T-BuONa-Mediated Transition-Metal-Free Autoxidation of Diarylmethanes to Ketones

Article abstract of DOI:10.1055/s-0036-1588928

Autoxidative sp³ C-H transformation of diarylmethanes has been demonstrated using O₂-mediation by t-BuONa. This protocol enables an alternative route for the access to diaryl ketones from benzyl derivatives in good to excellent yields under mild reaction conditions, without transition metal catalysts or additional chemical oxidants.

Full text of DOI:10.1055/s-0036-1588928

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© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–E
letter
A
J.-S. Li et al.
Letter
Synlett
t-BuONa-Mediated Transition-Metal-Free Autoxidation of Diaryl-
methanes to Ketones
Jiang-Sheng Li*
Fan Yang
O
t-BuONa, O2
R2
R2
R1
R1
Qian Yang
DMSO, 50 °C
Zhi-Wei Li
• C–H oxygenation
• O2 as sole oxidant
• No transition metal
up to 98% yield
22 examples
R1, R2 = Me, OMe, F,
Cl, Br, NO2
Guo-Qin Chen
Yu-Dong Da
Peng-Mian Huang
Chao Chen
Yuefei Zhang
Ling-Zhi Huang
Hunan Provincial Key Laboratory of Materials Protection for
Electric Power and Transportation, School of Chemistry and
Biological Engineering, Changsha University of Science &
Technology, Changsha 410114, P. R. of China
jslichem@gmail.com
jsli@csust.edu.cn
Received: 29.10.2016
Accepted after revision: 04.12.2016
Published online: 30.01.2017
useful intermediates in the construction of new C-based
chemical bonds or heterocycles.6 Traditionally, the synthet-
ic methods of aryl ketones6a include classical Friedel–Crafts
acylation of arenes,7 oxidation of secondary alcohols,8 CO
insertion reactions9 and transition-metal-catalyzed cou-
pling reactions.10 Notably, the latter has been well devel-
oped with the advancement of C–H activation strategies.
However, toxic or expensive metal catalysts, harmful oxi-
dants and harsh reaction conditions are still involved in
most of the transformations. In addition, the oxygenative
carbonylation of benzylic sp3 C–H bonds is an alternative
powerful tool to access aryl ketones. The well-documented
strategies employed for such a transformation include oxi-
dation using various stoichiometric chemical oxidants,11
photo-oxygenation mediated by light,12 and electrosynthe-
sis by electroxidative C–H activation.13 Recently, transition-
metal-free autoxidative coupling has been attracting exten-
sive interest from chemists.14 We envisioned that the au-
toxidative carbonylation of benzyl derivatives using O2
without transition metal catalysts or chemical oxidants
would occur. Herein, we demonstrate a transition-metal-
free oxygenation of benzylic sp3 C–H bonds by base using
an O2-promoted process, allowing the construction of dia-
ryl ketones from benzyl derivatives under mild conditions.
Initially, we set out to optimize the reaction conditions
for the autoxidative conversion of diarylmethanes to the
carbonylation products using diphenylmethane (1a) as the
model substrate. To achieve the optimal reaction parame-
ters, a series of factors such as temperature, bases, solvents,
molecular oxygen sources and so on were investigated, and
the results are summarized in Scheme 1 and Table 1. Con-
sidering the poor reactivity of diphenylmethane sp3 C–H
DOI: 10.1055/s-0036-1588928; Art ID: st-2016-w0736-l
Abstract Autoxidative sp3 C–H transformation of diarylmethanes has
been demonstrated using O2-mediation by t-BuONa. This protocol en-
ables an alternative route for the access to diaryl ketones from benzyl
derivatives in good to excellent yields under mild reaction conditions,
without transition metal catalysts or additional chemical oxidants.
Key words Ketones, benzyl derivatives, autoxidation, metal-free, oxy-
genation
Benzylic oxidative C–H functionalization is among the
most widely used and fundamental transformations in aca-
demic and industrial areas,1 which allows the production of
various carbonyl compounds such as aldehydes, ketones,
and carboxylic acid derivatives. For this oxidation process,
molecular oxygen (O2) is considered to be of the best choice
thanks to its low cost and the emission of water as a unique
by-product.2 However, it is not reactive enough to inert C–H
bonds and cannot be readily inserted into unactivated C–H
bonds. Thus, the presence of transition metal catalysts has
usually been required to promote oxidative reaction effi-
ciency,1 albeit with the problem of the metal residue and
environmental pollution.3 During the past decades, great
progress has been made in the benzylic C–H functionaliza-
tion;4 nevertheless, there remains a high need to seek sim-
ple, green and efficient synthetic methods for the accom-
plishment of such transformations.
Aryl ketones, in general, serve as structural units in nu-
merous pharmaceuticals, naturally-occurring products, and
organic functional materials.5 Also, they are widely used as
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

