Copper-catalyzed enantioselective hydroboration of unactivated 1, 1-disubstituted alkenes

Article abstract of DOI:10.1021/jacs.7b08379

We report an efficient and highly enantioselective hydroboration of aliphatic 1, 1-disubstituted alkenes with pinacolborane using a phosphine-Cu catalyst. The method allows facile preparation of enantiomerically enriched β-chiral alkyl pinacolboronates from a range of 1, 1-disubstituted alkenes with high enantioselectivity up to 99% ee. Unprecedented enantiodiscrimination between the geminal alkyl substituents was observed with functional group compatibility in the hydroboration. Furthermore, a catalyst loading as low as 1 mol % furnished the desired product without a decrease in yield or selectivity, demonstrating its efficiency in gram scale synthesis.

Full text of DOI:10.1021/jacs.7b08379

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Communication
Copper-Catalyzed Enantioselective Hydroboration
of Unactivated 1,1-Disubstituted Alkenes
Won Jun Jun Jang, Seung Min Min Song, Jong Hun Moon, Jin Yong Lee, and Jaesook Yun
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 12 Sep 2017
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Copper-Catalyzed Enantioselective Hydroboration of Unacti-
vated 1,1-Disubstituted Alkenes
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Won Jun Jang, Seung Min Song, Jong Hun Moon, Jin Yong Lee, and Jaesook Yun*
Department of Chemistry and Institute of Basic Science, Sungkyunkwan University, Suwon 16419, Korea.
Supporting Information Placeholder
regio- and enantioselectivity.8 The Mazet group reported an
ABSTRACT: We report an efficient and highly enantio-
Ir-catalyzed asymmetric hydroboration of α-methylstyrene in
92% ee.9a Co-catalyzed asymmetric hydroboration of 1,1-
disubstituted aryl alkenes by the Huang and Lu groups, re-
spectively, resulted in high enantioselectivity.10 Formal hy-
droboration approaches using a diboron reagent and metha-
nol under copper catalysis were recently reported for α-
substituted styrene derivatives12a and 1,1-diaryl alkenes12b de-
spite low boron atom-economy. However, all of these hy-
droboration methods were effective and enantioselective
only for the 1,1-disubstitued alkenes with a requisite aryl sub-
stituent. Recently, Takacs and co-workers described a car-
bonyl-directed, Rh-catalyzed asymmetric hydroboration of
1,1-dialkyl substituted alkenes with high enantioselectivity.13
However, in this case, the hydroboration requires a coordi-
nating carbonyl group as a directing group. Therefore, devel-
opment of the asymmetric hydroboration of simple 1,1-
dialkyl-substituted alkenes remains a challenge and is very
necessary.
selective hydroboration of aliphatic 1,1-disubstituted alkenes
with pinacolborane using a phosphine‒Cu catalyst. The
method allows facile preparation of enantiomerically en-
riched β-chiral alkyl pinacolboronates from a range of 1,1-
disubstituted alkenes with high enantioselectivity up to 99%
ee. Unprecedented enantiodiscrimination between the gemi-
nal alkyl substituents was observed with functional group
compatibility in the hydroboration. Furthermore, catalyst
loading as low as 1 mol % furnished the desired product
without decrease in yield or selectivity, demonstrating its
efficiency in gram scale synthesis.
Asymmetric hydroboration of alkenes has been developed
as one of the most powerful methods for the synthesis of
chiral alkylboron compounds.1 Due to a growing number of
versatile transformations of the C‒B bonds of the resulting
alkylboronic acid derivatives, their regio- and enantio-
selective preparation has become important.2 While stoichi-
ometric methods using chiral hydroborating reagents3 and
transition-metal catalyzed hydroboration4 of prochiral al-
kenes have been continuously developed, asymmetric hy-
droboration of 1,1-disubstituted alkenes imposes a great chal-
lenge in this research area due to the difficulty of discrimi-
nating two similar substituents at the geminal position.5,6
Scheme 1. Metal-Catalyzed Asymmetric Hydrobora-
tion of 1,1-Disubstituted Alkenes
a) Previous work: hydroboration of -substituted styrene derivatives
R
*
R
Bpin
Bpin
HBpin
B2Pin2
Ar
Ar
Ar
Rh, Ir, Co, Fe
For the asymmetric hydroboration of 1,1-disubstituted al-
kenes, strategic approaches have been designed by using an
equivalent chiral boron reagent or a chiral transition metal
catalyst. Since the pioneering work by Brown and co-
workers, a number of chiral organoboranes have been devel-
oped, but have shown low enantioselectivities.5 The Soder-
quist group reported that a chiral hydroborating agent, 9-
borabicyclo-[3.3.2]decane, was highly enantioselective for the
hydroboration of 1,1-disubstituted alkenes including 1,1-
dialkyl-substituted alkenes.7 However, high enantioselectivi-
ty over 90% ee was observed only when there was a signifi-
cant difference between the geminal substituents, such as t-
butyl and methyl.
