Palladium-catalyzed cyclization of o-alkynylphenols with allyl carbonates. A regioselective synthesis of 2-substituted-3-allylbenzo[b]furans

Article abstract of DOI:10.1055/s-1998-1758

2-Substituted-3-allylbenzo[b]furans 3 can be prepared from o-alkynylphenols 1 and allyl carbonates 2 through a palladium-catalyzed O-allylation/cyclization sequence. Two basic procedures have been developed: a stepwise method based on the isolation of O-allyl derivatives 4 and their subsequent cyclization to 3 (procedure A) and a one-pot reaction omitting the isolation of 4 (procedure B). The cyclization of 4 in the presence of the electron-rich sterically-encumbered ligand tris(2,4,6-trimethoxyphenyl)phosphine (ttmpp) exhibits remarkable regioselectivity in that 3-allylbenzofurans in which the benzofuryl unit is bound to the less substituted allyl terminus are formed almost exclusively. Some loss of the stereochemistry of the carbon-carbon double bond is observed.

Full text of DOI:10.1055/s-1998-1758

July 1998
SYNLETT
741
Palladium-Catalyzed Cyclization of o-Alkynylphenols with Allyl Carbonates. A Regioselective
Synthesis of 2-Substituted-3-allylbenzo[b]furans
Sandro Cacchi,* Giancarlo Fabrizi, Leonardo Moro
Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Università “La Sapienza”, P.le A. Moro 5, 00185 Roma, Italy
Fax: + 39 (6) 4991.2780; e-mail: cacchi@axrma.uniroma1.it
Received 20 March 1998
Abstract. 2-Substituted-3-allylbenzo[b]furans 3 can be prepared from
o-alkynylphenols 1 and allyl carbonates 2 through a palladium-
catalyzed O-allylation/cyclization sequence. Two basic procedures have
been developed: a stepwise method based on the isolation of O-allyl
derivatives 4 and their subsequent cyclization to 3 (procedure A) and a
one-pot reaction omitting the isolation of 4 (procedure B). The
cyclization of 4 in the presence of the electron-rich sterically-
encumbered ligand tris(2,4,6-trimethoxyphenyl)phosphine (ttmpp)
exhibits remarkable regioselectivity in that 3-allylbenzofurans in which
the benzofuryl unit is bound to the less substituted allyl terminus are
formed almost exclusively. Some loss of the stereochemistry of the
carbon-carbon double bond is observed.
The cyclization of alkynes containing proximate nucleophilic centers
promoted by organopalladium complexes is currently of great interest.
Scheme 2
1
However, while numerous approaches to carbo- and heterocycles based
and 3”e, in only 14% overall yield along with 2-phenylbenzo[b]furan
(38% yield) and other unidentified products.
on the activation of the carbon-carbon triple bond by σ-organopalladium
3
complexes have been described, the utilization of η -allylpalladium
2
Therefore, in light of the observed tendency of O-allyl derivatives 4 to
undergo a palladium-catalyzed cyclization to 3-allylbenzo[b]furans 3
(vide infra), we turned our attention to the development of a procedure
based on the O-allylation/cyclization sequence.
complexes has received little attention. Recently, we have found that 3-
allylindoles can be prepared from o-alkynyltrifluoroacetanilides through
3
3
a cyclization promoted by η -allylpalladium complexes. We now report
that this chemistry can be successfully applied to the synthesis of 2-
4
Two different experimental protocols have been developed: a stepwise
method, based on the preparation of stereo- and regioisomeric mixtures
of the O-allyl derivatives 4’ and 4” and their subsequent cyclization to
benzo[b]furans (procedure A), and a one-pot protocol omitting the
isolation of 4’ and 4” (procedure B).
substituted-3-allylbenzo[b]furans 3 from o-alkynylphenols 1 (Scheme
1).
