Palladium-catalyzed tandem heck-lactonization from o-iodophenols and enoates: Synthesis of coumarins and the study of the mechanism by electrospray ionization mass spectrometry

Article abstract of DOI:10.1021/jo1010922

The tandem Heck-lactonization reaction between enoates Z-1a,b, E-1a, E-2a-d, Z-2e, 2f, and o-iodophenols (4a-f) was studied in the presence of substoichiometric amounts of Pd(OAc)₂ or PdCl₂, under experimental conditions favoring the cationic mechanism (conditions A, B, and C), leading to coumarins 5a-f and 6a-e. Moderate to excellent yields were obtained under aqueous conditions (conditions A and B). Using electrospray ionization for transferring ions directly from solution to the gas phase, and mass spectrometry for structural assignments, key cationic palladium intermediates have been successfully intercepted and structurally characterized for the first time for this type of reaction.

Full text of DOI:10.1021/jo1010922

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Palladium-Catalyzed Tandem Heck-Lactonization from o-Iodophenols
and Enoates: Synthesis of Coumarins and the Study of the Mechanism
by Electrospray Ionization Mass Spectrometry
Talita de A. Fernandes,†,‡ Boniek Gontijo Vaz,§ Marcos N. Eberlin,§
Alcides J. M. da Silva,*,† and Paulo R. R. Costa*,†
†
ꢀ
´
^
Laboratorio de Quımica Bioorganica (LQB), Nucleo de Pesquisas de Produtos Naturais, Centro de Ciencias
da Sauꢀde, Bl H, Ilha da Cidade Universitaria, Universidade Federal do Rio de Janeiro, Rio de Janeiro,
ꢀ
^
ꢀ
‡
´
´
^
21941-590, Brazil, Instituto de Quımica, Departamento de Quımica Organica, Universidade Federal do Rio
de Janeiro, Rio de Janeiro, 21945-470, Brazil, and Laboratorio ThoMSon de Espectrometria de Massas,
§
ꢀ
´ ~
Instituto de Quımica, Universidade Estadual de Campinas, Campinas, Sao Paulo, 13083-970, Brazil
prrcosta@ism.com.br; lqb@nppn.ufrj.br; alcides@nppn.ufrj.br
Received June 5, 2010
The tandem Heck-lactonization reaction between enoates Z-1a,b, E-1a, E-2a-d, Z-2e, 2f, and
o-iodophenols (4a-f) was studied in the presence of substoichiometric amounts of Pd(OAc)2 or
PdCl2, under experimental conditions favoring the cationic mechanism (conditions A, B, and C),
leading to coumarins 5a-f and 6a-e. Moderate to excellent yields were obtained under aqueous
conditions (conditions A and B). Using electrospray ionization for transferring ions directly from
solution to the gas phase, and mass spectrometry for structural assignments, key cationic palladium
intermediates have been successfully intercepted and structurally characterized for the first time for
this type of reaction.
Introduction
intermediates can be generated in situ from the corresponding
aryl and heteroaryl iodides, bromides, triflates, diazonium
salts, or even chlorides in the presence of Pd reagents, whereas
nonconjugated olefins, olefins conjugated with both electron
withdrawing groups or electron donating groups, as well as
styrenes, can be used as acceptors of aryl-palladium species.2
Intramolecular Heck reactions are also powerful tools in
The arylation of olefins, discovered by Heck and Mizoroki,
is one of the most important palladium-catalyzed carbon-
carbon coupling reactions.1 Aryl- and heteroaryl-palladium
*To whom correspondence should be addressed. Phone: (þ55) 21 2562-
6791 or 2562-6793. Fax (þ55) 21-2562-6512.
(1) (a) Heck, R. F. J. Am. Chem. Soc. 1968, 90, 5518–5526. (b) Mizoroki,
T.; Mori, K.; Ozaki, A. Bull. Chem. Soc. Jpn. 1971, 44, 581. (c) Heck, R. F.;
Nolley, J. P., Jr. J. Org. Chem. 1972, 37, 2320–2322. (d) Cabri, W.;
Candiani, I. Acc. Chem. Res. 1995, 28, 2–7. (e) de Meijere, A.; Meyer,
F. E. Angew. Chem., Int. Ed. Engl. 1994, 33, 2379–2411. (f) Link, J. T.;
Overman, L. E. In Metal-catalyzed Cross-coupling Reactions; Diederich, F,
Eds.; Wiley-VCH: New York, 1998; pp 231-269. (g) Overman, L. E. Chem.
Rev. 2003, 103, 2945–2963.
(2) (a) Beletskaya, I. P.; Cheprakov, A. V. Chem. Rev. 2000, 100, 3009–
3066. (b) Alonso, F.; Beletskaya, I. P.; Yus, M. Tetrahedron 2005, 61, 11771–
11835. (c) Knowles, J. P.; Whiting, A. Org. Biomol. Chem. 2007, 5, 31–44. (d)
Negishi, E., Ed.; Handbook of Organopalladium Chemistry for Organic
Synthesis; John Wiley & Sons: New York, 2002. (e) Brase, S.; de Meijere,
A. In Metal-catalyzed Cross-coupling Reactions; Diederich, F., Stang, P. J.,
Eds; Wiley-VCH: New York, 1998; pp 99-166.
DOI: 10.1021/jo1010922
r
Published on Web 10/12/2010
J. Org. Chem. 2010, 75, 7085–7091 7085
2010 American Chemical Society

Fernandes et al.
