Rapid synthesis of homoallylic alcohol from aldehyde with allyltributylstannane under solvent-free conditions

Article abstract of DOI:10.1139/v11-138

A catalytic amount (2 mol %) of phosphotungstic acid (PTA) is sufficient to synthesize homoallylic alcohol in excellent yields from aldehyde with allyltributylstannane upon grinding under solvent-free reaction conditions. Easy handling, very short reactio

Full text of DOI:10.1139/v11-138

167
Rapid synthesis of homoallylic alcohol from
aldehyde with allyltributylstannane under solvent-
free conditions
Pranjal P. Bora, H. Atoholi Sema, Barisha Wahlang, and Ghanashyam Bez
Abstract: A catalytic amount (2 mol %) of phosphotungstic acid (PTA) is sufficient to synthesize homoallylic alcohol in ex-
cellent yields from aldehyde with allyltributylstannane upon grinding under solvent-free reaction conditions. Easy handling,
very short reaction time, solvent-free reaction conditions, and aqueous workup free isolation protocol may make our method
very useful for synthetic chemists.
Key words: aldehyde, allyltributylstannane, homoallylic alcohol, phosphotungstic acid, grinding.
Résumé : Une quantité catalytique (2 mol %) d’acide phosphotungstique (APT) est suffisante pour effectuer la synthèse
d’alcools homoallyliques, avec d’excellents rendements, par broyage d’un aldéhyde avec de l’allyltributylstannane dans des
conditions réactionnelles sans solvant. La facilité de la manipulation, son temps de réaction très court, les conditions réac-
tionnelles sans solvant et le protocole d’extraction en milieu aqueux pourrait en faire une méthode très précieuse pour les
chimistes de synthèse.
Mots‐clés : aldéhyde, allyltributylstannane, alcool homoallylique, acide phosphotungstique, broyage.
[Traduit par la Rédaction]
whereas transition-metal complexes such as AgOTf, ReBr
(CO)5, and Sc(OTf)3 are very expensive, although water-
tolerant. Although palladium and platinum complexes were
also used as catalysts,29–31 they are highly expensive and re-
quire high temperatures and long reaction times. The use of
solid-supported reagents, such as polymer-bound sulfonamide,32
silica–Bi(OTf)2,33 Ln–resins,34 pincer bis(oxazolinyl)phenyl
ligands on solid support,35 and titanium-exchanged ZSM-5
catalyst36 have also been reported, but those methods re-
quire catalyst preparation before use. Therefore, introduction
of a new catalyst system that can address these drawbacks,
as well as being efficient, selective, high yielding, and envi-
ronmentally friendly, cannot be overlooked. As most of
these reactions have considerably long reaction times and
use hazardous organic solvents, we were looking for a cata-
lyst to reduce the reaction time for this conversion and that
could work in the absence of any solvent. Here, we wish to
report PTA as an effective catalyst for the synthesis of ho-
moallylic alcohol from aldehyde with allyltributylstannane.
Introduction
In recent years, organic synthesis under solvent-free condi-
tions has been considered to be one of the most important
features for designing environmentally benign reaction proto-
cols.1,2 The solvent-free reactions can eliminate a commonly
encountered solubility problem associated with reactivity and
make the work-up process very easy by eliminating tedious
aqueous workup and subsequent removal of solvents. Ironi-
cally, solvent accounts for the high E factor3 of a reaction,
which is a measure of the total waste generated during a re-
action process. In recent years, phosphotungstic acid
(H3PW12O40) has found a lot of attention for being an envi-
ronmentally benign and commercially available solid com-
pound. Being a pseudoliquid,4 phosphotungstic acid (PTA) is
considered to be a better catalyst for organic transforma-
tions5–9 than conventional inorganic and organic acids.
