Copper-catalyzed monoborylation of silylalkynes; Regio- and stereoselective synthesis of (Z)-β-(borylvinyl)silanes

Article abstract of DOI:10.1002/adsc.201300881

The selective copper-catalyzed borylation of silylalkynes in the presence of a diboron reagent and methanol produced a variety of (Z)-β-(borylvinyl) silanes. The appropriate use of a selective ligand for copper allows the chemo-, regio-, and stereoselective monoborylation of silylalkynes. The β-regioselectivity of an N-heterocyclic carbene (NHC)-copper catalyst was investigated by DFT calculations.

Full text of DOI:10.1002/adsc.201300881

FULL PAPERS
DOI: 10.1002/adsc.201300881
Copper-Catalyzed Monoborylation of Silylalkynes; Regio- and
Stereoselective Synthesis of (Z)-b-(Borylvinyl)silanes
Yeong Mi Chae,a Jun Sung Bae,a Jong Hun Moon,a Jin Yong Lee,a,*
and Jaesook Yuna,*
a
Department of Chemistry and Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, Korea
Fax : (+82)-31-290-7075; e-mail: jinylee@skku.edu or jaesook@skku.edu
Received: September 30, 2013; Revised: December 12, 2013; Published online: March 3, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300881.
Abstract: The selective copper-catalyzed borylation selectivity of an N-heterocyclic carbene (NHC)-
of silylalkynes in the presence of a diboron reagent copper catalyst was investigated by DFT calcula-
and methanol produced a variety of (Z)-b-(borylvi- tions.
nyl)silanes. The appropriate use of a selective ligand
for copper allows the chemo-, regio-, and stereose- Keywords:
borylcupration;
(borylvinyl)silanes;
lective monoborylation of silylalkynes. The b-regio- copper; monoborylation; silylalkynes
Introduction
presence of alcohol via borylcupration and protonoly-
sis.[6] Based on the electronic natures of the copper-
Organoboron or organosilicon compounds are versa- boryl catalyst and substrate, selective monoborylation
tile reagents, and a variety of transformation reactions of silylalkynes was expected to provide borylated al-
greatly enhance their synthetic utility.[1] In particular, kenylsilanes with complementary selectivity to the
the stereocontrolled synthesis of alkene compounds previous methods.[7] In this paper, we report a simple
containing silyl or boryl groups[2] has attracted atten- and efficient protocol for the preparation of borylsily-
tion because of wide potential applications in various lalkenes through the chemo-, regio- and stereoselec-
organic reactions as vinyl synthons.
tive monoborylation of silylalkynes to afford (Z)-b-
Borylsilylalkenes are multifunctionalized alkenes (borylvinyl)silanes with high efficiency.
containing both silyl and boryl groups in one com-
For selective monoborylation, the catalyst should
pound. The regio- and stereoselective catalytic syn- be less reactive to the intermediate alkene product
thesis of such compounds is highly desirable, but it re- than the starting alkyne substrate. A phosphite
mains a challenge with regard to controlling the regio- [P
selectivity in alkyne functionalization. This is particu- to produce vicinal diborylalkanes from internal
(OEt)3]-copper catalyst[8] was previously reported
ACHTUNGTRENNUNG
larly true for internal substrates without strong steric silylalk
ACHTUNGTRENUNyNG nes with no detection of the monoborylated
or electronic biases.[2,3] Catalytic silaboration of termi-
À
nal alkynes with an Si B reagent and hydroboration
of silylacetylenes are representative methods for bor-
ylsilylalkene synthesis (Scheme 1). Metal-catalyzed si-
À
laboration (1,2-addition of Si B) to terminal alkynes
generally gives products in which the silyl residue is
attached to the internal carbon.[4] Conventional hy-
droboration methods often provide gem-dimetallic
alkene compounds containing boron and silicon at the
same carbon with moderate regioselectivity along
with overreacted 1,2-diboryl product.[5]
We previously found that a copper-boryl catalyst,
generated
(B2pin2) and a copper precursor, could produce
a formal hydroboration product from alkynes in the Scheme 1. Synthesis of borylsilylalkenes.
in-situ
from
(bispinacolato)diboron
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Yeong Mi Chae et al.
alkenyl product. This was attributed to the catalystꢁs cursors (IPr·HCl, SIPr·HCl) with diisopropylphenyl
higher activity for the intermediate alkene product.
groups gave very low conversions (entries 1 and 2).
