Enantioselective 1,4-addition of cyclopropylboronic acid catalyzed by rhodium/chiral diene complexes

Article abstract of DOI:10.1039/c5cc02140e

Rhodium-catalyzed asymmetric addition of cyclopropylboronic acids to electron-deficient alkenes such as alkenylsulfones, enones, enoates, and nitroalkenes proceeded to give high yields of the corresponding 1,4-addition products with high enantioselectivit

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Enantioselective 1,4-addition of
cyclopropylboronic acid catalyzed by
Cite this:DOI: 10.1039/c5cc02140e
rhodium/chiral diene complexes†
Received 13th March 2015,
Accepted 9th April 2015
Ryosuke Takechi and Takahiro Nishimura*
DOI: 10.1039/c5cc02140e
www.rsc.org/chemcomm
Rhodium-catalyzed asymmetric addition of cyclopropylboronic strained cyclopropene. The asymmetric addition of dicyclopropyl-
acids to electron-deficient alkenes such as alkenylsulfones, enones, zinc to aldehydes was reported by the use of a chiral amino alcohol.9
enoates, and nitroalkenes proceeded to give high yields of the The diastereoselective addition of cyclopropyllithium or magnesium
corresponding 1,4-addition products with high enantioselectivity.
bromide to imines is achieved using a chiral auxiliary on the
nitrogen.10 The Cu-catalyzed enantioselective addition of dicyclo-
Asymmetric conjugate addition of organometallic reagents to propylzinc to a b-disubstituted nitroalkene was reported to give
electron-deficient alkenes catalyzed by Rh complexes is now well- the addition product in low yield with low enantioselectivity.11,12
recognized to be one of the most reliable methods for carbon– Here we report that the asymmetric addition of cyclopropyl-
carbon bond formation introducing aryl and alkenyl groups with boronic acid to electron-deficient alkenes catalyzed by Rh/chiral
high enantioselectivity.1,2 On the other hand, asymmetric conju- diene complexes. To the best of our knowledge, this is the first
gate addition of alkyl groups has been developed using Ni and example of the metal-catalyzed asymmetric conjugate addition
Cu catalysts,3 and thus both catalytic systems perform a com- of cyclopropylboronic acids.13
plementary role in the transition metal-catalyzed asymmetric
We found that a Rh complex coordinated with a diene ligand
conjugate addition reactions.4 The catalytic conjugate addition has high catalytic activity in the addition of cyclopropylboronic
of simple alkyl metal reagents under the rhodium catalysis acid to an alkenylsulfone (Table 1). Thus, treatment of alkenyl-
using organometallic reagents is difficult because an intermediate sulfone 1a with cyclopropylboronic acid (2, 2.5 equiv.) in the
alkylrhodium(I) species having a b-hydrogen readily undergoes presence of [RhCl(cod)]2 (3 mol% of Rh) and K3PO4 (1 equiv.) in
elimination to give a hydridorhodium species and an alkene.4 toluene at 60 1C for 12 h gave the addition product 3a in 60%
As rare examples of the rhodium-catalyzed addition reaction of yield (entry 1, Table 1). An enantioselective addition was achieved
alkyl metal reagents, we reported asymmetric methylation of by the use of chiral diene ligands.14 A Rh complex coordinated
imines by the use of Me2Zn or trimethylboroxine as a methylating with a ferrocenyl (Fc)-substituted diene ligand based on the
reagent, where the b-hydrogen to be eliminated does not exist.5,6 tetrafluorobenzobarrelene (tfb) framework,15 which is a superior
von Zezschwitz and co-workers reported asymmetric 1,2- or catalyst in the asymmetric addition of arylboronic acids to alkenyl
1,4-addition of Me3Al to cyclic enones catalyzed by a rhodium/ sulfonyl compounds,16,17 displayed a high catalytic activity and
binap complex.7 The stereoselective alkyl transfer from potas- enantioselectivity to give 3a in 96% yield with 97% ee (entry 2).
