Formation of allylated quaternary carbon centers: Via C-O/C-O bond fragmentation of oxalates and allyl carbonates

Article abstract of DOI:10.1039/c9cc07072a

Disclosed herein emphasizes Fe-promoted cross-electrophile allylation of tertiary alkyl oxalates with allyl carbonates that generates all C(sp³)-quaternary centers. The reaction involves fragmentation of tertiary alkyl oxalate C-O bonds to give tertiary alkyl radical intermediates, addition of the radicals to less hindered alkene terminals, and subsequent cleavage of the allyl C-O bonds. Allylation with 2-aryl substituted allyl carbonates was mediated by Zn/MgCl₂, and Fe is used to promote the radical addition efficiency. By introduction of activated alkenes, a three-component radical cascade reaction took place.

Full text of DOI:10.1039/c9cc07072a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Formation of allylated quaternary carbon centers
via C–O/C–O bond fragmentation of oxalates and
allyl carbonates†
Cite this: Chem. Commun., 2020,
56, 454
Received 10th September 2019,
Accepted 2nd December 2019
ab
Haifeng Chen,a Yang Ye,a Weiqi Tong,b Jianhui Fangb and Hegui Gong
*
DOI: 10.1039/c9cc07072a
rsc.li/chemcomm
Disclosed herein emphasizes Fe-promoted cross-electrophile allylation reductive coupling of tertiary alkyl oxalates with allyl carbonates is
of tertiary alkyl oxalates with allyl carbonates that generates all feasible, which would involve two C–O bonds as the coupling
C(sp3)–quaternary centers. The reaction involves fragmentation of tertiary partners, and lead to the construction of all C(sp3)–quaternary
alkyl oxalate C–O bonds to give tertiary alkyl radical intermediates, centers. To our knowledge, formation of C–C bonds via the reaction
addition of the radicals to less hindered alkene terminals, and subse- of unactivated tertiary alkyl oxalates with a different C–O bond
quent cleavage of the allyl C–O bonds. Allylation with 2-aryl substituted partner remains unexplored.
allyl carbonates was mediated by Zn/MgCl2, and Fe is used to promote
On the other hand, all (sp3) quaternary carbon centers are an
the radical addition efficiency. By introduction of activated alkenes, a important class of structural motifs widely found in natural
three-component radical cascade reaction took place.
products and bioactive compounds.10–14 To access these congested
structural units, coupling of tertiary alkyl–MgX (X = halides) with
Transition metal-catalyzed cross-electrophile coupling of alkyl alkyl halides has shown considerable success under Cu and Co-
electrophiles has emerged as an important research area of catalyzed conditions.11,12 Alternatively, the coupling of tertiary alkyl
interest in the field of cross-coupling chemistry.1 The exploitation halides with allylic metallic nucleophiles (e.g., Zn and Mg) has also
of nickel and cobalt catalysts in particular has enabled a range of been revealed.13 To avoid the pre-preparation of organometallic
useful transformations. The scope of alkyl electrophiles has been reagents, we have developed the Ni-catalyzed reductive coupling of
largely expanded from the initial alkyl halides. Among them, the tertiary alkyl halides with allyl carbonates, allowing facile access to
engagement of carboxylate modified with NHP and alkyl pyrylium allylated quaternary centers.14 However, the allylation was limited
(Katritzky salts) and benzyl ammonium salts was noted, which to less hindered geminal dimethyl-derived tertiary alkyl halides or
featured radical decarboxylation and deamination processes.2,3 By special adamantyl bromide (in a Co-catalyzed case),15 wherein
contrast, reductive coupling of readily accessible unactivated alkyl addition of the alkyl radicals to the Ni centers within Z1- and
alcohols and their derivatives remains a formidable challenge.4–8 Z3-Ni-allyl intermediates was proposed (Scheme 1).14
Recently we have discovered that a combination of Zn and MgCl2
Herein we depict an unusual Fe-promoted reductive cou-
triggered Barton C–O bond radical scission of unsymmetrical pling of tertiary alkyl oxalates with allyl carbonates/acetates
tertiary alkyl oxalates, enabling efficient generation of unactivated affording all C(sp3) quaternary carbon centers up to moderately
tertiary alkyl radicals.9 Such a reduction process is attributed to high yields. This approach offers a valuable alternative to the
the coordination of MgCl2 with the oxalates which lowered the concurrent allylation methods: (1) the use of earth-abundant Fe
LUMO of the carbonyl groups, thus allowing the single electron as a promoter for reductive allylation has not been reported;
reduction of the carbonyl to take place. The process is further (2) unlike the Ni-catalyzed allylation, the present method is
promoted in the presence of a Ni catalyst, permitting reductive suitable for more hindered tertiary alkyl oxalates; (3) a unique
coupling with aryl halides to afford arylated all carbon quaternary mechanism involving double C–O bond fragmentation was
centers.9 Encouraged by these results, we investigated whether proposed, which is rare in radical addition chemistry; and (4) the
use of alcohol derivatives as the coupling partners is beneficial as
compared to that of less accessible halides.
