NHC·Alane Adducts as Hydride Sources in the Hydrodefluorination of Fluoroaromatics and Fluoroolefins

Article abstract of DOI:10.1002/ejic.201800589

We present herein the utilization of NHC-stabilized alane adducts of the type (NHC)·AlH₃ [NHC = Me₂Im (1), Me₂Im^Me (2), iPr₂Im (3), iPr₂Im^Me (4), Dipp₂Im (5)] and (NHC)·AliBu₂H [NHC = iPr₂Im (6), Dipp₂Im (7)] as novel hydride transfer reagents in the hydrodefluorination (HDF) of different fluoroaromatics and hexafluoropropene. Depending on the alane adduct used, HDF of pentafluoropyridine to 2,3,5,6-tetrafluoropyridine in yields of 15–99 % was observed. The adducts 1, 2, and 5 achieved a quantitative conversion into 2,3,5,6-tetrafluoropyridine at room temperature immediately after mixing the reactants. Studies on the HDF of fluorobenzenes with the (NHC)·AlH₃ adducts 1, 3, and 5 and (Dipp₂Im)·AliBu₂H (7) showed the decisive influence of the reaction temperature on the H/F exchange and that 135 °C in xylene afforded the best product distribution. Although the HDF of hexafluorobenzene yielded 1,2,4,5-tetrafluorobenzene in moderate yields with traces of 1,2,3,4-tetrafluorobenzene and 1,2,4-trifluorobenzene, pentafluorobenzene was converted quantitatively into 1,2,4,5-tetrafluorobenzene, with (Dipp₂Im)·AliBu₂H (7) showing the highest activity and reaching complete conversion after 12 h at 135 °C in xylene. The HDF of hexafluoropropene with (Me₂Im)·AlH₃ (1) occurred even at low temperatures and preferably at the CF₂ group with the formation of 1,2,3,3,3-pentafluoropropene (with 0.4 equiv. of 1) or 2,3,3,3-tetra-fluoropropene (with 0.9 equiv. of 1) as the main product.

Full text of DOI:10.1002/ejic.201800589

A Journal of
Accepted Article
Title: NHC-Alane Adducts as Hydride Sources in the
Hydrodefluorination of Fluoroaromatics and Fluoroolefines
Authors: Heidi 'Schneider, Andreas Hock, Alma D. Jaeger, Dieter
Lentz, and Udo Radius
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800589
Link to VoR: http://dx.doi.org/10.1002/ejic.201800589

10.1002/ejic.201800589
European Journal of Inorganic Chemistry
FULL PAPER
NHC-Alane Adducts as Hydride Sources in the
Hydrodefluorination of Fluoroaromatics and Fluoroolefines
Heidi Schneider[a], Andreas Hock[a], Alma D. Jaeger[b], Dieter Lentz[b]*, Udo Radius[a]*
Dedicated to Prof. Dr. Peter Jutzi on the occasion of his 80th birthday
Abstract: We present herein the utilization of NHC-stabilized alane
adducts of the type (NHC)∙AlH3 (NHC = Me2Im 1, Me2ImMe 2, iPr2Im 3,
iPr2ImMe 4, Dipp2Im 5) and (NHC)∙AliBu2H (NHC = iPr2Im 6, Dipp2Im
7) as novel hydride transfer reagents in the hydrodefluorination (HDF)
of different fluoroaromatics and hexafluoropropene. In dependence
on the alane adduct used, HDF of pentafluoropyridine to 2,3,5,6-
tetrafluoropyridine from 15-99 % is observed. The adducts 1, 2 and 5
achieve a quantitative conversion to 2,3,5,6-tetrafluoropyridine at
room temperature immediately after mixing the reactants. Studies on
the HDF of fluorobenzenes with the adducts (NHC)∙AlH3 1, 3 and 5
and (Dipp2Im)∙AliBu2H 7 showed the decisive influence of the reaction
temperature on the H/F exchange and that 135 °C in xylene affords
the best product dispersion. While the HDF of hexafluorobenzene
yields 1,2,4,5-tetrafluorobenzene in moderate yields with traces of
a little bit longer compared to the C-H bond (1.08 Å), the
introduction of an electron-withdrawing fluorine substituent at the
carbon atom is perfectly suited for modifying the electron density
distribution in a molecule without significantly increasing the steric
requirement or the overall geometry change. The lipophilicity,
acidity and metabolic stability of organic molecules can be
adjusted precisely through the targeted introduction of fluorine
substituents into specific positions.[3] Due to these well-defined
properties, the interest in new synthetic strategies for the
production of polyfluorinated molecules is steadily growing with
increasing demand for perfluorinated and partially fluorinated
compounds as components of pharmaceuticals, diagnostics,
polymers, or high-performance functional materials.[4] However,
access of such molecules via fluorination of known compounds is
often problematic since many organics just decompose in a
fluorine stream or when treated with hydrofluoric acid.
Furthermore, the selective introduction of fluoride (F-) or
fluoronium synthons (F+) is also often difficult to achieve,
especially if multiple substitution processes are required.
1,2,3,4-tetrafluorobenzene
and
1,2,4-trifluorobenzene,
pentafluorobenzene is converted quantitative to 1,2,4,5-
tetrafluorobenzene whereby (Dipp2Im)∙AliBu2H 7 shows the most
activity and reaches a complete conversion after 12 h at 135 °C in
xylene. The HDF of hexafluoropropene with (Me2Im)∙AlH3 1 occurs
however even at low temperatures and preferably at the CF2-group
with formation of 1,2,3,3,3-pentafluoropropene (with 0.4 eq. of 1) or
2,3,3,3-tetra-fluoropropene (with 0.9 eq. of 1) as main product.
Introduction
The element fluorine forms the strongest known σ-carbon bond to
a main group element, the bond dissociation energy associated
with the C-F bond lies in a range of 550 ± 100 kJ∙mol-1.[1] Due to
the high electronegativity of the fluorine substituent, C-F bonds
have some peculiarities compared to other C-E bonds,[2] such as
the high ionic bond fraction and a comparatively short C-F bond,
which leads to a low polarizability of the C-F bond. These
properties result in a remarkably high thermodynamic stability and
kinetic inertness of C-F bonds. Since a C-F bond (1.35 Å) is only
[a]
H. Schneider, A. Hock, Prof. Dr. U. Radius
Institut für Anorganische Chemie
Julius-Maximilians-Universität Würzburg
Am Hubland, D-97074 Würzburg (Germany)
E-mail: u.radius@uni-wuerzburg.de
http://www.ak-radius.de
[b]
A. D. Jaeger, Prof. Dr. D. Lentz
Institut für Chemie und Biochemie - Anorganische Chemie
Freie Universität Berlin
Fabeckstr. 34-36, D-14195 Berlin (Germany)
E-mail: dieter.lentz@fu-berlin.de
http://www.bcp.fu-
berlin.de/chemie/chemie/forschung/InorgChem/aglentz/index.html
[7]
Scheme 1. Examples for the main group element (boron,
aluminum[9]) mediated HDF of sp3-hybridized CFx groups.
silicon,[8] and
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201800589
European Journal of Inorganic Chemistry
FULL PAPER
Since perfluorinated compounds are often much more easily
accessible, the (selective) hydrodefluorination (HDF) of
perfluorinated organic compounds is for this purpose in many
cases a very efficient approach. The catalytic hydrodefluorination
of perfluorinated aromatics and olefins has been investigated
extensively over the last few years. In addition to microbial[5] and
heterogeneous catalytic hydrodefluorination[6], homogeneous
transition metal catalysis plays an important role. A unique variety
of mechanisms for HDF was observed, depending on the
transition metal, the ligands, and the substrate used.[4d]
[iBu2Al]+[B(C6F5)]- (Scheme 1), which converts 1-fluorohexane to
n-hexane. Similarly, Ozerov et al. [13] demonstrated that catalytic
amounts [iBu2Al]+[CHB11H5Br6]- and iBu2AlH as the hydride
source
lead
to
aluminum-mediated
HDF
of
4-
fluorobenzotrifluoride, which occurs selectively at the sp3-C-F
bond in the presence of a sp2-C-F bond to yield 4-fluorotoluene.