R
*
R
Ar
Cu
b) This work: Cu-catalyzed asymmetric hydroboration of 1,1-disubstituted alkenes
R1
R1
Bpin
HBpin
R2
R2
L*CuH
R1 = alkyl
R2 = alkyl, Ph, SiR'3, NR'2
Recently, we disclosed that bisphosphine-copper complex-
es are efficient catalysts for the highly regio- and enantio-
selective hydroboration of mono- and 1,2-disubstituted al-
kenes with pinacolborane.14,15 Herein, we report a copper-
catalyzed high regio- and enantioselective asymmetric hy-
droboration of unactivated 1,1-disubstituted olefins as a prac-
tical and general method for the synthesis of chiral primary
On the other hand, various transition metal-catalyzed sys-
tems such as Rh,8 Ir,9 Co,10 and Fe11 catalysts have been de-
veloped to avoid the use of stoichiometric chiral organo-
borane reagents (Scheme 1, a). Rh-catalyzed hydroboration of
1,1-disubstituted alkenes with catecholborane provided low
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alkylboronate esters. This hydroboration method is efficient
1
2
for non-aryl-containing, 1,1-dialkyl-substituted alkenes af-
fording products in high enantioselectivities.
3
4
5
6
We started our investigation by examining hydroboration
of 2,3-dimethyl-1-butene (1a) with moderately discriminating
substituents (2° alkyl vs. methyl) under various reaction con-
ditions (Table 1). Copper catalyst combined with racemic
7
8
bisphosphine
phosphino)benzene
ligands
such
as
and
1,2-bis(diphenyl-
4,5-bis(diphenyl-
(dppbz)
phosphino)-9,9-dimethylxanthene (xantphos) and a NHC-
copper catalyst did not show any reactivity (entries 1–3). Chi-
ral bisphosphine ligands, L1, L2, or L3 displayed no reactivity,
either (entries 4–6). With the bulky (R)-DTBM-Segphos lig-
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and (L4),14 16 the reaction eventually proceeded to the target
‒
product in a partial conversion but with good enantioselec-
tivity (entry 7). The use of copper(ǀ)-thiophene-2-carboxylate,
which is known to generate active Cu‒H species without
base,17 was ineffective in the hydroboration (entry 8). Addi-
tion of KOt-Bu slightly increased the yield, but led to an in-
complete reaction (entry 9). The results suggest that a base
plays an important role in the reactivity of the hydrobora-
tion. Finally, the use of KOt-Bu as the base with CuCl in-
creased the isolated yield from 36% to 94% yield, and the
desired product (S)-2a18 was obtained with good enantiose-
lectivity (entry 10). Therefore, we chose the conditions used
in entry 10 as optimal among the screened conditions.
Table 1. Optimization of Reaction Conditions
entry
[Cu]/ligand
CuCl/dppbz
base
yield (%)a
ee (%)b
1
NaOtꢀBu
0
‒
2
CuCl/xantphos NaOtꢀBu
0
‒
3
IMesCuCl
CuCl/L1
CuCl/L2
CuCl/L3
CuCl/L4
CuTCc/L4
CuTC/L4
CuCl/L4
NaOtꢀBu
NaOtꢀBu
NaOtꢀBu
NaOtꢀBu
NaOtꢀBu
‒
0
‒
4
0
‒
5
0
‒
aAbsolute configuration was assigned by analogy with 2a
and 2m, of which the absolute configurations were deter-
6
0
‒
7
36
0
86
‒
mined by comparison with the literature optical data after
8
b
c
oxidation. Reaction temperature was 40 °C. Reaction time
9
KOtꢀBu
KOtꢀBu
18
94
87
87
was 24 h.
