Photocatalytic atom transfer radical addition to olefins utilizing novel photocatalysts

Article abstract of DOI:10.3390/molecules24091644

Photocatalysis is a rapidly evolving area of research in modern organic synthesis. Among the traditional photocatalysts, metal-complexes based on ruthenium or iridium are the most common. Herein, we present the synthesis of two photoactive, ruthenium-based complexes bearing pyridine-quinoline or terpyridine ligands with extended aromatic conjugation. Our complexes were utilized in the atom transfer radical addition (ATRA) of haloalkanes to olefins, using bromoacetonitrile or bromotrichloromethane as the source of the alkyl group. The tailor-made ruthenium-based catalyst bearing the pyridine-quinoline bidentate ligand proved to be the best-performing photocatalyst, among a range of metal complexes and organocatalysts, efficiently catalyzing both reactions. These photocatalytic atom transfer protocols can be expanded into a broad scope of olefins. In both protocols, the photocatalytic reactions led to products in good to excellent isolated yields.

Full text of DOI:10.3390/molecules24091644

molecules
Article
Photocatalytic Atom Transfer Radical Addition to
Olefins Utilizing Novel Photocatalysts
Errika Voutyritsa, Ierasia Triandafillidi, Nikolaos V. Tzouras, Nikolaos F. Nikitas,
Eleftherios K. Pefkianakis, Georgios C. Vougioukalakis * and Christoforos G. Kokotos *
Laboratory of Organic Chemistry, Department of Chemistry, National and Kapodistrian University of Athens,
Panepistimiopolis 15771, Athens, Greece; errikav@chem.uoa.gr (E.V.); iertriant@chem.uoa.gr (I.T.);
nitzouras@gmail.com (N.V.T.); niknikitas@chem.uoa.gr (N.F.N.); pefkos99ele@gmail.com (E.K.P.)
* Correspondence: vougiouk@chem.uoa.gr (G.C.V.); ckokotos@chem.uoa.gr (C.G.K.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: Davide Ravelli
Received: 16 March 2019; Accepted: 25 April 2019; Published: 26 April 2019
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Photocatalysis is a rapidly evolving area of research in modern organic synthesis.
Among the traditional photocatalysts, metal-complexes based on ruthenium or iridium are the most
common. Herein, we present the synthesis of two photoactive, ruthenium-based complexes bearing
pyridine-quinoline or terpyridine ligands with extended aromatic conjugation. Our complexes were
utilized in the atom transfer radical addition (ATRA) of haloalkanes to olefins, using bromoacetonitrile
or bromotrichloromethane as the source of the alkyl group. The tailor-made ruthenium-based catalyst
bearing the pyridine-quinoline bidentate ligand proved to be the best-performing photocatalyst,
among a range of metal complexes and organocatalysts, efficiently catalyzing both reactions.
These photocatalytic atom transfer protocols can be expanded into a broad scope of olefins. In both
protocols, the photocatalytic reactions led to products in good to excellent isolated yields.
Keywords: photocatalysis; ATRA reactions; olefins; sustainable catalysis; ruthenium-based complexes
1. Introduction
Photocatalysis, the use of light to promote organic transformations, is a rather old concept that was
first conceived in the early 1900s by Ciamician [
receiving limited attention from researchers, as scientists were preoccupied with the thought that
radical chemistry is rather difficult to be controlled. In 2008, Macmillan [ ], Yoon [ ] and Stephenson [4]
1]. Since then, Photocatalysis spent almost a century
2
3
put forward the idea of photoredox catalysis, where transition-metal complexes could be employed as
photocatalysts, under mild reaction conditions. Since then, the field of Photocatalysis has expanded
significantly [5–12].
Olefins are very common building blocks in synthetic organic chemistry, due to their availability
and wide participation in addition reactions, leading to highly-useful products in a controlled
fashion. Also, alkyl halides are commonly employed in organic synthesis, since they bear a reactive
carbon-halogen bond, which can be homolytically cleaved with the use of radical initiators or
photocatalysts. Atom transfer radical addition (ATRA) of haloalkanes to unsaturated compounds
serves as a simple and useful method to form C-C and C-X bonds. Many successful ATRA reactions
have been reported in literature, most of which are metal-catalyzed, employing copper-, ruthenium-,
iridium-, palladium-, or nickel-based complexes [13–18]. Along the same lines, Bach, Nicewicz and
Melchiorre emphasized the ease of controlling photogenerated radicals, opening new avenues in the
field of photocatalysis [19–21].
Brominated compounds are valuable scaffolds in organic synthesis, being able to undergo
further transformation into useful functional groups. A well-known brominating reagent for ATRA
reactions is bromoacetonitrile, which has been widely used, by different research groups, making it a
Molecules 2019, 24, 1644; doi:10.3390/molecules24091644
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well-established precursor of addition products [22
–25]. However, the scope of olefins employed in
ATRA reactions with bromoacetonitrile, in the presence of a metal-based catalyst, is rather limited
(Scheme 1A). We have already contributed to this field with two studies, one focusing on expanding the
scope of olefins with iridium photocatalysts, just by altering the reaction mechanism utilizing sodium
ascorbate [26] (Scheme 1B) and by introducing a photoorganocatalytic alternative [27] (Scheme 1C).
The synthesis and study of new metal-based complexes that can be employed as photocatalysts is
essential, in order to further expand the substrate scope. Moreover, in order to widen the scope of
brominated addition products, the use of other alkyl halides, such as bromotrichloromethane, which is
used successfully in ATRA reactions, is required [28–30].
Scheme 1. Photocatalytic methodologies of atom transfer radical addition (ATRA) reactions with
bromoacetonitrile, (
A) previous work from other groups, (B) previous work from our group with an
iridium photocatalyst, (C) previous work from our group with a photoinitiator, (D) this work.
Using our experience in Photocatalysis [26,27,31–34] and combining it with the synthesis of
ruthenium-based photoactive complexes [35 39], we present the use of alternative ruthenium-based
–
photocatalysts, rather than the well-known and commercially available [Ru(bpy)3]Cl2, in the ATRA
reaction of bromoacetonitrile and bromotrichloromethane with olefins bearing various functional
groups (Scheme 1D).
These ruthenium photocatalysts bear bidentate pyridine-quinoline hybrid ligands or tridentate
terpyridine ligands with extended aromatic conjugation and anthracene units attached to them.
Such architectures can increase the extinction coefficients of the metal-to-ligand charge transfer
bands in the corresponding complexes, as well as their response to red light, as previously found
in dye-sensitized solar and singlet oxygen sensitization applications [36–39]. Moreover, as shown,
fundamental photophysical studies of these and analogous scaffolds, the anthracene moiety acts
as a secondary chromophore/quencher, thus improving the sensitizing ability of the corresponding
complexes [37,39].
2. Results
2.1. Synthesis of Photocatalysts
In this work, we report the synthesis of two ruthenium complexes and study their activity as
photocatalysts in ATRA reactions. The synthesis of the first ruthenium complex started with the Suzuki
reaction between pyridine-quinoline 1, boronic acid 2 and bromoanthracene 3 affording ligand 4.
In parallel, RuCl3 reacted with bipyridine 6 in DMF (dimethyl formamide) to give complex 7 in
62% yield. Finally, complex 7 reacted with bipyridine 5 in ethylene glycol, leading to the desired 8a
(Scheme 2) [36,37,39].

