Gallium Hydrides and O/N-Donors as Tunable Systems in C?F Bond Activation

Article abstract of DOI:10.1002/asia.201801030

The gallium hydrides (iBu)₂GaH (1 a), LiGaH₄ (1 b) and Me₃N?GaH₃ (1 c) hydrodefluorinate vinylic and aromatic C?F bonds when O and N donor molecules are present. 1 b exhibits the highest reactivity. Quantitative conversion to the hydrodefluorination (HDF) products could be observed for hexafluoropropene and 1,1,3,3,3-pentafluoropropene, 94 % conversion of pentafluoropyridine and 49 % of octafluorotoluene. Whereas for the HDF with 1 b high conversions are observed when catalytic amounts of O donor molecules are added, for 1 a, the addition of N donor molecules lead to higher conversions. The E/Z selectivity of the HDF of 1,1,3,3,3-pentafluoropropene is donor-dependent. DFT studies show that HDF proceeds in this case via the gallium hydride dimer–donor species and a hydrometallation/elimination sequence. Selectivities are sensitive to the choice of donor, as the right donor can lead to an on/off switching during catalysis, that is, the hydrometallation step is accelerated by the presence of a donor, but the donor dissociates prior to elimination, allowing the inherently more selective donorless gallium systems to determine the selectivity.

Full text of DOI:10.1002/asia.201801030

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Accepted Article
Title: Gallium Hydrides and O/N-donors as tunable systems in C-F
Bond Activation
Authors: Alma Dorothea Jaeger, Ruben Walter, Christian Ehm, and
Dieter Lentz
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Gallium Hydrides and O/N-Donors as Tunable Systems in C-F
Bond Activation
[
a]
[a]
[b]
[a]
Alma D. Jaeger, Ruben Walter, Christian Ehm,* and Dieter Lentz*
Dedicated to Prof. Dietmar Stalke on the occasion of his 60th birthday
Abstract: The gallium hydrides (iBu)
2
GaH (1a), LiGaH
4
(1b) and
example,
the
hydroalumination
of
1,4-
Me N∙GaH (1c) hydrodefluorinate vinylic and aromatic C-F bonds
3
3
bis(trimethylsilylethynyl)benzene with di(tert-butyl)aluminum
hydride yields the cis-product, which slowly isomerizes at
elevated temperatures to the thermodynamically favored trans-
product.[ However, the reaction with di(tert-butyl)gallium hydride
gave almost quantitatively the cis-product and rearrangement to
the trans-form was not observed even after continued heating.[8]
when O and N donor molecules are present. 1b exhibits the highest
reactivity. Quantitative conversion to the hydrodefluorination (HDF)
products could be observed for hexafluoropropene and 1,1,3,3,3,-
pentafluoropropene, 94 % conversion of pentafluoropyridine and 49 %
of octafluorotoluene. Whereas for the HDF with 1b high conversions
7]
are observed when catalytic amounts of O donor molecules are added, In continuation of our studies concerning the organocatalytic
for 1a the addition of N donor molecules lead to higher conversions.
The E/Z selectivity of the HDF of 1,1,3,3,3-pentafluoropropene is
donor dependent. DFT studies show that HDF proceeds in this case
hydrodefluorination (HDF) of several olefinic and aromatic
[
9]
fluorinated compounds with aluminum hydrides , we were
interested in exploring the reactivity of gallium hydrides for C-F
bond activation.
via the gallium hydride dimer-donor species and
a
hydrometallation/elimination sequence. Selectivities are sensitive to
the choice of donor, as the right donor can lead to an on/off switching
during catalysis, i.e. the hydrometallation step is accelerated by the
presence of a donor, but the donor dissociates prior to elimination,
allowing the inherently more selective donor-less gallium systems to
determine the selectivity.
The C-F bond is one of the strongest single bonds with a bond
dissociation energy (BDE) of approximately 120 kcal/mol.[10] The
introduction of fluorine into organic molecules increases their
thermodynamic and kinetic stability as well as their lipophilicity.