JOCArticle
organic synthesis.3 Several functional groups are tolerated in
the structure of these intermediates and this feature has
facilitated the use of the Heck reaction to synthesize a variety
of highly functionalized carbon skeletons.4 Enantioselective
Heck reactions in the presence of chiral ligands leading
to products in high enantiomeric excess have also been
described.5
Battistuzzi et al.,11 Ulgueri et al.,12 Fernandes et al.,13 and
Giguere et al.14 To gain further knowledge into the scope of
ꢁ
the tandem Heck-lactonization under the conditions pre-
viously studied by us,13 we report here results from the
reaction of enoates Z-1a,b, E-1a, E-2a-d, Z-2e, and 2f with
o-iodophenols (4a-f)15 as Figure 1 summarizes. From
electrospray ionization mass spectrometry (ESI-MS) mon-
itoring, a mechanistic rationalization is proposed for the
tandem Heck-lactonization reactions accomplished under
conditions favoring the cationic mechanism (use of Ag2CO3
as base or water as solvent). An intramolecular Heck reac-
tion from enoates 3 leading to coumarins or aurones is also
evaluated (Figure 1).
When aryl palladium intermediates are substituted at the
ortho-position by hydroxyl or amine groups, they react with
olefins by tandem or domino reactions leading to hetero-
cyclics. For example, dihydrobenzofurans are formed in
oxa-Heck reactions by oxyarylation of chromenes,6 dihydro-
naphthalenes, enol ethers, and other olefins with o-iodophe-
nols, whereas dihydrobenzopyrrolidines are obtained by
aza-Heck reactions by azaarylation of dihydronaphthalene
by o-iodoaniline.7
Coumarins display many biological activities such as
anticoagulant, antioxidant, antiproliferative, and anti-
HIV,8 and can also be used as intermediates in the synthesis
of cumestans and benzofurans.9 Several years ago, Heck
et al.10 failed to prepare coumarins by the reaction between
enoates and o-iodophenols. However, lactams were success-
fully prepared from enoates and o-iodoaniline under the
same conditions.10 More recently, coumarins were obtained
by the tandem Heck-lactonization reaction involving
enoates or cinnamates and o-halophenols, as reported by
FIGURE 1. Enoates (1-3) and o-iodophenols (4a-f) used in this
work.
Results
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R. J.; Thuilliez, A.; Tozer, M. J. J. Chem. Soc., Perkin Trans. 1 1997, 447–455.
(d) McDermott, M. C.; Stephenson, G. R.; Hughes, D. L.; Walkington, A. J.
Org. Lett. 2006, 8, 2917–2920. (e) Link, J. T.; Overman, L. E. In
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1994, 66, 1423–1430. (g) Overman, L. E. Chem. Rev. 2003, 103, 2945–2963.
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3736. (b) Zeni, G.; Larock, R. C. Chem. Rev. 2006, 106, 4644–4680.
(5) (a) Dounay, A. B.; Overman, L. E. Chem. Rev. 2003, 103, 2945–2963.
(b) Tietze, L. F.; Ila, H.; Bell, H. P. Chem. Rev. 2004, 104, 3453–3516. (c)
Shibasaki, M.; Boden, C. D. J.; Kojima, A. Tetrahedron 1997, 53, 7371–7393.
(d) Tietze, L.; Raschke, T. Synlett 1995, 597–598. (e) Ozawa, F.; Kobatake,
Y.; Hayashi, T. Tetrahedron Lett. 1993, 34, 2505–2508. (f) Loiseleur, O.;
Hayashi, M.; Schmees, N.; Pfaltz, A. Synthesis 1997, 1338–1345. (g) Honzawa,
S.; Mizutani, T.; Shibasaki, M. Tetrahedron Lett. 1999, 40, 311–314.
In recent studies by our research group, the tandem
Heck-lactonization reaction between enoates E-1, Z-1, E-2b
and o-iodophenols 4a-d leading to the corresponding cou-
marins (Scheme 1) was investigated under several conditions
and three procedures were selected as the more efficient:
condition A, condition B, and condition C (Table 1).16 In
this paper we extend the scope of this reaction including in
our study new conjugated esters (Z-1b, E-2a, E-2c, E-2d, and
E-2e) and o-iodophenols (4e,f). These new results are pre-
sented in Table1 along with those previously obtained,
which were included for comparative purposes.13
SCHEME 1. Reaction of Z-1a and Z-1b with o-Iodophenols
4a-f
(6) (a) Larock, R. C. U.S. Pat. Appl. No. 767,794, 1998. (b) Kiss, L.;
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Kurtan, T.; Antus, S.; Brunner, H. Arkivoc 2003, 69–76. (c) Sant’Anna, S. S.;
Evangelista, E. A.; Alves, R. B.; Raslan, D. S. Chem. Nat. Compd. 2005, 41,
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Horino, H.; Inoue, N. Bull. Chem. Soc. Jpn. 1971, 44, 3210. (f) Horino, H.;
Arai, M.; Inoue, N. Bull. Chem. Soc. Jpn. 1974, 47, 1683–1686.
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3766.
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Narvaez-Mastache, J. M.; Novillo, F.; Delgado, G. Phytochemistry 2008, 69,
451–456. (c) Musa, M. A.; Cooperwood, J. S.; Khan, M. O. F. Curr. Med.
