Enantioselective conjugate addition of alkylboranes catalyzed by a copper- N-heterocyclic carbene complex

Article abstract of DOI:10.1021/ja304481a

The first catalytic enantioselective conjugate addition of alkylboron compounds has been achieved. Reactions between alkylboranes and imidazol-2-yl α,β-unsaturated ketones proceeded with high enantioselectivity under the influence of a Cu(I) catalyst syst

Full text of DOI:10.1021/ja304481a

Communication
pubs.acs.org/JACS
Enantioselective Conjugate Addition of Alkylboranes Catalyzed by a
Copper−N-Heterocyclic Carbene Complex
Mika Yoshida, Hirohisa Ohmiya,* and Masaya Sawamura*
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
S
* Supporting Information
having 1-(1-naphthyl)ethyl groups at both N atoms imparted
moderate yields and enantioselectivities (46%, 41% ee in THF;
62%, 43% ee in toluene) (entries 3 and 4).
ABSTRACT: The first catalytic enantioselective conju-
gate addition of alkylboron compounds has been achieved.
Reactions between alkylboranes and imidazol-2-yl α,β-
unsaturated ketones proceeded with high enantioselectiv-
ity under the influence of a Cu(I) catalyst system, prepared
in situ from CuCl, a new chiral imidazolium salt as a
precursor for the N-heterocyclic carbene ligand, and
PhOK. Alkylboranes are widely obtained via alkene
hydroboration. A variety of functional groups are tolerated
in alkylboranes and α,β-unsaturated ketones.
Based on the results in the preliminary ligand screening, we
investigated various ring-unsaturated C2-symmetric NHC
ligands, featuring stereogenic centers at the positions α to the
N atoms, in toluene at 35−60 °C (Table 1, entries 5−9). To
our disappointment, replacing the Me group of L3 with an Et
group decreased the enantioselectivity (entry 5). Introducing
the adamantyl (L5)18 or mesityl (L6)19 group instead of the
naphthyl substituents in L3 resulted in loss of enantiocontrol or
complete inhibition of the reaction, respectively (entries 6 and
7). These results imply that introducing steric hindrance in
close proximity to the α-stereogenic center of N-substituents is
unfavorable. This prompted us to prepare a new NHC ligand,
in which steric hindrance is introduced at some distance from
the stereogenic center rather than within proximity. We
successfully improved the product yield (68%) and enantiose-
lectivity (59% ee) using the ligand (L7) with 3,5-di-tert-butyl-4-
methoxyphenyl (DTBM) substituents at 40 °C (entry 8).
Replacing the Me groups of L7 with Et groups resulted in
almost total loss of the enantiocontrol, indicating that the
enantiocontrol relies on the subtle balance of steric hindrance
associated with the substituents at the stereogenic centers
(entry 9).
nantioselective conjugate additions of organometallic
Ereagents to α,β-unsaturated carbonyl compounds under
the influence of chiral transition metal catalysts are efficient and
versatile methods for asymmetric C−C bond formation.1 In
recent decades, reactions using organoboron compounds as
organometallic reagents for conjugate additions have achieved
remarkable development, given their broad substrate scope and
functional group compatibility.2−5 Unfortunately, usable
organoboron reagents are limited to aryl-, alkenyl-, and
allylborons; the methodology has not yet been expanded to
the use of alkylborons.6,7
Here we report enantioselective conjugate addition of
alkylboranes (alkyl-9-BBN) to imidazol-2-yl α,β-unsaturated
ketones catalyzed by a Cu(I) complex with a chiral N-
heterocyclic carbene (NHC) ligand.6−10 To our knowledge,
this protocol is the first transition-metal-catalyzed enantiose-
lective conjugate addition of alkylborons. Alkylboranes are
widely obtainable through alkene hydroboration. A variety of
functional groups are tolerated in alkylboranes and α,β-
unsaturated ketones. The 2-acylimidazolyl moiety can be
transformed into various carboxylic functional groups.11,12
After our preliminary research with achiral NHC−Cu
catalysts,6c,d various chiral NHC ligands were examined for
catalytic activity and enantiocontrol in the copper-catalyzed
conjugate addition of the alkylborane 2a, which was prepared
from styrene (1a), to the trans-cinnamyl 1-methylimidazol-2-yl
ketone 3a (Table 1, entries 1−9).13,14 Copper−NHC catalysts
were prepared in situ from imidazolium salts (L1−L8), CuCl,
and t-BuOK. The ring-saturated NHC ligand (L1)15 with
stereogenic C centers in the ring did not form an active catalyst
(with t-BuOK) (entry 1). The N-substituted NHC ligand
(L2)16 with a hydroxyalkyl arm involving a stereogenic center
induced slight catalytic activity and enantioselectivity (entry 2),
but further investigation based on hydroxylated ligands was not
fruitful. The ring-unsaturated C2-symmetric NHC ligand (L3)17
Accordingly, we conducted substrate modification at the
imidazolyl moiety of an enone with the CuCl−L7 catalyst
system. As a result, the 1-methylbenzimidazol-2-yl ketone 3b
was found to be more reactive than the 1-methylimidazolyl
ketone 3a, giving the corresponding conjugate addition product
4ab in higher yield (76%) with higher enantioselectivity (76%
ee) (Table 1, entry 10).
