Rhodium-Catalyzed Enantioselective Hydroselenation of Heterobicyclic Alkenes

Article abstract of DOI:10.1021/acs.orglett.0c00762

A highly efficient Rh(I)/(S)-xyl-Binap catalytic system is developed for the asymmetric hydroselenation of various nonpolar olefins with diselenides. Under these mild reaction conditions, a wide range of heterobicyclic alkenes give selenol-incorporated adducts in excellent enantioselectivities (up to 97%) along with high yields (up to 96%) by overcoming self-promoted racemic hydroselenation. The strategy is also applied for kinetic resolution of unsymmetric oxabenzonorbornadiene.

Full text of DOI:10.1021/acs.orglett.0c00762

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Letter
Rhodium-Catalyzed Enantioselective Hydroselenation of
Heterobicyclic Alkenes
Sifeng Li, Qingjing Yang, Zhaoxiang Bian,* and Jun Wang*
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ABSTRACT: A highly efficient Rh(I)/(S)-xyl-Binap catalytic system is developed for the asymmetric hydroselenation of various
nonpolar olefins with diselenides. Under these mild reaction conditions, a wide range of heterobicyclic alkenes give selenol-
incorporated adducts in excellent enantioselectivities (up to 97%) along with high yields (up to 96%) by overcoming self-promoted
racemic hydroselenation. The strategy is also applied for kinetic resolution of unsymmetric oxabenzonorbornadiene.
Scheme 1. Hydroselenation of Alkenes
s an essential trace element for organisms, selenium was
1
Adiscovered over two hundred years ago. Selenium plays
an extremely important role in metabolism in the body2 and
disease chemoprevention and treatment.3 From the inves-
tigations on numerous stable organoselenium compounds that
were synthesized in recent decades, a large portion of them
could be used as fluorescent probes,4 antioxidants,5 electro-
philes,6 organocatalysts,7 which continually attracts interests
from chemists. To date, the carbon−selenium bond is the most
prevalent linkage in selenium-containing molecules. Among
numerous approaches to construct C−Se bond, hydro-
selenation of unsaturated carbon−carbon double bonds
provided the most efficient, atom-economic, and powerful
approaches for the preparation of organoselenium compounds.
In 1988, Hevesi reported the first hydroselenation of
conjugated enones in the presence of ZnCl2,8 although the
desired adduct was controlled by the substrates used. Then the
hydroselenation of α,β-unsaturated ketones was found to be
catalyzed by ceric ammonium nitrate (CAN)9 or β-cyclo-
dextrin,10 while the nitroalkenes11 and vinylsulfonamide12 were
self-promoted in the absence of any catalyst (Scheme 1A). The
substrates, such as those involving weakly activated or
nonelectronically activated CC bonds, were rarely reported.
In 2016, Ogawa revealed the hydroselenation of N-vinyl
lactams affording the corresponding N,Se-acetals as Markovni-
kov adducts (Scheme 1B).13 The hydroselenation of terminal
N-vinyl lactams was self-promoted, while Pd(II) catalyst was
essential for the efficient hydroselenation of internal N-vinyl
lactams. However, the direct asymmetric hydroselenation of
alkenes has been seldom discovered, probably because of the
high affinity of selenium with the transition metal.
Furthermore, the selenols used in hydroselenation are odorous,
poisonous, and easily oxidized in the air, which limited their
utilization in organic synthesis. Interestingly, when diphenyl-
phosphine oxide (Ph2P(O)H) was served as a reductant in the
selenation of unsaturated C−C bonds, the diaryl diselenides
were preferentially converted to the corresponding selenols.14
Herein, we present an example of asymmetric hydroselenation
of various nonpolar alkenes under the presence of
Received: February 28, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00762
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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a
Table 1. Initiation of Asymmetric Hydroselenation of
Oxabenzonorbornadiene
Table 2. Screening of Ligand, Solvent, and Temperature
a
b
c
time
(h)
yield
(%)
ee
entry
[Rh]
additive
(%)
d
1
48
6
8
36
8
48
8
4
4
8
0.3
0.5
3
4
30
81
91
70
80
82
90
89
77
83
91
92
97
91
91
82
2
3
4
5
6
7
8
9
10
11
12
13
14
15
[Rh(COD)Cl]2
[Rh(C2H4)Cl]2
[Rh(NBD)Cl]2
[Rh(coe)2Cl]2
none
none
none
none
85
82
80
83
86
86
87
87
89
94
94
95
87
68
[Rh(COD)OMe]2 none
[Rh(COD)acac]2
Rh(COD)2BF4
Rh(COD)2OTf
Rh(COD)2OTf
Rh(COD)2OTf
Rh(COD)2OTf
Rh(COD)2OTf
Rh(COD)2OTf
Rh(COD)2OTf
none
none
none
n-Bu4NBr
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
n-Bu4NI
b
c
entry
ligand
solvent
time (h)
yield (%)
ee (%)
e
1
2
3
4
5
6
7
8
L1
L2
L3
L4
L5
L6
L7
L8
L1
L1
L1
L1
L1
L1
L1
L1
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
THF
DME
DCE
toluene
dioxane
DMF
THP
3
2
91
95
86
95
85
93
e f
,
e fg
, ,
12
72
72
72
4
4
6
12
48
6
e fh
, ,
nr
a
trace
trace
83
87
83
80
60
93
90
Reaction conditions: Rh (5 mol %) and (S)-xyl-Binap (6 mol %)
dissolved in DCM with stirring for 30 min under argon, then the
additive (10 mol %, if applicable) was added and the mixture stirred
for an additional 10 min. 1a (0.2 mmol), 2a (0.24 mmol), and
84
92
97
96
84
95
92
b
Ph2P(O)H (2.4 equiv) were added into the mixture at 0 °C. Isolated
9
c
d
yields. Determined by HPLC analysis. No (S)-xyl-Binap was used.
