Fluorocyclization of N-Propargylamides to Oxazoles by Electrochemically Generated ArIF2

Article abstract of DOI:10.1021/acs.orglett.9b02884

A sustainable synthesis of 5-fluoromethyl-2-oxazoles by use of electrochemistry has been demonstrated. Hypervalent ArIF₂ is generated by direct electrochemical oxidation of iodoarene ArI in Et₃N·5HF and mediates the fluorocyclization

Full text of DOI:10.1021/acs.orglett.9b02884

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Fluorocyclization of N‑Propargylamides to Oxazoles by
Electrochemically Generated ArIF2
John D. Herszman,† Michael Berger,† and Siegfried R. Waldvogel*
Institute of Organic Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, 55128 Mainz, Germany
S
* Supporting Information
ABSTRACT: A sustainable synthesis of 5-fluoromethyl-2-oxazoles by use of
electrochemistry has been demonstrated. Hypervalent ArIF2 is generated by
direct electrochemical oxidation of iodoarene ArI in Et3N·5HF and mediates the
fluorocyclization of N-propargylamides to 5-fluoromethyl-2-oxazoles. The
stoichiometry in ArI turned out to be a key parameter in controlling the
product selectivity. This electrochemical protocol provides access to fluorinated
oxazoles starting from simply available N-propargylamides with yields up to 65%
and offers a green alternative over conventional reagent-based approaches.
Scheme 1. Conventionally vs Electrochemically Generated
ArIF2 for Fluorocyclization of N-Propargylamides
xazoles and oxazolines occur ubiquitously in structurally
Odiverse natural products and pharmaceutically active
ingredients.1 For this reason, oxazoles are an important scaffold
in medicinal chemistry. Drugs based on oxazoles have a wide
range of biological activities.2 Additionally, oxazole derivatives
are important intermediates in organic synthesis and ligands for
metal catalysis.3 Thus, the development of efficient synthetic
methodologies to oxazoles is very desirable. Various strategies
for the construction of oxazoles, particularly cyclization
reactions of N-propargylamides, have been developed either
by metal-catalyzed4 or iodine-mediated5 activation of the triple
bond. However, concerning bioactive compounds the sub-
stitution of hydrogen by fluorine such as in methyl groups as a
bioisostere has been become a standard tool in molecular editing
of drugs.6 The incorporation of fluorine can modulate the
metabolism of a drug and increase its potency. Thus, the
deoxyfluorination reaction is a widely used approach to
heterocycles containing a fluoromethyl group.7 Typically used
reagents are SF4, XeF2, Selectfluor, (PhSO2)2NF, or Et2NSF3
(DAST).8 These reagents prove to be very powerful but have
severe drawbacks: They are truly hazardous, difficult to handle,
and mostly expensive. Comparatively, the use of hypervalent
iodine reagents in fluorocyclization reactions provides a less
dangerous and efficient path to fluorinated heterocycles.9 Saito10
and Gilmour11 established straightforward strategies to
fluorinated oxazoles and oxazolines by fluorocyclization of the
respective N-allyl- or N-propargylamides mediated by hyper-
valent difluoroiodoarene ArIF2 (Scheme 1). This way, the
fluorocyclization provides the synthesis of the heterocycle and
the fluorination in only one step. Still, these methods involve
excess Selectfluor as terminal oxidant for the generation of ArIF2.
The use of reagent-based oxidizers leads to low atom economy
and production of a lot of reagent waste.
as a green oxidant and unstable intermediates are generated in
situ.12 This way, iodoarene ArI is directly oxidized to the
hypervalent ArIF2 at the anode.13 Electrochemical reactions
have many advantages over traditional, reagent-based syntheses.