Stepwise Synthesis of 2-Substituted-3-allylbenzo[b]furans
3
(procedure A). Though the employment of Pd(PPh ) has been found
3 4
to give good results, the O-allylation of o-alkynylphenols 1 has been
best carried out as follows: 1:2:Pd (dba) :dppb = 1:1.2:0.025:0.05 in
2
3
5,6
THF at 60 °C. The reaction gives rise to variable amounts of 4’ and
4”. However, no attempts have been made to control the product
selectivity. In fact, the regiochemistry of the C-C bond formed in the
cyclization step and the stereochemistry of the olefin fragment of 3-
allylbenzo[b]furans were found to be almost independent of the regio-
and stereochemistry of the O-allyl derivatives, as shown by the reactions
depicted in Scheme 3.
Scheme 1
As found with o-alkynyltrifluoroacetanilides, the reaction of o-
alkynylphenols 1 with allyl carbonates and Pd(PPh ) tends to afford
preferentially O-allyl products instead of the desired benzo[b]furans,
indicating that the nucleophilic attack of the oxygen on the η -
allylpalladium intermediate is faster than the organopalladium-
promoted cyclization. For example, treating 1a and 2e with Pd(PPh ) at
60 °C in THF for 6 h produced the O-allylation derivatives 4’e and 4”e
as an approximately 61:39 mixture in 82% yield (Scheme 2).
3 4
3
Attempting the cyclization reaction under the same conditions that give
the highest yields in the O-allylation met with failure. For example,
treatment of an approximately 60:40 regioisomeric mixture of 4’e and
3 4
4”e, our model system, with Pd (dba) and dppb in THF at 80 °C
2
3
produced o-phenylethynylphenol 1a in 71% yield and none of the
corresponding benzo[b]furan product was observed. The use of the
following conditions, K CO :Pd (dba) :ttmpp = 5:0.025:0.1 in DME at
Furthermore, whereas the reaction of o-alkynyltrifluoroacetanilides with
allyl carbonates in the presence of Pd (dba) and ttmpp [tris(2,4,6-
2
3
2
3
2
3
100 °C, produced regioselectively the corresponding 3-allyl
trimethoxyphenyl)phosphine] affords regioselectively 3-allylindoles in
high yield through a mechanism that does not involve the N-allylation
step, the extension of the same conditions to o-alkynylphenols appears
benzo[b]furan derivative (3’e:3”e, 96:4) in 81% yield (2 h). These
7
3
conditions have been generally employed for the cyclization step. High
yield (91%) but low regioselectivity (3’e:3”e, 70:30) were observed
to be flawed by competing side reactions. The trend is similar: 3-
allylbenzo[b]furans were in fact isolated and none of the O-allyl
products were formed. Yields, however, are low. For instance,
with Pd(PPh ) (2 h).
3 4
One-Pot Synthesis of 2-Substituted-3-allylbenzo[b]furans
3
subjection of 1a and 2e to the Pd (dba) /ttmpp combination (THF, 50
(procedure B). The inability of Pd (dba) /ttmpp (that gives the best
2
3
2
3
°C, 0.75 h) led to the isolation of the corresponding benzo[b]furans, 3’e
results in terms of regiochemistry of the new carbon-carbon bond in the

742
LETTERS
SYNLETT
cyclization step) and Pd (dba) /dppb (that gives the highest yields in the
2
3
O-allylation step) to promote, respectively, the O-allylation of o-
alkynylphenols and the cyclization of O-allyl derivatives led us to evolve
a one-pot protocol based on the utilization of Pd(PPh ) . The following
3 4
conditions (procedure B) have been found to be satisfactory:
1:2:Pd(PPh ) = 1:1.2:0.05 in THF at 60 °C till the disappearance of 1,
3 4
addition of K CO (5 equiv) (lower yields were obtained when it was
2
3
8
omitted), then raising the reaction temperature to 80 °C.
Using procedures A and B a variety of o-alkynylphenols and allyl
carbonates with a range of substitution patterns can be converted into 2-
substituted-3-allylbenzo[b]furans in good yields as summarized in the
Table. Procedure A has been found to be the method of choice when
steric differences between the two allylic termini are small, as it is in the
case of 2-hexen-1-yl and 1-octen-3-yl carbonates (Table, entries 1, 7, 13,
17, 21, 25, 28). Procedure B has been successfully employed when the
3
reaction proceeds through symmetric η -allylpalladium complexes
(Table, entries 4, 9, 14, 19, 23, 29) and even when the two allylic termini
are markedly different from a steric point of view (Table, entries 10-12,
15, 16, 18, 20, 22, 24, 26). The process is accompanied by some loss of
the stereochemistry of the carbon-carbon double bond.
