Aliphatic carbon-fluorine bond activation using (C5Me5)2ZrH2 [1]

Full text of DOI:10.1021/ja001006r

J. Am. Chem. Soc. 2000, 122, 8559-8560
8559
Communications to the Editor
employing isolated Cp*2ZrHF show that this species does not
disproportionate to give Cp*2ZrF2 and Cp*2ZrH2, as is believed
to occur with Cp2ZrHF.4c Reaction of Cp*2ZrHF with an
additional equivalent of 1-fluorohexane leads to quantitative
formation of Cp*2ZrF2, but elevated temperatures (120 °C) and
extended reaction times (10 days) are required. When the reaction
with 1 was repeated in the presence of cumene, a radical trap, no
change in the rate of reaction was observed.
Secondary and tertiary monofluorinated carbon centers also
react with Cp*2ZrH2. Fluorocyclohexane (1 equiv) reacts under
H2 to give cyclohexane in quantitative yield over a period of 4
days at 120 °C (eq 2). 1-Fluoroadamantane reacts with 1 (1 equiv)
Aliphatic Carbon-Fluorine Bond Activation Using
(C5Me5)2ZrH2
Bradley M. Kraft, Rene J. Lachicotte, and William D. Jones*
Department of Chemistry, UniVersity of Rochester
Rochester, New York 14627
ReceiVed March 21, 2000
Carbon-fluorine bonds are among the strongest and most inert
single bonds found in organic compounds.1 The extraordinary
properties of fluorocarbons have led to their increased use
commercially and, in turn, to an increased interest in the study
of C-F activation. In particular, the development of CFC disposal
methods remains a challenging task.2
A variety of early transition metal complexes have been shown
to be reactive toward aromatic C-F bonds such as those in
perfluorobenzene,2-4 while only a few metal complexes show well
characterized reactions with aliphatic fluorocarbons.5 The use of
early transition metal hydride reagents in C-F activation remains
largely unexplored. In fact, only two examples of C-F activation
by early transition metal hydrides have been documented.6 The
dihydride complex Cp*2ZrH2 is known to react with a variety of
unsaturated small molecules through what are commonly clas-
sified as electrophilic concerted addition reactions.7 We have
found that Cp*2ZrH2 reacts with many types of fluorinated
substrates to give hydrogenated organic products and Cp*2ZrHF.
Reaction of Cp*2ZrH2 (1) with 1-fluorohexane in cyclohexane-
d12 at ambient temperature under 1.3 atm H2 has been found to
produce hexane and Cp*2ZrHF (2) in quantitative yield (eq 1).
under H2 to give adamantane and 2, but longer reaction times
are required. The reaction was approximately 25% complete after
1 week at 120 °C.
Aliphatic fluorocarbons containing -CF2H and -CF3 groups
also undergo defluorination, although with more difficulty.
Reaction with 1,1-difluoroethane under H2 affords ethane and
Cp*2ZrHF. The reaction requires even higher temperatures (150
°C) reaching ∼90% completion after 1 day. The intermediate
fluorocarbon, 1-fluoroethane, is not observed. Defluorination of
1,1,1-trifluoropropane under H2 proceeds even more slowly (150
°C, >2 weeks). Again, no intermediate fluorocarbons (1,1-
difluoropropane or 1-fluoropropane) are observed. Under these
conditions, slow decomposition of Cp*2ZrH2 occurs to give a
yellow polymeric material of empirical formula believed to be
{(C5Me5)(C5(CH3)3(CH2)2)Zr}n.9 Although formation of Cp*2-
ZrHF was observed, propane could not be cleanly identified in
1
the H NMR spectrum.
