Full text of DOI:10.1021/acscatal.7b04032

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Article
Heterogeneous Dual Photoredox-Lewis Acid
Catalysis using a Single Bifunctional Nanomaterial
Gregory K Hodgson, and Juan C. Scaiano
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04032 • Publication Date (Web): 21 Feb 2018
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Heterogeneous Dual Photoredox-Lewis Acid Catalysis using a Single Bifunctional
Nanomaterial
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Gregory K. Hodgson and Juan C. Scaiano*
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Department of Chemistry and Biomolecular Sciences, Centre for Catalysis Research and
Innovation, University of Ottawa, 10 Marie Curie, Ottawa, Ontario, K1N 6N5, Canada.
Email: jscaiano@uottawa.ca
Abstract:
We report the heterogeneous dual photoredoxꢀLewis acid catalysis using a versatile and efficient
nanocomposite; samarium oxide nanoparticleꢀdecorated titanium dioxide. This emerging class of
nanomaterials harnesses the Lewis acidity of the lanthanide, eliminates product contamination by
the catalyst and can be excited with visible light. Useful intermolecular and intramolecular net
reductive and net neutral photoredox cyclization reactions are presented as examples of the
general efficacy of this reusable heterogeneous nanocatalyst in synthetically relevant organic
transformations.
Keywords: heterogeneous catalysis, photoredox, Lewis acid, nanomaterials, cyclizations,
samarium, lanthanides
Introduction
Heterogeneous and homogeneous catalysis are two sides of the same coin; they inherently offer
different advantages to the modern chemist. Often the best option is not immediately evident,
and ultimately the most suitable choice comes down to the specific requirements of the
application. The steady rise in popularity of homogeneous catalysis is primarily attributable to
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the relative ease with which both regioꢀ and stereoselectivity can be attained. Such control is
indeed a strong asset in pharmaceutical research and natural product synthesis. However, the
chemical industry has historically relied heavily upon heterogeneous catalysis for an abundance
of important organic transformations owing to the high efficiency stemming from phase
separation and catalyst reusability. While heterogeneous catalysts generally suffer from the
propensity to yield wider product distributions, the continuing push toward more
environmentally sustainable routes to synthetic building blocks and valueꢀadded chemicals has
seen a resurgence in the field of heterogeneous catalysis. Ideally it would be possible to combine
the advantages of homogeneous and heterogeneous catalysis into a single reusable system.
In recent decades, photoredox catalysis has emerged as a powerful tool in synthetic
organic chemistry, providing practical strategies for the preparation of a myriad of fine chemicals
under mild conditions.1 Merging photoredox catalysis with transition metal catalysis in dualꢀ or
triꢀcatalytic cycles has become a popular strategy for extending the utility of photoredox
catalysts,2 and pioneering work by Yoon and coꢀworkers has opened a new frontier in the form
of cooperative photoredoxꢀLewis acid homogeneous dual catalysis.3 Despite these developments,
few successful attempts have been made at heterogenizing dual photoredoxꢀtransition metal or
photoredoxꢀLewis acid catalysis. In light of this, we identified an opportunity to combine our
expertise in nanomaterials science, catalysis and photochemistry to devise novel heterogeneous
analogues of dual catalytic systems with the goal of mirroring established product yields and
regioselectivities while improving overall efficiency by designing readily separable, reusable,
bifunctional heterogeneous nanostructured catalysts.
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Lewis acids (LAs) often play critical, yet sometimes understated, roles in synthetic
organic chemistry and photochemistry. They can serve to activate heteroatomꢀcontaining
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substrates by increasing their electrophilicity and shifting their reduction potentials to more
positive values,4 rendering them more susceptible both to nucleophilic attack and to
photoreduction, respectively.2b,3,5 Several reports have also documented the ability of LAs to
impact reaction kinetics by stabilizing key photogenerated radical anion intermediates, leading to
alternative reaction mechanisms not observed in nonꢀphotochemical counterparts to these
catalytic systems.2b,3,6 Samarium and other lanthanides are wellꢀknown to form LA complexes
and some, such as SmI2, can become powerful reducing agents under appropriate conditions.7
Other Lewis acidic lanthanide complexes have also been shown to function as catalysts for
FriedelꢀCrafts acylation, the DielsꢀAlder reaction, alcohol esterification and the Mukaiyama
aldol addition reaction.8 Notably, lanthanide triflates, including Sm(OTf)3, have demonstrated
exceptional performance as LA catalysts, cocatalysts and as the basis for chiral additives in a
variety of homogeneous catalytic and photocatalytic reactions ranging from nucleophilic
additions9 to photoreductive cyclizations5b,6c to stereoselective [3+2] and [2+2]
photocycloadditions.4b,6d,6e,6g,6h
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Photochemical cyclization reactions are ubiquitous in synthetic organic chemistry; they
provide access to an array of valuable synthetic precursors and complex molecular scaffolds by
enabling otherwise challenging bondꢀformations.10 Intermolecular and intramolecular variations
of this class of reactions have given rise to a wealth of mechanistic knowledge and a multitude of
practical synthetic routes to the preparation of polycyclic systems, fourꢀmembered ring motifs
and key intermediates in natural product synthesis.10a,10b,11 In particular, the development of
photoreductive cyclizations and especially [2+2] photocycloadditions are regarded by some as
among the most significant contributions to the advancement of applied photochemistry in
organic synthesis.10a,11 Heterogenizing this class of catalytic reactions would therefore be a
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practical and worthwhile step forward for the academic and industrial chemistry communities
(Scheme 1).
