Conjugated polymers as photoredox catalysts: A new catalytic system using visible light to promote aryl aldehyde pinacol couplings

Article abstract of DOI:10.1016/j.tetlet.2012.06.144

The conjugated polymer poly-(p)-phenylene (PPP) was synthesized and used as a photoredox catalyst to promote pinacol coupling of aryl-aldehydes with visible light. The reaction required the use of a sacrificial electron donor (Et₃N), and was accelerated by the addition of Lewis and Bronsted acids. A distinct advantage of this photocatalytic system is the robust nature of the system, which is not overly sensitive to impurities, oxygen, or temperature, and proceeds cleanly with few side reactions. As a comparison with the PPP system, the reactivity of Ru(bpy)₃Cl₂, a popular photoredox catalyst was compared. The PPP system was superior to the Ru(bpy)₃Cl₂ for the pinacol couplings in both rate and yield.

Full text of DOI:10.1016/j.tetlet.2012.06.144

Tetrahedron Letters 53 (2012) 4942–4945
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Conjugated polymers as photoredox catalysts: a new catalytic system using
visible light to promote aryl aldehyde pinacol couplings
⇑
William D. Rouch, Miao Zhang, Ryan D. McCulla
Department of Chemistry, Saint Louis University, 3501 Laclede Avenue, Saint Louis, Missouri 63104, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The conjugated polymer poly-(p)-phenylene (PPP) was synthesized and used as a photoredox catalyst to
promote pinacol coupling of aryl-aldehydes with visible light. The reaction required the use of a sacrificial
electron donor (Et3N), and was accelerated by the addition of Lewis and Brønsted acids. A distinct advan-
tage of this photocatalytic system is the robust nature of the system, which is not overly sensitive to
impurities, oxygen, or temperature, and proceeds cleanly with few side reactions. As a comparison with
the PPP system, the reactivity of Ru(bpy)3Cl2, a popular photoredox catalyst was compared. The PPP sys-
tem was superior to the Ru(bpy)3Cl2 for the pinacol couplings in both rate and yield.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 7 May 2012
Revised 28 June 2012
Accepted 29 June 2012
Available online 16 July 2012
Keywords:
Pinacol
Poly-(p)-phenylene
Visible light
Photoredox catalyst
Aryl aldehydes
Conjugated polymer
Since most organic molecules lack the ability to absorb visible
light, photochemical reactions require the use of higher energy
UV light to induce the desired transformations. Since many com-
pounds absorb UV light, side-reactions and difficult product purifi-
cations are common for direct photolysis reactions. Photoredox
catalysts have been developed that can become ‘active’ in the vis-
ible spectrum and then facilitate charge-transfer to the organic in
question. The recent interest in the use of visible light photocata-
lytic systems for hydrogen production and waste water remedia-
tion are current examples of photoredox catalysis.
(h+ and eÀCB) can be used to oxidize or reduce adsorbed species
VB
with the proper redox potentials. To catalyze organic reactions,
photocatalytic systems are designed to take advantage of the reac-
tivity of the nascent radical anion (AꢀÀ) or radical cation (Dꢀ+) to
generate new products through unimolecular rearrangements or
reactions with other species at the surface or in bulk solution as
shown in Scheme 1. The majority of semiconductor photocatalysts
are inorganic materials. An extremely active area of research is the
use of titanium dioxide (TiO2) as a means of pollution remedia-
tion.3b,3c To a lesser extent, inorganic semiconductors have been
used in synthetic organic reactions.1a Compared to TiO2, which
only has a moderate conduction band potential (À0.7 V vs SCE),5
poly-(p)-phenylene (PPP) has a much higher conduction band
potential of (À1.7 V vs SCE).6 Electron transfer to an acceptor can
The use of photocatalysts in chemical synthesis has a venerable
history, and recently, the conjoining of photoredox catalysts and
visible light for applications in organic synthesis has been an
expanding area of research.1 Notably, organometallic ruthe-
nium(II) and iridium(III) polypyridine complexes have been used
for [2+2] cycloadditions, reductive dehalogenations, enantioselec-
tive trifluoromethylations, radical additions, and C–H bond activa-
tion.2 Additionally, small organic photosensitizers have been used
to promote electron transfer initiated radical additions into C@C
and C@O.3 While the aforementioned homogenous photoredox
catalysts have proven utility, heterogeneous catalysts can be desir-
able for certain applications.
