Hypervalent iodine reagents enable chemoselective deboronative/decarboxylative alkenylation by photoredox catalysis

Article abstract of DOI:10.1002/anie.201410176

Chemoselective C(sp³)-C(sp²) coupling reactions under mild reaction conditions are useful for synthesizing alkyl-substituted alkenes having sensitive functional groups. Reported here is a visible-light-induced chemoselective alkenylation through a deboronation/decarboxylation sequence under neutral aqueous reaction conditions at room temperature. This reaction represents the first hypervalent-iodineenabled radical decarboxylative alkenylation reaction, and a novel benziodoxole-vinyl carboxylic acid reaction intermediate was isolated. This C(sp³/sup>)-C(sp²) coupling reaction leads to aryl-and acyl-substituted alkenes containing various sensitive functional groups. The excellent chemoselectivity, stable reactants, and neutral aqueous reaction conditions of the reaction suggest future biomolecule applications.

Full text of DOI:10.1002/anie.201410176

Angewandte
Chemie
DOI: 10.1002/anie.201410176
Hypervalent Reagents
Hypervalent Iodine Reagents Enable Chemoselective Deboronative/
Decarboxylative Alkenylation by Photoredox Catalysis**
Hanchu Huang, Kunfang Jia, and Yiyun Chen*
3
2
À
Abstract: Chemoselective C(sp ) C(sp ) coupling reactions
under mild reaction conditions are useful for synthesizing
alkyl-substituted alkenes having sensitive functional groups.
Reported here is a visible-light-induced chemoselective alke-
nylation through a deboronation/decarboxylation sequence
under neutral aqueous reaction conditions at room temper-
ature. This reaction represents the first hypervalent-iodine-
enabled radical decarboxylative alkenylation reaction, and
a novel benziodoxole-vinyl carboxylic acid reaction intermedi-
3
2
À
ate was isolated. This C(sp ) C(sp ) coupling reaction leads to
aryl-and acyl-substituted alkenes containing various sensitive
functional groups. The excellent chemoselectivity, stable reac-
tants, and neutral aqueous reaction conditions of the reaction
suggest future biomolecule applications.
conditions, such as heating or use of strong oxidants.[7]
Recently, alkyl boronates were shown to undergo Michael
additions or couple with aryl bromides under mild photo-
redox conditions, but the iridium photocatalyst only applied
to benzyl or alkoxyalkyl boronates.[8] Our group recently
reported a photoredox system for general deboronative
alkynylation.[9] From the hypothesis that hypervalent iodine
enables alkyl radical addition and decarboxylation of vinyl
carboxylic acids, we envision a photoredox system with
hypervalent iodine will facilitate both deboronative radical
formation and decarboxylative radical alkenylation, thus an
3
2
À
T
he chemoselective C(sp ) C(sp ) coupling reaction under
mild reaction conditions is a useful synthetic transformation
for building alkyl-substituted alkenes.[1] With a latent and
removable carboxylate group to direct reactivity, vinyl
carboxylic acids are readily available, stable, and selective
alkene equivalents to build alkyl-substituted alkenes.[2] How-
ever, the reported alkyl radical addition to vinyl carboxylic
acids require transition-metal coordination to the carboxyl-
ate, followed by strong heating or strong oxidants to facilitate
the CO2 extrusion, reaction conditions which limit their
functional-group compatibility [Eq. (1)].[3] Hypervalent
iodine reagents (HIR) are known to demonstrate similar
reactivity to that of transition metals under mild reaction
conditions.[4] We hypothesized that a hypervalent-iodine-
bound vinyl carboxylic acid[5] might enable alkyl radical
addition and decarboxylation of vinyl carboxylic acids
[Eq. (2)].
3
2
À
unprecedented C(sp ) C(sp ) alkenylation reaction will be
achieved.
