RESEARCH
| REPORT
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Fig. 5. Mechanistic studies. (A) Radical scavenger experiments. (B) SEM images of BaTiO3 particles (i) before
and (ii) after ball milling (30 Hz; 60 min). (C) Thermographic image of the milling jar after the mechanoredox
arylation of 2a with 1a. See supplementary materials for details. GC-MS, gas chromatography–mass spectrometry.
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a regular shape of the BaTiO3 particles before
the reaction (approximate size, <75 mm) [Fig.
5B (i)]. Subsequently, the powder was sub-
jected for 60 min to ball milling at 30 Hz and
then analyzed by SEM. The resulting image
clearly shows a pronounced distortion of the
shape and a decrease in the size of the BaTiO3
particles [Fig. 5B (ii)]. These results suggest
that the mechanical stimulus provided by ball
milling is efficiently transferred onto the BaTiO3
particles, which would result in the generation
of localized electrochemical potentials on the
surface of the BaTiO3 particles that can be used
for the activation of the aryl diazonium salts.
To investigate whether the friction during
ball milling generates a thermal effect, the tem-
perature inside the milling jar during the mech-
anoredox arylation of 2a with 1a was measured
using thermography immediately after opening
the jar (Fig. 5C). The crude mixtures were pre-
pared under optimized conditions. The obtained
image showed that the temperature after aryla-
tion in the ball mill was around 30°C, which
discounts the possibility of thermal activa-
tion of the aryldiazonium salts generating aryl
radical species by the heat provided from ball
milling.
wrapped in a piece of a weighing paper and
placed in a zipper-locking plastic bag, followed
by striking with a hammer over 200 times.
Even under these crude conditions, the mech-
anoredox borylation product 5i was obtained
in 43% yield, as assessed by nuclear magnetic
resonance (NMR) integration.
The present mechanoredox reactions can be
carried out on gram scale without the use of
large amounts of dry and degassed organic
solvents in air, and the reactions do not re-
quire special operating conditions. This oper-
ational simplicity suggests that the present
approach may complement existing photo-
redox transformations in a practical and en-
vironmentally friendly manner. Beyond the
immediate benefits of this protocol, our strat-
egy could be applicable to light-sensitive or
light-absorbing substrates that cannot be sub-
jected to conventional photoredox systems.
ACKNOWLEDGMENTS
Funding: This work was supported by the Japan Society for the
Promotion of Science (JSPS) through KAKENHI grants 18H03907,
17H06370, and 19K15547; by the JST CREST grant number
JPMJCR19R1; and by the Institute for Chemical Reaction Design and
Discovery (ICReDD), which was established by the World Premier
International Research Initiative (WPI), MEXT, Japan. Y.P. thanks the
Otsuka Toshimi Scholarship Foundation for a scholarship. We thank
D. F. Toste and T. Shimada for advice on the preparation of this
manuscript. We thank Nippon Chemical Industrial Co., Ltd. for the
gift of piezoelectric materials. Author contributions: H.I. came up with
the original idea; K.K. and H.I. directed the project; K.K. and H.I.
designed the experiments; K.K. and Y.P. performed the experiments;
A.M. prepared ceramic materials used in this study; and K.K. and H.I.
wrote the manuscript with feedback from the other authors.
Competing interests: H.I. and K.K. are inventors on patent application
JP, 2019-163323 submitted by Hokkaido University that covers
mechanoredox reactions using piezoelectric materials and
REFERENCES AND NOTES
manufacturing methods using the reaction setup with the ball mill.
The authors declare no other competing interests. Data and materials
availability: All data are available in the supplementary materials.
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SUPPLEMENTARY MATERIALS
Materials and Methods
Figs. S1 to S9
Tables S1 to S5
NMR Spectra
References (45–58)
Movie S1
To demonstrate the robustness of the mech-
anoredox transformations, we conducted the
borylation of 1k with BaTiO3 in air using a
hammer (fig. S8 and movie S1). First, the re-
action mixture was prepared by gentle grind-
ing in a mortar. Subsequently, the mixture was
22 July 2019; accepted 30 October 2019
10.1126/science.aay8224
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