Visible-Light-Triggered Directly Reductive Arylation of Carbonyl/Iminyl Derivatives through Photocatalytic PCET

Article abstract of DOI:10.1021/acs.orglett.7b01677

The first visible-light-mediated radical-radical cross-coupling strategy that enables the direct arylation of carbonyl/iminyl derivatives in the presence of Et₃N has been realized. Such an atom-economical protocol furnishes a broad scope of arylation products such as secondary/tertiary alcohols and amines via a PCET process that facilitates the challenging reduction of C-X (X = O, N). Mechanistic investigation indicates two photocatalytic redox cycles were involved in the process, and Et₃N was proved to serve as a dual reductant and proton donor. Moreover, the isolated byproducts and controlled experiments could be considered as powerful supporting evidence for our hypothesis.

Full text of DOI:10.1021/acs.orglett.7b01677

Letter
pubs.acs.org/OrgLett
Visible-Light-Triggered Directly Reductive Arylation of Carbonyl/
Iminyl Derivatives through Photocatalytic PCET
Ming Chen, Xinxin Zhao, Chao Yang, and Wujiong Xia*
State Key Lab of Urban Water Resource and Environment, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China
S
* Supporting Information
ABSTRACT: The first visible-light-mediated radical−radical
cross-coupling strategy that enables the direct arylation of
carbonyl/iminyl derivatives in the presence of Et3N has been
realized. Such an atom-economical protocol furnishes a broad
scope of arylation products such as secondary/tertiary alcohols
and amines via a PCET process that facilitates the challenging reduction of CX (X = O, N). Mechanistic investigation indicates
two photocatalytic redox cycles were involved in the process, and Et3N was proved to serve as a dual reductant and proton donor.
Moreover, the isolated byproducts and controlled experiments could be considered as powerful supporting evidence for our
hypothesis.
ompounds with carbon−heteroatom double bonds (ke-
mediated photocatalytic platform have been highlighted but
limited tocoupling with alkyl radicals or the intra-/intermolecular
addition to unsaturated bonds.11,14 For example, In 2015,
Rueping’s group reported the reductive homocoupling of
aldehydes, ketones, and imines to the corresponding dimeriza-
tion products;15 in addition, their further research revealed a
photocatalytic approach to the access of 1,2-diamines and
alkamines via α-amino radicals and ketyl radicals16 (Scheme 1, i).
In respect to the reaction of double bonds, in 2003, Knowles
detailed an intramolecular ketyl−olefin cyclization via PECT.17
Recently, Ngai developed the first β-selective coupling of
alkenylpyridines with aldehydes and imines catalyzed by visible
light (Scheme 1, ii).18 These protocols not only demonstrate the
capacity of PECT to access the required radical synthons but also
provide alternatives for the reductive C−C coupling on
unsaturated carbon−heteroatom bonds. Nonetheless, to the
best of our knowledge, general photochemical strategies that
directly achieve the cross-coupling of ketyl and iminyl radicals
with aryl radicals or radical ion intermediates have never been
explored, which might be mainly due to the large steric hindrance
and high reduction potential of the substrates. Herein, we report
the first photocatalytic reductive arylation reaction of carbonyl/
iminyl derivatives. Such a concept was primarily based on the
PCET that enabled access to ketyl/iminyl radical and the feasible
generation of an aryl radical via a well-established photoredox
mode by using dicyanobenzene as a precursor.19
C
tones, aldehydes, and imines) are valuable synthetic
precursors in organic chemistry for the new C−C bond
construction.1 Among which one of their extraordinary
application is the preparation of alcohols or amines, which are
synthetically reactive intermediates and necessary building blocks
for many bioactive and pharmaceutical structures, by means of
reductive nucleophilic additions.2−4 Traditional technologies on
such transformations (especially for carbonyl compounds) rely
on air- and moisture-unstable organolithium, organomagnesium,
or organozinc reagents.5 In addition, the demands of functional
group compatibility also limit their wider application. To address
such drawbacks, some alternative strategies, such as the
organometallic reagents (Zn, Mg, In, Sn, Sm, Ti, etc.) engaged
Barbier−Grignard-type reactions6 which even performed in
water, the transition-metal complex (Rh, Pd, Ni) catalyzed
addition of nonmetallic reagents (such as boron, silicon
compounds) onto carbon−heteroatom double bonds,7 and the
phosphonium salt mediated reductive alkylation8 or arylation9
have been developed as more efficient tools. However, substrate
limitations and cost issues have inspired chemists to focus
continuous efforts on searching for less conventional and
ecocompatible techniques.