Heterogeneous photocatalysts are often semiconductors.4 Irra-
diation of a semiconductor promotes electrons from the valence
band to the conduction band, and the resulting charge carriers
⇑
Corresponding author. Tel.: +1 314 977 2838; fax: +1 314 977 2521.
E-mail addresses: rmcculla@slu.edu, rmccull2@slu.edu (R.D. McCulla).
Scheme 1. Semiconductor photocatalysis.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.06.144

W. D. Rouch et al. / Tetrahedron Letters 53 (2012) 4942–4945
4943
The photoreduction of 1 (5–6 mM) was performed in acetoni-
trile with 10 mg of either PPP-1 or PPP-2, 1 M Et3N, and a 0.75 M
protic equivalent of a Brønsted acid.11 Initially, the concentration
of the hydrogen donor from the Brønsted acid was kept at 75% of
the Et3N concentration so as to maintain excess of the sacrificial
electron donor. The basic environment also reduced the possibility
of pinacol rearrangement. The photolysis was carried out with
eight fluorescent bulbs with an emission range of 400–440 nm in
borosilicate test tubes. Prior to irradiation, the heterogeneous mix-
ture was sonicated to break-up the PPP into smaller, more uniform
particles followed by argon (Ar) sparging to remove oxygen. The
photolysis products were analyzed using GC, GC–MS, and HPLC,
and identified as 2 and 3 by comparison with authentic standards.9
Rates of product formation and reactant consumption were deter-
mined by sampling the photolysis reaction at four or five time
points. The reactions were zero-order with respect to 1 and 2 over
nearly the entire photolysis, which indicates the reaction is limited
by the photon flux. Photoreactions used for the comparison of reac-
tivity were carried out simultaneously. The reactivity of the PPP-1
was compared to PPP-2 using formic acid as the added acid. This
revealed that the PPP-2 was 13 times more active than the PPP-1
under these conditions. For the remaining studies, only virgin
PPP-2 was used in the model system to compare reactivity rates.
PPP can be easily recovered and reused from the reaction by simple
centrifugation or filtration. In general, the addition of a Brønsted
acid to the photocatalytic system made a dramatic increase in
the reaction rate; up to 39.5 times faster for oxalic acid versus no
acid. The reaction underwent almost exclusive pinacol coupling
with only a trace amount of benzyl alcohol observed. The overall
yield of 2 was determined by HPLC analysis and was normally
greater than 80%. It should be noted that the formation of 2 always
resulted in nearly equal concentrations of the meso and d/l prod-
ucts. The results for our model system with a series of Brønsted
acids are presented in Table 1.
Scheme 2. General reaction of benzaldehyde with PPP.
only occur easily if the reduction potential of the acceptor is below
the conduction band potential of the photocatalyst. Thus, PPP is ex-
pected to behave as a strong reducing photocatalyst.
In initial work by Yanagida and co-workers, benzaldehyde (1)
exclusively underwent a pinacol coupling reaction to form hydro-
benzoin (2) when PPP was irradiated with visible light in the pres-
ence of triethylamine (Et3N) as the sacrificial electron donor as
shown in Scheme 2.6a Upon repeating Yanagida’s experiments with
our PPP, the results showed that the reaction was very slow
(<0.05 mM/h). Formation of the ketyl (Ik) was proposed as the
key intermediate in the mechanism. Thus, we hypothesized the
addition of Brønsted or Lewis acids would increase the observed
rate of product formation.
