Catalytic asymmetric synthesis of tertiary alkyl fluorides: Negishi cross-couplings of racemic α,α-dihaloketones

Article abstract of DOI:10.1021/ja501815p

The development of new approaches to the construction of fluorine-containing target molecules is important for a variety of scientific disciplines, including medicinal chemistry. In this Article, we describe a method for the catalytic enantioselective syn

Full text of DOI:10.1021/ja501815p

Article
pubs.acs.org/JACS
Catalytic Asymmetric Synthesis of Tertiary Alkyl Fluorides:
Negishi Cross-Couplings of Racemic α,α-Dihaloketones
Yufan Liang†,‡ 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 development of new approaches to the construction
of fluorine-containing target molecules is important for a variety of
scientific disciplines, including medicinal chemistry. In this Article,
we describe a method for the catalytic enantioselective synthesis
of tertiary alkyl fluorides through Negishi reactions of racemic α-halo-
α-fluoroketones, which represents the first catalytic asymmetric cross-
coupling that employs geminal dihalides as electrophiles. Thus, selec-
tive reaction of a C−Br (or C−Cl) bond in the presence of a C−F
bond can be achieved with the aid of a nickel/bis(oxazoline) catalyst.
The products of the stereoconvergent cross-couplings, enantioenriched tertiary α-fluoroketones, can be converted into an array of
interesting organofluorine compounds.
this would enable the catalytic asymmetric synthesis of tertiary
alkyl fluorides.
INTRODUCTION
■
Motivated by potential applications in biomedical research and
other disciplines,1 substantial effort has been dedicated to the
development of methods for the preparation of organofluorine
compounds.2 In the case of alkyl fluorides, advances have been
described in the catalytic enantioselective synthesis of stereo-
genic centers that bear a fluorine substituent,3 particularly α
to a carbonyl group. Although most studies have addressed
the generation of secondary stereocenters,4 a few reports have
examined the establishment of tertiary centers; currently, the
latter methods are largely limited to either cyclic or doubly
activated acyclic α-fluorocarbonyl compounds.5,6 To the best
of our knowledge, no general catalytic enantioselective process
has been discovered for the synthesis of simple tertiary α-fluorinated
acyclic ketones.7−9
In view of the high interest in the enantioselective synthesis
of α-fluorocarbonyl compounds,4,5,8,9 as well as the excellent
functional-group compatibility of Negishi reactions,14 we chose to
examine the coupling of α-halo-α-fluoroketones with organozinc
reagents. In this Article, we describe the first catalytic asymmetric
cross-coupling that employs geminal dihalides as electrophiles,
specifically, a nickel/bis(oxazoline)-catalyzed stereoconvergent
Negishi arylation of racemic α-bromo-α-fluoroketones to generate
tertiary α-fluorinated acyclic ketones (eq 2).
During the past decade, we have developed an array of
nickel-catalyzed asymmetric cross-coupling methods, employing
racemic alkyl electrophiles as reaction partners; to date, all of
the electrophiles have been secondary, with Z = H (eq 1).10,11
RESULTS AND DISCUSSION
■
At the time that we began our investigation in 2011, we were
not aware of any precedent for selective nickel-catalyzed cross-
couplings of α-halo-α-fluoro compounds. However, during the
course of our studies, Ando reported diastereoselective Kumada
reactions of α-bromo-α-fluoro-β-lactams with aryl Grignard
reagents,15 employing a NiCl2·glyme/bis(oxazoline) catalyst
We have now begun to explore enantioselective couplings of
other families of electrophiles, beginning with geminal dihalides
(eq 1; Z = F, X = halide);12 such H → F substitutions can have a
dramatic impact on reactivity and/or ee.13 If a suitable catalyst
could achieve selective cleavage of the C−X bond, along with
efficient and highly enantioselective C−C bond formation, then
Received: February 20, 2014
Published: March 31, 2014
© 2014 American Chemical Society
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that we had described for enantioselective couplings of aryl
Grignard reagents with α-bromoketones.10c
Unfortunately, our attempts to apply our previous methods
for Kumada10c and Negishi10d arylations to the cross-coupling
of the racemic α-bromo-α-fluoroketone illustrated in Table 1
no erosion in ee (entries 9 and 10). Under our optimized
conditions, PhMgCl is not a useful coupling partner (entry 11),
whereas good enantioselectivity but less-efficient cross-coupling
is observed when Ph2Zn serves as the nucleophile (entry 12).
