Palladium-catalyzed benzylic-like nucleophilic substitution of benzofuran-, benzothiophene- and indole-based substrates by dimethyl malonate anion

Article abstract of DOI:10.1016/S0022-328X(03)00728-9

The palladium-catalyzed benzylic-like nucleophilic substitution of acetates derived from benzofuran, benzothiophene and indole was investigated. The asymmetric substitution on racemic 1-(2-benzofuryl) ethyl acetate gave disappointing results, but the subs

Full text of DOI:10.1016/S0022-328X(03)00728-9

Journal of Organometallic Chemistry 687 (2003) 353ꢀ364
/
www.elsevier.com/locate/jorganchem
Palladium-catalyzed benzylic-like nucleophilic substitution of
benzofuran-, benzothiophene- and indole-based substrates by
dimethyl malonate anion
Gaelle Primault, Jean-Yves Legros *, Jean-Claude Fiaud *
¨
Laboratoire de Catalyse Mole´culaire associe´ au C.N.R.S., UMR C8075, Institut de Chimie Mole´culaire et des Mate´riaux d’Orsay, Baˆtiment 420,
Universite´ de Paris-Sud, F91405 Orsay Cedex, France
Received 30 May 2003; received in revised form 22 July 2003; accepted 22 July 2003
Dedicated to Professor J.P. Geneˆt on the occasion of his 60th birthday
Abstract
The palladium-catalyzed benzylic-like nucleophilic substitution of acetates derived from benzofuran, benzothiophene and indole
was investigated. The asymmetric substitution on racemic 1-(2-benzofuryl)ethyl acetate gave disappointing results, but the
substitution product was obtained in 98% ee from (S)-1-(2-benzofuryl)ethyl acetate with overall retention of configuration.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Palladium; Benzylic-like nucleophilic substitution; Cꢀ/C bond formation; Benzofuran; Benzothiophene; Indole
1. Introduction
thalene (152 and 255 kJ molꢁ1, respectively [9]), the
formation of the h3-benzylpalladium in the first step of
We recently described the formation of benzylic
carbonꢀcarbon [1ꢀ5], carbonꢀhydrogen [6,7] and
carbonꢀnitrogen [8] bonds by palladium-catalyzed nu-
this transformation requiring partial lost of aromatic
stabilisation is less favorable for complex 7 than for its
naphthylmethyl counterpart 5.
/
/
/
/
cleophilic substitution of esters with fused six-membered
aromatic rings like naphthalene (1), phenanthrene (2),
quinoline (3) and isoquinoline (4) (Scheme 1). Optically
active substitution products were obtained from enan-
tiopure substrates in the presence of an achiral catalyst
[2,5] or by asymmetric catalysis from racemic esters
[3,5].
Furan is an heteroaromatic compound with a weak
resonance energy (67 kJ molꢁ1 [9]), so we expected that
furylmethanol derivatives could behave as compounds
1ꢀ
/
4. But furylmethyl methyl carbonate 8 gave only a
25%) of substitution product 9, the major
low yield (B
/
pathway being a degradation of the substrate in the
reaction conditions (Scheme 3) [10].
We postulated for this substitution a mechanism
involving a cationic h3-benzylic palladium complex 5
as depicted for the formation of 6 from 2-naphthyl-
methyl acetate (1a) (Scheme 2). The inertness of benzyl
acetate under the same reaction conditions strongly
We report in this article that introduction of an
additionnal benzenic ring (as for naphthylmethyl com-
pounds compared to benzyl substrates) allows the
extension of the palladium-catalyzed benzylic nucleo-
philic substitution reaction to derivatives of benzofuran
and to a lesser extent benzothiophene and indole. Some
preliminary experiments were also carried out in order
to obtain an enantiomerically enriched compound
including a benzofuran moiety.
support the analogy with the Tsujiꢀ/Trost reaction. If we
consider the resonance energies of benzene and naph-
* Corresponding authors. Tel.: ꢂ
/
33-1-6915-4736; fax: ꢂ33-1-6915-
/
It should be noticed that to our knowledge a single
example of this type of transformation was reported in
the literature albeit in low yield in the synthesis of
4680.
E-mail addresses: jylegros@icmo.u-psud.fr (J.-Y. Legros),
fiaud@icmo.u-psud.fr (J.-C. Fiaud).
0022-328X/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00728-9

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Scheme 1. Palladium-catalyzed nucleophilic substitution of fused six-membered aromatic ring substrates.
Scheme 2. Mechanism of palladium-catalyzed benzylic substitution.
inverto-yuehchuene using a N-protected indolylzinc
chloride as nucleophile [11].
propynol and but-3-yn-2-ol respectively, followed by in
situ cyclization [12]. The intermediate alcohols were
directly treated by acetic anhydride in the presence of
LiCl [13] to give acetates 10a and 11 (Scheme 4).
Enantiomerically pure (S)-11 was obtained from (S)-
but-3-yn-2-ol in comparable yield.
2. Preparation of the substrates
By using the same procedure, 4-chloro-2-bromophe-
nol gave compound 10b in only 15% yield, probably due
The benzofuran derivatives 10a and 11 were obtained
by a Sonogashira coupling between 2-iodophenol and

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Scheme 3. Attempted palladium-catalyzed substitution of carbonate 7.
(a) Two equivalents of NaCHE2, 2 mol% Pd(dba)2, 3 mol% dppe,
DMF, 80 8C, 24 h (B25%).
/
Scheme 5. Preparation of 2-(7-methoxy)benzofurylmethyl acetate 10c.
(a) BH3.THF, THF, 20 8C. (b) Ac2O (excess), LiCl (0.1 equivalents),
20 8C.
to a more difficult oxidative addition of palladium onto
the bromide compared to iodide. At higher (100 8C)
temperature and higher (10 mol%) catalyst loading, 10b
was obtained in a moderate 34% yield.