Next, the effect of a base was examined in the reaction
between 2a and 3b with the CuCl−L7 catalyst system in
toluene at 25 °C with a constant reaction time of 48 h (Table 1,
entries 11−14).20 Changing the alkoxide base from t-BuOK to
the smaller MeOK markedly increased the rate of substrate
conversion, but this change did not improve the enantiose-
lection (from 11% yield, 76% ee to 54% yield, 75% ee) (entries
11 and 12). Further screening on the different types of bases
showed PhOK to be optimal, affording the conjugate addition
product 4ab in 93% yield with an enantiomeric excess as high
as 85% ee (entry 13). The Cu loading could be reduced to 5
mol % in a slightly decreased yield with the high
enantioselectivity unchanged (89%, 85% ee, data not shown
Received: May 9, 2012
Published: June 29, 2012
© 2012 American Chemical Society
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a
Table 1. Copper-Catalyzed Conjugate Additions of
Alkylborane 2a under Various Conditions
Table 2. Scope of Alkenes (Alkylboranes)
a
temp yield
ee
(%)
b
c
entry SM
L
base
solvent (°C) (%)
1
2
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
3b
3b
3b
3b
3b
L1
L2
L3
L3
L4
L5
L6
L7
L8
L7
L7
L7
L7
L7
L7
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
MeOK
THF
THF
THF
tol
80
80
80
80
60
60
60
40
35
40
25
25
25
25
25
0
−
30
24
46
62
41
36
0
3
41(S)
43(S)
7(S)
2
4
5
tol
6
tol
7
tol
−
59
8
tol
68
68
76
11
54
93
12
85
9
tol
6
10
11
12
13
14
15
tol
76
tol
76
tol
75
PhOK
tol
85
2,6-Me2-C6H3OK
PhOK
tol
51
mes
87
a
Reaction was carried out with 3b (0.15 mmol), 2 (0.18 mmol), CuCl
(10 mol %), L7 (12 mol %), and PhOK (25 mol %) in toluene (0.6
mL; entries 1 and 6) or mesitylene (1.0 mL; entries 2−5) at 30 °C for
48 h. Alkylborane 2 was prepared in advance by hydroboration of 1
with the 9-BBN dimer at 60 °C for 1 h and used without purification.
b
c
Yield of the isolated product. Enantiomeric excess determined by
d
HPLC analysis. Reaction was carried out at 25 °C.
1−5). An ester moiety at the terminal of the aliphatic chain
(entry 6) was also tolerated in the reaction.
The reaction of the alkylboranes, which were prepared from
styrene or allylbenzene derivatives, proceeded with high
enantioselectivities (Table 2, entries 1−5). The sterically
more demanding alkylborane 2g, which was derived from the
terminal alkene 1g with a tertiary alkyl substituent, served as a
suitable substrate to afford the corresponding product 4gb
(entry 6).
