Visible-Light-Promoted Formation of C—C and C—P Bonds Derived from Evolution of Bromoalkynes under Additive-Free Conditions: Synthesis of 1,1-Dibromo-1-en-3-ynes and Alkynylphosphine Oxides

Article abstract of DOI:10.1002/cjoc.202000546

The controllable achievement of C—C and C—P bond formations is developed via visible-light-promoted bromoalkyne dimerization or its further transformation with secondary phosphine oxides. The 1,1-dibromo-1-en-3-ynes are formed when bromoalkyne is exposed to visible-light. While alkynylphosphine oxides are generated when bromoalkynes are mixed with secondary phosphine oxides.

Full text of DOI:10.1002/cjoc.202000546

Chinese Journal
of Chemistry
CJC
中 国 化 学 - An International Journal
Accepted Article
Title: Visible-Light-Promoted Formation of C–C and C–P Bonds Derived from the
Evolution of Bromoalkynes under Additive-Free Conditions: Synthesis of 1,1-
Dibromo-1-en-3-ynes and Alkynylphosphine Oxides
Authors: Hailong Xu, Rui Chen, Hongjie Ruan, Ruyi Ye, and Ling-Guo Meng*
This manuscript has been accepted and appears as an Accepted Article online.
This work may now be cited as: Chin. J. Chem. 2020, 38, 10.1002/cjoc.202000546.
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Chinese Journal
of Chemistry
Visible-light, C-C bond, C-P bond, Bromoalkynes,
1,1-Bibromo-1-en-3-ynes, Alkynylphosphine oxides
Report
CJC
中
国 化 学 - An International Journal
Visible-Light-Promoted Formation of C–C and C–P Bonds Derived from
the Evolution of Bromoalkynes under Additive-Free Conditions:
Synthesis of 1,1-Dibromo-1-en-3-ynes and Alkynylphosphine Oxides
Hailong Xu, Rui Chen, Hongjie Ruan, Ruyi Ye, and Ling-Guo Meng*
Key Laboratory of Green and Precise Synthetic Chemistry and Applications, Ministry of Education; School of Chemistry and Materials Science, Huaibei
Normal University, Huaibei, Anhui 235000, P. R.
Cite this paper: Chin. J. Chem. 2019, 37, XXX—XXX. DOI: 10.1002/cjoc.201900XXX
Summary of main observation and conclusion The controllable achievement of C–C and C–P bond formations is developed via visible-light-promoted
bromoalkyne dimerization or its further transformation with secondary phosphine oxides. The 1,1-dibromo-1-en-3-ynes are formed when bromoalkyne is
exposed to visible-light. While alkynylphosphine oxides are generated when bromoalkynes are mixed with secondary phosphine oxides.
prepared iodinated enynes in the presence of Ipy2BF4 and HBF4 at
Background and Originality Content
-80 °C (Scheme 1b).10 Another example of the head-to-tail
Finding a facile synthetic platform to construct chemical
coupling of iodoalkynes was observed by Hashimi via gold
catalysis (Scheme 1c).11 Han’s group achieved this coupling
transformation through multiple complicated reaction steps
(Scheme 1d).12 Goroff and coworkers studied a different
derivation of tetrabromobutatriene that could obtain brominated
bonds is an important and eternal challenge for chemists. In
particular, the construction of the C–C bond is of undoubted
importance in organic studies. Until now, innumerable reported
transition-metal catalyzed coupling synthetic strategies have been
applied in C–C bond formation to form diverse organic molecules.