This aspect is utilized in a wide range of applications, for example
in pharmaceuticals, agrochemicals, highly stable polymers like
Teflon, imaging agents or optoelectronics.[11] However, the
inertness of the C-F bond has also led to environmental
concerns.[ Therefore, the activation of the C-F bond is of high
interest in matters of degradation of highly fluorinated compounds,
but as well for the synthesis of pharmaceutic building blocks from
perfluorinated small molecules. Several systems, often transition
metal based, are known for the catalytic HDF of olefinic and
aromatic fluorinated compounds.[13] However, in many instances,
transition-metal-catalyzed HDF reactions show low turn-over
numbers (TON), caused by the strong affinity between transition
metals and fluorine atoms, which leads to catalyst deactivation. A
new approach is the use of silicon-, boron- and aluminum-based
Lewis acids for the heterolytic fluoride abstraction from aliphatic
fluorides.[14] Recently, Zhang reported an example for transition-
metal-free, photocatalytic HDF of several polyfluoroarenes by
pyrene-based photocatalyts[15]; and Ogoshi showed that the HDF
of polyfluoroarenes can be catalyzed by hydrosilicates.[16]
Here, we present the C-F bond activation of per- and
polyfluorinated substrates with gallium hydrides and O or N donor
molecules.
12]
Introduction
Gallium hydrides are applied in metal-organic chemical vapour
[
1]
[2]
depositon (MOCVD) technology , hydrogallation and more
recently as active Lewis pairs in C-H bond activation.[3]
Nonetheless, the chemistry of gallanes is scarcely scrutinized and
far less investigated compared to alanes and boranes.
3
Stabilization of gallane and alane (MH ) can be achieved by the
formation of adducts with Lewis bases like phosphanes or
amines.[
1c,4]
In general, aluminum has a stronger tendency to form
hyper-valent structures than gallium.[1c,4a] The less polarized Ga-H
bond results in a weaker Lewis acidity of gallanes, which usually
results in a lower reactivity compared to alanes.[1c,2b,4a,5] However,
earlier studies showed that gallane complexes are milder and
more selective reducing agents than similar alane complexes and
exhibit a higher selectivity in hydrogallation reactions than the
corresponding alanes in hydroalumination reactions.[2a,2b,4b,6] For
[
[
a]
b]
A. D. Jaeger, R. Walter, Prof. Dr. D. Lentz
Freie Universität Berlin
Institut für Chemie und Biochemie, Anorganische Chemie,
Fabeckstr. 34-36, 14195 Berlin (Germany)
E-mail: dieter.lentz@fu-berlin.de
Dr. C. Ehm
Università di Napoli Federico II
Dipartimento di Scienze Chimiche
Via Cintia, 80126 Napoli (Italy)
Results and Discussion
Hexafluoropropene (3) is hydrodefluorinated by diisobutylgallium
hydride (1a), lithium gallium hydride (1b) and the trimethylamine-
galliumhydride adduct (1c) in donor solvents at ambient
temperatures (Table 1). The HDF reactions lead to the formation
of the E- and Z-isomer of 1,2,3,3,3-pentafluoropropene (4a,b);
Supporting information for this article is given via a link at the end of
the document.
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Table 1. HDF of hexafluoropropene (3) with 1.1 equivalents of the gallium hydrides 1a-c at RT.
Gallium
hydride
Time
[h]
Donor
[mol%]
Conv.
[%]
Entry
Solvent
Donor additive
E/Z
TON
1
2
3
4
5
6
7
8
9
1a
19
21
16
16
21
21
18
0.25
18
15
22
18
16
21
22
toluene
diglyme
-
-
-
-
-
-
1a
-
2.0
1.9
1.8
1.8
1.9
1.6
3.2
2.8
1.5
1.6
1.6
2.9
2.9
1.7
76.5
95.5
5.1
-
-
1a
NEt
3
-
-
1a
toluene
toluene
toluene
toluene
diglyme
diglyme
diglyme
12
11
33
-
0.4
5.0
2.5
-
1a
NEt
3
54.7
81.1
0.8
1a
NEt
3
1b
1b
1b
1b
1b
1b
1c
-
-
-
-
-
99.7
100
97.0
96.4
86.6
14.2
80.7
29.7
-
-
-
10
11
12
13
14
15
NEt
3
-
-
toluene
toluene
toluene
diglyme
toluene
diglyme
10
11
-
10.7
8.9
-
NEt
3
-
-
1c
-
-
1c
diglyme
15
2.0
[
3
a] Reaction conditions: 1 mmol 3, 1.1 equiv. 1a-c in toluene, diglyme or NEt at RT. For full tables see the Supporting Information.