CFCs also react to give HFCs and subsequent hydrogenated
products.10 Dichlorofluoromethane reacts readily at room tem-
perature with 3 equiv of 1 to give fluoromethane, Cp*2ZrHCl,
and a small amount of Cp*2ZrCl2. Methane forms if the sample
is allowed to stand for 1 day at ambient temperature, with Cp*2-
ZrHF, Cp*2ZrF2, and Cp*2ZrFCl also being observed. Difluo-
rodichloromethane and difluorochloromethane were also found
to give initially the dechlorinated organic product, difluo-
romethane, upon reaction with 4 equiv of 1. As with other geminal
polyfluorinated substrates, subsequent defluorination is more
difficult, requiring heating to 120 °C under H2 over period of
>10 days.
The reaction proceeds to completion within 2 days, and no
intermediates are observed. Both Cp*2ZrH2 and Cp*2ZrHF have
been characterized by X-ray crystallography, and show the
expected bent metallocene geometries.8 Independent experiments
(1) Smart, B. E. Mol. Struct. Energ. 1986, 3, 141-191.
(2) Burdeniuc J.; Jedlicka, B.; Crabtree, R. H. Chem. Ber./Recl. 1997, 130,
145.
(3) There are several recent reviews on C-F activation by metal reagents,
see: (a) Kiplinger, J. L.; Richmond, T. G.; Osterberg, C. E. Chem. ReV. 1994,
94, 373. (b) Richmond, T. G. In Topics in Organometallic Chemistry; Murai,
S., Ed.; Springer: New York, 1999; Vol. 3, 243.
(4) Aromatic C-F references: (a) Deacon, G. B.; Koplick, A. J.; Raverty,
W. D.; Vince, D. G. J. Organomet. Chem. 1979, 182, 121. (b) Deacon, G.
B.; Mackinnon, P. I.; Tuong, T. D. Aust. J. Chem. 1983, 36, 43. (c) Edelbach,
B. L.; Rahman, A. K. F.; Lachicotte, R. J.; Jones, W. D. Organometallics
1999, 18, 3170. (d) Belt, S. T.; Helliwell, M.; Jones, W. D.; Partridge, M. G.;
Perutz, R. N. J. Am. Chem. Soc. 1993, 115, 1429. (e) Jones, W. D.; Partridge,
M. G.; Perutz, R. N. J. Chem. Soc., Chem. Commun. 1991, 264. (f) Hofmann,
P.; Unfried, G. Chem. Ber. 1992, 125, 659. (g) Lucht, B. L.; Poss, M. J.;
King M. A.; Richmond, T. G. J. Chem. Soc., Chem. Commun. 1991, 400. (h)
Crespo, M.; Martinez, M.; Sales, J. J. Chem. Soc., Chem. Commun. 1992,
822. (i) Cronin, L.; Higgitt, C. L.; Karch, R.; Perutz, R. N. Organometallics
1997, 16, 4920.
Allylic and olefinic fluorinated substrates react much more
quickly with 1. 3,3,3-trifluoropropene reacts with 4 equiv of Cp*2-
ZrH2 to give propane and Cp*2ZrHF (eq 3). Complete defluori-
(5) Aliphatic C-F references: (a) Kiplinger, J. L.; Richmond, Chem.
Commun. 1996, 1115. (b) Kiplinger, J. L.; Richmond, T. G. J. Am. Chem.
Soc. 1996, 118, 1805.
(6) (a) Burger, B. J. Ph.D. Thesis, California Institute of Technology, 1987.
(b) Booij, M.; Deelman, B.-J.; Duchateau, R.; Postma, D. S.; Meetsma, A.;
Teuben, J. H. Organometallics 1993, 12, 3531.