The scope of imidazolyl α,β-unsaturated ketone derivatives
(3) is shown in Table 3.21 The methoxy group was tolerated as
a para-substituent on the aromatic ring at the β-position (61%
yield, 93% ee, entry 1).22 The ketone 3d, bearing a 2-thienyl
group at the β-position, also underwent conjugate addition in
80% yield with 91% ee (entry 2).
a
Reaction was carried out with 3 (0.15 mmol), 2a (0.18 mmol), CuCl
(10 mol %), L1−8 (12 mol %), and a base (25 mol %) in THF (0.3
mL), mesitylene (1.0 mL), or toluene (0.6 mL) for 12 h (entries 1−8)
or 48 h (entries 9−15). Alkylborane 2 was prepared in advance by
hydroboration of 1 with the 9-BBN dimer at 60 °C for 1 h and used
without purification. Yield of the isolated product. Enantiomeric
excess determined by HPLC analysis.
b
c
in Table 1). A higher enantioselectivity (87% ee) for 4ab was
obtained when the reaction was conducted in mesitylene (entry
15). Bulkier alkoxide base 2,6-Me2-C6H3OK was much less
effective (entry 14), and other ArOK bases with varying
electronic nature did not lead to improvement in both reactivity
and enantioselectivity (see Supporting Information).
Various terminal alkenes were subjected to 9-BBN-hydro-
boration and were used for conjugate addition of the trans-
cinnamyl 1-methylbenzimidazolyl ketone 3b with the CuCl/
L7/PhOK catalyst system (Table 2). Alkenes having functional
groups such as ester, silyloxy, chloro, or methoxy moieties on
the aromatic ring were compatible with the protocol (entries
Alkyl groups were also acceptable as a β-substituent of the
α,β-unsaturated ketones (Table 3, entries 3−6). α,β-Unsatu-
rated ketones with a Me group at the β-position reacted with
high enantioselectivities (entries 3−5): the 4,5,6,7-tetrahydro-
benzimidazolyl substrate was used in this case because the
corresponding benzimidazolyl α,β-unsaturated ketone was
unstable. The reaction of the ketone having an Et group as
the β-substituent gave a slightly decreased product yield and
enantioselectivity (entry 6).
The conjugate addition product 4ab (87% ee) could be
readily converted into the corresponding methyl ester 5ab
through treatment with MeOTf in MeCN followed by the
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Journal of the American Chemical Society
Communication
a
phenoxyborane, alkyl−Cu addition across the C−C double
bond occurs to form a C-copper enolate (E) or an O-copper
enolate (E′).10b,24 Finally, the second B/Cu transmetalation
releases a boron enolate (F), regenerating the phenoxycopper-
(I) complex A for the next catalytic cycle.25,26
Table 3. Scope of Imidazolyl α,β-Unsaturated Ketones
According to the proposed catalytic cycle, the superiority of
PhOK over t-BuOK and MeOK in promoting the reaction may
be ascribed to the higher Lewis acidity of phenoxyborane over
the alkoxyboranes for activating the enone toward organo-
copper addition (D-TS).10b In addition, the compactness of
phenoxyborane relative to tert-butoxyborane may also contrib-
ute to the smooth catalyst turnover because the second
transmetalation step (from E/E′ to A and F) is sensitive to
steric effects. Similarly, the increase in product yield upon
replacing t-BuOK with MeOK can be explained by the steric
effect in the second transmetalation.
Models (D-TS-1 and D-TS-2) for enantiodiscrimination
based on the proposed catalytic cycle are depicted in Figure 2.
a
Reaction was carried out with 3 (0.15 mmol), 2 (0.18 mmol), CuCl
(10 mol %), L7 (12 mol %), and PhOK (25 mol %) in toluene (0.6
mL; entries 3−5), mesitylene (1.0 mL; entries 1 and 6) or a toluene/
mesitylene 1:1 mixture (1.0 mL; entry 2) at 30 °C for 48 h.
Alkylborane 2 was prepared in advance by hydroboration of 1 with the
b
9-BBN dimer at 60 °C for 1 h and used without purification. Yield of
c
the isolated product. Enantiomeric excess determined by HPLC
d
analysis. Reaction was carried out at 25 °C.
