Rhodium(III)-Catalyzed Directed C-H Dienylation of Anilides with Allenes Leads to Highly Conjugated Systems

Article abstract of DOI:10.1021/acs.orglett.9b00958

Allenes are unique coupling partners in transition-metal-catalyzed C-H functionalization leading to a variety of products via alkenylation, allenylation, allylation, and annulation reactions. The outcome is governed by both the reactivity of the allene and the formation and stability of the organometallic intermediate. An efficient Rh(III)-catalyzed, weakly coordinating group-directed dienylation of electronically unbiased allenes is developed using an N-acyl amino acid as a ligand. Further elaboration of the dienylated products to construct polycyclic compounds is also described.

Full text of DOI:10.1021/acs.orglett.9b00958

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Rhodium(III)-Catalyzed Directed C−H Dienylation of Anilides with
Allenes Leads to Highly Conjugated Systems
Chiranjit Ghosh, Prajyot Jayadev Nagtilak, and Manmohan Kapur*
Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal Bypass Road, Bhauri, Bhopal 462066,
Madhya Pradesh, India
*
S Supporting Information
ABSTRACT: Allenes are unique coupling partners in transition-metal-catalyzed C−H functionalization leading to a variety of
products via alkenylation, allenylation, allylation, and annulation reactions. The outcome is governed by both the reactivity of
the allene and the formation and stability of the organometallic intermediate. An efficient Rh(III)-catalyzed, weakly
coordinating group-directed dienylation of electronically unbiased allenes is developed using an N-acyl amino acid as a ligand.
Further elaboration of the dienylated products to construct polycyclic compounds is also described.
n recent years, transition-metal-catalyzed C−H functional-
izations, which enabled formation of new C−C as well as
diene systems are widely employed in the stereoselective
synthesis of polycyclic compounds as well as in natural product
synthesis. Glorius and co-workers reported an Rh-catalyzed
I
8
C−X bonds, have been extensively explored in an atom- and
1
step-economical manner. Unsaturated hydrocarbons such as
C−H dienylation with vinyl amides and allenyl carbinol
alkenes, alkynes, and allenes contain π(C−C)-bonds and
therefore have strong affinity to C−M bonds thus making
them versatile coupling partners in C−H functionalization
carbonates to obtain dendralene as well as arenes with diene
9a
substituents. The presence of the carbinol carbonate moiety
was deemed mandatory for β-oxygen elimination from a seven-
membered rhodacycle intermediate to yield the product. Fu
and co-workers also demonstrated a dienylation of vinyl
carbamates, aryl amides, and ketones with electron-deficient
2
,3
reactions.
Among these candidates, allenes are unique
coupling partners in transition metal catalysis since they have
three carbon centers of versatile reactivity based on both
4
electronic and steric factors. In recent years, the application of
(
preactivated for nucleophilic attack) allenes via β-hydride
2
−6
allenes in this field has shown great incremental growth.
9b
elimination from an allyl-Rh intermediate. We report herein
the dienylation of anilides with electronically unbiased allenes
and its extrapolation via synthesis of polycyclic frameworks and
substituted indole derivatives (Scheme 1).
Depending on the steric and electronic factors of allenes and
the nucleophilic nature of R−M (formed after C−H
activation), two different types of carbometalation intermedi-
ates are formed in a chemo- and regioselective fashion
Several directing groups, such as amides, imines, ketones,
carboxylic acid, etc., have been employed as effective groups to
direct the proximal C−H bond functionalization with
(
Scheme 1). Based on the nature of these two intermediates,
a variety of products can arise either via protonolysis, β-
5
,6
hydride elimination, or cyclization. In 2009, Krische and co-
6
,10
allenes.
In this regard, to the best of our knowledge, there
workers disclosed the first iridium-catalyzed C−H functional-
6a
is no report on the successful C−H dienylation of anilides with
ization with allenes, which was later explored by several
11
electronically unbiased allenes as the coupling partner. With
research groups including those of Ma, Ackermann, Glorius,
6
our group’s continued interest in exploring anilide-directed C−
Mascarenas, Cheng, Cramer, and others.
