Visible-Light-Mediated Ir(III)-Catalyzed Concomitant C3 Oxidation and C2 Amination of Indoles

Article abstract of DOI:10.1021/acs.orglett.9b00887

A visible-light-mediated concomitant C3 oxidation and C2 amination of indoles has been achieved at room temperature using an Ir (III) photocatalyst. This reaction proceeds without an isatin intermediate via the attack of a singlet oxygen at the C3 positio

Full text of DOI:10.1021/acs.orglett.9b00887

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Visible-Light-Mediated Ir(III)-Catalyzed Concomitant C3 Oxidation
and C2 Amination of Indoles
Gaurav Shukla, Tipu Alam, Hemant Kumar Srivastava, Ritush Kumar, and Bhisma K. Patel*
Department of Chemistry, Indian Institute of Technology Guwahati, North Guwahati 781039, India
S
* Supporting Information
ABSTRACT: A visible-light-mediated concomitant C3 oxidation and C2 amination of indoles has been achieved at room
temperature using an Ir (III) photocatalyst. This reaction proceeds without an isatin intermediate via the attack of a singlet
oxygen at the C3 position followed by C2 amination leading to difunctionalization of indoles.
ecently, photocatalytic functionalizations have emerged as
Rpowerful synthetic tools in modern organic chemistry/
synthesis.1 Compared to traditional functionalizations, the
photocatalytic strategy has emerged as an alternative method
for the rapid construction of highly functionalized structures by
amplifying their chemical diversity.2 In particular, the photo-
catalytic functionalizations of indoles have attracted consid-
erable attention since functionalized indole motifs have found
significant applications in biological and medicinal chemistry
and are present in various natural and artificial bioactive
products (Figure 1).3
catalysis.6 To the best of our knowledge, there is no reported
photocatalytic functionalization of indoles leading to selective
C3 oxidation and C2 amination. Our group is actively involved
in the design and development of operationally simple and
efficient methodologies for the functionalization of various
heterocycles.7 Herein, we report a visible-light-mediated
protocol for the selective difunctionalization of N-substituted
indoles using benzohydrazides in an oxygen atmosphere
leading to concurrent C3 oxidation and C2 amination of
indoles.
By judicious selection of photocatalysts and other reaction
parameters, a concomitant bis-functionalization of indoles has
been accomplished (Scheme 1b). While this manuscript was
under preparation, Borhan et al. published an elegant C3-
oxidation and C2-amination of indoles involving a Cu(I)
catalyst and an acyl nitroso reagent such as hydroxycarbamate
(Scheme 1a).8 This method has advantages in terms of
Scheme 1. Approaches toward Difunctionalization of
Indoles via Selective C3 Oxidation and C2 Amination
Figure 1. Representative bioactive products containing functionalized
indoles.
Among significant photocatalytic reactions of indoles, in
2017, Yoon et al. reported a Diels−Alder cycloaddition with
dienes leading to tetrahydro-1,4-ethanocarbazoles.4 A chemo-
divergent oxygenation of indole to isatins and formylformani-
lides under visible-light photocatalysis has been developed by
Jiang et al.5 The Stephenson group reported an efficient
addition of malonyl radicals to indoles under Ru-photo-
Received: March 12, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00887
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Substrate Scope for the Synthesis of 3-Oxindoles 3a−t
a
Reaction conditions: 1a−m,s,t (0.25 mmol), 2a−g (0.25 mmol), P1 (3 mol %), TBAI (30 mol %), CH3CN (5 mL), white LED (2 × 10 W, λmax
b
455 nm) at room temperature. Reaction carried out on a 1 mmol scale.
peroxide-free oxidation and use of hydroxycarbamate as the
source of both oxygen and the amide, making this protocol
advantageous over previous methodologies involving difuction-
alization of indoles.9 Nevertheless, this method has an inherent
limitation as it is applicable only to C2-protected indoles and
C2-free indoles failed to functionalize.
