The stereospecific preparation of two perfluoro-1,3-butadiene synthons; (E)-1-trimethylsilyl-1,2,3,4,4-pentafluoro-1,3-butadiene and (E)-1-tributylstannyl-1,2,3,4,4-pentafluoro-1,3-butadiene

Article abstract of DOI:10.1016/S0022-1139(02)00246-4

(E)-1-Trimethylsilyl-1,2,3,4,4-pentafluoro-1,3-butadiene (1) can be stereospecifically prepared by Pd(0)/CuI catalyzed cross-coupling of (Z)-1-tributylstannyl-1,2-difluoro-2-trimethylsilylethene with iodotrifluoroethene. The corresponding (E)-1-tributylst

Full text of DOI:10.1016/S0022-1139(02)00246-4

Journal of Fluorine Chemistry 119 (2003) 21±26
The stereospeci®c preparation of two per¯uoro-1,3-butadiene synthons;
(E)-1-trimethylsilyl-1,2,3,4,4-penta¯uoro-1,3-butadiene and
(E)-1-tributylstannyl-1,2,3,4,4-penta¯uoro-1,3-butadiene
Chongsoo Lim, Donald J. Burton*, Craig A. Wesolowski
Department of Chemistry, University of Iowa, Iowa City, IA 52242, USA
Received 21 June 2002; received in revised form 28 August 2002; accepted 28 August 2002
Dedicated to Professor Tatlow on the occasion of his 80th birthday
Abstract
(E)-1-Trimethylsilyl-1,2,3,4,4-penta¯uoro-1,3-butadiene (1) can be stereospeci®cally prepared by Pd(0)/CuI catalyzed cross-coupling of
(Z)-1-tributylstannyl-1,2-di¯uoro-2-trimethylsilylethene with iodotri¯uoroethene. The corresponding (E)-1-tributylstannyl-1,2,3,4,4-penta-
¯uoro-1,3-butadiene can be prepared via the stereospeci®c conversion of 1 with Bu3SnOSnBu3/KF (catalysis) to the corresponding
vinylstannane.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Fluorinated synthons; Fluorinated dienes; Fluorinated vinylsilanes; Fluorinated vinylstannanes; Palladium catalysis
1. Introduction
nated vinylcopper reagents [6] (Eq. (2)). The tri¯uorovinyl-
zinc reagents
One of the topics of recent interest in our laboratory has
been the development of stable ¯uorine-containing organo-
metallic synthons that would permit regiochemical and/or
stereochemical control for the introduction of ¯uoroalkenyl
units. The discovery that Zn(0) and Cd(0) could be directly
inserted into the carbon±bromine or carbon±iodine bond of
poly¯uorinated vinyl bromides or vinyl iodides [1,2] to give
a stable vinyl organometallic reagent was the stimulus for
our development of useful ¯uorinated organometallic syn-
thons. These ¯uorinated vinylic organometallic reagents
were readily cross-coupled with aryl iodides with Pd(0)
catalysis to provide a one-¯ask simple synthesis of a,b,b-
tri¯uorostyrenes and a stereospeci®c preparation of 1-phe-
nyl-F-propenes [3,4]. The scope of the a,b,b-tri¯uorostyr-
ene synthesis has been recently elaborated to include HFC-
134a as the precursor for the in situ, room temperature
preparation of the tri¯uorovinylzinc reagent [5] (Eq. (1)).
F2CˆCFCdBr ‡ CuꢁI† halideD!MF‰F2CˆCFCuŠ
(2)
RT
can be readily acylated with Cu(I) mediation and provide a
useful entry to a variety of tri¯uorovinylketones [7] (Eq. (3)).
When the vinylzinc reagent contained an a-halogen other
than
CuBr
!
