Picosecond Electron Transfer from Quantum Dots Enables a General and Efficient Aerobic Oxidation of Boronic Acids

Article abstract of DOI:10.1021/acscatal.8b01078

General visible light-mediated aerobic oxidation of boronic acids is unveiled using CdSe nanocrystal quantum dots (QDs) as the photoredox catalyst. This protocol requires mild reaction conditions and low catalyst loading (down to 10 ppm), and tolerates various functional groups. The resulting phenols and aliphatic alcohols are produced in good to high yield with turnover numbers as high as >62000. The reaction mechanism is probed using ultrafast transient absorption and luminescence spectroscopy. The existence of a rapid 350 ps initial electron transfer followed by a hole transfer is demonstrated.

Full text of DOI:10.1021/acscatal.8b01078

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Article
Picosecond Electron Transfer from Quantum Dots Enables
a General and Efficient Aerobic Oxidation of Boronic Acids
AMIT KUMAR SIMLANDY, Biswajit Bhattacharyya, Anshu Pandey, and Santanu Mukherjee
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01078 • Publication Date (Web): 02 May 2018
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Page 1 of 7
ACS Catalysis
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Picosecond Electron Transfer from Quantum Dots Enables a
General and Efficient Aerobic Oxidation of Boronic Acids
Amit Kumar Simlandy,† Biswajit Bhattacharyya,‡ Anshu Pandey*,‡ and Santanu Mukherjee*,†
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†Department of Organic Chemistry, Indian Institute of Science, Bangalore - 560012, India
‡Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore - 560012, India
Supporting Information Placeholder
the most attractive and versatile route to phenols and alcohols.
The reliability of this route makes this conversion particularly
attractive for late-stage applications in complex syntheses. While
a variety of oxidants have been reported,5 molecular oxygen re-
mains the most economical and sustainable oxidant.6 In 2012,
Xiao, Jørgensen and co-workers reported an aerobic oxidative
hydroxylation of boronic acids under visible light photoredox
catalysis using Ru(bpy)3Cl2 as the catalyst (Scheme 1A).7 Despite
high efficiency, the scope of this protocol is restricted only to aryl
boronic acids.
ABSTRACT: A general visible light-mediated aerobic oxidation
of boronic acids is unveiled using CdSe nanocrystal quantum dots
(QDs) as the photoredox catalyst. This protocol requires mild
reaction conditions, low catalyst loading (down to 10 ppm) and
tolerates various functional groups. The resulting phenols and
aliphatic alcohols are produced in good to excellent yields with
outstanding turnover numbers (up to >62000). The reaction
mechanism is probed using ultrafast transient absorption and lu-
minescence spectroscopy. The existence of a rapid 350 ps initial
electron transfer followed by a hole transfer is demonstrated.
KEYWORDS: quantum dots, photoredox catalysis, boronic acid,
aerobic oxidation, cadmium selenide, phenols, alcohols
In this article we show that the QD catalyzed oxidation is ame-
nable to aliphatic boronic acids as well (Scheme 1B). Consistent
with the enhanced substrate scope, our ultrafast studies confirm
that this QD catalyzed reaction proceeds through a somewhat
different reaction sequence. We further find the efficacy of the
reaction to be a strong function of QD band gap and architecture,
with wide band gap materials being significantly more efficient at
driving the reaction. A turnover number (TON) in excess of
62000 is observed under optimized reaction conditions.
Semiconductor nanocrystals, also known as quantum dots
(QDs), have recently attracted attention as photosensitizers that
can dictate outcome of organic reactions.1 Unlike conventional
photoredox agents, nanocrystals offer improved stability and larg-
er surface area, which in turn allows higher photon penetration
and consequently better catalyst turnover.2 It is further possible to
engineer redox activity of nanocrystals, thereby optimizing them
towards either selectivity or generality. Here we demonstrate the
ability of CdSe nanocrystals to sensitize the visible light mediated
aerobic oxidation of aliphatic and aromatic boronic acids for the
synthesis of aliphatic alcohols and phenols.
Figure 1a shows a typical absorption (red) and emission spec-
trum (green) of CdSe QDs. Transmission electron micrographs
(TEM) and high resolution transmission electron micrographs
(HRTEM) images further exemplify the narrow size distribution
of the QDs employed in this work (Figure 1b-c).
