Carbon-fluorine bond activation in fluoroolefins: Clear documentation of cooperative C-F bond activation by adjacent metal centers

Article abstract of DOI:10.1002/anie.200603972

(Chemical Equation Presented) F off: Fluoroolefins bridging a pair of iridium centers are susceptible to abstraction of a fluoride ion while those bound to a single metal center are not. The resulting bridging fluorovinyl groups also undergo fluoride abstraction to yield the corresponding vinylidenes, whereas terminal fluorovinyl groups are unreactive. Selective replacement of one fluorine substituent by either hydrogen or a methyl group can be effected (see scheme, Tf = trifluoromethanesulfonyl).

Full text of DOI:10.1002/anie.200603972

Angewandte
Chemie
DOI: 10.1002/anie.200603972
À
C F Activation
Carbon–Fluorine Bond Activation in Fluoroolefins: Clear
À
Documentation of Cooperative C F Bond Activation by Adjacent
Metal Centers**
D. Jason Anderson, Robert McDonald, and Martin Cowie*
One of the most exciting developments in organometallic
chemistry over the past few decades has been the activation of
otherwise inert chemical bonds by transition-metal com-
plexes.[1] In spite of substantial developments in this area,
many significant challenges remain, including the activation
of the very strong carbon–fluorine bond.[2] Studies on the
We previously established[7] that [Ir2(CH3)(CO)2(dppm)2]
[CF3SO3] (1; dppm = m-Ph2PCH2PPh2) yields fluoroolefin-
bridged products with olefins containing at least one pair of
geminal fluorine substituents (for example, 1,1-difluoro-
ethylene, trifluoroethylene, and tetrafluoroethylene) as
shown in Scheme 1.
À
activation of C F bonds are being driven, not only by the
fundamental challenges of effecting and understanding the
processes involved, but also by the important applications of
fluorocarbons in areas such as pharmaceuticals, pesticides,
polymers, and refrigerants,[3] as well as by the significant
challenges of converting potentially harmful chlorofluoro-
carbons into more environmentally benign compounds.[4]
A wide variety of transition-metal complexes have been
[2]
À
used to activate C F bonds, but in almost all cases these
approaches have been limited to the activation at a single
metal center. We are aware of only two studies in which
À
activation of C F bonds has been observed for bridging
fluorocarbyl ligands.[5,6]
Fluoroolefins bound in the bridging site between a pair of
metal centers can be viewed as resulting from the addition of
the dimetal unit across the olefinic bond to generate a 1,2-
dimetalated fluoroalkane with concomitant rehybridization
of the olefinic carbon atoms to sp3, as confirmed in the
structures of C2F4-bridged diiron[5] and diiridium[7] com-
pounds. In such a geometry the bridging fluoroolefin can be
viewed as analogous to two fluoroalkyl groups, for which the
lability of the a-fluorine atoms towards abstraction of a
fluoride ion has been demonstrated.[8] On this basis we
suggest that bridging fluoroolefins should also be susceptible
to abstraction of a fluoride ion by Lewis acids.
À
Scheme 1. Activation of C F bonds in bridging fluoroolefins. In all
the schemes the phenyl substituents on the phosphorus atoms are
omitted and the triflate counterions are not shown, while coordinated
triflate ions are shown as OTf.
Abstraction of a fluoride ion from compounds 2–4, to give
the respective fluorovinyl products 5–7 quantitatively, is
achieved by the addition of trimethylsilyl triflate at À208C.
Spectral data for all the compounds are given in the
Experimental Section and additional characterization data
are available in the Supporting Information. Although the
difluorovinyl-bridged product 6 is stable upon warming to
ambient temperature, the trifluorovinyl-bridged species 5
rearranges to the h1-trifluorovinyl product 8 at this temper-
Although olefin coordination at one metal center also
gives rise to some rehybridization, we suggest that it is less
than when it is bridging, and set out to determine if the
fluoroolefin bridging mode would result in enhanced activa-
À
tion of the C F bond.
[*] D. J. Anderson, Dr. R. McDonald, Prof. M. Cowie
Department of Chemistry
University of Alberta
Edmonton, AB T6G2G2 (Canada)
Fax: (+1)780-492-8231
À
ature, accompanied by activation of a C H bond of the
methyl group. The structure of 8 has been determined by X-
ray crystallography; although it is disordered, the connectivity
shown in Scheme 1 is clear (see the Supporting Information).
