Expedient Access to Unsymmetrical Diarylindolylmethanes through Palladium-Catalyzed Domino Electrophilic Cyclization-Extended Conjugate Addition Approach

Article abstract of DOI:10.1021/acs.orglett.5b01030

A palladium-catalyzed domino process to access unsymmetrical diarylindolylmethanes has been developed through the annulation of o-alkynylanilines followed by 1,6-conjugate addition with p-quinone methides (p-QMs) under relatively mild conditions. The broad substrate scope of this methodology was demonstrated through the use of a wide range of substituted o-alkynylanilines and p-quinone methides, and in most cases, the unsymmetrical diarylindolylmethanes could be prepared in moderate to excellent yields. Notably, this method does not require any amino group protection. Moreover, 100% atom economy makes this transformation attractive from a green chemistry perspective.

Full text of DOI:10.1021/acs.orglett.5b01030

Letter
pubs.acs.org/OrgLett
Expedient Access to Unsymmetrical Diarylindolylmethanes through
Palladium-Catalyzed Domino Electrophilic Cyclization−Extended
Conjugate Addition Approach
Virsinha Reddy and Ramasamy Vijaya Anand*
Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Sector 81, Knowledge City, S.
A. S. Nagar, Manauli (PO), Punjab 140306, India
S
* Supporting Information
ABSTRACT: A palladium-catalyzed domino process to access
unsymmetrical diarylindolylmethanes has been developed
through the annulation of o-alkynylanilines followed by 1,6-
conjugate addition with p-quinone methides (p-QMs) under
relatively mild conditions. The broad substrate scope of this
methodology was demonstrated through the use of a wide
range of substituted o-alkynylanilines and p-quinone methides, and in most cases, the unsymmetrical diarylindolylmethanes could
be prepared in moderate to excellent yields. Notably, this method does not require any amino group protection. Moreover, 100%
atom economy makes this transformation attractive from a green chemistry perspective.
Scheme 1. Approaches toward Unsymmetrical
Triarylmethanes
riarylmethanes are considered an attractive synthetic target
Tin organic synthesis due to their significant contributions in
the dye industry and medicinal chemistry.1,2 They have also
found applications in materials science as fluorescent probes3 and
photochromic agents.4 The symmetrical triarylmethanes, usually
called leuco dyes, are rather easy targets and could be effectually
accessed through a Brønsted5 or Lewis acid6 catalyzed Friedel−
Crafts reaction of aromatic aldehydes or their derivatives with
electron-rich arenes and heteroarenes. Nevertheless, the syn-
thesis of unsymmetrical triarylmethanes, which possess various
biological properties (Figure 1), still remains a relatively
challenging task.
Oshima,11 and Zhang12 independently. Watson13 and Jarvo14a
reported Ni-catalyzed synthesis of enantioenriched triaryl-
methanes through coupling of diarylmethanol derivatives with
arylboronic acids and boronic esters, respectively. Jarvo’s group
also developed Ni-catalyzed Kumada coupling of aryl Grignard
reagents with diarylmethyl ethers to access enantiomerically
enriched triarylmethanes.14b Very recently, the group of
Crudden and Nambo developed a protocol for the synthesis of
unsymmetrical triarymethanes15a and triarylacetonitriles15b
through Pd-catalyzed sequential arylation strategy. Crudden’s
group also developed a different approach involving enantiospe-
cific Suzuki−Miyura coupling of enantiomerically pure diben-
zylic boronic esters with aryl halides.16 Iron-catalyzed dehydro-
genative coupling of diarylmethanes with electron-rich aromatic
or heteroaromatic systems was also found to be effective for the
synthesis of triarylmethane derivatives.17 Very recently, Hirano
and Miyura reported a Pd-catalyzed C−H/C−O coupling of
Figure 1. Unsymmetrical triarylmethane-based drugs.
