Room Temperature Regioselective Catalytic Hydrodefluorination of Fluoroarenes with trans-[Ru(NHC)4H2] through a Concerted Nucleophilic Ru?H Attack Pathway

Article abstract of DOI:10.1002/anie.201610820

The efficient and highly selective room temperature hydrodefluorination (HDF) of fluoroarenes by the trans-[Ru(IMe₄)₄H₂] catalyst, 3, is reported. Mechanistic studies show 3 acts directly in catalysis without any ligand dissociation and DFT calculations indicate a concerted nucleophilic attack mechanism. The calculations fully account for the observed selectivities which corroborate earlier predictions regarding the selectivity of HDF.

Full text of DOI:10.1002/anie.201610820

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201610820
German Edition: DOI: 10.1002/ange.201610820
Hydrodefluorination
Room Temperature Regioselective Catalytic Hydrodefluorination of
Fluoroarenes with trans-[Ru(NHC)4H2] through a Concerted
À
Nucleophilic Ru H Attack Pathway
Mateusz K. Cybulski, David McKay, Stuart A. Macgregor,* Mary F. Mahon, and
Michael K. Whittlesey*
Abstract: The efficient and highly selective room temperature
hydrodefluorination (HDF) of fluoroarenes by the trans-
[Ru(IMe4)4H2] catalyst, 3, is reported. Mechanistic studies
show 3 acts directly in catalysis without any ligand dissociation
and DFT calculations indicate a concerted nucleophilic attack
mechanism. The calculations fully account for the observed
selectivities which corroborate earlier predictions regarding the
selectivity of HDF.
combined experimental and computational data to develop
a mechanistic framework for the logical design of new
catalysts with improved activity and regiocontrol. While our
first catalyst system, [Ru(IPr)(PPh3)2(CO)H2] (1),[5,6] showed
only modest activity, catalytic HDF of C6F6 at 708C did
proceed with a remarkably high and very unusual ortho-
regioselectivity to give 1,2,3,4-C6F4H2. DFT studies on the
HDF of C6F5H characterized two mechanisms based on the
nucleophilicity of a hydride ligand in the 5-coordinate
intermediate A formed via PPh3 loss from 1 (Scheme 2).[7,8]
T
he presence of partially fluorinated aromatic rings in many
high-value pharmaceuticals and agrochemicals (e.g. I and II,
Scheme 1)[1] has fuelled interest in the use of catalytic
Scheme 1. Examples of commercially important fluorinated molecules.
hydrodefluorination (HDF) as a route to such functionalities
by F/H substitution of perfluorinated substrates.[2–4] However,
to achieve this the development of more active and more
selective HDF catalysts is still required, as highlighted by the
very specific substitution patterns in I and II. This is
challenging as HDF becomes increasingly difficult as the
number of fluorine substituents decreases, and this difficult
process must be achieved with a high degree of regiocontrol.
Scheme 2. Nucleophilic hydride attack mechanisms in [Ru(IPr)(PPh3)2-
(CO)H2]-catalyzed HDF.
These were a concerted pathway in which F/H exchange
occurred in a single step, or, after fluoroarene coordination
À
(B), a stepwise pathway featuring insertion into the Ru H
À
Chemoselectivity is also an issue, as selective C F activation
bond, HF elimination and protonolysis of Ru–aryl intermedi-
ate D to release C6F4H2 and Ru–F species E. These two
pathways exhibited different kinetic selectivities, the con-
certed mechanism leading to the para-HDF product 1,2,4,5-
C6F4H2, while the stepwise process favored ortho-HDF and
formation of 1,2,3,4-C6F4H2. Overall, the stepwise pathway
proved more accessible and so accounted for the observed
regioselectivity.
À
must be targeted over potentially deactivating C H activa-
tion pathways.
In previous work on HDF catalysis using N-heterocyclic
carbene (NHC) ruthenium hydride complexes we have
[*] M. K. Cybulski, Dr. M. F. Mahon, Prof. M. K. Whittlesey
Department of Chemistry, University of Bath
Claverton Down, Bath, BA2 7AY (UK)
Having identified hydride nucleophilicity as a key element
in these Ru-catalyzed HDF reactions we turned to the more
electron-rich, trans-dihydride complex [Ru(IEt2Me2)2-
(PPh3)2H2] (2).[9,10] This did indeed give higher activity, with
C6F6 being converted to difluorobenzene through four HDF
cycles at 908C. However, this was counterbalanced by poorer
regioselectivity, with both the 1,2- and 1,4-isomers of C6F2H4
being formed. We reasoned that this may reflect the lability of
the PPh3 ligands in this system, resulting in a mixture of 5- and
E-mail: m.k.whittlesey@bath.ac.uk
Dr. D. McKay, Prof. S. A. Macgregor
Institute of Chemical Sciences, Heriot Watt University
Edinburgh, EH14 4AS (UK)
E-mail: S.A.Macgregor@hw.ac.uk
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201610820.
