Distinctive reactivity of N-benzylidene-[1,1'-biphenyl]-2-amines under photoredox conditions

Article abstract of DOI:10.3762/bjoc.16.114

A simple photocatalytic method was developed for the synthesis of unsymmetrical 1,2-diamines by the unprecedented reductive coupling of N-benzylidene-[1,1'-biphenyl]-2-amines with an aliphatic amine. The presence of a phenyl substituent in the aniline moiety of the substrate was critical for the reactivity. The reaction proceeded via radical–radical cross-coupling of α-amino radicals generated by proton-coupled single-electron transfer in the presence of an Ir photocatalyst. On the other hand, symmetrical 1,2-di-amines were selectively produced from the same starting materials by the judicious choice of the reaction conditions, showcasing the distinct reactivity of N-benzylidene-[1,1'-biphenyl]-2-amines. The developed method can be employed for the synthesis of various bulky vicinal diamines, which are potential ligands in stereoselective synthesis.

Full text of DOI:10.3762/bjoc.16.114

Distinctive reactivity of N-benzylidene-[1,1'-biphenyl]-2-amines
under photoredox conditions
Shrikant D. Tambe, Kwan Hong Min, Naeem Iqbal and Eun Jin Cho*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2020, 16, 1335–1342.
Department of Chemistry, Chung-Ang University, 84 Heukseok-ro,
Dongjak-gu, Seoul 06974, Republic of Korea
doi:10.3762/bjoc.16.114
Received: 30 March 2020
Accepted: 02 June 2020
Published: 18 June 2020
Email:
Eun Jin Cho* - ejcho@cau.ac.kr
* Corresponding author
This article is part of the thematic issue "Advances in photoredox
catalysis".
Keywords:
1,2-diamine; diversity; imine; photocatalysis; visible light
Guest Editor: T. Noël
© 2020 Tambe et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
A simple photocatalytic method was developed for the synthesis of unsymmetrical 1,2-diamines by the unprecedented reductive
coupling of N-benzylidene-[1,1'-biphenyl]-2-amines with an aliphatic amine. The presence of a phenyl substituent in the aniline
moiety of the substrate was critical for the reactivity. The reaction proceeded via radical–radical cross-coupling of α-amino radicals
generated by proton-coupled single-electron transfer in the presence of an Ir photocatalyst. On the other hand, symmetrical 1,2-di-
amines were selectively produced from the same starting materials by the judicious choice of the reaction conditions, showcasing
the distinct reactivity of N-benzylidene-[1,1'-biphenyl]-2-amines. The developed method can be employed for the synthesis of
various bulky vicinal diamines, which are potential ligands in stereoselective synthesis.
Introduction
The selective formation of distinct valuable compounds from tions [6-9]. In particular, the reactivities of N-benzylidenes have
the same starting material is a highly attractive divergent ap- been extensively explored under visible-light photocatalysis
proach, though it represents significant synthetic challenges. [10-16]. N-Benzylidenes can undergo facile single-electron
Recent advances in visible-light photocatalysis, mediated by reduction to generate α-amino radical intermediates, which can
visible-light-absorbing photosensitizers, have allowed ready participate in diverse processes, depending upon the nature of
access to complex molecules in a controlled manner, where the substrates and the reaction conditions (Scheme 1a). Various
subtle differences in the reaction conditions opened up distinct amine systems are generated from such intermediates via a wide
reaction pathways [1-5].
range of processes, including hydrogen atom abstraction
[17,18], the addition to unsaturated compounds [19-21],
Imines are versatile substrates that can be converted into radical–radical coupling [22-30], and cyclization reactions [31-
various azo compounds, depending upon the reaction condi- 33]. Among these diverse applications, the Rueping group has
1335

Beilstein J. Org. Chem. 2020, 16, 1335–1342.
Scheme 1: Photocatalytic transformations of imines.
reported excellent examples of the reductive umpolung homo- and are valuable ligands [39,40] in stereoselective organic
coupling of imines and heterocoupling with α-amino radicals synthesis. Despite the availability of a plethora of
for the synthesis of symmetrical and unsymmetrical vicinal di- synthetic methods for 1,2-diamines [41-45], the reported
amines (Scheme 1b) [23,26]. Notably, 1,2-diamines have photocatalytic synthetic methods are mainly limited to
widespread applications as core structures in a variety aniline-based substrates and do not encompass aliphatic
of natural products, pharmaceuticals, and agrochemicals [34-38] amines.
