Dicyanopyrazine-derived push-pull chromophores for highly efficient photoredox catalysis

Article abstract of DOI:10.1039/c4ra05525j

Here, we report dicyanopyrazine (DPZ)-derived push-pull chromophores, easily prepared and tunable organic compounds, as new kinds of photoredox catalysts. In particular, the DPZ derivative H, containing 2-methoxythienyl as electron-donating moiety, exhibits a broad absorption of visible light with an absorption edge up to 500 nm and excellent redox properties, and has been demonstrated as a desirably active and efficient photoredox catalyst in four challenging kinds of photoredox reactions. The amount of catalyst in most reactions is less than 0.1 mol% and even 0.01 mol%, representing the lowest catalyst loading in the current photoredox organocatalysis. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra05525j

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PAPER
Dicyanopyrazine-derived push–pull chromophores
for highly efficient photoredox catalysis†
Cite this: RSC Adv., 2014, 4, 30062
Yu Zhao,‡a Chenhao Zhang,‡a Kek Foo Chin,c Oldrich Pytela,b Guo Wei,a
ˇ
a
b
a
*
*
Hongjun Liu, Filip Bures and Zhiyong Jiang
ˇ
Here, we report dicyanopyrazine (DPZ)-derived push–pull chromophores, easily prepared and tunable
organic compounds, as new kinds of photoredox catalysts. In particular, the DPZ derivative H, containing
2-methoxythienyl as electron-donating moiety, exhibits a broad absorption of visible light with an
absorption edge up to 500 nm and excellent redox properties, and has been demonstrated as a desirably
active and efficient photoredox catalyst in four challenging kinds of photoredox reactions. The amount
of catalyst in most reactions is less than 0.1 mol% and even 0.01 mol%, representing the lowest catalyst
loading in the current photoredox organocatalysis.
Received 15th May 2014
Accepted 30th June 2014
DOI: 10.1039/c4ra05525j
www.rsc.org/advances
more likely reduce the cost and the potential contamination of
target products with toxic heavy metals;14 (2) tunable structure.
Introduction
The easily modied structure of photocatalyst will benet to
optimize the photophysical properties, such as the absorption
edge and intensity in the visible range as well as the redox
potentials. In comparison with the well-established metal
complexes, the organic photoredox catalyst with tunable struc-
ture is still decient;20–22 (3) high activity and efficiency. A
universal substance able to catalyse a variety of challenging and
unprecedented photoredox reactions is always desirable, which is
determined by the activity of photocatalyst. Meanwhile, the
amount of the catalyst loaded in the reaction is always an
important index to evaluate its efficiency. To the best of our
knowledge, few examples23–25 with less than 0.1 mol% of the
catalyst were presented to date unless using high power light
sources17,26 or in ow.27–29 Most important, the example of pho-
toredox reaction with less than 0.1 mol% organic catalyst is not
reported. In principle, the activity and efficiency of photocatalyst
could be improved by optimizing its photophysical properties.
Towards designing new reactions, mild and green conditions
are always favoured to minimize energy consumption on the
standpoint of green chemistry.1 In this context, photoredox
catalysis (photo-induced single electron transfer, SET), which
utilizes visible light as a naturally abundant and clean energy
source and does not need any special instrumentation nor
apparatus, has attracted considerable attention from synthetic
organic chemists.2–8 In particular, the visible light absorbing
photoredox catalysts, which are able to transform solar to
chemical energy, play the most important role in photocatalytic
reactions.
In recent years, various readily available compounds, such as
polypyridyl complexes of ruthenium9,10 and iridium11,12 as well as
organic dyes,13,14 have been utilized as the photoredox catalysts.
