Catalytic Dehydrogenation of (Di)Amine-Boranes with a Geometrically Constrained Phosphine-Borane Lewis Pair

Article abstract of DOI:10.1021/acscatal.8b00152

The o-phenylene bridged phosphine-borane iPr₂P(o-C₆H₄)B(Fxyl)₂ 2 was prepared. Despite ring strain, it adopts a closed form, as substantiated by NMR, XRD, and DFT analyses. However, the corresponding open form i

Full text of DOI:10.1021/acscatal.8b00152

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Catalytic Dehydrogenation of (Di)Amine-Boranes with a
Geometrically-Constrained Phosphine-Borane Lewis Pair
Maxime Boudjelel, Eric Daiann Sosa Carrizo, Sonia Mallet-Ladeira,
Stéphane Massou, Karinne Miqueu, Ghenwa Bouhadir, and Didier Bourissou
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 13 Apr 2018
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ACS Catalysis
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Catalytic Dehydrogenation of (Di)Amine-Boranes with a Geometri-
cally-Constrained Phosphine-Borane Lewis Pair
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Maxime Boudjelel, E. Daiann Sosa Carrizo, Sonia Mallet–Ladeira, Stéphane Massou, Karinne Mi-
b
a
a
queu, Ghenwa Bouhadir and Didier Bourissou *
a
CNRS / Université Paul Sabatier, Laboratoire Hétérochimie Fondamentale et Appliquée (LHFA, UMR 5069). 118 Route de
Narbonne, 31062 Toulouse Cedex 09 (France)
b
CNRS / UNIV PAU & PAYS ADOUR, Institut des Sciences Analytiques et de Physico-Chimie pour l’Environnement et
les Matériaux (IPREM UMR 5254). Hélioparc, 2 Avenue du Président Angot, 64053 Pau Cedex 09 (France)
c
Institut de Chimie de Toulouse (FR 2599). 118 Route de Narbonne, 31062 Toulouse Cedex 09 (France)
Supporting Information Placeholder
7
such main-group catalysts really valuable, their efficiency and
generality have to be significantly improved.
ABSTRACT: The o-phenylene bridged phosphine-borane
iPr P(o-C H )B(Fxyl) 2 was prepared. Despite ring strain, it
2 6 4 2
adopts a closed form, as substantiated by NMR, XRD and DFT
analyses. But the corresponding open form is only slightly higher
in energy. The dormant Lewis pair 2 proved to efficiently catalyze
the dehydrogenation of a variety of amine- and diamine-boranes
under mild conditions. The corresponding phosphonium-borate
iPr
diate of these dehydrogenation reactions. The propensity of 3 to
release H plays a major role in the catalytic turnover.
2 6 4 2
PH(o-C H )BH(Fxyl) 3 was authenticated as a key interme-
2
KEYWORDS: Phosphine-borane, Lewis pair, small molecule
activation, amine-borane, dehydrogenation, metal-free catalysis.
INTRODUCTION
The last 10-15 years have witnessed a spectacular development
of catalysis with main-group compounds. In particular, the ability
1
of Frustrated Lewis Pairs (FLP) to activate dihydrogen has
opened new avenues in catalytic hydrogenation. Spectacular
results have also been obtained in CO reduction.
2
Although comparatively much less developed, the use of FLP
to dehydrogenate amine-boranes has also emerged recently. Cata-
Chart 1. FLP applied to B–N dehydrogenation.
lytic dehydrogenation of amine-boranes is of great synthetic
Our group has a long standing interest in o-phenylene bridged
2
8
interest. The release of H
2
under mild and controlled conditions
phosphine-boranes. The proximity of the two Lewis pairs imparts
9
makes amine-boranes valuable materials for hydrogen storage. On
the other hand, dehydrogenation and dehydrogenative coupling
give straightforward access to a variety of BN building blocks
unique properties and favours cooperativity. Here, we report that
compound C (Chart 1) featuring a highly Lewis acidic boron
center is an efficient catalyst for the dehydrogenation of a variety
of amine- and diamine-boranes. It adopts a dormant closed form,
2
useful in synthesis and material science, with H as only by-
10
product. Transition metal (TM) based catalysts occupy a forefront
position in this field. With FLP, stoichiometric studies were first
but the corresponding open form is readily accessible in energy.