B
J.-S. Li et al.
Letter
Synlett
bonds to the direct insertion of molecular oxygen, the
strong base t-BuONa was chosen to deprotonate the sub-
strate so as to form a carbanion, which is very reactive to
molecular oxygen. Delightedly, the use of t-BuONa in anhy-
drous DMSO at room temperature under O2 balloon
smoothly delivered the carbonylation product 2a in moder-
ate yield (52%; Table 1, entry 1), albeit with part of 1a un-
converted. Thus, different reaction temperatures were at-
tempted. It turned out that with the temperature increas-
ing, the efficiency climbed up to the best yield (92%) at the
range of 50–60 °C (Table 1, entries 4 and 5), and the reac-
tion underwent to completion within less than one hour.
From the viewpoint of energy consumption, we chose 50 °C
as our preferred reaction temperature in this autoxidative
reaction.
as K2CO3 and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)
failed to give the desired product with 1a intact (Table 1,
entries 10 and 11), in spite of the prolonged reaction time.
It may result from their inability to effectively snatch hy-
drogen atom from 1a. Furthermore, tuning the amount of t-
BuONa (see Supporting Information) indicated that de-
creasing the amount (< 2 equiv) led to the remarkable ero-
sion of the yield due to the incomplete conversion of 1a.
Further increasing the base amount did not give higher
yield.
As for the solvents, the reaction underwent smoothly in
aprotic and highly polar solvents (Table 1, entries 1–9 and
12). For example, DMSO furnished the best yield (see Sup-
porting Information). By contrast, the transformation did
not occur in protic solvents or aprotic but lower polar sol-
vents (MeOH, MeCN, EtOAc, THF and CH2Cl2), with the
starting material quantitatively recovered (Table 1, entries
13–17).
Table 1 Screening Parameters for Autoxidative Ketone Formationa
O
The molecular oxygen source also exerted a significant
impact on the efficiency (Table 1, entry 18). The reaction in
an open flask for five hours provided a medium yield (Table
1, entry 18). It is worthwhile to note that extreme removal
of water or moisture in solvent, oxygen, and reaction sys-
tem favored the efficiency, probably due to the suppression
of the oxidative fission of benzophenone C(C=O)–C bond.15
Having established the optimal reaction conditions,16
we attempted to investigate the scope of the benzyl com-
pounds. The data listed in Scheme 1 reveals that this autox-
idative reaction underwent smoothly for the various sub-
strates tested, all giving good to excellent yields. It appears
that the presence of electron-withdrawing groups in-
creased the oxidation reactivity and selectivity of the ben-
zylic C–H bonds in the presence of a strong base (2h–l),
while the introduction of electron-donating substituents
decreased the reaction efficiencies (2b–g). This may result
from the acidity enhancement of the benzylic C–H bonds
by electron-withdrawing groups, and as a consequence the
ease to abstract the hydrogen atom by base, and thus the
susceptibility of carbanion to oxygen. Meanwhile, the elec-
tron-drawing substituents suppressed the oxidation of aro-
matic rings, whereas the electron-donating substituents
gave rise to an opposite impact. Unlike the electronic ef-
fects, however, steric effects exerted no significant influ-
ence. For instance, Me, OMe located at ortho- or para-posi-
tion offered similar yields (2b vs 2c, 2f vs 2g). Furthermore,
this protocol was successfully applicable to the substitu-
tions on the two arenes (2m–v). When both benzene rings
of the substrates possessed electron-donating groups such
as methyl, the reaction gave a lower but nonetheless good
yield (2c vs 2m).
Ph
Ph
Ph
Ph
Entry
Base
Oxidant Solvent
Time (h) Temp (°C) Yield (%)
1
2
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuOK
NaH
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
air
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
18
18
5
25
30
40
50
60
50
50
50
50
50
50
50
50
50
50
50
50
50
52
60
3
85
4
0.5
92
5
0.5
0.5
0.5
0.5
0.5
5
92
6
84
7
77
8
KOH
79
9
NaOH
80
10
11
12
13
14
15
16
17
18b
K2CO3
N.R.
N.R.
48
DBU
5
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
0.5
5
MeOH
MeCN
EtOAc
THF
N.R.
N.R.
N.R.
N.R.
N.R.
50
5
5
5
CH2Cl2
DMSO
5
5
a Reactions were carried out using diphenylmethane (0.2 mmol) and base
(0.4 mmol) in anhyd solvent (0.5 mL) under O2 balloon for the indicated
time.
b Carried out in an open flask.
Later on, screening other strong bases such as t-BuOK,
NaH, KOH and NaOH showed that slightly lower efficiencies
were observed (Table 1, entries 6–9). This suggests that the
addition of a strong base accelerated the oxidation reaction
without a long induction period as required in usual C–H
bond autoxidation. However, the use of weaker bases such
When an electron-withdrawing group was introduced
to the system bearing an electron-donating group, the yield
occurred between that of the system with only electron-do-
nating group and that with only electron-withdrawing one
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