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6-coordinate Ru species in solution. The former could access
both stepwise and concerted pathways, while for the latter,
the concerted process would be the only option. We now
report on catalytic HDF with a new catalyst, trans-[Ru-
(IMe4)4H2] (3).[11] In this system the use of four strongly bound
NHC ligands aims both to enforce coordinative saturation
and enhance hydride nucleophilicity. We show that 3 is
capable of taking C6F6 to 1,4-C6F2H4 at room temperature;
moreover the intermediate steps all occur in a highly selective
fashion. DFT calculations rationalize the observed outcomes.
The trans-dihydride complex 3 (Scheme 3) was reported
previously by Wolf upon reduction of [Ru(IMe4)4Cl2] with
LiAlH4, although it could only be obtained as an impure solid
in low yield.[12] If KC8/H2 is instead used as the reductant, 3
lectivity to 1. 1,2,4,5-C6F4H2 continued to react further, albeit
far more slowly, undergoing another two HDF cycles over ca.
1 month to ultimately give 1,4-C6F2H4 (entry 1). When the
HDF of C6F6 was performed at 908C, full conversion to 1,4-
C6F4H2 was complete in 10 h (entry 1). The formation of low
fluorine-content products was investigated using a range of
less fluorinated substrates (entries 2–5). HDF of 1,2,4,5-
C6F4H2 first formed 1,2,4-C6F3H3, which then reacted
onwards to give 1,4-C6F2H4 (entries 2 and 3). No further
reduction of 1,4-C6F2H4 to C6FH5 was observed, although
fluorobenzene could be formed from both the 1,2- and 1,3-
isomers of C6F2H4 (entries 4 and 5). No reduction to benzene
was observed.[16]
Variation of the silane reductant (entries 6–10) estab-
lished that those with mixed aryl/alkyl substituents
(PhMe2SiH, Ph2MeSiH), as well as secondary alkyl silanes
(Et2SiH2), performed similarly to Et3SiH, although lower
reactivity was found with aryl silanes (Ph3SiH, Ph2SiH2).[14]
Replacement of the IMe4 ligand by the less donating 1,3-
dimethylimidazol-2-ylidene (IMe2) ligand (Scheme 3) also
had a noticeable effect, [Ru(IMe2)4H2] (6; see the Supporting
Information (SI)) displaying lower activity than 3 (entries 11
and 12). This appeared to result from the relatively poor
solubility of the corresponding hydride fluoride complex,
[Ru(IMe2)4HF] (7; see the SI) in solution; even at 908C, a fine
yellow precipitate of 7 could be observed in catalytic HDF
reactions.
Scheme 3. Synthesis and hydrodefluorination chemistry of trans-[Ru-
(NHC)4H2].
can be isolated as an analytically pure yellow microcrystalline
solid in high (80%) yield (Scheme 3). The high symmetry of
the molecule led to a very simple 1H NMR spectrum
consisting of just three resonances at d = 3.37, 1.97 and
À8.14 ppm in a 24:24:2 ratio.
Given the coordinative saturation of both 3 and 6, the
potential for dissociation of an NHC from either ruthenium
complex was probed. The strength of metal–NHC bonds[17]
has led to carbenes being considered as innocent spectator
ligands which do not dissociate readily from metal centers.[18]
Indeed, no exchange between 3 and free IEt2Me2 (3 equiv)
was observed at room temperature, and so any involvement of
unsaturated species such as [Ru(IMe4)3H2] can be ruled out in
the HDF reactions in Table 1 conducted at room temperature.
However, upon heating at 908C, new hydride resonances
were observed in the same d = À8 ppm hydride region of the
proton NMR spectrum as 3, suggesting that carbene dissoci-
ation and exchange is possible at higher temperature.[19]
Upon addition of a stoichiometric amount of C6F6 to
a benzene solution of 3 at room temperature, rapid HDF took
place to afford [Ru(IMe4)4HF] (4) and C6F5H.[13] The X-ray
structure of 4 (ESI) confirmed the same trans-H-Ru-F
geometry as found in [Ru(IEt2Me2)2(PPh3)2HF] (5), albeit
À
with a lengthening of the Ru F distance (2.3070(18) ꢀ vs.
