Doubly Regioselective C-H Hydroarylation of Unsymmetrical Alkynes Using Carboxylates as Deciduous Directing Groups

Article abstract of DOI:10.1021/acs.orglett.7b00300

A catalyst system composed of [(C₆Me₆)RuCl₂]₂, potassium carbonate/guanidine carbonate, and mesitoic acid efficiently promotes the doubly regioselective C-H hydroarylation of unsymmetrical alkynes. The process involves carboxylate-directed ortho-C-H bond activation followed by regioselective addition to the alkyne C-C triple bond with concerted decarboxylation. This action of the carboxylate as a deciduous directing group ensures exclusive monovinylation with high selectivity for the (E)-1,2-diarylalkene.

Full text of DOI:10.1021/acs.orglett.7b00300

Letter
pubs.acs.org/OrgLett
Doubly Regioselective C−H Hydroarylation of Unsymmetrical
Alkynes Using Carboxylates as Deciduous Directing Groups
Agostino Biafora,§ Bilal A. Khan,*,‡ Janet Bahri,† Joachim M. Hewer,§ and Lukas J. Goossen*,†
†Fakultat fur Chemie und Biochemie, Ruhr-Universitat Bochum, Universitatsstrasse 150, 44801 Bochum, Germany
̈
̈
̈
̈
‡Department of Chemistry, University of Azad Jammu and Kashmir, 13100 Muzaffarabad, AJK, Pakistan
§FB Chemie und Forschungszentrum OPTIMAS, Technische Universitat Kaiserslautern, Erwin-Schroedinger-Strasse Geb 52-54,
67663 Kaiserslautern, Germany
̈
S
* Supporting Information
ABSTRACT: A catalyst system composed of [(C6Me6)RuCl2]2, potassium carbonate/guanidine carbonate, and mesitoic acid
efficiently promotes the doubly regioselective C−H hydroarylation of unsymmetrical alkynes. The process involves carboxylate-
directed ortho-C−H bond activation followed by regioselective addition to the alkyne C−C triple bond with concerted
decarboxylation. This action of the carboxylate as a deciduous directing group ensures exclusive monovinylation with high
selectivity for the (E)-1,2-diarylalkene.
directing groups18 that stay in place just long enough to direct
tyrenes are prevalent structures often encountered in
Sfunctional materials, pharmaceuticals, and natural and
one group into their ortho-position further improves the
synthetic products.1 Stoichiometric methods to access this
structural motif, including Wittig2 or Peterson olefinations3 and
insertions of alkynes into organometallic reagents,4 are waste-
intensive and require prefunctionalized substrates. Catalytic
alternatives such as the Mizoroki−Heck reaction5 and olefin
metathesis6 are more atom-economic but also require
prefunctionalized arenes. C−H vinylations of the Fujiwara−
Moritani type7 have emerged as a powerful alternative but rely on
stoichiometric oxidants.
versatility of this group. A deciduous-type reaction pathway, in
which the CO2 is released concomitantly to C−C bond
formation, intrinsically prevents unwanted double functionaliza-
tion, a typical side reaction in ortho-C−H functionalizations
(Scheme 1).19
Scheme 1. CO2H as Deciduous vs Removable Directing
Group (DG) in Catalytic Hydroarylations
C−H hydroarylations of alkynes compare favorably to the
above concepts, especially when the regioselectivity is controlled
effectively, e.g., by chelation assistance. Following early reports
on ruthenium-catalyzed carbonyl-directed hydroarylations of
alkynes,8 several transition-metal catalysts, including pre-
cious9−12 and first-row metals,13,14 have been found to efficiently
promote the insertion of alkynes into the C−H bond ortho to
various directing groups. However, most of these directing
groups, including phenol, ketone, pyridine, amide, and sulfoxide,
require additional chemical steps for their synthesis, removal, or
modification.
In this context, the use of carboxylates as directing groups is
particularly desirable because they are easily accessible at low cost
and in great structural diversity, can be transformed into a wealth
of other compound classes, may serve as leaving groups in
decarboxylative couplings, and are tracelessly removable by a
subsequent protodecarboxylation step.15,16 Over the years,
extensive research has led to the discovery of carboxylate-
directed substitutions of ortho-C−H atoms with (hetero)aryl,
alkyl, acyl, allyl, alkoxy, olefin, amine, amide, and halogen
groups.17 The discovery that carboxylates can act as deciduous
The development of carboxylate-directed regiospecific C−H
hydroarylations is challenging because of the weak coordinating
ability of the carboxylate group and the known reactivity of
alkynes to undergo carboxylate addition to the enol esters in the
20
presence of RuII and because carboxylate groups reduce the
electron density at the arene ring, thereby lowering its reactivity.
