A molten n-Bu4NOAc/n-Bu4NBr mixture as an efficient medium for the stereoselective synthesis of (E)- and (Z)-3,3-diarylacrylates

Article abstract of DOI:10.1055/s-2002-20460

The palladium-catalyzed reaction of neutral, slightly electron-rich and slightly electron-poor aryl iodides with methyl cinnamate in a molten n-Bu₄NOAc/n-Bu₄NBr 2:1.5 mixture provides a highly stereoselective route to (E)-3,3-diaryla

Full text of DOI:10.1055/s-2002-20460

LETTER
439
A Molten n-Bu4NOAc/n-Bu4NBr Mixture as an Efficient Medium for the Ste-
reoselective Synthesis of (E)- and (Z)-3,3-Diarylacrylates
Stereoselective
Sy
i
nthesis
a
of
(
E
)
-
and
n
(
Z
)-
,
-Dia
f
rtes anco Battistuzzi, Sandro Cacchi,* Giancarlo Fabrizi
Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Università degli Studi ‘La Sapienza’, P.le A. Moro5,
00185 Roma, Italy
Fax +39(06)49912780; E-mail: sandro.cacchi@uniroma1.it
Received 11 January 2002
stereochemistry is concerned, interesting results have
Abstract: The palladium-catalyzed reaction of neutral, slightly
been in some cases obtained. However, DMF or DMA are
electron-rich and slightly electron-poor aryl iodides with methyl
cinnamate in a molten n-Bu4NOAc/n-Bu4NBr 2:1.5 mixture pro-
vides a highly stereoselective route to (E)-3,3-diarylacrylates. Phos-
required as solvents,8–10 hindered tertiary amines are
needed as bases,9 or ortho substituents are necessary on
phine-free Pd(OAc)2 is employed as precursor of Pd(0) species. The the aryl units (either on the aryl acrylate ester or the aryl
iodide)10 and the development of a general, simple and
reaction of a variety of methyl 3-arylacrylates with phenyl iodide
under the same reaction conditions affords stereoselectively the cor-
responding (Z)-isomers. The catalyst system can be recycled several
times.
highly stereoselective process is still an important syn-
thetic target.
We now report our preliminary results on the reaction of
Key words: Heck reaction, stereoselectivity, molten salts, palladi-
aryl iodides with 3-arylacrylates 1 in a molten n-
Bu4NOAc/n-Bu4NBr mixture (Scheme). The reaction is
carried out in the presence of Pd(OAc)2 (no phosphine
ligands are required) and a variety of 3,3-diarylacrylates 3
can be prepared, usually in good to high yield, with high
stereoselectivity.
um, aryl halides
During our studies on the Heck reaction of disubstituted
olefins,1 -substituted2 and -substituted , -unsaturated
carbonyl compounds3 we have found that many variables
may influence the reaction outcome and that the presence
of acetate anions2d,3e–h may have a beneficial effect on the
rate, yield and stereoselectivity of the vinylic substitution
reaction. As to the latter, evidence was attained that the
presence of acetate anions in the Heck reaction of -sub-
stituted , -unsaturated carbonyl compounds may favor
the formation of vinylic substitution products with the
original -substituent on the same side of the carbonyl
group. This was the basis of our domino Heck arylation/
cyclization processes leading to the synthesis of quino-
lines,3h coumarins3h and cardenolides3g by using phos-
phine-free Pd(OAc)2.
Ar
O
X
X
Pd(OAc)2
ArI
OMe +
n-Bu4NOAc/n-Bu4NBr,
100 °C
O
OMe
1
2
3
Scheme
In initial experiments the influence on the reaction course
of solvents and bases was briefly investigated using meth-
yl cinnamate and p-iodoanisole as the model system.
Some of our results are summarized in Table 1.
