Pd-catalyzed decarboxylative arylation of silyl enol ester sp3 β-C-H bond under aerobic conditions

Article abstract of DOI:10.1039/c0dt01234c

Pd-catalyzed aerobic oxidative coupling of various benzoic acids with silyl enol esters proceeds via a combination of decarboxylation with sp³ β-C-H bond activation to give Heck-type products. Mechanistic studies reveal this coupling involves in situ generation of olefin from aerobic oxidation of silyl enolate, followed by decarboxylative Heck coupling.

Full text of DOI:10.1039/c0dt01234c

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/dalton | Dalton Transactions
Pd-catalyzed decarboxylative arylation of silyl enol ester sp3 b-C–H bond
under aerobic conditions†
Zhengjiang Fu, Shijun Huang, Jian Kan, Weiping Su* and Maochun Hong
Received 14th September 2010, Accepted 11th October 2010
DOI: 10.1039/c0dt01234c
Pd-catalyzed aerobic oxidative coupling of various benzoic
acids with silyl enol esters proceeds via a combination of
decarboxylation with sp3 b-C–H bond activation to give
Heck-type products. Mechanistic studies reveal this coupling
involves in situ generation of olefin from aerobic oxidation of
silyl enolate, followed by decarboxylative Heck coupling.
a Pd-catalyzed method for the synthesis of Heck-type products
via the combination of decarboxylation of benzoic acids with
the formal activation of sp3 b-C–H bond in silyl enolate of ester,
wherein dioxygen is utilized as the oxidant. Traditional Heck cou-
pling commonly employs a,b-unsaturated carbonyl compounds
as coupling partners that are usually synthesized through either
halogenation-dehalogenation sequences or dehydrosilylation of
the corresponding silyl enolates,15–18 whereas our protocol utilizes
silyl enolate of ester as starting material to directly form the Heck-
type products.
Transition-metal-catalyzed cross-coupling reactions are among
the most commonly used methods for generating C–C bonds.1
Their fundamental importance in synthetic organic chemistry has
given a great impetus to the development of a new version of cross-
coupling reaction, in which one or both of traditional coupling
partners such as electrophilic aryl halide or/and nucleophilic
organometallic reagent are replaced with distinct substrates. In
this context, the cross-coupling reaction via C–H bond activation
step, namely, the direct C–H bond functionalization, is a focus of
current catalysis studies,2–4 because such transformations provide
the most efficient synthetic pathways.
Recently, decarboxylative cross-coupling reactions of aromatic
carboxylic acids have emerged, some seminal studies include the
Pd-catalyzed Heck-type reaction of aromatic carboxylic acids,
Pd/Cu catalyzed arylation of aromatic carboxylic acids with
aryl halides and the Pd-catalyzed arylation of heteroaromatic
carboxylic acids with aryl halides.5–8 In these reactions, the
organometallic intermediates generated from the metal-mediated
decarboxylation served as either nucleophiles or electrophiles
depending on the nature of the metal salt used for decarboxylation,
exhibiting the potential of the aromatic carboxylic acid as a
versatile coupling component.5–14 The ready availability of aro-
matic carboxylic acid renders this synthetic approach particularly
attractive.
In contrast to arylation reactions of aromatic C–H bond
with aryl halides that have recently been attracted considerable
attentions, however, few examples of the direct C–H bond func-
tionalization with aromatic carboxylic acids via decarboxylation
have been reported.13 Our group recently reported a Pd/Ag-
catalyzed method for intermolecular decarboxylative direct C2
arylation of indoles with electron-rich benzoic acids and C3
arylation of indoles with electron-deficient benzoic acids.14 We
believe that the combination of decarboxylation with activation
of C–H bond for cross-coupling reaction would provide a new
synthetic access to C–C bond formation. In this paper, we report
Initially, we observed that the reaction of 2,6-dimethoxybenzoic
acid 1a with tert-butyldimethylsilyl (TBS) enolate of methyl
isobutyrate 2b in a mixed solvents of DMSO (5%, v/v) in DMF
in the presence of 20 mol% Pd(TFA)2 (TFA = trifluoroacetate)
˚
as catalyst, 4 A molecular sieves (MS) as drying agent and a
stoichiometric Ag2CO3 as oxidant under nitrogen atmosphere
furnished mono- and di-substituted Heck-type products rather
than a-arylated ester that can be generated from Pd-catalyzed
coupling of aryl bromide with silyl ketene acetal (entry 1,
Table 1).19 Interestingly, the reaction conducted under 1 atm of
dioxygen proceeded more efficiently than under nitrogen atmo-
sphere (entries 3–4). In our screening of oxidants, we observed
that metal salt oxidant was not necessary when the reaction
conducted under 1 atm of dioxygen (entry 4 versus 5). Under
otherwise identical conditions, 10 mol% Pd(OAc)2 provided much
higher yield than 10 mol% Pd(TFA)2, indicating that Pd(OAc)2
was superior to Pd(TFA)2 for this reaction (entry 7 versus 5).