Scheme 1. Oxidative Hydroxylation of Boronic Acids: (A)
Previous Reports and (B) Present Work
Figure 1. (a) Absorption and emission spectra of CdSe with
band gap of 2.35 eV. (b) TEM and (c) HRTEM images of the
same sample.
To evaluate the suitability of these QDs as photoredox catalyst
for the aerobic oxidation of boronic acids, we designed a model
reaction with 4-methoxyphenylboronic acid (1a) as the substrate
(Table 1). In a typical reaction, 0.08 mol% CdSe and 2.0 equiv of
i-Pr2NEt were dispersed in an acetone medium. It is observed that
under these conditions, QDs form a turbid dispersion. When the
reaction flask was irradiated with a blue LED of 450 nm wave-
length in air at ambient temperature, we were pleased to note the
formation of 4-methoxy phenol (2a) in 56% yield within 48 h
(entry 1). Despite rather modest yield, this experiment clearly
validated our hypothesis. The use of molecular oxygen instead of
The ubiquitous nature of phenols and aliphatic alcohols in natu-
ral products and various synthetic intermediates3 has made their
efficient and economic synthesis essential. Among various meth-
ods known,4 oxidative hydroxylation of boronic acids is arguably
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ACS Catalysis
Page 2 of 7
air led to significant improvement in yield (entry 2). While the
reaction was found to take place under the irradiation of white
LED (100W), 2a was formed with somewhat reduced yield (entry
3). We undertook further optimization of reaction conditions in
order to improve the chemical yield of the product. While increas-
ing the amount of i-Pr2NEt did not improve the yield, lowering
the same reduced the yield considerably (entries 4-5). Further
attempts to improve the yield by increasing the surface area of
QDs using silica coated CdSe also proved to be abortive (entry 6).
No product formation was observed when the reaction was carried
out in dark or in the absence of any one of the reaction compo-
nents (CdSe, base, O2) under otherwise standard conditions (en-
tries 7-10), thereby highlighting the essentiality of each of these
components. In addition, heating a mixture of 1a, i-Pr2NEt and
CdSe under oxygen in the absence of blue LED failed to furnish
any product (entry 11). These results unambiguously eliminate the
involvement of any thermal oxidation and support the photoredox
pathway.
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Figure 2. Chemical yield of the reaction as a function of the
band gap (1S exciton position) of CdSe quantum dots.
Having optimized the reaction conditions (Table 1, entry 2), the
generality of this QD-catalyzed aerobic oxidation protocol was
evaluated. As shown in Table 2, a range of aryl boronic acids
furnished the corresponding phenols in good to excellent yields
within reasonable time scales. Both electron-donating as well as
electron-withdrawing substituents at various positions of the aryl
ring were well tolerated (entries 1-9). It must be noted that the
formyl group, which is generally susceptible to oxidation condi-
tions, not only survived, but afforded para-salicylaldehyde 2i in
high yields within 16 h (entry 9). Inspired by this result, other
oxidizable functional groups (amine and acetamide) on the aryl
ring (1j-k) were also tested. While our reaction conditions tolerat-
ed these functional groups, the corresponding phenols (2j-k) were
formed in low to moderate yields (entries 10-11). In addition, both
naphthols can also be obtained with good yields, albeit after pro-
longed stirring (entries 12-13).
Table 1. Optimization of Reaction Conditionsa
entry
1
change from “standard conditions”
time (h) yield (%)b
air instead of O2
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48
60
48
48
48
48
48
48
48
48
56
85
2
none
3
100W white LED instead of blue LED
4.0 equiv of i-Pr2NEt
1.0 equiv of i-Pr2NEt
silica coated QD
no light
69
4
81
Table 2. Scope of Boronic Acidsa
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52
6
40
<5c
<5c
<5c
<5c
<5c
7
8
no CdSe
9
no i-Pr2NEt
entry
1
R (2)
time (h)
48
yield (%)b
85
TONc
1063
10
11
argon instead of O2
no light at 40 ºC
4-OMeC6H4 (2a)
2
Ph (2b)
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48
40
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72
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65
38
96
96
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81
925
988
aReactions were carried out on a 0.2 mmol scale. bYields correspond to the
c
1
3
isolated yield, unless stated otherwise. Conversion as determined by H
NMR spectroscopy.