presence of Au(III)/chiralphosphoric acid dual catalysis.15 To
the best of our knowledge, there is no report for the direct
synthesis of 3-oxoindole involving indole with benzohydrazide
and oxygen under photocatalysis. Encouraged by the synthesis
of N′-(1-isopropyl-3-oxoindolin-2-ylidene)benzohydrazide
(3f) via photocatalysis, further optimization of reaction
parameters was tuned to enhance the efficacy of the reaction
(Table S1). Use of different transition-metal-based photo-
catalysts such as Ir[dF(CF3)ppy]2(dtbpy))PF6 (P2) (20%)
and Ru(bpy)3(PF6)2 (P3) (10%) did not show any improve-
ment in the reaction yields (Table S1, entries 2 and 3). On the
other hand, organic dyes such as eosin B, eosin Y, and rose
bengal were completely ineffective, leaving the starting material
unreacted even after 24 h (Table S1, entries 4−6). Hence, it
was found that the catalyst [Ir(dtbbpy)(ppy)2]PF6 (P1) is
more suitable for the formation of 3f. Next, the catalyst loading
was varied, and it was found that 3 mol % of [Ir(dtbbpy)-
(ppy)2]PF6(P1) is ideal for the transformation, yielding
product 3f in 55% yield (Table S1, entries 7−9). Further, an
increase in the catalytic loading (3.5 mol %) did not show any
significant enhancement in the yield (56%) (Table S1, entry
10). Subsequently, the effect of solvents on the reaction was
examined. Solvent DCM (55%, Table S1, entry 11) was found
to be equally effective to that of DCE. The use of polar aprotic
solvents such as DMSO (40%) and DMF (45%) were found to
be inferior to that of DCE (Table S1, entries 12 and 13).
However, polar-protic solvents such as MeOH (60%) and
EtOH (65%) were superior to DCE (Table S1, entries 14 and
Our initial investigation began by using indole (1f) (0.25
mmol), benzohydrazide (2a) (0.25 mmol), [Ir(dtbbpy)-
(ppy)2]PF6 (P1, 1 mol %), and TBAI (10 mol %) in 1,2-
dichloroethane (DCE) at room temperature in an oxygen
atmosphere under an irradiation of visible light (2 × 10 W
white LEDs, λmax 455 nm). Interestingly, the reaction resulted
in the formation of a new product (3f, 30%). Spectroscopic
1
(IR, H, and 13C), HRMS, and crystallographic analysis of the
isolated product confirmed its structure to be (E)-N′-(1-
isopropyl-3-oxoindolin-2-ylidene)benzohydrazide (3f) (Table
S1, entry 1; see the Supporting Information). In contrast to
other preparations of 3-oxindoles, here formation of this
product (3f) is accompanied by regioselective C3-oxidation
and C2-amination.10 Oxindoles are an important class of
heterocycles and have attracted considerable attention in the
field of antimicrobial and anticancer research.11 The classical as
well as photocatalytic approaches for the synthesis of 2-
oxindoles frameworks are well documented,12,13 whereas there
are only limited reports on the synthesis of 3-oxindoles.10,14,15
The two well-established strategies are (i) a Pd-catalyzed,
MnO2/TBHP-mediated oxidative dearomatization of indoles14
and (ii) a redox annulation of nitroalkynes with indoles in the
B
DOI: 10.1021/acs.orglett.9b00887
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
15). Incidentally, the polar-aprotic solvent CH3CN was found
to be the best in terms of yield (70%) with fewer side products
(Table S1, entry 16). After the optimization of both catalyst
and solvent, next we scrutinized the loading of various
additives. When the loading of TBAI was increased from 10
to 15, 20, and 30 mol %, the yield of (3f) increased to 70%,
72%, and 75%, respectively (Table S1, entries 17−19).
