst 6. Both reacꢀ
tions exhibit good regioꢀ and stereoselectivity. More imꢀ
portantly, the ligandꢀcontrolled selectivity divergence is exactꢀ
ly the same as that observed with silyl alkynes, which is conꢀ
sistent with the theoretical prediction. However, replacement
t
i
of Bu with Pr in the alkyne results in almost complete loss of
regioselectivity.
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RPC11SC17 to J.S.), the National Science Foundation of China
The vinyl iodide products 12 and 13 are useful for subsequent
1
2
3
4
5
6
7
8
(21133002 to Y.ꢀD. W. and 21232001 to X. Z.), and the Shenzhen
Science and Technology Innovation Committee (KQTD201103 to
Y.ꢀD. W.).
crossꢀcoupling reactions towards various stereodefined multiꢀ
substituted alkenes. Finally, protodesilylation was also
achieved selectively on the triethoxysilyl group, providing Eꢀ
and Zꢀalkenes 14 and 15, respectively (eq. 5).26 It is worth
mentioning that, although the AgFꢀmediated protodesilylation
of 3h in MeOH as solvent is highly stereoselective, the same
reaction protocol cannot be extended to 4h, which gives a
mixture of Z/E isomers (Z/E = 3:2). Nevertheless, brief condiꢀ
tion optimization identified hexafluoroisopropanol as the solꢀ
vent of choice for excellent stereoselectivity (Z/E = 20:1).
REFERENCES
(1)
(a) Ojima, I.; Li, Z.; Zhu, J. In The Chemistry of Organosilꢀ
icon Compounds; Rappoport, S.; Apeloig, Y., Eds.; Wiley: New York,
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(2)
(a) Hiyama, T. In MetalꢀCatalyzed CrossꢀCoupling Reacꢀ
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p 421. (b) Denmark, S. E.; Sweis, R. F. In MetalꢀCatalyzed Crossꢀ
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10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
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38
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CONCLUSION
In summary, we have developed the first highly efficient
ligandꢀcontrolled regioꢀ and stereodivergent intermolecular
hydrosilylation processes of internal alkynes. In the presence
of cationic ruthenium complex 5 or 6, the hydrosilylation reacꢀ
tions of a variety of silyl alkynes and silanes proceed efficientꢀ
ly to form a range of vinyldisilanes with excellent but opposite
regioꢀ and stereoselectivity. The mild conditions also tolerate a
diverse set of functional groups. The remarkable and complete
switching of product regioꢀ and stereochemistry (α anti vs. β
syn addition) caused by subtle variation of the catalyst ligand
(Cp* vs. Cp) is unprecedented. The importance of the silyl
group in the alkyne substrates is twoꢀfold: it not only provides
a steric differentiation of the two sides of alkyne triple bond to
ensure excellent regioꢀ and stereoselectivity in the hydrosilylaꢀ
tion process, but also serves as an excellent masking group
that is easily convertible to other useful functional groups in a
stereodefined fashion. Thus, the present reactions provide an
attractive solution to the current limitations in regioꢀ and steꢀ
reoselective intermolecular hydrosilylations of internal alꢀ
kynes. DFT calculations combined with experimental eviꢀ
dence provide important insight into the reaction mechanism,
in which steric effect plays a critical role and the interplay
between the bulky silyl group and the ligand controls the reꢀ
markable regioꢀ and stereodivergence. Our proposed mechaꢀ
nism involving metallacyclopropeneꢀlike intermediates and
reductive silyl migration (instead of reductive elimination of
σꢀvinyl intermediates) leading to both syn and anti addition
products is unique. Similar mechanisms may also operate in
reactions catalyzed by other (middle) transition metal comꢀ
plexes as well as other alkyne transꢀaddition reactions cataꢀ
lyzed by rutheniumꢀbased catalysts.27
(3)
(a) Tamao, K.; Kumada, M.; Maeda, K. Tetrahedron Lett.
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7599.
(4)
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(6)
Lim, D. S. W.; Anderson, E. A. Synthesis 2012, 44, 983.