Mechanism of Cyclization and Regiochemistry of the New C-C
Bond. Very likely, as suggested for the related cyclization of the o-
3
alkynyl-N-allyltrifluoroacetanilides, the cyclization of the O-allyl
derivatives proceeds through the basic steps outlined in Scheme 4 for
one of the O-allyl isomers: (1) formation of 6’ and 6”, resulting from the
Scheme 3
palladium-promoted ionization of the Ο−C
bond and the
allyl
displacement of one ligand to the palladium by the carbon-carbon triple

July 1998
SYNLETT
743
bond; (2) intramolecular nucleophilic attack of the oxygen across the
activated carbon-carbon triple bond to afford 7’ and 7”; (3) reductive
elimination of Pd(0) through the transfer of the benzofuryl fragment to
9
the allyl group in a cis fashion, which gives the benzo[b]furan product
and regenerates the active catalyst.
The regioselectivity of the cis-migration step, favoring the isomeric
product 3’ over 3” (with all the ligands we examined), may reflect the
preferential formation of the geometrical isomer 7’ over 7”. Such a
preference may be influenced by steric and electronic factors.
Steric factors are expected to act so as to minimize steric strain and
appear to favor the location of the less sterically congested allylic
terminus cis to the 2-substituted-benzofuryl unit rather than to the
3
phosphine ligand. Previous work on η -allylpalladium intermediates
which react by transfer of a carbon fragment from the palladium to an
allylic carbon showed that the new C-C bond is formed preferentially at
10
the more crowded allyl terminus. This was assumed to indicate a
strong preference for the large triphenylphosphine to reside trans to the
more hindered allyl terminus. However, the less sterically demanding
phenyl and linear vinyl groups were investigated. In our case, it seems
that the steric requirements of the phosphine ligand are smaller than
those of the substituted benzofuryl unit.
Scheme 4

744
LETTERS
SYNLETT
Electronic factors associated with the donor-acceptor properties of the
4.
Typical procedure for the preparation of o-alkynylphenols 1 from
o-iodophenyl tetrahydropyranyl ether and 1-alkynes. To a solution
of o-iodophenyl tetrahydropyranyl ether (5.00 g, 16.45 mmol) and
ligands may create an asymmetric electronic distribution at the allylic
11
system.
Specifically, stronger π-acceptor ligands may enhance
electron donation from the allylic system to the metal and locate some
positive charge character on it. This positive charge character is
preferentially accommodated on the more substituted carbon atom.
Furthermore, it has been observed that π-acceptors prefer to relay their
phenylacetylene (2.16 mL, 19.74 mmol) in Et N (8 mL) and DMF
3
(2 mL) were added, under argon, PdCl (PPh ) (0.230 g, 0.33
2
3 2
mmol) and CuI (0.13 g, 0.66 mmol). The reaction mixture was
stirred at 45 °C for 2 h. Then, diethyl ether and water were added,
the organic layer was separated, dried (Na SO ) and concentrated
2 4
12
properties in a trans manner across the square planar complexes. By
these arguments, electron-withdrawing ligands are expected to favor the
formation of the η -allylpalladium complex 7” and, ultimately, the
under reduced pressure. The residue was dissolved in Me CO/
2
3
H O 80/20 (10 mL), TsOH (0.31g, 1.64 mmol) was added and the
2
benzo[b]furan derivative 3”; the amount of 7’, and then 3’, should
increase on going from electron-withdrawing to electron-donating
ligands. Accordingly, we observed that 3’e:3”e obtained by subjecting
an approximately 60:40 mixture of 4’e and 4”e to phosphine ligands
with different donor-acceptor properties increased in the order: (p-Cl-
solution was stirred overnight at room temperature. After usual
workup, the residue was purified by chromatography eluting with
a n-hexane/EtOAc (85/15 v/v) mixture to give 2.74 g (86% overall
1f
yield) of o-phenylethynylphenol 1a: mp 47-48 °C (lit. 47-48 °C).
