Palladium0-catalyzed isomerization of (Z)-1-functionalized-4-acetoxy-2-butenes: Solvent and substituent effects

Article abstract of DOI:10.1016/j.jorganchem.2009.09.028

The Pd(PPh₃)₄-catalyzed isomerization of (Z)-1,4-diacetoxy-2-butene, (Z)-1-(t-butyldimethylsilyloxy)-4-acetoxy-2-butene and (Z)-1-(t-butyldiphenylsilyloxy)-4-acetoxy-2-butene affords the corresponding (E)-isomers and 1,2-difunctional

Full text of DOI:10.1016/j.jorganchem.2009.09.028

Journal of Organometallic Chemistry 695 (2010) 62–66
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Palladium0-catalyzed isomerization of (Z)-1-functionalized-4-acetoxy-2-butenes:
Solvent and substituent effects
Anna Maria Zawisza a,b, Jacques Muzart b,
*
a Department of Organic and Applied Chemistry, University of Lodz, ul. Narutowicza 68, 90-136 Lodz, Poland
b Institut de Chimie Moléculaire, CNRS-Université de Reims Champagne-Ardenne, UFR Sciences, Boîte no. 44, BP 1039, 51687 Reims Cedex 2, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The Pd(PPh3)4-catalyzed isomerization of (Z)-1,4-diacetoxy-2-butene, (Z)-1-(t-butyldimethylsilyloxy)-4-
acetoxy-2-butene and (Z)-1-(t-butyldiphenylsilyloxy)-4-acetoxy-2-butene affords the corresponding (E)-
isomers and 1,2-difunctionalized-3-butenes. In THF, the formation of the (E)-isomers is mainly due to
Received 18 August 2009
Received in revised form 21 September
2009
Accepted 22 September 2009
Available online 29 September 2009
reaction from an
g g
1-allylpalladium intermediate while an 3-allylpalladium is the main key intermedi-
ate in DMF. The time to reach equilibrium between the products and their respective concentrations
depend on the nature of the substituents and the solvent.
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Catalysis
Allylpalladium
Coordination modes
1,4-Difunctionalized-2-butenes
Isomerization
Solvent effects
1. Introduction
tion of the g
1-allylpalladium intermediate by ligation of the acetate
unit, giving rise to the complex C0Ac possessing a 16-electron
configuration as depicted in Scheme 2. This hypothesis urges us
to investigate the Pd0-catalyzed isomerization of (Z)-1-(t-butyldi-
methylsilyloxy)-4-acetoxy-2-butene (1Sm) and (Z)-1-(t-butyldiphen-
ylsilyloxy)-4-acetoxy-2-butene (1Sp) under the same conditions.
Given the obtained results, we have also monitored the correspond-
ing reactivity of (E)-1-(t-butyldimethylsilyloxy)-4-acetoxy-2-butene
(2Sm) and 1-(t-butyldimethylsilyloxy)-2-acetoxy-3-butene (3Sm).
Here is reported this study.
The 1,3-transposition of allylic acetates in the presence of cat-
alytic amounts of palladium complexes is a well known reaction
[1], which has been extensively used in synthetic organic chemis-
try. We have previously reported the isomerisation of (Z)-1,4-
diacetoxy-2-butene (1Ac) using catalytic amounts of Pd(PPh3)4 or
PdCl2(MeCN)2 in THF and DMF [2,3]. With the Pd0 catalyst, we
have demonstrated that 1Ac was selectively isomerized to
(E)-1,4-diacetoxy-2-butene (2Ac
) in THF while both 2Ac and
1,2-diacetoxy-3-butene (3Ac) were simultaneously obtained in
DMF (Scheme 1) [2].1 A set of experiments, examined in the light
of Amatore and Jutand et al. results [4], has led to conclude to
2. Results
the involvement of an
g
1-allylpalladium complex in the former
The reaction of 1Sm induced by 0.05 equiv. of Pd(PPh3)4 in
refluxing THF afforded (E)-1-(t-butyldimethylsilyloxy)-4-acetoxy-
2-butene (2Sm) and 1-(t-butyldimethylsilyloxy)-2-acetoxy-3-bu-
tene (3Sm) (Eq. (1)). Monitoring the isomerization by GC (Fig. 1
and Table S1)2 showed that the quantity of 2Sm reached a maximum
after 15–20 min and then decreased, leading to an equilibrium
between the three isomers after 50–60 min with an 1Sm/2Sm/3Sm
ratio, noted ESm,THF, estimated to ca. 4:60:36.
solvent, and of an
g
3-allylpalladium complex in the latter as the
key intermediates from 1Ac. We have envisaged that the unexpected
behavior of 1Ac in THF could be due to the intramolecular stabiliza-
* Corresponding author. Fax: +33 3 26913166.
E-mail address: jacques.muzart@univ-reims.fr (J. Muzart).
1
For our previous reported studies, the Pd0-catalyzed isomerization of 1Ac was
monitored using 1H NMR [2]. In repeating the monitoring of this reaction using HPLC,
we have observed that this isomerization led to an equilibrium between 1Ac, 2Ac and
3Ac, the quantity of 1Ac being 2–3%.
2
Given the accuracy of the GC and HPLC analysis, the values of the proportions of
the isomers indicated in Tables S1–S6 are at 1.
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.09.028

