Asymmetric Desymmetrization of Oxetanes for the Synthesis of Chiral Tetrahydrothiophenes and Tetrahydroselenophenes

Article abstract of DOI:10.1002/anie.201910917

Chiral tetrahydrothiophenes and tetrahydroselenophenes are highly useful structural units. Described here is a new catalytic asymmetric approach for their synthesis. With a suitable chiral Br?nsted acid catalyst, an oxetane desymmetrization by a well-positioned internal sulfur or selenium nucleophile proceeded efficiently to generate all-carbon quaternary stereocenters with excellent enantioselectivities. Taming the sulfur and selenium nucleophile in the form of a thioester and selenoester, respectively, is crucial to the success of this work. This approach also allows the facile synthesis of chiral tetrahydrothiopyrans. Mechanistic studies, including DFT calculations, suggested an intramolecular acyl-transfer pathway. Utilities of the chiral products are also demonstrated.

Full text of DOI:10.1002/anie.201910917

Communication
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Generalized Synthetic Strategy for Transition-Metal-Doped
Brookite-Phase TiO2 Nanorods
Zhiyong Zhang,†,# Qiyuan Wu,‡,# Grayson Johnson,†,# Yifan Ye,§ Xing Li,∥ Na Li,∥ Meiyang Cui,†
Jennifer D. Lee,⊥ Chang Liu,† Shen Zhao,∥ Shuang Li,∥ Alexander Orlov,‡ Christopher B. Murray,⊥
Xu Zhang,∇ T. Brent Gunnoe,† Dong Su,*,∥ and Sen Zhang*,†
†Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States
‡Department of Materials Science and Chemical Engineering, State University of New York, Stony Brook, New York 11794, United
States
§Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
∥Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States
⊥Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
∇Department of Physics and Astronomy, California State University, Northridge, California 91330, United States
S
* Supporting Information
Such a doping strategy has been extensively studied for the
rutile- and anatase-phase TiO2 nanomaterials.33−36,41 In
ABSTRACT: We report a generalized wet-chemical
methodology for the synthesis of transition-metal (M)-
doped brookite-phase TiO2 nanorods (NRs) with
unprecedented wide-range tunability in dopant composi-
tion (M = V, Cr, Mn, Fe, Co, Ni, Cu, Mo, etc.). These
quadrangular NRs can selectively expose {210} surface
facets, which is induced by their strong affinity for
oleylamine stabilizer. This structure is well preserved with
variable dopant compositions and concentrations, leading
to a diverse library of TiO2 NRs wherein the dopants in
single-atom form are homogeneously distributed in a
brookite-phase solid lattice. This synthetic method allows
tuning of dopant-dependent properties of TiO2 nanoma-
terials for new opportunities in catalysis applications.
contrast, brookite TiO2, another polymorph of TiO2, is rarely
studied due to the challenge of synthesizing the pure brookite
phase. Recently, a high-yield synthesis of brookite-phase TiO2
nanorods (NRs) has been reported via the hydrolysis of
titanium chloride precursor,14 which promoted electron−hole
separation under solar irradiation due to the one-dimensional
(1-D) structure and therefore delivered a record-high photo-
catalytic activity toward photo-reforming of ethanol/glycerol
for hydrogen (H2) production.43 This success for the synthesis
of brookite-phase TiO2 NRs led to the present work, exploring
a robust approach to doping for further enhanced catalytic
properties. We found that single, dual, or multiple transition-
metal (M) dopants in a broad range (M = V, Cr, Mn, Fe, Co,
Ni, Cu, Mo, etc., as illustrated in Figure 1) can be
homogeneously doped into monodisperse single-crystalline
brookite-phase TiO2 NRs (M-TiO2). The resultant M-TiO2
NRs showed consistencies in physical dimensions and surface
facets, thereby creating a new class of well-defined TiO2
nanocrystals with tunable compositions as well as optical and
catalytic properties. Using Fe-TiO2 NRs as a model system, we
demonstrated that controlling the Fe-dopant level substantially
enhances the photocatalytic performance of TiO2 brookite-
phase nanomaterials.