Other designed photocatalysts with tailored structures include
platinum(II),15 gold(III),16 palladium(II)17 and copper(II)18,19
complexes and acridinium ions20,21 (Fig. 1). Obviously, the pho-
toredox catalyst simultaneously equipped with the following
features still remains highly desirable: (1) a readily available/
prepared organic compound. The organic compound should
aKey Laboratory of Natural Medicine and Immuno-Engineering of Henan Province,
Henan University, Kaifeng, Henan, 475004, P. R. China. E-mail: chmjzy@henu.edu.
cn; lip.bures@upce.cz
bInstitute of Organic Chemistry and Technology, University of Pardubice, Faculty of
´
Chemical Technology, Studentska 573, Pardubice, 53210, Czech Republic
cDivision of Chemistry and Biological Chemistry, Nanyang Technological University,
21 Nanyang Link, 637371, Singapore
† Electronic supplementary information (ESI) available: Procedures on the
preparation of DPZ derivatives, quantum chemical calculations, procedures for
photoredox catalysed reactions, characterization data of data and NMR spectra.
See DOI: 10.1039/c4ra05525j
‡ These authors contributed equally to this work.
Fig. 1 Selected examples of photoredox catalysts reported to date.
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donating (hetero)aromates at the pyrazine C5/C6 positions such
as in C–D or E–H (Fig. 2). The method used for the construction
of dicyanopyrazine moiety involves condensation of DAMN with
1,2-dicarbonyl compounds (benzils), affording DPZ derivatives
(C–G) in 89–92% yields. However, 5-methoxythienyl, as the most
electron-rich moiety, made this reaction sluggish and only 3%
yield of H was obtained. Our previous ndings30 prompted us to
attempt twofold cross-coupling reaction of 5,6-dichloropyr-
azine-2,3-dicarbonitrile with 5-methoxythiophen-2-ylboronic
acid pinacol ester (Fig. 2). Under the optimized conditions (see
the ESI†), this reaction afforded the desired product H in 83%
yield. Notably, the preparation of H in a gram scale could
conveniently be carried out via this efficient protocol (Fig. 2).
The properties of DPZ derivatives C–H were further studied
by electrochemical measurements, absorption spectra and DFT
calculations (Table 1, Fig. 3 and ESI†). The HOMO–LUMO
energies, their differences and the position of the longest-
wavelength absorption maxima can be considered as the most
important electronic parameters. The rst oxidation potential
respective the HOMO energy decrease with increasing electron
donating ability of the substituents and thus enhanced ICT. As a
result, the electrochemical gap (Eg) decreases from 3.70 to 2.82
eV. Likewise, the longest-wavelength absorption maxima lmax
shi bathochromically (Fig. 3) and the respective optical as well
as the calculated gaps EDg FT decrease signicantly. It is note-
worthy, that electrochemical, optical and calculated gaps show
the same trend and correlate very tightly (see ESI†). Hence, we
can consider both experiments and DFT calculation as reliable
tools describing the electronic parameters of C–H. As can be
seen, the electronic parameters of DPZ derivative H are excep-
tional. In contrast to G (Eg ¼ 3.27 eV; lmax ¼ 391 nm (3.17 eV),
EDg FT ¼ 3.37 eV, m ¼ 12.88 D), two additional methoxy substit-
uents in H lowered the electrochemical/calculated gap by 0.45/
0.39 eV, shied the lmax bathochromically by 57 nm and
increased the molar absorption coefficient as well as the ground
state dipole moment. Obviously, the electronic properties of
DPZ derivative H (Eg ¼ 2.82 eV, lmax ¼ 448 nm (2.78 eV), 3 ¼
21 500 molꢀ1 dm3 cmꢀ1, EDg FT ¼ 2.97 eV, m ¼ 18.26 D) are well
tuned and suited for the photoredox catalysis (e.g. Ru(bpy)3Cl2,
lmax ¼ 452 nm, Eg ¼ 2.62 eV). As expected, the HOMO and the
LUMO in H are clearly separated and localized on the donor and
acceptor parts of the molecule (Fig. 2, for C–G see ESI†).
Fig. 2 The structures and convenient/improved preparations of DPZ
derivatives A–H.
