Hydrogenations without hydrogen: Titania photocatalyzed reductions of maleimides and aldehydes

Article abstract of DOI:10.3390/molecules190915324

A mild procedure for the reduction of electron-deficient alkenes and carbonyl compounds is described. UVA irradiations of substituted maleimides with dispersions of titania (Aeroxide P25) in methanol/acetonitrile (1:9) solvent under dry anoxic conditions led to hydrogenation and production of the corresponding succinimides. Aromatic and heteroaromatic aldehydes were reduced to primary alcohols in similar titania photocatalyzed reactions. A mechanism is proposed which involves two proton-coupled electron transfers to the substrates at the titania surface.

Full text of DOI:10.3390/molecules190915324

Molecules 2014, 19, 15324-15338; doi:10.3390/molecules190915324
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Hydrogenations without Hydrogen: Titania Photocatalyzed
Reductions of Maleimides and Aldehydes
David W. Manley †, Luca Buzzetti †, Andrew MacKessack-Leitch † and John C. Walton †,*
EaStCHEM School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, UK;
E-Mails: dm53@st-andrews.ac.uk (D.W.M.); luca.buzzetti01@gmail.com (L.B.);
acml2394@gmail.com (A.M.-L.)
†
These authors contributed equally to this work.
* Author to whom correspondence should be addressed; E-Mail: jcw@st-and.ac.uk;
Tel.: +44-1334-463-864; Fax: +44-1334-463-808.
Received: 20 August 2014; in revised form: 16 September 2014 / Accepted: 17 September 2014 /
Published: 24 September 2014
Abstract: A mild procedure for the reduction of electron-deficient alkenes and carbonyl
compounds is described. UVA irradiations of substituted maleimides with dispersions of
titania (Aeroxide P25) in methanol/acetonitrile (1:9) solvent under dry anoxic conditions
led to hydrogenation and production of the corresponding succinimides. Aromatic and
heteroaromatic aldehydes were reduced to primary alcohols in similar titania photocatalyzed
reactions. A mechanism is proposed which involves two proton-coupled electron transfers
to the substrates at the titania surface.
Keywords: photocatalysis; reduction; titania; maleimides; aldehydes
1. Introduction
Semiconductor photoredox catalysis (SCPC) with titania under aerobic conditions is a well-established
method for homolytic oxidations and complete substrate mineralizations [1–3]. A few recent studies have
shown that, under anaerobic conditions, TiO2 SCPC can also be adapted for constructive procedures
that build organic structures. Examples include additions of enol-ethers to appropriate acceptors [4,5]
and addition-cyclizations of aromatic amines with alkenes [6–8]. We recently discovered that TiO2

Molecules 2014, 19
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SCPC with carboxylic acid precursors could result in dimerizations and, with suitable alkene
acceptors, both alkylations and tandem addition-cyclizations could be accomplished [9,10].
In our studies of TiO2 promoted reactions of a range of maleimides with carboxylic acids we
observed that the alkylation products were always accompanied by significant amounts of the
hydrogenated succinimides [10]. Absorption of a UV photon by the TiO2 particle leads to the creation
of an electron/hole pair (e−/h+) which migrates to the surface. Hole capture by the carboxylic acid led
to generation of the C-centered radical that alkylated the maleimide. It seemed probable, therefore, that
the maleimide also played the role of an electron sink, consuming electrons in two sequential
reduction-protonations, as well as acting as the radical acceptor. This gave us the idea that it might be
possible to design a TiO2 based SCPC protocol for hydrogenating maleimides and also other
electron-deficient unsaturated organic compounds. Such a procedure would offer several substantial
advantages. The TiO2 is cheap and non-toxic and can easily be removed by filtration at the end of a
reaction and re-cycled. Only soft UVA light (320–400 nm) is required, so standard Pyrex glassware can
be used with ordinary sunlamp sources. Potentially, the process would be mild and environmentally
friendly. In this paper, we describe the development of such a procedure and its successful application
to hydrogenations of maleimides and carbonyl compounds.
2. Results and Discussion
2.1. SCPC Reductions of Maleimides
N-Phenylmaleimide (2) has strong electron-withdrawing character, is symmetrical, photostable and
its photodimerization is negligible (under our experimental conditions). We employed this substrate
during initial investigations to test the viability of the process. Previous SCPC experiments in our lab
had shown that Aeroxide (Evonik) P25 TiO2, consisting of a 3:1 anatase/rutile mixture (particle size
~21 nm, surface area 50 m2·g−1, anatase excitation wavelength 385 nm, band gap 3.2 eV) was a good
choice of catalyst. Water and oxygen led to oxidative degradations so they were excluded by using
anhydrous solvents and purging the reaction mixtures with argon. For convenience Schlenk tubes were
used as reactors and were stirred and irradiated with light from two semi-circular banks of Phillips
sun-tanning tubes.
When N-phenylmaleimide was irradiated alone in a dispersion of P25 in CH3CN poor conversion
was observed. In the absence of a hole scavenger, the system was inefficient as the catalyst became
electron deficient over time. When the reaction was repeated in methanol, a known hole-scavenger for
TiO2, full conversion was achieved but nucleophilic ring-opening to the amide was observed to take
place in competition with the reduction. Thus, as a compromise, a 10% mixture of CH3OH in CH3CN
was trialed with P25 as a 1 mg·mL−1 dispersion (0.75 mole equiv.). This proved to be effective with 1c
being converted to N-phenylsuccinimide 2c in a pleasing yield of 94%. This approach was extended to
several commercially available maleimides and good to excellent yields (82%–94%) of the corresponding
succinimides were obtained (Scheme 1). Only when the reduction was carried out using the highly
electrophilic maleic anhydride 3 was a significant quantity of the ring opened product observed.

