Photoredox radical conjugate addition of dithiane-2-carboxylate promoted by an iridium(III) phenyl-tetrazole complex: a formal radical methylation of Michael acceptors

Article abstract of DOI:10.1039/c6sc03374a

A readily accessible iridium(iii) phenyl-tetrazole complex ([Ir(ptrz)₂(tBu-bpy)]⁺, 2; Hptrz = 2-methyl-5-phenyl-tetrazole; tBu-bpy = 4,4′-di-tert-butyl-2,2′-bipyridine) is shown to be a versatile catalyst for a new photocatalytic Michael reaction. Under light irradiation in the presence of 2, a dithiane 2-carboxylic acid, obtained by simple hydrolysis of a commercially available ethyl ester, generates a 1,3-dithiane radical capable of performing addition to a variety of Michael acceptors (e.g., unsaturated ketones, esters, amides and malonates). This broad scope reaction with high yields is a formal photo-redox addition of the elusive methyl radical and the adducts obtained can be starting materials for a variety of functionalized products. The excited-state oxidation potential of catalyst 2 allows selective formation of radicals only from α-heterosubstituted carboxylates. Chemical modification of this metal complex can tune the electrochemical properties, opening a route to new highly selective catalytic photo-oxidation reactions.

Full text of DOI:10.1039/c6sc03374a

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Photoredox Radical Conjugate Addition of Dithiane-2-carboxylate
Promoted by an Iridium(III) Phenyl-tetrazole Complex: a Formal
Radical Methylation of Michael Acceptors
Received 00th January 20xx,
Accepted 00th January 20xx
Andrea Gualandi,a Elia Matteucci,b Filippo Monti,c Andrea Baschieri,b Nicola Armaroli,c* Letizia
Sambri,b* and Pier Giorgio Cozzia*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A readily accessible iridium(III) phenyl-tetrazole complex ([Ir(ptrz)2(tBu-bpy)]+, 2; Hptrz= 2-methyl-5-phenyl-tetrazole; tBu-
bpy= 4,4′-di-tert-butyl-2,2′-bipyridine) is shown to be a versatile catalyst for a new photocatalytic Michael reaction. Under
light irradiation in the presence of 2, a dithiane 2-carboxylic acid, obtained by simple hydrolysis of the commercially
available ethyl ester, generates a 1,3-dithiane radical capable to perform addition to a variety of Michael acceptors (e.g.,
unsaturated ketones, esters, amides, malonates). This broad scope reaction with high yields is a formal photo-redox
addition of the elusive methyl radical; the adducts obtained can be starting materials for a variety of functionalized
products. The excited-state oxidation potential of catalyst 2 allows selective formation of radicals only from α-
heterosubstituted carboxylates. Chemical modification of this metal complex can tune the electrochemical properties,
opening the route to new highly selective catalytic photo-oxidation reactions.
acting as a strong oxidant in the first step of the photocatalytic
cycle.
Introduction
Photoredox catalysis has recently emerged as a mild and effi-
cient method for the generation of radicals, by taking ad-
vantage of the photophysical properties1 of suitable organic
dyes2 and transition metal complexes exhibiting tailored elec-
trochemical and photophysical properties.3,4 In particular,
MacMillan et al. investigated 1,4-conjugate additions (Michael
reaction)5 of radicals6 in connection with a series of electro-
philic olefins. In these reactions a photoredox-mediated CO2-
extrusion mechanism is operative and a broad array of Michael
Me
CF3
BF4
F
N
N
PF6
N
tBu
tBu
N
Ir
N
N
N
N
N
F
F
Ir
N
N
tBu
tBu
N
F
N
N
CF3
Me
acceptors have been used. Simple or
α-functionalized (N; O)
1
2
carboxylic acids are employed as Michael donors without the
need for organometallic mediated activation or propagation.
Recently, Overman et al. have also explored N-phthalimidoyl
oxalate derivatives of tertiary alcohols for the reductive cou-
pling of tertiary radicals with Michael acceptors, using visible
light and [Ru(bpy)3][PF6]2.7 The key relevant characteristics of
both the Overman and MacMillan radical generation method-
ologies were recently merged in a new powerful protocol.8 The
typical procedure entails the addition of 1-2 mol% of photo-
Figure 1. Structure for the iridium complexes 1 and 2.