formation of the other constitutional isomer 1,1,2,3,3-
pentafluoropropene, the 2 generation HDF-products E- and Z-
mol% diglyme yields just 5 % conversion to the HDF products
(entry 4). But when diglyme is replaced by 11 mol%
trimethylamine 55 % conversion is observed (entry 5). The
conversion can be further increased at higher temperatures (74 %
conversion at 80 °C, see SI) or when the amount of triethylamine
is increased (81 % conversion with 33 mol% NEt at RT, entry 6).
3
The HDF with 1c is weakly catalyzed by donor solvents, as just a
minor increase of the conversion is observed (entry 15).
nd
1,2,3,3-tetrafluoropropene (4d,e) and 2,3,3,3-tetrafluoropropene
(
5a) were observed in traces (see Supporting Information for full
tables). Whereas for the HDF of 3 with 1b nearly quantitative
conversion to the HDF products could be observed after just 15
min reaction time in diglyme (entry 8), the reaction with 1a yields
77 % conversion (entry 2) and the reaction with 1c 81 %
conversion to the HDF products after 21 h reaction time (entry 14).
However, when the reaction is conducted in triethylamine the
conversion increased to 96 % using 1a (entry 3). In non-
coordinating solvents the reactivity of the gallium hydrides is low;
no HDF products are observed for the reaction of 3 with 1a (entry
In conclusion, 1a shows a higher reactivity with N donors
compared to O donors for the HDF of 3, whereas just a small
difference could be observed between the HDF reactions of 3 with
1b in N or O donors. Slightly higher conversions are obtained with
O donors and 1b. The choice of solvent influences the E/Z
1), just traces for the reaction with 1b (entry 7) and 14 %
4
selectivity just for the ionic compound LiGaH (1b), which
conversion with 1c (entry 13) as this hydride already is a Lewis
acid-base adduct. When catalytic amounts of diglyme (10 mol%)
are added to 1b in toluene, the conversion increases to 96 %
increases from 1.6 to 3.2 when the reaction is conducted in
diglyme for 15 min.
Against our expectations the Lewis acid-base adduct 1c does not
show the expected HDF capability, although the conversion is
slightly increased by addition of 15 mol% diglyme. That might be
(
entry 11), i.e. the reaction is organocatalyzed by diglyme.
However, the reaction of 3 and 1a in toluene with addition of 12
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Table 2. Catalytic HDF of 1,1,3,3,3-pentafluoropropene (4f) with 1.1 equivalents gallium hydrides 1a-b at RT, overnight.
Entry
Gallium
hydride
Solvent
Donor additive
Donor
Main Products [%]
E/Z
Conv.
[%]
TON
[mol%]
5b
5c
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1b
1b
toluene
diglyme
-
-
-
-
-
-
-
-
-
47.3
53.3
5.2
4.0
1.9
0.7
1.6
4.7
0.9[b]
5.9
11.8
27.3
7.8
53.1
56.2
5.8
-
NEt
3
-
-
-
toluene
toluene
toluene
toluene
toluene
THF
46
36
123
-
0.1
1.0
0.7
-
NEt
NEt
-
3
33.1
84.0
1.7
20.6
17.8
10.1
15.5
35.2
90.1
2.8
3
diglyme
12
91.0
99.8
8.2
[
a] Conversion is the sum of all HDF products (major and minor) and therefore may differ from the sum of the main products. For
full tables see the Supporting Information. [b] Conversion of 5d.
3 3
due to the fact that Me N∙GaH (1c) is described as a mild
reducing agent, which e.g. selectively reduces the carbonyl group
in 4-bromophenylacyl bromide, but not the C-Br bond.[6] Therefore,
Table 3. Catalytic HDF of 7 and 9 with 1.1 equivalents of gallium hydrides
1a-b in toluene at RT, overnight.
1
c was not applied in further HDF studies. The best results for the
Donor[b]
mol%]
Main
Product [%]
Conv.