10c was obtained from the borane reduction followed
by acetylation of commercially available 7-methoxy-2-
benzofurancarboxylic acid (Scheme 5).
3. Palladium-catalyzed substitution reaction of esters
10
/
14
ꢀ
We first performed the palladium-catalyzed reaction
of acetate 10a under the conditions which were optimal
on naphthalene-derived substrates [2] (Scheme 9). We
observed that beside the expected substitution product
15a, a substantial amount of disubstitution compound
16 was produced. This double alkylation of a malonate
nucleophile was never observed on naphthalene sub-
strates but have some precedent in p-allylpalladium
chemistry [17,18]. However this behaviour is quite
surprising in the presence of an excess amount of
nucleophile as under our conditions. With less than
two equivalents of nucleophile, a total conversion of
acetate 10a could not be obtained even after 48 h of
reaction.
As described for benzofuran [14], lithiation of ben-
zothiophene, which occurred on 2 position [15], fol-
lowed
by
addition
of
DMF
gave
2-
benzothiophenecarboxaldelyde in good yield (Scheme
6). Reduction by sodium borohydride or reaction with
methyl Grignard reagent afforded after acetylation 12
and 13, respectively.
Esterification by diazomethane followed by LAH
reduction of commercially available 2-indolecarboxylic
acid afforded 2-hydroxymethylindole in good yield [16].
Acetylation was attempted as above (Ac2O in presence
of catalytic LiCl) but a very low yield of acetate 14a was
obtained, so we turned back to more classical conditions
(Ac2O, Et3N, catalytic DMAP). With one equivalent of
acetylating reagent, an inseparable mixture of unreacted
alcohol, expected 14a and diacetyled 14b was obtained
The ratio of the two products 15a and 16 was
1
determined from H-NMR spectrum. The selectivity in
favour of product 15a was slightly higher in THF than
in DMF. The reaction was complete in less than 4 h: the
reactivity of 10a is higher than naphthalene [1,2],
quinoline and isoquinoline-based [4] substrates which
required at least 24 h for total conversion.
1
in a 24:54:22 ratio from H-NMR spectrum. An excess
of acetic anhydride cleanly produced 14b (Scheme 7).
N-Methyl derivatives 14c and 14d were obtained by
reduction/esterification of commercial N-methylindole-
carboxaldehyde (Scheme 8).
Compounds 15a and 16 were difficult to separate (Rf
values of 0.52 and 0.62, respectively, eluent heptane/
Scheme 4. Preparation of benzofuran substrates 10aꢀ/b and 11 by Sonogashira coupling. (a) 3 mol% Pd(PPh3)2Cl2, 4 mol% CuI, Et3N, DMF, 60 8C.
(b) Ac2O (excess), LiCl (0.1 equivalents), 20 8C. (c) 10 mol% Pd(PPh3)2Cl2, 4 mol% CuI, Et3N, DMF, 100 8C.

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Scheme 6. Preparation of benzothiophene substrates 12 and 13 via lithiation. (a) n-BuLi, THF, 0 8C. (b) DMF (three equivalents), 20 8C. (c) for 12:
NaBH4, MeOH, 20 8C. (d) For 13: MeMgBr (1.1 equivalents), Et2O, 0 8C. (e) Ac2O (excess), LiCl (0.1 equivalents), 20 8C.
AcOEt 80:20). After careful flash-chromatography, 15a
could be isolated in only 37% yield.
sodium hydride for the deprotonation of dimethyl
malonate.
The formation of compound 16 involved the follow-
ing steps: (i) palladium-catalyzed substitution on 10a
On the other hand, compound 16 was isolated in 41%
yield from the palladium-catalyzed reaction of the
conjugated base of 15a (two equivalents) with acetate
10a.
producing 15a via intermediate 17 (Scheme 9); (ii) acidꢀ
/
base exchange between 15a and sodium dimethyl mal-
onate; (iii) second substitution reaction involving nu-
cleophilic attack of the conjugated base of 15a on
complex 17.
The reaction conditions determined above were
applied to the other primary substrates. Benzofurans
10b and 10c were both less reactive than unsubstituted
10a (Scheme 10). After 48 h, the conversion was only
67% for 10b and 70% for 10c. If the reaction was
selective in the case of the methoxy-substituted sub-
strate, giving 15c in 44% isolated yield, the formation of
15b in the former case was accompagnied by a reduction
One of the factors which governs the selectivity of the
reaction is undoubtedly the position of the equilibrium
between 15a and its conjugated base. Therefore, we
studied the influence of the initial malonate/deproto-
nated malonate ratio: the nucleophile was prepared by
adding n mmol of dimethyl malonate on a suspension of
NaH (2 mmol) in THF. The substitution reaction was
conducted on 1 mmol of substrate 10a (Table 1).
As expected, an increasing amount of dimethyl
malonate in the reaction medium induced a better
selectivity in favor of 15a, since the equilibrium (ii)
was shifted towards the left. The concentration of the
conjugated base of 15a was lowered and reaction
producing 16 slowed down.
of the substrate leading to 18 (15b/18 ratioꢃ
2 from 1H-
NMR) and the yield of isolated 15b was only 26%. Such
/
a reduction reaction was already observed by us on
quinolylmethyl acetates 3 (RꢃH) [4] although the
/
origin of the hydride source was not yet determined.
The product arising from double substitution analogous
to 16 was never detected in both cases.
The above proportions were determined by 1H-NMR,
so there is no significant difference between the use of
four and six equivalents of dimethyl malonate. In order
to avoid the presence of a too large excess of dimethyl
malonate at the end of the reaction (which could be
detrimental to the isolation of product 15a), we chose to
conduct the reactions with four equivalents in the
following.