ICy salts were less efficient than SIMes·HCl, resulting
in ~90% conversion, while no counterion effect was
observed (entries 5 and 6). Electron-donating phos-
phines resulted in comparable conversion, especially
Results and Discussion
Initial investigations on selective monoborylation with PCy3, but formed a diborylated product along
started with an alkyl-substituted silylalkyne (1a) by with 2a (entry 8).
screening various ligand–catalyst combinations
With an optimal reaction protocol using SIMes·HCl
(Table 1). Strongly electron-donating bulky ligands as the NHC precursor, various alkyl-substituted 1-tri-
were examined for reactivity and selectivity screens. methylsilyl-1-alkynes were examined in the copper-
The highest conversion was obtained using 1,3-dimesi- catalyzed boron addition reaction. Reactions of
tyl-4,5-dihydrolimidazolium chloride (SIMes·HCl) as simple (1b and 1c) or functionalized primary alkyl
an NHC ligand precursor, leading to the complete substrates with heteroatoms (1d–1f) proceeded
conversion of 1a in 3 h and formation of monobory- smoothly at room temperature in 3 h to provide
lated alkenylsilane (2a) with a boryl group attached monoACTHUNTGRNEUNGborylated products with excellent regio- and ste-
at the b-carbon from the TMS group as the only de- reoselectivity (Table 2, entries 1–5). The reactions of
tectable isomer (entry 4). Sterically bulkier NHC pre- more sterically congested substrates, such as secon-
dary alkyls (1g) or TIPS-substituted alkynes (1i), re-
quired higher reaction temperatures for efficient con-
version (entries 6 and 7). A tert-butyl-substituted silyl-
AHCTUNGTREGUNaNN lkyne gave low conversion even at 608C (entry 7).
Table 1. Optimization of reaction conditions.
An aryl-substituted silylalkyne (1j) resulted in com-
plete conversion but gave the monoborylated product
(2j) with formation of diborylated product 3j in a 4:1
ratio (entry 9).[9] This result is distinct from the reac-
tion catalyzed by a copper-phosphite catalyst,[8] which
produced only 3j without detection of the monobory-
lated product even at low conversion.
In a search for more selective catalysts for the mon-
oborylation of aryl-substituted silylalkynes, the Xant-
phos ligand was tested.[10] Reaction of 1j with a Xant-
phos–copper catalyst efficiently afforded the desired
monoborylated product at 508C in 24 h, indicating
lower reactivity than the SIMes–Cu catalyst (Table 3,
entry 1). Reactions of phenyl or electron-donating
aryl-substituted substrates with the Xantphos ligand
produced the expected (Z)-(b-borylvinyl)silanes as
the only detectable regioisomer (entries 1–4). In gen-
eral, reactivity and regioselectivity were affected by
the electronic nature of the aryl group. Silylalkynes
with electron-donating aryl substituents displayed low
reactivity, leading to incomplete conversion under the
given conditions. This indicates the electrophilic role
of alkynes in the reaction (entries 3 and 4). Substrates
with electron-withdrawing substituents showed de-
creased regioselectivity and formed (a-borylvinyl)si-
lane (4) compounds in greater amounts as the elec-
tron-withdrawing ability of aryl substituents increased
(entries 5–7). The b-regioselectivity (away from Si) of
catalysts was presumed to be under electronic rather
than steric control because electron-withdrawing aryls
could reverse the regiocontrol of TMS groups.[11] It is
noteworthy that an enyne substrate, 1q, also produced
the corresponding b-borylated alkenyl product (2q) in
good isolated yield with high regio-, and stereoselec-
tivity (entry 8).
Entry
L (Ligand)
Conv. [%][a]
Yield [%][b]
1
2
3
4
5
6
7
8
IPr·HCl
13
19
93
100
92
–
–
SIPr·HCl
IMes·HCl
SIMes·HCl
ICy·HCl
78
85
75
76
38
78
ICy·HBF4
92
[c]
P
(p-tolyl)3
54
[d]
PCy3
100[e]
[a]
Determined by GC analysis with internal standard.
