Quantum Dot-Catalyzed Photoreductive Removal of Sulfonyl-Based Protecting Groups

Article abstract of DOI:10.1002/anie.202005074

This Communication describes the use of CuInS₂/ZnS quantum dots (QDs) as photocatalysts for the reductive deprotection of aryl sulfonyl-protected phenols. For a series of aryl sulfonates with electron-withdrawing substituents, the rate of deprotection for the corresponding phenyl aryl sulfonates increases with decreasing electrochemical potential for the two electron transfers within the catalytic cycle. The rate of deprotection for a substrate that contains a carboxylic acid, a known QD-binding group, is accelerated by more than a factor of ten from that expected from the electrochemical potential for the transformation, a result that suggests that formation of metastable electron donor–acceptor complexes provides a significant kinetic advantage. This deprotection method does not perturb the common NHBoc or toluenesulfonyl protecting groups and, as demonstrated with an estrone substrate, does not perturb proximate ketones, which are generally vulnerable to many chemical reduction methods used for this class of reactions.

Full text of DOI:10.1002/anie.202005074

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Quantum Dot-Catalyzed Photoreductive Removal of Sulfonyl-
Based Protecting Groups
Authors: Kaitlyn Perez, Cameron Rogers, and Emily A Weiss
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202005074
Link to VoR: https://doi.org/10.1002/anie.202005074

10.1002/anie.202005074
Angewandte Chemie International Edition
Quantum Dot-Catalyzed Photoreductive Removal of Sulfonyl-Based Protecting Groups
Kaitlyn A. Perez, Cameron R. Rogers, and Emily A. Weiss*
Department of Chemistry, Northwestern University, 2145 Sheridan Rd., Evanston, IL 60208-
3113
*corresponding author. Email: e-weiss@northwestern.edu
Abstract: This communication describes the use of CuInS2/ZnS quantum dots (QDs) as
photocatalysts for the reductive deprotection of aryl sulfonyl-protected phenols. For a series of
aryl sulfonates with electron-withdrawing substituents, the rate of deprotection for the
corresponding phenyl aryl sulfonates increases with decreasing electrochemical potential for the
two electron transfers within the catalytic cycle. The rate of deprotection for a substrate that
contains a carboxylic acid, a known QD-binding group, is accelerated by more than a factor of ten
from that expected from the electrochemical potential for the transformation, a result that suggests
that formation of metastable electron donor-acceptor complexes provides a significant kinetic
advantage. This deprotection method does not perturb the common NHBoc or toluenesulfonyl
protecting groups and, as demonstrated with an estrone substrate, does not perturb proximate
ketones, which are generally vulnerable to many chemical reduction methods used for this class
of reactions.
Sulfonyls are viable protecting groups for phenols, because, unlike alkylsulfonates,
phenylsulfonates are stable to nucleophilic attack. Sulfonyl protection strategies for phenols are
perhaps not as widely used as they could be, however, because the deprotection requires harsh
conditions. Electrochemical cleavage of the S-O bond of a sulfonyl protected phenol requires
reduction with a potential of at least -1.4 V vs. SCE,[1] and thermal deprotection reactions employ
strong acids (H2SO4) or strong reducing agents (Na, Li or Mg) that are incompatible with
substituents or other protecting groups on the phenol.[2] Given that these deprotection reactions
have been accomplished via electrochemical deprotection[1a, 3] and direct photolysis,[4] and that
visible-spectrum photons have energies ranging from 3.10 eV (400 nm) to 1.77 eV (700 nm), the
deprotection of sulfonated phenols is a good candidate reaction for a photoredox approach.