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Scheme 1. P25 promoted hydrogenations of commercial maleimides and maleic anhydride.
a Yield determined by 1H-NMR; b 17% ring opened 4-oxobutanoic acid also observed.
To extend the scope of the method, a set of maleimides with substituents in the amide and aromatic
rings was prepared. An adaptation of the method due to Fles et al. [11] was used (Scheme 2).
The substituted maleic anhydride (3) and a functionalized aromatic amine were refluxed in toluene for
3 days. The maleamic acid derivative prepared in this way was then cyclized by heating with acetic
anhydride and sodium acetate to afford the required maleimides 5, albeit in low yields.
Scheme 2. Preparation of functionalized maleimides.
The maleimides 5 were then subjected to SCPC reduction under the optimized conditions described
above and all afforded the corresponding functionalized succinimides (pyrrolidine-2,5-diones) 6
(Table 1).
Lowest yields of the N-arylpyrrolidine-2,5-diones 6 were obtained for starting maleimides with
single substituents in the maleimide rings. The 3,4-dimethyl precursor 5c gave a better, but still very
modest, yield. Satisfactory yields were obtained for the 3,4-dimethyl-substrates with the electron
donating OMe substituents in the aromatic rings (5e,5f) but precursor 5g with the electron-withdrawing
4-chloro-substituent also gave a poor product yield. Conversions were high in most cases and it is
probable that significant product degradation occurred. Reduction of N-phenylphthalimide was also
examined using the same conditions but, not surprisingly, no reduction of the aromatic ring took place.

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Table 1. Titania semiconductor photoredox catalysis (SCPC) hydrogenations of
substituted maleimides a.
R4
R4
R4
R4
O
O
R1
R2
R1
UV, P25 (0.75 eq.)
R3
N
R3
N
R2
10% CH3OH/CH3CN
O
O
5b-g
6b-g
Irrad. Time
Yield of 6
Substrate
R1, R2, R3, R4
(h)
20
20
20
20
72
20
(%)
≤5
17
9
5b
5c
5d
5e
5f
Me, H, H, H
Me, Me, H, H
Ph, H, H, H
Me, Me, OMe, H
58
79
19
Me, Me, OMe, OMe
5g
Me, Me, Cl, H
a Yields determined by NMR.
2.2. SCPC Reductions of Carbonyl Compounds
Carbonyl compounds are also good electron acceptors and so SCPC reductions were investigated
for a representative range. When dispersions of benzaldehyde and titania were irradiated in various
solvents three products were observed, benzyl alcohol (8a) and the meso (9a') and D/L (9a)
diasterioisomeric set of 1,2-diphenylethane-1,2-diols (pinacols) (Scheme 3). Solvents containing 1%,
10%, 50% and 100% MeOH in MeCN were tested and the yield of 8a was found to be a maximum for
the 10% mixture.
Scheme 3. Products from the SCPC reaction of benzaldehyde.
The proportion of the alcohol to diol depended on the amount of P25 and the density of its
dispersion within the reaction medium. A set of experiments with dispersions ranging from zero P25 to
1.9 mg·mL−1 was carried out and the yields of the two product types as a function of this are shown in
Figure 1.
At light dispersions the process was selective for the diols which, as expected, were produced as a
1:1 mixture of the stereoisomers 9a and 9a'. The important result, however, was that irradiations with
dense dispersions were more selective for reduction of the aldehyde to benzyl alcohol. To find the
most efficient process a series of SCPC reactions was carried out with various catalysts. The same
reaction conditions of solvent (MeCN and MeOH, 9:1), catalyst (0.75 mole equiv.) and irradiation time
(20 h, except for zinc sulfide) were used for each system (Table 2).

Molecules 2014, 19
Figure 1. Product yields vs. P25 dispersion density in SCPC reactions of benzaldehyde.
15328
100
80
60
40
20
0
BnOH ylds.
Tot. Diol Ylds
0
0.5
1
1.5
2
P25 disp. in mg/mL
Table 2. Product yields a from SCPC reactions of benzaldehyde with various catalysts.
BnOH
(%)
63
Diols (9a + 9a')
Ratio
D/L: meso
1.0
Conversion
Catalyst
(%)
29
65
35
49
34
65
(%)
99
P25
Pt-P25 b
PC500
10
1.1
98
19
0.73
98
Coated Tube c
ZnS d
22
1.0
99
0
0.50
n.d.
ZnS e
0
0.51
99
a Yields determined by NMR; b P25 doped with Pt at loading of 0.1% (w/w); c NMR tube coated with anatase
TiO2 by a sol gel process; d Irradiation time 1 h; e Irradiation time 3 h.
The platinized titania (Pt-P25) was prepared by coating P25 with 0.1% Pt using the method of
Mills [12]. The Millenium PC500 (anatase, surface area ~300 m2·g−1, particle size 5–10 nm) had about
six times the surface area of P25. The coated tubes were NMR tubes coated internally with a fine layer
of anatase using the sol-gel process described by Mills and co-workers [13]. ZnS nanoparticles,
prepared by using the method described by Hu, [14] have a more negative reduction potential than
TiO2, (size 3–5 nm, ca. 156 m2·g−1 surface area) and so were also trialed. However, no reduction to
benzyl alcohol took place with this catalyst. Table 2 shows that P25 was selective for hydrogenation
and Pt-P25 was selective for dimerization. These catalysts also led to comparatively little product
degradation and so were chosen for subsequent work.
It has been known since the earliest days of photochemistry that UV irradiation of carbonyl
compounds produces pinacols [15]. The method was once widely exploited [16] but recent usage is
scant [17]. Recent methods of production of 1,2-diols from carbonyl compounds rely on catalysis by
metal complexes, notably of Ti [18–21]. The yields in Figure 1 show that 1,2-diol was formed by
direct UVA photolysis, under our conditions, in the absence of catalyst. However, from the small
increase in yield, it appeared at first that the presence of light dispersions of P25 or Pt-P25 might
increase the efficiency of dimerization. Series of irradiations of a selection of aldehydes and ketones
were carried out first with a light P25 dispersion (~0.25 mole equiv.) and then in the absence of
catalyst with UVA alone (Table 3). To ensure that the NMR product identifications were correct, five
of the reactions were scaled up and the product diols isolated and characterized (Table 3). This Table
shows that no hydrogenation to alcohol took place with any of the carbonyl substrates in the absence of