Received: January 30, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00300
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Organic Letters
Letter
The Ackermann group, the group of Hartwig and Zhao, and
our own group have independently developed Ru-catalyzed
carboxylate-directed C−H hydroarylations of internal alkyne-
s.19a−c All of these processes allow the decarboxylative
hydroarylation of diarylalkynes in high yields. However,
examples with alkylarylalkynes as coupling partners were
provided only by Hartwig and Zhao, and these reactions did
not proceed via a deciduous-type pathway. Selectivity for the
monovinylated, decarboxylated product was achieved by using a
2-fold excess of the arenecarboxylate and a powerful copper
protodecarboxylation catalyst. Satisfactory yields and selectivities
were obtained merely for a few substrates.19c
We herein report a catalyst system that requires only low Ru
loadings and no copper mediator to promote the regioselective
decarboxylative monovinylation using unsymmetrical alkynes. In
the search for an efficient protocol for the desired transformation,
we used the reaction of p-methoxybenzoic acid 1a and 1-phenyl-
1-propyne 2a as a model (Table 1).
(entry 2). Our conditions previously optimized for diarylalkynes,
namely 1a (0.5 mmol), 2a (1 equiv), [(p-cymene)RuCl2]2 (4
mol %, [Ru2]), guanidine carbonate (20 mol %), AcOH (1
equiv), and 2-picoline (20 mol %) in toluene, provided 3aa in
25% yield along with 21% of 4aa−6aa, which are products arising
from a competing nondeciduous directing mode of the
carboxylate group (entry 3). Screening of various catalysts,
additives, and solvents showed that the combination of a
[(C6Me6)RuCl2]2 ([Ru3]) catalyst and the polar aprotic solvent
NMP gave greater conversion and good regioselectivity (entry 4
and Table S1). Increasing the amount of 2a to 1.5 equiv further
improved the yield (entry 5). Interestingly, 2-picoline, which was
an important component of our original conditions, did not affect
the outcome here (entry 6). Higher yields were obtained when
the acetic medium was buffered with 5 mol % of guanidine
carbonate and 10 mol % of potassium carbonate (entries 8−11).
Substituting acetic by mesitoic acid shifted the reaction
completely toward the desired pathway, so that products 4−6
arising from competing pathways were no longer detected.
Within 16 h under optimal conditions, i.e., 1a (0.5 mmol), 2a
(0.75 mmol), [(C6Me6)RuCl2]2 (4 mol %), guanidine carbonate
(5 mol %), K2CO3 (10 mol %), and mesitoic acid (1 equiv) in
NMP (1 mL) at 120 °C, the monovinylated product 3aa was
obtained exclusively and with an impressive 3aa/3′aa
regioselectivity of 36:1 in favor of the less sterically hindered
alkyl-branched product (entry 12, method A). The regiochemical
preference is in agreement with findings by Fagnou, Miura,
Rovis, Li, Ackermann, Larock, and others on mechanistically
related oxidative annulation reactions.21
a
Table 1. Optimization of the Reaction Conditions
When a preformed o-vinylbenzoic acid (4ba) was subjected to
the reaction conditions, no decarboxylation was observed (see
the Supporting Information), which confirms that C−C bond
formation and decarboxylation indeed occur concertedly.
Further control experiments established that both base and
acid additive are required (Table S1).
The only drawback of this protocol was the long reaction time.
However, this can be shortened to only 5 min by employing
microwave irradiation (method B) after small adjustments to the
catalyst system (10 mol % of guanidine carbonate as the only
base and with the amount of mesitoic acid reduced to 0.5
equiv).22
With two effective sets of conditions in hand, the scope and
selectivity of the ruthenium-catalyzed decarboxylative C−H
hydroarylation of 2a with substituted benzoic acids 1 were
evaluated (Scheme 2). The scope extends from electron-rich to
electron-poor benzoic acids with various functional groups in the
ortho, meta, or para positions, as well as heterocyclic carboxylates.
