Iron catalysis and water: A synergy for refunctionalization of boron

Article abstract of DOI:10.1055/s-0033-1340500

A new catalytic system has been optimized to promote the conversion of boron species into others. FeCl₃ associated with imidazole and water favors boron refunctionalization under mild conditions. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1340500

LETTER
▌551
lIertteor n Catalysis and Water: A Synergy for Refunctionalization of Boron
Interconversion of Boron Substituents
John L. Wood, Ludovic D. Marciasini, Michel Vaultier, Mathieu Pucheault*
Institut des Sciences Moléculaires, UMR 5255 – 351 Cours de Libération – Bâtiment A11, Université de Bordeaux 1, 33405 Talence Cedex,
France
Fax +33(5)40006632; E-mail: m.pucheault@ism.u-bordeaux1.fr
Received: 08.10.2013; Accepted after revision: 06.12.2013
many others require harsh conditions eventually leading
Abstract: A new catalytic system has been optimized to promote
to side products on advance substrates. As part of our re-
the conversion of boron species into others. FeCl3 associated with
search focused on boron derivatives preparation, we
imidazole and water favors boron refunctionalization under mild
looked for an efficient method allowing interconverting
boron substituents, keeping in mind the simplicity, the
cost effectiveness, and the sustainability of the process.
conditions.
Key words: boron, iron, homogeneous catalysis, water, boronate
Trifluoro(organo)borates9 are stable derivatives formed
by fluorination4 of the corresponding boronic acids or
their esters. They are isolated by precipitation and purified
by recrystallization, and as such can be stored on the shelf
for an extended period of time. Nonetheless, these borates
have been used in numerous catalytic transformations, us-
ing palladium,10 copper,11 nickel,12 or rhodium13 complex-
es as catalysts. Despite their attractiveness, many groups
found that the corresponding boronic acids are often more
reactive than the borate due to the vacancy on the boron
atom.7,14
Boron derivatives have long been used in numerous syn-
thetic transformations, including as coupling partners in
organometallic catalyzed reactions such as the Suzuki–
Miyaura cross-coupling. Several methods are available to
prepare such derivatives, mostly by addition of a strong
organometallic reagent onto a trialkylborate. Alternative-
ly, the borylation of aryl halides or pseudo halides can be
promoted by transition-metal complexes leading usually
to pinacol boronates.1 C–H activation by late-transition-
metal complexes, such as iridium or rhodium, can afford
the same compounds.2 Many derivatives can be prepared
from boronic acids, however, it is not always trivial to in-
terconvert various groups on the boron without altering
the carbon–boron bond.
Classical methods, based on highly reactive Lewis acid
(SiCl4, TMSCl)15 are not tolerated by many functionalities
borne by the substrate. Lately some milder methods have
emerged to prepare boronic acids by the hydrolysis of the
corresponding trifluoroborates. Treatment of aryltrifluo-
roborates by alumina16 under microwave or classical heat-
ing, or with silica17 in a EtOAc–H2O mixture led to the
corresponding boronic acid. One of the simplest method
has been reported by Kabalka and involves the use of 1.1
equivalents of iron trichloride in a THF–H2O mixture.18
Intrigued by the role of iron chloride, supposedly forming
FeF3 which would displace the equilibrium, we finally
discovered that FeCl3 could be used as a catalyst to pro-
mote many interconversions of boron species, including
hydrolysis of trifluoroborates.
For example, the classical method for transforming boron-
ic acids into stable pinacol boronates stands in a classical
esterification process with water azeotropic removal us-
ing a Dean–Stark apparatus or molecular sieves on small
scale. Diaminonaphthyl boranes, used in the elegant
chemistry developed by Suginome,3 are similarly pre-
pared by reaction of boronic acids with 1,8-diaminonaph-
thalene. Until recently, trifluoro(organo)borates, easily
prepared by fluorination4,5 of the corresponding dialkoxy-
boranes, were not easily hydrolyzed, and many groups
have been looking for efficient methods to release the bo-
ronic acid from the borate. One other problem relies in the
generation of boronic acids by hydrolysis of pinacolboro-
nates. These derivatives, often obtained via transition-
metal-catalyzed borylation,6 are important stable building
blocks due to the steric bulkiness of the pinacol group
which prevents hydrolysis and protodeboronation. How-
ever, the down side of such derivatives is the poor reactiv-
ity in cross-coupling reactions as compared to the boronic
acids parental molecule. As direct hydrolysis is often in-
effective, therefore a fluorination–hydrolysis sequence
has been developed to afford the boronic acids.7,8 Overall,
some transformations occur without any problem, while
Assuming the mechanism of the hydrolysis would pro-
ceed through a Lewis acid–basic ligand exchange, we in-
vestigated a series of chloride salts, known for their Lewis
acidic properties, for example in Friedel–Crafts reactions.
