Conjugated polymers as photoredox catalysts: Visible-light-driven reduction of aryl aldehydes by poly(p-phenylene)

Article abstract of DOI:10.1002/ejoc.201200437

The use of visible light in photocatalysis has been intensively studied because of its natural abundance, ease of use, and promising potential for industrial applications. However, there are several challenges to utilizing visible light for organic functional group transformations. These challenges include the low absorptivity of most organic compounds in the visible spectrum, and side reactions are often prevalent in photochemical reactions. Visible-light-sensitive catalysts offer a means to overcome these obstacles. Conjugated polymers are semiconductors that offer a large range of redox potentials, they are stable, and they often absorb visible light. Despite these desirable properties for photocatalysis, only a limited number of organic reactions utilizing conjugated polymers as photocatalysts have been reported. In one such example, poly(p-phenylene) was used to induce the pinacol coupling reaction of benzaldehyde upon irradiation with visible light. In this work, visible light, thiols, and poly(p-phenylene) were employed to reduce aryl aldehydes to their respective alcohols to better characterize the reaction mechanisms of this system. The effects of varying reaction conditions on the rate of photocatalysis indicated interfacial electron transfer from the poly(p-phenylene) surface to the substrate as the initial productive step. Additionally, the chemoselective reduction of aryl aldehydes over aryl ketones and alkyl aldehydes was achieved with this system.

Full text of DOI:10.1002/ejoc.201200437

Job/Unit: O20437
/KAP1
Date: 19-09-12 18:03:16
Pages: 11
FULL PAPER
DOI: 10.1002/ejoc.201200437
Conjugated Polymers as Photoredox Catalysts: Visible-Light-Driven Reduction
of Aryl Aldehydes by Poly(p-phenylene)
[a]
[a]
[a]
Miao Zhang, William D. Rouch, and Ryan D. McCulla*
Keywords: Photocatalysis / Conjugation / Polymers / Chemoselectivity / Reduction
The use of visible light in photocatalysis has been intensively
studied because of its natural abundance, ease of use, and
promising potential for industrial applications. However,
gated polymers as photocatalysts have been reported. In one
such example, poly(p-phenylene) was used to induce the
pinacol coupling reaction of benzaldehyde upon irradiation
there are several challenges to utilizing visible light for or- with visible light. In this work, visible light, thiols, and
ganic functional group transformations. These challenges in-
clude the low absorptivity of most organic compounds in the
visible spectrum, and side reactions are often prevalent in
photochemical reactions. Visible-light-sensitive catalysts of-
fer a means to overcome these obstacles. Conjugated poly-
mers are semiconductors that offer a large range of redox
potentials, they are stable, and they often absorb visible
light. Despite these desirable properties for photocatalysis,
only a limited number of organic reactions utilizing conju-
poly(p-phenylene) were employed to reduce aryl aldehydes
to their respective alcohols to better characterize the reaction
mechanisms of this system. The effects of varying reaction
conditions on the rate of photocatalysis indicated interfacial
electron transfer from the poly(p-phenylene) surface to the
substrate as the initial productive step. Additionally, the
chemoselective reduction of aryl aldehydes over aryl ketones
and alkyl aldehydes was achieved with this system.
·
+
Introduction
cation (D ) to generate new products through unimolec-
ular rearrangements or reactions with other species at the
surface or in bulk solution. Thus, photocatalysts can be
used to promote both oxidative and reductive transforma-
tions of organic substrates.
The use of photocatalysts in chemical synthesis has a
venerable history, and recently, the conjoining of photore-
dox catalysts and visible light for applications in organic
[
1]
synthesis has become an expanding area of research. No-
tably, organometallic ruthenium(II) and iridium(III) poly-
pyridine complexes have been used for cycloadditions, re-
ductive dehalogenations, enantioselective alkylations, radi-
cal additions, and C–H bond activation.[ Additionally,
small organic photosensitizers have recently been used to
promote radical additions into C=C and C=O and oxidat-
ive C–C and C–P bond couplings.[ Although the afore-
mentioned homogeneous photoredox catalysts have proven
utility, heterogeneous photocatalysts can be desirable for
certain applications.
2]
3]
Scheme 1.
Heterogeneous photocatalysts are often
semi-
[
4]
conductors. Irradiation of a semiconductor promotes
The strong oxidation potential of the TiO valence band
2
electrons from the valence band to the conduction band, (2.5 V vs. NHE) allows TiO to photocatalytically degrade
2
+
–
and the resulting charge carriers (h
and e CB) can be many organic compounds, which has made the use of TiO2
VB
used to oxidize or reduce adsorbed species with the proper as a means to remediate pollution an extremely active area
[
5]
redox potentials (Scheme 1). To catalyze organic reactions, of research. Taking advantage of the strongly oxidative
photocatalytic systems are designed to take advantage of properties of TiO remains one of the most common appli-
2
–
·
the reactivity of the nascent radical-anion (A ) or radical- cations of heterogeneous photoredox catalysts in synthetic
[
6]
organic reactions. A variety of organic compounds such
[
a] Department of Chemistry, Saint Louis University,
[7]
[8]
[9]
as aliphatic alcohols, aromatic alcohols, alkenes, and
can undergo photo-oxidation catalyzed by
TiO . Fox and co-workers provided many synthetic and
3
501 Laclede Ave, 63103 St. Louis, MO, USA
[
10]
Fax: +1-314-977-2521
hydocarbons
E-mail: rmccull2@slu.edu
2
Homepage: http://www.slu.edu/~rmccull2/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200437.
mechanistic insights into photocatalytic properties of TiO2,
including the role of electron-transfer through the detection
Eur. J. Org. Chem. 0000, 0–0
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M. Zhang, W. D. Rouch, R. D. McCulla
FULL PAPER
of radical cations in the cis–trans isomerization of stil- through the addition of a hydrogen atom donor, namely, a
[
11]
bene.