The poly-(p)-phenylene polymers that were prepared by Yanag-
ida and co-workers in this study, were prepared according to pro-
cedures described by Yamamoto and co-workers.7,8 After
preparation of the Grignard reagent of 1,4-dibromobenzene, the
polymerization was catalyzed by nickel(II) dichloride bipyridine
to afford a yellow slurry. Soxhlet extraction of the solids for 30 h
with toluene yielded a yellow powder of lower molecular weight
polymer, PPP-1. Elemental analysis of several batches showed that
the extracted PPP consisted of 77.5% carbon, 5.2% hydrogen, and
13.5% bromine on average.9 Using GPC analysis, the Mw and Mn
were determined to be 500 Da and 363 Da, respectively corre-
sponding to a poly-dispersity of 1.38.9 If it is assumed that the
polymerization was terminated by the reaction by a protic impu-
rity, these values correspond with an average chain length of 5–6
phenyl units for PPP-1 with one end terminated by bromine. The
insoluble material remaining after Soxhlet extraction consisted of
higher molecular weight polymer, PPP-2, which was supported
by elemental analysis. The insoluble PPP-2 consisted on average
84.5% carbon, 5.0% hydrogen, and 7.6% bromine, which was consis-
tent with a PPP chain of 12 phenylene units for PPP-2. Since PPP-2
was completely insoluble in all examined organic solvents, the
molecular weight could not be determined by GPC. The UV Spec-
trum of PPP-1, which is available in the supporting information,
dissolved in chloroform shows a weak single absorption band that
extends past 400 nm with a maximum absorbance at 300 nm. As
shown in the supporting information, PPP-2 suspended in chloro-
form shows an absorbance band that extends past 400 nm with a
maximum absorbance at 364 nm.9
Since oxalic acid was the most effective in our standard reaction
conditions, the amount of oxalic acid was optimized. The optimum
molar ratio of Et3N/oxalic acid was found to be 5.7. The molar ratio
of Et3N-oxalic acid:1 showed no significant improvement in rate at
a ratio beyond 120. Semi-preparatory scale reactions, (100 mg of
1), were successful as well.12 The product could be isolated with
a simple work-up followed by silica gel purification (7:3 hex-
anes/EtOAc, 82 mg, 81% yield).12
The addition of an acid has the potential to increase the rate in
several ways. First, the pKa of the ketyl radical in aqueous solution
is 8.4–10.5,13 and thus, the addition of an acid (HA) would be ex-
pected to favor the formation of the ketyl radical (Ik(H+)) versus
the ketal anion (Ik), which is expected to disfavor non-productive
Table 1
The FT-IR spectrum of the PPP-2 showed an out-of-plane vibra-
tion of the C–H bonds in the p-phenylene units at 807 3 cmÀ1
(d(para)) and two out-of-plane C–H vibrations due to a terminal
phenyl group at 766 3 cmÀ1 (d(mono1) and 692 3 cmÀ1
(d(mono2) respectively. There was a medium to weak signal at
1075 3 cmÀ1 corresponding to the terminal p-bromophenyl
Photoredox system with Brønsted acids
Brønsted acid
Acida (mM)
Rel. rateb
2c (% Yield)
3c (% Yield)
None
HCl
Sulfuric
Nitric
Formic
0
750
375
750
750
750
750
750
375
250
1.0
9.3
17.8
7.4
14.4
14.2
5.8
4.7
39.5
18.3
76 7d
n.d.e
89
83
77
91
7
1
4
1
2
3
3
1
1
1
group
nyl and terminal bromophenyl groups as part of the chain. Since
the intensity of the (C–Br) band is still significant versus the
intensity of the combined (d(mono) bands suggests that there is
significant amount of the terminal bromophenyl present.
m(C-Br). This suggests that the PPP-2 has both terminal phe-
n.d.e
n.d.
Acetic
83 11
56 10
64 10
Trifluoroacetic
Methanesulfonic
Oxalic
3
3
2
2
m
90
91
5
5
n.d.
n.d.
a
Trimesic
Yamamoto and Kovacic have shown that the degree of polymeriza-
tion in PPP can be approximated by taking the relative intensity ra-
tio (R) of (d(para))/(d(mono1) + (d(mono2).10 The R ratio of PPP-2
prepared in this study was 2.4–2.6, which shows a relatively high
degree of polymerization as comparable to PPP polymers prepared
by Yamamoto (R = 1.9–3.9) and Kovacic (R = 0.6–8.4).
a
b
c
Theoretical H+ concentration available 750 mM.