We set out to examine the reaction of di-tert-butyl oxalate 1a
with methyl(2-phenylallyl)carbonate 2a. It was noted that the
a School of Materials Science and Engineering, Shanghai University, 99 Shang-Da
Road, Shanghai 200444, China. E-mail: hegui_gong@shu.edu.cn
b Center for Supramolecular Materials and Catalysis, and Department of Chemistry,
previously developed Ni-allylation method for tertiary halides
Shanghai University, 99 Shang-Da Road, Shanghai 200444, China
proved to be ineffective for oxalates (Scheme S11, ESI†).14,16
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9cc07072a
After extensive experiments, we identified that the optimized
454 | Chem. Commun., 2020, 56, 454--457
This journal is © The Royal Society of Chemistry 2020

View Article Online
Communication
ChemComm
When MgCl2 or Zn was omitted, no reaction occurred (entries 4
and 5). By contrast, the use of tert-butyl bromide to replace 1a
generated 3a in B10% yield (entry 6). Other ligands and metal
catalysts proved to be less effective (entries 7–12). Reduction of
Fe(acac)3 and ligand loading to half afforded 3a in 64% yield
(entry 13). Operation of the reaction at 25 1C gave 3a in 62%
yield. Replacement of Zn with TDAE offered a trace amount
of 3a (Table S5, ESI†).
With method A (Table 1, entry 1), we evaluated the scope of
unsymmetrical tertiary alkyl oxalates containing a t-butyl group
and a more sterically hindered t-alkyl group with respect to
t-butyl by reacting with allyl carbonate 2a (Fig. 1). Tertiary
oxalates containing geminal dimethyl groups generally gave
good results, as exemplified by 3–20, including those bearing
long-aliphatic chains derived from palmitate, laurate and oleate
(e.g., 9–11). For oxalates arising from naturally occurring
(ꢀ)-alpha-terpineol, lithocholic and cholic acids, and tetrahydro
linalool, up to moderately high yields were obtained for 17–19 and
22. The suitability of the present method for sterically more
hindered tertiary oxalates than the geminal dimethyl ones is
Scheme 1 Construction of allylated quaternary carbon centers via Ni and
Fe-mediated reductive coupling conditions (the proposed mechanism in
boxes).
conditions (method A) comprised the use of N,N-dimethyl- noteworthy. As such, 21–23 were obtained in preparatively useful
acetamide (DMA) as solvent, Fe(acac)3 (10 mol%) as a promoter, yields. It should be noted that the bromo analogs of these substrates
dtBBipy L1a (20 mol%) as a ligand, Zn powder (3.0 equiv.) as a were incompetent under our previous Ni-catalyzed allylation
reductant and MgCl2 (3.0 equiv.) as an additive.16 The reaction went conditions,14 indicating the unique reactivity of oxalates using an
to completion after 8 h at 45 1C, furnishing 3a in 84% yield Fe-promoted reductive allylation method. The oxalates arising from
(Table 1, entry 1). Control experiments showed that without Fe salt
and a ligand, 3a was still obtained in 38% yield (entries 2 and 3).
Table 1 Optimization for the coupling of di-tBu-oxalate 1a with methyl(2-
phenylallyl)carbonate 2aa
Entry
Variation from method A
Yieldb (%)
1
2
None
w/o Fe(acac)3
84c
41
3
4
5
6
7
8
9
w/o Fe(acac)3 and w/o dtBBipy
w/o MgCl2
w/o Zn
tert-Butyl bromide instead of 1a
L1b instead of L1a
L2a instead of L1a
L3 instead of L1a
38
ND
ND
B10
61
58
69
10
11
12
13
14
L4 instead of L1a
60
Trace
36
64
62
Ni(acac)2 instead of Fe(acac)3
Co(acac)2 instead of Fe(acac)3
5 mol% Fe(acac)3, 10 mol% dtBBipy
25 1C
a
Method A: 1 (0.15 mmol), 2a (0.3 mmol), Fe(acac)3 (10 mol%), ligand
(20 mol%), MgCl2 (300 mol%), Zn (300 mol%), DMA (1.0 mL), 45 1C.