The influence of the counter ion was also briefly
studied, the ratio 15a/16 could not be increased, since it
was 83/17 and 90/10, respectively, using potassium t-
butanoate and n-butyllithium as bases instead of
Scheme 8. Preparation of indole substrates 14cꢀd via reduction of N-
/
methylindolecarboxaldehyde. (a) NaBH4 (0.6 equivalents), MeOH,
20 8C. (b) for 14c: Ac2O (1.1 equivalents), Et3N, DMAP (0.1
equivalents), Et2O, 20 8C. (c) for 14d: t-BuCOCl (1.1 equivalents),
Et3N, DMAP (0.1 equivalents), Et2O, 20 8C.
Scheme 7. Preparation of indole substrates 14aꢀb via reduction of 2-indolecarboxylic acid. (a) CH2N2, Et2O, 20 8C. (b) LiAlH4 (one equivalent),
/
Et2O, 0 8C. (c) For 14a: Ac2O (excess), LiCl (0.1 equivalents), 20 8C. (d) For 14b: Ac2O (2.2 equivalents), Et3N, DMAP (0.1 equivalents), Et2O,
20 8C.

G. Primault et al. / Journal of Organometallic Chemistry 687 (2003) 353ꢀ
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357
Scheme 9. Palladium-catalyzed substitution of 10a. (a) Two equivalents of NaCHE2, 2 mol% Pd(dba)2, 3 mol% dppe, solvent, 60 8C. DMF, 24 h:
15a/16ꢃ73/27. THF, 4 h: 15a/16ꢃ80/20.
/
/
The reactivity of benzothiophene derivative 12 was
also considerably slower than for benzofuran one 10a
since after 48 h of reaction, the conversion reached only
selectivity since a new side reaction appeared: after
nucleophilic attack of malonate anion on carbonyl of
14c, the produced alkoxide performed a transesterifica-
tion reaction on another molecule of dimethyl malonate
to produce 23. This result indicated a very slow
palladium-catalyzed process on 14c. Like with sub-
strates 10b and 10c, no product resulting from double
substitution was detected. Similar results were recorded
in THF at 60 8C. To suppress the competitive transes-
terification process, the carbonyl of the substrate was
protected by a t-butyl group. The reaction of pivalate
14d gave clean and selective substitution and 22 was
isolated in 57% yield.
Finally the case of secondary substrates 11 and 13 was
considered (Scheme 13). The steric hindrance developed
by the added methyl group on the acetates disfavored an
a priori double substitution reaction, so we turned back
to the initial conditions (no excess of dimethyl malonate
with regard to base). In fact, the only observed side
reaction was as expected the elimination, producing
vinylheteroaromatic compounds 26 [20] and 27 [21] as in
1
39% (from H-NMR) and the selectivity in favor of the
monosubstitution product was poorer since the 19/20
ratio was 84/16 (Scheme 11). Here again, the order of
reactivity seems to be dictated by the relative stabilisa-
tion of the substrate since the resonance energies are in
the same order for benzo derivatives as for monocyclic
heterocycles [19].
The same reaction was conducted at higher (80 8C)
temperature in DMF. If the reactivity (complete con-
version) and the selectivity in favour of simple substitu-
tion (only 3% of double substitution product 20), were
good in these conditions, the isolated yield of 19 was
only 48% because of a partial de-methoxycarbonylation
reaction producing 21. The same process occurred on
10a when the reaction was performed in these conditions
(DMF at 80 8C).
We next examined the behaviour of indole derivatives
under the same reaction conditions. Substitution did not
take place on 14a and 14b and in both cases acetate 14a
was recovered from the reaction medium, resulting
probably from a deprotonation on the former and
from a N-deacetylation of the latter. Hydrolysis during
the work-up produced in both cases N-unprotected
substrate 14a. N-Methylated acetate 14c reacted to
give substitution product 22 (Scheme 12), but in low
case of quinoline and isoquinoline derivatives 3 (Rꢃ
/
Me) and 4 [4]. The elimination products 26 and 27
resulted probably from a base-promoted hydrogen
abstraction on the cationic h3-palladium intermediate
28 (Scheme 13). Here again, the substrate 11 bearing a
benzofuran aromatic system was much more reactive
than its benzothiophene counterpart 13. Moreover, the
selectivity on the latter compound was poorer and
elimination was the preferred pathway in THF at
60 8C. The reactivity and the selectivity were improved
in DMF at higher temperature, but the isolated yield of
25 was not so good.
Table 1
Influence of the initial malonate/deprotonated malonate ratio on the
selectivity of the palladium-catalyzed substitution reaction of 10a a
b
n
[CH2E2]/[NaCHE2]
15a/16
2
3
4
6
0
80/20
88/12
0.5
1
94/6 (52% isolated)
96/4
4. Obtention of optically active substitution product 24
2
The asymmetric palladium-catalyzed nucleophilic
substitution of racemic 1-(2-naphthyl)ethyl acetate (1b)
was recently performed with up to 74% ee if chiral
a
10a, (nꢁ2) equivlalents of CH2E2, two equivlalents of NaCHE2, 2
/
mol% Pd(dba)2, 3 mol% dppe, THF, 60 8C, 4 h.
Initial malonate/deprotonated malonate ratio.
b

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G. Primault et al. / Journal of Organometallic Chemistry 687 (2003) 353ꢀ364
/
Scheme 10. Palladium-catalyzed substitution of 10bꢀc. (a) Two equivalents of CH2E2, two equivalents of NaCHE2, 2 mol% Pd(dba)2, 3 mol% dppe,
/
THF, 60 8C, 48 h.
diphosphine ligands [(S,S)-BDPP and (R,R)-Me-DU-
PHOS] were used [5]. In view of the poor reactivity of 1-
(2-benzothienyl)ethyl acetate (13) compared to 1-(2-
benzofuryl)ethyl acetate (11), we focused on the latter
substrate in an asymmetric reaction but the results were
disappointing (Table 2). With (S,S)-BDPP, racemic 24
was produced in very good yield (82% isolated) and with
optically active chloride (R)-29. This latter was obtained
from (S)-2-(hydroxyethyl)benzofuran by a recent pub-
lished procedure [22] described with inversion of con-
figuration. The ee in this reaction was very poor
probably because of a low configurational stability of
chloride 29.
