Deoxyfluorination of acyl fluorides to trifluoromethyl compounds by FLUOLEAD/Olah's reagent under solvent-free conditions

Article abstract of DOI:10.3762/BJOC.16.254

A new protocol enabling the formation of trifluoromethyl compounds from acyl fluorides has been developed. The combination of FLUOLEAD and Olah's reagent in solvent-free conditions at 70 °C initiated the significant deoxyfluorination of the acyl fluorides and resulted in the corresponding trifluoromethyl products with high yields (up to 99%). This strategy showed a great tolerance for various acyl fluorides containing aryloyl, (heteroaryl)oyl, or aliphatic acyl moieties, providing good to excellent yields of the trifluoromethyl products. Synthetic drug-like molecules were also transformed into the corresponding trifluoromethyl compounds under the same reaction conditions. A reaction mechanism is proposed.

Full text of DOI:10.3762/BJOC.16.254

Deoxyfluorination of acyl fluorides to trifluoromethyl
compounds by FLUOLEAD®/Olah’s reagent under
solvent-free conditions
Yumeng Liang1, Akihito Taya2, Zhengyu Zhao2, Norimichi Saito3 and Norio Shibata*1,2,4
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2020, 16, 3052–3058.
1Department of Nanopharmaceutical Sciences, Nagoya Institute of
Technology, Gokiso, Showa-ku, Nagoya 466-5888, Japan,
2Department of Life Science and Applied Chemistry, Nagoya Institute
of Technology, Gokiso, Showa-ku, Nagoya 466-5888, Japan,
3Pharmaceutical Division, Ube Industries, Ltd., Seavans North Bldg,
1-2-1 Shibaura, Minato-ku, Tokyo 105-8449, Japan and 4Institute of
Advanced Fluorine-Containing Materials, Zhejiang Normal University,
688 Yingbin Avenue, 321004 Jinhua, China
https://doi.org/10.3762/bjoc.16.254
Received: 25 October 2020
Accepted: 04 December 2020
Published: 14 December 2020
This article is part of the thematic issue "Organo-fluorine chemistry V".
Guest Editor: D. O'Hagan
Email:
Norio Shibata* - nozshiba@nitech.ac.jp
© 2020 Liang et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
acyl fluorides; deoxyfluorination; fluorine; solvent-free; trifluoromethyl
group
Abstract
A new protocol enabling the formation of trifluoromethyl compounds from acyl fluorides has been developed. The combination of
FLUOLEAD® and Olah’s reagent in solvent-free conditions at 70 °C initiated the significant deoxyfluorination of the acyl fluo-
rides and resulted in the corresponding trifluoromethyl products with high yields (up to 99%). This strategy showed a great toler-
ance for various acyl fluorides containing aryloyl, (heteroaryl)oyl, or aliphatic acyl moieties, providing good to excellent yields of
the trifluoromethyl products. Synthetic drug-like molecules were also transformed into the corresponding trifluoromethyl com-
pounds under the same reaction conditions. A reaction mechanism is proposed.
Introduction
Due to an impressively wide effect of fluorine on the biological design of novel drugs [6,7]. One utility of the CF3 group is the
activity, the insertion of fluorine atoms or fluorine-containing replacement of a methyl group in biologically active molecules
functional groups into organic molecules has become a common to avoid the metabolic oxidation of a reactive methyl group in
strategy in pharmaceutical and agrochemical industries [1-5]. the parent molecules [8]. It should be noted that 19% out of 340
Among various fluorine-containing functional groups, the tri- marketed fluoro-pharmaceuticals [4] and 42% out of 424 regis-
fluoromethyl (CF3) group has received countless attention in the tered fluoro-agrochemicals [5] contain a CF3 moiety in their
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Beilstein J. Org. Chem. 2020, 16, 3052–3058.
structures (Figure 1), but this motif has never been found in of aryl, vinyl, and heteroaryl iodides with 2-(difluoromethoxy)-
nature [9]. Several notable examples of CF3-containing pharma- 5-nitropyridine [18]. A wide variety of acyl fluorides were effi-
ceuticals and agrochemicals include cinacalcet, efavirenz, travo- ciently obtained in high yields. We were fascinated by the syn-
prost, pexidartinib, fluoxetine, and upadacitinib (Figure 2) [4,5]. thetic versatility of acyl fluorides [19] to form other functional
groups during our research on acyl fluorides. Acyl fluorides,
Methodologies for the synthesis of trifluoromethyl compounds which are one type of acyl halides, show distinct stability in the
can be modestly divided into two general categories: the direct presence of moisture, indicating their suitable reactivity only in
introduction of a CF3 group into the target position by trifluoro- selected conditions [19].