Although these methodologies are extremely effective and use
cheap and abundant starting materials, there are still some
inherent limitations. Primarily, the commercially available irid-
ium photocatalyst
1 is expensive and for its preparation a Su-
zuki coupling is necessary.9 Then, the generation of primary
radicals by photocatalytic conditions after extrusion of CO2 is
challenging, and just one example where primary radicals are
intercepted by fluorinating agents is reported in literature.10
catalyst Ir[dF(CF3)ppy]2(tBu-bpy)[PF6] (1, Figure 1), which is
H
owever, to the best of our knowledge, the generation and
the reaction of primary radicals is still an open issue in photo-
catalytic Michael reactions. Furthermore, the addition of me-
thyl radical, recently reported by a keen methodology in
Minisci’s type reaction11,12 is still not developed in photocataly-
sis. In the above described context, the present study has two
main goals: (i) to demonstrate the use of an alternative, simply
prepared and tunable iridium(III) photocatalyst (2, Figure 1) for
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radical reactions, and (ii) to propose a new methodology for
effective photocatalytic methylation.
showed an efficiency comparable to catalyst 1. These com-
pounds were recently reported by some of us22 as emitting
materials in electroluminescent devices and here we show that
they can be successfully utilized also to promote photocatalyt-
ic reactions.
Notably,
Baran
has
recently
shown
that
zinc
bis[(phenylsulfonyl)methanesulfinate] can be effectively used
for introducing a methyl group through a two-steps synthetic
procedure, in which a radical mediated process is involved in
the first step.13 As a possible synthetic equivalent for a methyl
group, we propose herein the chameleonic dithiane group,
opening the route to the use of dithianecarboxylate as Michael
donor in photoredox catalysis. The formation of a secondary
radical, stabilized by the presence of two sulphur atoms can be
used as a versatile synthetic equivalent. Remarkably, the dithi-
ane can be not only transformed in a methyl group by treat-
ment with Raney®-nickel, but it is also possible to take ad-
vantage of its flexibility, allowing the installation of different
functional groups such as aldehydes and ketones, upon the
radical reaction.
Scheme 1. Optimized reaction conditions.
Notably, by careful tuning of the pristine ligand structure, this
class of complexes may exhibit remarkable opportunities for
enhancing the reaction selectivity (vide infra). The photoredox
radical conjugate addition of dithiane-2-carboxylate has been
used to test our approach, giving full conversions and high
isolated yields, when the reaction is conducted in DMSO and in
the presence of K2HPO4 as inorganic base (see ESI for details).
MacMillan has shown that the presence of the latter is crucial
to form the carboxylate, allowing the oxidation and afford the
release of CO2; the use of organic bases gave poorer results.
Among all iridium catalysts tested (see ESI for full detail, and
Optimization of the photocatalytic reaction and
scope
Dithianes, introduced by Corey and Seebach14 are widely used
in the synthesis of natural products.15 For instance, they were
exploited by Smith for linchpins applications to the synthesis of
complex natural molecules.16 However, the radical reactivity of
dithianes has been rarely reported in the literature, mainly
through the installation of a radical initiator group (phenylse-
leno, xanthates, TEMPO, chloro) at the 2-position for the gen-
eration of C-2 centred radicals.17 Direct radical addition of 1,3-
dithianes to alkenes was shown to occur in an intramolecular
fashion. Quite interestingly, Nishida and co-workers reported
an intramolecular photocatalytic addition for the 1,3-dithiane,
and this reaction was also reported with other radical intia-
tors.18 Recently, Leow and co-workers reported the photocata-
lytic addition of 1,3-benzodithioles to several Michael accep-
tors.19 However, only aryl or alkyl C2-substituted benzodithiole
could be used, while aryl dithiane was found completely unre-
active. On the other hand, Koike and Akita reported the reac-
tion of potassium 1,3-dithian-2-yl trifluoroborate with terminal
olefins bearing electron withdrawing groups20 as a synthetic
equivalent of a carbonyl groups.
for results obtained with the catalyst
1), complex [Ir(ptrz)2(tBu-
bpy)][BF4] (2, Figure 1) gave the better results in the optimized
reaction conditions (Scheme 1) also proving to be highly pho-
tostable in DMSO after prolonged irradiation (see ESI). Specifi-
cally, complex
2 is easily obtained through a two-steps synthe-
sis22 involving a facile silver-assisted cyclometalation reaction
of 2-methyl-5-phenyl-2H-tetrazole with IrCl3 and, notably, the
solvato complex intermediate can be a useful precursor for
other properly designed iridium(III) complexes.
