Titanium-catalyzed vinylic and allylic C-F bond activation-scope, limitations and mechanistic insight

Article abstract of DOI:10.1002/chem.201201125

The hydrodefluorination (HDF) of fluoroalkenes in the presence of a variety of titanium catalysts was studied with respect to scope, selectivity, and mechanism. Optimization revealed that the catalyst requires low steric bulk and high electron density; secondary silanes serve as the preferred hydride source. A broad range of substrates yield partially fluorinated alkenes, such as previously unknown (Z)-1,2-(difluorovinyl)ferrocene. Mechanistic studies indicate a titanium(III) hydride as the active species, which forms a titanium(III) fluoride by H/F exchange with the substrate. The HDF step can follow both an insertion/elimination and a σ-bond metathesis mechanism; the E/Z selectivity is controlled by the substrate. The catalysts' ineffieciency towards fluoroallenes was rationalized by studying their reactivity towards Group 6 hydride complexes. The broad application of the catalytic hydrofluorination of fluoroalkenes by the system [Cp₂TiF ₂]/silane is demonstrated. Isolated yields up to 79 % could be obtained for various substrates. Mechanistic studies indicate two competing reaction mechanisms. Copyright

Full text of DOI:10.1002/chem.201201125

FULL PAPER
DOI: 10.1002/chem.201201125
À
Titanium-Catalyzed Vinylic and Allylic C F Bond Activation—Scope,
Limitations and Mechanistic Insight
Moritz F. Kuehnel, Philipp Holstein, Meike Kliche, Juliane Krꢀger, Stefan Matthies,
Dominik Nitsch, Joseph Schutt, Michael Sparenberg, and Dieter Lentz*[a]
Dedicated to Prof. Dr. FranÅois Diederich on the occasion of his 60th birthday
Abstract: The hydrodefluorination
(HDF) of fluoroalkenes in the pres-
ence of a variety of titanium catalysts
was studied with respect to scope, se-
lectivity, and mechanism. Optimization
revealed that the catalyst requires low
steric bulk and high electron density;
secondary silanes serve as the pre-
ferred hydride source. A broad range
of substrates yield partially fluorinated
alkenes, such as previously unknown
(Z)-1,2-(difluorovinyl)ferrocene. Mech-
exchange with the substrate. The HDF
step can follow both an insertion/elimi-
nation and a s-bond metathesis mecha-
nism; the E/Z selectivity is controlled
by the substrate. The catalystsꢀ ineffie-
ciency towards fluoroallenes was ra-
tionalized by studying their reactivity
towards Group 6 hydride complexes.
anistic studies indicate a titanium
hydride as the active species, which
forms a titanium(III) fluoride by H/F
ACHTUNGTNER(NUNG III)
AHCTUNGTRENNUNG
À
Keywords: catalysis · C F activa-
tion · fluoroalkenes · hydrodefluori-
nation · titanium
Introduction
The introduction of fluorine into complex organic mole-
cules has been accomplished by employing various fluorina-
tion techniques.[7,8] While perfluorination is rather easily
achieved, the introduction of a defined substitution pattern
remains challenging; the use of fluorine-containing building
blocks circumvents this problem, but relies on the availabili-
ty of a suitable synthon. A promising approach to such
building blocks is based on derivatization of readily avail-
able perfluorinated compounds by selective cleavage of
carbon–fluorine bonds.[9]
Metal-catalyzed transformations of organic molecules are
fundamental to the synthetic chemist and cover a broad
range of substrates and functionalities.[1] Among the numer-
ous reactants, fluoroalkenes are somewhat underrepresent-
ed; this has often been attributed to their altered bond po-
larities and energies in comparison with their hydrocarbon
analogues.[2] Fluorine forms the strongest s bond to carbon
with a homolytic dissociation energy of (500Æ50) kJmolÀ1.[3]
This thermodynamic stability combined with an inherent ki-
netic inertness has allowed for technical applications of fluo-
rocarbons, for example, as refrigerants or chemically resist-
ant polymers,[4] but has also led to an accumulation of halo-
fluorocarbons in the upper atmosphere, where they have
had a deleterious impact on the ozone layer and contribute
to global warming.[5] On the other hand, the increase in hy-
drophobicity and metabolic stability induced by introduction
of fluorine into organic molecules has allowed for significant
advances in medicinal chemistry.[6] This ambiguous role of
fluorine chemistry has created a need for means of both se-
lective construction and deconstruction of carbon–fluorine
bonds.
A number of systems capable of catalytic aromatic,[10,11]
and aliphatic[12,13] C F bond activation have been reported
À
to date, whereas comparable processes involving fluoroal-
kenes have remained rare (Scheme 1).[10r,11z,14,15] Negishi-
type cross-coupling of fluoroalkenes with arylzinc reagents
was introduced by Saeki et al.;[11z] very recently, and Ohashi
et al. extended this approach and developed an efficient pro-
À
tocol for the synthesis of trifluorostyrenes via C F activation
of tetrafluoroethene.[15a,b] Rhodium-catalyzed silylation and
borylation of vinylic carbon–fluorine bonds has been ach-
ieved by the group of Braun.[14d,15c]
Despite the increasing importance of hydrofluoroolefins
(HFOs) as low-GWP (GWP=global warming potential) re-
placements for hydrofluorocarbons (HFCs), the simplest
À
possible C F activation of fluoroalkenes, namely the hydro-
defluorination (HDF) is still underdeveloped. Low-coordi-
nate iron fluorides were shown by Holland to act as precata-
lysts for the HDF of hexafluoropropene and trifluoropro-
pene (Scheme 1).[10r] Turnover numbers (TONs) up to 5
were achieved at 1008C. A slightly higher efficiency was re-
ported in the HDF of fluoroethene in the presence of Wil-
kinsonsꢀs catalyst.[14e]
[a] Dr. M. F. Kuehnel, P. Holstein, M. Kliche, J. Krꢁger, S. Matthies,
D. Nitsch, J. Schutt, M. Sparenberg, Prof. Dr. D. Lentz
Institut fꢁr Chemie und Biochemie—Anorganische Chemie
Freie Universitꢂt Berlin, Fabeckstr. 34–36, 14195 Berlin (Germany)
Fax : (+49)30-838-52424
E-mail: lentz@chemie.fu-berlin.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201125.
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Scheme 2. Titanium-catalyzed HDF of hexafluoropropene (4).
trace amounts of the allylic HDF product 5c and the two-
fold HDF product 6a; no HDF at the secondary carbon
atom is observed. The stereoselectivity is rather low (E/Z=
0.67) and is not influenced by the reaction temperature.
À
Scheme 1. Literature examples of catalytic fluoroalkene C F bond acti-
vation (L=(2,6-diisopropylphenyl)ACTHNUTRGNE{NUG (2E,4E)-4-[(2,6-diisopropylphenyl)i-
Optimization: A variety of high-boiling silanes 2a–2e were
evaluated as hydride source for the HDF of 4 in terms of
turnover frequency (TOF) and E/Z selectivity (Figure 1).
While the secondary silane 2a performs best with a TOF of
500 hÀ1, the primary silane 2b as well as the polymer 2e
slow down the reaction by almost one order of magnitude
with no influence on the stereoselectivity. Tertiary silanes 2c
and 2d fail to activate the catalyst.
mino]pent-2-en-2-yl}amide).
However, these limited efficiencies, the narrow substrate
scope and the high cost of most late-transition-metal-based
catalysts have rendered C F activation a rather academic re-
search area and prompted us to focus on early transition
metals. A few examples of Group 4 catalyzed C F activation
À
À
have appeared in the literature.[9a,10o,y,ad,ae,13g] The cleavage of
olefinic carbon–fluorine bonds by zirconocene and hafno-
cene hydrides has been thoroughly studied by Jones, albeit
in stoichiometric reactions.[9p,16] The concomitant formation
of stable zirconium and hafnium fluoro complexes is consid-
ered the driving force in these reactions, but also the main
obstacle in developing a catalytic process. In contrast, the ti-
tanium congener is more reactive towards various hydride
reagents.
Based on this difference in reactivity, we recently report-
ed our preliminary results on the titanium-catalyzed HDF of
fluoroalkenes.[17] In this paper, we present our detailed study
on the scope and mechanism of this catalyst system.
