Frustrated Lewis pairs: Reactivities of TMS protected amines and phosphines in the presence of B(C6F5)3

Article abstract of DOI:10.1039/b923196j

TMS protected amines in combination with B(C₆F₅) ₃ were found to activate H₂ and this is followed by a cleavage of the N-Si bond and the generation of TMSH. A TMS protected phosphine on the other hand reacts rap

Full text of DOI:10.1039/b923196j

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Frustrated Lewis pairs: reactivities of TMS protected amines and phosphines
in the presence of B(C6F5)3†
Felix Schulz,a Victor Sumerin,b Markku Leskela¨,b Timo Repob and Bernhard Rieger*a
Received 26th June 2009, Accepted 9th December 2009
First published as an Advance Article on the web 24th December 2009
DOI: 10.1039/b923196j
TMS protected amines in combination with B(C6F5)3 were
found to activate H2 and this is followed by a cleavage of the
N–Si bond and the generation of TMSH. A TMS protected
phosphine on the other hand reacts rapidly with B(C6F5)3 to
give the known compound tBu2P(C6F4)B(C6F5)2 by a facile
and efficient route.
Scheme 1 Weak Lewis bases cannot activate H2 in combination with
B(C6F5)3 under mild conditions.
Since the first publication on hydrogen activation by “frustrated”
pairs of Lewis acids and bases appeared,1 there has been consider-
able interest in exploring this exciting field. This is because on the
one hand these main group element systems have the potential to
become efficient green hydrogenation catalysts and on the other
hand they can be used as models for the research on non-metal
hydrogen storage materials.2
Because of the low basicity of the TMS protected amines with
aromatic substituents the combined LA/LB strength is evidently
not strong enough to cleave the H2 bond heterolytically under such
mild conditions.
We continued our studies by mixing the more basic TMS-
amine 3a which has an electron donating mesityl group with
B(C6F5)3. The toluene solution turned bright yellow, but by NMR
spectroscopy (1H, 11B, 13C, 19F), no LA/LB adduct or any other
reaction was detected. By exposing this solution to 1.5 bar H2
for 12 h at room temperature a white precipitate formed.17 The
isolated product dissolved well only in very polar solvents like
Not only has there been the introduction and study of a
wide variety of frustrated Lewis pairs (FLPs) which can cleave
dihydrogen,3-9 or activate C–C multiple bonds,7,9,10 but also their
successful use in hydrogenation catalysis under mild conditions has
been demonstratedonnumerous substrates.11-14 Those included the
reduction of imines, enamines, silyl enol ethers, protected nitriles,
aziridines and also an asymmetric hydrogenation of an imine.15
In continuation of our own work on the facile heterolytic cleav-
age of dihydrogen by simple and bulky amines in combination with
B(C6F5)3,3 we became interested in the behaviour of trimethylsilyl
(TMS) protected amines under equal conditions. There are two
properties of this group which should influence the performance
of the amine in the process of activating hydrogen: the TMS group
is (i) very bulky which means that most TMS protected amines
should not form a stable adduct together with B(C6F5)3, and (ii)
it reduces the Lewis basicity at the nitrogen center in comparison
with the analogous t-butyl substituted amine.16
We started our investigations with the TMS protected carbazole
1 and diphenylamine 2. Mixing of the two compounds separately
with B(C6F5)3 in toluene afforded colourless solutions. Multinu-
clear NMR studies (1H, 11B, 13C, 19F) showed no changes in the
spectra in comparison with the pure substances which indicates
that a stable Lewis acid (LA)/Lewis base (LB) adduct formation
did not appear in both cases. After exposing the solutions to a
hydrogen atmosphere (1.5 bar) for 24 h at room temperature or
at 110 ◦C no reaction could be detected by NMR (Scheme 1).
1
DMSO. In the H and 13C spectra no signals can be detected
that refer to the three methyl groups at Si. In addition 11B
NMR exhibits a single resonance at -0.98 ppm and the chemical
shift difference Ddp,m between the F atoms in meta and para
position of the C6F5 fragments is 6.59 ppm. These values are
characteristic of a four coordinate neutral adduct of B(C6F5)3.18,19
1
In fact the H spectrum is identical with the spectrum of pure
2,4,6-trimethylaniline in DMSO and the 11B and 19F spectra are
identical with the corresponding spectra of B(C6F5)3 in DMSO.