To support the general validity of our approach, we investigat-
ed a variety of Michael acceptors and the results obtained are
summarized in Scheme 2. Differently substituted Michael ac-
ceptors can participate in the dithiane conjugate addition pro-
tocol. The mild reaction conditions reported in Scheme 2 are
compatible with a wide range of functional groups (e.g., malo-
nates, esters, amides, ketones, aldehydes, sulfones, etc.), and
provide a versatile group for further functionalization and
transformation. Unsaturated ketones and aldehydes are well
tolerated in both cyclic and acyclic form (e.g., products 5n-q).
In addition, this protocol could be further applied to other
electrophilic alkenes, including α,β-unsaturated amides, sul-
fones, and malonates, as well as acrylates and fumarates, to
afford a variety of alkylated dithianes in good to excellent
yields. In term of substituents present in the Michael accep-
tors, β-substituents are tolerated, as well as α-aryl and α-alkyl
groups. It is also worth noting that the tailored synthesis of 2-
Based on the previous work by MacMillan5 and on the basis of
the recent report on the practical use of commercially availa-
ble dithiane carboxylate in organocatalytic reactions,21 we took
this cheap and commercially available starting material as a
suitable reagent for installing various unsubstituted or substi-
tuted dithianes by photocatalytic reactions. Accordingly, we
have carefully investigated the addition of dithiane carboxylate
(
4a) to alkyliden malonate (3a) (Scheme 1) in the presence of
different photocatalysts, by varying solvent and other reaction
conditions (see ESI for all complexes tested and full details).
Although the commercially available iridium catalyst
1 was
found to be effective, we have investigated the catalytic activi-
ty of alternative iridium complexes, which can be more easily
prepared, such as phenyl-tetrazole and pyridyl-triazole deriva-
tives (see ESI), without any Suzuki coupling. Only complexes
containing phenyl tetrazoles as cyclometalating ligands
2 | Chem. Sci., 2016, 00, 1-3
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R2
2 (1 mol%)
R2
R1
R4
S
R3
R4
K2HPO4 (1 equiv.)
S
R1
R3
+
COOH
DMSO (0.5 M)
O
S
O
S
Blue-Led, rt, 16-24 hours
5b-t
3a-q (2 equiv.)
4a-c (1 equiv.)
O
OMe
O
OEt
O
O
OEt
O
S
Ph
OMe
Ph
S
OEt
EtO
OEt
OEt
OEt
EtO
O
O
O
O
O
S
S
S
S
S
S
S
S
5f, 49%
5c, 85%
5d, 84%
5e, 91%
Ar
5b, 87%
O
Ph
S
OMe
OMe
OMe
OtBu
O
Ph
S
NH2
MeO
S
O
O
O
O
S
S
S
S
S
S
5h, 79%
S
5g, 90%
5i, 39%
5l, 45%
Ar = Ph, 5j, 90%
Ar = 4-Me-C6H4, 5k, 88%
Ph
O
Ph
SO2Ph
O
O
S
O
S
S
S
S
S
S
S
S
5o, 76%
5p, 93%
5m, 75%
5n, 85%
S
5q, 85%
Ph
S
Ph
Ph
OMe
Ph
Ph
Ph
O
O
S
Ph
O
S
S
S
S
5r, 89%
Scheme 2. Scope of the reaction with different Michael-acceptor derivatives.
5s, 87%
5t, 86%
substituted dithianes-2-carboxylate are possible (see ESI for tuted or unsubstituted dithiane can be used for further modifi-
details) with a wide (e.g., products 5r-t). In addition, it was cations.
possible to scale up the reaction without any problem from 0.2
mmol up to 2.4 mmol.