Figure 1. Influence of the employed silane 2a–2e on the HDF of 4 in the
presence of 1a. TOF determined by 19F NMR spectroscopy after 15 min
at room temperature.
The influence of the catalyst structure was studied by in-
troducing various substituents to the cyclopentadienyl li-
gands. As the substituentꢀs steric bulk increases, the reactivi-
ty is found to decrease substantially, while the selectivity is
only slightly affected (Figure 2). Interestingly, the trimethyl-
silyl derivative 1d is more active than its tert-butyl analogue
1c, although both moieties are similar in size. Apparently,
electron donation by the silyl group has a beneficial effect
on the HDF reaction; this conclusion is further supported
by the low TOFs achieved by acceptor-substituted titano-
cene 1h and diketiminate 1i; consistently, the simple titani-
um fluorides 1j and 1k exhibit no catalytic activity in the
absence of suitable donating ligands. Further optimization
of the E/Z selectivity was attempted by the synthesis of an
unsymmetrically substituted titanocene difluoride 1 f. How-
ever, apart from a higher activity than the symmetrically
substituted precatalyst 1d, no gain in selectivity was ob-
Results and Discussion
Owing to the limited number and high sensitivity of isolable
titanium hydride complexes, we followed Buchwaldꢀs in situ
preparation of a purported titanoceneACTHNUTRGNEUNG(III) hydride 3a by re-
acting air-stable titanocene difluoride (1a) with excess di-
phenyl silane (2a, Scheme 2) in ether-based solvents;[18] due
to its low vapor pressure, we chose diglyme in order to facil-
itate its separation from the gaseous substrates and products,
THF or dioxane give similar results. Stirring at ambient tem-
perature for 30 min or at elevated temperatures for a few
minutes results in a sudden color change from bright yellow
through purple to dark green. The resulting solution is
active for the catalytic hydrodefluorination of fluoroalkenes
(Scheme 2). Addition of hexafluoropropene (4) yields pen-
tafluoropropenes 5a and 5b as the main products besides
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Titanium-Catalyzed C F Bond Activation
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Figure 2. Influence of the employed titanium precatalyst 1a–1e on the
catalytic HDF of 4 using 2a as the hydride source (Dipp2NacNac=(2,6-
diisopropylphenyl)
yl}amide,
ACHTUNGTRENNUNG{(2E,4E)-4-[(2,6-diisopropylphenyl)imino]pent-2-en-2-
ebthi=1,2-bis(4,5,6,7-tetrahydro-1H-inden-1-yl)ethane-1,2-
diyl). TOF determined by 19F NMR spectroscopy after 15 min at room
temperature (1a,b,d,f) or at low conversion (1c,e,h,i,j); 1a,f reached com-
plete conversion within this time.
served. The solid-state structures of complexes 1e, 1 f, and
1h were determined by single-crystal X-ray diffraction
(Figure 3). Their structural parameters are very similar and
within the expected range. 1 f was found to crystallize in two
¯
polymorphs (P21/c) and (P1), which differ essentially in the
orientation of the silyl moiety.
Figure 3. Molecular structures of 1e (top left), 1h (top right). and 1 f
¯
(P21/c and P1, bottom left and right, respectively) determined by single-
Substrate scope: A number of fluorinated alkenes were sub-
jected to similar HDF conditions (Table 1). 1,1,3,3,3-Penta-
fluoropropene (5d) reacts with 2a to yield tetrafluoropro-
penes 6b–6d (Table 1, entry 2). The TOF of 200 hÀ1 is clear-
ly below the value of 500 hÀ1 observed for 4 under identical
conditions, but the selectivity is excellent; the E isomer of
6b is formed in 90% overall selectivity (entry 1). 3,3,3-Tri-
fluoropropene (7) reacts much slower (TOF=7.2 hÀ1) to
give difluoropropene (8) and monofluoropropenes 9a and
9b (entry 3); a competing hydrogenation is found to yield
1,1,1-trifluoropropane (10) and propane (11). This side reac-
tion may arise from the presence of dihydrogen formed by
dehydrocoupling of the silane, which is also catalyzed by
1a;[19] unlike the formation of 10, the hydrogenation leading
to 11 seems to poison the catalyst irreversibly, as indicated
by the stoichiometric yield of 11 with respect to 1a. At low
conversions, formation of 9a,b is observed before the com-
plete consumption of 7. Lowering the reaction temperature
to 08C has no significant effect apart from slowing down the
reaction.
The catalytic HDF of fluoroethenes 12, 13, and 14c pro-
ceeds significantly less smoothly. Even at elevated tempera-
tures, no HDF products of tetrafluoroethene (12; Table 1,
entry 4) were obtained; trifluoroethene (13) affords a mix-
ture of difluoroethenes 14a–14c, monofluoroethene (15),
and ethane (16), albeit with low efficiency (TON=4.2,
entry 5). The CF2 group is apparently more readily attacked
than the CFH group; this can be seen from the relative
yields of 14a and 14b with respect to 14c, resulting from
either a preferential formation of 14a and 14b or a faster
crystal X-ray diffraction (C principal ellipse, F shaded octant, Si cutout
ellipse, Ti plain ellipse; hydrogen atoms omitted for clarity, thermal ellip-
soids drawn at 50% probability).
subsequent hydrodefluorination of 14c. In either case, the
second and third HDF steps are faster than the first, result-
ing in the formation of 15 prior to the complete consump-
tion of the starting material. As observed in the HDF of 7,
a HDF/hydrogenation product 16 is formed in an exactly
stoichiometric amount. Similar results are observed starting
from 14c to give a mixture of 15 and 16 (TON=3.4;
entry 6).
Depending on the stoichiometry, 17 undergoes H/F ex-
change to give penta-, tetra-, tri- and difluorocyclobutenes
18–21 (Table ; entry 7). In order to ensure complete con-
sumption of all silane-bound hydrogen, longer reaction
times were employed. Preferential formation of pentafluoro-
cyclobutenes 18a and 18b over their HDF products 19a and
19b is achieved using 0.6 equivalents of 2a. The use of 1.2
equivalents of 2a furnishes tetrafluorocyclobutenes 19a and
19b as the main products (entry 8), whereas higher silane to
substrate ratios result predominantly in the formation of tri-
fluorocyclobutene 20 in addition to smaller amounts of 19a
and 19b, difluorocyclobutene 21, and a number of unidenti-
fied side products (entry 9).
The catalytic HDF of aryl-substituted fluoroalkenes re-
quires elevated temperatures in order to reach completion
within reasonable time. Trifluorostyrene (22) yields (1,2-di-
fluoroethenyl)benzenes 23a and 23b in good yields and
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Table 1. Substrate scope—reactions performed in diglyme solution at room temperature. Isolated yields of gaseous products were determined by integra-
tion of product resonances in the 19F NMR spectra relative to an internal standard.
Substrate
1a [mol%]
2a [equiv]
t [h]
Products
TON
1
0.64
1.0
0.25
125
4
5a,b 79%
E/Z=0.67
5c 0.9%
6a 1.1%
2
3
1.23
3.85
1.1
1.5
1
42
17
5d
7
6b,c 49%
E/Z=0.07
6d 1.9%
64
8 37%
9a,b 2.7%
E/Z=0.71
10 22%
11 3.5%
4
5
1.32
7.35
1.2
1.0
3
none
0
12
18
4.2
13
14a,b 4.1%
E/Z=0.36
15 8.4%
14c 0.2%
16 6.5%
15 2.2%
16 7.4%
6
7
14c
6.52
4.66
1.0
0.6
168
70
3.4
19
17
17
18a 71%
18b 6.8%
18b 3.3%
19a 56%
19a 5.9%
19b 10%
19b 1.1%
20 8.2%
21 trace
8
9
5.19
4.42
1.2
2.1
72
72
32
36
17
19a 16%
19b 6.6%
24a,b traces
26b trace
20 38%
24c trace
27 trace
10
5.00
1.1
10[a]
16
22
23a,b 79%
E/Z=0.50
11
12
4.97
0.86
1.2
1.1
66.5[b]
4
25
29
26a 16%
30 1.1%
28 trace
169
1.3
trace amounts of the (monofluoroethenyl)benzenes 24a–24c
after 10 h at 658C (Table ; entry 10). Previously unknown Z-
(1,2-difluoroethenyl)ferrocene (26a) was isolated in 16%
yield from the HDF of (trifluoroethenyl)ferrocene (25) at
908C; traces of the E isomer 26b, Z-(monofluoroethenyl)-
ferrocene (27) and ethenylferrocene (28) were detected in
the crude reaction mixture (entry 11). The overall low yield
is due to the competing thermal dimerization of 25.[20] Com-
pound 26a was characterized by single-crystal X-ray diffrac-
tion. It crystallizes in the monoclinic space group P21/c with
one molecule in the asymmetric unit. As observed in 25, the
cyclopentadienyl rings exhibit an eclipsed conformation and
the difluoroethenyl moiety is twisted out of the C5H4 plane
by 25.88 due to intermolecular hydrogen bonding (Figure 4).