In the photoreductive deprotection, a photosensitizer absorbs two successive photons, donates
two electrons in sequence to liberate a phenol and the corresponding sulfinate anion from the
protected species, and donates two holes to a sacrificial terminal reductant. Unlike direct
photolysis, photoredox catalysis does not require that a substrate be directly photoexcited, so UV
light-driven undesirable side reactions are minimized.[5] For more than 30 years,[6] photoredox
strategies using organic dyes and ruthenium- and iridium-based complexes as photocatalysts have
enabled deprotection of, for example, para-methoxybenzyl protected alcohols[7] and amides,[8] 4-
methoxybenzyl protected primary amines and alcohols,[9] and N-tosyl amides.[6a, 10] This strategy
has not been demonstrated for sulfonyl protected phenols, however, possibly because the only
commercially available inorganic complexes with excited-state oxidation potentials greater than -
1.4 V vs. SCE are fac-Ir(ppy)3, fac-Ir(dF-ppy)3, and [Ir(ppy)2(dtbbpy)]+, all of which have peak
absorption at wavelengths shorter than 400 nm.[11] Photons with energy above 400 nm often
directly photoexcite the substrate (in addition to the photosensitizer) and thereby enable parasitic
reactions. Recently, colloidal semiconductor quantum dots have been investigated as visible-light-
absorbing photoredox catalysts for organic transformations.[12]
This paper describes the photocatalytic reductive deprotection of a series of aryl sulfonyl-
protected phenols using copper indium sulfide/zinc sulfide core/shell quantum dots (CuInS2/ZnS
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10.1002/anie.202005074
Angewandte Chemie International Edition
QDs) as light harvesters and photo-redox catalysts. CuInS2/ZnS QDs have high excited-state
oxidation potentials, >-1.9 V vs. SCE, and excited-state lifetimes of ~30-500 ns.[13] While, as
mentioned above, these QDs are not unique in their ability to catalyze this class of reaction, we
highlight in this report two advantages of using CuInS2/ZnS QDs for photocatalyzed deprotections:
(i) their size can be tuned to absorb photons as low-energy as 730 nm (we use 532-nm illumination
in this study),[13b, 14] whereas the Ir compounds require blue or UV light, and (ii) while photoredox
reactions catalyzed by molecules require diffusion-limited collisions between substrates and
excited-state catalysts, we can design QDs to reversibly associate with suitably functionalized or
charged substrates.[15] Formation of metastable complexes of photocatalyst and substrate is
beneficial for increasing the yield of photoredox reactions and extending the scope to include
endergonic reactions, because this association increases the probability of electron transfer to the
substrate or intermediate upon photoexcitation of the catalyst.[16]
Figure 1 summarizes our proposed photocatalytic cycle for the 2-e- deprotection of phenyl aryl
sulfonates. CuInS2/ZnSQDs, synthesized using an adapted literature procedure[13b] and solubilized
in DMSO-d6 with a 3-mercapto-1-propanol ligand shell (details in the Supporting Information,
“SI”), are photoexcited by a 532-nm photon to form QD*. QD* then transfers one electron to the
substrate and one hole to the sacrificial reductant, triethylamine (TEA), to return to its ground
state. The S-O bond in the singly reduced substrate breaks to form a phenoxide ion, which is
protonated to form phenol, and a sulfonyl radical. The QD is then excited a second time to form
QD*, whereupon QD* transfers a hole to TEA and an electron to the sulfonyl radical to form a
sulfinate anion.
Figure 1. Reaction conditions and proposed catalytic cycle for the photocatalyzed 2-e- deprotection of
aryl sulfonates by CuInS2/ZnS QDs. Ar1 and Ar2 are aryl groups on the protected alcohol and the
protecting group, respectively, see Table 1. Sacrificial triethylammonium (TEA+) decomposes into
diethyl amine and acetaldehyde, see Figure S1 of the SI.
We synthesized a series of phenyl aryl sulfonates, Table 1, using adapted literature procedures
(see the SI),[17] and measured the potential necessary to liberate phenol from each phenyl aryl
sulfonate substrate from the potential corresponding to the peak of the irreversible cathodic wave
in its cyclic voltammogram (CV), Ep,c, Table 1 and the SI. The reductive wave at Ep,c is irreversible
for every substrate in the presence of our sacrificial reductant, TEA, and there are no other
2
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Table 1. CuInS2/ZnS QD-Catalyzed Deprotection of Phenyl Aryl Sulfonates.
