Expanding the Scope of Photocatalysis: Atom Transfer Radical Addition of Bromoacetonitrile to Aliphatic Olefins

Article abstract of DOI:10.1002/cctc.201800110

An efficient photocatalyzed bromocyanomethylation of alkenes is reported. Among a range of organocatalysts and metal complexes, Ir(ppy)₃ proved to be the best photocatalyst in promoting the addition of BrCH₂CN to olefins. This photocatalytic atom transfer protocol can be expanded into a wide substrate scope of aliphatic olefins bearing various functional groups, leading to the corresponding products in good to excellent yields. In addition, linchpin catalysis was developed, since the bromo group can undergo further transformation into useful functional groups, such as the synthesis of amino acids.

Full text of DOI:10.1002/cctc.201800110

Accepted Article
Title: Expanding the Scope of Photocatalysis: Atom Transfer Radical
Addition of Bromoacetonitrile to Aliphatic Olefins
Authors: Errika Voutyritsa, Ierasia Triandafillidi, and Christoforos G.
Kokotos
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Expanding the Scope of Photocatalysis: Atom Transfer Radical
Addition of Bromoacetonitrile to Aliphatic Olefins
Errika Voutyritsa,[a] Ierasia Triandafillidi,[a] and Christoforos G. Kokotos*[a]
various examples of the addition of haloalkanes to olefins
(Scheme 1, top), knowledge on the addition of bromoacetonitrile
to olefins is rather limited (Scheme 1, A-C). More specifically, in
2001, Oshima and coworkers treated bromo-compounds and
olefins with triethylborane in water to provide the corresponding
products in good yields. Only one example, utilizing
bromoacetonitrile, was demonstrated and the reaction employed
stoichiometric amount of the promoter (Scheme 1, A).[17] In 2014,
the group of Melchiorre developed a photoorganocatalytic
method for the atom transfer radical addition of haloalkanes onto
olefins (Scheme 1, B). In this case, only one example using
Abstract: An efficient photocatalyzed bromocyanomethylation of
alkenes is reported. Among a range of organocatalysts and metal
complexes, Ir(ppy)3 proved to be the best photocatalyst in promoting
the addition of BrCH2CN to olefins. This photocatalytic atom transfer
protocol can be expanded into a wide substrate scope of aliphatic
olefins bearing various functional groups, leading to the
corresponding products in good to excellent yields. In addition,
linchpin catalysis was developed, since the bromo-group can
undergo further transformation into useful functional groups, like the
synthesis of amino acids.
bromoacetonitrile
was
reported.[18a]
A
very
recent
photoorganocatalytic example, demonstrating a single example
with bromoacetonitrile was also reported by Cozzi and
coworkers.[18b] Also, two methods were developed for the
addition of bromoacetonitrile to styrene derivatives under
Photoredox catalysis (Scheme 1, C).[19,20] In both instances, the
olefin was only limited to styrene derivatives and the
intermediate radical was trapped by the alcoholic solvent,
Introduction
Radical generation from alkyl halides via homolytic cleavage of
the C-X bond leads to the incorporation of alkyl moieties in
carbon chains, providing useful intermediates, in Organic
Synthesis.[1] Photocatalysis allows the generation of these
radicals under mild conditions, following various mechanisms,
including hydrogen abstraction from alkanes or halide
abstraction from alkyl halides. These alkyl radicals are versatile
as reactive intermediates, in particular for C-C bond formation
events making this method appealing. Atom transfer radical
addition (ATRA) or atom transfer radical cyclization (ATRC) of
haloalkanes and halocarbonyls to unsaturated compounds,
forming C−C and C−X bonds, is reported with a range of metal
catalysts, such as ruthenium,[2] iron,[3] nickel,[4] palladium,[5]
iridium[6] and copper.[7] Photoredox catalysis has provided a
revolution in the field, since MacMillan,[8] Yoon[9] and
Stephenson[10] have proved the ability of Photoredox catalysts to
introduce novel organic transformations.[11] Around the same
time, Bach,[12] Nicewicz[13] and Melchiorre[14] through their
seminal work have developed photochemical reactions as useful
tools in Organic Chemistry. Bromonitriles are potentially useful
scaffolds for the synthesis of biologically active compounds and
have demonstrated important synthetic utilities.[15,16] The last few
years, photochemical atom transfer radical addition (ATRA) of
bromoacetonitrile to olefins starts to play a dominant role among
the methods reported for their synthesis. Although there are
evidence that this method proceeds via
a carbocation
intermediate and that the more versatile bromide intermediates
could not be isolated. We have recently turned our attention to
Photocatalysis,
both
in
Photoorganocatalysis[21a-e] and
Photoredox catalysis.[21f] Herein, we describe an expansion of
current knowledge into a synthetic protocol for the ATRA
addition of bromoacetonitrile utilizing tris[2-phenylpyridinato-
C2,N]iridium(III) as the photocatalyst onto a wide substrate
scope of aliphatic olefins bearing various functional groups
(Scheme 1, bottom).