Benzoic acids bearing ortho substituents generally gave excellent
yields (3ba−ka). Para-substituted benzoic acids afforded
monofunctionalized products (3aa,la−oa) exclusively and in
good yields. p-Toluic acid (1m) afforded 30% of non-
decarboxylated product 4ma along with 3ma, which presumably
result from a competing nondeciduous pathway. Extending the
reaction time to 48 h did not shift the product distribution further
toward 3ma (see the SI). This clearly indicates that the
decarboxylated product results from a concerted C−C bond
formation/decarboxylation process, and that once the non-
decarboxylated product is released, it does not re-enter the
catalytic cycle. The reactivity of meta-substituted acids was lower
(3pa,qa). Deactivating substituents such as nitro groups reduced
the yields (3wa). With 2-allyl benzoate, the side-chain double
bond isomerized into conjugation under the reaction conditions
(3da). The efficiency of the microwave method was generally
a
Method A: 1a (0.5 mmol), 2a (0.75 mmol), [Ru] (4 mol %), base,
b
acid (1 equiv), additive, NMP (1 mL), 120 °C, 16 h. Method A: 1a
(1 mmol), 2a (0.5 mmol), [Ru1] (10 mol %) in dioxane/mesitylene/
c
n-heptane (2:2:1), 80 °C, 48 h. Method A: 2a (0.5 mmol) in PhMe.
d
Method B: 1a (1 mmol), 2a (0.5 mmol), [Ru] (4 mol %), guanidine
carbonate (10 mol %), TMBA (0.5 equiv), NMP (2 mL), 180 °C μW,
5 min. Yields were determined by GC analysis after esterification with
MeI/K2CO3 using n-tetradecane as internal standard. [Ru1] = (p-
cym)Ru(OAc)2, [Ru2] = [(p-cym)RuCl2]2, [Ru3] = [(C6Me6)-
RuCl2]2. GuanCO3 = guanidinium carbonate. TMBA = 2,4,6-
trimethylbenzoic (mesitoic) acid.
When Hartwig and Zhao’s conditions were used, i.e.,
treatment a 2-fold excess of 1a with 2a in the presence of
[Ru1] and 20 mol % Cu(OAc)2 in dioxane/mesitylene/n-
heptane at 80 °C for 48 h,19c the desired styrene 3aa was formed
in 59% yield and a 3aa/3′aa regioselectivity of 12:1 (Table 1,
entry 1). Unsatisfactory results and formation of products 4−6 in
notable amounts were observed without the copper mediator
B
DOI: 10.1021/acs.orglett.7b00300
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Organic Letters
Letter
a
Scheme 2. Scope with Respect to the Benzoic Acids
a
Isolated yields. GC yields and product ratios of 3:3′:4, in parentheses, after esterification using n-tetradecane as internal standard.
comparable, although the higher reaction temperature somewhat
affected the regioselectivity. Only for unsubstituted benzoic acid
(1y) was nondecarboxylative hydroarylation a major side
reaction under thermal conditions, leading to the formation of
substantial amounts of disubstituted products. However, under
microwave conditions, the carboxylate acted as a deciduous
directing group again, and only monovinylated product 3ya was
observed.
Scheme 4. Proposed Mechanism for the Ruthenium-
Catalyzed Decarboxylative Hydroarylation
We next investigated the alkyne substrate scope in
combination with p-anisic (1a), p-fluorobenzoic (1n), and o-
toluic acid (1b) (Scheme 3). For alkylarylalkynes 2b−f, high
a
Scheme 3. Scope with Respect to the Alkynes
capable by itself of decarboxylating 4aa under these conditions.
CO2 extrusion can occur only in the presence of copper or silver
decarboxylation catalysts, as previously reported.16
In conclusion, an effective and broadly applicable C−H
hydroarylation of unsymmetrical alkynes has been developed on
the basis of the inexpensive and easy-to-handle catalyst
[(C6Me6)RuCl2]2. The concerted C−C bond formation/CO2
extrusion process ensures nearly exclusive formation of
monovinylated products and obviates a subsequent protode-
carboxylation step.
a
Isolated yields. GC yields and product ratios of 3:3′:4, in parentheses,
after esterification using n-tetradecane as internal standard.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b00300.
yields and excellent regioselectivities of the desired products
were achieved. Electron-poor propiolates (2b) were successfully
converted to β,β-diaryl acrylates, which are valuable synthons for
further decarboxylative couplings.18,23 Terminal alkynes did not
react under the reaction conditions.