[%]
Entry
Substrate
HS[a]
TON
catalytic HDF of 3 with 1a could be obtained with 30 mol%
triethylamine and for 1b with 10 mol% diglyme added to the
toluene mixture. These conditions were applied for the
subsequent HDF reactions.
[
Partially fluorinated propenes, as for example 1,1,3,3,3-
pentafluoropropene (4f), show lower reactivity for HDF, but the
selectivity is high. A remarkable E/Z ratio of 27 was observed for
the HDF reaction with 1a, viz. the thermodynamically more stable
E-isomer of 1,3,3,3-tetrafluoropropene (5b) is preferentially
formed. Such a high E/Z ratio is not observed in titanium-, rare-
7
8a
1
2
3
4
1a
1a
1b
1b
-
34
-
-
-
-
52.1
7.2
90.3
52.1
8.2
93.5
1.5
-
10.1
10
earth-metal-catalyzed or alane-induced HDF reactions;
maximum E/Z ratio of 15 was reported.[9,17] The conversion of the
HDF reactions with 1a in diglyme and NEt are similar (53-56 %,
entry 2,3), but the selectivity is much higher when the reaction is
conducted in NEt . Selectivity is lower, when catalytic amounts of
THF are used (entry 4). Interestingly, when 36 mol% NEt are
added to the toluene mixture of 1a 35 % conversion (entry 5)
could be observed, but when the amount of NEt is increased to
23 mol%, 90 % conversion (entry 6) could be observed, which is
a
3
3
9
10a
3
5
6
7
8
1a
1a
1b
1b
-
34
-
-
-
-
5.6
1.1
47.7
6.2
1.2
49.2
0.2
-
4.5
3
1
11
3
higher compared to the reaction in pure NEt . The reaction with
1
b shows almost quantitative conversion with 12 mol% diglyme
[
3
a] HS = hydride sources. [b] Donor additives are NEt for 1a and diglyme
(
entry 8).
for 1b. [c] The amount of the conversion is the sum of all HDF products
(major and minor) and therefore may differ from the sum of the main
products. For full tables see the Supporting Information.
To study the scope and limitations for the gallium hydride induced
HDF, 1a and 1b were reacted with polyfluorinated aromatic
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substrates (Table 3). The HDF of pentafluoropyridine (7) yields
the expected para-substituted HDF product 2,3,5,6-
tetrafluoropyridine (8a) as main product. In toluene without donor
additive 1a shows no reactivity, but 1b (8 % conversion, entry 3)
the same range.[18] Thus, this can be seen as evidence for the
hydrogen-fluorine-exchange from the gallane to the fluorinated
substrate. However, no well-defined gallium compounds could be
detected.
3
does. When 34 mol% NEt are added, 52 % conversion could be
observed for 1a (entry 2). The reaction with 1b can be catalyzed
by adding 10 mol% diglyme resulting in 94 % conversion (entry 4).
Octafluorotoluene (9) is predominantly hydrodefluorinated in
para-position, forming heptafluorotoluene (10a). The reactivity is
lower than for the other substrates. Just 6 % conversion could be
Mechanistic DFT Studies
Monomer-dimer-donor equilibria. We have recently shown that
HDF using aluminum hydrides and O or N donors proceeds via
the aluminum dimer donor adduct.[9] The donor is located far away
from the active center in the TS and the exact nature of the donor
thus does not influence the selectivity of the HDF reaction. The
dimerization energies for Group 13 hydrides[19] as well as metal-
hydride bond energies[10] decrease for the heavier group
homologues and we postulated that they therefore hold the
potential of offering higher selectivity.
3
observed when 1a with 34 mol% NEt were applied (entry 6).
Similar to the reaction of 7, the substitution reaction of 9 with 1b
could be catalyzed by 11 mol% diglyme with nearly 50 %
conversion (entry 8). In contrast to 1.2 % without adding diglyme
(
entry 7).