The traditional approach toward unsymmetrical triaryl-
methanes involves a Friedel−Crafts reaction of electron-rich
arenes with unsymmetrical diarylcarbinols or their derivatives
under Brønsted or Lewis acid catalyzed conditions (approach 1,
Scheme 1).1e,7 Recently, due to the broad substrate scope, the
cross-coupling-based approach has become a unique method for
the synthesis of triarylmethanes (approach 2, Scheme 1).
In line with an initial report by Molander’s group,8 Kuwano
and co-workers developed an approach to unsymmetrical
triarylmethanes through Pd-catalyzed Suzuki−Miyura coupling
of boronic acids with diarylmethyl carbonates.9 Another
approach based on Pd-catalyzed direct arylation leading to
triarymethanes was developed by the groups of Walsh,10
Received: April 9, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01030
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
oxazoles with diarylmethanol derivatives leading to oxazole
containing triarylmethanes.18
time to complete (entry 11). No product was observed in the
absence of the Pd catalyst (entry 12).
Having optimized conditions in hand (entry 7, Table 1),
substrate scope was evaluated using a wide range of p-quinone
methides (7a−r),20n and the results are summarized in Scheme 2.
In recent years, metal-catalyzed one-pot annulation of o-
alkynylanilines followed by trapping with suitable electrophiles
has become an extremely useful protocol for the construction of
2,3-substituted indoles.19 Based on this concept, we believed that
it is possible to access diarylindolylmethanes through metal-
catalyzed annulation of o-alkynylanilines followed by trapping
with p-quinone methides (p-QMs). The chemistry of p-quinone
methides is well explored in organic synthesis and physical
organic chemistry.20a−m Recently, a few enantioselective trans-
formations have also been reported using p-quinone methide as
an electrophile.20n,o However, to the best of our knowledge, the
synthesis of unsymmetrical diarylindolylmethane derivatives21
through Pd-catalyzed annulation of o-alkynylanilines followed by
1,6-conjugate addition with p-quinone methides (approach 3,
Scheme 1) is still unprecedented in the literature.
Scheme 2. Substrate Scope with Different p-Quinone
a
Methides
We commenced the optimization studies using readily
available o-alkynylaniline 4 and p-quinone methide 5 using a
diverse range of palladium catalysts under various reaction
conditions (Table 1). Although our initial attempts with
a
Table 1. Catalyst Screen and Optimization Studies
b
entry
catalyst
solvent
T (°C)
time (h)
yield (%)
1
PdCl2(PPh3)2
Pd(PPh3)4
Pd2(dba)3
Pd(OAc)2
Pd(TFA)2
PdCl2
DCE
DCE
DCE
DCE
DCE
DCE
DCE
MeCN
THF
PhMe
DCE
DCE
rt
rt
36
36
32
36
36
36
6
0
0
2
3
rt
0
4
rt
20
45
80
99
80
92
94
93
0
5
rt
a
Reaction conditions: 0.04 M solution of 4 in DCE. Yields reported
6
rt
are isolated yields.
7
8
PdCl2
70
70
70
70
70
rt
PdCl2
6
9
PdCl2
6
It is evident from Scheme 2 that this methodology worked very
well in the cases of p-quinone methides derived from electron-
rich (8a−f,m) as well as moderately electron poor (8g,h)
aromatic aldehydes, and in all cases, the expected diarylindolyl-
methanes were obtained in excellent yields (>90%). In the case of
p-quinone methide derived from a heteroaromatic aldehyde such
as thiophene-2-carboxaldehyde, the product 8i was isolated in
79% yield. The p-quinone methides derived from 2-naphthalde-
hyde and 4-phenylbenzaldehyde underwent smooth conversion
to their corresponding diarylindolylmethane derivatives 8k and
8l in 96 and 99% yields, respectively. This transformation was
also found to be effective for the synthesis of ferrocene (8j), 2-
fluorene (8o), and 4-(2-phenylethynyl)phenyl (8n) substituted
triarylmethane derivatives from their corresponding p-quinone
methides. Unfortunately, the diarylindolylmethane derivative 8p
was not formed in the case of p-quinone methide prepared from
4-nitrobenzaldehyde even after 24 h. Other p-QMs derived from
2,6-disubstituted phenols such as 2,6-diisopropylphenol (7q)
and 2,6-dimethylphenol (7r) also underwent smooth trans-
formation to their corresponding diarylindolyl derivatives 8q and
8r in 92 and 89% yields, respectively.