Homoallylic alcohols are considered as one of the most
ubiquitous building blocks in organic synthesis and their ap-
plications in the field of organic total synthesis and pharma-
ceutical research10–12 are well documented. With the advent
of the ring-closing metathesis (RCM) approach,13 homoal-
lylic alcohols and amines are finding new impetus in the syn-
thesis of bioactive natural products.14–16 Acid-catalyzed
reaction of aldehydes with allyltributylsatannanes is one of
the most frequently exercised protocols for generation of ho-
moallylic alcohols at room temperature. Among the Lewis
acids,17–28 BF3·Et2O, SnCl4, TiCl4, etc., are highly moisture
sensitive and hence require strictly anhydrous conditions,
Experimental
General remarks
All reagents were commercially available and used without
further purification. Most of the aldehydes were purchased
from Sigma-Aldrich. The IR spectra were recorded on a Per-
kinElmer 983 spectrophotometer. For column chromatogra-
1
phy, we employed Merck silica gel 60–120 mesh. H NMR
(400 MHz) and 13C NMR (100 MHz) spectra were recorded
Received 23 July 2011. Accepted 2 October 2011. Published at www.nrcresearchpress.com/cjc on 12 December 2011.
P.P. Bora, H.A. Sema, B. Wahlang, and G. Bez. Department of Chemistry, North Eastern Hill University, Shillong-793022, India.
Corresponding author: Ghanashyam Bez (bez@nehu.ac.in).
Can. J. Chem. 90: 167–172 (2011)
doi:10.1139/V11-138
Published by NRC Research Press

168
Can. J. Chem. Vol. 90, 2011
Table 1. Syntheis of homoallylic alcohols.
OH
O
SnBu3
PTA, rt
R
R
H
Time
(min)
10
Entry
Substrate
Product
Yield (%)
90
OH
1
O
1
OH
O
10
12
91
81
2
3
2
OH
O
MeO
3
MeO
OH
OH
O
OH
4
5
6
7
8
10
7
83
95
96
4
OH
O
Cl
5
Cl
OH
O
7
Br
6
Br
O2N
OH
O
5
97
O2N
7
OH
O
5
8
99
82
O2N
8
O2N
OH
O
O
O
O
O
9
9
OH
O
20a
No reaction
10
HO
10
OMe
HO
OMe
O
OH
10
10
89
88
11
12
O
O
11
OH
O
TBSO
12
TBSO
Published by NRC Research Press

Bora et al.
169
Table 1. (concluded).
Time
(min)
10
Entry
13
Substrate
THPO
Product
Yield (%)
83
OH
O
5
THPO
5
13
OH
14
O
10
93
14
OH
O
10
10
95
92
15
16
15
OH
O
16
OH
12
78
17
N
Boc
O
N
17
Boc
Note: Aldehyde–allyltributylstannane–PTA = 1:1.2:0.02, at room temperature. PTA, phosphotungstic acid.
a10 mol % of PTA.
on an AMX-400 MHz spectrometer using CDCl3 as solvent
and TMS as internal standard, unless otherwise stated. Mass
spectra were obtained from a Waters ZQ 4000 mass spec-
trometer by the ESI method and the elemental analyses of
the complexes were performed on a PerkinElmer-2400 CHN/S
analyzer. Please see the Supplementary data for procedures
and spectra.
Spectral data for selected compounds
1-(Benzo[d][1,3]dioxol-5-yl)but-3-en-1-ol (9)
1H NMR (400 MHz, CDCl3, ppm) d: 1.18 (s, 1H), 2.39 (t,
J = 6.8 Hz, 2H), 4.57 (t, J = 6.4 Hz, 1H), 5.05–5.06 (d, J =
4.4 Hz, 1H), 5.07–5.10 (d, J = 11.2 Hz, 1H), 5.60 (m, 1H),
5.87 (s, 2H), 6.65–6.80 (m, 3H). 13C NMR (CDCl3,
100 MHz, ppm) d: 43.8, 73.1, 100.9, 106.3, 108.0, 118.4,
119.2, 134.4, 137.9, 146.9, 147.7. ESI MS (m/z): 215 (M+ + 23).
Elemental anal. calcd for C11H12O3: C 68.74, H 6.29;
found: C 68.61, H 6.12.
Catalyst preparation
Phosphotungstic acid
Analytical grade H3PW12O40·nH2O was obtained commer-
cially and used as a catalyst for the reaction.
