Arylative cyclization of 2-isocyanobiphenyls with anilines: One-pot synthesis of 6-arylphenanthridines via competitive reaction pathways

Article abstract of DOI:10.1021/ol500923t

A transition-metal-free method for the synthesis of C6 phenanthridine derivatives by arylative cyclization of 2-isocyanobiphenyls with arylamines in one pot was developed. Mechanistic studies suggest that electrophilic aromatic substitution (S_EAr) of a nitrilium intermediate and homolytic aromatic substitution (HAS) of an imidoyl radical intermediate are two competitive reaction pathways involved in the annulation step.

Full text of DOI:10.1021/ol500923t

Letter
pubs.acs.org/OrgLett
Arylative Cyclization of 2‑Isocyanobiphenyls with Anilines: One-Pot
Synthesis of 6‑Arylphenanthridines via Competitive Reaction
Pathways
Zhonghua Xia, Jinbo Huang, Yimiao He, Jiaji Zhao, Jian Lei, and Qiang Zhu*
State Key Laboratory of Respiratory Disease, Guangzhou Institutes of Biomedicine and Health, Chinese Academy of Sciences, 190
Kaiyuan Avenue, Guangzhou 510530, China
S
* Supporting Information
ABSTRACT: A transition-metal-free method for the synthesis
of C6 phenanthridine derivatives by arylative cyclization of 2-
isocyanobiphenyls with arylamines in one pot was developed.
Mechanistic studies suggest that electrophilic aromatic
substitution (SEAr) of a nitrilium intermediate and homolytic
aromatic substitution (HAS) of an imidoyl radical intermediate
are two competitive reaction pathways involved in the annulation step.
he synthetic versatility of isocyanide is illustrated by the
groups of Yu6e and Zhou6f reported their synthesis of C6-
1
TUgi, Passerini, and related multicomponent reactions, alkylated and -trifluoromethylated phenanthridines via the
transition-metal-catalyzed imidoylation,2 and radical-based
imidoyl radical intermediates under photoredox neutral and
metal-free conditions, respectively. Intramolecular homolytic
aromatic substitution7 (HAS, step a, Scheme 1) of the imidoyl
radical on the pending phenyl ring, forming the cyclohexadienyl
radical intermediate, was proposed as a common step in these
reactions. Subsequently, there are two possible pathways
directed toward the formation of phenanthridine products.
One is single electron transfer (SET, step b) oxidation to afford
cyclohexadienyl cation followed by deprotonative aromatization
(step c), and the other is deprotonation of the cyclohexadienyl
radical first (step d) followed by SET (step e). Our recent study
disclosed that SET oxidation of biphenyl imidoyl radical to
biphenyl nitrilium (step f) was a competitive pathway to HAS
in an arylative cyclization of 2-isocyanobiphenyls using in situ
generated diazonium salts as arylation agents.
transformations as well. Addition of a carbon- or heteroatom-
centered radical to the terminal carbon of isocyanide generates
a geminal carbon radical intermediate, the imidoyl radical
(R1NC•R2).3 Subsequently, the resulting imidoyl radical can
add to double or triple carbon−carbon bonds, as well as onto
intramolecular aromatic rings to construct various nitrogen-
containing heterocycles. At least two chemical bonds are
formed sequentially in this process. This highly efficient bond-
forming strategy has been elegantly applied to the synthesis of
camptothecin by Curran.4 Recently, several groups have
successively reported their efforts to synthesize phenanthridine
derivatives5 through key biphenyl imidoyl radical intermediates,
formed by addition of various radicals to 2-isocyanobiphenyls
(Scheme 1). For example, in 2012, Chatani6a reported the first
example of using aryl/alkyl boronic acids as radical precursors
in the presence of 3 quivalents of Mn(acac)3 to synthesize 6-
aryl/alkyl phenanthridines. Later on, Studer and co-workers
demonstrated that trifluoromethyl,6b aroyl,6c and phosphoryl
radicals6d can react with 2-isocyanobiphenyls, affording
corresponding C6 diversified phenanthridines. Independently,
Diazonium salts,8 presynthesized or generated in situ from
anilines, are widely used as precursors of aryl radicals, as
