Synthesis of internal fluorinated alkenes via facile aryloxylation of substituted phenols with aryl trifluorovinyl ethers

Article abstract of DOI:10.1039/c1ob05041a

Nucleophilic addition-elimination of ortho- or para-substituted phenols to aryl trifluorovinyl ethers (TFVEs) in N,N-dimethylformamide was studied. Using sodium hydride as a base afforded vinyl substitution products R-Ar-O-CFCF-O-Ar-R′, where R or R′ = H,

Full text of DOI:10.1039/c1ob05041a

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PAPER
Synthesis of internal fluorinated alkenes via facile aryloxylation of substituted
phenols with aryl trifluorovinyl ethers†
Justin D. Moody,a Don VanDerveer,a Dennis W. Smith Jr.a,b and Scott T. Iacono*a,c
Received 7th January 2011, Accepted 7th March 2011
DOI: 10.1039/c1ob05041a
Nucleophilic addition–elimination of ortho- or para-substituted phenols to aryl trifluorovinyl ethers
(TFVEs) in N,N-dimethylformamide was studied. Using sodium hydride as a base afforded vinyl
substitution products R–Ar–O–CF CF–O–Ar–R¢, where R or R¢ = H, Br, OMe, tert-Bu, or Ph. The
vinyl substitution products produced mixtures of (Z)/(E)-isomers and this isomer ratio was influenced
by substitution with more sterically encumbered phenol nucleophiles. Reactions using caesium
carbonate afforded addition products R–Ar–O–CHFCF2–O–Ar–R¢ whereas upon dehydrofluorination
using sodium hydride produced vinyl substitution products. The preparation of vinyl substituted and
addition products proceeded in overall good isolated yields and were elucidated using 1H and 19
F
NMR, GC-MS, and X-ray analysis. Vinyl substituted products were inert to UV light and chemical
reactivity using common polymerization promoters. Thermal activation of the (Z)/(E)-fluoroolefin
(-CF CF-) was observed at an onset of 310 ◦C in nitrogen using differential scanning calorimetry
(DSC) producing insoluble network material. The synthesis, characterization, and mechanism for
stereoselectivity are discussed.
phase stability, which is one of the main goals for maintaining a
Introduction
vibrant LC display market.15,16 Continuing interest in these broad
areas warrants investigating new strategies for the development
functionalized of fluorinated alkenes.
Nucleophilic addition to perfluorinated alkenes is a well known
organofluorine transformation and continues to garner academic
and commercial interest.1–3 As a result, this methodology has
led to the development of next-generation hydrofluoroethers
(HCFCs) as leading alternatives to chlorofluorocarbons (CFCs),4,5
perfluorinated polyethers (PFPEs) for high service temperature
fluids,6 processable ring-containing fluoropolymers,7 intermedi-
ates/monomers for fluorinated polymers,8–11 and latent thermally
cross-linkable fluorinated aryl ether oligomers/polymers as addi-
tives for fluoroelastomers.12 Further utility of fluorinated alkenes
has more recently extended toward the development of quantita-
tive chemisorption of chemical warfare agents in environmental
samples.13 The design of peptide mimics for the development of
emerging biologically-active drug candidates has employed the
internal fluoroolefin moiety to enhance membrane transport.14
Use of internal fluorinated alkenes as a linking group for single
molecule liquid crystals (LC) has been shown to enhance nematic
The strong electron-withdrawing character of the fluorine atoms
on terminal perfluorinated alkenes exposes their adjacent carbons
to nucleophilic attack. The carbanion intermediate that results
from the nucleophilic addition to a trifluoroalkene is either
trapped by an accompanying electrophile or, in the absence of
an electrophile, eliminates a fluoride ion, resulting in vinyl or
allyl substitution (Scheme 1). In the case where the carbanion
is trapped by a proton source (E = H), dehydrofluorination with
aDepartment of Chemistry and Center for Optical Materials Science and
Engineering Technologies (COMSET), Clemson University, Clemson, SC,
29634, USA
bPresent address: Department of Chemistry, The University of Texas at
Dallas Richardson, TX, 275080, USA
cPresent address: Department of Chemistry, United States Air Force
Academy, USAF Academy, CO, 80840, USA. E-mail: scott.iacono@
usafa.edu
† CCDC reference numbers 656776, 656778, 656777 for compounds (E)-
3, (E)-6, and 10, respectively. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1ob05041a
Scheme 1 Nucleophilic addition to terminal fluorinated alkenes affording
addition or vinyl substitution products.
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Table 1 Aryloxylation of substituted phenols with aryl TFVEs
R
R¢
Producta
(Z)/(E)b
Yield [%]c
Br
Br
Br
OCH3
OCH3
Br
Br
Br
p-Br
(Z)/(E)-3
(Z)/(E)-4
(Z)/(E)-5
(Z)/(E)-5
(Z)/(E)-6
(Z)/(E)-7
(Z)/(E)-8
(Z)/(E)-9
1.26/1
1.31/1
1.26/1
1.28/1
1.27/1
1.41/1
1.69/1
1.95/1
75
72
60
86
64
69
50
67
H
p-OCH3
p-Br
p-OCH3
o-Br
o-C(CH3)3
o-Ph
a Average of two runs. b Determined by 19F NMR. c Isolated yield of (Z)/(E)-mixture.