A plausible reaction pathway derived from mechanistic
experiments (see the SI) is outlined in Scheme 4. Following
C−H activation, the ortho-ruthenated complex A coordinates the
alkyne substrate. Migratory insertion leads to the seven-
membered ruthenacycle B. Possible next steps involve either
decarboxylation to intermediate C, which is then protodemeta-
lated to product 3aa, or early protodemetalation of B, resulting in
the nondecarboxylated compound 4aa. Ruthenium is not
■
S
Detailed screening tables, experimental procedures,
analytical data for all the products, and NMR spectra
(PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: bkhan@ajku.edu.pk.
*E-mail: lukas.goossen@rub.de.
C
DOI: 10.1021/acs.orglett.7b00300
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Organic Letters
ORCID
Letter
J. Am. Chem. Soc. 2016, 138, 13076−13081. (c) Shibuya, T.; Shibata, Y.;
Noguchi, K.; Tanaka, K. Angew. Chem., Int. Ed. 2011, 50, 3963−3967.
(13) (a) Ilies, L.; Chen, Q.; Zeng, X.; Nakamura, E. J. Am. Chem. Soc.
2011, 133, 5221−5223. (b) Gao, K.; Lee, P.-S.; Fujita, T.; Yoshikai, N. J.
Am. Chem. Soc. 2010, 132, 12249−12251. (c) Cera, G.; Ackermann, L.
Top. Curr. Chem. (Z) 2016, 374, 57. (d) Wong, M. Y.; Yamakawa, T.;
Yoshikai, N. Org. Lett. 2015, 17, 442−445.
(14) (a) Nakao, Y.; Idei, H.; Kanyiva, K. S.; Hiyama, T. J. Am. Chem.
Soc. 2009, 131, 15996−15997. (b) Mukai, T.; Hirano, K.; Satoh, T.;
Miura, M. J. Org. Chem. 2009, 74, 6410−6413. (c) Nakao, Y.; Kashihara,
N.; Kanyiva, K. S.; Hiyama, T. J. Am. Chem. Soc. 2008, 130, 16170−
16171. (d) Tsukada, N.; Murata, K.; Inoue, Y. Tetrahedron Lett. 2005,
46, 7515−7517. (e) Gooßen, L. J. Angew. Chem., Int. Ed. 2002, 41,
3775−3778.
(15) (a) Goossen, L. J. Science 2006, 313, 662−664. (b) Goossen, L. J.;
Rodríguez, N.; Melzer, B.; Linder, C.; Deng, G.; Levy, L. M. J. Am. Chem.
Soc. 2007, 129, 4824−4833. (c) Gooßen, L. J.; Rodríguez, N.; Gooßen,
K. Angew. Chem., Int. Ed. 2008, 47, 3100−3120. (d) Rodríguez, N.;
Goossen, L. J. Chem. Soc. Rev. 2011, 40, 5030. (e) Inventing Reactions;
Goossen, L. J., Ed.; Topics in Organometallic Chemistry; Springer: New
York, 2013.
(16) (a) Zhang, Y.; Zhao, H.; Zhang, M.; Su, W. Angew. Chem., Int. Ed.
2015, 54, 3817−3821. (b) Luo, J.; Preciado, S.; Larrosa, I. J. Am. Chem.
Soc. 2014, 136, 4109−4112. (c) Cornella, J.; Righi, M.; Larrosa, I. Angew.
Chem., Int. Ed. 2011, 50, 9429−9432. (d) Bhadra, S.; Dzik, W. I.;
Gooßen, L. J. Angew. Chem., Int. Ed. 2013, 52, 2959−2962. (e) Qin, X.;
Sun, D.; You, Q.; Cheng, Y.; Lan, J.; You, J. Org. Lett. 2015, 17, 1762−
1765.
Lukas J. Goossen: 0000-0002-2547-3037
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Umicore for donating chemicals, the DFG (SFB/TRR-
88, “3MET”) and XC/1069 “RESOLV” for financial support, and
the AG Clusterchemie for providing technical equipment.
REFERENCES
■
(1) (a) Rosazza, J. P. N.; Huang, Z.; Dostal, L.; Volm, T.; Rousseau, B.
