Copper-Catalyzed Asymmetric Hydroboration of 1,1-Disubstituted Alkenes

Article abstract of DOI:10.1021/acs.orglett.7b01135

A mild and efficient approach for highly regio- and enantioselective copper-catalyzed hydroboration of 1,1-diaryl substituted alkenes with bis(pinacolato)diboron (B₂Pin₂) was developed for the first time, providing facile access to a

Full text of DOI:10.1021/acs.orglett.7b01135

Letter
pubs.acs.org/OrgLett
Copper-Catalyzed Asymmetric Hydroboration of 1,1-Disubstituted
Alkenes
Zining Wang, Xiaxia He, Rui Zhang, Ge Zhang, Guoxing Xu, Qian Zhang, Tao Xiong,* and Qian Zhang
Jilin Province Key Laboratory of Organic Functional Molecular Design & Synthesis, Department of Chemistry, Northeast Normal
University, Changchun 130024, China
S
* Supporting Information
ABSTRACT: A mild and efficient approach for highly regio- and
enantioselective copper-catalyzed hydroboration of 1,1-diaryl substituted
alkenes with bis(pinacolato)diboron (B2Pin2) was developed for the first
time, providing facile access to a series of valuable β,β-diaryl substituted
boronic esters with high enantiomeric purity. Moreover, this approach could
also be suitable for hydroboration of α-alkyl styrenes for the synthesis of
enantioenriched β,β-arylalkyl substituted boronic esters. Gram-scale reaction,
stereospecific derivatizations, and the application of important antimuscarinic
drug (R)-tolterodine for concise enantioselective synthesis further highlighted
the attractiveness of this new approach.
s ubiquitous substructures, gem-diaryl- and arylalkyl
substituted chiral tertiary carbon motifs are found in a
broad range of biologically active compounds and pharmaceut-
icals,1 such as podophyllotoxin,1c (R)-tolterodine,1d H1-anti-
histamine,1e (+)-sertraline,1f ramelteon,1g and (R)-naproxen1h
(Figure 1). Therefore, the development of approaches for
stoichiometric amounts of expensive and difficult-to-access chiral
organoboranes,5 transition-metal-catalyzed asymmetric hydro-
boration of alkenes with inexpensive and readily available
organoboronic reagents has provoked more significant attention,
and numerous transformations have been well disclosed in recent
years.6 For example, recently, amide-directed highly enantiose-
lective hydroboration of 1,1-dialkyl alkenes has been developed
with chiral Rh catalyst by Takacs and co-workers.7 In addition, on
the basis of the seminal works on Rh-catalyzed asymmetric
hydroboration of unactivated α-alkylstyrenes, reported by the
groups of Burgess,8 Suzuki,9 Hayashi and Ito10 and, more
A
́
recently, Mazet,11 Dieguez,12 and Tang13 have well disclosed
highly enantioselective hydroboration of α-alkyl- or amino-
substituted styrenes with noble chiral Ir or Rh catalysts. To avoid
using the traditional precious-metal catalysts, the groups of
Huang14 and Lu15 have realized the Co- or Fe-catalyzed
asymmetric hydroboration of α-alkyl substituted styrenes.
Additionally, the asymmetric hydrofunctionalization reactions
of unsaturated hydrocarbons through the use of low cost and
high earth abundance copper catalysts have received significant
attention in recent years, and various C−X (X: C, N, Si, B, etc.)
bonds have been successfully forged in an enantioselective
manner.16 In this content, Yun and Hoveyda have independently
reported Cu-catalyzed highly site- and enantioselective hydro-
boration of styrenes or 1,1-arylalkyl substituted alkenes (Scheme
1A).17 Despite these advances, to our knowledge, the asymmetric
hydroboration version of unactivated 1,1-diaryl substituted
alkenes, is still an ultimate challenge, owing to the hurdle in
discriminating the two enantiotopic faces of such prochiral
substrates. Indeed, so far, no effective catalytic system has been
exploited for asymmetric hydroboration of such substrate type
Figure 1. Representative bioactive molecules containing chiral tertiary
carbon motifs.
efficient enantioselective installation of these subunits has
attracted considerable attention, and a variety of strategies,
such as the asymmetric conjugate additions of organometallic
reagents2 and the cross-coupling benzylic electrophiles with
nucleophiles,3 has been successfully exploited. 1,1-Disubstituted
unactivated alkene is the readily available feedstock, and thus, the
corresponding asymmetric transformation represents one of
most ideal strategies to access such valuable chiral substructures.
