Enantioselective carbene insertion into the N-H bond of benzophenone imine

Article abstract of DOI:10.1039/c9sc03354h

Efficient enantioselective insertion of α-diazoesters into the N-H bond of N-sp²-hybridized benzophenone imine was realized by using Rh₂(esp)₂ and chiral guanidine cooperative catalysis. Both aliphatic and aromatic substituted α-amino esters were obtained in high yields (up to 99%) and good enantioselectivities (up to 95.54.5 er) under mild reaction conditions.

Full text of DOI:10.1039/c9sc03354h

Chemical
Science
View Article Online
View Journal | View Issue
EDGE ARTICLE
Enantioselective carbene insertion into the N–H
bond of benzophenone imine†
Cite this: Chem. Sci., 2019, 10, 10305
*
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Jian Yang,
Peiran Ruan,
Wei Yang,
Xiaoming Feng
and Xiaohua Liu
Efficient enantioselective insertion of a-diazoesters into the N–H bond of N-sp2-hybridized benzophenone
imine was realized by using Rh2(esp)2 and chiral guanidine cooperative catalysis. Both aliphatic and aromatic
substituted a-amino esters were obtained in high yields (up to 99%) and good enantioselectivities (up to
95.5 : 4.5 er) under mild reaction conditions.
Received 8th July 2019
Accepted 16th September 2019
DOI: 10.1039/c9sc03354h
rsc.li/chemical-science
stability of the carbene intermediates. This indicates one
limitation of these developed catalytic systems.
Introduction
Benzophenone imine, which is industrially produced by
condensation of benzophenone and ammonia, could serve as
an ammonia carrier and has been successfully applied to many
asymmetric catalytic amination reactions.7 The amination
products could be transformed into N-unprotected amino
compounds conveniently by one step acidic hydrolysis, and the
ammonia carrier precursor benzophenone could be recovered
and reused. We became interested in catalytic asymmetric car-
bene insertion into the N–H bond of N-sp2-hybridized benzo-
phenone imines because the products could be readily
transformed into optically active N-unprotected amino acids
(Scheme 1d).
Efficient synthesis of enantiomerically enriched a-amino acids
and their derivatives remains a hot topic in synthetic organic
chemistry and biology.1 The transition-metal-catalyzed enan-
tioselective insertion of an a-diazocarbonyl compound into an
N–H bond represents a potentially attractive route to chiral a-
amino acid derivatives.2 Signicant progress has been made on
asymmetric insertion reactions of N-sp3-hybridized N–H sour-
ces including primary and secondary anilines (Scheme 1a),3 as
well as those of tert-butyl carbamate4 (Scheme 1b). Recently,
enantioselective carbene insertion into the N–H bond of C3-
substituted indoles and carbazoles was achieved (Scheme
1c).5 With regard to the chiral catalyst variety, different tran-
sition metal complexes have proven to be useful, including
chiral dirhodium(II) prolinates by McKervey,6 copper
complexes with chiral ligands, such as bisoxazoline by
Jørgensen,3i spiro bisoxazolines by Zhou,3h bpy* by Fu,4d 2,20-
biimidazole by Zhou,5a a BINOL derivative by our group,3g
chiral palladium complexes liganded with guanidine by our
group,3a and PyBox by Vranken.5b In addition, cooperative
catalytic systems of dirhodium(II) carboxylates with cinchona
alkaloids3f or chiral phosphoric acids4c were well applied to the
asymmetric N–H bond insertion reaction and others.3b,i,4e,5a In
these cases, the coupling of a-diazo compounds and N–H
sources is interesting. It seems that insertion of alkyl a-diazo-
esters into anilines3 and aryl a-diazocarbonyl compounds into
tert-butyl carbamate4 and carbazoles5 yielded higher enantio-
selectivity than the opposite cross combination. This might be
due to the nucleophilicity of the nitrogen sources and the
Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of
Chemistry, Sichuan University, Chengdu 610064, China. E-mail: liuxh@scu.edu.cn
† Electronic supplementary information (ESI) available. CCDC 1915990 and
1935053. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c9sc03354h
Scheme 1 Asymmetric N–H insertion reactions.