The palladium-catalyzed nucleophilic substitution by
dimethyl malonate anion occurred stereospecifically in
agreement with global retention of configuration (note
that the modification of R, S descriptors was only the
consequence of different sequences in 11 and in 24
according to the CIP priority rules). The stereochemistry
is the same as on naphthylethyl substrates [2] and
a good selectivity (24/26ꢀ
/
93/7). (R,R)-MeꢀDUPHOS
/
was not the ligand of choice since it gave only a 6% yield
of 24 (5% ee) beside remaining acetate 11 (53%),
corresponding alcohol (29%) and some of the product
of transesterification (12%) as above on indole substrate
14c. Only (R)-TolꢀBINAP gave a little enantioselective
/
reaction since compound 24 was isolated with 29% ee in
16% yield (30% remaining 11 and 38% of 26). Recovered
11 was racemic and hence no kinetic resolution process
was operating to explain this result. The reaction
conducted in DMF at 90 8C allowed a total conversion
analogous to the well-known TsujiꢀTrost reaction
/
[23,24]. This result is in agreement with the inversion
observed in the reported synthesis of inverto-yuehchu-
kene [11] since organometallics and stabilized carba-
nions attack p-allylpalladium complexes with opposite
stereochemistry (retention and inversion with respect to
palladium, respectively) [24].
The poor enantioselectivity in the use of chiral
palladium catalysts and the good enantiomeric excess
from (S)-11 with an achiral one are consistent with the
following course for the transformation (Scheme 15).
and a better selectivity (24/26ꢃ
/
91/9), but 24 was
obtained in racemic form.
The acetate (S)-11, prepared in 67% yield from 2-
iodophenol and (S)-but-3-yn-2-ol (Scheme 4) led to
quasi-enantiopure 24 (Scheme 14). The absolute config-
uration of the major enantiomer was determined as (R)
by comparison with the product of a SN2 reaction on
Scheme 11. Palladium-catalyzed substitution of 12. (a) 2 eq. CH2E2, 2 eq. NaCHE2, 2 mol% Pd(dba)2, 3 mol% dppe, 48 h. THF, 60 8C: 12/19/20/
21ꢃ61/33/6/0. DMF, 80 8C: 0/70/3/27 (48% isolated).
/

G. Primault et al. / Journal of Organometallic Chemistry 687 (2003) 353ꢀ
/364
359
Scheme 12. Palladium-catalyzed substitution of 14aꢀ/b. (a) Two equivalents of CH2E2, two equivalents NaCHE2, 2 mol% Pd(dba)2, 3 mol% dppe,
DMF, 80 8C, 48 h. 14c (RꢃMe):14c/22/23ꢃ51/26/25. 14d (Rꢃ
/
/
/
t-Bu): 14d/22/23ꢃ0/100/0 (57% isolated).
/
The oxidative addition on (S)-11 [(R)-11] leads to
intermediate (R)-28 [(S)-28] (the configuration of inter-
mediate complex 28 refers to to exocyclic asymmetric
carbon atom). Both reactions have approximatly the
same rate (no kinetic resolution of the substrate is
observed). Nucleophilic attack of dimethyl malonate
anion on (R)-28 [(S)-28] produces (R)-24 [(S)-24]. The
two cationic intermediates (enantiomeric if L is dppe
and diastereomeric if L is a chiral ligand) do not
interconver (via a SN2 process involving a Pd(0) com-
plex [2]) at a substantial rate (compare to nucleophilic
attack) and the enantiomeric ratio of the substrate is
retained in the product.
elimination on secondary ones were the main observed
side reactions. The stereochemistry of the substitution
(overall retention of configuration) was determined.
Although the asymmetric synthesis on racemic 1-(2-
benzofuryl)ethyl acetate (11) was unsucessfull with the
chiral ligands tested, the substitution product 24 was
obtained with 98% ee from (S)-11 using an achiral
palladium catalyst when the non-catalyzed SN2 reaction
of chloride 29 showed considerable racemization. Unlike
the same reaction on naphthalene derivatives [2], the
interconversion of cationic intermediate complexes is
very slow compared to nucleophilic attack.
6. Experimental
5. Conclusion
The palladium-catalyzed nucleophilic substitution
was realized on some substrates containing a benzo-
furan, benzothiophene or indole aromatic moiety. The
isolated yields of substitution products were moderate to
good. Double substitution on primary acetates and
6.1. General
1H- and 13C-NMR spectra were recorded on a Bruker
AC-250 MHz spectrometer in CDCl3 with tetramethyl-
silane as an internal standard. Coupling constants are
Scheme 13. Palladium-catalyzed substitution of 11 and 13. (a) Two equivalents of NaCHE2, 2 mol% Pd(dba)2, 3 mol% dppe, 48 h.

360
G. Primault et al. / Journal of Organometallic Chemistry 687 (2003) 353ꢀ364
/
[27] was obtained as a colorless oil (2.66 g, 14 mmol,
70%).
Table 2
Asymmetric palladium-catalyzed substitution of racemic 11 a
1H-NMR 2.10 (3H, s), 5.19 (2H, s), 6.75 (1H, s), 7.18ꢀ
/
Chiral ligand
Conditions Recovered 11
(%)
24 (% ee) 26
7.21 (1H, m), 7.29 (1H, dd, Jꢃ
/
7.5 and 1.3), 7.47 (1H, d,
8.3 and 1.2). 13C-NMR 20.6,
Jꢃ8), 7.55 (1H, dd, Jꢃ
/
/
(S,S)-BDPP b
THF, 60 8C
THF, 60 8C 53
0
82 (0)
6 (5)
B
/
7
58.3, 106.8, 112.0, 121.2, 122.8, 124.7, 127.7, 151.7,
155.0, 170.3.