methylation [10,11], and the construction of a CF3 unit by trans-
forming other functional groups [12,13]. Our group has de- In addition to our direct cross-coupling reaction method, several
veloped various efficient methodologies for the electrophilic useful synthetic methods for the formation of acyl fluorides
[14,15], nucleophilic [16], and radical [17] trifluoromethylation have become available in recent years [19-23]. In this context,
reactions for more than a decade. In recent years, we also re- we were interested in the functional transformation of an acyl
ported the direct introduction of an acyl fluoride unit into aro- fluoride unit into a CF3 motif. Despite the current rich availabil-
matic compounds by the Pd-catalyzed cross-coupling reaction ity of acyl fluorides and a strong market demand for trifluoro-
Figure 1: Ratios of CF3-containing drugs in marketed fluoro-pharmaceuticals and registered fluoro-agrochemicals in the world.
Figure 2: Selected examples of CF3-containing biologically active molecules.
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methyl compounds, synthetic methods for the direct transfor- fluoromethylation and not as fluorination of acyl fluorides.
mation of acyl fluorides to trifluoromethyl compounds are rare Thus, the acyl fluoride moiety is commonly sacrificed as a
[24-26]. A seminal example is the work by Lal and co-workers “leaving group” in reactions. On the other hand, the transfor-
reported in 1999 (Scheme 1a) [24,25]. The acyl fluorides were mation to CF3 derivatives from carboxylic acids are tradition-
converted into the trifluoromethyl compounds in good yields ally examined. In 1960, Engelhardt [27] performed the deoxy-
using Deoxo-Fluor®, but they only provided two examples. In fluorination of carboxylic acids with sulfur tetrafluoride (SF4) at
2018, Schoenebeck and co-workers reported the decarbonyla- 120 to 150 °C in liquid hydrogen fluoride (HF) (Scheme 1c).
tive trifluoromethylation of acyl fluorides by trifluoromethyl tri- Very recently, Mykhailiuk [28,29] improved the method to use
ethylsilane (Et3SiCF3) under Pd catalysis at high temperature SF4 for the deoxofluorination of carboxylic acids with or with-
(Scheme 1b) [26], although the reaction was categorized as tri- out HF in the gram scale (Scheme 1c). Although Mykhailiuk’s
Scheme 1: Transformation of acyl fluorides to trifluoromethyl compounds. a) Deoxyfluorination of acyl fluorides by DeoxoFluor®. b) Decarbonylative
trifluoromethylation of acyl fluorides by Et3SiCF3. c) Deoxyfluorination of carboxylic acids by SF4. d) Deoxyfluorination of carboxylic acids by PhSF3
via acyl fluoride intermediates. e) Deoxyfluorination of acyl fluorides by FLUOLEAD®.
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Beilstein J. Org. Chem. 2020, 16, 3052–3058.
protocol is beneficial even for industrial applications, the use of pounds [27-29,35]. Based on these facts, we herein report the
toxic SF4 requires special apparatus with expert handling.
efficient deoxyfluorination protocol of acyl fluorides to the tri-
fluoromethyl compounds using FLUOLEAD® (Scheme 1e).
In 2010, Umemoto and co-workers developed a novel deoxyflu- The key to the successful transformation is the use of
orinating agent, FLUOLEAD® [30]. FLUOLEAD® is a nucleo- FLUOLEAD® combined with Olah’s reagent [36] in solvent-
philic fluorinating agent that can be used as a broad and general free conditions. A wide variety of acyl fluorides involving
substrate for the deoxyfluorination. FLUOLEAD® is a stable aryloyl, (heteroaryl)oyl, and even aliphatic acyl, as well as some
crystalline solid and has sufficient stability against moisture to drug-like molecules, were smoothly transformed into the corre-
be handled open to air without the emission of fumes, and it is sponding trifluoromethyl compounds in good to excellent yields
available in quantities that can exceed 100 kg from Ube by FLUOLEAD®/Olah’s reagent. Since both FLUOLEAD®
Industries, Ltd [31]. FLUOLEAD® is an attractive alternative to and Olah’s reagent are commercially available in industrial
the toxic sulfur tetrafluoride [32] and the explosive (diethyl- quantities, this would be an ideal method for the synthesis of tri-
amino)sulfur trifluoride (DAST) [33]. Although the 2010 report fluoromethyl compounds.
