Light-Induced Intramolecular Iodine-Atom Transfer Radical Addition of Alkyne: An Approach from Aryl Iodide to Alkenyl Iodide

Article abstract of DOI:10.1021/acs.orglett.9b03519

A light-induced intramolecular iodine-atom transfer radical addition was reported toward iodine-substituted fluorene derivatives. A thioxanthone derivative was employed as a visible light sensitizer for this 5-exo-dig exclusive radical addition, and the newly formed vinyl iodide compounds were further proved to be effective partners for several cross-coupling reactions.

Full text of DOI:10.1021/acs.orglett.9b03519

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Light-Induced Intramolecular Iodine-Atom Transfer Radical Addition
of Alkyne: An Approach from Aryl Iodide to Alkenyl Iodide
Yanbin Zhang,† Jincheng Xu, and Hao Guo*
†
Department of Chemistry, Fudan University, 2005 Songhu Road, Shanghai 200438, PR China
*
S Supporting Information
ABSTRACT: A light-induced intramolecular iodine-atom
transfer radical addition was reported toward iodine-
substituted fluorene derivatives. A thioxanthone derivative
was employed as a visible light sensitizer for this 5-exo-dig
exclusive radical addition, and the newly formed vinyl iodide
compounds were further proved to be effective partners for
several cross-coupling reactions.
−
I bonds are commonly considered as the most reactive
C−halogen bond in organic transformations, including
sources and alkenes and showed great potential in applications.
Consistent with the electron transfer process (VLPC), another
C
22
nucleophilic substitutions, cross-coupling reactions, and trans-
formations that involve radicals. However, the reactive nature
aspect of photochemistry is the energy transfer process. The
excited photocatalyst donates the excitation energy directly to
the substrate to excite the reactant. With the direct energy
transfer process, the Melchiorre group realized photo-organo-
catalyzed halogen ATRA to alkene employing 4-methoxyben-
zaldehyde as the photosensitizer via a direct energy transfer
1
of the C−I bond makes the synthesis of the C−I bond very
challenging because it requires accurate control to avoid
further cleavage of the newly generated C−I bond.
Vinyl iodide is one of the most useful scaffolds for direct
alkene functionalization. The vinyl C−I bond could easily
undergo oxidative addition to form a carbon−metal
23
process.
Nevertheless, previous works in photochemical activation
2
intermediate for varies transformations. Thus, the construc-
3
mainly focused on C(sp )−I bond cleavage-based I-ATRA;
tion of vinyl iodide has attracted a growing amount of attention
in the past few decades. One of the typical methods is to use a
substituted alkene with leaving groups [Br, B(OH) , TMS,
COOH, and H ] to react with the corresponding iodine
source, such as I , I , or NIS (Scheme 1A). Although many
studies were carried out for this type of transformation, the
substrate scope is greatly limited due to the leaving groups and
the atom economy of the reaction is low. Another way to
synthesize vinyl iodide is difunctionalization of alkynes.
Normally, an “initiating reagent (R source)” is employed
2
photoinduced C(sp )−I bond cleavage was reported only
2
4
3
3
4
5
recently. The C(sp )−I bond has a bond energy (∼55 kcal/
2
2
6
7
mol) that is lower than that of the C(sp )−I bond (∼65 kcal/
25
2
−
mol). Because of the high frequency of the C(sp )−I bond in
2
2
organic compounds, methods aiming at C(sp )−I bond
cleavage coupled with ATRA are highly desirable. A few issues
should be addressed with respect to this transformation. First,
8
2
the high bond energy of the C(sp )−I bond implies that a
3
higher excited energy is required, but a higher energy may
result in the unexpected decomposition of the substrate.
Second, after the I-ATRA process, the newly formed C−I bond
in the product remains reactive for further cleavage. Thus,
accurate control of C−I bond cleavage is necessary for effective
I-ATRA reaction. With these considerations in mind, we
envisioned that intramolecular I-ATRA from aryl iodide to
alkenyl iodide is reasonable to achieve with a proper
photosensitizer, because the Calkenyl−I bond is relatively more
and an iodinating reagent serves as a quenching reagent for the
formation of vinyl iodide (Scheme 1B).
Iodine-atom transfer radical addition (I-ATRA) shows a
high atom economy in C−I bond formation. Pioneering
9
10
research on I-ATRA reactions by Curran, Taguchi, and
1
1
others provided useful organic synthesis methods. However,
the classic I-ATRA reaction requires toxic and hazardous
initiators, such as AIBN, organotin, Et B, or peroxides.
