Copper(I)-Mediated Borofluorination of Alkynes

Article abstract of DOI:10.1021/acs.orglett.8b02195

The electrophilic fluorination of N-heterocyclic carbene (NHC) copper(I) vinyls results in fluoroalkene formation. Alkynes can be converted to cis-(β-fluorovinyl)boronates by a reaction with an (NHC)copper(I) boryl generated in situ, followed by N-fluorobenzenesulfonimide (NFSI). This sequence gives rise to anti-Markovnikov fluorination products from terminal alkynes. Oxidation of a cis-(β-fluorovinyl)trifluoroboronate yields an α-fluoroketone, whereas a palladium-catalyzed Suzuki-Miyaura coupling yields a tetrasubstituted monofluoroalkene.

Full text of DOI:10.1021/acs.orglett.8b02195

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Copper(I)-Mediated Borofluorination of Alkynes
Abraham J. Jordan,† Percie K. Thompson,†,‡,§ and Joseph P. Sadighi*,†
†School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States
‡Coretta Scott King Young Women’s Leadership Academy, 1190 Northwest Drive NW, Atlanta, Georgia 30318, United States
§Parker H. Petit Institute for Bioengineering and Biosciences, Georgia Institute of Technology, Atlanta, Georgia 30332, United
States
S
* Supporting Information
ABSTRACT: The electrophilic fluorination of N-heterocyclic
carbene (NHC) copper(I) vinyls results in fluoroalkene formation.
Alkynes can be converted to cis-(β-fluorovinyl)boronates by a
reaction with an (NHC)copper(I) boryl generated in situ, followed
by N-fluorobenzenesulfonimide (NFSI). This sequence gives rise to
anti-Markovnikov fluorination products from terminal alkynes.
Oxidation of a cis-(β-fluorovinyl)trifluoroboronate yields an α-
fluoroketone, whereas a palladium-catalyzed Suzuki−Miyaura
coupling yields a tetrasubstituted monofluoroalkene.
Scheme 1. Coinage Metals in C−F Bond Formation
pplications of fluorinated organic compounds in
1
materials,2 and agrochemicals3 are
Apharmaceuticals,
ever increasing due to the unique properties engendered by
carbon−fluorine bonds. Much effort has been dedicated to
furthering synthetic access to fluorine-containing organics.4
Transition-metal-catalyzed and -mediated processes provide
synthetic methods to construct C−F bonds while avoiding the
use of harsh fluorination reagents;5 the coinage metals copper,6
silver,7 and gold8 have proven notably useful in these efforts.
For example, C(sp2)−(main group element) bonds can be
converted to C−F bonds using electrophilic fluorine sources in
the presence of copper or silver salts (Scheme 1a).6b,d,7c−f,h
Additionally, gold(I) fluorides catalyze the trans-hydrofluori-
nation of alkynes using Et3N·3HF or DMPU·HF (Scheme
1b).8a,c,d Gold(III) fluorides have been shown to form alkyl−
fluoride bonds through reductive elimination (Scheme 1c).9
The significant utility of fluorinated alkenes has spurred
substantial advances in their synthesis during the past
decade.4b,10 Despite these advances, limitations in the scope,
selectivity, or safety of the fluorine source remain. The
reductive functionalization of alkynes represents an appealing
approach to fluorinated alkenes due to the synthetic
accessibility of alkynes. Gold hydrofluorination catalysis8 is a
useful approach to alkyne fluorination chemistry, but copper-
and silver-catalyzed hydrofluorination is limited to ynamides.11
In these systems, the C−F bond-forming step occurs by trans-
addition of the metal and fluoride across an alkyne (Scheme
1b).
co-workers to afford terminal bromoalkenes,12 and Buchwald
and co-workers recently demonstrated the oxidative anti-
Markovnikov hydroamination of alkynes.13 We envisioned an
analogous sequence involving alkyne insertion to form a
copper−carbon bond, followed by oxidative cleavage. Herein,
we present a new sequence for alkene C−F bond construction
Furthermore, the few methods developed for the fluorina-
tion of terminal alkynes yield internal fluorination products,
and a direct transformation of terminal alkynes to terminal
fluorinated alkenes remains elusive.7c The anti-Markovnikov
hydrobromination of alkynes has been developed by Lalic and
Received: July 13, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02195
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which occurs by the electrophilic fluorination of alkyne derived
copper(I) vinyls.
further functionalized to a variety of fluorinated products.
