Hydrogen-Transfer-Mediated Direct β-Alkylation of Aryl-1,8-naphthyridines with Alcohols under Transition Metal Catalyst Free Conditions

Article abstract of DOI:10.1021/acs.orglett.5b03699

By employing abundant and sustainable alcohols as the alkylating reagents, a new and direct alkylation method has been demonstrated. This method enables the selective alkylation of the less substituted pyridyl ring at the β-site of aryl-1,8-naphthyridines

Full text of DOI:10.1021/acs.orglett.5b03699

Letter
pubs.acs.org/OrgLett
Hydrogen-Transfer-Mediated Direct β‑Alkylation of Aryl-1,8-
naphthyridines with Alcohols under Transition Metal Catalyst Free
Conditions
Biao Xiong, Shudi Zhang, Huanfeng Jiang, and Min Zhang*
School of Chemistry & Chemical Engineering, South China University of Technology, Wushan Rd-381, Guangzhou 510641, People’s
Republic of China
*
S Supporting Information
ABSTRACT: By employing abundant and sustainable alcohols
as the alkylating reagents, a new and direct alkylation method has
been demonstrated. This method enables the selective alkylation
of the less substituted pyridyl ring at the β-site of aryl-1,8-
naphthyridines, affording the desired products in moderate to excellent yields upon isolation. The method proceeds under
transition-metal-free conditions in an atom- and step-economic fashion and liberates water as the sole byproduct. Mechanistic
investigations suggest the reaction undergoes a hydrogen-transfer-mediated alkylation mode.
1
3
ue to the extensive applications of alkylated heteroarenes
in both medicinal and material science, the alkylation of
restricted to applying the Friedel−Crafts reaction. Recently,
the Bruneau group has demonstrated an interesting β−C-H
D
3
14
heteroaromatic C−H bonds to access such compounds has
(sp ) alkylation of cyclic amines with aldehydes. However, the
selective β-alkylation of six-membered heteroarenes with
alcohols, to the best of our knowledge, is not known. As a
part of our continuous research interests in converting alcohols
into value-added products and developing new synthetic
methodologies under transition metal free conditions, we
1
long been an attractive subject of synthetic chemistry.
However, due to the heteroarenes possess many C−H bonds
with close dissociation energies, the control of site-selectivity
has, to date, presented a challenge. Pioneered by the Friedel−
15
2
16
Crafts alkylation, great efforts have been directed toward the
search for efficient approaches with alternative alkylating
recently reported a ruthenium-catalyzed straightforward syn-
agents. Representative examples mainly involve the directing
thesis of tetrahydronaphthyridines (THNADs) 3′ from o-
3
17
group-assisted C−H alkylation with alkyl halides, the metal-
aminopyridyl methanols 1′ and alcohols 2′ (Scheme 1, eq 1).
4
catalyzed hydroheteroarylation of olefins, carbene insertion
5
6
into heteroaromatic C−H bonds, radical alkylation including
Scheme 1. New Observation from a Previous Work
7
Minisci-type reactions, Catellani−Lautens-type coupling re-
8
9
actions and ring-opening alkylation. However, many of these
methods require the use of prefunctionalized or less environ-
mentally benign reagents in the presence of noble metal
catalysts, which could easily result in preparation difficulties and
detrimental influence on environment, and their applications to
large-scale production are therefore limited. Moreover, the
presence of noble metal catalysts would not only increase the
procedure cost but also could constitute severe drawbacks to
the methods, especially in pharmaceutical and industrial
applications. Hence, the development of step- and atom-
economic approaches to selectively alkylate heteroarenes even
under transition metal free conditions would be highly
desirable.
To gain insight into this transformation, treating intermediate
naphthyridine 1a with excess benzylic alcohol 2a gave the
transfer-hydrogenated product 3aa′ (eq 2), whereas the
absence of ruthenium complex resulted in an unexpected
alkylation compound 3aa in 32% yield (eq 3). Upon a thorough
investigation of this observation, we wish herein to report a new
Prompted by the abundance and sustainability of alcohols,
much attention has been focused on the utilization of such a
resource for various synthetic purposes, which mainly involve
the reduction of unsaturated chemical bonds with alcohols as
10
alternative hydrogen sources, the coupling reactions using the
in situ formed carbonyls arising from alcohol dehydrogen-
1
1
ation, the active C−H bond alkylation, or N-alkylation by
12
employing the hydrogen-borrowing strategy. To date, the
direct alkylation of heteroarenes with alcohols is generally
Received: December 29, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03699
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
alkylation mode, which enables us to selectively functionalize
the β-site of one pyridyl ring of the naphthyridines by using
alcohols as the alkylating reagents. The method proceeds in a
step- and atom-economic fashion, producing water as the sole
byproduct.