F2CˆCFZnX ‡ RCꢁO†Cl
F2CˆCFCꢁO†R
(3)
glyme RT
¯uorine, Cu(I)Br induced self-coupling and provided a high
yield entry to a variety of ¯uorine-containing butatrienes
[8,9] (Eq. (4)). The butatrienes could be readily prepared on
a multi-gram
CF3CꢁPh†ˆCꢁBr†ZnXC!uBrCF3ꢁPh†CˆCˆCˆCꢁPh†CF
3
DMF
E=Z
(4)
THF
CF3CFH2 ‡ 2LDA ‡ ZnCl215À20 ꢀC
!
‰F2CˆCFZnClŠ
(1)
scale and the E/Z isomers were readily separated by chro-
matography.
The chemistry described above was extended to the
preparation and functionalization of 2-metallated penta-
¯uoropropenes [10], per¯uoroallyl cadmium and copper
reagents [11], the preparation of b,b-di¯uoro-a-(tri¯uoro-
The vinylzinc and vinylcadmium reagents readily exchange
with Cu(I) halides and provide a useful entry to the ¯uori-
* Corresponding author. Tel.: ‡1-319-335-1363; fax: ‡1-319-335-1270.
E-mail address: donald-burton@uiowa.edu (D.J. Burton).
0022-1139/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 0 2 ) 0 0 2 4 6 - 4

22
C. Lim et al. / Journal of Fluorine Chemistry 119 (2003) 21±26
methyl)styrenes [12], and the preparation of the (E)- and (Z)-
hepta¯uorobutenyl-2-zinc reagent via zinc-induced dehalo-
genation and metallation of 2,2-dibromoocta¯uorobutane
[13].The (E)- and (Z)-1,2-di¯uoroethenylzinc reagents were
subsequently developed as a stereospeci®c entry either to
(Z)- or (E)-a,b-di¯uorostyrenes [14±17] (Eqs. (5) and (6));
and the vinylzinc chemistry was extended to include the
preparation of a-bromo-b,b-di¯uorostyrenes [18] and sub-
stituted 2,2-di¯uorostyrenes [19].
readily reacted with Cu(II) salts in a homo-coupling process
to stereospeci®cally form ¯uorinated dienes [23] (Eq. (9)).
(9)
Recently, this stannane chemistry was extended to the
stereoselective preparation of (E)-(1,2-di¯uoro-1,2-ethene-
diyl)bis(tributylstannane) [24] (Eq. (10)). This bis-organo-
metallic reagent permits bis-functionalization processes in
one step, as illustrated in the stereospeci®c preparation of
(E)-1,2-di¯uorostilbenes. This route is tolerant of many
functional groups compared to previously reported routes.
(5)
(6)
As an alternative to the vinylzinc reagent in Pd(0) cata-
lyzed cross-coupling processes, we also investigated the
stereospeci®c preparation of 1,2-di¯uoroethenylstannanes.
The key discovery in this route was the stereospeci®c
conversion of vinylsilanes to vinylstannanes [20,21]
(Eq. (7)). It was subsequently demonstrated that these
1,2-di¯uorovinylstannanes readily
(10)
Our interest has evolved to the preparation of synthons
containing multiple unsaturations. Our initial focus has been
on the stereospeci®c development of (E)- and (Z)-1-trialk-
ylsilyl-per¯uoro-1,3-butadiene synthons, which could be
stereospeci®cally converted to the corresponding 1-trialk-
ylstannyl per¯uoro-1,3-butadiene synthons via previously
reported chemistry [21]. The (Z)-1-tributylstannyl-1,2,3,4,
4-penta¯uoro-1,3-butadiene was recently reported [25]. In
this manuscript, we describe the stereospeci®c preparation
of (E)-1-trimethylsilyl-1,2,3,4,4-penta¯uoro-1,3-butadiene
and its stereospeci®c conversion to the corresponding (E)-
1-tributylstannyl-1,2,3,4,4-penta¯uoro-1,3-butadiene.