Ep,c[S/S2-]
Substrate
Ar1
Ar2
% Conversionb
% Yieldb
(V vs. SCE)a
1
2
3
-2.20
0
0
0
0
0
0
-2.20
-2.09
4
-2.02
0
0
5
6
7
-2.00
-1.81
-1.72
0
0
100
57
92
57
8
-1.72
78
78
9
-1.55
-1.48
-1.44
-1.50
-1.49
-1.48
100
87
91
83
10
11
12
13
14
100
69c,d
43c,d
100c
93
54c,d
43c,d
93c
15
-1.46
100c
85e
16
-1.45
98c
0c
aThe potentials at which we observed peak cathodic current (Ep,c) were determined from CVs of 10 mM of the
protected substrate under N2 with 100 mM tetrabutylammonium hexafluorophosphate, and 1 M TEA in DMSO,
using a glassy carbon working electrode, a Ag wire reference electrode, and a Pt wire counter electrode. The
potentials are referenced to SCE using a decamethylferrocene internal standard. The notation “Ep,c[S/S2-]”
indicates that two electrons are transferred during the wave, one to the substrate and one to the liberated sulfonyl
b
radical after S-O bond cleavage. The conversions and yields listed are NMR yields for samples of 10 μM
CuInS2/ZnS QDs, 0.001 M substrate, and 0.1 M TEA in DMSO-d6 illuminated with a 4.5-mW, 532-nm laser diode
d
for 48 hours. cYield and conversion determined after 24 hours. The reaction stopped progressing after 24 hours
even though starting material was still present. e Isolated yield for sample of 0.22 mM CuInS2/ZnS QDs, 0.022 M
substrate, 2.2 M TEA in DMSO-d6 illuminated with a 4.5-mW 532-nm laser diode for 24 hours.
3
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10.1002/anie.202005074
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reductive waves at potentials lower than the reduction potential of the QDs, Ered,QD, of ~-1.9 V vs.
SCE (see the SI),[13a] so we conclude that the wave includes both electrons within the catalytic
cycle in Figure 1.
Table 1 lists the 1H NMR-determined yields of phenol product and % conversions of substrates
1 - 11 for the QD-photocatalyzed deprotection reactions, after 48 hours of illumination with a 4.5-
mW, 532-nm laser diode. All NMR spectra are in the SI. For substrates where Ep,c > Ered,QD
(substrates 1-5), the yield of the deprotection is zero. For substrates where Ep,c < Ered,QD (substrates
6-11), the yield generally increases as Ep,c decreases, with the exception of substrate 6, discussed
below. The deprotection of 6-11 proceeds cleanly to produce phenol, as indicated by the close
agreement between % conversion and % yield.
We confirmed that both electrons required to accomplish the sulfonate deprotection reaction
originate from the QD (as opposed to TEA, which can act as a hydride donor), by performing the
deprotection of 11 using 1000 equiv 5,10-dihydro-5,10-dimethylphenazine (DMPZ) as a sacrificial
reductant instead of 1000 equiv TEA.[18] The yield of the reaction (after illumination with 532-nm
light for 2 hours) is 20% lower with DMPZ than with TEA, probably because DMPZ is larger and
has a lower probability of extracting the hole from the QD, but the deprotection clearly does not
require a hydride donor to occur. This result substantiates the mechanism we proposed in Figure
1.
Figure 2A shows the kinetics of the deprotection reactions for substrates 6-11. The QDs
remain stable during photocatalysis; we do not observe evidence of aggregation or etching over
the course of these reactions via optical spectroscopy (see the SI). We determined the initial rate
of each reaction from fits to these kinetic traces in the linear regime and plotted these initial rates
versus Ep,c in Figure 2B. The initial rate of the deprotection reaction generally increases as Ep,c
decreases. We also observe a positive correlation between the initial rate of the reaction and the
level at which % yield ([phenol]) plateaus (see the SI). We suspect the plateau is caused by
competition for catalytic sites between substrates and liberated sulfinate anions (i.e. given ample
time, sulfinate ions will bind to QD surface and block catalytic sites).
