Photo-Induced Dihydroxylation of Alkenes with Diacetyl, Oxygen, and Water

Article abstract of DOI:10.1002/hlca.202000228

Herein reported is a photo-induced production of vicinal diols from alkenes under mild reaction conditions. The present dihydroxylation method using diacetyl (= butane-2,3-dione), oxygen, and water dispenses with toxic reagents and intractable waste generation.

Full text of DOI:10.1002/hlca.202000228

DOI: 10.1002/hlca.202000228
COMMUNICATION
Photo-Induced Dihydroxylation of Alkenes with Diacetyl, Oxygen,
and Water
Yusuke Masuda,a Daichi Ikeshita,a and Masahiro Murakami*a
a
Department of Synthetic Chemistry and Biological Chemistry, Kyoto University, Katsura, Kyoto 615-8510,
Japan, e-mail: murakami@sbchem.kyoto-u.ac.jp
Dedicated to Professor E. Peter Kündig on the occasion of his 75th birthday
Herein reported is a photo-induced production of vicinal diols from alkenes under mild reaction conditions.
The present dihydroxylation method using diacetyl (= butane-2,3-dione), oxygen, and water dispenses with toxic
reagents and intractable waste generation.
Keywords: alkenes, diacetyl, dihydroxylation, photochemistry, water.
available, nontoxic, and easily disposable (Scheme 1,c).
Noteworthy is that not only styrene derivatives but
Introduction
A 1,2-diol motif is prevalent in natural products, also ordinary aliphatic alkenes are eligible substrates
pharmaceutical compounds, and functional materials. for the present photoinduced dihydroxylation reac-
Dihydroxylation of alkenes offers a straightforward tion.
access to 1,2-diols.[1–5] Osmium tetroxide is often
utilized as a catalyst for direct dihydroxylation despite
of its high toxicity (Scheme 1,a). Asymmetric variants of
the osmium-catalyzed reaction are also available.[6]
Another classical method is given by the Prevost–
Woodward reaction, in which stoichiometric amounts
of iodine and a silver salt are used, generating a lot of
wastes.[7,8] The sustainability of chemical processes has
become a major issue facing synthetic chemists. It is
highly desired to develop an alternative method of
dihydroxylation which dispenses with toxic reagents
and intractable waste generation. Lu reported a
dihydroxylation reaction which uses oxygen, water,
visible light, and an acridinium photoredox catalyst
(Scheme 1,b).[9] It proceeds through a radical pathway
under mild reaction conditions; however, the eligible
substrates are limited to styrene derivatives. Herein
described is a benign protocol for dihydroxylation
which uses diacetyl (=butane-2,3-dione), oxygen,
water, and visible light. All components are readily
Supporting information for this article is available on the
WWW under https://doi.org/10.1002/hlca.202000228
Scheme 1. Dihydroxylation reactions of alkenes.
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Helv. Chim. Acta 2021, 104, e2000228
Next examined were styrene derivatives (Scheme 4).
When styrene was subjected to the dihydroxylation
Results and Discussion
An acetonitrile/water (4:3 v/v, 7 mL) solution of cyclo- reaction, the corresponding diol 9 was obtained in
hexene (1, 0.30 mmol) and diacetyl (0.18 mmol) was 54% yield together with a small amount of α-
irradiated with blue light (470 nm) under oxygen hydroxyacetophenone (ca. 7% yield). It was assumed
(1 atm) at ambient temperature. After 24 h, the that the latter compound was produced by further
mixture was subjected to chromatographic purifica- oxidation of the diol 9. α-Substituted styrenes were
tion, which afforded trans-1,2-cyclohexanediol (2) in also suitable substrates. Sterically hindered α-isopro-
65% yield based on 1 (Scheme 2). Both light and pylstyrene participated in the reaction to afford the
oxygen were indispensable for the reaction. Other 1,2- 1,2-diol 12 in 47% yield. When α-cyclopropylstyrene
diketones such as 3,4-hexadione, benzil (1,2-diphenyl- was subjected to the dihydroxylation reaction, the 1,2-
ethane-1,2-dione), and 1,2-cyclohexanedione also gave diol 13 was obtained and no ring-opening product
2, albeit in lower yields (<5%–57%).
was detected. This result indicated that the reaction
The dihydroxylation reaction was examined using pathway involving a benzylic radical species was
other aliphatic alkenes (Scheme 3). 1-Methyl-1-cyclo- unlikely.[10] Halogenated styrene derivatives success-
hexene and 1-methyl-1-cyclopentene afforded the fully underwent the dihydroxylation reaction keeping
