Combinations of ethers and B(C6F5)3 function as hydrogenation catalysts

Article abstract of DOI:10.1002/anie.201303166

It works ether way: Labile adducts of dialkyl ethers with the electrophilic borane B(C₆F₅)₃ are shown to scramble HD to H₂ and D₂ and catalyze the hydrogenation of 1,1-diphenylethylene. Copyright

Full text of DOI:10.1002/anie.201303166

Angewandte
Chemie
Communications
Frustrated Lewis Pairs
It works ether way: Labile adducts of
dialkyl ethers with the electrophilic
L. J. Hounjet, C. Bannwarth, C. N. Garon,
C. B. Caputo, S. Grimme,*
borane B(C F ) are shown to scramble
6
5 3
HD to H and D and catalyze the hydro-
2
2
D. W. Stephan*
&&&& — &&&&
genation of 1,1-diphenylethylene.
Combinations of Ethers and B(C F )
5 3
6
Function as Hydrogenation Catalysts
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DOI: 10.1002/anie.201303166
Frustrated Lewis Pairs
Combinations of Ethers and B(C F ) Function as Hydrogenation
6
5 3
Catalysts**
Lindsay J. Hounjet, Christoph Bannwarth, Christian N. Garon, Christopher B. Caputo,
Stefan Grimme,* and Douglas W. Stephan*
Since its discovery in the 1960s, the electrophilic borane
B(C F ) to effect H activation, thereby affording a remark-
6 5 3 2
[1,2]
B(C F )
has been used extensively in catalysis. Perhaps
best-known as a co-catalyst or activator for ethylene poly-
ably simple and inexpensive hydrogenation catalyst. Indeed,
herein we describe such reactivity. Labile dialkyl ether/
B(C F ) combinations are shown to effect H activation to
6
5 3
[3–7]
merization,
this highly electrophilic species has also found
6
5
3
2
applications within a variety of Lewis acid-catalyzed trans-
formations. For example, in the early 2000s, Piers, Gevorgyan,
and co-workers pioneered its use in hydrosilylation chemis-
catalyze hydrogenations of 1,1-diphenylethylene and anthra-
cene.
The combination of B(C F ) with 2 equiv of Et O in
6
5
3
2
[8–13]
try.
hydrostannylation,
production and derivatization.
Since then, others have developed applications in
CD Cl exhibits two distinct chemical exchange processes by
2 2
[
14,15]
1
silane dehydrocoupling, and silicone
H NMR spectroscopy at reduced temperatures. Analysis of
[
16–25]
[39]
More recently, this elec-
a 2:1 mixture of Et O/B(C F ) between 25 and À908C
2
6
5 3
trophile has been exploited in “frustrated Lewis pair” (FLP)
chemistry, acting as the Lewis acid partner in conjunction with
bases to activate a wide variety of small molecules, including
reveals coalescing Et O methylene signals at À558C that are
2
consistent with lone pair inversion at oxygen in the adduct
°
À1
Et OÀB(C F ) of DG = 10.3 kcalmol . A similar coales-
2
6 5 3
[
26]
[27]
[28,29]
[30]
H , CO , olefins, alkynes, N O,
and NO, among
cence is also observed at À308C, corresponding to the rapid
2
2
31]
2
[
°
others. The application of this borane in FLP or “metal-
exchange of free and coordinated Et O with a DG =
2
[
32]
À1 [40]
free” hydrogenation catalysis has drawn much attention.
Initially, the substrates were limited to imines, protected
nitriles, and aziridines. Subsequently, enamines and silylenol
ethers were also shown to be viable substrates for FLP
reductions. More recently, combinations of B(C F ) with
10.5 kcalmol . In a subsequent experiment, a 1:1 mixture
of Et O/B(C F ) in CD Cl (0.14m) was exposed to 4 atm of
2
6
5
3
2
2
1
HD gas in a J-Young tube at ambient temperature. H NMR
analysis of the mixture after 15 min showed equal intensity
signals for H and HD. Integrations were also consistent with
6
5
3
2
weakly basic triaryl phosphines or triaryl amines were shown
the catalytic isotope equilibration of HD to a 2:1:1 statistical
mixture of HD/H /D (Figure 1B). It is noteworthy that such
[
33]
to be capable of catalyzing hydrogenations of olefins and
2
2
[
34]
polyaromatic systems. B(C F ) was then shown to effect
equilibration was not observed employing solutions of B-
(C F ) alone (Figure 1A).