1336

Beilstein J. Org. Chem. 2020, 16, 1335–1342.
We planned the synthesis of 1,2-diamine compounds having an photoredox cycle (Scheme 1c). It is likely that the presence of
aliphatic amine moiety by the intermolecular coupling of the additional phenyl group in the substrate stabilizes the
N-benzylidines with aliphatic amines that not only act as cou- α-amino radical intermediate and modulates its reactivity
pling partner but also as electron donors in the photoredox [46,47]. In addition to the cross-coupled 1,2-diamines, we envi-
cycle, and the results are reported herein. We began with the sioned the generation of other valuable structural motifs via the
reaction of simple N-benzylideneaniline. However, this sub- careful control of the reaction conditions for the reaction of the
strate did not furnish the desired products under several differ- biphenyl imine derivative N-benzylidene-[1,1'-biphenyl]-2-
ent photocatalysis conditions, including those reported by amine.
Rueping. We hypothesized that the structural modification of
the substrate might affect its reactivity, and to our delight, the Results and Discussion
introduction of an ortho-phenyl moiety on the N-benzylide- With the initial results in hand, we attempted the optimization
neaniline species provided the desired 1,2-diamine product, of the reaction conditions to improve the yield of the 1,2-di-
wherein the N,N-dicyclohexylmethylamine (Cy2NMe) acted as amine product (Table 1). Among the series of photocatalysts
both the coupling partner and an electron donor in the tested, ranging from the Ru/Ir polypyridyl complexes to organic
Table 1: Reaction optimization.a
entry
photocatalyst
variation
solvent
yield (%)b
2a
3a
1
[Ru(bpy)3]Cl2⋅6H2O
[Ru(bpz)3](PF6)2
Ir(dFppy)3
–
CH3CN
CH3CN
12
0
0
0
2
–
3
–
CH3CN
0
0
4
Ir(ppy)3
–
CH3CN
3
25
5
5
[Ir(dF(CF3)ppy)2(dtbpy)]PF6
[Ir(dtbbpy)(ppy)2]PF6
TTPP
–
CH3CN
38
60
0
6
–
CH3CN
11
0
7
–
CH3CN
8
crystal violet
–
CH3CN
0
0
9
eosin-Y
–
CH3CN
38
14
0
0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
[Ir(dtbbpy)(ppy)2]PF6
[Ir(dtbbpy)(ppy)2]PF6
no catalyst
Cy2NMe (1 equiv)
CH3CN
55
trace
0
no Cy2NMe
CH3CN
–
CH3CN
0
[Ir(dtbbpy)(ppy)2]PF6
[Ir(dtbbpy)(ppy)2]PF6
[Ir(dtbbpy)(ppy)2]PF6
[Ir(dtbbpy)(ppy)2]PF6
[Ir(dtbbpy)(ppy)2]PF6
[Ir(dtbbpy)(ppy)2]PF6
[Ir(dtbbpy)(ppy)2]PF6
[Ir(dtbbpy)(ppy)2]PF6
[Ir(dtbbpy)(ppy)2]PF6
[Ir(dtbbpy)(ppy)2]PF6
[Ir(dtbbpy)(ppy)2]PF6
[Ir(dtbbpy)(ppy)2]PF6
[Ir(dtbbpy)(ppy)2]PF6
no light
CH3CN
0
0
–
–
–
–
–
–
–
–
–
–
–
–
DCM
0
16
7
DMF
72 (71)
22
37
50
0
DCE
11
6
DMSO
dioxane
TFE
13
20
trace
8
acetone
EtOAc
22
36
4
CH3OH
75 (75)
29
10
14
DMF/CH3OH (1:1)
DMF/H2O (1:1)
CH3CN/H2O (1:1)
35
25
15
aReaction conditions: 1a (0.1 mmol), under argon atmosphere. bThe yields were determined by 1H NMR spectroscopy using 1,3,5-trimethoxyben-
zene as the internal standard, and the isolated yields are mentioned in parentheses.
