Copper-mediated C(sp3)-H azidation with Me3SiN3: Synthesis of imidazoles from ketones and aldehydes

Article abstract of DOI:10.1039/c6cc01863g

The efficient construction of 2,4,5-trisubstituted imidazoles, through a copper-mediated three-component reaction involving ketones, aldehydes, and Me₃SiN₃, has been developed. During the process, 4 C-N bonds were formed sequentially. Experimental results and DFT calculations suggested that azidation of the alpha methylene group of the ketone was the key C-N bond-forming step.

Full text of DOI:10.1039/c6cc01863g

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: Z. Xie, J. Deng, Z.
Qiu, J. Li and Q. Zhu, Chem. Commun., 2016, DOI: 10.1039/C6CC01863G.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/chemcomm

Page 1 of 4
ChemComm
View Article Online
DOI: 10.1039/C6CC01863G
Journal Name
RSCPublishing
COMMUNICATION
Copper-mediated C(sp3)-H azidation with Me3SiN3:
synthesis of imidazoles from ketones and aldehydes
Cite this: DOI: 10.1039/x0xx00000x
Zeqiang Xie, ‡a Jiaojiao Deng, ‡b, Zhiping Qiu,b Juan Li*b and Qiang Zhu*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
experimental results as well as DFT calculations (Scheme 1). In this
reaction, imidazole rather than the expected oxazole was obtained,
and two nitrogen atoms were introduced from Me3SiN3 by four CꢀN
bonds formation.
An efficient construction of 2,4,5-trisubstituted imidazoles,
through copper-mediated three-component reaction involving
ketones, aldehydes, and Me3SiN3, has been developed. During
the process,
4 C-N bonds were formed sequentially.
The scaffold of imidazole exists in many natural products and
pharmaceutical agents with broad biological activities, such as
antimicrobial, antiꢀneoplastic, antiꢀanxiety, and antiꢀinflammatory
and consequently, much attention has been paid to their synthesis.15
The synthesis of densely substituted imidazoles is mainly focused on
threeꢀcomponent condensation of 1,2ꢀdiketones, aldehydes, and
ammonia or modification of existing imidazole core by transitionꢀ
metalꢀcatalyzed crossꢀcoupling reactions.16 However, developing a
general procedure for imidazole synthesis starting from readily
available substance is still desirable.
Experimental results and DFT calculations suggested that
azidation of the alpha methylene group of ketone was the key
C-N bond-forming step.
In recent years, transitionꢀmetalꢀcatalyzed direct CꢀH bond
amination/amidation has been intensively studied due to the great
importance of nitrogenꢀcontaining compounds in almost all areas of
chemical research.1 In terms of nitrogen sources, nonꢀactivated NꢀH
free amines/amides under oxidative conditions or preꢀactivated NꢀX
species in the absence of oxidant are normally applied.2 Organic
azide is increasingly used as an alternative nitrogen source in
directing group assisted intermolecular CꢀH amination/amidation
catalyzed by Rh,3 Ir,4 Ru,5 Co,6 Pd,7 and Cu,8 in which N2 gas is
generated as the only byproduct under oxidantꢀfree conditions.9
Besides, functionalized organoazide is also applied in intramolecular
CꢀH amination/amidation, which offers a new approach for the
construction of Nꢀheterocycles. For instance, a variety of Nꢀ
heterocycles including pyrroles, indoles, carbazoles, and
benzoimidazoles were accessed starting from vinylꢀ or arylazides,
developed by Driver et al.10 Alternatively, Nꢀheterocycles could also
be constructed through oneꢀpot sequential CꢀH azidation/amination
using organic or inorganic azide followed by intramolecular CꢀN or
NꢀN bond formation. By using this strategy, Jiao et al developed an
efficient synthesis of pyrido[1,2ꢀb]indazoles via tandem C(sp2)ꢀH
azidation and denitrogenative NꢀN bond formation coꢀcatalyzed by
Pd and Fe (Scheme 1).11a The same group also developed a novel
Cuꢀcatalyzed anilinic NH2 directed C(sp2)ꢀH azidation to afford 2ꢀ
azidoꢀanilines which could be further transformed to benzimidazoles
with aldehydes.12a However, synthesis of Nꢀheterocycles through
azidation of C(sp3)ꢀH bond was scarce in literature.13 Herein, we
report a copperꢀmediated threeꢀcomponent reaction for the synthesis
of 2,4,5ꢀtrisubstituted imidazoles involving ketones, aldehydes, and
Me3SiN3. Unlike our recent report on the construction of
imidazo[1,5ꢀa]pyridines through amination of benzylic C(sp3)ꢀH
bond,14 azidation of the Alpha CꢀH bond of ketone was proposed as
the initial CꢀN bondꢀforming step, which was supported by
Scheme 1. CꢀH azidation for the construction of Nꢀheterocycles
.