6
5 3
[
35]
reductions of N-substituted anilines,
pyridines, and N-
6
5 3
[
36]
[37]
heteroatomic species. At the same time, Nikonov et al.
provided experimental and theoretical evidence that B(C F )
5 3
6
alone does not activate H . Interestingly, however, these
2
[
38]
authors and others showed that C H OÀBD undergoes H/
4
8
3
D exchange under pressures of H to produce C H OÀBH . In
2
4
8
3
very recent work, we described the reaction of H with the
2
epoxyborate salt, [tBu PH] [(C F ) BCH(C F )OB(C F ) ],
3
6
5
2
6
5
6 5 3
affording the borane–borate salt, [tBu PH][(C F )BCH -
3
6
5
2
[
31]
(
C F )OB(C F ) ]. This observation suggested the possibil-
6 5 6 5 3
ity that simple oxygen donors might also act in concert with
[*] Dr. L. J. Hounjet, Dr. C. N. Garon, C. B. Caputo,
Prof. Dr. D. W. Stephan
Department of Chemistry, University of Toronto
80 St. George Street, Toronto, Ontario, M5S 3H6 (Canada)
E-mail: dstephan@chem.utoronto.ca
Homepage: http://www.chem.utoronto.ca/staff/DSTEPHAN
1
Figure 1. H NMR spectra showing A) HD in the presence of B(C F )
5 3
6
after 60 min and B) complete isotope scrambling of HD by 1:1 Et O/
B(C F ) after 15 min in CD Cl .
2
C. Bannwarth, Prof. Dr. S. Grimme
6
5
3
2
2
Mulliken Center for Theoretical Chemistry, Institut fꢀr Physikalische
und Theoretische Chemie, Universitꢁt Bonn
Beringstrasse 4, 53115 Bonn (Germany)
These observations clearly indicate the ability of Et OÀ
2
B(C F ) to activate dihydrogen, thus suggesting the ability of
6
5 3
[
**] D.W.S. gratefully acknowledges financial support of the NSERC of
Canada and the award of a Canada Research Chair, and S.G.
acknowledges support from the DFG (FOR1175).
this species to act as a hydrogenation catalyst. To assess this
inference, a 0.4m solution of 1,1-diphenylethylene containing
2
0 mol% B(C F ) and 30 mol% ether in CD Cl was
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303166.
6
5
3
2
2
pressurized with 4 atm of H . Heating to 508C for 48 h
2
2
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also ineffective. These observations are consistent with the
lower Brønsted basicity of terminal olefins and suggest the
formation of a relatively stable tertiary carbocation by
protonation is required for reduction to proceed. In the case
of a-methylstyrene, p-methoxy-a-methylstyrene, and a,p-
dimethylstyrene, the reactions proceed to more than 99%
consumption of the olefin; however, the major products are
the Friedel–Crafts dimers with a minor product being the
hydrogenated alkane, demonstrating that Lewis acid cata-
lyzed dimerization is kinetically favored over hydrogenation.
The mechanism of hydrogenation (Scheme 2) is presum-
ably analogous to that reported for FLP hydrogenations of
olefins by phosphine/B(C F ) combinations in which hetero-
Scheme 1. Reaction of 1,1-diphenylethylene in presence of B(C F )
5 3
with (right) and without (left) diethyl ether.
6
resulted in 95% conversion of the olefin to the alkane,
Ph C(H)CH3 (Scheme 1; Table 1). Interestingly, the same
2
reaction carried out in C D Br at 1508C resulted in slower
6
5
hydrogenation of the olefin, although increasing the concen-
6
5 3
[
a]
Table 1: Ether/B(C F ) -catalyzed hydrogenation.
6
5 3
Substrate
B(C F )
5 3
Ether
(mol%)
T
[8C]
t
[h]
Conv.
[%]
6
[
mol%]
[
f]
Ph C=CH2
20
20
20
20
10
10
20
20
20
20
10
20
–
20
50
150
120
50
20
25
50
25
24
48
24
18
96
24
48
72
48
72
24
24
65
95
35
60
96
2
Ph C=CH2
Et O (30)
Et O (30)
O (160)
Et O (30)
Et O (20)
12C4 (20)
12C4 (20)
2
2
[
[
c]
c]
Ph C=CH2
2
2
Ph
2
C=CH
2
Et
2
Ph C=CH2
2
2
[
b]
Ph C=CH2
>99
56
>99
57
>99
34
2
2
Ph C=CH2
Scheme 2. Proposed mechanism of the hydrogenation of 1,1-diphenyl-
ethylene by Et O/B(C . (The addition and removal of Et O mole-
cules is not depicted.)
2
Ph C=CH2
F
6
)
5
2
2
3
2
Ph C=CH2
DB24C8 (20)
DB24C8 (20)
Et O (20)
DB24C8 (20)
2
Ph C=CH
50
20
80
2
2
[
b]
C H
1
4
10
[
2
b,d]
C H
59
1
4
10
lytic cleavage of hydrogen allows for protonation of the olefin
and hydride delivery to the resulting carbocation. Compar-
ing the experimental rates of H/D-exchange and hydrogena-
[
a] 0.4m Solutions of substrate in CD Cl , 4 atm of H . [b] 0.1m
[33]
2
2
2
Substrate, 100 atm of H . [c] Solvent C D Br. [d] Solvent C H Cl .
