Hydrogen Atom Transfer Reactions via Photoredox Catalyzed Chlorine Atom Generation

Article abstract of DOI:10.1002/anie.201810187

The selective functionalization of chemically inert C?H bonds remains to be fully realized in achieving organic transformations that are redox-neutral, waste-limiting, and atom-economical. The catalytic generation of chlorine atoms from chloride ions is one of the most challenging redox processes, where the requirement of harsh and oxidizing reaction conditions renders it seldom utilized in synthetic applications. We report the mild, controlled, and catalytic generation of chlorine atoms as a new opportunity for access to a wide variety of hydrogen atom transfer (HAT) reactions owing to the high stability of HCl. The discovery of the photoredox mediated generation of chlorine atoms with Ir-based polypyridyl complex, [Ir(dF(CF₃)ppy)₂(dtbbpy)]Cl, under blue LED irradiation is reported.

Full text of DOI:10.1002/anie.201810187

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Accepted Article
Title: HAT Reactions Via Photoredox Catalyzed Chlorine Atom
Generation
Authors: Louis Barriault, Terry McCallum, Samantha Rohe, and Avery
Morris
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201810187
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Dual Catalysis
DOI: 10.1002/anie.201((will be filled in by the editorial staff))
HAT Reactions Via Photoredox Catalyzed Chlorine Atom
Generation**
Samantha Rohe, Avery O. Morris, Terry McCallum, and Louis Barriault*
Abstract: The selective functionalization of chemically inert CH
bonds remains to be fully realized in achieving organic
transformations that are redox-neutral, waste-limiting, and atom-
economical. The catalytic generation of chlorine atoms from chloride
ions is one of the most challenging redox processes, where the
requirement of harsh and oxidizing reaction conditions renders it
seldom utilized in synthetic applications. We report the mild,
controlled, and catalytic generation of chlorine atoms as a new
opportunity for access to a wide variety of hydrogen atom transfer
(HAT) reactions owing to the high stability of HCl. The discovery of
the photoredox mediated generation of chlorine atoms with Ir-based
polypyridyl complex, [Ir(dF(CF3)ppy)2(dtbbpy)]Cl, under blue LED
irradiation is reported.
Examples of photocatalysts such as TBADT and Eosin Y
can perform HAT processes, however, the use of photoredox
HAT dual catalysis using additives such as thiols, quinuclidines,
amides, sulfonamides, and phosphates has become an actively
researched method for alkyl radical formation.[5] Largely
unexplored, the catalytic generation of chlorine atoms using
photoredox processes would represent an significant
advancement towards facile generation of chlorine atoms and
their applications towards HAT reactions in organic synthesis
and materials science.[6] To the best of our knowledge, few
examples exist of such a process.[7] Recently, advances in
IrIII/Ni0 dual photoredox HAT catalysis have enabled the cross-
coupling of alkanes (as solvent) with haloarenes through a
proposed halogen atom (Br or Cl) intermediate (Scheme 1, b).[8]
Chlorine atoms have traditionally been accessed by relatively
harsh reaction conditions such as photolysis or thermolysis
processes of chlorine gas (Scheme 1, a).[1] The requirement for
such conditions has rendered methodological advancements
limited to alkane chlorination reactions because many functional
groups are unstable to the conditions or the high reactivity of the
generated chlorine atom. Much progress has been made with
respect to understanding the highly reactive intermediate in
comparison to its more selective bromine counterpart with
respect to alkane halogenation reactions.[2] It has been generally
accepted that chlorine atoms are stabilized by solvents such as
benzene and pyridine, which can attenuate the highly reactive
chlorine atom and influence its ability to be more selective in
these processes. A mild and catalytic generation of chlorine
atoms would represent a breakthrough into new chemical space
and enable studies towards new organic transformations.
Scheme 1. Cl atom-mediated transformations in organic synthesis.
Advances in photoredox catalysis have led to the
discovery of creative new organic transformations as well as
innovative strategies that improve on classic methods of alkyl
radical initiation (tributylstannanes, AIBN, triethylborane,
peroxides).[3] The structural architecture of Nature’s light-
harvesting bio-complexes have inspired chemists ability to
harness the energy of a photon for its transformation into kinetic
energy.[4] Transition metal and organic dye based photocatalysts
enable the use of less energetic wavelengths to initiate
photochemical processes that circumvent degradation pathways
associated with direct excitation of organic substrates. Highly
reactive excited state complexes trigger reductive or oxidative
quenching processes that allow access to highly reactive
organic intermediates seldom accessed through other methods.
