Controllable Rh(III)-Catalyzed Annulation between Salicylaldehydes and Diazo Compounds: Divergent Synthesis of Chromones and Benzofurans

Article abstract of DOI:10.1021/acs.orglett.6b03355

A Rh(III)-catalyzed annulation between salicylaldehydes and diazo compounds with controllable chemoselectivity is described. AgNTf₂ favored benzofurans via a tandem C-H activation/decarbonylation/annulation process, while AcOH led to chromones through a C-H activation/annulation pathway. The reaction exhibited good functional group tolerance and scalability. Moreover, only a single regioisomer of benzofuran was obtained due to the in situ decarbonylation orientation effect.

Full text of DOI:10.1021/acs.orglett.6b03355

Letter
pubs.acs.org/OrgLett
Controllable Rh(III)-Catalyzed Annulation between Salicylaldehydes
and Diazo Compounds: Divergent Synthesis of Chromones and
Benzofurans
Peng Sun, Shang Gao, Chi Yang, Songjin Guo, Aijun Lin,* and Hequan Yao*
State Key Laboratory of Natural Medicines (SKLNM) and Department of Medicinal Chemistry, School of Pharmacy, China
Pharmaceutical University, Nanjing 210009, P. R. China
S
* Supporting Information
ABSTRACT: A Rh(III)-catalyzed annulation between salicylaldehydes and diazo compounds with controllable chemoselectivity
is described. AgNTf2 favored benzofurans via a tandem C−H activation/decarbonylation/annulation process, while AcOH led to
chromones through a C−H activation/annulation pathway. The reaction exhibited good functional group tolerance and
scalability. Moreover, only a single regioisomer of benzofuran was obtained due to the in situ decarbonylation orientation effect.
Scheme 1. Transition-Metal-Catalyzed C−H Activation of
Salicylaldehydes
n the last two decades, transition-metal-catalyzed C−H
Iactivation has been well studied and identified as one of the
most powerful tools in organic synthesis.1 Ideally, the structurally
diverse products could be achieved from the same materials via
selective C−H functionalization in a controllable manner.
Recently, advances have been achieved through the choice of
catalysts,2 ligands,3 directing groups,4 and electronic/steric
effects.5 Although the aforementioned methods have been
widely investigated, the continuous development of new
methods to precisely control the product distribution in C−H
activation is still a hot research topic and challenging problem for
chemists.
Salicylaldehydes are versatile building blocks in organic
synthesis, and numerous endeavors have been devoted to
functionalizing salicylaldehydes with alkenes, alkynes, and allenes
via a direct aldehydic C−H activation and/or annulation
process.6 The key to the success of these works was the
formation of a five-membered-ring organometallic intermediate
as shown in Scheme 1a. On the other hand, the synthetic
applications of the exotic oxa-metallacycles7 formed through
decarbonylative metalation of salicylaldehydes in annulation
have remained unexplored until now8 (Scheme 1b). To fulfill this
objective, predictable challenges might be met: (1) how to
regulate the selectivity to realize the facile decarbonylation in the
presence of the direct aldehydic C−H functionalization and (2)
whether the four-membered oxa-metallacycle generated from
decarbonylation exhibits sufficient stability to achieve the
subsequent annulation process. On the basis of our continuous
interest in transition-metal-catalyzed selective C−H activation
and diazo compounds9 and also to enrich the reaction type of
salicylaldehydes, herein we report our results on a Rh(III)-
catalyzed tandem C−H activation, decarbonylation, and
annulation between salicylaldehydes and diazo compounds10
toward the construction of benzofurans and chromones in a
controllable manner (Scheme 1c). It is necessary to point out
that in our reaction the decarbonylation plays an important role
in orientation to achieve a single regioisomer of benzofuran,
which overcomes the regioselective problems faced by employ-
ing phenols as coupling reactants.11 Moreover, the competitive
reaction between the formed metal carbenoid intermediate and
the phenolic hydroxyl group of salicylaldehydes has also been
well suppressed.12
We set out our investigation with salicylaldehyde 1a and ethyl
2-diazo-3-oxobutanoate 2a as model substrates. A preliminary
Received: November 9, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b03355
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Organic Letters
Letter
a
,b
attempt with 2.5 mol % of [RhCp*Cl2]2 and 20 mol % AgOAc in
2 mL of DCE at 50 °C offered the decarbonylation/annulation
product 3aa in 21% yield with the direct aldehydic C−H
activation/annulation product 4aa in 55% yield (Table 1, entry
Table 2. Substrate Scope for the Synthesis of Benzofurans
a
Table 1. Optimization of Reaction Conditions
b
entry
catalyst
additive
yield (3aa/4aa)
1
2
3
4
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
AgOAc
AgSbF6
AgBF4
21/55
52/31
43/0
c
AgNTf2
AgNTf2
AgNTf2
90 (87) /0
d
5
69/0
56/0
0/44
0/32
0/0
e
6
f
7
8
9
AcOH
f
PivOH
f
MSA
g
f
c
10
AcOH
0/82
11
12
AgNTf2
0/0
0/0
f
AcOH
a
Reaction conditions: 1a (0.5 mmol, 1.0 equiv), 2a (1.0 mmol, 2.0
equiv), catalyst (0.0125 mmol, 2.5 mol %), additive (20 mol %), DCE
b
(2 mL), 50 °C, 12 h under air atmosphere. Yields were determined
by 1H NMR using dibromomethane (δ = 4.80) as an internal standard.