As expected, with Et3N as the base low stereoselectivity
was achieved (Table 1, entries 1–3). Employment of an
Et3N/HOAc mixture increased the yield as well as the
stereoselectivity (Table 1, entry 4). Switching to KOAc in
DMF resulted in a further increase of the stereoselectivity
(Table 1, entry 5), that reached a satisfactory 94:6 E:Z
ratio11 by using n-Bu4NOAc in DMF (Table 1, entry 6). In
the latter case, however, a significant decrease of the reac-
tion rate was observed. Using n-Bu4NOAc melt both as
solvent and as base provided excellent stereoselectivity
(E/Z = 99:1), but the vinylic substitution product was iso-
lated in moderate yield (Table 1, entry 7), most probably
because of its instability in the reaction medium. This
view is supported by the observation that the vinylic sub-
stitution product and methyl cinnamate were recovered in
only 55 and 35% yield when they were treated with n-
Bu4NOAc in the presence of iodoanisole at 100 °C for 7
hours, omitting palladium acetate.12
Our continuing interest in this olefin chemistry prompted
us to explore the possibility of taking advantage of this ac-
etate effect to develop a simple and stereoselective syn-
thesis of 3,3-diarylacrylates from cinnamate esters. 3,3-
Diaryl acrylates have been employed as intermediates in
the preparation of a variety of pharmacologically active
compounds such as angiotensin II antagonists,4 platelet
activating factor (PAF) antagonists,5 and slow-reacting
substance of anaphylaxis (SRS-A) antagonists.6 This has
provided the stimulus to the employment of the Heck ary-
lation of cinnamate esters in the preparation of this class
of compounds, with the stereoselectivity being the major
task (the usually employed Wadsworth–Emmons
reaction4 affords a 1:1 stereoisomeric ratio). A variety of
reaction conditions have been used7–10and, as far as
Synlett 2002, No. 3, 04 03 2002. Article Identifier:
1437-2096,E;2002,0,03,0439,0442,ftx,en;G00602ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214

440
G. Battistuzzi et al.
LETTER
Table 2 The Palladium-catalyzed Reaction of Aryl Iodides with
Methyl Cinnamate in a Molten n-Bu4NOAc/n-Bu4NBr Mixturea
Table 1 Bases and Solvents in the Palladium-catalyzed Reaction of
p-Iodoanisole with Methyl Cinnamatea,b
Entry Aryl iodide 2
Time Vinylic
E/Z Ratioc
Entry Solvent
Base
Time
(h)
Vinylic
E:Z
Substitution Ratioe
Product
(h)
Substitution
Product 3
Yield %b
Yield %c,d
1
2
3
4
5
6
7
8
p-EtOOC-C6H4-I
p-Cl-C6H4-I
9
38
50
80
82
80
91
70
60
98:2
>99:1
>99:1
99:1
1
2
3
4
5
6
7
8
DMF
MeCN
–
Et3N
84f
84f
84f
48f
72f
168f
7g
40 (58)
27 (72)
25 (75)
87 (9)
65:35
60:40
64:36
75:25
85:15
94:6
6
Et3N
p-MeO-C6H4-I
p-Me-C6H4-I
3
Et3N
3.5
7
–
Et3N/HOAc
KOAc
m-Me-C6H4-I
p-MeCONH-C6H4-I
m-MeO-C6H4-I
p-F-C6H4-I
99:1
DMF
DMF
97 (2)
3
>99:1
>99:1
99:1
n-Bu4NOAc
84 (15)
63 (8)
9
n-Bu4NOAc (3 equiv)
99:1
5.5
n-Bu4NOAc/n-Bu4NBr
4g
61 (4)
99:1
(3 equiv:1.5 equiv)
a Reactions were conducted at 100 °C, under argon, in a n-Bu4NOAc/
n-Bu4NBr mixture with 0.1 g of methyl cinnamate using the following
molar ratios: methyl cinnamate:aryl iodide:Pd(OAc)2 = 1:1.5:0.05.
b Yields refer to single runs and are given for isolated products.
c E/Z ratios were calculated by NMR analyses in C6D6.
9
10
11
n-Bu4NOAc/n-Bu4NBr
(3 equiv:1.5 equiv)
6.5g
50 (3)
99:1
>99:1
97:3
n-Bu4NOAc/n-Bu4NBr
(2 equiv:1.5 equiv)
3.5g
7g
80 (3.5)
87 (10)
n-Bu4NOAc/n-Bu4NBr
(1.5 equiv:1.5 equiv)
Interestingly, no precipitation of palladium was usually
observed. Since this could provide access to recycling
procedures, we decided to assess this possibility and
found that the catalyst system can be recycled several
times without significant loss of activity. For example, the
same catalyst system was used to carry out three prepara-
tions of the Heck product from p-iodoanisole and methyl
cinnamate (80, 82 and 84% yield maintaining in each run
an E:Z > 99:1 ratio) and no precipitation of palladium was
observed.15
a Reactions in molecular solvents were conducted with 0.2 g of methyl
cinnamate in 2 mL of solvent, under argon, using the following molar
ratios: methyl cinnamate:p-iodoanisole:Pd(OAc)2 = 1:1.5:0.02.
b Reactions in molten salts were conducted, under argon, with 0.1 g of
methyl cinnamate using the following molar ratios: cinnamate:p-io-
doanisole:Pd(OAc)2 = 1:1.5:0.05.
c Yields refer to single runs and are given for isolated products.
d Figures in parentheses refer to the recovered methyl cinnamate.
e E/Z ratios were calculated by NMR analyses in C6D6.
f At 80 °C.