[a]
E. Voutyritsa, I. Triandafillidi, Prof. C. G. Kokotos
Laboratory of Organic Chemistry,
Department of Chemistry,
National and Kapodistrian University of Athens,
Panepistimiopolis 15771, Athens, Greece
E-mail: ckokotos@chem.uoa.gr
Supporting information for this article is given via a link at the end of
the document.
Scheme 1. Photocatalytic methodologies for the bromocyanomethylation of
olefins.
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10-12). Unlike previous literature precedent, no
alcoholic
addition was observed. We believe that the nature of the
substituent on the double bond (aryl vs aliphatic), that can
stabilize the intermediate radical formed, is crucial for the
reaction outcome. Also, the use of sodium ascorbate can alter
the reaction mechanism.
Results and Discussion
We began our investigations with the reaction between 1-
decene (1a) and bromoacetonitrile with a number of catalysts.
This reaction was chosen in order to overcome the current
literature limitation with the employment of styrene derivatives.
Utilizing metal complexes, as the photocatalyst, high yields were
obtained, with Ir(ppy)3 proving to be the best photocatalyst
(Table 1, entries 1-3). A range of organic photocalalysts were
also tested in the reaction, providing the desirable products in
lower yields (Table 1, entries 7-9). After identifying Ir(ppy)3 as
the optimum photocatalyst, we proceeded with the optimization
of the reaction conditions, studying the reaction’s performance
with and without the catalyst or sodium ascorbate and under
dark conditions (Table 1, entries 4-6). In the absence of the
photocatalyst, no reaction took place (Table 1, entry 4). When
no sodium ascorbate was employed, no product could be
identified (Table 1, entry 5). This is in complete agreement with
literature,[19,20] that aliphatic olefins do not lead to product
formation, when sodium ascorbate is absent. Moreover, when
the reaction was kept in the dark, 1-decene did not convert to
the corresponding product (Table 1, entry 6). Then, a variety of
solvents was tested. Acetonitrile combined with methanol proved
to be the best solvent system, leading to the higher yield (Table
1, entry 3). Others solvents led to lower yields (Table 1, entries
The next step was to explore the scope of the olefin partner
(Scheme 2). All tested double bonds were converted to the
corresponding products in excellent yields. Initially, the atom
transfer radical addition was performed with aliphatic olefins,
leading to the corresponding products in high yields (2a, 2b, 2e,
2f). Less reactive internal cyclic alkenes for ATRA reactions,
including 2-norbornene (1d) and cyclooctene (1c), were
competent substrates, leading to the desirable products in high
yields (2c and 2d). Allyl ethers proved to be very efficient
substrates, since the products were obtained in high yields (2i
and 2j). Terminal olefins bearing a wide array of functionalities
were efficiently transformed into the corresponding products in
good yields (2g, 2h and 2k-2o). Finally, in the case of 1p (a 1,1-
disubstituted double bond), after ATRA addition, an elimination
occurs leading to 2p.
In a recent study, Cismesia and Yoon challenged the ability of a
light-dark experiment as the sole evidence for photocatalytic
closed cycle mechanism versus a chain propagation cycle.[22] If a
closed catalytic cycle is involved, a maximum theoretical
quantum yield (Φ) of 1 is expected (every photon absorbed by
the photocatalyst produces a molecule of the product, although if
Table 1. Optimization of the reaction conditions for the photocatalytic atom
transfer radical addition of bromoacetonitrile to olefins.
Entry
Catalyst (mol%)
Sodium Ascorbate
(equiv.)
Yielda
(%)
Ru(bpy)3Cl2·6H2O (1)
Ru(phen)Cl2 (1)
Ir(ppy)3 (1)
2
2
2
-
78
62
98
0
1
2
3
Ir(ppy)3 (1)
4
-
2
2
2
2
2
2
2
2
0
5
Ir(ppy)3 (1)
0
6b
7
Eosin Y (10)
Benzophenone (10)
Benzoin (10)
Ir(ppy)3 (1)
32
0
8
26
66
48
28
9
10c
11d
12e
Ir(ppy)3 (1)
Ir(ppy)3 (1)
[a]
[c]
Isolated yield. [b] Reaction was kept in the dark. CH2Cl2 was used instead
[d]
[e]
of MeCN/MeOH.
MeCN was used instead of MeCN/MeOH.
MeOH was
used instead of MeCN/MeOH.
Scheme 2. Substrate scope of the reaction.
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non-productive pathways or other phenomena are involved, the
quantum yield will decrease). Although a quantum yield lower
than 1 does not exclude the possibility of a short chain
propagation. On the contrary, a quantum yield of Φ >> 1 would
mean a chain propagation process, since one-photon induced
initiation would lead to multiple molecules of the product. We
have calculated the quantum yield of the reaction was calculated
by dividing the moles of the product formed by the einsteins of
photons consumed, leading to a quantum yield of 2.49 under
identical reaction conditions that reaction was performed, hinting
that a chain propagation is followed.
Fluorescence quenching studies were then carried out in order
to understand the reaction mechanism. After irradiation of
Ir(ppy)3 at 372 nm, its fluorescence was measured at 518 nm.
