Bismuth triflate catalyzed allylation of aldehydes with allylstannane under microwave assistance

Article abstract of DOI:10.1002/ejoc.200700817

In the presence of a catalytic amount of bismuth triflate and under microwave irradiation, mixtures of aldehyde and allyl-stannane afforded smoothly the corresponding homoallylic alcohol. A wide variety of aldehydes were treated under these conditions. The reactions proceeded rapidly and afforded smoothly the corresponding homoallylic alcohol in good to very good yields using catalytic amounts of Bi(OTf)₃·4H₂O (0.5mol-%) and under microwave irradiation for a short time. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200700817

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200700817
Bismuth Triflate Catalyzed Allylation of Aldehydes with Allylstannane under
Microwave Assistance
[a]
[a]
Thierry Ollevier* and Zhiya Li
Dedicated to Barry M. Trost on the occasion of his 65th birthday
Keywords: Allylation / Bismuth / Lewis acids / Microwaves / Allyltin
In the presence of a catalytic amount of bismuth triflate and
under microwave irradiation, mixtures of aldehyde and allyl-
stannane afforded smoothly the corresponding homoallylic
alcohol. A wide variety of aldehydes were treated under
these conditions. The reactions proceeded rapidly and af-
forded smoothly the corresponding homoallylic alcohol in
good to very good yields using catalytic amounts of Bi(OTf)
4H O (0.5 mol-%) and under microwave irradiation for a
short time.
3
·
2
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Introduction
As a part of our ongoing interest in bismuth(III)-cata-
[
15]
lyzed Sakurai reactions,
we report herein our results in
The development of new methods for the preparation of
homoallylic alcohols is an important area of synthetic ef-
the bismuth(III)-catalyzed allylation of carbonyl com-
pounds by allyltributylstannane. This is the first disclosed
microwave-enhanced bismuth-catalyzed Sakurai reaction.
Homoallylic alcohols are obtained efficiently in the pres-
ence of 0.5 mol-% of Bi(OTf) ·4H O. To the best of our
[
1]
forts. Homoallylic alcohols are extremely important as
[
2]
precursors of biologically active molecules. Among the
variety of synthetic methods so far reported, the reaction
of carbonyl compounds with allylsilanes and -stannanes
provides an efficient route for the synthesis of homoallylic
3
2
knowledge, a truly catalytic version of this bismuth(III)-me-
diated reaction has never been reported. The bismuth(III)-
catalyzed allylation reaction of aldehydes has only been re-
ported in two very specific cases. A first report disclosed the
allylation using allyltributylstannane involving a carboxylic
acid, like benzoic acid, used in a stoichiometric quantity as
[
3]
alcohols. Catalytic allylation reactions have been reported
by several groups as an efficient method to prepare homoal-
[
4]
lylic alcohols.
Recently, several laboratories have disclosed significant
advances regarding rare-earth and lanthanide triflates as
catalysts for allylation reactions.[ High catalytic activity,
low toxicity, moisture and air tolerance make lanthanide
triflates valuable catalysts. However, the high cost of these
catalysts restricts their use. Bismuth compounds provide a
good alternative as they have recently attracted attention
[7f]
a co-catalyst. More recent work by Mohan reported the
5]
efficient preparation of homoallylic ethers and acetates
[7b,7c]
using allylsilanes in a Bi(OTf) -catalyzed process.
3
However, the direct bismuth(III)-catalyzed allylation of al-
dehydes with allylsilanes leading to homoallylic alcohols
has never been achieved because it is a slow reaction and
[
6]
due to their low toxicity, low cost, and good stability. Bis-
muth salts have been reported as catalysts for the Sakurai
[7b]
diallylation occurs as a major side reaction.
We have in-
deed shown that the Bi(OTf) -catalyzed allylation of benz-
3
[
7]
[8]
reaction, opening of epoxides, Mukaiyama aldol reac-
aldehyde led to a poor yield of the corresponding homoal-
lylic alcohol [with 1.5 equiv. allyltrimethylsilane, 5 mol-% of
Bi(OTf) ·4H O, 22 °C, 4 h, yield of homoallylic alcohol
[
9]
[10]
tion, Mannich-type reactions,
formation or deprotec-
[
11]
[12]
tion of acetals, Friedel–Crafts reactions, and Fries and
3
2
Claisen rearrangements.[ Bi(OTf) is particularly attract-
13]
3
25%, homoallylic alcohol/diallylation product = 10:1; with
ive because it is commercially available or can be easily pre-
1
2
.0 equiv. allyltributylstannane, 5 mol-% of Bi(OTf) ·4H O,
3 2
pared from commercially available compounds.[
14]
2 °C, 24 h, yield of homoallylic alcohol 30%].
Microwave-assisted organic synthesis is currently gaining
importance in synthetic chemistry largely due to the ad-
vancement in the technology that provides precision-con-
[
a] Département de Chimie, Université Laval, Québec
Québec, Canada G1K 7P4, Canada
Fax: +1-418-656-7916
[
16]
trolled microwave equipments.
Previous studies showed
E-mail: thierry.ollevier@chm.ulaval.ca
that various bismuth-catalyzed reactions can be conducted
Eur. J. Org. Chem. 2007, 5665–5668
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5665