Chem. Sci., 2019, 10, 10305–10309 | 10305
This journal is © The Royal Society of Chemistry 2019

View Article Online
Chemical Science
Edge Article
Table 1 Optimization of the reaction conditionsa
Previously, our group demonstrated enantioselective inser-
tion of a-diazocarbonyl compounds into the O–H bond of
carboxylic acids by the use of cooperative catalysis of Rh2(OAc)4
and a chiral guanidine.8 In this case, the chiral guanidine salt of
the carboxylic acid could act as a proton shuttle9 for the enan-
tioselective proton shi. In view of the strong basicity of
guanidine10 and the acidity of the N–H bond of benzophenone
imines albeit weak, we plan to extend the cooperative catalysis
of the dirhodium(II) complex and chiral guanidine to asym-
metric carbene insertion into the N–H bond of benzophenone
imine. In this text, we reported the details of such a process
which enables the convenient generation of both alkyl and aryl
substituted amino acetates in high yield and good enantiose-
lectivities under mild reaction conditions. With regard to
reaction partners and the type of proton-transfer carrier, this
method complements the studies of Fu4d and Zhou,4a–c using
either a chiral copper complex or dirhodium/chiral phosphoric
acids for enantioselective N–H insertion of tert-butyl carbamate
(Scheme 1b).
Metal
Yieldb
(%)
Entry
source
G
Solvent
erc
1
2
3
4
Pd2(dba)3
AgNTf2
G1
G1
G1
G2
G3
G4
G1
G1
G1
G1
G1
G1
G1
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
N.R.
83
84
82
88
89
96
87
88
90
93
96
96
—
50 : 50
75 : 25
50 : 50
50 : 50
50 : 50
75 : 25
82 : 18
74 : 26
74 : 26
82 : 18
90 : 10
90 : 10
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(OAc)4
Rh2(oct)4
Rh2(esp)2
Rh2(esp)2
Rh2(esp)2
Rh2(esp)2
5
6
7d
Results and discussion
8d
9d
We initially chose benzophenone imine 1a and ethyl 2-diazo-2-
phenylacetate 2a as the model substrate to optimize the reac-
tion conditions (Table 1). Several metal sources (see the ESI† for
details) were screened in the presence of chiral guanidine G111
in CH2Cl2 at 30 ꢀC, and it was found that Rh2(OAc)4 could
promote the reaction to give the N–H insertion product 3aa in
good yield with low enantioselectivity (84% yield, 75 : 25 er,
Table 1, entries 1–3). The combination of chiral guanidine with
Pd(0), which was efficient in the asymmetric carbene insertion
reaction of primary and secondary anilines,3a was completely
inefficient in this reaction (entry 1). Next, we used Rh2(OAc)4 to
explore different chiral guanidines. Aer exploring chiral
guanidines derived from various amino acids, such as L-proline
derived G2, L-pipecolic acid derived G3 and (S)-
tetrahydroisoquinoline-3-carboxylic acid derived G4, we ob-
tained enantiomerically enriched products only in the presence
10d,e
11d,f
12d,f,g
13d,g,h
a
Unless otherwise noted, all reactions were carried out with metal
source (5 mol%), G (10 mol%), 1a (0.1 mmol), and 2a (0.12 mmol) in
b
c
the solvent (0.5 mL) at 30 ꢀC for 5 h. Isolated yield. Determined by
d
e
˚
chiral HPLC analysis. 4 A MS (30 mg) was added. The reaction
f
g
time was 10 min. At 0 ꢀC for 8 h. 1b (0.1 mmol) was used.
h
Rh2(esp)2 (1 mol%) at 0 ꢀC for 24 h.
90 : 10 er, except for the 2-uorophenyl substituted one.