This protocol avoids the use of strong bases and allows the
direct addition of a versatile dithiane under very mild reaction
conditions. The methodology, by coupling the reaction with a
Raney®-nickel desulfurization realized on the crude reaction
mixture, gave a direct access to the corresponding methyl
group in a straightforward way (Scheme 3). When ketones are
present, the direct treatment can lead to their reduction, as
observed for 5q and 5s. A moderate to good diastereoselec-
tion is observed for the reaction. On the other hand, it is pos-
sible to transform the dithiane into the corresponding carbonyl
group by an easily deprotection reaction with well-established
conditions23 as it is reported with selected examples, giving the
desired products in high yields. (Scheme 4). Eventually, substi-
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Scheme 5. Selective generation of radical from carboxylate using the iridium
photocatalyst
2
Electrochemical and photophysical studies of the
catalytic system
Scheme 3. Introduction of methyl group by reaction with Raney®-nickel.
The photo-oxidation selectivity of complex
2, compared to the
iridium catalyst 1, can be easily understood by comparing the
excited-state redox potentials, estimated by combining photo-
physical and electrochemical data (see ESI for details).
The redox potentials of the two above mentioned iridium pho-
tocatalysts (i.e.,
1 and 2), together with those of the tetrabu-
tylammonium carboxylates of selected carboxylic acids (Figure
2) are gathered in Figure 2, Figure S3 and Table 1.
Figure 2. Tetrabutylammonium carboxylates used in electrochemical characteri-
zation
The electrochemical and photophysical experiments were car-
ried out on carboxylates – instead of pristine carboxylic acids –
because only after deprotonation by K2HPO4, the acid deriva-
tives are oxidized by the iridium photocatalyst, leading to CO2
release and their subsequent radical addition on Michael ac-
ceptors.5,6,7 The tetrabutylammonium was selected as counter-
ion to increase the solubility of the carboxylates in acetonitrile
and in order to have the same cation of the supporting electro-
lyte used for the electrochemical experiments.25
Scheme 4. Oxidative deprotection of dithiane adducts.
Photocatalyst selectivity
An interesting feature of the catalyst
oxidation of substrates. In fact, while the photocatalyst
able to discriminate between functionalized or unfunctional-
ized carboxylic acids, the complex is able to selectively oxi-
dize only -functionalized acids (e.g., 4a and 15 in Scheme 5).
2
is its selectivity towards
1
is not
The oxidation potential of the unfunctionalized cyclohex-
anecarboxylate 18 is approx. 0.2 V higher, if compared to that
of the more electron-rich heterocyclic compounds 17 and 19
(see Table 1). Therefore, a stronger oxidant is required to acti-
vate 18, while the two functionalized derivatives 17 and 19 are
more prone to oxidation.
2
α
Accordingly, no photo-generated radicals can be formed from
unfunctionalized derivatives like, for instance, cyclohexanecar-
boxylic acid (13 in Scheme 5).24
4 | Chem. Sci., 2016, 00, 1-3
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plex 1
, having E* ≈ + 0.7 V (i.e., well above the oxidation po-
red
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
tential of 18, see Table 1).
Stern–Volmer experiments
This scenario is also corroborated by the Stern–Volmer
quenching experiments carried out to explore in more detail
the intramolecular reductive quenching of the photocatalyst
phosphorescence operated by the carboxylate substrates. The
results are summarized in Table 2 and depicted in Figure 3. As
expected, both
1 and 2 are quenched by the presence of 17
due to a bimolecular quenching process having approximately
the same rate constant for both photocatalysts (kq ≈ 7·108 M–1
s–1, see Table 2).
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Applied potential vs. Fc+/Fc / V
Figure 3. Square-wave voltammograms of 17 (orange), 18 (black) and 19 (light blue)
carboxylate derivatives, 1 (green) and 2 (dark blue) iridium photocatalysts, and the
archetypal Michael acceptor 3a (red); sample concentration: 1 mM. Experiments were
carried out in room-temperature acetonitrile solutions recorded at a scan rate of 100
mV s–1 with a square-wave amplitude of 20 mV and a frequency of 25 Hz.