Proton-coupled electron transfer (PCET) has been recognized
as an elementary redox process in which both a proton and an
electron traveled in a concerted manner, and it plays a critical role
across biochemistry and material domains.10 Recently, the
applications of PCET in organic synthesis, especially in the
photochemical reactions, have become one of the most advanced
hotspots. Numerous excellent achievements have been made,11
e.g., the homolytic activation of O−H bonds of alcohols or N−H
bonds of amines12,13 and the direct reduction of π-bonds of
unsaturated carbon−heteroatom compounds,14 all of which were
inaccessible by using the classical HAT method.
With respect to the lower reduction potential of aldehydes
compared to ketones (benzaldehyde, Ered = −1.93 V vs SCE),20
our investigation was initially launched on the reaction between
the readily available benzaldehyde 1a and 1,4-dicyanobenzene
(1,4-DCB). After a series of screenings on photocalyst, solvent,
and other impact factors, we defined the reaction of 1a in the
presence of catalyst 3b (1.0 mmol %), 1,4-DCB (2.0 equiv), and
In particular, the synthetic applications of ketyl and iminyl
radicals derived from the reductive umpolung of carbonyls/
iminyls, in most cases with the aid of Lewis acids, on the PECT-
Received: June 3, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01677
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Previous Work and This Work
Table 1. Optimization of Reaction Conditions
b
entry
photocatalyst (mmol %)
solvent
yield (%)
1
2
lb (5)
MeCN
MeCN
MeCN
MeCN
MeOH
DMF
none
none
40
2b (5)
3b (5)
4b (5)
3b (5)
3b (5)
3b (5)
3b (5)
3b (1)
3b (0.5)
3
4
35
5
45
6
40
7
DMSO
THF
none
none
8
c
9
MeOH
MeOH
MeOH
MeOH
55 (31)
10
35
d
11
none
none
e
12
3b (1)
a
Reaction conducted with 0.2 mmol of 1a, 0.4 mmol of 1,4-DCB, 0.6
mmol of Et3N in 2.0 mL of solvent, 20 W blue LED, under N2
b
c
atmosphere unless otherwise noted. Isolated yield. Reaction system
d
under air atmosphere. Reaction conducted without photocatalyst.
triethylamine (3.0 equiv) in MeOH with the protection of
nitrogen under the irradiation of a 20 W blue LED at room
temperature as the optimized conditions (Table 1).
e
Reaction conducted in dark.
With the optimal conditions in hand, we then turned our
attention to evaluate the reactivity of aldehydes. As outlined in
Scheme 2, all the tested aromatic aldehydes were found to be
competent substrates to yield the corresponding alcohols within
7 h under mild conditions (1c−13c). Functional groups
regardless of electron-donating (methyl, methoxyl, phenyl, etc.)
or electron-withdrawing nature were well tolerated with the
reaction conditions. Delightfully, product 5c with p-methoxyl on
the benzene ring was obtained in a highest yield 96%. In addition,
substrates with large groups, such as phenyl and tert-butyl, could
furnish the corresponding products in 43% and 56% yield,
respectively (8c, 9c). Polysubstituted substrates 10a−12a were
also suitable for the reaction conditions to afford the products in
good yields (10c−12c, 62−76%). Notably, this protocol was also
applicable with other aromatic aldehydes such as naphthaldehyde
(13c).
benzoheterocyclones successfully demonstrated the compati-
bility of this protocol (29c−32c). Similar to the photochemical
behavior of polysubstituted benzaldehydes, the coupling reaction
of analogous ketone substrate was proven to be viable to form the
corresponding product in moderate yield (33c). In addition,
other tertiary aromatic alcohols such as heterocyclic and
polycyclic aromatic adducts could be easily implemented via
this method (34c, 35c).