Chem. 2008, 15, 2664–2679. (d) Chiang, C. C.; Mouscadet, J. F.; Tsai, H. J.;
Liu, C. T.; Hsu, L. Y. Chem. Pharm. Bull. 2007, 55, 1740–1743. (e) Kostova,
I.; Raleva, S.; Genova, P.; Argirova, R. Bioinorg. Chem. Appl. 2006, 68274.
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Tetrahedron 2008, 64, 3661–3666. (b) Lee, Y. R.; Suk, J. Y.; Kim, B. S. Org.
Lett. 2000, 2, 1387–1389. (c) Jin, Y. L.; Kim, S.; Kim, Y. S.; Kim, S. A.; Kim,
H. S. Tetrahedron Lett. 2008, 49, 6835–6837. (d) Yamashita, M.; Inaba, T.;
Nagahama, M.; Shimizu, T.; Kosaka, S.; Kawasaki, I.; Ohta, S. Org . Biomol.
Chem. 2005, 3, 2296–2304. (e) Yamashita, M.; Yadav, N. D.; Sawaki, T.;
Takao, I.; Kawasaki, I.; Sugimoto, Y.; Miyatake, A.; Murai, K.; Takahara,
A.; Kurume, A.; Ohta, S. J. Org. Chem. 2007, 72, 5697–5703.
(10) Cortese, N. A.; Ziegler, C. B.; Hrnjez, B. J.; Heck, R. F. J. Org. Chem.
1978, 43, 2952–2958.
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A. J. M.; Costa, P. R. R. Tetrahedron Lett. 2008, 49, 3322–3325.
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Tetrahedron Lett. 2009, 50, 4254–4257.
(15) Enoates E-1 and Z-1, and o-iodophenols 4a-f were prepared as
previously described: (a) Costa, J. S.; Dias, A. G.; Anholeto, A. L.;
Monteiro, M. D.; Patrocınio, V. L.; Costa, P. R. R. J. Org. Chem. 1997,
´
62, 4002–4006. (b) da Frota, L. C. R. M.; Canavez, R. C. P.; Gomes, S. L. S.;
Costa, P. R. R.; da Silva, A. J. M. J. Braz. Chem. Soc. 2009, 20, 1916–1920.
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Buarque, C. D.; Pinto, V. D.; Vaz, B. G.; Eberlin, M. N.; da Silva, A. J. M.;
Costa, P. R. R. J. Organom. Chem. 2010, 695, 2062–2067.
(11) Battistuzzi, G.; Cacchi, S.; De Salve, I.; Fabrizi, G.; Parisi, L. M. Adv.
Synth. Catal. 2005, 347, 308–312.
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6059.
7086 J. Org. Chem. Vol. 75, No. 21, 2010

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JOCArticle
TABLE 1. New Conditions and Yields for Reactions Shown in Scheme 1
TABLE 2. Conditions and Yields for Scheme 2
entry
4
olefin
product
conditiona
yield (%)
entry
4
olefin
product
conditiona
yield (%)
1
2
3
4
5
6
7
8
10
10
11
12
13
14
15
16
17
18
4a
4a
4a
4a
4a
4a
4b
4b
4b
4c
4c
4c
4d
4d
4e
4e
4f
Z-1a
Z-1a
Z-1a
Z-1b
Z-1b
E-1a
Z-1a
Z-1a
Z-1a
Z-1a
Z-1a
Z-1a
Z-1a
Z-1a
Z-1a
Z-1a
Z-1a
Z-1a
5a
5a
5a
5a
5a
5a
5b
5b
5b
5c
5c
5c
5d
5d
5e
5e
5f
A
B
C
A
C
B
A
B
C
A
B
C
A
C
A
C
A
C
73b
84b
68
56
42
7
78
90
23
59
51
68
50
52
31
traces
53
1
2
3
4
5
6
7
8
9
10
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
E-2a
E-2a
E-2b
E-2b
E-2c
E-2c
E-2d
Z-2e
2f
6a
6a
6b
6b
6c
6c
6d
6e
7
B
C
A
C
B
C
B
B
C
D
55
12
44
63
66
68
18
17
100
2f
7
aCondition A: 10 mol % of Pd(OAc)2, H2O, 3 equiv of Et3N, 80 °C, 40 h.
Condition B: 10 mol % of PdCl2, H2O, 3 equiv of Et3N, 80 °C, 40 h.
Condition C: 10 mol % of Pd(OAc)2, 20 mol % of PPh3, acetone, 3 equiv of
Ag2CO3, 70 °C, 40 h. Condition D: 10 mol % of Pd(OAc)2, MeCN, 3 equiv
of Et3N, 70 °C, 40 h.
4f
5f
SCHEME 3. Synthesis of E-3 and Z-3 and Their Intramolecular
Heck Reactions
aCondition A: 10 mol % of Pd(OAc)2, H2O, Et3N, 80 °C, 40 h.
Condition B: 10 mol % of PdCl2, H2O, Et3N, 80° C, 40 h. Condition C:
10 mol % of Pd(OAc)2, acetone, 3 equiv of Ag2CO3, 20 mol % of PPh3,
70° C, 40 h. b1 mol % was used in these cases.
