Traceless Directing-Group Strategy in the Ru-Catalyzed, Formal [3 + 3] Annulation of Anilines with Allyl Alcohols: A One-Pot, Domino Approach for the Synthesis of Quinolines

Article abstract of DOI:10.1021/acs.orglett.7b00715

A unique, ruthenium-catalyzed, [3 + 3] annulation of anilines with allyl alcohols in the synthesis of substituted quinolines is reported. The method employs a traceless directing group strategy in the proximal C-H bond activation and represents a one-pot Domino synthesis of quinolines from anilines.

Full text of DOI:10.1021/acs.orglett.7b00715

Letter
pubs.acs.org/OrgLett
Traceless Directing-Group Strategy in the Ru-Catalyzed, Formal
[
3 + 3] Annulation of Anilines with Allyl Alcohols: A One-Pot, Domino
Approach for the Synthesis of Quinolines
Gangam Srikanth Kumar, Pravin Kumar, and Manmohan Kapur*
Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal Bypass Road, Bhauri, Bhopal 462066,
MP, India
*
S Supporting Information
ABSTRACT: A unique, ruthenium-catalyzed, [3 + 3] annulation of anilines with allyl alcohols in the synthesis of substituted
quinolines is reported. The method employs a traceless directing group strategy in the proximal C−H bond activation and
represents a one-pot Domino synthesis of quinolines from anilines.
tilization of directing groups to guide site selectivity in
C−H activation reactions is probably the most widely
Scheme 1. Traceless Directing Group in One-Pot Domino
Synthesis of Quinolines
U
employed approach in the area of C−H functionalization. Of
the various methods or mechanisms put forward for C−H
activation, heteroatom-directed (or chelation-controlled) prox-
imal bond activation by transition-metal catalysts remains the
1
principal choice of synthetic chemists. The method obviously
suffers from limitations since installation and the inevitable
removal of the directing group lowers the step-economy of the
transformation. Often, the directing groups cannot be removed
from the products and continue to be a part of them, thereby
limiting the synthetic utility of such methods. Of late, the2
concept of traceless directing group has gained precedence,
whereby the step economy of the entire process of C−H
functionalization is maintained at a high level. In this regard,
transient directing groups too have made a mark in organic
often functioning as latent α,β-unsaturated carbonyl com-
pounds.
9
The alkene moiety of the allyl alcohols has been primarily
utilized for Heck-type coupling reactions and the allylic
hydroxyl functional group transforms very often into a carbonyl
group in the same transformation. Allyl alcohols have also been
employed for oxidative (or dehydrogenative) Heck-type
coupling reactions thereby resulting in oxidative alkylations of
1
0,11
10
11a
C−H bonds.
Of these, the works of Jiang, Glorius,
3
,4
11b
11c 11d
synthesis.
Jeganmohan,
Kim,
and Sundararaju
are noteworthy.
Nitrogen heterocycles are a ubiquitous part of a majority of
Allyl alcohols have also been employed as allylating agents in
pharmaceutically relevant molecules even as they are bio-
C−H functionalization reactions, thereby obviating the need for
5a
12a,13
logically very significant. Of these, quinolines are one of the
prefunctionalization of either of the coupling partners.
Saa
5b,c
most common N-heterocycles found in drug molecules. The
and Kim independently reported the use of the allyl acetates in
synthesis of substituted quinolines has been the subject of
widespread interest, spanning well over a century. Several
the rhodium-catalyzed C−H functionalization of anilides,
6
14
resulting in the synthesis of indoles. In this context, Stone
classical as well as modern syntheses of these heterocycles have
had reported a unique method wherein o-halo anilines were
7
,8
been described in literature. Of these, the Doebner−von
coupled with allyl alcohols in a two-step sequence to lead to
15
Miller modification of the Skraup-quinoline synthesis is a
quinolines. Ackermann and co-workers demonstrated the
synthesis of quinolines utilizing acetanilides and enones as
coupling partners in an ortho-selective C−H alkenylation−
7
a−e
unique reaction.