corresponding trans-1,2-diols 4 and 5 in 47% and 44% the carbonÀ halogen bond intact (14–17). Although a
yields, respectively. Methylidenecyclohexane gave 1,2-
diol 6 in 47% yield. Cyclic hemiacetal 7 was obtained
from 3,4-dihydropyran in 38% yield. On the other
hand, 2-benzyl-3-phenyl-1-propene gave rise to epox-
ide 8 instead of 1,2-diol.
Scheme 2. Visible light-induced dihydroxylation of alkenes with
diacetyl, oxygen, and water.
Scheme 3. Dihydroxylation of aliphatic alkenes.
Scheme 4. Dihydroxylation of styrene derivatives.
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carbonÀ iodide bond is particularly labile upon dihydroxylation reaction, which showed an opposite
photoirradiation,[11] that of 4-iodo-β-methylstyrene regioselectivity, giving 1-alkoxy-2-phenyl-2-propanol
remained under the reaction conditions; the diol 17 from 23.[9]
was obtained in 63% yield together with 4’-iodoaceto-
In order to gain mechanistic insights, the dihydrox-
phenone (ca. 29% NMR yield). Electron-donating ylation reaction of 1 was performed in water-18O. The
substituents such as methyl (18) and methoxy (19) product 2-18O incorporated only one 18O isotope,
groups were allowed on the phenyl ring. On the other which was confirmed by high resolution mass spec-
hand, 4-trifluoromethylstyrene failed to give a diol. (E)- trometry (Scheme 6,a). Next, we followed a course of
β-Methylstyrene and 1-phenyl-1-cyclohexene afforded the reaction of cyclohexene 1 by using gas chroma-
a mixture of diastereomers 21 and 22 because the tography, which detected cyclohexene oxide (26)
benzylic stereogenic center was labile enough to albeit as a very minor peak. We supposed intermedi-
epimerize via a benzylic cation under the reaction acy of the epoxide 26, and subjected isolated 26 to
conditions.[12]
the present reaction conditions. Ring-opening hydra-
Hydroxyalkoxylation of styrene derivatives took tion occurred, and the diol 2 was obtained in 91%
place when alcoholic solvents were used (Scheme 5). 2- yield (Scheme 6,b). Next, benzil (27) was used in place
Methoxy-2-phenyl-1-propanol (24) was produced from of diacetyl in the dihydroxylation reaction of alkene 1,
α-methylstyrene (23) in 39% yield when methanol and its end product was tracked down. Benzoic acid
was used as the solvent. The reaction in ethanol (28) was obtained together with the diol 2 (Sche-
afforded 2-ethoxy-2-phenyl-1-propanol (25) in 33% me 6,c).
yield, being in contrast to the previous photo-induced
It has been reported that alkenes are epoxidized
when irradiated with UV light in the presence of
diacetyl under an oxygen atmosphere.[13,14] Combining
this precedent result with the observation of the
epoxide 26 in the reaction of 1, we proposed the
reaction pathway involving intermediacy of an epox-
ide. Diacetyl is excited upon photoirradiation,[15,16] and
the following intersystem crossing generates a triplet
state of diacetyl A, which reacts with triplet oxygen to
furnish biradical species B (Scheme 7).[17] β-Scission of
B produces acetyl radical C and acetylperoxyl radical
D. The acetyl radical C couples with oxygen to
generate another acetylperoxyl radical D.[18] As a
whole, two acetylperoxyl radicals D arise from diacetyl.
Scheme 5. Hydroxyalkoxylation of alkene 23.
Scheme 6. Mechanistic experiments.
Scheme 7. Plausible reaction pathway.
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They epoxidize alkene 1 to afford the corresponding
Acknowledgements
epoxide E, which undergoes acid-mediated ring open-
ing by water, giving rise to the diol 2.[19,20] The This work was supported by JSPS KAKENHI Grant
generation of benzoic acid (28) in the experiment Number 19 K15562 (Y. M.).
shown in Scheme 6,c suggests that the generated
acetoxy radical species would abstract a hydrogen
atom from solvent or the substrate to generate acetic
acid, which acidifies the reaction media. Another
Author Contribution Statement
conceivable mechanistic scenario is that the acetoxy Y. M. and M. M. designed the experiments and wrote
radical would undergo a sequence of decarboxylation/ the manuscript. Y. M. and D. I. carried out the
radical coupling to generate ethane and related experiments.
compounds.[18]
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Received December 16, 2020
Accepted January 25, 2021
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