However, here, we focus on the C–C coupling reaction of alkynes
or their derivatives. Under conventional cognition,
buta-1,3-diynes or 1,4-enynes were usually obtained via the
head-to-head coupling of alkynes or their derivatives.1
Nevertheless, the head-to-tail coupling of alkynes or their
derivatives remains challenging to synthesize conjugated
1,3-enynes, which are important structural units in natural
products, biological molecular, organic composites and other
materials.2 Limited reports have realized the head-to-tail coupling
of alkynes to produce conjugated 1,3-enynes due to the
difficulties in regioselectivity.3 For example, the [Pd]-catalyzed
head-to-tail coupling of terminal acetylenes is the main and
effective synthetic strategy to synthesize 1,3-enynes;4 in addition,
the same synthetic target was achieved when the similar reaction
occurred in the presence of other transition-metal salts such as
[Fe]-,5 [Rh]-,6 [Au]-catalyst7 and actinide precatalysts8 (Scheme
1a). Meanwhile, 1,3-enynes can be produced from the
unavailable complicated synthetic precursor under harsh
conditions.9 Nonetheless, the above developed dimerization
reactions of terminal acetylenes cannot work without metal
catalysts and complicated conditions.
A glance at the literature shows that haloacetylenes have
been employed as an alternative precursor to achieve the
head-to-tail coupling reaction. In 1993, Barluenga et al
demonstrated that iodoalkynes can react with one another for
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First author et al.
Report
Scheme 1 Different routes to conjugate 1,3-enynes
3
—
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
< 5c
34c
work:
Previous
blue LED (420–425 nm)
blue LED (450–455 nm)
red LED (610–650 nm)
green LED (480–570 nm)
yellow LED (570–610 nm)
blue LED (450–455 nm)
blue LED (450–455 nm)
blue LED (450–455 nm)
blue LED (450–455 nm)
blue LED (450–455 nm)
blue LED (450–455 nm)
blue LED (450–455 nm)
blue LED (450–455 nm)
blue LED (450–455 nm)
blue LED (450–455 nm)
blue LED (450–455 nm)
blue LED (450–455 nm)
blue LED (450–455 nm)
blue LED (450–455 nm)
4
[TM]
ref
R
2 R
(a)
4-8
TM transition-metal
R
R
=
5
42c
BF
ref
4/HBF4
10
Ipy2
I
I
< 5c
(b)
(c)
6
2 R
I
R
[Au]
ref
7
< 5c
< 5c
40c
33c
33c
22c
< 5c
< 5c
< 5c
< 5c
< 5c
61
11
Br
Br
Br
n-BuLi, PhCHO;
1)
2)
8
Br PPh
C
PDC;
,
4
3)
3
(d)
(e)
Ph
Ph
Ph
Br
ref
12
9
Ph
Ph
DCE
ref
16
Mes-AcrClO4
ari, visible-light
as
10
11
12
13
14
15
16
17
18
19
20
21
22
EtOAc
Br
Ph
requried organic
dye
photocatalyst
acetone
work:
This
Br
Br
Ph
Br
CHCl3
toluene
THF
blue LED
Ph
N2
metal
catalyst
additive
(f)
Ph
Ph
Ph
Br
O
H
P(O)Ph2
PPh2
good
mild conditions
operation
regioselectivity
simple
EtOH
enynes via Suzuki coupling, but they only obtained 12% yield with
low chemoselectivity.13 According to the current development
and increasing demands in green and sustainable chemistry, it is
still significant for chemists to find a direct and convenient
synthetic approach to construct 1,3-enynes under mild, green and
feasible conditions.
DMF
DMSO
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
In recent years, remarkable progress has been made in
developing different visible-light-promoted transformations to
produce complicated and functional organic molecular, which
cannot be replaced by conventional methods in many cases.14
Bromoalkynes, containing the sp hybridization of the triple bond
and Br atom, are used as important building blocks in versatile
60 f
55 g
52 h
49i
Table 1 Optimization of reaction conditionsa
Br
Br
source
light
solvent, N2 48 h
2
Br
,
2a
1a
a (Reaction conditions: 1a (0.30 mmol), light source, solvent (2.0 mL), r.t.,
N2 atmosphere, 48 h.) b (Isolated yield.) c(the mixture was stirred for 24 h.)
d (5 W blue LED was used.) e (7 W blue LED was used.) f (The mixture was
stirred for 60 h.) g (The mixture was stirred for 72 h.) h (1a in CH3CN with
0.3 mol/L.) i(1a in CH3CN with 0.075 mol/L.)