10
PdCl2
6
c
11
PdCl2
36
36
12
a
Reaction conditions: 0.04 M solution of 4 in solvent. Pd catalyst (5
mol %) was used. Use of 1.2 equiv of 5 was found to be optimal.
Isolated yield. 2 mol % of PdCl2 was used. rt = 27−30 °C.
b
c
palladium catalysts such as PdCl2(PPh3)2, Pd(PPh3)4, and
Pd2(dba)3 in 1,2-dichloroethane (DCE) at room temperature
did not give any fruitful results (entries 1−3, Table 1), extensive
optimization experiments revealed that Pd(OAc)2 and Pd-
(TFA)2 were found to be effective for this transformation at rt,
although the yield of 6 was low to moderate (entries 4 and 5).
When PdCl2 was used as a catalyst in DCE at rt, the expected
product 6 was obtained in 80% yield (entry 6) after 36 h. When
the same reaction was carried out at 70 °C, 6 was isolated in
almost quantitative yield (entry 7) in just 6 h. The structure of 6
was unambiguously confirmed by NMR as well as X-ray analysis.
Further elaboration of optimization was carried out in other
solvents at 70 °C. However, in all those cases (entries 8−10), the
yield of 6 was found to be inferior when compared to entry 7.
Lowering the catalyst loading (2 mol %) did not decrease the
yield of the product considerably, but the reaction took a long
The scope of this methodology was also extended by treating 5
with a wide range of o-alkynylanilines (9a−p) under the
optimized reaction conditions, and the results are summarized
in Scheme 3. Irrespective of the electronic nature of the
B
DOI: 10.1021/acs.orglett.5b01030
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
On the basis of the above experiments and observations, a
plausible mechanism of this transformation has been proposed,
which can be found in the Supporting Information.
Scheme 3. Substrate Scope with Different Indole Precursors
Notably, we observed that careful monitoring of the reaction
between 4 and 5 under standard conditions revealed that the
amine addition product 11 was also formed in considerable
amounts (Scheme 4), but interestingly, the formation of 11 was
Scheme 4. Formation of Amine Addition Product 11
a
Reaction conditions: 0.04 M solution of 9 in DCE. Yields reported
are isolated yields.
found to be reversible. TLC analysis of the reaction mixture
indicated that the concentration of 11 was gradually decreasing,
and at the same time, the concentration of 6 was steadily
increasing during the course of the reaction. Although 11 was
unstable under acidic conditions, we could isolate some amounts
of 11 by purification through neutral alumina column. The amine
addition product 11 was also characterized by spectral
techniques. To confirm the reversible nature of this reaction, in
an independent experiment, 11 was treated with PdCl2 under
standard conditions, and as expected, it was completely
converted in to 6 in 2.5 h (Scheme 4).
In conclusion, an efficient one-pot protocol for the synthesis of
heavily substituted unsymmetrical diarylindolylmethane deriva-
tives has been developed through Pd-catalyzed annulation of o-
alkynylanilines followed by extended conjugate addition to p-
quinone methides. Broad substrate scope and 100% atom
economy are the key features of this methodology. Unlike most
of the reported methods for the synthesis of 2,3-substituted
indole derivatives, this protocol does not require any protection
of the amino group of o-alkynylanilines. An enantioselective
version of this methodology is currently under investigation.
substituents present in the alkyne moiety, the o-alkynylanilines
(derived from electron-rich as well as electron-deficient
arylalkynes) underwent smooth transformation to their corre-
sponding products (10a−f,i,j) in moderate to excellent yields.