Herein we report the photoredox mediated catalytic
generation of chlorine atoms and their ability to undergo
hydrogen atom transfer (HAT) reactions with a variety of
substrates such as alkanes, alcohols, ethers, ester, amides,
aldehydes, and silanes, for their applications in the redox-neutral
Giese-type addition to activated alkenes (Scheme 1, c). The
dual catalytic photoredox/HAT strategy targets hydridic C-H
bonds facilitated by the electrophilic nature of chlorine atoms,
enabling the genesis of relatively nucleophilic radical
intermediates that added to electrophilic alkenes. The selective
HAT transformation is supported with mechanistic studies that
indicate a closed catalytic cycle and the control effect of
coordinating solvent over the highly reactive chlorine atom.
[*]
[]
S. Rohe, A. O. Morris, Dr. T. McCallum, Dr. L. Barriault
Centre for Catalysis, Research and Innovation
Department of Chemistry and Biomolecular Sciences
University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada
E-mail: lbarriau@uottawa.ca
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/anie.201810187
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After establishing the optimal conditions for this
transformation, an investigation of the alkane coupling partner
tolerance for the dual photoredox HAT catalytic alkylation of
maleates/fumarates was performed (Table 2, a). Cyclopentane,
cyclooctane, and adamantane 1abad underwent the coupling
with 2a in 69%, 53%, and 61% (62:38 HA:HB) yields, respectively.
It should be noted that benzyl CH bonds on substrates such as
Table 1. Optimization of the reaction conditions.[a]
Table 2. Scope for Cl atom-mediated HAT reactions.[a]
TBACl
[mol%]
Conv.
[%]
Yield
[%]
Entry
X
[mol%]
1
2
PF6
PF6
PF6
PF6
Cl
2
2
10
5
100
100
100
20[b]
100
100
100
0[c]
80
84
84
0
3
1
20
---
5
4
1
5
1
89
84
86(68)
0
6
Cl
1
---
---
---
---
20
7
Cl
2
8
Cl
2
9
Cl
2
0[d]
0
10
---
---
0
0
[a]
See Experimental Section, reactions run between 60 and 80°C. 2a =
dimethyl maleate; 2d = dimethyl fumarate. [b] 2a (80%) as 1.73:1 Z:E (2a:2d).
[c] Reaction run at room temperature. [d] In absence of irradiation.
To begin, an initial screening of reaction conditions for the
photoredox mediated generation of chlorine atoms identified
[Ir(dF(CF3)ppy)2(dtbbpy)]PF6 (3-PF6, 2 mol%) with TBACl (10
mol%), cyclohexane (1aa, 3 equiv), and dimethyl maleate (2a, 1
equiv) in benzene (0.5 M) under 465 nm LED irradiation led to
4aaa in 80% yield (Table 1, entry 1, see Supporting Information).
Thrilled by this result, it was found that lowering the amount of
TBACl or photocatalyst were not detrimental to the yield of the
reaction (entries 2, 3, and 5). Little conversion was observed in
the absence of a chloride source, however, isomerization to the
fumarate (2d) was noticed (1.73:1 2a:2d) (entry 4). The
isomerization process was not observed in the absence of
photocatalyst which could be consistent with background energy
transfer mechanisms (entry 10). Considering that conditions
employing 1 mol% of 3-PF6 and 5 mol% of TBACl led to near
quantitative conversions of 1aa and 2a into 4aaa, we wondered
if [Ir(dF(CF3)ppy)2(dtbbpy)]Cl (3-Cl) could be used in this
transformation as a more convenient source of the photocatalyst
and chloride ion.[9] To our delight, 1 mol% of 3-Cl led to product
in 84% yield (entry 6). A slight increase in yield was observed
when using 2 mol% of the photocatalyst, giving product in 86%
yield (68% isolated, entry 7). This loading was used for all
further examples to ensure reactivity in challenging examples.
Finally, control experiments reveled that photocatalyst, light
irradiation, and heat were necessary for the transformation to
proceed (entries 810).
[a] 40%:34% 4aaa:4bfa.
toluene and mesitylene were ineffective in this transformation. n-
Butanol 1ae underwent the transformation in 36% yield without
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10.1002/anie.201810187
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COMMUNICATION
further lactone formation, however, cyclopentanol 1af underwent
the transformation in similar yield, giving lactonized product 4afa.
