.
Angewandte
Communications
Olefin Hydrogenation
Electrophilic Fluorophosphonium Cations in Frustrated Lewis Pair
Hydrogen Activation and Catalytic Hydrogenation of Olefins**
Thorsten vom Stein, Manuel PerØz, Roman Dobrovetsky, Daniel Winkelhaus,
Christopher B. Caputo, and Douglas W. Stephan*
[
17]
Abstract: The combination of phosphorus(V)-based Lewis
acids with diaryl amines and diaryl silylamines promotes
reversible activation of dihydrogen and can be further
exploited in metal-free catalytic olefin hydrogenation. Com-
bined experimental and density functional theory (DFT)
studies suggest a frustrated Lewis pair type activation mech-
anism.
phosphonium centers received lesser attention
although
Gabbaï and co-workers have used phosphonium centers for
[18]
enhanced fluoride ion sensing. On the other hand, we have
developed highly electrophilic phosphonium cations (EPCs).
These latter compounds proved to be highly Lewis acidic.
This arises from a low lying s* orbital, whereas classical
Group 13 Lewis acids derive their Lewis acidity from a vacant
p orbital. In an initial study we showed that an electrophilic P
center can be used in CO capture, in manner analogous to
that seen for FLPs. Utilizing this acidity we have also shown
that EPCs are more Lewis acidic than B(C F ) and effective
catalysts for hydrodefluorination of fluoralkanes,
imines, and ketones
dehydocoupling of silanes with carboxylic acids, alcohols,
Furthermore, concurrent catalytic
hydrogenation was achieved upon addition of olefins to
In these studies of
catalysis by the EPC [FP(C F ) ][B(C F ) ] 1, DFT computa-
[19]
T
he catalytic hydrogenation of unsaturated functionalities is
2
one of the most important fundamental tools in modern
[1]
chemical synthesis. While the field is still largely dominated
by metal-based catalytic systems, the discovery of reversible
hydrogen activation by frustrated Lewis pairs (FLPs) in 2006
6
5 3
[
20]
hydro-
[
21]
[22]
silylation of olefins, alkynes,
and
[2]
has paved the way for metal-free catalytic hydrogenation.
[23]
Over the past nine years, a number of FLP catalysts have
emerged for an increasing scope of substrates. To date, FLP
hydrogenation has been used to reduce unsaturated function-
thiols, and amines.
[23]
these dehydrocoupling reactions.
[
3]
[3]
[4]
alities, including imines, aziridines, enamines, silyl enol
6
5
3
6 5 4
[5]
[6]
[7]
[8]
ethers,
olefins,
alkynes,
polyaromatics,
and most
tions inferred intermediates including hydridophosphorane
[(C F ) PFH] and [Ph N(H)SiEt ] . We recognized that these
intermediates could also be generated by the activation of
hydrogen between 1 and Ph NSiEt . Herein, we demonstrate
that Lewis acidity of EPCs in combination with sterically
encumbered aryl-substituted amines can be exploited as FLPs
[
9]
[10]
+
recently ketones and aldehydes. The catalysts used for
these reductions have involved boron-based Lewis acids.
While the majority of studies employ highly electrophilic
6
5
3
2
3
2
3
[11]
boranes such as B(C F ) or related derivatives, a recent
6
5 3
development has been the use of carbene-stabilized bore-
[12]
nium-based catalysts. Another innovation exploits hydro-
boration of chiral-derived olefins by Piersꢀ borane HB-
for H activation and hydrogenation catalysis.
2
In an initial experiment, an equimolar mixture of p-
Tol NH and 1 was heated under an HD atmosphere. After
24 h at 1008C the formation of H was observed, indicating
reversible hydrogen activation. HD scrambling was subse-
quently shown to proceed at lower temperatures (608C)
albeit at lower rates of reaction (see the Supporting Informa-
tion, Section S3). These observations suggest that the combi-
[
13]
(
C F ) , affording a highly efficient catalyst for asymmetric
6
5
2
2
[14]
hydrogenations. In terms of the basic component of the
FLP, a variety of sterically demanding phosphines or amine
species are most common, although this has been extended to
include electronic deficient phosphines, as well as
2
[
6b, 9,15]
ethers.
Despite the apparent limitation of FLP hydrogenations to
largely boron-based Lewis acids, a growing range of Lewis
acids including Al, C, Si as well as Ti and Zr derivatives have
nation of 1 and p-Tol NH acts as an FLP to effect the
heterolytic activation of H2.
2
To probe the interactions of 1 and p-Tol NH, stoichio-
2
[15,16]
been investigated in FLP chemistry.
The Lewis acidity of
metric combination of the Lewis acid and base were
1
monitored by H NMR spectroscopy. The normally sharp
[
+]
[+]
[
*] Dr. T. vom Stein, Dr. M. PerØz, Dr. R. Dobrovetsky,
Dr. D. Winkelhaus, Dr. C. B. Caputo, Prof. Dr. D. W. Stephan
Department of Chemistry, University of Toronto
resonances for the amine are broadened dramatically. A very
broad resonance is observed at 7.10 ppm while the resonances
attributable to the p-Me fragment is not seen. A low-
temperature NMR study was also undertaken, however
8
0 St. George St, Toronto Ontario M5S3H6 (Canada)
E-mail: dstephan@chem.utoronto.ca
[
FP(C F ) Ph][B(C F ) ] 2 was used for solubility reasons.
+
6 5 2 6 5 4
[
] These authors contributed equally to this work.
At À908C the peaks arising from the p-Tol NH began to
2
[
**] The authors acknowledge NSERC of Canada for continuing support
of this research and D.W.S. is grateful for the award of a Canada
Research Chair. T.v.S. thanks the Alexander von Humboldt Foun-
dation for a Feodor von Lynen Research Fellowship. C.B.C. gratefully
acknowledges the receipt of an NSERC of Canada CGS-D Award.
resolve (Supporting Information, Section S3), although a lim-
iting spectrum was observed. These experimental data sup-
port the postulate of the generation of an encounter complex
analogous to that computed for the Lewis acid–base combi-
[24]
nation of tBu P and B(C F ) .
3
6 5 3
1
0178
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 10178 –10182