1337

Beilstein J. Org. Chem. 2020, 16, 1335–1342.
dyes, [Ir(dtbbpy)(ppy)2]PF6 was found to be the best and obtained in protic solvents (Table 1, entries 19 and 22), and in
afforded 2a in 60% yield, along with 11% of the homocoupled particular, CH3OH showed a good reactivity and formed 3a in
product 3a (Table 1, entries 1–9). The amount of Cy2NMe was 75% yield. Unexpectedly, the cyclized product with the te-
critical for achieving selectivity, and less than two equivalents thered phenyl ring proposed in Scheme 1c was not generated
of Cy2NMe gave greater amounts of the homocoupled product under any of the photocatalytic conditions evaluated.
3a (Table 1, entry 10). Control experiments showed that the
photocatalyst, amine base, and light source are integral aspects With the optimized conditions in hand, the generality of the
of the reaction (Table 1, entries 11–13). DMF was found to be transformations was investigated using a wide variety of phe-
the best solvent and yielded 2a selectively (Table 1, entries nyl-substituted N-benzylideneaniline derivatives (Scheme 2).
15–22). On the other hand, the homocoupled 3a was selectively First, the C–C cross-coupling process with Cy2NMe was
Scheme 2: Substrate scope for the radical cross-couplings. Reaction conditions: 1 (0.3 mmol), under argon atmosphere, isolated yields. aThe
debrominated product was also detected, and the yield represents the mixture of the brominated and debrominated product.
1338

Beilstein J. Org. Chem. 2020, 16, 1335–1342.
explored, with variations of the benzylidene moiety. The reac- A photochemical quantum yield, determined by using the stan-
tions with both electron-donating (2b–2e) and electron-with- dard ferrioxalate actinometry, was 83% for the radical cross-
drawing substituents (2i–2m) proceeded well. Several func- coupling between 1a and Cy2NMe (see Supporting Information
tional groups, such as benzylic ones (2b and 2c), ethers (2e), File 1 for details).
halogens (2i and 2l) [48-50], the medicinally important CF3
group (2m), and acetals (2p) were tolerated under these mild Next, the homocoupling reactions of substituted (E)-N-benzyl-
reaction conditions. The substitution pattern of the aryl groups, idene-[1,1'-biphenyl]-2-amines were studied in CH3OH
such as ortho (2c, 2e, and 2k), meta (2m and 2p), and para sub- (Scheme 3) [51-56]. It is noteworthy that the resulting symmet-
stitution (2b–2d, 2g, 2i, 2l, 2o, and 2p), did not have any signif- rical 1,2-diamine compounds are potential ligands for a variety
icant impact on the reaction outcome. The heteroaryl ring-bear- of organic transformations. The dimerization reactions
ing substrates (2n and 2o) also underwent the transformation, proceeded well, regardless of the electron density or position of
and furnished the valuable vicinal diamine products. The modi- the substituent on the benzylidene moiety. Interestingly, the
fications on the aniline moiety were also suitable, and sub- homocoupling process was highly stereoselective, resulting in
strates with both electron-donating (2q and 2r) and electron- the formation of only one diastereomer, probably due to the
withdrawing (2s) substituents underwent the cross-coupling bulky substitution pattern of the substrates. The structure of the
with excellent reactivities. We also tried the transformation with homocoupled 1,2-diamine product 3a was unambiguously con-
aliphatic amines other than Cy2NMe, such as TMEDA, TEA, firmed using X-ray crystallography [57].
and DIPEA. However, these reactions proceeded with a poor
chemoselectivity and resulted in the formation of mixtures of To further extend the diversity of the reactivity of the imines in
several types of amine compounds.
this process, we also attempted the use of a reducing agent in
Scheme 3: Substrate scope for the homocoupling. Reaction conditions: 1 (0.3 mmol), under argon atmosphere, isolated yield. aThe debrominated
product was also detected, and the yield represents the mixture of the brominated and debrominated product.