However, improvement in the yield (76%) was observed by
further increasing the TBAI loading to 40 mol %. Other
organic and inorganic halo additives such as tetrabutyl
ammonium bromide, sodium iodide, and lithium bromide
were less effective (Table S1, entries 20−23). Reaction without
the photocatalyst [Ir(dtbbpy)(ppy)2]PF6 (P1) did not provide
any trace of the product (3f) (Table S1, entry 24). The
reaction in the dark, under otherwise identical reaction
conditions, provided a trace (5%) of the desired product 3f
(Table S1, entry 25). Reaction in open air yielded the desired
product in 40% yield after 24 h; however, it was associated
with the formation of some fluorescent impurities (Table S1,
entry 26). When the reaction was performed under irradiation
of 2 W, 5 W, and 10 W white LED light for 24 h, the yield of
the product obtained was found to be 25%, 50%, and 60%,
respectively (Table S1, entries 27−29). The temperature of the
surrounding reaction mixture remained identical (28 °C) to
that of room temperature during the progress of the reaction
due to the use of a (2 × 10 W) LED as it was well ventilated,
suggesting the photochemical nature of the reaction. To see
the effect of wavelength of light, the reaction when performed
with light of various wavelengths such as red (632 nm), green
(513 nm), blue (430 nm), and white (449 nm, fwhm) LEDs of
5 W each. While red and green LEDs failed to give any
product, and the blue LED (45%) was nearly as effective as
that the white LED (50%) but was associated with the
formation of other minor side products (Table S1, entries 28
and 30−32). Thus, the effective wavelength for this trans-
formation lies in the region of 430−450 nm. Thus, the
optimum conditions for the synthesis of 3f were the use of 1f
(0.25 mmol), benzohydrazide (2a) (0.25 mmol), catalyst P1
(3 mol %), and TBAI (30 mol %) in 5 mL of CH3CN under an
atmosphere of oxygen by irradiation of 20 W white LEDs (λmax
455 nm) at room temperature (see Table S1 for further
details).
The scope and generality of the visible-light-mediated
reaction were extended to a variety of substituted indoles
and benzohydrazides under the optimized reaction conditions
(Table S1, entry 19). As shown in Scheme 2, a diverse array of
N-substituted and mono- and disubstituted (1,5 and 1,7)
indoles (1a−m,s,t) could be transformed to their correspond-
ing 3-oxindoles (3a−t) in good yields. At first, the effect of
various alkyl substituents (R2) at the indole nitrogen were
examined, and the results are summarized in Scheme 2. It was
found that the isolated yields of product improve as the alkyl
chain length increases. Alkyl substituents such as methyl (1a),
ethyl (1b), n-propyl (1c), n-butyl (1d), and n-pentyl (1e)
provided their corresponding bifunctionalized products 3a, 3b,
3c, 3d, and 3e in 65%, 68%, 72%, 74%, and 80% yields,
respectively. These primary alkyl groups are expected to
stabilize the nitrogen radicals to similar a extent and, hence,
should provide equal yields. The better isolated yields for
higher alkyl chains could be because of easy elution (isolation)
due to the enhanced hydrophobic character during silica gel
column chromatographic purification as highly nitrogenous
compounds often stick to the column. Similarly, secondary N-
alkyl substituents such as isopropyl- (1f) and cyclopentyl-
substituted (1g) indoles yielded their corresponding products
3f and 3g in 75 and 78% yields, respectively.
Next, the compatibility of various benzyl substituents (R2)
was tested as they are often prone to oxidation.16 Substrates
having N-benzyl substituents, bearing either electron-donating
p-CH3 (1h) or electron-withdrawing p-Cl (1i) or o-Br (1j)
groups, reacted efficiently with benzohydrazide (2a) to give
good yields of their products (3h, 72%; 3i, 70%; and 3j, 70%)
in almost identical yields without undergoing oxidation at the
benzylic position. An N-allyl-substituted indole (1k) under-
went the present photocatalytic bifunctionalization to give the
product 3k in 65% yield without affecting the oxidation
sensitive allylic group. After demonstrating the compatibility of
various N-protected indoles, we examined the nature of various
substituents (EDG and EWG) present on either the
homocyclic ring of indoles or the phenyl rings of
benzohydrazides. Irrespective of the nature of the substituents
[electron-donating 5-Me (1l) and electron-withdrawing 5-Br
(1m and 1s) or 7-F (1t) in the indolyl moiety and electron-
donating p-Me (2b), p-OMe (2c), or m-OMe (2d) and
electron-withdrawing p-CF3 (2e) and p-Br (2f) in the
benzohydrazide ring] or their positions of attachments all
provided moderate to good yields of their products (3l, 68%;
3m, 75%; 3n, 75%; 3o, 74%; 3p, 72%; 3q, 85%; 3r, 82%; 3s,
87%; and 3t, 85%), revealing no obvious electronic impact
from the substituents. The structures of all of the synthesized
(E)-N′-(1-alkyl/benzyl-3-oxoindolin-2-ylidene)-
1
benzohydrazides (3a−t) were fully characterized by IR, H
NMR, and 13C NMR spectroscopy and HRMS analysis.
