Correlating the Surface Basicity of Metal Oxides with Photocatalytic Hydroxylation of Boronic Acids to Alcohols

Article abstract of DOI:10.1002/anie.201805395

Photoredox catalysis provides opportunities in harnessing clean and green resources such as sunlight and O₂, while the acid and base surface sites of metal oxides are critical for industrial catalysis such as oil cracking. The contribution of metal oxide surfaces towards photocatalytic aerobic reactions was elucidated, as demonstrated through the hydroxylation of boronic acids to alcohols. The strength and proximity of the surface base sites appeared to be two key factors in driving the reaction; basic and amphoteric oxides such as MgO, TiO₂, ZnO, and Al₂O₃ enabled high alcohol yields, while acidic oxides such as SiO₂ and B₂O₃ gave only low yields. The reaction is tunable to different irradiation sources by merely selecting photosensitizers of compatible excitation wavelengths. Such surface complexation mechanisms between reactants and earth abundant materials can be effectively utilized to achieve a wider range of photoredox reactions.

Full text of DOI:10.1002/anie.201805395

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Accepted Article
Title: Correlating the Surface Basicity of Metal Oxides with
Photocatalytic Hydroxylation of Boronic Acids to Alcohols
Authors: Wan Ru Leow, Jiancan Yu, Bin Li, Benhui Hu, Wei Li, and
Xiaodong Chen
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10.1002/anie.201805395
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Correlating the Surface Basicity of Metal Oxides with
Photocatalytic Hydroxylation of Boronic Acids to Alcohols
Wan Ru Leow,[a] Jiancan Yu,[a] Bin Li,[a] Benhui Hu,[a] Wei Li,[b] and Xiaodong Chen*[a]
Abstract: Photoredox catalysis provides opportunities in harnessing
clean and green resources such as sunlight and O2, while the acid
and base surface sites of metal oxides are critical for industrial
catalysis such as oil cracking. Here, we elucidated the contribution of
metal oxide surfaces towards photocatalytic aerobic reactions, as
demonstrated through the hydroxylation of boronic acids to alcohols.
The strength and proximity of the surface base sites appeared to be
two key factors in driving the reaction; basic and amphoteric oxides
such as MgO, TiO2, ZnO and Al2O3 enabled high alcohol yields,
while acidic oxides such as SiO2 and B2O3 gave only low yields. The
reaction is tunable to different irradiation sources by merely selecting
photosensitizers of compatible excitation wavelengths. We believe
that such surface complexation mechanisms between reactants and
earth abundant materials can be effectively utilized to achieve a
wider range of photoredox reactions.
aluminas and zeolites are widely used to catalyze or promote
the catalysis of several important industrial reactions, such as oil
cracking and naptha reforming. The interface between such
surface sites and supported catalysts play a significant role in
promoting catalytic activity; for example, proton abstraction by
the surface base sites of Ag/Al2O3 can enhance alcohol
dehydrogenation,[15] while the activation of O2 by Lewis acid
sites on Au/Al2O3 can enable CO oxidation.[16] We believe that
the base sites on metal oxide surfaces can support
photosensitizers in achieving a wider range of reactions. It has
been suggested that the surface acidity of photocatalytic
[17]
semiconductors, such as that of TiO2
and HNb3O8,[18] can
increase the rate of visible-light-induced benzylalcohol oxidation.
However, there remains a need for greater understanding and
utilization of the surface properties of heterogeneous catalysts.
Previously, we have demonstrated that the surface complexation
of organic reactants with Al2O3 can support photocatalysts in
enabling the selective photocatalytic transformation of benzyl
alcohols to aldehydes with molecular O2 and visible light.[19] The
abstraction of proton from benzyl alcohol by the surface base
sites on Al2O3 resulted in an upshift in its HOMO for electron
abstraction by the photosensitizer. It is believed that such
surface complexation mechanisms between the reactants and
earth abundant materials can be effectively utilized to achieve a
wider range of photoredox reactions.