Among the various salts tested, aluminium chloride (85%
yield, Table 1, entry 1), copper(II) chloride (85% yield,
Table 1, entry 2), zinc chloride (85% yield, Table 1, entry
3), and iron(III) chloride (85% yield, Table 1, entry 4) dis-
played similar activity. Zinc chloride led to slightly purer
compounds, but iron(III) chloride was chosen because of
its relative environmental harmlessness and ease of use.
Initially at 16 hours, reaction duration was lowered down
to 15 minutes (Table 1, entries 5 and 6) as no further evo-
lution could be observed for extended reaction time.
SYNLETT 2014, 25, 0551–0555
Advanced online publication: 13.01.2014
DOI: 10.1055/s-0033-1340500; Art ID: ST-2013-B0948-L
© Georg Thieme Verlag Stuttgart · New York
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
Base influence was investigated, and no better base than
imidazole could be found. Classical amine base such as

552
J. L. Wood et al.
LETTER
BF3K
B(OH)2
Table 1 Catalytic System Optimization: Trifluoroborate Hydrolysis
FeCl3 (5 mol%), imidazole (3 equiv)
H2O (0.2 M), r.t., 15 min
R
R
BF3K
B(OH)2
cat. MXn, base
H2O (0.2 M), r.t., 15 min
B(OH)2
B(OH)2
B(OH)2
Entry Catalyst MXn Base
Solvent
Yield
(%)a
MeO
81%
(mol%)
(n equiv)
87%
85%
1
2
AlCl3 (10)
CuCl2 (10)
ZnCl2 (10)
FeCl3 (10)
ZnCl2 (10)
FeCl3 (10)
FeBr3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (10)
FeCl3 (0)
imidazole (3)
imidazole (3)
imidazole (3)
imidazole (3)
imidazole (3)
imidazole (3)
imidazole (3)
Et3N (3)
H2O
85b
82b
82b
84b
83c
83c
82
B(OH)2
B(OH)2
H2O
Cl
3
H2O
76%
85%
4
H2O
Scheme 1 Hydrolysis of aryltrifluoroborates under optimized condi-
tions
5
H2O
6
H2O
most importantly products were obtained in all cases as
impure materials.
7
H2O
Finally, catalyst loading and reaction concentration were
optimized to complete the study. We found out that the
optimal catalyst loading was 5 mol% (87% yield, Table 1,
entry 20). In short, 1 mol% catalyst is not enough to see
improvement on the reaction, 10 mol% and more of FeCl3
induce lower yields due to faster side reactions. Reaction
conditions were applied to the hydrolysis of few other tri-
fluoro(organo)borates (Scheme 1). Indeed, the optimized
catalytic systems afforded the corresponding boronic ac-
ids in good yields. Strikingly, no boroxine was formed un-
der these reactions conditions in contrast to classical
organometallic addition onto trialkylborates. However,
despite its intrinsic interest, we envisioned that other nu-
cleophiles could be used in place of water. In that case, it
would allow the transformation of trifluoro(organo)bo-
rates into other boron derivatives. Even if pinacol is not
known to be the most nucleophilic diol, it is nonetheless
the most employed substituent of boron. Therefore we de-
cided to adapt the conditions developed above to the syn-
thesis of pinacol boronates from trifluoro(organo)borates.
Indeed, under the same reaction conditions, simply adding
one equivalent of pinacol in the reaction mixture, reaction
of trifluorophenylborate afforded the pinacolphenylbo-
rane in 65% yield. Studies showed that water was required
to perform this reaction in good yield, indicating that the
reaction probably proceeded through a hydrolysis of the
borate into the boronic acid followed by esterification of
the intermediate acid.