In 2012, push–pull molecules based on 5,6-disubstituted
pyrazine-2,3-dicarbonitrile (dicyanopyrazine, DPZ) were pre-
sented and their nonlinear optical properties were investigated
by our group (e.g. DPZ derivatives A–B, Fig. 2).30 These organic
compounds can be readily prepared from available starting
materials such as diaminomaleodinitrile (DAMN) in excellent
yields and represent stable D–p–A chromophores with intra-
molecular charge-transfer (ICT), which resembles the MLCT
(metal to ligand charge transfer) in aforementioned metal
complexes. Furthermore, their redox potentials are similar to
known photoredox catalysts (e.g. E1/2(red) of eosin Y: ꢀ1.0 V,
rhodamine B: ꢀ0.8 V, A: ꢀ1.24 V, B: ꢀ0.87 V vs. saturated calomel
electrode [SCE]) while the absorption maxima locate at the visible
light area (lmax of A: 471 nm, B: 499 nm). Most importantly, their
optical and electrochemical properties can readily be tuned by
the variation of the appended electron-donors. Based on these
features, we envisioned that DPZ derivatives could probably serve
as aforementioned photoredox catalysts.
Results and discussion
Preparation and properties of DPZ
In this respect, DPZ derivatives C–H were designed and
synthesized, possessing either electron withdrawing or
Table 1 Electrochemical, optical and calculated data for DPZs C–H
lmax [nm (eV)]/3 ꢁ 10ꢀ3d
[molꢀ1 dm3 cmꢀ1
]
EDg FTe [eV]
me [D]
a
b
b
c
DPZ
E1/2(ox1) [V]
E1/2(red1)a [V]
EHOMO [eV]
ELUMO [eV]
Eg [eV]
C
D
E
F
G
H
+2.30
+2.27
+1.90
+1.95
+1.92
+1.37
ꢀ1.40
ꢀ1.41
ꢀ1.44
ꢀ1.36
ꢀ1.35
ꢀ1.45
ꢀ7.10
ꢀ7.07
ꢀ6.70
ꢀ6.75
ꢀ6.72
ꢀ6.17
ꢀ3.41
ꢀ3.39
ꢀ3.36
ꢀ3.44
ꢀ3.45
ꢀ3.35
3.70
3.68
3.34
3.31
3.27
2.82
340(3.65)/15.3
345(3.59)/13.2
383(3.24)/19.7
379(3.27)/17.4
391(3.17)/14.6
448(2.78)/21.5
3.93
3.92
3.58
3.54
3.37
2.97
11.38
7.43
10.80
12.69
12.88
18.26
a
b
E1/2(ox1)/E1/2(red1) are half-wave potentials of the rst oxidation/reduction measured in DCM versus SCE. EHOMO/LUMO ¼ E1/2(ox1/red1) + 4.80.31
c
d
e
E
HOMO–ELUMO electrochemical gap. Longest-wavelength absorption maxima (optical gap)/molar absorption coefficient (DCM). DFT
calculated HOMO–LUMO gap and ground state dipole moment (B3LYP/6-311++G(2df,p)//B3LYP/6-311++G(2df,p), scrf
dichloromethane)).
¼
(solvent
¼
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results depend on the efficiency of the ICT in the used catalyst.
Reduced HOMO–LUMO gap, bathochromically shied lmax and
enhanced absorption intensity of the catalysts resulted in an
increased conversion, which is being superior with catalyst H.
The reaction could be nished with complete conversion about
5 hours even if the amount of H was decreased to 0.1 mol%
(Table 2, entry 9). DMF as solvent cleanly provided the desired
product 3a with the same reactivity (Table 2, entry 10).
The reactions of a variety of N-aryltetrahydroisoquinolines 1
with nitromethane 2a as solvent were subsequently attempted
under established conditions, providing the corresponding
adducts 3a–g in 87–95% yields within 5 hours (Table 3, entries
1–7). Importantly, 85% yield of 3a was attained within 24 hours
even when 0.01 mol% of H was used under the irradiation with
9 W uorescent lamp. The reaction seems to be faster when
irradiated by sunlight [Table 3, entry 1, see items (iii) and (iv) in
footnote (b)]. The reactions of 1a with other nitroalkanes (2b–c)
gave adducts 3h–i in excellent yields aer 5 hours (Table 3,
entries 8 and 9).
