Coupling of C(sp3)-H bonds with C(sp2)-O electrophiles: mild, general and selective

Article abstract of DOI:10.1039/c6cc09685a

Herein is reported the mild and general coupling of amine/ether C(sp³)-H bonds with various kinds of C(sp²)-O electrophiles with high selectivity and efficiency. Valuable allylic/benzylic amines are generated in moderate to excellent yields. The utility of this transformation is demonstrated by a broad substrate scope (>50 examples), good functional group tolerance and facile product modification.

Full text of DOI:10.1039/c6cc09685a

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Coupling of C(sp3)–H bonds with C(sp2)–O
electrophiles: mild, general and selective†
Cite this:DOI: 10.1039/c6cc09685a
Yong-Yuan Gui,a Li-Li Liao,a Liang Sun,a Zhen Zhang,a Jian-Heng Ye,a Guo Shen,a
Zhi-Peng Lu,a Wen-Jun Zhouab and Da-Gang Yu*a
Received 6th December 2016,
Accepted 23rd December 2016
DOI: 10.1039/c6cc09685a
www.rsc.org/chemcomm
Herein is reported the mild and general coupling of amine/ether
C(sp3)–H bonds with various kinds of C(sp2)–O electrophiles with
high selectivity and efficiency. Valuable allylic/benzylic amines are
generated in moderate to excellent yields. The utility of this transforma-
tion is demonstrated by a broad substrate scope (450 examples), good
functional group tolerance and facile product modification.
Direct transformation of C–H bonds has become one of the most
powerful and useful tools in organic synthesis due to the high step
and atom economy.1,2 Outstanding progress has been made in the
direct transformation of C(sp2)–H bonds.1 However, C(sp3)–H bond
Scheme 1 Alkylation of C(sp2)–O bonds.
functionalization is much less developed due to the lower processes combining photoredox catalysis and transition metal-
reactivity and poorer selectivity.2 Among the limited options, catalysis show even greater potential to resolve the challenges of
the most frequently used electrophiles are organohalides, which modern organic synthesis.8,9 Notably, photoredox/nickel dual
can be expensive, toxic, and difficult to prepare, especially for the catalysis attracts much attention and has enabled the cross
electronically disfavored and complex examples.3 Therefore, the couplings of organohalides with alkyl and other nucleophiles.9
identification of more benign coupling partners is still a major Herein, we report the direct and general coupling of amine and
challenge for C(sp3)–H transformations.
ether C(sp3)–H bonds with various kinds of alkenyl/(hetero)aryl
Phenols and carbonyl-containing compounds, including ketones, C–O electrophiles under mild reaction conditions (Scheme 1B).
aldehydes, esters, and amides, are abundant and easily available Importantly, the reactions feature high selectivity, good yields,
in nature. The easily generated phenol/enol derivatives, such as and a broad scope of C–O electrophiles, which are easily derived
sulfonates, phosphinates and others, are attractive precursors in from phenols, ketones, aldehydes, lactones, and lactams.
cross couplings.4 Significant progress has been realized for
With the hypothesis in mind, we set out to test the coupling of
C(sp2)–C(sp2) bond formation via C(sp2)–O bond activation. N,N-dimethylaniline 1a10 with b-tetralone-derived tosylate 2a. The
However, the C(sp3)–C(sp2) bond formation with such alkenyl/aryl expected allylic amine product is representative of a molecular
electrophiles falls far behind. Although there are some alkylations framework that is an important scaffold in drugs11a and a versatile
of such electrophiles with organometallic reagents (Scheme 1A),5 intermediate in organic synthesis.11b,c Moreover, more complex
more atom- and step-economic alkylation via C(sp3)–H bond examples of such allylic amines are relatively difficult to access by
functionalization remains an unresolved challenge.6
other means.12 We screened a lot of parameters and were happy to
Recently, visible-light photoredox catalysis has proved to be a obtain the desired allylamine 3aa in 88% yield (Table S1, entry 1,
powerful tool to realize useful transformations.7 Dual catalytic ESI†). Other photocatalysts and nickel catalysts were also tested
(Table S1, entries 2–5, ESI†). The control experiments performed
a Key Laboratory of Green Chemistry & Technology of Ministry of Education,
College of Chemistry, Sichuan University, 29 Wangjiang Road, Chengdu 610064,
P. R. China. E-mail: dgyu@scu.edu.cn
in the absence of a photocatalyst, a nickel catalyst, a base or
light demonstrated the essential role of each component in this
reaction (Table S1, entries 6–9, ESI†).