By means of the electroorganic synthesis, toxic and expensive
chemical oxidizers can be replaced by electric current as an
inexpensive, renewable, and inherently safe reagent.14 It is a
powerful tool to increase atom economy by reducing the
amount of reagent waste on the side of oxidant using traceless
electric current. Thus, organic electrochemistry attracted much
attention as a green synthetic approach.15 Recently, we
demonstrated the electrochemical synthesis of 5-fluoromethyl-
2-oxazolines.16 However, an electrochemical approach to
fluorinated 2-oxazoles is not described yet in the literature.
Here, we present the synthesis of 5-fluoromethyl-2-oxazoles via
fluorocyclization of N-propargylamides by electrochemically
generated ArIF2 for the first time (see the postulated mechanism
in the Supporting Information). The electrolysis was conducted
In contrast, the electrochemical generation of iodine(III)
species is an attractive alternative because electric current is used
Received: August 14, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02884
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
in an undivided cell equipped with a simple two-electrode
arrangement using constant current conditions. The electrolytic
reactions were performed at platinum electrodes in a mixture
(1:1) of dichloromethane and triethylamine pentahydrofluoride
(Et3N·5HF) with a substrate concentration of 0.1 mol·L−1 and
stoichiometrically added PhI. Evaluation of the reaction
conditions was conducted within screening experiments in
small Teflon cells.17 Thus, N-propargylbenzamide (1a) served
as test substrate and the conversion to 5-fluoromethyl-2-
phenyloxazole (2a) as a benchmark reaction for the parameter
screening (Table 1). For optimization experiments the yield was
determined by 1H NMR with 1,3,5-trimethoxybenzene as
internal standard.
observed (Table 1, entry 2). The use of Py·9HF had an adverse
effect on the formation of 2a but promoted the side reaction to
byproduct 3a (Table 1, entry 3). In the absence of PhI no
formation of oxazole 2a was observed (Table 1, entry 4), which
confirms the unique role of the iodoarene mediator. Never-
theless, small amounts of byproduct 3a were obtained.
Consequently, the side reaction seemed to be independent of
the mediator. As the reaction was conducted with 2.0 equiv of
iodobenzene, the yield of oxazole 2a increased substantially
(51%), while the formation of byproduct 3a was almost fully
suppressed (Table 1, entry 5). The stoichiometry in
iodobenzene turned out to be a key parameter controlling the
product selectivity. Thus, we further investigated the effect of the
PhI stoichiometry on this electrochemical reaction (Figure 1).
Table 1. Parameter Screening for Optimizing the
Electrochemical Fluorocyclization of Substrate 1a to Oxazole
a
2a
b
b
entry
1
iodoarene (R = )
iodoarene (equiv)
2a (%)
3a (%)
H
H
H
1.0
1.0
1.0
32
0
6
16
0
33
7
c
2
d
3
e
4
0
5
6
7
8
9
10
11
12
H
H
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
51
41
16
54
55 (53)
30
3
4
30
0
0
0
0
0
f
Figure 1. Product selectivity by PhI stoichiometry: fluoromethyl
oxazole 2a vs methylene oxazoline 3a.
COMe
t-Bu
Me
Me
Me
H
g
Under otherwise identical conditions, we studied the product
formation with different equivalents of iodobenzene (0.1−3.0
equiv). Below stoichiometric amount of PhI a clear trend toward
the methylene oxazoline 3a could be observed. With 0.1 equiv of
PhI, 3a was obtained in 55%, whereas fluoromethyl oxazole 2a
was found in traces only. Although in the control experiment
without iodobenzene (Table 1, entry 4) little formation of the
methylene oxazoline 3a was observed, catalytic amounts of PhI
seem to prevent electrochemical degradation and allow 3a to
accumulate. In contrast, overstoichiometric amounts of PhI
promote the formation of oxazole 2a and repress the side
reaction progressively. Yet, more than 2.0 equiv of PhI did not
ameliorate the yield of 2a. Based on these findings, the following
experiments were conducted with 2.0 equiv of iodobenzene. The
ratios of product to side product dependent on the PhI
stoichiometry were further explored with electronic substitution
on the aryl moiety (see the Supporting Information).