Entry
Light source
Solvent
CH3CN
CH3CN
Yield (%)b
blue LED (420–470 nm)
blue LED (420–470 nm)
1
2
19c,d
21c,e
transformations under different conditions.15 Recently, we
designed some work related to photoredox chemistry;16
interestingly, the photoreaction could occur without additives, in
particular, bromoalkynes in the photoreaction can be used as
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Running title
Chin. J. Chem.
energetic photoactive molecules to achieve deep functional
transformations.16d, 16e We found a trace amount of head-to-tail
coupling product
visible-light-promoted bromoalkyne head-to-tail dimerization and
its further transformation with secondary phosphine oxides,
which provides a convenient route to controllably achieve the C–C
and C–P bond formation under photocatalyst- and additive-free
conditions.
((4,4-dibromobut-3-en-1-yne-1,3-diyl)dibenzene) was detected in
the study of the controllable reaction of bromoalkynes with
alcohols.16e To our delight, the head-to-tail coupling product could
be obtained with acceptable yield by adjusting the visible-light
wavelength and other conditions. Although Chen et al. reported
this reaction during our research (Scheme 1e),17 we retained the
efficiency of the head-to-tail coupling via simply adjusting the
reaction ambient atmosphere in the absence of photocalyst
(Scheme 1f); meanwhile, it should be speculated that the
initiation mode of this developed photo-coupling transformation
is also different. Further studies show that alkynylphosphine
oxides were formed when secondary phosphine oxides switched
off the head-to-tail coupling by trapping the alkynyl radical
derived from bromoacetylenes under
Results and Discussion
We commenced our investigation by dissolving phenylethynyl
bromide (1a) in CH3CN, and the mixture was illuminated by blue
LED (420–470 nm) for 24 h. Gratifyingly, the photoreaction
provided (4,4-dibromobut-3-en-1-yne-1,3-diyl)dibenzene (2a) in
19% yield, and no better yield was observed by changing power of
the light source (Table 1, entries 1 vs 2). Only a trace amount of
2a was observed when the light source was removed (Table 1,
entry 3). The yield of 2a was improved to 34% and 42% when the
wavelength of the blue LED was switched to 420–425 nm and
450–455 nm, respectively (Table 1, entries 4 and 5). When the
coupling reaction was exposed to other light sources, such as red,
green, or yellow LED light source, the reaction could not be
effectively triggered (Table 1, entries 6–8). Unsatisfactory yields
(22–40%) of desired product 2a were obtained when DCE, EtOAc,
acetone and CHCl3 were used as the solvent (Table 1, entries
9–12). Other solvents such as toluene, THF, EtOH, DMF, and
DMSO were tested, worse results were obtained (Table 1, entries
13–17). The yield of 2a was improved to 61% with prolonged
reaction time for 48 h (Table 1, entries 18 vs 19 and 20). When we
further changed the concentration of 1a in CH3CN, there was no
Scheme 2 Visible-light-induced head-to-tail coupling of
bromoacetylenesa,b
Br
Br
nm)
Br
blue LED
(450-455
CH3CN, N2, 48 h
Ar
2
Br
Ar
Ar
1
2
Br
Br
Br
Br
Br
OMe
Me
Bu
2a
61%
Br
2b
,
2c
50%
54%
,
,
MeO
Me
Bu
Br
Br
Br
Br
Br
Scheme 3 Control experiments
Br
Br
60 o
C
Pr
Et
or
2e
,
2f
41%
40%
2d
Pr
,
40%
Br
Et
,
without
light
48 h
2
Br
(a)
Br
Br
Br
Br
Br
N
CH3CN,
,
2
1a
1a
<
<
2a
5%
5%
,
tBu
F
Cl
TEMPO
equiv.)
(2.0
2g 60%
tBu
2h
2i
,
56%
58%
,
,
F
Cl
2a
,
(b)
N
48 h
CH3CN,
,
2
Br
Br
Br
Br
Br
Br
OMe
obvious effect on the yield of 2a (Table 1, entries 21 and 22).