We describe herein a one-pot, domino
approach for the synthesis of substituted quinolines via
proximal C−H bond activation in which a traceless directing-
group methodology has been employed in the ruthenium-
catalyzed [3 + 3] annulation of anilines with allyl alcohols
16
cyclization cascade. The present method utilizes allyl alcohols
as latent α,β-unsaturated carbonyl compounds in the ortho-
selective C−H functionalization of anilines, thereby leading to
substituted quinolines in a one-pot domino-type synthesis
(Scheme 1). This methodology provides a completely different
(
Scheme 1). The highlight of the synthesis is the use of a
product substitution pattern when compared to the Doebner−
von Miller synthesis. Allyl alcohols are extremely useful building
blocks in organic synthesis and have been widely employed as
coupling partners in C−H functionalization reactions, very
Received: March 10, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00715
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
traceless directing group without the need for any prefunction-
alization of any of the coupling partners. This method also
removes the need for rather labile α,β-unsaturated carbonyl
compounds.
worked well for most of the allyl alcohols that were scanned. A
notable exception was the 1-phenyl-prop-2-en-1-ol, which failed
to yield the desired product (3fa, Scheme 2). The reaction was
rather sensitive to steric crowding on the allyl alcohol, and
substituents on the alkene part of the allyl alcohol were not
good additions to the substrate scope (3ga, Scheme 2).
Unsubstituted allyl alcohol yielded quinolones (3ha−c, Scheme
2), which came as a surprise to us. However, such an example
of this type of outcome was reported by Glorius and co-workers
Taking inspiration from our previous work on C−H
12
allylation of indoles, we extensively scanned a variety of
reaction conditions, among which the cationic ruthenium
catalyst provided the best results (see Supporting Information
for further details). The reaction worked best only with the
ruthenium catalyst, and other catalysts that were scanned did
not yield the desired results. The use of copper acetate as co-
oxidant was essential for the transformation, and in particular
the hydrate form was essential for the annulation to proceed to
11a
under Rh(III) catalysis.
The substrate scope with unsub-
stituted allyl alcohol was not very promising since the reaction
was rather slow. Preforming the acetanilide in this case did not
help. We also attempted to apply the developed strategy to
acetyl directed ortho C−H functionalization of phenols and
thiophenols, but the desired products were not observed.
The reaction is postulated to proceed by a mechanism
depicted below in Scheme 3. The formation of acetanilide from
completion. Of all the additives scanned, AgSbF worked the
6
best. Of the solvents scanned, THF worked best for the
transformation. Although addition of AcOH resulted in good
yields of the product, the use of AgSbF (0.5 equiv) proved
6
sufficient to circumvent the use of protic media for the reaction.
Use of lower equivalents of AgSbF led to slower rates of
annulation.
Scheme 3. Proposed Mechanisms
6
The reaction had a broad substrate scope and worked very
well for most of the anilines scanned (Scheme 2). Electronic
Scheme 2. Substrate Scopea,c
the parent aniline installs the N-acyl group that would be the
traceless directing group in the transformation. Directed C−H
activation assisted by this weak-coordinating group leads to the
ruthenacycle A. It is also possible that the activation at the
ortho position is a result of a combination of an electrophilic
1
8,19b
C−H activation pathway
and the CMD (concerted
19
metalation deprotonation) pathway. The oxidation of allylic
alcohols to enones by transition metal catalysts is well
20
documented in literature. Carboruthenation of the enone
leads to the intermediate B which upon protonolysis leads to C.
This anilide then cyclizes onto the ketone functional group
followed by dehydration and aromatization to lead to the
product quinoline.
a
Unless otherwise mentioned all reactions were performed with 1a
(
0.3 mmol), 2a (0.6 mmol), catalyst (0.015 mmol), oxidant (0.6
mmol), and additive (0.15 mmol) in 1.5 mL solvent, in a sealed tube.
b
c
Regioisomeric ratio determined by GCMS and NMR. All yields are
d
isolated yields. Reaction performed on 1 mmol scale.
It is possible that carboruthenation with the allylic alcohol
results in D which after β-H elimination could lead to C
(Scheme 3, Pathway II). An alternate pathway involving a β-
hydride elimination to generate an oxidative-Heck product E
factors were well tolerated on the aniline. Steric factors were
also well tolerated, and the reaction worked well for ortho-
substituted anilines as well (3ee, 3ai, 3cf, Scheme 2).