To extend the scope of this reaction, we further employed
aromatic imines to demonstrate the compatibility of this
methology. As summarized in Scheme 3, series of imine
derivatives were effectively transformed into the desired amine
products (1e−6e) in acceptable yields. Remarkably, substrate 6d,
which was derived from an aromatic ketone, was also suitable for
the reaction conditions to furnish its corresponding product with
a quaternary center.
Encouraged by the satisfactory results using aldehydes, we
attempted to extend the substrate scope to ketones, which were
regarded as a big challenge due to their high reduction potentials
(acetophenone, Ered = −2.11 V vs SCE).20 Gratifyingly, the
reactions were carried out successfully when a broad range of
aromatic ketones were submitted to the standard conditions.
However, minor defects were found that showed most substrates
could not be fully consumed even with prolonged reaction times.
As shown in Scheme 2, a series of acetophenone derivatives with
diverse substituents on the benzene ring could effectively couple
with 1,4-DCB to provide functionalized products in moderate to
high yields (14c−22c). Replacing Rs (Rs = methyl) with other
chain or cyclic aliphatic groups, the transformations were also
amenable with the corresponding ketones (23c−28c). More-
over, benzocycloketones with different ring sizes as well as
Furthermore, we evaluated the generality of this protocol by
using other nitriles (2-methylterephthalonitrile and isonicotino-
nitrile) as potential coupling partners. It was found that the
methyl on 1,4-DCB did not hinder the reaction progress to
couple with both aldehydes and ketones, leading to the
photoinduced products in a similar fashion (Scheme 4, 1f−3f).
Notably, the arylation of aldehyde led to an inseparable alcohol
mixture (1f/1f′), while the ketone system afforded tertiary
alcohol as the only product (2f−3f), which might be due to the
steric effect of R. Additionally, we were delighted to find that
isonicotinonitrile could serve as a brilliant coupling partner and
deliver the corresponding products in good to high yields (4f−
5f).
To gain more insight into the mechanistic details, a series of
control experiments were conducted by using 1a as the
B
DOI: 10.1021/acs.orglett.7b01677
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Photocatalytic Arylation between Carbonyl
Derivatives and 1,4-DCB
Scheme 4. Photocatalytic Arylation Using Other Nitriles as
Coupling Partners
a
a,b
a
Reaction conditions: 0.2 mmol of aldehydes or ketones, 0.4 mmol of
nitriles, 0.6 mmol of Et3N, 1.0 mmol % of 3b in 2.0 mL of MeOH, N2
atmosphere, 20 W blue LED; isolated yields are shown; isolated yields
b
marked in red color based on recovered starting material. The ratio
determined by H NMR.
1
coupling protocol proceeded via a free radical pathway, and two
redox cycles might be involved in the catalytic process.
Therefore, a tentative visible-light catalytic mechanism was
proposed as shown in Scheme 5. Upon absorption of visible light,
Scheme 5. Proposed Reaction Mechanism
a
Reaction conditions: 0.2 mmol of a, 0.4 mmol of 1,4-DCB, 0.6 mmol
of Et3N, 1.0 mmol % of 3b in 2.0 mL of MeOH, under N2 atmosphere,
20 W blue LED; isolated yields are shown; isolated yields marked in
red were based on recovered starting material.
Scheme 3. Photocatalytic Arylation between Imines and 1,4-
a
DCB
the photocatalyst IrIII was initially excited into IrIII*, which
subsequently underwent a SET oxidation of the sacrificial
reductant Et3N to afford IrII and the radical cation A. On the
basis of previous reports,15 a three-electron bond (C) formed
between the generated Lewis acidic species A and the
nucleophilic CX system or a hydrogen bond (D) between
the tautomer B and CX bond favored the reduction of
carbonyl/iminyl catalyzed by IrII via a PCET process, thus leading
to the radical intermediate E, and returned the photocatalyst to its
ground state IrIII. On the other hand, a second redox cycle also
proceeded through the competitive formation of the cyanoaryl
radical anion F. In such a process, Et3N was as well identified as a
reductive quencher to reduce the excited IrIII* to IrII. However,
under the irradiation of light, 1,4-DCB or isonicotinonitrile might
perform a single-electron oxidation of the IrII to regenerate the
IrIII species to deliver the intermediate F. Finally, an
intermolecular radical−radical cross-coupling took place be-
tween E and F, thus affording the desired reduction product by
elimination of a cyanide anion. Additionally, the generation way
of compounds BP1 and BP2 could be clearly distinguished in this
mechanistic scheme.
a
Reaction conditions: 0.2 mmol of d, 0.4 mmol of 1,4-DCB, 0.6 mmol
of Et3N, 1.0 mmol % of 3b in 2.0 mL of MeOH, N2 atmosphere, 20 W
blue LED; isolated yields are shown; isolated yields marked in red
were based on recovered starting material.
representative substrate. (See SI, Scheme S1 for details). We have
respectively investigated the effect of TEMPO, inorganic base
and the amount of triethylamine on the reaction outcome.