A.M. Zawisza, J. Muzart / Journal of Organometallic Chemistry 695 (2010) 62–66
63
Switching from THF to DMF as solvent led at 70 °C to a fast reac-
tion: the proportions of 1Sm, 2Sm and 3Sm did not evolve after 1 min
(Fig. 2 and Table S2), the 1Sm/2Sm/3Sm noted ESm,DMF being ca.
4:70:26.
In THF, the amount of the (E)-isomer from 1Sp reached a maxi-
mum after 25–35 min and the 1Sp/2Sp/3Sp equilibrium
(ESp,THF ꢀ 2:66:32) was attained after ca. 80 min (Fig. 3 and Table
S3). In DMF, the equilibrium (ESp,DMF ꢀ 6:69:25) was attained after
ca. 7 min (Fig. 4 and Table S4).
ð1Þ
The isomerization of 2Sm and 3Sm has been examined only in
THF (Tables S5 and S6). The 2Sm ? 1Sm + 3Sm and 3Sm ? 1Sm + 2Sm
were observed with equilibria between the three compounds at-
tained after 90–220 min.
To resume the previous [2]1 and present results, the reaction
times to attain the equilibrium, noted TEY,solvent, and the corre-
AcO
sponding relative amounts of the three isomers (noted EY,solvent
from 1Ac, 1Sp, 1Sm, 2Sm and 3Sm, are listed in Table 1.
)
THF
DMF
2Ac
OAc
AcO
OAc
1Ac
3. Discussion
+ Pd(PPh3)4 (cat.)
70-72 °C
OAc
From the results assembled in Figs. 1 and 3 and those previously
obtained from 1Ac [2], it appears that the concentration, in THF, of
the (E)-isomer versus time from 1Ac, 1Sm and 1Sp went trough a
maximum before to decrease. Consequently, we assume that the
3Ac
OAc
Scheme 1.
PdL4
L
L
L: PPh3
PdL3
THF
OAc
OAc
AcO
OAc
PdL2
PdL2
1Ac
AcO
AcO
BAc
AAc
L
L
AcO
PdL3
AcO
L
AcO
O
Pd
PdL2
OAc
Me
OAc
2Ac
O
C'Ac
CAc
Scheme 2.
100
90
80
70
60
50
40
30
20
10
0
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240
t (min)
Fig. 1. Evolution over time of the proportions of 1Sm (j), 2Sm (N) and 3Sm (ꢀ) from the Pd(PPh3)4-catalyzed reaction of 1Sm in refluxing THF.