Fig. 3 UV-Vis absorption spectra of DPZ derivatives C–H in DCM (c ¼
2 ꢁ 10ꢀ5 M).
Next, we screened a series of CDC reactions between
different tertiary/secondary amines and various nucleophiles
under the established conditions (28 ꢂC, DMF as solvent,
ambient air and irradiation with 9 W uorescent lamp, Table 3,
DPZ derivatives in cross-dehydrogenative-coupling reactions
Since introduced by Stephenson and co-workers in 2010,32 the
visible light irradiated cross-dehydrogenative-coupling (CDC)
reaction of N-aryl-tetrahydroisoquinolines and nitroalkanes has
become the most favorable protocol to evaluate the efficiency of
catalysts.15,17,27,33–41 To probe the feasibility of DPZ derivatives in
photoredox catalysis, we initially performed the CDC reaction of
N-phenyl-tetrahydroisoquinoline 1a and nitromethane 2a in the
presence of 2.0 mol% A–H at 28 ꢂC in ambient air and irradiated
by a 9 W uorescent lamp (Table 2, entries 1–8). All DPZ
derivatives A–H could catalyse the reaction while H showed an
outstanding activity to access the desired adduct 3a with 96%
conversion in 2 hours (Table 2, entry 8). Obviously, the reaction
Table 3 Scope of the CDC reactions catalyzed by Ha
Table 2 Optimization of the reaction conditions of 1a and 2aa
Entry
DPZ catalyst (mol%)
T [h]
Conversionb [%]
1
2
3
4
5
6
7
8
A (2.0)
B (2.0)
C (2.0)
D (2.0)
E (2.0)
F (2.0)
G (2.0)
H (2.0)
H (0.1)
H (0.1)
2
2
2
2
2
2
2
2
5
5
12
7
10
8
38
49
76
96
100
100
9
10c
a
Detailed conditions for these reactions are described in the ESI.† b (i)
0.05 mol% of H was used, 9 W uorescent lamp, 28 ꢂC, t ¼ 12 h, yield ¼
86%; (ii) 0.01 mol% of H was used, 9 W uorescent lamp, 28 ꢂC, t ¼ 24 h,
yield ¼ 85%; (iii) 0.05 mol% of H was used, sunlight (30 ꢂC), t ¼ 3.5 h,
yield ¼ 83%; (iv) 0.01 mol% of H was used, sunlight (30 ꢂC), t ¼ 3.5 h,
yield ¼ 60%. c 0.15 mmol scale, DMF as solvent.
a
The reactions were performed with 0.05 mmol of 1a in 0.5 mL of 2a in
the presence of catalyst at 28 ꢂC irradiated by a 9 W household
uorescent lamp. Determined by 1H NMR of crude reaction mixture.
b
c
0.05 mmol of 1a and 0.5 mmol of 2a in 0.5 mL DMF under the given
reaction conditions.
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entries 10–15). The reaction27,35,36,41–43 of N-phenyl-tetra-
hydroisoquinoline 1a and TMSCN 2d was rstly processed in
the presence of 0.1 mol% of H, and the oxidative Strecker
adduct 3j was obtained within 2 hours in 87% yield (Table 3,
entry 10). With the same catalyst loading and the help of pyr-
rolidine/TFA, the powerful catalytic ability of H was further
demonstrated by the reaction16,33,36,38,40,44 of 1a and acetone 2e,
affording Mannich adduct 3k in 93% yield aer 4 hours (Table
3, entry 11). To introduce a challenging hetero-quaternary
stereogenic center adjacent to N-aryl-tetrahydroisoquinolines,
we attempted the rst example of oxidative Mannich reaction of
1a and 5H-oxazol-4-one 2f via photoredox catalysis (Table 3,
entry 12). It was discovered that, in the presence of only 0.01
mol% of H, the desired adduct 3l was obtained in 82% yield
with 10 : 1 dr aer 4 hours. Use of 0.01 mol% of H as catalyst
also resulted in the complete reaction34,36,45 between 1a and
diethyl phosphonate 2g, to furnish the P–C bond formation and
afford the product 3m in 81% yield within 7 hours (Table 3,
entry 13).