3
a
3b
reported in 2010 by Miller and Manners (Chart 1a). Soon after,
Slootweg and Uhl showed the ability of the geminal phosphine-
alane FLP A to catalyse the dehydrogenation of dimethylamine-
RESULTS AND DISCUSSION
The design and preparation of the target compound C face sev-
eral challenges. Introduction of the B(C F ) moiety by ionic
6 5 2
4
borane at 90°C in the melt (Chart 1b). Using xanthene as a rigid
coupling, the general route to prepare o-phenylene bridged phos-
linker between phosphine and borane sites, Aldridge further ad-
8
phine-boranes, suffers from competitive aromatic nucleophilic
vanced the field in 2016. Ammonia-borane H
dimethylamine-boranes MeH NBH (MAB) and Me
DMAB) were dehydrogenated in 1-2 days at 55°C using 1 mol%
3
NBH
3
, mono and
1
1
substitution of fluoride. So far, it has only been possible by
indirect means, namely electrocyclic ring closure of P,B-
2
3
2
HNBH
3
(
1
2
substituted hexatrienes. Inspired by the recent work of Wagner
5
6
of B. These studies point out the potential of FLP, but to make
13
2 n
on P(CH ) B FLP (n = 1,2), we turned our attention to the
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Page 2 of 6
B(Fxyl)
2
moiety (Fxyl = 3,5-(F
3
C)
2
C
6
H
3
) whose boron center is
features make the phosphine-borane 2 a promising candidate for
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both highly electro-deficient and relatively unhindered sterically.
the catalytic dehydrogenation of amine-boranes.
Lithiation of the o-bromo phenylphosphine 1 followed by electro-
To start with, the reaction of 2 with methylamine-borane
1
4
philic trapping with FB(Fxyl)
2
cleanly afforded the desired
18
(MAB) was investigated. Addition of one equivalent of
phosphine-borane 2 (Scheme 1). After work-up, 2 was isolated in
MeH
2
3 2
NBH to the phosphine-borane at 25°C leads to H re-
1
5
31
11
6
0% yield as a white solid (mp 92°C). The P and B NMR
11
lease and B NMR spectroscopy indicates complete conversion
of MAB within 1 h. According to P NMR, 2 is recovered at the
data indicate the presence of a strong P→B interaction in solution.
31
31
11
The P NMR signal of 2 appears at 29.8 ppm, while the
B
end of the reaction, which prompted us to react MAB with a
catalytic amount of phosphine-borane (5 mol%). After 12 h at
spectrum shows a sharp signal at 2.8 ppm. According to X-ray
diffraction analysis, 2 also adopts a monomeric closed form in the
solid state. The boron center is in tetrahedral environment
5
5°C, MAB is completely consumed and a mixture of borazane
(HMeNBH and borazine (MeNBH) (40 and 60% yield, respec-
2
)
3
3
(
CBC) = 345.8(5)°) and the P–B distance is short (2.072(2) Å),
0
1
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tively) is obtained. The activity of 2 towards MAB is relatively
modest but this result shows the ability of the phosphine-borane to
act as a dehydrogenation catalyst. We then explored its behaviour
towards various amine- and diamine-boranes.
1
6
despite the ring strain associated with the four-membered ring.
Table 1. Catalytic dehydrogenation of DMAB.
Scheme 1. Synthesis and molecular structure of 2 as deter-
mined by X-ray diffraction analysis; iPr and Fxyl simplified,
and H atoms omitted for clarity.
entry 2 (mol%) T (°C) additive time (h) yield (%)a
1
2
3
4
5
6
7
8
5
5
25
55
70
55
70
70
25
25
/
96
1.5
0.5
6
99
99
99
99
99
0
/
5
/
1
/
0.2
0
/
44
144
6
/
5
1 eq 5
1 eq 5
99
0
0
48
a
Reactions performed in benzene at 0.83 M. Yields determined
1
by H NMR using hexamethylbenzene as internal standard.