Recent studies towards aluminum hydrides in C-F bond activation
reactions showed that their reaction behavior coincide with the
gallium hydrides. No or almost no HDF products could be
To model the HDF reaction by computational chemistry, we chose
4f and the gallium hydride HGaMe (1M) and the donor THF. The
2
observed for the HDF reactions with (iBu)
2b) in toluene, but with addition of catalytic amounts of donor
2
AlH (2a)[9] or LiAlH
4
Gibbs free energy of two molecules 1M forming the gallium
hydride dimer 1D is predicted to be 18.4 kcal/mol or ~7 kcal/mol
lower than for the corresponding aluminum compound (Scheme
1). Additionally, THF binds only very weakly to the dimer 1D (-0.6
kcal/mol), forming 1D-THF. Due to the weak dimerization
energies of gallium hydrides, 1D-THF is unstable with respect to
the decomposition into 1M-THF and 1D (-2.2 kcal/mol), making
1M-THF the most stable gallium hydride donor species, even if
only sub-stoichiometric amounts of donor are added. This
represents a marked difference to the aluminum system, where
1D-THF was found to be most stable under similar conditions.
Bulkier R groups like iBu on Ga lead only to minute changes, but
the preference for 1M-THF over 1D-THF diminishes (-0.3
kcal/mol).
(
molecules resulting in high conversions (Table 4). Comparison of
the gallium hydrides 1a and 1b with corresponding aluminum
hydrides 2a and 2b demonstrate that the reaction behavior of the
ionic compounds 1b and 2b are similar: in the non-coordinating
solvent toluene the conversion is low, but addition of 10 mol%
diglyme increases the conversion up to 100 % for 3, 4f and 7
(
2
compare Table 1-3 with 4). In contrast, 1a is less reactive than
a. Whereas the HDF reactions with 2a can be catalyzed by
addition of 10 mol% diglyme, the HDF reactions with 1a give
higher conversions by addition of NEt instead of diglyme. But
even with ~ 30 mol% NEt added to the HDF reactions using 1a,
3
3
the conversions are still lower than with 2a (compare Table 1-3
with 4).
Table 4. HDF with 1.1 equivalents of aluminum hydrides 2a-b in toluene at
RT. Values for 2a were taken from reference [9].
Diglyme
mol%]
Conv.
[%]
Entry
Substrate
HS[a]
TON
[
1
2
3
4
5
6
7
8
3
2a
2b
2a
2b
2a
2b
2a
2b
10.8
10.5
12.4
12.5
11.7
10.1
10.9
10.4
99.8
100
59.3
100
90.5
100
6.1
9.5
3
11.0
4.8
4f
4f
7
8.3
7.7
7
10.2
0.6
9
Scheme 1. Gibbs free energies for the HGaMe
equilibria. T = 273 K.
2
monomer-dimer-donor
9
42.7
4.1
[
a] HS = hydride sources. For full tables see the Supporting Information.
Due to the low dimerization and donor coordination energies in
this system, we considered HDF to take place via all possible
gallium hydride species, i.e. 1M, 1M-THF, 1D and 1D-THF.
The 19F-NMR spectra of the residue of the reaction of 3 with 1a
displayed a broad signal at -160.7 ppm, which we assigned to a
gallium fluoride species as known shifts for Al-F compounds lie in
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Donor influence on the competition of σ-bond metathesis,
hydrometallation, S V. As HDF via 1D is associated with the
N
Coordination of a donor to 1M blocks the coordination vacancy at
Ga. The donor coordinates much weaker to Ga than to
corresponding Al system (~12 kcal/mol vs. 18 kcal/mol). Donor
coordination lowers the barrier for HM via TS4f-11(1M-THF) by
~ 5 kcal/mol to 21.1 kcal/mol (black traces). Similar to aluminum
systems, donor coordination renders the formation of SBM TS
highest barriers (> 32 kcal/mol, see supporting information), we
will not discuss this pathway here any further. The lowest
competing HDF pathways are shown in Figure 1. All TS are
formed directly from the reactants, as formation of adducts
between reactants is endergonic, similar to the corresponding
aluminum system.
In the absence of donor molecules, i.e. in HDF via 1M,
hydrometallation (HM) via TS4f-11(1M) is strongly preferred over
six-membered σ-bond metathesis TS (SBM TS) TS4f-5d(1M)
N N
impossible. Instead, a switch to S V character occurs. S V TS4f-
IP(1M-THF) (18.7 kcal/mol) is much lower in energy than HM and
HDF via 1M-THF would proceed via formation of an ion pair (IP)
from which elimination is likely unselective.