exemplified in the well-known Sandmeyer reactions9 and
Meerwein arylation.10 Recently, Grimaud11a and our group11b
successively described a novel preparation of arylcarboxyamides
from diazonium salts and isocyanides under transition-metal-
free conditions. It was believed that the reaction was sequenced
by aryl radical generation, radical addition to isocyanide, SET
oxidation of the resulting imidoyl radical, nucleophilic addition
to the nitrilium intermediate, and tautomerization in the case of
water as a nucleophile. This process suggests that when an
intra- or intermolecular radical acceptor is absent, oxidation of
imidoyl radical to nitrilium takes place. As these imidoyl
radicals (R1NC•R2) are rather electron rich,3 they have great
potential to be oxidized to the nitrilium ions (R1N+ ≡ CR2) by
SET process.11 It is anticipated that in a reaction of diazonium
Scheme 1. Reaction Pathways of Biphenyl Imidoyl Radical
Received: March 29, 2014
Published: April 22, 2014
© 2014 American Chemical Society
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Letter
salts with 2-isocyanobiphenyls where an intramolecular radical
trap is present, the intramolecular HAS will compete with
intermolecular SET followed by electrophilic aromatic
substitution (SEAr, Scheme 1). Our study demonstrates that
in situ formed aryl diazonium salts can react smoothly with 2-
isocyanobiaryls to afford 6-aryl phenanthridines in the absence
of transition metals. In addition, mechanistic studies suggest
that intermolecular SET followed by intramolecular SEAr via
the nitrilium intermediate is a competitive pathway to HAS.
Initially, 2-isocyano-5-methyl-1,1′-biphenyl 1a was chosen to
react with p-tolyldiazonium salt, generated in situ from p-
toluidine 2a and t-BuONO 3 in PhCF3 at 80 °C in argon (entry
1, Table 1). To our delight, the desired C6 tolyl phenanthridine
Next, the scope of anilines as well as heteroaromatic amines
was studied in reactions with 2-isocyano-5-methyl-1,1′-biphenyl
1a under the optimal reaction conditions as described in entry
11, Table 1. 2-Methyl-6-phenylphenanthridine 4b was obtained
in almost quantitative yield using unsubstituted aniline as an
arylation agent (Scheme 2). Anilines bearing electron-donating
a
Scheme 2. Scope of Arylamines
a
Table 1. Optimization of the Reaction Conditions
b
promoter
(mol %)
base
(1.1 equiv)
ratio of
1a:2a:3
temp
(°C)
yield
(%)
entry
1
2
3
4
5
6
7
none
1:1.2:1.5
1:1.2:1.5
1:1.2:1.5
1:3:3.5
80
80
11
22
41
64
62
55
81
BPO (20)
BPO (2)
BPO (2)
none
110
70
a
Reaction conditions: 1a (0.2 mmol), 2 (3 equiv) and 3 (3.5 equiv) in
1.0 mL of PhCF3 at 70 °C, in Ar, for 3 h.
NaOAc
1:3:3.5
70
none
1:3:3.5
70
OCH3 (4c) and tert-butyl (4d) as well as electron-withdrawing
Br (4e), COCH3 (4f), CO2Et (4g), and NO2 (4h) groups on
the para position reacted smoothly with 1a, delivering
corresponding arylated phenanthridines in good to excellent
yields. A substituent on the ortho position of aniline had a
negative effect on this one-pot arylative cyclization reaction (4k
vs 4a and 4i). It was notable that iodide also survived the
reaction (4l), making further diversification possible. The
synthesis of these iodine-containing phenanthridines is a
challenging task by using transition-metal-based methods.
Heterocycle fused anilines were also compatible with the
reaction conditions (4m−n). In addition, aromatic hetero-
cycles, such as pyridine, quinolone, and thiazole, can be
incorporated into the phenanthridine scaffold by using
corresponding heteroaromatic amines, albeit in lower yields
(4o−q).
Substitution effect on both aromatic rings of biphenyl
isocyanide 1 was then investigated in reactions with aniline 2b
(Scheme 3). 2-Isocyanobiphenyls containing substituents, such
as Cl, F, OMe, and Ph, on the para position of the
nonisocyanide phenyl ring reacted efficiently, affording
corresponding 6-phenyl phenanthridine derivatives in yields
ranging from 61 to 84% (4r−v). An electron-deficient pyridine
moiety in place of phenyl ring also cyclized to construct a
benzo[c][2,7]naphthyridine scaffold in low yield (26%, 4w).