(R,R)-Meꢀ
PHOS c
(R)-Tolꢀ
/
DU-
0
/
BINAP d
BINAP
THF, 60 8C 30
DMF,
90 8C
16 (29)
79 (0)
38
9
6.3. 2-(5-Chloro)benzofurylmethyl acetate (10b)
(R)-Tolꢀ
/
0
2-Bromo-4-chlorophenol (415 mg,
2
mmol),
a
11, two equivlalents of NaCHE2, 2 mol% Pd(dba)2, 2.5 mol%
chiral ligand, 48 h.
PdCl2(PPh3)2 (140 mg, 0.2 mmol, 10 mol%), CuI (18
mg, 0.09 mmol, 5 mol%), and Et3N (0.6 ml, 4 mmol)
were dissolved in 5 ml of DMF. After 15 min stirring,
propargyl alcohol (0.2 ml, 3.3 mmol) was added
dropwise over 5 min. The reaction mixture was stirred
one hour at r.t., then overnight at 100 8C. The treat-
ment, acetylation of the crude alcohol, and purification
were conducted as above for 10a, and compound 10b
was obtained as an oil (155 mg, 0.7 mmol, 34%).
b
(S,S)-BDPPꢃ/(2S,4S)-2,4-bis(diphenylphosphino)pentane.
c
(R,R)-Meꢀ
/
DUPHOSꢃ1,2-bis((2R,5R)-2,5-dimethylphosphola-
/
no)benzene.
d
(R)-Tolꢀ
/
BINAPꢃ(R)-2,2?-bis(di-p-tolylphosphino)-1,1?-bi-
/
naphthyl.
reported in Hz. Optical rotations were measured at
20 8C on a Perkin Elmer 241 polarimeter.
All reactions involving palladium catalysis were
carried out under argon using Schlenk techniques under
an argon atmosphere. Tetrahydrofuran (THF) was
distilled under argon from sodium/benzophenone under
nitrogen. Dimethylformamide (DMF) was dried over
CaH2 and distilled prior to use.
1H-NMR 2.10 (3H, s), 5.16 (2H, s), 6.69 (1H, s), 7.23
(1H, dd, Jꢃ
/
8.8 and 1.9), 7.37 (1H, d, Jꢃ8.8), 7.50 (1H,
/
d, Jꢃ
/
1.9). 13C-NMR 20.7, 58.3, 106.4, 112.3, 120.8,
125.0, 128.5, 129.2, 131.9, 153.4, 170.4. HRMS Calc. for
C11H9ClO3: 224.02406. Found: 224.0240.
Pd(dba)2 (dba denotes dibenzylideneacetone) [25] and
2-benzothiophenecarboxaldehyde [26] were prepared
according to reported procedures.
Byproducts 18, 20, 21 and 23 could not be isolated in
pure form and were identified by mass spectrometry.
6.4. 2-(7-Methoxy)benzofurylmethyl acetate (10c)
7-Methoxy-2-benzofurancarboxylic acid (480 mg, 2.5
mmol) was dissolved in 5 ml of THF under argon. At
0 8C, BH3.THF complex (2.5 ml, 2.5 mmol) was added
dropwise. After 6 h stirring at r.t., the solution was
cooled at 0 8C and 20 ml of HCl 1M were added. The
6.2. 2-Benzofurylmethyl acetate (10a)
aqueous phase was extracted by 3ꢄ20 ml of ether and
/
2-Iodophenol (4.4 g, 20 mmol), PdCl2(PPh3)2 (430
mg, 0.6 mmol, 3 mol%), CuI (150 mg, 0.8 mmol, 4
mol%), and Et3N (6 ml, 40 mmol) were dissolved in 10
ml of DMF. After 15 min stirring, propargyl alcohol
(1.2 ml, 20 mmol) was added dropwise over 5 min. The
reaction mixture was stirred 1 h at room temperature
(r.t.), then overnight at 60 8C, cooled and poured into 50
the organic phases were washed by 20 ml of a NaHCO3
saturated solution, dried (K2CO3) and concentrated.
The crude alcohol was dissolved in Ac2O (1 ml, 10
mmol), in the presence of LiCl (10 mg, 0.25 mmol).
After stirring overnight and dilution with 20 ml of ether,
the reaction mixture was washed twice with 10 ml of a
saturated CaCO3 solution, and the aqueous phases were
ml of water. The aqueous phase was extracted by 3ꢄ
ml of dichloromethane. The combined organic phases
were washed successively by 3ꢄ50 ml of 5 M NaOH
and 3ꢄ100 ml of water, dried over MgSO4 and
/
30
extracted by 3ꢄ15 ml of ether. The combined organic
/
phases were dried (MgSO4) and concentrated. The
residue was purified by flash chromatography (silica,
heptane/ethyl acetate 80:20). Compound 10c (313 mg,
1.4 mmol, 57%) was obtained as an oil.
/
/
concentrated.
The residual dark-brown oil was dissolved in Ac2O
(3.8 ml, 40 mmol) and LiCl (78 mg, 1.8 mmol) was
added. After stirring overnignt at r.t., the reaction
mixture was diluted with 20 ml of ether, washed twice
by 10 ml of a saturated Na2CO3 solution. The aqueous
1H-NMR 2.08 (3H, s), 3.99 (3H, s), 5.18 (2H, s), 6.75
(1H, s), 6.80 (1H, t, Jꢃ
/
4.4), 7.14 (2H, d, Jꢃ
4.4). 13C-
/
NMR 10.7, 55.8, 58.3, 106.6, 107.2, 113.4, 123.5, 129.4,
144.4, 145.2, 151.9, 170.3. HRMS Calc. for C12H12O4:
220.0736. Found: 220.0728.
phases were extracted by 3ꢄ15 ml of ether and the
/
combined ethereal phases were dried over MgSO4 and
concentrated. The crude product was purified by flash
chromatography (silica, heptane/ethyl acetate 80:20).