̃
12
H functionalization, we initiated our studies by probing
Dienes represent a class of highly valuable and important
building blocks, which are widely used in a range of reactions
such as cycloadditions, polymerizations, metathesis, and
Received: March 18, 2019
7
addition reactions. Electronically and stereochemically biased
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00958
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. C−H Functionalization Reactions with Allenes
and Present Work
Scheme 3. Substrate Scope for C−H Dienylation of
Substituted Pivaloylanilide and Diaryl Substituted
−
a e
Allenes
Scheme 2. Substrate Scope for C−H Dienylation of
Substituted Acetanilide and Substituted Aryl Allenes
−
a e
a
All yields are isolated yields and reactions were performed using
b
optimized conditions of Tables S1 and S2. Unless otherwise
mentioned, for all reactions: anilide (1 equiv) and allene (1.2
c
equiv) were used. For unsubstituted anilides and p-substituted
d
anilides: allene (1 equiv) and anilide (1.5 equiv) were used. For
diaryl substituted allenes: MeOH/toluene (1:2) solvent system was
e
used. Based on recovered starting material, sluggish reaction,
reactions are performed with anilide (1 equiv) and allene (1.5
equiv). NR = No reaction.
(
SI) for detailed optimization studies). The reaction
progressed smoothly in the presence of [Cp*RhCl ] as the
catalyst along with AgSbF as the additive to generate the
2
2
a
6
All yields are isolated yields and reactions were performed using
active cationic species.
b
optimized conditions of Table S1. Unless otherwise mentioned, for
all reactions: anilide (1 equiv) and allene (1.2 equiv) were used. For
c
In the present case, the use of a protic source seemed to
improve the efficiency of the transformation, and after
considerable optimization, pivaloyl glycine (Piv-Gly-OH) was
unsubstituted anilides and p-substituted anilides: allene (1 equiv) and
d
anilide (1.5 equiv) were used. For anilides with electron-withdrawing
13
t
e
found to be the best protic additive among various others.
substituents: bottle-grade BuOH was used as the solvent. Reaction
performed at 1 mmol scale.
Among the solvents scanned, methanol came across as a good
representative, and in many cases the MeOH−toluene mixture
worked well. Oxidants such as Cu(OAc) , Cu(OAc) ·H O,
2 2 2
AgOAc. Other transition-metal-derived catalysts (Pd or Ru)
2
2
2
various reaction conditions for the dienylation of anilides with
electronically unbiased aryl allenes through Rh(III)-catalyzed
C−H activation (see Table S1 in the Supporting Information
Ag O, Cu(OTf) , and O were found to be less efficient than
were found to be ineffective for this transformation. With the
B
DOI: 10.1021/acs.orglett.9b00958
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 4. Diels−Alder Reaction in Synthesis of Polycyclic
Compounds
Scheme 6. Control Experiments and Kinetic Isotope Effect
Study
Scheme 5. Synthesis of Indole Derivatives
a
Based on recovered starting material.
optimized conditions in hand, we investigated the effect of
diverse substituents on both the acetanilide and the aryl ring of
the allenes (Scheme 2). In general, the dienylation of various
anilides as well as substituted aryl allenes yielded moderate to
good transformations for almost all substrates. Electron-rich as
well as electron-neutral anilides were compatible with all
allenes possessing variable electronic nature under the
optimized conditions to deliver products with satisfying yields.
Halogen substituents on both the coupling partners were also
well-tolerated. The structure of the dienylated product was
confirmed unequivocally by X-ray crystallography of com-
pound 3o (CCDC 1880572). Interestingly, better yields were
observed for electron-deficient anilides when bottle-grade
hydroaminomethylation, and the construction of highly
8i−m
substituted organic frameworks.
After successfully synthe-
sizing several electronically biased diene systems, where one
double bond is electronically richer than the other one, we
t
9
BuOH was used as a solvent instead of MeOH.
explored the synthetic utility of the dienylated products.
Pivaloylanilides also furnished dienylation products in good
to moderate yields with good functional group tolerance
Treatment of dienophiles with suitable dienes would be useful
in constructing six-member carbocycles, as depicted in Scheme
4. In the case of unsymmetrical dienophiles, the selectivity in
this reaction was governed by secondary orbital effects. The
structure of the Diels−Alder product was confirmed
unequivocally by X-ray crystallography of compound 5a
(CCDC 1903553).
(Scheme 3). The reaction yielded a good outcome for the
different substituents on the alkyl chain of the allenes in the
toluene−MeOH solvent mixture system. Trifluoroacetanilide
and trisubstituted allenes were notable failures in this coupling
reaction. The poor-coordinating ability of the trifluoroacetani-
lide and the steric effects on the trisubstituted allene could be
attributed to these failures. Compared to the styryl allenes, a
relatively sluggish conversion was observed for nonstyrene-
based symmetric aliphatic allenes (3ai−3ap, Scheme 3).
