Practical Synthesis of Allyl, Allenyl, and Benzyl Boronates through SN1′-Type Borylation under Heterogeneous Gold Catalysis

Article abstract of DOI:10.1021/acscatal.0c03771

Efficient borylation of sp3 C-O bonds by supported Au catalysts is described. Au nanoparticles supported on TiO2 showed high activity under mild conditions employing low catalyst loading conditions without the aid of any additives, such as phosphine and bases. A variety of allyl, propargyl, and benzyl substrates participated in the heterogeneously catalyzed reactions to furnish the corresponding allyl, allenyl, and benzyl boronates in high yields. Besides, Au/TiO2 was also effective for the direct borylation of allylic and benzylic alcohols. A mechanistic investigation based on a Hammett study and control experiments revealed that sp3 C-O bond borylation over supported Au catalysts proceeded through SN1′-type mechanism involving the formation of a carbocationic intermediate. The high activity, reusability, and environmental compatibility of the supported Au catalysts as well as the scalability of the reaction system enable the practical synthesis of valuable organoboron compounds.

Full text of DOI:10.1021/acscatal.0c03771

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Research Article
Practical Synthesis of Allyl, Allenyl, and Benzyl Boronates through
SN1′-Type Borylation under Heterogeneous Gold Catalysis
Hiroki Miura,* Yuka Hachiya, Hidenori Nishio, Yohei Fukuta, Tomoya Toyomasu, Kosa Kobayashi,
Yosuke Masaki, and Tetsuya Shishido*
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ABSTRACT: Efficient borylation of sp3 C−O bonds by supported Au
catalysts is described. Au nanoparticles supported on TiO2 showed
high activity under mild conditions employing low catalyst loading
conditions without the aid of any additives, such as phosphine and
bases. A variety of allyl, propargyl, and benzyl substrates participated in
the heterogeneously catalyzed reactions to furnish the corresponding
allyl, allenyl, and benzyl boronates in high yields. Besides, Au/TiO2
was also effective for the direct borylation of allylic and benzylic
alcohols. A mechanistic investigation based on a Hammett study and control experiments revealed that sp3 C−O bond borylation
over supported Au catalysts proceeded through SN1′-type mechanism involving the formation of a carbocationic intermediate. The
high activity, reusability, and environmental compatibility of the supported Au catalysts as well as the scalability of the reaction
system enable the practical synthesis of valuable organoboron compounds.
KEYWORDS: gold, heterogeneous catalyst, borylation, allylic substitution, allenyl boronate, benzyl boronate, carbocation
Scheme 1. Proposed Mechanisms for C−B Bond Formation
in Allylic Borylation
1. INTRODUCTION
Organoboron compounds are versatile and essential reagents
in organic synthesis for obtaining value-added molecules such
as pharmaceuticals, agrochemicals, and functional materials.1
Particularly, extensive efforts have been devoted to exploit
novel protocols for synthesizing allyl and allenyl boronates,2
which are used as allyl, allenyl, and propargyl sources in
transition-metal-catalyzed cross-coupling.1e,3 In addition, val-
uable homoallyl- and homopropargyl alcohols and amines can
be synthesized from these organoboron compounds upon
treatment with carbon electrophiles such as aldehydes, ketones,
and imines.4 Catalytic borylation of sp3 C−X bonds (X = O
and halogen) under the influence of transition metals such as
Cu,5 Ni,6 Pd,7 and Fe8 is the most promising method for
preparing allyl, benzyl, and allenylboron compounds, in which
reductive elimination from π-allyl boryl metal species (Scheme
1a) or SN2′ reaction by copper-boryl species (Scheme 1b)
result in efficient C−B bond formation. Recent extensive
studies on the stereospecific synthesis of chiral organoboron
compounds further increased the importance of these
protocols in synthetic chemistry. Despite the significant utility
of the catalytic C−X bond borylation, major tools are still
limited to homogeneous catalyses based on metal complexes
which often face the difficulty of separating and recycling
catalysts as well as contamination by toxic metals. Moreover,
one reason why it is difficult to realize reactions at a scale
beyond lab scale is that additives such as phosphines and bases
are indispensable in most reported C−X bond borylation
under transition-metal catalysis. Although base-catalyzed
Received: August 29, 2020
Revised: November 20, 2020
Published: January 1, 2021
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borylation of allylic alcohols is also an attractive tool in allylic
borylation,9 the problem of limited substrate scope is still
unresolved. On the other hand, the use of heterogeneous
catalysis based on supported metal nanoparticles (NPs) can
provide a reliable solution thanks to their high stability,
reusability, and ease with which they can be separated from the
products. These features make it possible to synthesize
valuable chemicals on a practical scale in a green manner.