Enantioselective alkene hydroboration has attracted special
attention and emerged as a powerful protocol for enabling access
to the versatile optically pure organoboron compounds, which
are amenable to a wide variety of stereospecific transformations.4
Undoubtedly, compared with the early reports on employing
Received: April 13, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01135
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Letter
a,b
Scheme 1. Asymmetric Hydroboration of 1,1-Disubstituted
Alkenes
Table 1. Optimization of Reaction Conditions
and only two examples have been hitherto reported by Huang
and co-workers, but showed low enantioselectivities (8% and
54% ee).14 In actual fact, the catalytic asymmetric versions of 1,1-
disubstituted unactivated alkenes with high enantioselectivities
(>90% ee) have only been nicely achieved in asymmetric
hydrogenation,18 along with some sporadic reports on
asymmetric dihydroxylation19 and hydrosilylation.20 Herein, we
reported the first highly enantioselective hydroboration of 1,1-
diaryl substituted unactivated alkenes with chiral Cu catalyst,
furnishing the synthetic valuable boronic esters bearing chiral
β,β-diaryl substituted tertiary carbon motifs. Furthermore, this
method can also be applied to the asymmetric hydroboration of
1,1-arylalkyl substituted alkenes with high enantiomeric purity
(Scheme 1B).
We initiated our investigation using α-(o-methoxylphenyl)-
styrene 1a and B2Pin2 as model reactants (Table 1). Pleasing,
when the reaction was conducted at room temperature in the
presence of CuCl (5 mol %), NaOtBu (1.0 equiv), MeOH (2.0
equiv), and (R,R)-QuinoxP* L1 (5 mol %) in THF, affording the
desired chiral anti-Markovnikov hydroboration product 2a in
high yield with moderate level of enantioselectivity (Table 1,
entry 1). This promising result prompted us to further evaluate
the effectiveness of other chiral ligands, such as L2−L8 (Table 1,
entries 2−8). To our delight, good yield and high enantiose-
lectivity was obtained by employing (S,S)-BDPP L8 as the chiral
ligand (Table 1, entry 8). Then, we next proceeded to evaluate
the reactivities of various copper catalysts and the nature of the
proton source. The experimental results revealed that, through
the combination of CuCl and NaBArF with MeOH as the proton
supplier, the hydroboration product 2a could be furnished in
excellent yield and enantioselectivity (Table 1, entries 8−14).
Note that the hydroboration reactions occurred specifically in an
anti-Markovnikov fashion. The further screening of the base,
temperature, and solvent were also performed (see the
Supporting Information for details).
With the optimal reaction conditions in hand, we then
evaluated the reactions of various 1,1-disubstituted alkenes
(Scheme 2). 1,1-Diaryl alkenes bearing either weak or strong
electro-donating groups at ortho positions of the aromatic rings,
such as 1a−1g, worked well under the optimal conditions and
provided desired hydroboration products 2a−2g in moderate to
excellent yields with high enantiomeric ratios. Previous Co-
catalyzed asymmetric hydroboration of substrate 1b provided
chiral boronic ester 2b in low yield and moderate enantiose-
lectivity (19% yield and 54% ee).14 This copper catalytic system
showed high effectiveness for 1b, furnishing boronic ester 2b in
both excellent yield and enantioselectivity (86% yield and er =
98:2). We next examined whether this developed approach
would efficiently differentiate two different vinyl units in one
molecule, such as 1h. To our delight, this transformation
proceeded with excellent chemoselectivity and furnished chiral
a
Reaction conditions: 1a (0.15 mmol), B2pin2 (1.2 equiv), catalyst (5
mol %), ligand (5 mol %), NaOtBu (1.0 equiv), and alcohol (2.0
b
equiv) in 2 mL of THF at room temperate. Isolated yields.
c
Enantioselectivity of the purified product was determined by chiral
d
e
HPLC. er: enantiomeric ratio. CuX: Cu(CH3CN)4PF6. NaBArF (5
mol %) was added to the reaction. NaBArF: sodium tetrakis[3,5-
bis(trifluoromethyl)-phenyl]borate.
