Palladium-catalysed synthesis of allyl acetates from allenes

Article abstract of DOI:10.1016/j.tetlet.2010.05.136

Allyl acetates were synthesised from allenes utilising methodology based on the general reactivity of π-allyl palladium intermediates which participate efficiently in transformations involving nucleophiles. Reactions of allenes and aryl iodides in the presence of AcONa and Pd(OAc)₂/PPh₃ as the catalytic system afforded allyl acetates in moderate to good yields. Monosubstituted allenes, depending on their structure, produced either a separable mixture of two regioisomeric products or a single regioisomer. As allylic acetates can be easily hydrolysed, the methodology is applicable for the synthesis of allyl alcohols as well.

Full text of DOI:10.1016/j.tetlet.2010.05.136

Tetrahedron Letters 51 (2010) 4066–4068
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Palladium-catalysed synthesis of allyl acetates from allenes
b
Suren Husinec a, Milka Jadranin b, Rade Markovic b, Milos Petkovic c, Vladimir Savic c, , Nina Todorovic
*
a Institute for Chemistry, Technology and Metallurgy (ICTM), Centre for Chemistry, PO Box 815, Njegoseva 12, 11000 Belgrade, Serbia
b Faculty of Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia
c Faculty of Pharmacy, University of Belgrade, Vojvode Stepe 450, 11221 Belgrade, Serbia
a r t i c l e i n f o
a b s t r a c t
Article history:
Allyl acetates were synthesised from allenes utilising methodology based on the general reactivity of
-allyl palladium intermediates which participate efficiently in transformations involving nucleophiles.
Received 24 March 2010
Revised 10 May 2010
Accepted 28 May 2010
Available online 1 June 2010
p
Reactions of allenes and aryl iodides in the presence of AcONa and Pd(OAc)2/PPh3 as the catalytic system
afforded allyl acetates in moderate to good yields. Monosubstituted allenes, depending on their structure,
produced either a separable mixture of two regioisomeric products or a single regioisomer. As allylic ace-
tates can be easily hydrolysed, the methodology is applicable for the synthesis of allyl alcohols as well.
Ó 2010 Elsevier Ltd. All rights reserved.
Allyl acetates are important compounds in organic synthesis
and are widely used in C–C and C–X (X = heteroatom) bond forma-
tion. They have been extensively utilised in allylic alkylation
employing, in particular, palladium catalysis,1 but also other tran-
sition metals.2 In these transformations, they undergo metal-pro-
moted nucleophilic displacement with a range of C-, N-, S- and
O-nucleophiles. This very useful methodology has been further
expanded by umpolung using transmetallation of the intermediate
arise from the fact that the last step is an equilibrium and thus
may lead to side reactions of the -allyl palladium intermediates.
p
Intramolecular variations of this approach have been utilised for
the preparation of lactones.10
Our initial experiment was carried out in DMSO as the solvent
at 85–90 °C overnight (12 h) using allene 1, an excess of p-meth-
oxyiodobenzene 2 and Pd(OAc)2 (10 mol %)/PPh3 (20 mol %) as
the catalytic system (Scheme 2). Five equivalents of AcONa were
used in order to shift the equilibrium towards the product and also
to address solubility issues.
The expected product, comprising a separable mixture of com-
pounds 3 and 4, was isolated in 71% combined yield in the ratio
2.7:1. The major product 3 was isolated as a 1.4:1 mixture of
E- and Z-isomers.
p
-allyl palladium species, thus allowing reactions with a range of
electrophiles.3 Related processes of allylic acetates can be effected
by other transition metals, often with no need for transmetalla-
tion.4 An additional important synthetic methodology employing
allylic esters, including allyl acetates, is the Claisen-Ireland 3,3-sig-
matropic rearrangement which produces
c-unsaturated carboxylic
acids or derivatives.5
Most of the procedures for the preparation of allyl esters includ-
ing allyl acetates are based on the acylation of allyl alcohols.
Recently, the direct introduction of an acetoxy group, via Pd-acti-
vation of the C–H allylic position, emerged as an alternative syn-
thetic pathway.6 Substituted allyl acetates can be prepared from
allyl acetate itself employing highly regioselective Pd-catalysed
addition of boronic acids onto the double bond.7 Dienes have also
been shown to be suitable starting materials for the preparation of
allyl acetate derivatives via halo-acetoxylation catalysed by
palladium.8
Ar
_
Ar
Pd
+
R
ArX
R
R
AcONa
R
R
.
R
Pd(0)
OAc
Scheme 1.
OMe
OMe
BnO
.
Pd(OAc)2
Our interest in the chemistry of allyl acetates led us to study the
reactivity of allenes in the presence of nucleophilic acetates as a
method for their synthesis. This approach is based on well-estab-
1
2
+
+
OMe
PPh3
OAc
BnO
BnO
AcONa
DMSO
lished nucleophilic substitution reactions involving
p-allyl palla-
I
dium species as outlined in Scheme 1.9 Potential problems may
OAc
3
85-90 oC
71%
4
* Corresponding author. Tel.: +381 11 3951 243; fax: +381 11 3972 840.
E-mail address: vladimir.savic@pharmacy.bg.ac.rs (V. Savic).
Scheme 2.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.05.136