Conducting the Negishi reaction at room temperature or with
a single solvent causes a small drop in ee and a substantial loss
in yield (entries 13−15). The presence of water (0.1 equiv) has
essentially no impact on the course of the reaction (entry 16),
whereas running the reaction under air results in decreased
yield (entry 17).
Table 1. Catalytic Asymmetric Synthesis of Tertiary Alkyl
a
Fluorides: Effect of Reaction Parameters
Our optimized conditions can be applied to the catalytic
asymmetric Negishi arylation of a variety of racemic α-bromo-
α-fluoroketones, furnishing tertiary alkyl fluorides in generally
good ee (Table 2).16 The R group of the ketone can vary in
Table 2. Catalytic Asymmetric Synthesis of Tertiary Alkyl
a
Fluorides: Scope with Respect to the Electrophile
a
b
All data are the average of two experiments. A negative ee value
signifies that the major product of the reaction is the opposite (R)
c
enantiomer. The yields were determined through analysis by 19F
NMR spectroscopy, with the aid of an internal standard.
were unsuccessful (<2% yield). However, through the appro-
priate choice of reaction parameters, we were able to achieve
the desired α-arylation and to generate the tertiary alkyl
fluoride with very good enantioselectivity (97% ee; Table 1,
entry 1). NiCl2·glyme and bis(oxazoline) L* are commercially
available and air-stable.
Table 1 provides information on the impact of various reac-
tion parameters on the efficiency of this catalytic asymmetric
synthesis of tertiary alkyl fluorides. In the absence of NiCl2·
glyme or of ligand L*, essentially no carbon−carbon bond for-
mation is observed (entries 2 and 3). The cis phenyl substituent
in the 5 position of the oxazolines plays an important role in
enantioselectivity and yield (entry 4), as does the substitution
on the one-carbon linker that bridges the oxazolines (entries 5
and 6). A variety of pybox and 1,2-diamine ligands10b,d,e furnish
inferior results (e.g., entries 7 and 8). The use of less catalyst or
less nucleophile leads to a modest decrease in yield, although
a
b
All data are the average of two experiments. Yield of purified
product.
size, although a lower ee is observed with a bulky isopropyl
substituent (entries 1−6). High enantioselectivity is typically
obtained whether the aromatic group (Ar) is para-, meta-, or
ortho-substituted, and whether it is electron-rich or electron-
poor (entries 7−16); we have not previously observed high ee in
related nickel-catalyzed cross-couplings with ortho-substituted
Ar groups (entries 14 and 15; previously: ≤75% ee).10c,d,17
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Functional groups such as an olefin, alkyl chloride,18 aryl
methyl ether,19 and aryl fluoride20 are compatible with the reac-
tion conditions.
The scope of this method for the catalytic asymmetric syn-
thesis of tertiary alkyl fluorides is also fairly broad with
respect to the nucleophile (ArZnCl; Table 3).21 Thus, para- and
Table 3. Catalytic Asymmetric Synthesis of Tertiary Alkyl
a
Fluorides: Scope with Respect to the Nucleophile
Moreover, regioselective Baeyer−Villiger oxidation of the
enantioenriched α-fluoroketones can be achieved, thereby
providing access to either an alkyl ester or an aryl ester by con-
trolling the relative migratory aptitudes of the ketone sub-
stituents (eqs 6 and 7).26−28 This oxidation thereby enables, to
a
b
All data are the average of two experimens. Yield of purified product.
c
Reaction temperature: −20 °C.
our knowledge, the first asymmetric synthesis of acylated
fluorohydrins (eq 6),29 as well as an indirect method for the
catalytic enantioselective synthesis of tertiary α-fluoroesters
(eq 7) and amides.30
meta- (but not ortho-)22 substituted arylzinc reagents are suitable
cross-coupling partners, furnishing the desired α-fluoroketone
in good ee. Electron-rich as well as electron-poor nucleophiles can
be employed; in the case of the latter, a reaction temperature
of −20 °C, rather than −25 °C, is optimal. A silyl ether, aryl
chloride, and aryl bromide23 are compatible with the coupling
conditions.
This method for the selective cross-coupling of a geminal
dihalide electrophile is not limited to α-bromo-α-fluoroketones.