sium benzylic trifluoroborates to aldehydes was also reported Other tfb ligands substituted with phenyl (Ph) and benzyl (Bn),
by Aggarwal and co-workers, where it is proposed that the and bicyclo[2.2.2]octadienes L1 and L2,18 which are derived
reaction proceeds by direct migration of the benzylic group to from a natural product, were less effective in the present addition
the aldehyde without formation of a benzylrhodium species.8 reaction (entries 3–6). The use of a rhodium-bisphosphine complex
19
In this context, we focused on the use of a cyclopropyl- [RhCl((R)-binap)]2 did not give the addition product at all
rhodium(I) species for the conjugate addition, which may avoid (entry 7). Cyclopropylboronic acid neopentylglycolate 20 can
the b-hydrogen elimination leading to the formation of a highly also be used to give a 99% yield of 3a with 94% ee, although
the reaction requires a higher reaction temperature (80 1C,
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo,
entry 8). The absolute configuration of product 3a formed
by the use of (S,S)-Fc-tfb* was determined to be S by X-ray
crystallographic analysis.
Kyoto 606-8502, Japan. E-mail: tnishi@kuchem.kyoto-u.ac.jp
† Electronic supplementary information (ESI) available: Experimental procedures,
compound characterization data, and X-ray crystallographic data of compound 3a.
The results obtained for the enantioselective addition of
cyclopropylboronic acid (2) to several alkenyl sulfonyl compounds 1
CCDC 1047801. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c5cc02140e
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Table 1 Rh-catalyzed addition of cyclopropylboronic acid 2 to alkenyl-
sulfone 1aa
Table 3 Asymmetric cyclopropylation of enones, enoates, and nitroalkenesa
Entry L*
X
R
Yieldb (%) eec (%)
1d,e
2
L3
L3
L3
COPh
COPh
COPh
Ph (4a)
4-ClC6H4 (4b)
4-CF3C6H4 (4c) 95 (5c)
80 (5a)
89 (5b)
84
84
86
81
84
81
89
89
89
3
4d,e
5
Bn-tfb* CO2Et
L3
L3
Bn-tfb* NO2
Bn-tfb* NO2
Bn-tfb* NO2
Ph (4d)
CO2CH(CF3)2 Ph (4e)
CO2(t-Bu) CO2(t-Bu) (4f)
63 (5d)
70 (5e)
99 (5f)
92 (5g)
80 (5h)
Entry
L
Yieldb (%)
eec (%)
6
1
2
3
4
5
6
7
8f
cod (1,5-cyclooctadiene)
(S,S)-Fc-tfb*
(R,R)-Ph-tfb*
(S,S)-Bn-tfb*
(R)-L1
(R)-L2
(R)-Binap
60
96d
43
24
10
4
—
97
93
60
7
p-Tolyl (4g)
4-ClC6H4 (4h)
4-MeOC6H4 (4i) 70 (5i)
8e
9e, f
e
a
—
—
Reaction conditions: 4 (0.20 mmol), 2 (0.70 mmol), [Rh(OH)(L*)]2
(3 mol% of Rh), K3PO4 (for 4a–f; 1 equiv.) in 1,4-dioxane (0.8 mL) or
e
b
0
99
—
94
KHF2 (for 4g–i; 1 equiv.) in toluene (0.8 mL) at 60 1C for 12 h. Isolated
c
d
(S,S)-Fc-tfb*
yields. Determined by chiral HPLC analysis. 5 mol% of Rh was used.
e
f
For 24 h. At 80 1C.
a
Reaction conditions: alkenylsulfone 1a (0.10 mmol), 2 (0.25 mmol),
the Rh catalyst (3 mol% of Rh), K3PO4 (1 equiv.) in toluene (0.4 mL) at
60 1C for 12 h. Determined by 1H NMR analysis using 1,4-dimethoxy-
b
c
to give the corresponding addition products in good to high yields
with high enantioselectivity (entries 4–6). Not only alkenyl sulfones,
but also sulfonates (1h and 1i) and a sulfonamide (1j) can be
applicable with high enantioselectivity (entries 7–9).
benzene as an internal standard. Determined by chiral HPLC analysis.
d
Isolated yield. e Not determined. f Cyclopropylboronic acid neopentylglyco-
late 20 was used instead of 2 in the presence of methanol (3 equiv.) at 80 1C.