We started our investigation with the alkyl trifluoroborate
1 and vinyl carboxylic acid 2 using the hypervalent iodine
photoredox system under blue LED (lmax = 468 Æ 25 nm)
irradiation. Gratifyingly, we observed the desired alkenyla-
tion product 3 in 82% yield with [Ru(bpy)3](PF6)2/hydroxy-
benziodoxole (BI-OH; entry 1, Table 1). The use of other
radical acceptors[10] including vinyl boronates, terminal
alkenes, vinyl bromides, or vinyl sulfones was not effective
(see Table S1 in the Supporting Information). The widely
used noncyclic iodine(III) reagents[4a,c,d] PhIO, PhI(OAc)2,
and PhI(OCOCF3)2 gave no product (entries 2–4), although
they were previously shown to facilitate electrophilic halo-
genation,[5a] selenization,[5b] and azidation[11] of vinyl carbox-
ylic acids. In contrast, the milder oxidants, that is, cyclic
iodine(III) reagents[4b] methoxybenziodoxole (BI-OMe) and
acetoxybenziodoxole (BI-OAc) gave efficient results, while
the reactivity with vinyl carboxylic acids was unknown
(entries 5 and 6). The use of BI-OAc with DCE/H2O as
solvents gave an optimal 95% yield at room temperature
(87% yield upon isolation; entry 7). Photocatalyst, light
irradiation, and oxidant were all critical for the reaction
(entries 8–10).
Organoboronates are readily available and stable alkyl
radical building blocks,[6] but the chemoselectivity and func-
tional-group compatibility are limited by the radical reaction
[*] H. Huang, K. Jia, Prof. Dr. Y. Chen
State Key Laboratory of Bioorganic and Natural Products Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Road, Shanghai 200032 (China)
E-mail: yiyunchen@sioc.ac.cn
[**] Financial support was provided by the National Basic Research
Program of China 2014CB910304, National Natural Science
Foundation of China 21272260, 21472230, “Thousand Talents
Program” Young Investigator Award, and a start-up fund from the
State Key Laboratory of Bioorganic and Natural Products Chemistry
and Chinese Academy of Sciences. We thank the SIOC X-ray
crystallography facility for assistance.
We next explored the reaction mechanism by adding the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201410176.
radical quencher TEMPO[12] to the optimized reaction con-
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Table 1: Optimization of the alkenylation reaction.
(Scheme 1b). To test if the alkyl R radical added directly to
the free vinyl carboxylic acid, we used K2S2O8 to replace BI-
OAc and observed no alkenylation product (Scheme 1c).[13]
In a parallel experiment with the same reaction conditions,
K2S2O8 generated alkyl R radical and added to diphenyl-
ethylene (7) to yield 3.[7b,d] These results suggested BI-OAc
was not simply an oxidant for alkyl R radical formation, but
might contribute to an unknown reaction intermediate.
We then monitored the vinyl carboxylic acid 8 in the
reaction mixture by NMR spectrometry and observed the
formation of a new vinyl carboxylic acid complex 9 (Sche-
me 2a; see Scheme S8 for details). This unprecedented
complex 9 was stable and characterized by X-ray crystallog-
raphy. An oxygen–iodine bond (2.12 ꢀ) was shown between
the vinyl carboxylic acid and the benziodoxole moiety, which
was similar to the oxygen–iodine bond (2.12 ꢀ) in the
benziodoxole moiety. We also discovered that 9 could be
prepared in higher yield by refluxing under vacuum (Scheme
2b). Additionally, this stable complex 9 could replace BI-
OAc/vinyl carboxylic acid 8 as the alkenylation reagent for
obtaining the alkene 10 in 76% yield (Scheme 2c).
Entry
Conditions
Conversion [%][b]
Yield [%][b]
1
2
3
4
5
6
7
8
9
BI-OH, CH2Cl2/H2O
PhIO, CH2Cl2/H2O
>95
<5
12
82 (79)
0
0
0
76
93
95 (87)
0
0
0
PhI(OAc)2, CH2Cl2/H2O
PhI(OCOCF3)2, CH2Cl2/H2O
BI-OMe, CH2Cl2/H2O
BI-OAc, CH2Cl2/H2O
BI-OAc, DCE/H2O
entry 7, no [Ru(bpy)3](PF6)2
entry 7, no blue LED
entry 7, no BI-OAc
39
>95
>95
>95
<5
<5
<5
10
[a] Reaction conditions: 1 (0.15 mmol, 1.5 equiv), 2 (0.10 mmol,
1 equiv), [Ru(bpy)3](PF6)2 (0.002 mmol, 0.02 equiv), and oxidants
(0.15 mmol, 1.5 equiv) in 2.0 mL (1:1) organic solvent/H2O under
nitrogen with 4 W blue LED irradiation at 258C for 15 h, unless otherwise
noted. [b] Conversions and yields were determined by 1H NMR analysis.
