Metal-free Photochemical Atom Transfer Radical Addition (ATRA) of BrCCl3 to Alkenes

Article abstract of DOI:10.1002/ejoc.202001387

A simple, photochemical, and metal-free protocol for the atom transfer radical addition (ATRA) of bromotrichloromethane onto various alkenes is described. Among a range of organic molecules, phenylglyoxylic acid proved to be the most suitable photoinitiator to promote a sustainable process for the addition of bromotrichloromethane to olefins. This photochemical atom transfer radical 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.

Full text of DOI:10.1002/ejoc.202001387

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Metal-free Photochemical Atom Transfer Radical Addition (ATRA)
of BrCCl3 to Alkenes
Authors: Nikolaos F. Nikitas, Errika Voutyritsa, Petros L. Gkizis, and
Christoforos G. Kokotos
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202001387
Link to VoR: https://doi.org/10.1002/ejoc.202001387
01/2020

10.1002/ejoc.202001387
European Journal of Organic Chemistry
COMMUNICATION
Metal-free Photochemical Atom Transfer Radical Addition
(ATRA) of BrCCl3 to Alkenes
Nikolaos F. Nikitas,†[a] Errika Voutyritsa,†[a] Petros L. Gkizis[a] and Christoforos G. Kokotos*[a]
Dedicated to Prof. Ioannis Gallos on the occasion of his retirement
[a]
Nikolaos F. Nikitas, Dr. Errika Voutyritsa, Dr. Petros L. Gkizis and Prof. Christoforos G. Kokotos
Laboratory of Organic Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis 15771, Athens, Greece.
† denotes equal contribution
Corresponding author. E-mail: ckokotos@chem.uoa.gr homepage: http://www.chem.uoa.gr/?page_id=2898&lang=el
Abstract: A simple, photochemical and metal-free protocol for the
atom transfer radical addition (ATRA) of bromotrichloromethane onto
various alkenes is described. Among a range of organic molecules,
phenylglyoxylic acid proved to be the most suitable photoinitiator to
promote
a
sustainable
process
for
the
addition
of
bromotrichloromethane to olefins. This photochemical atom transfer
radical 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.
Introduction
Photon has been for many years considered as a “traceless
reagent”.[1] Thus, there is a continuous interest into harnessing
the power of light in the field of organic synthesis. Therefore, the
field of photochemistry, the use of light to promote organic
transformations, has received great attention by organic
chemists. The transformation of organic molecules utilizing the
power of light has become a powerful approach for the activation
of organic molecules and chemical bonds, enabling chemical
transformations that would otherwise be inaccessible.[2]
Ciamician was the first to perform organic reactions using light.[3]
Though, the true power of photochemistry was highlighted by
the establishment of photoredox catalysis by Macmillan,[4]
Yoon[5] and Stephenson,[6] where it was proven the ability of
photoredox catalysts to introduce novel organic transformations.
As a result, in the following years, more chemists took
advantage of photocatalysis for the construction of complex
molecules.[7]
The addition of bromoalkanes to alkenes is a very useful
reaction, as one can achieve in one reaction, the augmentation
of the carbon atom chain and the addition of moieties that can
be further functionalized with relative ease. In 1945, Kharasch
reported that halocarbons, such as CCl4, could be added to
olefins through an atom transfer radical addition (ATRA)
process.[8] A series of examples of ATRA reactions employing
haloalkanes have been reported in literature, most of which are
metal-catalyzed processes, employing ruthenium,[9] iron,[10]
nickel,[11] palladium,[12] iridium[13] and copper[14] based catalysts.
Photochemistry has also contributed in the development of
ATRA reactions, either employing metal-based photocatalysts[15]
or organic molecules.[16,17] Melchiorre, through his seminal work,
has developed ATRA reactions into a useful tool in organic
chemistry for various radical transformations, employing
anisaldehyde as the photocatalyst.[18]
Scheme 1. Synthetic methodologies for ATRA reactions with
bromotrichloromethane.
bromotrichloromethane and alkenes, in 2002, Oshima
developed a method for the atom transfer radical addition of
bromotrichloromethane onto olefins using a palladium complex
as the catalyst (Scheme 1, A).[19] In 2008, Majoral reported the
radical addition of bromotrichloromethane to terminal alkenes
using a ruthenium-based catalyst (Scheme 1, A).[20] In both
cases, the use of additives and the high excess of
bromotrichloromethane were required for the reaction to proceed.
