Bismuth triflate-catalyzed asymmetric allylation of aromatic aldehydes

Article abstract of DOI:10.1002/chem.201103646

Fits the BiL₂: A highly enantioselective method for the catalytic allylation of aromatic aldehydes was developed using Bi(OTf) ₃ and Trost's (R,R)-ProPhenol ligand (see scheme). Good yields and high enantioselectivities were obtained for a variety of aromatic aldehydes. The disclosed method is the first example of an enantioselective allylation reaction of aldehydes using a chiral Bi^III complex. A preliminary characterization of the Bi^III pre-catalyst is also reported. Copyright

Full text of DOI:10.1002/chem.201103646

DOI: 10.1002/chem.201103646
Bismuth Triflate-Catalyzed Asymmetric Allylation of Aromatic AHCUTNGRNEGUN ldehydes
[
a]
Zhiya Li, Baptiste Plancq, and Thierry Ollevier*
The allylation reaction of carbonyl compounds is well-rec-
ognized as one of the most powerful synthetic tools for fast
Table 1. Selected optimization experiments illustrating the effects of the
bismuth salt and additive used in the enantioselective allylation of ben-
zaldehyde.
[
a]
[1]
carbon–carbon bond formation. The enantioselective syn-
thesis of homoallylic alcohols is an essential objective in
asymmetric synthesis. Enantioenriched homoallylic alcohols
are important building blocks for the construction of biolog-
[2]
ically active compounds. In addition to the processes using
[3]
stoichiometric chiral reagents or mediators, several catalyt-
ic asymmetric methods have been developed, either by
[4]
using chiral Lewis and Brønsted acids or bases. With the
chiral Lewis acid approach, many reactions using allyltribu-
tylstannane involve various metal–chiral ligand complexes,
in which the catalyst loading is typically 10–20 mol%. This
drawback, in addition to the use of expensive or toxic
metals, strongly reduces the interest of such methods. How-
ever, catalytic enantioselective allylation by using a chiral
[
b]
Entry
BiX
3
Additive
[mol%]
Yield 3a
[%]
er
A
H
U
G
R
N
N
1
2
3
4
5
6
7
Bi
Bi
Bi
Bi
Bi
Bi
A
T
N
T
E
U
G
3
3
3
3
3
·4H
·4H
·4H
·4H
2
2
2
2
O
O
O
O
–
70
69
53
62
76
46
42
82:18
92:8
92:8
93:7
94:6
A
H
N
T
E
N
N
Proton sponge
Hꢁnigꢀs base
Hꢁnigꢀs base
Hꢁnigꢀs base
Hꢁnigꢀs base
Hꢁnigꢀs base
15
15
50
50
15
50
A
H
U
T
E
N
G
bismuth ACHTUNGTRENNUNG( III) complex has never been explored, even though
A
H
U
T
E
N
N
III
Bi -derived Lewis acid catalysts have gained widespread
use as efficient catalysts for numerous synthetic transforma-
A
H
U
G
R
N
U
A
H
U
G
R
N
U
G
3
·4H
2
O
89:11
88:12
[5]
BiBr3
tions. Bismuth salts have recently attracted attention due
to their low toxicity, low cost, and environmentally benign
character.
[a] Conditions: aldehyde (0.5 mmol), allyltributylstannane (1.2 equiv),
ꢃ MS (30 mg). [b] Determined by chiral HPLC analysis.
4
We describe herein a new method for the enantioselective
allylation of aromatic aldehydes using allyltributylstannane
level of enantioselectivity was already reached without the
use of a base (Table 1, entry 1). However, such additives led
to an increase in the enantioselectivity of the homoallylic al-
cohol 3a (Table 1, entries 2 and 3), with the two bases being
equally efficient. Upon bismuth salt screening, we found
and a novel chiral Bi
complex. This phenol ligand, initially reported by Trost,
has been used for various reactions.
Our initial studies began with the model coupling of ben-
zaldehyde with allyltributylstannane using catalytic
ACHTUNGTNRENUNG( OTf) –Trostꢀs (R,R)-ProPhenol 1a
3
[6]
[7]
a
that Bi
(Table 1, entries 4–7). Bi ACHTUNGTRENN(UNG OTf) used in conjunction with
3
ACHTUNGTNERNUNG( OTf) was more efficient than Bi ACHTUNGRETNNUN(G ONf) or BiBr
3
3
3
amount of a bismuth salt and (R,R)-ProPhenol ligand 1a.
The chiral complex was prepared by reacting BiX3 (5
mol%) and (R,R)-ProPhenol 1a (15 mol%) in dichlorome-
thane in the presence of molecular sieves. After 5.5 h of stir-
ring, benzaldehyde 2a was added to the pre-catalyst fol-
lowed by allyltributylstannane. The results are summarized
in Table 1. Additives, such as Proton sponge or Hꢁnigꢀs
base, were used to avoid any trace of triflic acid released
50 mol% Hꢁnigꢀs base afforded homoallylic alcohol 3a with
the highest enantioselectivity (Table 1, entries 4 and 5). Ad-
ditionally, the use of anhydrous Bi ACTHUNGTERNN(UNG OTf) led to an increase
3
in yield and a slightly better enantioselectivity was observed
(Table 1, compare entry 5 vs. entry 4).
The influence of the solvent and the ligand structure was
evaluated under the optimum conditions cited in Table 1.
Reactions conducted in THF, in PhMe or neat were not as
efficient as those in CH Cl (Table 2, entries 1–4). Under the
[8]
from the hydrolysis of Bi ACHTUNGETRNN(UNG OTf) in the medium. A good
3
2
2
best conditions in CH Cl , but using differently substituted
[
a] Dr. Z. Li, B. Plancq, Prof. Dr. T. Ollevier
2
2
Dꢂpartement de chimie
Universitꢂ Laval
(R,R)-ProPhenol ligands, the corresponding homoallylic al-
cohol was obtained with a slightly eroded enantioselectivity
1
045 avenue de la Mꢂdecine
(
Table 2, entries 1, 5–7). Additionally, it appeared that a 1:1
Quꢂbec (Quꢂbec) G1V 0 A6 (Canada)
Fax : (+001)418 656 7916
E-mail: thierry.ollevier@chm.ulaval.ca
or 1:2 versus 1:3 metal/ligand ratio led to a slight decrease
[9]
of the enantioselectivity.
In our optimized procedure, the chiral catalyst is prepared
by stirring a mixture of Bi(OTf) with 1a in a 1:3 ratio with
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103646.
AHCTUNGTRENNUNG
3
3144
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3144 – 3147