The removal of MS from the reaction system led to a decrease
in the yield (entry 6). MS presumably functioned as a water
scavenger to avoid hydrolysis of silyl enolate of ester. Trimethylsilyl
(TMS) enolate 2a as substrate was less effective than tert-
butyldimethylsilyl enolate 2b (entry 7 versus 8). Considering that
silyl enolate of esters are, to some degree, sensitive to protic acid,
we hypothesized that introducing a Brønsted base into the reaction
system would suppress protonolysis of silyl enolates of esters
caused by benzoic acids by tempering acidity of reaction medium,
and therefore improve the compatibility of silyl enolates of esters
with acid condition. Gratifyingly, addition of 0.2 equivalent of
weakly coordinating triphenylamine (Ph3N) to the reaction system
markedly improved the reaction, affording desired products in 84%
yield (entry 9). Increasing the amount of Ph3N to 1 equivalent
led to slight enhancement of the yield (entry 10). However, the
bases such as Et3N and K2CO3 that are much stronger than Ph3N
were ineffective to improve the yield (entries 11–13), presumably
because strong bases reduced the reactivity of Pd catalyst toward
decarboxylation step by coordinating more tightly to Pd, whereas
Ph3N binding strength to Pd was relatively weak.
State Key Laboratory of Structural Chemistry, Fujian Institute of Research
on the Structure of Matter, The Chinese Academy of Sciences, Fuzhou, Fu-
jian, 350002, China. E-mail: wpsu@fjirsm.ac.cn; Fax: (+86)591-83771575
† Electronic supplementary information (ESI) available: Experimental
procedures and spectroscopic characterization of the products. See DOI:
10.1039/c0dt01234c
Although it is known that the oxidation of Ph3N by Cu(II) salt
led to formation of tetraphenylbenzidine and other products via
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 11317–11321 | 11317
©

View Article Online
Table 1 Optimization studies for coupling of 2,6-dimethoxybenzoic acid 1a with silyl enolates of methyl isobutyrate 2a
Entry
Pdb (mol %)
Silyl enolate
Additive(equiv)
Oxidant(equiv)
Isolated yield(%)c
3 : 4d
1e
2e
3
Pd(TFA)2(20)
Pd(TFA)2(20)
Pd(TFA)2(20)
Pd(TFA)2(20)
Pd(TFA)2(10)
Pd(OAc)2(10)
Pd(OAc)2(10)
Pd(OAc)2(10)
Pd(OAc)2(10)
Pd(OAc)2(10)
Pd(OAc)2(10)
Pd(OAc)2(10)
Pd(OAc)2(10)
2b
2b
2b
2b
2b
2b
2b
2a
2b
2b
2b
2b
2b
Ag2CO3 (3)
Cu(OAc)2 (3)
Ag2CO3 (3)/O2
47
< 5
80
53
50
61
67
61
84
89
38
66
61
1 : 2.4
f
1 : 1.7
1 : 5 : 1
1 : 2.4
1 : 1.7
1 : 2 : 1
1.3 : 1
1 : 1.5
1.2 : 1
1 : 2.2
1 : 1.9
1 : 2.8
f
4
Cu(OAc)2 (3)/O2
O2, 1 atm
O2, 1 atm
O2, 1 atm
O2, 1 atm
O2, 1 atm
O2, 1 atm
O2, 1 atm
O2, 1 atm
O2, 1 atm
5
6g
7
8
9
10
11
12
13
Ph3N (0.2)
Ph3N (1.0)
Et3N (0.2)
Ph2Men (0.2)
K2CO3 (0.2)
◦
a
Conditions: 1a (0.2 mmol), 2 (0.4 mmol), 5% DMSO-DMF (2 mL), 4 A MS, 120 C, 24 h. b Relative to 1a. c Average of two runs. d The E/Z ratio was
˚
> 20 : 1, as determined by 1H NMR. e Reaction conducted under nitrogen atmosphere. f 1 atm O2. g Reaction run without MS.
