Ruthenium(II)-catalyzed ortho-C-H arylation of diverse N-heterocycles with aryl silanes by exploiting solvent-controlled N-coordination

Article abstract of DOI:10.1039/c7ob00818j

We report the first method for the direct, regioselective Ru(ii)-catalyzed oxidative arylation of C-H bonds in diverse N-heterocycles with aryl silanes by exploiting solvent-controlled N-coordination. The reaction takes advantage of the attractive feature

Full text of DOI:10.1039/c7ob00818j

Organic &
Biomolecular Chemistry
View Article Online
View Journal
COMMUNICATION
Ruthenium(II)-catalyzed ortho-C–H arylation of
diverse N-heterocycles with aryl silanes by
exploiting solvent-controlled N-coordination†
Cite this: DOI: 10.1039/c7ob00818j
Received 1st April 2017,
Accepted 11th April 2017
Pradeep Nareddy, Frank Jordan and Michal Szostak
*
DOI: 10.1039/c7ob00818j
rsc.li/obc
We report the first method for the direct, regioselective Ru(II)-cata- with organosilanes by N-coordination. The reaction manifold
lyzed oxidative arylation of C–H bonds in diverse N-heterocycles is highly desirable for the following reasons:
with aryl silanes by exploiting solvent-controlled N-coordination.
(1) the manuscript describes the first Ru-catalyzed arylation
The reaction takes advantage of the attractive features of organo- with arylsilanes exploiting N-coordination. This includes Ru(II)
silanes as coupling partners, providing proof of concept for and Ru(0) cycles,6 and could lead to the development of a
N-directed Ru(II)-catalyzed C–H arylation. This novel, operation- range of C–H arylation protocols by N-coordination in
ally-simple and versatile protocol utilizes the Ru(II)/CuF2 reagent common substrates.
system in which CuF2 serves as a dual activator/oxidant in non-
(2) the method shows much higher chemoselectivity than
coordinating solvents to accommodate for ligand N-coordination. other methods for arylation of N-heterocycles using Ru-cata-
This first Ru(II)-catalyzed N-directed Hiyama C–H arylation offers lysis. Specifically, our method tolerates aryl halides, such as
broad implications to achieve numerous C–H bond functionaliza- bromides which are beyond the scope of Ru(II)/(IV) cycle by
tions by versatile ruthenium(II) catalysis manifold.
N-coordination. In general, there are very few methods for
direct C–H arylation of N-heterocycles in the presence of aryl
bromides, and all of them limited to Pd and Rh catalysis.3,4
(3) we demonstrate that the reaction rate with organosilanes
using Ru(II)-catalysis by N-coordination is much higher than
by O-coordination,7–10 which may lead to the development of a
range of practical protocols for C–H arylation by this catalysis
manifold.
(4) we demonstrate that Ru(II)-catalyzed C–H arylation of
N-heterocycles using organosilanes proceeds with much
higher efficiency than when using organoboranes,7–10 which
may lead to complementary selectivity in Ru(II)-catalyzed C–H
arylation.
The Hiyama cross-coupling reaction is widely-recognized as
one of the most important methods for C–C bond formation
due to major advantages of organosilicon coupling partners,
including low toxicity, safe handling, accessibility and func-
tional group compatibility.1,2 The development of direct C–H
functionalization strategies has exerted a profound impact on
organic synthesis.3 The use of organometallic reagents pro-
vides multiple sources of alternative carbon nucleophiles
under mild, functional group tolerant, oxidative conditions
with functional group tolerance and unique selectivity un-
attainable by other catalytic manifolds.4
Pioneering work demonstrated the utility of direct Hiyama
C–H arylation based on Pd and Ni catalysis.5a–c More recently,
synthetically useful methods have been reported.5d–h However,
despite these advances, the use of organosilanes as viable
cross-coupling partners in C–H functionalization remains
severely underdeveloped due to (1) low nucleophilicity of
organosilanes;1a–d (2) incompatibility of silane activation and
metal re-oxidation steps within the C–H activation cycle.3,4
This manuscript reports the first Ru-catalyzed C–H arylation
Over the past decade, there have been remarkable advances
in ruthenium(II)-catalyzed C–H functionalizations owing to (1)
economic advantages of ruthenium; (2) operationally-simple
reaction protocols; (3) high functional group tolerance.6 The
groups of Jeganmohan7 and Pilarski8 have designed new cata-
lytic systems for C–H arylation with boronic acids (Fig. 1A).
