A phosphorus/aluminum-based frustrated lewis pair as an ion pair receptor: Alkali metal hydride adducts and phase-transfer catalysis

Article abstract of DOI:10.1002/anie.201201855

Helpful frustration: The geminal phosphorus/aluminum-based frustrated Lewis pair (Mes₂P)(tBu₂Al)C-C(H)Ph (Mes=2,4,6-Me ₃C₆H₂) forms stable adducts with alkali metal hydrides (LiH, NaH, KH). These molecular hydride complexes display enhanced reactivity, which was demonstrated by the catalytic transformation of chlorotriphenylsilane to the corresponding hydride through a frontside S _N2-f@Si pathway. Copyright

Full text of DOI:10.1002/anie.201201855

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Chemie
DOI: 10.1002/anie.201201855
Hydride Activation
A Phosphorus/Aluminum-Based Frustrated Lewis Pair as an Ion Pair
Receptor: Alkali Metal Hydride Adducts and Phase-Transfer
Catalysis**
Christian Appelt, J. Chris Slootweg,* Koop Lammertsma, and Werner Uhl*
Frustrated Lewis pairs (FLPs) based on main-group elements
have been the subject of enormous interest in recent years
owing to their metal-free dipolar activation of a multitude of
small molecules.[1] Yet, despite all of this progress, the
development of catalytic transformations based on FLPs is
still in its infancy. So far, only borane-based FLPs bearing
electron-withdrawing pentafluorophenyl substituents enable
the catalytic hydrogenation of unsaturated substrates.[2] The
most notable application for FLPs is in the catalytic tandem
hydride transfer/deoxygenative hydrosilylation of carbon
dioxide to methane.[3] In this reaction, the second reduction
step depends on Lewis acid-catalyzed activation of silanes by
B(C6F5)3 and a consecutive silyl transfer to the Lewis basic
Scheme 1. Synthesis of molecular alkali metal hydrides 2–4.
Mes=2,4,6-Me3C6H2.
carbonyl oxygen.[4]
À
À
The discovery of FLP catalysis with covalent H H and Si
H[5] bonds inspired us to explore catalysis with ionic hydrides,
as these have not yet been used in conjunction with FLPs, to
the best of our knowledge. Herein, we wish to report on the
propensity of FLPs to solubilize[6] and activate[7] the alkali
metal hydrides (LiH, NaH, KH).[8,9] These species are
important reducing agents, but with limited applicability
owing to their relatively high lattice energies,[10] poor sol-
ubility in organic solvents, and the poor purity of commer-
cially available material.[11] To this end, we selected our
recently reported, promising, and readily accessible geminal
P/Al-based FLP 1 (Scheme 1), which does not require
fluorinated substituents to enhance the Lewis acidity of the
acceptor site.[12]
Treatment of FLP 1 with solid lithium hydride and THF
(2 equiv) in toluene at room temperature for 15 h afforded,
after workup and crystallization, LiH adduct 2 (40% yield,
d31P = À7.9 ppm; Scheme 1). A direct interaction of the
lithium cation with the donor site of FLP 1 is indicated by
the large 1J(Li,P) coupling constant of 39.8 Hz[13] (d7Li =
À0.3 ppm, 1J(Li,H) = 14.8 Hz). The hydride resides on the
acceptor site, as can be deduced from the broad resonance at
d1H = 2.91 ppm.[14] The molecular structure of 2, obtained by
a single-crystal X-ray structure determination (Figure 1, top
left),[15] shows a Li-H-Al three-center two-electron bond
(Li1–H1 191(2), Al1–H1 164(2) pm) with the Li atom
connected
to
the
phosphorus
atom
(Li1–P1
271.1(3) pm)[13,16] and two molecules of THF. This compound
is the first structurally characterized monomeric lithium
hydride complex.[17] Interestingly, 2 is also obtained in 92%
yield upon treatment of 1 with super hydride Li[HBEt3], by
displacement of triethylborane.