As for the stereochemical outcome, equilibration subse-
quent to Heck arylation has been suggested to occur in the
case of reactions of cinnamate esters7–9 and this has been
invoked to account for the observed lack of selectivity.7
Under our conditions, however, it seems that the stereose-
lectivity is originated during the vinylic substitution reac-
tion. This view is supported by the fact that highly
stereoselective preparation of both (E) and (Z) stereoiso-
mers was achieved by selecting proper aryl iodides and
aryl acrylates and by the following experiment. A pure
sample of methyl (E)-3-(p-methoxyphenyl)-3-pheny-
lacrylate was subjected to reaction conditions producing
vinylic substitution products in the presence of methyl
cinnamate and p-acetamidophenyl iodide. Methyl (E)-3-
(p-methoxyphenyl)-3-phenylacrylate was recovered in
quantitative yield and its stereochemistry was maintained,
even under prolonged heating. The 3,3-diarylacrylate
product formed through the reaction of p-acetamidophe-
nyl iodide with methyl cinnamate was isolated in 90%
yield.
g At 100 °C.
Adding n-Bu4NBr to n-Bu4NOAc we were able to limit
this side reaction, still achieving a high E/Z ratio. A 2:1.5
n-Bu4NOAc/n-Bu4NBr mixture provided a reasonable
compromise between stereoselectivity and yield (Table 1,
entry 10) and this ratio was employed when other aryl io-
dides were treated with methyl cinnamate13 (Table 2) and
phenyl iodide was treated with a variety of methyl 3-ary-
lacrylates (Table 3). The latter were readily prepared via
vinylic substitution of methyl acrylate with aryl iodides.14
The reaction produces good results with a variety of neu-
tral, slightly electron-rich and slightly electron-poor aryl
iodides. With aryl iodides bearing strongly electron-with-
drawing substituents the reaction is inefficient (Table 1,
entry 1; the reaction of p-nitrophenyl iodide with methyl
cinnamate gave the Heck product in only 10% yield).
However, these substituents (Table 3, entries 1 and 7) –
along with other important functionalities – are well toler-
ated in the aromatic ring of the starting 3-arylacrylates.
Formation of small amounts of biaryl derivatives has been
observed in some cases.
In effect, stereoisomers might form during the Heck reac-
tion through the well-known elimination–readdition–
elimination of hydridopalladium species involving
-
Synlett 2002, No. 3, 439–442 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Stereoselective Synthesis of (E)- and (Z)- , -Diarylacrylates
441
scope of the present procedure and get a better under-
standing of the catalytic cycle.
Table 3 The Palladium-catalyzed Reaction of Phenyl Iodide with
Methyl 3-Arylacrylates in a molten n-Bu4NOAc/n-Bu4NBr mixturea
Entry
Methyl 3-Arylacry- Time
Vinylic
E/Z Ratioc
late 1 X
(h)
Substitution
Product 3
Yield %b
Acknowledgement
Work carried out in the framework of the National Project ‘Stereo-
selezione in Sintesi Organica. Metodologie ed Applicazioni’ sup-
ported by the Ministero dell’Università e della Ricerca Scientifica e
Tecnologica, Rome. Financial support of this research by the Uni-
versity ‘La Sapienza’, Rome, and by the Consiglio Nazionale delle
Ricerche (CNR) is also gratefully acknowledged.
1
2
3
4
5
6
7
8
9
p-COOEt
p-Cl
3.5
3
84
73
78
72
70
70
70
76
72
4:96
>1:99
2:98
p-OMe
p-Me
4
3.5
6
>1:99
1:99
References
m-Me
(1) (a) Arcadi, A.; Bernocchi, E.; Cacchi, S.; Marinelli, F.;
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G.; Pieper, H.; Guth, B.; Mueller, T.; Weisenberger, J. Eur
Pat. EP 587,134, 1994; Chem. Abstr. 1995, 122, 81372k.
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Tetrahedron 2001, 57, 6071.
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(10) Blettner, C. G.; König, W. A.; Stenzel, W.; Shotten, T.