by Umemoto provided a method for the deoxyfluorination of a
wide variety of substrates, including alcohols, ketones, and even Results and Discussion
carboxylic acids to the mono-, di-, and trifluoromethyl com- First, [1,1'-biphenyl]-4-carbonyl fluoride (1a) was selected as
pounds using FLUOLEAD® [30], they did not show the direct the benchmark substrate to optimize the reaction conditions (see
transformation of an acyl fluoride moiety to a CF3 unit. In 2012, Table S1 in Supporting Information File 1 for an extensive list
Umemoto and Singh examined the deoxyfluorination of of reaction conditions). The investigation of a range of parame-
carboxylic acids to trifluoromethyl compounds by phenylsulfur ters showed that the best results were achieved by the combina-
trifluoride (PhSF3) [34]. They suggested that the transformat- tion of 3 equiv FLUOLEAD® and 5 equiv nHF·pyridine in sol-
ion of carboxylic acids to the trifluoromethyl units consisted of vent-free conditions at 70 °C for 24 h, providing the product
two steps, including the generation of acyl fluoride intermedi- 4-(trifluoromethyl)-1,1'-biphenyl (2a) in 99% 19F NMR yield
ates (Scheme 1d). However, the direct transformation of acyl (Table 1, entry 1). Control experiments conducted in the
fluorides to trifluoromethyl compounds by PhSF3 did not occur. absence of either FLUOLEAD® or nHF·pyridine resulted in no
They explained the importance of the HF generation in the first desired product 2a (Table 1, entries 2 and 3). Moreover, other
step from acids to acyl fluorides and for the second step the fluorinating agents, such as DAST, DeoxoFluor, Xtalfluor-M®,
transformation of the acyl fluorides to the trifluoromethyl com- and tetrabutylammonium difluorotriphenylsilicate (TBAT),
pounds. Others have also suggested the critical role of HF for were less effective (Table 1, entry 4). When NEt3·(HF)3 was
transforming the carboxylic acids to the trifluoromethyl com- used instead of nHF·pyridine, the yield decreased to 37%
Table 1: Optimizing the reaction conditions for the conversion of 1aa.
entry
deviation from the standard conditions
yield (%)b
1
2
3
4
5
6
7
8
none
>99 (91)
0
no FLUOLEAD®
no nHF·pyridine
0
using DAST, DeoxoFluor, Xtalfluor-M, TBAT instead of FLUOLEAD®
using NEt3·(HF) 3 instead of nHF·pyridine
using MeCN, DMF, THF, toluene, or DCM, instead of neat
using 17 h, 3 h or 1 h instead 24 h
using 40 °C, 50 °C, 100 °C instead of 70 °C
25, 0, 0, 0
37
0, 0, 0, 65,26
55, 22, 9
9, 68, 85
aStandard conditions: 1a (0.3 mmol), FLUOLEAD® (0.9 mmol) and nHF·pyridine (1.5 mmol) in neat conditions at 70 °C for 24 h. bDetermined by
19F NMR spectroscopy; the number in parentheses refers to the isolated yield.
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Beilstein J. Org. Chem. 2020, 16, 3052–3058.
(Table 1, entry 5). Using a solvent to participate in the reaction in an excellent yield (99%). Substrates with two substituents,
reduced the reaction yield or even resulted in no reaction such as 3,5-dibutyl, 1-bromo-2-methyl, and 1,2-diethoxy groups
(Table 1, entry 6). Testing different reaction times indicated that (1j–l), furnished the desired products 2j–l in good to excellent
24 h were necessary (Table 1, entry 7). Lowering the tempera- yields (66–99%). The heteroaryl group of 2-thiophene (1m),
ture resulted in a decrease in the solubility of substrate 1a, while the alkyl substrates (1n and 1o) and vinylacyl fluoride (1p)
raising the temperature to 100 °C allowed a similar yield to be were attempted and gave the desired products 2m–p in good
obtained (Table 1, entry 8).
