Ruthenium-Catalyzed Olefin Cross-Metathesis with Tetrafluoroethylene and Analogous Fluoroolefins

Article abstract of DOI:10.1021/jacs.5b03342

This Communication describes a successful olefin cross-metathesis with tetrafluoroethylene and its analogues. A key to the efficient catalytic cycle is interconversion between two thermodynamically stable, generally considered sluggish, Fischer carbenes. This newly demonstrated catalytic transformation enables easy and short-step synthesis of a new class of partially fluorinated olefins bearing plural fluorine atoms, which are particularly important and valuable compounds in organic synthesis and medicinal chemistry as well as the materials and polymer industries.

Full text of DOI:10.1021/jacs.5b03342

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Communication
Ruthenium-Catalyzed Olefin Cross Metathesis with
Tetrafluoroethylene and Analogous Fluoroolefins
Yusuke Takahira, and Yoshitomi Morizawa
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b03342 • Publication Date (Web): 13 May 2015
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Ruthenium-Catalyzed Olefin Cross Metathesis with
Tetrafluoroethylene and Analogous Fluoroolefins
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Yusuke Takahira* and Yoshitomi Morizawa
Research Center, Asahi Glass Co., Ltd., 1150 Hazawaꢀcho, Kanagawaꢀku, Yokohama, Kanagawa 221ꢀ8755, Japan
Supporting Information Placeholder
electron negative substituents is not likely to be effective for
olefin metathesis.7 These two crucial drawbacks to olefin
metathesis explained why no successful catalytic metathesis
involving the difluorocarbene intermediate was reported, whereas
there have been a few successes via the monofluorocarbene
counterpart (Scheme 1b).8
ABSTRACT: This communication describes a successful olefin
cross metathesis with tetrafluoroethylene (TFE) and its analogues.
A key to the efficient catalytic cycle is interconversion between
two thermodynamically stable, generally considered sluggish,
Fischer carbenes. This newly demonstrated catalytic
transformation enables easy and shortꢀstep synthesis of a new
class of partially fluorinated olefins bearing plural fluorine atoms
which are particularly important and valuable compounds in
organic synthesis and medicinal chemistry as well as materials
and polymer industries.
Scheme 1. Olefin metathesis with fluoroolefins.
(a) Synthesis of ruthenium mono- and difluorocarbene complexes
N
N
N
N
Ph
Cl
Cl
X
F
F
F
or
Ru
Ru
+
Ph
Olefin metathesis is one of the most powerful and versatile
catalytic transformations to construct a new carbon−carbon
double bond, and has become a widely used synthetic tool in both
pure and applied chemistry.1 Despite ruthenium precatalysts
having excellent tolerance toward diverse functional groups, the
scarce successes underscore the incompatibility of directly
halogenated olefins.2 Focusing on directly fluorinated olefins,
commonly referred to as fluoroolefins, attempts at successful
olefin metathesis via fluorocarbene complexes have pointed out
two crucial drawbacks in catalytic transformation.3ꢀ8
Ruthenium monoꢀ and difluorocarbene complexes, G2-F4 and
G2-F2,5 respectively, have been prepared previously from the
parent benzylidene counterpart G2 with the corresponding
fluoroolefins via stoichiometric metathesis (Scheme 1a). Both
Cl
Cl
PCy3
PCy3
F
G2 (Cy = cyclohexyl)
G2-F (X = H)
G2-F2 (X = F)
(b) Crucial drawbacks in catalytic metathesis via fluorocarbenes
Thermodynamically
Slow initiation
unfavorable metathesis
L
L
Cl
Cl
G2-F
or
G2-F2
– PCy3
+ PCy3
X
F
Ru
Ru
R1 or R2
Cl
Cl
R1
R2
Plausible intermediate
(L = N-heterocyclic carbene)
(c) This work: Catalytic cross metathesis with fluoroolefins
F
F
X1
X2
X1
X2
F
F
Ru precatalyst
+
+
OR
OR
OR
Fluoroolefin
Enol ether
(1A-E)
complexes showed no phosphine dissociation,
a plausible
F
F
F
F
F
F
F
F
F
F
F
F
F
initiation step for catalytic cycles, even at elevated temperature
based on 31P NMR magnetization transfer experiments, which
indicated problematic initiation. Comparison of these catalytic
activities for ringꢀopening metathesis polymerization (ROMP) of
1,5ꢀcyclooctadiene (0.33 mol% G2-F or G2-F2, CD2Cl2, 25 °C,
1.25 h) indicated that the initiation of G2-F2 (only 9% conversion)
was much slower than that of G2-F (100% conversion), which
emphasized the extreme sluggishness of G2-F2. Another
drawback emerged through density functional theory (DFT)
calculations in regard to the Gibbs freeꢀenergy profiles of the
cross metathesis of 2ꢀnorbornene with several directly
halogenated olefins.6 The results indicated a large contribution of
halocarbene ligation, in particular, that of difluorocarbene, to
stabilize the whole of the complexes, and this would hinder
subsequent turnover. A recent study using coupled cluster theory
calculation also predicted that the type of complex involving two
F
F
Cl
F3C
TFE (1A)
CTFE (1B)
HFP (1C)
TrFE (1D)
VdF (1E)
TFE, tetrafluoroethylene; CTFE, chlorotrifluoroethylene; HFP, hexafluoropropylene;
TrFE,trifluoroethylene; VdF, vinylidene fluoride.