a base can also result in vinyl substitution by elimination via a
similar carbanion intermediate.17
TFVEs 1-bromo-4-(trifluorovinyloxy)benzene (1) and 1-methoxy-
4-(trifluorovinyloxy)benzene (2) in anhydrous DMF at 60 C for
◦
Reported additions to terminal trifluorovinyl perfluoroolefins
have included F-,18 AgF,19 N3-,20 and C-,21,22 O-,23–25 N-,21,24,26
or S-alkyl/aryl nucleophiles.21,24,27 Such nucleophilic additions
are primarily facilitated in basic conditions; other additions
have also included radical28,29 and Pd-catalyzed addition.4 Most
of the aforementioned reported examples have been limited
to nucleophile additions to fluoroalkyl or fluoroalkyl ether
substituted fluoroolefins. Herein, we describe the new synthe-
sis of aryl 1,2-difluorodioxyethylenes by the facile nucleophilic
addition of substituted phenols to aryl trifluorovinyl ethers
(TFVEs). Aryl TFVEs are well known to undergo thermal
[2 + 2] cyclodimerizations30 whereby difunctional aryl TFVEs
afford perfluorocyclobutyl (PFCB) aryl ether polymers used for
a multitude of high-performance material applications.31–34 Aryl
TFVEs are commercially available and can also be prepared in
two steps starting from phenolic precursors via alkylation with
1,2-dibromotetrafluoroethane (BrCF2CF2Br), followed by zinc-
mediated dehalogenation.32
We have previously reported the polymerization of bisphe-
nols to bis(trifluorovinyloxy)biphenyls affording high molecu-
lar weight, solution processable condensation polymers and
oligomers possessing hydro-1,2,2-fluoroethane (-CHFCF2-) or
1,2-difluoroethylene (-CF CF-) enchainment.12,35,36 In single
molecule studies, we have discovered that substituted phenol addi-
tions to monofunctionalized aryl TFVEs showed stereoselectivity
for the preparation of vinyl substituted isomers. This operationally
simple, modularly tailorable methodology afforded substituted
internal perfluorinated alkenes possessing functionalizable moi-
eties on the ortho/para positions on the aromatic ring. Their
synthesis, characterization, electronic/steric factors contributing
to stereoselectivity, and reactivity will be discussed.
1 h (Table 1). Excess NaH (4 equiv.) was used to deprotonate
substituted phenols in order to avoid the formation of addi-
tion products (e.g., Ar–O–CHFCF2–O–Ar¢) by protonation from
adventitious water from the solvent or unreacted phenol. The
optimized preparation of vinyl substituted products 3–9 proceeded
in quantitative conversion as observed by 19F NMR and in modest
to good isolated yields (50–86%) as a mixture of (E)- and (Z)-
isomers.
The preparation of the hydrofluorinated addition product 10
was achieved by the addition of p-bromophenol to 1-bromo-
4-(trifluorovinyloxy)benzene 1 in the presence of Cs2CO3 (0.5
equiv.) in DMF for 1 h (Scheme 2). As previously observed,
the conversion was quantitative based on 19F NMR and 10
was isolated in 73% yield. It was shown that aliphatic and
aromatic alcohols, particularly phenol, proceed via Pd-catalyzed
addition to perfluorinated alkenes to afford hydrofluorinated
ethers.4 Attempts to employ this strategy did not produce the
desired addition product 10, but rather afforded only unreacted
starting materials. The general utility of this methodology is
realized that by the choice of either NaH or Cs2CO3 as a base
selectively affords either vinyl substituted or addition products,
respectively. This is in agreement with previous studies for the
preparation of step-growth polymers from difunctional phenols
and aryl TFVEs.12
Scheme 2 Carbonate-catalyzed addition of 1,4-bromophenol with 1-bro-
mo-4-(trifluorovinyloxy)benzene 1 affording hydrofluorinated ether 10.
Fig. 1 shows the 19F NMR patterns in CDCl3 of the aryl
TFVE 1 starting material, vinyl substituted products (Z)/(E)-
3 and (Z)/(E)-5, and the addition product 10. Aryl TFVEs
typically produce a diagnostic AMX pattern, and in the case
of 1, with peaks at -120.5 ppm (dd, J = 95.5, 55.4 Hz,
cis-CF CF2, FA), -126.4 ppm (dd, J = 111.9, 95.5 Hz,
Results and discussion
Synthesis and characterization of vinyl substitution products
Vinyl substitution products 3–9 were prepared by the stoi-
chiometric addition of substituted sodium phenoxides to aryl
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Fig. 1 19F NMR spectra (in CDCl3) overlay of aryl TFVE 1, vinyl
substituted products (Z)/(E)-3 and (Z)/(E)-5, and hydrofluorinated
addition product 10.
trans-CF CF2, FM), and -136.8 ppm (dd, J = 111.9, 52.4 Hz,
CF CF2, FX). The vinyl substituted product isomer mixture of
(E)/(Z)-3 produced two singlet signals due to the asymmetry
of the isomers at -121.7 and -127.8 ppm for the (E)- and (Z)-
isomer, respectively. In comparison, the substitution of R = Br
with R = OMe desymmetrized the stereochemical environment
and produced peak signals for (E)/(Z)-5 at -120.5 and -122.3
ppm (d, J = 42.8 Hz, (Z)-CF CF) and -126.7 and -129.6 ppm
(d, J = 110.3 Hz, (E)-CF CF). Lastly, the addition hydro-1,2,2-
fluoroethane ether containing product 10 produced an AB pattern
at -85.4 and -85.9 ppm (d, J = 148.1 Hz) for the geminal fluorines
(CHFCF2) and -138.9 ppm (dt, J = 59.3 Hz, 8.2 Hz) for the
fluorine coupled with the proton (CHFCF2).