The reaction yielded a good outcome with para- and meta-
phenylenediamines (3ci, 3eh, 3ak, 3al, Scheme 2); however, it
failed to produce good outcomes for ortho-phenylenediamine
and pyridin-2-amine (3am, 3an, Scheme 2). The reaction
21a,b
(Fujiwara−Moritani type pathway)
can also be postulated
21c
(Scheme 3, Pathway III). This product could undergo an
olefin isomerization under the reaction conditions, followed by
condensation with the ketone functional group to directly lead
to the product quinoline. Incidently, when 3 equiv of 1-
B
DOI: 10.1021/acs.orglett.7b00715
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
phenylbut-3-en-2-ol were used under standard conditions,
ketone 4 was detected and isolated (Scheme 4), clearly
Scheme 5. Reversibility and Kinetic Studies
Scheme 4. Control Reactions
The Hammett reaction constant ρ was −0.69, which
indicates that the annulation of the anilide onto the ketone
may be involved in the rate-limiting step (Figure 1) and the fact
indicating that the oxidation of the allylic alcohol was the initial
step in this pathway. This points to a C−H alkylation pathway
(Scheme 3) being the predominant pathway. It could also be
possible that the pathway via the oxidative-Heck product
involves a rapid isomerization and cyclization to yield the
product. Each of these plausible intermediates resulted in the
desired product under the reaction conditions (Scheme 4). A
competition reaction involving the two plausible intermediates
resulted in a relative rate of 8:1, indicating that 3a″ may be a
short-lived intermediate (Scheme 4). N-Alkyl anilines did not
yield any desired product (Scheme 4).
The acyl directing group was necessary for the trans-
formation as was obvious from the reaction of the free aniline in
the absence of Ac O (Scheme 4), where only an N-allylation
2
reaction was observed. In order to study the plausible pathway
for the transformation, studies were conducted to determine
the reversibility of the C−H activation/metalation step. The
reaction yielded 70% of ortho-deuteration when the reaction
Figure 1. Hammett plot.
that the annulation is retarded by electron-withdrawing
substituents on the aniline. The fact that steric effects on the
allylic alcohol needed to be optimal indicated that two steps
were critical to the transformation, the carboruthenation and
the annulation steps.
was carried out in the presence of D O (Scheme 5). The high
2
level of deuteration observed at the ortho-position of the
recovered starting material clearly indicated that the metalation
was reversible. No deuteration was observed in absence of the
catalyst and additive, thereby ruling out H−D exchange via an
In summary, we have developed a unique, traceless
transient) directing group strategy for a one-pot domino
S Ar mechanism. Some deuteration was observed at the α-
E
(
position of the carbonyl which could be attributed to the
deuteration via keto−enol tautomerization (Scheme 5).
Oxidation of deuterated allyl alcohol 2ba (Scheme 5) led to
deuterium incorporation at the α-position, pointing to [Ru-D]
insertion into 5. The rate of this oxidation was much faster that
the observed rate of the overall transformation indicating that
Pathway II (Scheme 3) may not be a possibility. Studies were
synthesis of substituted quinolines from anilines and allyl
alcohols, utilizing a ruthenium-catalyzed C−H functionalization
strategy as the key step. The method is efficient, has high step
economy and a broad scope, and uses simple materials. The
transformation is quite unique since it incorporates several
chemical steps in one pot: installation of the directing group,
oxidation of the allyl alcohol, ortho-C−H functionalization,
annulation, removal of the directing group, and oxidation/
aromatization to the final product.
2
conducted to determine whether the cleavage of C(sp )−H was
involved in the rate-determining step. Although parallel
reactions indicated that the C−H bond cleavage was the rate-
limiting step, competition reactions indicated otherwise
ASSOCIATED CONTENT
Supporting Information
(
Scheme 5). The cleavage of the C−H bond bearing the
■
*
S
hydroxy functional group in the allylic alcohol was also not rate
limiting (Scheme 5). Studies undertaken to determine the effect
of substituents on the rate of the transformation indicated a
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b00715.
22
mild dependency on electronic nature of the substituent.
C
DOI: 10.1021/acs.orglett.7b00715
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Experimental procedures and full spectroscopic data for
Letter
Adhikary, A.; Kapur, M. Org. Lett. 2013, 15, 3310. (d) Yi, X.; Xi, C.