Molecules 2014, 19
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the P25 catalyst. The light dispersion of P25 gave very little alcohol and was selective for 1,2-diol
formation for all the carbonyls that did react. Good to moderate yields of diols were obtained for all the
substrates except the heterocycles and 2-naphthaldehyde. As expected, the diols were formed as a pair
of stereoisomers (D/L and meso) in a ratio of 1:1 (within measurement error limits) in each case.
Interestingly, the diol yields were greater in the absence of P25 for all the carbonyls except
benzaldehyde. We conclude from this that diol formation took place by the well-known photochemical
process and was independent of TiO2 catalysis.
Table 3. Comparison of carbonyl photolyses with and without P25 SCPC a.
Alcohol (%)
Alcohol (%)
Diol (%)
with P25 b
93 {75}
0
Diol (%)
Carbonyl
with P25
UVA Only
UVA Only
Benzaldehyde
2-Naphthaldehyde
4-Me-Benzaldehyde
4-MeO-Benzaldehyde
4-Cl-Benzaldehyde
4-CF3-Benzaldehyde
2-Thiophene-CHO
2-Furan-CHO
0
0
0
0
0
0
0
0
0
0
0
-
65
0
7
43 {43}
12
72
30
58
83
0
8
5
54 {50}
58 {49}
13
4
0
0
0
0
3-Benzofuran-CHO
Acetophenone
8
0
0
trace
66 {92}
-
a Yields by NMR except as otherwise indicated; b Isolated yields in parenthesis {%}.
The benzaldehyde reaction (Figure 1) implied that carbonyls could be reduced to alcohols by use of
denser dispersions of P25. Accordingly, a set of aldehydes and ketones was irradiated (20 h) in
MeCN/MeOH (9:1) solvent with a P25 dispersion of 1 mg·mL−1 (~0.75 equiv.) and the product yields
are recorded in Table 4.
Table 4. P25 SCPC reductions of carbonyl compounds to primary alcohols a.
Carbonyl
Alcohol (%)
Diol (%)
benzaldehyde
63
95
36
56
78
76
63
96
0
29
0
2-naphthaldehyde
4-Me-benzaldehyde
4-MeO-benzaldehyde
4-Cl-benzaldehyde
4-CF3-benzaldehyde
2-thiophene-CHO
3-benzofuran-CHO
Acetophenone
16
0
21
0
29
0
6
a Yields mol% by NMR.
Table 4 shows that TiO2 SCPC reductions of aldehydes to primary alcohols could be accomplished
in good to excellent yields for aromatic and heterocyclic aldehydes with both electron-withdrawing
and electron-releasing substituents. The selectivity for alcohol over 1,2-diol production was more
variable. It is probable this selectivity could have been increased by using denser dispersions of P25.

Molecules 2014, 19
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However, denser dispersions screen out the UVA and this leads to lower overall yields either because
of incomplete substrate consumption, or because of product degradation if longer irradiation times
were employed. The SCPC reductions of ketones to secondary alcohols were not successful. With
acetophenone (Table 4) and benzophenone very little reaction took place and essentially only minor
amounts of diols were observed; cyclohexanone also gave no cyclohexanol product.
In the absence of TiO2, absorption of a photon by the aldehyde generates a singlet diradical that
crosses over to the triplet (Scheme 4). The triplet abstracts an H-atom from solvent (SH) to afford
hydroxy-benzyl type radical 10. Two of these then combine to produce the 1,2-diol 9 as a 1:1 mixture
of stereo-isomers. The lower portion of Scheme 4 shows a schematic of a plausible surface process in
the presence of TiO2 catalyst. Absorption of a photon by the TiO2 generates an e−/h+ pair that migrates
to the surface of the catalyst particle. The TiO2 surface is plentifully populated with OH groups [22,23]
which, if deuteriated CD3OD is used, will rapidly exchange to OD groups. The aromatic aldehyde
H-bonds to the surface (11) where it readily accepts e− from the e−/h+ pair (12). The methanol from
the solvent also H-bonds to the surface (shown deuteriated in 11) where it donates an electron to a
surface h+ and also exchanges H for D as the radical cation is generated (13).
Scheme 4. Proposed surface mechanism for the TiO2 SCPC reduction of aryl aldehydes.
A surface proton (or deuteron) is transferred to the aryl radical anion in 12 thus generating the
resonance stabilized hydroxybenzyl radical in 13. The latter accepts a second e− from the TiO2 and the
resulting benzyl anion picks up a deuteron (or proton) from a neighboring surface OD(H) (14). Finally,
the benzyl type alcohol is released probably along with an hydroxymethyl radical.
We carried out an SCPC reduction of benzaldehyde using P25 and the conditions of Table 4 except
that CD3OD replaced the CH3OH in the solvent mixture. The 1H-NMR spectrum of the product benzyl
2
alcohol showed δH at 4.66 ppm (1:1:1 triplet, partly resolved J ≈ 2 Hz) and the H-NMR spectrum