Finally, we probed the catalyst systemꢀs applicability to
the HDF of tetrafluoroallene (29). Although small amounts
of trifluoroallene (30) are formed, the turnover number
hardly exceeds one, even after prolonged reaction times;
forcing conditions led to a slight increase, but mainly result-
ed in thermal dimerization of the starting material (Table ;
entry 12).[21]
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Titanium-Catalyzed C F Bond Activation
FULL PAPER
ent turnover frequencies observed in the catalytic HDF
upon variation of the silane. An additional equilibrium be-
tween dimeric and monomeric species was shown to exist in
the presence of a suitable donor;[25] notably, catalytic HDF
employing 1a only proceeds in donor solvents such as THF
or diglyme. Both 31a and 31b were detected in a solution of
1
the activated catalyst in toluene by H NMR spectroscopy at
low temperatures.
Independently prepared 31b reacts with excess 4 to give
one equivalent of its HDF products 5a–5c; upon standing,
a second equivalent of 4 is slowly consumed accompanied
by the formation of titanoceneACTHNUTRGNEUGN(III) fluoride (32, Scheme 4).
Figure 4. Molecular structure and packing diagram of Z-(1,2-difluoro-
ethenyl)ferrocene (26a) determined by single-crystal X-ray diffraction (C
principal ellipse, F shaded octant, Fe plain ellipse, H circles; thermal el-
lipsoids drawn at 50% probability).
Active species: The initial assumption of titanoceneACTHNUGTRNEUNG(III) hy-
dride (3a) as the catalytically active species is in evident
contradiction to a report by Bercaw and Brintzinger.[22]
While 3a is purple and prone to rapid decomposition in so-
lution above À708C, the active catalyst forms a dark green
solution and performs well at ambient temperature. This
fact prompted us to study the nature of the active species in
order to gain insight into the catalytic cycle.
Scheme 4. Stoichiometric HDF of 4 by different titanium hydride species.
A solution of the activated catalyst in THF was prepared
from 1a and 2b; upon cooling, crystals of [{Cp2Ti
(m-
This stepwise reaction process indicates that initially only
one of the two titanium centers is capable of a HDF reac-
tion, consistent with the presence of only one hydride
ligand. Ligand scrambling at elevated temperatures is neces-
sary to regenerate the hydride and allow for another HDF
step. The involvement of a titanium hydride was further
confirmed by an independent preparation of the highly sen-
SiH2Ph)}2] (31a) separated, which were isolated and identi-
fied by single-crystal X-ray diffraction. Compound 32a has
previously been characterized as an intermediate in the tita-
nium-catalyzed dehydrogenative coupling of primary and
secondary silanes.[19b,23] In solutions containing excess silane,
31a is known to be in equilibrium with the dimeric silyl hy-
dride complex [Cp2Ti
(m-SiH2Ph)
U
sitive parent dimeric titanoceneACTHNGUTERNNUG
(III) hydride (3a).[22] Com-
mixed-valence titanocene hydride [(Cp2TiH)
N
pound 3a reacts with excess 4 within minutes to give two
equivalents of its HDF products and 32 (Scheme 4). Com-
pounds 31a, 31b, and 32 can also be employed as precata-
lysts for the HDF of 4 with performances similar to 1a, thus
confirming them to be part of or closely linked to the cata-
lytic cycle.
These findings suggest
a
catalytic cycle, in which
titanocene(III) hydride reacts with the substrate to give
titanoceneACHUTNGTERN(NUNG III) fluoride (32) and the HDF product
ACHTUNGTRENNUNG
(Scheme 3). Compound 32 subsequently reacts with a silane
molecule to regenerate the active hydride under fluorosilane
formation. This cycle is consistent with the observed benefits
of electron-donating ligands on the precatalyst. Higher elec-
tron densitiy at the metal increases the hydridicity of the
active species and weakens the titanium–fluorine bond.
Compound 32 was crystallized from the reaction mixture
and identified by single-crystal X-ray diffraction (Figure 5)
and EPR spectroscopy.[26] Compound 32 crystallizes as
a trimer in the orthorhombic space group P212121 with five
molecules of THF in the asymmetric unit. The titanium and
fluorine atoms are coplanar and exhibit slightly alternating
Scheme 3. Proposed catalytic cycle for the titanium-catalyzed HDF of 4
(Do=donor solvent).
À
À
distances (Ti Fshort =2.053(5) ꢄ, Ti Flong =2.063(6) ꢄ, Ti-F-
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D. Lentz et al.
Figure 5. Molecular structure of trimeric [Cp2TiF] (32) determined by
single-crystal X-ray diffraction (C principal ellipse, F shaded octant, Ti
plain ellipse; hydrogen atoms and solvent molecules omitted for clarity,
thermal ellipsoids drawn at 50% probability).
Scheme 5. The b-hydride addition/b-fluoride elimination mechanism (IE)
for the HDF of 4.
Ti =158.5(4)8, F-Ti-F=81.4(2)8 and Cp-Ti-Cp=130.7(1)8).
This geometry resembles that of the related 1,1’-
dimethyltitanocene
G
group is highly favored, because it is the most electrophilic
site in the molecule.[16b] The lower TOFs observed in the
HDF of 5d and 7 agree well with this assumption, because
the substratesꢀ lower degrees of fluorination reduce their
electrophilicity. Attempts to observe an intermediate 33a–
33c at low temperature were not successful.
As 7 does not have vinylic fluorine substituents, its HDF
by means of the IE mechanism must involve b-fluoride elim-
ination from a stable CF3 group. Consistently, the TOF is
almost two orders of magnitude lower than for 4. In addi-
tion, long-lived intermediates 35a and 35b are expected to
be subject to hydrogenolysis (or silanolysis) to furnish the
observed hydrogenation product 10 (Scheme 6). The pri-
mary HDF product 8 features vinylic fluorine substituents
À
solid state (Ti F=2.091(3) ꢄ, Ti-F-Ti=109.57(8)8, F-Ti-F=
70.43(8)8 and Cp-Ti-Cp=128.9(1)8).[27] The shorter titani-
um–fluorine bonds in 32 are due to lower electron density at
the metal; the six-membered Ti3F3 ring is less sterically
crowded than the four-membered Ti2F2 ring, resulting in
a less acute Cp-Ti-Cp angle. Interestingly, a similar Ti3F3
ring is also observed in solid TiF4 (Ti Fshort =1.964(4) ꢄ, Ti
long =1.976(3) ꢄ, Ti-F-Ti=159.3(3)8 and F-Ti-F=
80.7(2)8).[28]
F
Mechanistic studies: Two distinct mechanisms for the hydro-
defluorination of alkenes by metal hydride complexes have
been discussed in the literature: The b-hydride addition/b-
fluoride elimination mechanism
(IE) and the s-bond metathesis
mechanism (SBM).[16b,c,e,29] The
former comprises an insertion
of the alkene double-bond into
the titanium-hydrogen bond to
yield an intermediate titanium
fluoroalkyl
(Scheme 5).
species
Early-transition-
33a
metal fluoroalkyl species are
unstable[30] and prone to under-
Scheme 6. Proposed mechanism for the HDF of 3,3,3-trifluoropropene (7).
go b-fluoride elimination with
formation of metal fluorides
and alkenes.[9p] Both b-addition and b-elimination reactions
are known to proceed in a syn stereoselectivity;[1a] product
formation must therefore include a rotation around the
carbon–carbon single bond to enable b-fluoride elimination
from rotamer 33b to give 5a and 5b or 33c to give 5c. For-
mation of 5c through elimination from 33c is found to occur
only to a minor extent; this may be ascribed to the higher
stability of CF3 groups in comparison with CF2H groups.[2c,31]
Addition of the hydride to the central carbon atom would
be necessary for the formation of 5d via intermediates 34a
and 34b, but is not observed. Despite the higher steric hin-
drance expected from addition of the titanocene fragment
to the secondary carbon atom, hydride addition to the CF2
and therefore undergoes rapid HDF to yield 9a and 9b
prior to the complete consumption of 7. The final product
propene removes titanium from the catalytic cycle by inser-
tion to yield a titanium propyl complex 37, which is incapa-
ble of b-fluoride elimination. Moreover, titanoceneACHTUNGTRENNUNG(III)
alkyl species are unstable towards decomposition into alka-
nes and h1-h5-cyclopentadienyl or fulvalenyl titanium oligo-
mers.[32] The observed stoichiometric formation of 11 sup-
ports this assumption. Similarly, the poor results employing
13 or 14c as substrates may be due to catalyst deactivation
by formation of the HDF/hydrogenation product 16 in stoi-
chiometric amount. It remains, however, unclear why tetra-
fluoroethene (12) does not react at all.