COMMUNICATION
Table 2. Influence of the solvent and the structure of (R,R)-ProPhenol
ligand on the model reaction.
action also proved to proceed in good yield and high enan-
tioselectivity using naphthyl carboxaldehydes as electro-
philes (Table 3, entries 7–9). Our conditions were further ap-
plied to a heteroaromatic aldehyde, such as 2-thionyl car-
boxaldehyde, affording the corresponding homoallylic
alcohol in a lower yield but in high enantioselectivity
[
a]
(
Table 3, entry 10).
H NMR analysis of the formation of Bi ACHTUNGTERNN(UNG OTf) -1a com-
3
1
plexes with different ratios of Bi
ACHTUNGENTRNUNG( OTf) and 1a was studied
3
(
Figure 1). We focused our study on a specific region of the
1
H NMR spectra (3.7–5.0 ppm), in which two out of the four
1
2
[b]
Entry
R
R
Solvent
Yield 3a
%]
er
[
1
2
3
4
5
6
7
Ph
Ph
Ph
Ph
1-naphthyl
Ph
Ph
Me
Me
Me
Me
Me
F
CH
2
Cl
2
76
55
5
47
30
61
82
94:6
THF
PhMe
–
CH
CH
CH
85:15
77:23
81:19
90:10
91:9
[c]
2
2
2
Cl
Cl
Cl
2
2
2
tBu
93:7
[
a] Conditions: aldehyde (0.5 mmol), allyltributylstannane (1.2 equiv),
4
ꢃ MS (30 mg). [b] Determined by chiral HPLC analysis. [c] Proton
sponge used as the additive.
powdered 4 ꢃ molecular sieves at room temperature in di-
chloromethane for 5.5 h; upon addition of Hꢁnigꢀs base, the
mixture was further stirred for 0.75 h.
The optimal conditions established for the enantioselec-
tive allylation of benzaldehyde 2a were applied to aldehydes
2
b–j (Table 3). The desired homoallylic alcohols 3b–j were
generated in mostly good yields with high enantioselectivi-
ties ranging from 93:7 to 96:4 enantiomer ratio (er). The re-
Table 3. Bismuth triflate-catalyzed enantioselective allylation of aromatic
[
a]
aldehydes.
[
b]
Entry
Ar
Product
Yield 3
%]
er
[
1
2
3
4
5
6
7
8
9
Ph
4-Me-C
4-MeO-C
2-MeO-C
4-nBuO-C
4-Cl-C
1-naphthyl
2-naphthyl
4-methyl-1-naphthyl
2-thionyl
3a
3b
3c
3d
3e
3 f
3g
3h
3i
76
69
56
76
54
77
84
78
79
65
94:6
95:5
93:7
93:7
94:6
94:6
96:4
94:6
95:5
95:5
6
H
4
6
H
H
4
6
4
6
H
4
6
H
4
1
0
3j
[
4
a] Conditions: aldehyde (0.5 mmol), allyltributylstannane (1.2 equiv),
ꢃ MS (30 mg). [b] Determined by chiral HPLC analysis.
Figure 1. Characterization of the chiral bismuth complexes.
Chem. Eur. J. 2012, 18, 3144 – 3147
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3145

