Correlating electronic and catalytic properties of frustrated Lewis pairs for imine hydrogenation

Article abstract of DOI:10.1016/j.jorganchem.2017.04.031

Combined computational and experimental work to probe Lewis acidity of some boranes to be used in FLP hydrogenation. Gutmann-Beckett method of estimating Lewis acidity has limited capacity for sterically congested boranes. Calculated hydride affinity is a more appropriate tool for gauging Lewis acidity and correlate their FLP hydrogenation utility.

Full text of DOI:10.1016/j.jorganchem.2017.04.031

Accepted Manuscript
Correlating electronic and catalytic properties of frustrated Lewis pairs for imine
hydrogenation
Éva Dorkó, Bianka Kótai, Tamás Földes, Ádám Gyömöre, Imre Pápai, Tibor Soós
PII:
S0022-328X(17)30269-3
10.1016/j.jorganchem.2017.04.031
JOM 19918
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 21 February 2017
Revised Date: 21 April 2017
Accepted Date: 25 April 2017
Please cite this article as: É. Dorkó, B. Kótai, Tamá. Földes, Áá. Gyömöre, I. Pápai, T. Soós, Correlating
electronic and catalytic properties of frustrated Lewis pairs for imine hydrogenation, Journal of
Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.04.031.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Correlating Electronic and Catalytic Properties of Frustrated
Lewis Pairs for Imine Hydrogenation
Éva Dorkó, Bianka Kótai, Tamás Földes, Ádám Gyömöre, Imre Pápai, and Tibor Soós
Research Centre for Natural Sciences, Hungarian Academy of Sciences, Institute of Organic Chemistry, Magyar
tudósok körútja 2, H-1117 Budapest, Hungary.
*Corresponding author: soos.tibor@ttk.mta.hu, papai.imre@ttk.mta.hu
Keywords: Lewis acidity, frustrated Lewis pairs, DFT, hydrogenation, catalysis
Dedicated to Prof. John A. Gladysz on his 65th birthday.
Introduction
The recently introduced concept of frustrated Lewis pair (FLP) [1] has become a fundamen-
tal strategy to develop main-group-element-based catalysis. An FLP is composed of a sterical-
ly overcrowded Lewis acid (LA) and a Lewis base (LB) that are unable to form a dative com-
plex because of steric hindrance. As a result, a highly reactive and cooperative Lewis acid-
base pairs evolve that can promote various unprecedented reactions, among which the hetero-
lytic activation of hydrogen excels. This remarkable capacity has been exploited in metal-free
hydrogenation in which onium (phosphonium, ammonium, etc.) borohydrides are generated
to reduce multiple bonds [2, 3, 4] such as of imines, enamines, carbonyls, silyl ethers and
α,β−enones.
The diverse reactivity of FLPs has often rendered them incompatible with many common
functionalities and solvents, therefore the success of the FLP hydrogenation hinges on the
selection of Lewis acidic and basic components. While the identity of the basic component
can influence certain catalytic behavior, the primary challenge is to select the right Lewis
acidic component for FLP hydrogenation. Therefore, methods to analyze and quantify the
impact of steric and electronic properties of Lewis acids on their reactivity are needed to make
a rational and informed decision upon FLP catalyst development. Along this line, the goal of
the present study is to fine-tune the electronic properties of boron-based Lewis acids and iden-
tify trends across them that could accelerate the FLP catalyst development for imine hydro-
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genation. Therefore, a series of boranes with default sterical setting has been synthesized and
their electronic properties were systematically varied and evaluated by various methods.
Previous efforts in our group have focused on the development of designer FLPs to increase
further the practicality of FLP hydrogenation. Using a deliberate structural design on boron-
based Lewis-acids, the size-exclusion design, we could secure orthogonal FLP reactivity in
hydrogenation, improve the functional group tolerance and even render FLP hydrogenation of
carbonyls moisture tolerant [5]. Given the success of FLPs with size-exclusion design we
were interested to fine-tune the electronic properties of this type of catalysts for a given target
of interest, for an imine reduction. Therefore two series of boranes having a general BX2Y
structure have been targeted, where Y refers to an aryl-ring having minimum two chlorines or
methyl groups in ortho positions and X refers to the aryl-rings with minimum two ortho fluo-
rine substituents. Accordingly, the steric congestion around the boron atom is a default and
practically the same as chlorine atom and methyl group are isosteric. Additionally, variation
of substituents in the meta and para positions of X or Y aryl rings allows to gauge the impact
of the electronic effect on Lewis acidity and reactivity.