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The observed low reactivity of the catalyst system towards
aryl alkenes 22 and 25 at room temperature is most proba-
bly due to steric repulsion between the titanocene moiety
and the geminal aryl substituent in the intermediate inser-
tion product.
The substrate-dependent variations in E/Z selectivity
demand a closer look at the conformation of the presumed
intermediate. b-Fluoride elimination requires a syn geome-
try of the metal fragment and the b-fluorine substituent, re-
sulting in an eclipsed conformation of either R1 (38a) or R2
(38b) with a vicinal fluorine substituent (Scheme 7). The rel-
ative sizes of R1 and R2 are therefore expected to determine
the preferred orientation and hence the selectivity. Consis-
tently, for R1 !R2 and R1 @R2 selectivity reaches up to 94
and 73%, respectively, while for R1 ꢀR2, almost no selectivi-
ty is observed.
Scheme 8. The s-bond metathesis mechanism (SBM) for the HDF of 4.
Nevertheless, unambiguous proof of the competition be-
tween both mechanisms is evident from the catalytic HDF
of 17. In this rigid cyclic substrate, stereochemical implica-
tions allow for a clear distinction between the mechanisms
by which a HDF product is formed.[16e] Addition of the tita-
nium hydride to 17 yields intermediate 42a (Scheme 9).
Scheme 9. Competing mechanisms in the HDF of hexafluorocyclobutene
(17).
Scheme 7. The origin of E/Z-selectivity explained by the IE mechanism
(percentages represent relative yields, Fc=ferrocenyl).
In contrast, the SBM mechanism (Scheme 8) does not in-
clude an intermediate. The HDF of 4 proceeds in one step
via a four-membered transition state 39a, 39b, or 40; transi-
tion states 39a and 39b should be similar in energy consis-
tent with the observed low E/Z-selectivity. For electronic
and steric reasons, 40 is expected to be unfavorable. Forma-
tion of 5c by SBM requires a six-membered transition state
Subsequent b-fluoride elimination must proceed in a syn ge-
ometry forming allylic HDF product 18b, which is detected
in the product mixture in small amounts. The main vinylic
HDF product 18a can only be formed by s-metathesis via
42b. The second HDF step follows a similar pattern: The
main product 19a is formed by SBM, whereas minor
amounts of 19b are formed by b-hydride addition/b-fluoride
elimination mechanism (IE); the side-product 18b also
yields 19b by SBM. Compound 19a does not contain any vi-
nylic fluorine atoms; further HDF must follow the IE path-
way, while its HDF product 20 is again subject to SBM to
yield 21.
Alternative mechanisms such as a radical mechanism or
an a-fluoride elimination followed by a rearrangement have
been discussed in the literature,[16e] but the absence of
dimers, oligomers, and hydrosilylation products render these
pathways unlikely.
41, resulting in a large F-Ti-H angle. TitanoceneACTHNUTRGNE(UNG III) halides
typically prefer L-Ti-L angles well below 908,[27,33] rendering
41 highly strained and energetically not feasible. Neither the
increased selectivity nor the lowered reactivity in the forma-
tion of 6a, 6b, 23a, 23b, 26a, and 26b are explained on the
basis of this mechanism, because steric bulk at the b-carbon
atom is unlikely to affect SBM.[16b] The HDF of 7 by SBM
involves an unfavorable six-membered transition state, con-
sistent with the low TOF; however, no explanation for the
competing hydrogenation is possible.
Chem. Eur. J. 2012, 00, 0 – 0
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D. Lentz et al.
The poor performance of the catalyst system in the HDF
of 29 starkly contrasts the high reactivity attributed to fluo-
rinated cumulenes. This is especially surprising, since 29 is
the only substrate for which the titanium-catalyzed reaction
differs significantly from a related stoichiometric reaction
involving zirconium hydride complexes.[34] This indicates
a substrate-specific deactivation pathway. Assuming an IE
mechanism, two hydrometalation products 43a and 43b are
possible (Scheme 10). While 43a can undergo b-fluoride
elimination to yield 30 and 32, 43b does not have a b-fluo-
rine substituent and cannot continue the catalytic cycle. The
Figure 6. Molecular structure of 45 determined by single-crystal X-ray
diffraction (C principal ellipse, F shaded octant, Cr plain ellipse, H cir-
cles, O cutout ellipse; thermal ellipsoids drawn at 50% probability).
sterically crowded primary carbon atom is strongly favored
over an addition to the secondary carbon atom. As the
steric trends in Groups 4 and 6 are very similar, the same se-
lectivity issues are likely to be responsible for the titanium
catalystꢀs poor performance.
Conclusion
In summary, we have developed a simple and inexpensive
Scheme 10. Top: Proposed intermediates in the titanium-catalyzed hydro-
defluorination of tetrafluoroallene (29). Bottom: Hydrometalation of 29
with Group 6 hydride complexes.
À
system for the catalytic C F activation of fluoroalkenes,
which exceeds all previously known systems in both activity
and scope. Titanocene difluoride (1a) as a precatalyst was
shown to effect the hydrodefluorination of a number of vi-
nylic and allylic fluorides with varying efficiencies and selec-
tivities. Optimization on the E/Z selectivity by introducing
substituents to the titanocene cyclopentadienyl ligands was
not successful, but demonstrated the importance of electron
donation to the metal. This is consistent with mechanistic
regioselectivity in this insertion step is therefore crucial to
the successful catalysis. In the stoichiometric HDF using the
Schwartz reagent, 30 was obtained in 47% yield.[34] Assum-
ing 60% selectivity here, the turnover number would con-
verge to 1.5, close to the experimental value of 1.2. As at-
tempts to isolate a catalyst deactivation product 43b failed,
we studied the reactivity of 29 towards Group 6 hydride
complexes (Scheme 10). As reported previously,[35]
[Cp(CO)3MoH] and 29 form a hydrometalation product 44a
by addition of the metal center to the central carbon atom;
compound 44a undergoes a 1,3-fluorine migration affording
44b. In contrast, the chromium congener forms insertion
product 45 by addition of the metal to a terminal carbon
atom and a subsequent 1,3-fluorine shift. Compound 45 was
characterized by single-crystal X-ray diffraction; it crystalli-
zes in the monoclinic space group P21/c with a single mole-
cule in the asymmetric unit (Figure 6). The bond angles and
studies indicating a titanium
ACHTUNGTRENNUNG
cies; upon reaction with the substrate, titanoceneAHCTUNGTRENNUNG
ride (32) is formed, which was characterized by X-ray crys-
tallography. The key hydrodefluorination step is likely to
follow both an alkene insertion/b-fluoride elimination and
a s-bond metathesis mechanism, as deduced from the ste-
reochemical outcome of the HDF of hexafluorocyclobutene
(17). Conformational aspects of the insertion/elimination
mechanism explain the observed substrate-dependent E/Z
selectivity.
À
distances are within the expected range; the distances Cr
Experimental Section
À
C1=2.105(2) ꢄ and Cr Cp=1.839(2) ꢄ are significantly
À
À
shorter than the distances Mo C2=2.252(2) ꢄ and Mo
Cp=2.000(4) ꢄ in 44a. Apparently, the chromium atomꢀs
smaller radius increases the steric bulk in its coordination
sphere so that an addition of the metal fragment to the less
Techniques: All reactions and manipulations were carried out in pre-
dried glassware under an argon atmosphere by using standard Schlenk-
type and vacuum-line techniques, or by working in an argon-filled
MBraun glove box model LAB master SP. The amount of gaseous com-
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À
Titanium-Catalyzed C F Bond Activation
FULL PAPER
Table 2. Selected X-ray crystallographic data.