We have reported direct arylation of N,N-dialkyl benzamides by
weak O-coordination.9 More recently, we have achieved the
first Ru(II)-catalyzed C–H arylation with organosilanes in
N,N-dialkyl
benzamide
substrates
exploiting
weak
O-coordination.10 This is the only successful example of Ru(II)-
catalyzed direct Hiyama C–H functionalization with organo-
silanes accomplished to date.11
Department of Chemistry, Rutgers University, 73 Warren Street, Newark, NJ 07102,
USA. E-mail: michal.szostak@rutgers.edu; Fax: (+1)-973-353-1264;
Tel: (+1)-973-353-532
Herein, we report the first method for the direct, regio-
selective Ru(II)-catalyzed oxidative Hiyama arylation of C–H
†Electronic supplementary information (ESI) available: Experimental details and
characterization data. See DOI: 10.1039/c7ob00818j
This journal is © The Royal Society of Chemistry 2017
Org. Biomol. Chem.

View Article Online
Communication
Organic & Biomolecular Chemistry
Table 1 Optimization of Ru(II)-catalyzed C–H arylation of 1a with
phenyltrimethoxysilanea
Entry Additive Oxidant
Solvent 3a : 3bb (%) Yieldb,c (%)
1
2
3
4
5
6
7
8
CuF2
CuF2
CuF2
CuF2
CuF2
CuF2
CuF2
CuF2
CuF2
AgF
AgF
AgF
AgF
KF
—
—
—
—
—
—
—
—
DCE
PhCH3 69 : 31
THF 75 : 25
i-PrOH 74 : 26
6 : 94
>95 (83)d
>95
>95
>95
70
<5
<2
<2
<2
40
<2
<2
<2
<2
<2
<2
DMF
CH3CN
CHCl3
NMP
DCE
DCE
DCE
86 : 14
—
—
—
—
9
AgF
10
11
12
13
14
15
16
Ag2O
Cu(OTf)2
Cu(OAc)2
>95 : 5
—
Fig. 1 (a) Ruthenium-catalyzed arylation using boronic acids (previous
studies), and (b) ruthenium(II)-catalyzed C–H arylation using organo-
silanes by N-coordination (this study).
DCE
—
—
Cu(OAc)2H2O DCE
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
DCE
DCE
DCE
—
—
—
CsF
CuF2
a 2-Phenylpyridine (1.0 equiv.), [RuCl2(p-cymene)]2 (5 mol%), AgSbF6
bonds in diverse N-heterocycles with aryl silanes by exploiting (20 mol%), PhSi(OMe)3 (3.0 equiv.), additive (3.5 equiv.), oxidant (2.0
equiv.), solvent (0.20 M), 140 °C, 20 h. b Determined by 1H NMR and
solvent-controlled N-coordination (Fig. 1B). Notable features of
our study include: (1) the first N-directed Hiyama cross-coup-
GC. c Yield of 3a and 3b. d Isolated yield of 3b. See ESI for details.
ling using cost-effective, versatile Ru(II) catalysis; (2) the use of
benign, functional group tolerant organosilanes, including tol-
erance for aryl halides; (3) Ru(II)/CuF2 reagent system in which
CuF2 serves as a dual activator/oxidant in non-coordinating cheap additive, CuF2, which serves as a dual silane activator
solvents to accommodate for ligand N-chelation;12 (4) syn- and Ru(0)-re-oxidant.15 Key optimization results are shown in
thesis of biaryl N-heterocyclic motifs that are prevalent in Table 1. Other solvents are inferior to DCE due to less efficient
natural products, pharmaceuticals and advanced organic arylation and decomposition (entries 2–8). Modest monoselec-
materials;13,14 (5) key mechanistic studies that support revers- tivity using PhCH3, THF, i-PrOH and DMF was observed.
ible cycloruthenation. Notably, we demonstrate that (i) the As anticipated, the efficiency of the reaction is strongly depen-
present Ru(II)-catalyzed Hiyama cross-coupling shows higher dent of the nature of the catalytic system used (entries 9–16).