Fluoroolefins are particularly important and valuable
compounds for the synthesis of many commercially successful
products in the materials and polymer industries.9 Thus far, only a
limited number of fluoroolefins have been used as monomers
because of a lack of suitable, inexpensive methods for their
preparation. The use of olefin metathesis involving inexpensive
fluoroolefins and a hydrocarbon counterpart will enable easy and
shortꢀstep synthesis of a wide range of functionalized fluoroolefin
monomers for exploitation in polymer chemistry, for example, as
well as possible new building blocks bearing plural fluorine atoms
in medicinal chemistry. TFE and its analogues are inexpensive,
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bulk organofluorine feedstock and are considered to be suitable
The successful catalytic metathesis of TFE (1A) with dodecyl
starting materials for this perspective.10
vinyl ether (2a) encouraged us to afford the corresponding
difluorinated product 3Aa (Table 1). A controlled experiment
highlighted the essential role of the ruthenium precatalyst in this
transformation (entry 1). Screening of a total of 19 ruthenium
precatalysts revealed that a class of precatalysts bearing a (2ꢀ
isopropoxyphenyl)methylidene moiety provided enhanced
catalytic activity for this transformation (entries 2−4). The
absence of a phosphine ligand was considered to contribute to the
superior results.15 G2, fastꢀinitiating G3, and sterically lessꢀ
hindered o-tol-HG2 served this reaction insufficiently. (entries
5−7).
During our investigations to develop new classes of catalytic
transformation with TFE, the simplest perfluoroolefin, we
discovered that G2 reacted with TFE under mild reaction
conditions to afford G2-F2 in excellent isolated yield, in the same
manner as with VdF (Scheme 2).5,11 This discovery led us to
accomplish the challenging catalytic cross metathesis with
fluoroolefins. We herein report a successful rutheniumꢀcatalyzed
cross metathesis with TFE and its analogues (Scheme 1c). A key
to the efficient catalytic cycle is interconversion between two
thermodynamically stable, generally considered sluggish, Fischer
carbenes.12
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Table 1. Screening of precatalysts.a-c
Scheme 2. Stoichiometric metathesis of G2 with TFE.
As described above, fluoroolefins serve as problematic
substrates for olefin metathesis, giving insufficient or no catalytic
turnover, and thereby hindering straightforward access to the
corresponding functionalized fluoroolefins. After failure of our
early attempts at successful catalytic cross metathesis with TFE,
we designed a peculiar catalytic cycle inspired by a pioneering
precedent.13 Grubbs and coꢀworkers have reported that a Fischer
carbene reacted with a stoichiometric amount of αꢀheteroatomꢀ
substituted olefin to result in an equilibriumꢀcontrolled mixture of
two Fischer carbenes (Scheme 3a). The ruthenium alkoxycarbenes
are representative Fischer carbenes and can be readily obtained
from the reaction of the parent alkylidenes with enol ethers (e.g.,
ethyl vinyl ether). The thermodynamic stability of alkoxycarbene
strongly hinders subsequent turnover in an olefin metathesis
manner, leading to the frequent use of enol ethers as a termination
agent for ROMP. We hence envisioned that the interconversion
between two Fischer carbenes, i.e., difluorocarbene and
alkoxycarbene, would be labile, and this system could catalyze the
cross metathesis of TFE and enol ethers: In the presence of TFE
and enol ether, difluorocarbene and alkoxycarbene would
catalytically interconvert to afford two difluorinated olefins
simultaneously (Scheme 3b). We anticipated that this mutual
characteristic of thermodynamic stability of difluorocarbene and
alkoxycarbene would contribute suitably to our catalytic system.14
Entry
Precatalyst
none
Yield of 3Aa/%d
TON
−
1
2
3
4
5
6
7
n.d.