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ilylation: A Comprehensive Review on Recent Advances; Marciniec,
B., Ed.; Springer: Berlin, 2009; Vol. 1, Chapters 2 and 3.
(7)
For selected examples dealing with terminal alkynes and
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(a) Andavan, G. T. S.; Bauer, E. B.; Letko, C. S.; Hollis, T.
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(a) Isobe, M. Nishizawa, R.; Nishikawa, T.; Yoza, K. Tetꢀ
rahedron Lett. 1999, 40, 6927. (b) Trost, B. M.; Ball, Z. T. J. Am.
Chem. Soc. 2004, 126, 13942. (c) Hamze, A.; Provot, O.; Alami, M.;
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J. Am. Chem. Soc. 2010, 132, 11926. (e) Rooke, D. A.; Ferreira, E. M.
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Chem. Soc. 1974, 96, 3684. (b) Kahle, K.; Murphy, P. J.; Scott, J.;
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mun. 2011, 47, 11104. (g) Ref. 9(e).
(11) (a) Tamao, K.; Maeda, K.; Tanaka, T.; Ito, Y. Tetrahedron
Lett. 1988, 29, 6955. (b) Marshall, J. A.; Yanik, M. M. Org. Lett.
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Denmark, S. E.; Pan, W. Org. Lett. 2002, 4, 4163. (f) Trost B. M.;
Ball, Z. T. J. Am. Chem. Soc. 2003, 125, 30. (g) Denmark, S. E.; Pan,
W. Org. Lett. 2003, 5, 1119.
(12) For selected examples with good stereoselectivity but unꢀ
predictable or low regioselectivity: (a) Tsipis, C. A. J. Organomet.
Chem. 1980, 187, 427. (b) Murphy, P. J.; Spencer, J. L.; Procter, G.
Tetrahedron Lett. 1990, 31, 1051. (c) Molander, G. A.; Retsch, W. H.
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Chem. Soc. 2001, 123, 12726. (e) Trost, B. M.; Ball, Z. T.; Jöge, T. J.
Am. Chem. Soc. 2002, 124, 7922. (f) Trost, B. M.; Ball, Z. T. J. Am.
Chem. Soc. 2005, 127, 17644. (g) BerthonꢀGelloz, G.; Schumers, J.ꢀ
ASSOCIATED CONTENT
Supporting Information
Experimental and computational details, computational and charꢀ
acterization data. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
wuyd@pkusz.edu.cn; sunjw@ust.hk.
Author Contributions
§ S.D. and L.ꢀJ.S. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Financial support was provided by HKUST (DAG11SC01 and
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M.; Bo, G. D.; Markó, I. E. J. Org. Chem. 2008, 73, 4190. (h) Ref. 7(a)
and 9(a).
(13) There have been only sporadic examples of hydrosilylation
p 1. (b) Ehlers, A. W.; Böhme, M.; Dapprich, S.; Gobbi, A.;
Höllwarth, A.; Jonas, V.; Köhler, K. F.; Stegmann, R.; Veldkamp, A.;
Frenking, G. Chem. Phys. Lett. 1993, 208, 111. (c) Hay, P. J.; Wadt,
W. R. J. Chem. Phys. 1985, 82, 299.
(22) (a) Lam, W. H.; Lin, Z. Organometallics 2003, 22, 473. (b)
Lam, W. H.; Jia, G.; Lin, Z.; Lau, C. P.; Eisenstein, O. Chem. Eur. J.
2003, 9, 2775.
(23) See the SI for more calculation results and/or discussions.
(24) (a) Although the relative free energy of B9ꢀTSβꢀsyn' is low
(22.0 kcal/mol), the rateꢀdetermining step of this pathway is the isomꢀ
erization with a very high barrier via B7ꢀTSβꢀsyn' (30.6 kcal/mol). (b)
Conventional reductive elimination of B6β to form B10βꢀsyn was
conceived, since this step is thermodynamically favorable (by ꢀ17.2
kcal/mol). However, it is unexpected that the conventional reductive
elimination of B6β is not possible, presumably due to geometrical
constraints. Instead, B8βꢀsyn' was found by IRC calculation. Agostic
interactions were found in both B6β and B8βꢀsyn' (2.03Å and 2.07Å
for Ru‒H bond length, respectively). Another significant structural
difference between these two intermediates is the Ru‒Cα‒Cβ angle
(88.0° vs. 142.0°).