In final optimization experiments, different current densities
and applied charge as well as different iodoarene mediators and
anode materials were tested. Both lower (10 mA/cm2) and
higher (60 mA/cm2) current density decreased the yield of 2a
(40% and 44%). A higher amount of applied charge also caused
lower yield (Table 1, entry 6). Using 4-acetylphenyl iodide as
mediator diminished the yield of 2a and led to more byproduct
3a (Table 1, entry 7). The 4-tert-butyl- and 4-methyl-substituted
mediators were more efficient (Table 1, entries 8 and 9) due to
lower oxidation potential. The yield dropped with boron-doped
diamond (BDD) as anode material18 (Table 1, entry 10).
However, glassy carbon19 achieved a good yield (Table 1, entry
11) but did not surpass previous observations. Concerning the
h
i
55
45
j
a
Standard reaction conditions: undivided cell, Pt electrodes, N-
propargylbenzamide (0.5 mmol), CH2Cl2 (2.5 mL), Et3N·5HF (2.5
b
mL), 3 F, 50 mA·cm−2, 15 h, rt. Determined by 1H NMR with 1,3,5-
c
trimethoxybenzene as internal standard. CH2Cl2/Et3N·3HF (1:1).
d
e
f
g
h
CH2Cl2/Py·9HF (1:1). No mediator. 4 F. Isolated yield. BDD as
i
j
anode. Glassy carbon as anode. Ex-cell experiment.
The control experiment without current flow (0 F, no
conversion of starting material) indicated, that the anodic
oxidation of the iodoarene based mediator is crucial for the
conversion of 1a. Under standard reaction conditions with
iodobenzene (1.0 equiv), applied charge of 3 F, current density
of 50 mA·cm−2, and reaction time (electrolysis + additional
stirring time) of 15 h in CH2Cl2/Et3N·5HF (1:1), the desired
product 2a was obtained in 32% yield (Table 1, entry 1). The
reaction time exceeded the electrolysis time since after
electrolysis starting material 1a was not fully converted.
Prolonging the stirring time increased the yield of 2a but with
more than 15 h no further improvement was observed. Thus, the
fluorocyclization of 1a takes longer than the anodic generation
of ArIF2. The nonfluorinated methylene oxazoline 3a was
identified as a byproduct. Therefore, we initially explored the
electrolyte system with different ratios and amine−HF sources.
Neither with higher amounts of Et3N·5HF (CH2Cl2/Et3N·5HF,
1:3) nor with lower amounts (3:1) could the yield of 2a be
increased. In Et3N·3HF, no conversion of starting material was
B
DOI: 10.1021/acs.orglett.9b02884
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
mass balance, just 55% of the starting material 1a was converted
to product 2a. Apart from uncharacterized byproducts the
electrochemical degradation and oligomerization of substrate
was the dominant side reaction. Therefore, the ex-cell
procedure, whereby substrate is added after anodic generation
of ArIF2, proved to be a powerful tool for indirect electrolysis in
order to avoid such side reactions.16 The ex-cell experiment
afforded a slightly lower yield of product 2a (Table 1, entry 12 vs
5), but 35% of 1a was recovered (82% brsm). Within the in-cell
experiment no starting material was recovered, but higher yield
of 2a was obtained due to in situ generation of unstable ArIF2.