T. Ollevier et al.
diastereotopic benzylic protons of the ligand can be ob-
served. When 10% Bi(OTf) was added to a solution of 1a
Bi(OTf) /1a 1:9), a new signal at d=4.6 ppm appeared.
This important deshielding of the benzylic protons would
suggest a strong interaction between Bi and ArOH. The rel-
ative integrations between this new signal and that of the
ligand appeared to be proportionally related in a 2:1 ratio
to the amount of bismuth introduced. These results indicat-
ed that two equivalents of 1a (Ligand (L)) and 1 equivalent
poured over MgSO
4
and filtered through a plug of Celite. The crude ma-
terial was purified by silica gel chromatography (Hexanes/EtOAc) to
afford the desired homoallylic alcohol. The enantiomeric excess of the
product was determined by chiral HPLC analysis. The HPLC conditions
and the spectral data of all compounds are provided in the Supporting
Information.
AHCTUNGTRENNUNG
3
(
ACHTUNGTRENNUNG
3
Acknowledgements
of Bi
from a mixture containing 25% Bi ACTHUNGTNERUNNG( OTf) (Bi ACHTUNGRETNN(GNU OTf) /1a 1:3)
3 3
ACHTUNGTRENNUNG( OTf) afforded a BiL complex. Adding more bismuth
3
2
This work was financially supported by the Natural Sciences and Engi-
neering Research Council of Canada (NSERC), the Fonds Quꢂbꢂcois de
la Recherche sur la Nature et les Technologies (FQRNT, Quꢂbec,
Canada), the Centre in Green Chemistry and Catalysis (CGCC), the
Canada Foundation for Innovation, and Universitꢂ Laval.
resulted in the appearance of another signal at d=4.3 ppm
corresponding to a BiL complex. The hypothesis of these
BiL and BiL complexes was further evidenced by HRMS
2
analyses of the mixture containing 50% Bi
ACTHUNGTRNENUN(G OTf) (Bi ACHTUNGTERNNNU(G OTf) /
3 3
1
a 1:1), showing BiL and BiL both as mono-cationic com-
2
[10]
plexes with the loss of one triflate counter-anion. NMR
results also allowed us to corroborate our results regarding
the Bi/L ratio. Indeed, the best selectivities were obtained
Keywords: allylation · asymmetric catalysis · bismuth ·
homoallylic alcohols · Lewis acids
with a Bi ACHTUNGTRENNUNG( OTf) /1a ratio of 1:3 (25% Bi ACHTUNGTERNN(GNU OTf) ), correspond-
3 3
ing to the presence of complex BiL only. Increasing the
amount of bismuth resulted in the appearance of complex
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2
BiL, which seems detrimental to the selectivity (Bi ACHTUGNTERNNN(UG OTf) /1a
3
9
77–980.
[9]
1
:1; 76:24 er).
In summary, the catalytic enantioselective allylation reac-
tion of various aromatic aldehydes with allyltributylstannane
has been achieved with Bi(OTf) and the chiral (R,R)-
[
2] For selected examples of asymmetric allylation reactions used in
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AHCTUNGTRENNUNG
3
ProPhenol ligand. To the best of our knowledge, this
method is the first allylation reaction of an aldehyde using a
III
chiral Bi complex. High enantioselectivities and good
chemical yields have been obtained with aromatic and heter-
oaromatic aldehydes. The conditions have also been applied
to aliphatic aldehydes. In preliminary experiments, enantio-
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selectivities up to 96:4 have been obtained.
Preliminary
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characterization including H NMR spectroscopy and mass
spectrometry data has been provided as first evidence of the
pre-catalyst structure. The process has the desirable feature
of using a low catalyst loading of an environmentally benign
Lewis acid. In addition, both the Lewis acid and the chiral
ligand are commercially available. The chiral ligand is also
easily recycled. Further studies to clarify the precise mecha-
nism are now in progress.
1
4
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General procedure: In a flame dried vial (12ꢅ35 mm, 0.5 dr) equipped
with triphenylbismuthine (11.1 mg, 0.025 mmol) in freshly distilled
CH Cl (0.25 mL) was added freshly distilled trifluoromethanesulfonic
2 2
acid (6 mL, 0.068 mmol). The solution was stirred for 4.5 h at room tem-
perature under argon atmosphere. Then this catalyst solution was trans-
ferred into a flame dried test tube filled with (R,R)-ProPhenol ligand 1a
49.1 mg, 0.075 mmol) and 4 ꢃ MS (30 mg) in freshly distilled CH
0.25 mL). The obtained mixture was stirred for 5.5 h at room tempera-
ture followed by addition of Hꢁnigꢀs base (44 mL, 0.25 mmol). After
.75 h, the aldehyde (0.5 mmol) and allyltributylstannane (185 mL,
.6 mmol) were added to the above mixture. The reaction was stirred at
room temperature for 65–70 h and quenched by addition of saturated
(
(
2 2
Cl
0
0
NaHCO
3
(0.5 mL). The contents were stirred for another 1 h and then
3146
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ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3144 – 3147