Finally, in 2001, Oshima reported
a method, in which
triethylborane/O2 was used as the initiator system in water to
provide the corresponding products in good yields.[21] Only one
example using bromotrichloromethane was demonstrated, and
the reaction was stoichiometric (Scheme 1, B). Later,
More specifically, regarding the ATRA reaction between
1
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10.1002/ejoc.202001387
European Journal of Organic Chemistry
COMMUNICATION
Table 2. Optimization of the reaction conditions for the photochemical atom
transfer radical addition of bromotrichloromethane to 1a.
Stephenson presented an iridium-based photochemical protocol
for the radical addition of haloalkanes onto olefins (Scheme 1,
C).[22] However, only one example using bromotrichloromethane
was presented and the use of an additive was required. Later,
he expanded the scope with a ruthenium-based catalyst.[14a]
Along the same lines, our group developed a photocatalytic
method, where custom-made ruthenium complexes could be
employed as the photocatalyst (Scheme 1, D).[23] In 2015, a
visible-light mediated transformation, without the use of a
photocatalyst, was reported by Zeitler. In that case, blue LEDs
were required to be used as the light source and only one
example was presented and the reaction was possible in the
presence of two equivalents of BrCCl3.[24]
Entry
Solvent
Yield (%)[a]
1
2
Pet. Eth.
CHCl3
98
86
72
75
0
3
Benzene
THF
4
Unfortunately, metal-based complexes can be expensive, and
some are not commercially-available. In order to solve this
problem, a number of organic dyes have been employed as
photocatalysts, which in many cases provide similar reactivities
to metal catalysts.[25] Photoorganocatalysis, the use of small
5
DMSO
6
t-Amyl alcohol
MeCN
83
90
96
0
7
8[b]
9[c]
10[d]
Pet. Eth.
Pet. Eth.
Pet. Eth.
organic molecules as catalysts, is
a
low-cost and
environmentally friendly alternative.[26,27] We have recently
developed a photochemical protocol that is easy to operate,
employing cheap household lamps as the irradiation source and
a cheap, organic, and commercially available molecule as the
catalyst.[28] Herein, we report a simple and versatile method for
the mild and green addition of bromotrichloromethane to alkenes,
using phenylglyoxylic acid as the promoter under photochemical
conditions (Scheme 1, E). The present transformation
functionalizes a diverse array of olefins and generates the
corresponding haloalkanes in high yields, using low catalyst
loading.
6
[a] Yield of isolated product, after column chromatography. [b] 1.2 equiv. of
BrCCl3 was employed. [c] The reaction was performed in the dark. [d] The
reaction was performed without the use of PhCOCOOH. CFL lamps: 2 x 85
Watts.
1-decene (1a), we examined various wavelengths of LEDs or
CFL household lamps as the light source (Table 1). We studied
the reaction in the presence and in the absence of
phenylglyoxylic acid (3a) in various wavelengths. More
specifically, under the irradiation of compact fluorescence lamps
(CFL), the presence of the phenylglyoxylic acid facilitated the
formation of the product, providing a quantitative yield, while in
the absence of 3a the reaction turned out to be sluggish (Table 1,
entry 1). On the other hand, under the use of LEDs, the
differences between the reaction yields, in the presence versus
the absence of phenylglyoxylic acid, are trivial (Table 1, entries
2-7). Also, it is apparent that increasing the wavelength of the
LED, the yield of the reaction drops (Table 1, entries 3-5), until
we reach 456 and 525 nm LEDs, where the product is not even
detected. The yield differences between our study and
literature[24] could be accounted in various factors, such as the
reagent stoichiometry, the different solvent and substrate, as
well as the difference of the power and source of the LED lamp.
Next, we turned our attention into optimizing the reaction
conditions, employing phenylglyoxylic acid as the photochemical
promoter (Table 2). Initially, a range of organic solvents was
tested; however, in all cases, the expected product 2a was
obtained in a lower yield (Table 2, entries 2-7). After identifying
petroleum ether as the appropriate solvent for the reaction, we
further proceeded with the optimization of the reaction conditions
(Table 2, entries 8-10).[29] In an attempt to lower the amount of
bromotrichloromethane, we employed almost stoichiometric
amounts, which led to slightly lower yield (Table 2, entry 8).
When the reaction was kept in the dark, 1-decene was not
converted into the corresponding product (Table 2, entry 9).
Finally, as stated before, in the absence of the phenylglyoxylic
acid, 2a was afforded in very low yield (Table 2, entry 10).
Results and Discussion
Our previous photochemical studies hinted that phenylglyoxylic
acid could be an excellent initiator for the atom transfer radical
addition (ATRA) of bromotrichloromethane to alkenes. Having in
mind the work of Zeitler[24] and in an effort to study the influence
of the wavelength of the irradiation source in the outcome of the
photochemical reaction between bromotrichloromethane and
Table 1. Optimization of the light source for the photochemical atom transfer
radical addition of bromotrichloromethane to 1a.