The yields of the product 10g and 10h were moderate in the
cases of indole precursors derived from 4-bromophenylacetylene
and 4-phenylphenylacetylene. The indole precursor prepared
from 3-ethynylthiophene was also converted to its corresponding
diarylindolylmethane 10k in excellent yield. The reaction worked
pretty well in the cases of indole precursors (9l and 9m) derived
from ethynylcyclopropane and ethynylcyclopentane, and the
products 10l and 10m were obtained in 95 and 82% yields,
respectively. We could also synthesize a few other diarylindolyl-
methanes (10n−p) in reasonable yields from p-quinone
methides derived from o-alkynylanilines (9n−p) having
substituents in the aniline ring.
At this stage, our attention was shifted to elucidate a reasonable
mechanism of this transformation. Initially, we believed that the
reaction proceeds via 2-substituted indole derivative (through
aminopalladation step), which then adds to p-QM in 1,6-fashion
to generate the diarylindolylmethane derivative. To get a better
understanding, a couple of control experiments were performed
in which 2-phenylindole (1 equiv) was treated with 5 (1.2 equiv)
in the presence or absence Pd catalyst at 70 °C in DCE (please
see the Supporting Information for the scheme). In the case of
reaction with Pd catalyst, the product 6 was obtained in
quantitative yield within 1 h. Unexpectedly, even in the case of
the reaction without Pd catalyst, 6 was obtained in 90% yield,
although the reaction time was approximately 6 times more than
that of the Pd-catalyzed reaction. Therefore, it is obvious that Pd
catalyst does help in accelerating the reaction. In the case of
reaction without Pd catalyst, we presume that the traces of HCl
present in DCE are responsible to effect this transformation by
activating the p-QM through hydrogen bonding. To confirm the
participation of HCl in the reaction, another experiment was
conducted where 2-phenylindole was treated with 5 in toluene
instead of DCE at 70 °C, but in this case, the product 6 was
observed only in trace quantities after 6 h. But, interestingly,
when one drop of 1 N aqueous HCl was added, the reaction was
completed in 1 h, and 6 was obtained in 71% isolated yield. These
experimental observations clearly suggest that HCl is playing
important role along with the Pd-catalyst in the 1,6-conjugate
addition step to generate the final product. It is also evident that
the reaction proceeds through 2-substituted indole intermediate.
ASSOCIATED CONTENT
■
S
* Supporting Information
General experimental procedures and characterization data of
the products. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.orglett.5b01030.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rvijayan@iisermohali.ac.in.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge IISER Mohali for financial support
and infrastructure. V.R. thanks the CSIR, New Delhi, for a
research fellowship. We also thank Mr. Hareram Yadav (IISER
Mohali) for his help in solving the crystal structure. The NMR
and HRMS facilities at IISER Mohali are gratefully acknowl-
edged.
C
DOI: 10.1021/acs.orglett.5b01030
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(18) Tabuchi, S.; Hirano, K.; Satoh, T.; Miura, M. J. Org. Chem. 2014,
79, 5401.
REFERENCES
■
(1) For reviews, see: (a) Duxbury, D. F. Chem. Rev. 1993, 93, 381.
(b) Muthyala, R.; Katritzky, A. R.; Lan, X. F. Dyes Pigm. 1994, 25, 303.
(c) Shchepinov, M. S.; Korshun, V. A. Chem. Soc. Rev. 2003, 32, 170.
(d) Nair, V.; Thomas, S.; Mathew, S. C.; Abhilash, K. G. Tetrahedron
2006, 62, 6731. (e) Mondal, S.; Panda, G. RSC Adv. 2014, 4, 28317.
(2) (a) Whitehead, C. W.; Whitesitt, C. A. J. Med. Chem. 1974, 17,
1298. (b) Wang, P.; Kozlowski, J.; Cushman, M. J. Org. Chem. 1992, 57,
3861. (c) Recanatini, M.; Cavalli, A.; Valenti, P. Med. Res. Rev. 2002, 22,
282. (d) Mibu, N.; Yokomizo, K.; Uyeda, M.; Sumoto, K. Chem. Pharm.