6.5. 1-(2-Benzofuryl)ethyl acetate 11
Following the same procedure as above for 10a,
substituting propargyl alcohol by but-3-yn-2-ol (1.6
After Kugelrohr distillation (115 8C, 0.1 mmHg), 10a
¨

G. Primault et al. / Journal of Organometallic Chemistry 687 (2003) 353ꢀ
/364
361
Scheme 14. Determination of the stereochemistry of the palladium-catalyzed substitution of (S)-11. (a) 3 mol% Pd(PPh3)2Cl2, 4 mol% CuI, Et3N,
DMF, 60 8C. (b) Ac2O (excess), LiCl (0.1 equivalents), 20 8C. (c) Two equivalents of NaCHE2, 2 mol% Pd(dba)2, 3 mol% dppe, DMF, 80 8C, 48 h.
(d) 2,4,6-trichloro-1,3,5-triazine, DMF, CH2Cl2, 20 8C, 12 h. (e) Two equivalents KCHE2, DMF, 20 8C, 72 h.
ml, 20 mmol), compound 11 [28] (2.36 g, 11.6 mmol,
58%) was obtained as a colorless oil. The two enantio-
mers were resolved by HPLC analysis with chiral
stationary-phase column WHELK 01 [hexane/isopropa-
mg of a white solid. The alcohol was stirred overnight in
Ac2O (1 ml, 10 mmol) in the presence of LiCl (10 mg,
0.25 mmol). After dilution (20 ml of ether) and washing
(3ꢄ/10 ml of a saturated Na2CO3 solution), the aqueous
nol 98/2, 1 ml minꢁ1, tꢃ
1H-NMR 1.65 (3H, d, Jꢃ
q, Jꢃ6.8), 6.67 (1H, s), 7.13ꢀ
(1H, m), 7.50ꢀ
7.55 (1H, m). 13C-NMR 18.0, 20.6, 65.1,
/
6.6 min, 10.1 min].
6.8), 2.08 (3H, s), 6.07 (1H,
7.34 (2H, m), 7.43ꢀ7.48
phases were extracted (3ꢄ
/
20 ml of ether). The com-
bined ethereal phases were dried (MgSO4) and concen-
/
/
/
/
trated to give 12 (229 mg, 1.1 mmol, 79%) as a white
/
solid, m.p. 78ꢀ
1H-NMR 2.10 (3H, s), 5.33 (2H, s), 7.28ꢀ
m), 7.71ꢀ
7.82 (2H, m). 13C-NMR 20.7, 61.1, 122.2,
/
79 8C (lit.: 79ꢀ
/
80 8C) [29].
103.9, 111.0, 120.9, 122.6, 124.2, 127.6, 154.5, 155.7,
/7.33 (3H,
170.0.
(S)-11 ([a]2D0ꢃ
67% yield from 2-iodophenol and (S)-but-3-yn-2-ol.
/
/
ꢁ
/
167 (c 1.2, MeOH)) was obtained in
123.6, 124.21, 124.24, 124.5, 138.6, 139.0, 140.2, 170.4.
6.7. 1-(2-Benzothiophenyl)ethyl acetate (13)
6.6. 2-Benzothiophenylmethyl acetate (12)
2-Benzothiophenecarboxaldehyde (486 mg, 3 mmol)
was dissolved in 10 ml of anhydrous ether. Methylmag-
nesium bromide (1.1 ml of a 3 M solution in ether, 3.3
mmol) was added dropwise at 0 8C to give a hetero-
geneous mixture. After TLC control, 20 ml of a cold 1
M HCl solution were added and the reaction mixture
2-Benzothiophenecarboxaldehyde (225 mg, 1.4 mmol)
was dissolved in 5 ml of methanol. NaBH4 (27 mg, 0.7
mmol) was added portionwise. After TLC control, 10 ml
of 1 M HCl were added and the reaction mixture was
extracted by 3ꢄ
/
20 ml of ether. The combined organic
phases were dried (MgSO4) and concentrated to give 212
was extracted by 3ꢄ20 ml of ether. The combined
/
Scheme 15. Stereochemical course of the palladium-catalyzed substitution of 11.

362
G. Primault et al. / Journal of Organometallic Chemistry 687 (2003) 353ꢀ364
/
organic phases were neutralized (20 ml of a saturated
NaHCO3 solution), washed by 2ꢄ20 ml of water, dried
6.10. 2-(N-Methyl)indolylmethyl acetate (14c)
/
(MgSO4), and concentrated. The crude alcohol was
stirred overnight in Ac2O (1 ml, 10 mmol) in the
presence of LiCl (13 mg, 0.3 mmol). After dilution (20
N-Methylindolecarboxaldehyde (795 mg, 5 mmol)
was dissolved in 20 ml of methanol. NaBH4 (114 mg,
3 mmol) was added portionwise. After TLC control, 10
ml of 1M HCl were added, then the reaction mixture
was neutralized by NaOH. The reaction mixture was
ml of ether) and washing (3ꢄ
Na2CO3 solution), the aqueous phases were extracted
(3ꢄ20 ml of ether). The combined ethereal phases were
/
10 ml of a saturated
/
extracted by 3ꢄ20 ml of ether. The combined organic
/
dried (MgSO4) and concentrated. The crude product
was purified by flash chromatography (silica, heptane/
ethyl acetate 90:10) to give 13 [28] (541 mg, 2.46 mmol,
82%) as an oil.
phases were dried (MgSO4) and concentrated. The crude
alcohol was disolved in 10 ml of ether, Et3N (1 ml, 6
mmol), DMAP (60 mg, 0.5 mmol), then dropwise Ac2O
(0.5 ml, 5.5 mmol) were added. The resulting mixture
was stirred overnight and hydrolyzed by a saturated
Na2CO3 solution. The organic phase was washed twice
by saturated Na2CO3, dried (MgSO4) and concentrated.