Continuing our interest in heterocycle synthesis with
azide,8aꢀb,14 we initiated the study by reacting 1,2ꢀ
diphenylethanone 1a with
pꢀmethylbenzaldehyde 2a in the
presence of Me3SiN3 under the reaction conditions we reported
recently.14 To our surprise, an imidazole derivative 3a, rather
than the expected oxazole product, was isolated in 32% yield
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 1

ChemComm
Page 2 of 4
View Article Online
COMMUNICATION
Journal Name
DOI: 10.1039/C6CC01863G
(entry 1, Table 1). Control experiments indicated that the studied (Scheme 2). Benzaldehydes substituted with either
copper salt was essential and PivOH was important for this electronꢀdonating OMe or electronꢀwithdrawing Cl, Br, NO2 in
transformation (entries 2ꢀ3). Screening of copper species and the para position reacted smoothly with 1a to provide the
acids revealed that the initial use of Cu(TFA)2xH2O and PivOH corresponding imidazoles 3cꢀ3f in 60ꢀ66% yields. The
was the best combination (entries 6ꢀ11). It was notable that oxidatively labile 3,4,5ꢀtrimethoxybenzaldehyde also survived
when the reaction was performed in air or O2, the formation of the reaction (3j, 73%). In addition, substituents in the ortho
3a was completely inhibited, and 1,2ꢀdiketone, the oxidation position of benzaldehydes did not affect the product formation
product of 1a, was isolated as the major byproduct. Considering
(3g, 3h), suggesting that the reaction was not sensitive to the
this side reaction, the amount of 1a was increased, and as a electronic nature nor the steric hindrance of aldehyde. 3,3ꢀ
result, the isolated yield of 3a was slightly improved (entry 12). Diphenylacrylaldehyde and thiopheneꢀ2ꢀcarbaldehyde were
When a solution of Me3SiN3 in DCB was introduced to the also compatible with the reaction conditions, delivering 2ꢀ
reaction mixture via a syringe pump during 1.25 h, the yield of vinylimidazole 3k and 2ꢀthiophenylimidazole 3l in 48% and 58%
3a was improved significantly to 61% (entry 13). Finally, the yields, respectively. Notably, aliphatic aldehydes could afford
novel imidazole synthesis reached a satisfactory level when 1.2 the corresponding alkylated imidazoles (3m, 3n) as well, albeit
equiv of Cu(TFA)2xH2O was used (82%, entry 14). It was in lower yields (20ꢀ34%). Next, reactions of benzaldehyde with
noteworthy that no byꢀproduct, derived from either Schmidt diverse substituted acetophenones were investigated. 1ꢀArylꢀ2ꢀ
reaction17 or Jiao’s reaction18 through CꢀC bond cleavage, was phenylethanones substituted with paraꢀOMe, Cl, or Br groups
detected.
reacted smoothly to furnish 2,4,5ꢀtriaryl imidazoles 3oꢀ3q as
Table 1. Optimization of the reaction condtions.a
mixtures of tautomers in good yields, which showed good
functional group compatibility. Besides, 1ꢀphenylꢀ2ꢀ
arylethanones substituted with electron donating (paraꢀOMe)
and electron withdrawing
(paraꢀester) groups gave the
corresponding products in good yields (3o, 3r). However, a
NO2 group in the same position gave the corresponding product
3s in a lower yield of 45%. In addition, propiophenone could
also be served as the reaction partner, giving 3u in synthetically
useful yield.
entry
Copper salts (eq.)