Moreover, two key byproducts, BP1 and BP2, were obtained in
the controlled experiments. These discoveries indicated this
C
DOI: 10.1021/acs.orglett.7b01677
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(i) Schmidt, F.; Stemmler, R. T.; Rudolph, J.; Bolm, C. Chem. Soc. Rev.
2006, 35, 454.
In conclusion, we have developed an atom-economical
radical−radical cross-coupling method that opened an alternative
door for carbonyl/iminyl derivatives to unexplored reactivities.
This versatile protocol provided a convenient appoach to achieve
the reductive arylation of the CX bond enabled by visible light,
leading to a broad range of secondary/tertiary alcohols and amine
products. Such a finding could be regarded as a very
complementary work to the addition of CX by an unsaturated
double bond or alkyl radical. Furthermore, the mechanistic
investigation showed that two redox cycles were involved in this
photoreaction, and the challenging reduction of the CX bond
was successfully realized by PECT. Notably, the speculation was
strongly supported by the isolated byproducts and controlled
experiments. With a view to the operational simplicity and mild
conditions, we believe this green, economic protocol will find
more extensive application in organic synthesis.
(6) (a) Li, C.-J.; Zhang, W.-C. J. Am. Chem. Soc. 1998, 120, 9102. (b) Li,
C.-J.; Meng, Y. J. Am. Chem. Soc. 2000, 122, 9538. (c) Breton, G. W.;
Shugart, J. H.; Hughey, C. A.; Conrad, B. P.; Perala, S. M. Molecules 2001,
6, 655. (d) Keh, C. C. K.; Wei, C.; Li, C.-J. J. Am. Chem. Soc. 2003, 125,
4062. (e) Imlinger, N.; Mayr, M.; Wang, D.; Wurst, K.; Buchmeiser, M.
R. Adv. Synth. Catal. 2004, 346, 1836. (f) Williams, D. R.; Berliner, M. A.;
Stroup, B. W.; Nag, P. P.; Clark, M. P. Org. Lett. 2005, 7, 4099. (g) Li, C.-
J.; Chen, L. Chem. Soc. Rev. 2006, 35, 68. (h) Sato, I.; Toyota, Y.; Asakura,
N. Eur. J. Org. Chem. 2007, 2007, 2608.
(7) For B reagents, see: (a) Yoshida, K.; Ogasawara, M.; Hayashi, T. J.
Am. Chem. Soc. 2002, 124, 10984. (b) Duan, H.-F.; Xie, J.-H.; Shi, W.-J.;
Zhang, Q.; Zhou, Q.-L. Org. Lett. 2006, 8, 1479. (c) Gois, P. M. P.;
́
Trindade, A. F.; Veiros, L.; Andre, V.; Duarte, M. T.; Afonso, C. A. M.;
Caddick, S.; Cloke, F. G. N. Angew. Chem., Int. Ed. 2007, 46, 5750.
(d) Morikawa, S.; Michigami, K.; Amii, H. Org. Lett. 2010, 12, 2520. For
Si reagents, see: (e) Tomita, D.; Wada, R.; Kanai, M.; Shibasaki, M. J. Am.
Chem. Soc. 2005, 127, 4138. (f) Lerebours, R.; Wolf, C. J. Am. Chem. Soc.
ASSOCIATED CONTENT
* Supporting Information
́
2006, 128, 13052. (g) Abid, I.; Gosselin, P.; Mathe-Allainmat, M.; Abid,
■
S.; Dujardin, G.; Gaulon-Nourry, C. J. Org. Chem. 2015, 80, 9980.
(h) Komiyama, T.; Minami, Y.; Hiyama, T. ACS Catal. 2017, 7, 631.