Transition metal catalysts could also be used for I-ATRA
3
25
stable than the Caryl−I bond according to bond energy, so
1
2
reactions. Methods involving other intermediates, such as
1
3
14
15
that the product will be stable under the reaction conditions
(Scheme 1D). To overcome the problems mentioned above, a
suitable photosensitizer must be carefully designed, which is
active enough to excite the substrate without unpredictable
decomposition but not too active to react with the newly
anion, cation, benzyne formation, and ligand-controlled
16
reductive elimination, have been reported but are relatively
rare. It has been reported that a radical intermediate could be
formed with the assistance of visible light photoredox catalysis
1
7
(
VLPC). Utilizing the photogenerated radical intermediate,
several groups developed intermolecular halogen ATRA via
1
8
19
20
21
Mn-based, Cu-based, Ru-based, or Ir-based VLPC
Scheme 1C). These reactions used a wide range of halogen
Received: October 4, 2019
(
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03519
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Synthesis of Vinyl Iodide
a
generated product. Herein, we report our recent results on
intramolecular I-ATRA of alkyne under visible light irradiation.
Initially, the visible light-induced I-ATRA reaction was
explored using 2-iodo-2′-[(4-methoxyphenyl)ethynyl]-1,1′-bi-
phenyl (1a) as a model substrate with a variety of visible light
photosensitizers. The key photophysical characteristics of the
selected visible light photosensitizers, including absorption
peaks (λmax), triplet energies (E ), and triplet lifetimes (τ ),
Table 2. Optimization of the Reaction Conditions
b
entry photosensitizer
light
concn (mM) time (h) yield (%)
T
T
26
c
are listed in Table 1. Commercially available organic dye
1
2
3
4
5
6
7
8
I
I
I
I
lamp
20
20
10
2
2
2
2
2
2
2
48
48
48
48
24
24
24
24
24
24
24
24
5 (93)
36 (61)
59 (40)
c
c
PLED
PLED
PLED
PLED
PLED
PLED
PLED
PLED
PLED
PLED
dark
a
Table 1. Screening of the Photosensitizer
c
93 (7)
c
c
c
d
I
90 (10)
38 (61)
54 (44)
94 (90)
92 (8)
19 (77)
II
III
IV
V
VI
−
IV
c
9
10
λmax
(nm)
E
τT
(μs)
yield
c
c
T
b
b
b
entry photosensitizer
(kJ/mol)
(%)
1
1
1
2
2
2
NR
NR
1
2
3
4
5
methylene blue
perylene
9-fluorenone
2,3-butanedione
thioxanthone
664
438
393
415
407
138
151
211
236
265
450
5000
100
145
95
NR
NR
NR
NR
a
All reactions were carried out using 1a (0.1 mmol for 2 mM
reactions, 0.2 mmol for 10 and 20 mM reactions) and a thioxanthone
derivative (5 mol %) in anhydrous CH Cl irradiated by the
corresponding light source under an argon atmosphere at rt.
Abbreviations: lamp, 23 W household lamp; PLED, purple LED
(for the detailed setup and emission spectra, see the Supporting
d
2
2
5 (93)
a
All reactions were carried out using 1a (0.2 mmol) and a
photosensitizer (5 mol %) in anhydrous CH Cl (10 mL) irradiated
2
2
b
1
by a 23 W household lamp under an argon atmosphere at rt for 48 h.
Information). Yield determined by H NMR analysis of the crude
reaction mixture using CH Br as the internal standard. Recovered
c
Abbreviations: lamp, 23 W household lamp; λmax, maximum
2
2
b
d
absorption wavelength; E , triplet energy; τ , triplet lifetime See
T
T
yield of 1a. Isolated yield.
c
1
ref 26. Yield determined by H NMR analysis of the crude reaction
mixture using CH Br (0.2 mmol) as the internal standard.
2
2
d
Recovered yield of 1a.
thioxanthone (for the emission spectra of a purple LED, see
Figure S1). A dramatically increased yield was observed (Table
2, entry 2). Concentration is also an important parameter in
photochemistry, which can greatly influence the efficiency of
absorbing light according to Lambert−Beer’s law. Thus, a
reaction with a 10-fold lower concentration was carried out,
affording 93% 2a with complete conversion (Table 2, entry 4).
Then, photosensitizers of different substituted thioxanthone
derivatives were tested, among which 4-methoxy thioxanthone
(IV) showed the best yield (Table 2, entry 8). Finally, control
experiments without a catalyst (Table 2, entry 11) or light
(Table 2, entry 12) were carried out. No reaction occurred
under either condition. These results indicated that both a
catalyst and light were necessary in this reaction. Thus,
condition A [1, IV (5 mol %), CH Cl , purple LED, and rt]
methylene blue was first tried in this reaction. Unfortunately,
no reaction took place (Table 1, entry 1). Other photo-
sensitizers with either a higher triplet energy or a longer triplet
lifetime were also tested, but no desired results were obtained
(Table 1, entries 2−4). When thioxanthone was employed as a
photosensitizer, the desired I-ATRA product 2a was formed,
although the reaction rate was very low (Table 1, entry 5).