Recently, several groups synthesized gem-borylfluoroalkenes,
which also could be functionalized further, by copper-catalyzed
borylation/defluorination of gem-difluoroalkenes (Scheme
3b).19 The present method introduces, rather than removes,
a carbon−fluorine bond while avoiding the use of hydrofluoric
acid.
Addition of NFSI to (IDipp)Cu[(E)-3-hexynyl], (IDipp =
1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene), generated
from [(IDipp)CuH]2,14 resulted in the formation of (E)-3-
fluorohex-3-ene (1), determined by the quartet in the 19F
NMR at δ − 108.3 ppm, in 35% yield (Scheme 2) The
With 3-hexyne as the substrate, reaction conditions were
then screened, and these results are summarized in Table S1 in
the Supporting Information. The common electrophilic
fluorinating reagent, SelectFluor, also resulted in considerable
product formation (Table S1), but NFSI resulted in higher
yields.20 Using (IDipp)CuO-t-Bu as the copper(I) source,
bis(neopentyl glycolato)diboron [B2(neo)2], and NFSI in
toluene solution, the yield of the borofluorination product of 3-
hexyne was 84% as judged by 19F NMR spectroscopy. The
copper-based product (IDipp)CuN(SO2Ph)2 (4) can be
isolated in good yield (vide infra).
Scheme 2. Fluorination of (NHC)copper(I) Vinyls
To confirm the formation of 3, a protiodeborylation of 3 was
carried out to generate (E)-3-fluoro-3-hexene (1). Addition of
aqueous potassium carbonate following the fluorination step
readily yields (E)-3-fluoro-3-hexene, judged by 19F NMR
spectroscopy (Scheme 4). Replacement of the boryl
insertion of phenylacetylene into [(IDipp)CuH]2 results in the
formation of both 1- and 2-phenylvinylcopper intermediates,
thus yielding both 1- and (E)-2-(fluorovinyl)benzene after
reaction with NFSI. Using the more sterically demanding
ligand 6Dipp, [1,3-bis(2,6-diisopropylphenyl)-3,4,5,6-tetrahy-
dropyrimidin-2-ylidene],15 we observed only the terminal
copper vinyl. Fluorination of this intermediate with NFSI
affords (E)-(2-fluorovinyl) benzene (2) in 30% yield; analysis
by GC−MS shows the major product to be the divinyl formed
by oxidative coupling (Scheme 3). This is the sole pathway
reported for the reaction of gold(I) vinyls with NFSI.16
Scheme 4. Borofluorination and Protiodeborylation
Scheme 3. Synthesis of Fluorovinylboronate Esters
substituent with a proton results in an upfield shift of the
fluorine resonance from δ −88.7 ppm to δ −108.3 ppm; the
additional coupling between the fluorine and the newly
installed vicinal proton turns the triplet resonance to a
pseudoquartet.
As hypothesized, (IDipp)CuO-t-Bu quickly reacts with
B2(neo)2 to yield a copper boryl,17 judged by 1H NMR
spectroscopy. The resulting copper boryl may be observed in
solution at ambient temperature, but decomposition is
substantial after 20 min (Scheme 5). In the presence of an
alkyne, the Cu−B bond readily adds across the CC bond of
the alkyne to generate the copper (2-borovinyl).21 This 1,2-
insertion must occur in a cis fashion, setting the geometry
between B and F. The copper borovinyls are stable at room
temperature, and the product of diphenylacetylene insertion
can be isolated cleanly. Subsequent addition of NFSI to the (2-
borovinyl)copper species yields (E)-(2-fluoro-1,2-
diphenylvinyl)boronic acid neopentyl glycolato ester (5) and
the (NHC)copper benzenesulfonimide (4), which can be
isolated in 93% yield. In addition to the fluorinated product
and (IDipp)CuN(SO2Ph)2, diphenylacetylene is reformed.