Bull. 2003, 51, 1325. (e) Palchaudhuri, R.; Nesterenko, V.;
Hergenrother, P. J. Am. Chem. Soc. 2008, 130, 10274. (f) Parai, M. K.;
Panda, G.; Chaturvedi, V.; Manju, Y. K.; Sinha, S. Bioorg. Med. Chem.
Lett. 2008, 18, 289.
(19) (a) Chen, Y.; Cho, C.-H.; Larock, R. C. Org. Lett. 2009, 11, 173.
(b) Hiroya, K.; Itoh, S.; Sakamoto, T. J. Org. Chem. 2004, 69, 1126.
(c) Alfonsi, M.; Arcadi, A.; Aschi, M.; Bianchi, G.; Marinelli, F. J. Org.
Chem. 2005, 70, 2265. (d) Ambrogio, I.; Arcadi, A.; Cacchi, S.; Fabrizi,
G.; Marinelli, F. Synlett 2007, 1775. (e) Guo, Y.-J.; Tang, R.-Y.; Li, J.-H.;
Zhong, P.; Zhang, X.-G. Adv. Synth. Catal. 2009, 351, 2615. (f) Oh, C.
H.; Karmakar, S.; Park, H.; Ahn, Y.; Kim, J. W. J. Am. Chem. Soc. 2010,
132, 1792. (g) Swamy, N. K.; Yazici, A.; Pyne, S. G. J. Org. Chem. 2010,
75, 3412. (h) Brand, J. P.; Chevalley, C.; Waser, J. Beilstein J. Org. Chem.
2011, 7, 565. (i) Gabriele, B.; Veltri, L.; Mancuso, R.; Salerno, G.; Costa,
M. Eur. J. Org. Chem. 2012, 2549. (j) Xu, C.; Murugan, V. K.; Pullarkat, S.
A. Org. Biomol. Chem. 2012, 10, 3875. (k) Wang, Q.; Huang, L.; Wu, X.;
Jiang, H. Org. Lett. 2013, 15, 5940. (l) Arcadi, A.; Pietropaolo, E.; Alvino,
A.; Michelet, V. Org. Lett. 2013, 15, 2766. (m) Janreddy, D.; Kavala, V.;
Kuo, C.-W.; Kuo, T.-S.; He, C.-H.; Yao, C.-F. Tetrahedron 2013, 69,
3323. (n) Hu, Z.; Luo, S.; Zhu, Q. Adv. Synth. Catal. 2015, 357, 1060.
(20) (a) Turner, A. B. Q. Rev., Chem. Soc. 1964, 18, 347. (b) Hart, D. J.;
Cain, P. A.; Evans, D. A. J. Am. Chem. Soc. 1978, 100, 1548. (c) Angle, S.
R.; Turnbull, K. D. J. Am. Chem. Soc. 1989, 111, 1136. (d) Peter, M. G.
Angew. Chem., Int. Ed. Engl. 1989, 28, 555. (e) Angle, S. R.; Arnaiz, D. O.
J. Org. Chem. 1990, 55, 3708. (f) Angle, S. R.; Arnaiz, D. O. J. Org. Chem.
1992, 57, 5937. (g) Baik; Joo Lee, H.; Koo, S.; Kim, B. H. Tetrahedron
Lett. 1998, 39, 8125. (h) Baik; Lee, H. J.; Jang, J. M.; Koo, S.; Kim, B. H. J.
Org. Chem. 2000, 65, 108. (i) Itoh, T. Prog. Polym. Sci. 2001, 26, 1019.
(j) Breugst, M.; Mayr, H. J. Am. Chem. Soc. 2010, 132, 15380.
(k) Toteva, M. M.; Richard, J. P. Adv. Phys. Org. Chem. 2011, 45, 39.