NMR yield using 2,5-dimethylfuran as the internal reference. Isolated
Fig. 1 Scope of oxalates using method A (Table 1, entry 1). Ratio in
parentheses refers to that of products arising from a more hindered tertiary
group to a t-butyl one.
b
c
yield.
This journal is ©The Royal Society of Chemistry 2020
Chem. Commun., 2020, 56, 454--457 | 455

View Article Online
ChemComm
Communication
moderate yields were detected for 2-vinyl, -methyl and -ester
decorated allylic partners, as manifested by the examples of
12l–n. By contrast, the unsubstituted allyl carbonate generated
12o in a poor yield, which was boosted to 49% using FePc/L4 as a
promoter.16 Finally, the coupling of di-tert-butyl oxalate with
1- and 3-phenyl substituted allylic carbonates indicated both that
reactions were not satisfactory. While 3-phenyl allylic carbonate
gave a trace amount of (E)-(4,4-dimethylpent-1-en-1-yl)benzene,
the 1-phenyl substituted substrate resulted in the same
product in 35% yield (Scheme S4, ESI†).16 Both reactions
generated a substantial amount of tert-butanol as the major
byproduct.16
To further explore the utility of this allylation strategy, we
sought to integrate the allylation approach into three-component
reactions (Fig. 3).17 When methyl acrylate was introduced, we were
pleased to isolate a three-component product 38a in 49% yield
(method B, Fig. 3). Screening of a range of activated alkenes
indicated that acrylates decorated with different alkyl groups,
acrylonitrile, N,N-diethylacrylamide and methyl 2-fluoroacrylate,
on the ester moieties were effective as evidenced by the formation
of 38b–g. An unexpected discovery was the capability of methyl
propiolate that generated conjugated trisubstituted alkene 38h in
27% yield. Other tertiary alkyl oxalates present on open-chain and
cyclic rings were also efficient, as exemplified by 39–42. A compet-
ing product arising from tert-butyl addition was negligible based
on the reaction of 1b, acrylate and 2a (Scheme S8, ESI†).16
In line with our previous evidence that alkyl oxalates under-
went radical process,9 we performed radical cyclization experi-
ments using allyl-tethered oxalates 1c–1e to react with methyl-
(2-phenylallyl)carbonate with method A. The cyclization products
43–45 were obtained in moderate yields (eqn (1)), supporting the
fact that radicals were involved in the allylation event.9
Fig. 2 Scope of allyl carbonates using method A. (a) Methyl 2-(((tert-butoxy
carbonyl)oxy)methyl)acrylate was used. (b) FePc (10%) and L4 (10%) were
used. Ratio in parentheses refers to that of products arising from a more
hindered tertiary group to a t-butyl one.
hydroxyl groups present on 5-, 6-, 7-, 12- and 15-memebered rings
furnished the allylated products in good to high yields, as exempli-
fied by 24–32. In these cases, the heterocyclic compounds were
found to be compatible (e.g., 26–28). Moderate yields for 33–37 were
obtained for more complex oxalates derived from testosterone,
2-adamantanol, (+)-cedrol, and cyclotryptamine derivatives respec-
tively. The reaction was suitable on a gram scale. The coupling of
the (2-phenyl) allyl carbonate with the oxalate 1b on a 5.0 mmol
scale delivered 12a in 67% yield, wherein B14% of oxalate was
recovered.16 The ratio of 12a to 3a was determined to be B10 : 1
which is lower than 11 : 1 for a 0.15 mmol scale reaction (Schemes
S5 and S6, ESI†), indicating high chemoselectivity for the unsym-
metrical oxalate.16
The reaction protocol also displayed good compatibility
across a range of allylic carbonates (Fig. 2). For instance, using
oxalate (1b) derived from 3-hydroxy-3-methylbutyl benzoate as
the coupling partner, 2-aryl-substituted allyl products bearing
methoxy phenyl (12b–d), 1,3-benzodioxole (12e), fluorinated
phenyl (12f–h), 2-naphthyl (12i), pyrenyl and thiophenyl (12j–k)
all furnished the coupling products in moderate yields. Low to
(1)
To gain further insight into the reaction details, treatment of 1b
with (Z)-methyl(2-phenylallyl-3-d)carbonate 2a-d with method A furn-
ished the mono-deuterated product 12a-d in 72% yield (eqn (2)). This
result is in sharp contrast to our previous Ni-catalyzed allylation
tertiary alkyl bromides, wherein a mixture of 1- or 3-deuterated allylic
products with a ratio of 1 : 2 was observed (Scheme S12, ESI†). Thus,
we reason that formation of p-allyl-Fe and its equilibria with Z1-allyl-
Fe is unlikely. This notion is in accordance with the fact that with the
removal of Fe and the ligand, the yield for 3a was 38%, indicating
that allylation is a radical addition and allyl C–O bond cleavage
process (Table 1, entry 3). We inferred that coordination of MgCl2
with allyl carbonate may play a key role in the allyl C–O bond
fragmentation (eqn (3)). Complexation of MgCl2 with 2a was con-
firmed by 1H NMR spectral analysis of a mixture of the two species
in DMSO-d6 wherein appreciable changes in chemical shifts for 2a
were observed upon addition of 1.5 equiv. of MgCl2 (Fig. S3, ESI†).