Although Morken and co-workers reported that Pd/C showed
activity for the conversion of an allyl chloride to an allyl
boronate,6a the development of a novel catalytic system that
shows high activity and reusability, as well as a wide substrate
scope is still largely unexplored. Meanwhile, in the last decade,
supported gold nanoparticles have emerged as efficient
heterogeneous catalysts for the synthesis of organoboron
compounds,10 which stimulated us to devise gold-based
catalytic system for C−X bond borylation. Herein, we describe
the catalytic borylation of sp3 C−O bonds over supported Au
catalysts. While the Au-catalyzed borylation of allyl, benzyl,
and propargyl esters required no aid of additives, a series of the
corresponding allyl, allenyl, and benzyl boronates were
obtained in high yields. A detailed investigation on the
reaction mechanism elucidated that the formation of C−B
bonds over Lewis acidic Au NPs proceeded in the SN1′ manner
involving the formation of a cationic intermediate (Scheme
1c), thereby leading to a high reaction efficiency and broad
substrate scope. Furthermore, supported Au catalysts showed
excellent reusability, scalability, and environmentally friendly
nature, which should be key characteristics for the practical
synthesis of organoboron compounds.
reported to be effective catalytic components under homoge-
neous conditions, their metal-NP variants were ineffective
(entry 2). A survey to identify the optimal solvent revealed that
1,4-dioxane was the solvent of choice for the present Au NP-
catalyzed borylation, while THF, toluene, acetonitrile, and 1,2-
dichloroethane were unsuitable (entries 3−5). The supported
Au catalyst showed high activity, and the reaction, even at 30
°C, gave 3a in 83% yield (entry 6). In addition to an acetoxy
t
(−OAc) group, carbonate (−OCO2Me and −OCO2 Bu) and
phosphate (−OPO(OEt)2) groups also served as suitable
leaving groups to give 3a in comparable yields (entry 7). In
contrast, 3a was not obtained at all in the reaction of cinnamyl
chloride (entry 8). The high catalytic activity of Au/TiO2 was
further well reflected in the reaction with a low catalyst loading
(substrate/Au = 5000), to provide 3a in satisfactory yield.
Notably, the turnover number of 3150 was the highest value
ever reported in the catalytic borylation of sp3 C−X bonds
(entry 9, data for comparison are shown in the Supporting
Information).
The scope of allylic substrates in the Au NP-catalyzed C−O
bond borylation was investigated (Table 2). The reactions of
primary allyl acetates provided the corresponding allyl
boronates in good to high yields, and boryl groups were
delivered predominantly at terminal positions (entries 2−5).
Although the reaction of the simplest allylic acetate required at
least 1 h to furnish 3b in moderate yield (entry 1), the
installation of substituents on the alkene moiety shortened the
time required for full conversion of substrate and increased
product yields (entries 2 and 3). The present heterogeneous
Au-catalyzed reaction can deliver a boryl group in terpene-
based scaffolds to provide 3e in excellent yields (entries 4 and
5). The E/Z configuration of the starting substrates (1e and
1f) did not affect the reaction efficiency, and the E/Z
configurations at alkene moieties were not transferred to the
corresponding allyl boronates. The borylation of secondary
and tertiary allyl acetates also proceeded efficiently to give the
corresponding allyl boronates in excellent yields (entries 6−
15). In the reaction of secondary allyl acetates bearing a
terminal alkene moiety (1g−1j), C−B bonds were formed
exclusively at the γ-position (entries 6−9). On the other hand,
the introduction of a methyl group at the γ-position of
secondary allyl acetate led to the formation of a mixture of
regioisomers (3k-α and 3k-γ) (entry 10). In contrast, a
substrate containing a phenyl substituent at the γ-position of
allyl acetate underwent C−B bond formation selectively at the
α-position to afford secondary allyl boronate 3l (entry 11). A
methyl substituent at the β-position was also tolerated in the
Au NP-catalyzed reaction to give the corresponding boronate
(entry 12). Such regiochemistry in the present borylation
enables us to envisage that C−O borylation over Au NPs
proceeds through reaction mechanism that is different from an
SN2′ mechanism, which is often seen in C−X bond borylations
under Cu catalysis.5 The terminal chloro group remained
intact during the catalytic borylation of 1p (entry 15).
