Synthesis of Acyl Fluorides via DAST-Mediated Fluorinative C-C Bond Cleavage of Activated Ketones

Article abstract of DOI:10.1021/acs.orglett.0c02603

A new protocol for preparation of acyl fluorides was developed by recognizing activated ketones as starting materials. The method provides a different scope compared with previously reported methods that employ carboxylic acids as substrates. A working hypothesis of pull-and-push-driven fluorinative C-C bond cleavage was successfully demonstrated by the simple addition of diethylaminosulfur trifluoride (DAST) derivatives to α-oximinoketones. The designed reaction system led to a highly efficient and chemoselective reaction. The wide availability of the ketones allowed for a range of synthetically useful aryloyl and aliphatic acyl fluorides including those containing chiral skeletons. The method is mild, fast, scalable, and potentially one-pot operative.

Full text of DOI:10.1021/acs.orglett.0c02603

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Letter
Synthesis of Acyl Fluorides via DAST-Mediated Fluorinative C−C
Bond Cleavage of Activated Ketones
Danhee Kim and Hee Nam Lim*
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ABSTRACT: A new protocol for preparation of acyl fluorides was
developed by recognizing activated ketones as starting materials. The
method provides a different scope compared with previously reported
methods that employ carboxylic acids as substrates. A working hypothesis
of pull-and-push-driven fluorinative C−C bond cleavage was successfully
demonstrated by the simple addition of diethylaminosulfur trifluoride
(DAST) derivatives to α-oximinoketones. The designed reaction system
led to a highly efficient and chemoselective reaction. The wide availability of the ketones allowed for a range of synthetically useful
aryloyl and aliphatic acyl fluorides including those containing chiral skeletons. The method is mild, fast, scalable, and potentially one-
pot operative.
Scheme 1. Prior Approaches and Proposed Work for the
Synthesis of Acyl Fluorides
cyl fluorides are versatile reagents in organic synthesis due
Ato their distinct reactivity and exceptional stability over
other types of acyl halides. The small size and intrinsic
electronic nature of the fluorine atom have often provided a
solution for challenging problems encountered in the
esterification and amidation of sterically demanding or
electronically deactivated alcohols or amines.1 In particular,
the use of an acyl fluoride in peptide syntheses has offered
advantages such as the suppression of byproduct formation and
minimal epimerization compared with that with conventional
coupling reagents.2 In addition to its direct reaction with
common nucleophiles, it has been employed as key building
blocks in transition-metal (TM)-catalyzed coupling reactions;
acyl fluorides can be classified as “RCO”, “R”, and “F” sources.
An overview of the applications of acyl fluorides in TM
catalysis was recently published by Ogiwara and Sakai.3
Such utility of acyl fluorides in organic synthesis led to the
development of a number of reliable protocols that focus on
the conversion of carboxylic acids to acyl fluorides (Scheme
1a). Since an early discovery using cyanuric fluoride by Olah
and coworkers,4 several seminal works have been reported by
employing hydrogen fluoride (HF)−pyridine/1,3-dicyclohex-
ylcarbodiimide (DCC),5 uronium-based reagents (e.g., tetra-
methylfluoroformamidinium hexafluorophosphate (TFFH)),6
diethylaminosulfur trifluoride (DAST)7 and its derivatives
(Deoxo-Fluor8 and XtalFluor9), and other sulfur-based
fluorinating reagents such as (Me4N)SCF3.10 Most recently,
a novel method using PPh3/NBS and Et3N/HF was presented
by Prakash and coworkers.11 Although these innovative
reagents are practical due to the low toxicity, generous
functional group scope, and minimal byproduct formation, the
methods only rely on the use of carboxylic acids as starting
materials.
Herein we report a simple protocol to convert activated
ketones to acyl fluorides (Scheme 1b). The method is an
unconventional approach, but it is operationally simple and
practical because it uses readily available ketones and sulfur-
based fluorination reagents. The use of ketones as a starting
material is a significant advantage because ketones are one of
the most common functional groups and are widely present in
many biologically important molecules.