MeO
Br
CF3
2j
2k
2l
57%
34%
43%
,
,
,
Br
F3C
According to the results of the explored reaction conditions,
the optimal reaction conditions were: 1a (0.3 mmol) dissolved in
Br
Br
Br
Br
Br
Br
F
Me
Cl
F
Me
Cl
CH3CN; the mixture was irradiated under blue LED (450–455 nm)
light for 48 h at room temperature in N2 (Table 1, entry 18). A
wide range of bromoacetylenes were employed to test the
efficiency of the reaction under the optimized conditions, and the
detailed results are listed in Scheme 2. Overall, substrates with
electron-rich groups showed comparable reactivity to the
electron-poor groups. For example, when the MeO, Me, Et, Pr, Bu
or tBu groups was at the para-position of the phenyl ring in
bromoalkynes, the corresponding products (2b–2g) were afforded
in 40–60% yields. Bromoalkynes 1 with electron-poor groups such
as F, Cl, Br and CF3 groups, also resulted in modest yields
(34–58%) of the corresponding gem-dibromonated conjugated
enynes products (2h–2k). The reactions of substrates 1 with a
substituted
2m
48%
Br
2n
2o
39%
,
,
43%
,
Br
Br
Br
Br
Br
Cl
Br
Cl
Br
S
S
2p
51%
,
2r
22%
,
2q
41%
,
a (Reaction conditions: 1 (0.30 mmol), CH3CN (2.0 mL), at room
temperature, under blue LED irradiation for 48 h in N2.) b (Isolated yield.)
visible-light irradiation. As we all know, alkynylphosphine oxides
structural unit found in bioactive and material molecules,18
however, most of reported synthetic mothods require harsh or
complicated conditions.19 Herein, we report a
Chin. J. Chem. 2019, 37, XXX－XXX
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First author et al.
Report
group, including MeO, Me, F or Cl on the meta-position of the
phenyl rings, generated the desired products (2l–2o) in 39–48%
yields. Further investigation indicated that 2-substituted aromatic
bromoalkynes, e.g., 1-(bromoethynyl)-2-chlorobenzene (1p) and
1-(bromoethynyl)-2-bromobenzene (1q), were also suitable for
the coupling reactions and showed a slight steric hindrance effect
(2p vs 2i; 2q vs 2j). Meanwhile, heteroaromatic bromoalkyne 1r
was also converted to the desired product 2r, although the yield
was lower. It should be noted that the modest yields of the
head-to-tail coupling in Scheme 2 were due to the incomplete
conversion of starting materials, and no other obvious byproduct
was observed.
Scheme 4 Visible-light-promoted coupling of bromoalkynes with
secondary phosphine oxides a,b
O
-
O
P
ue
bl LED 450 455
nm
)
r2
A
r2
A
r2
(
r1
r
+
B
r1
A
A
H
P
,
,
r2
CH2Cl2 N2 48 h
A
A
1
3
4
O
O
O
e
e
M
PPh2
PPh2
M
O
PPh2
,
,
39
,
c
4
a
4
4b
41% 80%
(
7
8
%
(
44 75
%
%
%
)
)
)
(
O
O
O
t
r
P
u
B
PPh2
F
PPh2
PPh2
,
,
,
,
41 60
% %
( )
e
4d
7
39% 8%
4
7
39% 9%
4f
(
)
(
)
To explore the head-to-tail coupling pathway, some control
experiments were performed, as shown in Scheme 3. The
reaction stopped when it was exposed to conventional heat
sources or in the absence of visible light (Scheme 3a), which
indirectly implied that the reaction cannot occur without
visible-light irradiation. Furthermore, by quenching the reaction
with radical scavengers such as TEMPO, only a trace amount of
the desired product 2a was observed, which shown that the
current transformation might be accomplished via a radical
pathway. But we did not observe TEMPO could trap the any
radical intermediate. Considering the results of control
experiments and previous correlated reports,16d,16e,20 the coupling
reaction is presumably initiated via the homolytic cleavage of