Org. Lett. 2015, 17, 5836. (e) Yadav, D. K. T.; Bhanage, B. M. RSC
Adv. 2015, 5, 51570. (f) Yan, Q.; Chen, Z.; Liu, Z.; Zhang, Y. Org.
Chem. Front. 2016, 3, 678. (g) Zhu, R.; Cheng, G.; Jia, C.; Xue, L.; Cui,
X. J. Org. Chem. 2016, 81, 7539. (h) Zheng, J.; Li, Z.; Huang, L.; Wu,
W.; Li, J.; Jiang, H. Org. Lett. 2016, 18, 3514. (i) Li, C.; Li, J.; An, Y.;
Peng, J.; Wu, W.; Jiang, H. J. Org. Chem. 2016, 81, 12189. (j) Neuhaus,
J. D.; Morrow, S. M.; Brunavs, M.; Willis, M. C. Org. Lett. 2016, 18,
all new compounds (PDF)
Crystal structure data (X-ray) for compound 3bb (CIF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: mk@iiserb.ac.in.
1
(
2
562.
9) (a) Werner, E. W.; Mei, T.-S.; Burckle, A. J.; Sigman, M. S. Science
012, 338, 1455. (b) Wen, Y.; Huang, L.; Jiang, H.; Chen, H. J. Org.
Notes
The authors declare no competing financial interest.
Chem. 2012, 77, 2029. (c) Chen, H.; Huang, L.; Fu, W.; Liu, X.; Jiang,
H. Chem. - Eur. J. 2012, 18, 10497. (d) Lumbroso, A.; Cooke, L. M.;
Breit, B. Angew. Chem., Int. Ed. 2013, 52, 1890. (e) Wang, L.; Huang,
X.; Wu, X.; Jiang, H. Org. Lett. 2013, 15, 5940. (f) Huang, L.; Qi, J.;
Wu, X.; Huang, K.; Jiang, H. Org. Lett. 2013, 15, 2330. (g) Vellakkaran,
M.; Andappan, M. M. S.; Nagaiah, K.; Nanubolu, J. B. Eur. J. Org.
Chem. 2016, 2016, 3575.
(10) (a) Huang, L.; Wang, Q.; Qi, J.; Wu, X.; Huang, K.; Jiang, H.
Chem. Sci. 2013, 4, 2665. (b) Qi, J.; Huang, L.; Wang, Z.; Jiang, H. Org.
Biomol. Chem. 2013, 11, 8009. (c) Zheng, M.; Chen, P.; Wu, W.; Jiang,
H. Chem. Commun. 2016, 52, 84.
ACKNOWLEDGMENTS
■
Funding from SERB-India (SB/S1/OC-10/2014) and CSIR-
India (02(205)/14/EMR-II) is gratefully acknowledged. G.S.
and P.K. thank CSIR-India and UGC-India, respectively, for
Research Fellowships. We thank CIF IISERB for the spectral
data and Mr. Lalit Mohan Jha, IISERB, for the X-ray data. We
also thank Director IISERB for funding and infrastructural
facilities.
(
11) (a) Shi, Z.; Boultadakis-Arapinis, M.; Glorius, F. Chem.
REFERENCES
■
Commun. 2013, 49, 6489. (b) Manoharan, R.; Jeganmohan, M.
Chem. Commun. 2015, 51, 2929. (c) Han, S. H.; Choi, M.; Jeong, T.;
Sharma, S.; Mishra, N. K.; Park, J.; Oh, J. S.; Kim, W. J.; Lee, J. S.; Kim,
I. S. J. Org. Chem. 2015, 80, 11092. (d) Kalsi, D.; Laskar, R. A.; Barsu,
N.; Premkumar, J. R.; Sundararaju, B. Org. Lett. 2016, 18, 4198.
(
1) (a) Kozhushkov, S.; Ackermann, L. Chem. Sci. 2013, 4, 886.
b) De Sarkar, S.; Liu, W.; Kozhushkov, S. I.; Ackermann, L. Adv.
(
Synth. Catal. 2014, 356, 1461. (c) Daugulis, O.; Roane, J.; Tran, L. D.
Acc. Chem. Res. 2015, 48, 1053. (d) Chen, Z.; Wang, B.; Zhang, J.; Yu,
W.; Liu, Z.; Zhang, Y. Org. Chem. Front. 2015, 2, 1107. (e) Gensch, T.;
Hopkinson, M. N.; Glorius, F.; Wencel-Delord, J. Chem. Soc. Rev.