The HDF of C-F bonds at sp2-hybridized carbon atoms of
polyfluorinated arenes using aluminum centered hydride sources
is currently not well explored. We have reported recently the
synthesis of mono NHC (=N-Heterocyclic Carbene) alane adducts
Only a few examples are known for the catalytic HDF of organic
fluorine compounds with the aid of main group element
compounds. In recent years, the use of particularly strong Lewis
acids, suitable Lewis acid base adducts or frustrated Lewis pairs
(FLPs) has rendered as an important tool for the C-F bond
breaking process. In order to compensate the C-F bond
dissociation energy, a thermodynamically strong main group
element-fluorine bond formation must be linked to C-F bond
cleavage, i. e. formation of an H-F, B-F, Al-F, Si-F or Sn-F bond.
The compounds of the elements boron,[7] silicon,[8] and
aluminum[9] have proven particularly suitability for this type of
reaction (Scheme 1) since they have the necessary Lewis acidity
and a particularly high fluorine affinity. In contrast to transition
of
the
type
(NHC)·AlH3
(NHC
=
1,3-bis-alkly-4,5-
(dihydrogen/dimethyl)-imidazolin-2-ylidene = Me2Im 1, Me2ImMe 2,
iPr2Im 3, iPr2ImMe 4, Dipp2Im 5) and (NHC)·AliBu2H (NHC = iPr2Im
6, Dipp2Im 7) (Scheme 2).
[14]
These NHC alane adducts are
crystalline, solid materials and are comparably stable and
manageable in comparison with other highly sensitive and
pyrophoric aluminum hydrides. Moreover, in contrast to other
hydride reagents, e.g. lithium aluminum hydride, they are
perfectly soluble in common organic solvents, such as benzene,
toluene and THF. We thus envisage for this type of reagents the
use in organic synthesis. Since we are interested in C-F bond
activation in general[15] and in HDF in particular,[16] we decided to
investigate the reactivity of these compounds towards
polyfluorinated aromatics and olefins.
metal complexes which exhibit
a
high selectivity for
hydrodefluorination of C-F bonds at sp2-hybridized carbon atoms,
these Lewis acidic compounds show complementary selectivity
and usually target fluorine-hydrogen exchange on sp3-hybridized
carbon atoms.[10]
Results and Discussion
The high potential, for example, of Lewis-acidic silylium cations
for the catalytic HDF of benzotrifluoride to toluene was
demonstrated by Müller et al. using the hydride-bridged disilyl
HDF of Pentafluoropyridine
cation [1,8-naphtyl(SiMe2)2(µ2-H)]+[B(C6F5)4]- [8a] and by Ozerov et
[8b]
al.
using
[Et3Si]+[B(C6F5)4]-
(TON
≤
45)
and
[Et3Si]+[CHB11H5Cl6]- [8c,d] (TON ≤ 2700).
For boron-based chemistry, Stephan and co-worker
demonstrated that the reaction of the boron-centered electrophile
B(C6F5)3 in conjunction with equimolar amounts of the hydride
source [tBu3PH]+[HB(C6F5)3]- converts 1-fluoropentane to n-
pentane (Scheme 1). In the absence of B(C6F5)3, no reaction was
observed, which indicates that fluoride abstraction proceeds from
borane attack. Stephan et al. also demonstrated that modified
+
substitution pattern of the phosphonium cations ([RnPF5-n
]
2+
[B(C6F5)4]- or [RnPF4-n
]
2[B(C6F5)4]-) also render these
compounds as suitable catalysts for the HDF of 1-
fluoropentane.[11]
In contrast, aluminum compounds such as lithium aluminum
hydride and di-iso-butylaluminum hydride were predominantly
used as an additional hydride source in the transition metal-
catalyzed HDF. Rosenthal et al., for example, employed di-iso-
butyl aluminum hydride as a hydride source in the zirconium
Scheme 2. HDF of pentafluoropyridine at room temperature using (NHC)·AlH3
(NHC = Me2Im 1, Me2ImMe 2, iPr2Im 3, iPr2ImMe 4, Dipp2Im 5) and (NHC)·AliBu2H
(NHC = iPr2Im 6, Dipp2Im 7) as hydride source.
mediated
pentafluoropyridine
fluorine-hydrogen
at room
exchange
temperature
and
converted
2,3,5,6-
to
tetrafluoropyridine.[12] However, the group of Nicolaou reported
already in 1984 that glycosyl fluoride can be hydrodefluorinated
in 90% yield by the reaction with equimolar amounts of AlH3 at
0 °C (Scheme 1).[9a] Rosenthal and Krossing et al. [9b] reported the
reaction of [Ph3C]+[B(C6F5)4]- with iBu2AlH to yield in situ
To calibrate our experiments, we first of all reacted pure lithium
aluminum hydride (LAH) with equimolar amounts of
pentafluoropyridine and hexafluorobenzene in THF at room
temperature. Immediately after the starting material was
combined, a strong exothermic reaction with gas evolution took
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10.1002/ejic.201800589
European Journal of Inorganic Chemistry
FULL PAPER
place in both cases. NMR spectroscopic studies of both reaction
solutions demonstrate the formation of a large number of new
components, with only 1,2,4,5-tetrafluorobenzene (yield
estimated from 19F NMR spectroscopy: 14%) being uniquely
identified as one of the products of the reaction with
hexafluorobenzene. As a conclusion, LAH leads to HDF at room
temperature, but this reaction is completely unselective.
In order to evaluate the potential of NHC alane adducts for the
hydrodefluorination of fluoroaromatics, the adducts (NHC)·AlH3
(NHC = Me2Im 1, Me2ImMe 2 iPr2Im 3, iPr2ImMe 4, Dipp2Im 5) and
(NHC)·AliBu2H (iPr2Im 6, Dipp2Im 7), containing one (6, 7) or three
(1 – 5) hydride substituents were tested for the transition-metal-
free HDF of pentafluoropyridine (see Scheme 2). The reaction
was performed at room temperature and monitored by means of
19F NMR spectroscopy. Immediately after the starting materials
were combined in C6D6 HDF takes place at the most nucleophilic
4-position of pentafluoropyridine for all NHC alane adducts tested.