Enoate Z-1a reacted with 4a leading to coumarin 5a in good
yields under conditions A and B, even in the presence of 1 mol %
of the catalyst (entries 1 and 2). A lower yield was obtained
under conditions C (entry 3) and poor yields were observed
when the amount of catalyst was reduced to 5 mol % (data not
shown). Enoate Z-1b was also allowed to react with 4a under
condition B, leading to coumarin 5a (entry 4), but the yield was
reduced to 56% (entry 4) probably due to the presence of the
bulky tBu ester group, which could hamper the lactonization
step. A poor yield was obtained in condition C (entry 5). In
contrast, only 7% of 5a was formed in the reaction of E-1 with
4a in condition B (entry 6). Enoate Z-1a also reacted with
p-chloro-o-iodophenol (4b) leading to coumarin 5b in good
yields under conditions A and B (entries 7 and 8), but in this
case the yield dropped to 23% under condition C (entry 9). For
the reactions of Z-1 with 4c,d, moderate to good yields were
obtained in conditions A, B, and C (entries 11-14). In the case
of o-iodophenols 4e,f, coumarins 5e and 5f were obtained in
moderated yields, only in condition A (entries 15-18).
condition C (entry 6) but 66% yield of arylcoumarin 6c was
obtained under condition B (entry 5). Arylcoumarin 6d could
also be obtained by reaction of Z-1a with 4a (entry 7). Despite the
Z-geometry, however, Z-2e furnished 6e in reduced yield (entry
8). Methylacrylate (2f), in contrast, afforded the cynnamate 7 in
low yield under condition C while under classical Heck condi-
tions 7 can be obtained in quantitative yields (entries 9 and 10).13
To confirm that Z enoates are more reactive in oxaaryla-
tion reactions than E enoates, a competitive reaction was
carried out with a 2:1 mixture of E-2a and Z-2a. Via NMR
monitoring (not shown), we were able to observe that Z-2a
was consumed much faster than E-2a.
SCHEME 2. Reactions of o-Iodophenol (4a) with Olefins E-2a-d,
Z-2e, and 2f
When the reactions between Z-1a and 4a-f were inter-
rupted before completion under conditions A, B, or C,
unreacted Z-1a was isolated without loss of its optical purity,
as shown by its [R] value.
Finally, we investigated the intramolecular Heck reaction
using enoates E-3 and Z-3 as substrates, which were prepared
respectively from E-1a and Z-1a (Scheme 3). The ester
groups of E-1a and Z-1a were hydrolyzed in basic medium
leading to E-8 and Z-8,17 respectively, and these acids were
To further investigate the scope of this interesting reac-
tion, enoates E-2a-d, Z-2e, and 2f were allowed to react with
4a (Scheme 2, Table 2). In contrast with enoate E-1a, E-2a led
to coumarin 6a in reasonable yield in condition B (entry 1),
but only 12% was obtained in condition C (entry 2). Coumarin
6b could be obtained in reasonable yield in conditions A and
C (entries 3 and 4). Cinnamates E-2c did not react with 4a in
(17) Fernandes, T. A.; Master Thesis, NPPN-UFRJ, 2008.
J. Org. Chem. Vol. 75, No. 21, 2010 7087

Fernandes et al.
JOCArticle
FIGURE 2. ESI(þ)-MS of the reaction solution of Z-1 with 4a (condition B, in the absence of PPh3:acetone, Pd(OAc)2, and Ag2CO3).
TABLE 3. Conditions and Yields for Scheme 3
accommodation of the incoming arylpalladium species in
the transition state of the cyclopalladation step. Among
E-enoates, however, the reactivity depended on the β-sub-
stituent. In E-2a and E-2b, Cβ is substituted by a CH2CH-
(Me)2 and a CH3 group, respectively, and their double bonds
are therefore less sterically hindered than that in E-1a, for
which the carbon attached at the double bond is disubstituted.
This reduced hindering could explain why E-2a and E-2b are
more reactive than E-1a. In cinnamates E-2c and E-2d, the
planar aryl groups do not afford an important steric hindrance at
the double bond carbons and, additionally, these electron donat-
ing groups may facilitate the reaction through electronic effects.
However, the electron poor olefin Z-2e led to 6e in poor yield.
Since better yields of coumarins were obtained under con-
ditions favoring the cationic mechanism (conditions A, B, and
C), these reactions were monitored by ESI-MS, which has been
shown to provide continuous snapshots of the changing
composition of reaction solutions of major organic reactions.23
entry
olefin
conditiona
product
yield (%)
1
2
3
4
E-3
E-3
Z-3
Z-3
C
B
C
B
8
5a
8
45
traces
32
29
5a
aCondition C: 10 mol % of Pd(OAc)2, 20 mol % of PPh3, acetone,
3 equiv of Ag2CO3, 70 °C, 40 h. Condition B: 10 mol % of PdCl2, H2O,
3 equiv of Et3N, 80 °C, 40 h.
allowed to react with o-iodophenol (4a) in the presence of a
mixture of PPh3, CCl4, and Et3N (E-7)18 or DCC (Z-7),19
leading to the respective E-3 and Z-3.20
In intramolecular Heck reactions, preference for the
exo-cyclization is usual even when the electronic effects favor
the endo-approach, mainly forming five-membered-ring com-
pounds.21 For example, Rizzi et al.3a recently reported that
the ester obtained from o-iodophenol and p-methoxy cinna-
mate cyclized to the corresponding aurone in the presence of
Pd(OAc)2 whereas Ashimori et al.22 showed that the amide
formed from R-methyl-Z-crotonate and o-iodoaniline furn-
ished an indole derivative in the presence of Pd2(dba)3. In our
study, as expected, E-3 led to aurone 9 (mixture of isomers, 1/1)
in acetone in the presence of Pd(OAc)2 and Ag2CO3 (condition B),
whereas in the presence of PdCl2 and Et3N in H2O (condition
D) only traces of coumarin 5a were found (Scheme 3, Table 3,
entries 1 and 2). From Z-3, both aurone 9 and coumarin 5a
were obtained in moderate yield, depending on the reaction
conditions employed (entries 3 and 4). It has been described
that aqueous media must exert influence on the regioselectivity
of intramolecular reactions.2a
FIGURE 3. Proposed cationic palladium intermediates formed in
the reaction of Z-1a with 4a in acetone, under condition C, in the
absence of PPh3.