The 1,3 diene system has been extensively utilized for
enantioselective borylation, hydroamination, hydroacylation,
We also envisioned the use of the amide as an internal
nucleophile for the construction of heterocycles of synthetic
14
value. Under iodo-amidation conditions, we successfully
synthesized highly substituted indoles, a very important
building block for natural products and pharmaceuticals
(Scheme 5). This reaction progresses via formation of the
C
DOI: 10.1021/acs.orglett.9b00958
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 7. Plausible Catalytic Cycle
oxidation regenerates the active Rh(III) catalyst for the next
cycle. Intermediate E was detected in ESI-MS (see the SI for
details).
In summary, we have developed a rhodium-catalyzed,
directed C−H dienylation of anilides using electronically
unbiased allenes as the coupling partners. The reaction features
excellent regioselectivity and a good substrate scope.
Diversified synthetic utility of the C−H dienylation product
was reflected in the construction of carbocycles via Diels−
Alder reactions as well as the synthesis of highly substituted
indole derivatives via an iodo-amidation method. The roles of
solvent and the nature of the amino acid ligand were found to
be important in the transformation.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00958.
General experimental procedures, characterization de-
1
13
tails, and copies of H and C NMR spectra of new
compounds (PDF)
iodonium ion at the more electron-rich and sterically crowded
olefin, followed by elimination of HI.
Accession Codes
CCDC 1880572 and 1903553 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
To understand the mechanistic pathway of the C−H
dienylation, a number of control experiments were performed.
During the optimization of the reaction conditions, we
observed the formation of Piv-Gly-OMe in the reaction
mixture. To investigate the plausible role of the ligand, the
methyl ester of the ligand was utilized under the optimal
reaction conditions in parallel reactions in MeOH and toluene
as solvents. We observed a decreased yield in MeOH, but a
decent increase in yields in toluene (Scheme 6A). This seemed
to indicate that both the acid and ester forms of the ligand
played an important role in the catalytic cycle.
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: mk@iiserb.ac.in.
ORCID
To investigate whether the C−H metalation step was rate-
Chiranjit Ghosh: 0000-0001-5941-7343
Manmohan Kapur: 0000-0003-2592-6726
Notes
limiting, we carried out kinetic isotope effect studies. A
competition reaction of a mixture of [D ]-4-methyl acetanilide
2
and 4-methyl acetanilide with the allene 2 gave a kinetic
isotope effect of 1.01 (by NMR) whereas parallel reactions
resulted in a kinetic isotope effect value of 0.94 (by GC).
These experiments indicate that the C−H activation is not
involved in the rate-limiting step (Scheme 6B). Next, to check
the reversibility of the C−H bond activation, the reaction was
performed in MeOH-d with added D O and a significant
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Funding from CSIR-India and SERB-India (EMR/2016/
004298/OC) is gratefully acknowledged. C.G. and P.J.N.
thank UGC-India and CSIR-India, respectively, for research
fellowships. We thank the CIF, IISERB, for the analytical data
and the Director, IISERB, for funding and the infrastructure
facilities.
4
2
amount of deuterium incorporation in both the product and
recovered anilide was observed (Scheme 6C). Second, a D−H
exchange experiment of the 99% deuterated anilide shows a
significant loss of the deuterium label (Scheme 6C). From
these two experiments it can be concluded that the C−H
activation step is a reversible one.
REFERENCES
■
(1) (a) Chen, X.; Engle, K. M.; Wang, D. H.; Yu, J.-Q. Angew. Chem.,
Int. Ed. 2009, 48, 5094. (b) Colby, D. A.; Bergman, R. G.; Ellman, J.
A. Chem. Rev. 2010, 110, 624. (c) Song, G.; Wang, F.; Li, X. Chem.
Soc. Rev. 2012, 41, 3651. (d) Wencel-Delord, J.; Glorius, F. Nat.