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Scheme 1. Homogeneous and heterogeneous dual catalytic strategies for photoreductive
cyclizations and [2+2] photocycloadditions.
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With all of these factors in mind, we set out to develop a multifunctional heterogeneous catalyst
comprising a nanostructured semiꢀconductor support decorated with Lewis acidic nanoparticles
(NP) that would allow for highly efficient preparation of a suite of useful photocyclization
products by way of heterogeneous dual photoredoxꢀLA catalysis. We reasoned that combining
the known single electron transfer (SET) properties of semiꢀconducting oxides such as ceria
(CeO2) and titania (TiO2) with the acidic properties and redox activity of samarium oxide
nanoparticles, on which we have previously reported,12 might induce a synergistic effect that
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would instill Lewis acidity into the nanocomposite material or enhance that of the support, slow
down counterproductive electronꢀhole recombination and potentially extend the absorbance of
the catalyst into the visible region. In this work, we describe the first example of purely
heterogeneous dual photoredoxꢀLA catalysis, using a single versatile nanocomposite comprised
of titanium dioxide decorated with samarium oxide NP (SmxOy@TiO2). This new heterogeneous
nanocatalyst is readily recoverable and fully reusable for intermolecular and intramolecular
photoreductive cyclizations and [2+2] photocycloaddition chemistry, eliminates product
contamination by the catalyst, paving the way for progress in the development of more efficient
heterogeneous analogues to homogeneous systems employing cooperative Lewis acid and
photoredox catalysis for organic synthesis.
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Results and Discussion
We elected to begin our investigation into heterogeneous dual photoredoxꢀLA catalysis with the
photoreductive cyclization of chalcones (Table 1). This family of molecules are biologically and
pharmaceutically relevant and are also of interest in synthetic organic chemistry.5b,13 For
example, the cyclopentanol derivatives of chalcones can be accessed via a LAꢀmediated
homogeneous photocatalytic cyclization that forms new CꢀC bonds in the process. The reaction
can be initiated by SET from an excited photocatalyst to the starting material to form a radical
anion intermediate stabilized by an appropriate LA. Dimerization gives rise to a dienolate which
then undergoes monoprotonation followed by a LAꢀmediated intramolecular aldol addition to
form the final cyclized product in a net reductive photoredox mechanism.1a,2b,5b In the reported
homogeneous system, the LA additive was further responsible for activating the substrate toward
2+
SET from the homogeneous photocatalyst Ru(bpy)3 (Ered ‒1.33 V vs. SCE) by shifting its
reduction potential14 from ‒1.49 V vs. SCE to a more positive value.
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We envisaged a scenario in which a nanostructured, samariumꢀdecorated semiꢀconductor
oxide might be capable of catalyzing this reaction heterogeneously. We began by
photochemically preparing two candidate nanocatalysts comprising samarium oxide NP
supported on titania (P25) and ceria, henceforth referred to as SmxOy@TiO2 and SmxOy@CeO2,
based on logical modifications to our previously reported work with larger, polydisperse,
colloidal samarium oxide nanoparticles (Sm2O3NP).12a In the present work, NP 1.2 ± 0.2 nm in
diameter were observed on titania but none could be identified on ceria microparticles by TEM,
likely due to a combination of small size, low Sm loading and the relatively large size (< 5 µm)
and thickness of the CeO2 microparticles (Figures S1ꢀS3). Characterization of these two novel
nanomaterials by EDS (Figures S7ꢀS8) detected samarium and XPS revealed the primary
oxidation state to be Sm(III) but could not definitively rule out the presence of SmO or metallic
samarium, hence the SmxOy label selected for the materials (Figures S11ꢀ12 and associated
discussion). Intriguingly, ICPꢀMS determined that although both materials were prepared with an
optimal loading of 5 mol% Sm, the final nanocomposites contained vastly different amounts of
samarium (4.7 and 0.29 wt% for TiO2 and CeO2, respectively, Table S1).
