Nickel-catalyzed formation of cyclopentenone derivatives via the unique cycloaddition of α,β-unsaturated phenyl esters with alkynes

Article abstract of DOI:10.1021/ja2059999

Oxygen-containing organic compounds, such as ethers, carboxylates, and carbamates, have recently received increasing attention because of their newly discovered applications as electrophiles in cross-coupling reactions via transition metal-catalyzed C-O bond activation. However, no cycloaddition reaction involving their C-O bond activation has been demonstrated thus far. The present study developed a Ni(0)-catalyzed unique [3+2] cycloaddition reaction of α,β-unsaturated phenyl esters with alkynes in ⁱPrOH to yield cyclopentenone derivatives.

Full text of DOI:10.1021/ja2059999

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Nickel-Catalyzed Formation of Cyclopentenone Derivatives via the
Unique Cycloaddition of α,β-Unsaturated Phenyl Esters with Alkynes
Masato Ohashi,*,†,‡ Tomoaki Taniguchi,† and Sensuke Ogoshi*,†
†Department of Applied Chemistry, Faculty of Engineering, and ‡Center for Atomic and Molecular Technologies, Osaka University,
Suita, Osaka 565-0871, Japan
S Supporting Information
b
been reported thus far. Therefore, the present study was focused
ABSTRACT: Oxygen-containing organic compounds, such
as ethers, carboxylates, and carbamates, have recently re-
ceived increasing attention because of their newly discov-
ered applications as electrophiles in cross-coupling reactions
via transition metal-catalyzed CÀO bond activation. How-
ever, no cycloaddition reaction involving their CÀO bond
activation has been demonstrated thus far. The present
study developed a Ni(0)-catalyzed unique [3+2] cycloaddi-
tion reaction of α,β-unsaturated phenyl esters with alkynes
in iPrOH to yield cyclopentenone derivatives.
on whether the dephenoxylated three-carbon unit derived from
the CacylÀO bond cleavage of α,β-unsaturated phenyl ester could
act as a coupling partner with the other unsaturated compounds.
When the stoichiometric reaction of 1b with 3-hexyne (4a)
was conducted in C6D6 at room temperature in the presence of
Ni(cod)2 and PCy3, the five-membered cycloaddition product 5
was formed in 39% yield (Scheme 1).8 Furthermore, a cyclopro-
pyl ring is not essential for the reaction; both phenyl cinnamate
(6a) and phenyl crotonate (6b) could be applied to the depheno-
xylative coupling reaction to yield the corresponding cyclopen-
tenone derivatives (7aa and 7ba) in 44 and 42% yield, respec-
tively (Scheme 1). Next, based on these stoichiometric findings,
the catalytic reaction of 6a with 4a by using iPrOH as a solvent
was examined with the anticipation that an alcoholic hydroxyl
group would serve as a hydrogen source (Table 1). As a result,
elevating the reaction temperature to 130 °C furnished the desired
product in 48% yield (run 2), whereas only 10% (the same
amount as catalyst) of 7aa was obtained at room temperature
(run 1). A quantitative formation of 7aa was achieved by the
addition of zinc powder (4 equiv, run 3), though a reaction
temperature of 130 °C was required (runs 4 and 5). The product
7aa was not generated at all in the absence of either Ni(cod)2 or
PCy3 (runs 6 and 7), but a transesterification reaction occurred in
run 7, giving isopropyl cinnamate. In addition, a significant
decrease in the yield of 7aa was observed with the use of 1 equiv
of 4a due to the undesired transesterification reaction (run 8).
The effects of both solvent and ligand on this reaction were
investigated. Although the use of sBuOH allowed the reaction to
ransition metal-catalyzed cycloaddition is one of the most
T
powerful strategies for one-step preparation of a variety of
cyclic compounds. Nickel(0) complexes have also been known
to catalyze cycloaddition reactions efficiently in various manners,
wherein a wide variety of combinations of unsaturated organic
compounds are used as a substrate.1 In the course of our con-
tinuous studies on development of nickel-catalyzed CÀC bond
formation reactions and isolation of nickelacycle key intermediates,2
we recently demonstrated a nickel-catalyzed [3+3] cyclodimer-
ization reaction of alkylidenecyclopropanes such as ethyl cyclo-
propylideneacetate (1a) via cleavage of a proximal CÀC bond to
yield 1,2-bis-exo-alkylidenecyclohexanes in excellent yields.3,4
However, a crucial difference was found in the reactivity between
the alkyl ester 1a and the corresponding phenyl ester 1b, which
gave a trace amount of the corresponding [3+3] cycloaddition
product 2b. To elucidate the difference in detail, a stoichiometric
reaction of 1b with Ni(cod)2 in the presence of PCy3 was carried
out. As a result, a small amount of a mixture containing the [3+3]
cycloaddition product 2b and the unanticipated spiro[2.5]octane
derivative 3 was obtained (eq 1),
t
give 7aa in moderate yield (run 9), neither BuOH nor PhOH
was effective (runs 10 and 11). The use of aprotic solvents such as
toluene and DME gave a small amount of 7aa (runs 12 and 13).
