Hydrodehalogenation of organohalides by Et3SiH catalysed by group 4 metal complexes and B(C6F5)3

Article abstract of DOI:10.1039/d0dt00360c

Catalytic hydrodehalogenation (HDH) of aliphatic organohalides such as trifluorotoluenes by Et₃SiH proceeds in the presence of readily available group 4 metal compounds: Cp′₂MX₂ (Cp′ = η⁵-C₅H₅ or η⁵-C₅Me₅; X = F, Cl, or Me; M = Ti, Zr, or Hf), CpTiCl₃ and TiCl₄ with a catalytic amount of B(C₆F₅)₃. The use of metallocenes in combination with the borane activator leads to a better selectivity of the reaction, i.e., suppression of Friedel-Crafts alkylations of arenes.

Full text of DOI:10.1039/d0dt00360c

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Hydrodehalogenation of organohalides by
Et3SiH catalysed by group 4 metal complexes
Cite this: DOI: 10.1039/d0dt00360c
and B(C6F5)3†
Received 31st January 2020,
Accepted 5th February 2020
David Dunlop, Jiří Pinkas, Michal Horáček, Naděžda Žilková and Martin Lamač
*
DOI: 10.1039/d0dt00360c
rsc.li/dalton
Catalytic hydrodehalogenation (HDH) of aliphatic organohalides reactive carbocation by halide abstraction is responsible for a
such as trifluorotoluenes by Et3SiH proceeds in the presence significant amount of Friedel–Crafts alkylation products
of readily available group 4 metal compounds: Cp’2MX2 (Cp’ = observed in the presence of arenes. In contrast, however, TiCl4-
η5-C5H5 or η5-C5Me5; X = F, Cl, or Me; M = Ti, Zr, or Hf), CpTiCl3 catalysed HDF of trifluorotoluenes with LiAlH4 as the reduc-
and TiCl4 with a catalytic amount of B(C6F5)3. The use of metallo- tant proceeded with high selectivity for the corresponding
cenes in combination with the borane activator leads to a better toluene derivatives, which indicated an important role of the
selectivity of the reaction, i.e., suppression of Friedel–Crafts alkyl- hydride source.8 In contrast, the use of TiCl4 (30 mol%) with
ations of arenes.
Et3SiH (10 equiv.) led to a successful HDF of substituted tri-
fluorotoluenes but along with a sequential intramolecular
Friedel–Crafts benzylation leading to polycyclic products.9
Ozerov and co-workers reported HDF of PhCF3 and related sub-
strates by Et3SiH/Ph3C+[B(C6F5)4]− which generated in situ the
strongly acidic silylium cation Et3Si+ or its adducts.10
Replacement of the [B(C6F5)4]− anion with an even more
robust carborane anion [HCB11H5Cl6]− led to a significant
increase of catalytic activity.11 The concept of electrophilic
organosilicon active sites for HDF was recently extended to
solid ZrO2-based materials with a silylated surface.12 In all
these cases, however, Friedel–Crafts products were dominant
for aromatic substrates such as PhCF3.13
The highly electrophilic borane B(C6F5)3, a strong yet rela-
tively robust Lewis acid, has been utilized in a number of cata-
lytic processes, including those based on FLP (frustrated Lewis
pair) chemistry.14 The mechanism of Si–H bond activation by
electron deficient boranes is well established.15 Concerning
the dehalogenation reactivity, Et3SiH/B(C6F5)3 combination
alone does not promote HDH of PhCF3 as previously noted by
Caputo et al.,16 although more reactive aliphatic monofluor-
ides were successfully dehalogenated.