2. RESULTS AND DISCUSSION
Initially, we examined the catalytic activity of a series of metal
NPs supported on TiO2 for the borylation of cinnamyl acetate
(1a) with bis(pinacolato)diboron (2) (Table 1). The reaction
in the presence of supported Au catalyst (2 mol %) at 60 °C
completed within only 15 min and delivered a boryl group
exclusively at the α-position, to give cinnamyl boronate (3a) in
94% yield (entry 1). In contrast, a supported Pt NP catalyst
showed no activity. Although Ni, Cu, and Pd have been
a
Table 1. Borylation of 1a Over Supported Catalysts
b
entry
variation from the standard condition
yield of 3a (%)
1
2
3
4
5
6
7
none
94 (93)
0
11
19
0
Ni, Cu, Pd, or Pt instead of Au
THF instead of 1,4-dioxane
toluene instead of 1,4-dioxane
1,2-DCE or CH3CN instead of 1,4-dioxane
at 30 °C for 1 h
83
t
−OCO2Me, −OCO2 Bu, or −OPO(OEt)2 as LG 72−83
Borylation of cinnamyl acetate (1a) with various diboron
compounds (2′−2‴) other than bis(pinacolato)diboron (2)
was investigated (Scheme 2). Although a higher reaction
temperature was required, the reaction of bis-
(neopentylglycolato)diboron (2′) and bis(hexyleneglycolato)-
diboron (2″) proceeded to furnish the corresponding cinnamyl
boronates (3a′ and 3a″) in 63 and 89% yields, respectively. In
contrast, tetrahydroxydiboron (2‴) was not suitable as a boron
source for the Au-catalyzed borylation.
instead of −OAc, at 30 °C for 1 h
8
9
−Cl as LG instead of −OAc
0.02 mol % of Au/TiO2 at 100 °C for 3 h
0
63
(TON = 3150)
a
Reaction condition: 1a (0.30 mmol), 2 (0.60 mmol), Au/TiO2 (2
b
mol %),1,4-dioxane (1.0 mL), 60 °C, 15 min under Ar. Determined
by H NMR with mesitylene as an internal standard. Isolated yield is
1
shown in parentheses.
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a b cb
, ,
Au/TiO2 also efficiently catalyzed benzylic borylation
Table 2. Synthesis of Allyl Boronate over Au/TiO2
without the aid of any additives (Table 3).5e,11,12 The reaction
a b c
, ,
Table 3. Benzylic Borylation over Au/TiO2 Catalyst
a
Reaction conditions: 4 (0.30 mmol), 2 (0.60 mmol), Au/TiO2 (2.0
mol %), 1,4-dioxane (1 mL). Determined by 1H NMR with
tetrachloroethane as an internal standard. Isolated yields are shown
b
c
in parentheses. 2 (0.90 mmol). Reaction at 100 °C.
of benzyl acetate (4a) with 2 furnished the corresponding
benzyl boronate (5a) in 69% yield. Electron-donating methoxy
and methyl substituents positively affected the yield of the
corresponding benzyl boronates (5b, 5c, and 5g). Although a
trifluoromethyl group on a phenyl group was not a suitable
substituent for the present borylation of a benzylic C−O bond,
fluoro and chloro groups were tolerated to afford the
corresponding benzyl boronates (5d and 5e) in high yields.
Naphthylmethyl and furylmethyl boronates (5h and 5i) were
also obtained in 89 and 75% yields, respectively. Notably, the
supported Au catalyst was remarkably effective for the
borylation of secondary benzyl acetates. Methyl, secondary,
and tertiary butyl groups as well as an aryl group were
compatible as second substituents at the benzylic position in
the substrates to furnish the corresponding secondary benzyl
boronates (5j−5m) in good to high yields. Besides, the Au
NP-catalyzed reaction allowed the delivery of a boryl group to
indene and chromane scaffolds (5n and 5o).