Additive
(1.0 eq)
1a/2a Yieldb
1
Cu(TFA)2xH2O (1.0)
No copper salt
PivOH
PivOH
No acid
PivOH
PivOH
PivOH
PivOH
PivOH
TFA
1/1
1/1
1/1
1/1
1/1
1/1
1/1
1/1
1/1
1/1
1/1
2/1
2/1
2/1
1.5/1
2.5/1
32 %
n.d.
2
3
Cu(TFA)2xH2O (1.0)
Cu(TFA)2xH2O (1.0)
Cu(TFA)2xH2O (1.0)
Cu(OAc)2 (1.0 )
21%
trace
n.d.
4c
5d
6
trace
trace
trace
21%
trace
trace
34%
61%
82%
54%
65%
7
CuCl2 (1.0)
8
CuI (1.0)
9
Cu(TFA)xH2O (1.0)
Cu(TFA)xH2O (1.0)
Cu(TFA)xH2O (1.0)
Cu(TFA)xH2O (1.0)
Cu(TFA)xH2O (1.0)
Cu(TFA)xH2O (1.2)
Cu(TFA)xH2O (1.2)
Cu(TFA)xH2O (1.2)
10
11
12
13e
14e
15e
16e
TsOH
PivOH
HOTf
PivOH
PivOH
PivOH
PivOH
a Reaction conditions: 1a (0.2ꢀ0.5 mmol), 2a (0.2 mmol), DCB (2 mL),
110 oC under Ar for 6 h. bYields of isolated product. c At 100 oC for 24 h. d
In O2 (balloon). e A solution of Me3SiN3 in 0.5 mL of DCB was injected
via a syringe pump during 1.25 h, then the mixture was stirred for 4.75 h.
DCB = o-dicholorobenzene. PivOH = pivalic acid. TFA = trifuloro acetic
acid.
TsOH
=
p-methybenzeneosulfonic
acid.
HOTf
=
Scheme 2. Substrate Scope. Reaction conditions:
mmol), PivOH (0.2 mmol), Cu(TFA)2.xH2O (0.24 mmol), DCB (2.0
1 (0.4 mmol), 2 (0.2
trifluoromethanesulfonic acid.
With the optimal reaction conditions in hand, the scope of mL) and Me3SiN3 (0.6 mmol, 3.0 equiv), a solution of Me3SiN3 in 0.5
mL of DCB was injected via a syringe pump during 1.25 h, then the
both aldehydes and ketones containing αꢀmethylene moiety was
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 3 of 4
Journal Name
ChemComm
View Article Online
COMMUNICATION
DOI: 10.1039/C6CC01863G
o
mixture was stirred stirred for 4.75 h, 110 C, in argon. a Starting from
b
1ꢀ(4ꢀmethoxyphenyl)ꢀ2ꢀphenylethanone.
methoxyphenyl)ꢀ1ꢀphenylethanone.
Starting from 2ꢀ(4ꢀ
b Department of Chemistry, Jinan University, Huangpu Road West 601,
Guangzhou 510632(China); Eꢀmail: tchjli@jnu.edu.cn
Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/c000000x/
To gain insights into the reaction mechanism, several
possible intermediates derived from ꢀmethylbenzaldehyde 2a
were investigated in reactions with 1,2ꢀdiphenylethanone 1a
under the same reaction conditions (Scheme 3). First, 4ꢀ
methylbenzonitrile 4a, a potential intermediate from Schmidt
reaction of 2a in the presence of Me3SiN3 and acid, was tested.