2.264(2) ꢀ). 4 exhibits approximate C4 molecular symmetry
around the H-Ru-F axis. The presence of the weakly
coordinated fluoride ligand trans
Table 1: [Ru(NHC)4H2]-catalyzed hydrodefluorination.[a]
to hydride is reflected in the low
À
frequency of the Ru H chemical
Entry
1
Cat.
Substrate
C6F6
Reductant
Et3SiH
Product
T [8C]
t [h]
TON
shift of 4 (d = À23.19 ppm). Addi-
tion of 5 equiv Et3SiH to 4 brought
about the rapid and clean reforma-
tion of 3 at room temperature
(Scheme 3).[14]
3
1,4-C6F2H4
25
740
10
10
80
80
40
20
20
20
80
80
18.5
25/90[b]
90
2
3
3
3
3
3
3
3
3
1,2,4,5-C6F4H2
1,2,4-C6F3H3
1,2-C6F2H4
1,3-C6F2H4
C6F6
Et3SiH
Et3SiH
Et3SiH
Et3SiH
PhMe2SiH
Ph2MeSiH
Ph3SiH
1,4-C6F2H4
1,4-C6F2H4
C6FH5
C6FH5
1,4-C6F2H4
1,4-C6F2H4
C6F5H (79%) +
1,2,4,5-C6F4H2 (21%)
1,4-C6F2H4
1,2,4,5-C6F4H2
1,2,4,5-C6F4H2
1,4-C6F2H4
90
9
4[c]
5[c]
6
120
120
25
90
25
157
539
740
17
Table 1 summarizes the results
of catalytic HDF with 3 (5 mol%)
in benzene with a silane as reduc-
tant. C6F6 underwent two HDF
cycles within ca. 5 min (TOF
> 480 hÀ1) at room temperature to
give the para-HDF product, 1,2,4,5-
C6F4H2. The reaction is therefore
notable not only for taking place at
room temperature,[15] but also in
7
C6F6
C6F6
8[d]
740
9
3
3
6
6
C6F6
C6F6
C6F6
C6F6
Et2SiH2
Ph2SiH2
Et3SiH
Et3SiH
25/90b
25
25
9
264
6
80
40
40
80
10
11
12
90
103
[a] Reaction conditions: 0.1m fluoroarene, 0.5m silane, 5 mol% 3 or 6, 0.5 mL C6H6, conversions
determined by 19F NMR spectroscopy. [b] Temperature raised to 908C after ca. 5 min at 258C.
that 3 exhibits a different regiose- [c] Solvent=toluene. [d] Product distribution is % of main products/total % of all HDF products.
2
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To address whether any dissociated IMe4 could therefore
play a similar role to that recently found for alkylphosphines
in catalyzing HDF,[20] free IMe4 was heated between 70–908C
with 1,2,4-C6F3H3 in the presence of Et3SiH. The addition
product 8 (ESI) and Et3SiF were formed in a 1:1 ratio
(Scheme 4). Activation at the 2-position (i.e. para to H rather
À
Scheme 4. Stoichiometric C F activation reactions of 1,2,4-C6F3H3 with
IMe4.
than para to F) was confirmed by structural characterization
of the imidazolium salt 9, which was formed when IMe4 and
1,2,4-C6F3H3 were heated together in the absence of any
silane (see the SI).[21] Crucially, heating 8 at 908C in both the
presence and absence of Et3SiH resulted in < 15% conversion
to 1,4-C6F4H2 upon fluoroarene elimination. This shows there
is only a low level of the NHC-mediated stoichiometric HDF
and confirms the need for Ru in the reactions of 3. Moreover,
the absence of 8 at the end of catalytic runs with 3 shows that
16e [Ru(IMe4)3H2] is not catalytically relevant even in the
high temperature HDF runs.
Figure 1. Computed profiles (wB97xD//BP86, free energies in benzene,
kcalmolÀ1) for the HDF of 1,2,4-C6F3H3 by [Ru(IMe4)4H2], 3. Selected
distances are shown in ꢀ and the inset provides additional information
for TS(3–4)2F.