Similarly, low valent Fe(I) or Fe(II) may function in the same way as
Fig. 3 Three component reaction (method B): oxalate (0.3 mmol), acrylate
(0.6 mmol), allyl carbonate (0.6 mmol), Fe (acac)3 (5%), L1a (10%), MgCl2
(300%), Zn (300%), DMA (1.0 mL), 45 1C.
456 | Chem. Commun., 2020, 56, 454--457
This journal is © The Royal Society of Chemistry 2020

View Article Online
Communication
ChemComm
Mg2+ so as to promote the reaction efficiency.18 The result of
addition of a tertiary alkyl radical to 1-phenyl substituted allyl
carbonate (35% yield, Scheme S4, ESI†) further supports the
mechanistic proposal in eqn (2) that may involve a possible
SN20-type process.
A. W. Yu, K. A. Johnson, A. K. Chatterjee, M. Yan and P. S. Baran,
Science, 2017, 356, eaam 7335; ( f ) T. Qin, L. R. Malins, J. T. Edwads,
R. R. Merchant, A. J. E. Novak, J. Z. Zhong, R. B. Mills, M. Yan, C. Yuan,
M. D. Eastgate and P. S. Baran, Angew. Chem., Int. Ed., 2017, 56, 260.
4 (a) T. Suga and Y. Ukaji, Org. Lett., 2018, 20, 7846; (b) T. Suga,
S. Shimazu and Y. Ukaji, Org. Lett., 2018, 20, 5389; (c) X.-G. Jia,
P. Guo, J. Duan and X.-Z. Shu, Chem. Sci., 2018, 9, 640.
¨
5 (a) M. van Gemmeren, M. Borjesson, A. Tortajada, S.-Z. Sun, K. Okura
and R. Martin, Angew. Chem., Int. Ed., 2017, 56, 6558; (b) X. B. Yan,
C. L. Li, W. J. Jin, P. Guo and X.-Z. Shu, Chem. Sci., 2018, 9, 4529.
(2)
´
´
6 H. R. Dieguez, A. Lopez, V. Domingo, J. F. Arteaga, J. A. Dobado,
M. M. Herrador, J. F. Q. del Moral and A. F. Barrero, J. Am. Chem.
Soc., 2010, 132, 254.
7 For reviews on C–O bond couplings, see: (a) J. Cornella, C. Zarate
and R. Martin, Chem. Soc. Rev., 2014, 43, 8081; (b) B. Su, Z.-C. Cao
and Z.-J. Shi, Acc. Chem. Res., 2015, 48, 886; (c) S. M. Pound and
M. P. Watson, Chem. Commun., 2018, 54, 12286; (d) B. M. Rosen,
K. W. Quasdorf, D. A. Wilson, N. Zhang, A. M. Resmerita, N. K. Garg
and V. Percec, Chem. Rev., 2011, 111, 1346; (e) E. J. Tollefson,
L. E. Hanna and E. R. Jarvo, Acc. Chem. Res., 2015, 48, 2344;
( f ) E. Bisz and M. Szostak, ChemSusChem, 2017, 10, 3964.
8 (a) R. J. Christopher and L. E. Overman, Acc. Chem. Res., 2016,
49, 1578; (b) D. Rackl, V. Kais, E. Lutsker and O. Reiser, Eur. J. Org.