Therefore it can be assumed that the adduct 3c is formed during
the reaction (Scheme 2). By the subsequent dissolution in DMSO
the coordinative B–N bond is cleaved in favour of the more stable
DMSO–B(C6F5)3 complex.
Scheme 2 Activation of H2, cleavage of the N–Si bond and formation of
a LA/LB adduct.
aWACKER-Lehrstuhl fu¨r Makromolekulare Chemie, Technische Univer-
sita¨t Mu¨nchen, Lichtenbergstraße 4, 85747 Garching bei Mu¨nchen, Germany.
E-mail: rieger@tum.de; Fax: +49 89 289 13562; Tel: +49 89 289 13570
bDepartment of Chemistry, Laboratory of Inorganic Chemistry, Uni-
versity of Helsinki, P.O. Box 55, 00014 Helsinki, Finland. E-mail:
timo.repo@helsinki.fi; Fax: +358 9 191 50198; Tel: +358 9 191 50194
A solution (C6D6) of the even more basic N-trimethylsilyl-t-
butylamine (4a) with B(C6F5)3 remained colourless and showed
also by NMR no sign of adduct formation. Interestingly this
frustrated pair clearly reacted over 12 h with hydrogen (1.5 bar)
† Electronic supplementary information (ESI) available: Experimental
details, and spectroscopic characterizations. See DOI: 10.1039/b923196j
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at room temperature (Scheme 2)17 and yielded the known adduct
4c20 which could be identified by multinuclear NMR. When the
sample was transferred directly out of the reaction vessel into
the NMR tube without evaporation of the solvent, in addition
to the signals for product 4c there appeared a clear multiplet at
4.15 ppm and a doublet at 0.02 ppm in the proton NMR spectrum
and also one signal at -2.75 ppm in the carbon spectrum. They
could be assigned to trimethylsilane (TMSH)21 which has a low
boiling point of 6.7 ◦C.22 Therefore only a small amount of TMSH
compared to 4c could be detected in solution. After evaporation
of the volatile compounds these signals disappeared completely.
As expected no LA/LB adduct could be detected by 1H, 11B, 13C,
and 19F NMR in the pale yellow solution of the extremely bulky
N-trimethylsilyldiisopropylamine (5a) and B(C6F5)3 in toluene-
D8. In contrast to previous literature,23 there is also no sign of
any other side reactions induced by a hydride abstraction in a
position to nitrogen. We assume that this unreactivity is caused
by the steric congestion of 5a. After stirring this mixture at room
temperature under 1.5 bar pressure of H2 for 12 h, NMR data
showed the expected 1 : 1 mixture of the salt 5c and the zwitterion
Scheme 4 Proposed mechanism for the activation of H2 by TMS
protected amines in combination with B(C6F5)3.
often referred to as a “bulky proton”,25 its behaviour differs
significantly from that of a real proton in the examined systems:
there are only a few examples of FLPs known where H2 can
evolve after activation.1,2,3,8,12 Even in those cases it has never
been reported to be fast and quantitative at room temperature.
Our investigations basically lead to the reverse reaction of the
previously reported B(C6F5)3 catalyzed dehydrogenative silation
of alcohols with tertiary silanes: the authors report that H2 evolves
while a Si–O bond is formed and in our case H2 is consumed while
a Si–N bond is broken followed by the formation of a tertiary
silane.26
=
iPrNH C(CH3)CH2B(C6F5)3 plus the additional side product
TMSH (Scheme 3). As described earlier by us,3 the former two
compounds are in an equilibrium with free diisopropylamine and
B(C6F5)3 at 110 ◦C. Thus, when exposing a solution of 5a and
B(C6F5)3 at 110 ◦C to a hydrogen atmosphere (1.5 bar), two
equivalents of H2 were consumed and after evaporation the pure
product 5c was obtained.