(7)
participated in Pd(0)/CuI catalyzed stereospeci®c cross-
coupling reactions with aryl halides and vinyl halides
[22] (Eq. (8)). Blumenthal demonstrated that these 1,2-
di¯uoroethenylstannanes
2. Results and discussion
Possible strategies for assembly of (E)-1-trimethylsilyl-
1,2,3,4,4-penta¯uoro-1,3-butadiene (1) via Pd(0) catalyzed
cross-coupling processes are outlined in Scheme 1.
(8)
Scheme 1.

C. Lim et al. / Journal of Fluorine Chemistry 119 (2003) 21±26
23
Both routes (a) and (c) require the preparation of iodotri-
¯uoroethene (2). This can be easily accomplished by iodi-
nation of [F2C=CFZnX] (4), which is readily prepared from
bromotri¯uoroethene [26]. Route (a) requires the synthesis
of (Z)-1-tributylstannyl-1,2-di¯uoro-2-trimethylsilylethene
(3). Our experience has taught us that Pd(0) cross-coupling
reactions of cis-hindered ¯uorinated vinylstannanes are
subject to steric hindrance. Hence, the trimethylsilyl analog
of 3 would be preferred over a more hindered silyl derivative
3 could be prepared by modi®cation of the triethylsilyl
analog methodology previously described by us [17]. In
route (b), the preparation of 4 is straightforward either from
bromotri¯uoroethene [26] or HFC-134a [5]. The (Z)-1-iodo-
1,2-di¯uoro-2-trimethylsilylethene (5) is not straightfor-
ward. As previously noted [17], the pre-generation of the
corresponding cis [LiFC=CFSiEt3] resulted in extensive
decomposition of the unstable lithium reagent. Better results
were obtained by iodination of the corresponding vinylstan-
nane. Consequently, the best route to 5 would be via
iodination of 3. Thus, the choice between routes (a) and
(b) is dictated by the preparation of either 2 or 5. Since the
preparation of 2 is easier to scale-up and involves only one
step, path (a) is an easy choice versus path (b). In path (c), the
zinc reagent 6 is prepared from 5 and requires the prepara-
tion of both 2 and 5; thus path (c) is the poorest route for the
preparation of 1. Thus, we elected to pursue route (a) as the
®rst choice to 1.
When 1 was initially prepared on a larger scale, we
attempted to isolate 1 via distillation at atmospheric pres-
sure. The boiling point of 1 is 109±110 8C. However, even at
this temperature, [2 ‡ 2] cyclization readily occurred [28]
and the isolated product was a mixture of 1 and the isomeric
cyclobutene (Eq. (13)).
(13)
However, distillation at reduced pressure permitted the
isolation of pure 1.
PdꢁPPh3†
CuI=DMFꢂ0ꢀC
4
3 ‡ 2
!
1distillation at reduced pressure
!
1
(14)
75%
When 1 was treated with Bu3SnOSnBu3 and catalytic KF
[21], the corresponding (E)-1-tributylstannyl-1,2,3,4,4-pen-
ta¯uoro-1,3-butadiene (9) was isolated in 67% yield
(Eq. (15)).
(15)
Attempts to form exclusively the cyclobutene product were
not successful. We found that the best conditions for cycli-
zation was to heat 1 in a sealed tube in tetradecane at 165±
170 8C. Cyclization did indeed occur; however, not only the
expected [2 ‡ 2] cyclized product was obtained, but an
isomeric cyclobutene was also formed (structure [29] of
this isomeric product assigned only by 19F NMR (19F
Compound 2 was readily prepared from bromotri¯uor-
oethene via the following route (Eq. (11)).
I2
F2CˆCFBr ‡ ZnD!MF4
!
2
(11)
DMF 69%
Intermediate 3 was prepared by modi®cation of previously
reported work [17] as outlined below (Eq. (12)).
(CDCl3) d À111.4 (m, 2F); d À117.1 (m,
4
3
2F); d À100.0 (tt, JFF ˆ 22:9 Hz, JFF ˆ 4:5 Hz, 1F).
