Tuning Catalytic Activity in the Hydrogenation of Unactivated Olefins with Transition-Metal Oxos as the Lewis Base Component of Frustrated Lewis Pairs

Article abstract of DOI:10.1021/acscatal.6b03313

The steric and electronic demands of the catalytic olefin hydrogenation of tert-butylethylene with oxorhenium/Lewis acid FLPs were evaluated. The sterics of the ligand were altered by installing bulkier isopropyl groups in the 2,6-positions of the diamidopyridine (DAP) ligand. Lewis acid/base adducts were not isolated for complexes with this ligand; however, species incorporating isopropyl groups were still active in catalytic hydrogenation. Modifications were also made to the Lewis acid, and catalytic reactions were performed with Piers' borane, HB(C₆F₅)₂, and the aluminum analogue Al(C₆F₅)₃. The rate of catalytic hydrogenation was shown to strongly correlate with the size of the alkyl, aryl, or hydride ligand. This was confirmed by a linear Taft plot with the steric sensitivity factor δ = -0.57, which suggests that reaction rates are faster with sterically larger X substituents. These data were used to develop a catalyst ((MesDAP)Re(O)(Ph)/HB(C₆F₅)₂) that achieved a TON of 840 for the hydrogenation of tert-butylethylene at mild temperatures (100 °C) and pressures (50 psi of H₂). Tuning of the oxorhenium catalysts also resulted in the hydrogenation of tert-butylethylene at room temperature.

Full text of DOI:10.1021/acscatal.6b03313

Research Article
pubs.acs.org/acscatalysis
Tuning Catalytic Activity in the Hydrogenation of Unactivated
Olefins with Transition-Metal Oxos as the Lewis Base Component of
Frustrated Lewis Pairs
Nikola S. Lambic, Roger D. Sommer, and Elon A. Ison*
Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-8204, United
States
S
* Supporting Information
ABSTRACT: The steric and electronic demands of the catalytic olefin hydrogenation
of tert-butylethylene with oxorhenium/Lewis acid FLPs were evaluated. The sterics of
the ligand were altered by installing bulkier isopropyl groups in the 2,6-positions of the
diamidopyridine (DAP) ligand. Lewis acid/base adducts were not isolated for
complexes with this ligand; however, species incorporating isopropyl groups were still
active in catalytic hydrogenation. Modifications were also made to the Lewis acid, and
catalytic reactions were performed with Piers’ borane, HB(C6F5)2, and the aluminum
analogue Al(C6F5)3. The rate of catalytic hydrogenation was shown to strongly
correlate with the size of the alkyl, aryl, or hydride ligand. This was confirmed by a
linear Taft plot with the steric sensitivity factor δ = −0.57, which suggests that reaction
rates are faster with sterically larger X substituents. These data were used to develop a
catalyst ((MesDAP)Re(O)(Ph)/HB(C6F5)2) that achieved a TON of 840 for the
hydrogenation of tert-butylethylene at mild temperatures (100 °C) and pressures (50
psi of H2). Tuning of the oxorhenium catalysts also resulted in the hydrogenation of
tert-butylethylene at room temperature.
KEYWORDS: frustrated Lewis pairs, transition metal oxos, hydrogenation, olefins, catalytic tuning
Scheme 1. Proposed Mechanism for FLP (Phosphorus/
Borane) Catalyzed Hydrogenation
INTRODUCTION
■
Since its discovery a decade ago, the chemistry of frustrated
Lewis pairs (FLPs) has emerged as a powerful method for
metal-free H2 activation and has been utilized in a variety of
catalytic reactions.1 The most common catalytic reaction
reported for FLPs is hydrogenation; as a result, many catalytic
hydrogenations of unsaturated substrates, such as imi-
nes,1d,h,i,l−n,p,q,2 silyl enol ethers,1i,m,3 activated olefins,1c,i,2b,4
and carbonyl-containing compounds2d,5 have been described in
the literature. However, the FLP-catalyzed hydrogenation of
simple olefins remains a major challenge.1c,2b,4g,j,o,6
FLP olefin hydrogenation usually features combinations of
boranes as the Lewis acid and main-group Lewis bases such as
amines and phosphines. Catalytic hydrogenation with these
FLPs is believed to proceed via initial heterolytic cleavage of H2
to generate, in the case of phosphorus−boron FLPs, a
hydridoborate/phosphonium salt. This species can protonate
an olefin, to generate a stable cation that can accept a hydride
from HB(C6F5)3, resulting in the generation of the product
(Scheme 1).1c,2b,4g,j,o,6 Electron-rich olefins or olefins that are
highly activated such as 1,1-diphenylethylene are typically
employed to stabilize the carbocation intermediate that results
from protonation. As a result, catalytic hydrogenation with
main-group FLPs are generally not effective with unactivated
olefins.