1339

Beilstein J. Org. Chem. 2020, 16, 1335–1342.
the reaction of 1a. Pleasingly, the reaction of 1a with 2 equiva- pling of B to selectively form the symmetrical diamine product
lents of the Hantzsch ester in CH3CN produced the reduced 3a.
amine product 4a in 67% yield (Scheme 4) [17,18,58-61].
Conclusion
We developed a divergent synthetic approach for the valuable
1,2-diamine motif from N-benzylidene-[1,1'-biphenyl]-2-amines
by slight alterations to the reaction conditions. The in situ-
generated α-amino radical intermediates successfully under-
went cross- and homocoupling to yield the unsymmetrical and
symmetrical 1,2-diamines, respectively. The presence of the
phenyl substituent at the aniline moiety was critical in the hete-
Scheme 4: Reduction of the imine 1a to the amine 4a.
rocoupling with aliphatic amines. Furthermore, the reduced
amine product was also obtained by employing the Hantzsch
Based on these observations, a plausible reaction mechanism ester. The developed method can be employed for the synthesis
was proposed for the developed transformation (Scheme 5). of bulky vicinal diamines with potential applications as ligands
Upon visible-light irradiation, the excited photocatalyst [IrIII]* for stereoselective synthesis.
is formed and is reductively quenched by single-electron
transfer from Cy2NMe, resulting in the generation of the highly Experimental
reducing [IrII] species and the radical cation A. To validate the An oven-dried resealable tube, equipped with a magnetic stir
reductive quenching pathway, we carried out Stern−Volmer bar, was charged with the N-benzylidene-[1,1'-biphenyl]-2-
quenching experiments (Figure S1, Supporting Information amine derivative (0.3 mmol), [Ir(dtbbpy)(ppy)2]PF6
File 1). The emission intensity of the excited Ir complex signifi- (0.003 mmol), and Cy2NMe (0.6 mmol). The reaction mixture
cantly decreased in proportion to the concentration of Cy2NMe, was purged with argon for 20 min. Then, degassed DMF or
while it was much less affected by the concentration of 1a, CH3OH was added to the reaction mixture under inert condi-
confirming the proposed working mode. The formation of 2a tions. The reaction mixture was stirred at ambient temperature
might be attributed to the proton-coupled electron transfer [62- for 2–15 h under visible-light irradiation with blue LEDs
66] from [IrII] to the imine 1a, where the radical cation A (18 W). The progress of the reaction was monitored by using
donates a proton to 1a to form the α-amino radical intermedi- TLC. Upon the completion of the reaction, the crude product
ates B and C, which undergo cross-coupling to give the desired was diluted with ethyl acetate and washed with brine. The
unsymmetrical vicinal diamine 2a. On the other hand, in organic layer was dried over anhydrous MgSO4 and concen-
CH3OH, 1a preferentially abstracts a proton from CH3OH trated in vacuo. The desired vicinal diamine product was
rather than from A, which, in turn, prevents the generation of purified by silica-gel column chromatography using hexane/
the α-amino radical C from Cy2NMe, resulting in the homocou- EtOAc as the eluent.
Scheme 5: Proposed mechanism.
1340

Beilstein J. Org. Chem. 2020, 16, 1335–1342.
13.Marzo, L.; Pagire, S. K.; Reiser, O.; König, B. Angew. Chem., Int. Ed.
2018, 57, 10034–10072. doi:10.1002/anie.201709766
14.Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev. 2013, 113,
5322–5363. doi:10.1021/cr300503r
Supporting Information
Supporting Information File 1
15.Romero, N. A.; Nicewicz, D. A. Chem. Rev. 2016, 116, 10075–10166.
doi:10.1021/acs.chemrev.6b00057
Additional experimental details and analytical data.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-16-114-S1.pdf]
16.Schultz, D. M.; Yoon, T. P. Science 2014, 343, 1239176.
doi:10.1126/science.1239176
17.Guo, X.; Okamoto, Y.; Schreier, M. R.; Ward, T. R.; Wenger, O. S.