Au/TiO2 catalyst was also effective for the synthesis of
allenyl boronates (Table 4).4a,5e,13,14 The reactions of
propargyl carbonates 6 and 2 in the presence of Au/TiO2
proceeded under mild reaction conditions to deliver a boryl
group at the γ-position, thus giving only the corresponding
allenyl boronates 7. A characteristic of the Au NP-catalyzed
reaction is its broad scope of propargyl substrates. The reaction
of both secondary and tertiary propargyl carbonates bearing
alkyl substituents gave the corresponding tri- and tetra-
substituted allenyl boronates (7a−7e) in high yields, in
which a terminus bulky tert-butyl group was tolerated in the
Au-catalyzed borylation. Furthermore, 1-boryl-1-silylallene
a
Reaction condition: 1 (0.3 mmol), 2 (0.6 mmol), Au/TiO2 (2 mol
b
%), 1,4-dioxane (1.0 mL), 60 °C under Ar. Determined by 1H NMR
with mesitylene as an internal standard. Isolated yield is shown in
parentheses. 0.9 mmol of 2 was used.
c
Scheme 2. Borylation of Cinnamyl Acetate with Various
Diboron over Au/TiO2 Catalyst
a b
,
derivative (7f) could be synthesized via borylation of γ-
14b,c,e
silylated propargyl carbonate. Although Ito14a and Szabo
́
independently demonstrated seminal Cu- and Pd/Cu-
catalyzed borylation of propargyl carbonates for the synthesis
of allenyl boronates, efficient borylation of aryl-substituted
substrates has not yet been explored satisfactorily. In this
a
Reaction conditions: 1a (0.30 mmol), 2 (0.60 mmol), Au/TiO2 (2.0
b
mol %),1,4-dioxane (1 mL). Isolated yields are given. Au/TiO2 (5.0
mol) was used.
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Table 4. Synthesis of Allenyl Boronates over Au/TiO2
Catalyst
Scheme 3. Borylation of Allylic and Benzylic Alcohols over
Au/TiO2 Catalyst
a b c
, ,
b
The present supported Au catalyst satisfies various essential
requirements for realizing the practical synthesis of organo-
boron compounds. The Au/TiO2 catalyst exhibited high
reusability for the borylation of allyl and propargyl esters,
and no significant decreases in product yields were confirmed
in four consecutive catalytic runs for both allyl and allenyl
boronate synthesis (Figure 1). Furthermore, hot filtration of
a
Reaction conditions: 6 (0.30 mmol), 2 (0.60 mmol), Au/TiO2 (2.0
1
mol %), 1,4-dioxane (1 mL). H NMR yields with mesitylene as an
internal standard are given, and the isolated are shown in parentheses.
b
c
0.90 mmol of 2 was used. Reaction by Au/TiO2 (5.0 mol %) at 70
°C.
Figure 1. Reusability of Au/TiO2 catalyst for sp3 C−O bond
borylationa,b. a Reaction condition for allyl boronate: 1a (0.5 mmol),
2 (1.0 mmol), Au/TiO2 (2.0 mol %), 1,4-dioxane (1.6 mL), 60 °C, 15
min. b Reaction condition for allenyl boronate: 6a (0.5 mmol), 2 (1.0
mmol), Au/TiO2 (2.0 mol %), 1,4-dioxane (1.6 mL), 60 °C, 1 h.
NMR yields are given.
regard, the Au/TiO2 catalyst was remarkably effective for the
synthesis of aryl-substituted allenyl boronates. Substrates with
an aryl group at the α-position provided allenyl boronates in
excellent yields (7g−7i). Moreover, aryl groups at both the α-
and γ-positions were compatible to furnish the bisaryl allenyl
boronate (7m). The reaction of primary propargyl carbonate
proceeded efficiently to provide boronates having a terminal
allene moiety (7o). The Au/TiO2 was also effective for the
borylation of a propargyl carbonate bearing a terminal sp C−H
bond to give allenyl boronate 7p in good yield.
the solid catalyst completely suppressed the further formation
of boronate products. Besides, atomic adsorption spectrometry
revealed no leaching of Au species into the reaction mixture
during the reaction of 1a and 2, proving the supported Au
catalysts can realize the environmentally friendly synthesis of
organoboron compounds.