This study was inspired by two seminal works of Momose
and Shibata: (1) In 1997, Momose disclosed the fluorinative
C−C bond cleavage of α,α-diakyl-substituted cyclic oximes
Received: August 4, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02603
© XXXX American Chemical Society
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using DAST as a bifunctional fluoride source.12 In this work,
oxime activation by an electrophilic trifluorosulfur was
followed by fluoride addition at the α-carbon of the oxime;
this concerted reaction constructed a fluorinated quaternary
carbon center instead of yielding a general Beckmann
rearrangement product. (2) In Shibata’s work, an interesting
C−C bond cleavage by the fluoride-initiated retro-Claisen
reaction was discovered when DAST derivatives were
subjected to 2-carboalkoxy-1-indanones (1,3-dicarbonyl).13
The unexpected formation of sulfenylated products was
featured. In this context, we anticipated that a ketone bearing
an α-oxime (1,2-dicarbonyl) might be an excellent substrate
for the synthesis of acyl fluorides because it has, in principle,
necessary components for C−C bond cleavage such as a
fluoride acceptor (activated ketone) and a leaving group
(activated oxime formed by electrophilic sulfur-based fluori-
nation reagents), as depicted in Scheme 1b.14 Compared with
the 1,3-dicarbonyl system, the designed 1,2-dicarbonyl system
was expected to differ in reactivity. For instance, because of the
high reactivity of ketones activated by α-oximes, it would offer
superior chemoselectivity over competing functional groups.
Moreover, the enhanced reactivity of ketones would allow a
broad ketone scope regardless of the original ketone reactivity.
However, a challenge was to avoid competitive Beckmann
rearrangement or Grob-like elimination, which can induce
undesirable side products.15
Fortunately, Prakash’s one-pot protocol11 could be successfully
applied to the given system (Table 1, entry 6). However, the
reaction did not proceed without preactivation of the oxime
moiety (Table 1, entry 7). We also attempted nucleophilic
fluoride addition to the oxime tosylate, but it was not fruitful
(Table 1, entry 8).
During the investigation of the substituent scope of arenes
(Scheme 2), careful handling was required for the precise
a
Scheme 2. Functional Group Scope
Setting out the research, the model substrate 1a was
prepared. α-Oximation of ketones is readily accessed in several
ways: direct formation at the ketone α-position by acid- or
base-assisted oximation using alkylnitrites,16 condensation
between hydroxylamine and 1,2-diketones,14b,17 or nitroso-
interception by silyl enol ethers.18 After surveying some
nucleophilic fluorinating reagents, we found that the simple
addition of DAST to 1a provided the acyl fluoride 2a in the
highest yield (Table 1, entry 1). It is notable that no side
products were found. Other sulfur-based fluorination reagents
also gave the desired product with comparable conversions
(Table 1, entries 2−4). In addition to SF4 derivatives, the
Ishikawa reagent worked in this system (Table 1, entry 5).
a
Isolated yield.
determination of product yield for a handful of volatile aryloyl
fluorides. The mild reaction conditions ensured the broad
scope of tolerable functional groups. Arenes bearing electron-
donating and -withdrawing functional groups such as methoxy
2a, halides 2b−d, trifluoromethyl 2e, and nitro 2f groups were
well suited. Gratifyingly, this method was highly chemo-
selective even for phenols, which are potential nucleophiles
and can undergo undesired oxidation to generate 2g.19 The
primary alcohol 1h also reacted to provide acyl fluoride 2h in
41% isolated yield, although it was not the sole product.20 In
addition, the substrate bearing a potentially competing ketone
group was converted to the corresponding product 2i with
complete selectivity. The benzyl bromide 2j, acetoxy 2k, and
arylester 2l functional groups were all compatible. Moreover,
arenes containing acid-sensitive unsaturated bonds were
tolerated under the present conditions, giving 2m and 2n in
good yields. The observed excellent chemoselectivity demon-
strates our premise that ketone is strongly activated by adjacent
oxime functionality.
Whereas a comparable functional group scope was displayed
for the preparation of acyl fluorides, the protocols of
Schoenebeck10 and Prakash11 appeared to be advantageous,
as demonstrated for the late-stage functionalization of active
pharmaceutical ingredients (APIs). Meanwhile, our method
was considered to have differentiated applications owing to its
use of cyclic ketones. To prove this point, we extended the
substrate scope from acyclic ketones to benzo-fused and
aliphatic cyclic ketones. By subjecting cyclic ketone substrates
a
Table 1. F-Source Screening
entry
fluorination reagents
time
yield (%)
b
1
2
3
4
5
6
7
8
DAST
Deoxo-Fluor
10 min
10 min
10 min
10 min
10 min
60 min
1 day
93 (87)
93
83
88
79
85
trace
<10
XtalFluor-E, Et3N/HF
Fluolead
Ishikawa’s reagent
PPh3/NBS, then Et3N/HF
Et3N/HF
TsCl, Et3N, then KF, 18-crown-6, THF
1 day
a
Determined by 19F NMR using fluorobenzene as the internal
b
standard. Isolated yield.