bromoalkynes. Alkynyl and bromine radical are generated during
the homolytic transformation and perform the unfinished
coupling process. According to the above speculation, we
attempted to find suitable substrates to couple with radicals
derived from bromoacetylenes, which might trigger another
cross-coupling reaction and interrupt the head-to-tail coupling of
bromoacetylenes. Here, secondary phosphine oxides realized this
assumption, and another coupling product (alkynylphosphine
oxide) was observed.21
e
M
Cl
O
O
O
PPh2
PPh2
PPh2
C
l
,
,
4i
36% 2%
)
4h
7
(
7
(
3
7
%
0%
4g
33% 60%
)
(
)
Cl
O
O
O
-
-
e
,
P 3 5 M 2C6H3
( )
2
PPh2
e
P 4 M C6H4
(
)
2
,
,
,
4
j
4l
32% 51%
50 82
4k
%
%
)
42 62
%
%
)
(
)
(
(
a [1 (0.20 mmol), 3 (0.60 mmol), CH2Cl2 (6.0 mL) at room temperature
under blue LED irradiation for 48 h in N2.] b (Isolated yields, and the yields
in brackets were given based on recovered starting materials. )
toluene, acetone, EtOAc and EtOH), and the best reaction
conditions: bromoalkynes 1 (0.20 mmol) and secondary
phosphine oxides 3 (0.6 mmol) were mixed in CH2Cl2 (6.0 mL), and
the mixture was irradiated under blue LED (450–455 nm) for 48 h
in N2. Different bromoalkynes and secondary phosphine oxides
were investigated, and the results are shown in Scheme 4 (the
isolated yield was calculated based on consumed 1). All
bromoalkynes with different substituents on the phenyl ring
smoothly reacted with diarylphosphine oxide to give the
corresponding products (4a−4l) in 51−82% yield. For
bromoalkynes, regardless of whether the electron-donation
group or electron-withdrawing group was located at the para- or
meta-position of the phenyl rings, the dehydrobromide coupling
reaction was compatible to give the desired
alkynyl(diaryl)phosphine oxides (4b−4i) in 60%−80% yields. When
the substituent moved to the ortho-position of the phenyl rings in
bromoalkynes, such as 1-(bromoethynyl)-2-chlorobenzene,
product 4j was afforded in 51% yield, which shown the steric
hindrance effect. Other diaryl-substituted phosphine substrates,
e.g., di(4-methylphenyl)- and di(3,5-dimethylphenyl)-phosphine
oxide, could also react with 1a and convert to the desired
products 4k and 4l, respectively.
Subsequently, the coupling of bromoalkynes with secondary
phosphine oxides was studied for the synthesis of
alkynylphosphine oxides under visible-light irradiation in the
absence of additive. The optimized conditions were determined
by exploring various conditions (details are shown in the ESI). The
obvious change in C–P bond formation was the use of CH2Cl2 as a
solvent by screening of different solvents (e.g. CH3CN, DCE,
According to the above experimental results and related
previous work, a plausible mechanism is outlined in Scheme 5.
First, bromoalkyne 1a was decomposed to form radical
intermediate A with the concurrent release of a Br• radical in the
presence of visible-light irradiation. Although this visible-light
promoted homolytic cleavage procedure would be speculated by
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Running title
Chin. J. Chem.
Scheme 5 Proposed mechanism
mL of dichloromethane was added bromoalkynes 1 (0.2 mmol).
The mixture was stirred in nitrogen atmosphere under the blue
LED (450-455 nm) light source for 48 h. The residue was then
purified by column chromatography on silica gel (petroleum
ether/EtOAc = 1:1, v/v) to give the pure product 4.
