Access to Stereodefined (E)-2-Silylallylboronates via Regioselective Chloroboration of Allenylsilanes

Article abstract of DOI:10.1021/acs.orglett.9b03720

A catalyst-free method for the highly regioselective chloroboration of allenylsilanes is described. In the presence of BCl₃ and 2,6-lutidine, chloroboration of allenylsilanes proceeds smoothly without any catalyst, and the product could be treated with pinacol to afford the corresponding pinacol borates in one-pot reaction. This reaction provides a direct approach to construct valuable 2-silylallylboronate frameworks with operational simplicity and high atom-economy.

Full text of DOI:10.1021/acs.orglett.9b03720

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Access to Stereodefined (E)‑2-Silylallylboronates via Regioselective
Chloroboration of Allenylsilanes
Zhantao Yang,†,‡ Tingjie Liu,†,§ Xixi Chen,†,§ Ranran Wan,† Yang Li,† Xianzhen Wang,†
Chun-Hua Yang,*,† and Junbiao Chang*,‡
†Henan Key Laboratory of New Optoelectronic Functional Materials, College of Chemistry and Chemical Engineering, Anyang
Normal University, 436 Xian’ge Road, Anyang 455000, P. R. China
‡School of Pharmaceutical Sciences, Zhengzhou University, Zhengzhou, Henan 450001, P. R. China
S
* Supporting Information
ABSTRACT: A catalyst-free method for the highly regioselective
chloroboration of allenylsilanes is described. In the presence of BCl3
and 2,6-lutidine, chloroboration of allenylsilanes proceeds smoothly
without any catalyst, and the product could be treated with pinacol to
afford the corresponding pinacol borates in one-pot reaction. This reaction provides a direct approach to construct valuable 2-
silylallylboronate frameworks with operational simplicity and high atom-economy.
Scheme 1. Synthetic Strategies toward 2-Silylallylboronates
ilylallylboronates are widely used due to their high stability
1
Sand lack of toxicity.
They have extensive functional
compatibility and can be efficiently transformed to polyfunc-
tional compounds through the reactions of C−Si and C−B
bonds. Consequently, a variety of methods have been
developed for the preparation of silylallylboronates.2 Among
them, there are some methods providing the preparation of 2-
silylallylboronates, such as silaboration, carboboration, hydro-
boration, and so on.
Silaboration of allenes is one of the most useful and
straightforward methods. However, this method usually gave a
product in which the boron moiety was added to the central
carbon atom and the silicon moiety was added to the
unsubstituted terminal carbon atom of allenes producing
allylsilanes.3 Only few reported the silaboration of allenes to
form allylborantes. Cheng et al. reported a method for the
preparation of various 2-silylallylboronates by palladium-
catalyzed silaboration of allenes with PhMe2SiBpin as the
reagent and an organic iodide as the initiator (Scheme 1a).4
This silaboration shows high regio- and stereoselectivity and
provides the results that are completely different from the
regiochemistry of the products reported previously. Stratakis et
al. presented the first example of silaboration of terminal
allenes catalyzed by Au nanoparticles with high yields and
stereoselectivity (Scheme 1a).5 The regioselectivity pattern of
this method is unusual, with the boryl group adding to the
terminal carbon atom. Ingleson et al. developed a method to
construct 2-silylallylboronates via carboboration of TMS-
substituted allenes (Scheme 1b).6 Ritter et al. reported a Fe-
catalyzed hydroboration of 2-dimethyl(phenyl)silylbutadiene.
This reaction is chemo-, regio-, and stereoselective and gives 2-
silylallylboronates (Scheme 1c).7 However, the use of precious
metals was often required for efficient catalytic turnover, and
these requirements limit the actual utility of these methods.
We are interested in the synthesis and application of
organoboron compounds,8 and we have focused on the
haloboration of C−C unsaturated bonds.9 Suzuki realized
bromoboration reaction of allenes with boron tribromide.10
Chloroboration of allenes is the addition of a chlorine atom
and a boron atom to a CC double bond, and it provides a
straightforward and atom-economical method for the synthesis
of versatile allylboronates and vinylboronates. Chloroboration
of allenylsilanes may provide a method to prepare
silylallylboronates, but controlling the regioselectivity of this
reaction is challenging. As far as is known, there are no reports
about chloroboration of allenylsilanes. Herein, we describe the
regioselective chloroboration of allenylsilanes (Scheme 1d).
