Ligand-Dependent-Controlled Copper-Catalyzed Regio- And Stereoselective Silaboration of Alkynes

Article abstract of DOI:10.1021/acs.orglett.9b02160

A copper-catalyzed highly regio- and stereoselective silaboration of alkynes was developed. In this work, direct cis-difunctionalization of alkynes was realized with silaboronate reagent and copper catalyst in aprotic solvents. The regiodivergent silabora

Full text of DOI:10.1021/acs.orglett.9b02160

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Ligand-Dependent-Controlled Copper-Catalyzed Regio- and
Stereoselective Silaboration of Alkynes
Meng Zhao,† Cui-Cui Shan,† Zi-Lu Wang, Chao Yang, Yao Fu,* and Yun-He Xu*
Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China
S
* Supporting Information
ABSTRACT: A copper-catalyzed highly regio- and stereoselective silaboration of alkynes was developed. In this work, direct
cis-difunctionalization of alkynes was realized with silaboronate reagent and copper catalyst in aprotic solvents. The
regiodivergent silaborations were controlled by tuning the copper catalysts and phosphine ligands used in reactions. This
protocol provides an efficient and practical method to synthesize the multisubstituted functionalized alkenes with specific
stereoselectivity.
o rapidly and efficiently construct various C−C and C−
regioselectivity was also observed by Stratakis’s group using
Thet bonds toward the preparation of complex molecules, gold nanoparticles supported on TiO2.15 Recently, the studies
toward trans vicinal silaboration of alkynoates and propargy-
lamides were realized by Sawamura16 and Santos,17 respec-
tively. Nagashima and Uchiyama elegantly developed a
dimethylzinc-catalyzed regioselective silaboration of terminal
alkynes.18 It is worth noting that a KOtBu-catalyzed
silaboration of aromatic alkenes was also established by Ito
and co-workers.19
Despite impressive progress toward the silaboration of
alkynes, the discovery of a sustainable and cheaper catalytic
system toward this kind of addition remains appealing yet
more challenging. Noticeably, so far, the copper-catalyzed
silaboronate reagents transformation is only limited to the
single utility of silyl functionality (Scheme 1, 1a and 1b).6,7
Based on our continued interest in synthesis of stereodefined
functionalized alkenes,20 herein we report a copper-catalyzed
silaboration of alkynes instead of protonation process as shown
in Scheme 1c.
To test the feasibility of copper-catalyzed silaboration of
alkynes, we initially carried out the reaction with 1-octyne (1a)
and silylboronate reagent (PhMe2Si-Bpin) 2 in the presence of
Cu2O (10 mol %) as catalyst, PCy3 (12 mol %) as ligand, and
2,6-di-tert-butylpyridine as additive in anhydrous cyclohexane
(Table 1). The desired silaboration products were detected in
4% total yield and without any regioselective identity (3a/4a =
50:50; Table 1, entry 1). When Cu(OAc)2 and CuTc were
used to catalyze this reaction, the yield and the regioselectivity
of the products were both improved significantly (entries 2 and
3). In particular, when 24 mol % of PCy3 was used, the major
the discovery of useful reagents and/or methodologies to
access the functionalized intermediates has always been the
ideal choice.1 Among them, the interelement compounds have
been developed as one of the most important classes of
precursors because they enable the transfer of their interele-
ment bonding partners to the unsaturated substrates.2 Of
particular interest is the study of Si−B compounds due to their