Generally, substrates with a halogen at the meta- and para-
position obviously gave higher enantioselectivity than
substrates with a halogen at the ortho-position. Electron-
donating groups were tolerated in this reaction, and the
desired products 3bh–3bj were generated in 98–99% yields with
84 : 16 to 88 : 12 er. 2-Naphthyl or heteroaromatic substituted a-
diazoesters were also suitable substrates, affording the related
products 3bk–3bm with moderate to good results (93–99%
yields, 78 : 22 to 90 : 10 er). It is noteworthy that ethyl (E)-2-
diazo-4-phenylbut-3-enoate 2n, which contains an alkenyl
substituent, underwent the reaction smoothly, but the product
3bn via [1,4]-H transfer was obtained. This result is different
from that of dirhodium(II) carboxylate/chiral spirophosphoric
acid cooperatively promoted N–H insertion of vinyl diazo-
acetates with tert-butyl carbamate in Zhou's work.4a
of L-ramipril derived G1 (Table 1, entry 3 vs. entries 4–6). The
˚
A
addition of
4
molecular sieves slightly beneted the
improvement of the yield (Table 1, entry 7). Aer careful
screening of the solvents, CHCl3 was proven to be the best
choice, and the N–H insertion product could be obtained in
87% yield and 82 : 18 er (Table 1, entry 8). Rh2(esp)2 exhibited
higher activity, with which a yield of 93% with 82 : 18 er could
be obtained at a reduced reaction temperature (Table 1, entries
9–11). The different substituents on phenyl of benzophenone
imines were evaluated (see the ESI† for details), and in the
presence of bis(4-uorophenyl)methanimine 1b as the model
substrate and 1 mol% of Rh2(esp)2, the product 3ba could be
obtained in 96% yield and 90 : 10 er (Table 1, entries 12 and 13).
With the optimized reaction conditions in hand, we turned
to evaluating the scope of a-diazoesters of this protocol. A
variety of a-aryl a-diazoesters 2 were investigated. As shown in
Table 2, a-diazoesters bearing halo-substituted phenyl groups at
different positions could be smoothly converted to the corre-
sponding products 3bb–3bg in 96%–99% yields with 85 : 15 to
Excited by the expected reaction prole, we next examined
the attractive scenario of preparing alkyl substituted amino
acids which are less enantioselective in the insertion reaction of
tert-butyl carbamate.4c We reoptimized the reaction conditions
between uoro-containing benzophenone imine 1b and 2-
diazopropanoate (see the ESI† for details). To our delight, the
10306 | Chem. Sci., 2019, 10, 10305–10309
This journal is © The Royal Society of Chemistry 2019

View Article Online
Edge Article
Chemical Science
Table 2 Substrate scope of a-aryl a-diazoestersa
Table 3 Substrate scope of a-alkyl a-diazoestersa
a
Unless otherwise noted, all reactions were carried out with Rh2(esp)2
˚
(1 mol%), G1 (10 mol%), 4 A MS (30 mg), 1b (0.1 mmol), and 2 (1.2
equiv.) in CHCl3 (0.5 mL) at
b
0
ꢀC for 24 h.
The absolute
conguration of the product 3ba was determined to be R aer the
a
transformation into the corresponding N-Boc-protected amine 7ba.12
Unless otherwise noted, all reactions were carried out with Rh2(esp)2
Isolated yield. d Determined by chiral HPLC analysis.
c
˚
(1 mol%), G1 (10 mol%), 5 A MS (30 mg), 1b (0.1 mmol), and 4 (2.0
b
equiv.) in CHCl3 (0.5 mL) at 50 ꢀC for 1 min. The absolute
conguration of the products 5ba and 5bf was determined to be S
aer further transformation into known compounds13,14 (see the ESI
c
d
for details). Isolated yield. Determined by chiral HPLC analysis.
cooperative catalysis of Rh2(esp)2 and chiral guanidine G1
occurred much more efficiently for alkyl substituted diazoesters
than for aryl substituted ones. All the reactions were completed
within one minute at increased reaction temperatures. Thus, we
examined the reaction of benzophenone imine 1b with various
a-alkyl a-diazoesters 4, with the results listed in Table 3. It was
found that the length of the a-alkyl chain attached to a-diazo-
esters had a limited inuence on the enantioselectivity (93 : 7 to
95.5 : 4.5 er) but the yield dramatically varied (30–96% yields)
due to the formation of b-H elimination byproducts.3d,15 The
reaction was also tolerable to a-alkyl a-diazoesters with impor-
tant functional groups, including phenyl, halogen, nitrile,
alkenyl, propargyl, ester, silyl ester, and ether, and the corre-
sponding products 5bf–5bp were obtained in moderate to good
yields (35–99% yields) with high enantioselectivities (85 : 15 to
95 : 5 er). Generally, the use of substrates with electron-
withdrawing groups gave obviously higher yields than the use
of those with electron-donating groups (5bf–5bn vs. 5bo–5bp)
because a-alkyl a-diazoesters containing electron-donating
groups easily underwent b-H eliminate to form the corre-
sponding olens. Notably, the protected isatin and oxindole
derived substrates could undergo the reaction smoothly and
afforded the desired products 5bq and 5br with excellent results
(99% yield, 95 : 5 and 95.5 : 4.5 er, respectively).
e
4p (4.0 equiv.) was added.