As is clear from inspection of Figure 2B, the initial rate of deprotection of 6 is enhanced by
more than a factor of ten from that expected from its Ep,c value. A direct comparison of the rate
constant for deprotection of 6 with that of the corresponding ester 9, which has a lower reduction
potential than 6 (by 0.26 V) leads to the tentative conclusion that the ability of 6 to associate with
unoccupied binding sites on the surface of the QD through its carboxylate group provides a kinetic
advantage in this reaction. None of the other substrates have substituents known to bind to the
surfaces of semiconductor QDs, which typically occurs through the coordination of surface metal
ions (by, e.g., COO-, S-) or dative binding of surface metal ions (by, e.g., NH2).[19] Importantly,
carboxylate groups such as the one on 6 is known, at least for CdSe QDs, to be reversible
binders,[15] so that once the deprotection occurs, the product will not permanently block its catalytic
site on the QD surface.
To support our proposal that increasing a substrate’s affinity for the QD surface increases the
initial rate of 2-e- deprotection of that substrate, we compared the performances of photocatalysts
CuInS2/ZnS and fac-Ir(ppy)3 for the deprotection of substrates 6 and 11, Figure 3. Because fac-
Ir(ppy)3 does not absorb 532 nm light, we used a 5-mW 405-nm laser as an excitation source; we
adjusted the catalyst loading to achieve the same optical density at 405 nm for the two
photocatalysts (1 mol% CuInS2 QDs; 9.7 mol% Ir(ppy)3). The photoredox steps for deprotection
of 11 should be diffusion controlled for both Ir(ppy)3 and the QDs, since 11 does not have a group
with affinity for the QD surface. Ir(ppy)3 generates a factor of six more phenol from 11 than does
4
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10.1002/anie.202005074
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Figure 2. (A) The concentration of phenol product in reaction mixtures with 10 μM CuInS2/ZnS QDs,
0.001 M substrate (labeled as in Table 1), and 0.1 M TEA in DMSO-d6 as function of time of illumination
with a 4.5-mW, 532-nm laser diode. The potentials listed in the legend correspond to the measured Ep,c[S/S2-
] value for each substrate. The lines are the fits used to acquire the initial rates of the reactions. (B) The rate
constants, as determined by the method of initial rates, for the deprotection reactions as a function of the
Ep,c[S/S2-] of the substrates, listed in Table 1.
the QD photocatalyst within the first 15 minutes of illumination. This result is expected, at least at
short reaction times, because Ir(ppy)3’s excited-state lifetime (~2 μs)[20] is ~50 longer than that
of the QDs (35 ns).[13a] A longer excited state lifetime increases the probability of a charge transfer
reaction before the photocatalyst returns to its ground state. The longer lifetime of Ir(ppy)3 makes
the results of the reaction of substrate 6, which does have a carboxylate binding group for the QD
surface but has no mechanism for generating a complex with Ir(ppy)3, even more notable: the QDs
generate a factor of 6.7 more phenol from 6 than does Ir(ppy)3 within the first 15 minutes of
illumination. This comparison not only supports our proposal that meta-stable association of the
substrate and the QD provides a kinetic advantage for this 2-e- reaction, but also that this kinetic
advantage more than compensates for its shorter excited-state lifetimewhen compared to Ir(ppy)3.
5
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10.1002/anie.202005074
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Figure 3. The yield of phenol product after the photocatalyzed deprotections of substrate 6 (black squares)
and substrate 11 (blue circles) using 1 mol % CuInS2 QDs (filled symbols) and 9.7 mol % fac-Ir(ppy)3
(open symbols) upon illumination from a 5-mW, 405-nm laser. 6 has a carboxylic acid substituent and 11
has no substituent with reported affinity for the QD surface.