1c
1h
1 f
1 f
26a
32
45
formula
Mr
crystal system
space group
a [ꢄ]
b [ꢄ]
c [ꢄ]
a [8]
b [8]
C12H14F2SiTi
272.22
monoclinic
P21/n
7.906(3)
14.771(6)
10.165(4)
90.00
98.626(9)
90.00
1173.7(9)
133(2)
4
C22H8F12Ti
548.18
monoclinic
C2/c
20.957(5)
5.7686(14)
16.298(4)
90.00
109.988(5)
90.00
1851.6(8)
133(2)
4
C13H18F2SiTi
288.26
monoclinic
P21/c
10.1318(17)
11.1388(19)
11.969(2)
90.00
99.710(4)
90.00
C13H18F2SiTi
288.26
C12H10F2Fe
248.05
monoclinic
P21/c
5.7200(10)
7.3943(13)
23.140(4)
90.00
90.919(4)
90.00
978.6(3)
133(2)
4
C50H70F3O5Ti3
951.76
orthorhombic
P212121
16.390(8)
16.837(7)
17.204(8)
90.00
90.00
90.00
4747(4)
133(2)
4
C11H6CrF4O3
314.16
monoclinic
P21/c
7.8156(11)
14.074(2)
10.6116(15)
90.00
98.016(3)
90.00
triclinic
¯
P1
6.058(3)
11.799(6)
19.594(10)
75.534(10)
81.612(11)
86.056(12)
1341.0(11)
133(2)
g [8]
V [ꢄ3]
T [K]
Z
1331.4(4)
133(2)
4
1155.8(3)
133(2)
4
4
m [mmÀ1
]
0.825
0.591
0.731
0.726
1.528
0.549
1.043
total reflns
18606
14381
14632
8571
15494
49085
17965
unique reflns
Rint
3593
2842
3931
0.0132
0.0211
0.0622
0.0223
0.0629
1.080
3760
0.0815
0.0748
0.1847
0.1252
0.2167
1.026
2994
8618
3535
0.0254
0.0285
0.0706
0.0359
0.0754
1.046
0.0354
0.0402
0.1066
0.0536
0.1133
1.090
0.0173
0.0303
0.0838
0.0340
0.0865
1.053
0.0178
0.0228
0.0609
0.0242
0.0617
1.059
0.1022
0.0713
0.1849
0.0970
0.2140
1.088
R1 [I>2s(I)]
wR(F2) [I>2s(I)]
R1 (all data)
wR(F2) (all data)
GOF on F2
pounds was determined using pVT techniques or by condensing the gas
into a weighed flask.
under a stream of cold nitrogen. Data collection, reduction and empirical
absorption correction were performed using the SMART, SAINT and
SADABS programs, respectively;[47] the SHELX program package[48] was
used for structure solution and refinement (see Table 2 for structural de-
tails). ORTEP[49] and POV-RAY[50] were employed for structure visuali-
zation. EPR spectra were recorded on a Bruker ER 200D-SRIC spec-
trometer equipped with a B-E 25 magnet, external standard diphenylpi-
crylhydroxyl (DPPH) g=2.0037. Mass spectra were measured on
a Varian MAT 711 spectrometer operating at 80 eV (EI) or an Agilent
6210 ESI-TOF, solvent flow rate 4 mLminÀ1, spray voltage 4 kV, drying
gas flow rate 105 Pa, all other parameters were adjusted for a maximum
abundance of the relative [M+H]+ (ESI).
Chemicals: Diglyme, 1,4-dioxane, THF, [D8]THF, and [D8]toluene were
freshly distilled from sodium/benzophenone ketyl or stored over sodium/
potassium alloy; diglyme for catalysis experiments was stored and manip-
ulated inside a glovebox. Acetonitrile, toluene, and pentane were dried
over activated alumina using an MBraun solvent system model MB SPS-
800. [Cp2TiF2] (1a),[36] [Cp2TiH] (3a),[22] [(Me3SiC5H4)2TiCl2],[37]
Me3SnF,[10r]
[(Me2SiC5H4)2TiCl2],[38]
[(C6F5C5H4)2TiCl2],[39]
[(Me3SiC5H4)CpTiCl2],[40] [(Dipp2Nacnac)TiF3] (1i),[41] hexafluorocyclobu-
tene (17),[42] (Trifluoroethenyl)benzene (22),[43] (trifluoroethenyl)ferro-
cene (25),[20] [Cp2Ti (m-H)TiCp2] (31b)[23a] tetrafluoroallene
ACHTUNGTRENNUNG(m-SiH2Ph)ACHTUGNTRENNUGN
(29),[35] and [Cp(CO)3CrH][44] were synthesized by literature procedures.
[Cp2TiF] (32) was synthesized by a modified literature procedure[45] using
zinc instead of aluminum powder. Ph2SiH2 (2a),[46] PhSiH3 (2b),[46]
PhMe2SiH (2c, Alfa Aesar), Et3SiH (2d, ABCR) and PMHS (2e, Alfa
Aesar) were purchased or synthesized, dried (calcium hydride 2a–2d,
molecular sieves 2e) and stored in a glovebox. [(EtC5H4)2TiCl2] (MCAT),
[(tBuC5H4)2TiCl2] (MCAT), [(R,R)-(ebthi)TiF2] (1g, Sigma–Aldrich), TiF3
(1j, ABCR), TiF4 (1k, ABCR), 1,1,3,3,3-pentafluoropropene (5d, Syn-
Quest Labs), trifluoroethene (13, SCM Specialty Chemicals), and 1,1-di-
fluoroethene (14c, SynQuest Labs) were obtained from commercial sour-
ces and used as received. Hexafluoropropene (4, Solvay Fluor), 3,3,3-tri-
fluoropropene (7, Hoechst) and tetrafluoroethene (12, Bundesanstalt fꢁr
Materialprꢁfung und -forschung) were obtained free of charge and used
as received.
Synthesis of bis(h5-ethylcyclopentadienyl)difluorotitanium (1b): In
a Schlenk flask equipped with a reflux condenser, dichlorobis(h5-ethylcy-
clopentadienyl)titanium (310 mg, 1.02 mmol) and trimethyltin fluoride
(384 mg, 2.10 mmol, 2.1 equiv) were refluxed in acetonitrile (20 mL) for
4 h.[27] After removal of the solvent in vacuo, the residue was sublimed at
1308C and 10À3 mbar to give 1b as a yellow powder. Yield: 85%; m.p.
1308C; 1H NMR (CDCl3): d=6.36 (m, 4H; C5H4), 6.01 (m, 4H; C5H4),
2.53 (q, 3J(H,H)=7.8 Hz, 4H; CH2), 1.14 ppm (t, 3J
ACTHNUTRGENNUG ACHUTNTGREN(NUGN H,H)=7.8 Hz, 6H;
CH3); 13C NMR (CDCl3): d=143.6 (s, C5H4), 116.3 (s, C5H4), 115.1 (s,
C5H4), 22.3 (s, CH2), 13.1 ppm (s, CH3); 19F NMR (CDCl3): d=61.2 ppm
(s, 2F; TiF2); IR (ATR): n˜ =3114 (w), 2966 (m), 2933 (w), 2869 (w), 1499
(m), 1456 (m), 1427 (w), 1402 (w), 1369 (m), 1312 (w), 1236 (w), 1092
(w), 1058 (m), 1031 (m), 954 (w), 917 (m), 842 (s), 821 (s), 686 (w), 608
(w), 562 (vs), 543 cmÀ1 (vs); MS (EI, 408C): m/z (%): 272.0849 (30) [M]+
(C14H18F2Ti requires 272.0856), 253 (19) [MÀF]+, 252 (31) [MÀHF]+,
232 (85) [MÀ2HF]+, 229 (23) [MÀC2H4ÀCH3]+, 179 (100)
CCDC-874358 (1c), 874361 (1h), 874360 (1 f; space group P21/c), 874359
¯
(1 f; space group P1), 874362 (26a), 874363 (32) and 874364 (45) contain
[MÀEtC5H4)]+,
159
(16)
[MÀEtC5H4ÀHF)]+,
158
(21)
the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Instrumentation: 1H, 13C, and 19F NMR spectra were recorded on
[MÀEtC5H5ÀHF)]+, 151 (51) [CpTiF2]+, 145 (11%), 132 (10) [CpTiF]+,
106 (5) [Et2Ti]+, 93 (29) [EtC5H4]+, 91 (45) [Et2TiÀCH3]+, 86 (11)
[TiF2]+, 78 (13) [EtTiH]+, 77 (29) [EtTi]+, 67 (6) [TiF]+, 65 (8) [C5H5]+;
elemental analysis calcd (%) for C14H18F2Ti: C 61.78, H 6.67; found:
61.48, H 6.61.