To determine an improved reaction system, we chose the
reaction of 1a with 2a as a model system. At the start of our
work, the effect of representative bases including organic and
inorganic ones, solvents, and different temperatures were
evaluated (see Table S1, Supporting Information). An optimal
yield of product 3aa was obtained at 130 °C and by using 1
equiv of NaOH, along with toluene as the solvent.
potential for further elaboration of functional ketones via direct
C−H bond oxidation.
18
Subsequently, we turned our attention to the variation of
aryl-1,8-naphthyridines. Thus, the combinations of different
substrates 1 with alcohols 2 were explored. Similar to the
results described in Scheme 2, all of the reactions selectively
furnished the β-alkylated products in moderate to excellent
isolated yields (Scheme 3, 3ba−hd). 2-Aryl(heteroaryl)-1,8-
Scheme 3. Variation of 1,8-Naphthyridines and Alcohols*
With the optimized reaction conditions in hand, we
examined the generality and limitations of this base-mediated
synthetic protocol. First, the reactions of 1a in combination
with a wide range of aryl(heteroaryl)methanols (see the
Supporting Information, Scheme S2, for the structures of
substrates) were examined. As shown in Scheme 2, all of the
Scheme 2. Variation of Alcohols*
*
a
Reaction conditions: same as described in Scheme 2. Isolated yield.
b
Reaction time.
naphthyridines underwent very efficient coupling reactions and
afforded the desired products in excellent yields (3ba−cb).
Further, aryl- and alkyl-disubstituted 1,8-naphthyridines (1d−f)
could afford the corresponding products in reasonable to high
yields (3da−dd,ea,fa−fb,fg). However, the reaction of 2,3-
dialkyl-1,8-naphthyridine 1g with alcohol 1a only yielded small
portion of desired product, indicating substrate 1 bearing an
aryl substituent is essential in affording a satisfactory yield. As
expected, 3-aryl-1,8-naphthyridine 1h demonstrated high
reactivity to couple with different alcohols and produced the
products in good to excellent yields (3ha−hb,hd).
To gain insight into the information on product formation,
the model reaction was interrupted after 2 h to analyze the
reaction intermediates. By means of GC−MS and NMR
analyses, both product 3aa, benzaldehyde 2a′, and tetrahe-
dronnaphthyridine 3aa′ were detected in 17%, 1%, and 1%
yields, respectively (Scheme 4, eq 4). Then, the reaction of 3aa′
with benzaldehyde 2a′ or benzylic alcohol 2a under standard
conditions failed to yield product 3aa (eq 5), indicating 3aa′
serving as a reaction intermediate can be ruled out. Further,
naphthyridine 1a also failed to couple with aldehyde 2a′ to give
3aa (eq 6). However, treating 1a with equimolar amount of 4-
methoxybenzaldehyde 2c′ and benzylic alcohol 2a gave both
products 3aa and 3ac in close yields (eq 7), showing the
alcohol plays a crucial role in activating the pyridyl ring. To
further clarify this phenomenon, the reaction employing a
*
Reaction conditions: unless otherwise stated, all reactions were
carried out at 130 °C under N atmosphere by using 1a (0.5 mmol), 2
2
a
(
0.6 mmol), NaOH (100 mol %), and toluene (1.5 mL). Isolated
b
yield. Reaction time.