Br
Br
Br
1a
A
B
2
Br
Br
Br
2a
4a
1a
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
A
O
O
Ph2P
C
O
PPh2
Ph2P
H
1a
3a
HBr
some control experiments in previous correlated
Acknowledgement
literature,16d,16e,17,20 we cannot give a reasonable explain for the
real internal reason to promote occurrence of this homolytic
procedure. Then, Br• combined with another 1a to generate
intermediate B, which coupled with intermediate A to produce
the head-to-tail coupling product 2a. It should be noted that
product 2a also might be afforded from the addition reaction of
vinyl radical B to 1a and concomitantly with concomitant
formation of Br•. On the other hand, if diphenylphosphine oxide
(3a) intervened in the coupling reaction, the binding of Br• to
bromoalkyne was inhibited; conversely, Br• was abstracted
hydrogen from 3a to give diphenylphosphine oxide radical C,
which reacted with intermediate A to afford phosphonoalkyne
product 4a. Meanwhile, it should be noted that 4a also might be
formed via the addition process of C to 1a However, we cannot
determine which is the main pathway for formation of
We gratefully acknowledge the Natural Science Foundation of
Educational Committee of Anhui Province (KJ2020A0045,
KJ2020B01) and the Scientific Research Project of Anhui Provincial
Education Department (KJ2015TD002) for financial support of this
work.
References
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1,1-dibromo-1-en-3-ynes and alkynylphosphine oxides products.
Conclusions
In summary, we have developed a practical and convenient
synthetic platform for the head-to-tail coupling of bromoalkynes
in the presence of visible-light irradiation without any additive,
and a series of gem-dibromonated conjugated enyne products is
obtained in modest yields with good group tolerance. Notably,
according to the reasonable logical assumption, this head-to-tail
coupling was successfully switched to a cross-coupling reaction,
and different alkynylphosphine oxides were produced via
combinations of bromoalkynes with secondary phosphine oxides
under similar reaction environments. Further studies of the
visible-light-driven bromoalkyne transformations are in progress
in our laboratory.
Vinylideneruthenium Complexes. Chem. Commun. 2005, 34,
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Experimental
General procedure for the synthesis of
1,1-dibromo-1-en-3-ynes 2: bromoalkynes 1 (0.3 mmol) was
dissolved in acetonitrile (2.0 mL) under nitrogen atmosphere. The
mixture was stirred under the blue LED (450-455 nm) irradiation
for 48 h. The residue was then purified by column
chromatography on silica gel (petroleum ether) to give the pure
product 2.
[2] (a) Zhang, W.; Zheng, S.; Liu, N.; Werness, J. B.; Guzei, I. A.; Tang, W.
Enantioselective Bromolactonization of Conjugated (Z)-Enynes. J.
Am. Chem. Soc. 2010, 132, 3664-3665. (b) Kong, J.-R.; Ngai, M.-Y.;
Krische, M. J. Highly Enantioselective Direct Reductive Coupling of
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(The following will be filled in by the editorial staff)
Manuscript received: XXXX, 2019
Manuscript revised: XXXX, 2019
Manuscript accepted: XXXX, 2019
Accepted manuscript online: XXXX, 2019
Version of record online: XXXX, 2019
Visible-Light-Induced Hydroxysulfurization and Alkoxysulfurization of
Styrenes in the Absence of Photocatalyst: Synthesis of
β-Hydroxysulfides and β-Alkoxysulfides. Adv. Synth. Catal. 2019, 361,
3217-3222.
Chin. J. Chem. 2019, 37, XXX－XXX
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Entry for the Table of Contents
Page No.
Visible-Light-Promoted Formation of C–C and
C–P Bonds Derived from the Evolution of
Bromoalkynes Under Additive-Free Conditions:
Synthesis of 1,1-Dibromo-1-en-3-ynes and
Alkynylphosphine Oxides
Br
Br
Ph
H
Br
blue LED
metal
Ph
N2
Ph
Ph
Ph
Br
catalyst
additive
O
P(O)Ph2
PPh2
good
mild conditions
operation
regioselectivity
simple
Visible-light-promoted synthesis of 1,1-dibromo-1-en-3-ynes and alkynylphosphine oxides via
controllable formation of C–C and C–P bonds under photocatalyst-free conditions.
Hailong Xu, Rui Chen, Hongjie Ruan, Ruyi Ye, and
Ling-Guo Meng*
www.cjc.wiley-vch.de
© 2019 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2019, 37, XXX－XXX
This article is protected by copyright. All rights reserved.