On the other hand, the unsubstituted carboxylate 18 is able to
quench the excited state of
1
with a kq that is more than three
(i.e., 1.0 vs. 0.3 108 M–1 s–1,
times higher than in the case of
see Table 2).
2
Table 1. Electrochemical data determined by cyclic voltammetry and square-
Table 2. Stern–Volmer experiments performed with the photocatalystsa
wave voltammetry.
Photocatalyst 1 a
Photocatalyst 2 a
c
d
d
Eox
Ered
[V]
ꢀEredox
[V]
E*
E*
red
ox
b
b
KSV
[mM–1
kq
KSV
[M–1
kq
[V]
[V]
---
---
---
---
[V]
---
---
---
---
]
[108 M–1 s–1
6.4 ± 0.2
]
]
[108 M–1 s–1
]
17
18
19
3ab
1
+ 0.39 a
+ 0.58 a
+ 0.37 a
---
---
---
17
18
3a
1.56 ± 0.05
0.89 ± 0.07
7.6 ± 0.6
---
---
0.24 ± 0.03
1.0 ± 0.1
0.039 ± 0.006
0.32 ± 0.05
---
---
Quenching not observed. c
Quenching not observed. c
– 2.42 b
– 1.73
– 1.89
---
a All the experiments were carried out in oxygen-free acetonitrile at 298 K with a
+ 1.32
+ 1.11
3.05
3.00
≈ – 1.1
≈ – 1.2
≈ + 0.7
≈ + 0.4
2
photocatalyst concentration of 0.015 mM and exciting at 330 nm. Data are re-
ported ± 95% confidence interval. In all the cases, the quality of the fitting is
aAll the measure were performed in room-temperature acetonitrile solution + 0.1
M TBAPF6. All redox potentials are referred to the ferrocene/ferrocenium couple,
b
assured by a R2 > 0.98. kq = KSV
/
τ0 , where τ0 is the unquenched excited-state
c
lifetime of the photocatalyst. There is no evidence of correlation between the
excited-state quenching of the photocatalyst and the increasing amounts of 3a up
to a concentration of 4 mM.
used as internal standard. bIrreversible redox process with an estimated error of ±
0.05 V. cꢀEredox = Eox – ꢀEred
.
E* = Eox – E00 and E* = Ered + E00 , where E00 is
d
ox red
the energy gap between the ground and excited states determined spectroscopi-
cally using data reported in Figure S4. Estimated error: ± 0.1 V.
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
It is worth noting that, despite both iridium complexes have
virtually the same redox gap (of around 3 eV), the oxidation
and reduction processes in
2
occur at more positive potentials
complex.
(approx. + 0.2 V) if compared to the polyfluorinated
1
This is due to the lack of electron-withdrawing fluorine sub-
stituents in the new tetrazole-based complex. It is well-known
that iridium-based photocatalysts (e.g.,
oxidizing species once excited to their lowest excited state.5,6,7
The excited-state redox potential of these molecules (E*
1 and 2) are strong
)
red/ox
can be roughly estimated using a simplified version of the so-
called Rehm-Weller equation:26
0.0
0.5
1.0
1.5
2.0
[X] / mM
2.5
3.0
3.5
4.0
E*red = Ered + E00 and E*ox = Eox – E00
Figure 4. Stern–Volmer plots showing the quenching of the excited-state lifetime of 1
(full circles and lines) and 2 (empty squares and dashed lines) photocatalysts in the
presence of increasing amounts of 17 (orange) or 18 (black) carboxylate derivatives.
Notably, no quenching is observed if the Michael acceptor 3a (red) is added to both
iridium complexes. Experiments were carried out in oxygen-free acetonitrile solution at
298 K with a photocatalyst concentration of 0.015 mM, exciting at 330 nm.
where E00 is the energy gap between the ground and excited
states determined spectroscopically (see Figure S4 for details
about its determination). Despite this approach can lead only
to estimates with uncertainties of 0.1 V or more,27 the E*
red
values of
1 and 2 (Table 1) do provide an explanation for the
As shown in Figure 4, the excited-state lifetime of both (
1) and
photo-oxidation selectivity of complex
2
, compared to the
. In fact, while the
2 is not high enough to
(
2) is virtually not affected by the presence of the Michael ac-
commercially available iridium catalyst
1
ceptor 3a (at least for concentrations up to 3.75 mM). This
excited-state reduction potential of
evidence is in accordance with the electrochemical data re-
oxidize 18 (i.e., + 0.4 V < + 0.58 V), this is not the case for com-
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ported in Table 1, showing that the excited-state redox poten-
tials of both the iridium photocatalysts cannot to promote any
redox process on 3a
.