Entry
Wavelength
Yield (%) with 3a[a]
Yield (%) in the absence of
3a[a]
1
2
3
4
5
6
7
CFL
370
390
427
440
456
525
100
82
66
64
34
nd
nd
6
79
66
59
60
nd
nd
Then,
the
photochemical
radical
addition
of
bromotrichloromethane to 1-decene (1a) was examined to
identify the appropriate photoinitiator (Scheme 2). A variety of
known photoinitiators (3a-i) led to moderate to excellent yields.
[a] Yield determined by 1H-NMR using internal standard, nd: not detected. CFL
lamps: 2 x 85 Watts, LED lamps: Kessil PR 160L 40 Watts.
2
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10.1002/ejoc.202001387
European Journal of Organic Chemistry
COMMUNICATION
various olefins (Scheme 3). All tested substrates were converted
into the corresponding products in good to excellent yields.
Initially, the atom transfer radical addition was performed with
aliphatic olefins, leading to the corresponding products in
excellent yields (2a and 2b). Internal cyclic alkenes, which are
less reactive for ATRA reactions, including cyclooctene and 2-
norbornene, were good substrates, leading to the desired
products in high yields (2c and 2d). The class of allyl ethers
proved to be a very efficient class of substrates, since the
products were obtained in high yields (2e-2j). Moreover, terminal
olefins bearing a wide array of functionalities were efficiently
transformed into the corresponding products in good to high
yields (2k-2n). Furthermore, short-aliphatic chain containing
halides, like allyl chloride and 4-bromo-1-butene, reacted well,
since the corresponding products were obtained in good yields
(2o and 2p). 1,1-Disubstituted alkenes reacted also well, leading
to 2q in excellent yield. Vinylcyclohexene, bearing two double
bonds, performed the reaction successfully (2r), verifying the
presence of a long-lived radical intermediate. Interestingly, only
the terminal double bond reacted in the reaction conditions.
Scheme 2. Initiator screening for the photochemical reaction between
bromotrichloromethane and 1-decene. Yield determined by 1H-NMR.
In all cases, a very low catalyst loading of 1 mol% was employed, Diallyl ether was also successfully employed leading to cyclic
leading to excellent yields. Phenylglyoxylic acid (3a) proved to
be the ideal promoter, providing 2a in quantitative yield.
Our following step was the exploration of the substrate scope of
the photochemical ATRA reaction of bromotrichloromethane with
product 2s.Cyclization was possible not only with oxygen-based
molecules, but also with carbon-containing molecules (product
2t). Finally, naturally-occurring substrates were employed
(Scheme 4). When limonene was employed as the substrate,
the radical addition occurred only in the less substituted double
bond, leading to product 2u, in a 50:50 mixture of diastereomers.
β-Pinene reacted well, leading to the corresponding ring-opened
product 2v in high yield.
Scheme 4. Limonene and β-pinene as substrates.
In an effort to shed some light in the reaction mechanism, UV-
Vis experiments were performed. The association of an electron-
rich molecule with an electron acceptor can lead to the formation
of an aggregate, called electron donor-acceptor (EDA) complex.
EDA complexes are known in literature, since the 1950s,[30,31] but
only recently Melchiorre and others have identified them as
active species in photochemical reactions. An EDA complex
sometimes is recognized, when upon addition of the two
components, an increase in the UV absorbance of the mixture is
observed.[31] However, our studies proved that there is no EDA
formation in this case.[29]
In
a recent report, Yoon employed the quantum yield
measurement as a mechanistic tool.[32] A closed photocatalytic
system lacks chain propagation and exhibits a maximum
theoretical quantum yield of Φ = 1. The chain processes can
potentially provide multiple equivalents of product from each
Scheme 3. Substrate scope of the photochemical ATRA of
bromotrichloromethane to alkenes.
3
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10.1002/ejoc.202001387
European Journal of Organic Chemistry
COMMUNICATION
Scheme 5. Mechanistic studies for the photochemical ATRA of
bromotrichloromethane to alkenes.