(8) (a) Deng, Z.; Liu, C.; Zeng, X.-L.; Lin, J.-H.; Xiao, J.-C. J. Org. Chem.
2016, 81, 12084. (b) Zeng, X.-L.; Deng, Z.-Y.; Liu, C.; Zhao, G.; Lin, J.-
H.; Zheng, X.; Xiao, J.-C. J. Fluorine Chem. 2017, 193, 17.
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b01677.
Experimental procedures and 1H and 13C NMR spectra for
new compounds (PDF)
(9) (a) Gu, Y.; Leng, X.; Shen, Q. Nat. Commun. 2014, 5, 5405.
(b) Deng, Z.; Lin, J.; Cai, J.; Xiao, J.-C. Org. Lett. 2016, 18, 3206.
(10) (a) Huynh, M. H. V.; Meyer, T. J. Chem. Rev. 2007, 107, 5004.
AUTHOR INFORMATION
(b) Costentin, C.; Drouet, S.; Robert, M.; Savea
́
nt, J.-M. Science 2012,
■
338, 90. (c) Urbanek, J.; Vohringer, P. J. Phys. Chem. B 2014, 118, 265.
̈
Corresponding Author
*E-mail: xiawj@hit.edu.cn.
ORCID
(d) Greene, B. L.; Wu, C.-H.; Vansuch, G. E.; Adams, M. W. W.; Dyer, R.
B. Biochemistry 2016, 55, 1813.
(11) For selected reviews, see: (a) Nguyen, L. Q.; Knowles, R. R. ACS
Catal. 2016, 6, 2894. (b) Gentry, E. C.; Knowles, R. R. Acc. Chem. Res.
2016, 49, 1546. (c) Skubi, K. L.; Blum, T. R.; Yoon, T. P. Chem. Rev.
2016, 116, 10035. (d) Miller, D. C.; Tarantino, K. T.; Knowles, R. R. Top.
Curr. Chem. 2016, 374, 30. (e) Hoffmann, N. Eur. J. Org. Chem. 2017,
2017, 1982.
Wujiong Xia: 0000-0001-9396-9520
Notes
The authors declare no competing financial interest.
(12) (a) Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. J. Org. Chem.
2016, 81, 6898. (b) Yayla, H. G.; Wang, H.; Tarantino, K. T.; Orbe, H. S.;
Knowles, R. R. J. Am. Chem. Soc. 2016, 138, 10794. (c) Amorati, R.;
Valgimigli, L.; Viglianisi, C.; Schmallegger, M.; Neshchadin, D.;
Gescheidt, G. Chem. - Eur. J. 2017, 23, 5299. (d) Tlahuext-Aca, A.; R.
Garza-Sanchez, A.; Glorius, F. Angew. Chem., Int. Ed. 2017, 56, 3708.
(13) (a) Choi, G. J.; Knowles, R. R. J. Am. Chem. Soc. 2015, 137, 9226.
(b) Miller, D. C.; Choi, G. J.; Orbe, H. S.; Knowles, R. R. J. Am. Chem. Soc.
2015, 137, 13492. (c) Tong, K.; Liu, X.; Zhang, Y.; Yu, S. Chem. - Eur. J.
2016, 22, 15669. (d) Choi, G. J.; Zhu, Q.; Miller, D. C.; Gu, C. J.;
Knowles, R. R. Nature 2016, 539, 268. (e) Chu, J. C. K.; Rovis, T. Nature
2016, 539, 272.
(14) (a) Rono, L. J.; Yayla, H. G.; Wang, D. Y.; Armstrong, M. F.;
Knowles, R. R. J. Am. Chem. Soc. 2013, 135, 17735. (b) Fuentes de Arriba,
A. L.; Urbitsch, F.; Dixon, D. J. Chem. Commun. 2016, 52, 14434.
(c) Wang, C.; Qin, J.; Shen, X.; Riedel, R.; Harms, K.; Meggers, E. Angew.
Chem., Int. Ed. 2016, 55, 685. (d) Xu, P.; Wang, G.; Zhu, Y.; Li, W.;
Cheng, Y.; Li, S.;Zhu, C. Angew. Chem., Int. Ed. 2016, 55, 2939. (e) Qi, L.;
Chen, Y. Angew. Chem., Int. Ed. 2016, 55, 13312. (f) Fava, E.; Nakajima,
M.; Nguyen, A. L. P.; Rueping, M. J. Org. Chem. 2016, 81, 6959.