i
Under the reaction conditions using PrOH as a solvent at
130 °C, the use of either PCyp3 (Cyp = c-C5H9) or IPr instead
of PCy3 as a ligand afforded 7aa in 90 and 82% yield, respectively
(runs 14 and 15). The use of other phosphine ligands such as
PPh3 and PnBu3, however, retarded the reaction (runs 16 and
17). Neither NiCl2/PCy3 nor Ni(acac)2/PCy3 catalyzed the
reaction under the same reaction conditions.
With the optimized reaction conditions represented in run 3
(Table 1), the scope of the Ni(0)-catalyzed dephenoxylative
cycloaddition reaction with respect to various alkynes was
investigated using 6a as an α,β-unsaturated phenyl ester. Treat-
ment of 6a with 4a in iPrOH at 130 °C for 3 h gave 7aa in 91%
and a “dephenoxylated fragment of 1b” constituted the six-mem-
bered ring in 3 (Chart 1). Although aryl carboxylates have recently
received increasing attention because they can participate in
catalytic cross-coupling reactions by way of CacylÀO activation5
as well as CarylÀO activation,6,7 no cycloaddition reaction of aryl
carboxylates involving their carbonÀoxygen bond cleavage has
Received: June 28, 2011
Published: August 30, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja2059999 J. Am. Chem. Soc. 2011, 133, 14900–14903
|

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Chart 1. Unanticipated Dephenoxylated Cyclodimerization
of 1b To Give a Spiro[2.5]octane Derivative 3
Table 2. Ni(0)-Catalyzed Dephenoxylative Cycloaddition
Reaction of α,β-Unsaturated Phenyl Ester 6 with Alkyne 4a
Scheme 1. Stoichiometric Dephenoxylative Cycloaddition of
α,β-Unsaturated Phenyl Esters with 3-Hexyne
Table 1. Ni(0)-Catalyzed Dephenoxylative Cycloaddition
Reaction of Phenyl Cinnamate 6a with 3-Hexyne 4aa
run
additive
ligand
solv temp (°C) conv (%)b yield (%)b
1
2
3
4
none
none
PCy3 iPrOH
PCy3 iPrOH
rt
12
>99
>99
51
10
130
130
90
48
zinc (4 equiv) PCy3 iPrOH
zinc (4 equiv) PCy3 iPrOH
>99 (91)
39
10
À
À
65
45
5
5c zinc (4 equiv) PCy3 iPrOH
6d zinc (4 equiv) PCy3 iPrOH
7e zinc (4 equiv)
8f zinc (4 equiv) PCy3 iPrOH
9g zinc (4 equiv) PCy3 sBuOH
10 zinc (4 equiv) PCy3 tBuOH
11 zinc (4 equiv) PCy3 PhOH
12 zinc (4 equiv) PCy3 toluene
13 zinc (4 equiv) PCy3 DME
14 zinc (4 equiv) PCyp3 iPrOH
rt
30
130
130
130
130
130
130
130
130
130
130
130
130
30
i
a General conditions: phenyl esters (1.0 mmol), PrOH (15 mL).
À
iPrOH
>99
>99
63
b Isolated yields. The values in parentheses were of the ratio of
1
c
regioisomers (determined by H NMR). GC yields by using C14H30
as an internal standard. d GC analysis of the crude product revealed the
formation of a mixture of regioisomers (major/minor = 97/3 in the case
of 4h, 99/1 in the case of 4i, and 76/24 in the case of 4j), and purification
by column chromatography led to the isolation of the corresponding
major isomers with the represented formula.