Molecules 2014, 19
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showed δD at 4.66 ppm (d, J ≈ 2 Hz) consistent with the structure PhCHDOD. The GC-MS supported
this structure in that the EI MS of the benzyl alcohol peak showed m/z 110 (M+). This result was good
evidence in support of the mechanism shown in Scheme 4.
3. Experimental Section
3.1. General Procedures
All solvents and reagents were purchased from commercial sources and used without further
purification unless stated otherwise. The acetonitrile and methanol were both distilled over calcium
hydride in an atmosphere of argon. Column chromatography was run using Silica 60A (particle size
40–63 µm) as the stationary phase. Thin layer chromatography was performed on pre-coated silica gel
plates (0.20 mm thick, Sil G UV253, Macherey-Nagel, Düren, Germany) and observed under UV
1
light. H- and 13C-NMR spectra were recorded using the Bruker AV 300 and Bruker AV II 400
instruments (Bruker, Karlsruhe, Germany). Chemical shifts are reported in parts per million (ppm)
from low to high frequency and referenced to residual solvent peaks unless otherwise stated. NMR
yields were calculated using a known volume of dibromomethane (2 µL) as an internal standard.
Coupling constants (J) are reported in hertz (Hz). Standard abbreviations indicating multiplicity were
used as follows: s = singlet, d = doublet, t = triplet, dd = double doublet, q = quartet, m = multiplet,
b = broad. Mass spectrometry was carried out by the services at the EPSRC National Mass
Spectrometry Service in Swansea, UK.
Zinc Sulfide was prepared in accordance with the procedure of Hu et al. [14]. Zinc acetate, thiourea
and polyvinylpyrrolidone were refluxed for 3 h at 150 °C in ethylene glycol. The resulting suspension
was then centrifuged to remove the nanoparticles, which were then dried overnight in a vacuum oven.
The polyvinylpyrrolidone is used to give smaller particle sizes for the ZnS.
3.2. General Procedure for Preparation of Maleimides
The aromatic amine (2.5 equiv.) and substituted maleic anhydride (1 equiv.) were refluxed in
toluene at 110 °C for 3 days. The mixture was then concentrated under reduced pressure and the
intermediate was washed with hydrochloric acid (1 M), before being dissolved in acetic anhydride
(35 mL) with sodium acetate (20 equiv.) and heated to 80 °C for 4 h. After cooling to room
temperature the acetic anhydride as removed in vacuo and the remaining solid was partitioned between
ethyl acetate (75 mL) and deionized water (3 × 100 mL). The combined organic extracts were then
washed with saturated aqueous sodium bicarbonate (100 mL) and dried over magnesium sulfate. The
solvent was removed under reduced pressure and the crude product was recrystallized from
cyclohexane to give the desired product.
3-Methyl-1-phenyl-1H-pyrrole-2,5-dione (5b). Aniline (2.45 mL, 26.76 mmol) and methyl maleic
anhydride (1.5 g, 13.38 mmol) in toluene (30 mL) were treated according to the general procedure. In
the second step sodium acetate (6.51 g, 79.4 mmol) and the intermediate were dissolved in acetic
anhydride (35 mL). After recrystallization in cyclohexane 5b was obtained as an off-white solid (1.3 g,
52%). 1H-NMR (400 MHz, CDCl3): δ 2.14 (3H, d, J = 2.14 Hz), 6.52 (1H, s), 7.35–7.49 (5H, m).

Molecules 2014, 19
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3,4-Dimethyl-1-phenyl-1H-pyrrole-2,5-dione (5c). From aniline (0.92 mL, 9.93 mmmol) and dimethyl
maleic anhydride (0.5 g, 3.97 mmol). After recrystallization in cyclohexane 5c was obtained as an
1
off-white solid (0.15 g, 19%). H-NMR (400 MHz, CDCl3): δ 2.09 (6H, s), 7.35–7.39 (3H, m),
7.45–7.50 (2H, m). Spectral data was consistent with literature [24].
1,3-Diphenyl-1H-pyrrole-2,5-dione (5d). From aniline (1.05 mL, 11.48 mmol) and N-phenyl maleic
anhydride (1.0 g, 5.74 mmol). Following column chromatography on silica gel (eluent: 20%–40% EtOAc
in petroleum 40/60) 5d was obtained as an off-white solid (0.2 g, 14%). 1H-NMR (400 MHz, CDCl3):
δ 6.91 (1H, s), 7.45–7.58 (8H, m), 8.00 (2H, m). Spectral data was consistent with literature [25].
1-(4-Methoxyphenyl)-3,4-Dimethyl-1H-pyrrole-2,5-dione (5e). From 4-methoxyaniline (1.22 g,
9.93 mmol) and dimethyl maleic anhydride (0.5 g, 3.97 mmol). After recrystallization from
1
cyclohexane 4 was obtained as a pale orange solid (0.23 g, 25%). H-NMR (400 MHz, CDCl3): δ 2.0
(6H, s), 3.75 (3H, s), 6.90 (2H, d, J = 9.0 Hz), 7.17 (2H, d, J = 9.0 Hz). Spectral data was consistent
with literature [26].
3,4-Dimethyl-1-(3,4,5-trimethoxyphenyl)-1H-pyrrole-2,5-dione (5f). From 3,4,5-trimethoxyaniline
(1.09 g, 5.95 mmol) and dimethyl maleic anhydride (0.5 g, 3.97 mmol). Following column
chromatography on silica gel (eluent: 20%–40% EtOAc in petroleum 40/60) 5f was obtained as an
1
off-white solid (0.17 g, 15%); mp. 141 °C. H-NMR (400 MHz, CDCl3): δ 2.09 (6H, s), 3.94 (6H, s),
13
3.96 (3H, s), 6.59 (2H, s). C-NMR (100MHz, CDCl3): δ 8.93, 56.17, 60.89, 77.35, 103.65, 127.51,
137.46, 153.35, 171.06. LR-ESIMS: m/z = 292 [MH]+; HR-EIMS: m/z = 292.1173 (calcd. for
C15H18N1O5, 292.1179).
1-(4-Chlorophenyl)-3,4-dimethyl-1H-pyrrole-2,5-dione (5g). From 4-chloroaniline (2.0 g, 15.7 mmol)
and dimethyl maleic anhydride (0.5 g, 3.97 mmol). Following column chromatography on silica gel
(eluent: 20%–40% EtOAc in petroleum 40/60) 5g was obtained as a pale brown solid (0.03 g, 3%);
1
mp. 142 °C. H-NMR (400 MHz, CDCl3): δ 2.10 (6H, s), 7.24–7.26 (2H, m), 7.33–7.36 (2H, m).
13C-NMR (100 MHz, CDCl3): δ 8.94, 126.78, 129.19, 130.51, 133.21, 137.64, 170.62. LR-ESIMS:
m/z = 236 [MH]+; HR-EIMS: m/z = 236.0472 (calcd. for C12H11Cl1N1O2, 236.0473).
3.3. General Procedure for SCPC Hydrogenation of Maleimides
The maleimide was added to a suspension of P25 in methanol (10%) and acetonitrile (90%).
The resulting mixture was then purged with argon for 15 min. The mixture was irradiated with eight
29 cm 15 W Philips Cleo tubes (λ = 350 nm). Following irradiation the P25 powder was removed via
filtration through Celite.
Succinimide (2a). Maleimide (24.5 mg, 0.25 mmol) and TiO2 (30 mg, 0.38 mmol) in CH3OH (3 mL)
and CH3CN (27 mL) were reacted in accordance with the general procedure. Following irradiation
1
for 17 h 2a was obtained as a white powder (22.8 mg, 91%). H-NMR (300 MHz, CDCl3, 296 K):
δ = 2.75 (s, 4H), 8.84 (br-s, 1H, NH); 13C-NMR (75 MHz, CDCl3, 299 K): δ = 29.6, 177.9. Consistent
with literature [27].