Increasing the amount of the added 1-decene, no changes in the
fluorescence were observed (Scheme 3, A). Increasing the
amount of the added bromoacetonitrile, a constant decrease in
the fluorescence was observed (Scheme 3, B). Increasing, also,
the amount of the added sodium ascorbate, a constant decrease
in the fluorescence was also observed (Scheme 3, C).
Comparing the slopes of the two diagrammes, ascorbate is a
better quencher than bromoacetonitrile.
When we attempted to employ styrene as the olefin partner, in
the exact same conditions, we could only observe
polymerization events to occur (Scheme 4, A). This is in
accordance with our previous postulation and findings that a
different mechanism is in place, when aryl vs alkyl substituent on
the olefin are employed and whether sodium ascorbate is added
or not.[21g] In order to verify this, we performed the reaction as
desribed in literature,[19] and indeed, the product bearing the
ether was obtained in 78% yield (Scheme 4, B). When sodium
bicarbonate was used, instead of sodium ascorbate, and
acetonitrile was employed as the solvent, the desired ATRA
product was obtained in low yield (Scheme 4, C). Thus, when
styrene derivatives are employed (aryl substituent on the olefin),
sodium ascorbate is not required for product formation. It seems
that in that case, once the intermediate radical B is formed, it is
Scheme 4. Test reactions with styrene.
oxidized to the corresponding carbocation, which is stabilized by
the adjacent aryl moiety (benzylic cation), closing the
Photoredox catalytic cycle, regenerating the iridium catalyst. The
carbocation formed reacts quickly with the nucleophilic alcoholic
solvent to yield the ether (Scheme 4, B). If the alcoholic solvent
is omitted, a low yield of the ATRA product is obtained (Scheme
4, C). On the other hand, the addition of sodium ascorbate
completely alters the reaction mechanism, and now styrene
derivatives lead to polymerization. However, this opens the door
for the successful employment of olefins bearing alkyl
substituents that could not be employed before. Additional
mechanistic experiments (employing TEMPO or BHT in the
reaction conditions led to no product formation) verifying the
radical nature of the process.
On the basis of these data, we propose the mechanism outlined
in Scheme 5. Upon irradiation, IrIII is excited and the excited
photocatalyst is reduced by sodium ascorbate, which reacts with
bromoacetonitrile forming electrophilic radical A and regenerates
the photocatalyst. The formed radical A reacts with the olefin
leading to new radical B. Then, propagation with
bromoacetonitrile leads to the desired products (Scheme 5).
Radical B cannot be oxidized to the corresponding carbocation,
since the R group is aliphatic and cannot stabilize it. We attribute
the fact that no product derived from nucleophilic attack of the
alcoholic solvent is observed to this event. Thus, the presence of
sodium ascorbate alters the reaction mechanism of the metal
photocatalyst making it prone to react with aliphatic olefins. The
A.
B.
C.
Scheme 3. Fluorescence studies. A) Fluorescence quenching of Ir(ppy)3
with progressive addition of 1-decene. B) Fluorescence quenching of Ir(ppy)3
with progressive addition of bromoacetonitrile. C) Fluorescence quenching of
Ir(ppy)3 with progressive addition of sodium ascorbate.
Scheme 5. Proposed reaction mechanism.
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Activation in Organic Synthesis (CHAOS) CA15106 is
acknowledged for helpful discussions.
Keywords: photocatalysis • radical addition • olefin •
bromoacetonitrile • amino acids
Scheme 6. Synthesis of 4-aminododecanoic acid (3).
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In a glass vial with a screw cap containing tris[2-phenylpyridinato-
C2,N]iridium(III) (1.5 mg, 0.0025 mmol) in acetonitrile (2 mL) and
methanol (1.5 mL), alkene (0.25 mmol), BrCH2CN (45 mg, 0.375 mmol)
and sodium ascorbate (100 mg, 0.50 mmol) were added consecutively.
The vial was sealed with a screw cap and left stirring under household
bulb irradiation (2 x 80W household lamps) for 24 hours. The desired
product was isolated after purification by column chromatography.
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The authors gratefully acknowledge the financial support
through the Latsis Foundation programme “EPISTHMONIKES
MELETES 2015” (PhotoOrganocatalysis: Development of new
environmentally-friendly methods for the synthesis of
compounds for the pharmaceutical and chemical industry) and
the Laboratory of Organic Chemistry of the Department of
Chemistry of the National and Kapodistrian University of Athens
for financial support. E.V. would like to thank the State
Scholarship Foundation (IKY) for financial support through a
doctoral fellowship. I.T. would like to thank the Hellenic
Foundation for Research and Innovation (ΕΛΙΔΕΚ) for financial
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Entry for the Table of Contents
FULL PAPER
Photocatalysis. A photocatalytic
method for the
Errika Voutyritsa, Ierasia Triandafillidi
and Christoforos G. Kokotos*
bromocyanomethylation of alkenes is
described.
Page No. – Page No.
Expanding the Scope of
Photocatalysis: Atom Transfer
Radical Addition of Bromoacetonitrile
to Aliphatic Olefins
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