K 2S 2O 8-mediated difunctionalization of C≡C bonds in water: A simple and efficient approach to α, α -dihaloacetophenones from phenylacetylenes and NaX

Article abstract of DOI:10.1080/00397911.2014.928938

A novel K₂S₂O₈-mediated oxy-1,1-dihalogenation of alkynes with NaX in the presence of water has been developed, affording α,α-dihaloacetophenones in moderate to good yields. The advantages of this reaction are mild reaction conditions, operational simplicity, and use of pure water as reaction medium. A plausible reaction mechanism is proposed on the basis of mechanistic studies.

Full text of DOI:10.1080/00397911.2014.928938

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K2S2O8-Mediated Difunctionalization
of C≡C Bonds in Water: A Simple
and Efficient Approach to α,α-
Dihaloacetophenones from
Phenylacetylenes and NaX
Jing-Yu Wang a , Qing Jiang a & Can-Cheng Guo a
a College of Chemistry and Chemical Engineering , Hunan
University , Changsha , China
Accepted author version posted online: 17 Jun 2014.Published
online: 20 Aug 2014.
To cite this article: Jing-Yu Wang , Qing Jiang & Can-Cheng Guo (2014): K2S2O8-Mediated
Difunctionalization of C≡C Bonds in Water: A Simple and Efficient Approach to α,α-
Dihaloacetophenones from Phenylacetylenes and NaX, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, DOI:
10.1080/00397911.2014.928938
To link to this article: http://dx.doi.org/10.1080/00397911.2014.928938
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Synthetic Communications1, 0: 1–9, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.928938
K2S2O8-MEDIATED DIFUNCTIONALIZATION OF
CꢀC BONDS IN WATER: A SIMPLE AND EFFICIENT
APPROACH TO a,a-DIHALOACETOPHENONES
FROM PHENYLACETYLENES AND NaX
Jing-Yu Wang, Qing Jiang, and Can-Cheng Guo
College of Chemistry and Chemical Engineering, Hunan University,
Changsha, China
GRAPHICAL ABSTRACT
Abstract A novel K2S2O8-mediated oxy-1,1-dihalogenation of alkynes with NaX in the
presence of water has been developed, affording a,a-dihaloacetophenones in moderate to
good yields. The advantages of this reaction are mild reaction conditions, operational
simplicity, and use of pure water as reaction medium. A plausible reaction mechanism is
proposed on the basis of mechanistic studies.
Keywords a,a-Dihaloacetophenones; oxyhalogenation; persulfate salts; phenylacety-
lenes; water
INTRODUCTION
The transformation of alkynes is an essential functional group interconversion
for organic synthesis.[1–5] The resulting compounds from alkyne functionalization
take a privileged position in drug discovery, supramolecular chemistry, polymer
chemistry, materials science, and biotechnology.[2] Among intensive research in
alkyne chemistry, vicinal difunctionalization of alkynes involving the simultaneous
installation of two different vicinal chemical bonds exemplifies a class of reactions
with significant synthetic potential to rapidly increase molecular complexity.[3] In this
regard, oxyhalogenation of alkynes has recently attracted attention because it installs
two versatile handles (carbonyl and halide) for further structural elaboration.[4,5]
Received February 6, 2014.
Address correspondence to Can-Cheng Guo, College of Chemistry and Chemical Engineering,
Hunan University, Changsha 410082, China. E-mail: ccguo@hnu.edu.cn
Color versions of one or more of the figures in the article can be found online at www.tandfonline.
com/lsyc.
1