Fig. 2 ORTEP representations (50% probability) of vinyl substituted
product (E)-3 (top), (E)-6 (middle), and addition product 10 (bottom). The
crystal of 10 produced a mixture of conformers which cause intermixing
of F- and H-atoms. The F-atoms are each given occupancy of 0.75.
unsaturated double bond lengths of these isomers are considerably
contracted compared to typical hydrocarbon alkene bonds and are
comparable to triple bond lengths. The result showed the F(1)–
C(7)–C(7A) bond angle for (E)-6 was decreased by 9.06◦ and the
O(1)–C(7)–C(7A) bond angle was increased by nearly the same
magnitude at 9.35◦ compared with bromine substituted (E)-3.
This may be due to the electron donation of the para-methoxy
substituted aryl groups to the fluorines low lying 2p orbitals.37
In all cases, the structures demonstrated that the O(1)–C(7)–
C(7A) bond angles were larger than the F(1)–C(7)–C(7A) angle
due to oxygen lone pair repulsions. Bond angles for both (E)-
isomers and the hydrofluorinated solid state structures indicate
relatively consistent sp2 and sp3 fluorocarbon C(7) hybridization,
respectively, compared with homoanalogous hydrocarbons.
(Z)- and (E)-isomers were difficult to separate using column
chromatography or vacuum distillation. However, in some cases
vinyl substituted (Z)- and (E)-isomers were easily separated by
fractional solvent precipitation. Specifically, dispersion of vinyl
substituted products 3 and 6 in methanol or hexanes resulted in the
exclusive precipitation of the (E)-isomer. Recrystallization of the
precipitated (E)-isomer from the slow evaporation of diethyl ether
at room temperature afforded X-ray quality crystalline solids.
Fig. 2 shows the ORTEP structures of (E)-3 and (E)-6. Crystalline
solids of the respective (Z)-isomers of 3 and 6 were difficult to
obtain; attempts to crystallize from a multitude of polar and
non-polar solvents produced only an amorphous solid or oil.
Differential scanning calorimetry (DSC) showed a broad melting
Electronic and steric effects on the stereoisomeric ratio of vinyl
substituted products
◦
point range of 120–147 C for (Z)/(E)-3 isomers. Broad melting
point ranges were similar for all other isomers with no distinct
melting endotherm of individual isomers. Finally, X-ray quality
crystalline solids of addition product 10 were elucidated from the
slow evaporation of diethyl ether.
The results in Table 1 indicated a preference for the formation
of the (Z)-isomer of the vinyl substituted products 3–9 as
indicated by 19F NMR. Substituted phenol additions to the
aryl TFVEs producing 3–6 showed quantitative conversion at
room temperature or 60 ◦C in DMF; however, the (Z)/(E)-
isomer ratios were unaffected. Furthermore, cation substitution
did not influence the stereoisomer outcome when using either
LiH or NaH as the base. Solvent effects were then investigated
facilitating sodium phenoxide additions to aryl TFVE 1 in THF
and benzene. These reactions produced lower conversions of the
˚
Selected bond length (A) mean values and bond angles for
(E)-3, (E)-6, and 10 are shown in Table 2. (E)-isomers of 3
and 6 showed nearly identical C(1)–O(1) bond lengths. However,
the C(7)–C(7A) unsaturated bond length for (E)-6 was shorter
by nearly 0.1, which profoundly influences the F(1)–C(7)–C(7A)
and O(1)–C(7)–C(7A) bond angles compared with (E)-3. The
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◦
˚
Table 2 Selected bond lengths (A) and angles ( ) of (E)-3, (E)-6, and 10
Entry
C(7)–C(7A)
C(7)–O(1)
C(7)–F(1)
F(1)–C(7)–C(7A)
O(1)–C(7)–C(7A)
(E)-3
(E)-6
10
1.288(7)
1.189(11)
1.513(7)
1.343(4)
1.321(5)
1.349(4)
1.358(4)
1.449(7)
1.381(4)
118.8(4)
109.7(6)
107.10(3)
126.2(5)
135.7(8)
111.3(5)
vinyl substituted product 4 in 60–88% and 15%, respectively.
Although conversions were not quantitative, the (Z)/(E)-isomer
ratio of the vinyl substituted product 4 was unaffected. Electronic
effects due to the donating/withdrawing ability of aryl ethers
have been shown to influence the formation of (Z)/(E) isomers.38
This was not the case in our observations as the nature of the
para-functionalized vinyl substituted products 3–6 also showed
no indication of preference of the respective (Z)- or (E)-isomer.
However, installing bromine, tert-butyl, and phenyl functional
groups in the ortho position of the phenol nucleophile produced
vinyl substituted products 7–9 with preferential formation of the
(Z)-isomer. The most pronounced effect was observed for the case
where R = Ph, where up to 54% more ortho-substituted (Z)-isomer
was formed of 9 compared with the para-substituted 3.