These data are similar to the reported data for this compound
by Fields et al. [29]. HRMS (mixture): for C7H9F5Si,
216.0394; Found: 216.0388)) (Eq. (16)). The two isomeric
cyclobutenes could not be separated.
(12)
Cross-coupling of the two precursors described above,
however, presented some problems. As usual, cross-cou-
pling of the vinylstannanes required not only Pd(0) catalysis,
but also CuI as a co-catalyst [22,24] (Liebeskind condi-
tions); otherwise, little or no cross-coupling product was
obtained. In addition to the cross-coupled process, a com-
petitive exchange process between 3 and the CuI occurs
to produce an unstable vinylcopper intermediate which
can lead to by-products that cannot be easily separated
from 1 [27]. These competitive processes are outlined in
Scheme 2.
(16)
Perhaps, cyclization in a ¯ow tube system would give the [2
‡ 2] product, but we did not pursue this option.
3. Conclusions
Temperature control and slow addition of 3 allowed us to
suppress these competitive side-reactions. If the temperature
and concentration are not controlled, 1 is still obtained but
will be contaminated with 7 and 8.
The (E)-1-trimethylsilyl-1,2,3,4,4-pent¯uro-1,3-butadiene
synthon (1) can be stereospeci®cally prepared by Pd(0)/
CuI catalyzed cross-coupling of (Z)-1-tributylstannyl-1,
2-di¯uoro-2-trimethylsilylethene with iodotri¯uoroethene.

24
C. Lim et al. / Journal of Fluorine Chemistry 119 (2003) 21±26
Scheme 2.
The corresponding (E)-1-tributylstannyl-1,2,3,4,4-penta-
¯uro-1,3-butadiene synthon can be stereospeci®cally pre-
pared from 1 via reaction of 1 with Bu3SnOSnBu3 and
catalytic KF. All precursors and materials for the preparation
of these synthons are readily available or easily prepared by
previously reported methods.
4.2. Materials
DMF was distilled under reduced pressure from CaH2.
Zinc was activated as described in previous work [4,26].
The [F2C=CFZnBr] was generated from F2C=CFBr as
previously described [4]. Tetrakis (triphenylphosphine)
palladium was prepared by Coluson's procedure [30].
CuI was puri®ed by the literature procedure [31]. (Z)-
1,2-Di¯uoro-1-tributylstannyl-2-trimethylsilylethene was
preparedby modi®cation of the procedure described for the
analogous triethylsilyl derivative [17]. All other materials
were obtained from commercial vendors and utilized
directly.
4. Experimental
4.1. General
All reactions were monitored by 19F NMR analysis of
the reaction mixture on a Bruker AC-300 MHz spectro-
meter. The 1H NMR spectra of ®nal products were
obtained on a Bruker AC-300 MHz (300.17 MHz) and
chemical shifts are reported in ppm with respect to internal
TMS. 19F NMR spectra were obtained on a Bruker AC-
300 MHz (282.41 MHz) spectrometer, and chemical shifts
are reported in ppm relative to internal CFCl3 or C6H5CF3.
13CNMR spectra were obtained on a Bruker AC-300 MHz
(75.49 MHz) spectrometer with CDCl3 as the lock solvent.
All 13Cspectra were broadband decoupled from hydrogen,
but not from ¯uorine. Chemical shifts are reported relative
to TMS or CDCl3 (lock solvent). Low resolution mass
spectral analyses were performed at 70 eV in the electron
impact mode on a single quadruple instrument interfaced
to a gas chromatograph ®tted with a DB-1 column
(0.25 mm i.d. Â 15 m). High resolution mass spectral ana-
lyses were performed by the University of Iowa Mass
Spectroscopy Facility at 70 eV in the electron impact
mode. Isolations utilizing column chromatography were
carried out on columns packed with silica gel. All fractions
were monitored by TLCusing UV light or iodine for
visualization.