Good yields were reported in Studer and Yu’s studies using
similar pyridine-containing substrates.6b,e The low-yielding
formation of 4w indicated that a different mechanism might
be involved in our case.15 When meta-methyl-substituted 2-
isocyanobiphenyl was subjected to the reaction, a 2.4:1 mixture
BPO (2) +
A1 (10)
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
1:3:3.5
70
8
BPO (2) + A2
(10)
1:3:3.5
1:3:3.5
1:3:3.5
1:3:3.5
70
70
70
70
76
81
71
89
9
BPO (2) + A3
(10)
10
BPO (2) + A4
(10
11 BPO (2) + A1
(20)
a
Reaction conditions: All reactions were performed with 1a (0.2
mmol), and the ratio of 1a:2a:3 is indicated, in 1.0 mL of PhCF3 at
70−110 °C as indicated, in Ar, for 3 h. Isolated yields of 4a.
b
4a was formed in 11% yield. A higher yield of 22% was
obtained when benzoyl peroxide12 (BPO, 20 mol %) was
present (entry 2). It was observed that the reaction proceeded
more efficiently in inert atmosphere than in air or oxygen (for
more details, see the Supporting Information).13 Increasing the
amount of diazonium salt relative to isocyanide 1a led to the
formation of 4a in much higher yield (64%) in the presence of
only 2 mol % of BPO at 70 °C (entry 4). Screening of bases
revealed that the combination of N,N′-dimethylethylenedi-
amine (DMEDA, A1; 10 mol %), and NaOAc (1.1 equiv)
improved the reaction dramatically,14 affording 4a in 81% yield
(entry 7). Other structurally related diamines A2−A4 cannot
improve the yield of 4a further (entries 8−10). Finally, a best
yield of 89% was achieved by increasing the amount of
DMEDA to 20 mol % (entry 11, for more details, see the
Supporting Information).
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Letter
a
Scheme 3. Scope of Biphenyl Isocyanides
Scheme 4. Mechanistic Studies
a
Reaction conditions: 1 (0.2 mmol), 2b (3 equiv) ,and 3 (3.5 equiv) in
b
1.0 mL of PhCF3 at 70 °C, in Ar, for 3 h. The ratio of the
regioisomers was 2.4:1 as determined by H NMR. The ratio was
2.8:1. Yield in 4.7 mmol scale of 1, and the reaction time was 4 h.
c
1
d
of two regioisomers was obtained in favor of the less steric
demanding C9 substituted isomer (4x). A similar ratio of 2.8:1
was determined in the case of chloro-substituted analogue (4y).
Biphenyl isocyanides containing a chloro group para or meta to
the isocyano moiety were also tolerated (4z and 4aa). A gram
scale reaction of 3-bromo-2-isocyano-5-methyl-1,1′-biphenyl
with aniline gave the desired product 4ab (1.05 g, 64%) in a
comparable yield with that of a small-scale experiment.
To confirm that aryl radical formation was the initial step of
this sequential arylative cyclization reaction, 4-nitrobenzen-
amine 2h reacted with t-BuONO 3 and 2,2,6,6-tetramethyl-1-
piperidinoxyl (TEMPO), a radical scavenger, in the absence of
2-isocyanobiphenyl 1a (Scheme 4a). 4-Nitrophenyl radical was
trapped by forming an adduct 5 in 48% yield under otherwise
identical conditions. Anilines 6 and 8 containing ortho ether
linked terminal alkenes were examined in reactions with 1a
under the standard conditions (Scheme 4b).16 Phenanthridine
derivatives 7 and 9, attached by tetrahydrobenzofuran and
tetrahydrobenzopyran with a methylene linkage, were obtained
as a result of aryl radical addition to the intramolecular C−C
double bond before reacting with isocyanide 1a. It is
noteworthy that three C−C bonds are formed sequentially in
the above one-pot process, providing a highly efficient approach
for the synthesis of multi heterocyclic systems containing a
phenanthridine moiety. These experiments undoubtedly
indicate that aryl radicals are generated in situ from anilines
and biphenyl imidoyl radical intermediates are formed by
addition of aryl radicals to 2-isocyanobiphenyls.