J. Ind. Microbiol. 1995, 15, 457−471. (b) Ali, M. S.; Mahmud, S.;
Perveen, S.; Ahmad, V. U.; Rizwani, G. H. Phytochemistry 1999, 50,
1385−1389. (c) Rukachaisirikul, V.; Rodglin, A.; Sukpondma, Y.;
Phongpaichit, S.; Buatong, J.; Sakayaroj, J. J. Nat. Prod. 2012, 75, 853−
858. (d) Hassam, M.; Taher, A.; Arnott, G. E.; Green, I. R.; van Otterlo,
W. A. L. Chem. Rev. 2015, 115, 5462−5569. (e) Roseblade, S. J.; Pfaltz,
A. Acc. Chem. Res. 2007, 40, 1402−1411.
(2) March, J. Advanced Organic Chemistry: Reactions, Mechanisms, And
Structure, 4th ed.; Wiley: New York, 1992.
́
(3) Kurti, L.; Czako, B. Strategic Applications of Named Reactions in
̈
Organic Synthesis: Background and Detailed Mechanisms; Elsevier
Academic Press: Amsterdam, 2005.
(4) (a) Xu, X.; Chen, J.; Gao, W.; Wu, H.; Ding, J.; Su, W. Tetrahedron
2010, 66, 2433−2438. (b) Fagnou, K.; Lautens, M. Chem. Rev. 2003,
103, 169−196. (c) Shirakawa, E.; Yamagami, T.; Kimura, T.; Yamaguchi,
S.; Hayashi, T. J. Am. Chem. Soc. 2005, 127, 17164−17165.
(5) Beletskaya, I. P.; Cheprakov, A. V. Chem. Rev. 2000, 100, 3009−
3066.
(17) (a) Pichette-Drapeau, M.; Gooßen, L. J. Chem. - Eur. J. 2016, 22,
18654−18677. (b) Shi, G.; Zhang, Y. Adv. Synth. Catal. 2014, 356,
1419−1442. (c) Biafora, A.; Krause, T.; Hackenberger, D.; Belitz, F.;
Gooßen, L. J. Angew. Chem., Int. Ed. 2016, 55, 14752−14755. (d) Mei,
R.; Zhu, C.; Ackermann, L. Chem. Commun. 2016, 52, 13171−13174.
(e) Simonetti, M.; Cannas, D. M.; Panigrahi, A.; Kujawa, S.; Kryjewski,
M.; Xie, P.; Larrosa, I. Chem. - Eur. J. 2017, 23, 549−553. (f) Huang, L.;
Weix, D. J. Org. Lett. 2016, 18, 5432−5435.
(6) (a) Grubbs, R. H.; Chang, S. Tetrahedron 1998, 54, 4413−4450.
(b) Furstner, A. Angew. Chem., Int. Ed. 2000, 39, 3012−3043.
̈
(7) (a) Moritani, I.; Fujiwara, Y. Tetrahedron Lett. 1967, 8, 1119−1122.
(b) Yokota, T.; Tani, M.; Sakaguchi, S.; Ishii, Y. J. Am. Chem. Soc. 2003,
125, 1476−1477. (c) Nishikata, T.; Lipshutz, B. H. Org. Lett. 2010, 12,
1972−1975.
(18) Tang, J.; Hackenberger, D.; Goossen, L. J. Angew. Chem., Int. Ed.
2016, 55, 11296−11299.
(19) (a) Huang, L.; Biafora, A.; Zhang, G.; Bragoni, V.; Gooßen, L. J.
Angew. Chem., Int. Ed. 2016, 55, 6933−6937. (b) Kumar, N. Y. P.;
Bechtoldt, A.; Raghuvanshi, K.; Ackermann, L. Angew. Chem., Int. Ed.
2016, 55, 6929−6932. (c) Zhang, J.; Shrestha, R.; Hartwig, J. F.; Zhao, P.
Nat. Chem. 2016, 8, 1144−1151. (d) Simonetti, M.; Larrosa, I. Nat.
Chem. 2016, 8, 1086−1088.
(8) (a) Kakiuchi, F.; Yamamoto, Y.; Chatani, N.; Murai, S. Chem. Lett.
1995, 24, 681−682. (b) Kakiuchi, F.; Sato, T.; Tsujimoto, T.; Yamauchi,
M.; Chatani, N.; Murai, S. Chem. Lett. 1998, 27, 1053−1054.
(c) Kakiuchi, F.; Uetsuhara, T.; Tanaka, Y.; Chatani, N.; Murai, S. J.
Mol. Catal. A: Chem. 2002, 182−183, 511−514.
