Stereoselective synthesis of (z)-perfluoroalkyl-1-iodo-1-triisopropyl silylalkenes: A convenient route to perfluoroalkyl-substituted unsaturated synthons

Article abstract of DOI:10.1080/00397910903161843

The quantitative and fully stereoselective synthesis of (Z)-perfluoroalkyl-1-iodo-1-triisopropylsilylalkenes and their easy, high-yielding transformation into the corresponding 1-iodo-2-perfluoroacetylenes and (Z)-2-perfluoroalkyl-1-iodo-1-alkenes, compounds of interest as synthons for classical organometallic chemistry or for metal-catalyzed coupling reactions, are discussed. Copyright

Full text of DOI:10.1080/00397910903161843

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Stereoselective Synthesis of (Z)-
Perfluoroalkyl-1-iodo-1-triisopropyl
Silylalkenes: A Convenient Route to
Perfluoroalkyl-Substituted Unsaturated
Synthons
Juan Francisco Chesa a , Dolores Velasco a & Francisco López-
Calahorra a
a Departament de Química Orgànica, Institut de Nanociéncies i
Nanotecnologia, Universitat de Barcelona, Barcelona, Spain
Published online: 17 May 2010.
To cite this article: Juan Francisco Chesa , Dolores Velasco & Francisco López-Calahorra (2010):
Stereoselective Synthesis of (Z)-Perfluoroalkyl-1-iodo-1-triisopropyl Silylalkenes: A Convenient Route
to Perfluoroalkyl-Substituted Unsaturated Synthons, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 40:12, 1822-1829
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Synthetic Communications1, 40: 1822–1829, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903161843
STEREOSELECTIVE SYNTHESIS OF
(Z)-PERFLUOROALKYL-1-IODO-1-TRIISOPROPYL
SILYLALKENES: A CONVENIENT ROUTE TO
PERFLUOROALKYL-SUBSTITUTED
UNSATURATED SYNTHONS
Juan Francisco Chesa, Dolores Velasco,
´
and Francisco Lopez-Calahorra
´
Departament de Quımica Organica, Institut de Nanociencies i
Nanotecnologia, Universitat de Barcelona, Barcelona, Spain
`
´
The quantitative and fully stereoselective synthesis of (Z)-perfluoroalkyl-1-iodo-1-
triisopropylsilylalkenes and their easy, high-yielding transformation into the corresponding
1-iodo-2-perfluoroacetylenes and (Z)-2-perfluoroalkyl-1-iodo-1-alkenes, compounds of
interest as synthons for classical organometallic chemistry or for metal-catalyzed coupling
reactions, are discussed.
Keywords: Perfluoroacetylenes; stereoselective synthesis; (Z)-1-iodoperfluoroalkenes
INTRODUCTION
The introduction of fluorine atoms into organic compounds significantly
changes the physical and chemical properties of these molecules. Partially fluorinated
organic compounds, particularly those containing perfluoroalkyl groups, have
widespread applications[1] as reagents for materials used in different branches of tech-
nology and in the design of medical and agricultural preparations.[2] Consequently, the
development of new or improved synthetic procedures for the introduction of poly-
fluorinated fragments into organic molecules has recently attracted much interest.
In a previous work,[3] we described a new group of 3-perfluoroalkyl-2-propyn-1-
ol acrylates, Rf(n)-CꢀC-CH2-OCO-CH¼CH2 [whereby Rf(n) is F(CF2)n], and their
subsequent conversion into the corresponding polymers. The 3-perfluoroalkyl-
2-propyn-1-ols were prepared via a three step-reaction procedure: first, addition of
perfluoroalkyl iodide to trimethylsilylacetylene yielded a perfluoalkyl-1-iodo-1-
trimethylsilylalkene; second, HI and trimethylsilyl group were simultaneously elimi-
nated with KtBuO; and last, the resulting terminal alkyne was reacted with BuLi and
paraformaldehyde. The main drawback of this method is that although the addition
Received May 26, 2009.
Address correspondence to Francisco Lopez-Calahorra, Departament de Quımica Organica,
´
Institut de Nanociencies i Nanotecnologia, Universitat de Barcelona, Martı i Franques 1-11, 08028,
´
`
´
Barcelona, Spain. E-mail: jfchesa@hotmail.com
´
`
1822