Furthermore, the synthetic utility of this reaction was
investigated. A gram-scale synthesis of the products 3ba and 5br
was carried out as shown in Scheme 2. Under the standard
reaction conditions, the corresponding product 3ba was
generated in 99% yield and 90 : 10 er (Scheme 2a). The optically
active N-unprotected 2-amino-2-phenylacetate 6ba could be
obtained easily by one step acidic hydrolysis in excellent yield
Scheme 2 Scaled-up version and further transformation.
Chem. Sci., 2019, 10, 10305–10309 | 10307
This journal is © The Royal Society of Chemistry 2019

View Article Online
Chemical Science
Edge Article
and enantioselectivity aer recrystallization. Meanwhile, the
benzophenone could be recovered in high yield from the
organic phase. The absolute conguration of the product 3ba
was determined to be R aer the transformation into the cor-
responding N-Boc-protected amine 7ba, whose structure was
determined by X-ray crystal analysis.12 It is noteworthy that the
absolute conguration of the products 5ba and 5bf was deter-
mined to be mainly S in comparison with the known chiral
compounds aer transformation (see the ESI† for details). An
enantiodivergent phenomenon16 was observed from the
substrate variation in these cases (3ba/5ba/5bf). Similarly, the
alkyl substituted amino acetate 6br was afforded at a gram-scale
with maintained enantioselectivity (Scheme 2b, 93% total yield,
94 : 6 er). This N–H insertion reaction of benzophenone imine
provides a new convenient route to various chiral a-amino
acids.
2014, 53, 1636; (b) G. Liu, J. Li, L. Qiu, L. Liu, G. Y. Xu,
B. Ma and J. T. Sun, Org. Biomol. Chem., 2013, 11, 5998; (c)
X. F. Xu, P. Y. Zavalij and M. P. Doyle, Angew. Chem., Int.
Ed., 2012, 51, 9829; (d) S.-F. Zhu, B. Xu, G.-P. Wang and
Q.-L. Zhou, J. Am. Chem. Soc., 2012, 134, 436; (e) H. Saito,
D. Morita, T. Uchiyama, M. Miyake and S. Miyairi,
Tetrahedron Lett., 2012, 53, 6662; (f) H. Saito, T. Uchiyama,
M. Miyake, M. Anada, S. Hashimoto, T. Takabatake and
S. Miyairi, Heterocycles, 2010, 81, 1149; (g) Z. R. Hou,
J. Wang, P. He, J. Wang, B. Qin, X. H. Liu, L. L. Lin and
X. M. Feng, Angew. Chem., Int. Ed., 2010, 49, 4763; (h)
B. Liu, S.-F. Zhu, W. Zhang, C. Chen and Q.-L. Zhou, J. Am.
Chem. Soc., 2007, 129, 5834; (i) S. Bachmann, D. Fielenbach
and K. A. Jørgensen, Org. Biomol. Chem., 2004, 2, 3044.
4 For asymmetric N–H insertion of tert-butyl carbamate, see:
(a) J.-X. Guo, T. Zhou, B. Xu, S.-F. Zhu and Q.-L. Zhou,
Chem. Sci., 2016, 7, 1104; (b) B. Xu, S.-F. Zhu, X.-D. Zuo,
Z.-C. Zhang and Q.-L. Zhou, Angew. Chem., Int. Ed., 2014,
53, 3913; (c) B. Xu, S.-F. Zhu, X.-L. Xie, J.-J. Shen and
Q.-L. Zhou, Angew. Chem., Int. Ed., 2011, 50, 11483; (d)
E. C. Lee and G. C. Fu, J. Am. Chem. Soc., 2007, 129, 12066;
(e) Y. Ni, X. Guo, W. H. Hu and S. Y. Liu, Chin. J. Org.
Chem., 2014, 34, 107.