8
H2O
73
9
pyridine (3)
piperidine (3)
K2CO3 (3)
H2O
68
10
11
12
13
14
15
16
17
18
19
20
21
22
H2O
55
H2O
71
KHCO3 (3)
H2O
72
imidazole (3)
imidazole (3)
imidazole (3)
imidazole (3)
imidazole (3)
imidazole (3)
imidazole (3)
imidazole (3)
imidazole (3)
imidazole (3)
MeCN–H2O
THF–H2O
acetone–H2O
MeOH–H2O
H2O
72
45
69
81
73
FeCl3 (1)
H2O
76
FeCl3 (2.5)
FeCl3 (5)
H2O
85
H2O
87
FeCl3 (5)
H2O
80d
75e
FeCl3 (5)
H2O
a Isolated yield as referred to analytically pure material.
b Reaction time: 16 h.
c Reaction time: 15 min.
d Reaction concentration: 0.1 M.
e Reaction concentration: 0.5 M.
Encouraged by this observation we envisioned that the
same catalytic system could be employed to transform bo-
ronic acids into 1,3,2-dioxaborolanes. Namely, the esteri-
fication of boronic acids into its esters can be tricky,
especially with low nucleophilic alcohols such as pinacol
or pinanediol. The classical method involves refluxing the
reaction mixture with azeotropic removal of the generated
water.19 Some milder alternatives have been described,17
using mechanochemistry20 for example.
triethylamine (73% yield, Table 1, entry 8), pyridine (68%
yield, Table 1, entry 9), piperidine (55% yield, Table 1,
entry 10) showed significantly lower yields, and in most
cases the product was contaminated by the ammonium
salt. Inorganic bases such as KHCO3 (71% yield, Table 1,
entry 11) or K2CO3 (85% yield, Table 1, entry 12) led to a
mere 70% yield. The addition of organic solvent to im-
prove the reaction rate or solubility had only a detrimental
effect on the reaction. Acetonitrile (72% yield, Table 1,
entry 13), tetrahydrofuran (45% yield, Table 1, entry 14),
acetone (69% yield, Table 1, entry 15), or methanol (81%
yield, Table 1, entry 16) led only to poorer conversion but
When the previous catalytic system was directly applied
to the esterification of boronic acids, isolated yields were
deceptively low (Table 2, entry 1). Indeed, pinacol is bare-
Synlett 2014, 25, 551–555
© Georg Thieme Verlag Stuttgart · New York

LETTER
Interconversion of Boron Substituents
553
ly soluble in neat water and reactivity is therefore de- ids were tested. The para-substituted arylboronic acids
creased. Replacement of water by anhydrous solvents led to 77–80% yield, and reaction with styryl boronic acid
turned out to be unsuccessful. For example, reaction in afforded 76% of the vinylboronate (Scheme 2).
acetonitrile led to a mere 60% yield (Table 2, entry 2). Us-
ing an aqueous 1:1 mixture of water and miscible solvents
FeCl3 (5 mol%)
imidazole (3 equiv)
O
B
HO
HO
such as acetonitrile (Table 2, entry 3), acetone (Table 2,
entry 4), or tetrahydrofuran (Table 2, entry 5) a clear im-
provement of conversion was observed. Finally, a 4:1 ace-
tonitrile–water mixture afforded the best conversions
(Table 2, entry 6). Reaction time was optimized, and after
a 30 minutes reaction time (Table 2, entry 7) the best
yields were obtained. Longer reaction times only provided
higher proportion of side products (Table 2, entry 8). An
excess of diol (Table 2, entries 9 and 10) or boronic acid
(Table 2, entry 11) showed no improvement. Finally, we
checked the requirement of adding iron chloride (Table 2,
entry 12) and three equivalents of base (Table 2, entries 13
and 14). In both cases, isolated yields were below 70%.
RB(OH)2
+
O
R
H2O–MeCN (1:4)
r.t., 30 min
B(pin)
O
O
B
B
O
Cl
O
Ph
Ph
78%
90%
86%
B(pin)
B(pin)
B(pin)
O
B
Ph
O
t-Bu
80%
77%
B(pin) O2N
76%
82%
O
B(pin)
B(pin)
HO
BocHN
F
Table 2 Catalytic System Optimization: Formation of Boronates
99%
46%
81%
O
O
O
O
B
pinacol
FeCl3, imidazole
B
B(OH)2
O2N
B
B
O
O
Ph
O
O
solvent, r.t.