Photoredox catalysis is a promising strategy for the
sustainable production of industrially-valuable products under
mild conditions and with minimal toxic reagents, which renders
them benign to health and the environment.[1] A prominent
subset of photoredox catalysis is selective aerobic photocatalytic
oxidations, which utilize clean and abundant resources such as
O2 and sunlight[2] to drive difficult organic transformations such
as the production of sulfoxides from thioanisoles[2b, 3] and imines
from benzylamines,[4] which are important synthesis
intermediates in the fine chemicals and pharmaceutical
industries. Recently, much interest has been dedicated towards
dual photocatalysis, in which photocatalysis is merged with other
types of catalysis such as organocatalysis[5] or transition metal
complex[6][7] catalysis, in order to extend the range of organic
transformations achievable. For example, Ru or Ir photocatalysts
are paired with Ni[6] or Pd catalysts[8] to achieve C-C bond
In this article, we elucidated the contribution of metal oxide
surfaces towards photocatalytic aerobic reactions, as
demonstrated through the selective hydroxylation of boronic
acids to alcohols. The hydroxylation of boronic acids to alcohols,
traditionally conducted with alkaline H2O2, plays
a useful
synthetic role towards reactions such as the preparation of chiral
aliphatic alcohols from alkenes[20] or from the homologation of
boronic esters,[21] as well as the one-pot synthesis of meta-
substituted phenols from benzene derivatives, which may be
difficult to achieve through other methods.[22] Here, we showed
that with the photocatalytic route can enable the reaction to
proceed under mild conditions, i.e., the use of molecular O2 as
the oxidant and visible light irradiation from a 7 W household
white light LED. The 9 metal oxides used are commercially
available, and their characterization data, such as SEM images,
XRD spectra and BET surface areas can be found in Figures
S1-3 and Table S1. The CO2-TPD of the metal oxides, which is
a standard technique for elucidating the surface basicity of
heterogeneous materials,[23] can be found in Figure S4 and
Table S2. The elucidated base sites are categorized based on
their strength; strong base sites are defined here as those
corresponding to CO2-TPD peak maxima ranging within 150-600
oC, while weak base sites are defined as those corresponding to
peak maxima within 50-100 oC. The density of base sites are
further characterized with respect to the metal oxides’ surface
area, in order to understand the contribution of the base sites’
proximity, and with respect to the metal oxides’ mass (Figure
S5). It was discovered that when amphoteric and basic metal
oxides (MOx) such as ZnO, Al2O3, TiO2 and MgO were coupled
with Ru(bpy)32+, high yields of 4-chlorophenol (4-ClPhOH, 2a)
9]
formations from substrates of various functional groups.[1a,
Photocatalysts can also be paired with Brønsted acids[10] or
bases[11] for proton-coupled electron transfer,[12] Lewis acids for
the activation of reactants to attack by nucleophiles,[13] and
Lewis bases such as secondary amines for the reductive
quenching of the photoexcited photocatalyst.[5, 14]
The utilization of surface properties in heterogeneous
catalysts to enable organic transformations constitutes a well-
established class of catalysis with wide applications. The acid
and base surface sites of aluminum oxide, silicon dioxide, silica-
[a]
[b]
Dr W. R. Leow, Dr J. Yu, Dr B. Li, Dr B. Hu, Prof X. Chen
Innovative Center for Flexible Devices, School of Materials Science
and Engineering
Nanyang Technological University
50 Nanyang Avenue, Singapore 639798, Singapore
E-mail: chenxd@ntu.edu.sg
Prof W. Li
Laboratory of Advanced Materials, Department of Chemistry
Fudan University
Shanghai 200433, P. R. China
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201805395
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could
be
obtained
from the
hydroxylation
of
4-
of aryl halides.[14b] The use of Ru(bpy)32+ only provided low yields
of alcohols (5.02-23.3%). The yield of 2f is lower than that of 2d
and 2e as the methoxy group provides more steric hindrance to
the B(OH)2 group in the ortho and meta positions. It can be seen
from Figure S8a that the rate of 2a production from 1a is almost
linear with time. Hence, the initial yield (defined arbitrarily as the
product yield in the 1st hour of reaction) may be reflective of the
chlorophenylboronic acid (4-ClPhB(OH)2, 1a) in 3 h (Figure 1).
