Tetrahedron Letters
Eosin Y as a direct hydrogen-atom transfer photocatalyst for the C3-H
acylation of quinoxalin-2(1H)-ones
a
a
b
b
a,
Hangcheng Ni a, , Yu Li , Xingzi Shi , Yi Pang , Congying Jin , Fei Zhao
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a Jinhua Branch, Sichuan Industrial Institute of Antibiotics, Chengdu University, 888 West Hai Tang Road, Jinhua 321007, China
b Pharmaceutical and Material Engineering School, Jinhua Polytechnic, 888 West Hai Tang Road, Jinhua 321007, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Visible light promoted eosin Y catalyzed selective C3-H acylation of quinoxalin-2(1H)-ones has been
developed in a green and sustainable manner. In contrast to the conventional anionic eosin Y-based pho-
toredox process, neutral eosin Y acts as the actual catalyst, which was responsible for the hydrogen-atom
transfer (HAT) process to generate the acyl radical with readily available aldehydes as the radical
precursor.
Received 22 December 2020
Revised 19 January 2021
Accepted 7 February 2021
Available online 12 February 2021
Ó 2021 Elsevier Ltd. All rights reserved.
Keywords:
Photocatalysis
Hydrogen-atom transfer
Aldehyde
Acylation
Quinoxalin-2(1H)-one
Introduction
Quinoxalin-2(1H)-ones are valuable heterocyclic compounds
due to their rich biological activities. [7] In recent years, a variety
Photocatalysis has emerged as a viable tool to realize cross-cou-
pling reactions over the past decade. [1] This subtle organic photo-
synthesis is generally achieved with various iridium and
ruthenium complexes as well as organic dyes as photocatalysts.
[2] In contrast to the classical two-electron pathways, single elec-
tron transfer (SET) or energy transfer (ET) process of the excited
photocatalysts occurs in these reactions to generate the reactive
radical species. However, new activation modes of the photoex-
cited catalysts are being explored by chemists aside from their
commonly utilized ones. [3]
Eosin Y, as an economic and ecological photocatalyst, has been
widely applied as the photoredox (Fig. 1) catalyst in organic syn-
thesis (Scheme 1a). [4] It was recognized that anionic forms of
eosin Y were responsible for the photocatalytic activity in the
majority of the previously reported photo-reactions whereas the
neutral forms were considered to be inactive. [5] However, recent
results reported by Wu’s and Wang’s groups indicated that the
excited neutral eosin Y act as the direct hydrogen atom transfer
(HAT) photocatalyst, which opened a new door for the use of eosin
Y in photocatalysis (Scheme 1b) [6].
of methods have been developed for the direct functionalization
of the versatile quinoxalin-2(1H)-one scaffold. [8] Among these,
C3-H acylation via radical process is emerging as an efficient way
for the preparation of C3-acylated quinoxalin-2(1H)-ones. [9]
However, novel and green methods are still demanding consider-
ing that harsh conditions [9] and unstable radical precursors [9]
are usually required for the previous methods. Herein, we reported
a visible light promoted eosin Y catalyzed C3-H acylation of
quinoxalin-2(1H)-ones. Compared to the unstable and complex
acyl radical precursors, [10] easily accessible aldehydes were uti-
lized as the radical precursors directly to generate the acyl radical
via HAT process (Scheme 1c).
Reaction conditions were optimized with quinoxalin-2(1H)-one
(1a) and benzaldehyde (2a) as the model substrates, and the
selected results were shown in Table 1 (see Table S1 for more
details). After extensive trials, low catalyst loading (Na2-eosin Y,
0.5%) in CH3CN under 20 W blue LED irradiation gave the desired
product in a 97% yield (entry 1–3, Table 1). Less or trace product
was formed with other photocatalysts or without catalyst (entry
4–6, Table 1). Yield decreased dramatically under air or with
K2S2O8 as the oxidant, and shorter reaction time gave the product
in a lower yield (entry 7–9, Table 1). Control experiment without
light gave no product, confirming the significance of light (entry
10, Table 1).
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Corresponding authors at: Jinhua Branch, Sichuan Industrial Institute of
Antibiotics, Chengdu University, 888 West Hai Tang Road, Jinhua 321007, China
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.