(25) Other possible mechanisms have been proposed for alkyne
hydrosilylations: (a) Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc.
1965, 87, 16. (b) Schroeder, M. A.; Wrighton, M. S. J. Organomet.
Chem. 1977, 128, 345.
(26) Fürstner, A.; Radkowski, K. Chem. Commun. 2002, 2182.
(27) (a) Frohnapfel, D. S.; Templeton, J. L. Coord. Chem. Rev.
2000, 206ꢀ7, 199. (b) transꢀaddition Alderꢀene reaction: Hansen, E. C.;
Lee, D. J. Am. Chem. Soc. 2005, 127, 3252. (c) transꢀ
hydrogermylation reaction: Matsuda, T.; Kadowaki, S.; Yamaguchi,
Y.; Murakami, M. Org. Lett. 2010, 12, 1056. (d) transꢀhydrogenation
reaction: Radkowski, K.; Sundararaju, B.; Fürstner, A. Angew. Chem.,
Int. Ed. 2013, 49, 355 and ref. 16c.
1
2
3
4
5
6
7
8
with internal silyl alkynes: (a) Chauhan, M.; Hauck, B. J.; Keller, L.
P.; Boudjouk, P. J. Organomet. Chem. 2002, 645, 1. (b) Sanada, T.;
Kato, T.; Mitani, M.; Mori, A. Adv. Synth. Catal. 2006, 348, 51.
(14) Complexes 5 and 6 are commercially available. They are
versatile catalysts in a range of reactions including hydrosilylation, as
demonstrated by Trost and others. For selected examples, see: (a)
Trost, B. M.; Machacek, M.; Schnaderbeck, M. J. Org. Lett. 2000, 2,
1761. (b) Trost, B. M.; Pinkerton, A. B. J. Am. Chem. Soc. 2000, 122,
8081. (c) Trost, B. M.; Machacek, M. R.; Ball, Z. T. Org. Lett. 2003,
5, 1895. (d) Ref. 11(f) and 12(dꢀf). (e) Hermatschweiler, R.; Fernanꢀ
dez, I.; Breher, F.; Pregosin, P. S.; Veiros, L. F.; Calhorda, M. J. Anꢀ
gew. Chem., Int. Ed. 2005, 44, 4397. (f) Matsuda, T.; Kadowaki, S.;
Murakami, M. Chem. Commun. 2007, 2627. (g) Kleinbeck, F.; Fettes,
G. J.; Fader, L. D.; Carreira, E. M. Chem. Eur. J. 2012, 18, 3598. (h)
Micoine, K.; Persich, P.; Llaveria, J.; Lam, M.ꢀH.; Maderna, A.; Loꢀ
ganzo, F.; Fürstner, A. Chem. Eur. J. 2013, 19, 7370. (i) ElMarrouni,
A.; Lebeuf, R.; Gebauer, J.; Heras, M.; Arseniyadis, S.; Cossy, J. Org.
Lett. 2012, 14, 314. (j) Micoine, K.; Fürstner, A. J. Am. Chem. Soc.
2010, 132, 14064. (k) Matsuda, T.; Kadowaki, S.; Yamaguchi, Y.;
Murakami, M. Org. Lett. 2010, 12, 1056. (l) Bressy, C.; Vors, J.ꢀP.;
Hillebrand, S.; Arseniyadis, S.; Cossy, J. Angew. Chem., Int. Ed. 2008,
47, 10137. (m) Lehr, K.; Mariz, R.; Leseurre, L.; Gabor, B.; Fürstner,
A. Angew. Chem., Int. Ed. 2011, 50, 11373. (n) Lacombe, F.; Radꢀ
kowski, K.; Seidel, G.; Fürstner, A. Tetrahedron 2004, 60, 7315.