The formation of this byproduct implies the formation of a
Encouraged by these results, we then considered the
fluorination of (NHC)copper(I) borovinyls due to the
synthetic utility of boronates. Initially, (IDipp)CuO-t-Bu,
bis(pinacolato)diboron [B2(pin)2]17 and 3-hexyne were
combined in THF before N-fluorobenzenesulfonimide
(NFSI) was added to the solution. Gratifyingly, the 19F
NMR showed a triplet at δ −90.5 ppm, consistent with the
formation of the (E)-(4-fluoro-3-hexen-3-yl)boronic acid
pinacol ester, albeit in only 19% yield. This 19F NMR chemical
shift is similar to those of related cis-(β-fluorovinyl)boronate
esters synthesized by Hara and co-workers from (2-fluoro-1-
alkenyl)iodonium salts (Scheme 3a).18 This motif could be
B
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1
species, and H NMR spectra show multiple sets of ligand
resonances. Furthermore, addition of (IDipp)CuO-t-Bu to 5
results in transmetalation to copper, forming a copper β-
fluorovinyl. Subsequent loss of diphenylacetylene is observed
by 1H NMR spectroscopy. We suspect the concomitant
formation of (IDipp)CuF but could not detect it by 19F NMR
spectroscopy. Although (IDipp)CuF is poorly soluble in C6D6,
we hypothesize that it reacts readily with (neo)B-O-t-Bu based
on the disappearance of the (neo)B-O-t-Bu resonances in the
1H NMR spectra.23 The copper β-fluorovinyl continues to lose
diphenylacetylene over the course of 24 h (Scheme 6).
Scheme 5. Isolation and Determination of Intermediates
Although 5 could be isolated in good yield, some
decomposition was evident over the course of a day on the
benchtop. In light of the stability and synthetic utility of
potassium trifluoroborate salts,24 we converted the boronic
ester products to the corresponding potassium trifluoroborate
salts as shown by Sakaguchi and co-workers.19b Extraction of
the crude reaction mixture with pentane separates the desired
product and organic byproducts from complex 4; treatment of
the concentrated extract with KHF2 in methanol converts the
boronate ester to the trifluoroborate salt, which is easily
separated from glycol and reformed alkyne.19b Generally, the
borofluorination of internal alkynes was more effective than
that of terminal alkynes, and alkyl alkynes gave higher yields
than aryl (Scheme 7). In the case of terminal alkynes, only the
terminal fluoroalkene was generated, but (IDipp)CuB(neo)
strong B−F bond, competitively with the desired C−F bond
formation, during the reaction of the (2-borovinyl)copper
intermediate with NFSI. The nature of the C−F bond-forming
step is unknown, but one could envision a concerted σ-bond
metathesis or an oxidative addition/reductive elimination
mechanism via a transient Cu(III) intermediate. Future studies
will explore this step. Compound 5 was generated in 75% yield
according to 19F NMR spectroscopy and could be isolated in
55% yield following recrystallization from pentane at −20 °C.
During the fluorinations of copper(I) vinyls in THF
solution, we observed the formation of 2-fluorotetrahydrofuran
as a minor byproduct, resulting in a complex multiplet at δ
−112.2 ppm in the 19F NMR spectra. Originally assigned as a
copper(III) fluoride complex in the copper(I)-mediated
fluorination of arylboron and aryltin bonds,6b,d this resonance
is consistent with that reported by Vincent and co-workers for
the NFSI-mediated fluorination of THF.22
Scheme 7. Scope of Alkynes Converted to cis-(β-
Fluorovinyl)trifluoroboronates
We have examined potential turnover steps for this reaction
but have not yet found conditions conducive to catalysis.