Yield of the isolated products are given within parentheses.
Based on the mechanistic investigations above, we
propose that vinyl carboxylic acid and BI-OAc generated
=
a benziodoxole vinyl carboxylic acid complex (BI-OOCCH
CHR’) in situ, which then oxidized the photoexcited [Ru-
(bpy)3]2+* to [Ru(bpy)3]3+ for reaction initiation
(Scheme 3).[14] The alkyl trifluoroborate (or boronic acid)
was deboronated by [Ru(bpy)3]3+ to an alkyl R radical and
regenerated [Ru(bpy)3]2+. The alkyl R radical adds to the
ditions (entry 7 in Table 1). After 20 hours, we observed
neither the desired alkenylation adduct 3 nor the alkene-
TEMPO adduct; only the alkyl–TEMPO adduct 4 was
isolated in 19% yield (Scheme 1a). The TEMPO experiments
with the vinyl carboxylic acid 2 alone or other combinations
did not yield the alkene–TEMPO adduct either (see
Scheme S5). These results excluded alkenyl radical formation
and confirmed the presence of alkyl R radical. We also found
this reaction was stereoconvergent as both trans-cinnamate
and cis-cinnamate exclusively yielded the trans-alkene prod-
uct 6, which additionally confirmed the radical mechanism
Scheme 2. Characterization and reactivity of the benziodoxole vinyl
carboxylic acid complex 9. Reaction conditions are those specified in
entry 7 of Table 1. Thermal ellipsoids are shown at 30% probability.[23]
Scheme 1. Mechanistic investigations of the alkenylation. Reaction
conditions are those specified in entry 7 of Table 1.
2
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Scheme 3. Mechanistic proposal of the alkenylation reaction.
a carbon atom[15] of BI-OOCCH CHR’, the adduct of which
=
undergoes benziodoxole-facilitated decarboxylation to
release the benziodoxole radical[16] and yields the alkene
product.[17] It is unclear at this point if a charge-separated
intermediate is formed by an internal single-electron transfer,
however the exclusive trans-alkene product formation may
suggest the formation of the carbocation intermediate.[18]
With the optimized reaction conditions, the deboronative/
decarboxylative alkenylation was tested on various substrates.
Primary, secondary, and tertiary alkyl trifluoroborates (or
boronic acids) bearing esters, ketones, or alkyl bromides all
deboronated smoothly to give good to excellent alkenylation
results (3, 11–18 in Scheme 4; for reaction conditions see
entry 7, in Table 1). Notably, the unactivated tertiary-alkyl-
substituted alkenes 13 and 18 are difficult to synthesize by
transition-metal-catalyzed alkyl Heck-type reactions.[19]
When the benziodoxole vinyl carboxylic acid complex was
pre-formed and used as the alkenylation reagent, similar
alkenylation yields were obtained (10, 16–18, 20–22).
The trans-alkene products (E/Z > 20:1) were exclusively
obtained from cinnamic acids having either electron-rich or
electron-deficient groups at the ortho-, meta-, and para-
positions (19–25). The oxidation-sensitive phenols, anilines,
or dimethylanilines were all compatible and gave alkene
products which are difficult to access using other alkyl Heck-
type reactions[10,19] (23–25). The sterically crowded 2,4,6-
trisubstituted cinnamic acid gave 26 in 71% yield. Cinnamic
acids with b substitution reacted smoothly to yield the trans-
alkene products, where functional groups such as Boc,
methoxy, aryl iodides, terminal alkenes, terminal alkynes,
alkyl azides, and heterocycles were all tolerated (27–34). The
compatibility of these transition-metal-sensitive functional
groups highlighted the excellent chemoselectivity compared
to other commonly-used alkyl Heck-type reactions.[19] This
alkenylation reaction was applicable to vinyl carboxylic acids
other than cinnamic acid analogues. We found that acyl-
substituted vinyl carboxylic acids yielded the trans-alkenes 35
and 36 smoothly, thus significantly expanding the synthetic
scope of the reaction.[3,19]
Scheme 4. Substrates scope and functional-group compatibility of the
alkenylation.[a] [a] Reaction conditions are those specified in entry 7 of
Table 1. Trifluoborates were used, unless otherwise noted. [b] Boronic
acids were used and DCE as the solvent. [c] E/Z>20:1 for all alkene
products. [d] For reaction conditions see Scheme 2c. The benziodoxole
vinyl carboxylic acid complex was used as the alkenylation reagent.