Molecules 2014, 19
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N-Methylsuccinimide (2b). N-Methylmaleimide (27.8 mg, 0.25 mmol) and TiO2 (30 mg, 0.38 mmol)
in CH3OH (3 mL) and CH3CN (27 mL) were reacted in accordance with the general procedure.
1
Following irradiation for 17 h 2b was obtained as an off white powder (25.5 mg, 90%). H-NMR
(300 MHz, CDCl3, 298 K): δ = 2.69 (s, 4H), 2.97 (s, 3H); 13C-NMR (75 MHz, CDCl3, 299 K):
δ = 24.8, 28.2, 177.3. Consistent with the literature [28].
1-Phenylpyrrolidine-2,5-dione (2c). N-Phenyl maleimide (0.0347 g, 0.2 mmol) and TiO2 (0.012 g,
0.15 mmol) were added to methanol (1.1 mL) and acetonitrile (10.9 mL). Following irradiation for
20 h, 2c was obtained as an off white solid (0.33 g, 94%). 1H-NMR (400 MHz, CDCl3): δ 2.90 (4H, s),
7.20–7.22 (2H, m), 7.30–7.35 (1H, m), 7.37–7.43 (2H, m). 13C-NMR (75 MHz, CDCl3, 297 K):
δ = 28.4, 126.5, 128.7, 129.2, 132.0, 176.3. Spectral data was consistent with literature [29].
N-Carboxymethylsuccinimide (2d). N-Carboxymethylmaleimide (38.8 mg, 0.25 mmol) and TiO2
(30 mg, 0.38 mmol) in CH3OH (3 mL) and CH3CN (27 mL) were reacted in accordance with the general
1
procedure. Following irradiation for 15 h 2d was obtained as colorless oil (32.1 mg, 82%). H-NMR
(400 MHz, CDCl3, 296 K): δ = 2.81 (s, 4H), 3.96 (s, 3H); 13C-NMR (75 MHz, CDCl3, 299 K):
δ = 28.6, 172.6, 176.4.
Succinic anhydride (4). Maleic anhydride (24.5 mg, 0.25 mmol) and TiO2 (30 mg, 0.38 mmol) in
CH3OH (3 mL) and CH3CN (27 mL) were reacted in accordance with the general procedure.
Following irradiation for 15 h, 4 was observed by 1H-NMR (60% w.r.t. a CH2Br2 standard). 1H-NMR
(300 MHz, CDCl3, 296 K): δ = 3.00 (s, 4H). 4-Oxobutanoic acid was observed in the same manner
1
(17%). H-NMR (300 MHz, CDCl3, 296 K): δ = 2.59–2.70 (m, 4H), 3.69 (s, 3H), 9.86 (br-s, 1H).
Spectral data for both compounds consistent with literature [30].
3-Methyl-1-phenylpyrrolidine-2,5-dione (6b). From 3-methyl-1-phenyl-1H-pyrrole-2,5-dione (0.037 g,
0.2 mmol) and TiO2 (0.12 g, 0.15 mmol). Following irradiation for 20 h, 6b was obtained as an off
white solid, 1H-NMR (wrt CH2Br2 internal standard) revealed 97% conversion of the starting material
1
and ≤5% yield of the product. H-NMR (400 MHz, CDCl3): δ 1.47 (3H, m), 2.55 (1H, m), 3.07–3.09
(1H, m), 3.15–3.19 (1H, m), 7.25–7.51 (5H, m). Spectral data was consistent with literature [31].
3,4-Dimethyl-1-phenylpyrrolidine-2,5-dione (6c). 3,4-Dimethyl-N-phenylmaleimide 5c (0.0402 g,
0.2 mmol) and TiO2 (0.12 g, 0.15 mmol) were added to a solution of methanol (1.1 mL) and
acetonitrile (10.9 mL). Following irradiation for 20 h, 6c was obtained as an off white solid, 1H-NMR
(wrt CH2Br2 internal standard) revealed 50% conversion of the starting material and 17% yield of the
1
product. H-NMR (400 MHz, CDCl3): δ 1.45 (6H, d, J = 7.0 Hz), 2.60 (2H, m), 7.31–7.32 (2H, m),
7.35–7.37 (2H, m), 7.47–7.49 (2H, m). Spectral data was consistent with literature [26].
1,3-Diphenylpyrrolidine-2,5-dione (6d). From 1,3-diphenyl-1H-pyrrole-2,5-dione (0.049 g, 0.2 mmol)
and TiO2 (0.12 g, 0.15 mmol). Following irradiation for 20 h, 6d was obtained as an off white solid,
1H-NMR (wrt CH2Br2 internal standard) revealed 99% conversion of the starting material and 9%
1
yield of the product. H-NMR (400 MHz, CDCl3): δ 3.02 (1H, dd, J = 23.4 and 4.8), 3.36 (1H, dd,
J = 9.9 and 18.7 Hz), 4.2 (1H, m), 7.37–7.51 (10H, m).