Catalytic activation of C–F bonds is a desirable aim of
research, addressing the degradation of persistent fluorinated
molecules but also providing tools for selective functionaliza-
tions utilizing these compounds.1
Group 4 metal complexes have been repeatedly demon-
strated to promote stoichiometric and catalytic C–F bond acti-
vations.2 Importantly, catalytic hydrodefluorination (HDF) of
sp2 and/or sp3 C–F bonds proceeded on in situ generated
group 4 metallocene hydride species such as “Cp2TiH” formed
by the reaction of Cp2TiF2 and Ph2SiH2. This Ti(III)-based cata-
lytic system mediated the partial HDF of perfluoroalkenes.3
The secondary silane acted as a hydride source regenerating
the active species. More reactive aluminium hydrides such as
iBu2AlH were utilized in a similar way for the zirconocene-cata-
lysed HDF of pentafluoropyridine.4 A β-diketiminate-stabilized
aluminium dihydride was also capable of C(sp2)–F bond HDF
in the presence of catalytic amounts of Cp2ZrCl2.5 Notably, cat-
ionic Zr(IV) and Hf(IV) metallocene complexes, generated from
dihalide precursors by Ph3C+[B(C6F5)4]− in the presence of
iBu3Al in excess, acted as catalysts in the cleavage of C–F
bonds in 3,3,3-trifluoropropene.6
We have recently reported a catalytic system for olefin
polymerization based on group 4 metal complexes activated by
a combination of hydrosilane R3SiH and B(C6F5)3.17 A hydride
transfer to the cationic metal centre was suggested as the
modus operandi of such catalysts, which was also in line with
previous observations of the reactivity of titanocenes with a
pendant SiH group.18 Herein, we describe the catalytic HDH of
Simple Lewis acids such as AlCl3 are also known to promote
HDH of alkyl halides by Et3SiH.7 Intermediate formation of a
J. Heyrovský Institute of Physical Chemistry, The Czech Academy of Sciences, v.v.i.,
Dolejškova 2155/3, 18223 Prague 8, Czech Republic.
E-mail: martin.lamac@jh-inst.cas.cz
aliphatic organohalides by Et3SiH using group
complex/B(C6F5)3 combination as the catalyst.
4 metal
†Electronic supplementary information (ESI) available: Experimental details.
See DOI: 10.1039/D0DT00360C
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Table 1 Catalyst screening and optimization of conditions for the HDF
of PhCF3
PhCF3
conversion %
Entry
Pre-catalyst
Solvent
1
2
3
4
5
6
7
8
Cp2TiCl2
Cp2TiF2
Cp*2TiCl2
Cp*2TiF2
CpTiCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
PhCl
68
81
24a
32a (76)b
99
TiCl4
99
50
20
95
Cp2ZrCl2
Cp2HfCl2
Cp2TiMe2
Cp*2TiF2
Cp*2TiF2
Cp*2TiF2
Cp*2TiCl2
Scheme 1 General scheme of catalytic HDF of trifluorotoluenes.
9
10
11
12
13
100 (82)c
100d
26
PhCF3
n-hexane
PhCl
12
Conditions (unless otherwise stated): 100 µL (0.81 mmol) PhCF3,
0.52 mL (4.0 equiv.) Et3SiH, 5 mol% B(C6F5)3, 5 mol% metal complex,
solvent (total volume 3 mL); 20 h reaction time, room temperature
(22 °C). a All Et3SiH was consumed by the reaction with the solvent
(see Fig. 2). b Reaction performed with 10 equiv. of Et3SiH; all Et3SiH
was consumed. c Reaction performed with
1 mol% Cp*2TiF2.
d Reaction performed in neat PhCF3; Et3SiH conversion to Et3SiF was
evaluated.
Fig. 1 HDF of PhCF3 by Et3SiH catalysed by Cp2TiCl2 and B(C6F5)3 in
various solvents. C–F bond conversion after 20 h at 22 °C (unless other-
wise indicated).
metal precursor. Importantly, the comparison of HDF activity
of particular catalysts was complicated by a concurrent hydro-
dechlorination (HDCl) of the solvent, which consumed part of
the silane. In the case of CHCl3, this side-reaction ultimately
led to a complete dechlorination, i.e., the formation of
methane (see ESI for details, Fig. S4†). The rate of HDF was
generally higher than that of HDCl, which is in line with
earlier observations.11b However, there were slight differences
within the series of catalysts as evident from kinetic profiles of
the reactions followed by NMR spectroscopy (Fig. 2). Thus,
Cp*2TiF2 being the most active among metallocene catalysts
also promoted the HDCl of CHCl3 at a higher rate, which
resulted in a complete consumption of Et3SiH and incomplete
conversion of PhCF3 (entry 4 in Table 1). This problem can be
partly overcome by increasing the amount of Et3SiH or by per-
forming the reaction in PhCl, in which Cp*2TiF2 is (unlike
Cp2TiCl2) sufficiently active to accomplish the transformation
of PhCF3 (entry 10 in Table 1). In conclusion, the permethyl-
ated titanocene dihalides Cp*2TiX2 had higher catalytic activity
Trifluorotoluene, PhCF3, was initially selected as a model
substrate for the HDF reaction (Scheme 1). Cp2TiCl2 was used
as a precatalyst together with B(C6F5)3 and tested in various
solvents (Fig. 1). Conversion of C–F bonds in PhCF3 was deter-
mined after 20 h at room temperature by GC and NMR. It was
found that the reaction proceeded at a reasonable rate only in
aliphatic halogenated solvents (in chloroform 68% conversion
was reached after 20 h), while in less polar solvents (including
halogenated aromatic ones) the reaction was very slow (only
traces of products were detected) and no conversion was
observed using coordinating solvents such as THF. The reac-
tion was accelerated by increasing the temperature (complete
conversion after 20 h was obtained when performing the reac-
tion at 60 °C in CHCl3) and can also be performed in neat
PhCF3, in which case a complete conversion of Et3SiH was
detected accompanied by the corresponding amount of
toluene. In all cases, toluene was identified as the major HDF
product, while no partially defluorinated compounds were
observed in the whole course of the reaction. Et3SiF was the
only fluorinated product detected and its amount corre-
sponded to the converted C–F bonds (see ESI† for experi-
mental details).