82%
HO
70%
77%
CO2Me
O
B
Entry FeCl3
Imidazole Solvent
Time Yield
(min) (%)a
H2N
O
O
(mol%) (equiv)
O
B
O
1
2
5
3
H2O
15
15
15
15
15
15
30
60
30
30
30
30
30
30
57
60
68
65
66
71
86
66
76
74b
55c
66d
61
65
97%
O2N
99%
5
3
MeCN
Scheme 2 Esterification of boronic acids
3
5
5
5
5
5
5
5
5
5
–
5
5
3
3
3
3
3
3
3
3
3
3
–
1
MeCN–H2O (1:1)
acetone–H2O (1:1)
THF–H2O (1:1)
Diazaborolanes21 have dragged a considerable interest
since Suginome reported the use of 1,8-diamino naphtha-
lene as boron masking group, leading to an elegant strate-
gy for an iterative Suzuki–Miyaura cross-coupling
sequence.3 However, despite its attractiveness, the synthe-
sis of these diazaborolanes requires high temperature (tol-
uene reflux for 2 h) with azeotropic removal of water
followed by a quick purification on silica gel.3,22
4
5
6
MeCN–H2O (4:1)
MeCN–H2O (4:1)
MeCN–H2O (4:1)
MeCN–H2O (4:1)
MeCN–H2O (4:1)
MeCN–H2O (4:1)
MeCN–H2O (4:1)
MeCN–H2O (4:1)
MeCN–H2O (4:1)
7
8
9
We thought that our method could be advantageously
adapted to prepare these compounds. Indeed, after a quick
optimization of reaction conditions, we found that high
temperature was not required for the transformation, but
the presence of imidazole was mandatory (Table 3, entry
2). Overall at room temperature, after two hours in the
presence of 5 mol% FeCl3, conversion was complete (Ta-
ble 3, entry 2).
10
11
12
13
14
a Isolated yield as referred to analytically pure material.
b Conditions: 1.05 equiv of pinacol were used.
c Conditions: 1.1 equiv of pinacol were used.
d Conditions: 0.9 equiv of pinacol were used.
Consequently, we quickly checked on various substrates
the generality of the system. In all cases, similar to those
reported in the previous sections, yields (87–95%) were
high enough to prove the utility of this method to prepare
diazaborolanes. Interestingly, the same catalytic system
allowed the direct conversion of the pinacolic ester into
the diaminonaphthalenyl derivative in 82% yield (Scheme
3).
With this optimized catalytic system in hand, we extended
the reaction to the use of other diols such as pinanediol,
neopentylglycol, or 2,4-dihydroxy-2-methyl pentane. The
corresponding boronates were obtained in 90%, 77%, and
82% yield, respectively. Similarly, few other boronic ac-
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 551–555

554
J. L. Wood et al.
LETTER
steps on the boron center. These steps are promoted by the
imidazole/imidazolium system acting as a acido-basic
buffer and by imidazole-stabilizing intermediate by form-
ing ate complexes. In most cases, a mixture of acetonitrile
and water turned out to be the optimal solvent system
leading to compounds in good yields at room temperature.
Table 3 Catalytic System Optimization: Diaminoborane Formation
NH2 NH2
B(OH)2
FeCl3, imidazole
solvent, temp
HN
B
+
N
H
Entry
FeCl3
(mol%)
Imidazole Temp
Time
(h)
Yield
(%)a
Acknowledgment
(equiv)
(°C)
L. M. and J. L.W. thank the Région d’aquitaine for funding. We gra-
tefully thank M. N. Rager of the Ecole Nationale Supérieure de Chi-
mie de Paris for NMR analyses. This work has been funded by the
CNRS, Université de Bordeaux 1 and Conseil Régional d’Aqui-
taine.
1
2
3
4
5
5
5
5
3
–
3
3
80
80
80
r.t.
2
2
1
2
89
57
89
93
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SupInpfioomgrattr
o
nSupprigI
o
tnnofrmat
a Isolated yield as referred to analytically pure material.
References
NH2 NH2
(1) Chow, W. K.; Yuen, O. Y.; Choy, P. Y.; So, C. M.; Lau, C.
P.; Wong, W. T.; Kwong, F. Y. RSC Adv. 2013, 3, 12518.
(2) Hartwig, J. F. Chem. Soc. Rev. 2011, 40, 1992.
(3) (a) Noguchi, H.; Shioda, T.; Chou, C.-M.; Suginome, M.