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1-(4-Methoxyphenyl)-3,4-Dimethylpyrrolidine-2,5-dione (6e). From 3, 4-dimethyl-N-4-methoxyphenyl
maleimide (0.046 g, 0.2 mmol) and TiO2 (0.12 g, 0.15 mmol). Following irradiation for 20 h, 5e was
1
obtained as an off white solid, H-NMR (wrt CH2Br2 internal standard) revealed 64% conversion of
1
the starting material and a 58% yield. H-NMR (400 MHz, CDCl3): δ 1.44 (6H, d, J = 6.9 Hz), 2.58
(2H, m), 3.84 (3H, s), 6.98 (2H, s), 7.19 (2H, s). Spectral data was consistent with literature. [26].
3,4-Dimethyl-1-(3,4,5-trimethoxyphenyl)pyrrolidine-2,5-dione (6f). From 3,4-dimethyl-N-(3,4,5-
trimethoxyphenyl) maleimide (0.058 g, 0.2 mmol) and TiO2 (0.12 g, 0.15 mmol). Following irradiation
1
for 72 h, 6e was obtained as an off white solid, (0.048 g, 79%). H-NMR (400 MHz, (CD3)2CO):
δ 0.92 (6H, d, J = 7.4 Hz), 2.82 (2H, dq, J = 2.9 and 16.8 Hz), 3.46 (6H, s), 3.48 (3H, s), 6.82 (2H, s).
1-(4-Chlorophenyl)-3,4-dimethylpyrrolidine-2,5-dione (6g). From 1-(4-chlorophenyl)-3,4-dimethyl-
1H-pyrrole-2,5-dione (0.0235 g, 0.1 mmol) and TiO2 (0.06 g, 0.075 mmol). Following irradiation for
20 h 6g was obtained as an off white solid, 1H-NMR analysis (wrt CH2Br2 internal standard) revealed
49% conversion of the starting material and a 19% yield; 1H-NMR (400 MHz, CDCl3): δ 1.34 (6H, d,
J = 7.3 Hz), 3.13 (2H, dq, J = 2, 16 Hz), 7.33–7.35 (2H, m), 7.42–7.44 (2H, m).
3.4. General Procedure for SCPC Reduction of Carbonyl Compounds
An oven dried Pyrex Schlenk tube was evacuated while still hot and then back-filled with argon.
This was repeated three times before the tube was allowed to cool to room temperature. The carbonyl
compound was added to a suspension of P25 in methanol (10%) and acetonitrile (90%) with a fast
stream of argon flowing before freshly distilled CH3CN was added to generate the desired dispersion
density (typically 1 mg·mL−1 unless otherwise stated). The resulting mixture was purged with argon
for 20 min. The stirred mixture was then irradiated while still under an atmosphere of argon with two
hemispherical banks of six 29 cm 15 W Philips Cleo tubes (λ = 350 nm) for the desired reaction time
at ambient temperature. Following irradiation the TiO2 powder was removed by filtration through a
Celite pad and the solvent removed under reduced pressure. Isolated yields were obtained from scale-up
1
reactions and, where indicated, yields were calculated from the H-NMNR spectra w.r.t. CH2Br2 as
internal standard. Characterization data for the compounds prepared using this procedure follows.
From Benzaldehyde. Benzyl alcohol: 1H-NMR (500 MHz, CDCl3): δ 4.70 (2H, s), 7.24–7.38 (5H, m).
1
1.2-diphenylethane-1.2-diol (D/L): H-NMR (500 MHz, CDCl3): δ 4.72 (2H, s), 7.12–7.24 (10H, m).
1.2-diphenylethane-1.2-diol (meso): 1H-NMR (500 MHz, CDCl3): δ 4.8 4(2H, s), 7.12–7.24 (10H, m).
Chemical shifts were in accordance with the literature [32].
1
From 2-Naphthaldehyde. (Naphthalen-6-yl)methanol: H-NMR (500 MHz, CDCl3): δ 4.86 (2H, s),
7.45–7.50 (3H, m), 7.80–7.86 (4H, m). Chemical shifts were in accordance with the literature [33].
1
From 4-Methylbenzaldehyde. 4-Methylphenylmethanol: H-NMR (500 MHz, CDCl3): δ 2.16 (3H, s),
1
4.64 (2H, s), 7.02–7.33 (4H, m). 1,2-Di(4-methylphenyl)ethane-1,2-diol (D/L): H-NMR (500 MHz,
CDCl3): δ 2.16 (3H, s) 4.66 (2H, s), 7.02–7.33 (8H, m). 1,2-Di(4-methylphenyl)ethane-1,2-diol
1
(meso): H-NMR (500 MHz, CDCl3): δ 2.16 (3H, s) 4.74 (2H, s), 7.02–7.33 (8H, m). Chemical shifts
were in accordance with the literature [32,34].

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From 4-Methoxybenzaldehyde. 4-Methoxyphenylmethanol: 1H-NMR (500 MHz, CDCl3): δ 3.84 (6H, s),
4.62(2H, s,), 6.75–7.17 (4H, m). 1,2-Di(4-methoxyphenyl)ethane-1,2-diol (D/L): 1H-NMR (500 MHz,
CDCl3): δ 3.84 (6H, s), 4.64 (2H, s), 6.75–7.17 (8H, m). 1,2-Di(4-methoxyphenyl)ethane-1,2-diol
(meso): 1H-NMR (500 MHz, CDCl3): δ 3.84 (6H, s), 4.73 (2H, s), 6.75–7.17 (8H, m). Chemical shifts
were in accordance with the literature [35].
From 4-Chlororbenzaldehyde. 4-Chlorophenylmethanol: 1H-NMR (500 MHz, CDCl3): δ 4.65 (2H, s),
7.00–7.31 (4H, m). 1,2-Di(4-chlorophenyl)ethane-1,2-diol (D/L): 1H-NMR (500 MHz, CDCl3): δ 4.59
(2H, s), 7.00–7.31 (8H, m). 1,2-Di(4-chlorophenyl)ethane-1,2-diol (meso): 1H-NMR (500 MHz, CDCl3):
δ 4.82 (2H, s), 7.00–7.31 (8H, m). Chemical shifts were in accordance with the literature [32,36].
1
From 4-(Trifluoromethyl)benzaldehyde. 4-(Trifluoromethyl)phenylmethanol: H-NMR (500 MHz,
CDCl3): δ 4.67 (2H, s), 7.21–7.60 (4H, m). 1,2-Di(4-trifluoromethyl)diphenylethane-1,2-diol (D/L):
1H-NMR (500 MHz, CDCl3): δ 4.72 (2H, s), 7.21–7.60 (8H, m). 1,2-Di(4-trifluoromethyl)diphenylethane
1
ethane-1,2-diol (meso): H-NMR (500 MHz, CDCl3): δ 4.95 (2H, s), 7.21–7.60 (8H, m). Chemical
shifts were in accordance with the literature [37,38].
1
From 2-Thiophenecarboxaldehyde. Thiophene-2-methanol: H-NMR (500 MHz, CDCl3): δ 4.83 (2H,
1
s), 7.81–7.28 (3H, m). 1.2-di(thiophenyl)ethane-1.2-diol (D/L): H-NMR (500 MHz, CDCl3): δ 5.06
1
(2H, s), 6.81–7.28 (6H, m). 1.2-di(thiophenyl)ethane-1.2-diol (meso): H-NMR (500 MHz, CDCl3): δ
5.13 (2H, s), 6.81–7.28 (6H, m). Chemical shifts were in accordance with the literature [36,39].
From 2-Benzofurancarboxaldehyde. 1-(Benzofuran-2-yl)methanol: 1H-NMR (500 MHz, CDCl3): δ 4.78
(2H, s), 6.40 (1H, s) 7.10–7.40 (6H, m). Chemical shifts were in accordance with the literature [40].
From Acetophenone. 1-Phenylethanol: 1H-NMR (500 MHz CDCl3): δ 4.75 (1H, s), 7.21–7.60 (4H, m).
1
2,3-Diphenylbutane-2,3-diol (D/L): H-NMR (500 MHz CDCl3): 1.39 (6H, s), 7.06–7.11 (10H, m).
1
2,3-Diphenylbutane-2,3-diol (meso): H-NMR (500 MHz CDCl3): 1.46 (6H, s), 7.06–7.11 (10H, m).
Chemical shifts were in accordance with the literature [35].
4. Conclusions
Our exploratory study demonstrated that both electron-deficient alkenes and aldehydes could be
effectively hydrogenated by P25 SCPC. The reactions took place at room temperature and required
only non-harmful UVA irradiation and dry solvents. These are mild processes because no toxic metals
or corrosive reagents, dangerous gases or forcing conditions are needed and the TiO2 catalyst could be
recovered and re-cycled. They represent an important step towards greener synthetic routes.
Acknowledgments
We thank the EPSRC for funding (Grant EP/I003479/1) and the EPSRC National Mass Spectrometry
Service, Swansea.