The yields of 2a were much lower when acidic oxides such as
B2O3 and SiO2 were coupled with Ru(bpy)32+. In the middle
region comprising Al2O3 of varying basic strength (Aldrich
11501-3), positive correlation appears to exist between the 2a
yields and the density of strong base sites w.r.t. surface area.
2+
The mere addition of basic or amphoteric oxide to Ru(bpy)3
initial reaction rate. The turnover frequency (TOF) of the reaction
2+
increased the turnover number (TON) and quantum yield (QY)
by >6.8 times, which at least 2.4 times that of the MOx alone
(Table S3-5). There appears to be no positive correlation
between the yields of the MOx-Ru(bpy)32+ surface complexation-
photocatalytic systems and the density of weak or all base sites
with respect to the MOx surface area (Figure S6). This suggests
that the strength of the base sites is a crucial factor towards
driving the hydroxylation of 1a. There also appears to be no
is 4.0~37 times greater for the Al2O3-Ru(bpy)3
surface
complexation-photocatalytic system compared to that of
Ru(bpy)32+ only (Figure S8b).
2+
positive relationship between the yields of the MOx-Ru(bpy)3
dual catalytic systems and the density of base sites with respect
to the MOx mass (Figure S7). This indicates that the proximity
between neighbouring base sites on the surface is important,
which suggests that two or more 1a molecules may possibly
interact during the reaction.
Figure 1. a) The selective photocatalytic hydroxylation of 1a to 2a under O2
and visible light. b) Yields of 2a generated by different metal oxide (MOx) and
2+
Ru(bpy)3 surface complexation-photocatalytic systems as a function of the
density of strong base sites w.r.t. surface area. Data point labels: i – no metal
oxide, ii - B2O3, iii - SiO2, iv - Al2O3 (Aesar 11502), v - Al2O3 (Aesar 11503), vi -
Al2O3 (Aesar 11501), vii - Al2O3 (Aldrich 517747), viii - ZnO, ix – MgO, and x -
TiO2. The lines serve as a guide to the eye.
The nature of the selective photocatalytic hydroxylation
was then investigated through a series of control experiments
with Al2O3 (Aldrich 517747) as the metal oxide of interest, as it
provided a good yield of 2a from 1a, and is an abundant and
2+
non-toxic material. The generality of the Al2O3-Ru(bpy)3
Figure 2. a) The selective photocatalytic hydroxylation of boronic acids (1a-h)
to alcohols (2a-h) catalyzed by the Al2O3-Ru(bpy)3 surface complexation-
photocatalytic system and Ru(bpy)32+ only. b) Initial yield of 2a vs Al2O3 mass.
c) Quenching of the Al2O3 surface base sites with acetic acid. The lines serve
as a guide to the eye. d) Yield of 2a catalyzed by Al2O3 in conjunction with
different photosensitizers.
surface complexation-photocatalytic system is demonstrated
through the high yields (56.3-96.0 %) of alcohols (2a-h)
achieved with 8 boronic acid derivatives (1a-h) in 3 h, as shown
in Figure 2a. This includes electron-rich phenols (2d-f), which
are difficult to obtain from the traditional nucleophilic substitution
2+
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Meanwhile, the initial yield of 2a increases linearly with the
mass of Al2O3 used, which indicates that the initial rate of the
reaction may be dependent on the adsorption of 1a on the Al2O3
surface sites (Figure 2b). This is further evidenced through
introduction of acetic acid, which would compete with 1a for
chemisorption on the base sites and result in the quenching of
the reaction (Figure 2c). The absence of either component of the
Al2O3-Ru(bpy)32+ surface complexation-photocatalytic system, or
the absence of any catalyst, would result in low yields of 2a
(Figure S9). The negligible yield of 2a obtained under N2
indicates that O2 acts the oxidant that accepts protons and
electrons from 1a to enable its hydroxylation to 2a. The low 2a
2+
yield in the dark suggests that Ru(bpy)3
acts as the
photosensitizer that absorbs visible light and facilitates electron
transfer between 1a and O2, while the strong base sites on the
Al2O3 surface facilitates proton transfer. This suggests that
Al2O3-photosensitizer
surface
complexation-photocatalytic
system can be extended to other photosensitizers, provided the
excitation wavelength of the photosensitizer corresponds to the
LED emission (Figures 2d and S10). 5,10,15,20-Tetrakis(4-
2+
methoxyphenyl)porphyrin (TMPP), Ru(bpy)3 and perylene-
3,4,9,10-tetracarboxylic dianhydride (PTCDA) possess excitation
wavelengths within 400~550 nm, and thus showed high yields of
(96.7±3.30) %, (96.5±2.47) % and (88.6±11.7) %, respectively.