We currently assume that the NHC–stabilized aluminum hydrides
attack the pyridine selectively and independently of the nature of
the adduct used in the 4-position of the aromatic compound,
forming a Meisenheimer intermediate.
The total conversion of pentafluoropyridine and the percentage
conversion of pentafluoropyridine to 2,3,5,6-tetrafluoropyridine
are plotted in Figure 1. The overall conversion of
pentafluoropyridine is in all cases good to excellent, with
exception of (iPr2Im)·AliBu2H 6. In general, reactions with iPr2Im
and iPr2ImMe as NHC ligands and AliBu2H as aluminum
component seem to be problematic, since they either suffer from
a rather low rate ((iPr2Im)·AlH3 3: 99 % after 6 h; (iPr2Im)·AliBu2H
6 86 % after 6 h or from a low selectivity of the reaction
((iPr2ImMe)·AlH3 4: 45 % 2,3,5,6-F4HC5N; (iPr2Im)·AliBu2H 6: 69 %
2,3,5,6-F4HC5N; (Dipp2Im)·AliBu2H 7: 13 % 2,3,5,6-F4HC5N).
However, for the adducts (Me2Im)·AlH3
1, (Me2ImMe)·AlH3 2 and (Dipp2Im)·AlH3
3 conversions are almost quantitative
shortly after the reactants are
assembled and very selective (93 –
99 % 2,3,5,6-F4HC5N formed). For the
reaction
(iPr2ImMe)·AlH3 4, and (iPr2Im)·AliBu2H 6,
the formation of by-product was
using
(iPr2Im)·AlH3
3,
a
observed in significant amount 17% (6)
18% (3) and 28% (4), respectively,
which could not be unambiguously
identified. This compound reveals two
resonances at -96.3 and -125.3 ppm
with an integration ratio of 1:1 in the 19
F
NMR spectra, and might be either an
alane adduct of the resulting 2,3,5,6-
tetrafluoropyridine,
C5F4HN)·AlH3
C5F4HN)·AliBu2H, respectively, or
i.e.
(2,3,5,6-
(2,3,5,6-
Figure 1. HDF of pentafluoropyridine at room temperature using (NHC)·AlH3 (NHC = Me2Im 1, Me2ImMe 2,
iPr2Im 3, iPr2ImMe 4, Dipp2Im 5) and (NHC)·AliBu2H (NHC = iPr2Im 6, Dipp2Im 7)
or
a
product arising from the nucleophilic
substitution of a fluoride substituent at
pentafluoropyridine with formation of a
salt [(NHC)·2,3,5,6-C5F4HN]+[AlH3F]- or
[(NHC)·2,3,5,6-
C5F4HN]+[AliBu2HF]-,
respectively, as reported by Kuhn et al.
[17]
for
perfluorinated
aromatics.
However, the stoichiometric reaction of
iPr2Im with pentafluoropyridine in C6D6
leads to the immediate formation of an
insoluble compound after assembly,
which indicates the formation of a salt
such as [(iPr2Im)-2,3,5,6-C5F4HN]+[H2F]-.
In contrast, the byproduct formed in the
reactions of 3,
4
and
6
with
pentafluoropyridine is soluble in C6D6.
The HDF of pentafluoropyridine using
(Dipp2Im)·AliBu2H 7 proceeds extremely
unspecific with a yield of 13% of 2,3,5,6-
tetrafluoropyridine and a large number
Figure 2. 1H NMR spectrum of the reaction of (iPr2Im)·AliBu2H 6 with pentafluoropyridine at room
temperature in C6D6 in comparison with the 1H NMR spectrum of (iPr2Im)·AliBu2H 6 in C6D6.
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10.1002/ejic.201800589
European Journal of Inorganic Chemistry
FULL PAPER
of unidentifiable by-products that have been formed.
products 1,2,4,5-tetrafluorobenzene (43%) and 1,2,3,4-
tetrafluorobenzene (10 %) were identified as reaction products.
However, increasing the temperature to 135 °C, the conversion
significantly improved to 99% after 72 h, but also the selectivity
Whereas all the fluoroaromatic compounds have been detected
in the 19F NMR spectra, the aluminum compounds formed defied
isolation and identification by NMR spectroscopy due to the
quadrupolar moment of the aluminum atom and the problems
completely changed to
tetrafluorobenzene, i.e.
a
100% formation of 1,2,4,5-
the formation of 1,2,3,4-
associated
with it. However, for the reaction of
pentafluoropyridine with 7 a broad resonance at -170.3 ppm for
the resulting NHC aluminum fluoride was detected in the 19F NMR
spectrum, which is located within the range of -155 to -180 ppm
typically observed for aluminum fluorides. [18] The integration ratio
of 1:4 (Al-F: 2,3,5,6-C5F4HN) confirms the abstraction of one
fluoride substituent from the perfluoro aryl by the aluminum
compound. Compared to (iPr2Im)·AliBu2H 6, the NHCs and iso-
butyl resonances of this adduct, presumably (iPr2Im)·AliBu2F, are
slightly upfield shifted in the 1H NMR spectrum (see Figure 2). The
iso-propyl methine protons of the NHCs reveal in this case a
doublet of septets at 5.18 ppm with a coupling constant JHF of 3.6
Hz, which accounts additionally for adduct formation of
(iPr2Im)·AliBu2F. Although we could not identify similar fluorinated
species for the reaction of pentafluoropyridine with (NHC)·AlH3
(NHC = Me2Im 1, Me2ImMe 2, iPr2Im 3, iPr2ImMe 4, Dipp2Im 5) and
(Dipp2Im)·AliBu2H 7, we assume that aluminum fluoride bond
formation takes also place in these cases.
tetrafluorobenzene is completely suppressed for the reaction at
higher temperatures. Further hydrodefluorination of 1,2,3,4-
tetrafluorobenzene or 1,2,4,5-tetrafluorobenzene could not be
observed in both cases. The formation of a fluorinated by-product
was not observed for the HDF of pentafluorobenzene using these
conditions, in sharp contrast to the reaction of
3 with
pentafluoropyridine. In total, the HDF of pentafluorobenzene with
(iPr2Im)·AlH3 3 leads selectively to the formation of 1,2,4,5-
tetrafluorobenzene without the formation of any side-products,
which encouraged us to further investigate other NHC adducts.