Molecules 2014, 19
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Author Contributions
JCW and DWM designed the research and wrote the paper; LB and AM performed the research and
analyzed the data. All authors read and approved the final manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
References
1. Linsebigler, A.L.; Lu, G.; Yates, J.T. Photocatalysis on TiO2 surfaces: Principles, mechanisms,
and selected results. Chem. Rev. 1995, 95, 735–758.
2. Ravelli, D.; Dondi, D.; Fagnoni, M.; Albini, A. Photocatalysis. A multi-faceted concept for green
chemistry. Chem. Soc. Rev. 2009, 38, 1999–2011.
3. Mills, A.; O’Rourke, C. Photocatalytic oxidation of toluene in an NMR tube. J. Photochem.
Photobiol. A 2012, 233, 34–39.
4. Künneth, R.; Feldmer, C.; Knoch, F.; Kisch, H. Heterogeneous photocatalysis. XIII.
Semiconductor-catalyzed photoaddition of olefins and enol ethers to 1, 2-diazenes: A new route to
allylhydrazines. Chem. Eur. J. 1995, 1, 441–448.
5. Keck, H.; Schindler, W.; Knoch, F.; Kisch, H. Heterogeneous photocatalysis. XVI. Type B
semiconductor photocatalysis: The synthesis of homoallyl amines by cadmium sulfide-catalyzed
linear photoaddition of olefins and enol/allyl ethers to N-phenylbenzophenoneimine. Chem. Eur. J.
1997, 3, 1638–1645.
6. Marinković, S.; Hoffmann, N. Efficient radical addition of tertiary amines to electron-deficient
alkenes using semiconductors as photochemical sensitizers. Chem. Commun. 2001, 1576–1578.
7. Marinković, S.; Hoffmann, N. Semiconductors as sensitizers for the radical addition of tertiary
amines to electron deficient alkenes. Int. J. Photoenergy 2003, 5, 175–182.
8. Marinković, S.; Hoffmann, N. Diastereoselective radical tandem addition-cyclization reactions of
aromatic tertiary amines by semiconductor-sensitized photochemical electron transfer. Eur. J.
Org. Chem. 2004, 3102–3107.
9. Manley, D.W.; McBurney, R.T.; Miller, P.; Howe, R.F.; Rhydderch, S.; Walton, J.C.
Unconventional titania photocatalysis: Direct deployment of carboxylic acids in alkylations and
annulations. J. Am. Chem. Soc. 2012, 134, 13580–13583.
10. Manley, D.W.; McBurney, R.T.; Miller, P.; Walton, J.C.; Mills, A.; O’Rourke, C. Titania-promoted
carboxylic acid alkylations of alkenes and cascade addition-cyclizations. J. Org. Chem. 2014, 79,
1386–1398.
11. Fles, D.; Vukovic, R.; Kuzmic, A.E.; Bogdanic, G.; Pilizota, V.; Karlovic, D.; Markus, K.;
Wolsperger, K.; Vikic-Topic, D. Synthesis and spectroscopic evidences of N-arylmaleimides and
N-aryl-2,3-dimethylmaleimides. Croat. Chem. Acta 2003, 76, 69–74.
12. Mills, A. Platinization of semiconductor particles. J. Chem. Soc. Chem. Commun. 1982, 6, 367–368.
13. Mills, A.; Elliott, N.; Hill, G.; Fallis, D.; Durrant, J.R.; Willis, R.L. Preparation and characterization
of novel thick sol-gel titania film photocatalysts. Photochem. Photobiol. Sci. 2003, 2, 591–596.