c
d
e
Isolated yields. 10 mol % of AgNTf2 was used. Room temperature.
f
g
2.0 equiv of additives. 80 °C.
1). When AgSbF6 was tested, 3aa could be produced in 52% yield
(Table 1, entry 2). The yield of 3aa was further increased to 90%
when AgNTf2 was used (Table 1, entry 4). Decreasing the
amount of AgNTf2 or reducing the reaction temperature led to
lower yields (Table 1, entries 5 and 6). To improve the yield of
4aa, some additives were then examined. When AcOH was used,
4aa could be furnished in 44% yield with a certain amount of 1a
and 2a unconverted (Table 1, entry 7). By further increasing the
temperature to 80 °C, the yield of 4aa was promoted to 82%
(Table 1, entry 10). Neither 3aa nor 4aa could be obtained
without the loading of [RhCp*Cl2]2 (Table 1, entries 11 and 12).
With the optimized conditions in hand, the substrate scope for
the synthesis of benzofurans was examined as shown in Table 2.
When 5-methoxysalicylaldehyde was investigated, 3ca was
obtained in 78% yield. 5-Halogen-substituted products 3da−fa
were prepared in 61−69% yields. In previous works,12 when 3-
substituted phenols were used as coupling reactants, it was
difficult to obtain a single regioisomer of 4-substituted or 6-
substituted benzofuran. Here, our methods provided a practical
way to settle this problem with the in situ decarbonylation
orientation effect. When 4-methylsalicylaldehyde was tested,
only 6-methylbenzofuran 3ha was obtained, and the structure
was identified by single-crystal X-ray analysis.13 Salicylaldehydes
with substituents at the C3 or C6 positions performed smoothly
to offer 3ja−la in 82−93% yields. 1-Hydroxy-2-naphthaldehyde
1p delivered 3pa in 78% yield. Considering that the benzofuran
skeleton is widespread in drugs and natural products, late-stage
modification of estrone and tyrosine were tested, and the desired
products 3qa and 3sa were prepared in 71% and 79% yields.
Compound 3ra,14 identified as a COX-1 and COX-2 inhibitor,
was also synthesized conveniently in 57% yield. When the ester
group of 2a was altered from an ethyl group to methyl, isopropyl,
a
Reaction conditions: 1 (0.5 mmol, 1.0 equiv), 2 (1.0 mmol, 2.0
equiv), [RhCp*Cl2]2 (0.0125 mmol, 2.5 mol %), AgNTf2 (20 mol %),
b
DCE (2 mL), 50 °C for 12 h under air atmosphere. Isolated yields.
benzyl, and allyl groups, 3ab−af were obtained in 81−90% yields.
When the R2 group was replaced with ethyl, n-propyl, and
cyclopropyl groups, 3ag−ai were prepared in 80−88% yields. 2-
Unsubstituted and 2-phenyl-substituted 3ak and 3al were
obtained in 72% and 69% yields. When the reaction was scaled
up to 10 mmol, 3aa could be prepared in 71% yield (1.45 g).