Conclusions
We have developed the rst catalytic asymmetric N–H insertion
of a-diazoesters with benzophenone imines. A cooperative
catalytic system of a dirhodium(II) complex and chiral guani-
dine showed high efficiency. Both aryl and alkyl substituted a-
amino esters were prepared in high yields and with good
enantioselectivities under mild reaction conditions. In partic-
ular, this method complements the substrate limitation in N–H
insertion of tert-butyl carbamate. We are currently extensively
studying the mechanism and the use of chiral guanidine for
other asymmetric reactions.
5 For asymmetric N–H insertion of C3-substituted indoles and
carbazoles, see: (a) H.-Q. Shen, B. Wu, H.-P. Xie and
Y.-G. Zhou, Org. Lett., 2019, 21, 2712; (b) V. Arredondo,
S. C. Hiew, E. S. Gutman, I. D. U. A. Premachandra and
D. L. Van Vranken, Angew. Chem., Int. Ed., 2017, 56, 4156.
´
6 C. F. Garcıa, M. A. McKervey and T. Ye, Chem. Commun.,
1996, 1465.
7 For selected examples, see: (a) L. Lykke, D. Monge,
M. Nielsen and K. A. Jgensen, Chem.–Eur. J., 2010, 16,
13330; (b) A. J. Lin, H. B. Mao, X. Zhu, H. M. Ge, R. X. Tan,
C. J. Zhu and Y. X. Cheng, Chem.–Eur. J., 2011, 17, 13676;
(c) H. Choubane, A. F. Garrido-Castro, C. Alvarado,
Conflicts of interest
There are no conicts to declare.
Acknowledgements
´
´
A. Martın-Somer, A. Guerrero-Corella, M. Daaou, S. Dıaz-
´
We thank the National Natural Science Foundation of China
Tendero, M. C. Maestro, A. Fraile and J. Alema, Chem.
(No. 21625205) for nancial support.
Commun., 2018, 54, 3399; (d) M. Y. Li, S. Berritt,
L. Matuszewski, G. G. Deng, A. Pascual-Escudero,
G. B. Panetti, M. Poznik, X. D. Yang, J. J. Chruma and
P. J. Walsh, J. Am. Chem. Soc., 2017, 139, 16327; (e)
M. J. O'Donnell, J. T. Cooper and M. M. Mader, J. Am.
Chem. Soc., 2003, 125, 2370; (f) B. M. Trost and Y. L. Wang,
Angew. Chem., Int. Ed., 2018, 57, 11025.
Notes and references
´
1 For selected reviews, see: (a) C. Najera and J. M. Sansano,
Chem. Rev., 2007, 107, 4584; (b) M. Calmes and J. Daunis,
Amino Acids, 1999, 16, 215; (c) V. Constantinou-Kokotou
and G. Kokotos, Amino Acids, 1999, 16, 273.
2 For selected reviews of N–H insertion, see: (a) A. Ford,
H. Miel, A. Ring, C. N. Slattery, A. R. Maguire and
M. A. McKervey, Chem. Rev., 2015, 115, 9981; (b)
D. Gillingham and N. Fei, Chem. Soc. Rev., 2013, 42, 4918;
(c) S.-F. Zhu and Q.-L. Zhou, Acc. Chem. Res., 2012, 45,
1365; (d) Z.-H. Zhang and J.-B. Wang, Tetrahedron, 2008,
8 F. Tan, X. H. Liu, X. Y. Hao, Y. Tang, L. L. Lin and X. M. Feng,
ACS Catal., 2016, 6, 6930.
9 (a) Y. Liang, H. L. Zhou and Z.-X. Yu, J. Am. Chem. Soc., 2009,
131, 17783; (b) Y.-Y. Ren, S.-F. Zhu and Q.-L. Zhou, Org.
Biomol. Chem., 2018, 16, 3087; (c) M.-L. Li, M.-Q. Chen,
B. Xu, S.-F. Zhu and Q.-L. Zhou, Acta Chim. Sin., 2018, 76,
883.
64, 6577; (e) C. J. Moody, Angew. Chem., Int. Ed., 2007, 46, 10 For selected reviews, see: (a) S. X. Dong, X. M. Feng and
9148; (f) T. Ye and M. A. McKervey, Chem. Rev., 1994, 94,
1091.
