Enantioselective nucleophile-catalyzed synthesis of tertiary alkyl fluorides via the α-Fluorination of ketenes: Synthetic and mechanistic studies

Article abstract of DOI:10.1021/ja5044209

The catalytic asymmetric synthesis of alkyl fluorides, particularly α-fluorocarbonyl compounds, has been the focus of substantial effort in recent years. While significant progress has been described in the formation of enantioenriched secondary alkyl flu

Full text of DOI:10.1021/ja5044209

Communication
pubs.acs.org/JACS
Enantioselective Nucleophile-Catalyzed Synthesis of Tertiary Alkyl
Fluorides via the α‑Fluorination of Ketenes: Synthetic and
Mechanistic Studies
Sarah Yunmi Lee,‡,† Stefan Neufeind,† and Gregory C. Fu*,‡,†
‡Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
†Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S
* Supporting Information
ABSTRACT: The catalytic asymmetric synthesis of alkyl
fluorides, particularly α-fluorocarbonyl compounds, has
been the focus of substantial effort in recent years. While
significant progress has been described in the formation of
enantioenriched secondary alkyl fluorides, advances in the
generation of tertiary alkyl fluorides have been more
limited. Here, we describe a method for the catalytic
asymmetric coupling of aryl alkyl ketenes with commer-
cially available N-fluorodibenzenesulfonimide (NFSI) and
C6F5ONa to furnish tertiary α-fluoroesters. Mechanistic
attempted to extend this halogenation strategy to an analogous
studies are consistent with the hypothesis that the addition
enantioselective fluorination of ketenes, building in part on the
pioneering work of Lectka on the cinchona alkaloid/palladium/
LiClO4-catalyzed synthesis of secondary alkyl fluorides from acid
of an external nucleophile (C6F5ONa) is critical for
turnover, releasing the catalyst (PPY*) from an N-acylated
intermediate. The available data can be explained by a
reaction pathway wherein the enantioselectivity is
determined in the turnover-limiting transfer of fluorine
chlorides and N-fluorodibenzenesulfonimide (NFSI; commer-
cially available),4 we obtained discouraging results (eq 2).
from NFSI to a chiral enolate derived from the addition of
PPY* to the ketene. The structure and the reactivity of the
product of this proposed elementary step, an α-fluoro-N-
acylpyridinium salt, have been examined.
We postulated that our failure to achieve the desired catalytic
asymmetric fluorination might be due to the stability of a
potential N-acylpyridinium intermediate (e.g., 1) toward
(SO2Ph)2N−. Consequently, we decided to determine if the
ecause of properties such as the high electronegativity and
Bthe small size of fluorine, as well as the relative stability of
the C−F bond, the incorporation of fluorine into organic
molecules, including stereoselective processes, has become a
widely used method in medicinal chemistry for altering the
bioactivity of drug candidates.1 The catalytic enantioselective α-
fluorination of carbonyl compounds has been a subject of
particular interest,2 and an array of versatile methods have been
described for the generation of such secondary alkyl fluorides.3,4
In contrast, except in the case of doubly activated molecules,2,5
there has been limited progress in the development of effective
processes that furnish tertiary α-fluorocarbonyl compounds,6−8
and we are not aware of any methods that directly afford esters,9
with the exception of kinetic resolutions.10 Here, we establish
that a planar-chiral nucleophilic catalyst (PPY*) can achieve the
coupling of a ketene,11 an electrophilic fluorine source,12 and an
alkoxide, thereby providing such tertiary alkyl fluorides with high
enantioselectivity (eq 1). We also present mechanistic studies
that help to illuminate the reaction pathway.
addition to the reaction mixture of a stoichiometric quantity of a
more reactive nucleophile might be beneficial. We were, of
course, cognizant of the possibility that the nucleophilic additive
might react directly with the ketene to generate an achiral enolate
and racemic product; fortunately, however, we were able to
identify a nucleophile that allowed us to achieve our objective
(Table 1).