The differentiation between terminal and bridging fluo-
rovinyl groups is made on the basis of the coupling patterns
involving the vinyl substituents in the NMR spectra. For
E-mail: martin.cowie@ualberta.ca
[**] This work was supported by the Natural Sciences and Engineering
Research Council (NSERC) of Canada through grants to M.C. and a
fellowship to D.J.A.; and by the University of Alberta. We wish to
thank Professor Robin Perutz for an advance copy of Ref. [2e].
example, the different fluorine–fluorine coupling constants
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
2
for the h1-trifluorovinyl group in 8 (3Jtrans = 115 Hz, Jgem
=
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3
90 Hz, Jcis = 40 Hz) are consistent with previous determina-
tions[9] and are all greater than the respective values for the
bridging group in 5 (100 Hz, 85 Hz, 20 Hz). Furthermore, in 8
the lone a-fluorine atom on the vinyl group displays coupling
to only the pair of adjacent 31P nuclei as well as to the adjacent
13CO and m-13CH2 groups, while the corresponding fluorine
atom in 5 displays coupling to the carbonyl groups on both
metal centers and to all four 31P nuclei, further supporting a
bridged arrangement for the latter compound. The decrease
in the coupling constants in the bridging mode has been noted
for perprotiovinyl moieties.[10] We are aware of only one
previous report of a bridging fluorovinyl ligand proposed on
the basis of IR spectroscopy;[11] no 19F NMR data were given
for this species.
Scheme 3. Comparison of the reactivity of m-h1:h2-C2F3 versus h1-C2F3.
Although the tetra- and trifluoroethylene adducts 2 and 3
are the only species observed at temperatures above À608C,
the 1,1-difluoroethylene adduct can be obtained as a mixture
of two isomers (4 and 4a) at À208C (Scheme 2).[7] The
enhancement in the reactivity over that of the group bound
to only one metal center. Facile removal of a fluoride ion from
the cis-difluorovinyl- and 1-fluorovinyl-bridged species 6 and
7, respectively, is also achieved to give the corresponding
monofluorovinylidene- and vinylidene-bridged products
=
[Ir2(CH3)(CF3SO3)(CO)2(m-C CHX)(dppm)2][CF3SO3]2
(X = F (10), H (11)).
Having converted the three fluoroolefins into the corre-
sponding fluorovinyl groups, it was of interest to attempt the
conversion of the latter into substituted fluoroolefins, thereby
completing the cycle for functionalization of the starting
fluoroolefins. This has been achieved in two ways: Reaction of
the di- and trifluorovinyl complexes 6 and 8 with H2 for
24 hours yields the corresponding olefins, cis-difluoroethyl-
ene and trifluoroethylene (Scheme 4), together with the
=
Scheme 2. Comparison of the reactivity of m-F2C CH2 versus
2
=
h -F2C CH2.
simultaneous appearance of both species with either bridging
or terminal olefin arrangements allows their reactivity to be
directly compared. Addition of trimethylsilyl triflate to a 2:1
mixture of 4 and 4a at À408C results in an instantaneous and
quantitative conversion of 4 into 7, while 4a remains
unchanged. This observation is the first clear documentation
of an enhanced reactivity of a bridging fluoroolefin over the
terminal (h2) binding mode.
Scheme 4. Hydrogenolysis of fluorovinyl groups.
We wondered whether the unusual bridging fluorovinyl
coordination mode would also give rise to greater reactivity
than that of the h1-binding mode so we set out to compare the
reactivity of these groups in compounds 5 and 8. The
attempted reactions of these species with Me3Si(SO3CF3)
are summarized in Scheme 3. Whereas Lewis acid promoted
abstraction of a fluoride ion from 5 occurs readily at
temperatures below 08C to yield the difluorovinylidene
known dicationic tetrahydride complex.[12] No evidence of
fluoroalkanes, which result from olefin hydrogenation, is
observed. Although compound 8 yields trifluoroethylene
quantitatively, 6 yields the expected cis-difluoroethylene
together with cis-difluoropropene in a 2:1 ratio; the latter
product results from a similar reductive elimination similar to
that described below in Scheme 5. Furthermore, the di- and
monofluorovinyl species 6 and 7 react with CO over 24 hours
to give cis-difluoropropene and 2-fluoropropene, respectively,
=
product
[Ir2(CH3)(CF3SO3)(CO)2(m-C CF2)(dppm)2]
[CF3SO3]2 (9), no reaction of 8 was observed at temperatures
up to 258C. Again, interaction of the unsaturated fluorocarbyl
unit with both metal centers gives rise to a significant
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where noted. In all cases, 31P and 13C NMR spectra are reported with
broadband 1H decoupling.