versatile displays in various organic transformations.3 Since the
̈
stable “monomeric” Si−B compound was reported by Noth in
1962,4 continuous efforts have been devoted to expanding the
diversity and utility of Si−B reagents.5 Currently, the
transition-metal-catalyzed protosilylation,6 silyl substitution,7
base-mediated boryl substitution,8 and silyl alkylation9 have
been developed to prepare various functionalized organo-
silicons or organoboronates. Nevertheless, due to the loss of
half of the interelement bonding partner during trans-
formation, there is a growing need to further improve its
atomic economy. Therefore, noble transition-metal-catalyzed
addition of Si−B reagents to alkynes is highly attractive
because they enable direct synthesis of silyl- and boryl-
disubstituted alkenes, which act as both vinylsilanes10 and
vinylborones.11 In 1996, Ito and Suginome discovered a
palladium−isonitrile catalytic system for highly regio- and
stereoselective silaboration of terminal and internal alkynes.5i,l
Following this seminal result, Tanaka,5k Pillot,5n Suginome,12
Spencer and Navarro,13 et al. successively developed different
palladium catalytic methodologies for silaboration of alkynes,
respectively. Importantly, in 2010, Suginome and co-workers
elegantly reported a palladium-catalyzed regiodivergent silabo-
ration of terminal alkynes by tuning the phosphine ligands
used in the reactions.14 The same remarkable switch in
Received: June 21, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02160
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(71% yield, 3a/4a = 18:82; Table 1, entry 5). To further
improve the regioselectivity of the product, different phosphine
ligands were investigated. No desired product was detected
when the bulky aryl-substituted phosphine ligands such as
JohnPhos and XantPhos were applied (Table 1, entries 6 and
7). With P(nBu)3 as ligand, 3a was obtained in 91% yield but
only with moderate regioselectivity (3a/4a = 75:25; entry 8).
It is worth noting that the product isomer 4a was formed as the
major product when the bulky and electron-rich P(tBu)3 was
used as ligand (79% yield, 3a/4a = 14:86; Table 1, entry 9).
Gratifyingly, when the loading of CuTc was reduced to 5 mol
%, 3a could be obtained in 90% isolated yield and with high
regioselectivity (3a/4a = 96:4; Table 1, entries 10 and 11).
Simultaneously, it was found that removing the 2,6-di-tert-
butylpyridine from the catalytic system had a negative effect on
the yield (Table 1, entry 11). We further optimized the
reaction conditions for the highly regioselective synthesis of
product 4a. The regioselectivity was improved when the bulky
phosphine PPh(tBu)2 was used in reaction. Next, different
ratios between copper catalyst and ligand, solvents, and
catalysts were examined (Table 1, entries 12−19). Finally, it
was found that the desired product 4a was obtained in 71%
isolated yield with high regioselectivity with copper iso-
caprylate as catalyst and PPh(tBu)2 as ligand in heptane (Table
1, entry 19).
Scheme 1. Copper-Catalyzed Activation and Application of
Silaboronate Reagents
With the optimized conditions in hand, we examined the
substrate scope of alkynes for the 1,2-silaboration reaction. The
results are summarized in Scheme 2. Various aliphatic alkynes
could give the 1,2-silaboration products in high yields with
high regioselectivities (Scheme 2, 3a−3b, 3d−3e). Functional
groups such as long carbon chain, ester, ketone, silyl ether,
product 2-boryl-1-silylalkene was obtained in an excellent yield