(Z)-PERFLUOROALKYL-1-IODO-1-TRIISOPROPYL SILYLALKENES
1823
of perfluoroalkyl iodide to trimethylsilylacetylene in a one-electron-transfer process
affords an equimolar mixture of Z=E-isomeric alkenes in good yield, in the next step
only the Z-isomer eliminates HI by an E2 process; in E1 conditions, both isomers
disappear but the yield in acetylene is less than 50%. Consequently, despite excellent
yields for the first and the third steps, the maximum possible global yield is 40%.
An alternative method[4] is the addition of perfluoroalkyl iodides to several
terminal alkynes at 10 to 15 ꢁC in aqueous acetonitrile in the presence of sodium
dithionite and sodium bicarbonate in a fully regioselective process. However, this
also leads to a mixture of Z=E adducts of the general formula Rf(n)-CH¼
CICRR0OH. The corresponding 3-perfluoroalkyl-2-propyn-1-ols are formed by sub-
sequent elimination of HI[5] from the Z-isomer in basic medium. When we employed
this method to form 3-perfluoroalkyl-2-propyn-1-ols starting from propargyl alcohol
and perfluoroalkyl iodides with four, six, or eight carbon atoms, we obtained
maximum global yields of 30%. Again, the problem is the lack of stereoselectivity
in the addition step.
In fact, this problem was described by Brace and Van Elswyk[6] several
years ago. They reported a Z=E ratio of 10:90 for the addition of 1-iodo-
heptafluoropropane to 1-hexyne to yield 1-perfluoropropyl-1-hexene and a Z=E
ratio of 30.8:69.2 for the analogous addition to 1-heptyne.
We report here a quantitative and fully stereoselective synthesis of (Z)-
perfluoroalkyl-1-iodo-1-triisopropylsilylalkenes. We also describe their ready trans-
formation into unsaturated synthons for the preparation of fluorinated molecules
via classical organometallic chemistry or metal-catalyzed coupling reactions.
RESULTS AND DISCUSSION
As previously explained, the problem in the addition reaction of perfluoroalkyl
iodides to alkynes is the stereoselectivity. This process consists of the direct incorpor-
ation of perfluoroalkyl radicals, generated in situ, into appropriate acetylenes by a
single-electron-transfer process (Scheme 1).
We thought that the geometry of the addition compound must be controlled by
the steric interaction between the perfluoroalkyl group and the R group. Thus we
tried adding various perfluoroalkyl iodides to several acetylenes. This reaction has
been widely studied,[7] and its radical process can been initiated photochemically,[8]
by radical initiators,[9] thermally,[10] or electrochemically.[11] We chose zinc (Zn;
purity ꢂ99%) as the electron donor[5,12] in freshly distilled methylene chloride, owing
Scheme 1. Mechanism of the addition of perfluoroalkyl radicals to acetylenes in a one-electron-transfer
reaction.

´
J. F. CHESA, D. VELASCO, AND F. LOPEZ-CALAHORRA
1824
to the easy purification of the crude product: only filtration and solvent evaporation
are required. The results obtained with several substituted alkynes are summarized in
Table 1.
The reaction proceeded with good yield in all cases. The stereoselectivity of the
process shows two opposite trends, depending on the substituent of the alkyne.
When the radical is hydrocarbonated and=or oxygenated, the E-isomer dominates;
however, with silyl derivatives, volume seems to dictate the isomeric ratio: the bulky
triisopropylsilyl group led to the Z-isomer in practically quantitative ratio. The
1
geometry of the isolated products was assigned by H NMR [in a one-dimensional
nuclear Overhauser effect spectroscopy (1D-NOESY) experiment]: in the Z-isomer,
the signal at 6.86 ppm corresponding to the olefinic proton shows nOe with the
tertiary proton in the isopropyl groups, whereas in the E-isomer, this effect is not
Table 1. Addition of perfluorinated perfluoroalkyl iodides to terminal alkynes
Alkyne
Producta
Yield (%)b
Z=E ratioc
87
45=55
87
25=75
89
93
95
82
94
11=89
50=50
50=50
50=50
Only Z detected
99
88
98=2
Only Z detected
aRf(n) ¼ perfluorinated chain of n carbon atoms.
bCalculated as isolated isomer mixture.
cCalculated from the 1H or 19F NMR spectra.