S. Husinec et al. / Tetrahedron Letters 51 (2010) 4066–4068
4067
Table 1
Table 2
Variation of the reaction conditionsa
Synthesis of allyl acetates from allene 1a
Entry
Catalyst
T (°C)
Solvent
Yieldb (%)
Entry
R
Ratio 6:7c
Yieldb (%)
a
b
c
d
e
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
Pd(OAc)2/PPh3
55–60
rt
85–90
Reflux
85–90
DMSO
DMSO
DMF
MeCN
47
40
37
—
a
2.6 (1.4:1):1
64
64
b
3 (1.2:1):1
1,4-Dioxane
—
Pd(OAc)2/
f
85–90
DMSO
45
c
1.4 (1.8:1):1
63
66
PCy2
OMe
d
1.7 (1.8:1):1
Pd(OAc)2/
g
85–90
85–90
DMSO
DMSO
40
47
Me2N
PCy2
h
Pd(OAc)2/BINAP
e
f
6.5 (0.8:1):1
5.2 (0.8:1):1
60
50
a
Reaction conditions: allene
(2.0 mmol), Pd(OAc)2 (0.04 mmol), ligand (0.08 mmol), solvent (6 mL), 12 h.
1 (0.4 mmol), p-MeOC6H4I (0.6 mmol), AcONa
b
Isolated combined yield after column chromatography.
NO2
g
2.7 (0.9:1):1
31
We attempted to optimise the reaction conditions further by
exploring the effect of various parameters on the reaction and
the results are summarised in Table 1. Performing the reaction
with allene 1 and iodide 2 under the conditions described, but at
lower temperatures (Table 1, entries a and b), afforded significantly
lower yields of the products. Replacing DMSO with solvents such
as DMF, MeCN or 1,4-dioxane (Table 1, entries c, d and e) had a
remarkable effect. While in DMF the yield of the combined prod-
ucts decreased to 37%, reactions carried out in MeCN and 1,4-diox-
ane did not afford the products at all. This suggests that the
solubility of AcONa is essential for the reaction to proceed and
the best results were obtained using DMSO. We also screened sev-
eral ligands (Table 1, entries f, g and h) varying their steric proper-
ties and the coordination mode under the optimal conditions. They
generally afforded lower yields than with Ph3P. This brief study
showed that the initial conditions, described in Scheme 2, are
superior and were used for further investigations.
O
a
Reaction conditions: allene 1 (0.4 mmol), ArI (0.6 mmol), AcONa (2.0 mmol),
Pd(OAc)2 (0.04 mmol), PPh3 (0.08 mmol), DMSO (6 mL), 12 h, 85–90 °C.
b
Isolated combined yield after column chromatography.
The E:Z ratio of compound 6 is given in parenthesis.
c
R
Pd(OAc)2
PPh3
Ph
OAc
N
Ph
N
S
O
.
S
O
O
O
AcONa
DMSO
85-90 oC
+
RI
8
9
Scheme 4.
Reaction of a range of aryl iodides with allene 1 afforded the ex-
pected allylic acetates in moderate to good yields (Scheme 3, Table
2). Aryl iodides with electron-donating substituents (Table 2, en-
tries a–d) were shown to be slightly more reactive than iodides
with electron-accepting moieties (Table 2, entries f and g). As ex-
pected, the major product in all cases was obtained via nucleo-
philic attack of the acetate on the less substituted terminus of
Table 3
Synthesis of allyl acetates from allene 8a
Entry
R
Ratio Z/E
Yieldb (%)
64
a
1:1
the
p-allyl palladium intermediate. The ratio of products 6 and 7
seemed to be influenced by the aryl substituent.
We also briefly investigated the reactivity of amido allene 8. Un-
der the typical conditions, compound 8 reacted with several aryl
iodides producing allylic acetates in good yields (Scheme 4, Table
3). Surprisingly, allene 8 did not afford the internal acetate related
to compound 7. This regioselectivity may be attributed to steric
hindrance caused by the sulfonamide moiety.
Using the above-mentioned method, allylic acetates can be pre-
pared from allenes in good yields, but, since they can be easily
hydrolysed, this method may also be useful for the preparation
b
c
1.1:1
1:1
68
63
MeO
a
Reaction conditions: allene 8 (0.4 mmol), ArI (0.6 mmol), AcONa (2.0 mmol),
Pd(OAc)2 (0.04 mmol), PPh3 (0.08 mmol), DMSO (6 mL), 12 h, 85–90 °C.
b
Isolated yield after column chromatography.
of allyl alcohols (Scheme 5), which are compounds of great impor-
tance in organic synthesis. Hydrolysis of acetates, such as 3 and 7b,
afforded the expected alcohols in good yields. The Z/E ratio of the
starting material 3 did not change during the hydrolyses.
In summary, we have described a procedure for the preparation
of allylic acetates from allenes. Under the optimised conditions
employing Pd(OAc)2/PPh3 as the catalytic system, allenes react
with aryl iodides in the presence of AcONa to afford the corre-
sponding allyl acetates in moderate to good yields. This method
BnO
Pd(OAc)2
.
R
R
PPh3
1
+
RI
5
OAc
+
BnO
BnO
AcONa
DMSO
OAc
6
7
85-90 o
C
Scheme 3.

4068
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MeOH/H2O
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OH
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BnO
BnO
10
3
NaOH
MeOH/H2O
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BnO
BnO
OH
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Acknowledgements
Financial support from the Serbian Ministry of Science (Grants
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