Chem. 2013, 5 (5), 369. (e) Ackermann, L. Acc. Chem. Res. 2014, 47,
On the basis of the above experiments and previous
6
literature reports, we propose a plausible reaction mechanism
for dienylation reactions (Scheme 7). The first step of the
transformation could be a combination of electrophilic
metalation and anilide directed ortho C−H activation to
provide the rhodacycle intermediate A. Coordination of the
allene to this rhodacycle followed by carborhodation via
migratory insertion into the allene results in an eight-
membered rhodacycle C, which is in equilibrium with π-allyl
rhodacycle intermediate D. This upon β-hydride elimination
delivers the targeted product 3, and AgOAc mediated
2
81. (f) Kuhl, N.; Schro
̈
der, N.; Glorius, F. Adv. Synth. Catal. 2014,
356, 1443. (g) Zhu, R. Y.; Farmer, M. E.; Chen, Y. Q.; Yu, J.-Q.
Angew. Chem., Int. Ed. 2016, 55, 10578. (h) Mishra, N. K.; Sharma, S.;
Park, J.; Han, S.; Kim, I. S. ACS Catal. 2017, 7, 2821. (i) Ackermann,
L. Chem. Rev. 2011, 111, 1315. (j) Gensch, T.; Hopkinson, M. N.;
Glorius, F.; Wencel-Delord, J. Chem. Soc. Rev. 2016, 45, 2900.
(k) Ping, L.; Chung, D. S.; Bouffard, J.; Lee, S.-G. Chem. Soc. Rev.
2017, 46, 4299.
D
DOI: 10.1021/acs.orglett.9b00958
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(
2) (a) Satoh, T.; Miura, M. Chem. - Eur. J. 2010, 16, 11212. (b) Le
(b) Heravi, M. M.; Vavsari, V. F. RSC Adv. 2015, 5, 50890.
(c) Heravi, M. M.; Ahmadi, T.; Ghavidel, M.; Heidari, B.; Hamidi, H.
́
RSC Adv. 2015, 5, 101999. (d) Chirkin, E.; Poree, F.-H. Curr. Org.
Bras, J.; Muzart, J. Chem. Rev. 2011, 111, 1170. (c) Patureau, F. W.;
Wencel-Delord, J.; Glorius, F. Aldrichimica Acta 2012, 45, 31.
d) Kozhushkov, S. I.; Ackermann, L. Chem. Sci. 2013, 4, 886.
(
Chem. 2016, 20, 2284. (e) Huang, Z.; Dong, G. Acc. Chem. Res. 2017,
50, 465. (f) Xiong, Y.; Sun, Y.; Zhang, G. Tetrahedron Lett. 2018, 59,
347. (g) Nunomoto, S.; Kawakami, Y.; Yamashita, Y. Bull. Chem. Soc.
Jpn. 1981, 54, 2831. (h) Nguyen, K. D.; Herkommer, D.; Krische, M.
J. J. Am. Chem. Soc. 2016, 138, 14210. (i) Liu, Y.; Fiorito, D.; Mazet,
C. Chem. Sci. 2018, 9, 5284. (j) Yang, X.-H.; Lu, A.; Dong, V. M. J.
Am. Chem. Soc. 2017, 139, 14049. (k) Chen, Q.-A.; Kim, D. K.; Dong,
V. M. J. Am. Chem. Soc. 2014, 136, 3772. (l) Oda, S.; Franke, J.;
Krische, M. J. Chem. Sci. 2016, 7, 136. (m) Fiorito, D.; Mazet, C. ACS
Catal. 2018, 8, 9382.
(9) (a) Wang, H.; Beiring, B.; Yu, D.-G.; Collins, K. D.; Glorius, F.
Angew. Chem., Int. Ed. 2013, 52, 12430. (b) Gong, T.-J.; Su, W.; Liu,
Z.-J.; Cheng, W.-M.; Xiao, B.; Fu, Y. Org. Lett. 2014, 16, 330.
(10) (a) Zeng, R.; Fu, C.; Ma, S. J. Am. Chem. Soc. 2012, 134, 9597.
(
3) (a) Fukutani, T.; Umeda, N.; Hirano, K.; Satoh, T.; Miura, M.
Chem. Commun. 2009, 5141. (b) Sun, Z.-M.; Chen, S.-P.; Zhao, P.
Chem. - Eur. J. 2010, 16, 2619. (c) Tian, C.; Massignan, L.; Meyer, T.
H.; Ackermann, L. Angew. Chem., Int. Ed. 2018, 57, 2383. (d) Li, Y.;
Xue, D.; Lu, W.; Fan, X.; Wang, C.; Xiao, J. RSC Adv. 2013, 3, 11463.
(
e) Zhang, H.-J.; Lin, W.; Su, F.; Wen, T.-B. Org. Lett. 2016, 18, 6356.