They also demonstrated that the oxidation of ole- thiol. The effect of varying the initial concentrations of the
fins to carbonyls occurred at the semiconductor–liquid starting reagents on the observed rates was investigated to
interface rather than in bulk solution.[12] Formylation of elucidate the underlying mechanism and to identify where
primary and secondary amines, which were frequently reaction optimization would have the greatest impact.
adopted as sacrificial electron donors, was the ultimate fate
[
13]
for amines in semiconductor photocatalytic systems.
Compared to TiO , which only has a moderate conduction
2
[14]
band potential (–0.5 V vs. NHE),
the conjugated poly-
mer poly(p-phenylene) (PPP) has a much higher conduction
band potential of –2.0 vs. NHE.[15] Electron transfer to an
acceptor can only occur easily if the reduction potential of
the acceptor is less negative than the conduction band po-
tential of the photocatalyst. Thus, PPP is expected to be a
much stronger reductive photocatalyst than TiO and most
2
Scheme 2.
other inorganic semiconductors.
Despite the ubiquitous application of conjugated poly-
mers as field-effect transistors, light-emitting diodes, and by Yamamoto and co-workers.
PPP was synthesized according to procedures described
[
18]
After preparation of the
photovoltaic cells, only a few examples of using conjugated Grignard reagent of 1,4-dibromobenzene, the polymeriza-
polymers as photocatalysts to degrade toxic organic pol- tion was catalyzed by nickel(II) dichloride bipyridine to af-
lutants have been investigated.[ The potential use of con- ford a yellow slurry containing PPP. Soxhlet extraction with
jugated polymer photocatalysts in synthetic applications toluene yielded a yellow powder. Elemental analysis of sev-
was illustrated by the visible-light-driven pinacol coupling eral batches showed that the extracted PPP consisted of
16]
[
15]
of benzaldehyde by using PPP as a photoredox catalyst.
74.5% carbon, 4.7% hydrogen, and 18.5% bromine on
Additionally, PPP and visible light has been used to reduce average. Using GPC analysis, the Mw and M was deter-
n
water, carbon dioxide, ketones, keto esters, and electron- mined to be 500 Da and 363 Da, respectively, correspond-
[
15,17]
poor alkenes while promoting cis–trans isomerization.
ing to a polydispersity of 1.38. If it is assumed that the
The photofixation of CO into benzophenone to yield benz- polymerization was terminated by the reaction by a protic
2
ylic acid was assisted by stronger reduction potentials of impurity, these values correspond with an average chain
[
17c]
PPP.
This has also spurred investigation into the photo- length of 5–6 benzene units with one end terminated by
[17k,17l]
chemical reduction of CO by PPP.
The change in the bromine (LMW-PPP). The insoluble material remaining af-
2
redox band potentials of conjugated polymers can have a ter Soxhlet extraction consisted of higher molecular weight
profound influence on the reaction mechanism. For exam- PPP (HMW-PPP), which was supported by elemental
ple, unlike TiO , there was no evidence for the involvement analysis. The insoluble PPP consisted, on average, of 84.5%
2
of radical ions in the photoinduced cis–trans isomerization carbon, 5.0% hydrogen, and 7.6% bromine, which would
of alkenes promoted by PPP.[
17a]
Similarly, the reduction of be consistent with a PPP chain of Ͼ7 phenylene units
maleate and fumarate to succinate was proposed to result (HMW-PPP). Because the HMW-PPP material was com-
from an initial one-electron reduction and subsequent dis- pletely insoluble, the molecular weight could not be deter-
[
17m]
proportionation.
In this work, a systematic investiga- mined by GPC. The UV spectrum of LMW-PPP dissolved
tion of the reaction mechanisms induced by PPP photoca- in chloroform shows a single absorption band that extends
talysis was undertaken. Using the photocatalysis of aryl al- past 400 nm with a maximum absorbance at 300 nm (Fig-
dehydes as a model system, the effect of varying the reac- ure 1). In the previous reports, the λmax for p-sexiphenyl as
tion conditions on the photocatalytic mechanisms were de- a solid was observed by reflectance spectrometry at 385 nm
[
19]
termined and ways to improve performance were identified. and when dissolved in THF at 317 nm. Therefore, it was
expected that as a solid the LMW-PPP would also absorb
more of the visible spectrum.
Results
Before examining the effect of a thiol hydrogen atom do-
The goal of this work was to examine how modifications nor, the effect of different catalyst loadings and sacrificial
to the photocatalytic conditions influenced reaction out- electron donor (i.e., Et N) concentrations on the pinacol
3
comes to gain insight into how to improve the system. In coupling of benzaldehyde were investigated. In a typical ex-
initial work by Yanagida and co-workers, benzaldehyde (1) periment, the photolysis was carried out with eight fluores-
exclusively underwent a pinacol coupling reaction to form cent bulbs with an emission range of 400–440 nm. Dispos-
hydrobenzoin (2) when PPP was irradiated with visible light able borosilicate glass test tubes (10 mL) were used as the
in the presence of triethylamine (Et N) as the sacrificial reaction vessels, and oxygen was removed by argon sparging
3
[
15]
electron donor.
As shown in Scheme 2, a ketyl (I ) was prior to irradiation. During photolysis, the reaction mixture
k
proposed as the key intermediate in the mechanism. The was stirred constantly. The concentration of the starting al-
hypothesis for this work was that putative I could be di- dehyde and the products was analyzed by using GC and
k
verted into a formal photoreduction to benzyl alcohol (3) GC–MS at 2 to 6 time points during irradiation. For these
2
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Conjugated Polymers as Photoredox Catalysts
Figure 1. Absorption spectrum of low molecular weight PPP
Figure 3. Concentration of benzaldehyde (1, open markers) and hy-
drobenzoin (2, solid markers) with 5 mg (diamond), 10 mg (tri-
angle), and 10 mg washed and reused (circle) LMW-PPP upon irra-
(LMW-PPP) dissolved in chloroform.