Eur. J. Org. Chem. 2020, 1288–1293. doi:10.1002/ejoc.201900777
18.Xi, Z.-W.; Yang, L.; Wang, D.-Y.; Pu, C.-D.; Shen, Y.-M.; Wu, C.-D.;
Peng, X.-G. J. Org. Chem. 2018, 83, 11886–11895.
doi:10.1021/acs.joc.8b01651
Supporting Information File 2
Crystal data for 3a.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-16-114-S2.cif]
19.Lee, K. N.; Lei, Z.; Ngai, M.-Y. J. Am. Chem. Soc. 2017, 139,
5003–5006. doi:10.1021/jacs.7b01373
20.Qi, L.; Chen, Y. Angew. Chem., Int. Ed. 2016, 55, 13312–13315.
doi:10.1002/anie.201607813
Funding
21.Wu, G.; Wang, J.; Liu, C.; Sun, M.; Zhang, L.; Ma, Y.; Cheng, R.; Ye, J.
Org. Chem. Front. 2019, 6, 2245–2249. doi:10.1039/c9qo00407f
22.Chen, M.; Zhao, X.; Yang, C.; Xia, W. Org. Lett. 2017, 19, 3807–3810.
doi:10.1021/acs.orglett.7b01677
This research was supported by Chung-Ang University
Research Scholarship Grants in 2019 and by the National
Research Foundation of Korea (NRF-2020R1A2C2009636).
23.Fava, E.; Millet, A.; Nakajima, M.; Loescher, S.; Rueping, M.
Angew. Chem., Int. Ed. 2016, 55, 6776–6779.
ORCID® iDs
Eun Jin Cho - https://orcid.org/0000-0002-6607-1851
doi:10.1002/anie.201511235
24.Hager, D.; MacMillan, D. W. C. J. Am. Chem. Soc. 2014, 136,
16986–16989. doi:10.1021/ja5102695
25.Jeffrey, J. L.; Petronijević, F. R.; MacMillan, D. W. C.
J. Am. Chem. Soc. 2015, 137, 8404–8407. doi:10.1021/jacs.5b05376
26.Nakajima, M.; Fava, E.; Loescher, S.; Jiang, Z.; Rueping, M.
Angew. Chem., Int. Ed. 2015, 54, 8828–8832.
Preprint
A non-peer-reviewed version of this article has been previously published
as a preprint doi:10.3762/bxiv.2020.35.v1
doi:10.1002/anie.201501556
References
1. Cheng, Q.-Q.; Yedoyan, J.; Arman, H.; Doyle, M. P. J. Am. Chem. Soc.
27.Okamoto, S.; Ariki, R.; Tsujioka, H.; Sudo, A. J. Org. Chem. 2017, 82,
9731–9736. doi:10.1021/acs.joc.7b01838
28.Okamoto, S.; Kojiyama, K.; Tsujioka, H.; Sudo, A. Chem. Commun.
2016, 52, 11339–11342. doi:10.1039/c6cc05867a
2016, 138, 44–47. doi:10.1021/jacs.5b10860
2. Iqbal, N.; Jung, J.; Park, S.; Cho, E. J. Angew. Chem., Int. Ed. 2014,
53, 539–542. doi:10.1002/anie.201308735
29.Patel, N. R.; Kelly, C. B.; Siegenfeld, A. P.; Molander, G. A. ACS Catal.
2017, 7, 1766–1770. doi:10.1021/acscatal.6b03665
30.Uraguchi, D.; Kinoshita, N.; Kizu, T.; Ooi, T. J. Am. Chem. Soc. 2015,
137, 13768–13771. doi:10.1021/jacs.5b09329
3. Liu, Z.; Xu, H.; Yao, T.; Zhang, J.; Liu, L. Org. Lett. 2019, 21,
7539–7543. doi:10.1021/acs.orglett.9b02810
4. Mose, R.; Preegel, G.; Larsen, J.; Jakobsen, S.; Iversen, E. H.;
Jørgensen, K. A. Nat. Chem. 2017, 9, 487–492.
doi:10.1038/nchem.2682
31.Fava, E.; Nakajima, M.; Nguyen, A. L. P.; Rueping, M. J. Org. Chem.