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Until now, literature reports describing samariumꢀsemiconductor hybrid materials have
been limited to the use of ionic samarium(III) as a dopant or coꢀdopant in TiO2,15 Gd2O3,16 and
BiFeO3.17 These studies have been mostly concerned with the degradation of organic dyes under
aerobic conditions; none involve Smꢀrelated Lewis acidity or employ inert atmosphere and
importantly, none of the materials contain samariumꢀbased NP. Indeed, SmxOy@TiO2 exhibited
exceptional performance in the photoreductive cyclization of chalcone 1a relative to
SmxOy@CeO2 (Table 1 entries 1 and 2) and to the homogeneous catalytic system (62%, 3 h)5b
under comparable optimized conditions (Scheme 1). Diffuse reflectance (DR) measurements
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showed that of the two nanomaterials, the absorbance of SmxOy@TiO2 extends further into the
visible region and has a much better overlap with the emission of the 400 nm LED used for
photoexcitation of the catalyst (Figure S15). Increasing the optimal loading of samarium on
CeO2 twoꢀfold only resulted in a small increase in the actual Sm loading (0.42 wt%), and the
yield of 2a was actually negatively impacted, decreasing to roughly 50% of that obtained using
unmodified commercial CeO2 with a particle size < 5 µm (Table 1 entries 3ꢀ4 and Figures S4,
S9, S13). We attempted to increase the samarium loading further by switching to a commercial
ceria nanopowder with a particle size (< 25 nm) more comparable to that of TiO2. The presence
of SmxOy NP 0.92 ± 0.2 nm in diameter was confirmed by a combination of TEM, EDS and XPS
analyses (Figures S5ꢀS6, S10, S14). The nanoscale CeO2 did perform marginally better than the
micronꢀrange CeO2 but although we were able to achieve a 3.3 wt% loading of samarium, the
material could not match the superior catalytic activity of SmxOy@TiO2 (Table 1 entries 5 and
6).
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Table 1. Heterogeneous dual catalytic photoreductive cyclization of transꢀchalcone.a
Entry
Catalyst
Yieldb 2a (%) Yieldb 3 (%)
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2
SmxOy@TiO2 (4.7 wt % Sm)
SmxOy@CeO2 (≤ 5µm) (0.29 wt% Sm)
SmxOy@CeO2 (≤ 5µm) (0.42 wt% Sm)
CeO2 (≤ 5 µm)
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CeO2 (< 25 nm)
6c SmxOy@CeO2 (< 25 nm) (3.3 wt% Sm)
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TiO2
TiO2 + Sm(OTf)3 additive
aReaction conditions: chalcone (1 mmol), catalyst (60 mg), iꢀPr2NEt (5 mmol), 15 mL dry MeCN, degassed with
argon 20 min, irradiated with a 400 nm LED (90 W); irradiation time was optimized for 100% conversion in entry 1
and coincidentally matched the optimal irradiation time in ref. 5b despite the different light sources used. Longer
irradiation times did not result in significantly higher conversion to 2a using either SmxOy@TiO2 or TiO2 alone.
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bDetermined by 1H NMR spectroscopy using an internal standard. cThe [2+2] cycloaddition product was detected in
d
≤ 5% yield. Reaction conducted in the presence of 1.2 mM (0.0185 mmol, 11.0 mg) Sm(OTf)3, corresponding to
the molar amount of samarium present in entry 1. In all cases no linear dimer was detected.
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These results are consistent with DR spectra, which illustrate that although the CeO2ꢀbased
materials do possess slightly smaller band gaps than TiO2 (Ebg = 3.2 eV vs. SCE),18
SmxOy@TiO2 was the only samarium oxideꢀdecorated nanocomposite to exhibit a change in the
spectral line profile with respect to the unmodified support (Figures S16ꢀ17). The addition of
samarium (4.7 wt%, 2.5 mol%) resulted in the appearance of a clear band from 400ꢀ450 nm
which resembles the known absorbance of colloidal Sm2O3NP (Figure S18).12b This effect has
also been observed when titania is doped with 0.05ꢀ1.5 mol% ionic Sm(III) using microwaveꢀ
assisted or autoꢀcombustion solꢀgel procedures.15a,15c Whether as a result of metal ion doping or
the formation of heterojunctions in semiꢀconductor nanocomposites, redꢀshifted absorbance is a
manifestation of a decrease to the band gap energy of the new overall material.15c,17,19 The DR of
3.3 wt% SmxOy@CeO2 (< 25 nm) is higher than that of SmxOy@TiO2 and extends comparably
far into the visible region (Figure S17); however, lower catalytic activity (Table 1 entry 6) and a
lack of the aforementioned spectral feature in the 400ꢀ450 nm region suggest that only in
SmxOy@TiO2 was the band gap of the nanocomposite successfully modified with respect to the
support (Figures S18ꢀ19).