are summarized in Table 2.20 The reaction of alkenyl sulfones
having 2-furyl (1b) and 2-thienyl (1c) at the b-position proceeded to
give the corresponding addition products 3b and 3c, respectively,
in high yields with high enantioselectivity (entries 1 and 2). A
pyridyl group on alkenyl sulfone 1d slowed the reaction to give 3d
in 51% yield at 80 1C for 12 h, but the enantioselectivity was high
(96% ee, entry 3). Alkenyl sulfones substituted with 2-methyl-1-
propenyl (1e), butyl (1f), and benzyl (1g)21 are also good substrates
The enantioselective addition of cyclopropylboronic acid pro-
ceeded toward the other electron-deficient alkenes than alkenyl
sulfones, where the higher enantioselectivity was observed with alkyl-
substituted tfb ligands than that with Fc-tfb* (Table 3). In the presence
of hydroxorhodium/chiral tfb catalysts,22 the addition to a,b-unsatura-
ted ketones 4a–c, esters 4d and 4e, and di-tert-butyl fumarate (4f)
proceeded to give the corresponding addition products in good
yields with 81–86% ee (entries 1–6), where ligand L3 substituted
with neopentyl groups displayed good enantioselectivity, except for
the addition to ethyl ester 4d. Nitroalkenes 4g–i were also applicable
acceptors by the use of KHF2 as a base23 instead of K3PO4 to give the
corresponding addition products with 89% ee (entries 7–9).
Table 2 Asymmetric cyclopropylation of alkenylsulfonyl compounds 1a
The addition of cyclobutylboronic acid (6) or cyclopentylboro-
nic acid (7) to alkenylsulfone 1a under the same reaction condi-
tions as for cyclopropylboronic acid (2) did not take place and 1a
was recovered intact (eqn (1)). On the other hand, the use of
n-butylboronic acid (8) gave only saturated sulfone 9 in 26% yield,
indicating that the reduction of 1a proceeds via the formation
of a hydridorhodium generated by b-hydrogen elimination of a
n-butylrhodium species (eqn (2)). The results also imply that the
transmetalation of the Rh with cyclobutyl- and cyclopentylboronic
acid does not take place under the present reaction conditions.
Entry
X
R
Yieldb (%)
eec (%)
1
2
p-Tolyl
p-Tolyl
p-Tolyl
p-Tolyl
p-Tolyl
p-Tolyl
2,6-Me2C6H3O
EtO
N-Morpholyl
2-Furyl (1b)
2-Thienyl (1c)
3-Pyridyl (1d)
CHQCMe2 (1e)
Butyl (1f)
Benzyl (1g)
Ph (1h)
97 (3b)
97 (3c)
51 (3d)
78 (3e)
94 (3f)
92 (3g)
80 (3h)
86 (3i)
96 (3j)
93
93
96
83
96
97
98
92
98
3d,e, f
4
5g
6g,h
7d,e
8
Ph (1i)
Ph (1j)
9
a
Reaction conditions: alkenylsulfone 1 (0.20 mmol), 2 (0.50 mmol),
[RhCl((S,S)-Fc-tfb*)]2 (3 mol% of Rh), K3PO4 (1 equiv.) in 1,4-dioxane
(for 1b–d, 1f, 1h; 0.8 mL) or toluene (for 1e, 1g, 1i, 1j; 0.8 mL) at 60 1C
b
c
d
(1)
for 24 h. Isolated yields. Determined by chiral HPLC analysis. At
e
f
80 1C. Performed with 3.5 equiv. of 2. Performed with 5 mol% of Rh.
g
h
For 12 h. Na3PO4 was used instead of K3PO4.
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Notes and references
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to methyl vinyl ketone (4j) in the presence of [Rh(OH)((S,S)-
Fc-tfb*)]2 proceeded to give the addition product 11 in 68%
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(4)
In summary, we have developed Rh-catalyzed asymmetric
addition of cyclopropylboronic acids to electron-deficient
alkenes. The Rh complexes coordinated with chiral diene
ligands based on a tetrafluorobenzobarrelene framework dis-
played high catalytic activity and enantioselectivity. The addi-
tion of a substituted-cyclopropyl group proceeded with the
stereoretention, indicating that the transmetalation and
the subsequent carborhodation proceed with the retention of
the configuration.
10 For selected examples, see: (a) P. W. Smith, P. A. Wyman, P. Lovell,
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This work was supported by JSPS KAKENHI Grant Number
24550117. We thank Prof. A. Osuka for X-ray crystallographic
analysis of compound 3a.
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