Silver-assisted difunctionalization of terminal alkynes: Highly regio- and stereoselective synthesis of bromofluoroalkenes

Article abstract of DOI:10.1002/adsc.201200250

The difunctionalization of terminal alk- ACHTUNGTRENUNGynes was achieved with silver fluoride (AgF) and N-bromosuccinimide (NBS) as halogen sources. The presence of the halide moiety greatly enhances the reactivity of the vinyl fluoride compounds that can probably can be transformed into various products that are difficult or even impossible to obtain via direct fluorination. Meanwhile, the monofluoro<.> alkenes were facilely synthesized via a highly chemo- and regioselective fluorination of electron-deficient C-C triple bonds using AgF as fluorinating reagent in good yields

Full text of DOI:10.1002/adsc.201200250

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DOI: 10.1002/adsc.201200250
Silver-Assisted Difunctionalization of Terminal Alkynes: Highly
Regio- and Stereoselective Synthesis of Bromofluoroalkenes
Yibiao Li,a Xiaohang Liu,a Deyun Ma,a Bifu Liu,a and Huanfeng Jianga,*
a
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, Peopleꢀs
Republic of China
Fax : (+86)-20-2223-6337; e-mail: jianghf@scut.edu.cn
Received: March 25, 2012; Revised: June 16, 2012; Published online: September 28, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200250.
Abstract: The difunctionalization of terminal alk-
ynes was achieved with silver fluoride (AgF) and
ciently synthesize difunctionalized fluoroalkenes. The
resulting vinyl fluoride compounds should be appeal-
ing to synthetic and medicinal chemists due to the
unique physical and biological properties imparted by
the fluorine atom.[5]
ACHTUNGTRENNUNG
N-bromosuccinimide (NBS) as halogen sources.
The presence of the halide moiety greatly enhances
the reactivity of the vinyl fluoride compounds that
can probably can be transformed into various prod-
ucts that are difficult or even impossible to obtain
via direct fluorination. Meanwhile, the mono-
fluoro<.>alkenes were facilely synthesized via
a highly chemo- and regioselective fluorination of
In 2007, Sadighi and co-workers reported that
Et3N·3HF could be regarded as fluorine source in
ꢀ
Au-catalyzed direct fluorination of C C triple bonds
[Scheme 1 (a)].[6,7] Inspired by this report, we envi-
sioned the possibility of the synthesis of bromofluor-
oalkenes from commercially available terminal al-
kynes in a one-pot manner. Our rationale based on
the facts that (i) Ag and Au have similar properties
when it comes to catalysis and (ii) bromination of ter-
minal alkynes is feasible under Ag catalysis. This syn-
thetic blueprint should be feasible if the addition of
a fluorine source does not interfere with the bromina-
tion step.
ꢀ
electron-deficient C C triple bonds using AgF as
fluorinating reagent in good yields.
Keywords: bromofluoroalkenes; difunctionaliza-
tion; fluorination; silver; terminal alkynes
Highly functionalized cis-haloalkenes have been
The development of efficient and sustainable methods prepared via the acetylation and iodation reaction of
for the synthesis of halogen-containing olefins is an haloalkynes in a regio- and stereoselective fashion in
important task in contemporary catalysis research be- our group.[8] Herein, we report a general, highly
cause of the central role of this class of compounds in regio- and stereoselective bromofluorination of termi-
biological systems and pharmaceutical applications. nal alkynes by NBS and AgF [Scheme 1 (b)]. Further-
Among different protocols for achieving this goal, di- more, we extended this methodology to other internal
ꢀ
functionalization of C C triple bonds to construct C- electron-deficient alkynes and successfully obtained
(sp ) X bonds represents a series of reactions with
2
À
significant synthetic potential.[1,2] Although our ongo-
ing studies on this subject have led to the synthesis of
a diverse set of haloalkene derivatives, such as 1-
chloro-1,4-dienes,[3a] 1-chlorovinylallenes,[3b] haloalk-
ACHTUNGTRENNUNG
enynes,[3c] 2-chloro-1,3-dienes,[3d] cyclobutenyl halid-
es[3e] and bromoacrylamides,[3f] it is a challenging issue
for us that the fluorine atom can be successfully at-
tached to the resulting olefin by transition metal cat-
alysis. As we know, few effective approaches are
available for the direct synthesis of fluoroalkene de-
rivatives without additional functional sites, and tran-
sition metal-catalyzed methods are particularly rare.[4]
Scheme 1. Reported method for the fluorination and the
Thus, we sought a mild and expedient protocol to effi- method presented herein.