Heck-Lactonization-Pd(II) Intermediates. The reaction of
Z-1 with 4a (condition C, in the absence of PPh3:acetone,
Pd(OAc)2, Ag2CO3) was monitored via direct infusion
ESI-MS and its tandem version (ESI-MS/MS) in the positive
ion mode. Samples were diluted in MeCN before recording
the spectra.
Discussion
For Heck-lactonization, enoate Z-1a proved to be more
reactive than its E-isomer, leading to coumarin 5a in better
yields under all reaction conditions studied. The Z-double
bond is less sterically hindered, which may allow a better
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Santos, L. S.; Eberlin, M. N.; Comasseto, J. V. Organometallics 2004, 23,
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procedure to synthesize esters E-9 and Z-9.
(21) Baldwin, J. E. J. Chem. Soc., Chem Commun. 1976, 734–736.
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L. E.; Poon, D. J. J. Am. Chem. Soc. 1998, 120, 6488–6499.
€
Sjoberg, P. J. R.; Larhed, M.; Nilsson, P. Tetrahedron 2008, 64, 1808–1812.
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FIGURE 4. Cationic palladium intermediates in the reaction of Z-1 with 4a (condition C in the presence of PPh3:acetone, Pd(OAc)2, Ag2CO3,
intercepted as MeCN adducts.
Interestingly, ESI(þ)-MS was able to provide the first
SCHEME 4. Mechanistic View for the Reaction of 4a with Z-1a
mechanistic detail for the reaction of Z-1a with 4a (Figure 2)
Based on the ESI(þ)-MS(/MS) Data
by intercepting a series of key cationic Pd(II)þ intermediates:
(a) 10a of m/z 322, which results from oxidative addition of
Pd[0] to 4a and triple-complexation with MeCN, and (b) 11a
of m/z 440 and 11b of m/z 481 originated from carbopallada-
tion of Z-1a and mono- and dicomplexation with MeCN.
The proposed structures of 10a and 11b are corroborated
by their characteristic Pd multi-isotopic patterns and by
ESI-MS/MS (see the Supporting Information). Cation 11b,
as expected, dissociates by two consecutive losses of MeCN
to form 11a of m/z 440 and 11c of m/z 399 or by the loss of the
reactant olefin to form H-Pd(MeCN)2þ of m/z 189, which in
turn loses MeCN to form H-Pd(MeCN)þ of m/z 148.
(Figure S1, Supporting Information). The intervention of
the 10b and 11c (Figure 3) in the actual reaction media likely
occurs via stabilization by chelating agents (other than
MeCN) present in the reaction solution, such as the acetone
solvent.
In the reaction carried out in the presence of PPh3,
analogous cationic Pd(II)þ intermediates now mainly coor-
dinated to PPh3 (andOdPPh3) were also intercepted (Figure 4).
Cation 10c of m/z 723, also originated from the oxidative
addition of Pd(0) to4a, is analogous to 10a and corresponds to
Heck-Matsuda reaction, using tellurides25 or diazonium
salts26 as substrates instead of aryl iodides, and for oxyaryla-
tion reactions with o-iodophenols,27 the major cationic
coordination to two PPh3 ligands. Cation 11d of m/z 663
originates from regioselective carbopalladation of the double
bound of Z-1a by 10c (or an analogous species), strongly
suggesting that the lactonization step occurs after the carbo-
palladation step. Note that 11 (see Scheme 4) was intercepted
in Figure 4 in a variety of coordinated forms: 11a, 11d, 11e, 11f,
and 11g. The OdPPh3 ligand likely originated from PPh3
oxidation by air, which occurs even when these reactions are
run under dry argon.24
palladium species were also detected and characterized by ESI-
MS(/MS). Therefore, ESI(þ)-MS(/MS) is shown herein to also
be able to intercept and characterize major cationic intermedi-
ates in Heck-lactonization reactions, further establishing in
detail the mechanism of this reaction.
Our next goal was to monitor the tandem Heck-lactoniza-
tion between Z-1a and 4a in water in the presence of PdCl2
and Et3N. The aim was to verify whether cationic inter-
mediates could also be intercepted and characterized by
ESI-MS(/MS) under these conditions. With use of Et3N,
the major cationic species detected from the aqueous reac-
tion solutions was the protonated base Et3NHþ. When Et3N
The intervention of cationic palladium intermediates in
Heck reaction with ArI as substrates has been suggested
when halide scavengers such as Ag2CO3 are used.2 For the
(24) (a) Kong, K. C.; Cheng, C. H. J. Am. Chem. Soc. 1991, 113, 6313–
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6315. (b) Herrmann, W. A.; Brossmer, C.; Ofele, K.; Beller, M.; Fischer, H.
J. Mol. Catal. 1995, 133–146. (c) Sakamoto, M.; Shimizu, I.; Yamamoto, A.
Chem. Lett. 1995, 1101–1102. (d) Morita, D. K.; Stille, J. K.; Norton, J. R.