C
J.-S. Li et al.
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t-BuONa, O2
O
Ar1
Ar2
Ar1
Ar2
DMSO, 50 °C
O
O
O
O
O
2b 89%
O
2c 88%
O
2a 92%
O
2e 82%
O
2d 88%
O
OMe
OMe
F
Cl
Cl
Br
2g 85%
2j 93%
2f 88%
2h 94%
2i 95%
O
O
O
O
O
Cl
Cl
NO2
Cl
2k 92%
2l 96%
2m 80%
2n 89%
2o 85%
O
O
OMe
O
Cl
O
OMe
Cl
O
Cl Cl
Cl
OMe
OMe
2p 89%
2q 89 %
2r 88%
2s 89%
2t 86%
O
O
F
F
Cl
Cl
2u 97%
2v 98%
Scheme 1 Substrate scope for autoxidative ketone formation. Reactions were carried out using 1 (0.4 mmol) and t-BuONa (0.8 mmol) in anhyd DMSO
(1.0 mL) under O2 balloon for 1 h.
(2d vs 2p, 2e vs 2o). When connected with both electron-
withdrawing groups, the yields turned out to be excellent
and amounted up to 98% (2v).
perature (up to 100 °C). Similarly, dibenzyl ether was an in-
ert substrate, which could not furnish its corresponding
benzyl benzoate.
In particular, the electron-donating groups such as Me
or OMe were well tolerated in this reaction although they
possess benzylic C–H bonds and are prone to oxidation. The
C-halo groups were compatible with this procedure and the
oxygenation products were obtained in excellent isolated
yields (2h–k,n–v). The NO2 group delivered the desired ke-
tone almost quantitatively (2l) due to its strong activation
of the benzylic C–H bonds and the nature of strongly re-
tarding the oxidation of aromatic rings. Importantly, the
compatibility of halo and nitro groups allow the possibility
for further post-functionalization based on the C-halo acti-
vation17 or hydrogen-borrowing strategies.18
With the successful oxygenation of diarylmethanes, we
had expected to expand this protocol to the oxygenative
carbonylation of the monoaryl systems. Much to our disap-
pointment, however, 1,2,3,4-tetrahydronaphthalene, 1-(4-
ethylphenyl)ethan-1-one, ethylbenzene, or 1-ethyl-4-nitro-
benzene were inert in such transformation, even on in-
creasing the amount of base or elevating the reaction tem-
To gain an insight into the mechanism for the autoxida-
tive reaction, several control experiments were conducted
(Scheme 2). First, the traditional radical scavenger 2,2,6,6-
tetramethyl-1-piperidinyloxyl (TEMPO) was used in this
transformation. Although the reaction was not completely
inhibited, the isolated yield was remarkably reduced to 30%
using two equivalents of TEMPO. Moreover, the TEMPO-ad-
1
duct was isolated and confirmed by H NMR and 13C NMR
spectra. This suggested that the reaction likely involved a
radical process. Second, when we monitored the reaction
process using TLC and GC–MS analysis, the presence of
benzhydrol was observed, which reminded us of the benzyl
alcohols being a possible intermediate in the autoxidative
formation of the ketones. Thus, we used benzhydrol as the
substrate under the standard conditions. It turned out that
the reaction completed within 30 minutes, and resulted in
an almost quantitative conversion into the corresponding
ketone 2a.
Finally, we attempted to carry out this standard reaction
under argon atmosphere using anhydrous air-degassed
DMSO, via three oxygen-evacuating/argon-refilling cycles.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