Our group recently reported an effective FLP where
oxorhenium complexes were featured as the Lewis base
component and B(C6F5)3 was the Lewis acid and has shown
that this system is capable of the hydrogenation of unactivated
olefins such as ethylene, propylene, and 1-hexene.7 The
mechanism for this reaction was elucidated with experimental
and computational data and proceeds by initial syn olefin
addition to the FLP, followed by H2 cleavage (Scheme 2). The
Received: November 21, 2016
Revised: December 5, 2016
Published: December 15, 2016
© XXXX American Chemical Society
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Scheme 2. Summary of the Proposed Mechanism for the
Catalytic Hydrogenation of Olefins with FLPs Generated
from Transition Metal Oxos
Scheme 3. Strategy for Examining the Sterics and Electronics
in Oxorhenium FLPs
RESULTS AND DISCUSSION
■
Effect of Sterics of the Diamidopyridine Ligand.
Synthesis of (DippDAP)Re(O)X Complexes. The DippDAP
ligand (2,6-bis(2,6-bis((2,6-diisopropylphenyl)amino)methyl)-
pyridine) was synthesized by an SN2 substitution on the
tosylated precursor with lithium (2,6-diisopropylphenyl)amide
according to Scheme 4. The synthesized ligand was obtained in
1
62% yield, and the H NMR data were consistent with the
previously reported spectrum.8b
generation of an acidic rhenium hydroxyl as a result of H2
splitting is the driving force for the final proton transfer that
releases the hydrogenated product. Most notably, this new
catalytic system was shown to be effective with low catalyst
loadings (5 mol %) and low hydrogen pressures (50 psi). This
is in contrast to the case for traditional FLPs, which typically
operate at high catalyst loadings (20 mol %) and much higher
H2 pressures (50 bar). Thus, by utilizing a system that involves
initial activation of the olefin followed by H2 activation, it
appears that we have addressed some of the challenges with
traditional FLPs and the hydrogenation of unactivated olefins.
In this paper, we carried out additional experimental studies
on this new FLP system. The modular nature of oxorhenium
Lewis acid/base adducts allows for steric and electronic
modifications on the diamidopyridine ligand, as well as the
Lewis acid. These variations would ultimately lead to further
improvements in the catalytic system and allow for an
understanding of the factors that affect catalytic activity
(Scheme 3).
The steric demands of the diamidopyridine ligand are
examined first by synthesizing a series of complexes that
incorporate the ligand DippDAP (DippDAP = 2,6-bis(2,6-
bis((2,6-diisopropylphenyl)amino)methyl)pyridine)8 and com-
paring their activities with those of the mesityl-substituted
version of the ligand in the original complex.7 Next, we
examined the effect of altering both the steric and electronic
demands of the borane component of the FLP by investigating
reactions with HB(C6F5)2, Piers’ borane.9 Finally, we examine
the electronic effect of varying the central atom in the group III
Lewis acids B(C6F5)3 and Al(C6F5)3.10 The results obtained
from these studies are critical for the rational design of new
catalysts for this new type of FLP.
Scheme 4. Synthesis of DippDAP Ligand
Rhenium complexes bearing the DippDAP ligand were
synthesized utilizing the procedure previously reported for
(MesDAP)Re(O)X type complexes (Scheme 5).11 Complex 1
was successfully used as a precursor for new complexes
incorporating a phenyl ligand (2) and a methyl ligand (3), as
well as a hydride ligand (4), whose synthesis was reported
elsewhere.8a X-ray-quality crystals of complex 2 were obtained
by slow diffusion of pentane into a concentrated methylene
chloride solution of the complex. The thermal ellipsoid plot of
complex 2 is depicted in Figure 1. Bond lengths and angles are
consistent with those in previously reported structures.7,11,12
The ligand environment around the rhenium center in
complexes 1−4 can be easily confirmed by 1H NMR
spectroscopy, since the methylene protons become diaster-
eotopic upon coordination of the ligand, leading to additional
coupling and splitting of the protons oriented syn or anti to the
rhenium oxo bond. Additionally, all complexes have two sets of
1
isopropyl resonances by H NMR spectroscopy, which further
confirms the different chemical environments imposed on these
protons, as a result of ligand coordination. The addition of
Lewis acids such as B(C6F5)3 to 1−4 resulted in the broadening
of peaks in the 19F NMR spectrum, reflecting the increased
steric protection of the rhenium oxo moiety by much bulkier
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Scheme 5. Synthesis of Oxorhenium DippDAP Complexes
Figure 2. Conversions determined by 1H NMR spectroscopy by
integrating the ratio of tert-butyl singlets of the reactant and product.