(15) There are some examples showing tendency of regioꢀ
/stereodivergence but not as dramatic: Ref. 12(d) and Fürstner, A.;
Bonnekessel, M.; Blank, J. T.; Radkowski, K.; Seidel, G.; Lacombe,
F.; Gabor, B.; Mynott, R. Chem. Eur. J. 2007, 12, 8762.
(16) We only found one single example of synꢀselective hydrosꢀ
ilylation with Ruꢀcatalyst, which is an intramolecular reaction:
Maifeld, S. V.; Tran, M. N.; Lee, D. Tetrahedron Lett. 2005, 46, 105.
For selected other metalꢀbased catalysts that give synꢀselectivity, see
Ref 12(aꢀc) and Takahashi, T.; Bao, F.; Gao, G.; Ogasawara, M. Org.
Lett. 2003, 5, 3479.
(17) (a) Chung, L. W.; Wu, Y.ꢀD.; Trost, B. M.; Ball, Z. T. J.
Am. Chem. Soc. 2003, 125, 11578. (b) Wu, Y.ꢀD.; Chung, L. W.;
Zhang, X.ꢀH. In Computational Modeling for Homogeneous and
Enzymatic Catalysis; Morokuma, K.; Musaev, D. G. Ed. WILEYꢀ
VCH Verlag GmbH & Co. KGaA: Weinheim, 2008, pp285. (c)
Chung, L. W. PhD. Dissertation, HKUST, 2006. (d) For the reaction
of butꢀ2ꢀyne with [CpRu(NCH)3]+, the reductive silyl migration TS
was computed to be only slightly higher in energy than the oxidative
hydrometalation TS by 0.1 kcal/mol without formation of the planar
σꢀvinyl intermediate (ref. 16c). (e) For isomerization involving metalꢀ
lacyclopropeneꢀlike intermediates, see: Tanke, R. S.; Crabtree, R. H. J.
Am. Chem. Soc. 1990, 112, 7984. (f) Crabtree, R. H. New J. Chem.
2003, 27, 771.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(18) Some recent DFT studies on reaction mechanism of hyꢀ
drosilylation: (a) Wu, Yin.; Karttunen, V. A.; Parker, S.; Genest, A.;
Rosch, N. Organometallics 2013, 32, 2363. (b) Gu, P.; Wang, Q.;
Wang, Y.; Wei, H. Organometallics 2012, 32, 47. (c) Tuttle, T.;
Wang, D.; Thiel, W.; Kohler, J.; Hofmann, M.; Weis, J. Dalton Trans.
2009, 30, 5894. (d) Zhang, X.ꢀH.; Chung, L. W.; Lin, Z.; Wu, Y.ꢀD. J.
Org. Chem. 2008, 73, 820. (e) Sridevi, V. S.; Fan, W. Y.; Leong, W.
K. Organometallics 2007, 26, 1157. (f) Chung, L. W.; Lee, H. G.; Lin,
Z.; Wu, Y.ꢀD. J. Org. Chem. 2006, 71, 6000. (g) Tuttle, T.; Wang, D.;
Thiel, W.; Köhler, J.; Hofmann, M.; Weis, J. Organometallics 2006,
25, 4504. (h) Beddie, C, Hall, M. B. J. Am. Chem. Soc. 2004, 126,
13564. (i) Sakaki, S.; Sumimoto, M.; Fukuhara, M.; Sugimoto, M.;
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kaki, S.; Mizoe, N.; Sugimoto, M. Organometallics 1998, 17, 2510.
(19) All calculations were carried out with Gaussian 09 proꢀ
grams: Frisch, M. J., et al. Gaussian 09; Gaussian, Inc.: Wallingford,
CT, 2009. Computational details can be found in the SI.
(20) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Becke, A.
D. J. Chem. Phys. 1993, 98, 5648. (c) Lee, C.; Yang, W.; Parr, R. G.
Phys. Rev. B 1988, 37, 785.
(21) (a) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical
Chemistry, Vol. 3; Schaefer, H. F., III, Ed.; Plenum: New York, 1976;
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ACS Paragon Plus Environment