.
Angewandte
Communications
Lewis Pairs
À
Elusive Silane–Alane Complex [Si H···Al]: Isolation,
Characterization, and Multifaceted Frustrated Lewis Pair Type
Catalysis**
Jiawei Chen and Eugene Y.-X. Chen*
Abstract: The super acidity of the unsolvated Al(C F )
with B(C F ) proved futile and only indirect spectroscopic
6 5 3
6
5
3
enabled isolation of the elusive silane–alane complex [SiÀ clues pointing to this intermediate could be obtained. Almost
H···Al], which was structurally characterized by spectroscopic
and X-ray diffraction methods. The Janus-like nature of this
adduct, coupled with strong silane activation, effects multi-
faceted frustrated-Lewis-pair-type catalysis. When compared
with the silane–borane system, the silane–alane system offers
unique features or clear advantages in the four types of
catalytic transformations examined in this study, including:
ligand redistribution of tertiary silanes into secondary and
quaternary silanes, polymerization of conjugated polar
alkenes, hydrosilylation of unactivated alkenes, and hydro-
defluorination of fluoroalkanes.
20 years after the original report, Piers and co-workers
successfully isolated and structurally characterized the
silane–borane adduct derived from Et SiH and 1,2,3-tris(pen-
3
tafluorophenyl)-4,5,6,7-tetrafluoro-1-boraindene, aperfluoro-
borole sophisticatedly tailored for higher Lewis acidity than
[
7]
B(C F ) .
6
5 3
As compared with B(C F ) , the congener alane Al(C F )
5 3
6
5
3
6
[
8]
is a stronger LA as gauged, for example, by double
activation of bridged metallocene dimethyls, fluoride ion
stable adduct formation with weakly basic
arenes, as well as by DFT calculations on the gas-phase
and the enthalpy of ion-pair formation in
solution for the methide abstraction reaction. Despite its
high Lewis acidity, the application of Al(C F ) in the area of
[
9]
[
10]
affinity,
[
11]
[12]
Lewis acidity
[
13]
H
ighly Lewis-acidic and chemically robust organoboranes,
especially B(C F ) , have proven their broad applications in
6
5
3
6
5 3
[
14]
catalysis for small-molecule transformation and macromolec-
FLP studies is much less explored. In 2002, we reported
[1]
ular synthesis. Such boranes continue to receive much
attention because of their recent success in frustrated Lewis
pair (FLP) chemistry which was pioneered by Stephan and
cleavage of a toluene CÀH bond with the Al(C F ) and 2,6-
6
5 3
[
15]
di-tert-butylpyridine pair. Subsequently, we and others have
[
16]
showed that the Lewis pair polymerization
is typically
[
2]
[17]
Erker. While earlier contributions emphasized the impor-
tance of orthogonal reactivity derived from sterically induced
separation of the Lewis pairs, accumulated evidence suggests
that electronically FLPs possessing a weak Lewis acid–Lewis
much more effective with the alane than with the borane.
Moreover, Krossing and co-workers noted that the analogous
aluminum super LA [Al{OC(CF ) } ] forms stable adducts
3
3 3
[
10]
with PhF and Me SiF. Interestingly, the fluorosilane adduct
3
[
3]
base (LA–LB) bond can be effective as well. Indeed,
[Me SiÀF···Al{OC(CF ) } ] was viewed as a Janus-like bifunc-
3
3 3 3
[
4]
a prototype can be traced back to 1996 when Parks and Piers
reported the first example of the B(C F ) -catalyzed hydro-
tional LA with a soft electrophilic Si site and a hard electro-
[
18]
philic Al site for different substrates.
6
5 3
silylation of C=O bonds with hydrosilanes by unusual SiÀH
We hypothesized that, as a result of the demonstrated
superior Lewis acidity and activity in many catalytic reactions
by the alane compared to the borane, Al(C F ) could lead to
[5]
bond activation rather than intuitive carbonyl activation.
Such a reaction was proposed to proceed through the
6
5 3
cleavage of the SiÀH bond with a dissociating carbonyl–
the isolable and characterizable simple silane–alane complex
borane Lewis pair (i.e., FLP-type bond activation). Oestreich
and co-workers further supported this hypothesis by proving
the inversion of the chirality at Si of a chiral probe after the
[R SiÀH···Al(C F ) ], and thus uncover its potentially unique
3
6 5 3
catalytic utilities. In this context, we communicate herein the
isolation and structural characterization of silane–alane
complexes with a hydride or fluoride bridge. Excitingly, the
silane–alane complex effectively catalyzes (or is involved in)
a variety of transformations, including four different types of
catalytic reactions: ligand redistribution of silanes, polymer-
ization of polar alkenes, hydrosilylation of unactivated
alkenes, and hydrodefluorination of fluoroalkanes.
[
6]
hydrosilylation step. Since then, much effort has been
invested in the direct spectroscopic and structural character-
izations of the proposed, yet elusive, silane–borane complex
containing the crucial [SiÀH···B] moiety, but the attempts
[
19]
[
*] Dr. J. Chen, Prof. Dr. E. Y.-X. Chen
Department of Chemistry, Colorado State University
Fort Collins, CO 80523-1872 (USA)
Mixing of Et SiH with [Al(C F ) (toluene) ] failed to
3
6
5
3
0.5
generate the silane–alane adduct because arenes such as
toluene are stronger donors than Et SiH, as shown for the
3
E-mail: Eugene.Chen@colostate.edu
[20]
isoelectronic silylium ions. Hence, it is crucial to use the
[
**] This work was supported by the United States National Science
Foundation (CHE-1150792). We thank Dr. Roger A. Lalancette for
the generous access to the SC-XRD facility at Rutgers-Newark and
Boulder Scientific Co. for the research gift of B(C F ) .
[19]
unsolvated Al(C F )
5 3
and avoid donor or even aromatic
6
solvents for the generation of the desired silane–alane
complex. Accordingly, addition of a slight excess of Et SiH
6
5
3
3
to a suspension of the unsolvated Al(C F ) in hexanes led to
6 5 3
immediate dissolution of Al(C F ) (the alane itself is
6
5 3
6
842
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 6842 –6846