2016, 81, 6959–6964. doi:10.1021/acs.joc.6b01006
32.Leitch, J. A.; Fuentes de Arriba, A. L.; Tan, J.; Hoff, O.; Martínez, C. M.;
Dixon, D. J. Chem. Sci. 2018, 9, 6653–6658. doi:10.1039/c8sc01704b
33.Rono, L. J.; Yayla, H. G.; Wang, D. Y.; Armstrong, M. F.;
Knowles, R. R. J. Am. Chem. Soc. 2013, 135, 17735–17738.
doi:10.1021/ja4100595
5. Park, D. D.; Min, K. H.; Kang, J.; Hwang, H. S.; Soni, V. K.; Cho, C.-G.;
Cho, E. J. Org. Lett. 2020, 22, 1130–1134.
doi:10.1021/acs.orglett.9b04646
6. Miyabe, H.; Yoshioka, E.; Kohtani, S. Curr. Org. Chem. 2010, 14,
1254–1264. doi:10.2174/138527210791330477
7. Belowich, M. E.; Stoddart, J. F. Chem. Soc. Rev. 2012, 41, 2003–2024.
doi:10.1039/c2cs15305j
34.Viso, A.; Fernández de la Pradilla, R.; García, A.; Flores, A.
Chem. Rev. 2005, 105, 3167–3196. doi:10.1021/cr0406561
35.Lucet, D.; Le Gall, T.; Mioskowski, C. Angew. Chem., Int. Ed. 1998, 37,
2580–2627.
8. Lei, J.; Huang, J.; Zhu, Q. Org. Biomol. Chem. 2016, 14, 2593–2602.
doi:10.1039/c6ob00087h
doi:10.1002/(sici)1521-3773(19981016)37:19<2580::aid-anie2580>3.0.
co;2-l
9. Otero, A.; Chapela, M.-J.; Atanassova, M.; Vieites, J. M.;
Cabado, A. G. Chem. Res. Toxicol. 2011, 24, 1817–1829.
doi:10.1021/tx200182m
36.Bogatcheva, E.; Hanrahan, C.; Nikonenko, B.; Samala, R.; Chen, P.;
Gearhart, J.; Barbosa, F.; Einck, L.; Nacy, C. A.; Protopopova, M.
J. Med. Chem. 2006, 49, 3045–3048. doi:10.1021/jm050948+
37.Kano, T.; Sakamoto, R.; Akakura, M.; Maruoka, K. J. Am. Chem. Soc.
2012, 134, 7516–7520. doi:10.1021/ja301120z
10.Cambié, D.; Bottecchia, C.; Straathof, N. J. W.; Hessel, V.; Noël, T.
Chem. Rev. 2016, 116, 10276–10341.
doi:10.1021/acs.chemrev.5b00707
11.Chatterjee, T.; Iqbal, N.; You, Y.; Cho, E. J. Acc. Chem. Res. 2016, 49,
2284–2294. doi:10.1021/acs.accounts.6b00248
12.Chen, J.-R.; Hu, X.-Q.; Lu, L.-Q.; Xiao, W.-J. Chem. Soc. Rev. 2016,
45, 2044–2056. doi:10.1039/c5cs00655d
38.Saibabu Kotti, S. R. S.; Timmons, C.; Li, G. Chem. Biol. Drug Des.
2006, 67, 101–114. doi:10.1111/j.1747-0285.2006.00347.x
39.Kizirian, J.-C. Chem. Rev. 2008, 108, 140–205. doi:10.1021/cr040107v
1341

Beilstein J. Org. Chem. 2020, 16, 1335–1342.
40.Bennani, Y. L.; Hanessian, S. Chem. Rev. 1997, 97, 3161–3196.
doi:10.1021/cr9407577
License and Terms
41.Cai, C.-Y.; Shu, X.-M.; Xu, H.-C. Nat. Commun. 2019, 10, 4953.
doi:10.1038/s41467-019-13024-5
This is an Open Access article under the terms of the
Creative Commons Attribution License
42.Ichikawa, S.; Dai, X.-J.; Buchwald, S. L. Org. Lett. 2019, 21,
4370–4373. doi:10.1021/acs.orglett.9b01592
(http://creativecommons.org/licenses/by/4.0). Please note
that the reuse, redistribution and reproduction in particular
requires that the authors and source are credited.