25
27
23
6
12.5
13.4
11.7
3.2
HG2
M51
M73SIPr
G2
2
1.1
G3
o-tol-HG2e
2
0.8
a
A total of 19 precatalysts were screened; the full list is
b
provided in the Supporting Information (SI).
Reaction
conditions: 1A (1 atm, ca. 0.12 mmol, ca. 2 equiv), 2a (0.06
mmol), 1,4ꢀbis(trifluoromethyl)benzene (0.01 mmol, internal
standard for determination of 19F NMR yield) and precatalyst
(0.0012 mmol, 2 mol%, except for entry 1) in C6D6 (0.6 mL) at
60 °C for 1 h in a screw cap NMR tube. c n.d., not detected; TON,
turnover number. d 19F NMR yield. e The precatalyst was partially
soluble in C6D6.
Scheme 3. Labile Fischer carbene interconversion.
(a) Pioneering precedent: Stoichiometric ligand exchange of two Fischer carbene complexes
L
L
Cl
Cl
E2
E1
Ru
Ru
E1
E2
Cl
Cl
PCy3
PCy3
Not only TFE (1A) but also analogous fluoroolefins were
capable of this transformation (Table 2). In the presence of
M73SIPr precatalyst, these fluoroolefins could convert to provide
the corresponding products under mild reaction conditions, in
moderate to good yields. When 1.0 mmol of 2a was used as the
starting material, product 3Aa was obtained in 64% isolated yield,
thereby showing this transformation was scalable and catalytic
(entry 1). Reaction with CTFE (1B) afforded a mixture of
difluorinated 3Aa (11%) and chlorofluorinated 4Ba (51%), thus
L = phosphine or N-heterocyclic carbene
E1 and E2 = -OR, -SR, or -NR2
(b) Our system: Catalytic metathesis with TFE via two Fischer carbene intermediates
R2
R2
F
F
OR1
L
L
Cl
Cl
F
F
Ru
Ru
OR1
Cl
Cl
F
F
F
F
F
F
OR1
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indicating turnover of 6.2 (entry 2). HFP (1C) and TrFE (1D) also
Related mechanistic studies ascertained that the expected
catalytic cycle was reasonable. G2-F2 reacted with ethyl vinyl
ether 2b to afford only G2-OEt and VdF, whereas neither G2-H2
gave the products 4Ca (22%) and 4Da (72%), respectively,
whereas no 3Aa was detected by 19F NMR in these cases (entries
3 and 4). VdF (1E) resulted in recovery of the starting material 2a
(entry 5).16 Neither 1,2ꢀbis(dodecyloxy)ethylene nor symmetric
fluoroolefins (the products of self metathesis from 1, e.g., 1,2ꢀ
dichloroꢀ1,2ꢀdifluoroethylene from 1B) was detected in the
reaction mixture under these reaction conditions. The products
were partially isolatable by careful chromatography, and all new
compounds were characterized by NMR spectroscopy and highꢀ
resolution mass spectrometry (HRMS).17,18 The stereochemistry
of 4Ba, 4Ca, and 4Da was assigned by NMR based on the
coupling constants between vinylic protons and fluorines.19
1
nor 3Ab was observed in both H and 19F NMR spectra, showing
complete regioselectivity of path I over path II (Scheme 5a). This
result strongly indicated that the difluorocarbene underwent
selective conversion to the alkoxycarbene according to step Aꢀi in
Scheme 4. Stoichiometric metathesis of G2-OEt highlighted a
significant contrast between TFE and VdF (Scheme 5b). When
VdF (X1 = X2 = H) was used as a reactant, the large energetic
drawback in conversion from a Fischer carbene [Ru]=CHOR to a
Schrock carbene [Ru]=CH2 would hinder step Bꢀii in Scheme 4,
thereby yielding nonꢀproductive metathesis through step Aꢀii. The
predominant formation of 4 over 3 shown in Table 2 might also
reflect a similar energetic advantage of cycle A over B. Further
experimental and computational studies are now underway.