2-MeC6H4 (2c)
3-MeC6H4 (2d)
4-(t-Bu)C6H4 (2e)
3-NO2C6H4 (2f)
4-CF3C6H4 (2g)
4-(CN)C6H4 (2h)
4-(CHO)C6H4 (2i)
3-(NH2)C6H4 (2j)
3-(NHAc)C6H4 (2k)
1-naphth (2l)
4
988
5
913
The reaction was also conducted with differently sized QDs in
order to arrive at the most optimal QD band gap for driving the
reaction.8 We employed QDs with band gaps of 2.3, 2.14, 2.03
and 1.91 eV. The lamp spectrum is shown in the supporting in-
formation (Figure S1).9 We observed that under these conditions,
reaction yields increase with increasing QD band gap (Figure 2).
In particular, we observed that the yield increases exponentially
from 40 to 85%, with a maximum yield over a 48 h reaction cycle
being observed for QDs with an S exciton position of 2.3 eV (540
nm). We further found that yields drop to 20% for core shell
quantum dots CdSe/CdS (1 ML). Details of this experiment are
provided in the supporting information Figure S2.9 This trend is
consistent with an activation energy barrier for electron transfer.10
It is nevertheless clear that the optimal results are observed in the
case of QDs with a band gap of 2.3 eV. CdSe QDs with still nar-
rower band gaps are unstable in the reaction medium.
6
1075
963
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1100
1025
400
9
10
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12
13
14
15
738
825
2-naphth (2m)
n-C8H17 (2n)
888
1013
813
c-pent (2o)
65d
b
aReactions were carried out on a 0.2 mmol scale. Yields correspond to the
isolated yield after column chromatography. cTON = Turnover number.
dNMR yield.
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ACS Catalysis
Our protocol is not restricted to aryl boronic acids and can be
Scheme 2. Oxidative Hydroxylation Reaction on a Larger
Scale
applied to the oxidation of aliphatic boronic acids as well. For
example, both long chain and alicyclic boronic acids were
smoothly converted to the corresponding alcohols in good to high
yields within reasonable reaction time (entries 14-15). Boronic
esters can also be used as substrates, albeit with a considerably
slower reaction rate.9
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4
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7
8
A noteworthy feature of our protocol is the high turnover num-
ber (TON) obtained for all the substrates, irrespective of their
steric and electronic nature.
Table 3. One-Pot Catalyst Recycling Experimenta
9
In order to gain insights on the stereochemical aspects of this
reaction, we subjected cis-1-phenyl-2-indanylboronic acid 3 to
our standard reaction conditions (Scheme 3). We were delighted
to find that the reaction proceeded with complete stereospecificity
to furnish cis-1-phenyl-2-indanol 4 in 55% yield. This observation
clearly points to a concerted oxidation pathway.
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Scheme 3. Stereospecificity of the Oxidative Hydroxylation
Reaction
cycle
1st
time (h)
24
overall yield (%)b
TON
2325
2nd
3rd
30
62
96
b
aEach batch of reaction was carried out on a 0.2 mmol scale. Yield corre-
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sponds to the isolated yield.
The practicality of our protocol was tested by performing a
one-pot recyclability experiment using 4-cyanophenylboronic acid
(1h) as substrate (Table 3). Here, new batches of 1h were added
after consumption of the previous batch (monitored by TLC).
Each reaction cycle was carried out with 0.2 mmol of 1h. At the
end of third cycle, the product 2h was isolated in 62% yield while
the overall TON was found to be 2325. Decrease in overall yield
and reaction rate for the subsequent batches are possibly due to
gradual decrease in effective catalyst loading.
The remarkably high turnover numbers as well as the broad
substrate scope prompted us to study the reaction mechanism in
greater detail.
We first examined the effects of treatment of QDs with the bo-
ronic acid. In general, it is observed that addition of boronic acid
to QDs in hexane strongly enhances their luminescence. In the
example shown in Figure 3a, the luminescence quantum yield
(QY) of the QDs increases from 8.4% (green curve) to 14.3%
(blue curve) upon addition of 20 µL of phenyl boronic acid to 2
mL of 0.04 mM QDs in hexane. This increase is not associated
with a shift in the QD spectrum, and is consequently consistent
with improved passivation of the QDs upon exposure to the bo-
ronic acid.11 This interpretation is supported by the lengthening of
the QD emission lifetime in time correlated single photon count-
ing (TCSPC) experiments. For example, Figure 3b shows the
emission lifetimes of CdSe QDs (green, 17 ns) as well as a solu-
tion of CdSe QDs treated with phenyl boronic acid (blue, 23 ns).