system with hypervalent iodine is compatible with various
biomolecules, including cell lysates, and we envision that the
use of boronates and carboxylic acids as stable reactants will
further improve its biocompatibility.[9]
With the excellent chemoselectivity and mild reaction
conditions, this visible-light-induced alkenylation reaction has
potential for applications for biomolecules if neutral aqueous
reaction conditions can be achieved.[20] To our delight, the
water-soluble vinyl carboxylic acid 38 reacted smoothly at
pH 7.4 in a phosphate saline buffer to deliver the alkene 39 in
73% yield [Eq. (3)]. It has been shown that the photoredox
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[10] a) G. J. Rowlands, Tetrahedron 2009, 65, 8603 – 8655; b) G. J.
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In conclusion, we have developed a visible-light-induced
deboronative/decarboxylative alkenylation, which repre-
sented the first hypervalent-iodine-enabled alkyl–alkene
coupling using aryl- or acyl-substituted[21] vinyl carboxylic
acids.[22] Compared to other commonly used alkyl Heck-type
[12] T. Vogler, A. Studer, Synthesis 2008, 1979 – 1993.
[13] K2S2O8 oxidizes alkyl boronates to alkyl R radicals and engages
in alkynylation with BI-alkyne, see Refs. [9] and [7b]. Its
inability for alkenylation also suggests the formation of a BI
complex similar to BI–alkyne. See Schemes S6, S15, and
Table S1.
3
2
À
reactions, this C(sp ) C(sp ) coupling reaction demonstrated
excellent chemoselectivity under mild reaction conditions,
and is useful for constructing aryl- and acyl-substituted alkyl
alkenes containing sensitive functional groups. The additional
reactivity and biomolecule applications of this novel photo-
redox/hypervalent iodine reagent system are under inves-
tigation in our laboratory.
[14] The reduction of BI-OCOR by photoexcited [RuII]* is very slow
(see Scheme S4), which implies that it is not the major reaction
pathway for generating benziodoxole radicals.
[15] A radical a-addition to vinyl carboxylates was observed before.
See: S. Martinet, A. Meou, P. Brun, Eur. J. Org. Chem. 2009,
2306 – 2311; Y. Abe, T. Sawada, M. Takafuji, T. Mashimo, M.
Ono, H. Ihara, Chem. Lett. 2010, 39, 174 – 175. The radical a-
addition was also confirmed by a 13C-labeling experiment; see
Scheme S16. A b-addition to Michael acceptors was observed in
the previous iridium photocatalyst system. See Refs. [8a,b],
which suggests the distinction between these two photoredox
systems.
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Received: October 16, 2014
Published online: && &&, &&&&
Keywords: alkenes · chemoselectivity ·
.
hypervalent compounds · photochemistry · ruthenium
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Hypervalent Reagents
H. Huang, K. Jia,
Y. Chen*
&&&& — &&&&
Hypervalent Iodine Reagents Enable
Chemoselective Deboronative/
Decarboxylative Alkenylation by
Photoredox Catalysis
Good visibility: A visible-light-induced
chemoselective deboronative/decarboxy-
lative alkenylation is reported. This novel
hypervalent-iodine-enabled radical reac-
tion proceeds through a benziodoxole
vinyl carboxylic acid intermediate (right).
Aryl-and acyl-substituted alkenes con-
taining sensitive functional groups are
constructed in good yields. Alkyl = 1’-, 2’-,
3’-alkyl, R = aryl, acyl.
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