64
A.M. Zawisza, J. Muzart / Journal of Organometallic Chemistry 695 (2010) 62–66
100
90
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
9
10
t (min)
Fig. 2. Evolution over time of the proportions of 1Sm (j), 2Sm (N) and 3Sm (ꢀ) from the Pd(PPh3)4-catalyzed reaction of 1Sm in DMF at 70 °C.
100
90
80
70
60
50
40
30
20
10
0
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240
t (min)
Fig. 3. Evolution over time of the proportions of 1Sp (j), 2Sp (N) and 3Sp (ꢀ) from the Pd(PPh3)4-catalyzed reaction of 1Sp in refluxing THF.
100
90
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
9
10
t (min)
Fig. 4. Evolution over time of the proportions of 1Sp (j), 2Sp (N) and 3Sp (ꢀ) from the Pd(PPh3)4-catalyzed reaction of 1Sp in DMF at 70 °C.
Table 1
Influence of the nature of Y and the solvent on the equilibria.
Substrate
Solvent
1Ac
1Sp
1Sm
2Sm
3Sm
Reaction time for the equilibrium = TEY,solvent
1y:2Y:3Y at equilibrium = EY,solvent
THF
DMF
THF
DMF
ꢀ45 min
ꢀ30 min
ꢀ2:60:38
ꢀ2:62:36
ꢀ80 min
ꢀ7 min
ꢀ55 min
ꢀ1 min
ꢀ100 min
ꢀ200 min
ꢀ2:66:32
ꢀ6:69:25
ꢀ4:60:36
ꢀ4:70:26
ꢀ4:59:37
ꢀ4:59:37