3
diation at 420 nm with 1 m Et N in acetonitrile.
experiments, the initial concentration of 1 was held con-
stant at ca. 5 mm, and the amounts of LMW-PPP and Et N
3
were varied. The only product identified was 2, and a 75%
mass balance was obtained. It also should be noted that a
nearly equal amount of meso and d/l isomers of 2 was al-
ways obtained. A linear relationship between the rate of
products of the irradiation of 1 in the presence of 2-mercap-
toethanol (BME), LMW-PPP, and Et N were quantified.
3
The photolysis products were analyzed by using GC and
GC–MS. Products were identified as 2, 3, and the corre-
sponding disulfide by comparison with authentic standards.
The results of a typical photocatalytic reaction containing
a thiol are depicted in Figure 4. The addition of BME in-
creased the rate of consumption of 1. The addition of other
hydrogen atom donors (Brønsted acids) was also investi-
gated to determine if the same results could be obtained.
The substitution of acids for the thiol in the reaction re-
sulted in the predominate formation of 2 rather than 3.
Subsequent study of the use of acid catalysis on this pho-
toredox system was performed but is beyond the scope of
this paper.
formation for 2 and the initial concentration of Et N was
3
found (Figure 2). As a practical matter, when less than 0.4 m
of Et N was used, the rate became too slow to measure
3
accurately, and at very high concentrations, the quantity of
Et N complicated the chromatographic analysis. Thus, the
3
remaining photocatalytic reactions were carried out at 1 m
Et N. As depicted in Figure 3, the rates of consumption of
3
1
and formation of 2 increased with the loading of LMW-
PPP. For all experiments, a control under the same reaction
conditions containing no PPP was irradiated for 24 h, upon
which less than 5% conversion of 1 was always observed.
Figure 2. Concentration of hydrobenzoin (2) after 15 h of irradia-
Figure 4. Concentration of 1 (triangle), 2 (diamond), and 3 (square)
3
tion at 420 nm with varying concentrations of the Et N, 5 mg of
at different time points upon 420 nm irradiation of 10 mg of LMW-
LMW-PPP, 5 mm 1 in acetonitrile.
3
PPP, 500 mm 2-mercaptoethanol, and 1 m Et N in acetonitrile.
Open markers: first photolysis with unused LMW-PPP; closed
markers: second run after LMW-PPP had been washed with aceto-
nitrile and reused.
The premise of this work was that modifications to the
photocatalysis conditions would influence the observed
products. Namely, the addition of hydrogen atom donors
was expected to reduce the ketyl intermediate to the corre-
Unexpectedly, it was found that when LMW-PPP was
sponding alcohol. Hydrogen abstraction from thiols by nu- reused, after sonicating and washing with acetonitrile, the
cleophilic radicals is known to proceed rapidly.[ Thus, the photocatalytic activity increased. To investigate the cause of
20]
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M. Zhang, W. D. Rouch, R. D. McCulla
FULL PAPER
this observation, the solubility of the LMW-PPP photocata-
lyst was examined. When LMW-PPP was sonicated and left
in acetonitrile overnight at 35 °C, 100% recovery of the
mass of LMW-PPP was obtained after filtration and dry-
ing, and no solid residue was observed upon evaporation of
the filtrate. This result indicated that LMW-PPP was insolu-
ble in acetonitrile. To ensure the observed reactivity was not
the result of some component of LMW-PPP being released
into solution, LMW-PPP was removed by filtration from
a typical photocatalytic reaction that had been allowed to
proceed for 12 h to ca. 60% completion. After being de-
gassed with argon, the filtrate was irradiated for another
8
h. Removal of the LMW-PPP resulted in the complete
loss of photocatalytic activity. Complete loss of photocata-
lytic activity was also observed if LMW-PPP was removed
Figure 5. Irradiation of 1 (8 mm) at 420 nm in the presence of PPP
10 mg), 2-mercaptoethanol (500 mm), and Et N (1 m) in acetoni-
(
3
by decantation after centrifugation immediately prior to ir- trile.
radiation. These control experiments indicated that the ob-
served photocatalytic activity arose from catalysis on the
surface of the LMW-PPP particles and was not the result sumption was observed from run to run. In the presence
of a molecular reductant or mediator. of 2-mercaptoethanol (500 mm), a very different result was
To determine if the change in photocatalytic activity obtained for a series of four 8-h irradiations reusing the
upon reuse was due to change in the LMW-PPP, the physi- same 10 mg of LMW-PPP throughout the series. Unlike in
cal properties of LMW-PPP were examined before and after the absence of a thiol, the fourth run was only 1.5 times
the photocatalytic reaction. The GPC data of PPP prior faster than the first, which was considerably slower than the
to and after photolysis were identical; however, elemental 7.5-fold expected increase observed when LMW-PPP was
analysis showed a significant loss of bromine: 11.6% bro- reused from a photocatalytic reaction that proceeded to
mine prior to photolysis and 5.8% after photolysis. The near completion (22 h). Elemental analysis of LMW-PPP
drop in bromine content to 5.8% was similar to the bro- after 8 h of (Ͻ33% conversion) revealed very little change
mine content typically found for the completely insoluble (Ͻ2%) in the content of carbon and bromine. However, for
material (HMW-PPP) remaining after Soxhlet extraction. It a separate sample that was allowed to proceed for 21 h
should be noted that HMW-PPP could not be detected by (90% conversion), a 10% increase in carbon content and
GPC, as it is completely insoluble in all organic solvents. concomitant loss of bromine was observed. Controls
This led to the postulation that during photocatalysis the showed no change in elemental content upon washing with
LMW-PPP was being transformed into a material similar acetonitrile or if 1 was removed from the reaction mixture.