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the corresponding “HF” cross-addition products. lytic systems, such as pyridinium/PdACHTUNTRGNEUNG(OAc)2, CuF2/
Functionalization of the bromide moiety is also show- AgBF4 and NiF2/AgNO3 were ineffective for bromo-
cased in this report. fluorination (entries 8–10). To our delight, when
Initial investigations focused on validating the bro- 1.5 equiv. of AgF were employed, the desired bromo-
mofluorination of terminal alkynes using various fluo- fluoroalkene 2a was obtained as major product in
rine sources. The reaction of phenylacetylene with 72% yield (entry 11). Further control experiments
NBS was chosen as the starting point (Table 1). When showed that the use of AgF can significantly improve
CsF was used as sole fluorine source in the absence of the yield of 2a (entry 12). The best result was ob-
catalysts, only a trace of the desired product was ob- tained when the reaction was carried out with
served (entry 1). Similar results were obtained with 2.5 equiv. of AgF in acetonitrile at 808C for 10 h
various common fluorine sources (KF, TBAF, CuF2, (entry 13). In such a case, the bromofluorination
NiF2 and N-fluoro-2,4,6-trimethylpyridinium tetra- product was formed in 95% yield in an exclusive cis
fluoroborate) (entries 2–6). We continued to examine fashion, and no phenylethynyl bromide 2a’ was de-
the reaction in the presence of Ag or Pd catalysts. tected. These results indicated that the reaction is
Traces of the desired bromofluorination product 2a mediated by a stoichiometric amount of AgF. A de-
were observed under the CsF/AgNO3 reaction crease in the temperature lowered the yield of the re-
system, but phenylethynyl bromide 2a’ was detected action (entry 14). Compared with the other solvents,
as the major by-product (86%) (entry 7). Other cata- acetonitrile is more suitable for the bromofluorination
(entries 15 and 16).
With these optimized reaction conditions in hand,
we set out to test the generality of this reaction. Grat-
ifyingly, as shown in Scheme 2, this approach to bro-
mofluoroalkenes is quite versatile. The aromatic al-
Table 1. Optimization of the reaction conditions for phenyl-
ACHTUNGTRENNUNG
acetylene.[a]
Entry Fluorine source Additive
Solvent Yield [%][b]
2a
2a’
1[c]
2[c]
3[c]
4[c]
5[c]
6[d]
7
CsF
KF
TBAF
CuF2
pyridinium
NiF2
CH3CN trace
CH3CN trace
CH3CN trace
CH3CN trace
CH3CN trace
CH3CN <5
CH3CN <5
<5
<5
<5
<5
<5
15
86
<5
83
93
15
8
CsF
AgNO3
Pd
AgBF4
AgNO3
8
9
10
11
pyridinium
CuF2
NiF2
AgF
AgF
AgF
AgF
AgF
AgF
(OAc)2 CH3CN <5
CH3CN <5
CH3CN <5
CH3CN 72
CH3CN 86
CH3CN 95
CH3CN 36
12[e]
13[f]
14[g]
15
–
62
43
91
THF
55
16
toluene <5
[a]
Reaction conditions: phenylacetylene (1.0 mmol), NBS
(1.1 mmol), fluorine source (1.5 mmol), solvent (2.0 mL)
and additive (5 mol%) at 808C for 10 h. NBS=N-bromo-
succinimide, Pyridinium=N-fluoro-2,4,6-trimethylpyridi-
nium tetrafluoroborate.
Isolated yield.
Starting material recovered.
2,2-Dibromo-1-phenylethanone as the major product in
56% yield.
2.0 mmol AgF.
[b]
[c]
[d]
[e]
[f]
2.5 mmol AgF.
Room temperature.
Scheme 2. Synthesis of bromofluoroalkenes with various ter-
minal alkynes.
[g]
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Silver-Assisted Difunctionalization of Terminal Alkynes
kynes with either electron-donating or electron-with-
drawing groups attached to the benzene rings, were
able to undergo bromofluorination smoothly and gen-
erated the corresponding products in good to excel-
lent yields (Scheme 2, 2a–2s). The reaction of alkynes
containing electron-rich groups provided slightly
higher yields than those conatining electron-poor
groups. The reaction tolerated a variety of substitu-
ents including Cl, Br, F, CN, CF3, NO2 and OMe
groups. Substituents at the ortho position of the
benzyl group did not affect the yield of the reaction
(Scheme 2, 2d and 2n). The steric hindrance of the
3,5-bis(trifluoromethyl)benzene ring has a negligible
effect on the yield in spite of the lowered stereoselec-
tivity (Z:E=77:23, Scheme 2, 2p). Besides the
arylalkACHTUNGTRENNUNGynes, aliphatic alkynes were also found to be
suitable substrates for the standard conditions. When
ethynylcyclopropane was employed, the desired
bromofluoroACHTUNGTRENNUNGalkene was formed in excellent isolated
Scheme 3. Synthesis of bromofluoroalkenes with various
diynes.
yield (Scheme 2, 2t). Cyanide-containing aliphatic al-
kynes has also been successfully employed without
protection, and the chemistry provides the desired tential of this transformation (Scheme 4). Similar to
bromofluoroalkenes in good yields (Scheme 2, 2v). ethyl 3-phenylpropiolate, phenyl alkynyl ketone was
Herein, simple filtration through a silica-gel plug was able to afford the corresponding fluorination products
sufficient to remove the residual catalyst and provide in good yields. It was found that structurally various
desired products. It is noteworthy that 1-ethynylcyclo- phenyl alkynyl ketone showed high reactivities with
hex-1-ene works effectively, and that gem-difluoroalk- AgF, affording the products in high Z/E selectivi-
ACHTUNGTRENNUNG
anes are not observed in our process despite the fact ties.[12,13] In addition, a heterocyclic compound, 3-
that gem-difluoroalkanes have been reported as the phenyl-1-(thiophen-2-yl)prop-2-yn-1-one, was equally
major products by addition of “XF” to alkenes amenable to the applied reaction conditions and
(Scheme 2, 2w).[9] Unfortunately, the reaction with
ethynyltrimethylsilane only afforded fluorotrimethyl-
silane as the product.
The bromofluorination of the 1,n-diyne system is
equally interesting. The major challenge in this reac-
tion is the control of chemo- and regioselectivity,
since examples of chemo- and regioselective fluorina-
tion involving alkynes are still limited.[10] It is ob-
ꢀ
served that an electron-rich internal C C triple bond
was tolerated under the present conditions. Thus, 1-
(buta-1,3-diynyl)-4-methylbenzene reacts with NBS
and AgF to generate the corresponding bromofluoro-
ACHTUNGTRENNUNGalkene in moderate yield with the internal alkyne
intact (3a). Moreover, similar results were observed
for other diyne or triyne compounds (3e, 3f and 3g)
with only the terminal triple bond being bromofluori-
nated (Scheme 3). On the other hand, bromofluorina-
ꢀ
tion occurred readily on both of the terminal C C
triple bonds when such substrates were used (3b, 3c
and 3d). The molecular structure of compound 3b was
established by X-ray crystallography, which enabled
us to confirm the reaction mechanism as a cis-addi-
tion process (see the Supporting Information).[11]
The above studies dealt only with terminal alkynes
as the reactive point in the substrates. For extensions,
we used other electron-deficient compounds, such as
Scheme 4. Synthesis of monofluoroalkenes with various elec-
substituted aryl alkynyl ketones, to investigate the po- tron-deficient alkynes.
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formed the hydrofluorination product 4g in good
yield.
To obtain further insight into the mechanism of the
present catalytic process, the direct fluorination of
The obtained bromofluoroalkenes are highly attrac- haloalkynes were performed, as shown in Scheme 6.
tive as intermediates for the preparation of more
functionalized fluoroalkenes. The synthetic versatility
of the bromofluorination products is demonstrated by
the transformations shown in Table 2. Under these re-
Table 2. Transformations of bromofluoroalkenes.[a]
Scheme 6. The direct fluorination of haloalkynes.
Entry
Bromofluoro-
alkene:
R1
Alkyne
R2
Prod-
uct
Yield
[%][b]
Gratifyingly, both bromoalkyne and chloroalkyne ex-
clusively gave the fluorination products in good yields
(Scheme 6, 2a–7a). In addition, the iodofluoroalkene
(8a) was found to be obtained in a 2:1 ratio with 1-(1-
fluoro-2,2-diiodovinyl)-4-methylbenzene (8b), sug-
gesting that iodoalkynes may be more active in com-
parison with bromoalkynes and chloroalkynes.
Next, two experiments were carried out to identify
the source of hydrogen in the products (Scheme 7).