2
J.-Y. WANG, Q. JIANG, AND C.-C. GUO
Although significant progresses have been made, some crucial issues have remained
fairly unaddressed: (1) the reaction is mainly limited to either oxybromination or
oxychlorination,[4,5a] (2) hazardous or low-atom-efficiency halogenating reagents
and environmentally undesirable organic solvents are often employed,[4,5] and (3)
special implements and transition metals are sometimes utilized.[4b,4d,4e,5b] Therefore,
the development of new, mild, and environmentally friendly approaches for selective
and direct oxidative conversion of alkynes to multifunctional organic compounds
still remains a highly desirable goal in organic synthesis. Herein, we report a
K2S2O8-mediated oxy-1,1-dihalogenation of alkynes with NaX in the presence of
water, which illustrates a convenient method toward simultaneous incorporation
of halide and ketone group to a carbon–carbon triple bond.
a,a-Dihaloacetophenones are an important class of intermediates in organic
synthesis and have been used in the synthesis of natural products and pharmaceuti-
cals.[6] In general, a,a-dihaloacetophenones are mainly obtained from acetophenones
with excess gaseous or volatile halogens, or their complex compounds.[7] However,
these methods involve the use of toxic, hazardous, or low-atom-efficiency halogenat-
ing reagents. Recently, direct conversion of alkynes to a,a-dihaloacetophenones has
attracted much attention because ketones are frequently prepared from the corre-
sponding alkynes.[8] Although oxidation of alkynes for the synthesis of a,a-dichlor-
oacetophenones or a,a-dibromoacetophenones is documented in the literature,[4]
a
common procedure that leads to both a,a-dichloroacetophenones and a,a-dibromoa-
cetophenones is much less abundant and more limited.[5] For instance, Pandit et al.
reported a cationic-surfactant-mediated direct transfer of halides to alkynes to afford
a,a-dihaloketones promoted by phenyliodonium diacetate.[5a] Subsequently, Liu et al.
described a oxy-1,1-dihalogenation of alkynes with N-halosuccinimides as the halo-
gen source using FeCl3 ꢁ H2O as the oxidant in tetrahydrofuran (THF)–H2O.[5b]
Recently, Madabhushi et al. reported a oxyhalogenation of terminal alkynes using
an oxone–KX system in CH3CN-H2O.[5c] However, these strategies suffer from
one or more limitations, including use of expensive and low-atom-efficiency reagents
and=or environmentally undesirable organic solvents and=or even transition metals.
Therefore, the development of new, mild, and environmentally friendly approaches
for the direct synthesis of a,a-dihaloacetophenones from phenylacetylenes still
remains a highly desirable goal in organic synthesis. Herein, we present a mild and
efficient method for the synthesis of a,a-dihaloacetophenones from phenylacetylenes
and NaX using K2S2O8 as the oxidant in the presence of pure water.
RESULTS AND DISCUSSION
In our previous studies, phenacyl bromides could be efficiently synthesized via
K2S2O8-mediated tandem hydroxybromination and oxidation of styrenes using KBr
as a bromine source in the presence of water.[9] We envisioned that a,a-dihaloaceto-
phenones can be generated in a similar fashion if the styrenes are replaced with
phenylacetylenes. When phenylacetylene was reacted with KBr in the presence of
K2S2O8 in H2O at 60 ꢂC for 12 h, we were delighted to see that our desired product,
a,a-dibromoacetophenone, was indeed formed in 85% yield (Table 1, entry 1). The
control experiment showed that K2S2O8 was necessary for the reaction to proceed
(Table 1, entry 2). Among the bromine sources screened, NaBr was found to be most

K2S2O8-MEDIATED OXYDIHALOGENATION OF ALKYNES
3
Table 1. Optimization of the reaction conditionsa
Entry
Oxidant
K2S2O8
None
Halogen source
Solvent
Yieldb (%)
1
2
NaBr
NaBr
KBr
H2O
H2O
H2O
H2O
H2O
H2O
H2O
85
0
3
K2S2O8
K2S2O8
K2S2O8
Na2S2O8
(NH4)2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
68
75
4
LiBr
5
NH4Br
NaBr
NaBr
NaBr
NaBr
NaBr
NaBr
NaBr
NaBr
NaBr
KCl
70
74
6
7
68
64
8
CH3OH-H2O(1=1)
CH3CN-H2O(1=1)
EtOH-H2O(1=1)
CH3OH
9
65
10
11d
12e
13f
14g
15
16
62
0 (85)c
0 (78)
0 (12)
68
CH3CH2OH
CH3CN
H2O
H2O
H2O
45
58
NaCl
aConditions: phenylacetylene (0.5 mmol), oxidant (2.5 equiv), halogen source (2.0 equiv), solvent (1 mL),
60 ꢂC, 12 h.
bYields are determined by GC.
cThe conversion is given in parentheses.
dtrans-1,2-Dibromo-1-phenylethene was obtained in 42%.
etrans-1,2-Dibromo-1-phenylethene was obtained in 35%.
ftrans-1,2-Dibromo-1-phenylethene was obtained in 9%.
g70 ꢂC.
effective (Table 1, entries 3–5). When other persulfate salts, such as Na2S2O8 or
(NH4)2S2O8, were used in place of K2S2O8, the desired product was obtained in poor
yields (Table 1, entries 6 and 7). Changing the solvent to CH3OH–H2O, CH3CN–
H2O or CH3CH2OH–H2O afforded 62–65% yield of product, whereas the use of
organic solvents alone as the solvent gave no product (Table 1, entries 8–13). When
the mixed solvent of other solvent–H2O was used as the reaction medium, only the
desired a,a-dibromoacetophenone was detected by gas chromatography (GC) and
the remaining starting material decomposed in those cases, whereas organic solvents
afforded trans-1,2-dibromo-1-phenylethene as the major product. Increasing the
reaction temperature to 70 ꢂC did not benefit the reaction (Table 1, entry 14). The
reaction between phenylacetylene and KCl in the presence of K2S2O8 in H2O at
60 ꢂC for 12 h provided the desired a,a-dichloroacetophenone in 45% yield
(Table 1, entry 15). After a series of optimizations, the best yield (58%) was achieved
when NaCl was used as chlorine source in the presence of K2S2O8 (Table 1, entry 16).
On the basis of these results, we determined the optimized conditions to be K2S2O8
(2.5 equiv), NaX (2.0 equiv), H2O, 60 ꢂC, and 12 h (for details, see the supplementary