Thus, the adducts (Me2Im)·AlH3 1, (iPr2Im)·AlH3 3, (Dipp2Im)·AlH3
5 and (Dipp2Im)·AliBu2H 7 were reacted with equimolar amounts
of the fluoroaromatics hexafluorobenzene, pentafluorobenzene
and 1,2,4,5-tetrafluorobenzene in xylene at 135 °C, and the
reactions were monitored by 19F NMR spectroscopy. Products
formation and product ratios were subsequently determined by
the integration of the 19F NMR resonances and confirmed by GC-
MS. These results and the corresponding reaction times required
are summarized in Figure 3. The four NHC alane adducts
employed do not show the desired product selectivity for the HDF
of hexafluorobenzene to yield pentafluorobenzene. Although
conversion of hexafluorobenzene is complete for 1, 3 and 5, the
reaction using 5 as hydride source affords aside from 1,2,3,4-
tetrafluorobenzene in 41% yield a variety of not further identified
fluorinated species. In the case of 1 and 3, mixtures of
HDF of fluorobenzenes
Regardless of their degree of substitution, the C-F bonds of
fluorinated benzene derivatives are much more difficult to activate
than those of the fluorinated pyridines. To investigate the potential
of NHC-stabilized alanes in the hydrodefluorination of
polyfluorobenzenes in some detail, the adducts (Me2Im)·AlH3 1
(NHC with the least steric demand), (iPr2Im)·AlH3 3 (NHC with
average steric demand), (Dipp2Im)·AlH3 5 (NHC with the highest
steric demand) and (Dipp2Im)·AliBu2H 7 (adduct with only one
hydride substituent) were reacted with different fluoroaromatics
pentafluorobenzene,
1,2,4,5-tetrafluorobenzene,
1,2,3,4-
tetrafluorobenzene and 1,2,4-trifluorobenzene have been
obtained, with a clear preference for the formation of 1,2,4,5-
tetrafluorobenzene ((Me2Im)·AlH3 1: 74%; (iPr2Im)·AlH3 3: 71%).
In contrast, the reaction of C6F6 with (Dipp2Im)·AliBu2H 7 leads
after 60 h at 135 °C to 51% pentafluorobenzene and traces (<1%)
of 1,2,4,5-tetrafluorobenzene and unreacted starting material
(hexafluorobenzene) in 48%. If pentafluorobenzene is used for
the HDF, the quantitative, selective formation of the
hydrodefluorination in the para position to the hydrogen
substituent can be achieved for (iPr2Im)·AlH3 3, (Dipp2Im)·AlH3 5
and (Dipp2Im)·AliBu2H 7. 1,2,4,5-Tetrafluorobenzene is also the
major product for the reaction with (Me2Im)·AlH3 1 (77%), but the
regioisomer 1,2,3,4-tetrafluorobenzene can be detected
additionally in a yield of 2%. The compound (Dipp2Im)·AliBu2H 7
shows the highest reactivity and selectivity of the four NHC-Alan
adducts used in the HDF of pentafluorobenzene. Multiple
hydrodefluorination seems to be effectively prevented here by the
absence of additional hydride hydrogen atoms. With none of the
NHC alane adducts the hydrodefluorination of 1,2,4,5-
tetrafluorobenzene was observed. However, if even traces of the
isomer 1,2,3,4-tetrafluorobenzene were observed in the reaction
mixtures, also products of a further fluorine-hydrogen exchange,
i.e. 1,2,4-trifluorobenzene, have been detected. To confirm the
finding, that 1,2,3,4-tetrafluorobenzene is prone to further HDF
whereas 1,2,4,5-tetrafluorobenzene is not, both arenes were
(hexafluorobenzene,
pentafluorobenzene,
1,2,4,5-
tetrafluorobenzene) (see Scheme 3 and Figure 3).
Scheme 3. Hydrodefluorination of hexafluorobenzene, pentafluorobenzene and
1,2,4,5-tetrafluorobenzene using (NHC)·AlH3 (NHC = Me2Im 1, iPr2Im 3,
Dipp2Im 5) and (Dipp2Im)·AliBu2H 7.
In order to determine the optimum reaction temperature for the
HDF, the adduct (iPr2Im)·AlH3
3
was reacted with
pentafluorobenzene (i) in toluene at 105°C and (ii) in xylene at
135°C and the process of the reaction was monitored using 19F
NMR spectroscopy. After 84 h at 105 °C, a conversion of
pentafluorobenzene of 53 % was observed, and the reaction
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10.1002/ejic.201800589
European Journal of Inorganic Chemistry
FULL PAPER
reacted with (Me2Im)·AlH3 1 in mesitylene at 135°C for 24 h.
Whereas for 1,2,4,5-tetrafluorobenzene no conversion could be
detected, for 1,2,3,4-tetrafluorobenzene a 19 % conversion to
1,2,3,4-tetrafluorobenzene was identified. In total, the reaction
sequence and the observed product selectivity of the HDF
reactions with NHC stabilized aluminum hydrides can be
summarized as shown in Scheme 4. The first HDF step leads to
pentafluorobenzene. In this compound the fluorine substituent in
para-position to the hydrogen substituent is activated and the
major amount of pentafluorobenzene reacts with further H/F
(xylene)), in addition to minor amounts of 1,2,4,5-
tetrafluorbenzene (9% to 13 %) and varying amounts of 1,2,3,4-
tetrafluorobenzene and 1,2,4-trifluorobenzene. In further
experiments, 3 and 5 were also reacted with three equivalents of
pentafluorobenzene, which also led to distributions of
tetrafluorobenzene with admixtures of 1,2,3-trifluorobenzene, as
described above.
exchange
to
give
1,2,4,5-tetrafluorobenzene.
This
tetrafluorobenzene is unreactive with respect to further HDF. As
a side reaction of the second HDF step, the hydride attacks at the
less activated ortho-positions, which provides minor amounts of
1,2,4-trifluorobenzene. In contrast to 1,2,4,5-tetrafluorobenzene
the 1,2,3,4-isomer contains F-substituents located in between two
ortho-positioned fluoride substituents and are thus activated with
respect to further HDF. H/F exchange at those positions leads to
the observed product 1,2,4-trifluorobenzene.
Figure 3. HDF of hexafluorobenzene, pentafluorobenzene and 1,2,4,5-
tetrafluorobenzene in xylene at 135°C using (NHC)·AlH3 (NHC = Me2Im 1: top
left, iPr2Im 3: top right, Dipp2Im 5: bottom left) and (Dipp2Im)·AliBu2H 7 (bottom
right) as the hydride source.
In order to obtain a more detailed insight into the reaction
sequence, (Dipp2Im)·AlH3 5 was reacted with pentafluorobenzene
in mesitylene-d12 at 135 °C, and the course of the reaction was
Scheme 4. Reaction sequence and product selectivity of the HDF with NHC-
stabilized aluminum hydrides.
1
monitored using H and 19F NMR spectroscopy over a period of
46 hours in 20 minutes intervals. Selected 19F NMR spectra are
shown in Figure 4. In the course of the reaction, the formation of
a new resonance at 139.3 ppm of 1,2,4,5-tetrafluorobenzene can
be detected in the 19F NMR spectra together with a continuous
decrease in the resonances of the pentafluorobenzene at 138.8
ppm, 154.1 ppm and -162.5 ppm. GC-MS studies of the reaction
solution after the reaction is completed confirm the formation of
1,2,4,5-tetrafluorobenzene as the main reaction product.