Whereas the addition of MeOH or PhNH2 did not result in a
significant amount of the desired tertiary alkyl fluoride (Table 1,
entries 1 and 2), the use of alkoxides led to a substantial quantity
(entries 3−5). In the case of sodium tert-butoxide, the product
In an earlier study, we reported that PPY* can catalyze the
asymmetric chlorination of ketenes by 2,2,6,6-tetrachlorocyclo-
hexanone to produce tertiary α-chloroesters.13 When we
Received: May 2, 2014
© XXXX American Chemical Society
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Journal of the American Chemical Society
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Table 1. Catalytic Asymmetric Synthesis of Tertiary Alkyl
a
Fluorides: Effect of Added Nucleophile
a
b
All data are the averages of two experiments. Determined through
c
GC analysis with the aid of an internal standard. Yield of purified
Figure 1. Transformations of an enantioenriched tertiary α-fluoroester.
product.
was racemic (entry 3); on the other hand, sodium phenoxide
furnished good enantioselectivity (entry 4), and a less
nucleophilic phenoxide, sodium pentafluorophenoxide, provided
excellent yield and ee (entry 5).14
Our optimized method is effective for the catalytic
enantioselective synthesis of tertiary α-fluoroesters from an
array of aryl alkyl ketenes (Table 2).15 The alkyl group of the
ketene can vary in size from Me to cyclopentyl (entries 1−5;
lower ee is observed with larger alkyl groups), and the aromatic
substituent can be electron-poor, electron-rich, para-substituted,
or meta-substituted (entries 6−9), as well as naphthyl or
heteroaryl (entries 10 and 11). On a gram scale, the fluorination
illustrated in entry 2 of Table 2 proceeded in 99% ee and 90%
yield.
Table 2. Catalytic Asymmetric Synthesis of Tertiary Alkyl
a
Fluorides
Figure 2. An outline of two of the possible mechanisms for the PPY*-
catalyzed enantioselective α-fluorination of ketenes: a “chiral enolate”
pathway (top) and a “chiral fluorinating agent” pathway (bottom).
The enantioenriched α-fluoroesters that are generated in this
catalytic asymmetric C−F bond-forming process can be
transformed in good yield into a variety of other tertiary alkyl
fluorides (Figure 1).
a
For the reaction conditions, see eq 1. All data are the averages of two
b
experiments. Yield of purified product (contains ≤5% of the non-
fluorinated ester). Catalyst loading: 10%.
c
Two of the possible pathways for the PPY*-catalyzed
enantioselective fluorination of ketenes to generate enantioen-
riched tertiary α-fluoroesters are illustrated in Figure 2. In one
mechanism, a PPY*-derived chiral enolate (2) is a key
intermediate (top), and in the other a PPY*-derived chiral
fluorinating agent (4) is critical (bottom).17 To gain insight into
the operative pathway, we determined the rate law for the
reaction of phenyl ethyl ketene, which is first order in PPY* and
in NFSI, and zeroth order in the ketene and in C6F5ONa.18 The
rate of product formation does vary with the identity of the
ketene, suggesting that the ketene is involved in the rate-
determining step.
Although our other chiral 4-(dimethylamino)pyridine
(DMAP)-catalyzed reactions of ketenes have generally not
been effective for dialkyl ketenes,16 in the case of this asymmetric
fluorination process, we have observed a promising level of
enantioselectivity (eq 3).
These observations are consistent with a “chiral enolate”
pathway (top of Figure 2; but not with a “chiral fluorinating
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Journal of the American Chemical Society
Communication
agent” pathway (bottom of Figure 2)19) wherein nucleophilic
addition of PPY* to the ketene affords enolate 2, this enolate is
the resting state of the catalytic cycle,20 and, in the turnover-
limiting step, enolate 2 is fluorinated by NFSI to provide
enantioenriched N-acylpyridinium salt 3. Reaction with the
phenoxide then furnishes the tertiary α-fluoroester and
regenerates the catalyst, PPY*.
In view of our earlier inability to achieve PPY*-catalyzed
asymmetric fluorination of ketenes with NFSI in the absence of
an added nucleophile (eq 2), we hypothesized that N-
acylpyridinium salt 3 (Figure 2) might be isolable. Indeed,
reaction of PPY*, phenyl benzyl ketene, and NFSI (1:1:1)
provided α-fluorinated acylpyridinium salt 5 (eq 4).21,22 This ion
pair is stable at room temperature under nitrogen for at least 6
months.