reactions proceeded smoothly, and the β-alkylation occurred
selectively on the sterically less hindered pyridyl ring, affording
the desired products in moderate to excellent yields upon
isolation (3aa−ah). It was found that the electronic property of
the substituents on the aryl ring of benzylic alcohols
significantly affected the reactions. Specifically, electro-rich
substitutents afforded the products (3ab,ac) in much higher
yields than those of electron-withdrawing ones (3ad−af). This
observation implies the reaction undergoing a nucleophilic
substitution mechanism is less likely. Heteroarylmethanol 2h
demonstrated remarkable activity in affording product 3ah
(
80% yield), and its structure was confirmed by X-ray
diffraction. Gratifyingly, aliphatic alcohols 2i and 2j were also
proven to be effective coupling partners, yielding the products
in reasonable yields (see 3ai,aj). It is noteworthy that all of the
obtained products possess a benzylic position, which has the
B
DOI: 10.1021/acs.orglett.5b03699
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 4. Control Experiments
would directly afford enamine D. Considering that position 5
has a higher deuterium ratio than that of position-7 (Scheme 4,
see eq 5), the latter is thus believed to be a favorable hydrogen-
transfer process. Further, a fast nucleophilic addition of D to
aldehyde followed by a dehydration step gives intermediate F.
Finally, the tautomerization of F would yield product 3. It is
noteworthy that a slow second transfer hydrogenation of D
with alcohol could produce trace of tetrahydronaphthyridine
(
THNAD).
In summary, we have disclosed a new alkylation mode to
functionalize N-heterocylces. By employing abundant and
sustainable alcohols as the alkylating reagents, it enables us to
directly alkylate the less substituted pyridyl ring at the β-
position of 2-arylnaphthyridines, affording the alkylated
products in reasonable to excellent yields upon isolation. The
method proceeds under transition metal free conditions in an
atom- and step-economic fashion and liberates water as the sole
byproduct. Mechanistic investigations suggest the reaction
undergoes a hydrogen-transfer-mediated alkylation mode.
Further investigations on gaining insight into the reaction
details involving DFT calculations and utilizing such a
alkylation mode to functionalize other heteroarenes are
ongoing in our laboratory and will be reported in due course.
deuterium-labeled n-butanol with 1a was tested (eq 8).
Different H, D exchange ratios were found at positions 3, 4,
ASSOCIATED CONTENT
■
5
, and 7 and the benzylic site, indicating the hydrogen atoms
*
S
Supporting Information
from alcohol are transferred to both pyridyl rings of
naphthyridine 3aa. Owing to the influence of substituent effect,
the in situ formed aldehyde selectively couples with the
sterically less hindered pyridyl ring of naphthyridine 1a.
Although the mechanism of this β-alkylation reaction has not
been fully elucidated, on the basis of the above-observed
findings, a hydrogen-mediated alkylation mode is proposed in
Scheme 5. The reaction initiates via a slow redox reaction of 1
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03699.
Experimental procedures and spectral data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Scheme 5. Possible Pathway for the Formation of 3
*
E-mail: minzhang@scut.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the “1000 Youth Talents Plan”, Science Foundation
for Distinguished Young Scholars of Guangdong Province
(
2014A030306018), the National Natural Science Foundation
of China (21472052), and the Fundamental Research Funds
for the Central Universities (2015PT018) for financial support.
REFERENCES
■
(
1) (a) Hu, X. L. Chem. Sci. 2011, 2, 1867. (b) Arockiam, P. B.;
Bruneau, C.; Dixneuf, P. H. Chem. Rev. 2012, 112, 5879.
(2) Selected examples: (a) Hu, Y. B.; Li, Y. N.; Zhang, S.; Li, C.; Li,
L. J.; Zha, Z.-G.; Wang, Z.-Y. Org. Lett. 2015, 17, 4018. (b) Taylor, R.
Electrophilic Aromatic Substitution; Wiley: Chichester, 1990. (c) Olah,
G. A. Fridel−Crafts Chemistry; Wiley: New York, 1973. (d) Rueping,
M.; Nachtsheim, B. J.; Scheidt, T. Org. Lett. 2006, 8, 3717. (f) Iovel, I.;
Mertins, K.; Kischel, J.; Zapf, A.; Beller, M. Angew. Chem., Int. Ed.
and 2 in which the deprotonation of alcohol 2 by NaOH gives
sodium alkoxide and H O and the sodium alkoxide interacts
2
with the imine unit in 1 to form a transition state A. Through a
reversible Meerwein−Pondorf−Verley−Oppenauer (MPV−
2
005, 44, 3913. (g) Mertins, K.; Iovel, I.; Kischel, J.; Zapf, A.; Beller,
M. Angew. Chem., Int. Ed. 2005, 44, 238.
3) Selected examples: (a) Huang, L. H.; Sun, X. D.; Li, Q.; Qi, C. Z.