Since equimolar transformations of the adducts (Me2Im)·AlH3 1,
(iPr2Im)·AlH3 3, and (Dipp2Im)·AlH3 5 with hexafluorobenzene
seemingly result in the transfer of at least two hydrogen atoms, it
was checked if
a
selective mono-hydrodefluorination to
pentafluorobenzene is possible just by a variation of the ratio of
(NHC)∙AlH3 to hexafluorobenzene. For this purpose, (iPr2Im)·AlH3
3 and (Dipp2Im)·AlH3 5 were reacted with three equivalents of
hexafluorobenzene, either in toluene at 105 °C or in xylene at
135 °C. In these cases, pentafluorobenzene was obtained as a
main product (3: 34% (toluene) and 37% (xylene); 5: 25%
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10.1002/ejic.201800589
European Journal of Inorganic Chemistry
FULL PAPER
Figure 4. Time dependent 19F NMR spectra (4 h intervals) of the HDF of pentafluorobenzene to 1,2,4,5-tetrafluorbenzene in mesitylene-d12 at 135 °C using
(Dipp2Im)·AlH3 5 as the hydride source.
In the corresponding proton NMR spectra (not shown), the
decrease of C6F5H (decrease in the intensity of the signal at δ =
6.38 ppm) and the formation of 1,2,4,5-tetrafluorobenzene (new
resonance at 6.61 ppm) can be confirmed.
well-defined NHC-aluminum compounds cannot be detected (with
the exception of (Dipp2Im)·AlH3 5). A conversion vs. time diagram
(based on the integration of 19F NMR spectra) and a ln[C6F5H] vs.
time diagram for the HDF of pentafluorobenzene to 1,2,4,5-
Furthermore, traces of the isomer 1,2,3,4-tetrafluorobenzene
(141.3 and 154.1 ppm) are present in solution, and, in addition, a
strongly broadened resonance occurs at -176.1 ppm with an
integration ratio of 1:4 compared to the integral of the resonance
of 1,2,4,5-tetrafluorobenzene, which is assigned to a aluminum
fluoride species.
Figure 6. ln[C6F5H] vs time diagram with linear regression (ln[C6F5H] = -0.054·t)
for the HDF of pentafluorobenzene (blue dots) to 1,2,4,5-tetrafluorobenzene
(red dots) in mesitylene-d12 at 135 °C using (Dipp2Im)·AlH3 7 as the hydride
source.
tetrafluorobenzene in mesitylene-d12 at 135°C using
(Dipp2Im)·AlH3 5 as the hydride source are given in Figures 5 and
6. The reaction follows first-order kinetics with a rate constant of
k = 5.4 × 10-2 h-1 and a half-life of 12.8 hours. We speculate that
at the beginning of the reaction an κ1-adduct between
(Dipp2Im)·AlH3 7 and pentafluorobenzene is formed (see Scheme
5, (i)) which initiates the fluorine-hydrogen exchange. The
nucleophilic hydride atom of the adduct attacks then the
fluoroarene at the positively polarized para-carbon atom to form a
Figure 5. Conversion vs. time diagram for the HDF of pentafluorobenzene (blue
dots) to 1,2,4,5-tetrafluorobenzene (red dots) in mesitylene-d12 at 135 °C using
(Dipp2Im)·AlH3 7 as the hydride source.
The strong broadening of the signal can be attributed to the
quadrupole moment of the aluminum core. As can be seen in
Figure 5, no clearly identifiable intermediate can be observed
during the reaction. However, apart from this fluorine resonance,
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10.1002/ejic.201800589
European Journal of Inorganic Chemistry
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Meisenheimer-like intermediate, while the negatively polarized
fluorine atom is transferred to the aluminum center (see Scheme
5: (ii)). Alternatively, it would be conceivable that the reaction is
initiated by the dissociation of the NHC from the alane at high
temperatures and the free NHC acts as the C-F bond activator
while the released AlH3 molecule serves as a hydride source and
fluoride acceptor. However, there is no hint for such an
mechanism in the proton NMR spectra shown above (Figure 4),
since only the resonances of the adduct (Dipp2Im)·AlH3 5 and the
two fluoroaromatic compounds pentafluorobenzene and 1,2,4,5-
tetrafluorobenzene can be detected.
further investigated the influence of the substitution pattern on the
HDF. As described above, the reaction of (Me2Im)·AlH3 1 with
equimolar amounts of 1,2,3,4-tetrafluorobenzene at 135 °C
affords a 19% conversion to 1,2,4-trifluorobenzene after 25 hours
(see Figure 3), which led to the suggestion that ortho-fluorine
substituents significantly activate the position of the nucleophilic
attack (see above). Consequently, we reacted (iPr2Im)·AlH3 3 and
(Dipp2Im)·AlH3 5 with 1,2,3-trifluorobenzene, a substrate without
any ortho-fluorine substituents, in a 1:1 ratio for 60 h at 135°C,
and could detect only traces (<1%) of 1,3-difluorobenzene by 19
NMR spectroscopy.
F
HDF of Hexafluoropropene
In order to study the HDF scope of the NHC-alane adducts
towards fluoroalkenes, hexafluoropropene and (Me2Im)∙AlH3
1
were chosen for reactivity studies. The reactions were conducted
at ambient temperature in toluene with a molar ratio of 0.4 and 0.9
of the alane, respectively. The reaction mixture was separated by
fractional condensation under vacuum and the conversion of the
products was determined by 19F NMR spectroscopy through
integration of the product resonances versus the internal standard
(fluorobenzene). The results are depicted in scheme 6. Using 0.4
equivalents of the alane resulted in a 93 % conversion to the
hydrodefluorinated products; with the E- and Z-isomer of
1,2,3,3,3-pentafluoropropene as major products and traces of
1,1,2,3,3-pentafluoropropene resulting from the attack of the CF3-
group.
Scheme 5. Reaction sequence and product selectivity of the HDF with NHC-
stabilized aluminum hydrides.
The high regioselectivity of the HDF of pentafluorobenzene to
1,2,4,5-tetrafluorobenzene using the adducts (Me2Im)·AlH3 1,
(iPr2Im)·AlH3 3, (Dipp2Im)·AlH3 5 and (Dipp2Im)·AliBu2H 7 may be
explained with the fact that the negative charge of the
Meisenheimer complex formed during the reaction is most
effectively stabilized by the -I effect (negative inductive effect) of
the ortho-fluorine and meta-fluorine substituents. Thus, the
nucleophilic attack on the fluorine atom in the para-position
relative to the hydrogen atom in pentafluorobenzene is kinetically
preferred because the negative charge of the Meisenheimer
complex is stabilized by two ortho-fluorine and two meta-fluorine
substituents. [19]
As already discussed above, the reaction of hexafluorobenzene
with (Me2Im)·AlH3 1 and (iPr2Im)·AlH3 3 leads in addition to the
formation of 1,2,4,5-tetrafluorobenzene to small amounts of
1,2,3,4-tetrafluorobenzene and 1,2,4-trifluorobenzene. Since
1,2,4,5-tetrafluorobenzene cannot be further hydrodefluorinated
with any of the NHC alane adducts (1, 3, 5, and 7) employed, we
Scheme 6. HDF of hexafluoropropene with 1 in toluene at room temperature.
Besides, small amounts of double HDF products were detected
as well as an unidentified product with a resonance at -76 ppm.
Using 0.9 equivalents of the alane gave a 100 % conversion with
2,3,3,3-tetrafluoropropene (79 %) as the main product. The
isomeric pentafluoropropenes are formed in just 14 % yield next
to small amounts of double and triple hydrodefluorinated products.
