Preparation of NHC borane complexes by lewis base exchange with amine- and phosphine-boranes

Article abstract of DOI:10.1021/jo101301d

A versatile new method for the preparation of NHC boranes starting from two stable, readily available reactants-an heterocyclic salt and an amine or phosphine-borane-is reported. It uses a Lewis base exchange at boron and provides easy access to new NHC boranes, in particular B-substituted borane ones.

Full text of DOI:10.1021/jo101301d

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boron and provides easy access to new NHC boranes, in
particular B-substituted borane ones.
Preparation of NHC Borane Complexes by Lewis
Base Exchange with Amine- and
Phosphine-Boranes
Malika Makhlouf Brahmi,† Julien Monot,†,‡
Marine Desage-El Murr,† Dennis P. Curran,*,‡
There is growing interest in the chemistry of borane com-
plexes of N-heterocyclic carbenes (NHC boranes). Work
focused first on structural determinations,1 then gradually
encompassed inorganic,2 organic (in particular as tin hydride
surrogates),3 and organometallic4 chemistries as well.
Further progress in NHC borane chemistry would be
expedited by the development of a general, operationally
simple and reliable pathway for the synthesis of such com-
plexes. Until now, the only access to NHC boranes has been
the complexation of isolated NHCs with solutions of reactive
boranes, like BH3-THF or BH3-SMe2.2b-f,5 This method-
ology suffers some drawbacks. For example, BH3-THF
cannot be stored over prolonged periods (as are most isolable
boranes) and BH3-SMe2 releases the foul-smelling and toxic
volatile dimethyl sulfide upon complexation.
Louis Fensterbank,*,† Emmanuel Lacote,*,† and
^
Max Malacria*,†
†UPMC University Paris 06, Institut parisien de chimie
ꢀ
moleculaire (UMR CNRS 7201), C 229, 4 place Jussieu,
75005 Paris, France, and ‡Department of Chemistry,
University of Pittsburgh, Pittsburgh, Pennsylvania 15260
curran@pitt.edu; louis.fensterbank@upmc.fr;
emmanuel.lacote@upmc.fr; max.malacria@upmc.fr
Received July 24, 2010
The formation of IPr-BH3 from BH3-THF is formally
an exchange at the boron atom of a weakly coordinated
Lewis base (THF) with the stronger NHCs. Boranes are
known to lead to a variety of highly stable complexes with
other, switchable, Lewis bases. In particular DABCO is used
to decomplex sensitive phosphines from borane.6 It thus
seemed interesting to examine whether more tightly asso-
ciated Lewis bases could also be exchanged with NHCs to
generate the NHC boranes. We selected easily accessible
amine-boranes7 and phosphine-boranes.8 We describe
herein a new access to NHC boranes by Lewis base exchange
at boron.
A versatile new method for the preparation of NHC
boranes starting from two stable, readily available reactants;
an heterocyclic salt and an amine or phosphine-
borane;is reported. It uses a Lewis base exchange at
The well-known 1,3-bis(2,6-diisopropylphenyl)imidazol-
2-ylidene carbene (IPr) was initially chosen as a representa-
tive NHC to probe exchanges with various amine-boranes.
IPr is generated by deprotonation of the corresponding
chloride imidazolium salt, and is an air-stable compound
that can be stored for weeks.9
In a typical experiment, a benzene solution of IPr and a
slight excess of Me3N-BH3 (1a, 1.1 equiv, Table 1, entry 1)
was refluxed overnight, and the conversion was monitored
by 11B NMR. A new resonance (-36 ppm) appeared upfield
from that of 1a (-7 ppm). NHC borane 2 was isolated in
93% yield after a quick filtration over silica gel. The same
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DOI: 10.1021/jo101301d Published on Web 09/14/2010
r
J. Org. Chem. 2010, 75, 6983–6985 6983
2010 American Chemical Society