4.2.1. Preparation of Iodotrifluoroethene (2), F2C=CFI
[F2C=CFZnBr] was prepared from bromotri-
¯uoroethene (42.97 g, 267 mmol) and activated zinc (20.9 g,
320 mmol) via the literature procedure [26]. After the for-
mation of the zinc reagent was completed, reduced pressure
was applied to the reaction mixture to remove any volatile
materials. The reaction mixture was then cooled to 5 8Cwith
an ice bath and iodine (101.5 g, 400 mmol) was slowly
added to the reaction mixture via a solids addition funnel,
maintaining the reaction temperature below 20 8C. The
reaction mixture was warmed to room temperature and
stirred at room temperature overnight. Flash distillation of
the reaction mixture, followed by distillation of the ¯ash
distillate on a 25 cm spinning-band column at atmospheric
pressure gave 38.31 g (184 mmol, 69%) of iodotri¯uor-
oethene, bp 31±42 8C(mainly 31±34 8C; lit. bp 27.5±
28.5 8C/627 Torr [32]). 19F NMR (CDCl3): d À87.8 (dd,
2JFF ˆ 64:1 Hz, 3JFF ˆ 51:2 Hz, 1Fa); d À113.3 (dd, 2JFF
ˆ
ˆ
64:1 Hz, JFF ˆ 128:4 Hz, 1Fb); d À150.2 (dd, JFF
3
3
51:2 Hz, JFF ˆ 129:0 Hz, 1Fc).
3

C. Lim et al. / Journal of Fluorine Chemistry 119 (2003) 21±26
25
4.2.2. Preparation of (E)-1-trimethylsilyl-1,2,3,4,4-
pentafluoro-1,3-butadiene (1), followed
by distillation at atmospheric pressure
water and extracted with pentane (3Â 150 ml). The pentane
extracts were combined, dried over sodium sulfate, and
distilled on a spinning band distillation column (over
CaH2) under reduced pressure to give 32.4 g (75%) of 1
as a colorless liquid: bp 51±55 8Cat 90 mmHg; 19F NMR
A 250 ml, two-necked, round-bottom ¯ask equipped with
a PTFE-coated magnetic stir bar, rubber septum, and nitro-
gen tee was charged sequentially with 50 ml of dry DMF,
Pd(PPh3)4 (1.7 g, 1.5 mmol) and F2C=CFI (16.6 g,
80.0 mmol). The mixture was stirred for 10 min; then Cu(I)I
(4.80 g, 25.0 mmol) was added to the mixture. The reaction
mixture was cooled to ꢂ10 8Cin a cold-water bath; then ( Z)-
1,2-di¯uoro-1-tributylstannyl-2-trimethylsilylethene (17.0 g,
40.0 mmol) was added in portions (via syringe) over 20 min.
After 10 min, the cold-water bath was removed and the dark
solution stirred at room temperature for 2 h. The reaction
mixture was ¯ash distilled (at 0.2 mmHg) at room tempera-
ture into a liquid nitrogen cooled 100 ml ¯ask. The ¯ash
distillate was transferred to a 250 ml separatory funnel
containing 150 ml pentane. The organic layer was washed
with water (2Â 15 ml) and 10 ml brine, dried over MgSO4
and concentrated by simple distillation. The concentrated
organic material was fractionally distilled through a 25 cm
spinning band column to give 6.36 g (72%) of a 41:59
mixture of 1 and 1,2,3,3,4-penta¯uoro-4-(trimethylsilyl)
cyclobutene as a clear, colorless liquid, bp 109±110 8C.