This demonstrates that the HDF occurs mainly at the CF2-group
of hexafluoropropene with fast formation of 1,2,3,3,3-
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10.1002/ejic.201800589
European Journal of Inorganic Chemistry
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pentafluoropropene. The thermodynamically less stable E-isomer
is preferentially formed. With an excess of the alane second and
third generation HDF products are formed. Attack of the CF3-
group yields CF2H-group containing compounds. Third
generation HDF seems to be rather slow, but explains the
formation of 2,3,3,3-tetrafluoropropene as major product if a large
excess (> 2 equivalents) of the hydride source was used. In order
to obtain an estimate of the activation parameters we conducted
kinetic studies at -80, -60, -40, -20 and 0 °C using an excess of
the alane. Assuming first order kinetics we got activation
parameters of ΔH‡ = 8.4 kcal mol-1 and ΔS‡ = 38 cal mol-1 K-1
(Figure 7, Figures S1-S5 and Tables S1-S6 of the Supporting
Information). The entropy contribution is rather high, indicating a
complex transition state with probably more than two molecules
involved.
selectively
and
quantitatively
converted
to
1,2,4,5-
tetrafluorobenzene. The most active hydride transfer reagent is
Scheme 7. HDF of hexafluorobenzene with 1 in toluene at room temperature.
here (Dipp2Im)·AliBu2H 7, for which completeness was reached
after 12 hours at 135 ° C in xylene.
The HDF of hexafluoropropene proceeds at ambient
temperatures. Depending on the amount of alane 1 different
product distributions are observed.
Our study on the HDF of fluoroaromatics using NHC alane
adducts demonstrate that these adducts are suitable as novel
hydride transfer reagents. Starting from lithium aluminum hydride
or di-iso-butyl aluminum hydride, the adducts (NHC)·AlH3 and
(NHC)·AliBu2H can be easily synthesized from the reaction with
the corresponding NHCs. These adducts are suitable reagents in
organic synthesis due to their high stability, their (comparably) low
moisture and air sensitivity and their good solubility in many
organic solvents, such as benzene, toluene and THF, they are
storable and much better to handle than most of the commonly
used pyrophoric alanes. And for the HDF of fluoroarenes they
show interesting reactivities and selectivities.
Figure 7. Eyring plot for HDF of hexafluoropropene with 1 at -80, -60, -40, -20
and 0 °C.
Conclusions
The results of our study are summarized in Scheme 6. We have
demonstrated here that the reaction of (NHC)·AlH3 (NHC = Me2Im
1, Me2ImMe 2, iPr2Im 3, iPr2ImMe 4, Dipp2Im 5) and (NHC)·AliBu2H
(NHC = iPr2Im 6, Dipp2Im 7) with equimolar amounts of
pentafluoropyridine at room temperature leads to the formation of
2,3,5,6-tetrafluoropyridine in yields between 15% and 99%. In
these reactions, the compounds (Me2Im)·AlH3 1, (Me2ImMe)·AlH3
2 and (Dipp2Im)·AlH3 5 seem to be particularly suitable for the
HDF of pentafluoropyridine, since quantitative conversion to
2,3,5,6-tetrafluoropyridine has been found immediately after the
reactants were mixed at room temperature. For the reactions of
(NHC)·AlH3 (NHC = Me2Im 1, iPr2Im 3, Dipp2Im 5) and
Experimental Section
General Information: All manipulations, unless otherwise stated, were
carried out under an atmosphere of argon using conventional Schlenk,
vacuum-line and glove-box techniques. All reactions were carried out in
oven-dried glassware. The amount of gaseous compounds was
determined by using pVT technique or by condensing the gas into a
weighted J. Young flask. Benzene, toluene, hexane, and THF were purified
by distillation from an appropriate drying agent (sodium with
benzophenones as indicator). C6D6 and mesitylene-d12 were purchased
from Sigma-Aldrich. The NHC alane adducts have been prepared as
reported previously.[14] Commercially available fluoroaromatics (ABCR)
were used directly without further purification. Hexafluoropropene (Solvay)
was obtained free of charge.
(Dipp2Im)·AliBu2H
7
with
the
fluorinated
aromatics
hexafluorobenzene and pentafluorobenzene in xylene at 135 °C
it has been found that the degree of fluorination shows a decisive
influence on the H/F exchange. Whereas the reaction of the
adducts
with
hexafluorobenzene
yields
1,2,4,5-
tetrafluorobenzene as the main product in moderate yields, with
minor amounts of 1,2,3,4-tetrafluorobenzene and 1,2,4-
trifluorobenzene formed, pentafluorobenzene is usually
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10.1002/ejic.201800589
European Journal of Inorganic Chemistry
FULL PAPER
Analytical methods: All NMR spectra for the HDF of pentafluoropyridine
2,3,5,6-Tetrafluorpyridin: 19F NMR (376.8 MHz, C6D6, 298 K): δ = -91.6 (m,
2 F), 140.1 (m, 2 F) Product of the reaction of (iPr2Im)·AliBu2H with
pentafluoropyridine:1H NMR (400.4 MHz, C6D6, 298 K): δ = 0.36 (m, 2 HA,
and fluorobenzenes were recorded at 298 K using a Bruker Avance 500
(1H NMR, 500.1 MHz; 19F NMR, 470.6 MHz) and a Bruker DPX 400 (19
F
3
NMR, 376.8 MHz). If not mentioned otherwise, the reaction products were
identified by means of the resonances found in the 19F NMR spectra and
their assignment was confirmed by GC-MS investigations. Yields were
determined by integration of the 19F NMR resonances. All NMR spectra for
the HDF of hexafluoropropene were recorded at 293 K using a JEOL JNM-
ECA400II (19F NMR, 376.0 MHz). 1H NMR chemical shifts are reported
relative to TMS and were referenced via residual proton resonances of the
corresponding deuterated solvent ( C6D5H, 7.16 ppm, C9D11H, 2.31, 6.87
ppm). 19F NMR spectra are reported relative to external CFCl3. GC-MS
analyses were performed using an Agilent 7890A gas chromatograph
(column: HP-5 MS 5% phenyl methyl siloxane, 30 m, ∅ 0.25 mm, film 0.25
μm; injector: 250 °C; oven: 40 °C (2 min), 40 °C to 280 °C (20 °C min–1);
carrier gas: He (1.2 mL min–1) equipped with an Agilent 5975C inert MSD
with triple-axis detector operating in EI mode and an Agilent 7693A series
auto sampler/injector.
iBu-CH2), 0.50 (ddd, 1 HB, iBu-CH2), 0.98 (d, 12 H, JHH = 6.8 Hz, NHCiPr-
3
3
CH3), 1.32 (d, 6 H, JHH = 6.49 Hz, iBu-CH3), 1.36 (d, 6 H, JHH = 6.5 Hz,
iBu-CH3), 2.26 (sept, 2 H, 3JHH = 6.5 Hz, iBu-CH), 5.18 (sept, 2 H, 3JHH
6.8 Hz, JHF = 3.6 Hz, NHCiPr-CH), 6.19 (s, 2 H, NCHCHN). 19F NMR (376.8
=
MHz, C6D6, 298 K): -170.3 ppm (s, br, 1F)
HDF of fluorobenzenes with (Me2Im)·AlH3 1: The fluoroaromatic
compound was added at room temperature to a solution of (Me2Im)·AlH3
1 (10 mg, 73.3 μmol) in 0.6 ml of xylene in a J. Young NMR tube. The
temperature of the reaction mixture was raised to 135 ° C and the reaction
was monitored using 19F NMR spectroscopy. Details are given in Table 2.