(7) (a) Bercaw, J. E. Advances in Chemistry Series, 167; American
Chemical Society: Washington, DC, 1978; p 136. (b) Manriquez, J. M.;
McAlister, D. R.; Sanner, R. D.; Bercaw, J. E. J. Am. Chem. Soc. 1978, 100,
2716. (c) Hillhouse, G. L.; Bercaw, J. E. J. Am. Chem. Soc. 1984, 106, 5472.
nation occurs within 10 min at room temperature! The interme-
diate fluorocarbon CF2dCH-CH3 was identified by GC/MS and
its characteristic 1H and 19F NMR spectra when the reaction was
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8560 J. Am. Chem. Soc., Vol. 122, No. 35, 2000
Communications to the Editor
repeated with 1 equiv of substrate. No other fluorocarbon species
are seen in the 19F NMR spectrum, where as little as a few percent
could be detected. Although propene was not observed directly,
the zirconium product Cp*2Zr(n-propyl)H is seen prior to its
decomposition to give propane.11 Formation of propane is
facilitated by addition of H2 to the propyl hydride complex.
In an even more remarkable reaction, perfluoropropene was
found to react with 7 equiv of Cp*2ZrH2 after 15 min at room
temperature to give Cp*2ZrHF and Cp*2Zr(n-propyl)H (eq 4).
Cp*2ZrH2 also reacts with 1 equiv of perfluorobenzene after 1
day at 85 °C to give a mixture of Cp*2ZrHF, Cp*2Zr(C6F5)H,
C6F5H, and Cp*2ZrF2 (2:1:1:0.1 ratio). Repetition of the reaction
in the presence of cumene gives the same ratio of products at the
same rate. Unlike the reaction with [Cp2ZrH2]2, no Cp*2Zr(C6F5)F
was observed.3c In comparison, 1-fluoro-naphthalene (1 equiv)
reacts cleanly under H2 over a period of 4 days at 85 °C to give
naphthalene and Cp*2ZrHF (eq 5).
The formation of a strong Zr-F bond is no doubt a main
driving force for all of these reactions. Surprisingly, reaction of
[Cp2ZrH2]2 with 1-fluorohexane does not lead to C-F activation,
suggesting the enhanced hydridic character in the Cp* analog
likely plays a major role in the mechanism. For the aliphatic
systems, the order of reactivity 1° > 2° > 3° suggests that fluorine
coordination to zirconium is necessary for hydrodefluorination.
The reactivity trends together with experiments performed in the
presence of a radical trap suggest that radical abstraction of
fluorine is not involved. Consequently, many of these reactions
are believed to proceed via a “bond metathesis pathway” involving
simultaneous fluorine coordination to the 16-electron zirconium
and hydridic attack on carbon. The olefins appear to react via an
insertion/â-F-elimination pathway, building upon the well-
established reactivity of the zirconium-hydride bond with olefins.
These pathways are distinct from the common σ-bond metathesis
reactions in that a lone pair on fluorine is likely to be involved
in the 6-electron rearrangements. A full mechanistic investigation
of these reactions is under study.
Again, propene is not observed, but rather Cp*2Zr(n-propyl)H as
an intermediate prior to its hydrogenolysis to give propane.
Reaction with only 1 equiv of 1 gives mainly the selectively
defluorinated product, (E)-CF3-CFdCHF.
Benzylic activation of a CF3 group proved more difficult.
R,R,R-Trifluorotoluene reacts with 3 equiv of 1 under H2 at 85
°C to give toluene and 2 only slowly over a period of >1 month,
but reaches completion over 1 week at 120 °C. This observation,
along with the trend observed with primary, secondary, and
terteriary mono-fluoroalkanes, suggests that the F/H exchange
does not occur via a radical chain process.
(8) Details of the structures will be reported elsewhere. For Cp*2ZrH2, the
rings are oriented at an angle of 144.6°, whereas for Cp*2ZrHF, the Cp*-
Zr-Cp* angle is 141.0°.
(9) Bercaw, J. E.; McAlister, D. R.; Erwin, D. K. J. Am. Chem. Soc. 1978,
100 (18), 5966. This material also forms if H2 is omitted from these reactions.
(10) CFC ) chlorofluorocarbon. HFC ) hydrofluorocarbon.
(11) Sanner, R. D.; Miller, F. D. Organometallics 1988, 7, 818.
Acknowledgment is given to the U.S. Department of Energy (FG02-
86ER13569) for their support of this work.
JA001006R