Different group 4 metal precatalysts were then tested under
optimized conditions (CHCl3 as the solvent), including
Cp2MX2 (M = Ti, X = Cl, F, Me; M = Zr, Hf, X = Cl), Cp*2TiX2
(Cp* = η5-C5Me5; X = Cl, F), CpTiCl3, and TiCl4 (Table 1). It was
found that the catalytic activity for HDF decreases in the order
Ti > Zr > Hf for Cp2MX2 complexes. A control experiment
Fig. 2 Kinetic profiles of the HDF of PhCF3 performed in CDCl3 with
showed no activity for the system that did not contain any different catalysts.
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than the unsubstituted counterparts and the difluorides were
more active than the corresponding dichlorides, which is in
line with previous observations.19 Higher activity of permethyl-
ated catalysts, especially in less polar solvents, could be
ascribed either to the enhanced stability of the corresponding
cationic active species, or even to their better solubility in com-
parison to those formed from unsubstituted Cp2TiX2 deriva-
tives. Decreasing the amount of catalyst to 1 mol% was
attempted for Cp*2TiF2 (entry 10, Table 1, value in parenth-
eses), which led to a conversion of 82% in 20 h and a
maximum TON value of 246. In comparison, the system based
on Et3Si+[B(C6F5)4]− gave a TON of 126 for the conversion of
PhCF3,10 and the related systems containing the carborane
anions were more active, giving TONs of up to 2000.11b On the
other hand, cationic group 4 metallocene complexes reported
Fig. 4 Catalytic HDF of trifluorotoluenes - substrate scope.
by Rieger et al. gave TONs of up to 117 for the conversion of Changing the catalyst to Cp*2TiF2 and performing the reaction
3,3,3-trifluoropropene.6
in PhCl at elevated temperature led to a 12% conversion of per-
Stronger Lewis acids CpTiCl3 and TiCl4 were even more fluorotoluene to 2,3,4,5,6-pentafluorotoluene in 20 h. Notably,
active than the metallocene complexes (entries 5 and 6 in in all these cases, aliphatic C–F bonds were transformed selec-
Table 1), however, the selectivity of the reaction toward toluene tively in the presence of aromatic C–X bonds, which were
decreased as determined by GC/GC-MS and NMR (Fig. 3). unreactive under the conditions studied.
Detectable amounts of aromatic Friedel–Crafts products were
The catalytic systems based on silylium cations or other
formed by alkylation of toluene by one to three molecules of Lewis acids are assumed to operate through a mechanism
the substrate with a concomitant dehalogenation (see ESI for based on the abstraction of the halide from the substrate by
details†). Interestingly, in an attempt to reproduce the con- the Lewis acid and formation of a carbocation, which then
ditions reported by Yamada et al. using only TiCl4 (30 mol% reacts with hydrosilane leading to the formation of the
was necessary for reproducible results) without B(C6F5)3,9 we product.1b,10,11 In our case (Scheme 2), we expect the activation
observed no conversion under moisture-free Schlenk con- of Et3SiH by borane and the simultaneous abstraction of the
ditions. When the reaction mixture was exposed to air, halide from the metal pre-catalyst through transition state A
however, the reaction was initiated and proceeded to a com- to give cationic species B with the hydridoborate anion
plete conversion of PhCF3, but the product distribution was [HB(C6F5)3]− This step was confirmed for group 4 metallocene
.
dominated by Friedel–Crafts products and toluene was and half-sandwich complexes in our previous studies.17a,b It
detected only in a small amount.
was also concluded that a subsequent hydride transfer from
The substrate scope of the metallocene-catalyzed HDF was the hydridoborate to the metal must occur in order to obtain a
studied in reactions with substituted trifluorotoluenes (Fig. 4). catalytically active species for the olefin polymerization.