The crude product was purified by flash chromatogra-
phy (silica, heptane/ethyl acetate 80:20) to give 14c (700
1H-NMR 1.67 (3H, d, Jꢃ
q, Jꢃ6.3), 7.23 (1H, s) 7.27ꢀ
(1H, m), 7.76ꢀ
7.79 (1H, m). 13C-NMR 21.2, 21.9, 68.1,
/
6.4), 2.07 (3H, s), 6.20 (1H,
7.35 (2H, m), 7.68ꢀ7.72
/
/
/
/
121.6, 122.3, 123.7, 124.3, 124.4, 139.3, 145.0, 170.1.
mg, 3.44 mmol, 69%) as a yellow solid, m.p. 73ꢀ
1H-NMR 2.08 (3H, s), 3.76 (3H, s), 5.26 (2H, s), 6.57
(1H, s), 7.08ꢀ7.12 (1H, m), 7.20ꢀ7.33 (2H, m), 7.58 (1H,
d, Jꢃ
7.8). 13C-NMR 20.8, 29.6, 58.0, 103.7, 109.2,
119.6, 120.9, 122.2, 127.0, 133.5, 137.9, 170.4. HRMS
Calc. for C12H13NO2: 203.0946. Found: 203.0935.
/
75 8C.
6.8. 2-Indolylmethyl acetate (14a)
/
/
/
2-Indolylmethanol [16] (2.27 g, 15.5 mmol) was
dissolved in Ac2O (3 ml, 31 mmol) and LiCl (67 mg,
1.5 mmol) was added. After stirring overnignt at r.t., the
reaction mixture was diluted with 20 ml of ether, washed
twice by 10 ml of a saturated Na2CO3 solution. The
6.11. 2-(N-Methyl)indolylmethyl pivalate (14d)
aqueous phases were extracted by 3ꢄ15 ml of ether and
/
the combined ethereal phases were dried over MgSO4
and concentrated. The crude product was purified by
flash chromatography (silica, heptane/ethyl acetate
80:20) and 14a was obtained as a yellow oil (315 mg,
1.7 mmol, 11%).
The same procedure as above for 14c, substituting
acetic anhydride by pivaloyl chloride, gave compound
14d (87%) as a yellow oil.
1H-NMR 1.20 (9H, s), 3.73 (3H, s), 5.25 (2H, s), 6.56
(1H, s), 7.08-7.11 (1H, m), 7.23ꢀ
/
7.31 (2H, m), 7.59 (1H,
7.8). 13C-NMR 26.3, 26.9, 29.5, 58.2, 103.3, 109.0,
1H-NMR 2.09 (3H, s), 5.21 (2H, s), 6.53 (1H, s), 7.08
d, Jꢃ
/
(1H, t, Jꢃ
7.9) 7.58 (1H, d, Jꢃ
/
7.4), 7.20 (1H, t, Jꢃ
/
7.5), 7.34 (1H, d, Jꢃ
/
119.4, 120.7, 121.9, 126.9, 133.8, 137.8, 177.7. HRMS
Calc. for C15H19NO2: 245.1416. Found: 245.1420.
/
7.7), 8.58 (1H, s). 13C-NMR 21.4,
60.1, 104.3, 111.5, 120.3, 121.3, 123.2, 127.9, 133.4,
136.9, 172.7.
6.12. General nucleophilic palladium-catalyzed procedure
2-Benzofurylmethyl acetate (10a) (190 mg, 1 mmol) in
1 ml of THF was added under argon to a solution of
Pd(dba)2 (11.5 mg, 0.02 mmol, 2 mol%) and dppe (12
mg, 0.03 mmol, 3 mol%) in 1 ml of THF. Dimethyl
malonate (0.46 ml, 4 mmol) was added dropwise on a
suspension of sodium hydride (48 mg, 2 mmol) in 3 ml
of THF. The resulting mixture was heated to 60 8C
6.9. 2-(N-Acetyl)indolylmethyl acetate (14b)
2-Indolylmethanol [16] (1.56 g, 10.6 mmol) was
dissolved in Et2O (15 ml). Triethylamine (2.2 ml, 13
mmol), DMAP (130 mg, 1 mmol) and dropwise Ac2O
(2.2 ml, 23 mmol) were added. After stirring overnignt
at r.t., the reaction mixture was hydrolyzed by 10 ml of a
saturated Na2CO3 solution. The organic phase was
washed twice by 10 ml of a saturated Na2CO3 solution,
dride over MgSO4 and concentrated. The crude product
was purified by flash chromatography (silica, heptane/
ethyl acetate 80:20) and 14b was obtained as a white
solid (2.03 g, 8.8 mmol, 83%).
during 15 min, when added by canula to the substrateꢀ
/
catalyst solution. The reaction mixture was stirred at
60 8C for 24 h, then diluted with ether (20 ml) and the
organic phase washed with 2ꢄ
saturated NaHCO3. The aqueous phases were extracted
with ether (3ꢄ20 ml) and the combined ethereal phases
/15 ml of a solution of
/
were dried (MgSO4) and concentrated. The crude
product was purified by flash chromatography (silica,
heptane/ethyl acetate 80:20) to give dimethyl 2-(2-
benzofurylmethyl)propanedioate 15a (136 mg, 0.52
mmol, 52%).