19
O)-type hydrogen transfer another transition state B is then
(
formed. The subsequent protonation of B with H O or alcohol
2
J. Org. Chem. 2014, 79, 6720. (b) Ilies, L.; Matsubara, T.; Ichikawa, S.;
Asako, S.; Nakamura, E. J. Am. Chem. Soc. 2014, 136, 13126. (c) Misal
Castro, L. C.; Chatani, N. Chem. - Eur. J. 2014, 20, 4548. (d) Fruchey,
E. R.; Monks, B. M.; Cook, S. P. J. Am. Chem. Soc. 2014, 136, 13130.
(e) Wang, H.; Yu, S. J.; Li, X. W. Org. Lett. 2015, 17, 2812.
followed by a thermodynamically favorable tautomerization of
C would liberate aldehyde and enamine D, respectively (1,2-H
addition: at positions 7 and 8). Similarly, a 1,4-H addition (at
positions 5 and 8) of the less substituted pyridyl ring of 1
C
DOI: 10.1021/acs.orglett.5b03699
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(
4) Selected examples: (a) Wu, K. K.; Wu, P.; Chen, J. P.; Sun, C. L.;
Crabtree, R. H. Chem. Rev. 2010, 110, 681. (i) Blank, B.; Kempe, R. J.
Am. Chem. Soc. 2010, 132, 924.
Yu, Z. K. Adv. Synth. Catal. 2015, 357, 3353. (b) Shibata, K.;
Yamaguchi, T.; Chatani, N. Org. Lett. 2015, 17, 3584. (c) Jin, H. M.;
Zhu, Z.-B.; Jin, N.; Xie, J.; Cheng, Y.-X.; Zhu, C. J. Org. Chem. Front.
(13) Selected examples: (a) Saha, S.; Alamsetti, S. K.; Schneider, C.
Chem. Commun. 2015, 51, 1461. (b) Zhang, Y. L.; Yang, N.; Liu, X. H.;
Guo, J.; Zhang, X. Y.; Lin, L. L.; Hu, C. W.; Feng, X. M. Chem.
Commun. 2015, 51, 8432. (c) Suarez, A.; Gohain, M.; Fernandez-
Rodriguez, M. A.; Sanz, R. J. Org. Chem. 2015, 80, 10421.
2
015, 2, 378. (d) Shibata, T.; Takano, H. Org. Chem. Front. 2015, 2,
383. (e) Lee, P. S.; Yoshikai, N. Org. Lett. 2015, 17, 22. (f) Wong, M.
Y.; Yamakawa, T.; Yoshikai, N. Org. Lett. 2015, 17, 442. (g) Deng, H.;
Li, H. J.; Wang, L. Org. Lett. 2015, 17, 2450. (h) Kommagalla, Y.;
Srinivas, K.; Ramana, C. V. Chem. - Eur. J. 2014, 20, 7884. (i) Andou,
T.; Saga, Y.; Komai, H.; Matsunaga, S.; Kanai, M. Angew. Chem., Int.
Ed. 2013, 52, 3213. (j) Sevov, C. S.; Hartwig, J. F. J. Am. Chem. Soc.
(14) (a) Yuan, K. D.; Jiang, F.; Sahli, Z.; Roisnel, T.; Bruneau, C.
Angew. Chem., Int. Ed. 2012, 51, 8876. (b) Sundararaju, B.; Achard, M.;
Sharma, G. V. M; Bruneau, C. J. Am. Chem. Soc. 2011, 133, 10340.
(
c) Sundararaju, B.; Tang, Z.; Achard, M.; Sharma, G. V. M; Toupet,
L.; Bruneau, C. Adv. Synth. Catal. 2010, 352, 3141.
15) (a) Xie, F.; Zhang, M.; Jiang, H. F.; Chen, M. M.; Lv, W.;
2
2
013, 135, 2116. (k) Ding, Z.; Yoshikai, N. Angew. Chem., Int. Ed.