The first series of triaryl-boranes has a mesityl- and two fluorophenyl-rings and twelve dif-
ferent boranes were envisioned (Scheme 1) [6, 7]. The mesityl-rings were chlorinated in the
meta-positions gradually (the non-chlorinated mesityl derivatives designated as 1, monochlo-
rinated 2, dichlorinated 3), and the fluorinated rings were changed systematically from 2,6-
difluorophenyl (a, F2) to 2,3,6-trifluorophenyl- (b, F3), 2,3,5,6-tetrafluorophenyl- (c, F4) and
perfluorinated-rings (d, F5). A second series of triaryl-boranes was also targeted (4a-c, 5b-c,
Scheme 1). These boranes have a chlorinated phenyl group (the 2,6-dichlorophenyl-ring was
designated as 4, 2,3,6-trichlorophenyl-ring as 5) and F2-F4 fluorinated fluorophenyl-rings (a–
c). It is important to note, that some of this borane variant have been successfully used for
reduction of carbonyls [5].
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Scheme 1. In the labelling, numbers refer to the chlorinated rings (1: mesityl-, 2: 3-chloromesityl-, 3: 3,5-
dichloromesityl-, 4: 2,6-dichlorophenyl-, 5: 2,3,6-trichloroophenyl-ring) and the letters to fluorinated rings (a:
2,6-difluorophenyl, b: 2,3,6-trifluorophenyl, c: 2,3,5,6-tetrafluorophenyl-, d: perfluorinated-ring). (two column
wide)
Results and discussion
The synthesis of mesityl-boranes was performed in three steps and based on the protocol we
have previously developed and reported [6, 7]. To generate a BX2Y type of borane library,
potassium mesityltrifluoroborate salts (7a-c) were selected as synthetic intermediates
(Scheme 2). Thus, bromomesityl-compounds (6a-c) were treated with n-butyllithium or acti-
vated magnesium, and then reacted with trimethylborate. The reaction was quenched with
hydrochloric acid, and the resulted boronic acid was treated with KHF2. It is worth noting that
these mesityl-fluoroborates 7a-c are easily available by this method in larger scale. Finally,
Grignard reagent (fluorophenyl-magnesiumbromide, 9a-d) was prepared from the appropriate
1-bromofluorobenzenes (8a-d, for details, see SI). Next, two equivalent of these Grignard
reagent (9a-d) was added dropwise to a suspension of one equivalent of corresponding mesit-
yl-BF3K salts (7a-c) in ether type solvent at 0°C. After work-up, the desired boranes (1a-3d)
could be isolated in serviceable 31-86% yields as white solids in 5-10 g scale. The 1H and 19F
NMR data clearly indicated that the aryl rings in boranes can rotate freely; the signals were
sharp and well separated at room temperature.
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Scheme 2: Preparation of mesitylboranes (for 6a, 7a: R1=R2=H; for 6b, 7b: R1=Cl, R2=H; for 6c, 7c:
R1=R2=Cl) (1.5 column wide)
Evaluation of Lewis acidity
After having these boranes 1–5 in hand, we embarked on the evaluation of the impact of
electronic properties on their acidity. However, unlike Brønsted acidity, the quantitative
measurement of Lewis acidity is more complicated because the Lewis acidity is situation de-
pendent (e.g. it is affected by the steric demand of a Lewis base) [8, 9, 10]. Nevertheless, sev-
eral methods have been developed to evaluate and rank Lewis acids. Methods include Gut-
mann’s acceptor numbers (AN) [11, 12], Drago, Marks, and Wayland’s equations [13, 14],
Childs-Marks scale [15], and Christe and co-worker’s fluoride affinities [16], and Evanseck’s
method [17]. Alternatively, the calculated hydride affinities are also considered as a reasona-
ble measure for the acidic strength, especially when Lewis acid catalysts are developed for
FLP mediated hydrogenation [18, 19, 20].