Received: October 21, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03720
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
We initially prepared allenylsilanes (Scheme 2). Allenylsi-
lanes were prepared from tetrahydro-2-(2-propynyloxy)-2H-
reaction (Table 2). When [Si] = SiMePh2 and R = a primary
alkyl group, the reaction proceeded smoothly (entries 1−4).
a
Scheme 2. Synthesis of Allenylsilanes
Table 2. Regioselective Chloroboration of Allenylsilanes
b
Entry
R
Yield
1
2
3
4
5
6
7
Me (1a)
Et (1b)
nC4H9 (1c)
nC8H17 (1d)
iPr (1e)
76%
71%
66%
68%
75%
75%
62%
pyran (S1) (see SI for details). The chloroboration of
allenylsilane 1e with BCl3 was studied, giving the results in
Table 1. As shown in this table, the reaction temperature was
Cy (1f)
iBu (1g)
Table 1. Optimization of the Reaction Conditions for
a
a
Reaction conditions: (1) chloroboration: 1 (0.1 mmol), BCl3 (0.15
mmol), 2,6-lutidine (0.15 mmol) and CH2Cl2 (1 mL) under argon
Chloroboration of Allenylsilane 1e
atmosphere; (2) esterification of the reaction mixture: pinacol (0.3
b
mmol) and NEt3 (0.3 mmol). Yield of isolated product.
When [Si] = SiMePh2 R = secondary alkyl substituents (entries
5−6), the reaction provided 2-silylallylboronates (2e) and
(2f), respectively. In addition, the substrate with an isobutyl
group could be subjected to the chloroboration reaction
providing the product (2g) with high regioselectivity (entry 7).
Then we studied the scope of allenes (Table 3). When R =
CH3 and [Si] = SiMe2Ph, SiPh3, or SiHPh2 (Table 3, entries
1−3), the reactions proceeded readily, providing the
corresponding (E)-2-silylallylboronates in good yields with
high regioselectivity. Moreover, R might be Et, iPr, Cy, or
nC8H17 (entries 4−9). For R = H, the desired chloroboration
product was not detected (entry 10). When dimethyl(5-
methylhexa-2,3-dien-2-yl)(phenyl)silane (1r) was subjected to
the reaction, the chloroboration product was not detected
when the substrate disappeared (entry 11). When mono-
subtituted allene (1s) and 1,1-disubtituted allenes (1t−1u)
were subjected to the reaction, the chloroboration reactions
did not proceed (Table 3, entries 12−14). These results
indicated that the silyl group was dedicated to the
chloroboration reaction. A gram scale reaction of 1n (1.7 g,
5 mmol) proceeded in 78% yield with the same selectivity
(entry 7). The regioselectivity of 2-silylallylboronates was
confirmed by X-ray diffraction analysis of (E)-2i (Figure 1).
Finally, to demonstrate the synthetic utility of our reaction,
several functional group transformation reactions were carried
out (Scheme 3). (E)-2-Silylallylboronate (2n) was transformed
to (E)-(3-chloro-1-iodo-4-methylpent-2-en-2-yl)-
triphenylsilane (3) by treatment with nBuLi/NIS.11 Allylbor-
onate 2n was treated with aqueous KHF2 and gave potassium
(E)-(3-chloro-4-methyl-2-(triphenylsilyl)pent-2-en-1-yl)-
trifluoroborate (4) in 83% yield. Iodinolysis of vinyl-Si of 2n
with iodine chloride afforded vinyl iodide (5).12
b
Entry
Temp.
Reaction time
Yield
c
1
2
3
4
5
rt.
3 h
3 h
3 h
12 h
3 h
nd
−25 °C
−60 °C
−78 °C
−78 °C → −60 °C
20%
70%
50%
d
80%
a
Reaction conditions: (1) chloroboration: 1e (0.1 mmol), BCl3 (0.15
mmol), 2,6-lutidine (0.15 mmol), and CH2Cl2 (1 mL) under argon
atmosphere; (2) esterification of the reaction mixture: pinacol (0.3
b
mmol) and NEt3 (0.3 mmol). Yields determined by 1H NMR
c
d
analysis. nd = not detected. The reaction was conducted at −78 °C,
and then slowly warmed to −60 °C and kept for 3 h at −60 °C.
very crucial for the reaction. When the reaction was conducted
at lower temperature, a higher conversion was obtained, and
the chloroboration product was not detected when allenylsi-
lane 1e was treated with BCl3/2,6-lutidine at room temper-
ature. When the temperature was adjusted to −25 °C, the
starting material was consumed. Then the reaction mixture was
treated with pinacol/NEt3 in one pot, and a new product was
isolated by column chromatography. NMR spectra confirmed
that this product was formed by chloroboration of the
allenylsilane (1e), but only in 20% yield (Table 1, entry 2).