In order to elucidate the mechanistic rationale for the preference
of (Z)-isomer formation, these results possibly indicate that a
steric effect is influencing their formation. It has been reported
most addition–elimination reactions to fluoroolefins proceed via a
E1CB-like mechanism1 and occur initially either by the formation
of a sp2-hybridized planar39,40 or sp3-hybridized tetrahedral41,42
carbanion. A competitive addition pathway via single-electron
transfer (SET) was ruled out based on previous reports43 and given
the fact that aryl TFVEs are benign to chemical radical initiation.44
In most cases, fluorocarbanion hybridization is predominately
influenced by repulsion or inductive effects.1,45 We initially invoke
the generation of the sp3-hybridized intermediate I shown in Fig. 3
as a result of anti-addition of the phenol nucleophile. Syn-addition
of the nucleophile is possible due to anion hyperconjugation by
the stabilizing interaction of the carbanion 2p orbital with the
s* Cb–F orbital.39 The anionic intermediates generated would
then facilitate syn-elimination of NaF to form the (Z)/(E)-vinyl
substituted products. However, syn-elimination may be unlikely
as the hyperconjugative stabilization would not overcome the
eclipsing steric interactions by initial addition followed by ensuing
elimination.41
Anti-elimination would involve comparing rotational energy
barriers of the intermediate I. Our observations pointing to the
formation of the preferred (Z)-isomer indicates the kinetically
favored intermediate is Ib. This elimination pathway involves
clockwise rotation of I between the two aryl ethers. Such a
rotation in Ib may be favored because of greater flexibility of the
adjoining aryl ether linkages compared with rotation by a more
sterically encumbered, shorter C–F bond shown as the proposed
intermediate Ia. This steric effect would be more pronounced if
ortho-substituted groups on the phenol nucleophile are present;
we have shown this to be experimentally the case as shown with
ortho-substituted vinyl addition products 7–9 (vide supra). The
intermediate Ib may be further stabilized by the gauche effect
whereby the s C–O bonding orbitals delocalize electron density to
the antibonding s* Cb–F orbitals.46 Syn-elimination can also be
considered since it would involve no rotation. If syn-elimination
were to occur, this would be indicative of favorable eclipsing of the
aryl ethers (Id) rather than the aryl ether eclipsing with fluorine
atoms (Ic).
To rule out p–p electronic interactions or the presence of phe-
nonium ion intermediates,47 addition to 1 with sodium ethoxide
(NaOEt) was performed. This afforded vinyl substituted product
11 with a higher 2.45/1 (Z)/(E) isomer ratio compared to phenol
nucleophiles (Scheme 3). This result indicates that smaller linear
aliphatic nucleophiles such as ethoxide desire rotation (for anti-
elimination) or eclipsing (for syn-elimination) with the carbanion
possessing the aromatic ether.
Scheme 3 Addition of aliphatic alcohol, sodium ethoxide, with aryl
TFVE 1.
Dehydrofluorination of addition product 10
Facile dehydrofluorination of addition product 10 proceeds using
an equivalent amount of base. In this case, elimination of NaF
using NaH in DMF at 60 ◦C produced 3 in nearly quantitative
yield (Scheme 4). The vinyl substituted product produced slightly
preferential formation of the (Z)-isomer in a (Z)/(E) ratio of
Fig. 3 Proposed mechanism of the addition–elimination of aryl TFVEs
by substituted phenols affording vinyl substituted (Z)- and (E)-isomers.
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Reactivity of vinyl substitution products
The chemical reactivity of vinyl substitution products of (Z)/(E)-
3 was investigated using typical polymerization protocols.49 At-
tempts to polymerize with 2,2-azobis(isobutyronitrile) (AIBN),
benzoyl peroxide (BPO), boron trifluoride etherate, and n-butyl
lithium resulted with retention of the unreacted (Z)/(E)-3.
Reactions with p-toluenesulfonic acid, I2, and UV radiation
failed to promote any evidence of isomerization. However,
thermal treatment of (Z)/(E)-3 using DSC analysis in nitrogen
revealed an exotherm at 310 ◦C, which produced an insoluble,
black glassy solid. These results were consistent with reported
crosslinking of aryl 1,2-difluorodioxyethylenes enchained in poly-
mer systems produced insoluble crosslinked networks.12 The
mechanistic details of thermal crosslinking are currently under
investigation.
Scheme 4 Dehydrofluorination of 10 using NaH affording isomeric
mixture (Z)/(E)-3.
1.16/1. This indicates the carbanion is relatively long-lived in
order to facilitate rotation in order to expel fluorine via syn-
or anti-elimination. Based on the single X-ray structure of 10
(Fig. 2, bottom image), the most stable conformation indicates
the aromatic ethers are pseudo-trans. Therefore, less (Z)-isomer
3 formation by dehydrofluorination (i.e., (Z)/(E) = 1.16/1)
compared with sodium p-bromophenoxide addition–elimination
(i.e., (Z)/(E) = 1.26/1) may be due to competitive syn- and anti-
elimination pathways.
Compound (Z)/(E)-3 possesses dibromofunctionality that was
found compatible with the Ni-catalyzed Yamamoto homocou-
pling protocol under mild conditions.50 Polymers prepared by
polymerizing aryl dihalides have produced a plethora of new
conjugated polymer systems, which continue to gain interest.50
19F NMR in DMSO-d6 showed the fluoroolefins were still intact
producing broad peaks associated with compound (Z)/(E)-3
with no observed isomerization. Thermal calorimetry◦ of the
polymer using DSC produced an exotherm onset at 260 C with
its maximum at 310 ◦C that is consistent with the exotherms
observed due to the thermal activation of 1,2-difluoroethylene
moiety.
Attempted fluorocarbanion trapping
The sequential trapping of carbanions with electrophiles upon
initial nucleophilic addition to fluoroolefins has been extensively
reported.39,41,48 This is evident by the inevitable formation of
trace amounts of hydro-1,1,2-trifluoroethane inevitable due to
adventitious water or from free hydroxyl (-OH) species. It would
seem obvious that the trapping of electrophiles could produce
a functionally diverse pool of fluorinated compounds from a
tandem, two-step addition–substitution methodology. As a model
system, initial attempts were performed in order to trap the
carbanion intermediate generated from the addition of sodium
phenoxide to 1 with labile electrophiles (denoted E) to produce
compounds with the general structure shown in Scheme 5.