radical cation step,20–21 we speculated that if Ph3N participated
in the catalytic process via a radical cation under our aerobic
oxidative conditions, there would be two possibilities for Ph3N:
1) conversion of Ph3N into tetraphenylbenzidine or other com-
pounds; 2) regeneration of Ph3N from catalytic recycle. For the
latter case, the radical cation formed from oxidation of Ph3N could
be trapped by adding nucleophile such as nBu4NBr.21 However,
we found that Ph3N can be recovered unchanged from reaction
system regardless of the presence or absence of silyl enolate of ester,
and Ph3N was isolated by column chromatography after reaction
the observed correlation between electronic nature of aromatic
carboxylic acid and its reactivity reflected the issue associated with
the decarboxylation step. Since Ag salts were effective catalysts
for protodecarboxylation of a wide range of benzoic acids,11
and we observed that Pd-catalyzed decarboxylation occurred for
electron-rich aromatic carboxylic acids, whereas decarboxylation
of electron-deficient aromatic carboxylic acids resulted from the
contribution of the silver salt in the decarboxylative coupling of
benzoic acids with indoles.14 Thus, a stoichiometric amount of
Ag2CO3 was required to replace Ph3N to promote the decarboxyla-
tive coupling of electron-deficient benzoic acid with silyl enol ester
2c, otherwise essentially no desired product could be obtained
under the standard conditions (entries 12–13). Similarly to others
and our observations,6,14,24 at least one ortho weakly coordinating
substitutent on the aromatic ring of aromatic carboxylic acid was
necessary for this reaction to occur, the reason was presumably
that the ortho weakly coordinating substituent stabilized the
transition structure of decarboxylation process by coordinating
to palladium center, and thus enhanced the formation rate of aryl
palladium intermediate. For example, 2-methoxy-4-methylbenzoic
acid (enties 9–10) participated in this reaction while 4-methoxy-
2-methylbenzoic acid was recovered unchanged from reaction
mixtures under the established conditions. Although most of
reactions formed two products, two products can be clearly
isolated by column chromatography. Di-substituted products have
been applied to synthesis of substituted indenes.22 The protocol
was not limited to 0.2 mmol scale reaction, which can also gave
the same yield when the reaction on 1 mmol scale was run (entry
1, Table 2).
worked up. The isolated Ph3N was characterized by 1H NMR, 13
C
NMR, mass spectrometry and measurement of its melting point.
Therefore, the first possibility can be excluded. Further experi-
ments showed that the reaction of Ph3N with nBu4NBr under our
aerobic oxidative conditions did not take place, indicating that
Ph3N did not undergo oxidation to generate radical cation under
our aerobic oxidative condition (see Electronic Supplementary
Information†). Based on these observations, we tend to believe
that Ph3N would act as a very weak base for this reaction
rather than an electron-transfer mediator for accelerating aerobic
oxidation, and there would exist the interaction between Ph3N and
benzoic acids through hydrogen bond, which may well prevent
silyl enolate of ester from decomposition caused by benzoic
acids.