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Table 2. Catalytic cross metathesis with fluoroolefinsa-d
Scheme 5. Mechanistic studies.
(a) Selective formation of G2-OEt over G2-H2
N
N
Yield/%e
Cl
F
Path I
Entry Fluoroolefin X1 X2
TON
Ru
+
OCH2CH3
F
Cl
4AaꢀEa
69f (64)
51 [39/61]
3Aa
N
N
VdF (1E)
PCy3
Cl
G2-OEt, 13% (1H NMR yield)
1
2
3
4
5
TFE (1A)
CTFE (1B)
HFP (1C)
TrFE (1D)
VdF (1E)
F
F
F
F
H
F
6.9
6.2
2.2
7.2
−
F
F
C6D6
Ru
+
60 °C, 6 h
OCH2CH3
Cl
11
Cl
2b
10 equiv
N
N
PCy3
CF3 n.d. 22 [25/75]
Cl
F
Path II
G2-F2
+
Ru
H
H
n.d. 72 [20/80]
F
OCH2CH3
3Ab
Cl
g
PCy3
n.d.
−
G2-H2, not detected
(b) Non-productive metathesis with VdF
a Reaction conditions: 1 (2 atm, ca. 10 mmol, ca. 10 equiv), 2a
(1.0 mmol) and M73SIPr precatalyst (0.1 mmol, 10 mol%) in
C6H6 (10 mL) at 60 °C for 1 h in an autoclave. b n.d., not detected;
TON, turnover number. c Isolated yields are given in parentheses.
d E/Z ratios are given in square brackets. e 19F NMR yield. f 4Aa is
identical to 3Aa. g 4Ea is identical to 2a.
F
F
F
N
N
F
F
F
OCH2CH3
Cl
TFE (1A)
From 1A: 3Ab, 81% (1H NMR yield)
or
+
or
Ru
F
C6D6
60 °C, 1 h
OCH2CH3
Cl
PCy3
F
VdF (1E)
OCH2CH3
From 1E: 2b, 89% (1H NMR yield)
G2-OEt
ca. 10 equiv
According to the simple principle regarding the interconversion
of two Fischer carbene intermediates shown in Scheme 3b, the
following two fundamental steps would compose the catalytic
cycles: (i) fluorocarbene to alkoxycarbene conversion (Scheme 4,
steps Aꢀi and Bꢀi) and (ii) its reverse counterpart (Scheme 4, steps
Aꢀii and Bꢀii). In this situation, cycles A and B involve [Ru]=CF2
and [Ru]=CX1X2 intermediates, respectively.
Ethenolysis, the cross metathesis with ethylene and another
olefinic counterpart featuring an internal carbon−carbon double
bond, is a practical and costꢀeffective manufacturing process to
provide highꢀvalue chemicals from bulk feedstock.20 We hence
introduced a method, “tetrafluoroethenolysis”, by which two
partially fluorinated olefins could be provided in an ethenolysis
manner with TFE. Indeed, in the presence of M73SIPr precatalyst,
2c reacted with TFE (1A) to convert into two terminal olefins,
3Ab (2.7 turnover determined by 19F NMR) and 5Ac,21 obviously
proving the feasibility of this transformation (Scheme 6). Notably,
an alkylꢀsubstituted product could be obtained via this
transformation according to the principle shown in Scheme 3b.
Scheme 4. Plausible catalytic cycles.
Scheme 6. “Tetrafluoroethenolysis”.
In conclusion, we demonstrated
a successful rutheniumꢀ
catalyzed cross metathesis with TFE and its analogous
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(5) (a) Trnka, T. M.; Day, M. W.; Grubbs, R. H. Angew. Chem. Int. Ed.
2001, 40, 3441−3444 and (b) Trnka, T. M. Ph.D. dissertation, California
Institute of Technology, Pasadena, California, 2003.