For reactions of organic halides with allenes via an oxidative addition,
see: (f) Zimmer, R.; Dinesh, C. U.; Nandanan, E.; Khan, F. K. Chem.
Rev. 2000, 100, 3067. (g) Chakravarty, M.; Swamy, K. C. K. J. Org.
Chem. 2006, 71, 9128. (h) Kim, S.; Seomoon, D.; Lee, P. H. Chem.
Commun. 2009, 1873. (i) Pavan, M. P.; Chakravarty, M.; Swamy, K.
C. K. Eur. J. Org. Chem. 2009, 2009, 5927. (j) Oh, C. H.; Jung, S. H.;
Bang, S. Y.; Park, D. I. Org. Lett. 2002, 4, 3325. (k) Chang, H.-M.;
Cheng, C.-H. J. Org. Chem. 2000, 65, 1767.
(
2
b) Wu, S.; Zeng, R.; Fu, C.; Yu, Y.; Zhang, X.; Ma, S. Chem. Sci.
015, 6, 2275.
11) Ma and co-workers had reported that anilides were not efficient
(
4) (a) Zimmer, R.; Dinesh, C. U.; Nandanan, E.; Khan, F. A. Chem.
Rev. 2000, 100, 3067. (b) Hashmi, A. S. K. Angew. Chem., Int. Ed.
000, 39, 3590. (c) Sydnes, L. K. Chem. Rev. 2003, 103, 1133.
d) Ma, S. Chem. Rev. 2005, 105, 2829. (e) Ma, S. Acc. Chem. Res.
(
directing groups for C−H functionalization with allenes for allylation
and annulation reactions. See ref 10a and 10b.
2
(
(
12) (a) Das, R.; Kapur, M. J. Org. Chem. 2017, 82, 1114.
b) Kumar, G. S.; Kumar, P.; Kapur, M. Org. Lett. 2017, 19, 2494.
13) (a) Cheng, G.-J.; Yang, Y.-F.; Liu, P.; Chen, P.; Sun, T.-Y.; Li,
G.; Zhang, X.; Houk, K. N.; Yu, J.-Q.; Wu, Y.-D. J. Am. Chem. Soc.
014, 136, 894. (b) Shi, B.-F.; Maugel, N.; Zhang, Y.-H.; Yu, J.-Q.
2
009, 42, 1679. (f) Lopez, F.; Mascaren
̃
as, J. L. Chem. - Eur. J. 2011,
(
17, 418. (g) Adams, C. S.; Weatherly, C. D.; Burke, E. G.; Schomaker,
(
J. M. Chem. Soc. Rev. 2014, 43, 3136.
5) Santhoshkumar, R.; Cheng, C.-H. Asian J. Org. Chem. 2018, 7,
151.
6) (a) Zhang, Y. J.; Skucas, E.; Krische, M. J. Org. Lett. 2009, 11,
248. (b) Thrimurtulu, N.; Dey, A.; Maiti, D.; Volla, C. M. R. Angew.
(
1
(
4
2
Angew. Chem., Int. Ed. 2008, 47, 4882. (c) Gair, J. J.; Haines, B. E.;
Filatov, A. S.; Musaev, D. G.; Lewis, J. C. Chem. Sci. 2017, 8, 5746.
(
2
d) Yang, Y.-F.; Hong, X.; Yu, J.-Q.; Houk, K. N. Acc. Chem. Res.
017, 50, 2853. (e) Lu, Y.; Wang, H.-W.; Spangler, J. E.; Chen, K.;
Cui, P.-P.; Zhao, Y.; Sun, W.-Y.; Yu, J.-Q. Chem. Sci. 2015, 6, 1923.
14) (a) Davies, S. G.; Figuccia, A. L. A.; Paul, A. M. F.; Roberts, M.;
Chem., Int. Ed. 2016, 55, 12361. (c) Kuppusamy, R.; Gandeepan, P.;
Cheng, C.-H. Org. Lett. 2015, 17, 3846. (d) Boobalan, R.;
Kuppusamy, R.; Santhoshkumar, R.; Gandeepan, P.; Cheng, C.-H.
ChemCatChem 2017, 9, 273. (e) Li, T.; Zhang, C.; Tan, Y.; Pan, W.;
Rao, Y. Org. Chem. Front. 2017, 4, 204. (f) Wang, H.; Glorius, F.
Angew. Chem., Int. Ed. 2012, 51, 7318. (g) Casanova, N.; Del Rio, K.