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For lanthanideꢀtitania hybrid materials, smaller band gaps arise from the presence of
intraꢀband gap electronic states corresponding to the 4f manifold, with the filled states near the
valence edge of TiO2 effectively forming the new highest occupied band of the nanocomposite
material.15a,15b,20 Similar effects on band gap energy have been reported for nanocomposites
comprising TiO2 decorated with various precious metal NP (e.g. Pt, Pd, Ir, Au)18b,21 as well as
firstꢀrow transition metal elements like Cu,22 and have been accredited to equilibration between
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the Fermi levels of the TiO2 conduction band and the NP on its surface.21c An important
distinction from the work reported herein is that samarium is more abundant than precious metals
and plays an additional role as a LA (Figure 1). In some materials, as DR suggests to be the case
for SmxOy@TiO2, visible light absorbance and charge transfer can be facilitated by the minor
constituent of the nanocomposite (SmxOy here) and charge separation managed by the native
semiꢀconductor component of the hybrid material.19b An additional benefit of nanocomposite
catalysts is that holeꢀscavenging oxygen vacancies generated at heterojunctions (or at metal ion
doping sites in other hybrid materials) can increase charge separation and slow down electronꢀ
hole recombination, making SET from the chargeꢀseparated photocatalyst to the substrate more
efficient.15a,19a,19b,23 When SmxOy@TiO2 was irradiated using solely visible (465 nm) or UV (365
nm) light, conversion of the starting material did eventually near completion, but yields of 2a
were lower in both cases (Table S2). From a practical perspective it is clear that decorating TiO2
with SmxOy NP allows the nanocomposite photocatalyst to perform efficient SET after excitation
by visible light, and that 400 nm represents an optimal excitation wavelength.
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Figure 1. Proposed mechanism for the heterogeneous net reductive photoredoxꢀLewis acid
catalytic reductive cyclization of transꢀchalcones.
The net reductive heterogeneous photocatalytic mechanism proposed in Figure 1 closely
resembles the previouslyꢀdescribed homogeneous catalytic mechanism,2b,5b but likely begins
with coordination of two molecules of substrate to a single SmxOy nanoparticle on the catalyst
surface such that the two reactants come into close proximity. As in the homogeneous analogue,
photoexcitation of the catalyst and two SET events are followed by radicalꢀradical coupling to
form a new carbonꢀcarbon bond. Subsequent monoprotonation of the dienolate followed by
intramolecular aldol addition furnishes the substituted cyclopentanol product. Interestingly, the
LA samarium triflate has been proposed to take on multiple roles in the homogeneous catalytic
version of this system. In addition to stabilizing the radical anion intermediate, it has been
reported that a single Sm(III) atom facilitates the ringꢀclosing final step by coordinating to both
the enol and carbonyl functionalities of the dienolate, rendering the reaction highly selective
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toward the thermodynamically favoured stereoisomer 2a. This is not necessarily the case in the
heterogeneous mechanism, as the surface of each SmxOy nanoparticle contains many closelyꢀ
spaced, nonꢀdiffusing Sm(III) atoms available for coordination. Contrary to homogeneous
catalysis in solution, when a solidꢀphase catalyst such as nanostructured TiO2 or SmxOy@TiO2 is
used, the involvement of multiple LA sites would be less sterically hindered than coordination to
a single atom. Nonetheless, the dienolate being anchored to a single NP rather than a single atom
prior to cyclization appears to have had the same effect. The diastereoselectivity of the
heterogeneously catalyzed reaction equaled that of the homogeneous system and was preserved
regardless of whether SmxOy@TiO2 or the relatively weak LA TiO2 was used (vide infra). It
therefore seems reasonable that the enhanced activity exhibited by SmxOy@TiO2 may be the
result of a combination of stronger Lewis acidity as well as a higher number or density of LA
sites relative to the unmodified support.