NaH and KH also reacted with FLP 1 at room temper-
ature, but required longer reaction times to break up the solid
metal hydride material to afford soluble adducts 3 (7 days,
40% yield) and 4 (2 days, 45% yield; Scheme 1). Both 3
(d31P = 0.9 ppm) and 4 (d31P = 1.0 ppm) display very broad
resonances in the 31P NMR spectrum, indicating a dynamic
interaction of the alkali metal cations with the P atom. Single-
crystal X-ray analysis unequivocally established that the
bonding of 2 differs from that of both 3 and 4. In contrast to 2,
the softer Lewis acidic alkali metals Na and K display no
interactions with the phosphorus atom in the solid state (3:
Na1–P1 359.8, 4: K1–P1 378.2 pm; Figure 1),[15,18] but instead
have short contacts with the P–mesityl ring (3: Na1–C61
281.3(2), 4: K1–C71 329.2(3) pm).[19] The coordination sphere
of the potassium ion in 4 is completed with four THF
molecules, whereas only one THF molecule coordinates to
the sodium atom in 3, creating a one-dimensional chain that is
held together by additional contacts between the Na cation
and the phenyl-ring of a neighboring molecule (Na1–C35
277.2(2), Na1–C36 302.2(2) pm; Figure 1).
[*] C. Appelt, Prof. Dr. W. Uhl
Institut fꢀr Anorganische und Analytische Chemie der Westfꢁlischen
Wilhelms-Universitꢁt Mꢀnster
Corrensstraße 30, 48149 Mꢀnster (Germany)
E-mail: uhlw@uni-muenster.de
Dr. J. C. Slootweg, Prof. Dr. K. Lammertsma
Department of Chemistry and Pharmaceutical Sciences
VU University Amsterdam
De Boelelaan 1083, 1081 HV Amsterdam (The Netherlands)
E-mail: j.c.slootweg@vu.nl
[**] This work was supported by the Deutsche Forschungsgemeinschaft.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201855.
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address this question, we selected the least solvated alkali
metal hydride adduct 3 and studied its ability to convert
chlorotriphenylsilane (5) to triphenylsilane (6).[21] Treatment
of 3 with chlorosilane 5 (1 equiv) in THFat room temperature
for 48 h quantitatively afforded silane 6 together with
precipitated sodium chloride and the THF adduct of FLP
1 (1·THF; Scheme 3). For comparison, sodium hydride alone
Scheme 3. Phase-transfer catalysis with FLP 1 by NaH adduct 3.
Figure 1. Molecular structures of 2 and 4, and polymeric structure of 3
(two units shown). Ellipsoids set at 30% probability; hydrogen atoms
(except H1), and CH2 groups of the THF molecules of 4 are omitted
for clarity. Selected average bond lengths [pm] and angles [8] for 2: P1–
C1 184.2(2), C1–Al1 205.7(2), Al1–H1 164(2), H1–Li1 191(2), Li1–P1
271.1(3); P1-C1-Al1 109.7(8), Al1-H1-Li1 128.5. 3: P1–C1 183.1(2), C1–
Al1 205.1(2), Al1–H1 154(2), H1–Na1 220(2), Na1–C61 281.3(2), Na1–
C35 277.2(2), Na1–C36 302.2(2), C61–P1 185.4(2); P1-C1-Al1
109.36(9), Al1-H1-Na1 155.7. 4: P1–C1 184.9(3), C1–Al1 206.8(3), Al1–
H1 174(5), H1–K1 257(5), K1–C71 329.2(3); P1-C1-Al1 109.2(1), Al1-
H1-K1 146.8.
proved to be unreactive under the same reaction conditions.