16
<1
À
14
8
18
27
>99
>99
31
90
82
9
iPrOH
(4e) allowed the reaction with 1a to afford 7ae (run 4). The
reactions with unsymmetrical internal alkynes, such as 4-methyl-
2-pentyne (4f) and 2-hexyne (4g), also proceeded to give 7af
(64%) and 7ag (52%), respectively, as a mixture of regioisomers
(runs 5 and 6). In contrast, the reactions using 1-phenyl-1-propyne
(4h) and 2-methyl-1-hexen-3-yne (4i) gave the corresponding
mixtures with excellent regioselectivity, and each major isomer
(7ah and 7ai) was isolated in 64 and 19% yield, respectively (runs
7 and 8). The regioselectivity of the reaction with 4h and 4i is much
better than that with 4f and 4g, probably due to the contribution
of either an η3-benzyl or an η3-allyl structure in a possible inter-
mediate.2l,9c On the other hand, of the terminal alkynes used,
only 1-hexyne (4j) reacted with 1a to furnish the corresponding
mixture of regioisomers (runs 9À11). In addition, the reactions
with silylacetylenes (4m,n) were all futile (runs 11À13).
15 zinc (4 equiv) IPr
16 zinc (4 equiv) PPh3 iPrOH
17 zinc (4 equiv) PnBu3 iPrOH
73
19
a General conditions: phenyl cinnamate (0.10 mmol), solvent (5 mL).
b The conversion of phenyl cinnamate and the yield of the product were
determined by GC analysis using C14H30 as an internal standard. The value
in parentheses was of isolated yield. c Run for 24 h. d Run in the absence of
Ni(cod)2. e Run in the absence of additional ligands. f Run using 1 equiv
of 3-hexyne. g Formation of 2-butanone was detected by GC analysis.
isolated yield (Table 1, run 3). The reactions with 2-butyne (4b)
and 4-octyne (4c) gave the corresponding cyclopentenone
derivatives 7ab and 7ac in 84 and 66% yield, respectively
(Table 2, runs 1 and 2), while the reaction with 5-decyne (4d)
gave only an 8% yield of 7ad (run 3). Use of diphenylacetylene
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COMMUNICATION
Scheme 2. Observation of the η3-Oxaallyl Phenoxynickel
Intermediate
the reaction using IPr, which is also a good ligand for the catalytic
reaction (Table 1, run 15), at À20 °C was found to generate a
phenoxynickel(II) intermediate (9) in which a cyclopenta-1,4-
dienolate fragment was coordinated to the nickel in η3-oxaallyl
fashion (Scheme 2).11 Complex 9 was found to be stable in
toluene below À20 °C,12 and its structure was confirmed on
the basis of characteristic 13C NMR resonances attributable to
the ipso-PhONi (δC 170.7)13 and the η3-oxaallyl moieties (δC
i
162.9 and 81.8).2l Treatment of 9 with PrOD (>98% D)
at À15 °C resulted in a clean formation of the corresponding
cyclopentenone with a deuterium (98% D) at the 5-position
(7aa-d1).
Scheme 3. A Plausible Reaction Mechanism
Based on these observations, a plausible mechanism for the
Ni-catalyzed dephenoxylative cycloaddition reaction is depicted
in Scheme 3.14,15 The oxidative cyclization of an α,β-unsaturated
phenyl ester 6 and an alkyne 4 with nickel(0) takes place to
give a nickelacyclopentene intermediate (A). The intermedi-
ate A would undergo β-phenoxy elimination16 to give a
transient ketene intermediate (B) which might be converted
into an η3-oxaallyl phenoxynickel species (C) corresponding
to 9 via insertion of the ketene moiety into the CÀNi bond.
Alcoholysis of C provides 7 along with the formation of a
nickel(II) alkoxide (D).
The β-hydrogen elimination followed by reductive elimination
would regenerate a nickel(0) species with concomitant generation
of acetone and phenol. Although acetone could not be detected
in any crude products of the optimized catalytic reactions, this
step is supported by the generation of 2-butanone (Table 1, run
9). Since a stoichiometric amount of 7aa was obtained below
room temperature (Scheme 2), a high reaction temperature
(130 °C) in this catalytic reaction would be required to regen-
erate a nickel(0) species. The role of zinc powder as an additive
might be explained as either accelerating the reductive elimina-
tion step of phenol or directly reducing the nickel(II) intermedi-
ate D to a nickel(0) species.