À
In a typical C F bond activation experiment, the fluoroolefin-
bridged complex (2, 3, or 4; 50 mg) was prepared in CH2Cl2 as
previously reported.[7] One equivalent of Me3Si(SO3CF3) was added
dropwise and the mixture stirred for 30 min. The resulting fluoro-
vinyl-bridged species (5, 6, or 7) was treated with an additional
equivalent of Me3Si(SO3CF3) to produce the vinylidene complexes (9,
10, or 11).
In a typical hydrogenation experiment, hydrogen gas (1–5 equiv)
was added slowly by a gas-tight syringe to a solution of 6 or 8 (50 mg)
in CD2Cl2 (0.7 mL) in an NMR tube at À788C. In the carbonylation
experiments, excess carbon monoxide gas was added slowly by a gas-
tight syringe to a solution of 6 or 7 (50 mg) in CD2Cl2 (0.7 mL) in an
NMR tube at À788C. NMR spectra (1H, 31P, 19F, and 13C) were
recorded from À808C to ambient temperature at 208C intervals.
Identification of the fluoroolefin products was established by
Scheme 5. Formation of fluoropropenes by reductive elimination of
fluorovinyl and methyl groups.
together with the known dicationic pentacarbonyl product[13]
(Scheme 5). The trifluorovinyl compounds 5 and 8 do not
liberate trifluoropropene upon exposure to CO.
comparison of the spectra to those of the known compounds.[18]
A
more complete description is given in the Supporting Information.
2
5: 31P NMR: d = 13.0 (t, 2JPP = 24 Hz, 2P), À8.0 ppm (t, JPP
=
Although, in principle, the stepwise conversions of
fluoroolefins into vinylidene-bridged species, as described
above, could lead to substitution of a pair of geminal fluorine
atoms, we have not yet succeeded in effecting such trans-
formations. All vinylidene species 9–11 are unreactive
towards H2 and react with CO to only give replacement of
the coordinated triflate group.
1
24 Hz, 2P); H NMR: d = 4.5 (brs, 2H), 3.6 (brs, 2H), 1.4 ppm (dt,
3JHP = 5.6, 5JHF = 6.0 Hz, 3H); 13C NMR: d = 161 (m), 178 (m),
À19 ppm (d, 4JCF = 13 Hz,); 19F NMR: d = À80 (brdd, 2JFF = 85,
2
3
3JFF = 20 Hz, 1F), À120 (dd, JFF = 85, JFF = 100 Hz, 1F), À131 (dd,
3JFF = 20, 3JFF = 100 Hz, 1F), À76 ppm (s, 3F). Spectra were recorded
at À208C.
6: 31P NMR: d = 9.0 (m, 1P), À2.5 (m, 2P), À19.0 ppm (m, 1P);
1H NMR: d = 6.0 (dd, 2JHF = 65, 3JHF = 10 Hz, 1H), 6.1 (brs, 1H), 5.6
(brs, 1H), 5.5 (m, 1H), 4.7 (m, 1H), 1.2ppm (t, 3JHP = 6.0 Hz, 3H);
13C NMR: d = 165 (brm), 172(brm), À23 ppm (brs); 19F NMR: d =
À23 (dd, 3JFF = 35, 3JFH = 10 Hz, 1F), À171 (dd, 3JFF = 35, 2JFH = 65 Hz,
1F), À77 ppm (s, 3F).
As noted in the most recent review on the subject,[2e]
À
“There are remarkably few examples of C F activation of
coordinated fluoroalkenes or fluorovinyl complexes.” Herein
À
we have demonstrated facile activation of a C F bond
7: 31P NMR: d = 1.6 (dt, 2JPP = 18.5 Hz, 3JPF = 45 Hz, 2P),
À5.5 ppm (t, 2JPP = 18.5 Hz, 2P); 1H NMR: d = 6.0 (d, 2JHH = 6 Hz,
involving both types of fluorocarbyl groups in what we
believe to be the first such study involving a closely related
series of fluoroolefins and their derived fluorovinyl groups.
One pivotal finding in this study is the greatly enhanced
reactivity of these groups when bridging a pair of metal
centers instead of being bound to a single metal center. As
such, this study also represents one of the few clear
demonstrations of metal–metal cooperativity in substrate
activation.