Ethers proved to be effective substrates in this transformation.
Common cyclic and acyclic substrates such as THF, pyran,
MTBE, iPr- and nBu- ether (1agah, 1ajal) underwent efficient
couplings at the α-position the most hydric position in yields
ranging from 55to 70%. Methoxycyclopentane 1ai gave 59%
yield of the corresponding product, isolated separately as a 3:1
ratio of the products deriving from HAT at the 3° and 1° positions.
Aryl methyl ethers 1amao provided the desired products in
5970% yields showing little difference in donating (OMe) and
withdrawing (F) substituents. 1,2-Dimethoxyethane 1ap gave
44% yield of the product corresponding to HAT at the 2° position
and 26% yield at the 1° position. Dioolane 1aq and dioxane 1ar
coupled efficiently in 70% (10:1 HA:HB, HB 1:1 d.r.) and 80%
yields, respectively. Interestingly, ketone 1as gave product in
63% yield chemoselectively towards the relatively nucleophilic
CH bond or hydridic position as opposed to the relatively
electrophilic α-keto CH bond. Ethyl acetate 1at also gave
product at the most hydridic position in 37% yield. Acyclic and
cyclic amides 1auaw furnished the desired product in some of
the best yields ranging 7496%. Notably, a variety of silanes
1axbb underwent efficient coupling in 5581% yields; an
interesting alternative to alkanes. Aryl and 1° aldehydes
(1bcbe) also engaged in the coupling reaction in good-to-
1ag:d8-1ag
3
---
---
58
75:25
3.0
(1:1)
[a] Isolated diastereomers in a 1:1 ratio. [b] Diastereomer 1 (less polar, 1:1 d.r.
with respect to D incorporation); Diastereomer 2 (more polar, 1:1 d.r. with
respect to D incorporation). Diastereomers assigned by 1H NMR analysis.
[c]
55:45 d.r.
Table 4. Solvent effects on the HAT reactions of 3-Cl and 3-Br with 1aa.
Entry
[M]
X
[mol%]
[%][a]
3°/1°
ArH
1
2
PhH
PhH
0.5
0.5
Cl
Br
Cl
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Br
Cl
Br
Cl
2
2
2
2
1
2
2
2
2
2
2
2
2
2
2
1
66
62
64
60
54
70
48
40
64
57
82
72
22
64
16
72
3.6
4.2
4.4
4.8
5.4
3.9
4.4
4.0
4.0
4.1
4.1
9.1
7.7
10.0
8.5
9.5
3
PhH
0.05
0.05
0.05
0.5
4
PhH
5
PhH
6
PhMe
PhOAc
PhCO2Me
PhC(O)Me
PhCF3
1ag
excellent
yields
(5299%).
The
treatment
of
7
0.5
cyclohexanecarboxaldehyde 1bf under the standard condition
gave a mixture of the desired ketone 4bfa along with the
decarbonylated product 4aaa in 34% and 40% yields,
respectively. Pivaldehyde gave only the decarbonylated product
4bga in 75% yield: a synthetically useful alternative to 2-
methylpropane in this HAT-based methodology. It should be
noted that when using phenylacetaldehyde, no addition of
benzyl substituents was observed. Quantitative conversion of
the triphenylphosphorane 1bh to the desired product 4bha was
observed in this transformation, however, separation from the
triphenylphosphine oxide by-products derived from the
decomposition of 1bh proved to be difficult. It was found that a
one-pot protocol could solve this problem, where benzaldehyde
was added after irradiation, giving 4bia in 51% yield (Table 2, b).
Notably, when attempting the photochemical transformation with
the parent cinnamaldehyde 1bi, no formation of the desired
product is observed. Finally, a variety of maleates and fumarates
2bf underwent coupling with 1aa in good-to-excellent yields
ranging 5887%.
8
0.5
9
0.5
10
11
12
13
14
15
16
0.5
0.5
Pyr
0.5
Pyr
0.5
Pyr
0.05
0.05
0.05
Pyr
Pyr
[a] Isolated yields where ratio of 4aia and 4aia’ were determined by 1H NMR
analysis.
Scheme 2. Photocatalyzed [2 + 2] cycloaddition with 5.
Table 3. Isotope labelling studies.
H2O
[equiv]
D2O
[mol%]
Yield[a]
[%]
KIE
[H/D]
Entry
4:d-4
RH/D
[a] 1H NMR yield using phenyltrimethylsilane as standard (isolated yield of 6).
