Communications
doi.org/10.1002/ejoc.202100434
Intermolecular Photoredox Coupling: Alternative to Norrish
Type II Reaction and Yang Cyclization in Ketones with
The expected compounds acetophenone (2) and ethene
(main products) as well as 1-phenylcyclobutanol (4) and 1-
tetralone (3) were found, all formed by intramolecular reaction
steps (HPLC and standard addition, MS-APCI, MS-ESI or GC). In
addition, other photoproducts were obtained (oxidized and
reduced species with respect to the H-atom balance), which
must have been formed by intermolecular reaction steps.[13,14]
Together they account for approx. 20% of the conversion,
similar to 1-phenylcyclobutanol[3] (Figure 1). They are referred
to in the following as “intermolecular photoproducts“. No
formation of oxetanes,[15] hydrols,[13,16] mixed recombination
products[17] (from phenacyl and ketyl radicals), and hydrogen[18]
has been observed. The time course of the stable components
shows the parallel formation of 1,2-dibenzoylethane (7) and
the sum of the pinacols (5, 8–12) (Figure 2). The 1,2-dibenzoyl-
ethane signal (7) is already observed (HPLC) in the initial phase
of the reaction after a very short irradiation time (2 min, the
total turnover would require at least 60 min). Variation experi-
ments of the reaction parameters (Figure 3) show reduced
intermolecular product formation at 0.005 M and none at all at
0.0005 M butyrophenone concentration. The experiments also
indicated complex mutual dependencies between turnover
time, irradiation intensity, temperature, solvent and the
presence of UVB radiation (through the use of quartz glass
between the light source and reaction volume). In the case of
the 0.05 M reactant concentration, intermolecular photoprod-
uct formation was generally observed. Qualitative comparative
experiments (using 0.05 M butyrophenone) showed an in-
creased formation of the intermolecular photoproducts at
rather long turnover time, low temperature, relatively high
radiation intensity and the use of UVB light sources and quartz
glass.
A new reaction pathway for the photoconversion of butyrophe-
none in acetonitrile was investigated. In addition to the classic
intramolecular photoreactivity of ketones with γ-CÀ H bonds
(Norrish type II fragmentation and Yang cyclization), intermo-
lecular generated species were isolated and characterized: 1,2-
Dibenzoylethane, 2-phenacylacetonitrile (oxidized species) and
pinacols (reduced species). They account for approx. 20% of the
converted starting material, similar to the Yang product. The
acetophenone enol intermediate, formed in situ via the Norrish
type II reaction, has been identified as an H-atom donor for the
main intermolecular reaction steps, and has been distinguished
from other conceivable mechanistic possibilities. Experimental
results with analogue compounds suggest that the intermolec-
ular product formation pathway may be of general relevance.
Butyrophenone (6) holds a special place in the history of
photochemistry as stated by Sundaresan, Jockusch and Turro.[1]
The intramolecular deactivation pathways by photoreactions
(Norrish type II fragmentation[2] to acetophenone enol and
ethene,[1,3,4,5,6] and Yang cyclization[7] to 1-phenylcyclobutanol
(4)[3,4,6,8,9]) are well established and can be found as common
textbook knowledge. The formation of 1-tetralone (3)[4,8] has
also been reported in the literature.
In contrast, to the best of our knowledge, 1,2-dibenzoyl-
ethane (7) has rarely been reported as a photoproduct in the
literature, and is formed in only very small amounts.[3,10,11] To
understand its photochemical formation from butyrophenone
(6), further studies were carried out by irradiation in
acetonitrile.[12]
The irradiation of the analogues hexanophenone and
octanophenone (both 0.05 M) provided also evidence of an
increased formation of 1,2-dibenzoylethane (7) compared to
the acetophenone system (2) (see below). First experiments to
irradiate the aliphatic analogue 2-pentanone show a hexane-
2,5-dione signal which is about a factor of 2 higher than the
corresponding signal found after acetone irradiation[17] (each
HPLC and standard addition).
[a] Dr. H. Hoffmann
Fachbereich Naturwissenschaften und Technik
Provadis School of International Management and Technology AG
Industriepark Höchst, Gebäude B 835, 65926 Frankfurt am Main, Germany
E-mail: heiko.hoffmann@provadis-hochschule.de
[b] Prof. Dr. M. W. Tausch
Fakultät für Mathematik und Naturwissenschaften
Bergische Universität Wuppertal, Gebäude V.11.027,
Gaußstraße 20, 42119 Wuppertal, Germany
E-mail: mtausch@uni-wuppertal.de
In developing the mechanism proposed in Scheme 1 for
the formation of intermolecular photoproducts, the following
experimental findings were also considered: 1. The irradiation
of acetophenone (2) (0.05 M) resulted in about a factor of
10 less intermolecular product formation than that from
butyrophenone (6) (HPLC, MS). 2. The irradiation of the
analogue propiophenone (0.05 M), which is α-alkyl-substituted
Supporting information for this article is available on the WWW under
© 2021 The Authors. European Journal of Organic Chemistry published by
Wiley-VCH GmbH. This is an open access article under the terms of the
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tations are made.
Eur. J. Org. Chem. 2021, 1–6
1
© 2021 The Authors. European Journal of Organic Chemistry
published by Wiley-VCH GmbH
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