Angewandte
Chemie
DOI: 10.1002/anie.200904520
Lewis Acid Catalysis
Taming the Silylium Ion for Low-Temperature Diels–Alder
Reactions**
Hendrik F. T. Klare, Klaus Bergander, and Martin Oestreich*
The silylium ion, a trivalent silicon cation, has a varied history
regarding its isolation and characterization in its free form in
the condensed phase.[1,2] An electron sextet at the silicon atom
renders these electron-deficient compounds extremely potent
Lewis acids, so strong that even weak Lewis bases (solvent
molecules), which are normally considered inert, will coor-
dinate.[3] Both steric shielding of the empty orbital at the
silicon atom,[4,5] and the clever design of weakly coordinating
anions[6,7] allowed the structural determination of a silylium
ion.[8] Aside from these elementary insights, an equally
fundamental question is posed: Are such reactive intermedi-
ates useful to synthetic chemistry? Yes, Ozerov and co-
workers[9] and likewise Mꢀller and co-workers[10] had identi-
3
ꢀ
Scheme 1. Fast generation of the silylium ion (1!2) and its slow
fied silylium ions to be catalytically active in C(sp ) F bond
activation.[11] Conversely, the self-evident use of silylium ions
in Lewis acid catalysis is elusive,[12–16] presumably because of
the irreversible formation of Lewis pairs.
decomposition through chloride-abstraction from CD2Cl2 (2!3).
Inspired by a seminal attempt by Corey et al.[17] and a
recent contribution by Manners and co-workers,[18] we set out
to employ an electron-rich late transition metal for intra-
molecular stabilization of a trivalent silicon cation; our goal
was realized by the ferrocene-based 2 (see the Supporting
Information for straightforward preparation of its precursor
1), which is decorated with both a small and a large group
(Scheme 1). Whereas the iron–silicon interaction attenuates
Lewis acidity, the deliberate choice of the substituents at the
silicon atom also seems to be vital.[19] These parameters bring
about the perfect balance of inherent Lewis acidity and steric
accessibility to the silicon atom, thereby securing reversible
coordination of Lewis basic functional groups essential for
catalytic turnover.
Lewis acid 2 is formed quantitatively at ꢀ788C within
Figure 1. 1H NMR spectrum (500 MHz) of 2 recorded in CD2Cl2 at
ꢀ408C.
seconds by conventional Bartlett–Condon–Schneider silicon-
to-carbon hydride transfer[20] (1!2, Scheme 1 and Figure 1),
[6a,21]
displaying remarkable chemical stability in CD2Cl2
at a
synthetically practical temperature window ranging from
ꢀ788C to ꢀ308C. Its formation by rapid hydride abstraction
(1!2), and decomposition by chloride abstraction from
CD2Cl2 (2!3) both proceed exceptionally cleanly, as moni-
tored by 1H NMR spectroscopy (see the Supporting Informa-
[*] H. F. T. Klare, Dr. K. Bergander,[+] Prof. Dr. M. Oestreich
Organisch-Chemisches Institut
1
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Corrensstrasse 40, 48149 Mꢁnster (Germany)
Fax: (+49)251-83-36501
E-mail: martin.oestreich@uni-muenster.de
tion for variable temperature H NMR spectroscopy). The
structure of 3 was detected by GC/MS measurements and
unambiguously assigned by comparison with an authentic
sample. We were pleased to see that the 29Si NMR spectrum
of 2 showed a diagnostic signal at d = 114.5 ppm, a significant
downfield shift (Figure 2). In comparison with the reported
29Si NMR chemical shifts (Figure 3),[3–6,10,18] the observed
value indicates to us that the iron-stabilized silylium ion is not
“quenched” by a solvent molecule and is therefore still a
reasonably strong Lewis acid.
[+] NMR spectroscopic measurements
[**] H.F.T.K. thanks the International Research Training Group Mꢁnster-
Nagoya (GRK 1143 of the Deutsche Forschungsgemeinschaft) for a
predoctoral fellowship (2007–2010).
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2009, 48, 9077 –9079
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9077