In each case we report the averaged lifetime described by the
expression shown in the supporting information.9
Table 4. Reaction with Lower Catalyst Loadinga
entry
1
x (mol %)
0.001
time (h)
72
yield (%)b
63
TON
62972
2
0.0001
96
8
83963
aReactions were carried out on a 0.4 mmol scale using 20W blue LED
source. bYield corresponds to the isolated yield.
High catalytic activity and photostability of the CdSe QDs as
displayed by high TON allowed us to reduce the loading of QDs.
As shown in Table 4, in the presence of as low as 0.001 mol% (10
ppm) of CdSe, the product was obtained in 63% yield with a TON
close to 63000 (entry 1). Further lowering of catalyst loading (to 1
ppm) compromised the reaction rate drastically. Nonetheless,
extraordinarily high TON is maintained. Such turnover numbers
are outstanding, even in the domain of photoredox catalysis1d and
are not the consequence of a chain process since such a possibility
has been eliminated through control experiments.9
Figure 3. (a) Emission spectra of the CdSe (green), CdSe with
20 µL phenyl boronic acid (blue) and CdSe with 20 µL phenyl
boronic acid and 20 µL Hunig’s base (red) with QYs of 8.4, 14.3
and 2.5% where excitation energy of 3.1eV. (b) Emission kinetics
of the same sample.
The scalability of this reaction is established by carrying out the
oxidation of 1h on a 2.0 mmol scale – 10 times that used for
demonstrating the scope of the reaction, using only 50 ppm of
CdSe (Scheme 2). We were pleased to find that 4-cyanophenol 2h
was formed in 70% yield with excellent TON.
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Further treatment of these boronic acid treated QDs with Hün-
Samples were illuminated with a 100 fs 400 nm pump pulse. An
800 nm pulse was employed as the gate. We observe that in case
of UPL, all samples show identical early-time luminescence dy-
namics. We note that the interpretation of UPL measurements is
somewhat complicated by the fact that these only interrogate the
emissive members of the ensemble, and further, the early time
dynamic is dominated by non-radiative decay that is intrinsic to
CdSe QDs. Nevertheless, our inability to detect the emergence of
a new transient significantly shorter than 350 ps in the acid and
base treated QDs is consistent with the occurrence of the hole
transfer step after the electron transfer.13b To summarize these
results, for the QD catalyzed reaction, the electron transfer from
the QDs occurs as the first step, while hole transfer occurs at a
later, unresolved timescale.14 This is illustrated in Scheme 4. The
requirement of both the boronic acid and the Lewis base (tertiary
amine) to accomplish the first electron transfer further suggests
the formation of a Lewis acid-base adduct A that acts as the elec-
tron acceptor. In the presence of molecular oxygen, the adduct A
collapses to furnish the boronic acid-superoxide radical anion
adduct B with concomitant release of the tertiary amine base. The
exact mechanism for this single-electron transfer (SET) step re-
quires further study. However, involvement of such Lewis acid-
base adducts in light-driven SET has been demonstrated by Van
der Eycken, Ley and co-workers.15 Another SET from tertiary
amine to the QD valence band restores the QDs to their neutral
ground state, and produces tertiary ammonium radical cation C. A
hydrogen atom abstraction from C by B generates the peroxy-
boronate intermediate D, which upon rearrangement and hydroly-
sis produces alcohols/phenols. The stereospecificity of the oxida-
tion as depicted in Scheme 3 supports the intermediacy of D and
the subsequent steps.
ig’s base leads to a drop in quantum yield from 14.3% (blue
curve, Figure 3a) to 2.5% (red curve, Figure 3a). We found that
this drop in QY is again correlated with a change in the lumines-
cence lifetimes as measured through TCSPC. As shown in Figure
3b, the addition of both base and boronic acid to QDs lowers the
lifetime to 9.3 ns. This decrease could be caused either by a
change in surface passivation or due to the initiation of the chemi-
cal reaction. In order to distinguish between these possibilities, we
further studied the electron and hole residence times in these QDs
through ultrafast spectroscopy.11,12 Electron residence times were
determined through ultrafast transient absorption (TA). In a typi-
cal experiment, QDs dispersed in hexane were pumped with fre-
quency doubled pulses of a 100 fs Coherent Libra laser. The band
edge dynamics was probed using a broadband white light probe.