With the optimized conditions in hand, we next examined the
scope of viable alkenyl tosylates, and the results are summarized in
Table 1. Gratifyingly, alkenyl tosylates, regardless of the electronic
b College of Chemistry and Chemical Engineering, Neijiang Normal University,
Neijiang 641112, P. R. China
† Electronic supplementary information (ESI) available: Experimental details and
characterization data. See DOI: 10.1039/c6cc09685a
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Table 1 Alkenylation with C(sp2)–O electrophiles derived from ketones
and aldehydesa
Table 2 Alkenylation with C(sp2)–O electrophiles derived from lactones
and lactamsa
a
b
1a (0.9 mmol), 2 (0.3 mmol). 1a (0.6 mmol), 2 (0.2 mmol), with
NiBr2ꢀglyme (5 mol%), Me4Phen (7.5 mol%).
Table 3 Arylation with aryl tosylates/triflatesa
a
12 h under the standard reaction conditions as in Table S1, entry 1, ESI.
b 5 mmol scale. c Alkenyl triflates were applied with NiBr2ꢀglyme (2 mol%),
Me4Phen (2 mol%), and DABCO (2.0 eq.). d E/Z = 2/1. e 72 h. Me4Phen =
3,4,7,8-tetramethyl-1,10-phenanthroline, DABCO
= 1,4-diazabicyclo[2.2.2]-
octane, dtbbpy = 4,40-di-tert-butyl-2,20-bipyrididyl.
nature of the aryl substituent (2b–2e), were well tolerated in the
reaction and gave the desired product in good yields (3ab–3ae,
71–83% yield). The reactions are highly chemoselective; C–Br13
and C–OMe bonds,4f,g which have been applied as electrophiles
in other nickel-catalysis, remained untouched. Vinyl tosylate 2f,
which also contained a phenyl tosylate moiety, resulted in a
diminished yield, mainly due to the hydrolysis of the C–O bond in
the phenyl tosylate motif. However, the presence of phenyl tosylate
provided an opportunity for a second C–O transformation.4 Besides
b-tetralone derived tosylates, we found that other simple cyclic
alkenyl tosylates 2g–2i with 6- to 8-membered rings worked nicely.
As expected, different substituted cyclic alkenyl tosylates 2j–2o also
showed high reactivity. Notably, the reaction could be executed on a
gram scale, delivering 3an in 84% yield. Importantly, the enol
triflates, easily derived from the corresponding bicyclic ketones,
including 2-norbornanone, tropinone, and pseudo-pelletierine,
showed good reactivity to give the desired products 3ap–3ar
(Table 1, bottom left), which are challenging to obtain. Notably,
3aq and 3ar might show unique bioactive properties based on the
core structures.14 Besides ketone derivatives, we were delighted to
find that the aldehyde derivatives, including styrenyl 2s and vinyl
tosylates 2t (Table 1, bottom right), also showed excellent to
moderate reactivity, respectively. It is notable that such reactions
are highly stereoselective, as the ratio of the product isomers 3as is
the same as the alkenyl tosylates 2s with full conversion.
a
Standard reaction conditions with NiBr2ꢀglyme (2 mol%), Me4Phen
b
(2 mol%), and DABCO (2.0 eq.). Isolated yields are given. 10.0 mmol
scale. Aryl triflates were applied.
c
As demonstrated in Table 3, a wide range of aryl tosylates worked
well to generate benzylic amines (5aa–5ag, 5ak–5al) in moderate
to high yields under similar reaction conditions. Although the
tosylates from electron-neutral phenols showed low reactivity,
their triflates (4h–4j) reacted smoothly to give the desired
products in moderate to good yields. Importantly, heteroaryl
tosylates (4m–4p) could also be applied in this reaction.
We then explored the substrate scope of anilines (Table 4).