(
Thomson, J. E. J. Org. Chem. 2016, 81, 6481. (b) Skladchikov, D. A.;
Suponitskii, K. Y.; Abdrakhmanov, I. B.; Gataullin, R. R. Zhurnal
Organicheskoi Khimii 2012, 48, 962. (c) Brock, E. A.; Davies, S. G.;
Lee, J. A.; Roberts, P. M.; Thomson, J. E. Org. Lett. 2012, 14, 4278.
P.; García-Fandin
, 3349. (h) Casanova, N.; Seoane, A.; Mascaren
Angew. Chem., Int. Ed. 2015, 54, 2374. (i) Cendon
Comanescu, C.; García-Fandin
Mascarenas, J. L. Org. Lett. 2017, 19, 1674. (j) Wu, L.; Meng, Y.;
Ferguson, J.; Wang, L.; Zeng, F. J. Org. Chem. 2017, 82, 4121.
o, R.; Mascarenas, J. L.; Gulías, M. ACS Catal. 2016,
̃ ̃
6
̃
as, J. L.; Gulías, M.
, B.; Casanova, N.;
́
̃
o, R.; Seoane, A.; Gulías, M.;
̃
(k) Tran, D. N.; Cramer, N. Angew. Chem., Int. Ed. 2010, 49, 8181.
(l) Tran, D. N.; Cramer, N. Angew. Chem., Int. Ed. 2013, 52, 10630.
(m) Ye, B.; Cramer, N. J. J. Am. Chem. Soc. 2013, 135, 636. (n) Zeng,
R.; Ye, J.; Fu, C.; Ma, S. Adv. Synth. Catal. 2013, 355, 1963. (o) Zeng,
R.; Wu, S.; Fu, C.; Ma, S. J. Am. Chem. Soc. 2013, 135, 18284.
(
(
p) Nakanowatari, S.; Ackermann, L. Chem. - Eur. J. 2015, 21, 16246.
q) Nakanowatari, S.; Mei, R.; Feldt, M.; Ackermann, L. ACS Catal.
2
017, 7, 2511. (r) Nakanowatari, S.; Muller, T.; Oliveira, J. C. A.;
̈
Ackermann, L. Angew. Chem., Int. Ed. 2017, 56, 15891. (s) Wang, C.;
Wang, A.; Rueping, M. Angew. Chem., Int. Ed. 2017, 56, 9935.
(t) Thrimurtulu, N.; Nallagonda, R.; Volla, C. M. R. Chem. Commun.
2017, 53, 1872. (u) Kuppusamy, R.; Santhoshkumar, R.; Boobalan,
R.; Wu, H.-R.; Cheng, C.-H. ACS Catal. 2018, 8, 1880. (v) Boobalan,
R.; Santhoshkumar, R.; Cheng, C.-H. Adv. Synth. Catal. 2019, 361,
1
(
140.
7) (a) Hopf, H. Classics in Hydrocarbon Chemistry: Syntheses,
Concepts, Perspectives; Wiley-VCH: Weinheim, 2000. (b) Hopf, H.
Nature 2009, 460, 183. (c) Hopf, H.; Sherburn, M. S. Angew. Chem.,
Int. Ed. 2012, 51, 2298. (d) Bojase, G.; Payne, A. D.; Willis, A. C.;
Sherburn, M. S. Angew. Chem., Int. Ed. 2008, 47, 910. (e) Rieder, C. J.;
Winberg, K. J.; West, F. G. J. Am. Chem. Soc. 2009, 131, 7504. (f) Li,
W.; Shi, M.; Li, Y. Chem. - Eur. J. 2009, 15, 8852. (g) Hopf, H.;
Yildizhan, S. Eur. J. Org. Chem. 2011, 2011, 2029. (h) Green, N. J.;
Lawrence, A. L.; Bojase, G.; Willis, A. C.; Paddon-Row, M. N.;
Sherburn, M. S. Angew. Chem., Int. Ed. 2013, 52, 8333. (i) Back, T. G.;
Clary, K. N.; Gao, D. Chem. Rev. 2010, 110, 4498.
(
8) (a) Nicolaou, K. C.; Snyder, S. A.; Montagnon, T.;
Vassilikogiannakis, G. Angew. Chem., Int. Ed. 2002, 41, 1668.
E
DOI: 10.1021/acs.orglett.9b00958
Org. Lett. XXXX, XXX, XXX−XXX