(Z)-PERFLUOROALKYL-1-IODO-1-TRIISOPROPYL SILYLALKENES
1825
Scheme 2. Two desilylation methods for (Z)-perfluoroalkyl-1-iodo-1-triisopropylsilylalkenes.
observed. Likewise, the 19F NMR spectra also enabled identification of each isomer:
the signals at approximately ꢃ109 ppm and ꢃ113 ppm correspond to the Z- and
E-isomers, respectively.
Desilylation can be achieved in two ways (Scheme 2). When a solution of
silylated Z-alkene in anhydrous ether was added to methanolic potassium hydroxide
at 0 ꢁC and the mixture was stirred for 1 h at room temperature, only (Z)-2-
perfluoroalkyl-1-iodoalkene was formed (based on the 19F NMR spectra) in practi-
cally quantitative yield. Silylate residue by-products had to be carefully removed by
column chromatography. An average yield of 70% was obtained for the isolated
products. The geometry of the pure compounds was determined based on the
1H NMR data: for the Z-isomer, the coupling constant JH1-H2 between the olefinic
protons is 7.2 Hz, whereas for the E-isomer, it is 14.8 Hz. Again, 19F NMR enables
identification of each isomer: the Z-compounds present a signal at approximately
ꢃ108 ppm, whereas the E-compounds present a signal at approximately ꢃ114 ppm.
The pure isomers of these compounds are interesting; some of these
compounds are currently only commercially available as mixtures of the Z- and
E-isomers.
Treatment of the silylated Z-alkene with tetrabutylammonium fluoride in
tetrahydrofuran (THF) for 2 h at room temperature, followed by addition of iodine
after 2 more h, provided the 2-perfluoroalkyl-1-iodo-acetylene in a yield of 70%,
which is twice the yield of previously described methods.[3,4,5]
Both (Z)-2-perfluoroalkyl-1-iodoalkenes and 2-perfluoroalkyl-1-iodo-acetylenes
are very interesting as synthons for classical organometallic chemistry or for metal-
catalyzed coupling reactions.
EXPERIMENTAL
General Methods
Infrared (IR) spectra were registered using a Perkin-Elmer Spectrum-One
instrument. 19F NMR, 1H NMR, and 13C NMR spectra were recorded on a Varian
Mercury-400 spectrometer with tetramethylsilane (TMS; 1H) and trifluoroacetic acid
(TFA; 19F) as internal standards. The purifications of the reactions were monitored
by gas chromatography=mass spectrometry (GC=MS) in a ThermoFinnigan Trace
DSQ instrument coupled with a Trace GC gas chromatograph. The molecular
weights of all prepared compounds were determined by MS spectrometry techniques;
the MS were determined by electronic impact (EI, 70 eV) and the high-resolution MS