Conditions: Re-X (X = Ph (1), Me (2), Cl (3), H (4); 0.0046 mmol),
and Lewis acid (0.0092 mmol), olefin (0.092 mmol) in a J. Young
tube.
reaction, while the reaction catalyzed by a hydride complex (4)
exhibited an induction period similar to that for the previously
reported MesDAP complexes.7 The activity of 4 can be
similarly attributed to the additional reactions observed for
MesDAPRe(O)(H), which resulted in the formation of more
active catalysts such as Re alkyls, cyclometalated products, or
pentafluorophenyl Re C6F5 derivatives.7
Significantly, it should be noted that adducts of B(C6F5)3
could not be isolated with complexes incorporating the
DippDAP ligand. Despite this observation, as shown in Figure
2, catalysis proceeded efficiently with these catalysts. This is
consistent with notion that oxorhenium/B(C6F5)3 Lewis acid
base adducts do not lie on the catalytic cycle for hydro-
genation.13 Instead, a loosely bound frustrated Lewis pair is
responsible for catalysis.
Figure 1. Thermal ellipsoid plot for 2 (50% probability ellipsoids).
Selected bond lengths (Å): Re−N1, 1.978(2); Re−N2, 2.077(2); Re−
N3, 1.983(2); Re−O1, 1.684(2); Re−C32, 2.082(3).
Altering the Steric and Electronic Demands of the
Lewis Acid Component of the FLP. Catalytic Hydro-
genation with Piers’ Borane. In order to further investigate the
effect of sterics and electronics on the catalytic hydrogenation
of tert-butylethylene, the substituents at the boron center were
altered. To do this, Piers’ borane HB(C6F5)2, a highly
electrophilic reagent effective in hydroboration of olefins, was
synthesized and utilized as the Lewis acid component of the
FLP.9 Replacement of one C6F5 group with a hydride
substituent would allow for significant tuning of the electronics
while still maintaining most of the necessary steric bulk
required for the proposed FLP reactivity. Piers’ borane was
synthesized according to a modified literature procedure.9
Metal-free olefin hydrogenation was achieved by Wang and
co-workers with Piers’ borane as a catalyst.4k The substrate
scope explored by this group is similar to that described in our
previous study;7 however, the conditions required to hydro-
genate many substrates were harsh (6 bar of H2, 120 °C, up to
72 h with 20 mol % of catalyst). The mechanism proposed
involves hydroboration of the substrate, followed by H2
cleavage via σ-bond metathesis. Given that HB(C6F5)2 can
serve as a catalyst, we carried out control studies with Piers’
borane to determine whether or not olefins can be hydro-
genated under the conditions described in eq 1 without the
oxorhenium catalyst.
isopropyl substituents on the amide ligand. However, unlike the
case for the analogous MesDAP complexes, adducts of Lewis
acids could not be isolated with the DippDAP ligand.
Hydrogenation of tert-Butylethylene using Catalysts 1−4.
In order to explore the effect of ligand sterics on the rates of
hydrogenation of our model substrate tert-butylethylene, kinetic
data were obtained and compared to those of the previously
reported (MesDAP)Re(O)X complexes.7 Complexes 1−4 were
tested as catalysts under an H2 atmosphere according to eq 1.
Data for catalysts 1−4 are outlined in Figure 2 and, in
general, are similar to the results reported for the MesDAP
ligand. Catalyst 2 (Re−Ph) was again shown to be the most
active catalyst (Figure 2). The observed rate constant, kobs, was
found to be 4.6 × 10−3 min−1, which is slightly lower than the
kobs value (7.5 × 10−3 min−1) for the MesDAP ligand.7
Complexes bearing small, sterically unhindered X-type ligands
such as methyl (2) and chloride (1) were poor catalysts in the
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These control experiments revealed that olefin hydro-
genation does not occur with Piers’ borane under the optimized
conditions employed here. However, when a variety of
oxorhenium complexes were added, hydrogenation proceeded
at overall improved rates for all complexes in comparison to the
analogous reaction with B(C6F5)3 as the Lewis acid component
(eq 2).7 The conditions were optimized to equimolar ratios of
Scheme 6. Proposed Mechanism for the Catalytic
Hydrogenation with Piers’ Borane as the Lewis Acid
Component
rhenium and borane, respectively, yielding overall better
conversions over time periods shorter than those for the
corresponding reactions with B(C6F5)3 (Figure 3).