The scope of the reaction with respect to α,β-unsaturated
phenyl esters was further examined. The reaction of 4a with 6b
gave the corresponding product 7ba in 86% isolated yield
(Table 2, run 14). A variety of (E)-disubstituted α,β-unsaturated
phenyl esters (6cÀf) also reacted with 4a to afford the corre-
sponding cyclopentenone derivatives (7caÀfa) in moderate to
high yields (runs 15À18). However, the reaction of phenyl
acrylate (6g) with 4a gave 7ga in low yield, probably due to
the rapid oligomerization of 6g (run 19). This catalytic system
was also successfully applied to phenyl methacrylate (6h),
yielding 7ha in moderate yield (run 20). However, trisubstituted
α,β-unsaturated phenyl esters, such as phenyl senecioate (6i)
and phenyl tiglate (6j), did not give the corresponding cyclo-
pentenones (runs 21 and 22).
It should be emphasized that the reactivity of phenyl ester 6b
was found to be quite different from those of the corresponding
ethyl ester and enal; the reaction of ethyl (E)-2-butenoate with
4a in the presence of a catalytic amount of Ni(cod)2 and PCy3 in
toluene gave an acyclic hexatriene, whereas the reaction of (E)-2-
butenal with 4a did not proceed under the same reaction
conditions.2l In addition, when vinyl cinnamate (8) was treated
with 4a under the optimized catalytic reaction conditions, the
corresponding devinyloxylative cycloaddition took place to give
7aa in 18% yield.10 The conversion of 8 was 71%, and the major
product was isopropyl cinnamate.
In summary, we demonstrated the unique reactivity of α,β-
unsaturated phenyl esters to serve as a three-carbon atom unit in
nickel-catalyzed cycloaddition reactions, as a result of the selec-
tive CacylÀO bond cleavage on nickel. A novel [3+2] cycloaddi-
i
tion reaction with alkyne in the presence of PrOH and zinc
powder was successfully applied to yield cyclopentenone deriva-
tives in excellent yield.
’ ASSOCIATED CONTENT
S
Supporting Information. Detailed experimental proce-
b
dures; analytical and spectral data for all new compounds. This
material is available free of charge via the Internet at http://pubs.
acs.org.
’ AUTHOR INFORMATION
Corresponding Author
ohashi@chem.eng.osaka-u.ac.jp; ogoshi@chem.eng.osaka-u.ac.jp
’ ACKNOWLEDGMENT
To elucidate the mechanism for this catalytic reaction, the
stoichiometric reaction of 4a and 6a with Ni(cod)2 was mon-
itored at low temperature by NMR spectroscopy. Although the
use of PCy3 as a ligand did not show any significant intermediates,
This work was supported by a Grant-in-Aid for Scientific
Research (No. 21245028) and a Grant-in-Aid for Scientific
Research on Innovative Areas “Molecular Activation Directed
toward Straightforward Synthesis” (No. 23105546) from MEXT.
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’ REFERENCES
(8) In contrast, 4a remained intact in the reaction with the ethyl ester
1a under the same reaction conditions, resulting in a rapid formation of
[3+2] and [3+3] cyclodimerization products of 1a. See ref 3 as well as:
Kawasaki, T.; Saito, S.; Yamamoto, Y. J. Org. Chem. 2002, 67, 4911–4915.
(9) (a) Hearth, A.; Li, W.; Montgomery, J. J. Am. Chem. Soc. 2008,
130, 469–471. (b) Li, W.; Hearth, A.; Montgomery, J. J. Am. Chem. Soc.
2009, 131, 17024–17029. (c) Liu, P.; McCarren, P.; Cheong, P. H.-Y.;
Jamison, T. F.; Houk, K. N. J. Am. Chem. Soc. 2010, 132, 2050–2057.
(10) See also related papers:(a) Yu, J.-Y.; Kuwano, R. Angew. Chem.,
Int. Ed. 2009, 48, 7217–7220. (b) Sun, C.-L.; Wang, Y.; Zhou, X.; Wu, Z.-
H.; Li, B.-J.; Guan, B.-T.; Shi, Z.-J. Chem.ÀEur. J. 2010, 16, 5844–5847.
(11) We previously isolated the oxidative cyclization product de-
rived from the reaction of an α,β-conjugated enone with an alkyne with
nickel(0). See ref 2l.