10
11
12
13
14
15
e
f
g
1.8 equiv of Ph2P(O)H was used. 2 mol % of catalyst was used. 0.2
h
equiv of BHT was used. 1.0 equiv of BHT was used. DCM =
dichloromethane. NaBArF = sodium tetrakis[3,5-bis(trifluoromethyl)
phenyl]borate. n-Bu4NBr = tetrabutylammonium bromide. n-Bu4NI =
tetrabutylammonium iodide. BHT = 2,6-di-tert-butyl-4-methylphenol.
6
72
6
nr
86
96
97
97
d
16
THP
2
diphenylphosphine oxide and diselenides for generating the
selenols in situ (Scheme 1C).
a
Reaction conditions: 1a (0.2 mmol), 2a (0.24 mmol), Rh-
(COD)2OTf (2 mol %), ligand (2.4 mol %), n-Bu4NI (10 mol %),
and Ph2P(O)H (0.36 mmol) were added into a Schlenk tube with
solvent (2 mL), and then the mixture was stirred at 0 °C for the
indicated duration. Isolated yields. Determined by HPLC analysis.
The reaction was carried out at room temperature. THF =
tetrahydrofuran. DME = dimethoxyethane. DCE = dichloroethane.
DMF = N,N-dimethylformamide. THP = tetrahydropyran.
Heterobicyclic alkenes are ideal substrates to be activated by
transition-metal complexes due to the angle strain and the
proximal heteroatom coordination.15 Recently, transition-
metal-catalyzed hydrofunctionalization of aza/oxabenzonor-
borna dienes with heteronucleophiles, including hydroamina-
tion,16 hydrothiolation,17 hydrophosphination,18 and hydro-
boration,19 have been developed successfully. However,
enantioselective hydroselenation for these bicyclic alkenes
has not been involved. Selenols are more nucleophilic and
acidic than thiols, thus providing distinct challenges and
opportunities for hydrofunctionalization.
Based our previous experience on asymmetric hydro-
functionalization of aza/oxabenzonorbornadienes with hetero-
nucleophiles,17d,18b we embarked on the asymmetric hydro-
selenation of nonpolar alkenes in the presence of rhodium
precursor (5 mol %) and (S)-xyl-Binap (6 mol %) in
dichloromethane at 0 °C by utilizing oxabicyclic alkenes 1a
and diphenyl diselenide 2a as standard substrates and
diphenylphosphine oxide as a reductant. It should be noted
that the self-promoted hydroselenation of oxabicyclic alkenes
1a was a competitive background reaction, and the racemic
adduct was obtained in 81% yield without any catalyst (Table
1, entry 1). In the primal reaction for screening the rhodium
precursors, a wide range of rhodium complexes including
[Rh(COD)Cl]2, [Rh(C2H4)Cl]2, [Rh(NBD)Cl]2, [Rh-
(coe)2Cl]2, [Rh(COD)OMe]2, [Rh(COD)acac]2, Rh-
b
c
d
(COD)2BF4, and Rh(COD)2OTf were investigated. Among
them, Rh(COD)2OTf gave relatively better results, 83% yield
and 87% enantioselectivity (Table 1, entry 9). An additional
improvement to 94% ee could be achieved via a halide
exchange protocol, where the initial OTf counterion was
replaced by an iodide counterion (n-Bu4NI) (Table 1, entry
11). The addition of n-Bu4NI also greatly accelerated this
process, reducing the reaction time to 20 min, and the yield
and ee were obtained in 92% and 94%, respectively. Decreasing
the amount of Ph2P(O)H to 1.8 equiv and lowering the
catalyst loading to 2 mol % have no significant effect on the
result, and the yield remained at 91% while the ee was retained
at 95% (entry 13). The use of 0.2 equiv of radical inhibitor
BHT did not obviously decrease the yield and enantioselec-
tivity, but the reaction slowed down and the ee was dropped to
68% when 1 equiv of BHT was added (entries 14 and 15).