Fig. 4 Test for the production of hydrogen peroxide in the reaction
mixture of 1a and 2a. (a) KI and starch in glacial acetic acid, colourless.
(b) The reaction mixture of 1a and 2a after reaction completed, brown.
(c) Addition of solution (b) to solution (a), dark red. (d) Addition of 30%
of H2O2 in solution (a). KI (0.2 M), aqueous acetic acid (0.2 M), starch (4
mg mLꢀ1), DPZ H (0.1 mmol, 14.14 mL, 2.5 mg mLꢀ1 in CH3NO2), 1a (0.1
mmol), CH3NO2 (1.0 mL), reaction time: 5 h, 28 ꢂC.
CH3CN and CD3CN solvent. The results of the kinetic
Over the past few years, the CDC reaction32,34,40 of N-arylpyr- measurements of the product conversions are indicated in
rolidines and nitromethane has been attempted in photoredox Fig. 5. From these dependencies we can conclude that singlet
catalysis under different conditions; however, the reported oxygen was likely not a key participant in the reaction as no
signicant increase in the reaction rate was observed in CD3CN.
Mechanistically, we suggest a plausible mechanism on the above
CDC reactions with DPZ derivative H as catalyst (Fig. 6), which is
reactions were always sluggish with poor yields, and N-arylpyr-
rolidines were thus recognized as the non-activated amines.
Hence, we were intrigued to conduct the formidable reaction of
N-(4-tert-butylphenyl)pyrrolidine 4 and nitromethane 1 (Table 3, a photoredox protocol as Stephenson previously disclosed for
entry 14). We were pleased that the desired adduct 5 could be the ruthenium or iridium complexes.32 This reaction mechanism
obtained in 76% yield aer 45 hours when 2.0 mol% of H was involves excitation of the DPZ H catalyst by visible light, reduc-
used. Due to the longest-wavelength absorption maxima of H tive quench (single-electron transfer, SET) from the amino-
reaching blue light area, the reaction was also tested under the substrate to form radical cation/anion R3Nc+ and DPZ Hcꢀ.
Subsequent reaction with the nitroalkane or eventually oxygen
([O]) regenerates the H-catalyst and formed [O]cꢀ deprotonates
the trialkylammonium radical cation R3Nc+ to iminium salt,
irradiation of blue Light Emitting Diode (LED, 4 W, house-
hold), and completed in 15 hours to deliver adduct 5 in 80%
yield. On the other hand, linear secondary amines exhibit very
poor activity in photocatalytic CDC reaction, and the singlet which undergoes nucleophilic attack of the nucleophile used.
oxygen protocol combined with a special photoreactor seems
On the basis of the above observations and the proposed
necessary.29 Accordingly, we turned our attention to the reaction mechanism, we can deduce the following fundamental struc-
of dibenzylamine 6 and TMSCN 2d (Table 3, entry 15). It was ture–catalytic activity relationships:
found that the reaction could be nished smoothly in 17 hours
by using 0.1 mol% of H, to afford the adduct 7 in 84% yield.
ꢃ Bathochromically shied absorption maxima (lmax), high
molar absorption coefficient (3) and small HOMO–LUMO gap
Mechanism of the photochemical CDC reactions
We were also interested in fundamental principles of DPZ
action in photoredox catalyzed reactions. In the above CDC
reactions, H2O2 can be envisaged as a reasonable by-product. As
Fig. 4 shown, hydrogen peroxide could clearly be detected in the
crude reaction mixture of 1a and 2a catalysed by H. The pres-
ence of H2O2 was conrmed by the iodide test (KI in glacial
acetic acid), in which a colour change to dark red was detected.