T. Ollevier, Z. Li
SHORT COMMUNICATION
under microwave irradiation.[17] We wish to disclose our re-
At first, we screened various solvents for the Sakurai re-
sults in this area, i.e, the development of an efficient and action of benzaldehyde with allyltributylstannane in the
mild, microwave-assisted bismuth-catalyzed Sakurai reac- presence of 1 mol-% of Bi(OTf) ·4H O (Scheme 2). Inter-
3
2
tion using allyltributylstannane.
estingly, when 1a was treated with 1 mol-% of Bi(OTf)3·
H O in CH Cl (1.5 equiv. allyltributylstannane, micro-
4
2
2
2
waves, 5 min), the corresponding homoallylic alcohol 3a
was isolated in low yield (16%) (Table 2, Entry 1). A less
polar solvent like toluene or ethereal solvents (THF, ether)
did not give good yields of the expected product (Table 2,
Entries 2–4). Among various solvents tested, acetonitrile
furnished the expected product in the highest yield using a
temperature of 160 °C (Table 2, Entry 5), a lower tempera-
ture leading to a decrease in yield (Table 2, Entry 6). Thus,
acetonitrile was selected as the solvent of choice.
Results and Discussion
Initial investigations in the allylation of aldehydes with
allyltributylstannane involved benzaldehyde as the model
substrate (Scheme 1, Table 1). Reaction times as short as
3–5 min allowed a full conversion, and the corresponding
homoallylic alcohol was obtained in a good yield (Table 1,
Entries 3 and 4). While BiCl gave almost no conversion
3
(Table 1, Entry 2), the corresponding homoallylic alcohol
3a could be obtained in good yield with 1 mol-% of Bi(OTf)3·
4
H O as the catalyst (Table 1, Entry 3). A lower loading of
2
Bi(OTf) ·4H O (0.5 mol-%) led to an increased yield (85%
3
2
of 3a) (Table 1, Entry 5). Other metal triflate catalysts, e.g.,
Sc(OTf)3 and Zn(OTf) , appeared to be as efficient as
2
Bi(OTf) ·4H O but at higher loading (Table 1, compare
3
2
3
Scheme 2. Bi(OTf) -catalyzed microwave-assisted allylation of
benzaldehyde: effect of solvent and temperature.
Entries 6–7 with Entry 5). As an increase of the quantity of
allyltributylstannane used led only to a slightly increased
yield (2.0 equiv. of allyltributylstannane, 82% of 3a;
1.5 equiv. of allyltributylstannane, 77% of 3a), 1.5 equiv. of
allyltributylstannane was systematically used for our opti-
mization studies (Tables 1 and 2).
Several examples of Bi(OTf) ·4H O-catalyzed Sakurai re-
3
2
actions of various aldehydes 1 with allyltributylstannane are
summarized in Table 3 (Scheme 3). A variety of differently
substituted benzaldehydes were treated in acetonitrile under
our optimized conditions, except regarding the quantity of
the allyltributylstannane used (only 1.0 equiv.). With both
electron-poor, electron-rich and o-substituted benzalde-
hydes, the corresponding homoallylic alcohols 3 were ob-
tained in moderate to excellent yields (Table 3, Entries 1–6).
In addition, heteroaromatic aldehyde 1g could also serve as
a substrate in this reaction, albeit giving the corresponding
homoallylic alcohol in a moderate yield (Table 3, Entry 7).
Our conditions proved to be mild enough to prevent furane
from side reactions such as polymerization frequently en-
Scheme 1. Metal salt catalyzed microwave-assisted allylation of
benzaldehyde.
Table 1. Metal salt catalyzed allylation of benzaldehyde under mi-
crowave assistance.
x mol-% Time [min] Yield 3a [%][a]
Entry
Catalyst
[
18]
countered with furane compounds.
Aliphatic aldehydes
1
2
3
4
5
6
7
–
BiCl
–
1
1
1
0.5
1
5
5
5
3
5
5
5
4[b]
7
77
80
85
86
85
led to a moderate to good yield of 3 (Table, Entries 8–10).
Given that the allylation reaction with 1a still proceeds
in the presence of a proton scavenger [1 equiv. of 1a,
3
Bi(OTf)
Bi(OTf)
Bi(OTf)
3
3
3
·4H
·4H
·4H
2
2
2
O
O
O
1
4
1
6
equiv. of allyltributylstannane 2, 0.5 mol-% of Bi(OTf) ·
3
Sc(OTf)
Zn(OTf)
3
1
1
8
8
H O, 1.5 mol-% of N ,N ,N ,N -tetramethylnaphthalene-
2
2
1
®
,8-diamine proton sponge , 160 °C, microwaves, 5 min,
1
[
a] Isolated yield. [b] Yield determined by H NMR spectroscopy.
9% of 3a; or 1 equiv. of 1a, 1 equiv. of allyltributylstan-
nane 2, 0.5 mol-% of Bi(OTf) ·4H O, 1.5 mol-% K CO ,
3
2
2
3
160 °C, microwaves, 5 min, 66% of 3a], a Lewis acid is
3
Table 2. Bi(OTf) -catalyzed allylation of benzaldehyde under mi-
clearly involved in the mechanism. However, when HOTf is
used as the catalyst, the reaction proceeds to afford the ex-
pected product 3a, indicating that HOTf could also be a
catalyst for the transformation (1 equiv. of 1a, 1 equiv. of
allyltributylstannane 2, 1.5 mol-% of HOTf, 160 °C, micro-
waves, 5 min, 81% of 3a). This observation suggests that
HOTf is likely to be involved to some extent as a Brønsted
acid as well. More detailed investigations on the mechanism
of this transformation are in progress.
crowave assistance: effect of solvent and temperature.
Entry
Solvent
CH Cl
PhMe
THF
Et O
2
MeCN
MeCN
Temp [°C]
Yield 3a [%][a]
1
2
3
4
5
6
2
2
160
160
160
120
160
80
16
23
16
7
77
11
[a] Isolated yield.
5666
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Eur. J. Org. Chem. 2007, 5665–5668