cinchona alkaloid-catalyzed α-fluorination of acid chlorides (via
monosubstituted ketenes) to afford secondary α-fluoroesters.4
In conclusion, we have described a method for the catalytic
enantioselective coupling of aryl alkyl ketenes (and a dialkyl
ketene) with NFSI and C6F5ONa to produce tertiary α-
fluoroesters, a family of target molecules that have rarely been
directly accessed via asymmetric catalysis. The addition of
C6F5ONa was critical to the success of this effort, as it enables
regeneration of the catalyst (PPY*) from a relatively stable N-
acylated intermediate, which we have synthesized and inves-
tigated independently. Our mechanistic studies are consistent
with a catalytic cycle wherein PPY* adds to the ketene and
generates a chiral enolate which, in the stereochemistry- and rate-
determining step, reacts with NFSI to furnish the enantioen-
riched N-acylpyridinium salt.
Although we were not able to obtain X-ray-quality crystals of
the sulfonimide salt of the N-acylpyridinium ion, anion exchange
of N(SO2Ph)2 for a carborane through treatment with
CsCB11H12 furnished a suitable crystal (Figure 3).23 The
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and compound characterization data.
This material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
gcfu@caltech.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Figure 3. ORTEP diagram of N-acylated (+)-PPY*. Thermal ellipsoids
are illustrated at the 35% probability level. For clarity, the carborane
counteranion, the hydrogen atoms, the solvent, and the second molecule
in the asymmetric unit are omitted, and only the major component of
the disorder in the Cp* group is shown.
Support has been provided by the National Institutes of Health
(National Institute of General Medical Sciences R01-GM57034)
and the Deutsche Forschungsgemeinschaft (fellowship to S.N.).
We thank Trixia M. Buscagan, Dr. Nathan D. Schley, Dr. Michael
K. Takase (X-ray Crystallography Facility; a Bruker KAPPA
APEX II X-ray diffractometer was purchased via NSF CRIF:MU
award CHE-0639094 to the California Institute of Technology),
Dr. David VanderVelde (NMR Facility), and Dr. Scott C. Virgil
(Caltech Center for Catalysis and Chemical Synthesis, supported
by the Gordon and Betty Moore Foundation) for assistance.
pyrrolidino group, the pyridine ring, and the carbonyl group
are approximately coplanar, consistent with extended con-
jugation of the π system, which decreases the electrophilicity of
the carbonyl group. With regard to the orientation around the
N−Ccarbonyl bond, the smaller group (carbonyl oxygen) is
positioned on the side of the fused cyclopentadienyl ring,
presumably to minimize unfavorable steric interactions. The
stereochemistry of the fluorine-bearing carbon of the N-
acylpyridinium ion is the same as for the major enantiomer of
the α-fluoroester that is generated by (+)-PPY* under our
standard fluorination conditions.
We have confirmed that N-acylpyridinium salt 5 is a chemically
and kinetically competent potential intermediate in the proposed
catalytic cycle that leads to the enantioenriched tertiary α-
fluoroester (top of Figure 2). Thus, upon treatment of this salt
with C6F5ONa in THF at −78 °C, the enantioenriched ester is
formed with retention of stereochemistry (eq 5). It is interesting
to note that this catalyst-regeneration issue from an N-acylated
catalyst intermediate is not encountered in “related” processes
such as the PPY*-catalyzed α-chlorination of ketenes13 or the
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(14) When C6F5OH was employed in place of C6F5ONa, essentially
none of the desired product was observed.
(15) Notes: (a) Under our optimized condition, solutions of the
ketene and the nucleophile are added dropwise via a syringe pump to a
solution of the catalyst and NFSI; Selectfluor reagent is not a suitable
substitute for NFSI; the ee of the product is constant during the course
of the reaction; an o-tolyl-substituted and a tert-butyl-substituted ketene
were not effective reaction partners. (b) Both enantiomers of PPY* are
available from Strem Chemicals.
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Brønsted-acid mode of reactivity parallels the chiral fluorinating agent
pathway (substitution of F-catalyst* with H-catalyst* at the bottom of
Figure 2)
(18) PPY* and t-BuONa react with phenyl ethyl ketene in THF-d8 at
−78 °C, whereas C6F5ONa does not.
(19) In THF-d8 at −78 °C, no reaction is observed when PPY* is
mixed with NFSI.
(20) We have not yet been able to identify the resting state of the
catalyst under our standard fluorination conditions.
(21) The yield was determined by 1H NMR spectroscopy with the aid
of an internal standard.
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