a
JEOL LAMBDA 400 or ECS 400 spectrometer (399.65, 100.40,
376.00 MHz, respectively). Chemical shifts are reported in ppm (d) rela-
tive to TMS (1H and 13C) and CFCl3 (19F), and were determined by refer-
ence to the residual solvent resonances (1H and 13C) or an external refer-
ence (19F). IR spectra were obtained from a Nicolet iS10 FTIR spectrom-
eter equipped with a Smart DuraSamplIR detector. Melting points are
given without correction. Single-crystal X-ray structure determination
was performed on a Bruker-AXS SMART 1000 fitted with a CCD detec-
tor. Crystals suitable for X-ray structure determination were selected
Synthesis of bis(h5-tert-butylcyclopentadienyl)difluorotitanium (1c): In
a Schlenk flask equipped with a reflux condenser, dichlorobis(h5-tert-bu-
tylcyclopentadienyl)titanium (375 mg, 1.04 mmol) and trimethyltin fluo-
ride (475 mg, 2.60 mmol, 2.6 equiv) were refluxed in acetonitrile (20 mL)
for 6 h. After removal of the solvent in vacuo, the residue was sublimed
at 1308C and 10À3 mbar to give 1c as a yellow powder. Yield: 85%; m.p.
1878C; 1H NMR (CDCl3): d=6.37 (m, 4H; C5H4), 6.17 (m, 4H; C5H4),
Chem. Eur. J. 2012, 00, 0 – 0
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D. Lentz et al.
1.22 ppm (s, 18H; CH3); 13C NMR (CDCl3): d=150.1 (s, C5H4), 115.6 (s,
C5H4), 114.8 (s, C5H4), 33.4 (s, C
(CH3)3), 30.4 ppm (s, CH3); 19F NMR
[MÀC6F5C5H4TiF2ÀHF]+, 205 (42) [MÀC6F5C5H4TiC5H4]+, 193 (100)
[M-C6F5C5H4TiF2]+, and smaller fragment ions.
AHCTUNGTRENNUNG
(CDCl3): d=62.2 ppm (s, 2F; TiF2); IR (ATR): n˜ =3109 (w), 3097 (w),
2954 (m br), 2905 (w), 2874 (w), 1491 (m), 1460 (m), 1417 (w), 1397 (w),
1383 (w), 1358 (m), 1276 (m), 1202 (w), 1158 (m), 1058 (w), 1045 (m),
1022 (w), 928 (w), 915 (w), 894 (m), 844 (m), 813 (s), 688 (m), 610 (m),
566 (s), 542 cmÀ1 (vs); MS (EI, 708C): m/z (%): 328.1490 (41) [M]+
(C18H26F2Ti requires 328.1482), 309 (7) [MÀF]+, 293 (29)
[MÀHFÀCH3]+, 274 (4) [MÀHF2ÀCH3]+, 207 (100) [MÀtBuC5H4]+, 185
(25) [MÀtBuTiF2]+, 172 (25) [MÀtBuC5H4ÀHFÀCH3]+, 121 (62)
[tBuC5H4]+, 105 (32) [tBuTi]+, 91 (14) [tBuTiÀCH3]+, and smaller frag-
ment ions; elemental analysis calcd (%) for C18H26F2Ti: C 65.86, H 7.98;
found: 85.87, H 7.95.
Catalytic hydrodefluorination: Substrates, conditions and products are
listed in Table 1 in the Supporting Information. A similar procedure was
applied for all substrates except 22 and 25:
A single-necked flask
equipped with a J. Young PTFE valve was charged with catalyst, silane,
and solvent. After degassing, the mixture was heated until the color
changed from yellow to purple to green (no color change was observed
when employing catalysts 1i–1k). The substrate was then condensed onto
the frozen solution, which was warmed to the desired temperature and
stirred for a given period of time under autogenous pressure. Fractional
condensation of the reaction mixture through two subsequent traps kept
at À788C (À308C for the HDF of 17) and À1968C, respectively, gave the
product mixture in the colder trap, which was vacuum transferred to a J.
Synthesis
of
bisACHTUNGTRENNUNG
Young NMR tube containing
a standard solution of fluorobenzene
(1d): In a Schlenk flask equipped with a reflux condenser, dichlorobisAHCTNUGTRENNNUG
(a,a,a-trifluorotoluene for the HDF of 12–14 and 17) in CDCl3. The
yields were determined from NMR spectra by integration of product res-
onances versus the internal standard.
(trimethylsilyl)cyclopentadienyl]-titanium (500 mg, 1.27 mmol) and trime-
thyltin fluoride (500 mg, 2.74 mmol, 2.2 equiv) were refluxed in acetoni-
trile (40 mL) for 7 h. After removal of the solvent in vacuo, the residue
was sublimed at 858C and 10À3 mbar. Recrystallization of the sublimate
HDF products were identified by NMR spectroscopy, using literature
data for 5a,[52] 5b,[52] 5c,[17a] 6a,[53] 6b,[53] 6c,[53] 6d,[17a] 6e,[54] 8,[55] 9a,[55]
9b,[55] 10,[54] 14a,[56] 14b,[56] 15,[56] 17,[57] 18a,[58] 19a,[58] 20,[58] 21,[59] 23a,b,[60]
24a,b,[61] 25[62] and 30[34] or by comparison with authentic samples of 4,
5d, 12, 13, 14c, 28 and 29.
1
from toluene gave 1d as a yellow powder. Yield: 89%. H NMR identical
to the literature;[51] 19F NMR (CDCl3): d=74.4 ppm (s, 2F; TiF2).
Synthesis of dimethylsilyleneACHTUNGTRNEUNG
[bis(h5-cyclopentadienylidene)]difluorotita-
nium (1e): In a Schlenk flask equipped with a reflux condenser, di-
chloro[dimethylsilylenebis(h5-cyclopentadienylidene)]titanium (390 mg,
1.28 mmol) and trimethyltin fluoride (850 mg, 4.65 mmol, 3.6 equiv) were
refluxed in acetonitrile (40 mL) for 7 h. After removal of the solvent in
vacuo, the residue was sublimed at 1108C and 10À3 mbar. Recrystalliza-
tion of the sublimate from toluene gave 1e as yellow crystals. Yield:
52%. 1H NMR identical to the literature;[51] 19F NMR (CDCl3): d=
68.8 ppm (s, 2F; TiF2).
Data
1H NMR (CDCl3): d=5.44 ppm
(dddd, 2J(H1,F5)=63.6 Hz, 4J(H1,F2)=
17.4 Hz, 3J(H1,F1)=4.8 Hz, 3J(H1,F3)=
1.4 Hz, 1H; H1); 19F NMR (CDCl3):
for
18b
(Scheme 11):
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
d=À113.1
(ddddd,
(F3,F1)=20.2 Hz,
ACTHNUTRGENNUG CAHTUNGTRENNUNG
(F3,F5)=17.5 Hz, 3J(F3,F2)=1.8 Hz, 3J-
2J
ACHTUNGTRENNUNG
198.8 Hz,
4J
ACHTUNGTRENNUNG
Scheme 11. Numbering
schemes for 1,2,3,3,4-penta-
fluorocyclobutene (18b) and
1,3,4,4-tetrafluorocyclobutene
(19b).