First, the Lewis acidity of boranes 1-5 was examined computationally. Density functional
theory was applied to compute hydride affinities (HA) defined as the Gibbs free energy of
hydride detachment from LA-hydride complex anions. The calculations were carried out us-
ing the B3LYP-D3 functional, and implicit solvent models with different polarity (toluene,
chloroform, dichloromethane) were tested to estimate the solvent effect [21].
The computed gas-phase HAs of the examined boranes are depicted in Figure 1 as a func-
tion of the number of F atoms in the X aryl rings. The obtained results reveal a systematic
trend as expected from the electronic nature of the F, Cl and CH3 aryl substituents. For in-
stance, the HA increases gradually with the number of fluorines. The effect is clearly more
enhanced for H/F substitutions at the meta-positions as compared to the para-substitution.
Boranes with chlorinated phenyl substituents show larger HAs, than the corresponding mesit-
yl analogues. The reduced acidity of mesityl-boranes is related to electron donation from the
4

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methyl groups. In both phenyl- and mesityl-boranes, H/Cl substitution increases HA by a few
kcal/mol.
100
95
90
85
80
75
70
65
2
3
4
5
x (number of fluorine atoms)
1a-d
2a-d
3a-d
4a-c
5b-c
Figure 1. Gasphase HAs of B(C6FxH5-x)2Y boranes (x: number of fluorine atoms on the two equivalent phenyl
rings). (one column wide)
Solvent effects are found to have a notable influence on the relative hydride affinities as il-
lustrated in Figure 2. Although the acidity trend with respect to the number of F substituents
remains very similar, the electronic effect of Cl substitution is not strictly apparent. This could
be partially associated with the fact that the hydride affinity values vary in a narrower energy
range as the polarity of solvent increases (see Figure 3). In addition, polar solvents such as
dichloromethane tend to reduce the stability of mesityl-borohydrides as compared to the phe-
nyl analogues, so these two series of boranes are well separated in the diagram shown in Fig-
ure 2.
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40
35
30
25
20
2
3
4
5
x (number of fluorine atoms)
1a-d
2a-d
3a-d
4a-c
5b-c
Figure 2. HA-s of B(C6FxH5-x)2Y boranes in DCM solvent, where x is the number of fluorines on the two equiv-
alent phenyl rings. (one column wide)
18.0
16.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
2
3
3
4
4
5
5
x (number of fluorine atoms)
gasphase
toluene
chloroform
dichloromethane
Figure 3. Relative hydride ion affinities for the B(C6FxH5-x)2Mes series compared to 1a (B(C6F2H3)2Mes) in
gasphase and toluene, chloroform, dichloromethane solvents. (one column wide)
For characterization of Lewis acid 1–5, the Gutmann-Beckett method was chosen, thus the
change in the 31P NMR chemical shift (∆δ) was measured between free Et3PO and that of the
Et3PO-Lewis acid adduct in CD2Cl2. The ∆δ values of the prepared boranes 1–5 were ana-
lyzed with respect to B(C6F5)3, which is the archetypal Lewis acid of the FLP chemistry. The
chemical shift of the dative complex of Et3PO-B(C6F5)3 was considered as 100% on this
acidity scale and 0% was the δ value of the free Et3PO base.