to HMW-PPP. This hypothesis was examined by evaluating Thus, the dramatic increase in the rate shown in Figures 4
the photoreduction of 1 with LMW-PPP, the recycled mate- and 5 was only observed when the initial photocatalysis was
rial obtained by washing the used LMW-PPP (R-LMW- carried out to near complete conversion of starting mate-
PPP), or HMW-PPP serving as the photoredox catalyst. As rial.
shown in Figure 5, the relative rates for the conversion of 1
Because one of the objectives of this work was to divert
increased roughly sixfold for HMW-PPP and R-LMW-PPP aldehydes into undergoing photoreductions, the photocatal-
compared to unused LMW-PPP. Additionally, whereas 3 ysis of 1 was carried out with 2-mercaptoethanol (BME),
was the major product for LMW-PPP, 2 was the major tert-butylthiol (tBuSH), 1-propanethiol (PrSH), benzyl
product when HMW-PPP or R-LMW-PPP was used as the mercaptan (BnSH), and ethyl 2-mercaptoacetate (EMA) by
photocatalyst. Thus, the photocatalytic activity of the R- using virgin LMW-PPP. Additionally, it was observed that
LMW-PPP was more similar to HMW-PPP than LMW- different batches of the PPP preparations had significant
PPP.
variability in their photocatalytic activity (Ϯ30%), as mea-
The increase in rate observed for recycled LMW-PPP (R- sured by the consumption rate of 1. Thus, unless stated
LMW-PPP) suggested that an acceleration in the rate otherwise, comparisons were only made with LMW-PPP
should occur during the course of the photolysis. However, from the same preparation, which was not reused, to reduce
no acceleration in the rate was detected for any single pho- the variance of the observed reaction rates. The relative
tocatalytic reaction performed during this study. One expla- rates and the product yields of 2 and 3 relative to the con-
nation for this observation was that the increase in photo- sumption of 1 in the presence of BME, PrSH, tBuSH,
catalytic activity was small over the course of a single pho- BnSH, or EMA are reported in Table 1. The average com-
tolysis to be observed. Thus, a series of 8-h photocatalytic bined product yield of 2 and 3 was near 70%, and every
reactions was performed by reusing the same LMW-PPP thiol was capable of inducing the photoreduction of 1 to 3.
material (after being washed and dried) in each run (i.e., The best yields of 3 were obtained with EMA; however,
the R-LMW-PPP obtained from the initial LMW-PPP). In compared to the absence of the additive thiol, BME, EMA,
the absence of a thiol, a gradual increase in the rate of con- and BnSH accelerated the reaction, whereas PrSH and
4
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Conjugated Polymers as Photoredox Catalysts
tBuSH had little effect or decreased the rate of reaction.
Photolysis with tBuSH added afforded a 1:3 alcohol to pin-
acol ratio. A Brønsted plot of the consumption rate of 1
and the pK values of the thiols (Figure 6) was used to de-
a
termine if there was any relationship with the acidity of the
thiols examined. Decent correlation between the reaction
rates and the pK values of the thiols was evident.
a
Table 1. Benzaldehyde (1) PPP photocatalysis with various thiols.[a]
Entry Thiol[a]
pK
a
Rel.
rate
Conv. (%) of
1 in 15 h
Yield (%)
2[c] 3[d]
[
b]
1
2
3
4
5
6
None
EMA
BnSH
BME
PrSH
–
8.0
9.4
9.6
1.0
4.5
1.9
2.2
1.0
0.8
21.2
95.2
40.2
46.5
21.2
16.9
83Ϯ14[e] none
[
f]
none
10Ϯ3
24Ϯ7
85Ϯ5
48Ϯ7
50Ϯ5
[f]
[f]
Figure 7. Concentration of products 2 (diamonds) and 3 (squares)
vs. the starting concentration of 2-mercaptoethanol (closed
markers) or tert-butylthiol (open markers) after 19 h of 420 nm ir-
radiation with 10 mg of LMW-PPP, 5 mm of 1, and 1 m Et N in
3
acetonitrile.
[f]
10.5
23Ϯ11 50Ϯ12
42Ϯ11 21Ϯ7
[f]
tBuSH 11.4
[a] 500 mm thiol concentration, 420 nm irradiation of 10 mg of
LMW-PPP, 5 mm of 1, and 1 m Et N in acetonitrile. [b] Averaged
3
relative rates for the conversion of 1 by using LMW-PPP from same
preparation. [c] Yield of 2 relative to the consumption of 1.
In a control experiment, it was unexpectedly observed
[
d] Yield of 3 relative to the consumption of 1. [e] 95% confidence that photoreduction of 1 occurred in the absence of the
[21]
interval. [f] Ref.
Et N sacrificial donor, albeit at a much slower rate. To ex-
3
amine if the thiols could serve as the sacrificial electron do-
nor, the photolysis was carried out at different concentra-
tions of BME without any Et N. Indeed, as shown in
3
Table 2, an increase in the relative rates of the formation of
3
was observed as the concentration of BME increased. A
similar trend was observed for 2; however, at very high con-
centrations of the thiol, 2 could not be detected. The ad-
dition of Et N (1 m) resulted in an eightfold increase in the
3
overall rate. These results indicated that BME acted as a
sacrificial electron donor.
Table 2. Comparison of rates with and without Et
3
N.
Entry
Thiol (mm)[a]
Rel. rate 2[b]
Rel. rate 3[b]
1
2
3
4
5
6
7
8
0
20
50
n.d.[c]
1.0
0.6
n.d.[c]
2.2
3.7
4.3
5.5
150
200
500
1.2
3.1
n.d.[
22.1
31.1
c]
10.1
Figure 6. Rate of consumption of 1 (log) vs. pK
a
of thiols used.