Determination of quantum yield.
In order to estimate the efficiency of our photocatalyst
2 and
to assess the potential presence of an important radical chain
contribution to the catalytic cycle, we evaluated the quantum
yield of the reaction between the Michael acceptor 3a and the
carboxylate 17, in the optimized conditions reported in the SI.
The reaction was irradiated at 334 nm with a 100 W Hg lamp,
see experimental section for further details. The choice of such
excitation wavelength was dictated by: (i) the higher molar
absorptivity of the photocatalyst, compared to 450 nm blue
LED excitation;22 (ii) the high reliability of the potassium ferri-
oxalate actinometer in this spectral region, which is not the
case for
λ
> 450 nm;28 (iii) the still high selectivity of excitation,
since all the reagents are optically transparent for
λ
> 300 nm
(see Figure S6).
The determined quantum yield of the reaction is 0.28
±
0.05.
This value indicates that a radical chain mechanism is unlikely
in our reaction but, however, could be not totally ruled out.29
Figure 5. Proposed reaction mechanism.
Proposed reaction mechanism
Conclusions
The above illustrated experimental findings corroborate the
reaction mechanism depicted in Scheme 6. Upon light absorp-
tion in the visible part of the electromagnetic spectrum, the
iridium photocatalyst (IrIII) is excited to its lowest electronic
excited state (*IrIII). This initial event is the only possible, since
all the other molecules are transparent to visible light (see
Figure S5 and S6). Then, the photoexcited complex is able to
In conclusion, we have developed a practical and effective
photocatalytic addition of dithiane-2-carboxylates to Michael-
type acceptors promoted by the iridium(III) complex 2, used
for the first time as powerful photocatalyst. The reaction has
broad scope and allows the introduction of dithiane in a varie-
ty of Michael acceptors (unsaturated ketones, esters, amides,
malonates, etc.) in high yields. The adducts can be further
functionalized with established chemistry by oxidative depro-
tection or alkylation. In particular, this photocatalytic reaction
can be used to introduce methyl group in unsaturated com-
pounds, after elimination of dithiane by Raney®-nickel. The
oxidize a suitable carboxylate derivative (I) inducing its decar-
boxylation and the formation of the corresponding radical spe-
cies (II). Subsequently, the addition of this radical to the Mi-
chael acceptor (III) affords the radical intermediate IV, which
undergoes a second SET event from the reduced iridium com-
plex (IrII). The photocatalyst is then restored and IV is convert-
photocatalyst 2 is easily accessible and its redox properties can
be finely tuned by chemical modification of the ligands, afford-
ing a brand new class of new Ir(III) photocatalysts. The electro-
ed into the carbanionic intermediate V. The latter can be easily
protonated leading to the final reaction products.
chemical potential of
2 allows tailored oxidations and opens
the way to the selective use in photocatalysis to activate sub-
strates. Studies on a stereoselective variant of the reaction
proposed here are under active investigations in our laborato-
ries.
Acknowledgements
A. G and P. G. C. are grateful to Fondazione Del Monte, Farb
funds University of Bologna (project SLAMM to A.G.) and EU-
Foundation through the TEC FP7 ICT-Molarnet project
(318516) for the partial financial support of this research. N.A.
and F.M. thanks the CNR for financial support through the pro-
jects PHEEL, N-CHEM, and bilateral CNR-CONICET. We are
grateful to Giandomenico Magagnano for preliminary results
6 | Chem. Sci., 2016, 00, 1-3
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ARTICLE
and Daniele Mazzarella for the preparation of tetrabu- source. The corrected spectra were obtained via a calibration
tylammonium salts.