Allylic alcohols should be used as ideal starting substrates in
the catalytic borylation of sp3 C−O bonds in terms of atom-
economic synthesis.5d,7h Au/TiO2 catalyst significantly pro-
moted the borylation of cinnamyl alcohol (8a) under mild
reaction conditions to give 3a in 81% yield (Scheme 3(1)). In
the catalytic borylation of 8a, the rapid formation of
cinnamyl−OBpin at the initial stage of the reaction and
subsequent formation of 3a accompanied by the consumption
of cinnamyl−OBpin were observed. This indicates that
cinnamyl−OBpin is the intermediate for the borylation of
allylic alcohol and OBpin species served as a good leaving
group for the Au-catalyzed borylation. Au NP enables us to
promote both the formation of an O−B bond and subsequent
C−O bond borylation,15 thereby achieving one-pot substitu-
tion of alcohol to a boryl group. Besides, the Au/TiO2 catalyst
was also effective for the borylation of benzylic alcohol (8b) to
furnish 5c in 86% yield (Scheme 3(2)).
The present Au catalytic system is highly scalable. Even at a
large scale (10 mmol of 1a) and under a low catalyst loading
condition (0.1 mol % as Au), the borylation of 2a proceeded
smoothly to afford 3a in 68% GC yield. If we consider the low
stability of allyl boronates to heat and acids,16 the process
without any purification such as distillation or silica gel
chromatography can avoid the loss of valuable boron reagents.
Notably, the in situ formed allyl boronate 3a could be used as
an allylating reagent without separating from the solid Au
catalyst. The subsequent addition of benzaldehyde and ethanol
allowed us to achieve a one-pot synthesis of a homoallylic
alcohol 9 at gram scale in a high yield (Scheme 4). The above
results demonstrate that the supported Au catalyst possesses
substantial potential for the green and practical synthesis of
valuable organoboron compounds and their derivatives.
To obtain mechanistic insight, we conducted a Hammett
study on the formation rate of benzyl boronates.17 The
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Scheme 4. One-Pot Synthesis of Homoallylic Alcohol from 1a via Au-Catalyzed Borylation under Large-Scale Condition
Hammett plot showed a linear correlation (R2 = 0.992)
between log (kx/kh) and the σ values of the respective para-
substituents, with a ρ value of −3.03 (Figure 2). Furthermore,
Scheme 6. Control Experiments on Borylation of sp3 C−O
Bonds over Au/TiO2 Catalyst
Figure 2. Hammett plot for the rate constants in the benzylic
borylation over Au/TiO2.
acetates18 as well as Mayer−Schuster rearrangement of
propargylic acetates,19 in which the formation of carbocationic
intermediates was proposed. Conversely, Au/TiO2 catalyst
showed no activity for the rearrangement of 1g to 1a under
conditions identical to those for standard C−O bond
borylations (Scheme 6(2)). Furthermore, thermal treatment
of (R)-4o with Au/TiO2 in the absence of diboron 2 resulted
in no decrease in its optical purity (Scheme 6(3)). These facts
suggest that C−O bond cleavage is unlikely to be taken place
through simple oxidative addition, which are often seen in
allylic substitution under Pd or Ni catalysis, but rather that Au
NPs and diboron would work in concert to form an allyl cation
as a key intermediate in the present borylation.
the intermolecular competitive reaction of allyl acetates
bearing different aryl groups at α-position (1h and 1i)
demonstrated that preferential conversion of the substrate
with an electron-donating substituent (Scheme 5). These facts
allowed us to postulate that C−O borylation over supported
Au catalysts proceeds through an SN1 or SN1′ mechanism
involving the formation of cationic (benzyl, allyl, and
propargyl) intermediates.