Org. Lett. 2008, 10, 377. (b) Noguchi, H.; Hojo, K.;
Suginome, M. J. Am. Chem. Soc. 2007, 129, 758.
(4) Vedejs, E.; Chapman, R. W.; Fields, S. C.; Lin, S.; Schrimpf,
M. R. J. Org. Chem. 1995, 60, 3020.
FeCl3 (5 mol%)
imidazole (3 equiv)
HN
B
RB(OH)2
+
H2O–MeCN (1:4)
r.t., 2–6 h
R
N
H
B(dan)
B(dan)
B(dan)
93%
t-Bu
94%
95%
(5) Lennox, A. J. J.; Lloyd-Jones, G. C. Angew. Chem. Int. Ed.
2012, 51, 9385.
B(dan)
B(dan)
(6) (a) Murata, M.; Oyama, T.; Watanabe, S.; Masuda, Y.
Synthesis 2000, 778. (b) Billingsley, K. L.; Buchwald, S. L.
J. Org. Chem. 2008, 73, 5589. (c) Marciasini, L.; Richy, N.;
Vaultier, M.; Pucheault, M. Chem. Comm. 2012, 48, 1553.
(d) Ma, Y.; Song, C.; Jiang, W.; Xue, G.; Cannon, J. F.;
Wang, X.; Andrus, M. B. Org. Lett. 2003, 5, 4635.
(e) Willis, D. M.; Strongin, R. M. Tetrahedron Lett. 2000,
41, 8683. (f) Wang, L.; Li, J.; Cui, X.; Wu, Y.; Zhu, Z.; Wu,
Y. Adv. Synth. Catal. 2010, 352, 2002. (g) Tang, W.;
Keshipeddy, S.; Zhang, Y.; Wei, X.; Savoie, J.; Patel, N. D.;
Yee, N. K.; Senanayake, C. H. Org. Lett. 2011, 13, 1366.
(h) PraveenGanesh, N.; Demory, E.; Gamon, C.; Blandin,
V.; Chavant, P. Y. Synlett 2010, 2403. (i) Lu, J.; Guan, Z.-
Z.; Gao, J.-W.; Zhang, Z.-H. Appl. Organomet. Chem. 2011,
25, 537. (j) Kawamorita, S.; Ohmiya, H.; Iwai, T.;
Sawamura, M. Angew. Chem. Int. Ed. 2011, 50, 8363.
(k) Billingsley, K. L.; Buchwald, S. L. J. Org. Chem. 2008,
73, 5589. (l) Broutin, P.-E.; Čerňa, I.; Campaniello, M.;
Leroux, F.; Colobert, F. Org. Lett. 2004, 6, 4419.
(m) Praveen Ganesh, N.; Chavant, P. Y. Eur. J. Org. Chem.
2008, 4690. (n) Euzenat, L.; Horhant, D.; Ribourdouille, Y.;
Duriez, C.; Alcaraz, G.; Vaultier, M. Chem. Commun. 2003,
2280. (o) Euzenat, L.; Horhant, D.; Brielles, C.; Alcaraz, G.;
Vaultier, M. J. Organomet. Chem. 2005, 690, 2721.
(p) Marciasini, L. D.; Richy, N.; Vaultier, M.; Pucheault, M.
Adv. Synth. Catal. 2013, 355, 1083. (q) Pucheault, M.;
Marciasini, L.; Vaultier, M. EP 123058547, 2012.
(7) Yuen, A. K. L.; Hutton, C. A. Tetrahedron Lett. 2005, 46,
7899.
Cl
O
87%
94%
B(dan)
B(dan)
O2N
99%
94%
NH2 NH2
FeCl3 (5 mol%),
imidazole (3 equiv)
HN
B
PhB(pin)
+
H2O-MeCN (1:4)
1 h, r.t.