Gratifyingly, we found that the anilines with different alkyl
groups, including ethyl (1b) and iso-propyl groups (1c), could
undergo the reaction in good yields and extremely high regio-
selectivities. No alkenylation occurred at secondary or tertiary
C–H bonds. When 1d was applied as the substrate, low conversion
was observed. However, the reaction was significantly improved to
Table 4 Aniline substrate scopea
Since esters/lactones and amides/lactams also exist widely in
natural and industrial products, we were happy to access the phos-
phinates and triflates and determine that they could also serve as
effective coupling partners (Table 2). For example, the phosphinate
2u and triflates 2v–2w, derived from 3-isochromanone and
2-piperidone, gave the desired products in moderate to good yields.
Notably, the products enol ethers 3au–3av and enamine 3aw are
highly challenging to obtain from the corresponding alkenyl halides.
To further extend the scope of the C(sp2)–O electrophiles, we
next turned our attention to phenol derivatives 4, which can be
easily obtained from inexpensive and easily available phenols.
a
12 h under the standard reaction conditions as in Table 1, entry 1.
Ir(ppy)2(dtbbpy)(PF6) as the photocatalyst. ppy = 2-phenylpyridine.
b
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give 3da in 85% yield using Ir(ppy)2(dtbbpy)(PF6) as a photo-
catalyst, which is likely due to the higher oxidative ability of the
excited Ir-catalyst to promote the SET oxidation.15 Furthermore,
different substituted N,N-dimethylanilines were tested to give
the desired products 3ea–3la in good to excellent yields. Many
functional groups, including chloro, ketone, amine and ether,
were well tolerated.
To our delight, some simple and challenging secondary C(sp3)–H
bonds can also be successfully coupled with aryl tosylate 4a to deliver
5ma–5na in moderate to good yields under a slightly changed
catalytic system (eqn (1)).16
(1)
Scheme 2 Functionalization of natural products and product diversifica-
tion. Please see the ESI† for details.
In order to gain more insights into this reaction, some
preliminary mechanistic experiments were carried out (Fig. 1,
see more information in the ESI†). When two equivalents of
2,2,6,6-tetramethylpiperding-1-oxyl (TEMPO) were added under
the standard conditions, no desired product 3aa was detected
(Fig. 1A). Moreover, adduct 6aa and cyclization product 6ab
were isolated in 23% total yield when tert-butyl acrylate was
added. These results indicated that the a-aminomethyl radical,
which was generated by single electron oxidation and deprotonation,
acted as a reactive intermediate in this transformation. Moreover,
stoichiometric reactions of the Ni(0) complex with alkenyl (2b) and
aryl tosylates (4f) were carried out (Fig. 1B). All such reactions showed
low conversion and gave less than 10% of deoxyprotonation
byproducts arising from C–O activation and further protonation.
Compared to the high efficiency of the catalytic reaction, these
low yields indicated that the direct oxidative addition to Ni(0)
was inefficient and might be less favoured in this reaction.
After developing this methodology, we further demonstrated
its utility by applying it toward the functionalization of natural
products, synthesis of some valuable intermediates and bioactive
compounds, which are difficult to obtain via other methods.
Besides the bicyclic ketones (Table 1), the natural products, estrone
4q0 and 7-hydroxyflavone 4r0, underwent smooth derivation and
subsequently coupled with amine 1a to give the desired products in
moderate to good yields (Scheme 2A). Furthermore, 3ag and 3an
were selected as model substrates for a few notable transformations
(Scheme 2B). For example, regioselective borylation and oxidation
afforded g-amino alcohol 7ag in 68% yield. Notably, 3ag and 3an
underwent oxidation with m-CPBA and tandem in situ [2,3]-
Meisenheimer rearrangement to generate alkoxylamines 8ag and
8an with an exocyclic double bond.17 Such products are challenging
to obtain otherwise. Moreover, selective bromination at the para
position of the aniline 3ag was realized to give 9ag in excellent
efficiency, which provides opportunities for further transformations.
In summary, we have developed a mild and general alkenylation/
(hetero)arylation of amine/ether C(sp3)–H bonds with various C(sp2)–
O electrophiles derived from ketones, aldehydes, lactones, lactams
and phenols via photoredox/nickel dual catalysis. We demonstrated
the utility of this method with a broad substrate scope, good
functional group tolerance, facile functionalization of complexes,
easy scalability, diverse product derivatizations, low catalyst loading,
high efficiency and excellent selectivity. Further mechanistic studies
and application of these transformations are underway in our
laboratory.