With the tentative reaction conditions in hand, a batch of
bulky bidentate phosphine ligands were tested for the better
yields and enantioselectivities. In the presence of 2 mol % of
B
https://dx.doi.org/10.1021/acs.orglett.0c00762
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a
Table 3. Scope of Diselenides for the Asymmetric
Hydroselenation of Oxabenzonorbornadiene
Scheme 2. Scope of Alkenes
a
b
c
entry
1
2
R
time (h)
yield (%)
ee (%)
Ph
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
2q
2
8
6
10
24
12
2
24
36
8
4
8
10
10
8
96
91
93
96
86
63
97
82
79
91
96
86
83
95
76
92
96
97
93
91
95
87
60
95
87
87
96
95
87
92
92
88
75
72
d
4-FC6H4
4-ClC6H4
4-BrC6H4
3
4
5
6
7
d
4-CF3C6H4
4-OHC6H4
4-OMeC6H4
3-OMeC6H4
2-OMeC6H4
4-tBuC6H4
4-i-PrC6H4
4-MeC6H4
3-MeC6H4
2-MeC6H4
4-PhC6H4
Bn
d
d
d
8
9
d
10
11
12
13
14
15
17
18
d
d
d
d
d
8
12
d
Me
a
Reaction conditions: 1a (0.2 mmol), 2a−2q (0.24 mmol), 2 mol %
of Rh(COD)2OTf, 2.4 mol % of (S)-xyl-Binap, 10 mol % of n-Bu4NI,
2.4 equiv of Ph2P(O)H, and THP (2 mL) were stirred at rt for the
b
c
indicated duration. Isolated yields. Determined by HPLC analysis.
d
5 mol % of catalyst was used.
a
Reaction conditions: oxabenzonorbornadienes 1a−1i or azabenzo-
norbornadienes 4a−4d (0.2 mmol), 2a (0.24 mmol), Rh(COD)2OTf
(2 mol %), (S)-xyl-Binap (2.4 mol %), n-Bu4NI (10 mol %), Ph2P(O)
H (2.4 equiv), and THP (2 mL) were stirred at rt for the indicated
Rh(COD)2OTf, Ph2P(O)H (1.8 equiv) and n-Bu4NI (10 mol
%), L2 (R)-DTBM-Segphos, L3 (R)-DM-Segphos, and Biphep
type ligands L7 and L8 all worked well, leading to a slightly
decreased enantioselective control (84−93%) compared to
(S)-xyl-Binap (Table 2, entries 1−3, 7, and 8), while the use of
L4 (R,R)-iPr-Duphos, L5 (R)-xyl-SDP, and L6 (R)-xyl-
phanephos blocked the hydroselenation at 0 °C (entries 4−
6). To further optimize the reaction, a series of solvents were
tested. To our delight, except for DMF, the reaction proceeded
well with the rest of the solvents employed (DCM, THF,
DME, DCE, toluene, dioxane, and THP (entries 9−13 and 15)
to give the yields in a range of 60−93% and the ee’s in a range
of 84−97%. THP was found to be the best solvent; when the
reaction was performed at room temperature, the yield was
increased from 92% to 96% and the enantioselectivity was kept
at 97% (entry 16). Thus, the optimal reaction conditions were
established under the presence of Rh(COD)2OTf (2 mol %),
(S)- xyl-Binap (2.4 mol %), and n-Bu4NI (10 mol %), and
oxabenzonorbornadiene reacted with diselenide (1.2 equiv)
and Ph2P(O)H (1.8 equiv) in THP at room temperature.
To test the compatibility of the optimized catalytic system,
various diselenides were used as a selenium resource in the
enantioselective hydroselenation of oxabenzonorbornadiene
1a. The aryl diselenides substituted with MeO or Me at the
ortho-, meta- or para-position were all well-tolerated and gave
satisfactory results as well (Table 3, entries 7−9 and 12−14).
The reaction protocol displayed a good functional group
tolerance in most cases, and both the electron-donating groups
(MeO, Me, i-Pr, t-Bu) (2g−2n) and the electron-withdrawing
groups (Cl, F, Br, CF3) (2b−2e) were found to be compatible
under these conditions, affording good yields (79−97%) and
high ee’s (87−95%). To our delight, the naked hydroxyl group
b
duration. Isolated yields and determined ees were indicated. D2O
c
(20 equiv) was used. 5 mol % of catalyst was used.