HDF via HM at 1D-THF can proceed via two different pathways
(TS4f-11_A(1D-THF) and TS4f-11_B(1D-THF) (red traces). The
former involves only a single Ga (23.9 kcal/mol), but the latter
proceeds cooperatively, involving both Ga centers, which lowers
the barrier by 3 kcal/mol to 20.9 kcal/mol.
(
Figure 1, blue traces). Formation of four-membered SBM TS is
associated with even higher barriers (see supporting information).
The lowest TS, HM, still possesses a rather sizable barrier (26.5
kcal/mol), indicating minimal reactivity at 273 K.
Figure 1. Lowest HDF reaction pathways for HDF of 4f in the 1M (blue traces), 1M-THF (black traces) and 1D-THF (red traces) and corresponding transition state
geometries. Hydrogen (white), carbon (grey), fluorine (yellow), aluminum (peach), oxygen (red). Structure labels for TS follow the notation TSreactant-product, e.g.
TS4f-5d leads from 4f to product 5d. IP = ion pair. Gibbs free energies in kcal/mol, 273 K, solvent toluene.
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3
colored C atoms and green colored F atoms) belongs to TS11-5b(1M-NEt ).
Ga-O distance 2.34 Å, Ga-N distance 2.71 Å.
Table 5. Relative Gibbs free energy differences of the transition states of the
elimination step in the systems 1M, 1M-THF, 1D and 1D-THF. The free
energy of TS11-5b has been set to 0. In kcal/mol, 273 K, solvent toluene.
The Ga-N distance must be increased much more than the Ga-O
distance in the two TS. Additionally, the TS is much more distorted
in the presence of the N-donor than in the presence of the O-
donor THF. As shown in Table 6, this leads to a dramatic increase
1M
1M-THF
1D
1D-THF
TS11-5b
TS11-5c
TS11-5d
0.0
2.3
6.8
0.0
0.8
5.5
0.0
2.4
6.4
0.0
1.8
6.3
of the elimination TS energies in the 1M-NEt
to the lowest elimination pathway (TS11-5b +10.9 kcal/mol, only
1.8 kcal/mol in 1M-THF). The donor effect is similar, albeit less
dramatic for the 1D-NEt system. This leads to a net preference
for elimination from the donor-less 1D system in the case of NEt
For THF, elimination via 1D-THF is preferred over the donor-less
3
system with respect
+
3
3
.
HM barriers in the 1M-THF and 1D-THF system have comparable
barriers (~21 kcal/mol) but unselective HDF via TS4f-IP(1M-THF)
would be preferred in presence of a donor (18.7 kcal/mol).
However, the four lowest barriers for HDF, either via 1M-THF or
pathways. The increased selectivity in the presence of NEt
experimentally observed.
3
was
1
D-THF all lie within ~2.5 kcal/mol and bulky R groups on Ga can
switch the preferred pathway, vide infra.
Donor influence on the selectivity
Table 6. Gibbs free energy differences of the transition states of the
elimination step in the systems 1M, 1M-donor, 1D and 1D-donor. Donor =
in
the
THF/NEt
on the same scale CF
point. In kcal/mol, 273 K, solvent toluene.
3
. The lowest absolute barrier has been set to 0; to bring all systems
hydrometallation/elimination pathway. HM leads in all systems
to several energetically close lying resting states 11. Elimination
via TS11-5(b-d) is associated with considerable barriers (see
Figure 1 and supporting information). Formation of 5b via
elimination is preferred by 0.8 kcal/mol over formation of 5c in the
2
CHCF + 1D + 1D-THF was chosen as a reference
3
THF/NEt
3
1M
1M-donor
1D
1D-donor
TS11-5b
TS11-5c
2.8/2.7
5.2/5.0
1.8/10.9
2.6/11.6
0.1/0.0
2.6/2.5
0.0/2.2
1.8/4.6
1M-THF system and by 1.8 kcal/mol in the 1D-THF system. Donor
less systems 1M and 1D offer much higher selectivity at the
elimination stage (~ 2.4 kcal/mol preference for formation of 5b,
Table 5). Elimination in the presence of THF proceeds via the 1D-
THF system (see Table 6).