J. Am. Chem. Soc. 1995, 117, 8576–8581. (e) Goodson, F. E.; Wallow, T. I.;
Novak, B. M. J. Am. Chem. Soc. 1997, 119, 12441–12453.
(25) Stefani, H. A.; Pena, J. M.; Gueogjian, K.; Petragnani, N.; Vaz,
B. G.; Eberlin, M. N. Tetrahedron Lett. 2009, 50, 5589–5595.
(26) Sabino, A. A.; Machado, A. H. L.; Correia, C. R. D.; Eberlin, M. N.
Angew. Chem., Int. Ed. 2004, 43, 2514–2518.
(27) Buarque, C. D.; Pinho, V. D.; Vaz, B. G.; Eberlin, M. N.; da Silva,
A. J. M.; Costa, P. R. R. J. Organomet. Chem. 2010, 695, 2062–2067.
J. Org. Chem. Vol. 75, No. 21, 2010 7089

Fernandes et al.
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FIGURE 5. ESI(þ)-MS of the reaction solution of the reaction of Z-1 with 4a in H2O in the presence of 10 mol % of PdCl2 and NaOH.
SCHEME 5. The Different Reaction Pathways for the Reaction
of 4a with Either E-2a-d or 2f
conformer C2. The C2 conformer has the required steric
relationships to undergo a cis-elimination of HPdþ and
lactonization, leading to coumarin 5a. The base present in
the reaction medium (CO32-) regenerates Pd(0) thus restart-
ing the catalytic cycle. A similar rationalization can be
proposed for the reactions in water.
On the basis of the mechanism of Scheme 4, we can
rationalize the formation of either 6 or 7 from enoates
E-2a-d and 2f (Scheme 5). When methyl acrylate 2f was
used in reaction with 4a, intermediate 12a (conformer C1,
R4 = H), formed by syn-carbopalladation, is in equilibrium
with C2, and after the elimination step leads to cinnamate 7
(Scheme 5). In intermediates 12b,c, conformer C1, R4 = Me
or iBu, formed from substituted enoates E-2a-d is, however,
more stable than conformer C2 (steric interactions between
R4 and CO2Et) and this conformer isomerizes to 14 via a
palladium enolate 13, which after the elimination step leads
to coumarins 6a,b. Precedents for epimerization of organo-
palladium intermediates in which the palladium is attached
to the R-position of aromatic rings and carbonyl groups are
known.28
Conclusions
The general scope of the Heck-lactonization involving
E- and Z-enoates was investigated. It was shown that this
reaction is sensitive to steric hindrance around the double
bound in the enoates. The intervention of cationic palladium
intermediates was evidenced by ESI(þ)-MS(/MS) for reac-
tions in the presence of Ag2CO3 or with H2O as solvent. A
series of coumarins disubstituted at the 6- and 4-positions
could be obtained in one step from enoates Z-1a, E-2a-d,
and Z-2e via a tandem Heck-lactonization reaction under
eco-friendly conditions employing water and avoiding the
use of silver salts and phosphines.
was replaced by NaOH, however, the same set of cationic
intermediates intercepted in reactions with acetone/Ag2CO3
(Figure 4) could also be intercepted in water, after dilution
with MeCN (Figure 5). Although these conditions are not
useful for preparative purposes (ester hydrolysis is expected)
these results strongly suggest the participation of cationic
palladium species 10b and 11c (Figure 3) as intermediates in
the oxyarylation of Z-1a by 4a under conditions A and B.
On the basis of the key information provided by ESI(þ)-
MS(/MS) monitoring, oxyarylation reactions accomplished
in the presence of Ag2CO3 (conditions C) or with water as
solvent (conditions A and B) can be rationalized to proceed
via a cationic mechanism, as Scheme 4 proposes for the
reaction between 4a and Z-1a in condition C. The reaction
proceeds as follows: oxidative insertion of Pd(0) to 4a in the
presence of Ag2CO3 leads to 10, and regioselective carbo-
palladation of the double bond in Z-1a by 10 leads to 11
(conformer C1, cis-addition), which is in equilibrium with
Experimental Section
General Procedures. 1H and 13C NMR spectra were acquired
at 200, 400, or 500 MHz for proton and 50 or 100 MHz for
(28) (a) Takahashi, T.; Jinbo, Y.; Kitamura, K.; Tsuji, J. Tetrahedron
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Lett. 1984, 25, 5921–5924. (b) Ludwig, M.; Stromberg, S.; Svessos, M.;
Akermark, B. Organometallics 1999, 18, 970–975. (c) Granberg, K. L.;
€
Backvall, J.-E. J. Am. Chem. Soc. 1992, 114, 6858–6863. (d) Kuwano, R.
Synthesis 2009, 1049–1061.
7090 J. Org. Chem. Vol. 75, No. 21, 2010

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carbon nuclei in CDCl3 or CH3OD. All reagents were used
without purification. All MS experiments were performed on a
hybrid quadrupole time-of-flight mass spectrometer (Q-TOF).
For typical electrospray ionization (ESI) conditions, the Teflon-
sealed microsyringe was put in a pump that delivered the reagent
solution into the ESI source at a flow rate of 10 μL min-1. ESI
and the mass spectrometer were operated in the positive ion
mode. Main conditions were capillary voltage, 3500 eV; cone
voltage, 35 eV; source temperature, 100 °C; and desolvation
temperature, 100 °C. The cationic species were subjected to
collision-induced dissociation (CID) with argon by using colli-
sion energies ranging from 5 to 45 eV.