Brahmi et al.
JOCNote
TABLE 1. Formation of NHC Borane Complexes from Amine-
Boranes
TABLE 2. Variation of the NHC Unit
entry
R1
R2
N(R3)3
NHCB, yield (%)
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
Me
Arb
Ar
Ar
Ar
Ar
Ar
Ar
iPr
Me3N (1a)
NH3 (1b)
Me2NH (1c)
t-Bu2NH (1d)
Et3N (1e)
pyridine (1f )
DMAP (1g)
Me3N (1a)
2, 93
-
a
a
-
a
-
2, 91
2, 87
2, 70
3, 89
aNo reaction. bAr = 2,6-iPrC6H3-.
compound was isolated in only 40% yield when the borane
source was BH3-THF.3b
A rapid screening of the commercially available amine-
10
boranes as well as the easily accessible DMAP-BH3
was examined next. No 11B signal of 2 was observed in the
crude mixture after one night with borane complexes of
ammonia or primary and secondary amines (Table 1, entries
2-4). The acidity of such amine-boranes may account for
these failures. On the other hand, good yields of 2 were
obtained from triethylamine-borane (1e), pyridine-borane
(1f, entry 6), and DMAP-borane (1g, entry 7). Overall, we
were pleased to observe that the highest yielding amine-
borane complex (1a) is also cheaper than the alternatives
(1 mmol costs $0.18, or 0.13h). The methodology was
applicable to other stable NHCs. It delivered 3 in 89% yield
(entry 8).
Most NHCs are prepared in situ so we looked for a more
convenient procedure that starts directly from the imidaz-
olium salts (Table 2). The latter were deprotonated in situ with
a THF solution of NaHMDS (1 M) for 1 h at -78 °C in THF.
Amine-borane 1a was then added in one portion and the
resulting solution was refluxed overnight (method A). To
compare this new method to the standard methodology, we
did a parallel series of experiments with BH3-THF solution
as boron source (method B).
aMethod A: NaHMDS (1 M in THF, 1.2 equiv), THF, -78 °C, 1 h
then Me3N-BH3 (1.1 equiv), THF, reflux overnight. Method B:
NaHMDS (1 M in THF, 1.2 equiv), THF, -78 °C, 1 h then BH3-THF
(1.1 equiv), THF, -78 °C to rt, overnight. bNo reaction.
boronic acids11 or boronates.12 The complexed boranes were
obtained by reduction of the corresponding boronate deriv-
atives with LAH in diethyl ether, followed by quenching by
pyridine (see the SI for detail).
The borane-pyridine complexes were then converted into
the corresponding NHC boranes by using the previously
established procedures, either from the isolated (method C)
or from the in situ prepared (method A) NHCs (see Table 3).
The yields were modest to satisfactory. Nonetheless there is
to date no other way to access compounds 13-14.
The reaction of IPr with (S)-Alpine-Boramine led to the
corresponding NHC borane 13e in 67% yield (entry 5),
which shows that B-substituted amine-boranes also under-
go the Lewis base exchange at boron.
Finally, we looked at phosphine-boranes as the borane
source. In a typical experiment, triphenylphosphine borane
was added to a benzene solution of iPr and led to 94% of 2,
after 24 h at 80 °C (see Scheme 1). The triphenylphos-
phine was fully recovered after a short purification. A similar
result was obtained with secondary di-tert-butylphosphine-
borane (81%).
For all the in situ generated complexes but three, the yields
with the new procedure were similar or slightly better than
when BH3-THF was used. Sterically hindered imidazolium
yielded more desired product with the BH3-THF solution
than with 1a (Table 2, entries 1, 3, and 11). The less tightly
bound and less hindered BH3-THF complex is probably
better suited when steric hindrance becomes the main ob-
stacle to reactivity.
Substituted boranes were examined next. The scattered
example of NHC boranes with a substituent at B were
prepared from stable boranes by direct complexation.2b-f
We felt that our Lewis base exchange methodology might
broaden the structural space that could be accessed since it
would extend it to boranes that cannot be isolated pure, yet
can be stabilized by amine complexation.
We selected two procedures from the literature to prepare
pyridine-B-substituted borane complexes from the corresponding
(11) Singaram, B.; Cole, T. E.; Brown, H. C. Organometallics 1984, 3,
774–777.
(12) Hawthorne, M. F. J. Am. Chem. Soc. 1958, 80, 4291–4293.
(10) Lesley, M. J. G.; Woodward, A.; Taylor, N. J.; Marder, T. B. Chem.
Mater. 1998, 10, 1355–1365.
6984 J. Org. Chem. Vol. 75, No. 20, 2010

Brahmi et al.
JOCNote
TABLE 3. Access to B-Substituted NHC Boranes
SCHEME 1. Phosphine/NHC Exchange
stable, readily available reactants;an heterocyclic salt and
an amine or phosphine-borane.13 NHCs proved to be
among the most tightly associated neutral ligands to boron.
Thus, Lewis base exchange at boron provides an easy access
to new substituted complexes, which should help in learning
more about the reactivity of the title compounds.
Experimental Section
General Procedure for the Synthesis of NHC Borane Com-
plexes from Isolable NHCs. Trimethylamine-borane (7 mmol,
1.1 equiv) was added to a solution of IPr (250 mg, 6.43 mmol) in
benzene (5 mL). The reaction mixture was refluxed overnight,
and concentrated in vacuo. The crude product was purified by
flash chromatography.
General Procedure for the Synthesis of NHC Borane Com-
plexes from in Situ Generated NHC (Method A). NaHMDS (1 M
in THF, 1.2 equiv) was added to a suspension of imidazolium
salt (250 mg, 1 equiv) in dry THF (5 mL) at -78 °C. The result-
ing mixture was stirred for 1 h at -78 °C then warmed to rt.
Trimethylamine-borane (1.1 equiv) was added and the result-
ing solution was refluxed overnight and concentrated in vacuo.
The crude product was purified by flash chromatography.
aMethod A: See Table 2. Method C: Borane-amine complex (1.1
equiv), benzene, reflux overnight.
On the other hand, the reaction with tricyclohexylphos-
phine stopped at around 30% conversion after 4.5 days. The
reverse reaction (PCy3 on 2) did not progress at all, so the
former poor conversion cannot be explained by an equilib-
rium reached in a reversible reaction. Nonetheless the ex-
change methodology might be another way to decomplex
specific phosphine-borane complexes.
Acknowledgment. This work was supported by grants
from the U.S. National Science Foundation (CHE-
0645998), UPMC, CNRS, and the French Agence Nationale
de la Recherche (ANR BLAN0309 Radicaux Verts, and
08-CEXC-011-01 Borane). We thank FR 2769 for technical
assistance (NMR, MS).
To conclude, we have introduced a versatile new method
for the preparation of NHC boranes starting from two
(13) An alternative way to NHC boranes has been proposed by Braunsch-
weig while this manuscript was under review. It involves carboxyl-
ate derivatives of NHCs as the NHC source, but uses borane-dimethyl
sulfide as the boron source. See: Bissinger, P.; Braunschweig, H.; Kupfer, T.;
Radacki, K. Organometallics 2010, 29, 3987-3990. This paper introduces
NHC boranes as ligands for transition metals.
Supporting Information Available: Procedures and charac-
terization of all new compounds, copies of spectra of products,
and CIF files of the crystal structures. This material is available
free of charge via the Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 75, No. 20, 2010 6985