Trapping with D2O or CF3SO2Cl failed to produce any substituted
product, where E = D or CF3SO2. 19F NMR spectroscopy revealed
only the formation of the vinyl ether product (Z)/(E)-3 and trace
amount of the hydro-1,2,2,-fluoroethane adduct. The addition of
the electrophiles was performed in a sequential fashion, whereas
addition of sodium phenoxide was added first to 1 at room
temperature, allowed to stir for 5 min, wherein the electrophile
was finally added. The evidence of trace amounts of hydro-1,2,2-
fluoroethane that indicate the rate of the nucleophilic addition
is quite rapid and the fluorocarbanion is short-lived, which is
consistent with a previous report.41 The lifetime of carbanions
generated by nucleophilic additions to fluoroolefins has been
shown to be very sensitive to the specific nature of nucleophile
substrates.39,48 Attempts to employ trapping at lower temperatures
(e.g., -78 ◦C) in THF or Et2O failed to produce any trapped
products in this study.
Conclusion
We have developed a methodology for the facile preparation of
substituted aryl 1,2-difluorodioxyethylenes 3–9 and aryl hydro-
1,2,2-fluorooxyethane 10 employing base-promoted nucleophilic
additions of phenols to aryl trifluorovinyl ethers 1 and 2. The
transformation demonstrates general utility using commercially
available materials that afforded either the vinyl substituted or
addition products by simple choice of the base. As such, a diverse
pool of functionalized internal fluoroolefins are now available
that possess functionalizable synthetic handles. We were able to
influence the selective formation of the (Z)-isomer upon addition
of sterically encumbered phenol nucleophiles to aryl trifluorovinyl
ether substrates. From this, we proposed that the formation of
vinyl substituted products are due to either syn- or anti-elimination
of fluorine from the generated carbanion intermediate which
is influenced by the steric nature of the appended nucleophile.
All other effects such as electronic, solvent, or temperature
failed to promote selectivity of either (Z)/(E)-isomers. Finally,
vinyl substituted products were benign to UV light exposure
and chemical treatment using common polymerization initiators;
thermal treatment using DSC analysis showed an exotherm
at an onset of 310 ◦C producing an insoluble material. This
may be due to the thermally activated self-initiation of the
fluoroolefin moiety which undergoes further reaction with other
fluoroolefins present ultimately producing network polymers. The
robust nature of the prepared substrates would be attractive
in the organic electronics whereby thermal stability is highly
desired.
Scheme 5 Nucleophilic addition of phenol to aryl TFVE followed by
attempted in situ trapping of electrophiles.
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layer was extracted with Et2O (2 25 mL). The combined organic
layers were washed with water (2 50 mL), dried over MgSO4,
filtered over a pad of silica gel, and concentrated under vacuum
affording the product mixture (Z)/(E)-3 as an oily yellow solid
(484 mg, 75%). 1H NMR (300 MHz, CDCl3) d 7.51–7.43 (m), 7.02
(d, J = 8.9 Hz), 6.95 (d, J = 8.9); 19F NMR (283 MHz, CDCl3)
d -121.7 (s, (Z)-CF CF, 2F), -127.8 (s, (E)-CF CF, 2F); GC-
EIMS (70 eV) m/z (% relative intensity) (Z)-3: 408, 406, 404 (M+,
10, 19, 11), 235, 233 (25, 26), 207, 205 (19, 19), 185, 183 (17, 17),
157, 155 (86, 91), 76, (89), 75 (89), 74 (24), 50 (100); (E)-3: 408,
406, 404 (M+, 10, 18, 9), 235, 233 (25, 23), 207, 205 (18, 20), 185,
183 (17, 16), 176, 175 (9, 11), 157, 155 (82, 85), 76 (85), 75 (88), 50
(100).
Experimental section
Reagents and general procedures
1-Bromo-4-(trifluorovinyloxy)benzene (1) is commercially avail-
able from Oakwood Products, Inc. and was donated along with
1-methoxy-4-(trifluorovinyloxy)benzene (2) by Tetramer Tech-
nologies, L.L.C. All other reagents and solvents were purchased
from Aldrich and purified according to established procedures.51
Anhydrous DMF was further dried by storage over anhydrous
MgSO4 under nitrogen atmosphere. All reactions and solvent
transfers were carried out under nitrogen atmosphere. Glassware
and syringes were flamed-dried and allowed to cool in a desiccator
prior to use. Syringes fastened with needles were flushed with
nitrogen prior to use. All reactions and solvent transfers were
carried out under an atmosphere of nitrogen. Air- and moisture-
free manipulations were carried out in a MBraun Labmaster glove
box under nitrogen circulation.