J = 8.4 Hz, 1 H), 6.76 (s, 1 H), 5.42 (t, J = 7.0 Hz, 1 H), 4.75-4.54
(m, 1 H), 3.97 (s, 3 H), 3.92-3.69 (m, 1 H), 1.59 (s, 3 H), 1.55 (s, 3
H); 13C NMR (50 MHz, CDCl3) δ165.4, 159.8, 156.4, 153.2, 132.7,
126.2, 125.4, 117.6, 117.0, 112.4, 110.7, 73.0, 69.3, 52.5, 25.9, 25.2;
HRMS (ESI) calcd for C16H17O6 [M þ H] 305.1025, found
305.1019.
(S)-4-(2,2-Dimethyl-1,3-dioxolan-4-yl)-8-methoxy-2-oxo-2H-
chromene-6-carbaldehyde (5e). Compound 5e was obtained as a
pale solid after purification by flash chromatography (EtOAc-
hexane 2:8): RD þ73.3 (c 1.0, CHCl3); mp 194-196 °C; 1H NMR
(200 MHz, CDCl3) δ 9.99 (s, 1 H), 7.60 (d, J = 1.5 Hz, 1 H), 7.51
(d, J = 1.5 Hz, 1 H), 6.80 (d, J = 1.2 Hz, 1 H), 5.41 (dt, J = 1.3,
6.9 Hz, 1 H), 4.63 (dd, J = 8.3, 7.1 Hz, 1 H), 4.05 (s, 3 H), 3.83 (dd,
J = 8.4, 6.7 Hz, 1 H), 1.59 (s, 3 H), 1.55 (s, 3 H); 13C NMR (50
MHz, CDCl3) δ 189.9, 159.0, 153.2, 148.6, 147.7, 132.3, 118.4,
117.9, 112.9, 111.5, 110.9, 73.1, 69.3, 56.4, 26.0, 25.1; HRMS (ESI)
calcd for C16H17O6 [M þ H] 305.1025, found 305.1019.
General Procedure for the Synthesis of Coumarins.29. A mix-
ture of o-iodophenols (1 equiv), enoate (3 equiv), palladium salt
(1-10 mol %), phosphine ligand (in some cases was not used),
and base was heated in the selected solvent for the indicated
temperature and time under nitrogen atmosphere. Then, the
reaction mixture was partitioned between ethyl acetate and
brine and the organic layer was filtered in diatomaceous earth,
dried over anhydrous sodium sulfate, and evaporated. The
resulting oil was purified by flash chromatography leading to
the coumarins in the indicated yield. The products obtained
were characterized by 1H NMR, 13C NMR, and mass spectro-
metry (high resolution).
(S)-4-(2,2-Dimethyl-1,3-dioxolan-4-yl)-6-(hydroxymethyl)-8-
methoxy-2H-chromen-2-one (5f). Compound 5f was obtained as
a pale solid after purification by flash chromatography
(EtOAc-hexane 1:1): RD þ66.9 (c 1.0, CHCl3); mp 139-141
°C; 1H NMR (200 MHz, MeOD) δ 7.23 (s, 1 H), 7.07 (s, 1 H),
6.57 (d, J = 1.3 Hz, 1 H), 5.52-5.37 (m, 1 H), 4.84 (s, 2 H),
4.74-4.56 (m, 2 H), 3.94 (s, 3 H), 3.74 (dd, J = 8.4, 6.6 Hz, 1 H),
1.54 (s, 3 H), 1.50 (s, 3 H); 13C NMR (50 MHz, MeOD) δ 162.7,
156.9, 148.9, 143.7, 140.0, 118.8, 113.9, 113.8, 111.8, 111.7, 74.6,
70.7, 64.5, 56.8, 26.5, 25.5; HRMS (ESI) calcd for C16H19O6
[M þ H] 307.1182, found 307, 1174.
(S)-4-(2,2-Ddimethyl-1,3-dioxolan-4-yl)-2H-chromen-2-one (5a).
Compound 5a was obtained as a yellow solid after purification by
flash chromatography (EtOAc-hexane 1:9): RD þ107 (c 1.0,
1
CHCl3); mp 132-135 °C; H NMR (200 MHz, CDCl3) δ 7.55
(ddd, J= 8.6, 7.0, 1.6 Hz, 1 H), 7.46-7.23 (m, 3 H), 6.72 (d, J=1.2
Hz, 1 H), 5.39 (dt, J = 1.2, 7.0 Hz, 1 H), 4.59 (dd, J = 7.2, 8.3 Hz,
Acknowledgment. We acknowledge the Rio de Janeiro State
Science Foundation (FAPERJ) and the Brazilian National
Science Council (CNPq) for financial support. P.R.R.C. and
M.N.E. thank CNPq, T.A.F. thanks FAPERJ and CAPES,
and B.G.V. thanks FAPESP for research fellowships.
1 H), 3.80 (dd, J = 6.9, 8.3 Hz, 1 H), 1.57 (s, 3 H), 1.53 (s, 3 H); 13
C
NMR (50 MHz, CDCl3) δ 160.5, 153.5, 153.3, 131.7, 124.2, 123.1,
117.3, 117.0, 111.5, 110.4, 73.0, 69.3, 25.9, 25.1. HRMS (ESI) calcd
for C14H15O4 [M þ H] 247.0970, found 247.0965.