10
The optimized reaction conditions were applied to a varie-
ty of 1,1-disubstituted alkenes (Table 2). It was found that the
enantioselectivities obtained in the hydroboration was corre-
lated with the steric difference between the two substituents
at the geminal carbon. The hydroboration of 1b and 1c fur-
nished the desired anti-Markovnikov addition products with
98% and 96% ee, respectively, much higher than that of 1d
(64% ee) and 1e (60% ee) possessing 1°ꢀalkyl and methyl sub-
stituent. Most of the entries in Table 2 proceeded with high
regioselectivity except for product 2f with an aryl substitu-
ent, of which a regioisomeric mixture (93:7) was formed with
moderate enantioselectivity for the major isomer. Successful
hydroboration of alkenes with a wide range of functional
groups were carried out. Acetal (1g), ketal (1h), silyl ether
(1i), benzyl ether (1j), ester (1n), and silyl groups (1k–1n)
were compatible with the standard reaction conditions. Es-
aIsolated yield. bEe was determined by HPLC analysis.
cCuTC = copper(I)-thiophene-2-carboxylate
Table 2. Copper-Catalyzed Enantioselective Hydrob-
oration of 1,1-Disubstituted Alkenes
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pecially, the hydroboration products (2g and 2h) containing
an acetal and ketal could be used as surrogates for asymmet-
*LCu
H
Me
Me
Me
1
2
3
4
5
6
7
8
CuL*
CuL*
ric aldol products with formaldehyde as electrophile,19 after
oxidation. Alkenes containing a silyl group afforded highly
enantio-enriched alkylboron products (2k–2n) containing a
silicon-stereogenic center. Moreover, the asymmetric hy-
droboration reaction of enamine substrates containing an
indole group furnished the chiral β-amino boronic esters (2o
and 2p) in 97% ee and 96% ee.20 Finally, we investigated the
efficiency of the current catalytic system in controlling dia-
stereoselectivity of enantiopure natural 1,1-disubstututed
alkenes as reaction substrates (1q and 1r). (+)-Valencene (1q)
and (-)-β-pinene (1r) were hydroborated to the products with
excellent diastereoselectivities (>20:<1 dr). Interestingly, the
formation of diastereomers in the hydroboration of 1r was
completely controlled by the catalyst applied, while that of
pinene was substrate-controlled to give the same diastere-
omer with two catalysts of the opposite chirality, indicating
that ent-L4 was mismatched.
i-Pr
i-Pr
i-Pr
Me
L*CuH
TS1
; favored
i-Pr
1a
*LCu
H
i-Pr
Me
TS2; disfavored
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The hydroboration of 1k was chosen for our investigation
1
Figure 1. Stereochemical models of the copper-catalyzed hy-
droboration based on DFT calculations.
on the kinetics and the reaction progress was followed by H
NMR. The hydroboration process consists of olefin insertion
to Cu‒H and transmetalation of the resulting alkyl copper
with HBpin to form product and the CuH catalyst.14b,16 Initial
rate experiments showed that the reaction was zero-order in
substrate and first order in catalyst and HBpin, suggesting
that the transmetalation is the turnover-limiting step
(Scheme 2, see the Supporting Information for details).
Scheme 3. Gram-Scale Synthesis and Transformation
of β-Chiral Alkylboron Products.
a) Gram-scale synthesis
1 mol % CuCl
1.1 mol %
2 mol % KOt-Bu
Me
Me
L4
Scheme 2. Reaction Profile of Hydroboration of 1k
and Observed Rate Orders
Bpin
HBpin
PhMe2Si
PhMe2Si
toluene, rt, 24 h
1.05 equiv
2k
1k
96% yield (1.46 g), > 99% ee
5 mmol scale
5 mol % Cu
5.5 mol % L4
component rate order
Me
b) Transf ormation of chiral boron products
HBpin
alkene
HBpin
CuCl/L
zero
first
first
20 mol % KOt-Bu
2k
PhMe2Si
(1.2 equiv)
toluene, rt
Me
1k
OH
PhMe2Si
100
80
60
40
20
0
3e
yield: 75%
ee: >99%
Me
1) vinyl bromide,
LDA
2) I2
Me
O
H
N
PhMe2Si
PhMe2Si
3d
3a
3) acetone
yield: 71%
ee: >99%
yield: 67%
ee: > 99%
Me
1) BCl3
2) benzyl azide
1) furan, n-BuLi
2) NBS
Bpin