Accordingly, we examined the chiral transfer of (R)-4-
chromanyl acetate [(R)-4o] to the corresponding benzyl
boronate 5o over Au/TiO2 catalyst. The reactions at both 60
and 30 °C resulted in the formation of 5o with a significantly
decreased optical purity (Scheme 6(1)). These results also
support the notion that racemization, probably due to the
formation of a planar carbocation, is involved in C−O bond
borylation. On the other hand, in the partial conversion of
benzyl acetate, its ee was maintained, indicating that the C−O
bond-cleaving step is irreversible. Note that the constant
optical purity of product 5o during the catalytic reaction means
that no racemization of 5o took place over the Au/TiO2
catalyst. Nolan and co-workers reported that the cationic Au(I)
complex promoted the catalytic rearrangement of allylic
Based on these results, we propose a reaction mechanism for
sp3 C−O bond borylation through an SN1′ pathway over
supported Au catalysts (Scheme 7). Initially, diboron 2 adsorbs
onto the surface of Au NPs, where the interaction of O atom at
boronate with Lewis acidic Au NPs would increase the Lewis
acidity of B atom.20 Subsequently, the ester and alkene
moieties in allyl esters coordinate to boronate and the Au
surface, respectively. This triggers cleavage of the C−O bond
to generate allyl cation with the simultaneous formation of
tetra-coordinated borate.21 Fernandez, Szabo, and co-workers
reported that diborane-base adducts could liberate nucleophilic
́
́
Scheme 5. Intermolecular Competitive Allylic Borylation
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Scheme 7. Possible Mechanism for Borylation of Allyl
Acetate over Au/TiO2 Catalyst
Scheme 8. Consideration on the Regioselectivity for Au-
Catalyzed Allylic Borylation of 1j, 1k, and 1l
boryl species, thus enabling the attack to electrophilic carbon
to form a C−B bond.22 The addition of such activated boron
species to an alkene moiety and subsequent bond isomer-
ization give allyl boronates.
This catalytic borylation should include the isomerization of
less stable allyl cations to a more stable structure, e.g., a
primary to secondary or a secondary to benzylic structure. The
sequences of allylic borylation of 1j−1l are summarized in
Scheme 8 as examples for explaining the regioselectivity of the
products in Table 1. As for the reaction of 1j, secondary allyl
cation directly formed via C−O bond cleavage is more stable
than possible primary allyl cation, which results in delivery of
the boryl group predominantly to the γ-position via
nucleophilic attack of borate in SN1′ manner to form 3j
(Scheme 8(1)). The introduction of a methyl group to alkene
moiety can form two possible secondary allyl cations with
comparable stability that consequently produce the mixture of
two regioisomers (Scheme 8(2)). In contrast, phenyl
substituent allows us to produce a benzylic cation, which is
much more stable than cinnamyl cation. Therefore, the
reaction of 1l selectively undergoes C−B bond formation at
the α-position of allyl acetate (Scheme 8(3)).
(Scheme 6(1)) forces us to anticipate that the reaction consists
of a competition of stereoablative and stereoretentive pathway.
A detailed mechanistic investigation on the stereospecific
borylation over supported Au catalysts is also currently
ongoing in our laboratory.
Additionally, the fact that electron-donating substituents and
an extended π-system stabilize carbocationic intermedia-
tes12b,23 can explain the higher formation rates of electron-
rich arylmethyl boronate (5b, 5c, and 5g), naphthylmethyl
boronate (5h), and bisarylmethyl boronate (5m) than those of
electron-poor arylmethyl boronates (5d−5f) and simple benzyl
boronate 5a (see Table 3). These considerations also support
the notion that the present sp3 C−O bond borylations proceed
in an SN1′ manner. In this mechanism, we surmise that Au NPs
activate boronate to enhance its Lewis acidity10 and stabilize
carbocations generated from allyl electrophiles, thereby
achieving significantly high reaction efficiency and wide
substrate scope. The high stability of byproduct RO-Bpin
due to the high dissociation energy of the O−B bond allows us
to deduce that the reaction is energetically downhill and has an
early transition state. We are currently carrying out a detailed
kinetic and theoretical study to elucidate the regioselectivity-
determining step and transition energies in each elemental
step. Besides, the fact that the complete loss of ee was not
confirmed in the borylation of chiral benzylic acetate (R)-4o
3. CONCLUSIONS
An efficient and versatile sp3 C−O bond borylation with a Au/
TiO2 catalyst was demonstrated. A variety of allyl, benzyl, and
propargyl substrates participated in the heterogeneous Au-
catalyzed reaction without any additives such as ligands and
bases to give the corresponding allyl, benzyl, and propargyl
boronates in high yields. In addition, Au/TiO2 catalyzed the
direct borylation of allylic and benzylic alcohols. The catalysis
by supported Au NPs has various important features related to
activity, reusability, environmental compatibility, and scal-
ability, which should be advantageous for the practical
synthesis of valuable organoboron compounds. A mechanistic
investigation including a Hammett study and control experi-
ments revealed that the borylation over a Au NP catalyst
proceeds through SN1′-type mechanism involving the for-
mation of a cationic intermediate, thereby leading to a high
reaction efficiency and broad substrate scope. Further
applications of supported Au catalysts toward the chiral
synthesis of organoboron compounds and other synthetic
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reactions as well as a theoretical study on the detailed reaction
mechanism are currently underway in our laboratory.