Figure 4a shows the dynamics observed when QDs are pumped
by a 400 nm pulse that generates 0.6 excitons per QD. As shown
in Figure 4a, the electron dwell times of QDs may be fit to biex-
ponentials with time constants of 12 ps (34%) and 8.3 ns (66%)
(green circle). Treatment with acid alone slows down this dynam-
ic, and we observe electron dwell times of 17 ps (38%) and 10.1
ns (62%) (blue circle). This slowing down of the electron decay
dynamic most likey arises from improved surface passivation of
the QDs. Once both acid and base are present in the medium, the
electron population decays significantly faster, with time con-
stants of 6.4 ps (34%) and 357 ps (66%) (red circle) correspond-
ing to an average lifetime of 353 ps. Our results are therefore
consistent with an electron transfer from the QDs on a 300 ps
timescale only if both reacting species are present. We further
verified this electron transfer criterion by checking the dynamics
in presence of base alone. This is highlighted in Figure 4b that
compares the dynamics of QDs treated with base alone (red hol-
low symbols) with the dynamics of untreated QDs (green solid
symbols). As evident from the figure, both samples show identical
dynamics. Since faster TA transients corresponding to electron
transfer are only observed when both acid and base are present,
we infer that the presence of both species is essential to trigger the
electron transfer sub-step of the photochemical reaction.13
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Scheme 4. Mechanistic Proposal for the Aerobic Oxidation
of Boronic Acids
In conclusion, we have demonstrated the ability of CdSe nano-
crystals to sensitize the visible light mediated aerobic oxidation of
boronic acids. This general and mild protocol allows for the effi-
cient oxidation of both aromatic as well as aliphatic boronic acids
for the synthesis of phenols and aliphatic alcohols in good to ex-
cellent yields. Requirement of low catalyst loading (down to 10
ppm) combined with excellent photostability of the QDs helped us
achieving outstanding turnover numbers (>62000) even in the
domain of photoredox catalysis. We further show that this reac-
tion occurs via an initial rapid (~350 ps) electron transfer from the
photoexcited QDs, followed by the later abstraction of the hole.
Considering the mildness and functional group tolerance, we ex-
pect our protocol to be applicable for the late-stage hydroxylation
in complex syntheses. In addition, QDs as photoredox catalyst
will no doubt make its mark in other visible light mediated bond
constructions, bond cleavages and rearrangements in the near
future.
Figure 4. (a) Transient bleach decay of CdSe (green), CdSe
with 20 µL phenyl boronic acid (blue) and CdSe with 20 µL phe-
nyl boronic acid and 20 µL Hunig’s base (red). (b)Transient
bleach decay of CdSe (green), CdSe with 20 µL Hunig’s base
(red) of the same sample. (c) Luminescence upconversion decay
of the CdSe (green), CdSe with 20 µL phenyl boronic acid (blue)
and CdSe with 20 µL phenyl boronic acid and 20 µL Hunig’s
base (red).
While TA spectroscopy provides insights into electron dynam-
ics, a full understanding of the mechanism also requires inferring
the dynamics of the hole. We note that a consistent trend in life-
times is observed in luminescence upconversion measurements
that allow us to resolve the early parts of the excitonic dynamics.
These experiments (Figure 4c) were performed on a Halcyone-
Femtosecond Fluorescence Spectrometer from Ultrafast Systems.
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Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
*sm@iisc.ac.in
*anshup@iisc.ac.in
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The authors declare no competing financial interests.
This work is funded by Council of Scientific and Industrial Re-
search (CSIR), New Delhi [Grant No. 02(0207)/14/EMR-II], Sci-
ence and Engineering Research Board (SERB), New Delhi [Grant
No. EMR/2016/005045] and Nano Mission [Grant No.
SR/NM/NS-1117/2012]. Spectroscopic studies were performed on
equipment purchased under an IRHPA grant [IR/S2/PU-
0005/2012]. A.K.S. thanks CSIR and B.B. thanks IISc Bangalore
for their respective doctoral fellowships. We are thankful to Mr.
Guru Pratheep Rajasekar and Ms. Arpita Mukherjee (Solid State
and Structural Chemistry Unit, IISc Bangalore) for their help with
some parts of ultrafast studies. Special thanks to Prof. M. Kevin
Brown (Department of Chemistry, Indiana University, USA) for a
gift of the precursor of compound 3.
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