B
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to the established reaction system, synthetically intriguing
building blocks were obtained, as illustrated in Scheme 3. The
3m−q were subjected to the C−C cleavage reactions; in this
way, the novel acylating agents 4m−q were obtained in good
to excellent yields. These examples strongly demonstrate the
power of this methodology that provides unprecedented chiral
acyl fluorides from a pool of readily available chiral cyclic
ketones. As a set of final examples, steroidal compounds were
deconstructed to provide interesting acyl fluorides 4r and 4s,
respectively, which can potentially give stereochemically
enriched fragments after relevant manipulations.
a
Scheme 3. Cyclic Ketone Scope
Next, the synthetic utility of the methodology was
investigated. The protocol was scalable, as shown in Scheme
4a. We anticipated that the acidic medium of the oximation
Scheme 4. Further Scope and Application
a
Isolated yield.
resulting acyl fluorides contain a pendant nitrile group; due to
the presence of the nitrile, they can be potentially utilized as
1,5-, 1,6-, 1,7-, and 1,8-dicarbonyl equivalents with orthogonal
reactivity. The nitrile functionality is used in many organic
transformations including conversion to amines and various
carboxylic acid derivatives21 as well as in the “Click” reaction.22
Indanone derivatives 3a−c were successfully converted to
the corresponding acyl fluorides in good yields, regardless of
the position of the ketone moiety. Whereas the corresponding
acyl fluoride 4c was not fully isolated due to its instability in
silica gel columns, the crude product was directly subjected to
a bulky amine nucleophile such as diisopropylamine, giving 4c′
in 68% yield. Other benzofused cycles, 3d and 3e, were also
compatible and showed clean conversion. 2-Arylbenzoyl- and
naphthoyl fluorides 4f and 4g were obtained in good yields. It
was notable that 3h was converted to Z-acryloyl fluoride 4h
without isomerization. The reaction of 3j gave cyanomethyl
ether 4j. In addition to aryloyl fluorides, aliphatic acyl fluorides
were obtained with high efficiency. Through the use of a
cyclohexanone-based substrate, 5-cyano-4-phenylpentanoyl
fluoride 4k (1,6-dicarbonyl equivalent) was obtained. A
cyclooctanone substrate was cleaved to afford the useful acyl
fluoride 4l bearing a cyanoalkyl substituent. Among the
commercially available chiral ketones, the selected substrates
step would be compatible with DAST. Indeed, a two-step
reaction was found to be operative in one pot (Scheme 4b).
The acyl fluoride 2a was a good acylating source in some
transformations (Scheme 4c). For instance, the Friedel−
Crafts-type acylation of electron-rich thiophene provided
ketone 5 in excellent yield.23 Under the same conditions, 2a
could be used for the acylation of alkenes; the use of 1,1-
diphenylethylene as the coupling partner afforded an α,β-
unsaturated ketone 6.24 In addition, 2a could be coupled to
C
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electron-deficient heterocycles. A C−H coupling reaction25
provided diaryl ketone 7, and the decarbonylative coupling
reaction via the direct C−H activation of the quinoline gave 2-
arylquinoline 8.26 Beyond intermolecular coupling reactions,
the bifunctional molecule 4a could participate in base-
promoted annulations to give previously unreported bicyclic
compounds such as isocoumarin 9 and 1,3-naphthalenediol 10.
Under acidic conditions, 3-chloro-isoquinolone 11 was
obtained (Scheme 4d).
AUTHOR INFORMATION
Corresponding Author
■
Hee Nam Lim − Eco-Friendly New Materials Research Center,
Therapeutics&Biotechnology Division, Korea Research Institute
of Chemical Technology(KRICT), Daejeon 34114, Republic of
Korea; orcid.org/0000-0002-0051-1578;
Email: heenam@krict.re.kr
Author
On the basis of the prior mechanistic proposals of fluoride-
mediated reactions,12,15d a plausible reaction mechanism was
proposed (Scheme 5a). The oxime activation is initiated by the
Danhee Kim − Eco-Friendly New Materials Research Center,
Therapeutics&Biotechnology Division, Korea Research Institute
of Chemical Technology(KRICT), Daejeon 34114, Republic of
Korea; Department of Chemistry, Korea University, Seoul
02841, Republic of Korea
Scheme 5. Proposed Mechanism of the Fluorinative C−C
Bond Cleavage
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02603
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the KRICT, Republic of Korea
(KK2061-24) and the National Research Foundation of
Korea (NRF-2019R1C1C1004970) is greatly appreciated.
Prof. Hyun S. Ahn (Yonsei University) and Prof. Michael C.
Young (The University of Toledo) are acknowledged for
proofreading the manuscript.
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02603.
Experimental procedures, characterization data, copies of
NMR spectra for all new products (PDF)
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