016, 45, 2900.
2) (a) Rousseau, G.; Breit, B. Angew. Chem., Int. Ed. 2011, 50, 2450.
b) Zhang, F.; Spring, D. R. Chem. Soc. Rev. 2014, 43, 6906. (c) Sun,
H.; Guimond, N.; Huang, Y. Org. Biomol. Chem. 2016, 14, 8389.
3) (a) Yamashita, M.; Hirano, K.; Satoh, T.; Miura, M. Org. Lett.
009, 11, 2337. (b) Huang, C.; Chattopadhyay, B.; Gevorgyan, V. J.
Am. Chem. Soc. 2011, 133, 12406. (c) Cornella, J.; Righi, M.; Larrosa,
I. Angew. Chem., Int. Ed. 2011, 50, 9429. (d) Liu, G.; Shen, Y.; Zhou,
Z.; Lu, X. Angew. Chem., Int. Ed. 2013, 52, 6033. (e) Luo, J.; Preciado,
S.; Larrosa, I. J. Am. Chem. Soc. 2014, 136, 4109. (f) Liu, X.; Li, X.; Liu,
H.; Guo, Q.; Lan, J.; Wang, R.; You, J. Org. Lett. 2015, 17, 2936.
(12) (a) Kumar, G. S.; Kapur, M. Org. Lett. 2016, 18, 1112. For
selected contributions from our group, see: (b) Tiwari, V. K.; Kamal,
N.; Kapur, M. Org. Lett. 2015, 17, 1766. (c) Das, R.; Kapur, M. Chem. -
Asian J. 2015, 10, 1505. (d) Das, R.; Kapur, M. Chem. - Eur. J. 2016,
2
(
(
2
2
2, 16986. (e) Pawar, G. G.; Brahmanandan, A.; Kapur, M. Org. Lett.
016, 18, 448. (f) Das, R.; Kapur, M. J. Org. Chem. 2017, 82, 1114.
(
2
(
g) Tiwari, V. K.; Kamal, N.; Kapur, M. Org. Lett. 2017, 19, 262.
(13) (a) Suzuki, Y.; Sun, B.; Sakata, K.; Yoshino, T.; Matsunaga, S.;
Kanai, M. Angew. Chem., Int. Ed. 2015, 54, 9944. (b) Bunno, Y.;
Murakami, N.; Suzuki, Y.; Kanai, M.; Yoshino, T.; Matsunaga, S. Org.
Lett. 2016, 18, 2216.
(
14) (a) Cajaraville, A.; Lop
013, 15, 4576. (b) Kim, M.; Park, J.; Sharma, S.; Han, S.; Han, S. H.;
Kwak, J. H.; Jung, Y. H.; Kim, I. S. Org. Biomol. Chem. 2013, 11, 7427.
́ ́
ez, S.; Varela, J. A.; Saa, C. Org. Lett.
2
(
2
g) Zhang, Y.; Zhao, H.; Zhang, M.; Su, W. Angew. Chem., Int. Ed.
015, 54, 3817. (h) Zhou, S.; Wang, J.; Chen, P.; Chen, K.; Zhu, J.
Chem. - Eur. J. 2016, 22, 14508. (i) Agasti, S.; Dey, A.; Maiti, D. Chem.
Commun. 2016, 52, 12191. (j) Xie, F.; Yu, S.; Qi, Z.; Li, X. Angew.
Chem., Int. Ed. 2016, 55, 15351.
(
(
(
15) Stone, M. T. Org. Lett. 2011, 13, 2326.
16) Li, J.; Ackermann, L. Org. Chem. Front. 2015, 2, 1035.
17) Crystal Structure submitted to Cambridge Cystallographic Data
Centre. CCDC deposition number: 1497813 (compound 3bb).
18) (a) Tiwari, V. K.; Pawar, G. G.; Jena, H. K.; Kapur, M. Chem.
(
4) (a) Huang, X.; Huang, J.; Du, C.; Zhang, X.; Song, F.; You, J.