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100
90
80
70
60
50
40
30
20
10
0
2
3
4
5
x (number of fluorine atoms)
1a-d
2a-d
3a-d
4a-c
5b-c
Figure 4. Relative Lewis acidity of B(C6FxH5-x)2Y boranes via Gutmann-Beckett method, measured with
Et3PO in CD2Cl2, where x is the number of fluorines on the two equivalent phenyl rings. (one column wide)
To explore the consequence of successive replacement of hydrogen with F or Cl, the rela-
tive Lewis acidities of 1–5 were measured in CD2Cl2 solvent using the Gutmann-Beckett
method (Figure 4). Most importantly, unlike the computed hydride affinity (Figure 2), a non-
linear trend has been established by the Gutmann-Beckett method that can be rationalized by
the enhanced steric congestions around the boron center when complexed with Et3PO. For
mesityl-series 1–3, the data indicates that the meta-chlorine substituents on the mesityl ring
has only a minor impact on the Lewis acidity in case of F4- and F5-rings, similar to the trend
found in HA (Figure 1 and 2), but has an affect in the case of F3-ring. For F2-ring deriva-
tives, an anomaly was observed in the relative Lewis-acid strength; the most chlorinated com-
pound 3a (Cl2MesF2) somehow appeared to be the weakest acid in the 1a–3a series. As the
Gutmann-Beckett method can only gauge the relative acidities of complexed boranes, in case
of very weak dative complexes the obtained data might be less appropriate for comparisons.
Interestingly, the acidity values were almost the same for the chloromesityl (2b-d) and di-
chloromesityl (3b-d) rings. These data suggest that the electronic tuning of the bulkier aro-
matic rings (Y vs X) has a lower impact on Lewis acidity. As the sterically overcrowded aryl
rings (Y) are enforced to be perpendicular to the plan of three ipso-C atoms, there is a lower
level of conjugation with the boron center, which presumably results in the observed trend.
For the chlorophenyl series 4–5, the variation of relative Lewis acidity is proved to be negli-
gible. Additionally, the hydride ion affinity predicts that the chlorophenyl-series 4–5 well
separate from the mesityl series 1–3, however, in the Gutmann-Beckett method, the obtained
Lewis acidity values of the mesityl and chlorophenyl series appear to be much closer.
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The above study indicates that even Et3PO can exert such a steric repulsion upon dative ad-
duct formation with sterically overcrowded boranes 1–5, that the Lewis acid scale is signifi-
cantly perturbed. The contribution of this steric effect upon dative bond formation (the facial-
strain) was then demonstrated by applying a more bulky Lewis base, such as Ph3PO. When
this sterically hindered base was applied as a reference base for Gutmann-Beckett method,
significantly larger differences in relative acidities of the boranes could be observed (Figure
6).
100
90
80
70
60
50
40
30
20
10
0
2
3
4
5
x (number of fluorine atoms)
1a-d
2a-d
3a-d
4a-c
5b-c
Figure 6. Lewis acidity of B(C6FxH5-x)2Y boranes via Gutmann-Beckett method, measured with Ph3PO in
CD2Cl2, where x is the number of fluorines on the two equivalent phenyl rings. (one column wide)
Hydrogenation experiments
To compare the Lewis acidity and hydrogenation capacity of boranes 1–5, all boranes were
evaluated in the same benchmark reaction. The test reaction was the reduction of N-
benzylidene-tert-butylamine to N-benzyl-tert-butylamine, in benzene solvent, with DABCO,
as the LB (Scheme 3).
Scheme 3. The benchmark reaction for borane reactivity evaluation (one column wide)
Most importantly, all borane/DABCO pairs were effective catalysts for reduction of N-
benzylidene-tert-butylamine (Table 1). Data also indicates that chlorophenyl boranes 4–5
have higher catalytic capacity compared to mesityl boranes 1–3, since conversions were com-
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plete within 3 hours in presence of 4–5. This general trend correlates well with the calculated
hydride affinity scale in DCM (Figure 2.), but shows deviation from trend determined by the
Gutmann-Beckett method (Figure 4.).