013, 52, 8574. (l) Nakao, Y.; Kashihara, N.; Kanyiva, K. S.; Hiyama,
(
Zheng, A. B.; Jian, X. J. Green Chem. 2015, 17, 279. (b) Xie, F.; Zhang,
M.; Chen, M. M.; Lv, W.; Jiang, H. F. ChemCatChem 2015, 7, 349.
T. Angew. Chem., Int. Ed. 2010, 49, 4451. (m) Colby, D. A.; Bergman,
R. G.; Ellman, J. A. Chem. Rev. 2010, 110, 624.
(c) Lv, W.; Xiong, B.; Tan, Z. D.; Jiang, H. F.; Zhang, M. Asian J. Org.
(
5) Selected examples: (a) Cui, X.; Xu, X.; Jin, L. M.; Wojtas, L.;
Chem. 2015, 4, 1127−1131. (d) Chen, M. M.; Zhang, M.; Xiong, B.;
Tan, Z. D.; Lv, W.; Jiang, H. F. Org. Lett. 2014, 16, 6028. (e) Chen, M.
M.; Zhang, M.; Xie, F.; Wang, X. T.; Jiang, H. F. ChemCatChem 2014,
Zhang, X. P. Chem. Sci. 2015, 6, 1219. (b) Mishra, N. K.; Choi, M.; Jo,
H.; Oh, Y.; Sharma, S.; Han, S. H.; Jeong, T.; Han, S.; Lee, S. Y.; Kim,
I. S. Chem. Commun. 2015, 51, 17229. (c) Choi, M.; Park, J.; Mishra,
N. K.; Lee, S.-Y.; Kim, J. H.; Jeong, K. M.; Kim, J.; Jung, Y. H.; Kim, I.
S. Tetrahedron Lett. 2015, 56, 4678.
6
, 2993. (f) Xie, F.; Chen, M. M.; Wang, X. T.; Jiang, H. F.; Zhang, M.
Org. Biomol. Chem. 2014, 12, 2761. (g) Zhang, M.; Neumann, H.;
Beller, M. Angew. Chem. 2013, 125, 625. (h) Zhang, M.; Fang, X.;
Neumann, H.; Beller, M. J. Am. Chem. Soc. 2013, 135, 11384.
(
6) Selected examples: (a) Wu, X. J.; See, J. W. T.; Xu, K.; Hirao, H.;
Roger, J.; Hierso, J. C.; Zhou, J. R. Angew. Chem., Int. Ed. 2014, 53,
3573. (b) Kandukuri, S. R.; Bahamonde, A.; Chatterjee, I.; Jurberg, I.
D.; Escudero-Adan, E. C.; Melchiorre, P. Angew. Chem., Int. Ed. 2015,
4, 1485. (c) Verbelen, B.; Rezende, L. C. D.; Boodts, S.; Jacobs, J.;
Meervelt, L. V.; Hfokens, J.; Dehaen, W. Chem. - Eur. J. 2015, 21,
(16) (a) Xiong, W. F.; Qi, C. R.; He, H. T.; Ouyang, L.; Zhang, M.;
1
Jiang, H. F. Angew. Chem., Int. Ed. 2015, 54, 3084. (b) Xiong, W. F.;
Qi, C. R.; Peng, Y. B.; Guo, T. Z.; Zhang, M.; Jiang, H. F. Chem. - Eur.
J. 2015, 21, 14314. (c) Xie, F.; Yan, F. X.; Chen, M. M.; Zhang, M.
RSC Adv. 2014, 4, 29502. (d) Zhang, M.; Jiang, H. F.; Liu, H. L.; Zhu,
Q. H. Org. Lett. 2007, 9, 4111.
5
12667. (d) Arora, A.; Teegardin, K. A.; Weaver, J. D. Org. Lett. 2015,
17, 3722. (e) Thirumoorthi, N. T.; Jia Shen, C.; Adsool, V. A. Chem.
(
17) Xiong, B.; Li, Y.; Lv, W.; Tan, Z. D.; Jiang, H. F.; Zhang, M. Org.
Lett. 2015, 17, 4054.
18) De Houwer, J.; Abbaspour Tehrani, K.; Maes, B. U. W. Angew.
Chem., Int. Ed. 2012, 51, 2745.
19) Xu, Q.; Chen, J. H.; Tian, H. W.; Yuan, X. Q.; Li, S. Y.; Zhou, C.