Using 5 mol% of 2, dimethylamine-borane (DMAB), the
4
,5
benchmark substrate in FLP chemistry, is slowly dehydrogenat-
ed at 25°C (Table 1). The reaction is complete in 4 days and
quantitatively leads to the cyclic dimer 4 (entry 1). Among com-
mon solvents, the best results were obtained in benzene and tolu-
ene. Lower rates and yields were observed in ethereal solvents
Figure 1. Reaction profile computed for the heterolytic split-
ting of H by the phosphine-borane 2 at B97D/6-31G** level
2
of theory. Distances in Å.
1
5
(
Table S3). Complete dehydrogenation of DMAB requires only
No sign of a reaction between 2 and H
2
was detected even after
1.5 h at 55°C and 0.5 h at 70°C (entries 2 and 3). It is possible to
decrease the catalytic loading of 2. The reaction takes only 6 h at
1 mol% and 55°C (entry 4). For comparison, the xanthene-derived
FLP B requires 48 h of reaction under similar conditions. Im-
portantly, a control experiment without 2 (entry 6) showed no sign
days at 50°C and 2 atm pressure. The phosphine-borane also
remained unchanged when treated with a 1:1 mixture of H and
under the same conditions, but according to H NMR spectros-
copy, some H/D scrambling occurred (HD detected among the
2
1
5b
D
2
15
dissolved gas, 15% conversion). These experiments suggest that
the fixation and splitting of H by 2 is thermodynamically unfa-
of a reaction even after prolonged heating, substantiating the
1
9
catalytic role of the phosphine-borane. The ability of H
2
accep-
2
vourable and slow but accessible kinetically. To gain more insight
into the energy balance and activation barrier of the reaction, DFT
calculations were performed at B97D/6-31G** level of theory
tors to significantly improve the efficiency of dehydrogenation
2
0
and dehydrocoupling reactions, prompted us to envision the use
of an additive. Gratifyingly, the addition of 1 equiv. of the al-
21-23
(Figure 1). The optimized geometry of the closed form 2 nicely
dimine PhHC=NtBu 5 proved very beneficial.
The reaction is
matches that determined crystallographically (P–B distance 2.094
considerably speeded up and now proceeds within only 6 h even
at 25°C (entry 7). According to the control experiment performed
without 2 (entry 8), the combination of the phosphine-borane and
aldimine is really critical for the reaction to proceed so efficiently.
1
5
Å). In the corresponding open form 2’, the ring strain is re-
lieved, the boron center is trigonal planar and the P•••B distance is
long (2.890 Å). 2’ is located only 1.6 kcal/mol above the closed
structure 2, which can thus be viewed as a dormant form. Con-
The phosphine-borane proved also an efficient catalyst for the
2
sistent with experimental observations, the activation of H is
dehydrogenation of iPr
substrate slows down the reaction but the monomeric amino-
2
3
HNBH . The steric demand of the
1
7
endergonic (G 12.7 kcal/mol) and requires a fairly large activa-

tion barrier (G 26.2 kcal/mol). Conversely, the phosphonium-
borane iPr N=BH was quantitatively obtained within 12 h at
2
2
15
borate 3 should be prone to release H
2
: the reaction is downhill in
70°C using 5 mol% of 2. We then turned to cyclic amine-
energy and its activation barrier is only 13.5 kcal/mol. These
boranes which have recently attracted interest as hydrogen storage
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materials.24 As reported by Liu et al, the five and six-membered
rings 6a and 6b (Scheme 2) can release two equivalents of H per
substrate molecule to give trimeric products (borazines 7a,b).
Thermal activation requires harsh conditions (150°C, 1-2 h), but
with transition metal halides as catalysts, 6b can be dehydrogena-
diazaborolidines by a sequence of dehydrogenation and dehydro-
2
5
1
2
3
4
5
6
7
8
9
2
genative coupling (Table 2). To date, only two catalysts have
2
been disclosed for this reaction. The first one, namely [RuH
(PCy ], was reported by Alcaraz and Sabo-Etienne in
Working at 25°C with 2.5 mol% Ru catalyst, the reac-
2
( -
H
2012.