43.Mwenda, E. T.; Nguyen, H. M. Org. Lett. 2017, 19, 4814–4817.
doi:10.1021/acs.orglett.7b02256
44.Xiao, L.; Zhao, Y.; Qiao, S.; Sun, Z.; Santoro, O.; Redshaw, C.
Dalton Trans. 2020, 49, 1456–1472. doi:10.1039/c9dt04332b
45.Yu, L.; Somfai, P. Angew. Chem., Int. Ed. 2019, 58, 8551–8555.
doi:10.1002/anie.201902642
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
46.Aurrecoechea, J. M.; Suero, R. ARKIVOC 2004, No. 14, 10–35.
doi:10.3998/ark.5550190.0005.e02
(https://www.beilstein-journals.org/bjoc)
47.McNally, A.; Prier, C. K.; MacMillan, D. W. C. Science 2011, 334,
1114–1117. doi:10.1126/science.1213920
The definitive version of this article is the electronic one
which can be found at:
48.Kim, H.; Lee, C. Org. Lett. 2011, 13, 2050–2053.
doi:10.1021/ol200455n
doi:10.3762/bjoc.16.114
49.Narayanam, J. M. R.; Tucker, J. W.; Stephenson, C. R. J.
J. Am. Chem. Soc. 2009, 131, 8756–8757. doi:10.1021/ja9033582
50.Kim, H.; Lee, C. Angew. Chem., Int. Ed. 2012, 51, 12303–12306.
doi:10.1002/anie.201203599
51.Machrouhi, F.; Namy, J.-L. Tetrahedron Lett. 1999, 40, 1315–1318.
doi:10.1016/s0040-4039(98)02673-2
52.VellemÄe, E.; Tsubrik, O.; Mäeorg, S.; Mäeorg, U. J. Chem. Res. 2006,
149–150. doi:10.3184/030823406776330792
53.Jiang, H.-F.; Huang, X.-Z. J. Supercrit. Fluids 2007, 43, 291–294.
doi:10.1016/j.supflu.2007.07.005
54.Kumar, A.; Samuelson, A. G. Chem. – Asian J. 2010, 5, 1830–1837.
doi:10.1002/asia.200900711
55.Rao, C. N.; Hoz, S. J. Am. Chem. Soc. 2011, 133, 14795–14803.
doi:10.1021/ja205885q
56.Zhu, Y.; Buchwald, S. L. J. Am. Chem. Soc. 2014, 136, 4500–4503.
doi:10.1021/ja501560x
57.CCDC 1991125 contain the supplementary crystallographic data for
this article, which can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
58.Cheng, X.; Huang, W. Synlett 2016, 28, 148–158.
doi:10.1055/s-0036-1588129
59.Jung, J.; Kim, J.; Park, G.; You, Y.; Cho, E. J. Adv. Synth. Catal. 2016,
358, 74–80. doi:10.1002/adsc.201500734
60.Park, G.; Yi, S. Y.; Jung, J.; Cho, E. J.; You, Y. Chem. – Eur. J. 2016,
22, 17790–17799. doi:10.1002/chem.201603517
61.Wang, P.-Z.; Chen, J.-R.; Xiao, W.-J. Org. Biomol. Chem. 2019, 17,
6936–6951. doi:10.1039/c9ob01289c
62.Gentry, E. C.; Knowles, R. R. Acc. Chem. Res. 2016, 49, 1546–1556.
doi:10.1021/acs.accounts.6b00272
63.Hoffmann, N. Eur. J. Org. Chem. 2017, 1982–1992.
doi:10.1002/ejoc.201601445
64.Jiang, H.; Studer, A. CCS Chem. 2019, 38–49.
doi:10.31635/ccschem.019.20180026
65.Pannwitz, A.; Wenger, O. S. Chem. Commun. 2019, 55, 4004–4014.
doi:10.1039/c9cc00821g
66.Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.; Kent, C. A.;
Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D. G.; Meyer, T. J.
Chem. Rev. 2012, 112, 4016–4093. doi:10.1021/cr200177j
1342