Typical procedure for the preparation of o-alkynylphenols 1 from
o-iodophenyl tetrahydropyranyl ether, trimethylsilylacetylene and
aryl halides or vinyl triflates. To a solution of o-iodophenyl
tetrahydropyranyl ether (2.00 g, 6.56 mmol) and
C H ) P (53:47) < Ph P (70:30) < (p-MeO-C H ) P (87:13). This result
6
4 3
3
6 4 3
is in agreement with previous observations on the thermolysis of a series
3
of Pd(η -CH CHCHMe)(C H Cl )(PR ) complexes which showed that
2
6
3
2
3
the bond formation at the less substituted allyl end increases with
trimethylsilylacetylene (1.12 mL, 7.89 mmol) in Et N (4 mL) and
3
DMF (1 mL) were added, under argon, PdCl (PPh ) (0.092 g,
2 3 2
0.13 mmol) and CuI (0.050 g, 0.26 mmol). The reaction mixture
was stirred at 45 °C for 2 h. After the usual workup the residue
9
decrease in the electron-withdrawing ability of the ligand. Presumably,
13
the strong electron-donating power of ttmpp can account for most of
its remarkable effect on the regiochemistry of the new carbon-carbon
bond. The steric encumbrance of the ligand, however, by virtue of the
likely influence on the electronic properties and geometrical features of
was dissolved in MeOH (5 mL), K CO (0.091 g, 0.66 mmol) was
2
3
added and the reaction mixture was stirred overnight at room
temperature. Then, diethyl ether and water were added, the
14
the square planar complexes, might also play a role.
organic layer was separated, dried (Na SO ) and concentrated
In conclusion, the present palladium-catalyzed allylation/cyclization of
2
4
under reduced pressure. The residue was dissolved in Et N (4 mL)
and DMF (1 mL). Then, p-iodoanisole (1.85 g, 7.89 mmol),
o-alkynylphenols provides a valuable approach to the synthesis of 2-
substituted-3-allylbenzo[b]furans. The overall high yields generally
3
15
PdCl (PPh ) (0.092 g, 0.13 mmol) and CuI (0.050 g, 0.26 mmol)
were added under an argon atmosphere. The reaction mixture was
stirred for 2 h at 45°C. After usual work-up, the residue was
observed and the almost complete regioselectivity toward the formation
of 3-allylbenzofuran in which the benzofuryl unit is bound to the less
substituted allyl terminus suggest a vast synthetic potential for the
preparation of this class of compounds.
2
3 2
dissolved in Me CO/H O 80/20 (10 mL), TsOH (0.130 g, 0.66
2
2
mmol) was added and the solution was stirred overnight at room
temperature. After this time, ethylacetate was added and the
Acknowledgment. The authors gratefully acknowledge the Ministero
dell’Università e della Ricerca Scientifica (MURST) for financial
support of this research. The authors are also greatly indebted to Dr.
Luciana Turchetto of the Istituto Superiore di Sanità for obtaining the
mass spectra of new products.
organic mixture was washed with water, dried (Na SO ) and
2
4
concentrated under reduced pressure. The residue was purified by
chromatography eluting with a n-hexane/EtOAc (87/13 v/v)
mixture to give 0.94
g (61% overall yield) of o-(p-
methoxyphenyl)ethynylphenol 1c: mp 53-55 °C; IR (KBr) 3427,
-1
1
2213, 835, 761 cm ; H NMR δ 7.50-7.40 (m, 3 H), 7.25 (t, J =
13
References and Notes
7.0 Hz, 1 H), 7.01-6.86 (m, 4 H), 5.98 (s, 1 H), 3.81 (s, 3 H);
C
NMR δ 160.1, 156.5, 133.2, 131.6, 130.2, 120.4, 114.7, 114.5,
1.
For a recent leading reference, see: (a) Cacchi, S.; Fabrizi, G.;
Marinelli, F.; Moro, L.; Pace, P. Synlett 1997, 1363. (b) Cacchi, S.;
Fabrizi, G.; Moro, L. J. Org. Chem. 1997, 62, 5327. (c) Bouyssi,
D.; Cavicchioli, M.; Balme, G. Synlett 1997, 944. (d) Arcadi, A.