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Scheme 2. Synthesis of complex 8a.
For the synthesis of 8b, acetyl pyridine 9 reacted with acid 10 and RuCl3 furnishing compound
11 (Scheme 3). In parallel, acetyl pyridine 9 reacted with boronic acid 12 and the bromoanthracene
3 afforded compound 13. Finally, complex 13 reacted with complex 11, in the presence of
N-methylmorpholine, leading to the final complex 8b in 85% yield (Scheme 3) [35,38].
Scheme 3. Synthesis of complex 8b.
2.2. Photocatalytic Reactions
As mentioned earlier, the ATRA reaction between olefins and bromoacetonitrile is a very useful
organic transformation that can be used as a benchmark reaction for testing new photocatalysts.
The ruthenium and iridium complexes previously described in literature were employed in this ATRA
reaction, providing very good results. We began our studies with the reaction between 1-decene and
bromoacetonitrile (Table 1). Commercially available photocatalysts, Ir(ppy)3 (8c) [26] and Ru(bpy)3Cl2
(8d) [2–4] were tested, for comparison purposes (Table 1, entries 1 and 2). By utilizing only 1 mol%
catalyst loading, iridium complex 8c, the most expensive of the two commercially available metal-based
photocatalysts tested, allowed a product formation in a quantitative yield, while ruthenium complex