Considering that thioxanthone has the highest triplet energy
among these tested photosensitizers to enable the I-ATRA
reaction and the absorbance maxima of thioxanthone lies in the
visible region, thioxanthone was chosen as the photosensitizer
for the following optimizations.
With the original screening of the proper photosensitizer, we
continued to optimize the reaction conditions (Table 2). A
purple light-emitting diode (LED) was used instead of a 23 W
household lamp with consideration of the absorption spectra of
2
2
was applied as the optimized condition for further studies.
With the optimized conditions in hand, the substrate scope
of this visible light-induced I-ATRA reaction was explored. As
B
DOI: 10.1021/acs.orglett.9b03519
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
shown in Scheme 2, substrates with a strong electron-donating
group (1a), a weak electron-donating group (1b), or no
reaction gave the desired product in good to excellent yield. A
slightly decreased yield was obtained for 1k−n. Similarly, no
other C−X bond cleavage occurred during the I-ATRA process
(1l−n). For the substrate bearing one strong electron-
withdrawing group and two strong electron-donating groups
a
Scheme 2. Substrate Scope
(
1o), the I-ATRA reaction yielded the corresponding product
2
3
with a good yield. The substrate with only Ar and Ar (1p−r)
did not show reactivity under optimized conditions (for a
detailed discussion, see Scheme S1). A gram-scale reaction
using 1a was carried out and gave a good yield.
With those readily available compounds in hand, we also
tested some coupling reactions with 2a. As shown in Scheme 3,
Scheme 3. Diversification of I-ATRA Product 2a
using 2a as a coupling partner, Suzuki coupling was achieved in
95% yield and the product was 3a. Sonogashira coupling of 2a
with TMSA also gave 3b in an excellent yield, and Kumada
coupling was also feasible for 2a. Thus, the formed vinyl iodide
products are viable substrates for the coupling reactions,
indicating the potential application of the I-ATRA product.
To gain insight of the mechanism, several control experi-
ments were designed (Table 3). First, to define the exact role
a
All reactions were carried out using 1 (0.1 mmol) and IV (5 mol %)
in anhydrous CH Cl (50 mL) irradiated by a purple LED under an
argon atmosphere at rt for 24 h. Isolated yield reported. Gram-scale
reaction. Reaction carried out at 60 °C.
2
2
b
a
c
Table 3. Mechanistic Studies
3
substitution (1c) in Ar showed decent reactivity. Halogen
atoms, like Br (1d), Cl (1e), and F (1f), were introduced onto
3
Ar , affording similar excellent yields of the desired products.
Notably, the C−Br bond normally shows a reactivity similar to
that of the C−I bond in the aspect of C−X bond cleavage of
both organometallic catalysis or photoinduced cleavage, but in
this case, the C−Br bond was tolerated while the C−I bond
proceeded the I-ATRA reaction (2d). This observation
indicated a selectivity between the C−Br bond and the C−I
bond in this reaction. A methoxycarbonyl group (1g), as a
entry
additional conditions
purged with oxygen
styrene (25 mol %)
ethyl 4-(dimethylamino)benzoate (25 mol %)
TEMPO (5 equiv)
yield (%)
b
1
2
3
4
5
3 (22)
b
49 (41)
55 (35)
17 (30)
b
b
(n-Bu) NBr (5 equiv)
52:40 2a:4a
3
4
strong electron-withdrawing group, was introduced onto Ar
and gave a decreased yield. A cyano group (1h) dramatically
inhibited this reaction. A higher temperature was required for
this transformation. Next, some trisubstituted substrates were
applied under the optimized condition. With an electron-
a
All reactions were carried out using 1a (0.1 mmol) and IV (5 mol %)
in anhydrous CH Cl (50 mL) irradiated by a purple LED under an
2
2
argon atmosphere at rt for 24 h unless otherwise noted. Yield
determined by H NMR analysis of the crude reaction mixture using
CH Br as the internal standard. Recovered yield of 1a.