[
d]
[c]
Photolysis carried out by using 420 nm irradiation of 10 mg of
0
n.d.
500[
d]
35.0
LMW-PPP, ≈5 mm benzaldehyde, 500 mm thiol, and 1 m Et
acetonitrile.
3
N in
[
a] 2-Mercaptoethanol concentration. [b] Relative rates for the for-
mation of the products irradiated with 420 nm light, 10 mg of
LMW-PPP in acetonitrile. [c] Not detected. [d] 1 m Et N.
3
The effect of thiol concentration on photolysis on the
reaction was also examined. Identical solutions were pre-
For interfacial charge transfer to compete with facile
pared except the starting concentration of the thiol was var- charge carrier recombination, pre-adsorption of the sub-
ied from 20 to 500 mm. The samples were allowed to react strate to the surface was assumed necessary. To examine the
for 19 h followed by analysis of the products. As depicted effect of adsorption of 1 to LMW-PPP, a set of photocata-
in Figure 7, no definitive trend was observed for 2. How- lytic reactions with increasing starting concentrations of 1
ever, there was a linear correlation between the thiol con- and constant BME (250 mm) were performed. Unlike when
centration and the amount of 3 formed after 19 h of photol- the concentration of Et N, PPP, or thiol was varied, a rela-
3
ysis. Because there was a linear relationship with respect to tively large variance in the rates was observed when the con-
thiol concentration, the change in the concentration of 3 centration of 1 was varied, even within the same LMW-PPP
can be directly correlated to the rate of formation. Consis- preparation. In Figure 8, each data point represents the
tent with the data in Table 1, the magnitude of the slope for average of a minimum of four runs at equal starting concen-
BME was greater than the slope for tBuSH.
trations of 1. The rate data was collected from three dif-
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M. Zhang, W. D. Rouch, R. D. McCulla
FULL PAPER
ferent PPP preparations, which likely accounts for some of creased the overall rate of reaction compared to benzalde-
the error. Additionally, at high concentrations of 1, the hyde, and in the case of moderately deactivating 4-acetyl-
change in concentration was small relative to the starting benzaldehyde, the corresponding alcohol was detected as a
concentration, which also increased the observed error. As minor product. In all cases, the addition of a thiol increased
shown in Figure 8, varying the concentration of 1 resulted the rate of consumption of the aryl aldehyde.
in saturation kinetics. Namely, the reactions exhibited a
Table 3. PPP photocatalysis of 4-substituted benzaldehydes in the
change from pseudo-first-order at low substrate concentra-
tion to near zero-order at high concentration. This is typical
presence and absence of 2-mercaptoethanol (BME).[
a]
for heterogeneous photocatalytic reactions and does not ne-
cessitate a change in mechanism.[
18]
Langmuir–Hinshel-
wood kinetics are commonly used to describe these photo-
catalytic systems, which assumes that the substrate par-
titioning between the surface and solution are at equilib-
rium. Thus, the rate is dependent upon the extent of surface
Entry
R
BME
Rel.
rate
Conv. (%)
of SM in
Yield (%)
[b]
[c]
pinacol[d] alcohol[e]
(mm)
coverage, θ, as defined in Equation (1), where K is the ad-
c
2
3 h
sorption coefficient. The observed rate is then proportional
to the extent of surface coverage as defined by Equation (2),
where kLH is the concentration-independent photocatalytic
[
[
f]
f]
76Ϯ7[g]
n.d.[h]
1
2
3
4
5
6
7
8
9
H
H
0
500
0
500
0
500
0
1.0
1.6
0.6
1.1
0.4
0.9
1.7
2.2
2.0
2.2
–
43.2
69.2
25.9
47.6
17.3
38.9
73.5
95.1
86.5
95.1
–
19Ϯ1
67Ϯ10
11Ϯ2
44Ϯ3
51Ϯ10
n.d.
83Ϯ6
n.d.
[h]
Me
Me
MeO
MeO
F
[
22]
rate constant and [1] is the initial concentration of 1.
A
0
least-squares curve fit was used to determine Kc
[h]
–
5
–1
n.d.
98Ϯ2
(
1
(
8.8ϫ10 m ) and kLH [kLH (benzaldehyde)
=
n.d.[
h]
89Ϯ12
18Ϯ5
6
–1
5 –1
.2ϫ10 s ; k
(hydrobenzoin) = 1.5ϫ10 s ; k
LH LH
F
500
0
70Ϯ15
18Ϯ7
84Ϯ1
n.d.
100Ϯ1
5
–1
benzyl alcohol) = 4.1ϫ10 s ] for the substrates and prod-
[i]
COMe
COMe 500
OH
OH
52Ϯ28
[
[
[
h]
h]
h]
ucts.
10
n.d.
n.d.
n.d.
[h]
1
1
0
500
θ = K
c
[1]
0
/(1 + K
c
[1]
0
)
(1)
12
1.9
82.2
[a] Photocatalysis carried out with 420 nm, 10 mg of LMW-PPP,
rate = kLHθ = kLH
K
c
[1]
0
/(1 + K
c
[1]
0
)
(2) and 1 m Et N. The LMW-PPP was all from the same preparation
3
for all runs. [b] 2-Mercaptoethanol concentration. [c] Relative rates
for the consumption of 1. [d] Yield of the corresponding pinacol
coupling product relative to the conversion of aldehyde. [e] Yield
of the corresponding benzyl alcohol relative to the conversion of
aldehyde. [f] Discrepancy in rates from Table 1 a result of using
different PPP from different preparations. [g] 95% confidence inter-
val. [h] Not detected. [i] Calibration curve sensitivity was estimated
as the average value of the other pinacol products.