We next examined the substrate scope of this reaction under the
standard conditions of entry 9 in Table 1, and some adjustments
were required to the reaction conditions in some cases due to
the variations in reactivity toward decarboxylation of electron-
deficient benzoic acids. As shown in Table 2, this protocol tolerated
alkoxy, alkyl, amino, chloro, nitro and bromo substituents as
well as substitution patterns on aromatic rings. Good yields were
obtained with electron-rich aromatic carboxylic acids (entries 1–
8, Table 2), including a benzoic acid bearing amino and chloro
substituents on the aromatic ring and a heterocyclic carboxylic
acid such as 3-methyl-2-benzofuran carboxylic acid (entries 11
and 14), and moderate yields were obtained with less electron-
rich aromatic carboxylic acids (entries 9–10, 12–13 and 15),
Further studies were performed to gain some insights into the
reaction mechanism. Firstly, 53% isolated yield of protodecar-
boxylation product was obtained when 2,6-dimethoxybenzoic acid
1a was conducted under our aerobic oxidative conditios in the
absence of silyl enolate of ester 2 (Scheme 1), the observation
supported that a catalytic amount of Pd(OAc)2 enabled to pro-
mote decarboxylation of aromatic carboxylic acid under aerobic
conditions.23
11318 | Dalton Trans., 2010, 39, 11317–11321
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
Table 2 Pd-catalyzed decarboxylative coupling of benzoic acids with silyl enolate of ester 2a
Entry
1e
Ar–COOH
Silyl enolate
3 + 4b
Isolated yield (%)c,d
84 (1 : 1.6)
1a
2b
3ab + 4ab
2
3
1a
1b
2c
2b
3ac + 4ac
3bb + 4bb
65 (1 : 1.3)
80 (1 : 1.3)
4
5
1b
1c
2c
2b
3bc + 4bc
3cb + 4cb
75 (2 : 8 : 1)
81 (1 : 2.5)
6
7
1c
1d
2c
2b
3cc + 4cc
3db + 4db
85 (1 : 3.7)
86 (1 : 1.8)
8
9
1d
1e
2c
2b
3dc + 4dc
3eb + 4eb
77 (1 : 1.4)
36 (1 : 1 : 1)
10
11
1e
1f
2c
2c
3ec
3fc + 4fc
27 (27 : 0)
72 (1 : 1.4)
12f
13f
1g
1h
2c
2c
3gc
3hc
35 (35 : 0)
32 (32 : 0)
14
1i
1j
2b
2b
3ib + 4ib
3jb + 4jb
63 (1 : 8)
15g
37 (1 : 2.4)
Conditions: 1 (0.2 mmol), 2 (0.4 mmol), Pd(OAc)2 (10 mol%), Ph3N (20 mol%), 4 A MS, 5% DMSO-DMF (2 mL), 1 atm O2, 120 ◦C, 24 h, except as
noted. b Average of two runs. c The E/Z ratio was > 20 : 1, as determined by 1H NMR. d the value in parentheses referred to the ratio of 3/4. e 1 mmol 1a
was used. f 3 equiv Ag2CO3 instead of Ph3N. g Reaction conducted at 90 ◦C.
a
˚
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 11317–11321 | 11319
©

View Article Online
catalytic species reacted with aromatic carboxylic acid substrate
to form intermediate A, whereafter intermediate A proceeded
a decarboxylation to liberate CO2 and generate key reactive
Ar–Pd(II) intermediate B. The olefin was generated from Pd-
catalyzed aerobic oxidation of silyl enolate of ester 2b, which
inserted into Ar–Pd(II) B to give Pd(II)-alkyl intermediate C. C
underwent b-hydride elimination to produce the desired product
and active Pd(0) species. Finally, peroxopalladium species D was
generated from oxygenation of Pd(0), and D subsequently gave
the active Pd(II) catalytic species via protonolysis to complete
the catalytic cycle, whereas the concomitant formation of H2O2
rapidly disproportionated into O2 and H2O under this effective
Scheme 1 Protodecarboxylation of 2,6-dimethoxybenzoic acid 1a under
aerobic oxidative conditions.