curve supplied with the instrument. The photoluminescence
quantum yields (PLQY) in solution were obtained from the
corrected spectra on a wavelength scale (nm) and measured
according to the approach described by Demas and Crosby30
using an air-equilibrated water solution of quinine sulfate in 1
N H2SO4 as reference (PLQY = 0.546).31 The emission lifetimes
Experimental section
General procedure: In a Schlenk tube with rotaflo® stopcock
under argon atmosphere at r.t., catalyst
2 (1.7 mg, 0.002
(τ) were measured through the time-correlated single photon
counting (TCSPC) technique using an HORIBA Jobin Yvon IBH
mmol), 1,3-dithiane-2-carboxylic acid 4a (0.2 mmol, 0.032g)
and K2HPO4 (0.2 mmol, 0.034 g), were dissolved in 400 μL
DMSO. After 2 min, Michael acceptor (0.4 mmol) was added.
The reaction mixture was carefully degassed via freeze-pump
thaw (three times), and the vessel refilled with argon. The
Schlenk tube was stirred and irradiated with a blue LED posi-
tioned approximately at 10 cm distance from the reaction ves-
sel. After 16 h of irradiation, NaHCO3 sat. (2 mL) was added
and the mixture was extracted with ethyl acetate (4 x 5 mL).
The collected organic layers were dried over Na2SO4, filtered
and concentrated under reduced pressure to give crude prod-
ucts. Column chromatography on silica (cyclohexane:ethyl
acetate or cyclohexane:Et2O) afforded pure compounds.
FluoroHub controlling a spectrometer equipped with a pulsed
NanoLED (λexc = 330 nm; FWHM = 11 nm) as excitation source
and a red-sensitive Hamamatsu R-3237-01 PMT (185– 850 nm)
as detector. The analysis of the luminescence decay profiles
was accomplished with the DAS6 Decay Analysis Software pro-
vided by the manufacturer, and the quality of the fitting was
assessed with the χ2 value close to unity and with the residuals
regularly distributed along the time axis. Samples were excited
at 340 nm for the evaluation of PLQYs and at 330 nm for
termination. Experimental uncertainties are estimated to be ±
10% for determinations, ±20% for PLQY, ±2 nm and ±5 nm
τ de-
τ
for absorption and emission peaks, respectively. Stern-Volmer
quenching experiments were performed at room-temperature
in oxygen-free conditions (argon-saturated environment) using
3 ml of acetonitrile solution containing the appropriate iridium
photocatalyst (with a concentration of 1.5·10–5 M) and increas-
ing amounts of quencher. For the determination of the photo-
catalytic quantum yield, the reaction was carried out in spec-
trofluorimetric grade DMSO and placed in a Suprasil® quartz
cuvettes with a 2.00 mm path length. The reaction mixture
was excited at 334 nm, using a 100 W Hg lamp equipped with
an appropriate dichroic filter. The photon flux was estimated
using the ferrioxalate actinometer, following the procedure
reported by Montalti et al.28 The conversion of the reaction
was determined by 1H-NMR.
Electrochemistry
Voltammetric experiments were performed using a Metrohm
AutoLab PGSTAT 302 electrochemical workstation in combina-
tion with the NOVA software package. All the measurements
were carried out at room temperature in acetonitrile solutions
with a sample concentration of approx. 1 mM and using 0.1 M
tetrabutylammonium hexafluorophosphate (electrochemical
grade, TBAPF6) as the supporting electrolyte. Oxygen was re-
moved from the solutions by bubbling argon for 20 minutes.
All the experiments were carried out using a three-electrode
setup (BioLogic VC-4 cell, with a cell volume of 5 ml) with a
glassy-carbon working electrode (1.6 mm diameter), the
Ag/AgNO3 redox couple (0.01 M in acetonitrile with 0.1 M
TBAClO4 supporting electrolyte) as reference electrode and a
platinum wire as counter electrode. At the end of each meas-
urement, ferrocene was added as the internal reference. Cyclic
voltammograms (CV) were typically recorded at a scan rate of
200 mV s–1, but several rates were used to check reversibility
(in the range between 50 and 2000 mV s–1). Osteryoung
square-wave voltammograms (OSWV) were recorded with a
Notes and references
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4
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ARTICLE
Chemical Science
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8 | Chem. Sci., 2016, 00, 1-3
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