Further, the structure of one of the representatives (E)-N′-(1-
isopropyl-3-oxoindolin-2-ylidene)benzohydrazide (3f) was un-
ambiguously established by single-crystal X-ray diffraction
analysis (see Figure S1 for further details).
Reaction with an alkyl hydrazide gave a very poor yield of
the product, which is also associated with an inseparable
mixture of uncharacterized impurities, whereas sulfonyl
hydrazide failed to give the anticipated oxidative amination
product (see Scheme S2 for details). Unlike benzimidazoles,
this photocatalytic process was not successful for the
functionalization of analogous heterocycles, such as benzothia-
zoles, benzoxazoles, and many other N-bearing heterocycles
(Scheme S3). Thus, as far as further functionalization is
concerned, the bifunctionalized product could be hydrolyzed
to an isatin analogue albeit in low yield (Scheme S4).
To gain insight into the mechanism of this regioselective bis-
functionalization, a series of control experiments were
conducted. To ascertain the origin of the carbonyl oxygen in
the product, whether it is originating from O2 or from other
sources (moisture from the solvent) in the reaction mixture, a
standard reaction of 1a and 2a was performed under a nitrogen
atmosphere. The reaction failed to proceed efficiently, giving
just 5% yield of the product 3a after 24 h. Further, to rule out
the possibility of water as the source of carbonyl oxygen of the
indole ring, a standard reaction was carried out in the presence
of 5 equiv of H218O. The HRMS analysis of the isolated
product revealed no traces of 18O incorporation, suggesting
that water is not the source of the carbonyl oxygen in the
product. These observations suggest that molecular oxygen is
the only possible oxygen source of the carbonyl oxygen as in its
absence the reaction is unsuccessful. To prove the photo-
catalytic radical pathway, a standard experiment between 1a
and 2a was carried out in the presence of TEMPO (1 equiv)
C
DOI: 10.1021/acs.orglett.9b00887
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 3. Plausible Mechanism for the Formation of 3f Supported by DFT-Calculated Energy Profile
a
Relative energies (kcal/mol) and bond distances (Å) are calculated at the M06/6-31+G(d) level of theory.
under otherwise identical conditions, which provided 3a in
only a trace amount. Most oxidations of indoles proceed via an
isatin (indoline-2,3-dione) intermediate.17 Thus, a question
arises whether this oxidative bifunctionalization is proceeding
via a similar isatin intermediate? An isatin analogue, 1-
methylindoline-2,3-dione in lieu of 1a, was reacted with 2a
under identical reaction conditions and gave no trace of 3a,
confirming isatin not to be the intermediate in this reaction
(see Scheme S1 for further details). On the basis of the control
experiments carried and from the DFT calculations, a plausible
mechanistic path is depicted in Scheme 3. In order to clearly
understand the mechanism of the reaction we performed DFT
calculations and modeled the reaction profile at the M06/6-
31+G(d) level of theory. In the first step, indole (1f) is
oxidized to an indole radical cation (A) through a reductive
quenching process by the photocatalyst Ir(III)* (P1).
Simultaneously, the molecular oxygen is reduced to a
The intermediate C is around 1.56 kcal/mol more stable
compared to intermediate B. As expected, the barrier for
removal of a molecule of water is very low (1.90 kcal/mol)
from the intermediate C to form intermediate D, which is
associated with higher stabilization. Finally, there is removal of
two hydrogens, one each from indole and benzohydrazide
(2a). The final step seems to be a facile one with an activation
barrier of 4.89 kcal/mol. Moreover, the final product 3f is
14.91 kcal/mol more stable compared to the intermediate D.