Figure 2. Models for enantiodiscrimination.
addition of MeOH and DBU (eq 1).11,12 The absolute
configuration of 4ab was determined to be R by the optical
rotation of 6ab (87% ee) obtained by reduction with LiAlH4.23
The models involve two additional assumptions: first, the
NHC−Cu coordination axis is coplanar with the developing
Cu−C(α) and C(R1)−C(β) σ-bonds in D-TS, and second, the
substituents at the stereogenic centers in L7 are arranged such
that the aryl groups are placed away from the enone-
coordinated Cu-atom and hence the two methyl groups
constitute a C2-type quadrant around the copper center (the
first and third quadrants being sterically more demanding).
According to these assumptions, the imidazolylcarbonyl group
and the O-bound 9-BBN-OPh moieties should encounter larger
steric repulsion with the chiral ligand in D-TS-2, which leads to
the minor enantiomer, than in D-TS-1. On the other hand, the
β-substituent (R2) of the enone should have less steric
interactions with the chiral ligand in both TSs than the
imidazolylcarbonyl moiety has. These considerations are
consistent with the experimental observations that the sterically
more demanding imidazolyl groups (e.g., 3a vs 3b) induced
higher enantioselectivities while the steric demand of the β-
substituent have less impact on the enantiocontrol.
A possible catalytic cycle for the present copper catalysis is
illustrated in Figure 1. Initially, the reaction of CuCl, L7, and
PhOK forms a phenoxycopper complex (A). B/Cu trans-
metalation between A and alkylborane 2 forms an alkylcopper-
(I) species (B) and phenoxyborane (9-BBN-OPh).10 The
alkylcopper(I) species B forms a π-complex (C) with enone 3.
Then, with the assistance of Lewis acidic activation with
In summary, we demonstrated the enantioselective conjugate
addition of alkylborons (alkyl-9-BBN) to imidazol-2-yl α,β-
unsaturated ketones catalyzed by a copper(I)−chiral N-
heterocyclic carbene (NHC) complex. This is the first catalytic
enantioselective conjugate addition of alkylboron derivatives.
The chiral NHC−Cu catalysis allowed versatile and enantio-
controlled sp3-alkylation at the β-position of α,β-unsaturated
carbonyl compounds. The availability of alkylboranes through
in situ alkene hydroboration and the broad functional group
compatibility in both alkenes and α,β-unsaturated ketones are
attractive features from a synthetic viewpoint. The 2-
acylimidazolyl moiety can be transformed into various
carboxylic functional groups.
Figure 1. Possible catalytic cycle.
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Journal of the American Chemical Society
Communication
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ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
ohmiya@sci.hokudai.ac.jp; sawamura@sci.hokudai.ac.jp
■
Notes
The authors declare no competing financial interest.
(12) Examples of the conversion of 2-acylimidazoles to other groups
such as carboxylic acids, esters, amides, and ketone derivatives:
(a) Evans, D. A.; Song, H.-J.; Fandrick, K. R. Org. Lett. 2006, 8, 3351.
(b) Evans, D. A.; Fandrick, K. R.; Song, H.-J.; Scheidt, K. A.; Xu, R. J.
Am. Chem. Soc. 2007, 129, 10029.
(13) A review on NHCs: N-Heterocyclic Carbenes in Transition Metal
Catalysis; Glorius, F., Ed.; Topics in Organometallic Chemistry, Vol. 21;
Springer: Heidelberg, 2007.
ACKNOWLEDGMENTS
■
This work was supported by CREST, JST to M.S. and by
Grants-in-Aid for Young Scientists (A), JSPS and by the Uehara
Memorial Foundation to H.O. We thank MEXT for their
financial support through the Global COE grant (Project No.
B01: Catalysis as the Basis for Innovation in Materials Science).
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with NHC−Cu(I) alkoxide systems: (a) Shintani, R.; Takatsu, K.;
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K.; Takeda, M.; Hayashi, T. Angew. Chem., Int. Ed. 2011, 50, 8656.
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