Secondly, the GC-MS analysis revealed that under dioxygen
heating compound 2b with 10 mol% Pd(OAc)2 in 5% DMSO-
DMF at 120 ◦C for 4 h dominantly produced methyl methacrylate
in 90% conversion and that the presence of Ph3N decelerated
the conversion of 2b into methyl methacrylate, therefore, we
concluded that Ph3N had no beneficial effect on accelerating
aerobic oxidation of 2b to methyl methacrylate from this ob-
servation. The Pd-catalyzed aerobic oxidation of silyl enol ether
into a,b-unsaturated carbonyl compound conducted in DMSO
has previously been reported by Larock and co-workers,16 and
the similar conversions have also been reported by Saegusa17 and
Tsuji18 with other oxidants such as benzoquinone. Moreover, The
reaction of compound 1a with methyl methacrylate in the presence
of 10 mol% Pd(OAc)2 under dioxygen afforded decarboxylative
Heck coupling products in 97% yield (Scheme 2).24
catalytic conditions,25 and H2O was absorbed by 4 A MS. On
˚
the other hand, as for the entries 12 and 13 of Table 2, we
believed that the decarboxylation of electron-deficient benzoic
acids resulted from the contribution of Ag2CO3 rather than
Pd catalyst.11,14 Accordingly, formation of di-substituted product
could be explained by the possibility that the decarboxylative Heck
reaction was much faster than areobic oxidation of silyl enolate of
ester 2b into methyl methacrylate, however, it’s frustrated to obtain
di-substituted compound as the sole product by adding excessive
aromatic carboxylatic acid (relative to silyl enolate of ester 2b)
into reaction sytem. Unfortunately, Our attempt to obtain mono-
substituted compound as the major product also failed by adding
1a into reaction system after oxidation reaction of silyl enolate of
ester 2b was run for 4 h under aerobic conditions, and this one-
pot sequential method instead led to a decrease in the yield due
in part to decomposition of palladium catalyst in the first step
reaction.
In conclusion, we have established a new protocol for Pd-
catalyzed Heck-type coupling of aromatic carboxylic acid with
silyl enolate of ester via the combination of sp3 b-C–H bond cleav-
age and decarboxylation by using dioxygen as the oxidant. Pre-
liminary mechanistic studies disclosed that this reaction involved
in situ generation of a,b-unsaturated ester from silyl enolate of
ester under aerobic conditions, followed by decarboxylative Heck
coupling. Efforts are currently underway to improve substrate
Scheme 2 Pd-catalyzed coupling of 2,6-dimethoxybenzoic acid 1a with
methyl methacrylate.
Although the details of this protocol still remain elusive, these
observations pointed to a possible mechanism that involved in situ
generation of methyl methacrylate from the aerobic oxidation of
silyl enolate of ester 2b, followed by decarboxylative olefination of
aromatic carboxylic acid (Scheme 3). Initially, electrophilic Pd(II)
Scheme 3 Proposed catalytic cycle for aerobic oxidative decarboxylative olefination of aromatic carboxylic acid with silyl enolate of ester 2b.
11320 | Dalton Trans., 2010, 39, 11317–11321 This journal is The Royal Society of Chemistry 2010
©

View Article Online
scope of this reaction further and investigate the mechanism more
detailedly.
This work was financially supported by the 973 Program
(2006CB932903, 2009CB939803), NSFC (20821061, 20925102),
“The Distinguished Oversea Scholar Project”, “One Hundred
Talent Project” and Key Project from CAS.
2009, 11, 2341; J. Moon, M. Jang and S. Lee, J. Org. Chem., 2009, 74,
1403; A. A. N. Magro, G. R. Eastham and D. J. Cole-Hamilton, Dalton
Trans., 2009, 4683; S.-L. Zhang, Y. Fu, R. Shang, Q.-X. Guo and L.
Liu, J. Am. Chem. Soc., 2010, 132, 638; K. Xie, Z. Yang, X. Zhou, X.
Li, S. Wang, Z. Tan, X. An and C.-C. Guo, Org. Lett., 2010, 12, 1564;
C. Feng and T.-P. Loh, Chem. Commun., 2010, 46, 4779; F. Zhang and
M. F. Greaney, Angew. Chem., Int. Ed., 2010, 49, 2768; M. Zhang, J.
Zhou, J. Kan, M. Wang, W. Su and M. Hong, Chem. Commun., 2010,
46, 5455; L. Xue, W. Su and Z. Lin, Dalton Trans., 2010, 39, 9815.