Relative rates for at least 80% consumption of 1.
% Yield determined by HPLC relative to consumption of 1 at 80–99% conversion
of 1.
95% Confidence level.
not detected.
d
e

4944
W. D. Rouch et al. / Tetrahedron Letters 53 (2012) 4942–4945
exclusive formation of 2 when PPP was present. However, the
overall rate for this system (pyridine/oxalic acid) was less than
5% of the rate observed for the Et3N:oxalic acid system. This indi-
cated that the effect of oxalate was minimal in the observed rate
for the Et3N/oxalic acid system.
The following mechanism is proposed for the use of Brønsted
acids, which is shown in Scheme 3. The reaction involves the forma-
tion of a ketyl anion (Ik) which is converted to a ketyl radical
(Ik(H+)). The formation of 2 could be initiated from the coupling
of two Ik(H+) or by coupling Ik(H+) with Ik or 1. The acid anion
can assist the reaction by either electron transfer to the Et3NÅ+ or
h+
to regenerate Et3N or fill h+
preventing charge
VB
VB
recombination.
The addition of Lewis acids to the system was also investigated
to determine their catalytic ability in the model system. An equi-
molar equivalent of a Lewis acid was first added to 1 in acetonitrile
to form a metal–carbonyl complex, to which Et3N (1 M) and PPP-2
(10 mg) were then added. Upon complexation of the Lewis acid
with the carbonyl, the reactivity of the carbonyl carbon was
expected to increase. The results for the addition of four different
Lewis acids are shown in Table 2. The addition of Lewis acids to
the model system did increase the reaction rates of 1 up to 7.8
times faster for aluminum chloride versus no acid. The reaction
also underwent almost exclusive pinacol coupling; however, the
overall yields were poor. Further investigation using Lewis acid
catalysis was not pursued.
Scheme 3. Proposed mechanism for acid photocatalytic system.
Table 2
Photoredox system with Lewis acids
Lewis Acid
Acid (mM)
Rel. Ratea
2b (% yield)
3b (% yield)
The reactivity of PPP-2 was compared to the photoredox cata-
lyst Ru(bpy)3Cl2 to compare its reactivity in our model system.
Upon excitation with visible light, Ru(bpy)3Cl2 populates a metal
to ligand charge transfer state that is easily reductively quenched
None
NiCl2
FeCl3
AlCl3
MgCl2
0
6
6
6
6
1.0
1.0
1.2
7.8
2.3
76 7c
n.d.
50 10
53 10
57 10
n.d.d
n.d.
6
4
3
2
10
8
+
to form Ru(bpy)3+. Since the reduction potential of Ru(bpy)3
(À1.3 V vs SCE)1b is below that of 1 (À1.6 V vs SCE)6, it was hypoth-
esized that the Ru-catalyst would not effectively reduce 1 (À1.6 V
a
c
Relative rates for at least 70% consumption of 1 (Ni no reaction).
% Yield determined by HPLC or GC relative to consumption of 1 at 70–90%
conversion of 1.
vs SCE)6 as was observed with PPP-2 (eÀ À1.7 V vs SCE). Using
b
CB
95% Confidence level.
Not detected.
d
three different conditions: oxalic acid-Et3N catalytic system, 1 M
Et3N, and two-fold excess of Et3N, the reduction of 1 with 7–
10 mg of Ru(bpy)3Cl2 was not successful as shown in Table 3. Deg-
radation of 1 with Ru(bpy)3Cl2 did occur, but the by-products could
not be identified as the mixture was complicated by HPLC. PPP-2
has a high reduction potential and therefore offers a broad scope
as to potential compounds that can be reduced.
oxidations that regenerate 1. Secondly, the ability of an acid to in-
crease the rate of reaction could occur as a result of electron trans-
fer from the acid anion (AÀ) to either the Et3NÅ+or h+VB. This
electron transfer would decrease the amount of non-productive
eÀCB/h+VB recombinations. Examples of this have been shown using
a dual electron donor system of ascorbic acid and 1,5-dimethoxy-
naphthalene.2k To test this second possibility, an experiment was
performed to determine the effect of oxalate on the reaction sys-
tem in the presence of a non-electron donating base (pyridine).