Thus, all of the proposed steps are highly favorable, which
leads to a stable bifunctionalized product. Some of the selected
interatomic distances found in the geometries of various
transition states are depicted in Scheme 3 (see Figure S2 for
further details). The reaction mixture was colorless at the
beginning and turned violet as the reaction progressed. This is
because of the oxidation of the iodide from tetrabutylammo-
nium iodide to molecular iodine due to the generation of
hydrogen peroxide in the medium. Formation of H2O2 has
been detected by two other methods (see Supporting
Information). All of these experiments support the formation
of H2O2 in the medium.
•−
superoxide ion O2 by Ir(II), and the catalyst is regener-
ated.18a Next, we compared the possibility of attack of O2•− on
two possible sites (C3 and C2) of the radical cation A through
TS-1A and TS-1B. Clearly, TS-1A, i.e., attack at the C3-site,
has a lower barrier and lower stabilization compared to TS-1B
involving a C-2 attack as depicted in Scheme 3. Further, the
forward reaction is preferred for TS-1A, and reverse reaction is
preferred for TS-1B. Thus, the formation of transition state
In summary, a mild and efficient photocatalytic regiose-
lective method has been developed for the selective
difunctionalization of indoles by visible-light-induced photo-
redox Ir(III) catalysis. A wide range of indoles, as well as
benzohydrazides, participate competently in the free-radical
reaction to afford structurally diverse oxindoles in good yields.
On the basis of control experiments, a plausible mechanism has
been proposed which is well supported by DFT calculations.
•−
TS-1B via the C-2 attack of O2 is ruled out. The highly
reactive O2•− then attacks at the C-3 site of the intermediate A
to form a peroxy iminium species B. The benzohydrazide 2a
attacks at the C-2 site of intermediate B to form intermediate
C through TS-2. This is perhaps the rate-determining step for
this reaction, having an activation barrier of 22.14 kcal/mol.
D
DOI: 10.1021/acs.orglett.9b00887
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
W.; Cheng, Y.; Zhu, C. Chem. Commun. 2016, 52, 4761.
(d) MacMillan, K. S.; Boger, D. L. J. Am. Chem. Soc. 2008, 130,
16521. (e) Kaushik, N. K.; Kaushik, N.; Attri, P.; Kumar, N.; Kim, C.
H.; Verma, A. K.; Choi, E. H. Molecules 2013, 18, 6620. (f) Wu, W.;
Xiao, M.; Wang, J.; Li, Y.; Xie, Z. Org. Lett. 2012, 14, 1624.
(g) Umehara, A.; Ueda, H.; Tokuyama, H. Org. Lett. 2014, 16, 2526.
(h) Baran, P. S.; Corey, E. J. J. Am. Chem. Soc. 2002, 124, 7904.
(i) Deluca, V.; Balsevich, J.; Tyler, R. T.; Eilert, U.; Panchuk, B. D.;
Kurz, W. G. W. J. Plant Physiol. 1986, 125, 147. (j) Schiff, P. L. Am. J.
Pharm. Educ. 2006, 70, 98. (k) Jadhav, A. P.; Ali, A.; Singh, R. P. Adv.
Synth. Catal. 2017, 359, 1508. (l) Bhaskara Rao, V. U.; Kumar, K.;
Singh, R. P. Org. Biomol. Chem. 2015, 13, 9755.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00887.
■
S
Optimization table; spectra and spectral data for all
compounds; X-ray crysatallographic data for 3f (PDF)
Accession Codes
CCDC 1901162 contains 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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(4) Pitre, S. P.; Scaiano, J. C.; Yoon, T. P. ACS Catal. 2017, 7, 6440.
̌
(5) Zhang, C.; Li, S.; Bures, F.; Lee, R.; Ye, X.; Jiang, Z. ACS Catal.