2
)
2
3 2
)
25a,b
ted at 80°C (complete conversion within ~0.5 h using 5 mol% of
tion takes from 3 h to 3 d. The other catalyst, reported by Robert-
son and Mulvey, is a group 1 complex, namely a lithio-
24b
2
FeCl for example). Pleasingly, we found that the phosphine-
25c
borane 2 is able to dehydrogenate both 6a and 6b under mild
conditions. For the five-membered ring substrate, complete con-
version is achieved within 3.5 h at 80°C using 5 mol% of 2 and
dihydropyridine. In this case, the reaction is carried out at 70°C
with 5 mol% catalyst and takes from 6 h to 2 d. With the phos-
phine-borane 2 (Table 2), the diamine-borane 8a is rapidly con-
verted into the corresponding 1,3,2-diazaborolidine 9a at 70°C
15
the borazine is obtained in 97% yield.
(the reaction is complete within 1 h at 5 mol%). The catalytic
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
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0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
loading can be reduced (at 1 mol%, the reaction is complete with-
in 12 h) and using the aldimine 5 as additive (2 equiv.), the reac-
tion works as well at 25°C. Good results were also obtained with
substrates 8b,c featuring more sterically demanding substituents
at the nitrogen atoms. For example, complete reaction of the tBu
substrate 8b is achieved within only 5 h at 70°C using 1 mol% of
2
, and within 4.5 h at 25°C using 5 mol% of 2 and 2 equiv. of 5.
Scheme 2. Dehydrogenation of cyclic amine-boranes 6a,b
catalysed by 2.
The dehydrogenation of 8b was scaled up at 6.4 mmol and 1,3,2-
diazaborolidine 9b was isolated in 83% yield by simple distilla-
15
Table 2. Catalytic dehydrogenation of the diamine-boranes
tion. Again, no sign of a reaction is observed in the absence of
phosphine-borane (with or without the aldimine additive). These
results further substantiate the ability and efficiency of the phos-
phine-borane 2 to catalyze dehydrogenation and dehydrogenative
coupling of amine-boranes. It works well with a variety of sub-
strates and reactions proceed under mild conditions.
8
a-c.
Finally, we tried to identify the features that make 2 a good cat-
alyst for such dehydrogenation reactions. To substantiate the
cooperativity of the P and B sites, and the key role of the geomet-
ric constraints induced by the o-phenylene linker, the catalytic
activity of 2 was compared with that of a related bimolecular
substrate 2 (mol%) T (°C) additive time (h) yield (%)a
5
70
70
70
70
25
25
25
25
70
70
70
70
25
25
25
70
70
70
70
25
25
/
1
90
90
90
0
15
2.5
1
/
6
Lewis pair. Using the iPr
2 3
PhPB(Fxyl) adduct as catalyst,
dehydrogenation of DMAB is very slow: only 35% conversion
after 18 h at 70°C and 5 mol% cat. loading, while the reaction is
complete within 0.5 h with 2.
/
8
8a
0
/
8
R = Me
5
2 eq 5
6
91
90
0
In situ NMR studies were also performed. In most cases, com-
plex spectra with multiple broad signals were obtained, suggesting
the coexistence of different species. Varying the conditions, we
could eventually characterize a key intermediate upon reacting 2
with 3 equivalents of DMAB in dichloromethane. Under these
conditions, dehydrogenation is complete after 2 h, but freezing the
reaction mixture at –80°C just after mixing provided valuable
2.5
0
2 eq 5
24
24
72
1.5
2.5
5
2 eq 5
5
/
90
90
90
90
0
5
/
3
1
information. P NMR spectroscopy revealed the presence of two
2.5
1
/
species resonating at  57.1 and 21.7 ppm in approximately 70:30
8
b
/
15
31
11
ratio.
Multi-nuclear NMR analyses including
P{ B},
1
1
31
1
11
R = tBu
0
/
5
B{ P}, H/ B TOCSY experiments enabled us to authenticate
the two formed products as rotamers of the phosphonium-borate
5
2 eq 5
4.5
24
180
2.5
3.5
4
90
99
0
15
1
1
3
.