(1) For reviews, see:(a) Montgomery, J. Acc. Chem. Res. 2000,
33, 467–473. (b) Ikeda, S.-I. Angew. Chem., Int. Ed. 2003, 42, 5120–
5122. (c) Montgomery, J. Angew. Chem., Int. Ed. 2004, 43, 3890–3908.
(d) Moslin, R. M.; Miller-Moslin, K.; Jamison, T. F. Chem. Commun.
2007, 4441–4449. (e) Rayabarapu, D. K.; Cheng, C.-H. Acc. Chem. Res.
2007, 40, 971–983. (f) Montgomery, J.; Sormunen, G. J. Top. Curr.
Chem. 2007, 279, 1–23. (g) Kimura, M.; Tamaru, Y. Top. Curr. Chem.
2007, 279, 173–207. (h) Jeganmohan, M.; Cheng, C.-H. Chem.Eur. J.
2008, 14, 10876–10886. (i) Ng, S.-S.; Ho, C.-Y.; Schleicher, K. D.;
Jamison, T. F. Pure Appl. Chem. 2008, 80, 929–939.
(2) (a) Ogoshi, S.; Oka, M.; Kurosawa, H. J. Am. Chem. Soc. 2004,
126, 11802–11803. (b) Ogoshi, S.; Ueta, M.; Arai, T.; Kurosawa, H.
J. Am. Chem. Soc. 2005, 127, 12810–12811. (c) Ogoshi, S.; Nagata, M.;
Kurosawa, H. J. Am. Chem. Soc. 2006, 128, 5350–5351. (d) Ogoshi, S.;
Tonomori, K.-i.; Oka, M.-a.; Kurosawa, H. J. Am. Chem. Soc. 2006,
128, 7077–7086. (e) Ogoshi, S.; Ikeda, H.; Kurosawa, H. Angew. Chem.,
Int. Ed. 2007, 46, 4930–4932. (f) Ogoshi, S.; Arai, T.; Ohashi, M.;
Kurosawa, H. Chem. Commun. 2008, 1347–1349. (g) Ohashi, M.;
Kishizaki, O.; Ikeda, H.; Ogoshi, S. J. Am. Chem. Soc. 2009, 131, 9160–
9161. (h) Ogoshi, S.; Haba, T.; Ohashi, M. J. Am. Chem. Soc. 2009,
131, 10350–10351. (i) Tamaki, T.; Nagata, M.; Ohashi, M.; Ogoshi, S.
Chem.ÀEur. J. 2009, 15, 10083–10091. (j) Ogoshi, S.; Hoshimoto, Y.;
Ohashi, M. Chem. Commun. 2010, 46, 3354–3356. (k) Ohashi, M.; Saijo,
H.; Arai, T.; Ogoshi, S. Organometallics 2010, 29, 6354–6540. (l)
Ogoshi, S.; Nishimura, A.; Ohashi, M. Org. Lett. 2010, 12, 3450–3452.
(3) Ohashi, M.; Taniguchi, T.; Ogoshi, S. Organometallics 2010,
29, 2386–2389.
(4) A unique role of 1a as a three-carbon source has also been
developed in Ni-catalyzed [3+2+2], [4+3], and [4+3+2] cycloaddition
reactions.(a) Saito, S.; Masuda, M.; Komagawa, S. J. Am. Chem. Soc.
2004, 126, 10540–10541. (b) Komagawa, S.; Saito, S. Angew. Chem., Int.
Ed. 2006, 45, 2446–2449. (c) Saito, S.; Takeuchi, K. Tetrahedron Lett.
2007, 48, 595–598. (d) Maeda, K.; Saito, S. Tetrahedron Lett. 2007,
48, 3173–3176. (e) Saito, S.; Komagawa, S.; Azumaya, I.; Masuda, M.
J. Org. Chem. 2007, 72, 9114–9120. (f) Komagawa, S.; Yamasaki, R.;
Saito, S. J. Synth. Org. Chem. Jpn. 2008, 66, 974–982. (g) Yamasaki, R.;
Sotome, I.; Komagawa, S.; Azumaya, I.; Masu, H.; Saito, S. Tetrahedron
Lett. 2009, 50, 1143–1145. (h) Komogawa, S.; Takeuchi, K.; Sotome, I.;
Azumaya, I.; Masu, H.; Yamasaki, R.; Saito, S. J. Org. Chem. 2009,
74, 3323–3329. (i) Fukusaki, Y.; Miyazaki, J.; Azumaya, I.; Katagiri, K.;
Komagawa, S.; Yamasaki, R.; Saito, S. Tetrahedron 2009, 65, 10631–
10636. (j) Yamasaki, R.; Terashima, N.; Sotome, I.; Komagawa, S.; Saito,
S. J. Org. Chem. 2010, 75, 480–483. (k) Saito, S.; Maeda, K.; Yamasaki,
R.; Kitamura, T.; Nakagawa, M.; Kato, K.; Azumaya, I.; Masu, H. Angew.