The substrate scope for the synthesis of chromones was then
examined, and the results are shown in Table 3. 5-Methyl- and 5-
methoxysalicylaldehydes gave 4ba and 4ca in 85% and 90%
yields. Salicylaldehydes with halogen groups at the C5 position
offered 4da and 4fa in 83% and 79% yields. Compound 4ga with
a strong electronic-withdrawing group (NO2) was obtained in
50% yield. The C3- or C4-substituted salicylaldehydes offered
the corresponding products in 80−87% yields. 6-Hydroxybenzo-
[d][1,3]dioxole-5-carbaldehyde delivered 4ma in 70% yield.
Substrates with phenyl and 2-thienyl groups at the C5 position
yielded 4na and 4oa in 68% and 60% yields. When 1-hydroxy-2-
naphthaldehyde 1p was examined, 4pa was obtained in 58%
yield. Estrone and tyrosine derivatives 4qa and 4sa with a
chromone skeleton were prepared in 80% and 91% yields.
Methyl, tert-butyl, and benzyl 2-diazo-3-oxobutanoates yielded
4ab, 4ad, and 4ae in 81%, 77%, and 82% yields. Alkyl- and aryl-
substituted diazo compounds in the R2 position offered 4ag, 4ah,
B
DOI: 10.1021/acs.orglett.6b03355
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Letter
a,b
Table 3. Substrate Scope for the Synthesis of Chromones
Scheme 2. Plausible Reaction Pathway
membered oxa-metallacycle B, which reacts with 2a to offer the
Rh−carbene complex C with the release of N2. Then the
intermediate D is generated after the migratory insertion process,
which is protonated to form E with the regeneration of the active
rhodium catalyst. Later, the intramolecular dehydration
condensation of E yields the product 3aa. When AcOH was
used as the additive (Scheme 2b), the hydroxyl-directed C−H
activation of 1a affords the intermediate F, which reacts with 2a
directly to produce a five-membered-ring Rh−carbene complex
G. Next, the migratory insertion yields intermediate H, which is
then protonated to form I with the regeneration of the active
catalyst. Chromone 4aa is achieved through the intramolecular
dehydration condensation of intermediate I.
In conclusion, we have developed a rhodium-catalyzed
annulation between salicylaldehydes and diazo compounds
with controllable chemoselectivity. AgNTf2 favored benzofurans
via a tandem C−H activation/decarbonylation/annulation
process, while AcOH led to chromones through a C−H
activation/annulation pathway. The reaction exhibited good
functional group tolerance and scalability, and only a single
regioisomer of benzofuran was obtained due to the in situ
decarbonylation orientation effect. To expand the application of
our method, late-stage modification was conducted to provide
estrone and tyrosine derivatives with chromone and benzofuran
skeletons in high efficiency, and two reported bioactive
molecules were also conveniently synthesized. The preliminary
mechanism study illustrated that a four-membered oxa-metalla-
cycle might be generated during the C−H activation/decarbon-
ylation step.
a
Reaction conditions: 1 (0.5 mmol, 1.0 equiv), 2 (1.0 mmol, 2.0
equiv), [RhCp*Cl2]2 (0.0125 mmol, 2.5 mol %), AcOH (2.0 equiv),
b
DCE (2 mL), 80 °C for 12 h under air atmosphere. Isolated yields.
and 4al in 80%, 79%, and 49% yields. Ethyl 2-diazo-3-
oxopropanoate 1k gave 4ak in 90% yield. Compound 4aj with
a chloro group in the R2 position was obtained in 70% yield.
When 3-diazopentane-2,4-dione 2m was tested, 4am was
obtained in 94% yield. Flavone derivative 4an15 with
antityrosine-kinase activity could be prepared in 64% yield.
When the model reaction was conducted on a 10 mmol scale, 4aa
was obtained in 1.74 g (74% yield).