To demonstrate the orthogonality of this photocatalyzed deprotection method, we synthesized
substrates that contain two protecting groups (compounds 12, 13, 14), one 4-acetylphenyl sulfonate
group and another protecting group that, thermodynamically, should not be redox-active with
respect to the photoexcited QD. The synthetic procedures for these substrates are in the SI. Table
1 shows that, after 24 hours of illumination of the QD-substrate mixtures with 532-nm light, the
% conversion matches the % yield for deprotection of 13 and 14; we do not observe any
perturbation of the tert-butyloxycarbonyl (Boc) or toluenesulfonyl (Ts) protecting groups on those
substrates. Although QDs successfully liberate phenol from 12, the % conversion of starting
material is 15 percentage points greater than the % yield of desired product. We suspect the benzyl
ether groups on the substrate and the desired, deprotected product may partially decompose when
subjected to these reductive conditions, see SI for more details. Additionally, as discussed above,
we suspect that the % conversion of substrates 12 and 13 is limited by binding of the sulfinate
anion product to the surface of the QD, which poisons the catalyst.
Common reducing agents for aryl sulfonyl deprotections, including magnesium, SmI2, LiAlH4,
and sodium amalgam[2] can also cause unwanted reductions of carbonyl groups. To demonstrate
the functional group tolerance of the photocatalytic deprotection route, we attempted photo-
deprotections of 15, derived from estrone, and 16, derived from vanillin, which contain carbonyl
groups that would be reduced in the presence of any of the chemical reducing agents listed above.
Table 1 shows that, upon illumination with 532-nm light for 24 h, we achieve selective cleavage
of 15 to liberate estrone without perturbation of the ketone. In contrast, we observe no formation
of vanillin from 16 even though 98% of starting material has disappeared. Given the disappearance
of the sharp aldehyde peak after illumination (see the SI), we suspect the QD reduces the aldehyde
to form a ketyl radical, which then couples to another radical to form C–C products.[12b]
In summary, photo-redox catalysis is a simple and effective route to the reductive deprotection
of aryl sulfonyl-protected phenols using green (532-nm) light and CuInS2/ZnS QDs as
photocatalysts, as long as the potential needed for the required two electron transfers, Ep,c, is less
than the measured reduction potential of the QD (~-1.9 V vs. SCE). The initial rate of deprotection
6
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Angewandte Chemie International Edition
increases as Ep,c becomes more positive. We find that, if a substrate contains a QD-binding group,
here a carboxylic acid, the rate of deprotection is greatly accelerated from that expected from the
Ep,c value of the substrate, likely because the substrate can associate with a surface of a QD, and
the rates of the two electron transfer reactions are not diffusion-controlled. The kinetic advantage
provided by reversible association of QD and substrate more than compensates for the shorter
excited-state lifetime of CuInS2/ZnS QDs when we compared their performance to that of fac-
Ir(ppy)3. The QD-catalyzed photodeprotection method is selective for the removal of sulfonyl
protecting groups with electron-withdrawing substituents in the presence of Boc and tosyl groups,
and does not perturb proximate ketones, which are generally vulnerable to many chemical
reduction methods used for this class of reactions. The orthogonality and selectivity of this method
makes it potentially attractive for accomplishing deprotection steps within multi-step syntheses of
complex molecules. Furthermore, it might be useful in light-controlled syntheses of polymers that
proceed through successive protection and deprotection reactions.[21] Finally, photocatalyzed
deprotection reactions may be a route to unique ligand shell structures on QDs that cannot be
accessed by ligand exchange; such structures could be useful for a wide variety of chemical sensing
and biological tagging applications.[22]
Acknowledgements. We thank Prof. R. Thomson, J. Rote, and L. Redfern for helpful discussions.
This work was supported by the Department of Defense (DoD) National Defense Science &
Engineering Graduate (NDSEG) Fellowship to K.A.P, and by the National Science Foundation
under CHE-1664184. C.R.R. thanks the International Institute for Nanotechnology at
Northwestern University for a fellowship. We made use of IMSERC at Northwestern University,
which has received support from the NIH (1S10OD012016-01 / 1S10RR019071-01A1); the Soft
and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the State
of Illinois and the International Institute for Nanotechnology (IIN).
Keywords: Copper Indium Sulfide, Deprotection, Phenylsulfonates, Photocatalysis, Quantum
dots
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Entry for Table of Contents:
Quantum dots photocatalyze the reductive deprotection of phenyl aryl sulfonates. The rate of
deprotection is enhanced for substrates that bind to the surface of the quantum dots.
8
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