Under similar conditions, a racemic α-chloro-α-fluoroketone
also reacts with an arylzinc reagent to generate a tertiary alkyl
fluoride in good ee (eq 3). In contrast, in the case of related
cross-couplings, α-chlorocarbonyl compounds were not suitable
reaction partners.10c−e
CONCLUSIONS
■
In summary, we have developed the first catalytic asymmetric
cross-coupling method that employs geminal dihalides as
electrophiles. Specifically, we have established that nickel/
bis(oxazoline)-catalyzed stereoconvergent Negishi reactions
of racemic α-halo-α-fluoroketones provide access to
enantioenriched tertiary alkyl fluorides, thereby complement-
ing earlier catalytic asymmetric methods for the synthe-
sis of organofluorine compounds, which have typically
focused on the generation of secondary alkyl fluorides. The
α-fluoroketones that are produced in these Negishi couplings
can be transformed into a variety of interesting families
of organofluorine target molecules. Additional investigations
of catalytic enantioselective cross-couplings of alkyl electrophiles
are underway.
The enantioenriched organofluorine cross-coupling products
can be derivatized with high stereoselectivity. For example,
nucleophilic additions of allyl and aryl nucleophiles to the
carbonyl group proceed with >20:1 dr to generate densely func-
tionalized adducts (eqs 4 and 5).24,25
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(6) For examples of the catalytic asymmetric synthesis of other
families of tertiary alkyl fluorides, see: (a) Phipps, R. J.; Hiramatsu, K.;
Toste, F. D. J. Am. Chem. Soc. 2012, 134, 8376−8379. (b) Shunatona,
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and compound characterization data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
H. P.; Fruh, N.; Wang, Y.-M.; Rauniyar, V.; Toste, F. D. Angew. Chem.,
̈
Int. Ed. 2013, 52, 7724−7727. (c) Wu, J.; Wang, Y.-M.; Drljevic, A.;
Rauniyar, V.; Phipps, R. J.; Toste, F. D. Proc. Natl. Acad. Sci. U.S.A.
2013, 110, 13729−13733.
AUTHOR INFORMATION
(7) For examples of methods for the synthesis of racemic products,
see: (a) Wang, W.; Jasinski, J.; Hammond, G. B.; Xu, B. Angew. Chem.,
Int. Ed. 2010, 49, 7247−7252. (b) Guo, Y.; Tao, G.-H.; Blumenfeld,
A.; Shreeve, J. M. Organometallics 2010, 29, 1818−1823. (c) Zhang,
W.; Hu, J. Adv. Synth. Catal. 2010, 352, 2799−2804. (d) Guo, C.;
Wang, R.-W.; Guo, Y.; Qing, F.-L. J. Fluorine Chem. 2012, 133, 86−96.
(8) For a pioneering study, although with limited scope with regard
to the diversity of product structures, see: Shibatomi, K.; Futatsugi, K.;
Kobayashi, F.; Iwasa, S.; Yamamoto, H. J. Am. Chem. Soc. 2010, 132,
5625−5627.
■
Corresponding Author
gcfu@caltech.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support has been provided by the National Institutes of Health
(National Institute of General Medical Sciences: R01-GM62871).
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),
and Dr. Scott C. Virgil (Caltech Center for Catalysis and
Chemical Synthesis, supported by the Gordon and Betty Moore
Foundation) for assistance.
(9) For an example with esters, see: Tengeiji, A.; Shiina, I. Molecules
2012, 17, 7356−7378.
(10) For a few examples and leading references, see: (a) Do, H.-Q.;
Chandrashekar, E. R. R.; Fu, G. C. J. Am. Chem. Soc. 2013, 135,
16288−16291. (b) Wilsily, A.; Tramutola, F.; Owston, N. A.; Fu, G. C.
J. Am. Chem. Soc. 2012, 134, 5794−5797. (c) Lou, S.; Fu, G. C. J. Am.
Chem. Soc. 2010, 132, 1264−1266. (d) Lundin, P. M.; Esquivias, J.; Fu,
G. C. Angew. Chem., Int. Ed. 2009, 48, 154−156. (e) Dai, X.; Strotman,
N. A.; Fu, G. C. J. Am. Chem. Soc. 2008, 130, 3302−3303.
(11) For an example of an application in the total synthesis of a
natural product (carolacton), see: Schmidt, T.; Kirschning, A. Angew.