The M-TiO2 NRs were synthesized via an organic solution
colloidal hydrolysis method {see the Supporting Information
(SI)}. Typically, TiCl4 precursor dissolved in oleic acid (OAc)
is decomposed in a mixture of 1-octadecene (ODE) and
oleylamine (OAm). At high temperature (290 °C), OAm and
OAc can react to generate a small amount of water, resulting in
slow hydrolysis and condensation of TiCl4 along with the
release of HCl. Unlike in the previous method,43,44 we
incorporated M-oleate precursor in our synthesis by mixing it
with TiCl4-OAc to produce M-TiO2. It is known that M-
entral to functional nanomaterials is the ability to
Csynthetically control nanocrystals physical dimensions,
compositions, and structures with atomic-scale precision.1−7
TiO2 is one of the most studied metal oxides nanomateri-
als8−15 due to its unique semiconducting and oxygen-carrying
properties and important applications in optoelectronics,16
batteries,17−19 heterogeneous catalysis,20,21 and photocataly-
sis.22−25 For example, TiO2 can provide active lattice oxygen
species and strong interactions with metal nanoparticles,
allowing the modulation of kinetics for many heterogeneous
catalytic reactions.26−28 TiO2 can also absorb photons to
generate excited electrons and holes, driving redox reactions
for photocatalytic water splitting and photo-reforming
processes.29−32 To maximize the benefits of these applications,
an emerging aspect in the studies of TiO2 nanostructures is to
dope them with foreign elements, including cations (e.g., Fe,
Sn)33,34 and/or anions (e.g., C, N, F),35−39 which can tune
lattice oxygen activity as well as improve TiO2’s electronic
structure, surface acidity/basicity, and photon absorption,
enabling the enhanced catalytic properties.40−42
Received: June 15, 2019
Published: September 19, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.9b06389
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
tri-M-TiO2 (Figure S4), and pristine TiO2 (Figures S1A and
S2B) samples have a consistent NR morphology with an
average width of 4.2
0.5 nm and a length of 30−50 nm.
These M-TiO2 NRs preserve the brookite phase, as indicated
by their powder X-ray diffraction (XRD) patterns (JCPDS No.
29-1360, orthorhombic), with the characteristic brookite (121)
peak at 2θ = 30.81° (Figure S5). By adjusting the precursor
M/Ti molar ratio, the M dopant level can be controlled (see
SI). TEM images in Figures 2A and S1H confirm that the Fe
atomic concentration can be increased up to 10% (out of all
cations Fe + Ti, obtained with energy-dispersive X-ray
spectroscopy (EDS)) without any obvious morphology
change. But we noticed that higher concentrations of M-oleate
precursor do not result in further increased doping levels.
Instead, M remains in the solution, probably because the
stability of the brookite-phase TiO2 lattice does not favor the
incorporation of a high concentration of dopants.
Figure 1. Diverse M-TiO2 brookite-phase NRs synthesized by the
High-resolution TEM (HRTEM) images of the representa-
tive Fe-TiO2 (Figure 2B) and pristine TiO2 NRs (Figure S6)
suggest that these NRs are single crystalline and grow along the
⟨001⟩ direction. Fast Fourier transform of HRTEM images in
Figure 2B indicates that the side facets might be {210} planes.
Thanks to their monodisperse size and morphology, the TiO2
and Fe-TiO2 NRs can be easily aligned into ordered assemblies
at the interface of air/diethylene glycol via the previous
method (Scheme S1).46 The TEM images (Figures 2C and
S7) viewed along the NR longitudinal direction, based on their
vertical assemblies, show a rhombic NR cross-section shape
with an angle of ∼80°/100°, confirming that the NRs expose
four {210} planes as the side facets.
Density functional theory (DFT) calculations (SI) were
performed to evaluate the binding between surfactants/ions
(OAm, OAc, and Cl−) and two TiO2 facets, (001) and (210),
to understand the formation mechanism of quadrangular NRs
(Figure 2D). We found, in agreement with Gong et al.,47 that
the (001) plane of brookite-phase TiO2 is not stable and
undergoes a 1×1 reconstruction both under vacuum and in the
presence of surfactant (Figure S8). Moreover, the adsorption
energies (Ead) of OAm, OAc, and Cl− on the (210) plane were
calculated to be −0.93, −0.44, and −0.72 eV, all stronger
compared to those on the (001) plane (−0.07, −0.24, and
−0.06 eV, respectively) (Table S1). As a result, (210) planes
can be preferentially exhibited because of the stronger binding
with the surfactants/ions, and such a structure is preserved
when only a low concentration of dopant is incorporated. It is
worth noting that OAm is present in a much larger amount
than OAc and Cl− in our synthesis and therefore played the
most important role in stabilizing the (210) plane due to the
strongest Ead (−0.93 eV).
reported synthetic methodology.
oleates, such as Fe-oleate, can be rapidly decomposed to
initiate metal oxide nucleation only at temperatures higher
than 310 °C.45 Our reaction temperature (290 °C) and the
presence of HCl minimize the possibility of MOx formation. As
a result, M is doped into the TiO2 matrix, producing M-TiO2
that can be well-dispersed in nonpolar solvent and exhibit
distinct colors relative to undoped TiO2. As summarized in
Figure 1, this strategy can be generalized to all first-row
transition-metal dopants and even second-row metals (e.g.,
Mo) as well as binary and ternary combinations.