(6) (a) Fomine, S.; Ortega, J. V.; Tlenkopatchev, M. A. J. Mol. Catal. A
Chem. 2007, 263, 121−127 and (b) Fomine, S.; Tlenkopatchev, M. A.
Appl. Catal. A Gen. 2009, 355, 148−155.
(7) Vasiliu, M.; Arduengo, A. J.; Dixon, D. A. J. Phys. Chem. C 2014,
118, 13563−13577.
fluoroolefins. This newly demonstrated catalytic transformation
indicates that fluoroolefins are no longer exotic substances of
olefin metathesis. Furthermore, these findings prove the feasibility
of a new synthetic methodology for organofluorine chemistry,
such as cross metathesis with two fluoroolefins and ROMP with a
cyclic fluoroolefin via Fischer carbene interconversion. Further
investigations related to this work are now in progress and will be
reported in due course.
(8)
A number of successes with α,ωꢀdiene, one of whose
carbon−carbon double bonds bears a fluorine atom, indicated certain
compatibility for ringꢀclosing metathesis, see ref. 3 and references therein.
(9) For example, see: (a) Kirsch, P. Modern Fluoroorganic Chemistry:
Synthesis, Reactivity, Applications; WileyꢀVCH: Weinheim, Germany,
2004, (b) Chambers, R. D. Fluorine in Organic Chemistry; Blackwell:
Oxford, U.K., 2004, and (c) Uneyama, K. Organofluorine Chemistry;
Blackwell: Oxford, U.K., 2006.
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ASSOCIATED CONTENT
Supporting Information
Experimental procedures and compound characterization data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(10) Recently, Ogoshi and coꢀworkers have intensively studied
transitionꢀmetalꢀcatalyzed transformation of fluoroolefins, see: (a) Ohashi,
M.; Kambara, T.; Hatanaka, T.; Saijo, H.; Doi, R.; Ogoshi, S. J. Am. Chem.
Soc. 2011, 133, 3256−3259, (b) Ohashi, M.; Saijo, H.; Shibata, M.;
Ogoshi, S. Eur. J. Org. Chem. 2013, 443−447, (c) Ohashi, M.; Kamura,
R.; Doi, R.; Ogoshi, S. Chem. Lett. 2013, 42, 933−935, (d) Ohashi, M.;
Shibata, M.; Saijo, H.; Kambara, T.; Ogoshi, S. Organometallics 2013, 32,
3631−3639, (e) Saijo, H.; Sakaguchi, H.; Ohashi, M.; Ogoshi, S.
Organometallics 2014, 33, 3669−3672, (f) Ohashi, M.; Ogoshi, S.
Catalysts 2014, 4, 321−345, (g) Ohashi, M.; Shibata, M.; Ogoshi, S.
Angew. Chem. Int. Ed. 2014, 53, 13578−13582, and (h) Saijo, H.; Ohashi,
M.; Ogoshi, S. J. Am. Chem. Soc. 2014, 136, 15158−15161.
(11) Unfortunately, no perfluororuthenacyclobutane species that was a
plausible intermediate for olefin metathesis with TFE was detected in the
product mixture. For other perfluorometallacyclobutanes, see: (a) Iron
complexes; Karel, K. J.; Tulip, T. H.; Ittel, S. D. Organometallics 1990, 9,
1276−1282, (b) cobalt complexes; Harrison, D. J.; Lee, G. M.; Leclerc, M.
C.; Korobkov, I.; Baker, R. T. J. Am. Chem. Soc. 2013, 135, 18296−18299,
and (c) Vicic and coꢀworkers have reported platinum complexes during
the preparation of this manuscript; Xu, L.; Solowey, D. P.; Vicic, D. A.
Organometallics in press, DOI: 10.1021/acs.organomet.5b00045.
(12) The term “Fischer carbene” refers to a metal complex bearing a
divalent carbene ligand featuring at least one αꢀheteroatom substitutent
(e.g., halogen atom, −OR, −SR, or −NR2) with loneꢀpair electrons.
(13) Louie, J.; Grubbs, R. H. Organometallics 2002, 21, 2153−2164.
(14) Likewise, Macnaughtan has proposed a similar system involving
interconversion between acetoxycarbene and other Fischer carbenes in her
Ph.D. dissertation, see ref. 4c.