(l) Nigst, T. A.; Ammer, J.; Mayr, H. Angew. Chem., Int. Ed. 2012, 51,
1353. (m) Breugst, M.; Corral Bautista, F.; Mayr, H. Chem. - Eur. J. 2012,
18, 127. (n) Chu, W.-D.; Zhang, L.-F.; Bao, X.; Zhao, X.-H.; Zeng, C.;
Du, J.-Y.; Zhang, G.-B.; Wang, F.-X.; Ma, X.-Y.; Fan, C.-A. Angew. Chem.,
Int. Ed. 2013, 52, 9229. (o) Caruana, L.; Kniep, F.; Johansen, T. K.;
Poulsen, P. H.; Jørgensen, K. A. J. Am. Chem. Soc. 2014, 136, 15929. For
selected reviews on o-quinone methides, see: (p) Amouri, H.; Le Bras, J.
Acc. Chem. Res. 2002, 35, 501. (q) Pathak, T. P.; Sigman, M. S. J. Org.
Chem. 2011, 76, 9210. (r) Willis, N. J.; Bray, C. D. Chem. - Eur. J. 2012,
18, 9160.
(3) For reviews, see: (a) Kim, H. N.; Lee, M. H.; Kim, H. J.; Kim, J. S.;
Yoon, J. Chem. Soc. Rev. 2008, 37, 1465. (b) Beija, M.; Afonso, C. A. M.;
Martinho, J. M. G. Chem. Soc. Rev. 2009, 38, 2410.
(4) Irie, M. J. Am. Chem. Soc. 1983, 105, 2078.
(5) For selected examples, see: (a) Shirakawa, S.; Kobayashi, S. Org.
Lett. 2006, 8, 4939. (b) Kumar, S.; Malik, V.; Kaur, N.; Kaur, K.
Tetrahedron Lett. 2006, 47, 8483. (c) He, Q. L.; Sun, F. L.; Zheng, X. J.;
You, S. L. Synlett 2009, 1111. (d) Wilsdorf, M.; Leichnitz, D.; Reissig, H.
U. Org. Lett. 2013, 15, 2494.
(6) For selected reports, see: (a) Yu, L.; Chen, D.; Li, J.; Wang, P. G. J.
Org. Chem. 1997, 62, 3575. (b) Yadav, J. S.; Reddy, B. V. S.; Murthy, C.
V. S. R.; Kumar, G. M.; Madan, C. Synthesis 2001, 783. (c) Nagarajan, R.;
Perumal, P. T. Tetrahedron 2002, 58, 1229. (d) Ji, S.-J.; Zhou, M.-F.; Gu,
D.-G.; Jiang, Z.-Q.; Loh, T.-P. Eur. J. Org. Chem. 2004, 1584. (e) Nair, V.;
Abhilash, K. G.; Vidya, N. Org. Lett. 2005, 7, 5857. (f) Podder, S.;
Choudhury, J.; Roy, U. K.; Roy, S. J. Org. Chem. 2007, 72, 3100.
(g) Periasamy, M.; Kishorebabu, N.; Jayakumar, K. N. Tetrahedron Lett.
2007, 48, 1955. (h) Liu, C. R.; Li, M. B.; Yang, C. F.; Tian, S. K. Chem.
Commun. 2008, 1249. (i) Prakash, G. K. S.; Panja, C.; Shakhmin, A.;
Shah, E.; Mathew, T.; Olah, G. A. J. Org. Chem. 2009, 74, 8659.
(j) Dhiman, S.; Ramasastry, S. S. V. Org. Biomol. Chem. 2013, 11, 8030.
(k) Beltra,
10, 2206.
(7) (a) Esquivias, J.; Gom
́
J.; Gimeno, M. C.; Herrera, R. P. Beilstein J. Org. Chem. 2014,
(21) For recent reports on Brønsted acid catalyzed enantioselective
synthesis of diarylindolylmethanes, see: (a) Sun, F. L.; Zheng, X. J.; Gu,
Q.; He, Q. L.; You, S. L. Eur. J. Org. Chem. 2010, 47. (b) Zhuo, M. H.;
Jiang, Y. J.; Fan, Y. S.; Gao, Y.; Liu, S.; Zhang, S. Org. Lett. 2014, 16, 1096.