´
J. F. CHESA, D. VELASCO, AND F. LOPEZ-CALAHORRA
1826
(HMRS) spectra have been recorded in an L=CMSD-TOF instrument (Agilent
Technologies).
General Procedure for the Addition of Iodoperfluoroalkanes to Alkynes
Zinc (5.5 g, 75.8 mmol), 1-iodoperfluoroalkane (75.5 mmol), and triisopropylsi-
lylacetylene (17 mL, 75 mmol) were added to 20 mL of CH2Cl2. The mixture was
boiled and stirred for 2 h, then filtered, and the solvent was removed under low
pressure to provide (Z)-perfluoro-1-iodo-1-triisopropylsilyl-1-alkene (see Table 1
for yield).
(Z)-3,3,4,4,5,5,6,6,6-Nonafluoro-1-iodo-1-triisopropylsilyl-1-hexene.
Yellow oil. IR (film, t, cmꢃ1): 1660 (st C¼C), 1147 (st C-F), 882 (st Si-C). 1H NMR
(400 MHz, CDCl3, d ppm): 6.86 (1H, t, JH-F ¼ 13.3 Hz, H alkene), 1.41 (3H, m, CH
isopropyl), 1.14 (18H, d, J ¼ 7.4 Hz, CH3 isopropyl). 19F NMR (376 MHz, CDCl3, d
ppm): ꢃ84.42 (3F, t, J ¼ 9.4 Hz, F6), ꢃ112.53 (2F, q, JH-F ¼ 12.3 Hz, F3), ꢃ127.22
(2F, m, F4), ꢃ129.20 (2F, m, F5). MS (EI): 528 (M); 485 [M -CH(CH3)2], 359
[M -(CF2)2CF3].
(Z)-3,3,4,4,5,5,6,6,7,7,8,8,8-Tridecafluoro-1-iodo-1-triisopropylsilyl-1-
octene. Yellow oil. IR (film, t, cmꢃ1): 1661 (st C¼C); 1146 (st C-F), 882 (st Si-C).
1H NMR (400 MHz, CDCl3, d ppm): 6.86 (1H, t, JH-F ¼ 12.0 Hz, H alkene), 1.43
(3H, m, CH isopropyl), 1.14 (18H, d, J ¼ 8.0 Hz, CH3 isopropyl). 19F NMR
(376 MHz, CDCl3, d ppm): ꢃ83.35 (3F, m, F8), ꢃ111.41 (2F, q, JH-F ¼ 12.9 Hz,
F3), ꢃ124.18 (2F, m, F4), ꢃ125.35 (4F, m, F5 and F6), ꢃ128.63 (2F, m, F7). MS
(EI): 628 (M); 585 [M -CH(CH3)2], 359 [M -(CF2)4CF3]. HRMS: 628.3261.
(Z)-3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-Heptadecafluoro-1-iodo-1-triiso-
propylsilyl-1-decene. Yellow oil. IR (film, t, cmꢃ1): 1661 (st C¼C), 1151 (st C-F),
879 (st Si-C). 1H NMR (400 MHz, CDCl3, d ppm): 6.86 (1H, t, JH-F
¼
13.3 Hz, H2), 1.41 (3H, m, H10), 1.14 (18H, d, J ¼ 7.5 Hz, H20). 19F NMR
(376 MHz, CDCl3, d ppm): ꢃ83.65 (3F, t, J ¼ 9.8 Hz, F10), ꢃ111.55 (2F, q, JHF
¼
13.0 Hz, F3), ꢃ124.29 (2F, m, F4), ꢃ124.77 (4F, m, F5 and F6), ꢃ125.61 (4F, m,
F7 and F8), ꢃ128.98 (2F, m, F9). MS (EI) 728 (M), 685 [M-CH(CH3)2], 359
[M -(CF2)6CF3].
General Procedure for the Preparation of 1-Iodo-2-perfluoroacetylenes
The corresponding (Z)-perfluoro-1-iodo-1-triisopropylsilyl-1-alkene (23 mmol)
was dissolved in anhydrous Et2O (23 mL). Under an N2 atmosphere, 1 M Bu4NF
(69 mL) in THF was added dropwise while the solution was maintained at 0 ꢁC.
The reaction mixture was stirred for 2 h at room temperature. Iodine (11.7 g, 46 mmol)
was added, and the mixture was stirred for an additional 2 h. The mixture was then
diluted with Et2O (250 mL) and washed once with Na2S2O3 1 M (300 mL), twice with
saturated aq. NaCl. (300 mL), and once with H2O (300 mL). The organic layer was
dried over anhydrous MgSO4 and then filtered, and the solvent was removed under
reduced pressure. The crude was purified by flash silica-gel chromatography (hexane
as eluent) to obtain the corresponding 1-iodo-2-perfluoroacetylene (yield: 70 to 75%).