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We attribute the slightly higher catalytic activity of CeO2 (< 25 nm) relative to CeO2 (< 5
µm) to a difference in surface area to mass ratio, a wellꢀknown phenomenon in nanocatalysis.
This may also contribute to the low samarium loadings achieved using ceria microparticles as
supports, but based on the 3.3 wt% Sm achieved with the smaller ceria nanopowder versus 4.7
wt% with the comparablyꢀsized TiO2, a more general explanation could be that samarium simply
has a greater affinity for titania over ceria. The latter is intrinsically a more reducible support
than TiO2 and therefore might reasonably be expected to act as a stronger LA, yet even as the
surface area to mass ratio of ceria was increased toward that of TiO2 its reactivity only
approached but still did not match that of TiO2 (Table 1 entries 4, 5 and 7). This could be due to
faster electronꢀhole recombination. However, for SmxOy@CeO2 we speculate that the
combination of Ce(IV), Sm(III) and Sm(II) might give rise to an internal pathway for nonꢀ
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radiative energy decay in which photoexcitation of SmxOy could foreseeably initiate a charge
transfer transition loop and impede SET to the substrate (Scheme 2).
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Scheme 2. Possible charge transfer transition loop in samariumꢀdecorated ceria, explaining the
nonꢀradiative dissipation of energy after light excitation.
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According to reported ground state reduction potentials of Sm(III) (EredIII/II ≈ ‒1.6 V) and Ce(IV)
(EredIV/III ≈ ‒1.8 V),20a Scheme 2 may be feasible. Additionally, polydisperse colloidal Sm2O3NP
are thought to exist as a dynamic mixed oxides of primarily Sm2O3 with localized surface
regions of the less stable SmO,12a,12c and when irradiated, at certain loadings their presence may
impact the ability of ceria to cycle between CeO2 and the less stable Ce2O3 state.
It is important to note that ICPꢀMS showed that SmxOy@TiO2 does not leach samarium
and therefore that the nanomaterial functions as a purely heterogeneous catalyst (Table S1 and
associated calculation). In all cases minor amounts of pinacol coupling of 1a were observed and
in some instances traces of the [2+2] photocycloaddition product were also detected. The
reaction did not proceed significantly in the absence of the amine, the catalyst or in the dark at
35°C (the temperature reached by the reaction mixture under irradiation; see Table S2 and
associated discussion). As is typical in heterogeneous photocatalysis using TiO2ꢀbased materials,
the role of the amine is primarily to turn over the photocatalyst by acting as a sacrificial electron
donor to holes (h+) trapped on the catalyst surface. However, the αꢀaminoalkyl radical formed
after amine oxidation and rapid deprotonation (Ered ‒1.12 V vs. SCE)24 could also have
contributed to reduction of 1a (Ered ‒1.49 V) once the latter was activated by the LA samarium.
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In any event, SmxOy@TiO2 outperformed SmxOy@CeO2 and unmodified TiO2 even when titania
was used together with Sm(OTf)3 in an equivalent molar quantity of samarium (Table 1 entries 1,
7 and 8). Based on these encouraging results, SmxOy@TiO2 was selected for further exploration
of its potential as a heterogeneous photoredoxꢀLA catalyst.
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We next investigated the scope of the reaction (Table 2). To our satisfaction,
SmxOy@TiO2 was able to catalyze the photoreductive cyclization of a series of monoꢀ and
disubstituted chalcone derivatives bearing electronꢀdonating and electronꢀwithdrawing
2+ 5b
substituents in a similar fashion to Ru(bpy)3
.
Compared to the homogeneously catalyzed
system, our heterogeneous dual catalytic strategy produced significantly higher yields of the
photoreductive coupling product for the parent chalcone as well as for substrates bearing either
an electronꢀwithdrawing group (EWG) at the 4’ꢀposition or an electronꢀdonating group (EDG) at
the 4’ꢀ or at the 4ꢀposition (1a-c and 1f, Table 2 entries 1ꢀ3 and 6). In a similar trend to that
observed in the homogeneously catalyzed reaction, SmxOy@TiO2 is less tolerant of EWGs at the
4ꢀposition but unlike the system described by Xia and coworkers, here the yield remains low
even when EWGs are simultaneously present at both the 4’ꢀ and the 4ꢀpositions (1d-e, Table 2
entries 4 and 5).