K.; Liu, J. P. Angew. Chem., Int. Ed. 2014, 53, 225.
Commun. 2015, 51, 3139.
7) Minisci-type reactions: (a) Bowman, W. R.; Storey, J. M. D.
(
(
Chem. Soc. Rev. 2007, 36, 1803. (b) O'Hara, F.; Blackmond, D. G.;
Baran, P. S. J. Am. Chem. Soc. 2013, 135, 12122. (c) Fujiwara, Y.;
Dixon, J. A.; O’Hara, F.; Funder, E. D.; Dixon, D. D.; Rodriguez, R. A.;
Baxter, R. D.; Herle, B.; Sach, N.; Collins, M. R.; Ishihara, Y.; Baran, P.
S. Nature 2012, 492, 95.
(
(
8) Selected examples: (a) Shen, P. X.; Wang, X. C.; Wang, P.; Zhu,
R. Y.; Yu, J. Q. J. Am. Chem. Soc. 2015, 137, 11574. (b) Lei, C. H.; Jin,
X. J.; Zhou, J. S. Angew. Chem. 2015, 127, 13595.
(
9) Selected examples: (a) Cheng, G. L.; Li, T. J.; Yu, J. Q. J. Am.
Chem. Soc. 2015, 137, 10950. (b) Gao, K.; Paira, R.; Yoshikai, N. Adv.
Synth. Catal. 2014, 356, 1486.
(
10) Selected examples: (a) Zuo, W.; Lough, A. J.; Li, Y. F.; Morris,
R. H. Science 2013, 342, 1080. (b) Wienhoefer, G.; Westerhaus, F. A.;
Junge, K.; Beller, M. J. Organomet. Chem. 2013, 744, 156.
(
c) Werkmeister, S.; Bornschein, C.; Junge, K.; Beller, M. Chem. -
Eur. J. 2013, 19, 4437. (d) Jagadeesh, R. V.; Wienhoefer, G.;
Westerhaus, F. A.; Surkus, A.-E.; Junge, H.; Junge, K.; Beller, M. Chem.
-
2
Eur. J. 2011, 17, 14375. (e) Horn, S.; Albrecht, M. Chem. Commun.
011, 47, 8802.
(
11) Selected examples: (a) Hille, T.; Irrgang, T.; Kempe, R. Chem. -
Eur. J. 2014, 20, 5569. (b) Ruch, S.; Irrgang, T.; Kempe, R. Chem. -
Eur. J. 2014, 20, 13279. (c) Khusnutdinova, J. R.; Ben-David, Y.;
Milstein, D. J. Am. Chem. Soc. 2014, 136, 2998. (d) Michlik, S.; Kempe,
R. Nat. Chem. 2013, 5, 140. (e) Srimani, D.; Ben-David, Y.; Milstein,
D. Angew. Chem., Int. Ed. 2013, 52, 4012. (f) Balaraman, E.; Khaskin,
E.; Leitus, G.; Milstein, D. Nat. Chem. 2013, 5, 122. (g) Michlik, S.;
Kempe, R. Angew. Chem., Int. Ed. 2013, 52, 6326.
(
12) Selected examples: (a) Pena-Lopez, M.; Neumann, H.; Beller,
M. Chem. Commun. 2015, 51, 13082. (b) Wang, D. W.; Zhao, K. Y.;
Xu, C. Y.; Miao, H. Y.; Ding, Y. Q. ACS Catal. 2014, 4, 3910.
(c) Cano, R.; Yus, M.; Ramon, D. J. Tetrahedron Lett. 2013, 54, 3394.
(d) Michlik, S.; Hille, T.; Kempe, R. Adv. Synth. Catal. 2012, 354, 847.
(e) Zhang, M.; Imm, S.; Baehn, S.; Neumann, H.; Beller, M. Angew.
̈
Chem., Int. Ed. 2011, 50, 11197. (f) Imm, S.; Bahn, S.; Zhang, M.;
Neubert, L.; Neumann, H.; Klasovsky, F.; Pfeffer, J.; Haas, T.; Beller,
M. Angew. Chem., Int. Ed. 2011, 50, 7599. (g) Guillena, G.; Ramon, D.
J.; Yus, M. Chem. Rev. 2010, 110, 1611. (h) Dobereiner, G. E.;
D
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