Synthesis of (h5-cyclopentadienyl)
fluorotitanium (1 f): In a Schlenk flask equipped with a reflux condenser,
dichloro(h5-cyclopentadienyl)[h5-(trimethylsilyl)cyclopentadienyl]titanium
AHCTUNGTRENNUNG
[h5-(trimethylsilyl)cyclopentadienyl]di-
AHCTUNGTRENNUNG
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHUTNGRENNUG CAHTUNGTRENNUNG
(F4,F5)=15.3 Hz, 3J
(1.57 g, 4.89 mmol) and trimethyltin fluoride (1.78 g, 9.78 mmol,
2.0 equiv) were stirred in toluene (30 mL) at 808C for 4 h. After removal
of the solvent in vacuo, the residue was sublimed at 1308C and
10À3 mbar. Recrystallization of the sublimate from toluene/pentane (1:3)
gave 1 f as yellow crystals. Yield: 90%; m.p. 1428C (polymorph P21/c),
10.8 Hz, 1F; F4), À122.5 (ddddd, 4J-
A
(F1,F4)=17.5 Hz,
3J
(F1,F2)=10.8 Hz, 3J(F1,H1)=4.1 Hz,
ACHUTGTNRENNUG CAHTUNGTRENNUGN
3J
(F1,F5)=2.3 Hz, 1F; F1), À127.4 (dtt, 4J
G
ACHTUNGTRENNUNG
(F2,F1)=
10.8 Hz, 3J
A
ACHTUNGTRENNUNG
1448C (polymorph P1); 1H NMR ([D8]toluene): d=6.86 (m, 2H;
¯
63.4 Hz,
3J
A
3J
(F5,F4)=15.3 Hz,
3J
ACHTUNGTRENNUNG
C5H4SiMe3), 6.60 (m, 2H; C5H4SiMe3), 6.54 (s, 5H; C5H5), 0.27 ppm (s,
9H; CH3); 13C NMR ([D8]toluene): d=132.9 (s, CSiMe3), 126.1 (s, C5H5),
117.7 (s, C5H4SiMe3), 116.5 (s, C5H4SiMe3), À1.1 ppm (s, SiMe3);
19F NMR ([D8]toluene): d=93.0 ppm (s, 2F; TiF2); IR (ATR): n˜ =3125
(w), 3090 (w), 3083 (w), 2956 (m), 1450 (m), 1400 (m), 1369 (m), 1244
(s), 1186 (m), 1034 (s), 1011 (m), 906 (m), 837 (vs), 821 (vs), 752 (s), 697
(m), 630 (s), 608 (vs), 577 cmÀ1 (vs); MS (ESI-TOF): m/z calcd for
C13H18F2TiSiNa: 311.0523; found: 311.0604 [M+Na]+; m/z calcd for
C26H36F4Ti2Si2Na: 599.1148; found: 599.1303 [M2 +Na]+.
ACTHNUTRGNEUNG
4J(F5,F2)=1.8 Hz, 1F; F5); supported by 19F COSY.
Data for 19b (Scheme 11): 1H NMR (CDCl3): d=5.85 (ddddd, 4J-
(H1,F3)=11.9 Hz, 4J(H1,F4)=8.7 Hz, 3J(H1,F1)=7.8 Hz, 3J(H1,F2)=
0.9 Hz, 3J(H1,H2)=0.9 Hz, 1H; H1), 5.38 ppm (dddd, 2J(H2,F2)=62.3 Hz,
4J (H2,H1)=0.9 Hz, 1H; H2);
(F1,H2)=18.9 Hz, 3J(F1,F4)=
A
E
N
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
(H2,F1)=19.2 Hz, 3J(H2,F3)=1.8 Hz, 3J
AHCTUNGTREUNNGN ACHTUNGTRENNUNG ACTUHNGTRENNUGN
19F NMR (CDCl3): d=À102.1 (dddd, 4J
N
ACHTUNGTRENNUNG
8.5 Hz, 3J
A
(F1,F2)=5.0 Hz, 1F; F1), À111.0 (dddd, 2J-
AHCTUNGTERNNUGN ACHTUGTNRENNUG ACHTUNGERTNNUNG ACHTUNGTRENNUNG
(F3,F4)=205.6 Hz, 3J(F3,F2)=18.0 Hz, 4J(F3,H1)=11.7 Hz, 3J(F3,H2)=
1.8 Hz, 1F; F3), À119.6 (dddd, J
(F4,F3)=205.6 Hz, J(F4,F2)=14.8 Hz, J-
ACHUTGTNRENNUG CAHTUNGTRENNUGN
2
3
4
Synthesis of bis
ACHTUNGTRENNUNG
(F4,H1)=8.7 Hz, 3J
(F4,F1)=8.5 Hz, 1F; F4), À182.8 ppm (ddddd, 2J-
(1i): In a Schlenk flask equipped with a reflux condenser, dichlorobisAHCTNUGTRENNNUG
(F2,H2)=62.5 Hz, 3J
A
(F2,F4)=14.8 Hz, 4J(F2,F1)=
ACHUTGTNRENNUG ACHTUNGTRENNUNG
(pentafluorophenyl)cyclopentadienyl] titanium (279 mg, 0.48 mmol) and
trimethyltin fluoride (252 mg, 1.38 mmol, 2.9 equiv) were refluxed in ace-
tonitrile (5 mL) for 12 h. Upon cooling to room temperature, yellow crys-
tals of 1i separated, which were filtered off, washed with acetonitrile
(10 mL) and sublimed (1908C, 10À3 mbar). Yield: 19%; m.p. 2198C;
1H NMR ([D8]THF): d=6.76 ppm (m, 4H; C5H4); 13C{1H} NMR
3
1
5.0 Hz, JACTHUNGTRENNNUG
A
E
N
ACHTUNGTRENNUNG
A
E
N
ACHTUNGTRENNUNG
A
E
N
ACHTUNGTRENNUNG
ACTHNUTRGNEUNG
([D8]THF): d=117.4 (s, C5H4), 116.3 ppm (s, C5H4); 13C{19F} NMR
A
E
ACHTUNGTRENNUNG
2
2
3
186 Hz, JACTHUNGTRENU(NNG C,F)=30 Hz, JACHUTTGNRENNUG(C,F)=26 Hz, JACTHUNTGRENNUGN
([D8]THF): d=145.1 (s, C6F5), 140.2 (s, C6F5), 137.9 ppm (s, C6F5);
19F NMR ([D8]THF): d=110.4 (brs, 2F; TiF2), À149.0 (brd, J=18.2 Hz,
4F; C6F5), À157.6 (tt, J=20.9 Hz, J=1.8 Hz, 2F; C6F5), À165.2 ppm (m,
4F; C6F5); IR (ATR): n˜ =3152 (w), 3118 (vw), 1660 (w), 1622 (w), 1534
(m), 1491 (s), 1415 (m), 1385 (w), 1342 (w), 1321 (vw), 1263 (vw), 1235
(w), 1136 (vw), 1106 (w), 1086 (w), 1056 (s), 979 (s), 931 (w), 911 (m),
879 (w), 860 (m), 824 (s), 782 (s), 676 (vw), 618 (w), 607 (w), 574 (s),
560 cmÀ1 (s); MS (EI, 1508C): m/z (%): 547.9893 (7) [M]+ (C22H8F12Ti
requires 547.9914), 529 (1) [MÀF]+, 336 (3) [MÀC6F4C5H4]+, 317 (53)
Catalytic hydrodefluorination of trifluoroethenylbenzene (22): A single-
necked flask equipped with a J. Young PTFE valve was charged with 1a
(10.8 mg, 0.05 mmol, 5 mol%), 2a (203 mg, 1.10 mmol, 1.1 equiv), and
THF (5.0 mL). After degassing, the mixture was heated until the color
changed to blue-green and 22 (158 mg, 1.00 mmol) was added by syringe.
The reaction mixture was degassed again and stirred at 658C for 10 h.