On the basis of our control experiments (see the Supporting
Information for details) and previous reports,16 the plausible
reaction pathway for the Rh(III)-catalyzed annulation between
salicylaldehydes and diazo compounds with controllable chemo-
selectivity is proposed as shown in Scheme 2. When AgNTf2 is
used as an additive (Scheme 2a), the cationic complex
Cp*Rh(III) is generated, participating in the directed C−H
activation of salicylaldehyde to offer intermediate A. Catalyst
with a weaker coordinating counterion (such as Tf2N−) prefers
the decarbonylation, while the stronger counterions (Cl−, AcO−)
are unlikely to fulfill this role.17 We speculated that the Ag+ may
also play an important role in promoting the decarbonylation
process, while the specific role is unclear at the present stage. A
sequential decarbonylation process occurs to generate the four-
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b03355.
C
DOI: 10.1021/acs.orglett.6b03355
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
1H and 13C NMR spectra for all new compounds (PDF)
Letter
Atesi
̧
n, T. A.; Chavez, G.; Arman, H. D.; Larionov, O. V. ACS Catal.
2015, 5, 167. (d) Guo, P.; Joo, J. M.; Rakshit, S.; Sames, D. J. Am. Chem.
Soc. 2011, 133, 16338.
X-ray crystallographic data for 3ha (CIF)
(6) For selected examples, see: (a) Stemmler, R. T.; Bolm, C. Adv.
Synth. Catal. 2007, 349, 1185. (b) Nagamoto, M.; Nishimura, T. Chem.
Commun. 2015, 51, 13791. (c) von Delius, M.; Le, C. M.; Dong, V. M. J.
Am. Chem. Soc. 2012, 134, 15022. (d) Murphy, S. K.; Coulter, M. M.;
Dong, V. M. Chem. Sci. 2012, 3, 355. (e) Wang, H.; Xie, F.; Qi, Z.; Li, X.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: ajlin@cpu.edu.cn.
*E-mail: hyao@cpu.edu.cn; cpuhyao@126.com.
ORCID
Org. Lett. 2015, 17, 920. (f) Shi, Z.; Schroder, N.; Glorius, F. Angew.
̈
Chem., Int. Ed. 2012, 51, 8092. (g) Kuppusamy, R.; Gandeepan, P.;
Cheng, C.-H. Org. Lett. 2015, 17, 3846. (h) Yang, J.; Yoshikai, N. Angew.
Chem., Int. Ed. 2016, 55, 2870. (i) Du, X.-W.; Stanley, L. M. Org. Lett.
2015, 17, 3276.
Hequan Yao: 0000-0003-4865-820X
Notes
(7) For selected examples of four-membered oxa-metallacycle, see:
(a) Pattanayak, S.; Chattopadhyay, S.; Ghosh, K.; Ganguly, S.; Ghosh,
P.; Chakravorty, A. Organometallics 1999, 18, 1486. (b) Bag, N.;
Choudhury, S. B.; Pramanik, A.; Lahiri, G. K.; Chakravorty, A. Inorg.
Chem. 1990, 29, 5013. (c) Dauth, A.; Love, J. A. Chem. Rev. 2011, 111,
2010.
(8) Only one example was shown in a rhodium-catalyzed oxidative
coupling between salicylaldehydes and internal alkynes reported by
Miura’s group; see: Shimizu, M.; Tsurugi, H.; Satoh, T.; Miura, M.
Chem. - Asian J. 2008, 3, 881.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Generous financial support from the National Natural Science
Foundation of China (NSFC 21572272 and NSFC21502232),
the Natural Science Foundation of Jiangsu Province
(BK20140655), and the Foundation of State Key Laboratory
of Natural Medicines (ZZYQ201605) is gratefully acknowl-
edged.
(9) (a) Su, Y.; Zhou, H.; Chen, J.; Xu, J.; Wu, X.; Lin, A.; Yao, H. Org.
Lett. 2014, 16, 4884. (b) Su, Y.; Gao, S.; Huang, Y.; Lin, A.; Yao, H.
Chem. - Eur. J. 2015, 21, 15820. (c) Gao, S.; Wu, Z.; Wu, F.; Lin, A.; Yao,
H. Adv. Synth. Catal. 2016, DOI: 10.1002/adsc.201600696. (d) Sun, P.;
Wu, Y.; Yang, T.; Wu, X.; Xu, J.; Lin, A.; Yao, H. Adv. Synth. Catal. 2015,
357, 2469. (e) Jiang, H.; Gao, S.; Xu, J.; Wu, X.; Lin, A.; Yao, H. Adv.