p
1
(a) R. Hili, A. K. Yudin, Nat. Chem. Biol. 2006, 2, 284; (b) Amino
However, only 1,2ꢀdiketone 5a, an oxidation product of 1a
,
was detected together with unconsumed 1a and 4a. Similar
phenomena were observed with 4ꢀmethylbenzamide 4b, 4ꢀ
methylbenzoic acid 4c, or 4ꢀmethylbenzoyl azide 4d as
reactants in place of aldehyde 2a, ruling out the possibility of
their being reaction intermediates (Scheme 3). Reaction of
1,2ꢀdiketone 5a with aldehyde 2a failed to lead to the
formation of 3a. When 2ꢀazidoꢀ1,2ꢀdiphenylethanꢀ1ꢀone 5b
was subjected to the standard reaction with 2a, the expected
imidazole 3a was isolated in 74% yield. It is known that
copper salt can promote the transformation of ketone or
aldehyde to its imine derivative in the presence of azide.19
Therefore, azidation of the methylene group in 1,2ꢀ
diphenylethanone or its imine intermediate is proposed as the
key step in the current imidazole synthesis. However, direct
amination of the methylene group, similar to our recent report
Group Chemistry: From Synthesis to the Life Sciences (Ed., A.
Ricci,), WileyꢀVCH, Weinheim, Germany, 2007; (c) P. A. Gale,
Acc. Chem. Res. 2006, 39, 465.
2
(a) S. H. Cho, J. Y. Kim, J. Kwak, S. Chang, Chem. Soc. Rev. 2011,
40, 5068; (b) T. R. M. Rauws, B. U. W. Maes, Chem. Soc. Rev.
2012, 41, 2463; (c) Z. Shi, C. Zhang, C. Tang, N. Jiao, Chem. Soc.
Rev. 2012, 41, 3381; (d) C. Zhang, C. Tang, N. Jiao, Chem. Soc.
Rev. 2012, 41, 3464; (e) G. Song, F. Wang, X. Li, Chem. Soc. Rev.
2012, 41, 3651; (f) J. Bariwal, E. Van der Eycken, Chem. Soc. Rev.
2013, 42, 9283; (g) M. L. Louillat, F. W. Patureau, Chem. Soc. Rev.
2014, 43, 901.
3
(a) J. Y. Kim, S. H. Park, J. Ryu, S. H. Cho, S. H. Kim, S. Chang, J.
Am. Chem. Soc. 2012, 134, 9110; (b) J. Ryu, K. Shin, S. H. Park, J.
Y. Kim, S. Chang, Angew. Chem., Int. Ed. 2012, 51, 9904; (c) K.
Shin, Y. Baek, S. Chang, Angew. Chem., Int. Ed. 2013, 52, 8031;
on the synthesis of imidazo[1,5ꢀa]pyridine through benzylic
C(sp3)ꢀH bond amination,14 cannot be ruled out. To confirm
that C(sp3)−H azidation product 5b was a plausible reaction
intermediate, density functional theory (DFT) calculations
were conducted.20
(d) J. Jeong, P. Patel, H. Hwang, S. Chang, Org. Lett. 2014, 16
,
4598; (f) H. Hwang, J. Kim, J. Jeong, S. Chang, J. Am. Chem. Soc.
2014, 136, 10770.
4
(a) J. Kim, J. Kim, S. Chang, Chem. Eur. J. 2013, 19, 7328; (b) V.
S. Thirunavukkarasu, K. Raghuvanshi, L. Ackermann, Org. Lett.
2013, 15, 3286; (c) M. Bhanuchandra, M. R. Yadav, R. K. Rit, M.
R. Kuram, A. K. Sahoo, Chem. Commun. 2013, 49, 5225; (d) M. R.
Yadav, R. K. Rit, A. K. Sahoo, Org. Lett. 2013, 15, 1638; (e) Q. Z.
Zheng, Y. F. Liang, C. Qin, N. Jiao, Chem. Commun. 2013, 49
5654; (f) K. Shin, J. Ryu, S. Chang, Org. Lett. 2014, 16, 2022; (g)
E. Brachet, T. Ghosh, I. Ghosh, B. Konig, Chem. Sci. 2015, , 987.
,
6
Scheme 3. Investigation of possible reaction intermediates.
5
6
(a) J. Ryu, J. Kwak, K. Shin, D. Lee, S. Chang, J. Am. Chem. Soc.
2013, 135, 12861; (b) J. Kim, S. Chang, Angew. Chem., Int. Ed.