Isolated yield.
p-tolyl=p-CH3C6H4.
Cy=cyclohexyl.
4% of vicinal diboronate product (3a) was detected
(2a:3a=96:4 by GC ratio).
[b]
[c]
[d]
[e]
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Copper-Catalyzed Monoborylation of Silylalkynes
Table 2. Monoborylation of alkyl-substituted silylalkynes.
Table 3. Monoborylation of aryl-substituted silylalkynes.
Entry Ar
Conv. Ratio
Yield of
[%][a]
ACHTUNGTRENNUNG
Entry Substrate (1)
Time Conv.
[h]
Yield
[%][b]
1
2
Ph (1j)
p-CH3C6H4 (1k)
3,5-dimethylphenyl (1l) 90
p-CH3OC6H4 (1m)
p-PhC6H4 (1n)
p-ClC6H4 (1o)
100
100
>99:1 91
>99:1 93
>99:1 78
>99:1 65
9:1
9:1
[%][a]
3
1
2
3
100
79
86
4[d]
5
80
95
100
100
82[e]
90[e]
87[e]
6
7
3
100
p-CF3C6H4 (1p)
1.2:1
8
92
>99:1 72
3
4
3
3
100
100
71
72
[a]
[b]
Determined by GC analysis with internal standard.
Regioisomeric ratio of 2:4 determined by GC and NMR
analysis of crude reaction mixture.
[c]
[d]
[e]
Isolated yield of 2.
5
3
3
100
100
83
82
Reaction was conducted at 708C.
Isolated yield of regioisomeric mixture.
6[c]
both insertion pathways of the alkyne indicate that
TS1 is the preferred reaction pathway compared to
TS2 (7.31 and 10.68 kcalmolÀ1 for b- and a-boron ad-
dition). These results are in good agreement with the
experimentally obtained regioselectivity results.
The natural bond orbital (NBO) populations of Cu
and B for the catalyst, complex RC1/RC2 and TS1/
TS2 were also calculated. The calculated NBO popu-
lations of Cu and B for catalyst were 0.415 and 0.483,
respectively. These charges changed to 0.608/0.617
(Cu) and 0.574/0.486 (B) in complex RC1/RC2 and to
0.624/0.588 (Cu) and 0.883/0.906 (B) in TS1/TS2, im-
7[c]
8[c]
6
6
3
7
–
70
52
51[e]
9
100[d]
[a]
Determined by GC analysis. 2 (2b–2i) was the only de-
tectable product except for substrate 1j.
Isolated yield of 2.
[b]
[c]
[d]
[e]
Reaction was conducted at 608C.
2j:3j=80:20 by GC.
Isolated yield of 2j.
À
plying a significant charge transfer from the Cu B s-
bond to the alkyne substrate (C C p*) during the in-
À
sertion. In Figure 2, orbital energies of the catalyst
and substrate were calculated and the most dominat-
À
To investigate the origin of the observed b-regiose- ing orbital interaction is due to the Cu B s-bond
lectivity of internal silylalkynes, DFT calculations (HOMO) of the catalyst and the p* orbital (LUMO)
were carried out with M06-2X/6-31G* basis sets using of the substrate. Thus, the Bpin moiety of the catalyst
a suite of Gaussian 09 programs.[12] Coordination of could be preferably transferred to the Cb of the sub-
alkyne (1) to the copper center could form two differ- strate because Cb has larger a MO coefficient than Ca
ent metal-h2-alkyne complexes (RC1 or RC2), and in the LUMO of the substrate,[14] suggesting the ob-
À
subsequent insertion of the alkyne to the Cu B bond served b-regioselectivity.
produces regioisomeric borylalkenyl copper inter-
Boron addition to other types of silylalkynes was
mediates I1 and I2. RC1 was calculated to be more briefly investigated with the SIMes-Cu catalyst
stable than RC2 by 0.67 kcalmolÀ1, and it has shorter (Scheme 2). Selective monoborylation of terminal si-
À
À
Cu Cb and Ca(b) B bonds (1.92 and 2.92 ꢂ, respec- lylalkynes was not possible, resulting in a mixture of
tively) than those of RC2 (Figure 1).[13] Thus, the C C mono- and diborylated products under reaction condi-
ꢀ
triple bond of 1 interacts with the copper center more tions using 1 equiv, of B2pin2. This result, however,
strongly in RC1. Calculated activation energies for proves that the current copper catalytic system is still
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Yeong Mi Chae et al.