Subsequently, water (5 mL) was added and the mixture was extracted
with pentane (3ꢅ5 mL). The combined extracts were washed with water,
dried over sodium sulfate and filtered through a pad of silica. Fractional
condensation through two subsequent traps kept at À788C and À1968C,
[MÀC6F5C5H4]+,
300
(3)
[MÀC6F5C5H4ÀF]+,
231
(82)
[MÀC6F5C5H4TiF2]+, 212 (36) [MÀC6F5C5H4TiF2ÀF]+, 211 (52)
&
10
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ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
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À
Titanium-Catalyzed C F Bond Activation
FULL PAPER
respectively, afforded
(2:1:0.0072:0.0050:0.0094) in the colder trap. Yield: 79%.
a
mixture of 23a, 23b, 24a, 24b and 24c
hydridochromium (475 mg, 2.35 mmol) by vacuum transfer. The reaction
mixture was stirred at room temperature overnight, filtered over a glass
frit and the frit was washed with additional pentane. The filtrate was
cooled to À808C overnight to precipitate 45 as yellow crystals. Yield:
28%; m.p. 988C (decomp); 1H NMR ([D8]toluene): d=5.31 (dq, 3J-
Catalytic hydrodefluorination of trifluoroethenylferrocene (25): A single-
necked flask equipped with a J. Young PTFE valve was charged with 1a
(48.1 mg, 0.22 mmol, 5 mol%), 2a (946 mg, 5.13 mmol, 1.2 equiv) and di-
oxane (2.0 mL). After degassing, the mixture was heated until the color
changed to blue-green. A solution of 25 (1.18 g, 4.43 mmol) in dioxane
(35 mL) was added. The reaction mixture was degassed again and stirred
at 908C for 66.5 h. Subsequently, the solvent was removed in vacuo and
the crude product was purified by column chromatography on silica gel
with pentane as eluent to give 26a. Yield: 16%; m.p. 638C; 1H NMR
A
ACHTUNGTREN(NUGN H,F)=7.7 Hz, 1H; HC=CF3), 3.99 ppm (brs, 5H;
A
G
ACHTUNGTRENNUNG
(C,F)=9.58 Hz, CF3), 118.0 (qd, 2J(C,F)=32.6 Hz, 3J
AHCTUNGTREUNNGN ACHTUNGTRENNUNG ACHTUNGTRENNUNG
3.35 Hz, =CH), 90.3 ppm (s, C5H5); 19F NMR ([D8]toluene): d=À13.8
(qd, 3J
(F,H)=46.0 Hz, 4J
A
ACHUTNGRENNUG CAHTUNGTERN(NUGN F,H)=7.9 Hz, 3F; CF3); IR (ATR): n˜ =3135 (w), 2963
(F,F)=14.2 Hz, 3J
2
3
(CDCl3): d=6.54 (dd, JACHTUNGTRENNUNG(H,F)=74.0 Hz, JACHUTNGTRENN(UNG H,F)=17 Hz, 1H; CF=CFH),
(vw), 2872 (vw), 2112 (vw), 2028 (m), 1968 (s), 1943 (vs br), 1592 (s),
1433 (m), 1371 (vw), 1283 (s), 1244 (m), 1124 (m), 1097 (s), 1064 (m), 949
(m), 873 (w), 842 (m), 768 (m), 645 (m), 632 (m), 597 (vs), 554 cmÀ1 (s);
MS (EI, 408C): m/z (%): 313.9673 (4) [M]+ (C11H6F4CrO3 requires
313.9658), 286 (2) [MÀCO]+, 258 (6) [M+À2CO]+, 230 (14) [M+
À3CO]+, 211 (6) [M+À3COÀF]+, 136 (100) [CpCrF]+, 117 (5) [CpCr]+,
90 (3) [CrF2]+, 71 (18) [CrF]+, 52 (13) [Cr]+.
4.35 (brs, 2H; C5H4), 4.28 (brs, 2H; C5H4), 4.23 ppm (s, 5H; C5H5);
13C NMR (CDCl3): d=148.0 (dd, 1J
A
E
1
2
C2F2H), 130.8 (dd, J
(C,F)=253.0 Hz, J
G
2J
65.3 ppm (t, J
(F,F)=16.2 Hz, 3J
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG ACHTUNGTRENNUNG
(F,F)=25.4 Hz, 3J
ACHTUNGTRENNUNG
G
3
3
2J
A
R
ACHTUNGTRENNUNG
IR (ATR): n˜ =3117 (w), 3083 (vw), 2955 (w), 2924 (m), 2850 (w), 1747
(vw), 1702 (m), 1537 (w), 1476 (m), 1381 (m), 1308 (m), 1277 (m), 1225
(w), 1130 (s), 1103 (m), 1072 (s), 1029 (m), 999 (m), 901 (s), 867 (w), 818
(s). 788 (s). 766 (s), 659 (m), 571 cmÀ1 (m); MS (EI, 308C): m/z (%):
248.0094 (100) [M]+ (C12H10F2Fe requires 248.0100), 153 (24)
[MÀFeFÀHF]+, 152 (13) [MÀFeÀ2 HF]+, 140 (124) [C5H5FeF]+, 124 (6)
Acknowledgements
[M]2+
,
108 (6) [MÀC5H5FeF]+, 89 (51) [MÀC5H5FeF2]+, 63 (10)
This work was financially supported by the Deutsche Forschungsgemein-
schaft as part of the graduate school program GRK 1582/1 “Fluor als
Schlꢁsselelement”. We thank Solvay Fluor (Germany) for a donation of
hexafluoropropene and tetrafluoroethane, Hoechst for a donation of tri-
fluoropropene and the Bundesanstalt fꢁr Materialprꢁfung und -forschung
for a gift of tetrafluoroethene. M.F.K. is grateful to Prof. Dr. Uwe Rosen-
thal for helpful discussions and to the GDCh division Fluorchemie for
a conference scholarship. We thank Cindy Glor, Luisa Losensky, and
Christian Wende for their contributions. We thank the Humboldt-Univer-
sitꢂt zu Berlin for performing elemental analyses.
[C2F2H]+, and smaller fragment ions; elemental analysis calcd (%) for
C12H10F2Fe: C 58.10, H 4.06; found: 59.01, H 4.21.
Data for 26b, identified from the crude product: 1H NMR (CDCl3): d=
7.26 (dd, 2J
ACHTUNGTRENNUNG(H,F)=75.8 Hz, J=3.2 Hz, 1H; CF=CFH), 4.57 (brs, 2H;
C5H4), 4.34 (brs, 5H; C5H5), 4.31 ppm (brs, 2H; C5H4); 19F NMR
(CDCl3): d=À161.4 (d, 3J
(F,F)=126.4 Hz, 1F; C5H4CF=CFH), À179.5
A
3
2
(dd, JACHTUNGTRENNUNG(F,F)=126.4 Hz, JAHCTUNGTREN(NUGN F,H)=75.6 Hz, 1F; C5H4CF=CFH).
Data for 27, identified from the crude product: 1H NMR (CDCl3): d=
2
3
3
6.55 (dd, J
N
ACHTUNGTRENN(UNG H,H)=5 Hz, 1H; ArCH=CFH), 5.33 (dd, J-
A
ACHTUNGTRENNUNG
4.35 (brs, 2H; C5H4), 4.21 ppm (brs, 5H; C5H5); 19F NMR (CDCl3): d=
2
3
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51%, 5b: 38%, 28a: 11%.
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containing 3a (5.2 mg, 15 mmol) held at À1968C. The tube was warmed
to À808C, shaken and allowed to warm to room temperature within
10 min. NMR and EPR spectra were recorded, the relative amounts of
starting material and HDF products were determined by integration of
19F NMR resonances: 10 min (RT): 4: 0%, 5a: 56%, 5b: 31%, 6a: 12%;
EPR: broad singlet at g=1.979(3), identical to literature data for 32.[26]
ACHTUNGTRENNUNG
(h5-Cyclopentadienyl)[h1-{(Z)-1,3,3,3-tetrafluoropropenyl}]tricarbonyl-
chromium (45): pentane (20 mL) and 29 (5.0 mmol, 2.1 equiv) were
added to a Schlenk flask charged with tricarbonyl(h5-cyclopentadienyl)-
Chem. Eur. J. 2012, 00, 0 – 0
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Received: April 3, 2012
Published online: && &&, 0000
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These are not the final page numbers!

À
Titanium-Catalyzed C F Bond Activation
FULL PAPER
À
C F Bond Activation
M. F. Kuehnel, P. Holstein, M. Kliche,
J. Krꢁger, S. Matthies, D. Nitsch,
J. Schutt, M. Sparenberg,
D. Lentz* ...................... &&&& — &&&&
Titanium-Catalyzed Vinylic and Allylic
The broad application of the catalytic
hydrofluorination of fluoroalkenes by
the system [Cp2TiF2]/silane is demon-
strated. Isolated yields up to 79%
could be obtained for various sub-
strates. Mechanistic studies indicate
two competing reaction mechanisms.
À
C F Bond Activation—Scope, Limita-
tions and Mechanistic Insight
Chem. Eur. J. 2012, 00, 0 – 0
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!