A.M. Zawisza, J. Muzart / Journal of Organometallic Chemistry 695 (2010) 62–66
65
L: PPh3
PdL4
Y = OAc, OSiMe2t-Bu, OSiPh2t-Bu
AcO
L
Y = Ac, Sm, Sp
PdL2
Y
Y
PdL3
Y
L
PdL2
BY
Y
CY
(c)
AcO
(a)
Y
AcO
1Y
PdL2
AcO
AcO
AY
(e)
L
PdL3
2Y
(d)
AcO
PdL2
Y
(b)
L
PdL2
AcO
L
B'Y
OAc PdL3
Y
DY
PdL3
L
Y
OAc
PdL3
3Y
Y
Scheme 3.
formation of 2Ac from an
g
1-allylpalladium rather than from an g3
-
As shown in Table 1, the TEY,solvent depends on Y, in particular
when DMF is the solvent. Indeed, the reaction time in DMF for
the equilibrium varies from a few min when Y = SiMe2t-Bu or
SiPh2t-Bu to 30 min when Y = OAc. The differences are much lower
in THF, in which the main reactive pathway is 1Y ? AY ? BY ?
CY ? 2Y. A possible explanation of the greatest TEAc,DMF could be
the already suspected stabilization of CAc by the acetate unit lead-
ing to C0Ac (Scheme 2). The absence of such stabilization when 1Sm
and 1Sp are the substrates, would facilitate the transformation of
allylpalladium is not due to the possible stabilization of the former
complex by coordination of the OAc unit leading to C0Ac (Scheme
2). As from 1Ac, the shape, in DMF, of the curves corresponding to
the concentration of (E)-isomers 2Sm and 2Sp did not indicate the
presence of a maximum before the equilibrium (Figs. 2 and 4).
Given these observations, a mechanistic scheme common to 1Ac
,
1Sm and 1Sp can be proposed in taking into account the following
Amatore and Jutand remark: ‘‘ions pairs [(g
3-CH2CHCH2)Pd-
(PPh3)2+AcOꢁ] are formed in THF whereas free ions are formed in
DMF” [4,5].
the g g
1-allylpalladium into the 3-allylpalladium, i.e. the
CSm ? DSm and CSp ? DSp pathways, hence a faster equilibrium.
In THF, the main reactive pathway at the beginning of the trans-
formation of 1Y is the formation of 2Y via tight ion pairs AY, BY and
CY as intermediates (Scheme 3, path a). According to previous re-
sults [2] and those from 1Sm, 1Y evolves more rapidly than 2Y. Con-
sequently, a relative high conversion of 1Y is attained before one
observes 1,3-transposition of 2Y into 3Y (path b). Concurrent
reactive pathways leading to 3Y could involve the CY ? DY trans-
formation (path c) and reaction from AY with possibly B0Y as inter-
mediate (paths d and e).
We have previously shown that, in DMF, 2Ac and 3Ac are con-
comitantly produced from 1Ac, the 2Ac/3Ac ratio being ca. 1.7
throughout the entire reaction [2]. While equilibrium from 1Sm is
obtained too rapidly (Fig. 2) to make valuable comments on the
formation of 2Sm and 3Sm, the reaction from 1Sp is, according to
Fig. 4, rather similar to that from 1Ac. Nevertheless, the 2Sp/3Sp ratio
decreased with time from ca. 13 after 1 min of reaction, to 2.8 at
the equilibrium (Table S4).3 The separation of the ions in DMF
[4,5] led the CY ? DY transformation (path c) to effectively compete
with the CY ? 2Y step (path a). Consequently, 2Y and 3Y can be both
produced from DY. The non-linearity with time of the 2Sp/3Sp ratio
led us, however, to suspect that some 2Sp is produced via path a even
in DMF. Since the 2Sp/3Sp ratio is always widely >1, the formation of
3Sp from AY via path d or e is, at the best, a very limited reactive
process.
The dependence of the 2Y/3y ratios with the nature of Y is par-
ticularly observed in DMF (Table 1). This can be explained in con-
sidering the DY ? 2Y + 3Y transformation. Due to its polarity [6],
DMF solvates efficiency the acetate anion [7] yielding bulky nucle-
ophilic species. Consequently, steric repulsions between these spe-
cies and Y increase with the size of this latter leading to a decrease
of the DY ? 3Y reaction at the benefit of the DY ? 2Y reaction,
hence 2Ac/3Ac ratio lower than the 2Sp/3Sp and 2Sm/3Sm ratios. The
acetate anion being less prone to solvation in THF, the OAc/Y inter-
actions are lower and, consequently, the difference in the two reac-
tive pathways occurring from DY is also decreased.
The nature of the solvent may also affect the equilibrium
(EY,solvent) between the three isomers (Table 1). This is highlighted
from 1Sp and 1Sm, and could be due to the coordinating properties
of DMF towards the transition metals [4,5,8,9]. It is known that
DMF can substitute coordinated PPh3 [10], and that the equilibria
attained from the Pd0-catalyzed 1,3-transposition of allylic ace-
tates is ligand dependent [11]. Thus, coordination of DMF to the
palladium intermediates depicted in Scheme 3 can have an effect
on equilibria and reaction rates.
In conclusion, the mechanism of the Pd0-catalyzed isomeriza-
tion of allylic acetates depends on the dissociating, solvating and
coordinating properties of the solvent.
Acknowledgement
3
The isomerization of 1Sp and corresponding GC monitoring have been carried out
twice: similar results have been obtained from one experiment to the other.
We are grateful to CNRS for a temporary position to A.M.Z.

66
A.M. Zawisza, J. Muzart / Journal of Organometallic Chemistry 695 (2010) 62–66
[6] For scales of solvent polarity, see: C. Reichardt, Solvents and Solvent Effects in
Appendix A. Supplementary material
Organic Chemistry, 2nd ed., VCH, Weinheim, 1988. pp. 339–413;
J. March, Advanced Organic Chemistry, 4th ed., John Wiley & Sons, New York,
1992. pp. 360–362.
Supplementary data associated with this article (Tables S1–S6.
Synthesis of 1Sm, 1Sp. Isomerization and analysis procedures.) can
be found, in the online version, at doi:10.1016/j.jorganchem.
2009.09.028.
[7] A.J. Parker, Chem. Rev. 69 (1969) 1–32.
[8] V. Gutmann, Coordination Chemistry in Non-Aqueous Solutions, Springer,
Wien, 1968. pp. 152–154;
T. Hosokawa, T. Nomura, S.-I. Murahashi, J. Organomet. Chem. 551 (1998) 387–
389;
M. Aresta, C. Pastore, P. Giannoccaro, G. Kovács, A. Dibenedetto, I. Pápai, Chem.
Eur. J. 13 (2007) 9028–9034;
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