Interestingly, when 10 equiv of H2O was added to the
reaction between 2h and 1a under the standard conditions, the
normal cyclization product 4h was obtained in a reduced 40%
yield, together with 26% of amidated product 10. Although
both intramolecular HAS and SEAr can account for the
formation of phenanthridine product 4h, nucleophilic addition
of water to a corresponding nitrilium intermediate was the
reasonable explanation for amide formation (Scheme 4c).11 To
gain more insight into the reaction, a series of 2,6-diaryl p-
tolylisocyanides 11 in which one of the aryl rings were electron
biased were designed for intramolecular competition reaction
(Scheme 4d).17 The preference of the aryl ring to be cyclized
will hint at the nature of the cyclization precursor. The ratio of
cyclization products 12 to 13 was determined by H NMR of
1
the crude reaction mixture after routine workup. The Hammett
plot of log 12/13 versus Hammett parameter σmeta was a curve
with negative ρ value rather than a straight line (see the
Supporting Information). Such Hammett curve suggests that a
positive transition-state intermediate is involved in the
annulation step but cannot be explained by a single reaction
mechanism.18 In other words, both HAS and SEAr are highly
likely involved in cyclization.
Based on mechanistic studies and previous work on
aryldiazonium salts8 and isocyanides,1,3 two competing reaction
pathways involved in the annulation step were proposed (step a
and step f in Scheme 1, also see the Supporting Information).
Initially, aryldiazonium ion was formed in situ from anilines and
t-BuONO. Then, the aryl radical was generated by decom-
position of aryldiazonium ion with concurrent releasing of N2
and t-butoxyl radical (t-BuO•). Many factors might promote
this process,8 such as BPO,12 NaOAc, and heat. Subsequently,
the resulting aryl radical added to the terminal divalent carbon
of 2-isocyanobiphenyl 1, giving the N-biphenyl-2-yl imidoyl
radical A.3 Next, two competitive reaction pathways were
involved in the transformation of intermediate A. One was the
intramolecular HAS, as proposed by other authors in the
synthesis of phenanthridine derivatives.6 The cyclohexadienyl
radical intermediate B could undergo SET oxidation first
followed by deprotonation (steps b and c) or deprotonation
first followed by SET (steps d and e). Both pathways are
possible in this process. The other route is SET oxidation to
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Letter
nitrilium ion D11 by excess of nitrite. The nitrilium ion D could
distort to structure E, then followed by intramolecular SEAr on
the pending aryl group.19 Deprotonative aromatization of the
resulting cyclohexadienyl cation F delivered the final phenan-
thridine product G (steps f, g, and c).
Angew. Chem., Int. Ed. 2013, 52, 13289. (f) Wang, Q.; Dong, X.; Xiao,
T.; Zhou, L. Org. Lett. 2013, 15, 4846. (g) Gu, L.; Jin, C.; Liu, J.; Ding,
H.; Fan, B. Chem. Commun. 2014, 50, 4643. (h) Zhu, T.-H.; Wang, S.-
Y.; Tao, Y.-Q.; Wei, T.-Q.; Ji, S.-J. Org. Lett. 2014, 16, 1260. (i) Liu, J.;
Fan, C.; Yin, H.; Qin, C.; Zhang, G.; Zhang, X.; Yi, H.; Lei, A. Chem.
Commun. 2014, 50, 2145. (j) Xiao, T.; Li, L.; Lin, G.; Wang, Q.;
Zhang, P.; Mao, Z.-W.; Zhou, L. Green Chem. 2014, DOI: 10.1039/
C3GC42517G. (k) Janza, B.; Studer, A. Org. Lett. 2006, 8, 1875.
(7) (a) Studer, A.; Curran, D. P. Angew. Chem., Int. Ed. 2011, 50,
5018. (b) Studer, A. In Radicals in Organic Synthesis, 1st ed.; Renaud,
P., Sibi, M. P., Eds.; Wiley-VCH: Weinheim, 2001; Vol. 2, pp 62−80.
(c) Bolton, R.; Williams, G. H. Chem. Soc. Rev. 1986, 15, 261.
(d) Vaillard, S. E.; Schulte, B.; Studer, A. In Modern Arylation Methods;
Ackermann, L., Ed.; Wiley-VCH: Weinheim, 2009; pp 475−511.