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8d led to a diminished 78% yield. A range of photocatalysts were then tested, with our tailor-made
ruthenium-based photocatalyst having the pyridine-quinoline ligand of extended aromatic conjugation
8a affording very good yields (Table 1, entries 3, 4, 8, 9). It has to be noted that 8a outperformed the
commercially available Ru(bpy)3Cl2 catalyst, being slightly inferior in catalytic efficiency compared
to 8d (iridium-based complex) (Table 1, entry 3 vs. 2). Studying the reaction conditions, the use of
sodium ascorbate proved to be essential (Table 1, entry 5). Also, when the reaction was kept in dark,
no conversion was detected (Table 1, entry 6). Unlike previous literature precedent [24,25], no alcoholic
addition took place. We believe that the use of sodium ascorbate alters the reaction mechanism,
bypassing this reaction route.
Table 1. Optimization of the reaction conditions for the photocatalytic atom transfer radical addition of
bromoacetonitrile to olefins.
Catalyst
Loading (mol %)
Sodium Ascorbate
(Equiv.)
Entry
Catalyst
Yield (%) a
1
2
Ir(ppy)3 (8c)
1
1
2
2
2
2
-
98
78
84
63
0
Ru(bpy)3Cl2 (8d)
3
8a
1
4
8b
1
5
8a
1
6 b
8a
-
1
2
2
2
2
0
7
-
0
8
Thioxanthone (8e)
Eosin Y (8f)
10
10
43
32
9
a Isolated yield. The reaction was performed with 1-decene (35 mg, 0.25 mmol), bromoacetonitrile (60 mg, 0.50 mmol),
catalyst (0–10 mol%, 0.00–0.025 mmol), sodium ascorbate (0–100 mg, 0–0.50 mmol), acetonitrile (2 mL) and methanol
b
(1.5 mL) under household bulb irradiation for 24 h. The reaction was performed in the dark.
Following the optimization of the reaction conditions, we tested the substrate scope of the reaction,
as far as the aliphatic olefin is concerned (Scheme 4). All types of double bonds tested led to the
corresponding addition products in good to excellent isolated yields. Initially, aliphatic double bonds
were tested, leading to the desired products in very good to excellent yields (Scheme 4, 15a, 15b, 15e–h).
The usually less reactive disubstituted double bonds, like cyclooctene (14c) and norbornene (14d),
were also examined (Scheme 4, 15c and 15d), providing very good results. Furthermore, allyl ethers
proved to be efficient substrates for this type of atom transfer radical addition as well, since the desired
products were obtained in very good yields (Scheme 4, 15i and 15j). Finally, substrates bearing different
functional groups, such as ester (Scheme 4, 15k), amide (Scheme 4, 15l), free alcohol (Scheme 4, 15m)
and halogens (Scheme 4, 15n), were tested, affording the corresponding products in good to very good
yields. Also, the 1,1-disubstituted double bond 4-methylenecyclohexyl)benzene (14o) was examined
furnishing two products (Scheme 4, 15oA and 15oB), where the main product was the product of
elimination of HBr. Trisubstituted naturally occurring double bonds, like limonene (14p) and
(14q) were also tested on this protocol, furnishing various products (Scheme 4, 15p and 15q). In the case
of limonene, the result of the reaction was a mixture of three products. Regarding -pinene, the reaction
β-pinene
β
afforded two different fractions, where the first was a mixture of two compounds (15qA and 15qB) and
compound 15qC. During the optimization process, the use of other addition reagents, such as iodoacetic

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acid and nonafluoro-1-iodobutane, was also examined. The reaction furnished the corresponding
product of addition to 1-decene (Scheme 4, 15r and 15s), but in lower yields than the additional product
of bromoacetonitrile (Scheme 4, 15a), so it was a rational choice to continue further with the latter.
Scheme 4. Substrate scope of the photocatalytic reaction between bromoacetonitrile and olefins.
In a similar manner, we also studied the reaction between bromotrichloromethane and aliphatic
olefins (Scheme 5). The reaction conditions employed were adopted from the literature [40]. Using these
conditions, we tested a number of substrates (Scheme 5). When 1-decene was employed, the targeted
product was obtained in excellent yield (Scheme 5, 16a). A more challenging disubstituted alkene,
cyclooctene, was tested and proved to be a competent substrate, affording the desired product in good
yield (Scheme 5, 16e). Olefins bearing different functional groups were also studied and gave the
corresponding products in very good yields (Scheme 5, 16b–d, 16f, 16g).

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Scheme 5. Substrate scope of the photocatalytic reaction between bromotrichloromethane and olefins.
2.3. Mechanistic Studies
2.3.1. Phosphorescence Quenching Studies
To shed light on the mechanism of these transformations, a series of phosphorescence quenching
studies were carried out (Scheme 6). Regarding the reaction with bromoacetonitrile, after irradiation of
the photocatalyst at 370 nm, its phosphorescence was measured at 516 nm. Upon increasing the amount
of the added 1-decene, no changes in the phosphorescence were observed (Scheme 6A). Increasing the
amount of the added bromoacetonitrile, a constant decrease in the phosphorescence was observed
(Scheme 6B). Finally, increasing the amount of the added sodium ascorbate, a constant decrease in the
phosphorescence was also observed (Scheme 6C). From these data, we conclude that sodium ascorbate
is important for the success of the reaction and is a better quencher than bromoacetonitrile.
(A)
(B)
(C)
) Phosphorescence quenching studies of 8a with 1-decene.
Scheme 6.
(A
(
B) Phosphorescence
quenching studies of 8a with bromoacetonitrile. (
sodium ascorbate.
C) Phosphorescence quenching studies of 8a with