Table 2. Experimental Details for the HDF of fluorobenzenes with
(Me2Im)·AlH3 1.
HDF of pentafluoropyridine: Pentafluoropyridine (in a stoichiometric
ration of 1:1) was added at room temperature to (NHC)AlR3 dissolved in
0.6 ml of C6D6 in a J. Young NMR tube. The reaction was monitored 5 min
after addition by 19F NMR spectroscopy and afterwards after 1, 2, 3, 4, 5,
and 6 hours. Details are given in Table 1.
Alane
Arene
:
Solvent
/ Temp.
Reaction
timer
Substrate
Product
C6F6
C6F5H :18 %
9.2 µl
(14.8 mg,
79.3 µmol)
1,2,4,5-C6F4H2: 74 %
1,2,3,4-C6F4H2: 3 %
1,3,4-C6F3H3: 4 %
xylene
135 °C
1 : 1
1 : 1
25 h
25 h
C6F5H
8.8 µl
(13.3 mg,
79.3 µmol)
1,2,4,5-C6F4H2: 77 %
1,2,3,4-C6F4H2: 2 %
xylene
135 °C
Table 1. Experimental Details for the reaction of (NHC)·AlR3 with
pentafluoropyridine
1,2,4,5-
C6F4H2
8.9 µl
(11.9 mg,
79.3 µmol)
C6F5N
used
2,3,5,6-
C5F4N
Reaction
time
(NHC)·adduct used
Conversion
99 %
xylene
135 °C
1 : 1
-
25 h
25 h
8.7 µl
13.4 mg,
79.3 µmol)
(Me2Im)·AlH3 1
10 mg (79.3 µmol)
99 %
93 %
75 %
45 %
99 %
69 %
13 %
5 min
5 min
6 h
1,2,3,4-
C6F4H2
8.5 µl
(11.9 mg,
79.3 µmol)
7.1 µl
(11.0 mg,
64.9 µmol)
xylene
135 °C
(Me2ImMe)·AlH3 2
10 mg (64.9 µmol)
1 : 1
1,3,4-C6F3H3 19 %
99 %
99 %
99 %
99 %
86 %
99 %
9.0 µl
(13.9 mg,
82.3 µmol)
(iPr2Im)·AlH3 3
15 mg (82.3 µmol)
7.8 µl
(12.1 mg,
71.3 µmol)
(iPr2ImMe)·AlH3 4
15 mg (71.3 µmol)
5 min
5 min
6 h
7.1 µl
(10.9 mg,
64.5 µmol)
(Dipp2Im)·AlH3 5
27 mg (64.5 µmol)
7.5 µl
(11.5 mg,
67.9 µmol)
(iPr2Im)·AliBu2H 6
20 mg (67.9 µmol)
5.6 µl
(8.6 mg,
50.9 µmol)
(Dipp2Im)·AliBu2H 7
27 mg (50.9 µmol)
5 min
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10.1002/ejic.201800589
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HDF of fluorobenzenes with (iPr2Im)·AlH3 3: The fluoroaromatic
compound was added at room temperature to a solution of (iPr2Im)·AlH3 3
(15 mg, 82.4 μmol) in 0.6 ml of xylene or toluene in a J. Young NMR tube.
The temperature of the reaction mixture was raised to 135°C (xylene) or
105°C (toluene) and the reaction was monitored by 19F NMR spectroscopy.
Details are given in Table 3.
Table 4. Experimental Details for the HDF of fluorobenzenes with
(Dipp2Im)·AlH3 5.
Table 3. Experimental Details for the HDF of fluorobenzenes with
(iPr2Im)·AlH3 3.
Alane:
Arene
Solvent
Temp.
Reaction
time
Substrate
Product
C6F6
7.4 µl
(12.0 mg,
64.5 µmol)
1,2,4,5-C6F4H2: 41 %
C6F6 was completely
consumed
xylene
135 °C
1 : 1
1 : 3
1 : 1
1 : 3
1 : 1
1 : 1
60 h
48 h
48 h
48 h
60 h
12 h
Alane :
Arene
Solvent
/ Temp.
Reaction
time
Substrate
Product
C6F6
8.3 µl
(13.3 mg,
71.7 µmol)
xylene
135 °C
C6F5H: 25 %
C6F6
9.5 µl
(15.3 mg,
82.4 µmol)
1,2,4,5-C6F4H2: 71 %
1,2,3,4-C6F4H2: 8 %
1,3,5-C6F3H3: 21 %
xylene
135 °C
1 : 1
60 h
84 h
12 h
C6F5H
7.2 µl
(10.8 mg,
64.5 µmol)
xylene
135 °C
1,2,4,5-C6F4H2: 99 %
1,2,4,5-C6F4H2: 15 %
-
C6F5H: 34 %
toluene
105 °C
1,2,4,5-C6F4H2: 9 %
1,2,3,4-C6F4H2: 2 %
1,3,5-C6F3H3: 3 %
C6F5H
8.0 µl
(12.0 mg,
71.7 µmol)
C6F6
28.5 µl
(45.9 mg,
247 µmol)
xylene
135 °C
1 : 3
C6F5H: 37%
xylene
135 °C
1,2,4,5-C6F4H2: 13 %
1,2,3,4-C6F4H2: 3 %
1,3,5-C6F3H3: 5 %
1,2,4,5-C6F4H2
9.9 µl
(13.3 mg,
64.5 µmol)
xylene
135 °C
toluene
105 °C
1,2,4,5-C6F4H2: 43 %
1,2,3,4-C6F4H2: 10 %
84 h
72 h
84 h
36 h
C6F5H
9.1 µl
(13.8 mg,
82.4 µmol)
1 : 1
1,3,5-C6F3H3
6.7 µl
(8.5 mg,
xylene
135 °C
xylene
135 °C
1,2,4,5-C6F4H2: 99 %
1,3-C6F2H4: < 1 %
64.5 µmol)
xylene
105 °C
1,2,4,5-C6F4H2: 22 %
1,2,3,4-C6F4H2: 5 %
C6F5H
27.4 µl
(41.5 mg,
247 µmol)
1 : 3
xylene
135 °C
1,2,4,5-C6F4H2: 41%
1,3,5-C6F3H3: 5 %
Time dependent NMR spectroscopy on the HDF of C6F5H with
(Dipp2Im)·AlH3 5 to 1,2,4,5-C6F4H2: Pentafluorobenzene (7.2 μl, 10.8
mg, 64.5 μmol) is added at room temperature to
a solution of
1,2,4,5-C6F4H2
9.3 µl
(12.4 mg,
82.4 µmol)
(Dipp2Im)·AlH3 5 (27 mg, 64.5 μmol) in 0.6 ml of mesitylene-d12, dissolved
in a J. Young NMR tube. The reaction solution is then heated to 135 ° C.