(Z)- and (E)-1-(4-Bromophenoxy)-1,2-difluoro-2-phenoxy-
ethene ((Z)/(E)-4)
Sodium phenoxide (184 mg, 1.58 mmol), sodium hydride
(76 mg, 3.16 mmol) and 1-bromo-4-(trifluorovinyloxy)benzene (1)
(400 mg, 1.58 mmol) were used following the procedure outlined
for the preparation of (Z)/(E)-3 affording the product mixture
Instrumentation
1H, and 19F NMR data were obtained on a JEOL Eclipse 300+
NMR Spectrometer and chemical shifts were reported in parts
per million (d ppm). 1H NMR was internally referenced to tetram-
ethylsilane (d 0.0), 13C NMR chemical shifts were reported relative
to the center peak of the multiplet for CDCl3 (d 77.0 (t)), and
19F NMR was referenced to CFCl3. Gas chromatography (GC)
coupled with electron ionization mass spectrometry (EIMS) was
performed on a Shimadzu GC17A gas chromatograph coupled
with a Shimadzu QP5000 mass spectrometer (EI at 70 eV) with
1
(E)/(Z)-4 as a yellow oil (373 mg, 72%). H NMR (300 MHz,
CDCl3) d 7.53–7.39 (m), 7.20–7.01 (m); 19F NMR (283 MHz,
CDCl3) d -120.9 and -122.5 (d, J = 41.1 Hz, (Z)-CF CF), -127.1
and -128.7 (d, J = 111.9 Hz, (E)-CF CF); GC-EIMS (70 eV) m/z
(% relative intensity) (Z)-4: 328, 326 (M+, 22, 24), 157, 155 (31,
42), 77 (100), 76 (32), 75 (29), 51 (64), 50 (43); (E)-4: 328, 326 (M+,
21, 21), 157, 155 (32, 42), 77 (100), 76 (32), 75 (29), 51 (61), 50 (44).
(Z)- and (E)-1-(4-Bromophenoxy)-1,2-difluoro-2-(4-methoxyphen-
oxy)ethene ((Z)/(E)-5)
initial temperature of 60 ◦C at a ramp of 10 C min-1. Melting
points and thermal analysis was performed on a TA Q1000
differential scanning calorimetry (DSC) instrument at 10 ◦C min-1
in nitrogen.
◦
p-Methoxyphenol (196 mg, 1.58 mmol), sodium hydride
(152 mg, 6.32 mmol) and 1-bromo-4-(trifluorovinyloxy)benzene
(1) (400 mg, 1.58 mmol) were used following the procedure
outlined for the preparation of (Z)/(E)-3 affording the product
Single crystal X-ray
1
mixture (Z)/(E)-5 as a yellow oil (340 mg, 60%). H NMR (300
Intensity data were collected using a Rigaku Mercury CCD
detector and an AFC8S diffractometer. Data reduction including
the application of Lp and absorption corrections used the Crys-
talClear program. The structure was solved by direct methods and
subsequent Fourier difference techniques, and refined anisotropi-
cally, by full-matrix least squares, on F2 using SHELXTL 6.10.52
Hydrogen atom positions were calculated from ideal geometry
with coordinates riding on the parent atom. Crystallographic
data for structures (E)-3, (E)-6, and 10 have been deposited in
the Cambridge Crystallographic Data Center with publication
numbers CCDC 656776, 656778, 656777, respectively.†
MHz, CDCl3) d 7.50–7.43 (m), 7.12–6.84 (m), 3.79 (d, J = 8.9 Hz,
-OCH3); 19F NMR (283 MHz, CDCl3) d -120.5 and -122.3 (d,
J = 42.8 Hz, (Z)-CF CF), -126.7 and -129.6 (d, J = 110.3 Hz,
(E)-CF CF); GC-EIMS (70 eV) m/z (% relative intensity) (Z)-5:
358, 356 (M+, 31, 31), 185 (37), 157 (54), 155 (32), 154 (37), 135
(36), 123 (52), 107 (44), 92 (63), 77 (100), 76 (37), 75 (31), 64 (45),
63 (31), 50 (43); (E)-5: 358, 356 (M+, 31, 31), 185 (40), 157 (58), 155
(35), 154 (35), 135 (38), 123 (52), 107 (45), 92 (69), 77 (100), 76 (38),
75 (33), 64 (44), 63 (31), 50 (45). (Z)/(E)-5 can also be prepared in
86% isolated yield using the above procedure using p-bromophenol
(339 mg, 1.96 mmol), sodium hydride (152 mg, 6.32 mmol), and
1-methoxy-4-(trifluorovinyloxy)benzene (2) (400 mg, 1.96 mmol).
(Z)- and (E)-1,2-Bis(4-bromophenoxy)-1,2-difluoroethene
((Z)/(E)-3)
(Z)- and (E)-1,2-Difluoro-1,2-bis(4-methoxyphenoxy)ethene
((Z)/(E)-6)
A solution of p-bromophenol (274 mg, 1.58 mmol) dissolved in
DMF (4 mL) was added dropwise to a stirred suspension of NaH
(152 mg, 6.32 mmol) in DMF (2 mL) at room temperature. The
flask was placed in a preheated oil bath at 60 ◦C and sparged
with nitrogen for 30 min. 1-Bromo-4-(trifluorovinyloxy)benzene
(1) (400 mg, 1.58 mmol) was transferred into the solution via
syringe in a single portion. After 1 h, the mixture was treated with
DI H2O (25 mL). The organic layer was separated and the aqueous
p-Methoxyphenol (243 mg, 1.96 mmol), sodium hydride (152 mg,
6.32 mmol) and 1-methoxy-4-(trifluorovinyloxy)benzene (2)
(400 mg, 1.96 mmol) were used following the procedure outlined
for the preparation of (Z)/(E)-3 affording the product mixture
1
(Z)/(E)-6 as a yellow oil (384 mg, 64%). H NMR (300 MHz,
CDCl3) d 7.11–7.02 (m), 6.91–6.78 (m), 3.78 (d, J = 5.82 Hz, -
OCH3); 19F NMR (283 MHz, CDCl3) d -122.2 (s, (Z)-CF CF),
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-128.5 (s, (E)-CF CF); GC-EIMS (70 eV) m/z (% relative
intensity) (Z)-6: 308 (M+, 40), 185 (68), 157 (43), 135 (63), 126
(25), 123 (24), 107 (58), 92 (63), 77 (100), 64 (42), 63 (23); (E)-6:
308 (M+, 35), 185 (66), 157 (41), 154 (20), 135 (61), 126 (24), 123
(23), 107 (58), 92 (66), 77 (100), 64 (44), 63 (25).