Transmission electron microscopy (TEM) and scanning
TEM (STEM) images show that the as-synthesized mono-M-
TiO2 (Figures 2A, S1B−G, and S2A), bi-M-TiO2 (Figure S3),
The dopant distribution within NRs was first studied by
looking at STEM high-angle annular dark field (HAADF)
images coupled with electron energy loss spectroscopy (EELS)
elemental mappings (Figures 3A−C and S9−S11). It is clearly
seen that the dopants, including Fe mono-dopant, bi-M, and
tri-M, are homogeneously distributed within NRs. To further
uncover the atomic arrangement and bonding configuration of
M, we performed X-ray absorption near-edge structure
(XANES) and extended X-ray absorption fine structure
(EXAFS) studies. Figure 3D displays the Fe K-edge XANES
spectra of Fe-TiO2 NRs as well as reference samples, including
Fe, FeO, Fe3O4, and Fe2O3. The Fe-TiO2 spectrum is clearly
distinct from those of metallic Fe, excluding the existence of
the interstitial dopant of Fe in the TiO2 lattice. Meanwhile, the
Figure 2. (A) TEM and (B) HRTEM images of the as-synthesized
Fe-TiO2 (10% Fe). (C) TEM image of vertically aligned Fe-TiO2
(10% Fe) assembly. (D) Optimized OAm adsorption on brookite-
phase TiO2 (210) and (001) planes. Insets in B and C illustrate the
corresponding TiO2 atomic models, with purple and pink atoms being
Ti and O, respectively. In the DFT models, the color scheme is red
(O), gray (C), white (H), blue (N), and teal (Ti).
B
DOI: 10.1021/jacs.9b06389
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Communication
Figure 4. (A) UV−vis diffuse reflectance spectra and (B) photo-
catalytic H2 production of TiO2 and Fe-TiO2 NRs with different
dopant concentrations.
edge of TiO2 toward longer wavelengths. Therefore, the
present synthetic strategy offers an effective method to
improve the visible-light absorption of pristine TiO2 NRs.
To illustrate one potential impact of our general synthesis,
we selected the photocatalytic H2 production via methanol
reforming as a model reaction and investigated the perform-
ance of Fe-TiO2 NRs (SI). The pristine TiO2 brookite-phase
NRs, after removal of the surfactants using the NOBF4
treatment50 and photo-deposition of 1 wt% Pt co-catalyst,44,51
show a hydrogen production rate of ∼57 μmol/h under 300 W
xenon lamp illumination (Figure 4B). Interestingly, all the Fe-
TiO2 NRs with different Fe dopant levels show higher H2
production rates under the same conditions. The H2
production rate of the 3.7% Fe-TiO2 reaches ∼140 μmol/h,
corresponding to a ∼2.5 times enhancement compared to that
of pristine TiO2 NRs. Such an enhancement is encouraging,
given the fact that the pristine TiO2 brookite-phase NRs have
already been demonstrated as one of the most active
photocatalysts.43,44 Increasing the Fe doping level beyond
3.7% results in decreased activity. The enhanced activity of Fe-
TiO2 probably originates from the improved light utilization in
the visible range. However, the dopant atoms can also act as
charge recombination sites that compromise the charge
separation efficiency, which becomes an issue at higher Fe
amounts.40 Thus, an optimal doping concentration for
photocatalytic hydrogen production is observed to be 3.7%
Fe. Meanwhile, it was observed that the H2 production rate
decreased only slightly after a 16 h reaction (Figure S14). The
Fe-TiO2 structure and morphology are also well-maintained
after catalysis, as indicated by the XRD patterns, TEM images,
and EELS elemental mappings (Figures S15 and S16),
demonstrating the excellent stability of the Fe-TiO2 NR
photocatalyst.
This Communication highlights a generalized synthetic
strategy for doped M-TiO2 NRs with consistent brookite
phase, quadrangular 1-D morphology, and controllable dopant
compositions and concentrations. Photocatalytic H2 produc-
tion based on the resultant Fe-TiO2 NRs demonstrates that Fe-
doping can lead to a substantial catalytic activity enhancement,
probably due to a dopant-induced optical absorption improve-
ment. The unique capability of our synthetic approach in
preparing more improved photocatalysts will be further
investigated in our follow-up study of M-TiO2 NRs with
diverse M compositions/concentrations. In addition, the
present M-TiO2 NRs may allow additional opportunities in
heterogeneous catalysis, serving as a new type of either
catalytic materials with single-atom M sites or catalyst supports
Figure 3. (A−C) HAADF-EELS elemental mappings of Fe-TiO2
(10%) NRs (scale bar: 2 nm). (D) Fe K-edge XANES and (E)
Fourier transform EXAFS spectra of Fe-TiO2 (10%) and reference
standards.