(c) Qi, S.; Liu, Y.; Ding, J. Y.; Han, F. S. Chem. Commun. 2014, 50, 8605.
(d) Saha, S.; Alamsetti, S. K.; Schneider, C. Chem. Commun. 2015, 51,
1461. (e) Wang, Z.; Ai, F.; Wang, Z.; Zhao, W.; Zhu, G.; Lin, Z.; Sun, J. J.
Am. Chem. Soc. 2015, 137, 383.
́
́
ez Arrayas, R.; Carretero, J. C. Angew. Chem.,
Int. Ed. 2006, 45, 629. (b) Thirupathi, P.; Kim, S. S. Tetrahedron 2010,
66, 2995. (c) Rueping, M.; Nachtsheim, B. J. Beilstein J. Org. Chem. 2010,
6. (d) Hikawa, H.; Suzuki, H.; Yokoyama, Y.; Azumaya, I. J. Org. Chem.
2013, 78, 6714. (e) Singh, P.; Dinda, S. K.; Shagufta; Panda, G. RSC Adv.
2013, 3, 12100.
(8) Molander, G. A.; Elia, M. D. J. Org. Chem. 2006, 71, 9198.
(9) Yu, J.-Y.; Kuwano, R. Org. Lett. 2008, 10, 973.
(10) (a) McGrew, G. I.; Temaismithi, J.; Carroll, P. J.; Walsh, P. J.
Angew. Chem., Int. Ed. 2010, 49, 5541. (b) Zhang, J.; Bellomo, A.;
Creamer, A. D.; Dreher, S. D.; Walsh, P. J. J. Am. Chem. Soc. 2012, 134,
13765. (c) Bellomo, A.; Zhang, J.; Trongsiriwat, N.; Walsh, P. Chem. Sci.
2013, 4, 849. (d) Zhang, J.; Bellomo, A.; Trongsiriwat, N.; Jia, T.;
Carroll, P. J.; Dreher, S. D.; Tudge, M. T.; Yin, H.; Robinson, J. R.;
Schelter, E. J.; Walsh, P. J. J. Am. Chem. Soc. 2014, 136, 6276.
(11) Niwa, T.; Yorimitsu, H.; Oshima, K. Org. Lett. 2007, 9, 2373.
(12) Xia, Y.; Hu, F.; Liu, Z.; Qu, P.; Ge, R.; Ma, C.; Zhang, Y.; Wang, J.
Org. Lett. 2013, 15, 1784.
(13) Zhou, Q.; Srinivas, H. D.; Dasgupta, S.; Watson, M. P. J. Am.
Chem. Soc. 2013, 135, 3307.
(14) (a) Taylor, B. L. H.; Harris, M. R.; Jarvo, E. R. Angew. Chem., Int.
Ed. 2012, 51, 7790. (b) Harris, M. R.; Hanna, L. E.; Greene, M. A.;
Moore, C. E.; Jarvo, E. R. J. Am. Chem. Soc. 2013, 135, 3303.
(15) (a) Nambo, M.; Crudden, C. M. Angew. Chem., Int. Ed. 2014, 53,
742. (b) Nambo, M.; Yar, M.; Smith, J. D.; Crudden, C. M. Org. Lett.
2015, 17, 50.
(16) Matthew, S. C.; Glasspoole, B. W.; Eisenberger, P.; Crudden, C.
M. J. Am. Chem. Soc. 2014, 136, 5828.
(17) (a) Li, Y.-Z.; Li, B.-J.; Lu, X.-Y.; Lin, S.; Shi, Z.-J. Angew. Chem., Int.
Ed. 2009, 48, 3817. (b) Guo, S.; Li, Y.; Wang, Y.; Guo, X.; Meng, X.;
Chen, B. Adv. Synth. Catal. 2015, 357, 950.
D
DOI: 10.1021/acs.orglett.5b01030
Org. Lett. XXXX, XXX, XXX−XXX