We thank the ‘‘1000-Youth Talents Plan’’, a Start-up Grant
from Sichuan University, the ‘‘973’’ Project from the MOST of
China (2015CB856600), and the National Natural Science Founda-
tion of China (21502124) for financial support. We thank Prof. Jason
J. Chruma (Sichuan University) for helpful manuscript revisions. We
also thank the comprehensive training platform of the Specialized
Laboratory in the College of Chemistry at Sichuan University for
compound testing. In memory of Professor Zhitang Huang.
Notes and references
1 (a) L. Ackermann, Chem. Rev., 2011, 111, 1315–1345; (b) B.-J. Li and
Z.-J. Shi, Chem. Soc. Rev., 2012, 41, 5588–5598; (c) K. M. Engle,
T.-S. Mei, M. Wasa and J.-Q. Yu, Acc. Chem. Res., 2012, 45, 788–802;
(d) S. R. Neufeldt and M. S. Sanford, Acc. Chem. Res., 2012, 45,
936–946; (e) N. Kuhl, M. N. Hopkinson, J. Wencel-Delord and
F. Glorius, Angew. Chem., Int. Ed., 2012, 51, 10236–10254;
( f ) G. Rouquet and N. Chatani, Angew. Chem., Int. Ed., 2013, 52,
11726–11743; (g) Y. Segawa, T. Maekawa and K. Itami, Angew. Chem.,
Int. Ed., 2015, 54, 66–81.
Fig. 1 Mechanistic studies.
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2 (a) O. Baudoin, Chem. Soc. Rev., 2011, 40, 4902–4911; (b) H. Li,
B.-J. Li and Z.-J. Shi, Catal. Sci. Technol., 2011, 1, 191–206; (c) C. Liu,
H. Zhang, W. Shi and A. Lei, Chem. Rev., 2011, 111, 1780–1824.
3 H. H. Hodgson, Chem. Rev., 1947, 40, 251–277.
4 For reviews, see: (a) B.-J. Li, D.-G. Yu, C.-L. Sun and Z.-J. Shi, Chem. –
Eur. J., 2011, 17, 1728–1759; (b) B. M. Rosen, K. W. Quasdorf,
D. A. Wilson, N. Zhang, A.-M. Resmerita, N. K. Garg and V. Percec,
Chem. Rev., 2011, 111, 1346–1416; (c) J. D. Sellars and P. G. Steel,
Chem. Soc. Rev., 2011, 40, 5170–5180; (d) M. Tobisu and N. Chatani,
Top. Organomet. Chem., 2013, 44, 35–53; (e) S. I. Kozhushkov,
H. K. Potukuchi and and L. Ackermann, Catal. Sci. Technol., 2013,
3, 562–571; ( f ) J. Cornella, C. Zarate and R. Martin, Chem. Soc. Rev.,
2014, 43, 8081–8097; (g) M. Tobisu and N. Chatani, Acc. Chem. Res.,
2015, 48, 1717–1726.
J. Am. Chem. Soc., 2013, 135, 5505–5508; (i) X.-Z. Shu, M. Zhang, Y. He,
H. Frei and F. D. Toste, J. Am. Chem. Soc., 2014, 136, 5844–5847;
( j) D. C. Fabry, J. Zoller, S. Raja and M. Rueping, Angew. Chem., Int.
Ed., 2014, 53, 10228–10231; (k) S. Kim, J. Rojas-Martin and F. D. Toste,
Chem. Sci., 2016, 7, 85–88; (l) H. Huo, K. Harms and E. Meggers, J. Am.
Chem. Soc., 2016, 138, 6936–6939.
9 For selected examples, see: (a) J. C. Tellis, D. N. Primer and
G. A. Molander, Science, 2014, 345, 433–436; (b) Z. Zuo, D. T. Ahneman,
L. Chu, J. A. Terrett, A. G. Doyle and D. W. C. MacMillan, Science, 2014,
345, 437–440; (c) J. Xuan, T.-T. Zeng, J.-R. Chen, L.-Q. Lu and W.-J. Xiao,
Chem. – Eur. J., 2015, 21, 4962–4965; (d) O. Gutierrez, J. C. Tellis,
D. N. Primer, G. A. Molander and M. C. Kozlowski, J. Am. Chem. Soc.,
2015, 137, 4896–4899; (e) S. Z. Tasker and T. F. Jamison, J. Am. Chem. Soc.,
´
2015, 137, 9531–9534; ( f ) V. Corce, L.-M. Chamoreau, E. Derat, J.-P.