1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTCDA), which
absorbs light below 365 nm, provided only a low yield of
(9.59±7.59) %. Rose Bengal was able to effect significant yield
of (50.2±11.8) %, but its lack of stability detracts from a higher
yield even though its excitation wavelength is within the range of
the LED. Without Al2O3, the photosensitizers produced only low
yields of 0.65-47.7 %. This suggests that the Al2O3-
photosensitizer surface complexation-photocatalytic system can
be programmed for compatibility with light sources of different
wavelength ranges simply by using a suitable photosensitizer.
Al2O3 can also be paired with dyes such as Rose Bengal to
realize a transition metal-free catalytic system that is benign to
the environment.
Figure 3. a) Importance of the proton of the boronic acid group towards the
reaction; the yield of 2a with non-anhydrous DMF versus that with anhydrous
DMF, as well as the yield of 2a produced from 1i, as catalysed by Al2O3-
Ru(dcbpy)2(bpy)2+. b) Initial yield of 2a with different percentages of H2O in the
solvent. The line serves as a guide to the eye. Proposed reaction equations c)
without and d) with the involvement of H2O.
Next, we investigated the potential source of proton that
constitutes the OH group of the alcohol product. In the traditional
route of boronic acid hydroxylation, water is thought to be the
source of the proton. The ipso-carbon of B(OH)2 is proposed to
migrate from the B atom to an O atom in hydrogen peroxide, and
the resultant intermediate undergoes hydrolysis to form
24]
alcohol.[22a,
However, our photocatalytic reaction is able to
proceed to high yield in anhydrous solvent, which suggests that
H2O does not take part in the reaction as an O or H donor
(Figure 3a). Figure 3b shows that the initial yield of 2a is
reduced with a larger percentage of H2O in the solvent. This
suggests that the reaction may not involve hydrolysis of the
intermediate as in Figure 3c, and that H2O may even compete
with 1a for adsorption on the Al2O3 surface. When we scrutinize
2+
the components of the reaction system (Al2O3, Ru(bpy)3
,
boronic acid, DMF), the only possible source of H appears to be
the B(OH)2 group. This is confirmed by the observation that
negligible 2a yield was obtained with 4-chlorophenylboronic acid
pinacol ester 1i (Figure 3a). The atom balanced equation (Figure
3d) appears to be consistent with our aforementioned
suggestion that two or more boronic acid molecules are required
to be in proximity during the reaction.
Scheme 1. Proposed mechanism for the selective photocatalytic hydroxylation
2+
of boronic acids catalysed by Al2O3-Ru(bpy)3
photocatalytic system under O2 and visible light.
surface complexation-
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Keywords: photocatalysis • surface base sites • boronic acids •
alcohols • hydroxylation
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Entry for the Table of Contents
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W. R. Leow, J. Yu, B. Li, B. Hu, W. Li, X.
Chen*
Page No. – Page No.
Correlating the Surface Basicity of
Metal Oxides with Photocatalytic
Hydroxylation of Boronic Acids to
Alcohols
Elucidating the contribution of metal oxide surfaces towards photocatalytic
aerobic reactions. The strength and proximity of the base sites on metal oxide
surfaces, when coupled with photosensitizer, are crucial factors driving the selective
photocatalytic aerobic hydroxylation of boronic acids to alcohols.
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