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Regarding the reaction with bromotrichloromethane, the same phosphorescence quenching
experiments were carried out (Scheme 7). After irradiation of 8a at 370 nm, its phosphorescence
was measured 516 nm. When the amount of the added 1-decene was increased, no change
in phosphorescence was observed (Scheme 7A). Increasing the amount of the added
bromotrichloromethane, a constant decrease in the phosphorescence was observed (Scheme 7B).
(A)
(B)
Scheme 7.
(A
) Phosphorescence quenching studies of 8a with 1-decene. (
B) Phosphorescence quenching
studies of 8a with bromotrichloromethane.
2.3.2. Quantum Yield Measurement
In an effort to further understand the mechanism of the reaction, we also measured the quantum
yield of both reactions, in order to discover if they progress through a closed photocatalytic cycle or via
a propagation mechanism [41]. The quantum yield of both reactions proved to be higher than 1, (
Φ1 = 9
and 2 = 3), indicating that in both reactions, a propagation mechanism was taking place. On the basis
Φ
of these data, we propose the mechanisms presented in Scheme 8. Both of these proposed catalytic
cycles begin with the excitation of the photocatalyst after the absorption of visible light irradiation.
The following step is the one that differentiates the two cycles. In the presence of an electron donor,
such as sodium ascorbate, the left photocatalytic cycle occurs (Scheme 8A). The cycle begins with the
reductive quenching of the photocatalyst by sodium ascorbate, leading to Ru+, followed by a single
electron transfer (SET) between the photocatalyst and bromoacetonitrile, leading to alkyl radical I.
Radical I will subsequently react with the double bond, giving a second radical species II, which via
propagation leads to product formation. In the second cycle (B), in the first step, the excited Ru2+
catalyst will participate in an oxidative quenching through a SET procedure, resulting to the formation
of radical III. Then, radical III will react with the olefin, furnishing radical species IV. In this step,
radical IV will either lead to the propagation route or can participate to a SET with the ruthenium
photocatalyst leading to carbocation V, which will be attacked by a bromine anion giving the final
product (Scheme 8B). However, quantum yield experiments suggest that the first cycle is more likely
taking place.

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Scheme 8. Proposed reaction mechanism.
Synthetic procedures for the preparation of the photocatalysts, reaction optimization,
compound characterization data, 1H- and 13C-NMR of compounds, full mechanistic studies including
UV-Vis and phosphorescence studies can be found at Supplementary Materials.
3. Conclusions
In conclusion, we report the synthesis of two ruthenium complexes, the unprecedented evaluation
of their photocatalytic efficiency in two well-known ATRA reactions, and the synthesis of a wide
variety of brominated products in good to excellent yields. The reactions took place under mild
conditions, using common household lamps and no inert atmosphere, with no need for dry solvent,
and employing a low catalyst loading. Mechanistic experiments were carried out, shedding some light
on the mechanism of the reactions.
Supplementary Materials: The following are available online at http://www.mdpi.com/1420-3049/24/9/1644/s1,
Synthetic procedures for the preparation of the photocatalysts, reaction optimization, compound characterization
data, 1H- and 13C-NMR of compounds, full mechanistic studies including UV-Vis and phosphorescence studies.
Author Contributions: Conceptualization, C.G.K.; Exchange of ideas and discussions, G.C.V. and C.G.K.;
Synthesis of photocatalysts, N.V.T., E.K.P. and G.C.V.; Reaction optimization and substrate scope, E.V., I.T.,
N.F.N. and C.G.K.; Mechanistic studies, E.V., C.G.K.; Compound characterization, E.V., I.T., N.V.T. and N.F.N.;
writing—original draft preparation, N.F.N.; writing—review and editing, N.F.N., G.C.V. and C.G.K.; supervision
and project administration, G.C.V. and C.G.K.; funding acquisition, E.V., I.T., N.F.N., G.C.V. and C.G.K.
Funding: The authors gratefully acknowledge financial support through the John S. Latsis Foundation programme
“EPISTHMONIKES MELETES 2015” (PhotoOrganocatalysis: Development of new environmentally-friendly
methods for the synthesis of compounds for the pharmaceutical and chemical industry). I.T. would like to thank
the Hellenic Foundation for Research and Innovation (EΛI∆EK), while E.V. and N.F.N. would like to thank the
State Scholarships Foundation (IKY) for financial support. Also, the special account for research grants (E
the National and Kapodistrian University of Athens is acknowledged for the support.
ΛKE) of
Acknowledgments: The authors would also like to thank Prof. V. Constantinou from the Agricultural University
of Athens for access to the fluorescence spectrometer and Dr. Maroula Kokotou for her assistance in acquiring
HRMS data. Moreover, the contribution of COST Action C-H Activation in Organic Synthesis (CHAOS) CA15106
is gratefully acknowledged.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are available from the authors.
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