5 For reviews, see: (a) R. Jana, T. P. Pathak and M. S. Sigman, Chem. Rev.,
2011, 111, 1417–1492; (b) B. Su, Z.-C. Cao and Z.-J. Shi, Acc. Chem. Res.,
2015, 48, 886–896. For selected examples, see: (c) A. Fu¨rstner and L. Turet,
Angew. Chem., Int. Ed., 2005, 44, 3462–3466; (d) A. L. Hansen, J. P. Ebran,
T. M. Gøgsig and T. Skrydstrup, J. Org. Chem., 2007, 72, 6464–6472;
(e) B.-J. Li, L. Xu, Z.-H. Wu, B.-T. Guan, C.-L. Sun, B.-Q. Wang and Z.-J. Shi,
Goddard, C. Ollivier and L. Fensterbank, Angew. Chem., Int. Ed., 2015, 54,
11414–11418; (g) M. S. Oderinde, M. Frenette, D. W. Robbins, B. Aquila
and J. W. Johannes, J. Am. Chem. Soc., 2016, 138, 1760–1763; (h) C. L. Joe
and A. G. Doyle, Angew. Chem., Int. Ed., 2016, 55, 4040–4043;
´ ˆ
´
(i) C. Leveque, L. Chenneberg, V. Corce, C. Ollivier and L. Fensterbank,
Chem. Commun., 2016, 52, 9877–9880.
˜
´
J. Am. Chem. Soc., 2009, 131, 14656–14657; ( f ) M. Giannerini, M. Fananas- 10 For recent photo-catalyzed transformations of N-methylanilines, see:
Mastral and B. L. Feringa, Nat. Chem., 2013, 5, 667–672; (g) M. Tobisu,
T. Takahira, T. Morioka and N. Chatani, J. Am. Chem. Soc., 2016, 138,
6711–6714; (h) L. Guo, C.-C. Hsiao, H. Yue, X. Liu and M. Rueping, ACS
Catal., 2016, 6, 4438–4442. For recent two elegant works via photo/nickel
dual catalysis, see: (i) N. R. Patel and G. A. Molander, J. Org. Chem., 2016,
81, 7271–7275; ( j) L. Fan, J. Jia, H. Hou, Q. Lefebvre and M. Rueping,
Chem. – Eur. J., 2016, 22, 16437–16440.
6 Only one example of alkenylation of amine C–H bonds with an enol
triflate was reported in 68% yield, see: D. T. Ahneman and
A. G. Doyle, Chem. Sci., 2016, 7, 7002–7006.
(a) Y. Miyake, K. Nakajima and Y. Nishibayashi, J. Am. Chem. Soc.,
2012, 134, 3338–3341; (b) S. Zhu, A. Das, L. Bui, H. Zhou, D. P. Curran
and M. Rueping, J. Am. Chem. Soc., 2013, 135, 1823–1829; (c) H. Zhou,
P. Lu, X. Gu and P. Li, Org. Lett., 2013, 15, 5646–5649; (d) C. K. Prier
and D. W. C. MacMillan, Chem. Sci., 2014, 5, 4173–4178; (e) P. Zhang,
T. Xiao, S. Xiong, X. Dong and L. Zhou, Org. Lett., 2014, 16,
3264–3267; ( f ) H. B. Hepburn and P. Melchiorre, Chem. Commun.,
2016, 52, 3520–3523; (g) E. Fava, A. Millet, M. Nakajima, S. Loescher
and M. Rueping, Angew. Chem., Int. Ed., 2016, 55, 6776–6779.