with high selectively (96% yield, 3a/4a = 90:10; Table 1, entry
4). In contrast, an inverse regioselectivity of the silaboration
product was observed when 6 mol % of PCy3 was employed
a
Table 1. Optimization of Reaction Conditions of Silaboration Products
b
c
entry
[Cu] (mol %)
Cu2O (10)
Cu(OAc)2 (10)
CuTc (10)
CuTc (10)
CuTc (10)
CuTc (10)
CuTc (10)
CuTc (10)
CuTc (10)
CuTc (5)
CuTc (5)
CuTc (10)
CuTc (10)
CuTc (10)
CuTc (6)
copper distearate (6)
Cu(CH3CN)4BF4 (6)
copper isocaprylate (6)
copper isocaprylate (6)
ligand (mol %)
solvent (extra dry)
yield (%) (3a + 4a)
3a/4a (%)
1
2
3
4
5
6
7
8
9
PCy3 (12)
PCy3 (12)
PCy3 (12)
PCy3 (24)
cyclohexane
cyclohexane
cyclohexane
cyclohexane
cyclohexane
cyclohexane
cyclohexane
cyclohexane
cyclohexane
cyclohexane
cyclohexane
cyclohexane
cyclohexane
heptane
4
50:50
45:55
81:19
90:10
18:82
89
84
96
71
PCy3 (6)
JohnPhos (24)
XantPhos (12)
P(nBu)3 (24)
P(tBu)3 (24)
PCy3 (24)
91
79
95
75:25
14:86
94:6
96:4
27:73
7:93
7:93
7:93
6:94
26:74
5:95
10
11
d
e
PCy3 (24)
PPh3 (6)
96 (90)
d
12
73
64
65
72
71
61
73
d
13
PPh(tBu)2 (6)
PPh(tBu)2 (6)
PPh(tBu)2 (6)
PPh(tBu)2 (6)
PPh(tBu)2 (6)
PPh(tBu)2 (6)
PPh(tBu)2 (6)
d
14
d
15
heptane
heptane
heptane
heptane
d
16
d
17
d
18
df
,
g
19
heptane
88 (71)
5:95
a
Reaction conditions unless otherwise specified: The mixture of 1 (0.2 mmol), 2 (0.3 mmol), copper catalyst, 2,6-di-tert-butylpyridine (10 mol %),
b
and ligand in extra dry solvent was stirred at 60 °C for 12 h under argon atmosphere (see the SI). Yield of (3a + 4a) was determined by 1H NMR
c
d
1
with use of mesitylene as internal standard. Ratio of 3a/4a was determined by crude H NMR analysis. Without 2,6-di-tert-butylpyridine.
e
f
g
Isolated yield of 3a was given. 0.4 mmol 2 (2.0 equiv) was used in the reaction. Isolated yield of 4a was given. CuTc = copper(I) thiophene-2-
carboxylate.
B
DOI: 10.1021/acs.orglett.9b02160
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Copper-Catalyzed 1,2-Silaboration of Alkynes
Scheme 4. Copper-Catalyzed Silaborative Carbocyclizations
of 1,6- and 1,7-Diynes
a
a
b
1
Yields were determined by H NMR analysis. Isolated yields.
Scheme 5. Copper-Catalyzed 2,1-Silaboration of Terminal
a
Alkynes
a
The mixture of 1 (0.2 mmol), 2 (0.3 mmol), CuTc (5 mol %), and
PCy3 (24 mol %) in extra dry cyclohexane was stirred at 60 °C for 12
b
c
h under argon atmosphere (see the SI). Isolated yields of 3. Yields
of 3 in parentheses were determined by 1H NMR analysis with use of
mesitylene as internal standard. The regioselectivities (3/4) were
d
1
determined by crude H NMR.
ether, tosyl, phthalimidyl, halide, and cyano were all well
tolerated (Scheme 2, 3f−3o), and the alkynes bearing a
terminal double bond (Scheme 2, 3q) or cyclohexenyl group
(Scheme 2, 3r) were also competent substrates for this 1,2-
silaboration reaction. In addition, internal alkyne 1s also
delivered the desired product 3s in a moderate yield and with
good regioselectivity (3s/4s = 85:15). However, aromatic
alkynes such as phenylacetylene only afforded the correspond-
ing desired products in poor yields, along with the formation of
protosilylation and gem-diborylated vinylsilane products
(Scheme 3).21 Of note, trace amounts of gem-diborylated
a
The mixture of 1 (0.2 mmol), 2 (0.4 mmol), copper isocaprylate (6
mol %), and PCy3 (6 mol %) in extra dry heptane was stirred at 60 °C
b
c
for 12 h under argon atmosphere. Isolated yields of 4. Yields of 4 in
parentheses were determined by 1H NMR analysis with use of
d
mesitylene as internal standard. The regioselectivities (3/4) were
determined by crude H NMR.