Synth. Catal. 2016, 358, 188. (f) Sun, P.; Wu, Y.; Huang, Y.; Wu, X.; Xu,
J.; Yao, H.; Lin, A. Org. Chem. Front. 2016, 3, 91. (g) Wu, Y.; Sun, P.;
Zhang, K.; Yang, T.; Yao, H.; Lin, A. J. Org. Chem. 2016, 81, 2166.
(10) For selected examples on rhodium-catalyzed C−H functionaliza-
tion with diazo compounds, see: (a) Chan, W.-W.; Lo, S.-F.; Zhou, Z.;
Yu, W.-Y. J. Am. Chem. Soc. 2012, 134, 13565. (b) Shi, Z.; Koester, D. C.;
Boultadakis-Arapinis, M.; Glorius, F. J. Am. Chem. Soc. 2013, 135, 12204.
(c) Cheng, Y.; Bolm, C. Angew. Chem., Int. Ed. 2015, 54, 12349.
(11) For selected examples, see: (a) Guo, X.; Yu, R.; Li, H.; Li, Z. J. Am.
Chem. Soc. 2009, 131, 17387. (b) Liu, Y.; Qian, J.; Lou, S.; Xu, Z. J. Org.
Chem. 2010, 75, 6300. (c) Lu, B.; Wang, B.; Zhang, Y.; Ma, D. J. Org.
Chem. 2007, 72, 5337. (d) Yuan, H.; Bi, K.-J.; Li, B.; Yue, R.-C.; Ye, J.;
Shen, Y.-H.; Shan, L.; Jin, H.-Z.; Sun, Q.-Y.; Zhang, W.-D. Org. Lett.
2013, 15, 4742. (e) Kuram, M. R.; Bhanuchandra, M.; Sahoo, A. K.
Angew. Chem., Int. Ed. 2013, 52, 4607. (f) Liu, G.; Shen, Y.; Zhou, Z.; Lu,
X. Angew. Chem., Int. Ed. 2013, 52, 6033.
(12) For selected review and examples, see: (a) Gillingham, D.; Fei, N.
Chem. Soc. Rev. 2013, 42, 4918. (b) Maier, T. C.; Fu, G. C. J. Am. Chem.
Soc. 2006, 128, 4594. (c) Chen, C.; Zhu, S.-F.; Liu, B.; Wang, L.-X.;
Zhou, Q.-L. J. Am. Chem. Soc. 2007, 129, 12616. (d) Xie, X.-L.; Zhu, S.-
F.; Guo, J.-X.; Cai, Y.; Zhou, Q.-L. Angew. Chem., Int. Ed. 2014, 53, 2978.
(13) Crystallographic data for 3ha have been deposited with the
Cambridge Crystallographic Data Centre as deposition no. CCDC
1450291. Detailed information can be found in the Supporting
Information.
REFERENCES
■
(1) For selected reviews, see: (a) Shilov, A. E.; Shul’pin, G. B. Chem.
Rev. 1997, 97, 2879. (b) Colby, D. A.; Bergman, R. G.; Ellman, J. A.
Chem. Rev. 2010, 110, 624. (c) Bergman, R. G. Nature 2007, 446, 391.
(d) Wencel-Delord, J.; Droge, T.; Liu, F.; Glorius, F. Chem. Soc. Rev.
̈
2011, 40, 4740. (e) Jia, C.; Kitamura, T.; Fujiwara, Y. Acc. Chem. Res.
2001, 34, 633. (f) Davies, H. M. L.; Manning, J. R. Nature 2008, 451,
417. (g) Giri, R.; Shi, B.-F.; Engle, K. M.; Maugel, N.; Yu, J.-Q. Chem. Soc.
Rev. 2009, 38, 3242. (h) Song, G.; Wang, F.; Li, X. Chem. Soc. Rev. 2012,
41, 3651. (i) Kakiuchi, F.; Murai, S. Acc. Chem. Res. 2002, 35, 826.
(2) For selected examples on catalyst controlled selectivities, see:
(a) Gao, X.; Wu, B.; Huang, W.-X.; Chen, M.-W.; Zhou, Y.-G. Angew.