2016, 6, 6853.
(6) Furst, L.; Matsuura, B. S.; Narayanam, J. M. R.; Tucker, J. W.;
Stephenson, C. R. J. Org. Lett. 2010, 12, 3104.
(7) (a) Gogoi, A.; Guin, S.; Rout, S. K.; Patel, B. K. Org. Lett. 2013,
15, 1802. (b) Gogoi, A.; Modi, A.; Guin, S.; Rout, S. K.; Das, D.;
Patel, B. K. Chem. Commun. 2014, 50, 10445.
AUTHOR INFORMATION
Corresponding Author
■
(8) Zhang, J.; Kohlbouni, S. T.; Borhan, B. Org. Lett. 2019, 21, 14.
(9) (a) Wu, J.; Dou, Y.; Guillot, R.; Kouklovsky, C.; Vincent, G. J.
Am. Chem. Soc. 2019, 141, 2832. (b) Xu, D.; Sun, W. W.; Xie, Y.; Liu,
J. K.; Liu, B.; Zhou, Y.; Wu, B. J. Org. Chem. 2016, 81, 11081.
(10) (a) Zhang, M.; Duan, Y.; Li, W.; Cheng, Y.; Zhu, C. Chem.
Commun. 2016, 52, 4761. (b) Chuang, C.-P.; Tsai, A.-I.; Lin, C. H.
Heterocycles 2005, 65, 2381.
(11) (a) Kamal, A.; Ramakrishna, G.; Raju, P.; Rao, A. V.;
Viswanath, A.; Nayak, V. L.; Ramakrishna, S. Eur. J. Med. Chem. 2011,
46, 2427. (b) Romagnoli, R.; Baraldi, P. G.; Prencipe, F.; Oliva, P.;
Baraldi, S.; Salvador, M. K.; Lopez-Cara, L. C.; Bortolozzi, R.;
Mattiuzzo, E.; Basso, G.; Viola, G. Eur. J. Med. Chem. 2017, 134, 258.
(c) Ribeiro, C. J. A.; Amaral, J. D.; Rodrigues, C. M. P.; Moreira, R.;
Santos, M. M.M. Bioorg. Med. Chem. 2014, 22, 577. (d) Rindhe, S. S.;
Karale, B. K.; Gupta, R. C.; Rode, M. A. Indian J. Pharm. Sci. 2011, 73,
292.
(12) (a) Jang, Y. J.; Yoon, H.; Lautens, M. Org. Lett. 2015, 17, 3895.
(b) Bhunia, S.; Chang, C.-J.; Liu, R.-S. Org. Lett. 2012, 14, 5522.
(13) (a) Ju, X.; Liang, Y.; Jia, P.; Li, W.; Yu, W. Org. Biomol. Chem.
2012, 10, 498. (b) Zhang, M.; Fu, Q.-Y.; Gao, G.; He, H.-Y.; Zhang,
Y.; Wu, Y.-S.; Zhang, Z.-H. ACS Sustainable Chem. Eng. 2017, 5, 6175.
(14) Guchhait, S. K.; Chaudhary, V.; Rana, V. A.; Priyadarshani, G.;
Kandekar, S.; Kashyap, M. Org. Lett. 2016, 18, 1534.
*Fax: (+91)361-258-2349. E-mail: patel@iitg.ac.in.
ORCID
Hemant Kumar Srivastava: 0000-0001-6589-6854
Ritush Kumar: 0000-0002-8384-3868
Bhisma K. Patel: 0000-0002-4170-8166
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
B.K.P. acknowledges the support of this research by the
Department of Science and Technology (DST/SERB) (EMR/
2016/007042), G.S. thanks SERB/DST (PDF/2016/001643)
for the National-Postdoctoral Fellowship, and T.A. thanks IIT
Guwahati for a fellowship. H.K.S. thanks DST/SERB SB/S2/
RJN/004-2015 and the Param-Ishan supercomputing facility at
IIT Guwahati. CIF, IIT Guwahati is highly acknowledged.