reactivity than the Ru(II)-catalyzed direct C–H arylation with In agreement with our hypothesis, the combined use of
boronic acids; (ii) N-coordination leads to significantly other well-established Ru(0)-re-oxidants (Ag2O, Cu(OTf)2,
improved reactivity than O-coordination10 under oxidative Cu(OAc)2)7–9,11 and silane activators (AgF, KF, CsF) gave
Ru(II)-Hiyama conditions. This first Ru(II)-catalyzed N-directed inferior results (entries 9–16).5 Notably, the attempted C–H
Hiyama C–H arylation offers broad implications to arylation of 1a with phenylboronic acid under various Ru(II)
achieve numerous C–H bond functionalizations by versatile conditions gave the desired product with low conversions
ruthenium(II) catalysis manifold that is beyond the scope of (vide infra, see ESI†), suggesting the high potential of the
classic Ru(0)/(II) arylations.11j
We initiated our investigations by studying arylation of 1a of N-coordinating substrates.
Ru(II)-catalyzed Hiyama cross-coupling in C–H functionalization
with trimethoxyphenylsilane (Table 1). At the outset, we
With the optimized conditions in hand, the scope of this
hypothesized that by fine-tuning the coordinating properties new Ru(II)-catalyzed Hiyama C–H arylation of N-heterocycles
of the solvent to accommodate for N-chelation may enable the was surveyed (Table 2). We were delighted to find that the devel-
first direct Ru(II)-catalyzed C–H arylation of N-heterocycles oped Ru(II)/CuF2 catalyst system is effective for arylation of a
with aryl silanes. After extensive optimization, we were wide range of N-heterocycles with high arylation selectivity,
delighted to find that the use of cationic Ru(II) catalyst, including medicinally-relevant pyridines, pyrazoles and quino-
[RuCl2(p-cymene)]2 (5 mol%), AgSbF6 (20 mol%), trimethoxy- lines.16 The example with substoichiometric amount of 2-phenyl-
phenylsilane (3 equiv.), and CuF2 (3.5 equiv.) in DCE (1,2- pyridine (entry 2, 3a) illustrates that monoarylation is possible
dichlorethane) cleanly delivered the desired diarylation with high selectivity, a process difficult to achieve with aryl
product in 83% isolated yield (Table 1, entry 1). It is worth halides.11 The arylation proceeds in high yields for electron-
noting that the reaction is efficiently promoted by a single and donating and withdrawing substrates (entries 3–5, 3c–3e). In
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Organic & Biomolecular Chemistry
Communication
Table 2 Ruthenium(II)-catalyzed C–H arylation of N-heterocycles with
phenyltrimethoxysilanea,b
Next, the substrate scope with respect to the aryl silane
component was evaluated (Table 3). The scope with respect to
the aryl silane component is also broad. We were pleased to
find that trimethoxyphenylsilane was similarly effective as tri-
methoxyphenylsilane without modification of the reaction
conditions (entry 1 vs. entry 2).5c Electron-rich (entries 3 and 4)
and electron-deficient silanes (entry 5) are suitable cross-
coupling partners, affording the desired mono-arylation
product with high arylation selectivity. Especially noteworthy
is the finding that fluoro (entry 6), chloro (entry 7) and bromo
(entry 8) substituents on the silane component can be
employed, providing valuable handles for further manipu-
lation. Of note, these results provide the first example of an
aryl bromide tolerated in C(sp2)–H direct Hiyama cross-coup-
ling using N-coordinating groups.5 Selectivity for halides
(entries 7 and 8) is complementary to Ru(II)/carboxylate
systems,6,11 paving the way for sequential techniques to con-
struct polyarenes.