1H-NMR 2.14 (3H, s), 2.79 (3H, s), 5.48 (2H, s), 6.68
(1H, s), 7.21ꢀ
(1H, d, Jꢃ
8.2). 13C-NMR 20.9, 26.9, 61.0, 110.7, 114.5,
120.7, 121.2, 122.6, 123.2, 124.5, 129.4, 136.0, 170.3.
/
7.34 (2H, m), 7.54 (1H, t, Jꢃ7.3), 7.79
/
/

G. Primault et al. / Journal of Organometallic Chemistry 687 (2003) 353ꢀ
/364
363
6.12.1. Dimethyl 2-(2-benzofurylmethyl)propanedioate
(15a) (52% isolated yield)
6.12.7. Dimethyl 2-(1-(2-
benzofuryl)ethyl)propanedioate (24) (82% isolated yield
using two equivalents of dimethyl malonate with (S,S)-
BDPP as palladium ligand)
1H-NMR 3.73 (6H, s), 3.62ꢀ
/3.88 (3H, m), 6.45 (1H,
s), 7.16ꢀ
/
7.23 (2H, m), 7.35ꢀ
/
7.40 (1H, m), 7.45ꢀ7.49
/
(1H, m). 13C-NMR 27.7, 50.1, 52.6, 103.7, 110.7, 120.5,
122.5, 123.6, 128.3, 154.4, 154.6, 168.6. HRMS Calc. for
C14H14O5: 262.0841. Found: 262.0835.
1H-NMR 1.41 (3H, d, Jꢃ
s), 3.66ꢀ3.85 (2H, m), 6.45 (1H, s), 7.12ꢀ
7.37ꢀ7.40 (1H, m), 7.45ꢀ
7.48 (1H, m). 13C-NMR 16.5,
/
6.3), 3.60 (3H, s), 3.74 (3H,
/
/7.21 (2H, m),
/
/
33.7, 52.4, 56.0, 102.4, 110.7, 120.5, 122.4, 123.5, 128.2,
154.4, 158.8, 168.0. HRMS Calc. for C15H16O5:
276.0998. Found: 276.0997. The two enantiomers were
resolved by HPLC analysis with chiral stationary-phase
column WHELK 01 [hexane/isopropanol 98/2, 1 ml
6.12.2. Dimethyl 2,2-di(2-
benzofurylmethyl)propanedioate (16) (41% isolated
yield from the reaction of (10a) with two equivalents of
sodium salt of 15a)
1H-NMR 3.44 (4H, s) 3.81 (6H, s), 6.57 (2H, s), 7.19ꢀ
/
minꢁ1, tꢃ
(R)-24: [a]2D0ꢃ
98% ee.
/12.6 min, 16.5 min].
7.23 (4H, m), 7.35ꢀ
/
7.42 (2H, m), 7.45ꢀ
/
7.56 (2H, m).
/
ꢁ/34 (c 1.7, MeOH) for a sample with
13C-NMR 31.4, 52.7, 57.0, 105.9, 110.8, 120.4, 122.6,
123.8, 128.2, 153.3, 154.8, 170.1. HRMS Calc. for
C23H20O6: 392.1260. Found: 392.1266.
6.12.8. Dimethyl 2-(1-(2-
benzothiophenyl)ethyl)propanedioate (25) (48% isolated
yield using two equivalents of dimethylmalonate in DMF
at 80 8C)
6.12.3. Dimethyl 2-(2-(5-
chloro)benzofurylmethyl)propanedioate (15b) (26%
isolated yield)
1H-NMR 1.44 (3H, d, Jꢃ
/
6.8), 3.56 (3H, s), 3.76 (3H,
3.97 (2H, m), 7.08 (1H, s), 7.15ꢀ7.19 (1H, m),
7.68 (1H, m), 7.72ꢀ7.77 (1H,
1H-NMR 3.72 (6H, s), 3.63-3.90 (3H, m), 6.40 (1H, s),
s), 3.63ꢀ
/
/
7.09ꢀ
/
7.31 (2H, m), 7.42 (1H, d, Jꢃ
2). 13C-NMR 27.8,
/
7.25ꢀ7.30 (1H, m), 7.64ꢀ
/
/
/
50.1, 52.8, 103.5, 111.8, 120.2, 123.9, 127.5, 128.1, 128.5,
156.1, 168.6. HRMS Calc. for C14H13ClO5: 296.0452.
Found: 296.0444.
m). 13C-NMR 20.3, 36.0, 52.4, 52.5, 59.1, 121.0, 122.1,
123.1, 123.8, 124.1, 138.9, 139.4, 147.2, 167.9, 168.1.
HRMS Calc. for C15H16O4S: 292.0769. Found:
292.0764.
6.12.4. Dimethyl 2-(2-(7-
methoxy)benzofurylmethyl)propanedioate (15c) (44%
isolated yield)
1H-NMR 3.40 (2H, d, Jꢃ
t, Jꢃ
6.4 and 2.4), 7.06ꢀ
52.8, 56.0, 104.3, 106.0, 113.0, 123.3, 128.2, 128.5, 136.0,
154.6, 168.7. HRMS Calc. for C15H16O6: 292.0947.
Found: 292.0943.
/
7.6), 3.72 (6H, s), 3.88 (1H,
7.6), 3.97 (3H, s), 6.45 (1H, s), 6.73 (1H, dd, Jꢃ
7.10 (2H, m). 13C-NMR 27.8, 50.3,
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/
/
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/
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/
7.6), 3.69 (6H, s), 3.74 (3H,
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/
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/
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/
/
7.28 (1H, m), 7.51 (1H, d, Jꢃ
/
7.7). 13C-NMR 25.9, 29.4, 51.0, 52.8, 99.4, 108.8, 119.3,
120.0, 121.0, 127.5, 136.5, 137.3, 168.9. HRMS Calc. for
C15H17NO4: 275.1158. Found: 275.1147.
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