DFT calculations were used to account for the selectivity
of the various HDF reactions in Table 1.[22] As a stepwise
HDF process based on initial NHC/fluoroarene substitution
can be ruled out experimentally, the calculations focused on
the concerted mechanism and applied this to the full range of
fluoroarenes C6F6ÀnHn (n = 0–5). The results obtained with
1,2,4-C6F3H3 are typical and details are provided in Figure 1.
The lowest-energy pathway involves attack of the hydride
ligand at the C2 position of the arene and proceeds with a free
energy barrier (relative to 3 + free 1,2,4-C6F3H3) of 16.2 kcal
molÀ1. The transition state involved, TS(3–4)2F, features
a near-linear {Ru···Ha···C2} moiety (171.98) and elongated
The alternative HDF at the C1 and C4 positions of 1,2,4-
C6F3H3 proceed via transition states TS(3–4)1F and TS(3–4)4F
at + 19.7 kcalmolÀ1 and + 21.4 kcalmolÀ1, respectively. These
display similar geometries to TS(3–4)2F, although with some-
what longer Ru···Ha and shorter C1/C4···Ha distances. These
later geometries (in terms of H-transfer) are consistent with
the higher computed barriers which indicate a clear kinetic
preference for HDF at the 2-position, in line with experiment
where only that process is observed (Table 1, entry 3).
The DFT study was extended to the HDF of other
fluoroarenes by 3 starting with C6F6. Results are shown in
Figure 2a as calculated barriers (relative to 3 and the
appropriate fluoroarene in each case) for each HDF step.
As seen previously,[7b] there is a general increase in the barrier
as the number of F-substituents is reduced and this is reflected
in the more forcing conditions that are required experimen-
tally to achieve HDF with lower fluorinated substrates. The
pattern of the F-substituents also directs the selectivity. We
have previously shown that the concerted mechanism is
favored most by the presence of ortho-F substituents which
À
Ru···H and C2 F2 distances of 1.90 ꢀ and 1.41 ꢀ, respec-
a
À
tively. As this occurs, the new C2 H bond begins to form
b
À
(1.64 ꢀ) and a shortening of the trans Ru H distance is seen
(1.65 ꢀ) in response to the weakening of the Ru H
interaction. The orientation of the approaching fluoroarene
(as defined by the C6 plane) is offset by approximately 408
relative to the best-fit plane containing Ru and the four C2
carbons of the IMe4 ligands. TS(3–4)2F exhibits a Meisen-
a
À
À
heimer-type geometry with elongation of the Cipso Cortho
bonds (see inset, Figure 1), although H-transfer onto C is
À
À
À
more progressed than the C F bond cleavage, the C2 F2
bond being only 0.06 ꢀ longer than in free 1,2,4-C6F3H3. The
Ru···F2 distance is also rather long (3.70 ꢀ), but character-
ization via IRC calculations confirms that F2 does move onto
the metal center to generate 4 and release the 1,4-C6F4H2
product all in one step.[23] This HDF process is extremely
exergonic (DG = À49.9 kcalmolÀ1).
cause a weakening of the target C F bond; meta-F substitu-
ents also reduce barriers (although to a lesser extent), while
para-F substituents can actually cause a slight increase in the
barrier.[7b] These patterns are borne out here, with C6F5H
reacting at the 4-position (this having two ortho-F and two
meta-F substituents) and, as seen in Figure 1, 1,2,4-C6F3H3
reacts at the 2-position (its ortho-F and meta-F substituents
trumping the 1-position which has one ortho-Fand one para-F
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Conflict of interest
The authors declare no conflict of interest.
Keywords: density functional calculations ·
homogeneous catalysis · hydrodefluorination ·
N-heterocyclic carbenes · ruthenium
Figure 2. Selectivity of HDF for a range of fluoroarene substrates:
a) C6F6 gives 1,4-C6F2H4; b) 1,2,3,4-C6F4H2 and 1,2,3,5-C6F4H2 give 1,4-
C6F2H4; and c) 1,3,5-C6F3H3, 1,2,3-C6F3H3 and 1,2-C6F2H4 give C6FH5. In
each case the calculated barrier is indicated in kcalmolÀ1 (wB97xD//
BP86, free energies in benzene).
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of 25.6 kcalmolÀ1 and so, experimentally, the catalytic HDF of
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Acknowledgements
We thank the EPSRC (grants EP/J009962/1, EP/J010677/1,
DTA) for financial support and Drs John Lowe and Anneke
Lubben for experimental assistance.