Fe-TiO2 shows white line characteristics similar to those of
FeOx, indicating the high valence state of Fe (+3) in the Fe-
TiO2 NRs. Additionally, the observed spectra present different
profiles in the range of 7120−7160 eV, implying that the state
of Fe in Fe-TiO2 differs from that in FeOx. We ascribe these
differences to the replacement of Fe in the TiO2 lattice, which
is due to the similarity in the ionic radii of Fe3+ (0.064 nm)
and Ti4+ (0.068 nm).48 Doping of Fe in the TiO2 lattice alters
the hybridization of Fe with O, inducing a different Fe−O
bond environment compared with that in FeOx. Thus, it is
clear that Fe is atomically doped into the TiO2 without
forming any FeOx segregation.
Moreover, the Fe K-edge EXAFS spectrum of Fe-TiO2 along
with the spectra of reference samples are presented in Figure
3E. We found that Fe-TiO2 does not have the Fe−Fe bond due
to the absence of a peak at 2.21 Å, which is an indicator of Fe−
Fe bonding observed in metallic Fe foil. Instead, a peak at 1.49
Å, due to the Fe−O bond, is observed in the Fe-TiO2
spectrum. The peak position is different for FeO (1.57 Å),
Fe3O4 (1.43 Å), and Fe2O3 (1.42 Å). More obviously, the
second and third nearest neighboring coordination shells of Fe,
in the range of 2−4 eV of Fe-TiO2, are much weaker compared
to those in the FeOx spectra. This reveals again that the
existence of Fe metal and Fe oxide secondary phases in the
samples can be excluded, and it indicates that Fe is distributed
in the TiO2 network as single atoms. The EXAFS studies of
other M-TiO2 (Figure S12) also show that single-atom M
occupies the Ti site in TiO2, confirming the universality of
atomic doping in the present synthesis.
As illustrated in previous studies on rutile and anatase TiO2,
the t2g state of M dopants can interact with Ti, generating
additional occupied states in the bandgap of TiO2 and tuning
the electronic structure of TiO2.49 As shown in Figures 1 and
S13, our M-TiO2 NR suspensions in hexanes exhibit different
colors from TiO2 (white), suggesting similar electronic
structure modification in brookite-phase TiO2. The absorption
edge of brookite-phase TiO2 NRs is at ∼380 nm in the
ultraviolet−visible (UV−vis) diffuse reflectance spectrum
(Figure 4A), corresponding to a band gap of ∼3.3 eV. For
Fe-TiO2, it can be seen that the Fe dopant shifts the absorption
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.9b06389.
■
S
Materials and methods, Scheme S1, Table S1, and
Figures S1−16 (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*dsu@bnl.gov
*sz3t@virginia.edu
ORCID
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Growth and Photoelectrochemistry of Highly Oriented, Crystalline
Anatase TiO2 Nanorods on Transparent Conducting Electrodes.
Chem. Mater. 2015, 27, 4180.
Zhiyong Zhang: 0000-0001-7936-9510
Grayson Johnson: 0000-0001-5229-1135
Jennifer D. Lee: 0000-0003-2644-3507
Xu Zhang: 0000-0002-6491-3234
T. Brent Gunnoe: 0000-0001-5714-3887
Dong Su: 0000-0002-1921-6683
Sen Zhang: 0000-0002-1716-3741
Author Contributions
́
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#Z.Z., Q.W., and G.J. contributed equally to this work.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the U.S. National Science
Foundation (CBET-1805022). Partial work on electron
microscopy was carried out at the Center for Functional
Nanomaterials, which is a U.S. Department of Energy Office of
Science Facility, at Brookhaven National Laboratory under
Contract No. DE-SC0012704. We appreciate experimental
support from Sirine Fakra at the Advanced Light Source. The
Advanced Light Source is supported by the Director, Office of
Science, Office of Basic Energy Sciences, of the U.S.
Department of Energy under Contract No. DE-AC02-
05CH1123. C.B.M. and J.D.L. acknowledge partial support
provided by the Vagelos Institute of Energy Science and
Technology at the University of Pennsylvania.
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DOI: 10.1021/jacs.9b06389
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX