Zinc-Catalyzed Stereo- and Regioselective 1,4-Hydrative Fluorination of 3-En-1-ynamides with Selectfluor

Article abstract of DOI:10.1002/adsc.201501021

Zinc-catalyzed 1,4-oxofluorinations of 3-en-1-ynamides with Selectfluor in acetonitrile/water proceeded with high regio- and stereoselectivity, giving E-configured γ-fluoro-α,β-unsaturated amides efficiently. Our control experiments indicate that kinetica

Full text of DOI:10.1002/adsc.201501021

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DOI: 10.1002/adsc.201501021
Zinc-Catalyzed Stereo- and Regioselective 1,4-Hydrative
Fluorination of 3-En-1-ynamides with Selectfluor
Appaso Mahadev Jadhav,a Deepak B. Huple,a Rahulkumar Rajmani Singh,a
and Rai Shung Liua,*
a
Department of Chemistry, National Tsing-Hua University, Hsinchu, 101, Sec. 2, Kuang-Fu Rd., Hsinchu 30013, Taiwan
E-mail: rsliu@mx.nthu.edu.tw
Received: November 10, 2015; Revised: January 13, 2016; Published online: March 8, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201501021.
Abstract: Zinc-catalyzed 1,4-oxofluorinations of 3-
en-1-ynamides with Selectfluor in acetonitrile/water
proceeded with high regio- and stereoselectivity,
mediates (I) before proceeding to a-fluoro ketones
and a,a-fluoroaryl ketones.
g-Fluoro-a,b-unsaturated carbonyl species consti-
tute a class of versatile intermediates in organic syn-
thesis, and are important functionalities in biologically
active compounds.[6] Common methods for the syn-
thesis of these compounds include electrophilic fluori-
nations of conjugated enol ethers[7] and Wittig-type
giving
E-configured
g-fluoro-a,b-unsaturated
amides efficiently. Our control experiments indicate
that kinetically unstable C-bound zinc dienolates
are chemically reactive to undergo SE2’-electrophil-
ic fluorinations whereas the detectable O-bound di-
enolates preferably undergo protodemetalation re-
actions instead.
Keywords: SE2’-electrophilic fluorinations; 1,4-oxo-
fluorinations; Selectfluor; zinc dienolates
Considerable interest has focused on fluoroorganic
compounds due to their potential use in material
chemistry, fine chemicals, agrochemicals, polymer and
fiber-related industries. The presence of a fluorine
atom in the organic compound can largely alter its
biological functions; such organofluoro compounds
are hence pharmaceutically important.[1] Despite
many synthetic methods, syntheses of organofluoro
compounds under mild conditions are still limited.[2]
The advent of gold catalysis has inspired considerable
interest in the 1,2-hydrative fluorination of alkynes to
produce a-fluoro ketone derivatives using Select-
fluor[3] as the fluorine source.[4,5] Nevado and co-
worker reported gold-catalyzed hydrative fluorina-
tions on alkynes to afford a-fluoro ketones [Eq.
(1)].[4] Hammond and co-worker reported remarkable
hydrative fluorination/aryl cascade reactions of al-
kynes using Selectfluor and boronic acid to yield a,a-
fluoroaryl ketones [Eq. (2)].[5] The mechanisms of
these reactions involve initial oxidations of neutral
À
Au(I) complexes to F Au(III) species, enabling the
alkyne hydrations to form a-oxo fluorogold inter-
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Appaso Mahadev Jadhav et al.
reactions of a-fluoroaldehydes and ketones,[8] vinylo- 1,2-oxofluorination products 2a and 2a’ in a combined
gous fluorinations of vinyldiazo esters.[9] We are 81% yield (2a/2a’=2.6:1) that were inseparable on
aware of no examples aiming at g-fluoro-a,b-unsatu- a silica column (entry 1). The structure of 2a’ was as-
À
rated amides.
signed based on an observed C F coupling of its
We envisaged that metal-catalyzed hydrative fluori- amide carbonyl (JC,F =24 Hz). In entry 2, we followed
nations of 3-en-1-ynamides might be a convenient the methods of Nevado[4] and Hammond[5] to treat 3-
tool to access g-fluoro-a,b-unsaturated amides be- en-1-ynamide 1a with Ph3PAuCl (5 mol%) and Select-
cause possibly existing C- and O-bound dienolates (II fluor to attain a complete conversion, yielding E-con-
and II’) would direct the 1,4- and 1,2-regioselectivities figured 2a in only 37% yield together with hydration
of their electrophilic activations. We recently reported product 3a in 24% yield. Fluorogold(III) intermediate
zinc-catalyzed 1,2-hydrative aldol reactions of 3-en-1- [I, Eqs. (1) and (2)] is not a viable route for the de-
ynamides via the aldol reactions of their O-bound sired 1,4-hydrative fluorinations. We thus employed
zinc dienolates intermediates (II’) that were detecta- cationic gold catalysts including Ph3PAuCl/AgNTf2
ble in solution.[10] In contrast with our previous re- and P(t-Bu)2(o-biphenyl)AuCl/AgNTf2 which afford-
sults, the present work reports 1,4-regioselectivity in ed the same product 2a in good yields (63–65%, en-
the zinc-catalyzed hydrative fluorinations of 3-en-1- tries 3 and 4). We tested the reactions with less acidic
ynamides to furnish E-configured 4-fluoro-2-en-1- IPrAuCl/AgNTf2 [IPr=1,3-bis(diisopropylphenyl)imi-
amides 2a with excellent regio- and stereoselectivity. dazol-2-ylidene] that gave the desired 2a in 48% yield
The occurrence of such a 1,4-regioselectivity is attrib- together with a hydration product 3a and unreacted
uted to the SE2’ electrophilic fluorinations of C-bound 1a in minor proportions (16–17%); the reaction time
zinc-dienolates (II) that are undetectable in solution. was long (ca. 4 h) although this catalyst was reported
Table 1 shows the realization of a 1,4-oxofluorina- to be very efficient for alkyne hydration (entry 5).[12]
tion of 3-en-1-ynamide 1a[11] with Selectfluor Notably, the yield of 2a was greatly improved to 85%
(2.0 equiv.) that is soluble in water. In the absence of with Zn(OTf)2 after a brief period (10 min, entry 6)
catalyst, the reaction of 3-en-1-ynamide 1a with Se- whereas ZnCl2 maintained the high efficiency to de-
lectfluor in CH3CN/water (3:1) at 258C gave 1,4- and liver the same product 2a in 83% (entry 7). The use
of AgOTf was troubled with the reaction chemoselec-
tivity to yield compounds 2a, by-product 3a and un-
reacted 1a in 62%, 12%, and 15%, respectively
(entry 8). Other metal catalysts such as Cu(OTf)2,
In(OTf)3, and Sc(OTf)3, gave the desired product 2a
in moderate yields (47–52%) along with undesired 3-
alkenylamide 3a in 12–16% (entries 9–11). The E-con-
figured geometry of 4-fluoro-2-en-1-amide 2a was
Table 1. Catalyst screening for 1,4-oxofluorination.
1
confirmed by H NOE effects. We hypothesize that
selective formation of product 2a is due to a rapid
Zn(II)-catalyzed hydration of alkynes to yield C-
bound zinc dienolates II (see Scheme 1). In the ab-
sence of metal catalyst, F+ can coordinate with alkene
or alkyne before the attack of water, further affording
1,4- and 1,2-oxofluorination reactions.
Entry Catalysts[a]
Time [h] Compounds (yields)[c]
2a/2a’
3a
1a
We prepared various 3-en-1-ynamides 1b–r to
assess the scope of applicable substrates; the results
are presented in Table 2. Initially, 3-en-1-ynamides
1b–1q were prepared in E-configurations with R2
being larger than R1. The 1,4-oxofluorination reac-
tions were performed with Zn(OTf)2 (5 mol%) fol-
lowing the same procedure as in Table 1 (entry 6).
The resulting 4-fluoro-2-en-1-amides 2b–2p were ob-
tained as single diastereomeric products (entries 1–
15) whereas compounds 2q and 2r were present with
two isomeric products. For 3-en-1-ynamides 1b–e bear-
ing various sulfonamide groups including cyclopropyl
tosylsulfonamide, methyltosylsulfonamide, benzyl-
tosylsulfonamide, and oxazolidine, their zinc-cata-
lyzed reactions gave the desired 1,4-oxofluorination
products 2b–2e in satisfactory yields (76–87%, en-
1
2
3
4
5
6
7
8
–
0.6
2.0
81(2.6:1)
37/–
65/–
63/–
48/–
85/–
83/–
62/–
47/–
–
24
–
–
16
–
–
–
–
–
17
–
–
15
–
–
Ph3PAuCl
Ph3PAuCl/AgNTf2 2.0
LAuCl/AgNTf2 2.0
IPrAuCl/AgNTf2 4.0
Zn(OTf)2
ZnCl2
0.15
0.50
2.0
2.0
2.0
2.0
–
AgOTf
12
16
14
12
9
10
11
Cu(OTf)2
Sc(OTf)3
In(OTf)3
50/–
52/–
–
[a]
L=P(t-Bu)2(o-biphenyl);
phenyl)imidazol-2-ylidene).
[1a]=0.13M.
Product yields are reported after purification by using
a silica gel column.
IPr=1,3-bis(diisopropyl-
[b]
[c]
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Zinc-Catalyzed Stereo- and Regioselective 1,4-Hydrative Fluorination
Scheme 1. Control experiments.
tries 1–4). The reactions were compatible with sub- mides 1o and 1p bearing R3 =H, R2 =CH2OH and
strates 1f and 1g bearing R2 =4-ClC6H4, and 4- CH2OMe to test the tolerance of a functional group,
BrC6H4, yielding the expected 1,4-oxofluorinatiton their Zn(II)-catalyzed 1,4-oxofluorination reactions
products 2f and 2g in satisfactory yields (83–84%, en- gave the desired 2o and 2p in 83 and 86% yields, re-
tries 5 and 6). We prepared additionally 3-en-1-yna- spectively (entries 14 and 15). The reaction was fur-
mides 1h and 1i containing R1 =n-propyl and phenyl, ther compatible with 2-en-1-ynamide 1q bearing
giving the desired 2h and 2i in 82 and 83% yields re- a vinyl group (R2 =R3 =H); two isomeric products E-
spectively (entries 7 and 8). To further expand the 2q and Z-2q’ were obtained in E- and Z-configura-
substrate scope, we tested the reactions on starting tions, respectively, with corresponding yields of 67%
1
species 1j–1l bearing a cycopentenyl, cyclohexenyl and 11% (entry 16). H NOE was used to determine
and cycloheptenyl ring, respectively (R1, R2 =C5H6, the olefin geometries of the two isomers. The reaction
C6H8 and C7H10); the corresponding products 2j– is less compatible with substrate 1r bearing a 1,2-sub-
l were obtained with satisfactory yields (72–89%, en- stituted alkene, yielding 1,4- and 1,2-oxofluorination
tries 9–11). These zinc-catalyzed reactions were ex- products 2r and 2r’ in a combined 78% yield
tendable to substrates 1m and 1n bearing a dihydro- (entry 17).
naphthalene ring, the desired products 2m and 2n re-
Scheme 1 presents data to assist in an understand-
sulted in 77% and 67% yields, respectively (entries 12 ing of the reaction mechanism. Treatment of a 1:1
and 13). We performed X-ray diffraction studies to molar ratio of 3-en-1-ynamide 1a and a hydration
characterize the molecular structure of 2n, confirming product 3c, together with Selectfluor (2 equiv.) and
its E-configuration.[13] We also prepared 3-en-1-yna- Zn(OTf)2 (5 mol%) in MeCN/H2O (3:1) for a brief
Scheme 2. Rationale for the stereo- and regioselectivity.
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Appaso Mahadev Jadhav et al.
Table 2. 1,4-Oxofluorination of various 3-en-1-ynamides.
period (258C, 10 min) delivered 2a in 85% yield
whereas species 3c was recovered exclusively. This in-
formation indicates that a hydration product like 3c is
unlikely to be an intermediate for this 1,4-oxofluori-
nation reaction. We examined also the reactivity of
species 3a toward Selectfluor (1.5 equiv.) in MeCN/
H2O (3:1) at 508C, the reaction proceeded smoothly
to yield 4-oxo-2-en-1-amide 4a in 85% yield in-
stead.[11c]
Scheme 2 provides a plausible mechanism to ration-
alize the excellent regio- and stereocontrol of the 1,4-
oxofluorination reactions of 3-en-1-ynamide 1q. In
our previous work,[10] we identified an O-bound dien-
olate C/C’ via a slow hydration of 3-en-1-ynamide 1q
with Zn(OTf)2, this O-bound zinc dienolate reacted
with aldehydes at the Ca-addition, inconsistent with
a distinct Cg-addition regioselectivity observed for
our fluorination products 2. We postulate that C-
bound dienolate B is actually active despite being un-
detectable. The SE2’-reaction of this enolate (B) with
Selectfluor likely proceeds in two possible conforma-
tions with state D being more favorable than state D’
because the latter has a large and proximate amide
group to impede Selectfluor from approaching the
olefin group according to an SE2’ model, in which the
À
Zn C bond is parallel to the electrophile to exert
a hyperconjugation.
To verify our hypothesis, we followed our reported
procedure[10] to generate O-bound zinc dienolate C
through the treatment of 3-en-1-ynamide 1q with
H2O (5 equiv.) in CD3CN (258C, 16 h) to attain a com-
plete consumption of initial 1q; the resulting solution
contained O-bound zinc dienolate C and 2-en-1-
amide 3q’ and 3-en-1-amide 3q in 40%, 53% and 7%
yields, respectively, as depicted in Scheme 3. A subse-
quent treatment of this solution provided mainly two
hydration products 3q’ and 3q in 54% and 46% yields,
respectively. This information suggests that O-bound
zinc dienolate C is inactive toward Selectfluor in the
presence of HOTf, but preferably formed the hydra-
tion product 3q. With this information, we envisage
that initial hydration product 3q’ arises from C-bound
enolate B, further supporting our postulated mecha-
nism.
This work continues our interest in the zinc-cata-
lyzed hydrative functionalizations of 3-en-1-ynamides
via dienolate intermediates. In contrast with our re-
ported 1,2-regioselectivity of the hydration and hydra-
tive aldol reactions of these 3-en-1-ynamides, the
present 1,4-oxofluorinations proceeded through 1,4-
addition regioselectivity with high E-stereoselectivity,
yielding diverse 4-fluoro-2-en-1-amides efficiently.[14]
As a result of a series of control experiments to con-
firm the intermediacy of dienolate intermediates,[15]
we postulate C-bound dienolates to control the regio-
and stereochemical course for the final fluorination
reactions.
[a]
Reaction
conditions:
1
(0.27 mmol),
Zn(OTf)2
(0.013 mmol), Selectfluor (0.54 mmol).
1a (0.13M).
Product yields are reported after separation on a silica
gel column.
[b]
[c]
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Zinc-Catalyzed Stereo- and Regioselective 1,4-Hydrative Fluorination
Scheme 3. Reactivity of the O-bound zinc dienolate.
References
Experimental Section
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A glass vial was charged with an MeCN/H2O (2 mL, 3:1) so-
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1H NMR (600 MHz, CDCl3): d=7.74 (dd, J=6.8, 1.8 Hz,
2H), 7.37~7.35 (m, 3H), 7.29 ~7.26 (m, 4H), 6.64 (dd, J=
2.6, 1.3 Hz, 1H), 5.68(d, J=47.2 Hz, 1H), 3.82(t, J=6.7 Hz,
2H,) 2.40 (s, 3H), 1.73–1.68 (m, 5H), 1.38–1.34 (m, 2H),
0.94 (t, J=7.4 Hz, 3H); 13C NMR (150 MHz, CDCl3): d=
165.9, 151.4 (d, J=18.1 Hz), 144.5, 137.0, 136.4 (d, J=
20.5 Hz), 129.6, 129.2, 128.7, 127.6, 127.0 (d, J=5.4 Hz),
117.0 (d, J=11.1 Hz), 96.0 (d, J=179.2 Hz), 46.6, 32.0, 21.6,
19.9, 15.0, 13.6 (d, J=17.6 Hz); HR-MS: m/z=403.1612,
calcd. for C22H26FNO3S: 403.1617.
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Acknowledgements
The National Science Council, Taiwan supported this work.
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