PhMe2Si
2k
0
2
4
6
8
10
12
time (h)
ee: >99%
1)
MgBr
2) I2
Pd(OAc)2, rac-Binap
To gain insight into the origin of the enantioselectivity, we
carried out DFT calculations of transition states for the inser-
tion step of the alkene 1a into L4Cu-H, based on the hydrob-
oration mechanism. The calculated, enantiodiscriminating
transition state models of the hydroboration of 1a are shown
in Fig. 1. The favored transition state (TS1) leading to the
major enantiomer is lower than that of the minor enantiomer
by ~5 kcal/mol.21 This difference in energy of the transition
states apparently results from steric repulsion between the
isopropyl group of the alkene and an aromatic substituent of
the chiral phosphine ligand. In addition, the distance be-
tween the copper and the alkene carbon of TS1 (2.01 Å) is
noticeably shorter than that of TS2 (2.20 Å), which indicates
NaOH, PhBr
Me
Me
PhMe2Si
PhMe2Si
3c
3b
yield: 74%
ee: >99%
yield: 91%
ee: >99%
The hydroboration protocol was suitable for large scale
synthesis. A 1.0 mol % loading of the copper catalyst was
sufficient to perform the reaction on a 5 mmol scale (Scheme
3, a). Despite the reduced amounts of catalyst and ligand, 2k
was produced in 96% yield and 99% ee. To demonstrate the
utility of the resulting chiral organoboron product, we also
carried out transformation of the C‒B bond into various
a more favorable interaction with the alkene
.
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functional groups. Heteroarylation22a and the Suzuki cou-
Moteki, S. A.; Wu, D.; Chandra, K. L.; Reddy, D. S.; Takacs, J. M. Org.
Lett. 2006, 8, 3097.
(5) For a review, see: Thomas, S. P.; Aggarwal, V. K. Angew. Chem.
Int. Ed. 2009, 48, 1896.
1
2
3
4
5
6
7
8
pling of 2k afforded products 3a and 3b, respectively.
Alkenylation22b of 2k using Grignard reagent produced 3c in
91% yield, and C‒N bond formation22c furnished the corre-
sponding benzyl-protected amine 3d. Finally, 3e was gener-
ated by alkynylation.22d In all of these reactions, the original
ee was preserved without deterioration.
(6) Representative examples of enantioselective hydrogenation or
hydrofunctionalization of 1,1-disubstituted alkenes: (a) Die
́guez, M.;
Mazuela, J.; Pamies, O.; Verendel, J. J.; Andersson, P. G. J. Am. Chem.
̀
Soc. 2008, 130, 7208. (b) Monfette, S.; Turner, Z. R.; Semproni, S. P.;
Chirik, P. J. J. Am. Chem. Soc. 2012, 134, 4561. (c) Song, S.; Zhu, S.-F.;
Yu, Y.-B.; Zhou, Q.-L. Angew. Chem. Int. Ed. 2013, 52, 1556. (d) Zhu,
S.; Buchwald S. L. J. Am. Chem. Soc. 2014, 136, 15913. (e) Chen, J.;
Cheng, B.; Cao, M.; Lu, Z. Angew. Chem. Int. Ed. 2015, 54, 4661.
In summary, we developed a copper-catalyzed regio- and
enantioselective hydroboration of unactivated 1,1-dialkyl-
substituted alkenes with pinacolborane. The DTBM-
Segphos‒copper catalyst was most effective with a wide
range of 1,1-alkene substrates with alkyl, silyl, and amine sub-
stituents, furnishing useful β-chiral alkylboronates in high
enantioselectivities. This hydroboration method is unique
and complementary to other metal-catalyzed hydroborations
efficient for 1,1-disubstituted aryl alkenes. Moreover, the pro-
tocol could be easily scaled up to gram-scale synthesis, and
useful transformations of the resulting alkylboronate to vari-
ous compounds proved its utility.
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na, J. R.; Matos, K.; Soderquist, J. A. J. Am. Chem. Soc. 2008, 130, 9218.
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(b) Sato, M.; Miyaura, N.; Suzuki, A. Tetrahedron Lett. 1990, 31, 231.
(c) Hayashi, T.; Matsumoto, Y.; Ito, Y. Tetrahedron: Asymmetry 1991, 2,
601.
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Magre, M.; Biosca, M.; Pàmies, O.; Diéguez, M. ChemCatChem. 2015,
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2014, 136, 15501. (b) Chen, J.; Xi, T.; Ren, X.; Cheng, B.; Guo, J.; Lu, Z.