1-(4-tert-Butyldimethylsilyloxyphenyl)but-3-en-1-ol (12)
1H NMR (400 MHz, CDCl3, ppm) d: 0.00 (s, 6H), 0.78 (s,
9H), 1.42 (s, 1H), 2.30 (t, J = 6.8 Hz, 2H), 4.48 (t, J =
6.8 Hz, 1H), 4.93 (d, J = 5.2 Hz, 1H), 4.93 (d, J = 5.2 Hz,
1H), 4.97 (d, J = 14.0 Hz, 1H), 5.60 (m, 1H), 6.62 (d, J =
8.8 Hz, 2H), 7.02 (d, J = 8.4 Hz, 2H). 13C NMR (CDCl3,
100 MHz, ppm) d: –4.4, 18.2, 25.6, 43.7, 73.0, 118.2, 119.9,
126.9, 134.6, 136.5, 115.0. ESI MS (m/z): 215 (M+ + 23).
Elemental anal. calcd for C16H26O2Si: C 69.01, H 9.41;
found: C 69.02, H 9.37.
Phosphotungstic acid/SiO2
To a solution of H3PW12O40.nH2O (0.5 g, 0.5 equiv by
weight) in methanol (25 mL) was added silica gel (60–120
mesh, 4.5 g, 0.9 equiv by weight) and the mixture was stirred
at room temperature for 6 h. Evaporation of methanol under
reduced pressure gave a dry white powder. The catalyst was
stored in a container for further use.
General procedure for phosphotungstic acid-catalyzed
synthesis of homoallylic alcohol
12-(Tetrahydro-2H-pyran-2-yloxy)dodec-1-en-4-ol (13)
1H NMR (400 MHz, CDCl3, ppm) d: 1.25–1.30 (m, 10H),
1.45–1.84 (m, 10H), 2.09–2.33 (m, 2H), 3.34–3.4 (m, 2H),
3.47–3.52 (m, 1H), 3.63 (s, 1H), 3.69–3.75 (m, 2H), 3.84–
3.89 (m, 1H), 4.57 (d, J = 4 Hz, 1H), 5.11–5.15 (m, 1H),
5.77–5.88 (m, 1H). 13C NMR (100 MHz, CDCl3, ppm) d:
19.7, 25.5, 25.6, 26.2, 29.4, 29.5, 29.6, 29.7, 30.7, 36.8,
41.3, 62.3, 67.6, 70.6, 98.8, 118, 134.9. ESI MS (m/z):
307.1 (M+ + 23). Elemental anal. calcd for C17H32O3: C
71.79, H 11.34; found: C 70.43, H 11.62.
A mixture of the aldehyde (1 mmol), allyltributylstannane
(1.2 mmol), and phosphotungstic acid (0.02 mmol) was
ground with a pestle in a mortar for the specified time (see
Table 1). After complete conversion of the starting material,
the crude mixture was dissolved in dichloromethane and di-
rectly loaded into a chromatography column charged with
silica gel 60–120 mesh and eluted with a petroleum ether /
ethyl acetate mixture as eluent in the appropriate ratio to ob-
tain the pure product.
Published by NRC Research Press

170
Can. J. Chem. Vol. 90, 2011
Scheme 1. Synthesis of homoallylic alcohols.
Non-1-en-4-ol (14)
1H NMR (400 MHz, CDCl3, ppm) d: 0.62 (t, J = 6.8 Hz,
3H), 1.02–1.10 (m, 4H), 1.13–1.20 (m, 4H), 1.74 (s, 1H),
1.79–1.91 (m, 1H), 2.01–2.06 (m, 1H), 3.36 (m, 1H), 4.86
(m, 2H) 5.57 (m, 1H). 13C NMR (100 MHz, CDCl3, ppm) d:
13.9, 22.5, 25.3, 31.8, 36.7, 41.8, 70.6, 117.8, 134.9. ESI
MS (m/z): 151.1 (M+ + 23). Elemental anal. calcd for
C9H18O: C 76.00, H 12.76; found: C 75.29, H 12.53.