Tetrahedron Lett. 1999, 40, 2101.
(11) The stereochemistry of vinylic substitution products
obtained via our model reaction was established by NOE
experiments on pure isomers. That of the other vinylic
substitution derivatives has been assigned based on these
data.
(12) Interestingly, methyl 3-(p-methoxyphenyl)-3-
phenylacrylate and methyl cinnamate were recovered in
only 30 and 20% yield, respectively, when they were
separately subjected to n-Bu4NOAc at 100 °C for 7 h
omitting all the other reactants. The corresponding acids
were isolated in 70 and 67% yield.
p-NHCOMe
p-NO2
m-OMe
p-F
5
1:99
3
1:99
5
>1:99
>1:99
6
a Reactions were conducted at 100 °C, under argon, in a n-Bu4NOAc/
n-Bu4NBr mixture with 0.1 g of methyl cinnamate using the follow-
ing molar ratios: methyl cinnamate:aryl iodide:Pd(OAc)2 =
1:1.5:0.05.
b Yields refer to single runs and are given for isolated products.
c E/Z ratios were calculated by NMR analyses in C6D6.
alkylpalladium adducts. An alternative working hypothe-
sis considers isomerization through equilibration between
-palladated esters and palladium O-enolates. The pres-
ence of palladium O-enolates have been invoked in sever-
al cases to account for experimental data16 (they have also
been prepared via the reaction of ArPdBr(bidentate phos-
phine ligand) with the enolate of isobutyrophenone)17 and
may be responsible for the lack of stereoselectivity in this
type of reaction.
Under our conditions, however, acetate anions might play
a key role in controlling the stereoselectivity. Their pres-
ence might favor the irreversible displacement of palladi-
um from -alkylpalladium adducts (possibly through
basic intramolecular attack of the acetate moiety of the
putative -alkylpalladium acetate intermediates on the -
hydrogen),3e thus suppressing both the possible isomer-
ization mechanisms.
Several other details of the mechanism of the present re-
action are likely to be affected by the nature of the reaction
medium. For example, large ammonium cation have been
shown to stabilize palladium-nanoparticles18 and halide
and acetate anions have been reported to stabilize palladi-
um(0) through the formation of anion associated anionic
palladium(0) species.19,20 These effects might play a role
in our procedure, but we have not investigated this point.
To sum up, we have developed a simple, highly efficient
procedure for the stereoselective preparation of both (E)-
and (Z)-isomers of 3,3-diarylacrylates. Recycling of the
catalyst system represents an interesting promising future
of our protocol. Further work is under way to evaluate the
Synlett 2002, No. 3, 439–442 ISSN 0936-5214 © Thieme Stuttgart · New York

442
G. Battistuzzi et al.
LETTER
3.83 (s, 3 H), 3.79 (s, 3 H); 13C NMR: = 167.8, 161.4,
144.5, 129.7, 127.1, 115.3, 114.3, 55.4, 51.6; MS: m/z
(relative intensity) = 192(73) [M+], 161(100), 133(27). Anal.
Calcd for C11H12O3: C, 68.74; H, 6.29. Found: C, 68.60; H,
6.37.
(13) A typical procedure for the preparation of 3 is as follows:
to a stirred solution of methyl cinnamate (0.104 g, 0.641
mmol), p-iodoanisole (0.225 g, 0.962 mmol),
tetrabutylammonium acetate (0.387 g, 1.284 mmol) and
tetrabutylammonium bromide (0.310 g, 0.962 mmol) at
100 °C under argon, palladium diacetate (0.007 g, 0.032
mmol) was added. The mixture was stirred at the same
temperature for 3 h. Then it was diluted with ethyl acetate,
washed with water, dried over Na2SO4, and evaporated
under vacuum. The residue was chromatographed on silica
gel eluting with n-hexane/ethyl acetate (92/8 v/v) to afford
0.134 g (78%) of methyl 3-(p-methoxyphenyl)-3-
phenylacrylate (E:Z > 99:1). A sample was further purified
through preparative HPLC to give pure methyl (E)-3-(p-
methoxyphenyl)-3-phenylacrylate: Mp 70–71 °C; IR (KBr):
1724 cm–1; 1H NMR: = 7.43–7.38 (m, 3 H), 7.30–7.20 (m,
4 H), 6.89–6.82 (m, 2 H), 6.35 (s, 1 H), 3.82 (s, 3 H), 3.62 (s,
3 H); 13C NMR: = 166.6, 160.9, 156.9, 139.1, 133.1, 129.8,
129.1, 128.1, 127.9, 114.7, 113.8, 55.4, 51.2; MS: m/z
(relative intensity) = 268(100) [M+], 237(81), 165(47),
135(36). Anal. Calcd for C17H16O3: C, 76.12; H, 6.01.