Considering three plausible pathways, such as singlet-oxygen
oxidation, photoredox reaction or both of these two reactions, we
subsequently conducted the study on solvent effects since the
lifetime of singlet oxygen highly depends on the solvent used.
The singlet oxygen lifetime should be signicantly longer in
deuterated solvents in comparison to their protonated counter-
parts.46–48 Therefore, a reaction of 1a and 2a was performed in
Fig. 5 Photocatalytic CDC reaction of 1a and 2a (or 2a-d3) to give
adduct 3a in the presence of DPZ derivative H; reaction kinetics was
monitored by using 1H NMR spectroscopy; CH3NO2 (-), CD3NO2 ( );
DPZ H (0.05 mmol, 7.0 mL, 2.5 mg mLꢀ1), 1a (0.5 mmol), CH3NO2 or
ꢂ
the presence of 0.1 mol% of DPZ derivative H at 25 C in both CD3NO2 (0.5 mL), reaction time: 6 h, 25 ꢂC.
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Fig. 6 Plausible mechanism of CDC reaction catalyzed by DPZ H.
(Eg) of the catalyst improve its excitation ability by low-energy
visible light (compare for H and C–G, Table 1).
Fig. 7 Photocatalytic oxidation, oxidative hydroxylation and reductive
dehalogenation.
ꢃ Principal changes in the properties of DPZ derivatives C–H
are caused by the donor-centered HOMO (EHOMO range from
ꢀ7.10 to ꢀ6.17 eV, Table 1).
2.0 mol% of Ru complex as catalyst, in which 36 W uorescence
ꢃ The LUMO localized on the dicyanopyrazine acceptor lamp was used as light source.51 Moreover, Acid Red 87, the
remained almost unaltered in all derivatives (ELUMO range from organic compound, has also been demonstrated as the photo-
ꢀ3.45 to ꢀ3.39 eV, Table 1).
redox catalyst, but the reaction became slower.51 Scaiano
group52 gave a metal-free example of oxidative hydroxylation of
aryl boronic acids with methylene blue as catalyst; the catalyst
ꢃ Reduced HOMO–LUMO gap of the catalyst facilitates the
generation of radicals (SET, Fig. 6).
ꢃ The conversion of CDC reaction depends linearly and amount could be decreased to 1.0 mol% and the reactions
tightly on the electrochemical as well as optical HOMO–LUMO became faster, but two 90 W warm white LEDs as the light
gaps of the used X-shaped push–pull DPZs (see the correlation source and pure oxygen atmosphere were necessary. Therefore,
in Fig. S12 in the ESI†).
we investigated this demanding reaction with the DPZ deriva-
ꢃ Planar and polariable p-system of the catalyst help to tive H as catalyst. As eqn (2) shown (Fig. 7), the oxidative
stabilize the radical anion e.g. DPZ Hcꢀ (Fig. 6) through hydroxylation of 4-methoxy-phenylboronic acid 10 was per-
delocalization.
formed with 1.0 mol% of H in ambient air and irradiated by a 9
W uorescent lamp, and the desired product 11 was obtained in
ꢃ Well-tuned ICT helps to reduce HOMO–LUMO gap (see
charge separation in H, Fig. 2) but does not cause high electron 96% yield aer 13 hours. The reaction could be accelerated by
saturation of the cyano acceptors.
utilizing 4 W LEDs as the light source, in which the product 11
ꢃ Higher electron saturation of the cyano groups would have could be obtained in 92% yield within 4.5 hours.
detrimental effect on the formation of the radical anion e.g. DPZ
Hcꢀ (Fig. 6).