1
2
1ag THF
---
10
---
65[b]
72[c]
0:100
100:0
---
---
Given the exciting results found during the investigation of
this transformation, mechanistic studies were conceived to
develop an understanding of the underlying mechanistic
d8-1ag
10
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10.1002/anie.201810187
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pathways. First of which, isotope labelling experiments were
performed to assess where deuterium incorporation may occur
(Table 3). These studies also allowed for the evaluation of the
extent to which chain reactions may occur. THF 1ag in the
presence of D2O (10 equiv) gave solely the deuterium
incorporated d-4aga without the observation of any scrambling
in 65% yield (Entry 1). This result is indicative of 1) the radical
intermediate formed after coupling with 2a must then be reduced
as opposed to undergoing chain reaction with a THF equivalent
(leading to incorporation of an H), 2) the product is not in
equilibrium with any other intermediates, and 3) ether substrates
have been responsible for this lack of desired product formation.
When considering that the triplet energy of 3-PF6 is 61 kcal/mol,
cinnamate 5 (55 kcal/mol) known to undergo photocatalyzed [2 +
2] cycloadditions was used as acceptor with and without
cyclohexane, where cyclobutane 6 was isolated in excellent
yield in both cases (6:6’, 7:1 d.r., Scheme 2).[11] In an attempt to
mitigate the background triplet-triplet energy transfer pathways,
the use of acridinium salts (react through the singlet state) or the
Cl salt of an oxidizing Ir-complex [Ir(dF(CF3)dtbbpy)2(5,5’-
dCF3bpy)]PF6 with a relatively low triplet energy (48 kcal/mol),
were found to be ineffective in the coupling of 1aa with a variety
of alkenes.[6e, 7]
proceed via
a
closed cycle mechanism. The opposite
experiment was performed using d8-1ag in the presence of
water (10 equiv) and gave the expected H-incorporated product
in 72% yield, further validating the hypothesis (Entry 2).
Furthermore, a competition experiment using a 1:1 mixture of
1ag:d8-1ag gave a 3:1 mixture of products derived from the
addition favouring the 1ag (KIE = 3, Entry 3). This result
supports the occurrence of a HAT process.[2e]
After establishment that the reaction pathway likely
proceeds through a closed cycle, we wondered if the ratio of
products observed in the reaction of 1ai could be increased
selectively towards the 3° position (Table 4). Using a solvent
that can attenuate the high reactivity of the chlorine atom would
After obtaining several mechanistic data, a product forming
pathway may be proposed (Figure 1). Upon excitation of 3,
M*/M-
reductive quenching of chloride may occur (3*, E1/2
= +1.21
V vs SCE; TBACl, E1/2 = +2.03 V vs SCE),[8b, 9] albeit under
large uphill thermodynamic requirements, giving chlorine atom
and IrII (3-A). This interaction may be facilitated by the
requirement of heat for the transformation to proceed and the
proximity of the chloride ion. Stern-Volmer fluorescence
quenching experiments found this process to be feasible when
red
considering kinetics (3-PF6, kqTBACl = 1.1 ± 0.1 x 106 M-1s-1; 3-Cl,
TBACl
kq
= 8.2 ± 0.9 x 105 M-1s-1). Several alkene types may
undergo background activation through triplet-triplet energy
be
a
valuable methodological advancement in HAT
transfer processes and was elegantly demonstrated in Hong’s
recent study (3-PF6, kq
cyclohexeneone
transformations. First, the comparison of 3-Cl and 3-Br
counterions were evaluated at various concentrations in
benzene (Entries 15). The reactions proceeded in 5466%
yields with 3-Br being slightly more selective at low
concentration (4.8 vs 4.4, Br:Cl) however, 3-Cl giving efficient
reactivity with a 5.4 selectivity at low concentration and 1 mol%
catalyst loading (3-Br gave little conversion at 1 mol%). To our
surprise, little change to the yield and selectivity of the
transformation was observed when varying the electronics of the
arene-based solvent (Entries 610). Spanning functional groups
such as Me, OAc, CO2Me, C(O)Me, and CF3, yields and
selectivity ranged 4070% and 3.94.4 (3-Cl 2 mol%, 0.5 M),
respectively, the results were similar to those of benzene; an
unexpected result given the body of literature that propose the
π-complexation of chlorine atoms.[2,10] Using 1ai as the solvent
similarly gave the product in 82% yield with a selectivity of 4.1
(Entry 11). One may conclude that little complexation of chlorine
=
3.2 x 108 M-1s-1).[12]
Quenching of 2a was found to be slower than chloride in our
study (3-PF6, kq2a = 6.3 ± 0.7 x 105 M-1s-1; 3-Cl, kq2a = 7.0 ± 1.6 x
105 M-1s-1) and may explain why maleates and fumarates are
tolerated under the described conditions. Upon generation of the
electrophilic chlorine atom, complexation to a solvent such as
pyridine may take place (Pyr-Cl), where the chlorine atom can
undergo HAT with a variety of substrates, giving nucleophilic
alkyl radical I.[10] This radical can add efficiently to an activated
alkene, giving intermediate II. When coupled with alkanes,
silanes, and ethers, intermediate II undergoes SET with 3-A (II,
red
M/M-
E1/2 = 0.6 V vs SCE; 3-A, E1/2
= 1.37 V vs SCE),[5o, 7,9,13,
14]
regenerating 3 and furnishing the desired product. It should
be noted that substrates such as aldehydes are known to
undergo chain reaction processes and this transformation is
likely no exception.[15]
Figure 1. Proposed mechanism.