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Table 2. Substrate scope for the heterogeneous photoreductive cyclization of chalcones 1a-f
catalyzed by SmxOy@TiO2.a
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7
Entry
Substrate
Product
Time (h) Yieldb (%)
dr
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1
2
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6
1a, R1 = Ph, R2 = Ph
3
5
3
3
3
3
3
70 (62)
67 (47)
90 (84)
27 (78)
32 (59)
65 (54)
41 (62)
>10:1
>10:1
>10:1
>10:1
>10:1
>10:1
>10:1
2a
2b
2c
2d
2e
2f
1b, R1 = 4ꢀMeOꢀPh, R2 = Ph
1c, R1 = 4ꢀClꢀPh, R2 = Ph
1d, R1 = 4ꢀClꢀPh, R2 = 4ꢀFꢀPh
1e, R1 = Ph, R2 = 4ꢀFꢀPh
1f, R1 = Ph, R2 = 4ꢀMeOꢀPh
7c 1a, R1 = Ph, R2 = Ph
2a
aReaction conditions: transꢀchalcone (1 mmol), SmxOy@TiO2 (60 mg), iꢀPr2NEt (5 mmol), 15 mL dry MeCN,
degassed with argon 20 min, irradiated with a 400 nm LED (90 W). bDetermined by 1H NMR spectroscopy using an
c
internal standard. Yields in parenthesis refer to the analogous homogeneous system from ref. 5b. Control reaction
using unmodified TiO2 as the catalyst.
In addition to the convenience of easy separation of our heterogeneous catalyst from the reaction
mixture by simple centrifugation, avoidance of product contamination by the catalyst and the
absence of any additional additives to remove, another major advantage of heterogeneous
catalysis is the potential for catalyst reusability. We were pleased to observe that SmxOy@TiO2
can be recovered and used to catalyze the photoreductive cyclization of 1a for at least five cycles
at the same laboratory bench scale (1 mmol starting material) without showing any evidence of
diminished catalytic activity. In cycles two through five the reaction still reached completion
after three hours of irradiation, suggesting that reusing the catalyst had no obvious impact upon
the reaction kinetics (Figure 2). This high level of reusability goes a long way to demonstrating
the advantages offered by heterogeneous catalysis. Aside from the obvious environmental benefit
of reducing dependence on precious metals, from a purely economic perspective the increased
2+
costꢀeffectiveness of replacing a system comprising commercially available Ru(bpy)3 and
Sm(OTf)3 (as in ref. 5b) with SmxOy@TiO2 on an equivalent scale is over 90ꢀfold for just the
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first use of the readily recoverable catalyst, including the cost of the samarium precursor lost
during the preparation of the nanocatalyst (Table S3 and associated discussion).
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Figure 2. Reusability study of SmxOy@TiO2 in the heterogeneous photoreductive coupling of
chalcone 1a to form the cyclopentanol derivative 2a. Reaction conditions were identical to those
summarized in Table 1 and Table 2, including reaction time and scale, and the recovered catalyst
was used without any additional preꢀtreatment.
Turnover number (TON), the number of moles of substrate converted to product per mole of
active sites prior to catalyst deactivation, and similarly turnover frequency (TOF), are important
metrics routinely used to characterize and compare the efficacies of different catalysts. However,
obtaining reliable values for TON and TOF can be difficult for many nanostructured
heterogeneous catalysts, including SmxOy@TiO2, due to the presence of an unknown number of
active sites. NPs might possess multiple active sites and their catalytic activities could differ
greatly as a result of nonꢀuniform size or spatial distributions on the support. In addition, many
of the atoms making up each NP do not represent potential active sites because they are not
located on the NP surface. In the case of SmxOy@TiO2 the problem is compounded further
because the precise stoichiometry of the samarium oxide nanoparticle surface is uncertain.
Therefore, commenting on turnover as a function of moles of samarium atoms or numbers of
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NPs does not allow for an easy and direct comparison to the known TONs or TOFs of other
catalysts. For nanocomposites catalysts such as SmxOy@TiO2, where the support possesses some
degree of catalytic activity on its own, the presence of additional or cooperative active sites on
the support must also be considered. Even the accurate molecular weight of the nanocomposite
catalyst is an elusive quantity, and so incorporating moles of catalyst into TON and TOF
calculations is not an option. Estimates of TON and TOF should therefore be made and
interpreted with caution so as to avoid unreasonable comparisons with other heterogeneous or
homogeneous catalysts. Based upon these considerations, estimating TON and TOF of
heterogeneous catalysts as functions of substrate and catalyst mass may become a useful
convention.25 By this method, TON was 187 with respect to mass of samarium and 9 with
respect to total mass of the nanocomposite. However, both values should be regarded as low
estimates because they are based upon just the first five cycles of use rather than until complete
catalyst deactivation. Similarly, TOF was 12 hꢀ1 and 0.6 hꢀ1 as a function of mass of samarium or
total catalyst mass, respectively (calculations available in the Supporting Information).