In the photocatalysis of 4-acetylbenzaldehyde with thiols,
only 4-acetylbenzyl alcohol was detected, which was veri-
fied by comparison to an authentic standard. Because 4-
(1-hydroxyethyl)benzaldehyde was not detected, the LMW-
PPP photocatalytic system was used to achieve the chemo-
selective reduction of an aryl aldehyde in the presence of an
aryl ketone. In the absence of a thiol, GC–MS analysis re-
vealed four different peaks with a m/z consistent with the
pinacol coupling of 4-acetylbenzaldehyde. This was taken
Figure 8. Rates for the loss of 1 (triangles) and the formation of 2
(
diamonds) and 3 (squares) at different starting concentrations of as an indication that the meso and d/l isomers of two out
. Photocatalysis carried out with 420 nm irradiation, 10 mg of of the three possible pinacol couplings (aldehyde/aldehyde,
1
3
PPP, 250 mm of 2-mercaptoethanol (BME), and 1 m Et N. Error
aldehyde/ketone, ketone/ketone) were formed. Attempts to
isolate these products were thwarted by instability of these
products towards column chromatography. The instability
bars represent 95% confidence interval for 2 and 3 and 80% for 1.
Curves were obtained by a least square fit of Equation 2.
To study electronic effects on the photocatalytic redox of these products also contributed to the larger variation
system, a series of 4-substituted benzaldehydes were photo- reported in entry 9 of Table 3. The calibration curve sensi-
lyzed with and without BME (500 mm). Except for 4-acetyl- tivity of our GC analysis for the various pinacol coupling
benzaldehyde, the products of the reaction were identified products was fairly consistent, and thus, the concentration
as the corresponding pinacols and benzyl alcohols by com- of these products was estimated by averaging the calibration
parison to authentic standards. As seen in Table 3, electron- curves of the other pinacols. Additionally, it was examined
donating substituents decrease the yield and rate of pinacol if benzaldehyde could be reduced in the presence of an alde-
formation in the absence of the thiol, and no alcohol was hyde with a higher reduction potential, hexanal, to see if
observed. Conversely, electron-withdrawing substituents in- selectivity could be achieved. A series of mixed aldehyde
6
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/KAP1
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Conjugated Polymers as Photoredox Catalysts
Scheme 3.
experiments with benzaldehyde and hexanal, where the were expected to reduce benzaldehyde (–1.69 vs. NHE).[24]
amount of hexanal was varied from equimolar to 10-fold As with any photochemical reaction, the rate of photoredox
excess, were performed. As shown in Scheme 3, the only catalysis increases with greater photon flux, and therefore,
products obtained from these experiments were 2 and 3, greater absorption efficiency would also be expected to in-
and no hexanol or pinacol coupling products involving crease rates. Spectral characterization (UV/Vis) of dissolved
hexanal were observed. For both cases shown in Scheme 3, LMW-PPP in chloroform shown in Figure 1 revealed only
the aryl aldehyde was selectively reduced consistent with its minimal absorption in the visible region. The low ab-
less negative reduction potential. This indicates that other sorbance in the visible ranges indicated PPP photocatalysts
chemoselective reductions could be achieved by tailoring would benefit by increasing the efficiency of absorption in
the redox potential of PPP or other conjugated polymer this region. For most semiconductor photocatalysts, energy
photoredox catalysts.
wasting electron-hole annihilation is the dominant non-pro-
ductive processes competing with productive interfacial
charge transfer. Therefore, efficient trapping of charge
carriers is essential to photocatalysis. The first-order depen-
[
25]
Discussion
The structural variety, distinctive redox potentials, and dence of the rate of formation of 2 in respect to Et N up to
3
the absorption of visible light make conjugated polymers 2 m suggested that non-productive processes involving h+VB
attractive photoredox catalysts. Given the previously re- were occurring. Because pre-adsorption was an expected
ported pinacol coupling of 1 by PPP photocatalysis and requirement to compete with electron-hole annihilation, the
visible light, aryl aldehydes were a reasonable model system high concentrations of Et N required for catalytic activity
3
to begin the investigation of conjugated polymers as pho- suggested that association of the amine with the catalytic
[
15]
toredox catalysts.
working hypothesis that the addition of a hydrogen donor binding would be expected to more efficiently trap h VB.
was able to trap putative ketyl I leading to a formal photo- The addition of thiols to the photolysis mixture diverted
The proceeding results support the surface was weak. A sacrificial electron donor with stronger
+
k
reduction of 1 (Scheme 2). Because electron-hole annihila- aryl aldehydes into undergoing a formal photoreduction
tion is energetically favorable, pre-adsorption of 1 to the rather than the pinacol coupling reaction observed in the
surface of PPP was an expected necessity for efficient in- absence of thiols. As shown for the proposed mechanism in
terfacial electron transfer. The observed saturation kinetics Scheme 4, reduction of the aldehyde by a light-promoted
with respect to 1 shown in Figure 8 was consistent with the conduction band electron was expected to generate putative
expectation of heterogeneous catalysis.[ Additionally, the ketyl intermediate I (Scheme 4, line 5). The self-coupling
23]
k
loss of any photocatalytic activity upon the removal of PPP and protonation of I would yield 2 (Scheme 4, line 7). An
k
supports the postulation that the reduction of 1 occurs by argument against I reacting with 1 from the bulk solution
k
interfacial electron transfer at the surface and not due to a leading to 2 is the independence of the product ratio with
mediator or some miniscule amount of dissolved LMW- respect to the starting concentration of 1. When the thiol
PPP. Analysis of the Langmuir–Hinshelwood kinetics found concentrations are held constant, the ratio of 2 to 3 should
–
5
–1
a very low value for K (10 m ). This low K suggested increase proportionally with the initial concentration of 1 if
c
c
that binding of 1 to the catalytic surface was weak, and pinacol coupling arose from a reaction of I with bulk 1.