2
(CDCl3): d À92.9 (dddd (appears as ddt), JFF ˆ 50:5 Hz,
4JFF ˆ 5:0 Hz, JFF ˆ 5:0 Hz, JFF ˆ 28:4 Hz, 1Fa); d
5
3
2
4
5
À107.9 (dddd, JFF ˆ 50:5 Hz, JFF ˆ 18:2, JFF
ˆ
ˆ
3:4 Hz, JFF ˆ 118:9 Hz, 1Fb); d À126.1 (dddd, JFF
3
4
4
3
3
5:0 Hz, JFF ˆ 18:2 Hz, JFF ˆ 23:6 Hz, JFF ˆ 37:0 Hz,
1Fc); d À132.4 (ddm, JFF ˆ 5:1 Hz, JFF ˆ 3:4 Hz,
5
5
3JFF ˆ 23:6 Hz, JFF ˆ 4:3 Hz, 1Fd); d À172.1 (dddd,
4
3
3
4
3JFF ˆ 28:4 Hz, JFF ˆ 118:9 Hz, JFF ˆ 37:0 Hz, JFF
ˆ
4:3 H, 1Fe). 1H NMR (CDCl3): d 0.25 (s). 13C NMR (CDCl3):
d1.2(s, CE);d124.6(dm, 1JCF ˆ 231:9 Hz, CB);d145.9(dm,
1JCF ˆ 265:3 Hz, CC); d 158.8 (ddm, JCF ˆ 283:2 Hz,
1
289.3 Hz, CA); d 163.3 (dm, 1JCF ˆ 292:7 Hz, CD). HRMS:
Calcd. for C7H9F5Si, 216.0394; Found: 216.0395; Calcd. for
12C613CH9F5Si, 217.0427; Found: 217.0403.
4.2.4. Preparation of (E)-1-tributylstannyl-1,2,3,4,4-
pentafluoro-1,3-butadiene (9), (E)-F2C=CF±CF=CFSnBu3
1
19F NMR and H NMR data for 1; cf. 4.2.3. 19F NMR
(CDCl3) data for
: d À112.3 (ddd, J ˆ 198:2,
22.3, 22.3 Hz, 1F); À116.0 (dm, J ˆ 198:4 Hz, 1F); À121.9
(m, 1F); À128.3 (ddd, J ˆ 11:8, 3.6, 3.6 Hz, 1F); À132.5
(dd, J ˆ 24:0, 4.2 Hz, 1F). 1H NMR (CDCl3): d 0.27 (s, 9H).
13CNMR: d 3.8 (s).
A 250 ml, three-necked, round-bottomed ¯ask equipped
with a PTFE-coated magnetic stirring bar, thermometer,
50 ml dropping funnel, rubber septum and nitrogen tee
was charged with KF (0.54 g, 9.2 mmol) and 20 ml of
benzene. Using either a heat gun or oil bath, traces of water
in the KF were removed by azeotropic distillation with
benzene. After removal of most of the liquids from the
mixture, the remaining mixture was subjected to full vacuum
until complete dryness was obtained. Then, 100 ml of dry
DMF and Bu3SnOSnBu3 (19.2 g, 32.3 mmol) were added to
the KF and the reaction mixture was cooled to ꢂ 0 8Cwith
an ice-water bath. Then a solution of 1 (10 g, 46.2 mmol) in
20 ml DMF was added dropwise (via the dropping funnel)
over 3±4 h. After the addition of 1 was completed, the ice-
water bath was removed, and the reaction mixture allowed to
warm to room temperature overnight (12 h). 19F NMR of the
reaction mixture veri®ed the reaction was completed. Any
volatile by-products and DMF were removed by ¯ash dis-
tillation under vacuum. The residue (after ¯ash distillation)
was directly applied to a silica gel column for separation
(Rf ˆ 0:6 in hexane). After column chromatography separa-
tion and evaporation of solvent, 13.4 g (30.8 mmol, 67%) of
9 was obtained as a colorless, transparent liquid. 19F NMR
4.2.3. Preparation of (E)-1-trimethylsilyl-1,2,3,4,4-
pentafluoro-1,3-butadiene (1),
(E)-F2C=CF-CF=CFSiMe3
A 1 l, three-necked, round-bottomed ¯ask equipped with a
PTFE-coated magnetic stirring bar, thermometer, 500 ml
pressure-equalized dropping funnel, rubber septum and nitro-
gen tee was charged with 11.6 g (10 mmol) of Pd(PPh3)4 and
400 ml of dry DMF. Then, iodotri¯uoroethene (41.6 g,
200 mmol, 18.2 ml) was added dropwise to the solution.