DEDICATION
■
(15) Liu, R.-R.; Ye, S.-C.; Lu, C.-J.; Zhuang, G.-L.; Gao, J.-R.; Jia, Y.-
X. Angew. Chem. 2015, 127, 11357.
(16) (a) Guin, S.; Rout, S. K.; Banerjee, A.; Nandi, S.; Patel, B. K.
Org. Lett. 2012, 14, 5294. (b) Rajamanickam, S.; Majji, G.; Santra, S.
K.; Patel, B. K. Org. Lett. 2015, 17, 5586.
(17) (a) Luo, J.; Gao, S.; Ma, Y.; Ge, G. Synlett 2018, 29, 969.
(b) Zi, Y.; Cai, Z.-J.; Wang, S.-Y.; Ji, S.-J. Org. Lett. 2014, 16, 3094.
(c) Bredenkamp, A.; Mohr, F.; Kirsch, S. F. Synthesis 2015, 47, 1937.
Dedicated to Professor Vinod K. Singh on the occasion of his
60th birthday.
REFERENCES
■
(1) (a) Arora, A.; Weaver, J. D. Acc. Chem. Res. 2016, 49, 2273.
(b) Beeler, A. B. Chem. Rev. 2016, 116, 9629. (c) Wang, L.-M.;
Jenkinson, K.; Wheatley, A. E. H.; Kuwata, K.; Saito, S.; Naka, H. ACS
Sustainable Chem. Eng. 2018, 6, 15419. (d) Prier, C. K.; Rankic, D. A.;
MacMillan, D. W. C. Chem. Rev. 2013, 113, 5322. (e) Qvortrup, K.;
Rankic, D. A.; MacMillan, D. W. C. J. Am. Chem. Soc. 2014, 136, 626.
(f) Arora, A.; Weaver, J. D. Org. Lett. 2016, 18, 3996. (g) Yadav, A. K.;
Singh, K. N. Chem. Commun. 2018, 54, 1976. (h) Liu, K.; Zou, M. Z.;
Lei, A. W. J. Org. Chem. 2016, 81, 7088.
́
́
(18) (a) Condie, A. G.; Gonzalez Gomez, J. C.; Stephenson, C. R. J.
J. Am. Chem. Soc. 2010, 132, 1464. (b) Shiraishi, Y.; Kanazawa, S.;
Sugano, Y.; Tsukamoto, D.; Sakamoto, H.; Ichikawa, S.; Hirai, T. ACS
Catal. 2014, 4, 774.
(2) (a) Yang, X.; Li, L.; Li, Y.; Zhang, Y. J. Org. Chem. 2016, 81,
12433. (b) Senaweera, S.; Weaver, J. D. J. Am. Chem. Soc. 2016, 138,
2520. (c) Carreira, E. M.; Hastings, C. A.; Shepard, M. S.; Yerkey, L.
A.; Millward, D. B. J. Am. Chem. Soc. 1994, 116, 6622. (d) Wei, G.;
Basheer, C.; Tan, C.-H.; Jiang, Z. Tetrahedron Lett. 2016, 57, 3801.
́
́
(e) Toth, B. L.; Tischler, O.; Novak, Z. Tetrahedron Lett. 2016, 57,
̈
4505. (f) Hering, T.; Meyer, A. U.; Konig, B. J. Org. Chem. 2016, 81,
6927. (g) Douglas, J. J.; Sevrin, M. J.; Stephenson, C. R. J. Org. Process
Res. Dev. 2016, 20, 1134−1147. (h) Romero, N. A.; Nicewicz, D. A.
Chem. Rev. 2016, 116, 10075.
(3) (a) Li, M.; Yang, J.; Ouyang, X.-H.; Yang, Y.; Hu, M.; Song, R.-J.;
Li, J.-H. J. Org. Chem. 2016, 81, 7148. (b) Zhu, M.; Zhou, K.; Zhang,
X.; You, S.-L. Org. Lett. 2018, 20, 4379. (c) Zhang, M.; Duan, Y.; Li,
E
DOI: 10.1021/acs.orglett.9b00887
Org. Lett. XXXX, XXX, XXX−XXX