Figure 1. Optimized structures and energy profile calculated for a- and b-addition to Si pathways of 1 (R=CH2CH3) cata-
lyzed by SIMesCu–Bpin catalyst. Hydrogen atoms were omitted for clarity. Relative free energies are given in kcalmolÀ1
and bond lengths in ꢂ.
more effective for the monoborylation than the previ- provides synthetically versatile, multifunctionalized
ous phosphite-copper catalyst. The reaction of a prop- (b-borylalkenyl)silanes with high chemo-, regio-, and
argylic silylalkyne produced monoborylated product stereoselectivity. This reaction protocol uses inexpen-
(2t) with high b-regioselectivity without formation of sive copper and NHC precursors to offer an efficient
the other regioisomer and diborylated product.
approach to the stereocontrolled synthesis of multi-
Synthetic applications of (borylvinyl)silanes are functionalized alkenes that can be further derivatized
shown in Scheme 3. The successive coupling of 2j to various organic compounds. The utility of resulting
with haloarenes[15] in the presence of palladium cata- compounds in coupling reactions and functional-
lysts gave coupling product 6.[16] Selective oxidation[17] group transformations was shown. Moreover, DFT
of the boryl group in 2i afforded ketone 7 in 77% calculations revealed that the regioselectivity of this
yield, demonstrating the stability of the triisopropyl study is under electronic control.
group on silicon. Furthermore, the boryl group in 2b
could be transformed to the corresponding trifluoro-
borate salt 8,[18] which can be further used for various
coupling reactions.
Experimental Section
General Procedure
Conclusions
A mixture of CuCl (2.5 mg, 0.025 mmol), NaO-t-Bu (9.6 mg,
0.1 mmol), and SIMes·HCl (10.2 mg, 0.03 mmol) in anhy-
drous THF (0.5 mL) was stirred for 30 min in a Schlenk
In summary, we have demonstrated that a copper-cat-
alyzed selective boron addition to internal silylalkynes tube under an atmosphere of nitrogen. Bis(pinacolato)dibor-
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Copper-Catalyzed Monoborylation of Silylalkynes
Figure 2. Frontier molecular orbitals calculated for SIMesCu–Bpin and substrate 1 (R=CH2CH3).
Scheme 2. Boron addition to other silylalkynes
on (152.4 mg, 0.6 mmol) in THF (0.5 mL) was added. The
reaction mixture was stirred for 10 min and silylalkyne (1)
(0.5 mmol) was added to the reaction mixture, followed by
MeOH (40.5 mL, 1 mmol). The reaction tube was washed
with further THF (1.0 mL), sealed, and stirred at room tem-
perature. The reaction was monitored by TLC and GC.
Upon completion of the reaction, the reaction mixture was
filtered through a pad of Celite and concentrated. The prod-
uct was purified by chromatography on silica gel.
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Yeong Mi Chae et al.
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a) K. Semba, T. Fujihara, J. Terao, Y. Tsuji, Chem. Eur.
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À
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Acknowledgements
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Sasaki, M. Sawamura, Angew. Chem. 2008, 120, 7534–
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This research was supported by the Basic Science Research
Program through the National Research Foundation of
Korea (NRF) funded by the Ministry of Education, Science,
and Technology (NRF-2011-0009533). J.Y.L. acknowledges
the financial support by an NRF grant (NRF-2007-0056343)
funded by the Korea government (MEST).
[8] H.-Y. Jung, J. Yun, Org. Lett. 2012, 14, 2606–2609.
[9] The formation of 3j (2j:3j=92:8, in 85% conversion)
was still observed even when a reduced amount of
B2pin2 (1 equiv.) was used under otherwise the same
conditions.
[10] Xantphos=9,9’-dimethyl-4,5-bis(diphenylphosphino)-
AHCTUNGTREGUNxNN anthene. One example of the monoborylation of
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