10 L. J. Gooßen, W. R. Thiel, N. Rodr´ıguez, C. Linder and B. Melzer, Adv.
Synth. Catal., 2007, 349, 2241; L. J. Gooßen, F. Rudolphi, C. Oppel
and N. Rodr´ıguez, Angew. Chem., Int. Ed., 2008, 47, 3043; Z. Duan, S.
Ranjit, P. Zhang and X. Liu, Chem.–Eur. J., 2009, 15, 3666; R. Shang,
Y. Fu, Y. Wang, Q. Xu, H.-Z. Yu and L. Liu, Angew. Chem., Int. Ed.,
2009, 48, 9350; W. Jia and N. Jiao, Org. Lett., 2010, 12, 2000.
11 L. J. Gooßen, C. Linder, N. Rodr´ıguez, P. P. Lange and A. Fromm,
Chem. Commun., 2009, 7173; J. Cornella, C. Sanchez, D. Banawa and
I. Larrosa, Chem. Commun., 2009, 7176; P. Lu, C. Sanchez, J. Cornella
and I. Larrosa, Org. Lett., 2009, 11, 5710; J. Cornella, P. Lu and I.
Larrosa, Org. Lett., 2009, 11, 5506; L. J. Gooßen, P. P. Lange, N.
Rodr´ıguez and C. Linder, Chem. Eur. J., 2010, 16, 3906.
12 Z.-M. Sun and P. Zhao, Angew. Chem., Int. Ed., 2009, 48, 6726; Z.-M.
Sun, J. Zhang and P. Zhao, Org. Lett., 2010, 12, 992.
13 A. Voutchkova, A. Coplin, N. E. Leadbeater and R. H. Crabtree, Chem.
Commun., 2008, 6312; H.-P. Bi, W.-W. Chen, Y.-M. Liang and C.-J. Li,
Org. Lett., 2009, 11, 3246; C. Wang, I. Piel and F. Glorius, J. Am.
Chem. Soc., 2009, 131, 4194; H.-P. Bi, L. Zhao, Y.-M. Liang and C.-J.
Li, Angew. Chem., Int. Ed., 2009, 48, 792.
14 J. Zhou, P. Hu, M. Zhang, S. Huang, M. Wang and W. Su, Chem.
Eur. J., 2010, 16, 5876.
15 B. Miller and H.-S. Wong, Tetrahedron, 1972, 28, 2369; A. E. Green,
J.-C. Muller and G. Ourisson, Tetrahedron Lett., 1972, 13, 3375; P. L.
Stotter and K. A. Hill, J. Org. Chem., 1973, 38, 2576.
Notes and references
1 Metal-Catalyzed Cross-coupling Reactions, ed. A. de Meijere and
F. Diederich, Wiley-VCH, Weinheim, 2nd edn, 2004.
2 L.-C. Campeau and K. Fagnou, Chem. Commun., 2006, 1253; I. V.
Seregin and V. Gevorgyan, Chem. Soc. Rev., 2007, 36, 1173; G. P.
McGlacken and L. M. Bateman, Chem. Soc. Rev., 2009, 38, 2447;
D. Balcells, E. Clot and O. Eisenstein, Chem. Rev., 2010, 110, 749.
3 N. R. Deprez, D. Kalyani, A. Krause and M. S. Sanford, J. Am. Chem.
Soc., 2006, 128, 4972; B.-J. Li, S.-L. Tian, Z. Fang and Z. Shi, Angew.
Chem., Int. Ed., 2008, 47, 1115; N. Chernyak and V. Gevorgyan, J. Am.
Chem. Soc., 2008, 130, 5636; L. Ackermann, A. Althammer and S.
Fenner, Angew. Chem. Int. Ed., 2009, 47, 201; R. B. Bedford, R. L.
Webster and C. J. Mitchell, Org. Biomol. Chem., 2009, 7, 4853; A.
Ishikawa, Y. Nakao, H. Sato and S. Sakaki, Dalton Trans., 2010, 39,
3279.
4 X. Chen, X.-S. Hao, C. E. Goodhue and J.-Q. Yu, J. Am. Chem. Soc.,
2006, 128, 6790; H. M. Mbuvi and L. K. Woo, Organometallics, 2008,
27, 637; R. J. Phipps, N. P. Grimster and M. J. Gaunt, J. Am. Chem.