2
3
1H), 5.4 (dd, JHH = 6, JHF = 14 Hz, 1H), 3.1 (m, 2H), 4.3 (m, 2H),
0.2ppm (t, 3JHP = 5.6 Hz, 3H); 13C NMR: d = 164 (brd, 3JCF = 58 Hz),
172(t, 2JCP = 11 Hz), 5 ppm (s); 19F NMR: d = À211 (brt, 3JFP = 45 Hz,
1F), À77 ppm (s, 3F). Spectra were recorded at À208C.
2
2
8: 31P NMR: d = À0.5 (t, JPP = 26 Hz, 2P), À28.5 ppm (t, JPP
=
26 Hz, 2P); 1H NMR: d = 6.4 (q, 3JHP = 6.0 Hz, 2H), 5.0 (m, 4H),
À12.2 ppm (brs, 1H); 13C NMR: d = 164 (dt, 2JCP = 11 Hz, JCF
=
4
22 Hz), 166 (brm), 38 ppm (brs); 19F NMR: d = À92(dd, 2JFF = 90,
2
3
3JFF = 40 Hz, 1F), À121 (dd, JFF = 90, JFF = 115 Hz, 1F), À123 (dd,
À
Although selective activation of a single C F bond and
3JFF = 40, 3JFF = 115 Hz, 1F), À76 ppm (s, 3F).
9: 31P NMR: d = À6.2(t, 2JPP = 21 Hz, 2P), À20.6 ppm (m, 2JPP
=
=
subsequent functionalization has been observed with
fluoroarenes,[14] it has not previously been observed with
fluoroolefins. Unlike recent studies[15] in which multiple
hydrodefluorinations of fluoroolefins has occurred, our
system allows regioselective replacement of a single fluorine
atom by a hydrogen atom to give trifluoroethylene from
tetrafluoroethylene, as well as cis-difluoroethylene from
trifluoroethylene. It is also believed to be the first report of
the regioselective transformation of fluoroethylene molecules
into the respective fluoropropenes by replacement of a
fluorine atom by a methyl group.
21 Hz, 2P); 1H NMR: d = 4.1 (m, 2H), 2.8 (m, 2H), 1.9 ppm (t, JHP
3
9.0 Hz, 3H); 13C NMR: d = 151 (brs), 174 (brs), 38 ppm (s);
19F NMR: d = À69 (d, JFF = 97 Hz), À86 (d, JFF = 97 Hz), À76 ppm
2
2
(s, 3F).
10: 31P NMR: d = À5.0 (t, JPP = 23 Hz, 2P), À21.0 ppm (t, JPP
=
2
2
1
2
23 Hz, 2P); H NMR: d = 8.6 (d, JHF = 85 Hz, 1H), 4.3 (m, 2H), 2.8
(m, 2H), 2.15 ppm (t, 3JHP = 9.0 Hz, 3H); 13C NMR: d = 153 (t, 2JCP
12.5 Hz), 176 (brs), 41 ppm (s); 19F NMR: d = À107 (d, 2JFH = 85 Hz),
À76 ppm (s, 3F).
=
11: 31P NMR: d = À10.0 (t, 2JPP = 17 Hz, 2P), À20.0 ppm (t, 2JPP
=
17 Hz, 2P); 1H NMR: d = 6.01 (s, 1H), 5.99 (s, 1H), 4.1 (m, 2H), 2.9
(m, 2H), 1.25 ppm (brt, 3JHP = 4.9 Hz, 3H); 13C NMR: d = 164 (brs),
170 (brs), 0.2ppm (s); 19F NMR: d = À77 ppm (s, 3F). Spectra were
recorded at 08C.
Experimental Section
All solvents were dried, distilled, and stored under dinitrogen.
Reactions were carried out under argon using standard Schlenk
techniques. 1,1-Difluoroethylene was supplied by Lancaster Syn-
thesis; trifluoroethylene was supplied by SynQuest Fluorochemicals,
or prepared by a literature method.[16] Tetrafluoroethylene was
prepared as reported.[17] NMR experiments were carried out on
Bruker AM400, Varian Unity 400, 500, or 600 spectrometers. The
NMR data reported below were recorded at 258C in CD2Cl2, except
Received: September 27, 2006
Revised: February 24, 2007
Published online: April 5, 2007
Keywords: bond activation · bridging ligands ·
.
cooperative phenomena · fluorinated ligands · iridium
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Communications
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