(8) (a) Galli, C. Chem. Rev. 1988, 88, 765. (b) Mo, F.; Dong, G.;
Zhang, Y.; Wang, J. Org. Biomol. Chem. 2013, 11, 1582. (c) Hari, D. P.;
In summary, we have developed a transition-metal-free
synthesis of C6 phenanthridine derivatives by arylative
cyclization of 2-isocyanobiphenyls using readily available and
inexpensive anilines as arylating agents in one pot. A range of
anilines as well as heteroaromatic amines react with 2-
isocyanobiphenyls bearing various functional groups to afford
diversified C6-aryl phenanthridines in good to excellent yields.
This reaction proceeds through key biphenyl imidoyl radical
intermediates formed by addition of aryl radicals to 2-
isocyanobiphenyls. Mechanistic studies reveal that a new
reaction pathway which involves SET of biphenyl imidoyl
radical to the corresponding nitrilium intermediate followed by
SEAr is taking place as a competitive pathway to previously
reported HAS. This method provides a rapid approach to
phenanthridine derivatives under mild conditions and evidence
of a new mode of action of imidoyl radicals as well.
Konig, B. Angew. Chem., Int. Ed. 2013, 52, 4734. (d) Zollinger, H.
̈
Angew. Chem., Int. Ed. Engl. 1978, 17, 141.
(9) (a) Sandmeyer, T. Ber. Dtsch. Chem. Ges. 1884, 17, 1633.
(b) Sandmeyer, T. Ber. Dtsch. Chem. Ges. 1884, 17, 2650.
(10) (a) Meerwein, H.; Buchner, E.; van Emsterk, K. J. Prakt. Chem.
1939, 152, 237. (b) Heinrich, M. R. Chem.Eur. J. 2009, 15, 820.
(c) Doyle, M. P.; Siegfried, B.; Elliott, R. C.; Dellaria, J. F. J. Org. Chem.
1977, 42, 2431. (d) Raucher, S.; Koolpe, G. A. J. Org. Chem. 1983, 48,
2066.
ASSOCIATED CONTENT
* Supporting Information
■
(11) (a) Basavanag, U. M. V.; Dos Santos, A.; El Kaim, L.; Gam
́
ez-
S
Montano, R.; Grimaud, L. Angew. Chem., Int. Ed. 2013, 52, 7194.
̃
Experimental procedure and characterization of new com-
pounds (1H and 13C NMR spectra). This material is available
free of charge via the Internet at http://pubs.acs.org.
(b) Xia, Z.; Zhu, Q. Org. Lett. 2013, 15, 4110. (c) Russell, G. A.;
Rajaratnam, R.; Chen, P. Acta Chem. Scand. 1998, 52, 528.
(12) Although BPO decomposes at temperatures higher than 100 °C,
it is also used as a radical promoter at room temperature; see: (a) Mo,
F.; Jiang, Y.; Qiu, D.; Zhang, Y.; Wang, J. Angew. Chem., Int. Ed. 2010,
49, 1846. (b) Qiu, D.; Wang, S.; Tang, S.; Meng, H.; Jin, L.; Mo, F.;
Zhang, Y.; Wang, J. J. Org. Chem. 2014, 79, 1979.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: zhu_qiang@gibh.ac.cn.
Notes
(13) In fact, the heterolytic dediazoniation of aryldiazonium salts
prevails under O2, while the homolytic one predominates under N2,
see ref 8a,d.
The authors declare no competing financial interest.
(14) (a) Thome,
Int. Ed. 2013, 52, 7509. (b) Thome,
1892. (c) Beyer, A.; Reucher, C. M.; Bolm, C. Org. Lett. 2011, 13,
2876.
́
I.; Besson, C.; Kleine, T.; Bolm, C. Angew. Chem.,
I.; Bolm, C. Org. Lett. 2012, 14,
́
ACKNOWLEDGMENTS
This work was supported by the National Science Foundation
of China (21272233).
■
(15) The low-yielding formation of 4w is consistent with the
formation of nitrilium ion D rather than only imidoyl radical A in the
annulation step (see Scheme 1), since imidoyl radical A is rather
electron rich, whereas nitrilium ion D is the opposite.
(16) (a) Morimoto, H.; Tsubogo, T.; Litvinas, N. D.; Hartwig, J. F.
Angew. Chem., Int. Ed. 2011, 50, 3793. (b) Annunziata, A.; Galli, C.;
Marinelli, M.; Pau, T. Eur. J. Org. Chem. 2001, 1323. (c) Dai, J.-J.;
Fang, C.; Xiao, B.; Yi, J.; Xu, J.; Liu, Z.-J.; Lu, X.; Liu, L.; Fu, Y. J. Am.