Bismuth Triflate-Catalyzed Asymmetric Allylation
COMMUNICATION
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+
4
0, 4649–4707; d) S. Kobayashi, T. Ogino, H. Shimizu, S. Ishikawa,
[10] BiL: HRMS (ESI-TOF) calcd for C H BiF N O S : 1145.2347
4
5
46
6
2
9 2
+
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[LBi
A
H
U
G
R
N
U
G
2
] ; found: 1145.2353. BiL
2
: HRMS (ESI-TOF) calcd for
+
+
C
88
H
6
N
4
O
12
S
2
: 1783.5856 [L
2
Bi
A
H
U
G
R
N
U
G
2
] ; found: 1783.5871.
[11] The same conditions have been applied without further optimization
to aliphatic aldehydes. In preliminary experiments, the reaction of
allyltributylstannane with cyclohexanecarboxaldehyde and 2-(benzy-
loxy)acetaldehyde furnished the corresponding homoallylic alcohol
with good selectivities (88:12 and 96:4, respectively) and unopti-
mized yields (45 and 28%, respectively).
1
1456–11457; i) P. Rubenbauer, E. Herdtweck, T. Strassner, T. Bach,
Angew. Chem. 2008, 120, 10260–10263; Angew. Chem. Int. Ed. 2008,
4
7, 10106–10109; j) T. Ollevier, Z. Li, Adv. Synth. Catal. 2009, 351,
3
251–3259; k) T. Ollevier, J.-E. Bouchard, V. Desyroy, J. Org.
Received: November 18, 2011
Chem. 2008, 73, 331–334; l) M. Rueping, B. J. Nachtsheim, W. Ieaw-
Published online: February 15, 2012
Chem. Eur. J. 2012, 18, 3144 – 3147
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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