The solution was then slowly heated to 40 8C, and a clear
solution was obtained. The reaction mixture was cooled to
À5 to 0 8Cwith an ice-water bath. Copper(I) iodide (19.0 g,
100 mmol) was then added all at once. Then, a solution of (Z)-
1,2-di¯uoro-1-tributylstannyl-2-trimethylsilyl ethene (100 g,
235 mmol) in 300 ml DMF was transferred to the dropping
funnel and added dropwise (slowly) over ꢂ3 h. The reaction
mixture was stirred an additional 1±2 h at 0 8C ; then1 was
¯ash distilled under reduced pressure from the reac-
tion mixture. The ¯ash distillate was added to 300 ml of cold
2
3
(CDCl3): d À94.1 (dddd, JFF ˆ 53:7 Hz, JFF ˆ 28:0 Hz,
4JFF ˆ 11:5 Hz, JFF ˆ 2:5 Hz, 1Fa); d À109.2 (dddd,
5
3
4
5
2JFF ˆ 53:7 Hz, JFF ˆ 117:2 Hz, JFF ˆ 20:8 Hz, JFF
ˆ
b
5
3
2:9 Hz, 1F ); d À123.0 (m, JFF ˆ 2:5 Hz, JFF ˆ 2:8 Hz,
4JFF ˆ 1:2 Hz, JFF ˆ 2:8 Hz, JSnÀF ˆ 76:9 Hz (tin satel-
5
2
lite peaks: isotope 7.60% 117Sn and 8.58% 119Sn), 1Fd); d

26
C. Lim et al. / Journal of Fluorine Chemistry 119 (2003) 21±26
4
4
3
[13] P.A. Morken, R.F. Campbell, D.J. Burton, J. Fluorine Chem. 66
(1994) 81±85.
À127.6 (dddd, JFF ˆ 20:8 Hz, JFF ˆ 11:5 Hz, JFF
ˆ
ˆ
2:8 Hz, JFF ˆ 36:0 Hz, 1Fc); d À172.9 (dddd, JFF
3
3
[14] C.R. Davis, D.J. Burton, Tetrahedron Lett. 37 (1996) 7237±7240.
[15] C.R. Davis, D.J. Burton, J. Org. Chem. 62 (1997) 9217±9222.
[16] Q. Liu, D.J. Burton, Tetrahedron Lett. 41 (2000) 8045±8048.
[17] S.A. Fontana, C.R. Davis, Y.-B. He, D.J. Burton, Tetrahedron 52
(1996) 37±44 (Symposium in Print).
3
3
4
28:0 Hz, JFF ˆ 117:2 Hz, JFF ˆ 36:0 Hz, JFF ˆ 1:2 Hz,
1Fe).
[18] B.V. Nguyen, D.J. Burton, J. Org. Chem. 63 (1998) 1714±1715.
[19] B.V. Nguyen, D.J. Burton, J. Org. Chem. 62 (1997) 7758±7764.
[20] L. Xue, L. Lu, S.D. Pedersen, Q. Liu, R.M. Narske, D.J. Burton,
Tetrahedron Lett. 37 (1996) 1921±1924.
Acknowledgements
We thank the National Science Foundation for ®nancial
support of this work.
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Org. Chem. 62 (1997) 1064±1071.
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[25] Preparation and utility of a new fluorinated synthon, (Z)-1-tri-n-
butyl-stannyl-1,2,3,4,4-pentafluoro-1,3-butadiene, Presented in the
``Fluorine Award Symposium'' at the 219th American Chemical
Society National Meeting, San Francisco, CA, March 2000, Abstract
FLUO #38.
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