4
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[9] M. K. Cybulski, I. M. Riddlestone, M. F. Mahon, T. J. Woodman,
M. K. Whittlesey, Dalton Trans. 2015, 44, 19597 – 19605.
[10] IEt2Me2 = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene.
[11] IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene.
[12] R. Wolf, M. Plois, A. Hepp, Eur. J. Inorg. Chem. 2010, 918 – 925.
[13] 4 was also formed cleanly upon reaction of 3 with the less
fluorinated substrate 1,2,4-C6F3H3, although more slowly (see
the SI).
[14] See the SI for a study of the stoichiometric reaction of 3 with
Ph3SiH.
[15] Multiple HDF steps typically require forcing conditions. See
Refs. [3f] and [3n].
[16] HDF of monofluoarenes has been reported only rarely. a) R. J.
Young, V. V. Grushin, Organometallics 1999, 18, 294 – 296; b) S.
Kuhl, R. Schneider, Y. Fort, Adv. Synth. Catal. 2003, 345, 341 –
344; c) Ref. [3j]; d) Ref. [3k]; e) Ref. [3q].
[20] A. Arꢅvalo, A. Tlahuext-Aca, M. Flores-Alamo, J. J. Garcꢆa, J.
Am. Chem. Soc. 2014, 136, 4634 – 4639.
[21] Similar reactivity has been found for more fluorinated sub-
strates: a) N. Kuhn, J. Fahl, R. Boese, G. Henkel, Z. Naturforsch.
B 1998, 53, 881 – 886; b) E. Mallah, N. Kuhn, C. Miaichle-
Mçssmer, M. Steimann, M. Strçbele, K.-P. Zeller, Z. Natur-
forsch. B 2009, 64, 1176 – 1182; c) S. Styra, M. Melaimi, C. E.
Moore, A. L. Rheingold, T. Augenstein, F. Breher, G. Bertrand,
Chem. Eur. J. 2015, 21, 8441 – 8446; d) Z. R. Turner, Chem. Eur.
J. 2016, 22, 11461 – 11468; e) Y. Kim, E. Lee, Chem. Commun.
2016, 52, 10922 – 10925.
[22] DFT calculations were run with Gaussian 03/09 and were based
on geometries optimized with the BP86 functional. Energies
were re-computed with the wB97XD functional and quoted free
energies include a correction for C6H6 solvent (PCM approach).
See the SI for full details.
[17] a) J. Huang, L. Jafarpour, A. C. Hillier, E. D. Stevens, S. P.
Nolan, Organometallics 2001, 20, 2878 – 2882; b) R. Dorta, E. D.
Stevens, N. M. Scott, C. Costabile, L. Cavallo, C. D. Hoff, S. P.
Nolan, J. Am. Chem. Soc. 2005, 127, 2485 – 2495.
[18] a) T. Weskamp, F. J. Kohl, W. Hieringer, D. Gleich, W. A.
Herrmann, Angew. Chem. Int. Ed. 1999, 38, 2416 – 2419; Angew.
Chem. 1999, 111, 2573 – 2576; b) J. Schwarz, V. P. W. Bçhm,
M. G. Gardiner, M. Grosche, W. A. Herrmann, W. Hieringer, G.
Raudaschl-Sieber, Chem. Eur. J. 2000, 6, 1773 – 1780.
[19] For examples of NHC substitution: a) P. B. Hitchcock, M. F.
Lappert, P. L. Pye, J. Chem. Soc. Dalton 1978, 826 – 836; b) L.
Titcomb, S. Caddick, F. G. N. Cloke, D. J. Wilson, D. McKer-
[23] Test calculations including the effect of benzene solvent in the
optimization procedure gave similar results. See the SI for
details.
Manuscript received: November 5, 2016
Revised: December 11, 2016
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Hydrodefluorination
The reason for selectivity: Efficient and
selective hydrodefluorination of fluoro-
arenes by a trans-[Ru(NHC)4H2] catalyst is
reported. DFT calculations indicate that
the observed selectivities are fully con-
sistent with a concerted nucleophilic
attack mechanism without any prior
ligand dissociation.
M. K. Cybulski, D. McKay,
S. A. Macgregor,* M. F. Mahon,
M. K. Whittlesey*
&&& — &&&
Room Temperature Regioselective
Catalytic Hydrodefluorination of
Fluoroarenes with trans-[Ru(NHC)4H2]
À
through a Concerted Nucleophilic Ru H
Attack Pathway
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!