Scheme 6. Proposed reaction mechanism.
photon absorbed, therefore the quantum yield of a chain
reaction is Φ >> 1. Using potassium ferrioxalate as the
actinometer,[32,33] we calculated the quantum yield of the reaction
(Φ = 11), indicating a chain propagation mechanism.[29] Further
optimization and mechanistic studies proved that the reaction
has the characteristics of a radical process, since addition of
BHT or TEMPO prevents the reaction from occurring (Scheme 5,
A and B).[29]
bond leading to radical (IV), which reacts with another molecule
of bromotrichloromethane, leading to product 2a and
trichloromethyl radical (III), which takes part in a radical
propagation chain process, which is supported by the quantum
yield calculated [Φ > 1].[29] Thus, the role of phenylglyoxylic acid
is merely that of an initiator.
Next, we studied the photochemical reaction by NMR
spectroscopy.[29] Previous studies in our group proposes that the
excited triplet state of phenylglyoxylic acid decomposes to
different products upon irradiation in different solvents.[28] The
Conclusions
In conclusion, we have developed a green, metal-free and
efficient
method
for
the
radical
addition
of
1
process was monitored by 13C- and H-NMR spectroscopy and
bromotrichloromethane to olefins. Various sources of irradiation,
CFL lamps and LED lamps of various wavelengths, were tested
both in the presence and absence of phenylglyoxylic acid. In
CFL lamps, the presence of PhCOCOOH is crucial for product
formation, while in LED lamps the yields were comparable. After
optimization, the optimum reaction conditions were found and
applied in a variety of olefins. Mechanistic studies revealed that
phenylglyoxylic acid can be employed in a very low catalyst
loading (1 mol%) for the introduction of a low-cost, simple,
photochemical protocol for the synthesis of a wide variety of
bromo-compounds in excellent yields.
our observations state that excited phenylglyoxylic acid, in
petroleum ether, photodecomposes mainly to benzaldehyde.[29]
The products of the photodecomposition are observed both in
the spectra from the irradiation of phenylglyoxylic alone and of
the reaction mixture.[29] According to literature precedents[28] and
our own observations,[29] this means that excited PhCOCO2H is
decomposing to benzoyl radical, which is responsible for the
initiation of the process. Furthermore, in the absence of either
the catalyst or light, the reaction does not proceed. Finally, on-
off experiments were carried out and proved that constant
irradiation is necessary for the reaction to proceed (Figure 1).[29]
Based on the above experiments,
a proposed reaction
Experimental Section
mechanism is depicted in Scheme 6. Phenylglyoxylic acid upon
irradiation, is excited to its singlet and triplet state which can
then lead to its decomposition to benzoyl radical (I).[28,29] In the
presence of bromotrichloromethane, the benzoyl radical (I) can
abstract the bromine atom furnishing benzoyl bromide (II) and
trichloromethyl radical (III) via a halogen atom transfer (XAT).[34]
The latter radical can then react with an electron-rich double
Green Photochemical Atom Transfer Radical Addition (ATRA) of
BrCCl3 to Olefins
In a glass vial with a screw cap containing phenylglyoxylic acid (1.5 mg,
0.01 mmol, 0.01 equiv.) in Petroleum Ether (2 mL), alkene (1.00 mmol,
1.00 equiv.) and BrCCl3 (296 mg, 1.50 mmol, 1.50 equiv.) were added
consecutively. The vial was sealed with a screw cap and left stirring
under household bulb irradiation (2 x 85 Watts) for 18 h. The desired
product was isolated after purification by column chromatography.
Acknowledgements
The authors gratefully acknowledge the Hellenic Foundation for
Research and Innovation (HFRI) for financial support through a
grant, which is financed by 1st Call for H.F.R.I. Research
Projects to Support Faculty Members & Researchers and the
procurement of high-cost research equipment grant (grant
number 655 N. F. N. would like to thank the State Scholarships
Foundation (IKY) for financial support through a doctoral
fellowship, which is co-financed by Greece and the European
Union (European Social Fund-ESF) through the Operational
Figure 1. On-off experiments for the photochemical reaction between pentanal
and 1-decene.
4
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10.1002/ejoc.202001387
European Journal of Organic Chemistry
COMMUNICATION
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Keywords: Photochemistry · Green Chemistry · ATRA · Olefins
· Bromotrichloromethane
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10.1002/ejoc.202001387
European Journal of Organic Chemistry
COMMUNICATION
Photochemistry
Nikolaos F. Nikitas, Errika Voutyritsa,
Petros L. Gkizis and Christoforos G.
Kokotos *
Page No. – Page No.
Metal-free Photochemical Atom
Transfer Radical Addition (ATRA)
of BrCCl3 to Alkenes
Photochemistry. Phenylglyoxylic acid can be employed in a very low catalyst
loading (1 mol%) for the introduction of a low-cost, simple, photochemical protocol
for the ATRA reaction of BrCCl3 with olefins.
Institute and/or researcher Twitter usernames: @KokotosCG
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