Chem., Int. Ed. 2012, 51, 1063−1066.
REFERENCES
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(16) Under our standard conditions: (a) On a gram-scale, the
asymmetric Negishi cross-coupling illustrated in entry 2 of Table 2
furnished the desired tertiary alkyl fluoride in 97% ee and 66% yield
(1.41 g of product). (b) Hydrodebromination of the electrophile was a
side reaction (the secondary alkyl fluoride was produced in <5% ee).
(c) During the course of a cross-coupling, no kinetic resolution of the
electrophile was detected (<5% ee), and the ee of the product was
constant. (d) The cross-coupling product was stable (no further
reaction and no erosion in ee). (e) A cyclic ketone (α-tetralone-
derived) and a 2-thienyl ketone were not suitable electrophiles.
(17) A ketone in which the aromatic group (Ar) was mesityl did not
undergo cross-coupling under our standard conditions.
(4) For early examples of catalytic, asymmetric α-fluorinations of
aldehydes, see: (a) Marigo, M.; Fielenbach, D.; Braunton, A.;
Kjærsgaard, A.; Jørgensen, K. A. Angew. Chem., Int. Ed. 2005, 44,
3703−3706. (b) Steiner, D. D.; Mase, N.; Barbas, C. F., III. Angew.
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D. A.; Zhang, N.; Resmerita, A.-M.; Garg, N. K.; Percec, V. Chem. Rev.
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fluorides, see: Tobisu, M.; Xu, T.; Shimasaki, T.; Chatani, N. J. Am.
Chem. Soc. 2011, 133, 19505−19511.
(21) Our standard conditions were not effective for the cross-
coupling of a thienylzinc reagent or a Knochel-type arylzinc reagent:
Krasovskiy, A.; Malakhov, V.; Gavryushin, A.; Knochel, P. Angew.
Chem., Int. Ed. 2006, 45, 6040−6044.
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10165. (d) Belanger, E.; Cantin, K.; Messe, O.; Tremblay, M.; Paquin,
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(22) Essentially no carbon−carbon bond formation was observed
when o-tolylzinc chloride was employed as the nucleophile under our
standard conditions.
(23) Nickel-catalyzed Negishi reactions of aryl halides are well-
́
established; for examples, see: Phapale, V. B.; Cardenas, D. J. Chem.
Soc. Rev. 2009, 38, 1598−1607. Interestingly, under the conditions
described in eq 2, essentially no coupling of PhZnCl with an
aryl iodide or an aryl bromide was observed (≥95% recovery of
4-bromotoluene and 4-iodotoluene).
(24) The stereochemistry of each product (eqs 4 and 5) has been
determined by X-ray crystallography (see the Supporting Informa-
tion). Reduction of the ketone with NaBH4 also proceeded with high
diastereoselectivity (>20:1) and in good yield (>99%). However, we
have not yet been able to establish the relative stereochemistry of the
major diastereomer.
(25) For a review of the effect of a fluorine substituent on the
conformation of molecules, see: Hunter, L. Beilstein J. Org. Chem.
2010, 6, No. 38.
(26) For the Baeyer−Villiger reactions illustrated in eqs 6 and 7, the
selectivity is ≥20:1 for the indicated bond cleavage.
(27) For a discussion of the effect of a fluorine substituent on
migratory aptitude in Baeyer−Villiger reactions, see: (a) Crudden, C.
M.; Chen, A. C.; Calhoun, L. A. Angew. Chem., Int. Ed. 2000, 39,
2851−2855. (b) Grein, F.; Chen, A. C.; Edwards, D.; Crudden, C. M.
J. Org. Chem. 2006, 71, 861−872.
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fluoro-4,7,7-trimethyl-3-oxabicyclo[4.1.0]heptane-2-ones, Method for
Their Production and Their Use in the Production of Cyclopropane
Carboxylic Acids. WO 2001034592 A1, May 17, 2001. (c) Ishibashi,
M. Method for Screening Therapeutic Agent for Glomerular Disorder.
U.S. Patent 20090041719 A1, Feb. 12, 2009.
(30) The aryl ester (eq 7) can readily be converted into an amide by
treatment with an amine (e.g., pyrrolidine: 70% isolated yield). We are
not aware of reports of direct methods for the catalytic
enantioselective synthesis of tertiary α-fluorinated acyclic amides.
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