(
Angew. Chem., Int. Ed. 2013, 52, 12970. (b) Preshlock, S. M.; Plattner,
D. L.; Maligres, P. E.; Krska, S. W.; Maleczka, R. E., Jr; Smith, M. R.,
III Angew. Chem., Int. Ed. 2013, 52, 12915. (c) Wu, J.; Xiang, S.; Zeng,
J.; Leow, M.; Liu, X.-W. Org. Lett. 2015, 17, 222. (d) Geng, K.; Fan, Z.;
Zhang, A. Org. Chem. Front. 2016, 3, 349.
5) (a) Katritzky, A. Chem. Rev. 2004, 104, 2125. (b) Michael, J. P.
Nat. Prod. Rep. 2008, 25, 166. (c) Andrews, S.; Burgess, S. J.; Skaalrud,
D.; Kelly, J. X.; Peyton, D. H. J. Med. Chem. 2010, 53, 916.
6) (a) Bharate, J. B.; Vishwakarma, R. A.; Bharate, S. B. RSC Adv.
015, 5, 42020. (b) Chelucci, G.; Porcheddu, A. Chem. Rec. 2017, 17,
00. (c) Ramann, G.; Cowen, B. Molecules 2016, 21, 986.
7) (a) Skraup, Z. H. Ber. Dtsch. Chem. Ges. 1880, 13, 2086.
b) Friedlander, F. Ber. Dtsch. Chem. Ges. 1882, 15, 2572. (c) Doebner,
O.; Miller, W. V. Ber. Dtsch. Chem. Ges. 1883, 16, 2464. (d) Combes,
A. Bull. Soc. Chim. Fr. 1883, 49, 89. (e) Denmark, S. E.; Venkatraman,
S. J. Org. Chem. 2006, 71, 1668. (f) Wu, Y.-C.; Liu, L.; Li, H.-J.; Wang,
D.; Chen, Y.-J. J. Org. Chem. 2006, 71, 6592. (g) Korivi, R. P.; Cheng,
C. J. Org. Chem. 2006, 71, 7079. (h) Cai, S.; Zeng, J.; Bai, Y.; Liu, X. J.
Org. Chem. 2012, 77, 801. (i) Zhou, W.; Lei, J. Chem. Commun. 2014,
Commun. 2014, 50, 7322. (b) Pawar, G. G.; Tiwari, V. K.; Jena, H. K.;
Kapur, M. Chem. - Eur. J. 2015, 21, 9905 and references cited therein.
(
c) Midya, S. P.; Sahoo, M. K.; Landge, V. G.; Rajamohanan, P. R.;
Balaraman, E. Nat. Commun. 2015, 6, 8591.
19) (a) Gorelsky, S. I.; Lapointe, D.; Fagnou, K. J. Am. Chem. Soc.
008, 130, 10848. (b) Ma, W.; Mei, R.; Tenti, G.; Ackermann, L.
Chem. - Eur. J. 2014, 20, 15248.
20) (a) Bouziane, A.; Carboni, B.; Bruneau, C.; Carreaux, F.;
(
2
(
(
(
2
2
(
Renaud, J.-L. Tetrahedron 2008, 64, 11745. (b) Johnson, T. C.; Morris,
D. J.; Wills, M. Chem. Soc. Rev. 2010, 39, 81. (c) Li, B.; Darcel, C.;
Dixneuf, P. H. Chem. Commun. 2014, 50, 5970.
(
21) (a) Moritani, I.; Fujiwara, Y. Tetrahedron Lett. 1967, 8, 1119.
b) Fujiwara, Y.; Moritani, I.; Danno, S.; Asano, R.; Teranishi, S. J. Am.
Chem. Soc. 1969, 91, 7166. For related Ru-catalyzed alkenylation, see:
c) Ackermann, L.; Wang, L.; Wolfram, R.; Lygin, A. V. Org. Lett.
012, 14, 728.
(
(
(
2
(
22) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
5
(
2
0, 5583.
8) (a) Zhang, Y.; Wang, M.; Li, P.; Wang, L. Org. Lett. 2012, 14,
206. (b) Yan, R.; Liu, X.; Pan, C.; Zhou, X.; Li, X.; Kang, X.; Huang,
G. Org. Lett. 2013, 15, 4876. (c) Tiwari, V. K.; Pawar, G. G.; Das, R.;
D
DOI: 10.1021/acs.orglett.7b00715
Org. Lett. XXXX, XXX, XXX−XXX