2014, 53, 2203; (c) T. Kang, Y. Kim, D. Kee, Z. Wang, S. Chang, J.
Am. Chem. Soc. 2014, 136, 4141; (d) K. Shin, S. Chang, J. Org.
Chem. 2014, 79, 12197; (e) H. Kim, J. Park, J. G. Kim, S. Chang,
Org. Lett. 2014, 16, 5466.
In conclusion, we developed an efficient method for the
synthesis of triꢀsubstituted imidazoles starting from simple
acetaphenone derivatives and aldehydes, which are readily
available. In this process, two nitrogen atoms derived from
Me3SiN3 were formally inserted to the target molecule by 4 Cꢀ
N bonds formation. Azidation of the sp3 hybridized CꢀH bond
is the key step for this multiple CꢀN bondꢀforming sequence,
suggested by experimental results and DFT calculations.
(a) B. Sun, T. Yoshino, S. Matsunaga, M. Kanai, Adv. Synth. Catal.
2014, 356, 1491; (b) B. Sun. T. Yoshino, S. Matsunaga, M. Kanai,
Chem. Commun. 2015, 51, 4649.
7
8
Z. Hu, S. Luo, Q. Zhu, Sci. Chin. Chem. 2015, 58, 1349.
(a) J. Peng, M. Chen, Z. Xie, S. Luo, Q. Zhu, Org. Chem. Front.
Acknowledgement
We are grateful for the financial support of the National Science
Foundation of China (21272233, 21472190, 21573095), the
Fundamental Research Funds for the Central Universities (Grant
No. 21615405) and the highꢀperformance computing platform of
Jinan University.
2014, 1, 777; (b) J. Peng, Z. Xie, S. Luo, Q. Zhu, Org. Lett. 2014,
16, 4702; (c) R. R. Donthiri, V. Pappula, N. N. K. Reddy, D.
Bairagi, S. Adimurthy, J. Org. Chem. 2014, 79, 11277; (d) Y. M.
Badiei, A. Dinescu, X. Dai, R. M. Palomino, F. W. Heinemann, T.
R. Cundari, T. Warren, Angew. Chem., Int. Ed. 2008, 47, 9961.
(a) S. Cenini, E. Gallo, A. Caselli, F. Ragaini, S. Fantauzi, C.
Piangiolino, Coord. Chem. Rev. 2006, 250, 1234; (b) S. Brase, C.
Notes and references
9
‡
These authors contributed equally to this work.
a State Key Laboratory of Respiratory Disease, Guangzhou Institutes of
Biomedicine and Health, Chinese Academy of Sciences, 190 Kaiyuan
Avenue, Guangzhou 510530 (China), Fax: (+86) 20ꢀ3201ꢀ5299.
Eꢀmail: zhu_qiang@gibh.ac.cn
Gil, K. Knepper, V. Zimmerman, Angew. Chem., Int. Ed. 2005, 44
,
5188; (c) S. Lang, J. A. Murphy, Chem. Soc. Rev. 2006, 35, 146; (d)
K. Shin, H. Kim, S. Chang, Acc. Chem. Res. 2015, 48, 1040.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 3

ChemComm
Page 4 of 4
View Article Online
COMMUNICATION
Journal Name
DOI: 10.1039/C6CC01863G
10 (a) M. Shen, T. G. Driver, Org. Lett. 2008, 10, 3367; (b) T. G.
Driver, Org. Biomol. Chem. 2010, , 3831; (c) B. J. Stokes, T. G.
8
Driver, Eur. J. Org. Chem. 2011, 4071; (d) H. Dong, R. T. Latka, T.
G. Driver, Org. Lett. 2011, 13, 2726; (e) A. L. Pumphrey, H. Dong,
T. G. Driver, Angew. Chem., Int. Ed. 2012, 51, 5920; (f) Q. Nguyen,
K. Sun, T. G. Driver, J. Am. Chem. Soc. 2012, 134, 7262; (g) Q.
Nguyen, T. Nguyen, T. G. Driver, J. Am. Chem. Soc. 2013, 135
620.