in a NMR spectrometer and monitored by 19F NMR spectroscopy.
xylene
135 °C
1 : 1
1 : 1
-
60 h
36 h
1,3,5-C6F3H3
8.5 µl
(10.9 mg,
82.4 µmol)
HDF of fluorobenzenes with (Dipp2Im)·AliBu2H 7: The fluorobenzene is
added at room temperature to a solution of (Dipp2Im)·AliBu2H 7 (45 mg,
84.8 μmol) in 0.6 ml of xylene in a J. Young NMR tube. The reaction
solution is then heated to 135 °C and the reaction is monitored by 19F NMR
spectroscopy. Details are given in Table 5.
xylene
135 °C
1,3-C6F2H4: <1 %
HDF of fluorobenzenes with (Dipp2Im)·AlH3 5: The fluoroaromatic
compound was added at room temperature to a solution of (Dipp2Im)·AlH3
5 (stoichiometry 1: 1 = 27 mg, 64.5 μmol; stoichiometry 1: 3 = 10 mg, 23.9
μmol) in 0.6 ml of xylene in a J. Young NMR tube. The temperature of the
reaction mixture was raised to 135°C and the reaction was monitored 19
F
NMR spectroscopy. Details are given in Table 4.
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10.1002/ejic.201800589
European Journal of Inorganic Chemistry
FULL PAPER
Table 5. Experimental Details for the HDF of fluorobenzenes with
(Dipp2Im)·AliBu2H 7.
Table 6. Reaction conditions, conversion, E/Z and for HDF of
hexafluoropropene with (Me2Im)∙AlH3 1 in toluene at rt.
Alan :
Arene
Solvent
Temp.
/
Reaction
time
Substrate
Product
C6F6
9.8 µl
(15.8 mg,
84.8 µmol)
C6F5H: 51 %
1,2,4,5-C6F4H2: <1 %
xylene
135 °C
1 : 1
1 : 1
12 h
12 h
Solvent
/ Temp.
Reaction
time
Entry
1
1 [eq.]
0.9
Products
8a: 9.0 %; 8b: 5.0 %; 8c: -; 8d:
0.6 %, 8e: 2.1 %, 9a: 79.3 %,
9b: 1.7 %, 10a: 0.4 %, 10b:
1.8 %
C6F5H
9.4 µl
(14.3 mg,
84.8 µmol)
Toluene
25 °C
xylene
135 °C
21 h
20 h
1,2,4,5-C6F4H2: 99 %
8a: 54.5 %; 8b: 33.3 %; 8c:
0.9 %; 8d: 0.6 %, 8e: 1.0 %,
9a: 0.1 %, 9b: 2.3 %, 10a: -,
10b: -
1,2,4,5-
C6F4H
9.5 µl
(12.7 mg,
84.8 µmol)
Toluene
25 °C
2
0.4
xylene
135 °C
1 : 1
-
60 h
60 h
1,3,5-
C6F3H3
8.8 µl
(11.2 mg,
84.8 µmol)
Kinetic experiments: For a stock solution, 1 (0.26 g, 2.0 mmol) and
toluene (3 ml) were mixed in the glove box. 500 µl stock solution was filled
in a J. Young NMR tube together with a glass capillary tube filled with
acetone-d6. The solution was degassed and hexafluoropropene (51 mg,
0.34 mmol) was added by vacuum transfer. To determine ΔH‡ and ΔS‡,
the reaction progress was monitored by 19F NMR spectroscopy at -80, -60,
-40, -20 and 0 °C, respectively. The conversion of the substrate was
determined from 19F NMR spectra by integration of product and starting
material resonances. Experimental data is given in the SI (Table S1-S6
and Figure S1-S5).
xylene
135 °C
1 : 1
-
Analytic and Spectroscopic Data of the fluorobenzenes:[40]
Hexafluorobenzene: 19F NMR (376.8 MHz, C6D6, 298 K): δ = -162.9 (6 F).
GC/MS (t = 0.77 min): m/z 186 [M]+ Pentafluorobenzene: 19F NMR (376.8
MHz, C6D6, 298 K): δ = -139.1 (m, 1 Fmeta), -154.0 (m, 1 Fipso), -162.9 (m,
1 Fortho). GC/MS (t = 0.86 min): m/z 168 [M]+ 1,2,4,5-Tetrafluorobenzene
19F NMR (376.8 MHz, C6D6, 298 K): δ = -139.8 (m, 4 F). GC/MS
(t = 0.94 min): m/z 150 [M]+ 1,2,3,4-Tetrafluorobenzene 19F-NMR (376.8
MHz, C6D6, 298 K): δ = -139.8 (m, 2 F), 156.8 (m, 2 F). GC/MS
(t = 1.06 min): m/z 150 [M]+ 1,2,4-Trifluorobenzene 19F-NMR (376.8 MHz,
Acknowledgements
Financial support from the Julius-Maximilians-Universität
Würzburg, the Deutsche Forschungsgemeinschaft (DFG RA
720/13-1) and the GRK 1582 (Fluorine as a Key Element) is
gratefully acknowledged. ADJ and DL thank Janis M. Berkemeyer
(FU Berlin) for his contributions to the kinetics of the HDF of
hexafluoropropene.
C6D6, 298 K): δ = -115.3 (m, 1 Fpara), -143.1 (m, 1 Fipso), -155.9 (m, 1 Fortho
)
GC/MS (t = 1.00 min): m/z 132 [M]+ 1,3,5-Trifluorobenzene 19F-NMR
(376.8 MHz, C6D6, 298 K): δ = -110.1 (m, 1 Fortho) GC/MS (t = 0.80 min):
m/z 132 [M]+
HDF of hexafluoropropene with (Me2Im)∙AlH3 1:. A single-necked flask
equipped with a J. Young valve was charged with alane 1 and toluene and
the mixture was degassed. Hexafluoropropene was condensed into the
flask. The corresponding reaction conditions were applied. The conversion
of the substrate was determined by integration of the 19F NMR product
resonances versus the internal standard (fluorobenzene). The products
were identified by 19F NMR spectroscopy (benzene-d6), using available
literature data for 8a-b[20], 8c[21], 8d-e[22], 9a-b[23], 10b[24] or by comparison
with an authentic sample (10a).
Keywords: N-Heterocyclic Carbenes•Alane Lewis Base
Adducts• Hydrodefluorination•Fluoroarenes• Hexafluoropropene.
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The utilization of NHC-stabilized alane
adducts of the type (NHC)∙AlH3 (NHC
= Me2Im 1, Me2ImMe 2, iPr2Im 3,
iPr2ImMe 4, Dipp2Im 5) and
(NHC)∙AliBu2H (NHC = iPr2Im 6,
Dipp2Im 7) in the hydrodefluorination
(HDF) of different fluoroaromatics and
hexafluoropropene is presented.
These adducts (NHC)∙AlH3 and
(NHC)∙AliBu2H are new and easy to
handle hydride transfer reagents.
NHC-Alane Adducts as Hydride
Sources in the Hydrodefluorination of
Fluoroaromatics and Fluoroolefines
H. Schneider, A. Hock, A. D. Jaeger, D.
Lentz*, U. Radius*
Page No. – Page No.
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