Synlett 1997, 941. (e) Arcadi, A.; Anacardio, R.; D’Anniballe, G.;
Gentile, M. Synlett 1997, 1315. (f) Arcadi, A.; Cacchi, S.; Del
Rosario, M.; Fabrizi, G.; Marinelli, F. J. Org. Chem. 1996, 61,
9280. (g) Cavicchioli, M.; Decortiat, S.; Bouyssi, D.; Gore, J.;
Balme, G. Tetrahedron 1996, 52, 11463. (h) Arcadi, A.; Rossi, E.
Tetrahedron Lett. 1996, 37, 6811. (i) Saulnier, M.G.; Frennesson,
D.B.; Deshpande, M.S.; Vyas, D.M. Tetrahedron Lett. 1995, 36,
7841. (j) Arcadi, A.; Cacchi, S.; Carnicelli, V.; Marinelli, F.
Tetrahedron 1994, 50, 437. (k) Balme, G.; Bouyssi, D.
Tetrahedron 1994, 50, 403. (l) Arcadi, A.; Cacchi, S.; Larock,
R.C.; Marinelli, F. Tetrahedron Lett. 1993, 34, 2813. (m) Bouyssi,
D.; Gore, J.; Balme, G.; Louis, D.; Wallach, J. Tetrahedron Lett.
1993, 34, 3129.
114.2, 110.1, 96.4, 81.8, 55.4; MS m/e (relative intensity) 324
+
(M , 100), 209 (97), 181 (71); Anal. Calcd for C
H O : C,
15 12 2
80.33; H, 5.40. Found: C, 80.39; H, 5.45.
5.
6.
Goux, C.; Lhoste, P.; Sinou, D. Synlett 1992, 725.
Typical procedure for the O-allylation of o-alkynylphenols 1 (First
step of procedure A). To a solution of o-phenylethynylphenol 1a
(0.20 g, 1.03 mmol) and 2-hexen-1-yl ethyl carbonate 2e (0.21 g,
1.24 mmol) in anhydrous THF (4 mL) were added, under argon,
Pd dba (0.024 g, 0.02 mmol) and dppb (0.022 g, 0.05 mmol).
2
3
The reaction mixture was stirred at 60 °C for 1 h. After this time,
ethylacetate was added and the organic mixture was washed with
water, dried (Na SO ) and concentrated under reduced pressure.
2
4
The residue was purified by chromatography eluting with a n-
hexane/EtOAc (99/1 v/v) mixture to give 0.155g (55 % yield) of
4e, as an about 84:16 E:Z mixture, and 0.104 g (36 % yield) of 4e;
-1
1
4e: oil; IR (liquid film) 2221, 752, 690 cm ; H NMR δ 7.65-7.50
2
Mandai, T.; Ohta, K.; Baba, N.; Kawada, M.; Tsuji, J. Synlett
1992, 671. Tsuda, T.; Ohashi, Y.; Nagahama, N.; Sumiya, R.;
Saegusa, T. J. Org. Chem. 1988, 53, 2650.
(m, 3 H), 7.45-7.22 (m, 4 H), 7.00-6.87 (m, 2 H), 5.95 (dt, J = 15.3
Hz, J = 6.4 Hz, 1 H, -H C-CH=CH-), 5.77 (dt, J = 15.3 Hz, J = 5.2
2
Hz, 1 H, -H C-CH=CH-), 4.70 (d, J = 5.4 Hz, 0.32 H, isom. Z),
2
3.
Cacchi, S.; Fabrizi, G.; Pace, P. J. Org. Chem. 1998, 62, 1001.