In difference to aluminum systems, the donor is only weakly
bound in the present gallium systems. Weaker Lewis bases
(
3
diglyme) or bulkier Lewis bases (NEt ) than THF are therefore
expected to bind even weaker in the TS, making elimination at the
donor-less 1M or 1D preferred and thus increase selectivity. This
is shown exemplary for TS11-5b(1M-THF) and TS11-5b(1M-
3
NEt ) in Figure 2.
Figure 2. TS11-5b (1M-donor) for THF and NEt
grey), fluorine (yellow), aluminum (peach), oxygen (red), nitrogen (indigo).
Shown as an overlay, the more distorted CFH=CHCF fragment (dark grey
3
. Hydrogen (white), carbon
(
Figure 3. Lowest HDF reaction pathways for HDF of 4f with HAliBu
white), carbon (grey), fluorine (yellow), aluminum (peach), oxygen (red). Gibbs
free energies in kcal/mol, 273 K, solvent toluene.
2
. Hydrogen
3
(
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Influence of bulky R groups on gallium. The real system
possesses bulkier iBu groups, which shift the monomer-dimer
equilibrium. This influences the balance between the HM and S V
N
pathways. The preference for 1M-THF over 1D-THF is reduced
by almost 2 kcal/mol (Scheme 1d), which favors all pathways
proceeding via 1D-THF. Re-optimization of the lowest lying 1M-
THF and 1D-THF TS with iBu groups shows that HM via TS4f-
was added in portions. After the mixture warmed to room temperature, it
was stirred for 1 hour, filtered and the solvent was removed in vacuo. 67 %
of 1a was obtained as a colorless liquid and characterized according to the
literature.[22] iBu
GaCl was synthesized and characterized as described by
2
Kovar[23], but instead of benzene the reaction was conducted in toluene.
Catalytic hydrodefluorination
1
1_B(1D-THF) is slightly preferred (-0.6 kcal/mol) over
Reaction conditions and substrates are listed in table 1-4 and in table S1-
unselective S V via TS4f-IP(1M-THF) (Figure 3). Unselective and
N
4
of the Supporting Information. A single-necked flask equipped with a J.
selective HDF pathways lie energetically very close and the
nature of the fluoro-olefin will influence selectivities. Only if a
Young valve was charged with 1a, 1b, 1c, 2a or 2b, organocatalyst and
solvent. The substrate was added with a syringe and the mixture was
degassed. Gaseous substrates were condensed into the flask to the prior
degassed mixture. The corresponding reaction conditions were applied.
The crude reaction mixture was purified by fractional condensation under
vacuum to a trap kept at -80 °C (for liquid substrates) or through two
subsequent traps kept at -80ꢀ°C and -196ꢀ°C, respectively, for gaseous
substrates. Fluorobenzene was added to the contents of the trap (liquid
substrates) and a defined amount of that mixture was added to an NMR
fluoro-olefin can stabilize a negative charge, unselective S
pathways are favored. This is experimentally observed for C
Already C H prefers an HM HDF pathway.
N
V
3 6
F .
3 5
F
In summary, donor molecules lower the barrier for HDF at gallium
hydride systems. Selectivities are substrate dependent but can be
high for substrates with marginally stabilized anions, as HM
becomes the dominant HDF pathway. In such a case, the
selectivity can be tuned via the choice of donor. This is an indirect
effect, as the presence of a donor during elimination decreases
selectivity. The right choice of donor allows to accelerate the initial
HM but forces the donor to dissociate before the selectivity
determining elimination step.
tube containing
6 6
C D . The contents of the second trap (gaseous
substrates) were condensed into a NMR tube containing a standard C
6 6
D
solution of fluorobenzene. The conversion of the substrates was
determined by NMR spectra by integration of product resonances versus
the internal standard (fluorobenzene). The products were identified by 19F
NMR spectroscopy (benzene-d
4 [17a], 4d-e[25], 5a-c[26], 5d[17a], 8a[27], 8b[28], 8c[29], 10a[30], 10b[31], 10c[29]
0d[32], 10e[33] or by comparison with an authentic sample (4f, 8d, 6).