The yield of the chloroboration product was 70% at −60 °C
and was accompanied by the consumption of the starting
material and formation of other products. The chloroboration
product was formed as the only product in 50% yield at −78
°C, and the conversion did not increase with longer the
reaction time. We reasoned that the starting material
vigorously reacted with BCl3 at high temperature, but when
the reaction was conducted at −78 °C, and then slowly
warmed to −60 °C, the reaction could be finished in 3 h at
−60 °C. Thus, we successfully established the standard
reaction conditions as allenylsilane (1.0 equiv), BCl3 (1.5
equiv), 2,6-lutidine (1.5 equiv), and CH2Cl2 mixed at −78 °C
and then slowly warmed to −60 °C where it was kept for 3 h.
The reaction mixture was esterified with pinacol/NEt3 for the
exclusive formation of 2-silylallylboronate.
Results in Table 3 showed that the chloroboration of
nonsilyl substituted allenes did not proceed. These results
indicated that a silyl group was dedicated to the chloroboration
reaction. In order to examine the reaction mechanism, DFT
calculations were carried out using silylated allene (1h) (see
SI). Based on our preliminary studies, we propose a possible
mechanism for the chloroboration of allenylsilanes (Scheme
4). When BCl3 is mixed with silylated allene (1h),
chloroboration of silylated allene undergoes with silyl group
With the standard reaction conditions in Table 1 in hand, we
turned our attention to the scope of the chloroboration
B
DOI: 10.1021/acs.orglett.9b03720
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 3. Regioselective Chloroboration of Allenylsilanes
Using Various Silyl Substituents
Scheme 3. Derivatization of Chloroboration Products
a
Scheme 4. Proposed Mechanism
It is in accordance with the mechanism. Then sigmatropic 1,3-
boron shift of allylBCl2 A forms the allylBCl2 species B, which
is less hindered and more thermodynamically stable.6,13 This
can be transformed to boronate ester 2h upon the treatment
with pinacol. There is another possible mechanism for this
reaction (see SI for details).
In summary, a regioselective chloroboration of allenylsilanes
is reported. This provides a direct and one-pot approach to
construct valuable 2-silylallylboronate frameworks with opera-
tional simplicity and high atom-economy under metal-free
conditions. This is the first reported chloroboration of silylated
allenes and leads unexpectedly to regio-reversed silylallylbor-
onates with an E geometry.
a
Reaction conditions: (1) chloroboration: 1 (0.1 mmol), BCl3 (0.15
ASSOCIATED CONTENT
* Supporting Information
mmol), 2,6-lutidine (0.15 mmol) and CH2Cl2 (1 mL) under argon
atmosphere; (2) esterification of the reaction mixture: pinacol (0.3
mmol) and NEt3 (0.3 mmol). Yield of isolated product. 5 mmol
scale. nr = no reaction. nd = not detected.
■
S
b
c
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b03720.
d
e
Representative experimental procedures, characteriza-
tion data of substrates and products, and NMR spectra
(PDF)
Accession Codes
CCDC 1951271 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: changjunbiao@zzu.edu.cn.
*E-mail: 09yangchunhua@163.com.
Figure 1. Structure of (E)-2i.
ORCID
migration producing the intermediate allylBCl2 A.6 The
chloroboration of nonsilyl substituted allenes did not proceed.
Zhantao Yang: 0000-0003-1575-3665
Chun-Hua Yang: 0000-0002-3770-9887
C
DOI: 10.1021/acs.orglett.9b03720
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Junbiao Chang: 0000-0001-6236-1256
Author Contributions
§Tingjie Liu and Xixi Chen contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge financial support from the National Natural
Science Foundation of China (21801006, 81703353), China
Postdoctoral Science Foundation (2018T110744), Natural
Science Foundation of Henan Province (182300410295), the
project of Science and Technology Department of Henan
Province (182102210202), and the Industrial Research Project
of Technology Bureau of Anyang.
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DOI: 10.1021/acs.orglett.9b03720
Org. Lett. XXXX, XXX, XXX−XXX