1
corresponding 2,1-silaboration products were also obtained in
good yields and with high regioselectivities in most of the cases
(Scheme 5). Various functional groups were also well tolerated
(Scheme 5, 4f−4k, 4m, and 4n). A slightly decreased
regioselectivity was observed when phthalimidyl- and chloro-
substituted terminal alkynes were used. Unfortunately, internal
alkynes failed to work under these inverse silaboration
conditions.
Scheme 3. Copper-Catalyzed 1,2-Silaboration of Aromatic
a
Alkyne
In order to probe the mechanism of competitive
protosilylation and gem-diborylsilylation reactions of aryl-
substituted alkynes, the deuterated alkyne 3u′ was prepared
and subjected to this reaction under the standard reaction
conditions. Thus, 88% deuterium incorporation was observed
at the presumed site of protonation in 5u′-D (Scheme 6, eq 1).
The nonideal deuterium incorporation in the vinylsilane
product could be attributed to protonation from water that
originated from the reagents. This result indicates that the
alkynyl proton may serve as the source of proton in this
system.23 This observation is consistent with a higher yield
observed for the vinylsilane 5t (Scheme 3), as alkynyl proton
of phenylacetylene displays higher acidity than the electron-
a
Yield of isolated product.
vinylsilane products were also observed in some cases such as
the substrates bearing halide, cyano, or cyclohexenyl
functionality. Moreover, the silaborative carbocyclizations of
diynes 1v and 1w were also developed under the optimized
reaction conditions, producing the corresponding products in
good yields (Scheme 4).22
The scope of alkynes under the inverse silaboration
conditions was also investigated (Scheme 5). Similarly, the
C
DOI: 10.1021/acs.orglett.9b02160
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
The stereodefined and regiodivergent alkenes bearing silyl and
boryl functionalities were obtained with high efficiency under
mild conditions. The silaboration/carbocyclization products of
1,6- and 1,7-diynes were also successfully achieved. Further
studies on expanding the synthetic utility of this method are in
progress in our laboratory.
Scheme 6. Controlled Experiments for Evidence of
Silylcupration
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02160.
Supporting spectra and characterization data (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
rich 4-benzyloxy phenylacetylene. Next, when the pregenerated
(phenylethynyl)copper 1x was directly used to react with
silylborane 2 in the presence of 24 mol % of PCy3 ligand, the
desired gem-diborylated vinylsilane 7t was isolated in 26% yield
(Scheme 6, eq 2), indicating transmetalation might occur
between the (phenylethynyl)copper 1x and silylborane 2 to
form a alkynlboronate V intermediate. Furthermore, the direct
addition of the compound V was proved effective to form the
gem-diborylated vinylsilane 7t in 73% yield under the standard
conditions (Scheme 6, eq 3).
*E-mail: fuyao@ustc.edu.cn
*E-mail: xyh0709@ustc.edu.cn.
ORCID
Yao Fu: 0000-0003-2282-4839
Yun-He Xu: 0000-0001-8817-0626
Author Contributions
†M.Z. and C.-C.S. contributed equally to this work.
Notes
The authors declare no competing financial interest.
A plausible mechanistic rationale is depicted in Scheme 7.
First, the LCuX catalyst reacts with silylborane 2 to generate
ACKNOWLEDGMENTS
■
We gratefully acknowledge the funding support of the National
Natural Science Foundation of China (21871240, 21672198),
the Anhui Provincial Natural Science Foundation
(1708085MB29), and the Fundamental Research Funds for
the Central Universities (WK2060190082). Prof. Jin.-Bo Zhao
of Northeast Normal University is acknowledged for valuable
discussions and draft polishing work.
Scheme 7. Proposed Mechanism for the Copper-Catalyzed
Silaboration Reaction of Phenylacetylene
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DOI: 10.1021/acs.orglett.9b02160
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