Complex 4 does not react with B2(neo)2 to regenerate a
copper boryl but does undergo ligand exchange with potassium
or sodium tert-butoxide to regenerate the copper alkoxide
(Scheme 6). Unfortunately, both (IDipp)CuO-t-Bu and
(IDipp)CuB(neo) react with NFSI to generate multiple
vibrant colors, which we suspect is due to formation of Cu(II)
Scheme 6. Cu−F Elimination from Copper cis-β-Fluorovinyl
C
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must be generated prior to addition of the terminal alkyne to
avoid deprotonation by the copper(I) alkoxide to form a
copper(I) acetylide.25 Addition of excess terminal alkyne can
also lead to formation of a copper(I) acetylide, as the
vinylcopper(I) intermediate can deprotonate the alkyne.25
Interestingly, the borofluorination of phenylpropyne yielded a
5:1 mixture of the β- and α-cis-fluorovinyl boronic esters, but
following conversion to the trifluoroborate salt the ratio of
isolated product increased to a 20:1 mixture of β- to α-cis-
fluorovinyl boronate, presumably due to differential solubility.
The borofluorination of the doubly propargylic substrate 1,4-
dimethoxy-2-butyne resulted in multiple products, and
conversion to potassium trifluoroborate salts was quite low.
Additionally, no product was detected when dimethyl
acetylenedicarboxylate was used under normal conditions.
Knowing that the byproduct (IDipp)CuN(SO2Ph)2 (4) can
undergo anion exchange to yield (IDipp)CuO-t-Bu, we
explored the possibility of reusing 4 as our initial copper
source. Sodium tert-butoxide and 4 were mixed in tetrahy-
drofuran solvent prior to addition of a solution of alkyne and
diboron reagent in toluene. Following the standard conditions
after the addition of the alkyne and B2(neo)2, the potassium
cis-(β-fluorovinyl)trifluoroboronate product of phenylpropyne
was isolated in good yield (Scheme 7). In addition, the
borofluorination of 3-hexyne was also achieved at a 1 mmol
scale starting with a combination of (IDipp)CuCl and sodium
tert-butoxide to generate (IDipp)CuO-t-Bu in situ. The
byproduct 4 was isolated in 71% yield from this reaction.
With the cis-(β-fluorovinyl)trifluoroboronates in hand,
various methods for carbon−carbon bond functionalization
were examined to gauge the synthetic utility of these
compounds. To date, we have not observed successful
conversion to vinylaldehydes using established rhodium
catalysis26 nor to vinylamines via Chan−Evans−Lam cou-
pling.27 This may be in part due to β-fluoride elimination from
the metal cis-(2-fluorovinyl) intermediate (vide supra). The
potentially vulnerable C−F bond is retained, however, through
other elaborations. Initial attempts to oxidize the cis-(2-
fluorovinyl)trifluoroboronates following the established
routes28 resulted in slow reaction times and a mixture of
products. Addition of silica gel to the common oxidizing
mixture of aqueous NaOH/H2O2 led to the formation of the
α-fluoroketone in good yield (Scheme 8). Presumably, the
trifluoroborate is first converted to the boronic acid which is
more susceptible to oxidation, allowing a clean transformation.
In contrast to the attempted Cu- or Rh-catalyzed couplings,
a Pd-catalyzed Suzuki−Miyuara coupling proceeded effectively.
Under conditions established by Molander and co-workers,29
phenylation of a cis-(β-fluorovinyl)trifluoroboronate afforded
the corresponding tetrasubstituted fluoroalkene.
We have explored the formation of fluorovinyl products by
the reaction of an electrophilic fluorine source with copper(I)
vinyl complexes, which are readily generated by 1,2-insertion
of alkynes into copper(I) hydrides or boryls. The sequence of
alkyne insertion followed by electrophilic fluorination enables a
one-pot synthesis of cis-(β-fluorovinyl)boronates that can be
readily isolated as their potassium trifluoroborate salts. This
method affords potential nucleophiles for the construction of
more elaborate fluorovinyl-containing systems.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02195.
Experimental details and spectral characterization (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: joseph.sadighi@chemistry.gatech.edu.
ORCID
Joseph P. Sadighi: 0000-0003-1304-1170
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Georgia Institute of Technology College of
Science and School of Chemistry and Biochemistry for
generous support of this research. Professor Jake D. Soper
(Georgia Institute of Technology) allowed us the use of his
group’s FTIR spectrometer. We thank Project ENGAGES for
support of P.K.T. and the Coretta Scott King Young Women’s
Leadership Academy for their cooperation.
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