TBHP-promoted direct oxidation reaction of benzylic Csp3-H bonds to ketones

Article abstract of DOI:10.1039/c7ra00352h

A metal-free oxidation system employing tert-butyl hydroperoxide (TBHP) has been developed for selective oxidation of structurally diverse benzylic sp³ C-H bonds. This low-cost methodology allows for rapid generation of synthetically and biologically valued arylketones in good to excellent yields from readily available alkylarenes and diarylmethanes.

Full text of DOI:10.1039/c7ra00352h

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TBHP-promoted direct oxidation reaction of
3
benzylic Csp –H bonds to ketones†
Cite this: RSC Adv., 2017, 7, 15176
a
a
c
b
*
*
Jiajing Tan, Tianyu Zheng, Yuqi Yu and Kun Xu
Received 9th January 2017
Accepted 27th February 2017
A metal-free oxidation system employing tert-butyl hydroperoxide (TBHP) has been developed for selective
oxidation of structurally diverse benzylic sp3 C–H bonds. This low-cost methodology allows for rapid
generation of synthetically and biologically valued arylketones in good to excellent yields from readily
available alkylarenes and diarylmethanes.
DOI: 10.1039/c7ra00352h
rsc.li/rsc-advances
C–H bond activation reactions have become one of the most also known as potential carcinogens, and long-term exposure
important classes of reactions in organic synthesis, the synthetic to those hazardous metals might lead to central nervous
scope and utility of which has advanced considerably during the system disorders.8 Consequently, less toxic, cost-effective and
past decade.1 Undoubtedly, there are fundamental benets for sustainable oxidants like peroxides or molecular oxygen, have
integrating such a strategy in organic synthesis, especially from drawn increasing research interest.
the vantage point of environmental benignity and high atom/step
In recent years, considerable progress has been made in this
economy. Furthermore, the construction of complex organic eld.9 The group of Pandey and Lei independently reported
molecules nowadays can be designed in a more direct and a C–H aerobic oxidation approaches via photo-redox chemistry
effective fashion as these C–H functionalization transformations to synthesize aryl ketones.10 In addition, Li and Wang's group
are an established piece in synthetic organic chemists' toolbox.1 achieved the benzylic C–H oxidation by developing a recyclable
Among this progress, the direct oxidation of C–H bonds for the TEMPO catalyst.11 Moreover, in 2016, Stahl et al. employed the
synthesis of functionalized molecules has been of great interest electrochemical N-hydroxyphthalimide (NHPI) system to achieve
in both academic and industrial settings.2 Many C–H oxidation benzylic oxygenation reaction to prepare (hetero)aryl ketones.12
methods have thus been developed to date.
Very recently, our group also reported a simple and efficient
Aryl ketones are widely recognized as key functional and potassium tert-butoxide promoted (hetero)benzylic C–H oxida-
structural motifs in a large plethora of signicant molecules, tion reaction using oxygen as oxidant.13 Our approach provided
including natural products, active pharmaceutical ingredients, a ready access to versatile diarylketones under mild conditions.
agrochemicals and advanced materials (Fig. 1). Also, they While similar to most of other benzylic oxidation protocols,
frequently serve as useful precursors for the synthesis of alkylbenzenes were unfortunately not amenable substrates in
complicated organic molecules.3 The classical methods to that report.14 Therefore, further research is highly necessary, as
assemble such motifs involve the Friedel–Cras acylation4 and direct oxidative C–H functionalization of diverse substrates are
transition metal catalyzed coupling reactions,5 which oen still challenging and reported examples are limited in terms of
suffer from poor selectivity and require harsh reaction condi- substrate types. Since one of our lab's long-term goal is to
tions as well as toxic or expensive metal salts. In this way, direct
3
benzylic Csp –H oxidation reactions, as we proposed, could be
a more ideal synthetic pathway.6 However, the literature survey
suggested that most of the existing methods relied on less
sustainable oxidants, such as potassium permanganate or
chromium acid derivatives.7 The resulting vast metal residue
not only caused operational challenges upon scale-up, but were
aDepartment of Organic Chemistry, Faculty of Science, Beijing University of Chemical
Technology, Beijing 100029, China. E-mail: tanjj@mail.buct.edu.cn
bCollege of Chemistry and Pharmaceutical Engineering, Nanyang Normal University,
Nanyang, Henan 473061, P. R. China. E-mail: xukun@nynu.edu.cn
cChina Academy of Engineering Physics, P. O. Box 919, Mianyang, Sichuan, 621900,
China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra00352h
Fig. 1 Selected aryl ketones.
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Table 1 Reaction condition optimizationa
Entry
Oxidant
Yieldb (%)
1
2
3
4
(NH4)2S2O8
K2S2O8
m-CPBA
DTBP
n.r.
n.r.
n.r.
n.r.
5
6
7
8
CHP
Dicumyl peroxide
BPO
TBHP
TBHP
68%
89%
82%
91%
31%
51%
9c
10d
TBHP
a
Ethylbenzene (1 mmol) and oxidant (12 mmol) were reacted at 130 ꢀC
b
for 10 h. Hexane is the solvent if the oxidant is solid. Determined by
c
GC with internal standard mesitylene. TBHP (3 mmol) was used
instead. d Reaction temperature was 100 ꢀC.
develop efficient oxidation methods to construct valuable mole-
cules, we herein would like to report our recent efforts in devel-
oping a simple and efficient tert-butyl hydroperoxide (TBHP)
promoted benzylic sp3 C–H oxidation reaction to make aryl
ketones.15
We began our studies by evaluating the oxygenation of
ethylbenzene (1a) to acetophenone (2a), which proved to be
challenging in our prior work.13 Key results of condition
optimization are briefed in Table 1. The initial search for
suitable oxidation conditions indicated that persulfates were