AUTHOR INFORMATION
Corresponding Author
yusukeꢀtakahira@agc.com
Notes
CAUTION! The International Agency for Research on Cancer
(IARC) classifies TFE into “Group 2A: Probably carcinogenic to
humans”,22 and hence all manipulations using TFE must be
carried out with care.
CAUTION! Under the representative reaction condition,
fluoroolefins may ignite in the presence of oxygen, and hence air
must be completely removed from the operating system (e.g.,
NMR tube and autoclave).
The authors declare the following competing financial interests:
The authors are employees of Asahi Glass Co., Ltd. (AGC) and
patent applications on this work have been filed.
ACKNOWLEDGMENTS
Prof. Graham Sandford (Durham University) is acknowledged for
helpful discussions and critical comments during the preparation
of this communication. The authors are also grateful to colleagues
at AGC: Dr. Takashi Okazoe, Mr. Kunio Watanabe, and Mr.
Daisuke Jomuta for initial support and continuous encouragement
on this work, Mr. Hiroshi Hatano and Mr. Nobuyuki Otozawa for
helpful discussions, Mr. Yasunori Sasaki for technical assistance
with handling of fluoroolefins, Mr. Tatsuya Miyajima and Ms.
Yuki Nakamura for assistance with NMR, and Mr. Toshifumi
Kakiuchi, Dr. Yoji Nakajima, and Ms. Aya Serita for assistance
with HRMS.
(15) In the case of cross metathesis using 1ꢀhaloethylene and 1,2ꢀ
dihaloethylene, Grela and Johnson reached
a similar conclusion
independently, see: (a) Sashuk, V.; Samojłowicz, C.; Szadkowska, A.;
Grela, K. Chem. Commun. 2008, 2468−2470 and (b) ref. 4b.
(16) We have separately confirmed that catalytic cross metathesis
between enol ether and VdF actually occurred, see eqs. S1 and S2 in the
SI.
(17) Although the preparation of 3Aa has been reported previously, the
NMR data are not available, see: Yang, Q.; Njardarson, J. T. Tetrahedron
Lett. 2013, 54, 7080−7082.
REFERENCES
(18) Only (E)ꢀ and (Z)ꢀ4Ba were inseparable chromatographically, see
the SI.
(19) For compound characterization data, see the SI.
(1) Handbook of Metathesis; Grubbs, R. H., Ed.; WileyꢀVCH:
Weinheim, Germany, 2003.
(2) Olefin Metathesis: Theory and Practice; Grela, K., Ed.; Wileyꢀ
VCH: Weinheim, Germany, 2014.
(3) In contrast to directly fluorinated olefins, i.e., fluoroolefins,
perfluoroalkylꢀsubstituted ethylenes showed certain compatibility for
olefin metathesis, see: Fustero, S.; SimónꢀFuentes, A.; Barrio, P.; Haufe,
G. Chem. Rev. 2015, 115, 871−930 and references therein.
(4) (a) Macnaughtan, M. L.; Johnson, M. J. A.; Kampf, J. W.
Organometallics 2007, 26, 780−782, (b) Macnaughtan, M. L.; Gary, J. B.;
Gerlach, D. L.; Johnson, M. J. A.; Kampf, J. W. Organometallics 2009, 28,
2880−2887, and (c) Macnaughtan, M. L. Ph.D. dissertation, The
University of Michigan, Ann Arbor, Michigan, 2009.
(20) For an example of ethenolysis and analogous alkenolysis, see:
Nickel, A.; Ung, T.; Mkrtumyan, G.; Uy, J.; Lee, C. W.; Stoianova, D.;
Papazian, J.; Wei, W. H.; Mallari, A.; Schrodi, Y.; Pederson, R. L. Top.
Catal. 2012, 55, 518−523.
(21) Authentic NMR data for 5Ac in C6D6 have not been reported
previously, and we hence tentatively determined the formation by NMR
and HRMS analyses of the reaction mixture. See the SI for details.
(22) BenbrahimꢀTallaa, L.; LaubyꢀSecretan, B.; Loomis, D.; Guyton, K.
Z.; Grosse, Y.; El Ghissassi, F.; Bouvard, V.; Guha, N.; Mattock, H.;
Straif, K. Lancet Oncol. 2014, 15, 924−925.
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