Chem., Int. Ed. 2010, 49, 1830–1833. (l) Saito, S.; Yoshizawa, T.;
Ishigami, S.; Yamasaki, R. Tetrahedron Lett. 2010, 51, 6028–6030.
(5) Examples of catalytic transformation reactions involving the
CacylÀO activation of aryl carboxylates:(a) Kakino, R.; Shimizu, I.;
Yamamoto, A. Bull. Chem. Soc. Jpn. 2001, 74, 371–376. (b) Goossen,
L. J.; Paetzold, J. Angew. Chem., Int. Ed. 2002, 41, 1237–1241. (c)
Goossen, L. J.; Paetzold, J. Angew. Chem., Int. Ed. 2004, 43, 1095–1098.
(6) Examples of catalytic transformation reactions involving the
CarylÀO activation of aryl carboxylates:(a) Guan, B.-T.; Wang, Y.; Li,
B.-J.; Yu, D.-G.; Shi, Z.-J. J. Am. Chem. Soc. 2008, 130, 14468–14470. (b)
Quasdorf, K. W.; Tian, X.; Garg, N. K. J. Am. Chem. Soc. 2008,
130, 14422–14423. (c) Li, B.-J.; Li, Y.-Z.; Lu, X.-Y.; Liu, J.; Guan, B.-T.;
Shi, Z.-J. Angew. Chem., Int. Ed. 2008, 47, 10124–10127. (d) Li, Z.;
Zhang, S.-L.; Fu, Y.; Guo, Q.-X.; Liu, L. J. Am. Chem. Soc. 2009, 131,
8815–8823. (e) Li, B.-J.; Xu, L.; Wu, Z.-H.; Guan, B.-T.; Sun, C. L.;
Wang, B. Q.; Shi, Z.-J. J. Am. Chem. Soc. 2009, 131, 14656–14657. (f)
Shimasaki, T.; Tobisu, M.; Chatani, N. Angew. Chem., Int. Ed. 2010, 49,
2929–2932.
(12) Detailed spectral data for 9 are found in the Supporting
Information.
(13) Liang, L.-C.; Chien, P.-S.; Lee, P.-Y.; Lin, J.-M.; Huang, Y.-L.
Dalton Trans. 2008, 3320–3327.
(14) Alternative mechanism via oxidative addition of the CacylÀO
bond to Ni(0) at the initial stage of the reaction ruled out on the basis of
the fact that both Ni(0) complexes, [(η2-(E)-PhCHdCHCO2Ph)Ni-
(PCy3)2] and [(η2-(E)-PhCHdCHCO2Ph)2Ni(PCy3)], were intact in
solution at room temperature. See Supporting Information.
(15) The other alternative mechanism, which involves alcoholysis
of A followed by ester carbonyl insertion into the NiÀC bond and β-
phenoxy elimination, might be unlikely because it does not involve the
formation of intermediate C. See Supporting Information as well as
related [3+2] cycloaddition references:(a) Herath, A.; Montgomery, J.
J. Am. Chem. Soc. 2006, 128, 14030–14031. (b) Chang, H.-T.; Jayanth,
T. T.; Wang, C.-C.; Cheng, C.-H. J. Am. Chem. Soc. 2007, 129, 12032–12041.
(16) Komiya, S.; Shindo, T. J. Chem. Soc., Chem. Commun. 1984,
1672–1673.
(7) See also reviews on CÀO activation of aryl carboxylates in cross-
coupling reactions:(a) Goossen, L. J.; Goossen, K.; Paetzold, J.; Stanciu,
C. Angew. Chem., Int. Ed. 2009, 48, 3569–3571. (b) Yu, D.-G.; Li, B.-J.;
Shi, Z.-J. Acc. Chem. Res. 2010, 43, 1486–1495.
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