The fact that 1 was regenerated, suggested the feasibility of
catalytically converting chlorosilane 5. Indeed, treating a sus-
pension of 5 with commercially available NaH (1.5 equiv) in
THF using 1 (10 mol%) under reflux conditions for 48 h
1
resulted in full conversion according to H NMR spectrosco-
py, affording 6 in 76% yield after a vacuum distillation
workup (Scheme 3). As expected, the same reaction without
the FLP phase-transfer catalyst proved to be much less
effective[22] and even the use of AltBu3 as a Lewis acid
(29 mol%) only resulted in a 30% conversion of 5 to 6 under
similar conditions, which underscores the cooperative
action[23,24] of the donor and acceptor sites of FLP 1 in the
reaction.
To provide insight into the stability of the different
coordination modes, we optimized the geometries of mono-
mers 2 and 4 with two molecules of THF at the M06-2X/6-
31G(d) level of theory (2,4–P vs. 2,4–Ar; Scheme 2).[20] In
To gain insight into the underlying mechanism of the
catalytic transformation, we resorted to M06-2X/6-31G(d)
calculations[20] on model structures where the phenyl sub-
stituent and all the Me groups of 1·THF were replaced by
H atoms (model species labeled with ’). We found the
substitution reaction to be exothermic by 24.5 kcalmolÀ1
(Figure 2). First, after break-up of the polymer chain,
monomer 3’ and chlorosilane 5 form reactant complex 7’
(DE = À18.2 kcalmolÀ1), which undergoes a frontside nucle-
ophilic substitution at silicon (SN2-f@Si)[25] affording product
complex 8’ (DDE° = 17.7, DDE = À0.9 kcalmolÀ1). In the
pentacoordinate transition species (TS7’–8’) the hydride
attacks the silicon center at an angle of 77.68 (H1-Si1-Cl1)
to the leaving chloride, in contrast to the 1808 common for
backside SN2 substitutions. Subsequently, 8’ rearranges to the
more stable 9’ (DDE = À23.2 kcalmolÀ1; Figure 2),[26] which is
a van der Waals complex of 10’ (the NaCl adduct of 1’) and
triphenylsilane 6. Finally, dissociation of 6 (DDE = 18.0, DE =
À24.5 kcalmolÀ1) and precipitation of NaCl generates FLP 1’,
which then reenters the catalytic cycle. This FLP-mediated
nucleophilic substitution at silicon proceeds with retention of
configuration[25] and holds promise for the development of
SN2-f@Si reactions for silicon-stereogenic silanes with con-
servation of stereochemistry.
Scheme 2. Relative M06-2X/6-31G(d) energies (in kcalmolÀ1) for the
two different coordination modes (M–P vs. M–Ar) of 2,4·2THF.
accord with the molecular structures, 2–P, which has a lith-
ium–phosphorus interaction, was found to be more stable
than 2–Ar (DE = 0.7 kcalmolÀ1); whereas the bigger potas-
sium ion favors coordination to the arene ring (4–Ar, DE =
À2.1 kcalmolÀ1).[19]
So far, we have demonstrated that FLP 1 can solubilize
metal hydrides (LiH, NaH, KH), but do the resulting
molecular hydrides also display enhanced reactivity? To
2
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bond lengths [pm] and angles [8] for 7’: Al1–H1 167.0, H1–Na1 212.9,
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Received: March 8, 2012
Published online: && &&, &&&&
Keywords: alkali metals · density functional calculations ·
.
frustrated Lewis pairs · hydrides · silanes
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4
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Communications
Hydride Activation
C. Appelt, J. C. Slootweg,*
K. Lammertsma, W. Uhl*
&&&& — &&&&
A Phosphorus/Aluminum-Based
Frustrated Lewis Pair as an Ion Pair
Receptor: Alkali Metal Hydride Adducts
and Phase-Transfer Catalysis
Helpful frustration: The geminal phos-
complexes display enhanced reactivity,
phorus/aluminum-based frustrated
which was demonstrated by the catalytic
transformation of chlorotriphenylsilane
to the corresponding hydride through
a frontside SN2-f@Si pathway.
=
Lewis pair (Mes2P)(tBu2Al)C C(H)Ph
(Mes=2,4,6-Me3C6H2) forms stable
adducts with alkali metal hydrides (LiH,
NaH, KH). These molecular hydride
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!