Most diagnostic are the JP–H and JB–H coupling constants:
2.5
0
2 eq 5
4
76 / 81 Hz for the major species 3a and 402 / 75 Hz for the
minor one 3b. Slow exchange between 3a and 3b at the NMR
2 eq 5
3
1
31
11 11
timescale is apparent from P/ P and B/ B EXSY experi-
ments. The coexistence of two rotamers of 3 is somewhat surpris-
ing, but consistent with the presence of four local minima close in
energy according to DFT calculations (Figure 2). It is hardly
possible at this stage to identify precisely the structures of two
rotamers observed experimentally. But the computed NMR data
reveal large variations in the NMR chemical shifts of the P and H
5
/
95
95
95
0
2
1
0
5
0
.5
/
8c
/
R = Bn
/
4
2 eq 5
2 eq 5
36
72
80
0
1
atoms as well as of the JPH coupling constant, consistent with the
values determined experimentally. To confirm our interpretation,
the phosphonium-borate 3 was independently synthesized by
a
Reactions performed in benzene at 0.77 M. Yields determined
reacting 2 consecutively with triflic acid and triethylsilane
1
by H NMR using hexamethylbenzene as internal standard.
26
(
Scheme 3). The NMR data show two species matching exactly
those characterized in situ. In addition, crystals of 3 were grown at
–25°C and its molecular structure was confirmed by X-ray dif-
fraction analysis. The solid-state structure of 3 determined by X-
Recent studies have also pointed out the ability of diamine-
boranes 8 to release two molecules of H
2
affording 1,3,2-
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ray diffraction resembles most rotamer 3. Compared to 2, the
dbouriss@chimie.ups-tlse.fr
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
P…B distance is now 3.195(3) Å and the PCC and CCB bond
angles open to 119.7(2) and 123.2(2)°, respectively [vs 105.0(2)
and 97.5(1)° in 2]. The proton at phosphorus and the hydride at
boron could be located in the difference Fourier map without
position constraints. The long H•••H distance (2.38 Å) suggests
the absence of hydrogen bonding. Upon warm up, solutions of 3
Notes
The authors declare no competing financial interests.
ASSOCIATED CONTENT
Supporting Information
spontaneously and rapidly release H
2
with recovery of the phos-
phine-borane 2 (50% conversion after 10 min at 25°C according
3
1
to P NMR).
The Supporting Information is available free of charge via the
Internet at http://pubs.acs.org. Experimental procedures, charac-
terization of the catalyst and intermediates (PDF). Crystallograph-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
ic data for 2, 2.BH and 3 (CIF).
ACKNOWLEDGMENT
The Centre National de la Recherche Scientifique (CNRS), the
Université Paul Sabatier (UPS) and the Agence Nationale de la
Recherche (ANR-15-CE07-0003) are acknowledged for financial
support of this work. M.B. thanks the “Région Occitanie” for his
Ph.D. funding. UPPA, MCIA (Mésocentre de Calcul Intensif
Aquitain) and CINES under allocation A003080045 made by
Grand Equipement National de Calcul Intensif (GENCI) are
acknowledged for computational facilities. E. Daiann Sosa Carri-
zo thanks CDAPP for funding part of his post-doctoral contract.
Scheme 3. Synthesis and molecular structure of 3 as deter-
mined by X-ray diffraction analysis; iPr and Fxyl simplified,
and H atoms, except the ones at P and B, omitted for clarity.
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2
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(
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Amine–Boranes and Phosphine–Boranes. Top. Organomet. Chem. 2015,
49, 153.
Of note, when Lewis pairs such as 2 are reacted with a large
excess of amine-boranes, BH
potential side-reaction. Such a process is not observed with 2.
The corresponding BH adduct does not form easily and it was
3
3
transfer (from N to P) may occur, a
2
7
1
5
(3) (a) Miller, A. J. M.; Bercaw, J. E. Dehydrogenation of amine-
actually not detected in the catalytic runs.
boranes with a frustrated Lewis pair. Chem. Commun. 2010, 46, 1709. (b)
Whittell, G. R.; Balmond, E. I.; Robertson, A. P. M.; Patra, S. K.;
Haddow, M. F.; Manners, I. Reactions of Amine- and Phosphane-Borane
Adducts with Frustrated Lewis Pair Combinations of Group 14 Triflates
and Sterically Hindered Nitrogen Bases. Eur. J. Inorg. Chem. 2010, 3967.
(4) Appelt, C; Slootweg, J. C.; Lammertsma, K.; Uhl, W. Reaction of a
P/Al-Based Frustrated Lewis Pair with Ammonia, Borane, and Amine-
Boranes: Adduct Formation and Catalytic Dehydrogenation. Angew.
