UPDATES
Visible Light-Induced Radical Cyclization of Ethyl 2-(N-arylcarbamoyl)-2-Chloroiminoacetates
drous Na2SO4. The solvent was removed under reduced
pressure, and the residue was purified by silica gel chroma-
tography (using ethyl acetate/petroleum ether as the eluent)
to give the product(s).
yl)iminyl radicals, we subjected compounds 1v–1x to
the current irradiation conditions, and the results are
illustrated in Scheme 8. The selectivity was low for
the reaction of 1v, whereas with 1x as the substrate,
2x-2 was largely favored over 2x-1. In the case of 1w,
on the other hand, the intramolecular attack on the
phenyl ring bearing a para-methoxy became more fa-
vorable. The selectivities of these two methods are es-
sentially the same in the cases of 1v and 1x, but dif-
fered from that of the reaction of 1w. This discrepan-
cy might be caused by the severe decomposition of
2w, as the combined yield of 2w-1 and 2w-2 was low
under the conditions of method A. Nonetheless, these
results, in combination with those shown in Scheme 5,
indicate that the a-(aminocarbonyl)iminyl radicals are
electrophilic, and thus their attack on the more elec-
tron-rich aryl ring is favored.
Method B: To
equipped with
a
10-mL round-bottom Pyrex flask
a
magnetic stirring bar were added
1 (0.25 mmol, 1.0 equiv.), Na2CO3 (54 mg, 0.5 mmol), and
DMF (2.5 mL, bubbled with argon for 15 min before use).
The mixture was irradiated under stirring with two 45W
household fluorescent lamps (distance approx. 3 cm) at
room temperature under an argon atmosphere. After the re-
action was complete as indicated by TLC (generally 12–
24 h), the reaction mixture was poured into saturated aque-
ous NH4Cl solution (10 mL), and was extracted with ethyl
acetate (10 mL4). The combined organic layers were
washed with brine (30 mL6) and dried with anhydrous
Na2SO4. The solvent was removed under reduced pressure,
and the residue was purified by silica gel chromatography
(using ethyl acetate/petroleum ether as the eluent) to give
the product(s).
In summary, we have developed two new photo-
chemical protocols for the generation of iminyl radi-
cals from easily available 2-(N-arylcarbamoyl)-2-
À
chloroiminoacetates via N Cl cleavage. The first one
features the use of Ru(phen)3Cl2 as photoredox cata-
lyst, which under visible light irradiation can induce
Acknowledgements
À
the N Cl cleavage by way of a single electron trans-
The authors thank the National Natural Science Foundation
of China (No. 21372108) for financial support.
fer. The second one uses DMF as the solvent, and re-
À
quires only visible light to cleave the N Cl bond. The
thus formed a-(aminocarbonyl)iminyl radicals then
undergo cyclization to afford quinoxalin-2(1H)-one
products in good yields. The current protocols have
the merits of high efficacy, easy operation and envi-
ronmental friendliness. We hope that the reactions de-
scribed herein will not only be highly valuable for the
synthesis of quinoxalin-2(1H)-ones, but also help to
promote the usefulness of the iminyl radical-based
methods in the synthesis of nitrogen heterocycles.
References
[1] a) A. G. Fallis, I. M. Brinza, Tetrahedron 1997, 53,
17543–17594; b) L. Stella, Nitrogen-centered radicals,
in: Radicals in Organic Synthesis (Eds.: P. Renaud,
M. P. Sibi), John Wiley & Sons, Ltd., 2001, pp 407–426;
c) S. Z. Zard, Chem. Soc. Rev. 2008, 37, 1603–1618;
d) S. Chiba, Bull. Chem. Soc. Jpn. 2013, 86, 1400–1411.
[2] a) M. Kitamura, K. Narasaka, Bull. Chem. Soc. Jpn.
2008, 81, 539–547; b) J. C. Walton, Acc. Chem. Res.
2014, 47, 1406–1416.
[3] a) R. Alonso, P. J. Campos, B. García, M. A. Rodríguez,
Org. Lett. 2006, 8, 3521–3523; b) R. Alonso, A. Cabal-
lero, P. J. Campos, M. A. Rodríguez, Tetrahedron 2010,
66, 8828–8831.
[4] a) H. Jiang, X. An, K. Tong, T. Zheng, Y. Zhan, S. Yu,
Angew. Chem. 2015, 127, 4127–4131; Angew. Chem. Int.
Ed. 2015, 54, 4055–4059; b) X.-D. An, S. Yu, Org. Lett.
2015, 17, 2692–2695.
[5] a) V. A. Mamedov, N. A. Zhukova, Progress in Qui-
noxaline Synthesis (Part 1), in: Progress in Heterocyclic
Chemistry, Vol. 24, Elsevier Ltd. 2012, pp 55–87;
b) Progress in Quinoxaline Synthesis (Part 2). in: Prog-
ress in Heterocyclic Chemistry, Vol. 25, Elsevier Ltd.,
2013, pp 1–45.
Experimental Section
Ethyl 2-(N-arylcarbamoyl)-2-chloroiminoacetates were pre-
pared from ethyl 3-arylamino-2-imino-3-oxopropanoates[9]
following the literature method.[11]
General Procedures for the Photoinduced Reactions
of 1
Method A: To a 10-mL round-bottom Pyrex flask equipped
with a magnetic stirring bar were added 1 (0.25 mmol,
1.0 equiv.), Ru(phen)3Cl2 (3.6 mg, 0.005 mmol), Na2CO3
(54 mg, 0.5 mmol), and MeCN (2.5 mL, bubbled with argon
for 15 min before use). The mixture was irradiated under
stirring with two 45 W household fluorescent lamps (dis-
tance approx. 3 cm) at room temperature under an argon at-
mosphere. After the reaction was complete as indicated by
TLC (generally 12–24 h), the reaction mixture was poured
into saturated aqueous NH4Cl solution (10 mL), and was ex-
tracted with ethyl acetate (10 mL4). The combined organic
layers were washed with brine (30 mL) and dried with anhy-
[6] For recent examples of synthetic studies on quinoxalin-
2(1H)-one derivatives, see: a) D. Chen, Z.-J. Wang, W.
Bao, J. Org. Chem. 2010, 75, 5768–5771; b) D. Chen, W.
Bao, Adv. Synth. Catal. 2010, 352, 955–960.
[7] G. Bencivenni, T. Lanza, R. Leardini, M. Minozzi, D.
Nanni, P. Spagnolo, G. Zanardi, J. Org. Chem. 2008, 73,
4721–4724.
Adv. Synth. Catal. 2015, 357, 3696 – 3702
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3701
Li, Dianjun