Chem., 2017, 2130.
9 (a) Y. Ye, H. Chen, J. L. Sessler and H. Gong, J. Am. Chem. Soc., 2019,
141, 820; (b) M. Gao, D. Sun and H. Gong, Org. Lett., 2019, 21, 1645.
10 (a) A. Joshi-Pangu, C.-Y. Wang and M. R. Biscoe, J. Am. Chem. Soc.,
2011, 133, 8478; (b) S. Thapa, A. Kafle, S. K. Gurung, A. Montoya,
P. Riedel and R. Giri, Angew. Chem., Int. Ed., 2015, 54, 8236;
(c) S. L. Zultanski and G. C. Fu, J. Am. Chem. Soc., 2013, 135, 624;
(d) C.-Y. Huang and A. G. Doyle, J. Am. Chem. Soc., 2015, 137, 5638;
(e) J. J. Dunsford, R. Ewan, E. R. Clark and M. J. Ingleson, Angew.
Chem., Int. Ed., 2015, 54, 5688.
(3)
In summary, a new allylation protocol that afforded all C(sp3)
quaternary centers has been developed. The mild Fe-promoted
conditions are suitable for a wide set of unactivated tertiary alkyl
oxalates and allylic carbonates decorated with different substitu-
ents. MgCl2 not only serves as a Lewis-acid to promote reduction
of C–O bonds of oxalates to generate tertiary alkyl radicals as
described previously, but also plays a key role in allyl C–O bond
cleavage.
This work was financially supported by the Chinese NSF
(No. 21871173 and 21572127).
11 (a) J. Terao, H. Todo, S. A. Begum, H. Kuniyasu and N. Kambe, Angew.
Chem., Int. Ed., 2007, 46, 2086; (b) P. Ren, L. A. Stern and X. L. Hu,
Angew. Chem., Int. Ed., 2012, 51, 9110; (c) C.-T. Yang, Z.-Q. Zhang,
J. Liang, J.-H. Liu, X.-Y. Lu, H.-H. Chen and L. Liu, J. Am. Chem. Soc.,
2012, 134, 11124; (d) T. Iwasaki, H. Takagawa, S. P. Singh, H. Kuniyasu
and N. Kambe, J. Am. Chem. Soc., 2013, 135, 9604.
12 (a) A. M. Lauer, F. Mahmud and J. Wu, J. Am. Chem. Soc., 2011,
133, 9119; (b) X. Han, Y. Zhang and J. Wu, J. Am. Chem. Soc., 2010,
132, 4104; (c) B. Breit, P. Demel, D. Grauer and C. Studte, Chem. – Asian
J., 2006, 1, 586.
Conflicts of interest
There are no conflicts to declare.
Notes and references
1 For reviews, see: (a) D. J. Weix, Acc. Chem. Res., 2015, 48, 1767; (b) J. Gu,
X. Wang, W. Xue and H. Gong, Org. Chem. Front., 2015, 2, 1411;
(c) C. E. I. Knappke, S. Grupe, D. Gartner, M. Corpet, C. Gosmini and
J. A. von Wangelin, Chem. – Eur. J., 2014, 20, 6828; (d) T. Moragas, 13 (a) S. K. Ghorai, M. Jin, T. Hatakeyama and M. Nakamura, Org. Lett.,
´ ´
A. Correa and R. Martin, Chem. – Eur. J., 2014, 20, 8242; (e) J. Y. Nedelec,
2012, 14, 1066; (b) H. Someya, H. Ohmiya, H. Yorimitsu and
K. Oshima, Org. Lett., 2008, 10, 969; (c) Y. Mitamura, Y. Asada,
K. Murakami, H. Someya, H. Yorimitsu and K. Oshima, Chem. – Asian
J., 2010, 5, 1487; (d) T. Tsuji, H. Yorimitsu and K. Oshima, Angew.
Chem., Int. Ed., 2002, 41, 4137; (e) H. Ohmiya, T. Tsuji, H. Yorimitsu
and K. Oshima, Chem. – Eur. J., 2004, 10, 5640.