O
OH
SnBu3
H
PTA, rt, 5 min
O2N
O2N
Table 2. Optimization of catalyst for the allylation of aldehyde.
Catalyst
(mol %)
p-Nitrobenzaldehyde
Hexanal time
(min)a
Entry
time (min)a
1-Phenylhexa-1,5-dien-3-ol (16)
1H NMR (400 MHz, CDCl3, ppm) d: 1.86 (s, 1H), 2.26–
2.39 (m, 2H), 4.27 (q, J = 6.4 Hz, 1H), 5.07 (d, J = 4.4 Hz,
1H), 5.10 (d, J = 10.8 Hz, 1H), 5.77 (m, 1H), 6.16 (dd, J =
16.0, 6.4 Hz, 1H), 6.52 (d, J = 16 Hz, 1H), 7.11–7.30 (m,
5H). 13C NMR (100 MHz, CDCl3, ppm) d: 42.0, 71.7,
118.5, 126.5, 127.6, 128.5, 130.3, 131.5, 134.0, 136.6. ESI
MS (m/z): 173 (M+ – 1), 172 (100). Elemental anal. calcd
for C12H14O: C 82.72, H 8.10; found: C 82.49, H 7.85.
1
2
3
4
5
1.0
1.5
2.0
4.0
5.0
120
15
5
5
5
300
25
10
9
9
Note: Aldehyde–allyltributylstannane, 1:1.2.
aThe time for complete conversion.
prise, the reaction was completed within 5 min to give al-
most quantitative yield (Scheme 1).
tert-Butyl 2-(1-hydroxybut-3-enyl)pyrrolidine-1-carboxylate
(17)
1H NMR (400 MHz, CDCl3, ppm) d: 1.40 (s, 9H), 1.68
(m, 2H), 1.81 (m, 2H), 2.09 (m, 2H), 3.18 (m, 3H), 3.45 (m,
1H), 3.82 (s, 1H), 5.01 (d, J = 9.2 Hz, 1H), 5.04 (d, J =
15.6 Hz, 1H), 5.83 (m, 1H). 13C NMR (CDCl3,
100 MHz, ppm) d: 24.2, 27.4, 28.4, 37.1, 48.0, 62.6, 72.7,
79.9, 116.7, 135.6. ESI MS (m/z): 264 (M+ + 23). Elemental
anal. calcd for C13H23NO3: C 64.70, H 9.61, N 5.80; found:
C 64.41, H 9.85, N 6.19.
Correlation of the catalyst ratio and reaction time
The fact that reaction proceeded slowly in the solution
phase in the presence of PTA and silica-supported PTA sug-
gests that the collision among the reactants upon grinding of
the reaction mixture with PTA might have contributed to the
shorter reaction time. It was observed that the reaction does
not occur at room temperature, when a mixture of p-nitrobenz-
aldehyde and allyltributylstannane is ground in the absence
of the catalyst. This established that there is no catalytic ef-
fect produced by the mortar and the pestle that might have
contributed towards the shorter reaction time for comple-
tion. When complete conversion of the starting aldehyde
was studied for the same reaction under catalytic conditions
by varying the catalyst ratio, we found that 2 mol % of the
catalyst is sufficient to complete the reaction within 5 min
(Table 2). It was also observed that an amount of catalyst
<2 mol % leads to increased reaction time for complete
conversion, whereas an increased concentration of the cata-
lyst up to 5 mol % could not reduce the reaction time sig-
nificantly. We also tried to evaluate the reactivity pattern of
aliphatic aldehyde by reacting a mixture of hexanal and al-
lyltributylstannane in the presence of a catalytic amount of
PTA (2 mol %) and found that the reaction takes a compa-
ratively longer time.
Results and discussion
Catalyst testing
In pursuance of our recent interests in PTA-catalyzed reac-
tions,37,38 we sought to use it for the allylation of aldehyde
with allyltributylstannane. To achieve the stated objective,
we stirred a mixture of p-nitrobenzaldehyde (1 mmol) with
allyltributylstannane (1.2 equiv) in acetonitrile in the pres-
ence of 5 mol % of PTA. Upon constant monitoring by
TLC, it was observed that the reaction was completed within
5 h to give 92% yield of the desired product.