Entry
Conversion (%) in
Catalyst
0.5h
0
24
36
7
1h
0
32
43
10
3
35
42
15
2h
0
3h
0
4h
0
87
99
87
9
99
99
80
15
43
44
27
3
6h
0
24h
0
1
2
3
4
5
6
7
8
9
-
99
99
99
48
99
99
99
64
68
20
5
81
91
70
6
99
99
99
16
99
99
99
1d
1c
1b
1a
2d
2c
2b
2a
3d
3c
3b
3a
4c
4b
4a
5c
5b
1
20
24
9
0
5
6
2
0
53
37
8
32
22
48
60
26
80
88
40
0
3
6
21
65
57
50
8
61
99
99
99
40
-
10
12
17
8
23
29
11
2
33
38
17
2
11
12
13
14
15
16
17
18
0
68
56
20
48
33
99
75
32
65
57
99
94
47
99
90
99
99
56
99
99
-
-
-
80
-
99
-
-
-
Table1. Conversion of imine in FLP hydrogenation. (two column wide)
For comparison, catalytic conversions to amine after 2 hours were gathered (Figure 7). The
data indicates that F4-ring containing 1c, 2c, 3c boranes (entries 3, 7, 11) were more effective
catalysts for imine reduction than the 1d, 2d, 3d boranes with perfluorophenyl rings (entries
2, 6, 10). However, the Lewis acidity of these boranes was almost identical in both Gutmann-
Beckett methods and HA calculations. This discrepancy can be explained by the lower cata-
lyst stability of the F5 series, as they can decompose in an aromatic nucleophilic substitution
reaction [7].
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100
80
60
40
20
0
2
3
4
5
x (number of fluorine atoms)
1a-d
2a-d
3a-d
4a-c
5b-c
Figure 7. Conversions in 2 hours with B(C6FxH5-x)2Y boranes as catalysts, where x is the number of fluorines
on the two equivalent phenyl rings (one column wide)
In the case of 1a, 2a, 3a F2 series (entries 5, 9, 13) and 1b, 2b, 3b F3 series (entries 4, 8,
12), the catalytic capacities were also in agreement with the calculated hydride ion affinity
and Gutmann-Beckett method results. The Lewis acidity trends and the conversions were sim-
ilar in case of the 1a-d mesityl and 2a-d chloromesityl-boranes (entries 2-5 vs 6-9). In case of
3a-d boranes with dichloromesityl rings, however, the conversions were reduced (entries 10-
13), and no full conversion was achieved within 6 hours. Similar behaviour can be seen in the
catalytic performance of the 4–5 chlorophenyl boranes, while the Lewis acidities were rather
similar according to both hydride ion affinity calculations and Gutmann-Beckett measure-
ments, the reactivity of 5b,c boranes with trichlorinated rings were markedly lower compared
to 4a–c boranes with dichlorinated rings (entries 14-16 vs 17,18). This might be explained
that the hydride donation capacity of the stronger Lewis acids is lower.
Conclusions:
Gradually substituting meta- and para-hydrogen atoms to fluorines or chlorines, a series of
seventeen triaryl-boranes with a general BX2Y structure was generated for FLP hydrogena-
tion and comprehensively characterized using combined experimental and theoretical meth-
ods. As a consequence of structural design, this series of boranes has a default sterical setting
around the boron center and their electronic properties were systematically varied. As demon-
strated, the Gutmann-Beckett method has a limited capacity to gauge the Lewis acidity of
these highly congested Lewis acids and correlate their FLP hydrogenation utility. Neverthe-
less, this study reveals that the calculated hydride affinity is a useful tool to quantify the elec-
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tronic effects on Lewis acidity and predict the hydrogenation capacities of these boranes. As a
general trend, the hydrogen-fluorine replacement in meta-positions resulted in a significant
enhancement of Lewis acidities, however, the H/Cl replacements on the bulkier aromatic ring
have only negligible effects. These observations reveal important properties affecting Lewis
acidity and FLP reactivity and can guide future catalyst developments.
Acknowledgements
This work was supported by the National Research, Development and Innovation Office (K-
116150 and K-115660).
Appendix A. Supplementary data
All measurement and synthesis data, and information about chemical compounds and com-
putational details are available in SI.
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Highlights
1. Combined computational and experimental work to probe Lewis acidity of some
sterically highly congested boranes to be used in FLP hydrogenation.
2. Gutmann-Beckett method of estimating Lewis acidity has limited capacity for
these bulky boranes.
3. Calculated hydride affinity represents a more appropriate parameter for gauging
Lewis acidity and correlate their FLP hydrogenation utility.