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ASSOCIATED CONTENT
Supporting Information
(11) Chen, J.; Xi, T.; Lu, Z. Org. Lett. 2014, 16, 6452.
(12) (a) Corberán, R.; Mszar, N. W.; Hoveyda, A. H. Angew. Chem.
Int. Ed. 2011, 50, 7079. (b) Wang, Z.; He, X.; Zhang, R.; Zhang, G.; Xu,
G.; Zhang, Q.; Xiong, T.; Zhang, Q. Org. Lett. 2017, 19, 3067.
(13) (a) Smith, S. M.; Hoang, G. L.; Pal, R.; Khaled, M. O. B.; Pelter,
L. S. W.; Zeng, X. C.; Takacs, J. M. Chem. Commun. 2012, 48, 12180.
(b) Shoba, V. M.; Thacker, N. C.; Bochat, A. J.; Takacs, J. M. Angew.
Chem. Int. Ed. 2016, 55, 1465.
(14) (a) Noh, D.; Chea, H.; Ju, J.; Yun, J. Angew. Chem. Int. Ed.
2009, 48, 6062. (b) Noh, D.; Yoon, S. K.; Won, J.; Lee, J. Y.; Yun, J.
Chem. Asian J. 2011, 6, 1967.
(15) Copper-catalyzed hydroboration of 1,2-disubstitued alkenes by
others: Xi, Y.; Hartwig, J. F. J. Am. Chem. Soc. 2016, 138, 6703.
(16) For the effect of phosphine ligands on catalytic activity in
copper-catalyzed hydroboration, see: Won, J.; Noh, D.; Yun, J.; Lee, J.
Y. J. Phys. Chem. A 2010, 114, 12112.
(17) (a) Lee, H.; Lee, B. Y.; Yun, J. Org. Lett. 2015, 17, 764. (b) Jang,
W. J.; Lee, W. L.; Moon, J. H.; Lee, J. Y.; Yun, J. Org. Lett. 2016, 18,
1390.
Experimental procedures, characterization of products and
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*jaesook@skku.edu.
ORCID
Jaesook Yun: 0000-0003-4380-7878
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This research was supported by National Research Founda-
tion of Korea (NRF) grants (NRF-2016R1A2B4011719 and NRF-
2016R1A4A1011451), funded by the Korean government
(MEST). The authors thank Prof. Y. H. Rhee for helpful dis-
cussions.
(18) The absolute configuration of 2a was determined by compar-
ing optical rotation of the corresponding hydroxy compound with
literature value. (a) Brown, H. C.; Imai T.; Desai, M. C.; Singaram, B.
J. Am. Chem. Soc. 1985, 107, 4980. (b) Tietze, L. F.; Raith, C.; Brazel, C.
C.; Hölsken, S.; Magull, J. Synthesis 2008, 2, 229.
(19) (a) Palomo, C.; Oiarbide, M.; Garcίa. Chem. Soc. Rev. 2004, 33,
65. (b) Meninno, S.; Lattanzi, A. Chem. Rec. 2016, 16, 2016.
(20) Catalytic asymmetric hydroboration of enamine substrates
has not been reported yet. N-alkyl-enamines were not reactive under
our catalytic conditions. The hydroboration of enamides with a chi-
ral Rh catalyst was recently reported: Hu, N.; Zhao, G.; Zhang, Y.;
Liu, X.; Li, G.; Tang, W. J. Am. Chem. Soc. 2015, 137, 6746.
(21) See the Supporting Information for details of all calculations.
(22) (a) Bonet, A.; Odachowski, M.; Leonori, D.; Essafi, S.; Ag-
garwal, V. K. Nat. Chem. 2014, 6, 584. (b) Aggarwal, V. K.; Binanzer,
M.; de Ceglie, M. C.; Gallanti, M.; Glasspoole, B. W.; Kendrick, S. J.
F.; Sonawane, R. P.; Vázquez-Romero, A.; Webster, M. P. Org. Lett.
2011, 13, 1490. (c) Hupe, E.; Marek, I.; Knochel, P. Org. Lett. 2002, 4,
2861. (d) Wang, Y.; Noble, A.; Myers, E. L.; Aggarwal, V. K. Angew.
Chem. Int. Ed. 2016, 55, 4270.
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(4) For a review, see: (a) Carroll, A.-M.; O'Sullivan, T. P.; Guiry, P. J.
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C. M.; Hleba, Y. B.; Chen, A. C. J. Am. Chem. Soc. 2004, 126, 9200. (c)
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