As the surface area of PTA is smaller in comparison with
its polyoxometalates, we wanted to know whether silica-
supported PTA could reduce the reaction time. While using
a PTA/SiO2 reagent system, we initially opted to carry out
the reaction in acetonitrile to mix the reacting species prop-
erly and found that the reaction was completed in 5 h to
give 83% isolated yield of the desired product. Since the
solvent is one of the most important contributors to the re-
action rate, we wanted to use solvent-free conditions solely
for carrying out the reaction.39 When p-nitrobenzaldehyde
(1 mmol) and allyltributylstannane (1.2 equiv) were stirred
with the PTA/SiO2 reagent system without using any sol-
vent, complete conversion of the starting aldehyde took
~5 h. As these observations did not warrant any improve-
ment over solution-phase stirring with PTA itself, we
wanted to see the effect of grinding a mixture of aldehyde
and allyltributylstannane with 5 mol % of the catalyst. To
that effect, a mixture of p-nitrobenzaldehyde (1 mmol) and
allyltributylstannane (1.2 equiv) was ground with a pestle in
a mortar in the presence of 5 mol % of PTA. To our sur-
Recovery of the catalyst
Since catalyst recovery is one of the most crucial compo-
nents in catalytic reactions in general, we studied the reus-
ability of the catalyst for successive batches for the reaction
of p-nitrobenzaldehyde and allyltributylstannane using our re-
action protocol. After completion of the reaction, diethyl
ether was added to dissolve the product mixture and filtered
to get the solid PTA residue. The PTA residue was washed
thoroughly with diethyl ether and dried in an oven at 100 °C
for 1 h and then cooled to room temperature. The recovered
PTA catalyst obtained as mentioned was used for second,
third, and fourth batches of the same reactants under similar
reaction conditions without loss of appreciable activity in
terms of reaction time and yield.
Published by NRC Research Press

Bora et al.
171
Generalization of the method
(SAIF)-NEHU is gratefully acknowledged for the analytical
data.
Having standardized the reaction parameters, such as the
catalyst and the process, we set out to explore the general
applicability of our method for both aromatic as well as ali-
phatic aldehydes14 and its compatibility towards acid-sensitive
functional groups (Table 1). It was observed that reaction
worked exceedingly well for aromatic aldehydes having
either +M or –M effect on the phenyl rings. The presence
of phenolic an –OMe group in the para position of the al-
dehyde group did not have any noticable effect on the yield
of the product, which is evident from the excellent yields of
the products (Table 1, entry 3). Ironically, the vanillin (Ta-
ble 1, entry 10) did not react despite the addition of
10 mol % of the catalyst under similar reaction conditions.
It might be due to the poor electrophilicity of the aldehyde
group resulting from the presence of the p-OH and m-OMe
groups having +M effect on the phenyl ring. Several func-
tional groups such as OTHP, methylenedioxy, etc., were
hardly affected by the reaction conditions, as reflected by
their excellent yields. From its excellent yield, we con-
cluded that the OTBS (Table 1, entry 12) group is very stable
under our reaction conditions. We did not observe any signif-
icant differences in reactivity for m- and p-nitrobenzaldehydes
(Table 1, entries 7 and 8), as they took almost a similar re-
action time for complete conversion. It is interesting to note
that the aliphatic aldehydes (Table 1, entries 13–15) also
gave excellent yields of their corresponding homoallylic al-
cohols upon grinding with allyltributylstannane in the pres-
ence of PTA. Excellent yield of homoallylic alcohol from
cinnamaldehyde (Table 1, entry 16) shows that 1,2-addition
is prefered in a,b-unsaturated aldehydes. N-Boc-pyrrolidine-
2-carbaldehyde also generated its corresponding homoallylic
alcohol (Table 1, entry 17) in good yield to verify the fact
that the amide group, a to the aldehyde functionality, hardly
has any negative impact on the efficiency of the reaction
system.
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