,
11 (a) Q. Z. Zheng, P. Feng, Y. F. Liang, N. Jiao, Org. Lett. 2013, 15
.
4262; (b) F. Xie, Z. S. Qi, X. W. Li, Angew. Chem. Int. Ed. 2013,
52, 45; (c) N. Khatun, A. Modi, W. Ali, B. K. Patel. J. Org. Chem.
2015, 80, 9662.
12 (a) C. Tang, N. Jiao, J. Am. Chem. Soc. 2012, 134, 18924; (b) Y.
Peng, W. Wan, G. Ma, W. Gao, H. Jiang, S. Zhu, J. Hao, Chem.
Commum. 2014, 50, 5733; (c) M. Devulapally, S. Pradeep, P.
Tharmalingam, J. Org. Chem. 2015, 80, 1644.
13 For examples of azidation of C(sp3)ꢀH bond: (a) A. Sharma, J. F.
Hartwig, Nature, 2015, 517, 600; (b) X. Huang, T. M. Bergsten, J.
T. Groves, J. Am. Chem. Soc. 2015, 137, 5300; (c) X. Zhang, H.
Yang, P. Tang, Org. Lett. 2015, 17, 5828.
14 Z. Xie, J. Peng, Q. Zhu. Org. Chem. Front. 2016, 3, 82.
15 (a) M. R. Grimmett, In Comprehensive Heterocyclic Chemistry; A.
R. Katritzky, C. W. Rees, Eds.; Pergamon: Oxford, UK, 1984; Vol.
5
, pp 345−499; (b) M. R. Grimmet, In Comprehensive Heterocyclic
Chemistry II; A. R. Katritzky, C. W. Rees, E. F. V. Scriven,; Vol. 3
(Eds. Pergamon) Oxford, UK, 1996; Vol. 3, pp 79ꢀ220; (c) J.
Dupont, R. F. de Souza, P. A. Z. Suarez, Chem. Rev. 2002, 102
,
3667; (d) F. Bellina, S. Cauteruccio, R. Rossi, Tetrahedron 2007, 63
,
4571.
16 For selected synthesis of imidazole derivatives: (a) C. P. Caaveiro, J.
P. Sestelo, M. M. Martínez, L. A. Sarandeses, J. Org. Chem. 2014,
79, 9586; (b) R. Rossi, F. Bellina, M. Lessi, Adv. Synth. Catal. 2012,
354, 1181; (c) L. Revesz, F. E. Di Padova, T. Buhl, R. Feifel, H.
Gram, P. Hiestand, U. Manning, R. Wolf, A. G. Zimmerlin, Bioorg.
Med. Chem. Lett. 2002, 12, 2109; (d) Y.ꢀM. Wang, H.ꢀH. Zhang, C.
Li, T. Fan, F. Shi, Chem. Commun. 2016, 52, 1804; (e) Q.ꢀH. Li, L.
Wei, X. Chen, C.ꢀJ. Wang, Chem. Commun. 2013, 49, 6277.
17 Named Organic Reactions, Vol. 2 (Eds.: T. Laue, A. Plagens), John
Wiley & Sons: Chichester, England, New York, 2005, pp. 320.
18 (a) F. Chen, T. Wang, N. Jiao, Chem. Rev. 2014, 114, 8613; (b) T.
Wang, N. Jiao, Acc. Chem. Res. 2014, 47, 1137; (c) C. Tang, N.
Jiao, Angew. Chem., Ed. Int. 2014, 53, 6528; (d) P. Feng, X. Sun, Y.
Su, X. Li, L. H. Zhang, X. Shi, N. Jiao, Org. Lett. 2014, 16, 3388; (e)
Y. Liang, Y. F. Liang, N. Jiao, Org. Chem. Front. 2015, 2, 403.
19 (a) Z. Li, X. Huang, F. Chen, C. Zhang, X. Wang, Ning Jiao, Org.
Lett. 2015, 17, 584; (b) F. C. Jia, C. Xu, Z. W. Zhou, Q. Cai, D. K.
Li, A. X. Wu, Org. Lett. 2015, 17, 4236.
20 See supporting information for detail.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012