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We expanded our investigation into the photocatalytic activity of SmxOy@TiO2 to
include the intramolecular [2+2] photocycloaddition of bis(enones) 4a-c and found that
SmxOy@TiO2 is indeed a versatile heterogeneous catalyst. The [2+2] cycloaddition of
unsaturated enones is often employed as a key step in natural product synthesis and is an
important tool in the construction of synthetic building blocks.10a,10b,11 Elegant strategies for the
2+
homogeneous photocatalytic version of this reaction using Ru(bpy)3 have been studied
extensively by Yoon,6cꢀe and nonꢀphotochemical routes have been investigated by Krische10d,26
and Bauld.27 We have also reported in the past that the reaction can be photocatalyzed
heterogeneously, by platinized titania (Pt@TiO2).18b
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Utilizing the Lewis acidity of SmxOy@TiO2 allowed us to make substantial
improvements to the efficiency of the heterogeneously catalyzed reaction (Table 3). We obtained
higher yields of the [2+2] cycloadducts 5a and 5b in a much shorter reaction time (71% 5a and
72% 5b in 1 h here vs. 51% 5a and 42% 5b in 15 h using Pt@TiO2 in ref. 18b), under conditions
that are now on par with the homogeneous catalytic system reported by Yoon (Scheme 1).
Compared to Pt@TiO2, we were also able to increase the selectivity for the desirable [2+2]
cycloadduct to 78% for 5a and 81% for 5b, and greatly improve the catalyst reusability up to at
least three cycles without significant loss of activity (Figure S20). Using the previously described
method, TON (based on 3 cycles) and TOF were 117 and 39 hꢀ1 by mass of samarium and were 6
and 2 hꢀ1 by total nanocomposite mass, respectively.
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Table 3. Heterogeneous intramolecular [2+2] cycloaddition of bis(enones) 4a-c.a
Catalyst &
Entry
Substrate
Product Selectivity (%) Yieldb (%)
dr
Conversionb (%)
1a
1b
1c
78
10
12
57
22
17
81
4
71
9
1.2:1
>10:1
>10:1
1.3:1
>10:1
>10:1
1.8:1
>10:1
>10:1
‒
4a
X = CH2
R = Ph
4a
X = CH2
R = Ph
4b
X = CH2
R = 4ꢀClꢀPh
4c
X = CH2CH2
R = Ph
5a
6a
7a
5a
6a
7a
5b
6b
7b
5c
6c
7c
SmxOy@TiO2
91
11
47
18
14
72
4
13
0d
13e
54
2a
2b
2c
TiO2
83
3a
3b
3c
4ac
4bc
4cc
SmxOy@TiO2
89
15
0
SmxOy@TiO2
73
18
74
1.2:1
1.4:1
aReaction conditions: substrate (0.17 mmol), catalyst (20 mg), DABCO (5 equiv), 5 mL dry MeCN, degassed with
argon 20 min, irradiated with a 400 nm LED (90 W). Longer irradiation times did not result in significantly higher
conversion to the [2+2] cycloadduct using either SmxOy@TiO2 or TiO2 alone. bDetermined by 1H NMR
c
spectroscopy using an internal standard unless otherwise noted. Irradiation time not optimized for conversion.
dIsolated yields. eYield determined by 1H NMR spectroscopy performed on an impure fraction of the crude product
mixture obtained after flash chromatography. DABCO = 1,4ꢀdiazabicyclo[2.2.2]octane.
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Similar to the photoreductive cyclization of chalcones, SmxOy@TiO2 significantly outperformed
unmodified TiO2 in terms of yield but also in selectivity for 5a (Table 3 entries 1a and 2a).
Higher yield and selectivity for the [2+2] cycloadduct over the reductive coupling and [4+2]
heteroꢀDielsꢀAlder products (6a and 7a, respectively) relative to both TiO2 and Pt@TiO2 is most
likely due to coordination to samarium, mirroring results observed by Yoon upon addition of a
LA.6c,6e Diastereoselectivity was again unaffected by modification of the TiO2 support with
samarium, and remained high for 6a-b and 7a-b, matching results reported using Pt@TiO2 or the
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homogeneous catalyst Ru(bpy)3 in the presence of a LA additive. However, here the
diastereometric ratio was much lower for the [2+2] cycloadducts (Table 3). Conversion of a
symmetric bis(enone) with identical aryl substituents possessing an EDG at the 4ꢀposition (R =
4ꢀMeOꢀPh) was less effective and not quantified.