D
J.-S. Li et al.
Letter
Synlett
transformation smoothly undergoes under mild reaction
conditions without transition metal catalysts or additional
chemical oxidants. Extensive investigation on the autoxida-
tive reactions of other sp3 C–H bonds is still underway.
(a) Radical-trapping experiment
O
Ph
TEMPO (2.0 equiv)
standard conditions
+
Ph
Ph
N
Ph
Ph
Ph
O
1a
2a 30%
(b) Benzhydrol as an intermediate
TEMPO-adduct
O
OH
standard conditions
Acknowledgment
Ph
Ph
Ph
Ph
2a 98%
This work was supported by the National Natural Science Foundation
of China (21202010 & 21376031), the Hunan Provincial Natural Sci-
ence Foundation of China (2015JJ3012), the Scientific Research Fund
of Hunan Provincial Education Department (16B003), the Hunan Pro-
vincial Science and Technology Project (2013FJ3076), the Hunan Pro-
vincial Key Laboratory of Materials Protection for Electric Power and
Transportation (2015CL05), Changsha University of Science & Tech-
nology, P. R. of China.
(c) O2-free experiment
t-BuONa, Ar
no 2a
Ph
Ph
DMSO, 50 °C, 24 h
Scheme 2 Control experiments
The TLC and GC–MS analyses showed that no reaction oc-
curred, with only the starting material detected. This
means that in this transformation, molecular oxygen is in-
dispensable.
Supporting Information
Base-mediation suggests that this autoxidative carbon-
ylation most likely involves an anion-radical oxidation rath-
er than a pure free-radical process. Based on the document-
ed reports12a,19 and our experimental observations, a tenta-
tive mechanism is proposed for this base-promoted
autoxidative formation of diaryl ketones,20 as shown in
Scheme 3. In the case of diphenylmethane (1a), the t-BuO
anion initially abstracts the hydrogen atom from 1a to form
a carbanionic intermediate 3, which is oxidized by oxygen
to intermediate free radical 4. The radical 4 would be rapid-
ly converted into a peroxy radical 5 in the presence of ex-
cess oxygen. Undoubtedly, this rapid consumption of 4 ac-
counts for the absence of the homocoupled product. Then,
the peroxy radical 5 can be readily transformed into a hy-
droperoxidate intermediate 6 by the proton abstraction
from the species such as t-BuOH or 1a in the reaction sys-
tem. Next, this intermediate 6 loses one water molecule to
yield the desired ketone 2 or otherwise converts to the ben-
zyl alcohol 7, which undergoes further autoxidative trans-
formation mediated by base to finally produce the ketone 2.
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0036-1588928.
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References and Notes
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OO
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Scheme 3 Plausible mechanism
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In conclusion, we have demonstrated an autoxidative
oxygenation of diarylmethanes sp3 C–H bonds in the pres-
ence of O2-mediation by t-BuONa. This protocol allows for
an alternative method for ready access to diaryl ketones
from benzyl derivatives in good to excellent yields. The
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J.-S. Li et al.
Letter
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(16) Typical Procedure for t-BuONa-Mediated Autoxidation of
Diarylmethanes to Ketones: To a predried 5-mL round-bottom
flask diarylmethane 1a (0.4 mmol, 67 mg, 1 equiv), anhyd
DMSO (0.4 mol/L), and t-BuONa (0.8 mmol, 77 mg, 2 equiv)
were subsequently added as soon as possible. The reaction
system was sealed by a rubber septum with a needle connected
to an O2 balloon, and then stirred at 50 °C for 1 h. During this
period, the reaction system suffered complex color changes.
The usual workup afforded the desired ketone 2a as a white
solid (yield: 92%, 67 mg); mp 48–49 °C. 1H NMR (400 MHz,
CDCl3): δ = 7.81 (d, J = 7.7 Hz, 4 H), 7.59 (t, J = 7.3 Hz, 2 H), 7.48
(t, J = 7.5 Hz, 4 H). 13C NMR (101 MHz, CDCl3): δ = 196.8, 137.6
(2 × C), 132.4 (2 × C), 130.1 (4 × C), 128.3 (4 × C). EI–MS: m/z =
182.0 [M]+.
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E