Preliminary mechanistic studies were conducted (Schemes
1–3). (1) In agreement with silane activation, intermolecular
competition studies with different arylsilanes revealed that
electron deficient silanes react preferentially (Scheme 1A). (2)
Electron-withdrawing groups on the arene facilitate C–H ary-
lation (Scheme 1B). Selectivity trends observed in the aryla-
tion of electronically-differentiated substrates (Table 1)
provide additional insights into the arylation selectivity. (3)
Deuterium incorporation studies revealed reversibility of the
cycloruthenation step (Scheme 2).6 Most notably, competition
experiments reveal that N-coordination leads to significantly
improved reactivity than O-coordination10 under oxidative
Ru(II)-Hiyama conditions (Scheme 3). Moreover, the utility of
this new Hiyama direct coupling is highlighted by the fact
that the attempted Ru(II)-catalyzed arylation of N-heterocycles
with boronic acids proceeds with low efficiency (Scheme 4,
see ESI† for details). Further studies to elucidate the mechan-
a N-heterocycle (1.0 equiv.), [RuCl2(p-cymene)]2 (5 mol%), AgSbF6
(20 mol%), PhSi(OMe)3 (3.0 equiv.), CuF2 (3.5 equiv.), DCE (0.20 M),
140 °C, 20 h. b Isolated yields. c PhSi(OMe)3 (0.50 equiv.). d PhSi(OMe)3
(1.0 equiv.). Selectivity is indicated in brackets. See ESI for full details.
these examples, significant electronic effects favoring arylation ism are ongoing.
of electron-deficient arenes (3e vs. 3d) are observed (vide infra).
In summary, we have developed the first direct
The sterically-demanding ortho-methyl substrate underwent ruthenium(II)-catalyzed oxidative C–H Hiyama cross-coupling
arylation in good yield (entry 6, 3f). Moreover, monoarylation method for regioselective arylation of diverse N-heterocycles
is selectively achieved with (1) substrates bearing ortho-substi- with aryl silanes by N-coordination. This strategy is character-
tuents on the pyridine ring (entry 7, 3g); (2) substrates featur- ized by low toxicity of arylsilanes, the functional group toler-
ing meta-substitution on the arene17 (entry 8, 3h), representing ance for aryl halides, and broad substrate scope with respect to
another strategy to achieve selective mono-arylation. both components. The protocol provides an operationally-
Furthermore, substrates bearing other heterocycles such as simple method to produce a wide range of valuable hetero-
sterically-demanding naphthalene (entry 9, 3i), electronically- cyclic biaryls that are particularly common in natural products
biased benzothiophene (entry 10, 3j), as well as directing and pharmaceuticals under mild oxidative conditions with
groups such as pyrazole (entry 11, 3k), and quinoline (entry 12, 3l) user-friendly Ru(II). This reaction employs CuF2 as a dual
were successfully arylated to deliver the C–H arylation pro- silane activator/Ru re-oxidant, and no added ligand or additive
ducts in high yields. Of note, our results provide the first is necessary. We anticipate that this study will set the stage for
example of a direct arylation of 2-pyrimidyl-benzothiophene the discovery of numerous valuable C–H bond functionali-
(3j), providing a modular access to benzothiophenes that are zation reactions with versatile ruthenium(II) catalysts.
prevalent in functional material applications.18 Taken
Financial support was provided by Rutgers University. The
together, these results clearly demonstrate that this Ru(II)-cata- Bruker 500 MHz spectrometer used in this study was sup-
lyzed N-directed Hiyama protocol might set the stage for the ported by the NSF-MRI grant (CHE-1229030). We thank SEED
development of numerous C–H arylation methods with versa- Grant (Rutgers University) to the Center for Sustainable
tile Ru(II) catalysts.
Synthesis for partial support of this project.
This journal is © The Royal Society of Chemistry 2017
Org. Biomol. Chem.

View Article Online
Communication
Organic & Biomolecular Chemistry
Table 3 Ruthenium(II)-catalyzed C–H arylation of N-heterocycles with various organosilanesa
Entry
1
Organosilane
2
Product
3
Yielda (%)
83
2a
3a
2
2b
3a
79
3
2c
3m
67
4
5
6
7
8
2d
2e
2f
3n
3o
3p
3q
3r
59
67
75
70
57
2g
2h
a See Table 2. Selectivity: 3m (4 : 1), 3o (2.5 : 1), all other examples >98 : 2. See ESI for full details.
Scheme 2 Deuterium incorporation studies.
Scheme 1 Mechanistic studies.
Scheme 3 Selectivity studies: N- vs. O-coordination.
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Organic & Biomolecular Chemistry
Communication
S. L. Kozushkov and L. Ackermann, Adv. Synth. Catal.,
2014, 356, 1461. In March 2017, the average prices of Rh,
Ir, Pd and Ru were 875, 690, 769, and 42 US$/oz. https://tax-
freegold.co.uk (3/3/2017).