3 For asymmetric N–H insertion of primary and secondary
anilines, see: (a) Y. Zhu, X. H. Liu, S. X. Dong, Y. H. Zhou,
W. Li, L. L. Lin and X. M. Feng, Angew. Chem., Int. Ed.,
X. H. Liu, Chem. Soc. Rev., 2018, 47, 8525; (b) W. D. Cao,
X. H. Liu and X. M. Feng, Chin. Chem. Lett., 2018, 29, 1201;
(c) X. Fu and C.-H. Tan, Chem. Commun., 2011, 47, 8210;
(d) X.-Y. Cui, C.-H. Tan and D. Leow, Org. Biomol. Chem.,
2019, 17, 4689. For selected examples, see: (e)
10308 | Chem. Sci., 2019, 10, 10305–10309
This journal is © The Royal Society of Chemistry 2019

View Article Online
Edge Article
Chemical Science
A. Hjelmencrantz and U. Berg, J. Org. Chem., 2002, 67, 3585; 12 CCDC 1915990 (7ba). (See the ESI† for details) contains the
(f) Z. P. Yu, X. H. Liu, L. Zhou, L. L. Lin and X. M. Feng, supplementary crystallographic data for this paper.
Angew. Chem., Int. Ed., 2009, 48, 5195; (g) S. X. Dong, 13 F. Ulatowski and J. Jurczak, J. Org. Chem., 2015, 80, 4235.
X. H. Liu, X. H. Chen, F. Mei, Y. L. Zhang, B. Gao, L. L. Lin 14 P. A. Evans, J. E. Robinson and J. D. Nelson, J. Am. Chem. Soc.,
and X. M. Feng, J. Am. Chem. Soc., 2010, 132, 10650; (h)
1999, 121, 6761.
S. X. Dong, X. H. Liu, Y. Zhu, P. He, L. L. Lin and 15 For selected examples of b-H elimination of metal carbenes,
X. M. Feng, J. Am. Chem. Soc., 2013, 135, 10026; (i)
T. F. Kang, P. Zhao, J. Yang, L. L. Lin, X. M. Feng and
X. H. Liu, Chem.–Eur. J., 2018, 24, 3703; (j) Y. Tang,
Q. G. Chen, X. H. Liu, G. Wang, L. L. Lin and X. M. Feng,
Angew. Chem., Int. Ed., 2015, 54, 9512; (k) Q. G. Chen,
Y. Tang, T. Y. Huang, X. H. Liu, L. L. Lin and X. M. Feng,
see:(a) D. F. Taber, R. J. Herr, S. K. Pack and J. M. Geremia, J.
Org. Chem., 1996, 61, 2908; (b) Y. Yasutomi, H. Suematsu and
T. Katsuki, J. Am. Chem. Soc., 2010, 132, 4510; (c) T. Goto,
K. Takeda, N. Shimada, H. Nambu, M. Anada, M. Shiro,
K. Ando and S. Hashimoto, Angew. Chem., Int. Ed., 2011,
50, 6803.
Angew. Chem., Int. Ed., 2016, 55, 5286; (l) Y. Tang, J. Xu, 16 (a) G. Zanoni, F. Castronovo, M. Franzini, G. Vidari and
J. Yang, L. L. Lin, X. M. Feng and X. H. Liu, Chem, 2018, 4,
1658; (m) S. S. Guo, P. Dong, Y. S. Chen, X. M. Feng and
X. H Liu, Angew. Chem., Int. Ed., 2018, 57, 16852.
E. Giannini, Chem. Soc. Rev., 2003, 32, 115; (b) T. Tanaka
and M. Hayashi, Synthesis, 2008, 3361; (c) G. Zhan, W. Du
and Y.-C. Chen, Chem. Soc. Rev., 2017, 46, 1675; (d)
´
11 CCDC 1935053 (G1). (See the ESI† for details) contains the
supplementary crystallographic data for this paper.
I. P. Beletskaya, C. Najera and M. Yus, Chem. Rev., 2018,
118, 5080; (e) W. D. Cao, X. M. Feng and X. H. Liu, Org.
Biomol. Chem., 2019, 17, 4689.
This journal is © The Royal Society of Chemistry 2019
Chem. Sci., 2019, 10, 10305–10309 | 10309