Since it is well-known in the literature that bulky phosphines
4-9
can also cleave dihydrogen in combination with B(C6F5)3 the
question arose whether this is also feasible with TMS protected
phosphines. By mixing trimethylsilyldi-t-butylphosphine 6a with
B(C6F5)3 in benzene at room temperature, the solution turned
bright orange, and after subsequent evaporation it was found
by NMR spectroscopy that it converts cleanly into the previ-
ously reported intramolecular boron–phosphorus compound 6b5
(Scheme 5), which is able to activate hydrogen and catalyzes
the hydrogenation of imines. This finding is in agreement with
previous results on the reaction of TMS protected phosphines
with polyfluoroarenes.27-29 After the nucleophilic attack of the
phosphorous at the carbon atom in para position on the electron
deficient C6F5-ring a strong Si–F bond (561 kJ mol-1)30 forms◦and
fluorotrimethylsilane (TMSF) with a low boiling point of 16.4 C31
evolves. The mechanistic study of the reaction of (CH3)2PTMS
with polyfluorarenes suggested a SNAr pathway.28
Scheme 3 Activation of a second equivalent of H2 after cleavage of the
N–Si bond.
We conclude from these observations that H2 activation by TMS
protected amines in combination with B(C6F5)3 is in principle
possible if the Lewis basicity of the amine is strong enough and if
the amine is bulky enough to form a frustrated pair. The proposed
mechanism of the activation is shown in Scheme 4. Dihydrogen
is split heterolytically by 3a, 4a and 5a in combination with
B(C6F5)3 and the salts 3b, 4b and 5b are formed. One additional
experimental observation supports this assumption: after 10 h of
reacting 4a and B(C6F5)3 with H2, 19F NMR showed two sets
of signals comprising of 3 peaks. One set can be attributed to
the product 4c and the other one is consistent with the anion
[HB(C6F5)3]-.24
The resulting salts 3b, 4b, and 5b are not stable in contrast
to the salts of unprotected amines reported earlier3,13,14 and
decompose. TMSH is released and stable LA/LB adducts are
formed in the case of 3 and 4 which cannot activate H2 even at
elevated temperatures (110 ◦C). When 5 is employed the bulky
diisopropylamine and B(C6F5)3 are formed which react further
as described before (Scheme 3).3 Although the TMS group is
Scheme 5 Reaction of a TMS protected bulky phosphine with B(C6F5)3.
Accordingly, the TMS-phosphine shows a different behaviour
than secondary phosphines. If R2PH is heated together with
B(C6F5)3 in a 1 : 1 stoichiometry, nucleophilic attack at the same
C atom also takes place, but then a rearrangement occurs and the
corresponding zwitterion R2PH(C6F4)BF(C6F5)2 is formed.19
As has been investigated earlier the TMS protected phosphines
offer a much greater reactivity than the respective amines.29 This is
in accordance with our own studies: Even by heating TMS-amines
◦
with B(C6F5)3 to 110 C we could not find similar reactivity due
to their lower nucleophilicity.
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12 V. Sumerin, F. Schulz, M. Atsumi, C. Wang, M. Nieger, M. Leskela¨, T.
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17 The reaction can be sped up significantly at higher temperatures.
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Conclusions
It is shown that various TMS protected amines with a high
enough steric congestion and Lewis basicity can activate H2
in combination with B(C6F5)3 under mild conditions. By doing
so the N–Si bond is quantitatively cleaved and the amines
are deprotected.32 This represents a new metal free hydrogen
activation by N-TMS amines and B(C6F5)3 with the simultaneous
generation of TMSH which could be used in further hydrosilation
reactions for example. In combination with the previously reported
B(C6F5)3 catalyzed hydrosilation of imines33 a novel method for the
reduction of imines could be developed. Another application could
be the effective and clean deprotection of sensitive organometallic
compounds since it was demonstrated earlier14 that H2 activation is
feasible with these substances. In addition, the described reaction
of TMS protected phosphines with B(C6F5)3 offers an efficient
access to the frustrated Lewis pair tBu2P(C6F4)B(C6F5)2. This
route may also work for other R2P(C6F4)B(C6F5)2 species and
thereby facilitate the ongoing investigation of their potential to
activate small molecules.1,5,6,9
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3
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1922 | Dalton Trans., 2010, 39, 1920–1922
This journal is
The Royal Society of Chemistry 2010
©