7 R. K. Chinnagolla and M. Jeganmohan, Org. Lett., 2012, 14,
5246.
8 C. Sollert, K. Devaraj, A. Orthaber, P. J. Gates and
L. T. Pilarski, Chem. – Eur. J., 2015, 21, 5380.
9 P. Nareddy, F. Jordan, S. E. Brenner-Moyer and M. Szostak,
ACS Catal., 2016, 6, 4755.
10 P. Nareddy, F. Jordan and M. Szostak, Chem. Sci., 2017, 8,
3204.
Scheme 4 Ru(II)-catalyzed arylation of N-heterocycles using organo-
boranes. Representative examples are shown (see ESI†).
11 Select examples of Ru-catalyzed functionalization: (a) S. Oi,
S. Fukita, N. Hirata, N. Watanuki, S. Miyano and Y. Inoue,
Org. Lett., 2001, 3, 2579; (b) L. Ackermann, Org. Lett., 2005,
7, 3123; (c) L. Ackermann, A. Althammer and R. Born,
Angew. Chem., Int. Ed., 2006, 45, 2619; (d) I. Özdemir,
S. Demir, B. Cetinkaya, C. Gourlaouen, F. Maseras,
C. Bruneau and P. H. Dixneuf, J. Am. Chem. Soc., 2008, 130,
1156; (e) R. K. Chinnagolla and M. Jeganmohan, Chem.
Commun., 2014, 50, 2442; (f) R. K. Chinnagolla, A. Vijeta
and M. Jeganmohan, Chem. Commun., 2015, 51, 12992;
(g) K. Padala and M. Jeganmohan, Org. Lett., 2011, 13,
6144; (h) H. Li, W. Wei, Y. Xu, C. Zhang and X. Wan, Chem.
Commun., 2011, 47, 1497; (i) K. Devaraj, C. Sollert, C. Juds,
P. J. Gates and L. J. Pilarski, Chem. Commun., 2016, 52,
5868; ( j) F. Kakiuchi, Y. Matsuura, S. Kan and N. Chatani,
J. Am. Chem. Soc., 2005, 127, 5936; (k) F. Hu and
M. Szostak, Org. Lett., 2016, 18, 4186; (l) F. Hu and
M. Szostak, Chem. Commun., 2016, 52, 9715;
(m) M. K. Lakshman, A. C. Deb, R. R. Chamala, P. Pradhan
and R. Pratap, Angew. Chem., Int. Ed., 2011, 50, 11400;
(n) M. Simonetti, G. J. P. Perry, X. C. Cambeiro, F. Julia-
Hernandez, J. N. Arokianathar and I. Larrosa, J. Am. Chem.
Soc., 2016, 138, 3596; (o) L. Huang and D. J. Weix, Org.
Lett., 2016, 18, 5432; (p) A. Biafora, T. Krause,
D. Hackenberger, F. Belitz and L. J. Gooßen, Angew. Chem.,
Int. Ed., 2016, 55, 14752; (q) R. Mei, C. Zhu and
L. Ackermann, Chem. Commun., 2016, 52, 13171;
(r) M. Simonetti, D. M. Cannas, A. Panigrahi, S. Kujawa,
M. Kryjewski, P. Xie and I. Larrosa, Chem. – Eur. J., 2017,
23, 549.
Notes and references
1 Reviews: (a) S. E. Denmark and C. S. Regens, Acc. Chem.
Res., 2008, 41, 1486; (b) S. E. Denmark and J. H. C. Liu,
Angew. Chem., Int. Ed., 2010, 49, 2978; (c) Y. Nakao and
T. Hiyama, Chem. Soc. Rev., 2011, 40, 4893;
(d) S. E. Denmark and A. Ambrosi, Org. Process Res. Dev.,
2015, 19, 982. Select examples: (e) L. Zhang and J. Wu,
J. Am. Chem. Soc., 2008, 130, 12250; (f) S. K. Gurung,
S. Thapa, A. S. Vangala and R. Giri, Org. Lett., 2013, 15,
5378.