The activated 4-(trifluoromethyl)anisole was transformed with Formation of cationic hydride D through neutral complex C
a complete conversion in 20 h using Cp2TiCl2 as the catalyst was suggested. Related titanium cationic hydride complexes,
with a concomitant cleavage of the MeO- group and its conver- although highly reactive in nature, were even isolated as
sion to Et3SiO-, which is in line with the known B(C6F5)3-cata- reported in the literature and their reactivity towards
lysed reactions of alkyl ethers.20 On the other hand, 1-fluoro-4- C-halogen bonds was observed.21 All cationic species of types
(trifluoromethyl)benzene gave only 34% conversion, and per- B and D may coordinate molecules present in the mixture
fluorotoluene showed no reactivity under these conditions. such as the formed silyl halides, organohalide substrates, or
solvents. The σ-bond metathesis transition state E is then a
plausible route of the HDH process (Path 1). As long as the
reaction is performed in a halogenated solvent such as chloro-
form, a reduction pathway to Ti(III) species, which was
observed in solvents such as toluene,18 should be suppressed.
An alternative mechanism (Path 2) is based on the fact that
cationic complex B as a strong Lewis acid may interact with
the halogenated substrate and activate it for the reduction by
the previously formed [HB(C6F5)4]−. As reported by Stephan
et al., stoichiometric reactions of the salt [tBu3PH]+[HB
(C6F5)3]− with 1 equiv. of B(C6F5)3 and fluoroalkanes led to the
corresponding alkanes and [tBu3PH]+[FB(C6F5)3]− which
demonstrated the ability of the hydridoborate anion to act as a
reductant in HDF reactions.16 We tried to confirm the role of
Fig. 3 Selectivity of the HDF of PhCF3 by Et3SiH to toluene using
different catalysts.
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Acknowledgements
The Czech Science Foundation is acknowledged for financial
support of this research (GAČR grant no. 17-13778S). We thank
Jan Jurka for technical assistance.
References
1 (a) H. Amii and K. Uneyama, Chem. Rev., 2009, 109, 2119;
(b) T. Stahl, H. F. T. Klare and M. Oestreich, ACS Catal.,
2013, 3, 1578; (c) M. F. Kuehnel, D. Lentz and T. Braun,
Angew. Chem., Int. Ed., 2013, 52, 3328; (d) T. Ahrens,
J. Kohlmann, M. Ahrens and T. Braun, Chem. Rev., 2015,
115, 931; (e) W. L. Huang and P. L. Diaconescu,
Organometallics, 2017, 36, 89.
2 M. Klahn and U. Rosenthal, Organometallics, 2012, 31,
1235.
3 (a) M. F. Kuhnel and D. Lentz, Angew. Chem., Int. Ed., 2010,
49, 2933; (b) M. F. Kuehnel, P. Holstein, M. Kliche,
J. Kruger, S. Matthies, D. Nitsch, J. Schutt, M. Sparenberg
and D. Lentz, Chem. – Eur. J., 2012, 18, 10701.
4 U. Jager-Fiedler, M. Klahn, P. Arndt, W. Baumann,
A. Spannenberg, V. V. Burlakov and U. Rosenthal, J. Mol.
Catal. A: Chem., 2007, 261, 184.
5 S. Yow, S. J. Gates, A. J. P. White and M. R. Crimmin,
Angew. Chem., 2012, 124, 12727.
6 Friedel-Crafts type products 3,3-(difluoroallyl)aromates
were obtained from the consecutive reaction with appropri-
ate aromatic substrates: D. Lanzinger, I. M. Hohlein,
S. B. Weiss and B. Rieger, J. Organomet. Chem., 2015, 778,
21.
Scheme 2 Suggested mechanism of hydrodehalogenation catalysis.
the [HB(C6F5)3]− anion by replacing it with an inert [B(C6F5)4]−
one. Thus, we performed the HDF of PhCF3 catalysed by
Cp2TiMe2/Ph3C+[B(C6F5)4]− without B(C6F5)3. When using sub-
stoichiometric amounts of trityl borate (as a methyl-abstract-
ing agent) to prevent the formation of the silylium cation from
Et3SiH (see ESI† for details) the reaction showed no conver-
sion. This demonstrated the important role of the anion [HB
(C6F5)3]− in the catalysis. It might be argued that the decreased
selectivity of the reaction in the presence of stronger Lewis
acids suggests a preference for mechanism 2, which can lead
to the formation of carbocationic intermediates, eventually
giving rise to Friedel–Crafts products.
In conclusion, catalytic dehalogenation of aliphatic organo-
halides by Et3SiH can be performed with readily available
group 4 metallocene dihalides (such as Cp2TiCl2), CpTiCl3 or
TiCl4 as catalysts in combination with B(C6F5)3. A high degree
of selectivity (i.e., suppression of Friedel–Crafts by-products) in
the case of aromatic substrates such as PhCF3 can be achieved
in contrast to some previously reported catalytic systems.