ineffective oxidants (entries 1–2). We then moved to organic
oxidizing agents. A wide array of peroxides and peroxy acids
was thus examined (entries 3–8), among which, we were glad
to nd that the desired product 2a could be obtained with
a superior 91% yield in the presence of TBHP (entry 8). Further
investigation revealed that the amount of oxidizing agents is
crucial to the reaction outcome, as the desired oxidation
product was obtained with only 31% yield when 3 equivalents
of TBHP was employed instead. It's also worth mentioning
Scheme 1 Substrate scope.ab a Alkylarene or diarylmethane (1 mmol),
TBHP (hexane solution, 2 ml, ꢁ12 equiv.), 130 ꢀC in pressure tube.
b
Isolated yields. c GC yields.
a decreased reaction temperature is detrimental to the rate of products in 81% and 87% yields respectively. Furthermore, dia-
the oxidation step as well as the chemical yield, which is rylmethanes were compatible substrates, and reacted under the
probably due to a reduced rate of radical generation step via standard conditions to give the diarylketones (2h–2l) in good to
thermal decomposition. Only moderate yield was attained excellent yields. In general, both the electron-donating and
when the transformation was performed at 100 ꢀC (51% yield, electron-withdrawing substitution groups had little effects on the
entry 10).
chemical yields of oxidation products (2h–2l). In addition, uo-
With these optimized conditions in hand, we next began an rene (1m) proved to be suitable substrate, giving uorenone (2m)
exploration of the substrate scope. As revealed in Scheme 1, the in 81% yield. Notably, this oxidation protocol was discovered to
benzylic C–H oxidation transformation could smoothly convert be slightly sensitive to the steric hindrance at ortho-position of
alkylarenes to the corresponding aryl ketones in excellent yields aromatic ring as 1-napthyl substituted diarylmethane substrate
(2a–2c). 1,4-Diacetylbenzene (1d), which is a common building (1n) displayed lower reactivity to give only 75% yield to the
blocks for uorophore type compound,16 could be obtained desired ketone product 2n. Given the importance and synthetic
from corresponding starting material in 81% yield. Next, cyclic challenges of nitrogen-containing heterocycles, we were
alkylarenes such as 2,3-dihydro-1H-indene (1e) and 1,2,3,4-tetra- delighted to nd that pyridyl substituted substrate also reacted
hydronaphthalene (1f) were employed to deliver the target ketone under this metal-free oxidation conditions to give the heteroaryl
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ketone 2g and 2o in good yields. Noteworthy, these pyridine-
containing substrates oen showed poor reactivity under metal-
mediated benzylic oxidation reaction, which is probably due to
the product inhibition via chelation.12 Finally, these optimized
reaction conditions were examined for the oxygenation of phar-
maceutically relevant imidazole substrates. 2-Benzoyl-1H-
benzimidazole (2p), the core structural motif of potent phos-
phodiesterase inhibitor AMG 580, could be directly prepared in
70% yield.3l
To expand the synthetic utility of this oxidative protocol, we
then tested other benzylic sp3 C–H compounds as substrates.
Under the identical reaction conditions, 2-phenylacetophenone
(2q) underwent the oxidation reaction to give the benzoic acid
(3q) instead of the desired benzyl product in 70% isolated yields
(Scheme 2).10b
The scale-up synthesis of 2h also highlighted the utility and
operational simplicity of this procedure. When the reaction was
run at one-gram scale with diphenylmethane (1h), the product
(2h) could still be obtained in a comparable 94% isolated yield
(Scheme 3).
Scheme 5 Proposed mechanism.
was stopped aer 5 hours, the oxidation product 2h and radical
adducts 4 were both observed in 66% and 30% yields respectively.
When intermediate 4 was subjected to the standard reaction
conditions, the corresponding product 2h was also obtained in
91% yield. When the diphenylmethanol (5) was subjected to the
standard reaction conditions, the desired ketone product 2h was
also obtained with 95% yield. These results indicated that this
transformation might involve benzylic peroxides and/or benzylic
alcohols as the reaction intermediates.
To gain insight into this oxidative reaction, control experi-
ments were employed as shown in Scheme 4. Firstly, the reaction
On the basis of the above-mentioned experimental results and
previous reports,17 a plausible reaction mechanism was proposed
in Scheme 5. First, the decomposition of TBHP generated the
oxygen-based radicals, which then underwent hydrogen
abstraction of substrate A to generate intermediate B. The radical
intermediate B can either react with cOH or cOOtBu to form the
oxygenated intermediate C or D. The intermediate D either
eliminated one molecule of tBuOH to directly form ketone E, or
transformed into the benzyl alcohol C, which then underwent
further oxidation to give nal aryl ketone product E.
Scheme 2 Oxidative reaction of 2-phenylacetophenone.
Scheme 3 Scale-up synthesis.
Conclusions
In summary, a simple metal-free TBHP system has been iden-
tied for direct oxidation of benzylic methylene groups to the
corresponding aryl ketones. The radical-based reaction pathway
tolerates a large variety of benzylic C–H bonds, giving the
desired product in good to excellent yields. More importantly,
the methodology is amenable to challenging heterocyclic
substrates, such as pyridines and imidazoles. The synthetic
utility of this protocol was further showcased in the gram-scale
synthesis of diphenylketone. Further studies on both reaction
scope and reaction mechanism are currently under investiga-
tion in our group.
Acknowledgements
We are grateful to Fundamental Research Funds for the Central
Universities in Beijing University of Chemical Technology, and
the Natural Science Foundation of China (21602119, U1504208),
Scheme 4 Control experiments.
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