2 Reports
on
mutagenicity
of
boronic
acids:
(a) M. M. Hansen, R. A. Jolly and R. J. Linder, Org. Process
Res. Dev., 2015, 19, 1507; (b) M. R. O’Donovan, C. D. Mee,
S. Fenner, A. Teasdale and D. H. Phillips, Mutat. Res.,
Genet. Toxicol. Environ. Mutagen., 2011, 724, 1.
3 Select reviews: (a) J. Q. Yu, Science of Synthesis: Catalytic
Transformations via C–H Activation, Thieme, Stuttgart, 2015;
(b) R. Rossi, F. Bellina, M. Lessi and C. Manzini, Adv. Synth.
Catal., 2014, 356, 17; (c) J. Wencel-Delord and F. Glorius,
Nat. Chem., 2013, 5, 369; (d) G. Rouquet and N. Chatani,
Angew. Chem., Int. Ed., 2013, 52, 11726; (e) C. S. Yeung and
V. M. Dong, Chem. Rev., 2011, 111, 1215; (f) T. Lyons and
M. Sanford, Chem. Rev., 2010, 110, 1147.
4 Review on C–H arylation with organometallics: R. Giri,
S. Thapa and A. Kafle, Adv. Synth. Catal., 2014, 356, 1395.
5 (a) S. Yang, B. Li, X. Wan and Z. J. Shi, J. Am. Chem. Soc., 12 (a) K. M. Engle, T. S. Mei, M. Wasa and J. Q. Yu, Acc. Chem.
2007, 129, 6066; (b) H. Zhou, Y. H. Xu, W. J. Chung and
T. P. Loh, Angew. Chem., Int. Ed., 2009, 48, 5355;
(c) H. Hachiya, K. Hirano, T. Satoh and M. Miura, Angew.
Res., 2012, 45, 788; (b) J. F. Hartwig, Organotransition Metal
Chemisty: From Bonding to Catalysis, University Science
Books, 2010.
Chem., Int. Ed., 2010, 49, 2202; (d) W. Li, Z. Yin, X. Jiang 13 (a) J. Hassan, M. Sevignon, C. Gozzi, E. Schulz and
and P. Sun, J. Org. Chem., 2011, 76, 8543;
(e) N. Senthilkumar, K. Parthasarathy, P. Gandeepan and
C. H. Cheng, Chem. – Asian J., 2013, 8, 2175; (f) M. Z. Lu,
P. Lu, Y. H. Xu and T. P. Loh, Org. Lett., 2014, 16, 2614;
(g) J. He, R. Takise, H. Fu and J. Q. Yu, J. Am. Chem. Soc.,
2015, 137, 4618; (h) S. Zhao, B. Liu, B. B. Zhan,
W. D. Zhang and B. F. Shi, Org. Lett., 2016, 18, 4586.
M. Lemaire, Chem. Rev., 2002, 102, 1359; (b) A. J. Burke and
C. S. Marques, Modern Arylation Methods, Wiley-VCH,
Weinheim, 2015; (c) J. A. Garcia-Lopez and M. F. Greaney,
Chem. Soc. Rev., 2016, 45, 6766; (d) Y. Chen and
R. C. Larock, in Modern Arylation Methods, ed.
L. Ackermann, Wiley-VCH, New York, 2009, pp. 401–473.
14 J. A. Joule and K. Mills, Heterocyclic Chemistry, Wiley, 2010.
6 (a) P. B. Arockiam, C. Bruneau and P. H. Dixneuf, Chem. 15 K. M. Engle, T. S. Mei, X. Wang and J. Q. Yu, Angew. Chem.,
Rev., 2012, 112, 5879; (b) S. De Sarkar, W. Liu,
Int. Ed., 2011, 50, 1478.
This journal is © The Royal Society of Chemistry 2017
Org. Biomol. Chem.

View Article Online
Communication
Organic & Biomolecular Chemistry
16 R. D. Taylor, M. Maccoss and A. D. G. Lawson, J. Med.
Chem., 2014, 57, 5845.
M. F. Mahon, G. Kociok-Köhn, M. K. Whittlesey and
C. G. Frost, J. Am. Chem. Soc., 2011, 133, 19298.
17 Seminal example of Ru(II)-catalyzed meta-functionalization: 18 Y. Li, Organic Optoelectronic Materials, Springer,
O. Saidi, J. Marafie, A. E. W. Ledger, P. M. Liu,
2015.
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2017