Chem. Soc. 2013, 135, 8436.
REFERENCES
■
(1) (a) Domling, A.; Ugi, I. Angew. Chem., Int. Ed. 2000, 39, 3168.
̈
(b) Zhu, J. Eur. J. Org. Chem. 2003, 1133. (c) Domling, A. Chem. Rev.
̈
2006, 106, 17. (d) Lygin, A. V.; de Meijere, A. Angew. Chem., Int. Ed.
2010, 49, 9094.
(2) (a) Vlaar, T.; Maes, B. W.; Ruijter, E.; Orru, R. V. A. Angew.
Chem., Int. Ed. 2013, 52, 7084. (b) Lang, S. Chem. Soc. Rev. 2013, 42,
4867. (c) Qiu, G.; Ding, Q.; Wu, J. Chem. Soc. Rev. 2013, 42, 5257.
(3) (a) Minozzi, M.; Nanni, D.; Spagnolo, P. Curr. Org. Chem. 2007,
11, 1366. (b) Nanni, D. In Radicals in Organic Synthesis, 1st ed.;
Renaud, P., Sibi, M. P., Eds.; Wiley-VCH: Weinheim, 2001; Vol. 2, pp
44−61.
(4) (a) Curran, D. P.; Liu, H. J. Am. Chem. Soc. 1992, 114, 5863.
(b) Curran, D. P.; Ko, S.-B.; Josien, H. Angew. Chem., Int. Ed. Engl.
1995, 34, 2683.
(5) (a) Fuchino, H.; Kawano, M.; Yasumoto, K. M.; Sekita, S. Chem.
Pharm. Bull. 2010, 58, 1047. (b) Giordani, R. B.; de Andrade, J. P.;
Verli, H.; Dutilh, J. H.; Henriques, A. T.; Berkov, S.; Bastid, J.;
Zuanazzia, J. A. S. Magn. Reson. Chem. 2011, 49, 668. (c) Li, K.;
Frankowski, K. J.; Frick, D. N. J. Med. Chem. 2012, 55, 3319.
(6) (a) Tobisu, M.; Koh, K.; Furukawa, T.; Chatani, N. Angew. Chem.,
(17) (a) Stokes, B. J.; Richert, K. J.; Driver, T. G. J. Org. Chem. 2009,
74, 6442. (b) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91,
165.
(18) Hammett plots have been interpreted as evidence for dual
reaction mechanisms. For leading reports, see: (a) Swansburg, S.;
Buncel, E.; Lemieux, R. P. J. Am. Chem. Soc. 2000, 122, 6594.
(b) Ohshiro, H.; Mitsui, K.; Ando, N.; Ohsawa, Y.; Koinuma, W.;
Takahashi, H.; Kondo, S. i.; Nabeshima, T.; Yano, Y. J. Am. Chem. Soc.
2001, 123, 2478. (c) Moss, R. A.; Ma, Y.; Sauers, R. R. J. Am. Chem.
Soc. 2002, 124, 13968. (d) Zdilla, M. J.; Dexheimer, J. L.; Abu-Omar,
M. M. J. Am. Chem. Soc. 2007, 129, 11505.
(19) (a) Ishikawa, T.; Shimooka, K.; Narioka, T.; Noguchi, S.; Saito,
T.; Ishikawa, A.; Yamazaki, E.; Harayama, T.; Seki, H.; Yamaguchi, K.
J. Org. Chem. 2000, 65, 9143. (b) Talaty, E. R.; Young, S. M.; Dain, R.
P.; Van Stipdonk, M. J. Rapid Commun. Mass Spectrom. 2011, 25, 1119.
Int. Ed. 2012, 51, 11363. (b) Zhang, B.; Muck-Lichtenfeld, C.;
̈
Daniliuc, C. G.; Studer, A. Angew. Chem., Int. Ed. 2013, 52, 10792.
(c) Leifert, D.; Daniliuc, C. G.; Studer, A. Org. Lett. 2013, 15, 6286.
(d) Zhang, B.; Daniliuc, C. G.; Studer, A. Org. Lett. 2014, 16, 250.
(e) Jiang, H.; Cheng, Y.; Wang, R.; Zheng, M.; Zhang, Y.; Yu, S.
2549
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