ACKNOWLEDGMENTS
■
This study was supported in part by the Program for Element
Strategy Initiative for Catalysts & Batteries (ESICB) (Grant
JPMXP0112101003), Grants-in-Aid for Scientific Research
(B) (Grant 17H03459), Early-Career Scientists (Grant
19K15362), and Scientific Research on Innovative Areas
(Grant 17H06443) commissioned by MEXT of Japan. The
XAFS experiment at SPring-8 was carried out under the
approval (Proposal No. 2019A1131) of the Japan Synchrotron
Radiation Research Institute (JASRI). The authors thank Ono
of Tokyo Metropolitan University for providing valuable
technical support during the HAADF-TEM observations.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c03771.
Optimization of reaction conditions for C−O bond
borylation, characterization of supported Au catalysts,
1
and H NMR and 13C NMR spectra of the substrates
and products (PDF)
REFERENCES
■
AUTHOR INFORMATION
■
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Corresponding Authors
Hiroki Miura − Department of Applied Chemistry for
Environment, Graduate School of Urban Environmental
Sciences and Research Center for Hydrogen Energy-Based
Society, Tokyo Metropolitan University, Tokyo 192-0397,
Japan; Elements Strategy Initiative for Catalysts & Batteries,
Kyoto University, Kyoto 615-8520, Japan; orcid.org/
0000-0002-2488-4432; Email: miura-hiroki@tmu.ac.jp
Tetsuya Shishido − Department of Applied Chemistry for
Environment, Graduate School of Urban Environmental
Sciences, Research Center for Hydrogen Energy-Based Society,
and Research Center for Gold Chemistry, Tokyo
Metropolitan University, Tokyo 192-0397, Japan; Elements
Strategy Initiative for Catalysts & Batteries, Kyoto University,
Kyoto 615-8520, Japan; orcid.org/0000-0002-8475-
4226; Email: shishido-tetsuya@tmu.ac.jp
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Yuka Hachiya − Department of Applied Chemistry for
Environment, Graduate School of Urban Environmental
Sciences, Tokyo Metropolitan University, Tokyo 192-0397,
Japan
Hidenori Nishio − Department of Applied Chemistry for
Environment, Graduate School of Urban Environmental
Sciences, Tokyo Metropolitan University, Tokyo 192-0397,
Japan
Yohei Fukuta − Department of Applied Chemistry for
Environment, Graduate School of Urban Environmental
Sciences, Tokyo Metropolitan University, Tokyo 192-0397,
Japan
Tomoya Toyomasu − Department of Applied Chemistry for
Environment, Graduate School of Urban Environmental
Sciences, Tokyo Metropolitan University, Tokyo 192-0397,
Japan
Kosa Kobayashi − Department of Applied Chemistry for
Environment, Graduate School of Urban Environmental
Sciences, Tokyo Metropolitan University, Tokyo 192-0397,
Japan
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Yosuke Masaki − Department of Applied Chemistry for
Environment, Graduate School of Urban Environmental
Sciences, Tokyo Metropolitan University, Tokyo 192-0397,
Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.0c03771
Notes
́
The authors declare no competing financial interest.
976. (d) Mao, L.; Bertermann, R.; Emmert, K.; Szabo, K. J.; Marder,
764
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