to excellent yields (52–93%). Moreover, drug-like derivatives
were next investigated as substrates to assess further the utility
With the optimized conditions in hand, the reaction scope was of this method. The acyl fluoride of the probenecid derivative
explored, and the results are summarized in Scheme 2. Various 1q was successfully transformed into the corresponding
acyl fluorides were investigated in the presence of 3 equiv of trifluoromethyl compound in an excellent yield of 97%
FLUOLEAD® and 5 equiv of nHF·pyridine. We first examined under the same conditions. The deoxyfluorination of the acyl
the deoxyfluorination of biarylacyl fluorides 1a and 1b with a fluoride in febuxostat (1r) required a higher reaction tempera-
para- or ortho-substitution. Both substrates gave excellent ture (100 °C), and gave the corresponding CF3 product 2r in a
yields of the desired product 2 (95–99%). Phenyl substrates 73% yield. The gram-scale transformation of 1a to 2a was also
with a functional group, such as bromo, butyl, cyclohexyl, and performed under the same conditions providing a similar result
methyl at the para-position afforded the desired products 2c–f (87% isolated yield). In all cases, the yields of the CF3 products
in good to high yields (54–89%). Naphthoyl fluorides 1g and 1h 2 were high to excellent, as determined by NMR, while the
gave the desired products 2g and 2h in excellent yields (99%). isolated yields decreased due to the high volatility of the prod-
Benzoyl fluoride (1i) also furnished trifluoromethylbenzene (2i) ucts 2.
Scheme 2: The substrate scope of acyl fluorides. Reaction conditions: 1 (0.3 mmol), FLUOLEAD® (0.9 mmol, 3.0 equiv) and nHF·pyridine (1.5 mmol,
5.0 equiv) in neat conditions at 70 °C for 24 h. Yields in parentheses were determined by 19F NMR spectroscopy. aAt 100 °C. bUsing 1a (1.0 g,
5.0 mmol).
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Beilstein J. Org. Chem. 2020, 16, 3052–3058.
A mechanism of the deoxyfluorination of the acyl fluorides hydrous Na2SO4, filtered, and concentrated under reduced pres-
with FLUOLEAD®/nHF·pyridine is proposed in Scheme 3. sure. The yield was determined by 19F NMR analysis of the
First, FLUOLEAD® is activated with (HF)n via hydrogen crude mixture by using C6H5OCF3 (40.0 μL, 0.3 mmol,
bonding to provide an activated form I, which induces a nucleo- 1.0 equiv) as an internal standard. The residue was purified by
philic attack from the carbonyl oxygen of 1 to I providing an silica gel flash chromatography (n-hexane) to afford the title
intermediate III via transition state II. The intermediate III is compounds.
further activated by (HF)n to form IV, which finally induces the
deoxyfluorination via V furnishing the trifluoromethylated
products 2 with the release of ArS(O)F and (HF)n.
Supporting Information
Supporting Information File 1
Conclusion
Optimization of the reaction conditions, general procedure
and product characterization data.
We have reported an efficient protocol for the deoxyfluorina-
tion of acyl fluoride substrates, using FLUOLEAD® as the
deoxofluorinating agent in the presence of Olah’s reagent, to
generate trifluoromethyl compounds. FLUOLEAD® combined
with Olah’s reagent, nHF·pyridine, was most effective for this
functional transformation reaction. The reaction of acyl fluo-
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-16-254-S1.pdf]
rides afforded the desired products smoothly in good to high Funding
yield in solvent-free conditions. An extension of this deoxyfluo- This work was supported by JSPS KAKENHI grants JP
rination strategy to drug-like molecules was demonstrated to 18H02553 (KIBAN B, NS).
show the usefulness of this transformation. The present protocol
also expands the utility of FLUOLEAD® in organic synthesis.
ORCID® iDs
Yumeng Liang - https://orcid.org/0000-0002-2846-0792
Norio Shibata - https://orcid.org/0000-0002-3742-4064
Experimental
General procedure: An oven-dried narrow-mouth FEP tube
(Nalgene®, 10.0 mL) containing a magnetic stirring bar was
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