Chem., Int. Ed. 2015, 54, 11956. (b) Yu, Z.; Ma, B.; Chen, M.; Wu, H.-H.;
Liu, L.; Zhang, J. J. Am. Chem. Soc. 2014, 136, 6904. (c) Deprez, N. R.;
Kalyani, D.; Krause, A.; Sanford, M. S. J. Am. Chem. Soc. 2006, 128, 4972.
(d) Phipps, R. J.; Grimster, N. P.; Gaunt, M. J. J. Am. Chem. Soc. 2008,
130, 8172. (e) Phipps, R. J.; Gaunt, M. J. Science 2009, 323, 1593.
(f) Wagner, A. M.; Hickman, A. J.; Sanford, M. S. J. Am. Chem. Soc. 2013,
135, 15710.
(3) For selected examples on ligand-controlled selectivities, see:
(a) Wang, M.; Zhang, X.; Zhuang, Y.-X.; Xu, Y.-H.; Loh, T.-P. J. Am.
Chem. Soc. 2015, 137, 1341. (b) Ding, D.; Mou, T.; Feng, M.; Jiang, X. J.
Am. Chem. Soc. 2016, 138, 5218. (c) Ueda, K.; Yanagisawa, S.;
Yamaguchi, J.; Itami, K. Angew. Chem., Int. Ed. 2010, 49, 8946.
(d) Sanhueza, I. A.; Wagner, A. M.; Sanford, M. S.; Schoenebeck, F.
Chem. Sci. 2013, 4, 2767. (e) Lyons, T. W.; Hull, K. L.; Sanford, M. S. J.
Am. Chem. Soc. 2011, 133, 4455. (f) Yanagisawa, S.; Ueda, K.; Sekizawa,
H.; Itami, K. J. Am. Chem. Soc. 2009, 131, 14622. (j) Xu, K.; Thieme, N.;
Breit, B. Angew. Chem., Int. Ed. 2014, 53, 7268.
(4) For selected reviews on directing-group-controlled selectivities,
see: (a) Engle, K. M.; Mei, T.-S.; Wasa, M.; Yu, J.-Q. Acc. Chem. Res.
2012, 45, 788. (b) Wang, C.; Huang, Y. Synlett 2013, 24, 145. For
selected examples, see: (c) Deng, Y.; Yu, J.-Q. Angew. Chem., Int. Ed.
2015, 54, 888. (d) Liu, Y.-J.; Xu, H.; Kong, W.-J.; Shang, M.; Dai, H.-X.;
Yu, J.-Q. Nature 2014, 515, 389. (e) Stuart, D. R.; Villemure, E.; Fagnou,
K. J. Am. Chem. Soc. 2007, 129, 12072. (f) Robbins, D. W.; Boebel, T. A.;
Hartwig, J. F. J. Am. Chem. Soc. 2010, 132, 4068. (g) Xu, L.; Zhang, C.;
He, Y.; Tan, L.; Ma, D. Angew. Chem., Int. Ed. 2016, 55, 321. (h) Yang, Y.;
Qiu, X.; Zhao, Y.; Mu, Y.; Shi, Z. J. Am. Chem. Soc. 2016, 138, 495.
(i) Yang, Y.; Li, R.; Zhao, Y.; Zhao, D.; Shi, Z. J. Am. Chem. Soc. 2016,
138, 8734.
(14) Yadav, P.; Singh, P.; Tewari, A. K. Bioorg. Med. Chem. Lett. 2014,
24, 2251.
(15) (a) Thakur, A.; Vishwakarma, S.; Thakur, M. Bioorg. Med. Chem.
2004, 12, 1209. (b) Cushman, M.; Nagarathnam, D. J. Nat. Prod. 1991,
54, 1656.
(16) Shi, Z.; Schroder, N.; Glorius, F. Angew. Chem., Int. Ed. 2012, 51,
̈
8092.
(17) Phan, D. H. T.; Kim, B.; Dong, V. M. J. Am. Chem. Soc. 2009, 131,
15608 and references cited therein.
(5) For selected examples on electronic/steric effect controlled
selectivities, see: (a) Beck, E. M.; Grimster, N. P.; Hatley, R.; Gaunt, M.
J. J. Am. Chem. Soc. 2006, 128, 2528. (b) Cheng, C.; Hartwig, J. F. Science
2014, 343, 853. (c) Stephens, D. E.; Lakey-Beitia, J.; Atesin, A. C.;
D
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