Bismuth Triflate Catalyzed Allylation of Aldehydes
Table 3. Bi(OTf)
microwave assistance.
3
-catalyzed allylation of various aldehydes under alternative to other Lewis acids and as a valuable surrogate
for HOTf since the latter is very corrosive and difficult to
handle. Because of its numerous benefits, this method for
the straightforward synthesis of homoallylic alcohols using
Bi(OTf) ·4H O catalysis should find utility in the synthesis
3
2
of biologically active compounds. Research is on its way to
demonstrate other significant applications of this Bi(OTf)3·
4
H O-catalyzed Sakurai reaction.
2
Experimental Section
General Procedure for the Microwave-Enhanced Bismuth Triflate-
Catalyzed Allylation of Aldehydes: All reactions were performed
using a 400 W Biotage Initiator microwave synthesis instrument.
In the capped microwave reactor, the aldehyde (0.5 mmol), the allyl-
3 2
tributylstannane (0.5 mmol), and Bi(OTf) ·4H O (0.0025 mmol)
were mixed together in dry MeCN (1 mL) under nitrogen. The
solution was then brought up to 160 °C for 5 min under microwave
irradiation. The resulting mixture was concentrated directly in a
rotary evaporator to afford the crude product which was purified
by column chromatography [eluent: hexane/EtOAc (95:5) or tolu-
ene]. Spectral data agree with those previously reported in the lit-
erature.[
19]
Acknowledgments
This work was financially supported by the Natural Sciences and
Engineering Research Council of Canada (NSERC), the Fonds
Québécois de la Recherche sur la Nature et les Technologies
(FQRNT, Québec, Canada), the Canada Foundation for Innova-
tion, and the Université Laval. We thank Rhodia (Lyon, France)
and Sidech s. a. (Tilly, Belgium) for a generous gift of chemicals.
We appreciate the help and support provided by R. Kadri (Biotage
AB).
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Received: September 4, 2007
Published Online: October 19, 2007
5668
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Eur. J. Org. Chem. 2007, 5665–5668