We subsequently explored the reductive dehalogenation of a-
bromoacetophenones. In comparison with the excellent work
ꢃ These suppositions can be demonstrated as follows: DPZ by Zeitler and co-workers [2.5 mol% eosin Y, 1 W high-power
LEDs (l ¼ 530 nm), 18 h, 83% yield],14 the reductive dehaloge-
nation of bromoacetophenone 12 could be nished within 1.5
bathochromically shied absorption maxima than H, but their hours in the presence of 0.1 mol% of H under the irradiation by
catalytic activity is very low (Table 2). This is most likely caused a 9 W house-hold uorescent lamp, to access the product 13 in
by high D–A coupling between NMe2 and CN groups and thus 87% yield (Fig. 7, eqn (3)).
higher contribution of the quinoid/zwitterionic form with
derivatives bearing strong electron NMe2 donors A (Eg ¼ 2.26
eV) and B (Eg ¼ 1.91 eV) showed lower Eg values and more
negatively charged cyano groups (see Fig. S13 in the ESI†).
Conclusion
H catalyses other challenging reactions
Dicyanopyrazine-derived (DPZ) push–pull chromophores have
To further evaluate the performance of DPZ derivative H, other been demonstrated as a new kind of organic catalysts for pho-
challenging or unprecedented photoredox reactions were toredox catalysis. 2-Methoxythienyl as an electron-donating
successively attempted (Fig. 7). We rstly carried out an oxida- moiety provides a DPZ derivative (H) with a broad absorption
tion of N-methyl-2-(phenylamino)acetamide 8 using 1.0 mol% edge reaching up to 500 nm as well as the excellent redox
of H in DMF at 28 ꢂC under oxygen atmosphere and irradiation properties. Moreover, all the aforementioned features are well
by a 9 W uorescent lamp (eqn (1)). We found that the reaction balanced and enable the DPZ derivative H to become the
was nished within 7 hours to give unsymmetric oxalamide 9. desirably active and efficient photoredox catalyst, which was
Indeed, despite the well-established photocatalytic oxidation of demonstrated by more than four kinds of photocatalytic organic
tertiary amines,49,50 the transformation of activated secondary reactions. Without the aid of high power light sources or in
amine to amide has not been reported yet.
ow, the amount of catalyst in most reactions is always less than
In 2012, Jørgensen and Xiao and co-workers introduced the 0.1 mol% and even 0.01 mol%, which represents the lowest
rst oxidative hydroxylation of aryl boronic acids by utilizing catalyst loading in the current photoredox organocatalysis.
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Other excellent performances on the DPZ derivative H-catalyzed 24 J. D. Nguyen, E. M. D'Amato, J. M. R. Narayanam and
reactions include lower power light source, air atmosphere C. R. J. Stephenson, Nat. Chem., 2012, 4, 854.
while short reaction time and higher yields. Further investiga- 25 M. A. Ischay, M. S. Ament and T. P. Yoon, Chem. Sci., 2012, 3,
tions, which involve the use of DPZ derivatives as catalyst in 2807.
novel photocatalytic reactions, are ongoing in our labs and will 26 S. Samanta, S. Das and P. Biswas, J. Org. Chem., 2013, 78,
be reported in due course.
11184.
27 J. W. Tucker, Y. Zhang, T. F. Jamison and C. R. J. Stephenson,
Angew. Chem., Int. Ed., 2012, 51, 4144.
28 M. Neumann and K. Zeitler, Org. Lett., 2012, 14, 2658.
Acknowledgements
This research was supported by the Czech Science Foundation 29 D. B. Ushakov, K. Gilmore, D. Kopetzki, D. T. McQuade and
(13-01061S), NSFC (nos. 21072044, U1204207) and the Program
P. H. Seeberger, Angew. Chem., Int. Ed., 2014, 53, 557.
ˇ
ˇ
´
´
´
for New Century Excellent Talents in University of Ministry of 30 F. Bures, H. Cermakova, J. Kulhanek, M. Ludwig, W. Kuznik,
ˇ
˚ˇ
Education (NCET-11-0938).
I. V. Kityk, T. Mikysek and A. Ruzicka, Eur. J. Org. Chem.,
2012, 529.
31 B. W. D'Andrade, S. Datta, S. R. Forrest, P. Djurovich,
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