Chem. Int. Ed. 2013, 52, 4256.
(5) (a) Mo, Z.; Kolychev, E. L.; Rit, A.; Campos, J.; Niu, H.; Aldridge,
S. Facile Reversibility by Design: Tuning Small Molecule Capture and
Activation by Single Component Frustrated Lewis Pairs. J. Am. Chem.
Soc. 2015, 137, 12227. (b) Mo, Z.; Rit, A.; Campos, J.; Kolychev, E. L.;
Aldridge, S. Catalytic B–N Dehydrogenation Using Frustrated Lewis
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(6) For amine-borane dehydrogenation with Zr-based FLP, see: (a)
Chapman, A. M.; Haddow, M. F.; Wass, D. F. Frustrated Lewis Pairs
beyond the Main Group: Cationic Zirconocene–Phosphinoaryloxide
Complexes and Their Application in Catalytic Dehydrogenation of Amine
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Dehydrocoupling of Amine–Boranes using Cationic Zirconium(IV)–
Phosphine Frustrated Lewis Pairs. ACS Catal. 2016, 6, 6601.
CONCLUSION
In conclusion, the phosphine-borane 2 proved to be an efficient
catalyst for the dehydrogenation of amine-boranes, surpassing
known metal-free systems in activity and generality. Two key
features of this Lewis pair have been highlighted: (i) the presence
of a strong but geometrically-constrained PB interaction, and
(ii) the propensity of the corresponding phosphonium-borate 3 to
release dihydrogen. The mechanism of amine-borane dehydro-
genation by 2 remains unclear at this stage. The non-observation
of the linear condensation product Me
2 2 2 3
NBH NMe BH is not in
favor of a chain-growth mechanism, as proposed for the xanthene-
5
b
derived phosphine-borane B. We hypothesize that the reaction
proceeds by hydrogen transfer from the amine-borane to 2 (con-
certed proton transfer from N to P and hydride transfer from B to
2
8,29
B),
2
and that the imine additive facilitates the H release from
the phosphonium-borate 3. Future work from our group will seek
to explore further the behaviour of phosphine-borane 2 towards
small molecule activation and as ambiphilic ligand for transition
3
0
metals.
(7) For B-N dehydrocoupling/dehydrogenation with low coordinate Sn
AUTHOR INFORMATION
Corresponding Author
and bis borane compounds, see: (a) Lu, Z.; Schweighauser, L.; Hausmann,
H.; Wegner, H. A. Metal-Free Ammonia–Borane Dehydrogenation Cata-
lyzed by a Bis(borane) Lewis Acid. Angew. Chem. Int. Ed. 2015, 54,
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Formaldehyde Adduct. ACS Catal. 2015, 5, 2513.
H
2
acceptors in the BCF-promoted quinolone synthesis: Fasano, V.;
Radcliffe, J. E.; Ingleson, M. J. Mechanistic Insights into the B(C F ) -
6 5 3
Initiated Aldehyde–Aniline–Alkyne Reaction To Form Substituted
Quinolines. Organometallics 2017, 36, 1623.
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Li, S.; Li, G.; Meng, W.; Du, H. A Frustrated Lewis Pair Catalyzed
Asymmetric Transfer Hydrogenation of Imines Using Ammonia Borane.
J. Am. Chem. Soc. 2016, 138, 12956.
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(23) For dihydrogen transfer from ammonia-borane and poly(amine-
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a four-membered heterocyclic
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Erker, G. Formation of Thermally Robust Frustrated Lewis Pairs by
Electrocyclic Ring Closure Reactions. Angew. Chem. Int. Ed. 2016, 55,
5
526.
13) (a) Samigullin, K.; Georg, I.; Bolte, M.; Lerner, H. W.; Wagner,
(
M. A Highly Reactive Geminal P/B Frustrated Lewis Pair: Expanding the
Scope to C-X (X=Cl, Br) Bond Activation. Chem. Eur. J. 2016, 22, 3478.
b) Wang, L.; Samigullin, K.; Wagner, M.; McQuilken, A. C.; Warren, T.