4
J.-Y. WANG, Q. JIANG, AND C.-C. GUO
material). Although phenylacetylene reacted with NaBr or NaCl in the presence
of K2S2O8 to afford the corresponding a,a-dibromoacetophenone and a,a-
dichloroacetophenone, the reaction of phenylacetylene with KI did not gave the
corresponding a,a-diiodoacetophenone under the same reactions; instead it provided
(E)-1,2-diiodophenylethene as the main product (see Scheme S1 in the Supporting
Information).
With the optimized conditions in hand, we next explored the scope of the reac-
tion between various alkynes and NaBr, and the results are summarized in Scheme 1.
Diverse phenylacetylenes bearing both electron-donating groups and electron-
withdrawing groups provided the desired a,a-dibromoacetophenones in yields
ranging from 61 to 78% (2a–2j). It is worth noting that F (2b), Cl (2c), and Br (2d)
substituents on the phenyl ring were well tolerated, which enable potential
Scheme 1. Oxy-1,1-dibromination of alkynes with NaBr in the presence of K2S2O8. Conditions: alkynes
(0.5 mmol), K2S2O8 (2.5 equiv), NaBr (2.0 equiv), H2O (1 mL), 60 ꢂC, 12 h. Isolated yields.

K2S2O8-MEDIATED OXYDIHALOGENATION OF ALKYNES
5
applications in further functionalization.[10] In addition, this reaction was also
applicable to internal alkynes, 1-phenyl-1-propyne, and 1-phenyl-1-butyne, as demon-
strated by the formation of the desirable products in good yields (2k and 2l). While the
results of phenylacetylenes were all favorable, the reaction of aliphatic alkynes did not
give the corresponding a,a-dibromoketones; instead they afforded dibromides as the
main products. For instance, when 1-heptyne was subjected to the same reaction
conditions, trans-1,2-dibromo-1-heptene was obtained in 58% yield (2m).
Subsequently, we investigated the scope of the reaction with respect to alkynes
and NaCl. As shown in Scheme 2, the efficiency of oxy-1,1-dichlorination is much
lower than the corresponding oxy-1,1-dibromination. Phenylacetylene derivatives
with electron-withdrawing substituents afforded the desired a,a-dichloroacetophe-
none in 35–49% yield (3b–3d), whereas styrene derivatives bearing electron-donating
substituents provided the desired a,a-dichloroacetophenone in 40–46% yield (3e–3g).
Notably, F (3b), Cl (3c), and Br (3d) substituents can also be tolerated in this
reaction. Moreover, 1-phenyl-1-propyne and 1-phenyl-1-butyne, which are internal
alkyne compounds, gave the corresponding 3h and 3i in 37 and 34% yields, respect-
ively. Unfortunately, the reaction of 1-heptyne did not give the corresponding
Scheme 2. Oxy-1,1-dichlorination of alkynes with NaCl in the presence of K2S2O8. Conditions: alkynes
(0.5 mmol), K2S2O8 (2.5 equiv), NaCl (2.0 equiv), H2O (1 mL), 60 ꢂC, and 12 h. Isolated yields.