J. Perichon and M. Troupel, Top. Curr. Chem., 1997, 185, 141.
2 (a) R. M. Montero, V. R. Yatham, H. Yin, J. Davies and R. Martin,
Org. Lett., 2019, 21, 2947; (b) T. Moragas, M. Gaydou and R. Matrtin,
Angew. Chem., Int. Ed., 2016, 55, 5053; (c) H. Yue, C. Zhu, L. Shen,
Q. Geng, K. J. Hock, T. Yuan, L. Cavallo and M. Rueping, Chem. Sci.,
2019, 10, 4430; (d) J. Yi, S. O. Badir, L. M. Kammer, M. Ribagorda 14 H. F. Chen, X. Jia, Y. Y. Yu, Q. Qian and H. G. Gong, Angew. Chem.,
and G. A. Molander, Org. Lett., 2019, 21, 3346; (e) R. Mao, A. Frey, Int. Ed., 2017, 56, 13103.
J. Balon and X. Hu, Nat. Catal., 2018, 1, 120; ( f ) S. Ni, C.-X. Li, 15 (a) X. Qian, A. Auffrant, A. Felouat and C. Gosmini, Angew. Chem.,
Y. Mao, J. Han, Y. Wang, H. Yan and Y. Pan, Sci. Adv., 2019,
5, eaaw9516; (g) J. Liao, C. H. Basch, M. E. Hoerrner, M. R. Talley,
B. P. Boscoe, J. W. Tucker, M. R. Garnsey and M. P. Watson, Org.
Lett., 2019, 21, 2941; (h) C. H. Basch, J. Liao, J. Xu, J. J. Piane and
M. P. Watson, J. Am. Chem. Soc., 2017, 139, 55313.
Int. Ed., 2011, 50, 10402; (b) Y.-L. Lai and J.-M. Huang, Org. Lett.,
2017, 19, 2022; (c) Y. Dai, F. Wu, Z. Zang, H. You and H. Gong, Chem.
– Eur. J., 2012, 16, 808; (d) L. L. Anka-Lufford, M. R. Prinsell and
D. J. Weix, J. Org. Chem., 2012, 77, 9989.
16 See the ESI† for details.
3 (a) J. Wang, B. P. Cary, P. Beyer, S. H. Gellman and D. J. Weix, Angew. 17 (a) H. Cao, H. Jiang, H. Feng, J. M. C. Kwan, X. Liu and J. Wu, J. Am.
Chem., Int. Ed., 2019, 58, 12081; (b) L. Huang, A. M. Olivares and
D. J. Weix, Angew. Chem., Int. Ed., 2017, 56, 11901; (c) K. M. M. Huihui,
J. A. Caputo, Z. Melchor, A. M. Olivares, A. M. Spiewak, K. A. Johnson,
T. A. DiBenedetto, S. Kim, L. K. G. Ackerman and D. J. Weix, J. Am.
Chem. Soc., 2016, 138, 5016; (d) S. Ni, N. M. Padial, C. Kingston,
J. C. Vantourout, D. C. Schmitt, J. T. Edwards, M. M. Kruszyk,
R. R. Merchant, P. K. Mykhailiuk, B. B. Sanchez, S. Yang, M. A. Perry,
G. M. Gallego, J. J. Mousseau, M. R. Collins, R. J. Cherney, P. S. Lebed,
Chem. Soc., 2018, 140, 16360; (b) X. Zhao, H. Y. Tu, L. Guo, S. Zhu,
F. L. Qing and L. Chu, Nat. Commun., 2018, 9, 3488; (c) L. Guo,
F. Song, S. Zhu, H. Li and L. Chu, Nat. Commun., 2018, 9, 4543;
(d) J. He, C. Chen, G. C. Fu and J. C. Peters, ACS Catal., 2018,
´
´
8, 11741; (e) A. Garcıa-Domınguez, R. Mondal and C. Nevado, Angew.
Chem., Int. Ed., 2019, 58, 12286; ( f ) Q. Zhao, J. Chen, X. Zhou, X. Yu,
J. Chen and W. Xiao, Chem. – Eur. J., 2019, 25, 8024; (g) Y. Zhao,
J. Chen and W. Xiao, Org. Lett., 2018, 20, 224.
J. S. Chen, T. Qin and P. S. Baran, J. Am. Chem. Soc., 2019, 141, 6726; 18 C. W. Cheung, F. E. Zhurkin and X. Hu, J. Am. Chem. Soc., 2015,
(e) C. Li, J. Wang, L. M. Barton, S. Yu, M. Tian, D. S. Peters, M. Kumar,
137, 4932, the discussion of the role of Fe is detailed in the ESI† (p. S12).
This journal is ©The Royal Society of Chemistry 2020
Chem. Commun., 2020, 56, 454--457 | 457