(15) Nakajima, M.; Fava, E.; Loescher, S.; Jiang, Z.; Rueping, M. Angew.
Chem., Int. Ed. 2015, 54, 8828.
ACKNOWLEDGMENTS
■
We are grateful for financial support from China NSFC (Nos.
21372055, 21472030, and 21672047) and SKLUWRE (No.
2018DX02).
REFERENCES
■
(1) Mukherjee, S.; Yang, J. W.; Hoffmann, S.; List, B. Chem. Rev. 2007,
107, 5471.
(2) (a) Salvi, L.; Kim, J. G.; Walsh, P. J. J. Am. Chem. Soc. 2009, 131,
12483. (b) Kim, H. Y.; Walsh, P. J. Acc. Chem. Res. 2012, 45, 1533.
(c) Long, J.; Zhang, S.-F.; Wang, P.-P.; Zhang, X.-M.; Yang, Z.-J.; Zhang,
Q.; Chen, Y. J. Med. Chem. 2014, 57, 7098.
(3) (a) Burns, N. Z.; Hackman, B. M.; Ng, P. Y.; Powelson, I. A.;
Leighton, J. L. Angew. Chem., Int. Ed. 2006, 45, 3811. (b) Itoh, J.; Han, S.
B.; Krische, M. J. Angew. Chem., Int. Ed. 2009, 48, 6313.
(4) (a) Legros, J.; Meyer, F.; Coliboeuf, M.; Crousse, B.; Bonnet-
́ ́
Delpon, D.; Begue, J.-P. J. Org. Chem. 2003, 68, 6444. (b) Keinicke, L.;
Fristrup, P.; Norrby, P.-O.; Madsen, R. J. Am. Chem. Soc. 2005, 127,
15756. (c) Buesking, A. W.; Baguley, T. D.; Ellman, J. A. Org. Lett. 2011,
13, 964. (d) Schrittwieser, J. H.; Velikogne, S.; Kroutil, W. Adv. Synth.
Catal. 2015, 357, 1655.
(16) Fava, E.; Millet, A.; Nakajima, M.; Loescher, S.; Rueping, M.
Angew. Chem., Int. Ed. 2016, 55, 6776.
(17) Tarantino, K. T.; Liu, P.; Knowles, R. R. J. Am. Chem. Soc. 2013,
135, 10022.
(18) Lee, K. N.; Lei, Z.; Ngai, M.-Y. J. Am. Chem. Soc. 2017, 139, 5003.
(19) (a) Pirnot, M. T.; Rankic, D. A.; Martin, D. B. C.; MacMillan, D.
W. C. Science 2013, 339, 1593. (b) Qvortrup, K.; Rankic, D. A.;
MacMillan, D. W. C. J. Am. Chem. Soc. 2014, 136, 626.
(5) For Li reagents, see: (a) Bloch, R. Chem. Rev. 1998, 98, 1407.
(b) Tomioka, K.; Shioya, Y.; Nagaoka, Y.; Yamada, K. J. Org. Chem. 2001,
66, 7051. (c) Wu, G.; Huang, M. Chem. Rev. 2006, 106, 2596. (d) Vidal,
́
C.; García-Alvarez, J.; Hernan
Angew. Chem., Int. Ed. 2016, 55, 16145. For Mg reagents, see:
(e) Seyferth, D. Organometallics 2009, 28, 1598. (f) Collados, J. F.; Sola,
R.;Harutyunyan, S.R.;Macia, B. ACSCatal. 2016, 6, 1952. (g)Bieszczad,
́ ́
-Gomez, A.; Kennedy, A. R.; Hevia, E.
̀
́
B.; Gilheany, D. G. Angew. Chem., Int. Ed. 2017, 56, 4272. For Zn
(20) Roth, H. G.; Romero, N. A.; Nicewicz, D. A. Synlett 2016, 27, 714.
reagents, see: (h) Pu, L.; Yu, H.-B. Chem. Rev. 2001, 101, 757.
D
DOI: 10.1021/acs.orglett.7b01677
Org. Lett. XXXX, XXX, XXX−XXX