H.; Daniliuc, C. G.; Kehr, G.; Erker, G. An Ethylene-Bridged
Phosphane/Borane Frustrated Lewis Pair Featuring the -B(Fxyl) Lewis
2
Acid Component. Chem. Eur. J. 2016, 22, 11015.
(
(14) Samigullin, K.; Bolte, M.; Lerner, H. W.; Wagner, M. Facile
Synthesis of (3,5-(CF BX (X = H, OMe, F, Cl, Br): Reagents for
the Introduction of a Strong Boryl Acceptor Unit. Organometallics 2014,
33, 3564.
3 2 6 3 2
) C H )
methylene-Bridged Frustrated Lewis Pair Mes
Activate Dihydrogen? Chem. Eur. J. 2016, 22, 5988.
(27) Fixation of BH has also been authenticated as a possible deactiva-
2 2 2 2 6 5 2
PCH CH CH B(C F ) Not
(
(
15) See Supporting Information for details.
16) The o-phenylene Mes P/B(C
3
F
5
)
2
derivative displays
a
tion pathway for TM catalysts, see: (a) Denney, M. C.; Pons, V.; Hebden,
T. J.; Heinekey, D. M.; Goldberg, K. I. Efficient Catalysis of Ammonia
Borane Dehydrogenation. J. Am. Chem. Soc. 2006, 128, 12048. (b) Glüer,
A.; Förster, M.; Celinski, V. R.; Schmedt auf der Günne, J.; Holthausen,
M. C.; Schneider, S. Highly Active Iron Catalyst for Ammonia Borane
Dehydrocoupling at Room Temperature. ACS Catal. 2015, 5, 7214.
(28) For such a hydrogen transfer between N,B compounds, see: Leit-
ao, E. M.; Stubbs, N. E.; Robertson, A. P. M.; Helten, H.; Cox, R. J.;
Lloyd-Jones, G. C.; Manners, I. Mechanism of Metal-Free Hydrogen
Transfer between Amine-Boranes and Aminoboranes. J. Am. Chem. Soc.
2012, 134, 16805.
2
6
12
significantly longer P-B distance of 2.203(6) Å.
(17) For a detailed DFT study of the factors influencing the thermody-
namics of H activation by FLP, see: Rokob, T. A.; Hamza, A.; Pápai, I.
Rationalizing the Reactivity of Frustrated Lewis Pairs: Thermodynamics
of H Activation and the Role of Acid−Base Properties. J. Am. Chem. Soc.
2009, 131, 10701.
18) With ammonia-borane, one equivalent of H
is rapidly released, but no catalytic turnover is observed.
19) For catalyst-free B-N dehydrocoupling, see: (a) Helten, H.;
2
2
(
2
per phosphine-borane
(
Robertson, A. P. M.; Staubitz, A.; Vance, J. R.; Haddow, M. F.; Manners,
I. "Spontaneous" Ambient Temperature Dehydrocoupling of Aromatic
Amine-Boranes. Chem. Eur. J. 2012, 18, 4665. (b) Romero, E. A.; Peltier,
J. L.; Jazzar, R.; Bertrand, G. Catalyst-free dehydrocoupling of amines,
alcohols, and thiols with pinacol borane and 9-borabicyclononane (9-
BBN). Chem. Commun. 2016, 52, 10563.
(29) The activation of amine-boranes by FLPs A and B was proposed
4
,5b
to start by heterolytic splitting of the N–H and B–H bonds, respectively.
(30) (a) Amgoune, A.; Bourissou, D. σ-Acceptor, Z-type ligands for
transition metals. Chem. Commun. 2011, 47, 859. (b) Bouhadir, G.;
Bourissou, D. Complexes of ambiphilic ligands: reactivity and catalytic
applications. Chem. Soc. Rev. 2016, 45, 1065.
(20) For recent examples in BCF and FLP-catalyzed reactions, see: (a)
Yin, Q.; Klare, H. F. T.; Oestreich, M. Catalytic Friedel–Crafts C−H
Borylation of Electron-Rich Arenes: Dramatic Rate Acceleration by
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H
P
B
P
H
B
F
F
Metastable
Dormant Lewis Pair
phosphonium-borate
Key intermediate
Dehydrogenation catalyst
Table of Contents artwork
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