Still, the optimized reaction conditions in Table 1, entry 9,
exceeded the conventional approach by Saito.10 With the
developed method, we explored the scope of the electrochemical
fluorocyclization of N-propargylamides varying the substitution
pattern at the aryl moiety (Table 2). The unsubstituted substrate
1a gave 5-fluoromethyl-2-phenyloxazole (2a) in 53% yield
(Table 2, entry 1). The electrosynthesis of compound 2a was
also conducted on a 2.5 mmol scale with 49% isolated yield. In
comparison to the unsubstituted scaffold 2a, the methyl- and
tert-butyl-substituted derivatives 2b and 2c were obtained in
lower yields (Table 2, entries 2 and 3). As the electron-rich aryl
moiety led to lower yields (2d vs 2a), electron-poor derivatives
afforded the respective oxazoles in higher yields (2e vs 2a and 2f
vs 2b). Although methoxy derivative 2d was obtained in only
37% yield, with the conventional method that transformation of
1d was not even possible.10 The nitro-substituted substrate 1g
gave 47% of product 2g. Due to possible cathodic side reaction
of the nitro group this reaction was also conducted in an ex-cell
experiment. Thus, fluoromethyloxazole 2g was obtained in 59%
yield (Table 2, entry 7). Halogenated substrates 1h−k with
different substitution patterns and halogen atoms gave the
desired products 2h−k in 38−49% yields (Table 2, entries 8−
11). The thienyl and furanyl oxazoles 2l and 2m were obtained
in low yield even if performing the ex-cell procedure (Table 2,
entry 12 and 13). Naphthyl oxazole 2n was obtained in 32%
(Table 2, entry 14). The cinnamamide 1o gave styryl oxazole 2o
in 37% yield (Table 2, entry 15).
Table 2. Synthesis of 5-Fluoromethyl-2-oxazole Derivatives
by Electrochemical Fluorocyclization
a
Beyond variation of the substitution pattern at the aryl moiety,
also α-amide-substituted N-propargylbenzamides and non-
aromatic derivatives were tested but did not undergo
fluorocyclization. A substituent R ≠ H in the α position to the
nitrogen might slow down or even inhibit isomerization to the
oxazole. In the crude NMR spectra obtained from the
nonaromatic derivatives mainly starting material was found. N-
Propargylbenzamides with a substitution at the terminal
position of the alkyne gave 4,5-dihydrooxazolyl ketones instead
of the expected fluorinated oxazoles. Those nonsuccessful
substrates are reported with more detailed data in the
Supporting Information.
a
Standard reaction conditions: undivided cell, Pt electrodes, N-
propargylbenzamide (0.5 mmol), 4-methyliodobenzene (2.0 equiv),
CH2Cl2 (2.5 mL), Et3N·5HF (2.5 mL), 3 F, 50 mA·cm−2, 15 h, rt.
Isolated yields, ex-cell studies in parentheses.
To conclude, the synthesis of 5-fluoromethyl-2-oxazoles by
ArIF2-mediated fluorocyclization of N-propargylamides with
electric current as green oxidant has been established. This
electrosynthesis provides access to fluorinated oxazoles, for
example, as attractive building blocks in medicinal chemistry.
The electrochemical protocol is applicable to a variety of
substrates affording oxazoles involving a fluoromethyl group
with yields up to 65%. In comparison to the reagent-based
path,10 this electrochemical approach permits the conversion of
even electron-rich N-propargylamides for the first time. The
simple setup using undivided cells in a two-electrode arrange-
ment with no reagent-based oxidants needed is sustainable and
inherently safe, enabling easy scale up.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b02884.
■
S
General experimental procedure, synthetic details, and
characterization including NMR and MS spectra for all
products, byproducts. and precursors (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: waldvogel@uni-mainz.de. Tel: +49 6131/39-26069.
C
DOI: 10.1021/acs.orglett.9b02884
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
ORCID
Letter
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A.; Zhdankin, V. V.; Saito, A. Asian J. Org. Chem. 2016, 5, 1314−1317.
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(12) (a) Roesel, A. F.; Broese, T.; Majek, M.; Francke, R.
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Winterson, B.; Alharbi, H.; Folgueiras-Amador, A. A.; Genot, C.;
Siegfried R. Waldvogel: 0000-0002-7949-9638
Author Contributions
†J.D.H. and M.B. contributed equally.
́
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Notes
Wirth, T. Angew. Chem., Int. Ed. 2019, 58, 9811−9815.
The authors declare no competing financial interest.
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J.D.H. acknowledges the Friedrich Ebert Foundation for
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