(9) (a) Arockiam, P. B.; Bruneau, C.; Dixneuf, P. H. Chem. Rev. 2012,
112, 5879−5918. (b) Hashimoto, Y.; Hirano, K.; Satoh, T.; Kakiuchi, F.;
Miura, M. Org. Lett. 2012, 14, 2058−2061. (c) Manikandan, R.;
Jeganmohan, M. Org. Biomol. Chem. 2015, 13, 10420−10436.
(d) Hashimoto, Y.; Hirano, K.; Satoh, T.; Kakiuchi, F.; Miura, M. J.
Org. Chem. 2013, 78, 638−646. (e) Cheng, K.; Yao, B.; Zhao, J.; Zhang,
Y. Org. Lett. 2008, 10, 5309−5312. (f) Itoh, M.; Hashimoto, Y.; Hirano,
K.; Satoh, T.; Miura, M. J. Org. Chem. 2013, 78, 8098−8104.
(g) Manikandan, R.; Jeganmohan, M. Org. Lett. 2014, 16, 912−915.
(10) (a) Hong, P.; Cho, B.-R.; Yamazaki, H. Chem. Lett. 1979, 8, 339−
342. (b) Hong, P.; Cho, B.-R.; Yamazaki, H. Chem. Lett. 1980, 9, 507−
510. (c) Schipper, D. J.; Hutchinson, M.; Fagnou, K. J. Am. Chem. Soc.
2010, 132, 6910−6911. (d) Shibata, Y.; Otake, Y.; Hirano, M.; Tanaka,
K. Org. Lett. 2009, 11, 689−692. (e) Katagiri, T.; Mukai, T.; Satoh, T.;
Hirano, K.; Miura, M. Chem. Lett. 2009, 38, 118−119. (f) Parthasarathy,
K.; Jeganmohan, M.; Cheng, C.-H. Org. Lett. 2008, 10, 325−328.
(g) Colby, D. A.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc. 2008,
130, 3645−3651. (h) Patureau, F. W.; Besset, T.; Kuhl, N.; Glorius, F. J.
Am. Chem. Soc. 2011, 133, 2154−2156.
(20) (a) Goossen, L. J.; Paetzold, J.; Koley, D. Chem. Commun. 2003,
706−707. (b) Bruneau, C.; Dixneuf, P. H. Angew. Chem., Int. Ed. 2006,
45, 2176−2203. (c) Miura, H.; Tsutsui, K.; Wada, K.; Shishido, T. Chem.
Commun. 2015, 51, 1654−1657.
(21) (a) Guimond, N.; Gouliaras, C.; Fagnou, K. J. Am. Chem. Soc.
2010, 132, 6908−6909. (b) Hyster, T. K.; Rovis, T. J. Am. Chem. Soc.
2010, 132, 10565−10569. (c) Mochida, S.; Umeda, N.; Hirano, K.;
Satoh, T.; Miura, M. Chem. Lett. 2010, 39, 744−746. (d) Ackermann, L.;
Lygin, A. V.; Hofmann, N. Angew. Chem. 2011, 123, 6503−6506. (e) Li,
B.; Feng, H.; Xu, S.; Wang, B. Chem. - Eur. J. 2011, 17, 12573−12577.
(f) Larock, R. C.; Yum, E. K.; Refvik, M. D. J. Org. Chem. 1998, 63,
7652−7662.
(22) Goossen, L. J.; Manjolinho, F.; Khan, B. A.; Rodríguez, N. J. Org.
Chem. 2009, 74, 2620−2623.
(23) Zhang, L.; Hang, Z.; Liu, Z.-Q. Angew. Chem., Int. Ed. 2016, 55,
236−239.
(11) (a) Kuninobu, Y.; Kawata, A.; Takai, K. J. Am. Chem. Soc. 2005,
127, 13498−13499. (b) Kuninobu, Y.; Tokunaga, Y.; Kawata, A.; Takai,
K. J. Am. Chem. Soc. 2006, 128, 202−209. (c) Tsuchikama, K.; Kasagawa,
M.; Hashimoto, Y.-K.; Endo, K.; Shibata, T. J. Organomet. Chem. 2008,
693, 3939−3942.
(12) (a) Tsukada, N.; Mitsuboshi, T.; Setoguchi, H.; Inoue, Y. J. Am.
Chem. Soc. 2003, 125, 12102−12103. (b) Liu, Z.; Derosa, J.; Engle, K. M.
D
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