1,2-Bis(4-bromophenoxy)-1,1,2-trifluoroethane (10)
1-Bromo-4-(trifluorovinyloxy)benzene (1) (400 mg, 1.58 mmol),
p-bromophenol (274 mg, 1.58 mmol), and Cs2CO3 (258 mg,
0.79 mmol) in DMF (4 mL) and DI H2O (10 L) were placed
◦
in a preheated oil bath at 60 C and allowed to stir for 1 h. The
mixture was then treated with DI H2O (25 mL). The organic layer
was separated and the aqueous layer was extracted with Et2O
(2 25 mL). The combined organic layers were washed with water
(2 50 mL), dried over MgSO4, filtered over a pad of silica gel, and
concentrated under vacuum affording 10 as an oily yellow solid
(493 mg, 73%). 1H NMR (300 MHz, CDCl3) d 7.52–7.48 (m), 7.13
(d, J = 8.9 Hz), 7.04 (d, J = 8.6 Hz), 5.84 (dt, J = 58.4 Hz, 3.1 Hz,
CHFCF2); 19F NMR (283 MHz, CDCl3) d -85.4 and -85.9 (AB
pattern, Jab = 148.1 Hz, CHFCF2), -138.9 (dt, J = 59.26 Hz, 8.2
Hz, CHFCF2); GC-EIMS (70 eV) m/z (% relative intensity) 428,
426, 424 (M+, 49, 100, 51), 205, 203 (54, 56), 157, 155 (92, 99), 145,
143 (35, 34), 76 (48), 75 (41), 64 (39), 63 (55), 50 (43).
(Z)- and (E)-1-(2-Bromophenoxy)-2-(4-bromophenoxy)-1,2-
difluoroethene ((Z)/E)-7)
o-Bromophenol (274 mg, 1.58 mmol), sodium hydride (152 mg,
6.32 mmol) and 1-bromo-4-(trifluorovinyloxy)benzene (1)
(400 mg, 1.58 mmol) were used following the procedure outlined
for the preparation of (Z)/(E)-3 affording the product mixture
1
(Z)/(E)-7 as a yellow oil (441 mg, 69%). H NMR (300 MHz,
CDCl3) d 7.52–7.00 (m); 19F NMR (283 MHz, CDCl3) d -120.9
and -121.2 (d, J = 42.8 Hz, (Z)-CF CF), -126.9 and -127.8 (d,
J = 111.9 Hz, (E)-CF CF); GC-EIMS (70 eV) m/z (% relative
intensity) (Z)-7: 408, 406, 404 (M+, 12, 22, 12), 235, 233 (19, 18),
207, 205 (17, 18), 185, 183 (19, 19), 157, 155 (97, 100), 152 (14),
76 (83), 75 (91), 74 (21), 63 (13), 51 (15), 50 (96); (E)-7: 408, 406,
404 (M+, 12, 23, 11), 235 (17, 18), 207, 205 (16, 16), 185, 183 (18,
19), 157, 155 (94, 100), 126 (12), 76 (86), 75 (85), 74 (20), 63 (12),
51 (14), 50 (95).
(Z)- and (E)-1-(4-Bromophenoxy)-2-(ethyloxy)-1,2-difluoroethene
((Z)/(E)-11)
Anhydrous sodium ethoxide (57 mg, 0.830 mmol) and 1-bromo-
4-(trifluorovinyloxy)benzene (1) (200 mg, 0.790 mmol) were used
following the procedure outlined for the preparation of (Z)/(E)-3
affording the product mixture (Z)/(E)-11 as a yellow oil (190 mg,
86%). 1H NMR (300 MHz, CDCl3) d 7.42 (d, J = 8.9 Hz), 6.96 (d,
J = 8.9 Hz), 4.10 and 3.99 (q, J = 6.8 Hz, -CH2CH3), 1.25 and 1.28
(q, J = 6.8 Hz, -CH2CH3); 19F NMR (283 MHz, CDCl3) d -120.9
and -126.9 (d, J = 39.5 Hz, (Z)-CF CF), -127.3 and -133.6 (d,
J = 108.6 Hz, (E)-CF CF); GC-EIMS (70 eV) m/z (% relative
intensity) (Z)-11: 280, 278 (M+, 19, 18), 252, 250 (28, 26), 158 (40),
156 (42), 157, 155 (96, 94), 77 (100), 76 (65), 75 (62), 74 (18), 50
(63); (E)-11: 280, 278 (M+, 17, 17), 252, 250 (29, 29), 158 (40), 156
(43), 157, 155 (93, 100), 77 (98), 76 (63), 75 (64), 74 (17), 50 (62).