When 1-ethynyl-4-methylbenzene with the alkynyl
1
2
3
4
5
6
7
8
9
2a: Ph
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
5a
5b
5c
5d
5e
5f
5g
5h
5i
92
91
88
86
93
85
84
86
87
88
2b: 4-Me-C6H4
2c: 3-Me-C6H4
2d: 2-Me-C6H4
2e: 4-Et-C6H4
2g: 4-MeO-C6H4
2i: 4-Cl-C6H4
2m: 4-F-C6H4
2o: 4-CF3-C6H4
2b: 4-Me-C6H4
TMS
4-Cl-C6H4
10
5j
[a]
Reaction conditions: bromofluooalkenes (1.0 mmol), ter-
minal alkynes (1.6 mmol), PdCl2A(PPh3)2 (5 mol%), CuI
(10 mol%) in 2 mL
CTHUNGTRENNUNG
(10 mol%), Et3N (3.0 mol), Ph3P
CH3CN at 708C for 10 h.
ACHTUNGTRENNUNG
[b]
Isolated yields are given.
action conditions, various 1-fluoro-1,3-enynes could
be stereoselectively prepared from bromofluoroal-
kenes and terminal alkynes, which are difficult to
obtain via direct fluorination of the 1,3-diyne. To
Scheme 7. Control experiments.
demonstrate the synthetic utility of bromofluoroal- proton deuterated was used as a substrate only 2b
kenes, we showed that the Pd/Cu-catalyzed coupling was obtained in 87% isolated yield. Moreover, under
of bromofluoroalkenes with terminal alkynes gave 1- the optimized condition, 1.0 equiv, D2O instead of
fluoro-1,3-enyne products in good yields. Since 1- H2O was added and a successful bromofluorination
fluoro-1,3-enyne products are found in materials sci- afforded the mixture 2b-D and 2b-H in 88% yield
ence and biologically active products, the presented with a 2.7:1 ratio. These results provided evidence
synthetic scheme should be applicable to access this that this chemistry presumably proceeds through the
class of compounds, starting with readily available ter- haloalkyne intermediates, which can be readily pre-
minal alkynes.[14] It is noteworthy to point out that 2f pared from the bromination of terminal alkynes in
also reacted with phenylboronic acid to give 1-((Z)-1- the the presence of Ag catalysis.[15] As we know, cat-
fluoro-2-phenylvinyl)-4-methoxybenzene in 91% yield ionic silver species have been regarded as effective
(Scheme 5).
catalysts for electrophilic activation of alkynes toward
Scheme 5. Transformations of bromofluroralkene.
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Silver-Assisted Difunctionalization of Terminal Alkynes
a sealed tube. The reaction mixture was stirred at 808C for
10 h. After completion of the reaction, the solution was di-
luted with ethyl acetate, washed with water and brine. The
organic layer was then dried over anhydrous Na2SO4 and
the solvent was removed under vacuum. The residue was fi-
nally purified by column chromatography on silica gel using
petroleum ether-ethyl acetate mixture as eluent.
Acknowledgements
We thank the National Natural Science Foundation of China
(20932002 and 21172076), National Basic Research Program
of China (973 Program) (2011CB808600), Doctoral Fund of
Ministry of Education of China (20090172110014), Guang-
dong Natural Science Foundation (10351064101000000) and
the Fundamental Research Funds for the Central Universities
(2010ZP0003) for financial support.
Scheme 8. Tentative mechanism for the bromofluorination
reaction.
a variety of nucleophiles.[16] Consequently, possible
mechanisms were proposed based on the previous lit-
erature and our experimental results (Scheme 8).[6,7,17]
First, the bromoalkyne is formed by Ag-catalyzed
bromination of terminal alkynes. Then, the silver
cation is attacked by the triple bond of bromoalkyne
to form a p-complex B, which is then converted to the
corresponding vinyl-sliver intermediate D by trans-ad-
dition of AgF to bromoalkyne. Protonation of the
vinyl-sliver intermediate D gives (Z)-2-bromo-1-fluo-
roalkene as the product and silver oxide.[18] The high
regio- and stereoselectivity of the product originates
from the back-side attack of fluoride (C to D).[8d] The
bromide atom is regarded as both an activating and
regio-directing group. Another mechanism including
nucleophilic addition of fluorine anion to bromoal-
kyne form vinyl-sliver intermediate D also cannot be
ruled out.[8]
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[11] The cif file for the compound 3b has been deposited in
Cambridge Crystallographic Data Center. CCDC
806256 contains the supplementary crystallographic
data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif or
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 2683 – 2688