[
product: olefin dimer.
2
6
5
2
4
2
e] Conversion [%] calculated with respect to substrate. [f] Major
tion demonstrates that the activation of H , and thus the
2
equilibrium concentration of the ion pair, [Et O···H···OEt ]
2
2
[
HB(C F ) ], is not rate-determining for the olefin reduction.
6 5 3
tration of Et O to 160 mol% afforded faster and higher
The somewhat accelerated conversions observed when phos-
phines were employed as bases could be attributed to the
simultaneous use of a J-Young NMR tube rotation technique
2
conversion. Increasing the pressure of H also enhances the
2
reduction rate. Using 10 mol% B(C F ) and 20 mol% Et O
6
5
3
2
under 100 atm of H pressure at 208C, the reduction of Ph C= (that we did not employ), which was found to increase the
2
2
[33]
CH2 is essentially quantitative in 24 h, whereas at 4 atm,
effective concentration of H in solution.
2
heating to 508C for 96 h is required for complete reduction.
It is indeed surprising that ether and borane act as an FLP
Although somewhat less effective than Et O, the crown
to effect the activation of H with such ease. To understand
2
2
ethers [12]crown-4 (12C4) or dibenzo[24]crown-8 (DB24C8)
the mechanism of this remarkably facile H activation process,
2
also enabled the hydrogenation of Ph C=CH in the presence
a DFT study using state-of-the-art quantum chemical meth-
2
2
[
42–46]
of B(C F ) . Anthracene could also be hydrogenated to 9,10-
ods was conducted
(see the Supporting Information for
6
5 3
dihydroanthracene using DB24C8/B(C F ) in up to about
details). First, gas-phase structure optimizations at the
dispersion-corrected DFT level, using large triple-zeta AO
basis sets (TPSS-D3/def2-TZVP), followed by single-point
energy calculations at the high double-hybrid level (B2PLYP-
D3/def2-QZVP), were performed. On top of that, thermo-
statistical corrections from ZPVE-exclusive energy (DE) to
free energy (at T= 298.15 K, 1 atm pressure) and corrections
for solvation free energy by the accurate (DFT-based)
6
5 3
6
0% yield, although more forcing conditions (heating to 808C
under 100 atm of H in 1,2-dichloroethane) were required.
2
Using (Me Si) O or Ph O instead of Et O only resulted in the
3
2
2
2
previously reported Friedel–Crafts-type dimer of the olefin
[
41]
(
Scheme 1). It is also noted that this same dimer is formed
on treatment of Ph C=CH with 20 mol% B(C F ) alone,
2
2
6
5 3
under H . Thus, it appears that the decreased basicity of
2
[47–50]
(
Me Si) O or Ph O preclude H activation, thus permitting the
COSMO-RS model were applied.
Various H activated
2
3
2
2
2
dimerization pathway to prevail in the absence of a donor. An
attempt to hydrogenate a series of olefins, including cis-
stilbene, 1-hexene, cyclohexene, methylenecyclohexane, and
tert-butylethylene as well as 1,2-diphenylacetylene using
Et O/B(C F ) gave no reduction after 24 h. Efforts to
structures were investigated and the most likely candidates
selected (Figure 2). Consideration of all corrections from gas-
phase energy to DG in solution (DG ) is essential for an
R
[
51,52]
understanding of the mechanism. Previous results
have
À1
showed that the barrier to H -splitting is only a few kcalmol
2
6
5
3
2
reductively open cis/trans-1,2-diphenylcyclopropane was
higher than the zwitterionic intermediate. Thus, only the
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with an accessible barrier to the “encounter complex” in
which H is activated. Subsequent dissociation of this complex
2
+
À
affords the ions [Et O···H···OEt ] and [HB(C F ) ] in
2
2
6
5 3
sufficient concentration to effect hydrogenation. The syn-
thetic utility and variants of this remarkably simple hydro-
genation catalyst are the subject of on-going efforts.
Received: April 16, 2013
Published online: && &&, &&&&
Keywords: boranes · ethers · frustrated Lewis pairs ·
.
hydrogenation · organocatalysis
Figure 2. (Free) energy diagram with DFT-D3 optimized structures for
hydrogen activation by B(C F ) and Et O (n=1, 2) leading to the ions
6
À
5
3
2
+
À1
À1
[
Et OH···OEt ] [HB(C F ) ] (DG (kcalmol ): black; DE (kcalmol ):
2 2 6 5 3 R
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2
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R
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www.angewandte.org
5
These are not the final page numbers!