Other bases were tried such as tetramethyl and tetrabutyl ammo-
nium hydroxide but the solubilities of the resulting salts were low.
The photoreduction of 1 (5–6 mM) was performed in 9:1 metha-
nol-water with 10 mg of PPP-2, 1 M pyridine, and a 0.75 M protic
equivalent of oxalic acid. It was found that this did result in the
In summary, the conjugated polymer, poly-(p)-phenylene was
synthesized and used to promote homolytic pinacol couplings of
aryl-aldehydes with visible light. Analysis of the polymer showed
that the PPP-2 had an average chain length of 12 benzene rings.
The reaction requires the use of a sacrificial electron donor (Et3N)
and is catalyzed by the presence of certain Lewis or Brønsted acids,
with Brønsted acids being the most effective. Yields of the pinacol
were as high as 90%, with oxalic acid being the most effective acid
tried in the study. The yields of the Lewis acid catalyzed reaction
were low, 57% and reactivity was lower. As a comparison, the reac-
Table 3
PPP-2 versus Ru(bpy)3Cl2 in 50 mM 1
Catalyst/Polymer
PPP-2b
PPP/cat. (mg)
Et3N (mM)
Oxalic acid (mM)
Pinacola (% yield)
81
Trace
Trace
Trace
Trace
Alcohol (% yield)
25
7
25
25
7
1000
95
1000
1000
1000
175
0
0
175
175
5
n.d.c
n.d.
n.d.
n.d.
n.d.
d
Ru(bpy)3Cl2
Ru(bpy)3Cl2
Ru(bpy)3Cl2
d
d
e
Ru(bpy)3Cl2
a
b
c
Yield determined by HPLC at 99% conversion of 1.
Reaction complete (99% conversion of 1) in 26 h.
not detected.
Allowed to react 40.5 h.
Allowed to react 45.5 h.
d
e

W. D. Rouch et al. / Tetrahedron Letters 53 (2012) 4942–4945
4945
6. (a) Shiragami, T.; Kabumoto, A.; Ishitani, O.; Pac, C.; Yanagida, S. J. Phys. Chem.
1990, 94, 2068; (b) Pavlishchuk, V.; Addison, A. Inorg. Chim. Acta 2000, 298, 97.
7. Hasegawa, K.; Wen, C.; Kotani, T.; Kanbara, T.; Kagaya, S.; Yamamoto, T. J.
Mater. Sci. Lett. 1999, 18, 1091.
tivity of a popular photoredox catalyst Ru(bpy)3Cl2 was compared
to PPP-2. The Ru-catalyst did not reduce benzaldehyde as expected
while PPP-2 reduced benzaldehyde efficiently. The explanation for
the rate increases on the addition of acid to the reaction centers on
the prevention of back-electron transfer from the ketyl intermedi-
ate by either stabilization or filling of the valence band hole in the
polymer.
A distinct advantage of this photocatalytic system is that it is ro-
bust and not overly sensitive to impurities and other factors. It is
also very clean with few side reactions. All reagents were used as
supplied from the vendors eliminating the need for elaborate puri-
fication or drying of reagents. No elaborate set-ups or equipment
are needed to perform the reactions. The reactions will work using
regular glass or borosilicate glass reactors. The reactions were not
overly sensitive to oxygen as a simple Ar sparging of the reaction
mixture was sufficient to remove oxygen.