11 (a) A. Stu¨tz, Angew. Chem., Int. Ed., 1987, 26, 320–328; (b) S. Nag and
S. Batra, Tetrahedron, 2011, 67, 8959–8965; (c) E. M. Skoda,
G. C. Davis and P. Wipf, Org. Process Res. Dev., 2012, 16, 26–34.
7 For recent reviews, see: (a) C. K. Prier, D. A. Rankic and D. W. C.
MacMillan, Chem. Rev., 2013, 113, 5322–5363; (b) Y. Xi, H. Yi and
A. Lei, Org. Biomol. Chem., 2013, 11, 2387–2403; (c) D. P. Hari and 12 For reviews, see: (a) N. R. Candeias, F. Montalbano, P. M. S. D. Cal
¨
B. Konig, Angew. Chem., Int. Ed., 2013, 52, 4734–4743; (d) X.-J. Dai,
and P. M. P. Gois, Chem. Rev., 2010, 110, 6169–6193; (b) T. A. Ramirez,
B. Zhao and Y. Shi, Chem. Soc. Rev., 2012, 41, 931–942. For photo-
catalyzed alkenylation of amines with electron-deficient alkenyl
sulfones or fluorides, see: (c) M. Nagatomo, M. Watanabe, K. Tao,
S. Kamijo and M. Inoue, Chem. Sci., 2014, 5, 4339–4345; (d) J. Xie,
J. Yu, M. Rudolph, F. Rominger and A. S. K. Hashmi, Angew. Chem.,
Int. Ed., 2016, 55, 9416–9421.
X.-L. Xu and X.-N. Li, Chin. J. Org. Chem., 2013, 33, 2046–2062;
(e) D. M. Schultz and T. P. Yoon, Science, 2014, 343, 1239176;
( f ) J. W. Beatty and C. R. J. Stephenson, Acc. Chem. Res., 2015, 48,
1474–1484; (g) J.-R. Chen, X.-Q. Hu, L.-Q. Lu and W.-J. Xiao, Chem.
Soc. Rev., 2016, 45, 2044–2056; (h) J.-P. Goddard, C. Ollivier and
L. Fensterbank, Acc. Chem. Res., 2016, 49, 1924–1936. For selected
examples, see: (i) J. J. Murphy, D. Bastida, S. Paria, M. Fagnoni and 13 Metal-Catalyzed Cross-Coupling Reactions and More, ed. A. de Meijere,
¨
P. Melchiorre, Nature, 2016, 532, 218–222; ( j) G. J. Choi and
R. R. Knowles, J. Am. Chem. Soc., 2015, 137, 9226–9229.
8 For reviews, see: (a) M. N. Hopkinson, B. Sahoo, J.-L. Li and F. Glorius,
Chem. – Eur. J., 2014, 20, 3874–3886; (b) E. Jahn and U. Jahn, Angew.
Chem., Int. Ed., 2014, 53, 13326–13328; (c) C. Vila, ChemCatChem, 2015,
S. Brase and M. Oestreich, Wiley-VCH, Weinheim, 2014.
14 (a) D. O’Hagan, Nat. Prod. Rep., 2000, 17, 435–446; (b) G. Grynkiewicz
and M. Gadzikowska, Pharmacol. Rep., 2008, 60, 439–463; (c) A. Brachet,
˜
O. Munoz, M. Gupta, J.-L. Veuthey and P. Christen, Phytochemistry, 1997,
46, 1439–1442.
7, 1790–1793; (d) Y.-Y. Gui, L. Sun, Z.-P. Lu and D.-G. Yu, Org. Chem. 15 J. D. Slinker, A. A. Gorodetsky, M. S. Lowry, J. Wang, S. Parker,
Front., 2016, 3, 522–526; (e) J. C. Tellis, C. B. Kelly, D. N. Primer,
M. Jouffroy, N. R. Patel and G. A. Molander, Acc. Chem. Res., 2016, 49,
R. Rohl, S. Bernhard and G. G. Malliaras, J. Am. Chem. Soc., 2004,
126, 2763–2767.
1429–1439; ( f ) K. L. Skubi, T. R. Blum and T. P. Yoon, Chem. Rev., 2016, 16 (a) D. R. Heitz, J. C. Tellis and G. A. Molander, J. Am. Chem. Soc.,
116, 10035–10074. For selected examples, see: (g) D. Kalyani,
K. B. McMurtrey, S. R. Neufeldt and M. S. Sanford, J. Am. Chem. Soc.,
2016, 138, 12715–12718; (b) B. J. Shields and A. G. Doyle, J. Am.
Chem. Soc., 2016, 138, 12719–12722.
2011, 133, 18566–18569; (h) B. Sahoo, M. N. Hopkinson and F. Glorius, 17 J. Meisenheimer, Ber. Dtsch. Chem. Ges., 1919, 52, 1667–1677.
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