k
thus, poor adsorption of 1 to the PPP surface was likely a However, the ratio of 2 to 3 did not change once surface
significant factor limiting the rate of photocatalysis.
saturation was reached in Figure 8 where the rate for the
In the absence of a thiol, the consumption of 1 was fairly formation of I would be constant. The addition of a thiol
k
inefficient, and thus, improvement of the photocatalytic was expected to provide a competing pathway to self-cou-
properties of PPP is desirable. The conduction band poten- pling, as hydrogen abstraction from thiols by carbon-cen-
[
20]
tial of PPP prepared in the same fashion was reported as tered radicals is diffusion controlled. The formation of 3
2.0 V vs. NHE, and thus, electrons in the conduction band was consistent with I abstracting a hydrogen atom from
–
k
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
7

Job/Unit: O20437
/KAP1
Date: 19-09-12 18:03:16
Pages: 11
M. Zhang, W. D. Rouch, R. D. McCulla
FULL PAPER
the added thiol (Scheme 4, line 9). As shown in line 11 cilitate the reductive couplings reactions in lines 8 and 10
(
Scheme 4), the resulting sulfenyl radicals form disulfides (Scheme 4). These results indicate that the observed acceler-
[26]
at diffusion controlled rates.
The rate-limiting step was ation in the overall rate upon the addition of acidic thiols
expected to be interfacial electron transfer, and thus, an un- was largely due to their ability to protonate I and only
k
anticipated result was the increase in the overall rate of the partially by acting as sacrificial electron donor (Scheme 4,
reaction for some of the examined thiols (Table 1, entries 2– line 3). As expected based on their pK values, the overall
a
4
). A potential explanation was that these thiols prevented rate was largely unaffected by the addition of tBuSH and
charge carrier recombination, which was possible because PrSH, which was consistent with the assumption that they
-mercaptoethanol (BME) acted as the sacrificial electron do not substantially protonate Ik.
donor in the absence of Et N. However, this explanation As expected, an increase in the concentration of a thiol
2
3
was deemed unlikely because the conversion of 1 in the ab- led to an increase in the rate of formation of 3 as shown in
sence of Et N occurred at only 10% of the rate observed Figure 7. Because the bond strength of the S–H bond was
3
under our standard conditions (Table 2 entries 6 & 8), expected to correlate with the pK , it was expected that thi-
a
whereas the rate doubled when BME was added (Table 2, ols with lower pK values would be better hydrogen atom
a
–
[28]
entries 7 & 8). If the addition of thiols only prevented e CB/ donors and yield greater ratios of 3 to 2. This trend was
+
h
recombination, a much smaller increase in the rate of observed for BME, BnSH, and EMA and also can be used
VB
consumption of 1 would have been anticipated.
to explain PrSH vs. tBuSH. However, PrSH and BME
yielded a similar ratio of 3 relative to 2, which was reflective
of their different mechanisms. For the more acidic thiols
(
BME, BnSH, EMA), their ability to protonate I acceler-
k
ates the overall reaction with a concomitant increase in the
+
amount of I (H ). Because pinacol coupling (Scheme 4,
k
line 8) increases to the second power of the concentration
+
I (H ) and hydrogen abstraction from the thiol increases
k
+
linearly (Scheme 4, line 10), an increase in the rate of I (H )
k
generation favors the formation of 2. Likewise, the same
+
explanation of increased I (H ) formation can be applied
k
to explain the increased amount of 2 compared to 3 for
HMW-PPP in the presence of BME, as HMW-PPP is more
active (Figure 5). Propanethiol and tert-butylthiol were not
expected to protonate I , and thus, they have little or a
k
negative effect on the amount of I being created. Thus, the
k
addition of PrSH or tBuSH simply provides a hydrogen
atom abstraction pathway that competes with pinacol cou-
pling of I . The increased sterics and increased S–H bond
k
dissociation for tBuSH most likely hinders the hydrogen ab-
straction pathway leading to 3, which accounts for 2 being
the major product.
In the absence of thiols, electron-donating groups slow
the rate of aldehyde consumption and the formation of the
corresponding hydrobenzoin. This was consistent with the
interfacial charge transfer becoming less energetically favor-
able. Group 16 hydrogen atom donors react faster with nu-
cleophilic radicals, and thus, electron-rich ketyl intermedi-
ates were expected to react faster with thiols, which hinders
[
20a]
pinacol coupling and back electron transfer.
Conversely,
thiols were not able to completely prevent pinacol coupling
of benzaldehyde and 4-fluorobenzaldehyde. A plausible ex-
Scheme 4. Proposed mechanism.
As shown in Figure 6, a Brønsted plot demonstrated a planation is that electron-withdrawing substituents ease the
linear relationship between the overall rates and the pKa initial reduction leading to I , which again increases the ef-
k
values of the examined thiols. The pK value of the proton- fective concentration of Ik.
a
+
ated ketyl radical [I (H )] has been estimated to lie between
Another interesting result was the improved photocata-
k
8
.4 and 10.5, and thus, protonation of I by the examined lytic activity observed for the reused LMW-PPP (R-LMW-
k
[27]
thiols was possible.