β,β-diaryl boronic ester 2h in good yield and excellent
enantioselectivity. 1-(1-Naphthyl)styrene 1i and 1j bearing
strong electro-withdrawing group (CF3) at the ortho position
of aromatic ring were also the good substrates for these
transformations, providing desired boronic ester 2i and 2j in
good yields with high enantioselectivities. Furthermore, hetero-
cycles, such as thiophene and indole, which are frequently found
in many bioactive molecules, were also accommodated and
converted into the products 2k and 2l with excellent
enantiocontrol, although low yield was observed for 1l under
this transformation conditions. Additionally, alkenes 1m−1p
bearing multi-substitutents at one or two aromatic rings could
also be efficiently converted into corresponding boronic esters
2m−2p in good yields with high level of enantioselectivities. By
increasing the steric congestion in the vicinity of the alkenyl units
by installation of the tert-butyldimethylsilyl (TBS) group, which
is widely used as protecting group in the synthesis of complex
molecules, to our delight, the transformation proceeded
smoothly to give the target product in both excellent yield and
enantioselectivity. Moreover, the preliminary experimental
results showed that the cyclic internal olefin 1r and 1s could
be successfully transformed into the corresponding structurally
important cyclic secondary boronic esters 2r and 2s in good
yields and enantiocontrols. 1,1-Diaryl alkene 1t and 1u without
ortho-site substituent were also examined under the present
B
DOI: 10.1021/acs.orglett.7b01135
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Letter
olefins showed good reactivities and provided chiral boronic
esters 3g and 3h in good to nearly quantitative yields with
impressive enantioselectivities. The absolute configurations of
hydroboration products 2b and 3c was determined by means of
the comparison of HPLC data with reported data.14,22
Scheme 2. Copper-Catalyzed Asymmetric Hydroboration of
1,1-Disubstituted Alkenes
a,b
To illustrate the potential and versatile synthetic utility of the
present approach, Gram-scale synthesis and stereospecific
transformations of enantioenriched boronic esters to construct
an array of chiral hydrocarbon building blocks were conducted
(Scheme 3). The chiral boronic ester 2b and target molecules 4
Scheme 3. Gram-Scale Synthesis and Applications of Chiral
Boronic Ester
and 5 were obtained in excellent yields without erosion of
enantiomeric excess. To further highlight the applicability of this
newly developed method, we completed the enantioselective
synthesis of an intermediate that had previously been converted
to the important antimuscarinic drug (R)-tolterodine, a
therapeutic used for the treatment of urinary incontinence.
Briefly, a three-step sequence involving asymmetric hydro-
boration, boronate homologation/oxidation providing the
clinically used therapeutic precursor 6 in good yield with
excellent enantiocontrol, followed by an amination process could
furnish (R)-tolterodine (Scheme 3).23
a
Reaction conditions: 1 (0.15 mmol), B2pin2 (1.2 equiv), catalyst (5
mol %), ligand (5 mol %), NaOtBu (1.0 equiv), NaBArF (5 mol %),
and MeOH (2.0 equiv) in 2 mL of THF at room temperate. Isolated
b
In summary, we have successfully developed a unified, mild,
and efficient approach for copper-catalyzed asymmetric hydro-
boration of 1,1-disubstituted alkenes with readily available
B2Pin2. This copper catalysis system exhibited high efficiency
for 1,1-diaryl- and arylalkyl olefins, thus allowing access to a range
of valuable chiral β,β-diaryl substituted boronic esters in good
yields and high level of regio- and enantioselectivities for the first
time. The optical active boronic esters were not only envisioned
as versatile building blocks for further stereospecific derivatiza-
tions but also suitable for Gram-scale synthesis, as well as
application for enantioselective synthesis of an important
urological drug (R)-tolterodine. Additional studies of asym-
metric transformations for other types of alkenes are currently
underway in our laboratory.
yields.
catalytic conditions, and the desired products could be obtained
in good yields, while the enantioselectivities dropped signifi-
cantly. The above results suggest that the ortho position
substituent of the 1,1-diaryl alkene plays a key role for effective
discriminating the two enantiotopic faces and is essential for
inducing the high enantioselectivity.21
To further highlight this method, 1,1-disubstituted arylalkyl
alkenes were examined under the optimal conditions. To our
delight, aryl ethenes bearing different alkyl chain-length at the α-
position were valid substrates and gave the corresponding
hydroboration products 3a, 3b, and 3c in moderate to excellent
yields with high enantioselectivities. In addition, substrate
bearing two symmetrical alkenyl units was also examined; as a
result, the asymmetric transformation gave monohydroboration
product 3d exclusively in good yield with excellent enantiocon-
trol, without observation of over hydroboration reaction under
this conditions. Benzyl substituted styrene and ferrocenyl olefin
can also undergo the transformations to yield the corresponding
chiral boronic esters in good level of enantiocontrol. Addition-
ally, the hydroboration of exocyclic 1,1-disubstituted arylalkyl
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b01135.
Experiment procedure and characterization data (PDF)
C
DOI: 10.1021/acs.orglett.7b01135
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Letter
(7) (a) Smith, S. M.; Takacs, J. M. J. Am. Chem. Soc. 2010, 132, 1740.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: xiongt626@nenu.edu.cn.
ORCID
Tao Xiong: 0000-0002-2516-084X
Notes
(9) Sato, M.; Miyaura, N.; Suzuki, A. Tetrahedron Lett. 1990, 31, 231.
(10) Hayashi, T.; Matsumoto, Y.; Ito, Y. Tetrahedron: Asymmetry 1991,
2, 601.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(11) Mazet, C.; Ger
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We acknowledge NSFC (21672033, 21372041), Jilin Province
Natural Science Foundation (20160520140JH), and Changbai
Mountain Scholarship Program.
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