Scheme 3. Strategy for Kinetic Resolution
Scheme 4. Proposed Mechanism
C
https://dx.doi.org/10.1021/acs.orglett.0c00762
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Letter
(entry 6) was also tolerated in this system, and 63% yield and
60% ee were obtained within 12 h. Apart from aromatic
diselenides, the aliphatic diselenides also proceeded well in this
catalyst system with 5 mol % of Rh, affording good ee’s (75%
ee and 72% ee) with excellent yields (entries 17 and 18).
To further investigate the substrate scope, a broad range of
heterobicyclic olefins were tested (Scheme 2). Delightedfully,
all of the phenyl-functionalized oxabenzonorbornadienes were
well compatible with standard conditions, maintaining good
yields (81−96%) and high enantioselectivities (94% to 97%)
(3aa−3ia). Furthermore, the azabenzonorbornadienes also
tolerated the the rhodium system, yielding high yields (78−
93%) and good enantioselectivities (67−73% ee) by using 5
mol % of catalyst (5aa−5da). When the deuterated water (20
equiv) was added to the standard system, 74% of deuteration
on the vertical position was detected by NOESY. The exo-
configuration of the exclusive addition product 3aa was further
confirmed by X-ray crystal structure analysis. To demonstrate
the practicability of our method, the scaled-up reaction was
conducted by using 1.2 mmol of oxabenzonorbornadiene 1a.
The reaction was completed in 6 h under the standard reaction
conditions, and the product 3aa was obtained in 91% yield
(328 mg) without any loss of enantioselectivity.
Experimental procedures, characterization data, and
1
copies of H 13C NMR, NOESY, and COSY spectra
and HPLC charts (PDF)
Accession Codes
CCDC 1976783 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
■
Zhaoxiang Bian − School of Chinese Medicine, Hong Kong
Baptist University, Kowloon, Hong Kong; Email: bianzxiang@
gmail.com
Jun Wang − Shenzhen Grubbs Institute and Department of
Chemistry, Southern University of Science and Technology,
Shenzhen 518055, China; orcid.org/0000-0001-9723-
4054; Email: wang.j@sustech.edu.cn
Authors
When the C−O bridge monomethylated oxabenzonorbor-
nadiene 1j was subjected to Rh(COD)2OTf/(S)-xyl-Binap
conditions, kinetic resolution occurred (Scheme 3). Intrigu-
ingly, the newly formed selenol attacked the C−C double bond
closer to the substituent group to form product 3ja in 49%
yield and 87% ee after 12 h, while the substrate 1j was
recovered in 84% ee with 48% yield (s factor is 38). This result
indicated that the reaction condition could be applied to the
kinetic resolution of unsymmetric oxabenzonorbornadienes.
On the basis of previous study for Rh(I)-catalyzed
hydrothiolation,20 a possible pathway for this hydroselenation
is shown in Scheme 4. Chiral Rh(I) complex A is formed by
the ligand and ion exchange of Rh(COD)2OTf with (S)-xyl-
Binap and n-Bu4NI. Oxidative addition of selenol with Rh(I)
complex A provides intermediate B. The substrate 1a
coordinates to intermediate B to form the less sterically
encumbered intermediate coordinated Rh complex C, and
then the insertion of the CC into the Rh−Se bond provides
the intermediate D. Reductive elimination affords the product
3aa and regenerates the active Rh complex A. The catalytic
cycle consists of oxidative addition, coordination, syn-
selenometalation, and reductive elimination.
In summary, a rhodium catalytic system consisting of
Rh(COD)2OTf/(S)-xyl-Binap and n-Bu4NI was successfully
established in the mild conditions for the highly enantiose-
lective hydroselenation of heterobicyclic alkenes, which
overcame the presumed poison of selenium to rhodium and
the competition of the self-promoted hydroselenation. Mean-
while, this system could be well applied to the kinetic
resolution of the bridge mono methylated oxabenzonorborna-
diene. This methodology unlocked the asymmetric hydro-
selenation of nonpolar olefins. We believe this study will have
versatile applications in the synthesis of bioactive organo-
selenium compounds.
Sifeng Li − Shenzhen Grubbs Institute and Department of
Chemistry, Southern University of Science and Technology,
Shenzhen 518055, China; School of Chinese Medicine, Hong
Kong Baptist University, Kowloon, Hong Kong
Qingjing Yang − Shenzhen Grubbs Institute and Department of
Chemistry, Southern University of Science and Technology,
Shenzhen 518055, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00762
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the National
Natural Science Foundation of China (NSFC 21971102) and
S h e n z h e n B a s i c R e s e a r c h P r o g r a m
(JCYJ20170817112532779).
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D
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