Found: C, 76.01; H, 6.12.
(15) Recycles were carried out by extracting the reaction mixture
with diethyl ether and adding n-Bu4OAc to the mixture of
the remaining ammonium salts to restore the proper amount
of acetate anions (during the vinylic substitution reaction
acetate anions are converted into acetic acid, which is lost in
the extraction).
(16) (a) Ito, Y.; Hirao, T.; Saegusa, T. J. Org. Chem. 1987, 43,
1011. (b) Minami, I.; Nisar, M.; Yuhara, M.; Shimizu, I.;
Tsuji, J. Synthesis 1987, 992. (c) Nokami, J.; Mandai, T.;
Watanabe, H.; Ohyama, H.; Tsuji, J. J. Am. Chem. Soc.
1989, 111, 4126. (d) Lemke, F. R.; Kubiak, C. P. J.
Organomet. Chem. 1989, 373, 391. (e) Genet, J. P.; Blart,
E.; Savignac, M. Synlett 1992, 715. (f) Ogoshi, S.;
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Chem. 1993, 58, 9. (g) Sodoeka, M.; Ohrai, K.; Shibasaki,
M. J. Org. Chem. 1995, 60, 2648. (h) Sodoeka, M.;
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Synlett 1997, 463. (i) Shim, J.-G.; Nakamura, H.;
Yamamoto, Y. J. Org. Chem. 1998, 63, 8470. (j) Garg, N.;
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(18) (a) Reetz, M. T.; Maase, M. Adv. Mater. 1999, 11, 773.
(b) Reetz, M. T.; Westermann, E. Angew. Chem. Int. Ed.
2000, 39, 165.
(19) Amatore, C.; Jutand, A. Acc. Chem. Res. 2000, 33, 314.
(20) For other references dealing with the stabilization of
palladium catalysts by halide salts, see: (a) Herrmann, W.
H.; Brossmer, C.; Öfele, K.; Reisinger, C.-P.; Priermeier, T.;
Beller, M.; Fisher, H. Angew. Chem., Int. Ed. Engl. 1995, 34,
1844. (b) Herrmann, W. A.; Boehm, V. P. W. J. Organomet.
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Herrmann, W. A. Chem.–Eur. J. 2000, 6, 1017; see also
ref. 5.
Selected NMR and MS data for (Z)-3-(p-methoxyphenyl)-3-
phenylacrylate: 1H NMR: = 7.36–7.29 (m, 5 H), 7.20–7.15
(m, 2 H), 6.94–6.89 (m, 2 H), 6.29 (s, 1 H), 3.85 (s, 3 H), 3.65
(s, 3 H); 13C NMR: = 166.6, 159.8, 157.1, 141.5, 130.9,
130.8, 129.4, 128.6, 128.3, 116.2, 113.2, 55.2, 51.2; Anal.
Calcd for C17H16O3: C, 76.15; H, 6.02. MS: m/z (relative
intensity)= 268(100) [M+], 237(77), 165(53), 135(30).
(14) A typical procedure for the preparation of cinnamate esters
is as follows: to a solution of methyl acrylate (422 L, 4.68
mmol), p-iodoanisole (365 mg, 1.56 mmol), and Et3N (653
L, 4.68 mmol) in DMF (2 mL) Pd(OAc)2 (7 mg, 0.03
mmol) was added. The solution was stirred at 80 °C for 24 h
under argon. After cooling, the reaction mixture was diluted
with ethyl acetate, washed with water, dried over Na2SO4,
and evaporated under vacuum. The residue was
chromatographed on silica gel eluting with n-hexane/ethyl
acetate (90/10 v/v) to afford 279 mg (93% yield) of methyl
3-(p-methoxyphenyl)acrylate: Mp 87–88 °C; IR (KBr):
1717 cm–1; 1H NMR: = 7.64 (d, 1 H, J = 15.94 Hz), 7.50–
7.43 (m, 2 H), 6.92–6.86 (m, 2 H), 6.30 (d, 1 H, J = 16 Hz),
Synlett 2002, No. 3, 439–442 ISSN 0936-5214 © Thieme Stuttgart · New York