1
1
2
3
b
donating group in Ar , Ar , and Ar (1i and 1j), the I-ATRA
2
2
C
DOI: 10.1021/acs.orglett.9b03519
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
of catalyst in this I-ATRA reaction, three triplet quenchers that
are especially sensitive to thioxanthone were added to the
reaction system. Oxygen could quench the reaction
effectively. On the other hand, 75% of 1a decomposed into
some undefined compounds under an oxygen atmosphere
Scheme 4. Proposed Mechanism
2
6
(
Table 3, entry 1). Oxygen might be the triplet quencher but
could also inhibit this reaction by oxidative decomposition of
1
a due to its oxidative property. When styrene was applied,
4
9% of 2a was formed and 41% of 1a was recovered (Table 3,
entry 2). The reaction in the presence of another triplet
quencher, ethyl 4-(dimethylamino)benzoate, showed a similar
result (Table 3, entry 3). These results demonstrated that the
addition of triplet quenchers for thioxanthone restrained this I-
ATRA reaction effectively, which strongly indicated that the
triplet excited state of IV was likely to be involved in this
transformation.
After obtaining a clear understanding of the role of the
catalyst, we focused on how this reaction proceeded. Upon
excitation of 1a, the C−I bond was likely to undergo
homolysis, giving the radical intermediates. Thus, 5 equiv of
TEMPO was added under condition A; 17% of 2a was formed,
and 30% of 1a was recovered (Table 3, entry 4). Although the
reaction was strongly inhibited, it was still confusing that even
5
equiv of TEMPO could not completely inhibit the reaction,
so we considered that the radical might be the predominant
but not the only intermediate. Heterolysis might also occur
during the initial transformation.
to yield 2. The bromo anion could also couple with 8 to
generate the corresponding bromide 4.
24g,h
Hence, a competitive
In conclusion, we developed a visible light-induced I-ATRA
2
reaction between the bromo anion and iodine anion was
designed, using 5 equiv of tetrabutylammonium bromide
reaction of C(sp )−I bonds using thioxanthone derivatives as
the photosensitizer. This photochemical method enabled the
2
(
TBAB) as the bromo anion source. Bromo-substituted
cleavage of one C(sp )−I bond and formation of another
2
product 4a was formed, and the 2a:4a ratio was 52:40
C(sp )−I bond under visible light irradiation without any
24h
(
Table 3, entry 5). This result proved that there was indeed
transition metal catalyst. Further study indicated that the
catalyst played the role of a visible light triplet photosensitizer.
24g,h
heterolysis that involved a cation during the reaction.
2
Upon careful comparison of the ratio of the “radical
intermediate” and “cation intermediate”, they seemed not to
be consistent in the TEMPO experiment and TBAB
experiment (Table 3, entries 4 and 5, respectively). These
two control experiments seemed to have a contrast between
the ratio of the radical intermediate and cation intermediate.
An equilibrium between homolysis and heterolysis could
possibly explain this observation. The equilibrium would tune
to heterolysis to form the ion intermediate and gave more 4a
with 5 equiv of TBAB. Thus, a mechanism including both
homolysis and heterolysis was reasonable for this I-ATRA
reaction.
Both homolysis and heterolysis of the C(sp )−I bond were
likely involved in this transformation. Further study of the
applications of this I-ATRA reaction to organic synthesis is in
progress in our laboratory.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b03519.
Experimental details and characterization data for the
products (PDF)
With the evidence presented above and literature precedent,
a complete mechanism was proposed as shown in Scheme 4.
Upon visible light irradiation, IV underwent an n−π*
transition to its singlet excited state and relaxed to its first
singlet excited state, IV (S ). Due to its high instability, IV (S )
AUTHOR INFORMATION
Corresponding Author
■
1
1
*
E-mail: Hao_Guo@fudan.edu.cn.
proceeded via intersystem crossing (ISC) to its first triplet
ORCID
excited state, IV (T ). This is a relatively long lifetime
1
intermediate, which was also the key excited state in this
reaction. Then, an energy transfer occurred between IV (T1)
Yanbin Zhang: 0000-0002-4087-420X
Hao Guo: 0000-0003-3314-4564
Author Contributions
2
6
and substrate 1 to form triplet 1* (T ), and IV returned to its
1
ground state for another catalytic cycle. 1* (T ) could proceed
†
1
Y.Z. and J.X. contributed equally to this work.
Notes
via reversible homolysis to generate radical 5 and an iodine
radical. Intramolecular radical addition of 5 produced cyclized
alkenyl radical 6, which would couple with the iodine radical to
furnish product 2. Meanwhile, a reversible heterolysis
The authors declare no competing financial interest.
24g,h
ACKNOWLEDGMENTS
The authors greatly acknowledge the financial support from
Shanghai Science and Technology Committee
occurred, producing cation 7 and an iodine anion.
The
■
intramolecular CC bond might attack the cation to form
cyclized alkenyl cation 8. The iodine anion could couple with 8
D
DOI: 10.1021/acs.orglett.9b03519
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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