(Z)- and (E)-1-(4-Bromophenoxy)-2-(2-tert-butylphenoxy)-1,2-
difluoroethene ((Z)/(E)-8)
o-tert-Butylphenol (238 mg, 1.58 mmol), sodium hydride
(152 mg, 6.32 mmol) and 1-bromo-4-(trifluorovinyloxy)benzene
(1) (400 mg, 1.58 mmol) were used following the procedure
outlined for the preparation of (Z)/(E)-3 affording the product
mixture (Z)/(E)-8 as a yellow oil (304 mg, 50%). 1H NMR
(300 MHz, CDCl3) d 7.51–6.97 (m), 1.45 (s, -C(CH3)3), 1.35 (s,
-C(CH3)3); 19F NMR (283 MHz, CDCl3) d -120.7 and -122.3 (d,
J = 42.8 Hz, (Z)-CF CF), -126.8 and -129.2 (d, J = 108.6 Hz,
(E)-CF CF); GC-EIMS (70 eV) m/z (% relative intensity) (Z)-8:
384, 382 (M+, 6, 6), 157, 155 (4, 4), 117, 115 (4, 4), 105 (12), 92 (8),
91 (100), 77 (6), 76 (5), 75 (5), 65 (3), 55 (22), 51 (3), 50 (5); (E)-8:
384, 382 (M+, 6, 7), 157, 155 (4, 4), 117, 115 (4, 4), 105 (11), 92 (8),
91 (100), 77 (5), 76 (5), 75 (5), 55 (22), 51 (3), 50 (6).
General procedure for the isolation of (Z)- and (E)-isomers
Dispersion of the vacuum dried (Z)- and (E)-isomer mixture in
MeOH (10–20 mL) precipitated exclusively the (E)-isomer as a
white or yellow solid, which was filtered and dried in vacuum.
This process was typically repeated 5–10 times to separate the (E)-
isomer affording >90% pure isolated (Z)-isomer as a yellow oil.
X-ray quality white crystals were produced by recrystallization of
the combined precipitates of the (E)-isomers of 3 and 6 from slow
evaporation in diethyl ether at room temperature.
(Z)- and (E)-1-(4-Bromophenoxy)-2-(2-phenylphenoxy)-1,2-
difluoroethene ((Z)/(E)-9)
o-Phenylphenol (269 mg, 1.58 mmol), sodium hydride (152 mg,
6.32 mmol) and 1-bromo-4-(trifluorovinyloxy)benzene (1)
(400 mg, 1.58 mmol) were used following the procedure outlined
for the preparation of (Z)/(E)-3 affording the product mixture
Dehydrofluorination of 10
1
(Z)/(E)-9 as a yellow oil (429 mg, 67%). H NMR (300 MHz,
1,2-Bis(4-bromophenoxy)-1,1,2-trifluoroethane (10) (100 mg,
CDCl3) d 7.80–7.20 (m), 7.15–6.74 (m); 19F NMR (283 MHz,
CDCl3) d -120.2 and -122.3 (d, J = 42.8 Hz, (Z)-CF CF), -127.3
and -128.8 (d, J = 111.9 Hz, (E)-CF CF); GC-EIMS (70 eV) m/z
(% relative intensity) (Z)-9: 404, 402 (M+, 8, 9), 231 (13), 230 (26),
200 (42), 199 (54), 183 (10), 181 (17), 157, 155 (9, 9), 153 (51), 152
(100), 151 (26), 127 (8), 77 (7), 76 (15), 75 (15), 51 (8), 50 (13);
(E)-9: 404, 402 (M+, 9, 8), 231 (12), 230 (25), 200 (40), 199 (56),
183 (9), 181 (17), 157, 155 (8, 9), 153 (50), 152 (100), 151 (24), 127
(7), 76 (16), 75 (16), 51 (8), 50 (15).
1.58 mmol), sodium hydride (14 mg, 0.587 mmol) in DMF (3 mL)
were placed in a preheated oil bath at 60 C and allowed to stir
◦
for 24 h. The mixture was then treated with DI H2O (10 mL). The
organic layer was separated and the aqueous layer was extracted
with Et2O (2 10 mL). The combined organic layers were washed
with water (2 10 mL), dried over MgSO4, filtered over a pad of
silica gel, and concentrated under vacuum. 19F NMR (283 MHz,
CDCl3) peak integration showed 71% conversion to (Z)/(E)-3 in
a 1.16 : 1 isomer ratio with the remaining as unreacted 10.
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24 D. C. England, L. R. Melby, M. A. Dietrich and R. V. Lindsey, Jr., J.
Am. Chem. Soc., 1960, 82.
Acknowledgements
25 A. E. Feiring, S. Rozen and E. R. Wonchoba, J. Fluorine Chem., 1998,
89, 31–34.
26 G. G. Furin, L. S. Pressman, L. M. Pokrovsky, A. P. Krysin and K.-W.
Chi, J. Fluorine Chem., 2000, 106, 13–24.
27 C. M. Timperley, M. J. Waters and J. A. Greenall, J. Fluorine Chem.,
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28 O. Paleta, V. Crkva and J. Kvcˆala, J. Fluorine Chem., 1996, 80, 125–134.
29 V. Crkva and O. Paleta, J. Fluorine Chem., 1999, 94, 141–156.
30 B. K. Spraul, S. Suresh, J. Jin and D. W. Smith, Jr., J. Am. Chem. Soc.,
2006, 128, 7055–7064.
We thank the National Science Foundation (NSF DMR 0514622)
for financial support. JDM also acknowledges CU for financial
support through the REU program of the NSF (CHE 0453318).
We also thank Dr Rama V. Rajagopal (CU) and Dr Chris Topping
(Tetramer Technologies, L.L.C.) for technical assistance.
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The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 4842–4849 | 4849
©