Interestingly, Yoon reported that bis(enone) 4c, bearing an additional methylene unit in
the carbon tether, exclusively underwent the [4+2] heteroꢀDielsꢀAlder cycloaddition to form
7c.6g While we also did not detect any trace of the [2+2] cycloadduct 5c in our analogous
heterogeneously catalyzed system, we did isolate the reductive coupling product 6c with 18%
selectivity (Table 3 entries 4aꢀc). Yoon suggested that 6c could be the result of overꢀreduction
and reductive cleavage of 7c at longer irradiation times (>24 h). Although the quantities of
reagents were only given in equivalents in that report, our system was irradiated for much less
time (1 h), albeit with a different light source (90 W 400 nm LED at 2 mm vs. 200 W tungsten
filament bulb at 30 cm in ref. 6g). In the homogeneously catalyzed system the addition of 10
equivalents of H2O resulted in a dramatic increase in the yield and selectivity for 7c, and the time
required to reach complete conversion decreased from 9.5 to 1 h.6g We instead observed an
opposing trend, with 10 equivalents of H2O resulting in a decrease in conversion from 73% to
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less than 60% in 1 h. These differences in reactivity do much to highlight the inherent divide
between homogeneous and heterogeneous catalytic systems, but could be viewed as an
advantage or disadvantage depending upon the specific application.
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Figure 3. Proposed mechanism for the heterogeneous net neutral photoredoxꢀLA dual catalytic
intramolecular [2+2] photocycloaddition of symmetric aryl bis(enones).
A mechanism for the heterogeneous catalytic intramolecular [2+2] photocycloaddition of
symmetric bis(enones) is proposed in Figure 3. Similar to the photoreductive cyclization of
chalcones, the proposed heterogeneous catalytic mechanism for the major product closely
follows that of the homogeneous catalytic system.1a,2b SET from the photoexcited nanocomposite
catalyst to the LAꢀactivated substrate forms the key radical anion intermediate stabilized by the
LA SmxOy NP. Subsequent intramolecular Michael addition leads to closing of the fiveꢀ
membered ring followed by cyclobutanation to afford the samariumꢀcoordinated ketyl radical,
which then gives up an electron to yield the cycloadduct 5 (one diastereomer shown for clarity).
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Unlike the photoreductive cyclization of chalcones then, the intramolecular [2+2]
photocycloaddition of bis(enones) is a net neutral redox process and the possibility of a chain
component in the overall mechanism should not be ignored. In principle, oxidation of the ketyl
radical in the final step of the mechanism could proceed via SET to quench h+ in the
photocatalyst, or by reduction of another molecule of substrate to propagate a chain reaction.
Since heterogeneous catalytic reaction mechanisms are often difficult to study and to know with
certainty, Figure 3 focuses instead on what is likely to be the dominant pathway to the major
[2+2] photocycloaddition product. Further work to elucidate the precise nature of the
mechanism, extend the scope of the reaction and improve diastereoselectivity is currently
underway in our laboratory and may be the subject of a future report.
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Conclusion
In summary, we have described the efficacy and versatility of the first member of a new class of
bifunctional nanomaterials, SmxOy@TiO2, in the first two examples of heterogeneous dual
photoredoxꢀLewis acid catalysis. The nanocomposite material utilizes visible light to perform
both net reductive and net neutral photoredox processes requiring LAs. It efficiently catalyzes
the intermolecular photoreductive cyclization of a series of chalcones as well as the
intramolecular [2+2] photocycloaddition of bis(enones), two classes of substrates and cyclization
reactions with proven track records of high synthetic utility. Further, SmxOy@TiO2 is easily
recovered, reusable for several cycles, functions as a purely heterogeneous catalyst without
leaching, and may yet possess even more general applicability for heterogeneous dual
photoredoxꢀLA catalysis. This work demonstrates the potential for realizing systems that
combine all the benefits of homogeneous and heterogeneous catalysis, and should set in motion a
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wide range of new collaborative efforts across synthetic organic, industrial and catalysis
research.
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Supporting Information
Experimental details, instrumentation, characterization of materials and organic compounds
(PDF)
Acknowledgement
The authors wish to thank the Natural Sciences and Engineering Research Council of Canada,
the Canada Foundation for Innovation, and the Canada Research Chairs program for generous
financial support, including a Doctoral Post Graduate Scholarship for G. K. Hodgson. Thanks are
due to M. Grenier, Y. Liu, S. Curtis and S. Mommers for their relevant technical expertise.
Special thanks to S. Impellizzeri for valuable insights and a special thanks as well to S. Pitre for
productive discussions.
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