4.58 (dd, J = 9.9 Hz, J = 5.2 Hz, 1.68 H, -H C-CH=CH-), 2.07
2

July 1998
SYNLETT
745
(bq, J = 6.4 Hz, 2 H), 1.42 (q, J = 7.3 Hz, 2 H), 0.90 (t, J = 7.3 Hz,
EtOAc mixture to provide 0.220 g (91 % yield) of 3f: oil; IR
13
-1 1
3 H); C NMR δ 159.4, 134.9, 134.1, 133.5, 131.7, 129.7, 128.3,
128.1, 124.9, 123.8, 120.7, 112.8, 98.6, 86.1, 69.5 (O-CH , 4’e E
(liquid film) 1458, 745, 695 cm ; H NMR δ 7.78 (d, J = 8.4 Hz,
2 H), 7.55-7.18 (m, 7 H), 6.10 (ddd, J = 17.8 Hz, J = 10.9 Hz, 5.4
Hz, 1 H), 5.18 (d, J = 10.9 Hz, 1 H), 5.04 (d, J = 17.8 Hz, 1 H),
2
isomer), 65.1 (O-CH , 4’e Z isomer); MS m/e (relative intensity)
2
+
13
276 (M , 6), 247 (6), 194 (100); Anal. Calcd for C
H
O: C,
3.64 (d, J = 5.4 Hz, 2 H); C NMR δ 154.1, 151.7, 135.2, 131.0,
20 20
86.91; H, 7.30. Found: C, 86.95; H, 7.35; 4''e: oil; IR (liquid film)
130.4, 128.7, 128.3, 127.0, 124.4, 122.5, 119.8, 116.2, 113.1,
111.1, 28.5; MS m/e (relative intensity) 234 (M , 100), 194 (45);
-1
1
+
2221, 750, 695 cm ; H NMR 7.61-7.15 (m, 7 H), 7.00-6.88 (m, 2
H), 5.95 (ddd, J = 17.2 Hz, J = 10.7, J = 6.4 Hz, 1 H, H C=CH-
Anal. Calcd for C
6.09.
H O: C, 87.14; H, 6.03. Found: C, 87.20; H,
17 14
2
CH-), 5.32 (d, J = 17.2 Hz, 1 H, H C=CH-CH-), 5.06 (d, J = 10.7
2
Hz, 1 H, H C=CH-CH-), 4.72 (q, J = 6.4 Hz, 1 H, H C=CH-CH-),
2.00-1.50 (m, 4 H), 1.00 (t, J = 7.2 Hz, 3 H); C NMR 159.3,
138.2, 133.3, 131.6, 129.4, 128.4, 128.0, 124.0, 120.8, 116.5,
2
2
9.
Kurosawa, H.; Shiba, K.; Hirako, K.; Kakiuchi, K.; Ikeda, I. J.
Chem. Soc., Chem. Commun. 1994, 1099. Trost, B.M.; Weber, L.;
Strege, P.E.; Fullerton, T.J.; Dietsche, T.J. J. Am. Chem. Soc. 1978,
100, 3416.
13
115.0, 114.0, 86.3, 80.3, 37.9, 18.5, 14.1; MS m/e (relative
+
intensity) 276 (M , 10), 194 (100); Anal. Calcd for C
H O: C,
20 20
10. Keinan, E.; Sahai, M. J. Chem. Soc., Chem. Commun. 1984, 648.
Temple, J.S.; Riediker, M.; Schwartz, J. J. Am. Chem. Soc. 1982,
104, 1310. Hayasi, Y.; Riediker, M.; Temple, J.S.; Schwartz, J.
Tetrahedron Lett. 1981, 22, 2629.
86.91; H, 7.30. Found: C, 86.87; H, 7.31.
7.
Typical procedure for the cyclization of allylic o-alkynylphenyl
ethers 4 (Second step of procedure B). To a solution of an
approximately 60:40 4’e (84:16, E:Z):4”e mixture (0.100 g, 0.36
mmol) in anhydrous DME (2 mL) were added, under argon,
11. Aranyos, A.; Szabó, K.J., Castaño, A.M.; Bäckvall, J.-E.
Organometallics 1997, 16, 1058. Togni, A.; Burckhardt, U.;
Gramlich, V.; Pregosin, P.S.; Salzmann, R. J. Am. Chem. Soc.
1996, 118, 1031. Szabó, K.J. Organometallics 1996, 15, 1128.
Szabó, K.J. J. Am. Chem. Soc. 1996, 118, 7818. Ward, T.R.
Organometallics 1996, 15, 2836. Castaño, A.M.; Aranyos, A.;
Szabó, K.J.; Bäckvall, J.-E. Angew. Chem. Int. Ed. Engl. 1995, 34,
2551. Hoffmann, H.M.R.; Otte, A.R.; Wilde, A.; Menzer, S.;
Wiliams, D.J. Angew. Chem. Int. Ed. Engl. 1995, 34, 100.
Åkermark, B.; Krakenberger, B.; Hansson, S.; Krakenberg, B.;
Vitagliano, A. J. Organomet. Chem. 1987, 335, 133. Åkermark,
B.; Krakenberger, B.; Hansson, S.; Vitagliano, A.
Organometallics 1987, 6, 620. Åkermark, B.; Vitagliano, A.
Organometallics 1985, 4, 1275. Åkermark, B.; Hansson, S.;
Krakenberger, B.; Vitagliano, A.; Zettreberg, K. Organometallics
1984, 3, 679.
K CO (0.250 g, 1.81 mmol), Pd dba (0.008 g, 0.009 mmol) and
2
3
2
3
ttmpp (0.019 g, 0.036 mmol). The reaction mixture was stirred at
100 °C for 2 h and worked-up as before to afford a residue which
was purified by chromatography eluting with a n-hexane/EtOAc
(98/2 v/v) mixture to provide 0.081g (81 % yield) of 3’e and 3’’e
as a 96:4 mixture. The presence of 3''e was indicated by a
multiplet (ddd) at 6.45-6.30 (H C=CH-CH-) and a multiplet at
2
5.20-5.15 ppm (H C=CH-CH-); 3'e was obtained as an about
2
-1
1
87:13 E:Z mixture: IR (liquid film) 1453, 786, 697 cm ; H NMR
δ 7.95-7.30 (m, 9 H), 5.85-5.50 (m, 1.92 H), 3.75 (d, J = 6.5 Hz,
0.26 H), 3.68 (d, J = 7.0 Hz, 1.66 H), 2.35 (m, 0.26 H), 2.10 (dd, J
= 12.0 Hz, J = 6.0 Hz, 1.74 H), 1.68-1.24 (m, 2 H), 1.08 (t, J = 7.1
Hz, 0.39 H, 3’e Z isomer), 0.95 (t, J = 7.1 Hz, 2.61 H, 3’e E
13
isomer); C NMR δ 154.1, 151.4, 132.2, 131.2, 130.6, 128.7,
128.5, 128.0, 126.8, 124.7, 122.5, 119.9, 114.2, 111.1, 34.7, 27.5,
22.6, 13.7; MS m/e (relative intensity) 276 (M , 96), 233 (80), 194
(100).
+
12. Appleton, T.G.; Clark, H.C.; Manzer, L.E. Coord. Chem. Rev.
1973, 10 , 335. Faller, J.W.; Mattina, M.J. Inorg. Chem. 1972, 11,
1296. Faller, J.W.; Incoura, M.J. J. Organomet. Chem. 1969, 19,
P13.
8.
Typical procedure for the one-pot synthesis of 3-
allylbenzo[b]furans
3 (Procedure B). To a solution of o-
phenylethynylphenol 1a (0.200 g, 1.03 mmol) and allyl ethyl
carbonate 2f (0.140 g, 1.24 mmol) in anhydrous THF (3 mL) was
13. Wada, M.; Higashizaki, S. J. Chem. Soc., Chem. Commun. 1984,
482.
added, under argon, Pd(PPh ) (0.061 g, 0.05 mmol). The mixture
3 4
14. Tolman, C.A. Chem. Rev. 1977, 77, 313.
was stirred at 60°C for 1 h till the disappearance of 1a. Then,
K CO (0.710 g, 5.15 mmol) was added and the mixture was
stirred at 80°C for 3.5 h. Usual workup gave a residue which was
purified by chromatography eluting with a (99/1 v/v) n-hexane/
15. A similar approach to the synthesis of 2-substituted-3-allyl-
benzo[b]furans has been developed independently by Balme and
coworkers: Synlett 1998, 746.
2
3