6
J.-Y. WANG, Q. JIANG, AND C.-C. GUO
a,a-dichloroketone, and only trace amounts of dichloride product (3j) were detected
by NMR (see Fig. S1 in the Supporting Information).
To shed light on the mechanism of the reactions, some information has been
gathered. When phenacyl chloride and bromide were introduced into the standard
conditions, the desired a,a-dichloroacetophenone and a,a-dibromoacetophenone
were not detected by GC [Eq. (1)]. The reaction of b-bromostyrene under the same
conditions afforded only a trace amount of a,a-dibromoacetophenone [Eq. (2)].
These results indicate that phenacyl halide and b-halostyrene seem not to serve as
intermediates in the reaction. Also, when the reaction was carried out in the presence
of BHT (2,6-di-tert-butyl-4-methyl phenol) [Eq. (3)], which was a traditional radical
scavenger, a,a-dibromoacetophenone was obtained in 82% yield, thus demonstrating
that a radical pathway might not be involved in the present reaction system. Futher-
more, Table 1 showed that the reaction did not proceed at all in the absence of
K2S2O8. This result shows that K2S2O8 plays a role as the oxidant. In addition, the
results that we obtained from Table 1 suggested that carbonyl oxygen of a,a-dihaloa-
cetophenone originates from water, not from the molecular oxygen of air. Finally, the
presence of the yellow color during the reaction suggests the formation of bromine.[9]
Based on these results and related reports,[4,5] a plausible mechanism for the
present process is proposed in Scheme 3. Initially, oxidation of the halide ion by
the persulfate ion generates molecular halogen, which is trapped by water to give
hypohalous acid (HOX). Next, hypohalous acid converts into dihalo monoxide
(X2O)[5c,11] and undergoes electrophilic addition onto the phenylacetylene to produce
a three-membered cyclic halonium ion intermediate A. The cyclic intermediate
undergoes ring opening by the nucleophile (XOꢃ, hydroxy or halide ion) to yield
the corresponding substituted products B, C, and D. This also further explained
the results of Schemes 1 and 2. When substrates were aliphatic alkynes, halonium
ion intermediate undergoes ring opening by halide ion to give vicinal dihalo
substituted product, whereas when substrates were phenylacetylenes, halonium ion

K2S2O8-MEDIATED OXYDIHALOGENATION OF ALKYNES
7
Scheme 3. Proposed reaction mechanism.
intermediate undergoes ring opening by XO- (path a) and water (path b) to provide
vicinal substituted products B and C. Finally, the intermediates B and C are
converted into the corresponding a,a-dihaloacetophenone.
CONCLUSIONS
In conclusion, we have developed a highly attractive and operationally simple
oxy-1,1-dihalogenation method to construct a,a-dichloroacetophenones and a,a-
dibromoacetophenones. This method is of great value from the viewpoint of green
chemistry and organic synthesis due to use of inexpensive halogen sources,
K2S2O8 as oxidant, and H2O as solvent. Further mechanistic studies and develop-
ment of relevant reactions are currently under investigation.
EXPERIMENTAL
All reagents and solvent were purchased commercially and used without
further purification. Mass spectra were measured on a mass instrument (EI).
1H NMR spectra were recorded on 400 MHz in CDCl3, and 13C NMR spectra were
recorded on 101 MHz in CDCl3 using Tetramethylsilane (TMS) as internal standard.

8
J.-Y. WANG, Q. JIANG, AND C.-C. GUO
Multiplicities are indicated as s (singlet), d (doublet), t (triplet), q (quartet), and
1
m (multiplet), and coupling constants (J) are reported in hertz. Copies of H NMR
and 13C NMR spectra are provided as Supporting Information.
K2S2O8 (2.5 equiv.), NaX (2.0 equiv.), phenylacetylene (0.5 mmol), and H2O
(1 mL) were added to a 25-mL Schlenck tube. The reaction mixture was warmed to
60 ꢂC (oil bath) and stirred for 12 h. The reaction was cooled to room temperature,
and ethyl actate (5 mL) and water (2 mL) were added. The organic layer was sepa-
rated, and the aqueous phase was extracted with ethyl actate (10 mL ꢄ 2). The com-
bined organic layers were dried over anhydrous Na2SO4, filtered, and concentrated.
The residue was purified by column chromatography to give desired product
1
2,2-dibromo-1-phenylethanone 2a. Compound 2a was a pale yellow oil. H NMR
(CDCl3, 400 MHz) d 6.75 (s, 1 H), 7.50 (t, J ¼ 8.0 Hz, 2 H), 7.63 (t, J ¼ 8.0 Hz,
1 H), 8.07 (d, J ¼ 8.0 Hz, 2 H); 13C NMR (CDCl3, 101 MHz) d 39.91, 129.00,
129.72, 130.84, 134.53, 186.00. The spectral data agreed with those in the literature.[5a]
FUNDING
We are grateful for the financial support from the National Natural Science
Foundation of China (21372068, J1210040, J1103312) and the Presidential Scholar-
ship for Doctoral Students, Hunan University.
SUPPORTING INFORMATION
Supplemental data for this article can be accessed on the publisher’s website.
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