), using available literature data for 4a-b[24]
,
6
c
1
,
Calculations. Starting geometries were generated by taking the optimized
structures of the corresponding aluminum systems and replacing Al with
Ga. All structures were fully optimized at the M06-2X(PCM)[34] /cc-pVDZ
level of theory using Gaussian09[35] coupled to an external optimizer (PQS,
OPTIMZE routine)[36] within the BOPT software package.[37] An fine grid
and standard SCF convergence quality settings (Scf=tight) for Gaussian
single point calculations were used. The nature of each stationary point
was checked with an analytical second-derivative calculation (no
imaginary frequency for minima, exactly one imaginary frequency for
transition states, corresponding to the reaction coordinate). Solvent
influence (toluene, ε=2.3741) was modelled explicitly during the
optimization and SP step, using the polarizable continuum model (PCM)
implemented in the Gaussian 09 software suite. The M06-2X functional
has been shown to yield accurate results for systems involving group 13
systems.[9,38]
Conclusions
Gallium hydrides activate vinylic and aromatic C-F bonds in the
presence of catalytic amounts of O or N donor molecules.
Conversions up to 100 % for the HDF of 3 and 4f and high
selectivity could be obtained for 4f, 7 and 9. LiGaH
the highest conversions followed by (iBu) GaH (1a) and
Me N∙GaH (1c). The selectivity of the HDF of 4f is donor
4
(1b) exhibited
2
3
3
dependent. DFT could demonstrate that the right choice of donor
allows to accelerate the HDF reaction and to tune the selectivity.
A high selectivity can be achieved by choosing weakly bound
donors, which do not bound to the elimination TS, as the
selectivity of the elimination step is higher in absence of a donor.
The weak donor coordination energies of gallium hydrides are a
pre-requisite for this on/off switch during organocatalysis.
Vibrational analysis data derived at this level of theory were used to
calculate thermal corrections (enthalpy and entropy, 298 K, 1 bar) for all
species considered. Final single-point energies (SP) were calculated at the
M06-2X(PCM)[39] level of theory employing triple-ζ Dunning basis sets (cc-
pVTZ) from the EMSL basis set exchange library,[40] to minimize BSSE
contributions.[41]
Experimental Section
All preparations and reactions were performed using standard Schlenk-
type and vacuum line techniques, or by working in an argon-filled glove
box. The amount of gaseous compounds was determined by using pVT
techniques or by condensing the gas into a weighted J. Young flask.
Diglyme, THF, n-pentane, diethyl ether and toluene were distilled from
sodium or potassium. Trimethylamine hydrochloride, 2b (Sigma Aldrich),
Acknowledgements
The authors would like to thank Prof. P. H. M. Budzelaar
(University of Naples, Federico II., Italy) for generous donation of
CPU time. Support from the Deutsche Forschungsgemeinschaft
3
4f (Syn-Quest Labs), GaCl (abcr) were obtained from commercial sources
and used as received. 7, 9 were purchased from abcr and distilled from
calcium hydride. 3 (Solvay) was obtained free of charge. 1b,c[20] were
synthesized as described in the corresponding literature. 1a was
(
DFG) Le423/17-1 and the GRK 1582 (Fluorine as a Key Element)
synthesized from iBu
mmol) was dissolved in Et
2
GaCl analog the method of Petrov[21]. iBu
O, cooled to -80 °C and then LiH (114.6 mmol)
2
GaCl (27.3
is gratefully acknowledged.
2
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1
31, 11203-11212; e) C. Douvris, C. M. Nagaraja, C.-H. Chen, B. M.
Keywords: homogeneous catalysis • DFT • gallium hydrides •
hydrodefluorination • organocatalyst
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O and N donor solvents activate
gallium hydride bonds and enable the
HDF of per- and polyfluorinated
olefins and arenes under mild
A. D. Jaeger, R. Walter, C. Ehm*, D.
Lentz*
Page No. – Page No.
conditions. DFT studies elucidate the
preferred mechanisms in respect to
different alkyl groups at Ga and
donors. The selectivity can be tuned
via the choice of donor.
Gallium Hydrides and O/N-donors as
tunable systems in C-F Bond
Activation
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