(S)-6-Chloro-4-(2,2-dimethyl-1,3-dioxolan-4-yl)-2H-chromen-2-
one (5b). Compound 5b was obtained as a white solid after
purification by flash chromatography (EtOAc-hexane 1:19): RD
Supporting Information Available: ESI(þ)-MS/MS of m/z
322 “fished” from the reaction solution containing Z-1, 4a,
Pd(OAc)2, and Ag2CO3 in acetone diluted with MeCN (p S1);
ESI(þ)-MS/MS of m/z 481 “fished” from the reaction solution
containing Z-1, 4a, Pd(OAc)2, and Ag2CO3, PPh3 in acetone
diluted with MeCN (p S2); ESI(þ)-MS/MS of m/z 661 “fished”
from the reaction solution containing Z-1, 4a, Pd(OAc)2, and
Ag2CO3, PPh3 in acetone diluted with MeCN (p S3); ESI(þ)-
MS/MS of m/z 677 “fished” from the reaction solution contain-
ing Z-1, 4a, Pd(OAc)2, and Ag2CO3, PPh3 in acetone diluted
with MeCN (p S4); ESI(þ)-MS/MS of m/z 939 “fished” from the
reaction solution containing Z-1, 4a, Pd(OAc)2, and Ag2CO3,
PPh3 in acetone diluted with MeCN (p S5); ESI(þ)-MS/MS of
m/z 957 “fished” from the reaction solution containing Z-1, 4a,
Pd(OAc)2, and Ag2CO3, PPh3 in acetone diluted with MeCN
(p S6); ESI(þ)-MS/MS of m/z 723 “fished” from the reaction
solution containing Z-1, 4a, Pd(OAc)2, and Ag2CO3, PPh3 in
acetone diluted with MeCN (p S7); 1H NMR of 5a (p S8); APT
of 5a (p S9); high resolution mass of 5a (p S10); 1H NMR of 5b
(p S11); APT of 5b (p S12); high resolution mass of 5b (p S13); 1H
NMR of 5c (p S14); APT of 5c (p S15); high resolution mass of
5c (p S16); 1H NMR of 5d (p S17); APT of 5d (Figure S18); high
resolution mass of 5d (p S19); 1H NMR of 5e (p S20); APT of 5e
(p S21); high resolution mass of 5e (p S22); 1H NMR of 5f
(p S23); APT of 5f (p S24); high resolution mass of 5f (p S25); 1H
NMR of 6a (p S26); APT of 6a (p S27); 1H NMR of 6b (p S28);
1H NMR of 6c (p S29); 1H NMR of 6d (p S30); high resolution
mass of 6d (p S31); 1H NMR of 6e (p S32); and high resolution
mass of 6e (p S33). This material is available free of charge via
the Internet at http://pubs.acs.org.
1
þ96.9 (c 1.0, CHCl3); mp 129-130 °C; H NMR (200 MHz,
CDCl3) δ 7.50 (dd, J = 2.3, 8.8 Hz, 1 H), 7.38 (d, J = 2.3 Hz, 1 H),
7.32 (d, J = 8.8 Hz, 1 H), 6.73 (d, J = 1.2 Hz, 1 H,), 5.31 (dt, J =
1.2, 6.9 Hz, 1 H), 4.59 (dd, J = 7.2, 8.4 Hz, 1 H), 3.81 (dd, J = 6.7,
8.4 Hz, 1 H), 1.57 (s, 3 H), 1.53 (s, 3 H); 13C NMR (50 MHz,
CDCl3) δ 159.9, 152.3, 152.0, 131.7, 129.7, 122.9, 118.8, 118.3,
112.8, 110.7, 72.9, 69.2, 26.0, 25.1; HRMS (ESI) calcd for
C14H14ClO4 [M þ H] 281.0581, found 281.0575.
(S)-4-(2,2-Dimethyl-1,3-dioxolan-4-yl)-6-nitro-2H-chromen-2-
one (5c). Compound 5c was obtained as a white solid after
purification by flash chromatography (EtOAc-hexane 1:9):
RD þ109.6 (c 1.0, CHCl3); mp 154-156 °C; 1H NMR (200
MHz, CDCl3) δ 8.50-8.36 (m, 2 H), 7.52 (d, J = 9.2 Hz, 1 H),
6.82 (s, 1 H), 5.41 (t, J = 6.8 Hz, 1 H), 4.72-4.59 (m, 1 H),
3.93-3.81 (m, 1 H), 1.61 (s, 3 H), 1.56 (s, 3 H); 13C NMR (50
MHz, CDCl3) δ 158.8, 157.2, 152.5, 143.7, 126.5, 119.8, 118.5,
117.3, 113.9, 111.1, 73.0, 69.0, 25.9, 24.9; HRMS (ESI) calcd for
C14H14NO6 [M þ H] 292.0821, found 292.0815.
(S)-Methyl 4-(2,2-Dimethyl-1,3-dioxolan-4-yl)-2-oxo-2H-
chromene-6-carboxylate (5d). Compound 5d was obtained as a
yellow solid after purification by flash chromatography (EtOAc-
hexane 1:9): RD þ88.9 (c 1.0, CHCl3); 1H NMR (200 MHz,
CDCl3) δ 8.21 (dd, J = 1.8, 8.8 Hz, 1 H), 8.13 (s, 1 H), 7.42 (d,
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