Given the redox potentials of Ik, PPP), which correlated with a significant decrease in the
+
+·
h
(
, and Et N , back electron transfer from I to PPP bromine content of the material. This indicated that the
VB
3
k
h+ ) or Et N would not be surprising. Protonation of LMW-PPP was being modified at the molecular level. Com-
+·
VB 3
I decreases the oxidation potential of the ketyl and con- parison of photocatalytic and physical properties revealed
k
+
comitantly increases the lifetime of I (H ), which would fa- that R-LMW-PPP was more similar to the HMW-PPP ma-
k
8
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/KAP1
Date: 19-09-12 18:03:16
Pages: 11
Conjugated Polymers as Photoredox Catalysts
terial than starting unused LMW-PPP from which it was Photocatalytic reactions were carried out by using a LZ4-X
Luzchem Photoreactor with eight fluorescent bulbs generating ap-
proximately 70 Wm from 400–440 nm. As noted in the text, start-
ing concentrations of the aryl aldehydes were varied from 1 to
obtained. Therefore, a reasonable explanation for the in-
–
2
creased rate of photocatalysis was the transformation of
LMW-PPP into a material similar to HMW-PPP by the for-
mation of C–C bonds between the PPP units during the
course of the reaction. In the presence of a thiol, the trans-
formation of LMW-PPP into a material similar to HMW-
PPP accelerated after near complete conversion of the start-
250 mm; however, typical experiments were run at 5 mm. Solutions
of the starting aryl aldehyde, triethylamine (1 m), thiol (0–500 mm),
and PPP (10 mg) were placed into a borosilicate test tube for pho-
tolysis. Prior to photolysis, all of the samples were sparged with
argon for at least 30 min and sonicated to break up the hetero-
ing material, and thus, much of the transformation of PPP geneous PPP powder. During the photoreaction, the samples were
occurred after the reaction was complete. An increasing rate stirred and the temperature was held constant at 25–30 °C. The
of PPP transformation at high conversion of 1 was consis- products analysis was performed by periodic GC analysis by using
a Hewlett–Packard 5890 Series II GC fitted with a flame ionization
tent with the observation that no increase in the rate of
detector and DB-5 column. The internal standards used for GC
photocatalysis was observed over the course of a single re-
quantifying the aryl aldehydes and products were 2-octanol and
action during this study. Because the relative rates were de-
dodecane. For products not amenable to GC analysis, an Agilent
termined for reactions proceeding to less than 40% conver-
1
100 Series HPLC fitted with a UV/Vis diode array detector and
sion with unused LMW-PPP from the same preparation,
the relative rates provided a valid method of evaluating the
effects of modifications to the reaction conditions on pho-
tocatalysis.
Eclipse XDB-C18 column was used. Control reactions were per-
formed where one variable (light, PPP, or Et N) was removed. Less
than 5% loss of the starting material was observed for all of these
controls.
3
General Methods: Ultravisible absorbance spectra were collected
with a Shimadzu PharmaSpec UV-1700. GPC analysis was per-
formed by using Agilent 1100 Series HPLC fitted with a UV/Vis
diode array detector and Shodex LF-404 temperature-controlled
column by using chloroform. A Bruker 400 MHz Broadband
NMR spectrometer with a direct probe was used to collect proton
signal for all of the prepared compounds. A Shimadzu GC–MS
QP2010S was used to collect molecular fragmentation information
for initial identification of possible photoproducts. For elemental
Conclusions
Irradiation of PPP with visible light in the presence of a
sacrificial electron donor was able to catalyze the one-elec-
tron reduction of aryl aldehydes to generate a ketyl interme-
diate. The ketyl intermediate undergoes a pinacol coupling
reaction on the surface of the photocatalyst competitively
with non-productive back electron transfer. In the presence analysis, compounds were sent to Atlantic Microlabs (Norcross,
of thiol hydrogen atom donors, the ketyl intermediate can GA, USA) for C, H, and Br content.
be diverted into a formal photoreduction by abstracting a
Supporting Information (see footnote on the first page of this arti-
1
13
hydrogen atom from the thiol to form an alcohol. The cle): Copies of the H NMR, C NMR, and mass spectra of com-
chemoselective reduction of aryl aldehydes over alkyl alde- pounds 1,2-bis(4-fluorophenyl)-1,2-ethanediol, 1,2-bis(4-meth-
hydes and aryl ketones was also achieved. As expected, aryl oxyphenyl)-1,2-ethanediol, and 1,2-bis(4-methykphenyl)-1,2-eth-
anediol.
aldehydes with lower reduction potentials were more reac-
tive towards photocatalysis. The conjugated polymer photo-
catalysts could be reused by sonicating and rinsing with
acetonitrile, which resulted in an increase in photocatalytic
Acknowledgments
activity. The rate of photocatalysis was hindered by inef- The authors thank the donors of the Beaumont Faculty Develop-
ficient absorbance, poor binding to surface by the substrate, ment Fund for support of this work.
and an unoptimized sacrificial electron donor.
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[
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[
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methoxyphenyl)-1,2-ethanediol, and 1,2-bis(4-fluorophenyl)-1,2-
[
[
[
30]
ethanediol.
Irradiations: The photolysis reactions were carried out in HPLC-
grade acetonitrile, which was used without further purification.
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
9

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/KAP1
Date: 19-09-12 18:03:16
Pages: 11
M. Zhang, W. D. Rouch, R. D. McCulla
FULL PAPER
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Received: April 4, 2012
Published Online: ᭿
10
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O20437
/KAP1
Date: 19-09-12 18:03:16
Pages: 11
Conjugated Polymers as Photoredox Catalysts
Visible-Light Photocatalysis
Visible-light irradiation of poly(p-phenyl-
ene) in the presence of a sacrificial electron
donor catalyzed the one-electron reduction
of aryl aldehydes to generate ketyl inter-
mediates. The ketyl intermediates undergo
pinacol coupling reactions, and in the pres-
ence of thiol hydrogen atom donors, they
can be diverted into a formal photoreduc-
tion to form alcohols.
M. Zhang, W. D. Rouch,
R. D. McCulla* ............................... 1–11
Conjugated Polymers as Photoredox Cata-
lysts: Visible-Light-Driven Reduction of
Aryl Aldehydes by Poly(p-phenylene)
Keywords: Photocatalysis / Conjugation /
Polymers / Chemoselectivity / Reduction
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
11