Tetrahedron Letters
Mild oxidation of benzyl alcohols to benzyl aldehydes or ketones
catalyzed by visible light
Shujian Ren , Dongping Cheng b, , Xiaonian Li a, , Xiaoliang Xu a,
a
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a
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, PR China
College of Pharmaceutical Science, Zhejiang University of Technology, Hangzhou 310014, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Induced by visible light, mild oxidation condition to prepare benzyl aldehydes or ketones have been
developed by using bromotrichloromethane as photochemical oxidant. This method avoids high temper-
ature, pressure and peroxidation with only visible light as the green driving force.
Ó 2021 Elsevier Ltd. All rights reserved.
Received 21 April 2021
Revised 25 May 2021
Accepted 10 June 2021
Available online xxxx
Keywords:
Visible light photocatalysis
Bromotrichloromethane
Alcohol oxidation
Introduction
predecessors have made some achievements, it is far from enough
for such an important and fundamental reaction.
The oxidation of alcohols to aldehydes and ketones was the
most important reaction in pharmaceutical and fine chemical
industry [1]. Numerous methods have been developed to achieve
this goal in the past few decades. Among the oxidation methods
Bromotrichloromethane is a kind of simple polyhalomethanes,
whose bond dissociation energy of C-Br bond is about 55.3 kcal/mol
[43]. Therefore, the C-Br bond can be splitted directly by lighting or
heating, which produces a free radical initiator or propagator. The
free radical produced by bromotrichloromethane can induce
hydrogen atom transfer process, which promotes the functional-
ization of CAH bond [44]. Since Kharasch first reported its applica-
tion under photocatalysis in 1947 [45], bromotrichloromethane
has been widely used in photochemical oxidation/bromination
due to its excellent photochemical properties, especially in the
reaction involving Ru/Ir complex photocatalysts [46,47]. With the
deepening of the research, it was found that bro-
motrichloromethane as photochemical oxidant has following
advantages: (1) it can split to bromine radicals and trichloromethyl
radicals under the irradiation of specific wavelength light without
any photocatalyst; (2) low price; (3) trichloromethane is the final
by-product, which is convenient for the separation and purification
of the products. The above advantages made bro-
motrichloromethane to become the most widely used oxidant in
halogenated oxides. We hypothesized that the photocatalytic oxi-
dation of alcohols can be realized by using bromotrichloromethane
as mild oxidant.
2
of alcohols to aldehydes, stoichiometric oxidants such as MnO ,
hypochlorite, activated dimethylsulfoxide etc. are the most com-
monly used [2–9], and oxygen has been developed as a green oxi-
dant in recent years [10–16]. However, peroxidation was
frequently occurred in these processes. Although metal-free
method with tempo derivatives as catalyst have been proposed
[
17–24], co-catalyst is required and separation of product from
tempo is tedious. In recent years, visible light catalysis developed
rapidly and a series of achievements have been realized because
of its unique single electron transfer (SET) and hydrogen atom
transfer (HAT) mechanism [25–34]. For the oxidation of alcohols
to aldehydes and ketones by photocatalysis, some progress had
also been achieved [35–42]. Das reported the oxidation of alcohol
catalyzed by 9-fluorenone in 2018 (Scheme 1a) [35]; Mitsunuma
and Kanai disclosed the catalytic acceptorless dehydrogenation of
aliphatic alcohols through a ternary hybrid catalytic system in
2
020 [42], which solved the challenge of oxidation of aliphatic
alcohols under visible light catalysis (Scheme 1b). Although
Result and discussion
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In order to realize the viability of the reaction, a model reaction
employing benzyl alcohol (1a, 0.5 mmol), bromotrichloromethane
0
040-4039/Ó 2021 Elsevier Ltd. All rights reserved.