(Z)-PERFLUOROALKYL-1-IODO-1-TRIISOPROPYL SILYLALKENES
1827
3,3,4,4,5,5,6,6,6-Nonafluoro-1-iodo-1-hexyne. Yellow oil. IR (film, t, cmꢃ1):
2196 (st CꢀC), 1148 (st C-F). 19F NMR (376 MHz, CDCl3, d ppm): ꢃ83.81 (3F, t,
J ¼ 9.5 Hz, F6), ꢃ100.22 (2F, m, F3), ꢃ125.91 (2F, m, F4), ꢃ128.28 (2F, m, F5). MS
(EI): 370 (M), 351 (M -F), 201 (M -CF2CF2CF3), 127 (I); 69 (CF3).
3,3,4,4,5,5,6,6,7,7,8,8,8-Tridecafluoro-1-iodo-1-octyne. Yellow oil. IR (film,
t, cmꢃ1): 2196 (st CꢀC), 1146 (st C-F). 19F NMR (376 MHz, CDCl3, d ppm): ꢃ83.68
(3F, m, F8), ꢃ100.03 (2F, m, F3), ꢃ124.24 (2F, m, F4), ꢃ125.11 (2F, m, F5),
ꢃ125.71 (2F, m, F6), ꢃ129.01 (2F, m, F7). MS (EI): 470 (M), 201 [M -(CF2)4CF3],
69 (CF3). HRMS: 469.8835.
3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-Heptadecafluoro-1-iodo-1-decyne.
Yellow oil. IR (film, t, cmꢃ1): 2198 (st CꢀC), 1151 (st C-F). 19F NMR (376 MHz,
CDCl3, d ppm): ꢃ83.34 (3F, t, J ¼ 9.9 Hz, F10), ꢃ99.70 (2F, m, F3), ꢃ123.70
(2F, m, F4), ꢃ124.45 (4F, m, F5 and F6), ꢃ124.75 (2F, m, F7), ꢃ125.29 (2F, m,
F8), ꢃ128.67 (2F, m, F9). MS (EI): 570 (M); 201 [M -(CF2)6CF3], 69 (CF3).
General Procedure for the Preparation of (Z)-2-Perfluoroalkyl-1-iodo-1-
alkenes
Potassium hydroxide (KOH; 779 mg, 13.1 mmol) was dissolved in MeOH
(3 mL). The solution was cooled to 0 ꢁC under an N2 atmosphere, and the corre-
sponding (Z)-perfluoro-1-iodo-1-triisopropylsilyl-1-alkene (4.4 mmol) in anhydrous
Et2O (4 mL) was added dropwise. The reaction mixture was stirred for 1 h at room
temperature, diluted with Et2O (20 mL), and washed twice with H2O (20 mL). The
organic layer was dried over anhydrous MgSO4 and then filtered, and the solvent
was removed to afford a crude reaction mixture with silylate residue by-products.
The crude was purified by silica-gel chromatography (hexane as eluent) to obtain
(Z)-2-perfluoroalkyl-1-iodo-1-alkene (yield: 68 to 75%).
(Z)-3,3,4,4,5,5,6,6,6-Nonafluoro-1-iodo-1-hexene. Yellow oil. IR (film, t,
1
cmꢃ1): 1559 (st C¼C); 1238 (st C-F). H NMR (400 MHz, CDCl3, d ppm): 6.42
(1H, dt, JH1-F ¼ 2.1 Hz, JH1-H2 ¼ 7.2 Hz, H1), 4.51 (1H, dt, JH1-H2 ¼ 7.2 Hz, JH2-F
¼
14.6 Hz, H2). 19F NMR (376 MHz, CDCl3, d ppm): ꢃ81.56 (3F, t, J ¼ 9.7 Hz, F6),
ꢃ106.72 (2F, q, J ¼ 13.5 Hz, F3), ꢃ125.16 (2F, m, F4), ꢃ125.18(2F, m, F5). MS
(EI): 372 (M), 303 (M -CF3), 203 (M -CF2CF2CF3), 69 (CF3).
(Z)-3,3,4,4,5,5,6,6,7,7,8,8,8-Tridecafluoro-1-iodo-1-octene. Yellow oil. IR
1
(film, t, cmꢃ1): 1558 (st C¼C); 1238 (st C-F). H NMR (400 MHz, CDCl3, d ppm):
6.39 (1H, dt, JH1-F¼ 2.0 Hz, JH1-H2 ¼ 7.2 Hz, H1), 4.49 (1H, dt, JH1-H2 ¼ 7.2 Hz,
JH2-F ¼ 14.8 Hz, H2). 19F NMR (376 MHz, CDCl3, d ppm): ꢃ83.66 (3F, t,
J ¼ 9.8 Hz, F8), ꢃ108.92 (2F, q, J ¼ 13.5 Hz, F3), ꢃ124.61 (2F, m, F4), ꢃ125.74
(2F, m, F5), ꢃ126.70 (2F, m, F6), ꢃ128.96 (2F, m, F7). MS (EI): 472 (M), 403
(M -CF3), 203 (M -CF2CF2CF2CF2CF3), 69 (CF3). HRMS: 471.8995.
3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-Heptadecafluoro-1-iodo-1-decene.
1
Yellow oil. IR (film, t, cmꢃ1): 1560 (st C¼C), 1239 (st C-F). H NMR (400 MHz,
CDCl3, d ppm): 6.39 (1H, dt, JH1-F¼ 2.0 Hz, JH1-H2 ¼ 7.2 Hz, H1), 4.48 (1H, dt,
JH1-H2 ¼ 7.1 Hz, JH2-F ¼ 14.7 Hz, H2). 19F NMR (376 MHz, CDCl3, d ppm): ꢃ83.21

´
J. F. CHESA, D. VELASCO, AND F. LOPEZ-CALAHORRA
1828
(3F, t, J ¼ 9.7 Hz, F10), ꢃ108.50 (2F, q, J ¼ 13.5 Hz, F3), ꢃ123.97 (2F, m, F4),
ꢃ124.35 (2F, m, F5), ꢃ124.37 (2F, m, F6), ꢃ125.16 (2F, m, F7), 126.24 (2F, m, F8),
128.14 (2F, m, F9). MS (EI): 572 (M), 503 (M -CF3), 203 (M -CF2CF2CF2CF2CF3),
69 (CF3).
ACKNOWLEDGMENT
´
Financial support for this work from the Ministerio de Educacion y Ciencia
(Spain), through project CTQ2006-15611-C02-02=BQU, is gratefully acknowledged.
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