Soc., 2008, 130, 8172; P. B. Arockiam, C. Fischmeister, C. Bruneau and
P. H. Dixneuf, Angew. Chem., Int. Ed., 2010, 49, 6629; C. He, S. Guo,
L. Huang and A. Lei, J. Am. Chem. Soc., 2010, 132, 8273; W. Zhou,
L. Zhang and N. Jiao, Angew. Chem., Int. Ed., 2009, 48, 7094; K. Gao,
P.-S. Lee, T. Fujita and N. Yoshikai, J. Am. Chem. Soc., 2010, 132,
12249; X.-Z. Shu, Y.-F. Yang, X.-F. Xia, K.-G. Ji, X.-Y. Liu and Y.-M.
Liang, Org. Biomol. Chem., 2010, 8, 4077.
16 R. C. Larock, T. R. Hightower, G. A. Krause, P. Hahn and D. Zheng,
Tetrahedron Lett., 1995, 36, 2423.
5 O. Baudoin, Angew. Chem., Int. Ed., 2007, 46, 1373; L. J. Gooßen,
N. Rodr´ıguez and K. Gooßen, Angew. Chem., Int. Ed., 2008, 47,
3100.
6 A. G. Myers, D. Tanaka and M. R. Mannion, J. Am. Chem. Soc., 2002,
124, 11250; D. Tanaka and A. G. Myers, Org. Lett., 2004, 6, 433; D.
Tanaka, S. P. Romeril and A. G. Myers, J. Am. Chem. Soc., 2005, 127,
10323.
17 Y. Ito, T. Hirao and T. Saegusa, J. Org. Chem., 1978, 43, 1011.
18 J. Tsuji, K. Takahashi, I. Minami and I. Shimizu, Tetrahedron Lett.,
1984, 25, 4783; I. Minami, K. Takahashi, I. Shimizu, T. Kimura and J.
Tsuji, Tetrahedron, 1986, 42, 2971.
19 X. Liu and J. F. Hartwig, J. Am. Chem. Soc., 2004, 126, 5182.
20 K. Sreenath, C. V. Suneesh, V. K. R. Kumar and K. R. Gopidas, J. Org.
Chem., 2008, 73, 3245.
7 L. J. Gooßen, G. Deng and L. M. Levy, Science, 2006, 313, 662; L. J.
Gooßen, N. Rodr´ıguez, B. Melzer, C. Linder, G. Deng and L. M. Levy,
J. Am. Chem. Soc., 2007, 129, 4824.
8 P. Forgione, M.-C. Brochu, M. St-Onge, K. H. Thesen, M. D. Bailey
and F. Bilodeau, J. Am. Chem. Soc., 2006, 128, 11350.
21 M. Kirchgessner, K. Sreenath and K. R. Gopidas, J. Org. Chem., 2006,
71, 9849.
22 P. Shanmugam and P. Rajasingh, Chem. Lett., 2005, 34, 1494.
23 J. S. Dickstein, C. A. Mulrooney, E. M. O’Brien, B. J. Morgan and
M. C. Kozlowski, Org. Lett., 2007, 9, 2441.
9 J.-M. Becht and C. Le Drian, Org. Lett., 2008, 10, 3161; Z. Wang,
Q. Ding, X. He and J. Wu, Org. Biomol. Chem., 2009, 7, 863; M.
Miyasaka, A. Fukushima, T. Satoh, K. Hirano and M. Miura, Chem.–
Eur. J., 2009, 15, 3674; P. Hu, J. Kan, W. Su and M. Hong, Org. Lett.,
24 Z. Fu, S. Huang, W. Su and M. Hong, Org. Lett., 2010, 12,
DOI: 10.1021/ol102158n.
25 B. V. Popp and S. S. Stahl, J. Am. Chem. Soc., 2007, 129, 4410; B. A.
Steinhoff, S. R. Fix and S. S. Stahl, J. Am. Chem. Soc., 2002, 124, 766.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 11317–11321 | 11321
©