7 M. P. Doyle, C. C. McOsker and C. T. West, J. Org. Chem.,
1976, 41, 1393.
8 T. Akiyama, K. Atobe, M. Shibata and K. Mori, J. Fluorine
Chem., 2013, 152, 81.
9 T. Yamada, K. Saito and T. Akiyama, Adv. Synth. Catal.,
2016, 358, 62.
10 V. J. Scott, R. Çelenligil-Çetin and O. V. Ozerov, J. Am.
Chem. Soc., 2005, 127, 2852.
11 (a) C. Douvris and O. V. Ozerov, Science, 2008, 321, 1188;
(b) C. Douvris, C. M. Nagaraja, C.-H. Chen, B. M. Foxman
and O. V. Ozerov, J. Am. Chem. Soc., 2010, 132, 4946.
12 D. B. Culver and M. P. Conley, Angew. Chem., Int. Ed., 2018,
57, 14902.
13 Interestingly, a related hydride-bridged disilyl cation with a
1,8-naphthalenediyl backbone catalysed the HDF of PhCF3
with Et3SiH selectively to toluene, which can be accounted
for the close proximity of two silyl moieties performing
both the halogen abstraction and reduction of the formed
carbenium ion: R. Panisch, M. Bolte and T. Müller, J. Am.
Chem. Soc., 2006, 128, 9676.
14 (a) G. Erker, Dalton Trans., 2005, 1883; (b) W. E. Piers, Adv.
Organomet. Chem, Academic Press, 2004, vol. 52, pp. 1–76;
(c) D. W. Stephan and G. Erker, Angew. Chem., Int. Ed.,
2015, 54, 6400; (d) D. W. Stephan and G. Erker, Frustrated
Conflicts of interest
There are no conflicts to declare.
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Lewis Pairs I: Uncovering and Understanding, Springer,
Berlin, 2013; (e) D. W. Stephan and G. Erker, Frustrated
Lewis Pairs II: Expanding the Scope, Springer, Berlin, 2013.
15 (a) M. C. Lipke, A. L. Liberman-Martin and T. D. Tilley,
M. Večeřa, R. Gyepes, J. Pinkas, M. Horáček, J. Merna and
M. Lamač, ChemCatChem, 2017, 9, 3160; (c) V. Varga,
J. Pinkas, I. Císařová, J. Kubišta, M. Horáček, K. Mach and
R. Gyepes, Eur. J. Inorg. Chem., 2018, 2018, 2637.
Angew. Chem., Int. Ed., 2017, 56, 2260; (b) M. Oestreich, 18 T. Strašák, J. Sýkora, M. Lamač, J. Kubišta, M. Horáček,
J. Hermeke and J. Mohr, Chem. Soc. Rev., 2015, 44, 2202; R. Gyepes and J. Pinkas, Organometallics, 2013, 32, 4122.
(c) A. Y. Houghton, J. Hurmalainen, A. Mansikkamäki, 19 (a) P. Arndt, A. Spannenberg, W. Baumann, V. V. Burlakov,
W. E. Piers and H. M. Tuononen, Nat. Chem., 2014, 6, 983;
(d) W. E. Piers, A. J. V. Marwitz and L. G. Mercier, Inorg.
Chem., 2011, 50, 12252; (e) D. J. Parks, J. M. Blackwell and
W. E. Piers, J. Org. Chem., 2000, 65, 3090.
U. Rosenthal, S. Becke and T. Weiss, Organometallics, 2004,
23, 4792; (b) X. Verdaguer, U. E. W. Lange, M. T. Reding
and S. L. Buchwald, J. Am. Chem. Soc., 1996, 118, 6784.
20 V. Gevorgyan, M. Rubin, S. Benson, J.-X. Liu and
Y. Yamamoto, J. Org. Chem., 2000, 65, 6179.
21 (a) K. Ma, W. E. Piers, Y. Gao and M. Parvez, J. Am. Chem.
Soc., 2004, 126, 5668; (b) K. Ma, W. E. Piers and M. Parvez,
J. Am. Chem. Soc., 2006, 128, 3303.
16 C. B. Caputo and D. W. Stephan, Organometallics, 2012, 31,
27.
17 (a) V. Varga, M. Lamač, M. Horáček, R. Gyepes and
J. Pinkas, Dalton Trans., 2016, 45, 10146; (b) V. Varga,
This journal is © The Royal Society of Chemistry 2020
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