8. Poly-(p)-phenylene PPP; general procedure: 1,4-dibromobenzene (11.8 g) and
magnesium turnings (1.22 g) were combined in dry THF (40 mL) under dry
argon atmosphere and stirred at room temperature for 1 hour. 50.0 mg of
Ni(bpy)Cl2 was added at room temperature, which turned the reaction to a
dark red–brown color. The polymerization was highly exothermic, and the
reaction was allowed to proceed at reflux for another 16–30 h before being
cooled to room temperature. Reaction quenched into 600 mL of EtOH followed
by filtration. The solids were then reconstituted with 100 mL 0.1 M HCl(aq),
filtered, and washed with water and EtOH prior being dried under reduced
pressure. The sample was then purified by Soxhlet extraction with toluene for
30 h. The extract was concentrated under reduced pressure to yield 300 mg of a
yellow powder, PPP(1). The insoluble solids were dried under reduced pressure
to yield 2.3 g of a yellow powder, PPP(2).
9. General Methods: Ultra-visible absorbance spectra of PPP-1 were collected with
a shimadzu pharmaspec UV-1700. Ultra-visible absorbance spectra of PPP-2
were collected with a Olis RSM 1000UV/VIS spectrophotometer using an Olis
Clarity sample holder. GPC analysis was performed using Agilent 1100 Series
HPLC fitted with a UV–Vis diode array detector and Shodex LF-404 temperature
controlled column using chloroform. FT-IR performed using a Shimadzu 8400S
using KBr pellets. Bruker 400 MHz Broadband NMR with direct probe was used
to collect proton signal for the isolated compounds. Shimadzu GC–MS
QP2010S, was used to collect molecular fragmentation information for initial
identification of possible photoproducts. For elemental analysis, compounds
were sent to Atlantic Microlabs (Norcross, GA, USA) for C, H, and Br content.
10. (a) Yamamoto, T. Prog. Polym. Sci. 1992, 17, 1153; (b) Kovacic, P.; Oziomek, J. J.
Org. Chem. 1964, 29, 100.
Acknowledgments
The authors thank the donors to the Beaumont Faculty Develop-
ment Fund for support of this work and the assistance of Dr. Eu-
gene Pinkhassik for assistance with characterizing the polymers.
11. Photocatalytic reactions; general procedure: Carried out using a LZ4-X Luzchem
Photoreactor with eight fluorescent bulbs generating approximately 70 W m-2
from 400 to 440 nm. Prior to photolysis, all of the samples were sparged with
argon for at least 30 min and sonicated to break up the heterogeneous PPP
powder. During the photoreaction, the samples were stirred and the
temperature held constant at 25–30 °C. The product analysis was performed
by periodic GC analysis using a Hewlett-Packard 5890 Series II GC fitted with a
flame ionization detector and DB-5 column. The internal standard used for GC
quantifying the aryl aldehydes and products was dodecane. For products not
amenable to GC analysis, an Agilent 1100 Series HPLC fitted with a UV–Vis
diode array detector and Phenomenex 4.6 Â 150 mm C–4 column was used.
For determining reaction rates, control reactions were performed where one
variable (light, PPP, Et3 N, or the acid) was removed and as comparison to
determine the effect of the change.
12. Photo-reduction (semi-prep scale) of benzaldehyde with Brønsted acid 2; general
procedure: In a 25 mL glass vial, 1370 mg of Et3 N was added to 20 mL of ACN
to which 213 mg of oxalic acid was added. 107 mg of 1 was added along with
25 mg of PPP-2. The vial was sealed and sonicated for 30 minutes and sparged
with Ar for 30 min. Reaction was photolyzed for 24–36 h, until completed by
HPLC. Reaction was filtered to remove catalyst and concentrated to dryness
under reduced pressure. Residue dissolved in 5 mL ethyl acetate and 5 mL of
water, layers cut, and organics washed with 5 mL sat. bicarbonate (aq) and
dried over magnesium sulfate. Purified by silica gel column chromatography
using 7:3 ethyl acetate/hexanes. Yield: 82 mg (81% yield). White solid; 1H NMR
(CDCl3, 400 MHz) d 2.16 (t, 1H), 2.80 (t, 1H), 4.63 (t, 1H), 4.75 (t, 1H), 7.01–7.18
(m, 10H); 13C NMR (CDCl3, 400 MHz) d 78.12, 79.13, 126.95(2), 127.10(2),
127.95(2), 128.15(2), 128.27(2), 139.82, 139.76); GC–EIMS m/z 214, 108, 107,
79, 77.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
06.144.
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