Molecules 2014, 19
15337
14. Hu, J.; Ren, L.; Guo, Y.; Liang, H.; Cao, A.; Wan, L.; Bai, C. Mass production and high
photocatalytic activity of ZnS nanoporous nanoparticles. Angew. Chem. Int. Ed. 2005, 44, 1269–1273.
15. Ciamician, G.; Silber, P. Chemical actions of light. Ber. Dtsch. Chem. Ges. 1900, 33, 2911–2913.
16. Turro, N.J. Modern Molecular Photochemistry; Benjamin: NY, USA, 1978; p. 363 et seq.
17. Nagwanshi, R.; Bakhru, M.; Jain, S. Photodimerization of heteroaryl chalcones: Comparative
antimicrobial activities of chalcones and their photoproducts. Med. Chem. Res. 2012, 21, 1587–1596.
18. McMurry, J.E. Carbonyl-coupling reactions using low-valent titanium. Chem. Rev. 1989, 89,
1513–1524.
19. Okamoto, S.; He, J.-Q.; Ohno, C.; Oh-iwa, Y.; Kawaguchi, Y. McMurry coupling of aryl aldehydes
and imino pinacol coupling mediated by Ti(O-i-Pr)4/Me3SiCl/Mg reagent. Tetrahedron Lett.
2010, 51, 387–390.
20. Kim, Y.; Do, Y.; Park, S. Mechanistic study of half-titanocene-based reductive pinacol coupling
reaction. Bull. Korean Chem. Soc. 2011, 32, 3973–3978.
21. Scheffler, U.; Mahrwald, R. A one-pot cross-pinacol coupling/rearrangement procedure.
Helv. Chim. Acta 2012, 95, 1970–1975.
22. Deiana, C.; Fois, E.; Coluccia, S.; Martra, G. Surface structure of TiO2 P25 nanoparticles: Infrared
study of hydroxy groups on coordinative defect sites. J. Phys. Chem. C 2010, 114, 21531–21538.
23. Salazar, C.; Nanny, M.A. Influence of hydrogen bonding upon the TiO2 photooxidation of
isopropanol and acetone in aqueous solution. J. Catal. 2010, 269, 404–410.
24. Matuszak, N.; Muccioli, G.G.; Labar, G.; Lambert, D.M. Synthesis and in vitro evaluation of
N-substituted maleimide derivatives as selective monoglyceride lipase inhibitors. J. Med. Chem.
2009, 52, 7410–7420.
25. Bestmann, H.J.; Schade, G.; Luetke, H.; Moenius, T. Cumulated ylides. XVII. Synthesis of cyclic
compounds from triphenyl[(phenylimino)ethenylidene]phosphorane and oxocarboxylic acids—A
novel method for anellation. Chem. Ber. 1985, 118, 2640–2658.
26. Hegasy, M.F.; Shishido, K.; Hirata, T. Asymmetric hydrogenation of the C-C double bond of
1- and 1,2-methylated maleimides with cultured suspension cells of Marchantia polymorpha.
Tetrahedron-Asymmetry 2006, 17, 1859–1862.
27. Walling, C.; El-Taliawi, G.M.; Zhao, C. Radical chain carriers in N-bromosuccinimide brominations.
J. Am. Chem. Soc. 1983, 105, 5119–5124.
28. Ito, R.; Umezawa, N.; Higuchi, T. Unique oxidation reaction of amides with pyridine-N-oxide
catalyzed by ruthenium porphyrin: Direct oxidative conversion of N-acyl-L-proline to
N-acyl-L-glutamate. J. Am. Chem. Soc. 2004, 127, 834–835.
29. Kantak, A.A.; Potavathri, S.; Barham, R.A.; Romano, K.M.; Deboef, B. Metal-free intermolecular
oxidative C-N bond formation via tandem C-H and N-H bond functionalization. J. Am. Chem. Soc.
2011, 133, 19960–19965.
30. Kocz, R.; Roestamadji, J.; Mobashery, S. A convenient triphosgene-mediated synthesis of
symmetric carboxylic acid anhydrides. J. Org. Chem. 1994, 59, 2913–2914.
31. Stueckler, C.; Reiter, T.C.; Baudendistel, N.; Faber, K. Nicotinamide-independent asymmetric
bioreduction of C=C-bonds via disproportionation of enones catalyzed by enoate reductases.
Tetrahedron 2010, 66, 663–667.

Molecules 2014, 19
15338
32. García-Muñoza, A.; Ortega-Arizmendia, A.; García-Carrillob, M.; Díazb, E.; Gonzalez-Rivasa, N.;
Cuevas-Yañez, E. Direct, metal-free synthesis of benzyl alcohols and deuterated benzyl alcohols
from p-toluenesulfonylhydrazones using water as solvent. Synthesis 2012, 44, 2237–2242.
33. Murai, N.; Yonaga, M.; Tanaka, K. Palladium-catalyzed direct hydroxymethylation of aryl halides
and trifates with potassium acetoxymethyltrifuoroborate. Org. Lett. 2012, 145, 1278–1281.
34. Ashikari, Y.; Nokami, T.; Yoshida, J. Oxidative hydroxylation mediated by alkoxysulfonium ions.
Org. Lett. 2012, 14, 938–941.
35. Uchiyama, M.; Matsumoto, Y.; Nakamura, S.; Ohwada, T.; Kobayashi, N.; Yamashita, N.;
Matsumiya, A.; Sakamoto, T. Development of a catalytic electron transfer system mediated by
transition metal ate complexes: Applicability and tunability of electron-releasing potential for
organic transformations. J. Am. Chem. Soc. 2004, 126, 8755–8759.
36. Li, Y.G.; Tian, Q.S.; Zhao, J.; Feng, Y.; Li, M.J.; You, T.P. Asymmetric pinacol coupling of
aromatic aldehydes catalyzed by a new titanium Schiff base complex. Tetrahedron-Asymmetry
2004, 15, 1707–1710.
37. Wei, Y.; Xue, D.; Lei, Q.; Wang, C.; Xiao, J. Cyclometalated iridium complexes for transfer
hydrogenation of carbonyl groups in water. Green Chem. 2013, 15, 629–634.
38. Lacour, J.; Londez, A. Diastereoselective synthesis of C2-symmetric hexacoordinated phosphate
anions (Hyphats) with predetermined chirality from 1,2-diaryl-ethane-1,2-diols. J. Organomet. Chem.
2002, 643, 392–403.
39. Chen, Y.Z.; Ni, C.W.; Teng, F.L.; Ding, Y.S.; Lee, T.H.; Ho, J.H. Construction of polyaromatics via
photocyclization of 2-(fur-3-yl)ethenylarenes, using a 3-furyl group as an isopropenyl equivalent
synthon. Tetrahedron 2014, 70, 1748–1762.
40. Wan, W.C.; Chen, W.; Liu, L.X.; Li, Y.; Yang, L.J.; Deng, X.Y.; Zhang, H.B.; Yang, X.D.
Synthesis and cytotoxic activity of novel hybrid compounds between 2-alkylbenzofuran and
imidazole. Med. Chem. Res. 2014, 23, 1599–1611.
Sample Availability: Samples of some compounds are available from the authors.
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