Ph
N
H
Scheme 3 Conversion of boronic aicds into diazaborolanes
Overall we developed a simple cost effective but yet effi-
cient method to interconvert boron substituents, that is,
fluorine, alkoxy, and amino. The iron(III) chloride–imid-
azole–water system displayed a unexpected versatility to
transform aryl boronic derivatives into one another. If po-
tassium 4-methoxyphenyltrifluoroborate and FeCl3 in a
1:1 mixture led to the formation of the boronic acid in
CD3CN, no change in 11B NMR was witnessed when add-
ing FeCl3 to boronic acid. Imidazole reacts with the bo-
ronic acid (δ = 28 ppm) by making a weak complex (δ =
20.3 ppm) but without forming the borate. Imidazole and
iron chloride unsurprisingly form an iron–imidazole com-
plex as witnessed by a weak downfield shift of imidazole
protons upon complexation. The mechanism is thought to
proceed via classical activation by this iron complex as
Lewis acids, followed by sequential addition–elimination
(8) Inglis, S. R.; Woon, E. C. Y.; Thompson, A. L.; Schofield,
C. J. J. Org. Chem. 2010, 75, 468.
(9) (a) Molander, G. A.; Ellis, N. Acc. Chem. Res. 2007, 40, 275.
(b) Darses, S.; Genêt, J.-P. Chem. Rev. 2007, 108, 288.
(c) Stefani, H. A.; Cella, R.; Vieira, A. S. Tetrahedron 2007,
63, 3623.
Synlett 2014, 25, 551–555
© Georg Thieme Verlag Stuttgart · New York

LETTER
Interconversion of Boron Substituents
555
(10) (a) Darses, S.; Genêt, J.-P.; Brayer, J.-L.; Demoute, J.-P.
Tetrahedron Lett. 1997, 38, 4393. (b) Darses, S.; Michaud,
G.; Genêt, J.-P. Eur. J. Org. Chem. 1999, 1875.
(11) Joubert, N.; Baslé, E.; Vaultier, M.; Pucheault, M.
Tetrahedron Lett. 2010, 51, 2994.
Lennox, A. J. J.; Lloyd-Jones, G. C.; Murray, P. M. Angew.
Chem. 2010, 122, 5282.
(15) Kim, B. J.; Matteson, D. S. Angew. Chem. Int. Ed. 2004, 43,
3056.
(16) Kabalka, G. W.; Coltuclu, V. Tetrahedron Lett. 2009, 50,
6271.
(12) Han, F.-S. Chem. Soc. Rev. 2013, 42, 5270.
(13) (a) Pucheault, M.; Darses, S.; Genêt, J.-P. Eur. J. Org.
Chem. 2002, 3552. (b) Pucheault, M.; Darses, S.; Genêt,
J.-P. Tetrahedron Lett. 2002, 43, 6155. (c) Pucheault, M.;
Darses, S.; Genêt, J. P. J. Am. Chem. Soc. 2004, 126, 15356.
(d) Pucheault, M.; Michaut, V.; Darses, S.; Genêt, J.-P.
Tetrahedron Lett. 2004, 45, 4729. (e) Navarre, L.;
Pucheault, M.; Darses, S.; Genêt, J.-P. Tetrahedron Lett.
2005, 46, 4247. (f) Pucheault, M.; Darses, S.; Genêt, J.-P.
Chem. Commun. 2005, 4714.
(14) (a) Amatore, C.; Jutand, A.; Le Duc, G. Angew. Chem. Int.
Ed. 2012, 51, 1379. (b) Molander, G. A.; Ito, T. Org. Lett.
2001, 3, 393. (c) Molander, G. A.; Biolatto, B. J. Org. Chem.
2003, 68, 4302. (d) Butters, M.; Harvey, J. N.; Jover, J.;
(17) Molander, G. A.; Cavalcanti, L. N.; Canturk, B.; Pan, P.-S.;
Kennedy, L. E. J. Org. Chem. 2009, 74, 7364.
(18) Blevins, D. W.; Yao, M.-L.; Yong, L.; Kabalka, G. W.
Tetrahedron Lett. 2011, 52, 6534.
(19) Sugihara, J. M.; Bowman, C. M. J. Am. Chem. Soc. 1958, 80,
2443.
(20) (a) Schnurch, M.; Holzweber, M.; Mihovilovic, M. D.;
Stanetty, P. Green Chem. 2007, 9, 139. (b) Kaupp, G.;
Naimi-Jamal, M. R.; Stepanenko, V. Chem. Eur. J. 2003, 9,
4156.
(21) Pailer, M.; Fenzl, W. Monatsh. Chem. 1961, 92, 1294.
(22) Slabber, C. A.; Grimmer, C.; Akerman, M. P.; Robinson, R.
S. Acta Crystallogr., Sect. E: Struct. Rep. Online 2011, 67,
o1995.
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