atoms is evident considering these solvents or that
a
background chain reaction may be at play. Pleasingly, when
switching to pyridine (an efficient complex forming solvent), the
selectivity soared to 9.1 (Entry 12). Further, the selectivity of 3-
Br was found to be lower than 3-Cl, 7.7, which demonstrates
that a chlorine atom can be rendered less reactive than its mild
bromine atom comparison (Entry 13): a result that challenges
conventional wisdom with respect to chlorine atom reactivity.
Lowering the concentration, the selectivity was improved to 10.0
with 3-Cl, besting 3-Br (Entries 1416). These results show
great potential to furthering our knowledge in selective HAT
transformations and will be explored further on more complex
substrates in the future.
Other electron-rich-, styryl-, and enone-based alkenes
were examined, however, little reactivity or decomposition was
observed (see SI). As observed in Table 1 (entry 4), we
hypothesized that triplet-triplet energy transfer processes may
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10.1002/anie.201810187
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COMMUNICATION
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In summary, the catalytic generation of highly reactive
chlorine atoms using photoredox mediated activation of chloride
has been achieved. The chlorine atoms engaged with a variety
of hydridic CH bonds including alkanes, alcohols, ethers, ester,
amides, aldehydes, and silanes, for the redox-neutral Giese-type
addition of alkyl radicals to activated alkenes. Mechanistic
studies showed a closed catalytic cycle with ethers, the
attenuation of chlorine atoms to be more selective than bromine
atoms with pyridine as the solvent, and that background energy
transfer pathways were at play. These results challenge the
currently accepted information about chlorine atoms and future
studies evaluating these highly reactive intermediates in the
context of synthesis will be reported in due course.
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Photoredox generation of chlorine atom was reported during
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Chem. Int. Ed. 2018, In Press.
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Acknowledgements
We thank the Natural Sciences and Engineering Research
Council (NSERC) for the Discovery grant to L.B. T.M. thanks
NSERC for a Ph.D. scholarship (Alexander Graham Bell CGSD).
S.R. thanks the government of Ontario for an OGS scholarship.
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Wu, Angew. Chem. Int. Ed. 2017, 56, 15407.
Keywords: Hydrogen atom transfer • Photoredox catalysis •
Chlorine atom • Alkyl radicals • CH functionalization
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10.1002/anie.201810187
Angewandte Chemie International Edition
COMMUNICATION
This article is protected by copyright. All rights reserved.

10.1002/anie.201810187
Angewandte Chemie International Edition
COMMUNICATION
Radical Reactions
S. Rohe, A. O. Morris, T. McCallum, L.
Barriault* __________ Page – Page
HAT Reactions Via Photoredox
Catalyzed Chlorine Atom Generation
A HAT trick. The generation of highly reactive chlorine atoms through photoredox
mediated reductive quenching of *[Ir(dF(CF3)ppy)2(dtbbpy)]+ with chloride is reported.
The CH functionalization of a variety of alkanes, alcohols, ethers, ester, amides,
aldehydes, and silanes through hydrogen atom transfer (HAT) processes with chlorine
atoms underwent efficient redox-neutral coupling reactions with activated alkenes. The
catalytic nature of the polarity matched transformation was further investigated through
mechanistic studies.
This article is protected by copyright. All rights reserved.