The kinetics of the catalytic hydrogenation of tert-butyl-
ethylene with the adduct (C6F5)2(3,3-dimethylbutyl)B·(O)Re-
1
(MesDAP)(Ph) employed as a catalyst were examined by H
NMR spectroscopy (see the Supporting Information for
details). The observed rate constant for this reaction, kobs
=
2.1(4) × 10−3 min−1, was approximately 7 times slower than
the rate constant obtained when Piers’ borane was employed as
the Lewis acid component, 1.5(2) × 10−2 min−1. While these
results do not preclude the involvement of (C6F5)2(3,3-
dimethylbutyl)B·(O)Re(MesDAP)(Ph) as a catalyst, the data
suggest that this species is not the primary component
responsible for catalysis, as the catalytic rate with this species
should be comparable to the rates obtained when Piers’ borane
was directly employed.
Figure 3. Conversions determined by 1H NMR spectroscopy by
integrating the ratio of tert-butyl singlets of the reactant and product.
Conditions: Re-Ph (0.031 M), Lewis acid (0.061 M), and olefin (0.61
M) in a J. Young tube.
Synthesis of Oxorhenium Alane Lewis Acid/Base Adducts.
In order to evaluate the electronic demands of the Lewis acid
component and determine its effect on the rates of hydro-
genation, we also synthesized traditional Lewis acid/base
adducts of (MesDAP)Re(O)X with tris(pentafluorophenyl)-
alane, Al(C6F5)3 (Scheme 7).
The conversion rates were found overall to be slightly faster
in comparison to those in studies with B(C6F5)3 as the Lewis
acid component. For example, the kobs value for the Re-Ph/
B(C6F5)3 pair was 7.5 × 10−3 min−1, while kobs for Re-Ph/
HB(C6F5)2 was 1.2 × 10−2 min−1.
Mechanism with Piers’ Borane. Since Piers’ borane
HB(C6F5)2 contains a reactive B−H bond and has been
shown to be active in olefin hydroboration, we examined
whether the mechanism with complexes employing this Lewis
acid was different.
Complexes 5−9 were isolated in modest to good yields (34−
70%). All complexes exhibit the characteristic 1H NMR
resonances for the diastereotopic methylene backbone, which
appear as two doublets. Other resonances are consistent with
1
the previously reported H NMR shifts for similar complex-
es.7,8,12a For example, the hydride ligand in complex 8 has a
Hydroboration of tert-butylethylene with Piers’ borane
occurs within minutes at room temperature. Therefore, the
mechanism depicted in Scheme 6 begins with the hydro-
boration of tert-butylethylene to produce the new borane 3,3-
dimethylbutyl-B(C6F5)2. Addition of the FLP across the double
bond in tert-butylethylene then occurs, followed by the
subsequent addition of H2, which eventually leads to the
generation of product in a mechanism similar to that proposed
earlier by us for oxorhenium/B(C6F5)3 FLPs.7
Scheme 7. Synthesis of Lewis Acid/Base Adducts with
Al(C6F5)3
Since the alkyl borane 3,3-dimethylbutyl-B(C6F5)2 was easily
generated from tert-butylethylene and Piers’ borane, reactions
with adducts of this Lewis acid and (MesDAP)Re(O)Ph were
examined.
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characteristic downfield shift in benzene-d6 to 11.50 ppm,
consistent with the coordination of the Lewis acid at the
rhenium oxo.8a Interestingly enough, all complexes have
identical 19F NMR resonances at −123 ppm (t, 2F) −154
ppm (t, 1F), and −162 ppm (m, 2F), which are only slightly
shifted from those of the parent Al(C6F5)3.8a The synthesized
complexes are not stable in the solid state, as traces of free
oxorhenium complexes are observed in the matter of a few
days.
X-ray Crystal Structure of 5. X-ray-quality crystals of 5 were
obtained via the slow diffusion of pentane into a concentrated
toluene solution at −40 °C. The thermal ellipsoid plot for 5 is
shown in Figure 4. The structure and unit cell are similar to
Figure 5. Variable-temperature spectrum of 5 from −80 to +100 °C.
phenyl ligand (Figure 5).15 From the coalescence temperature
and the chemical shift differences, a free energy of activation,
ΔG⧧, of 13.5 kcal mol−1 was calculated. For the borane system
this rotation was estimated to be 14.1 kcal mol−1.7 The smaller
barrier to rotation calculated in the aluminum system is
consistent with the observation from the crystal structure for 5
and provides further evidence that aluminum adducts are less
sterically hindered.
Influence of Steric Parameters of the Re-X Fragment. The
effect of the variation of the sterics of the X-type ligand bound
to rhenium on the rates of hydrogenation of tert-butylethylene
was investigated. Rate data were collected for complexes 5−9
according to eq 3, and the pseudo-first-order rate constants
were extracted (Figure 6).
Figure 4. Thermal ellipsoid plot (50% probability ellipsoids) for
complex 5. Selected bond lengths (Å) and angles (deg): Re−O1,
1.765(4); Re−N1, 1.961(5); Re−N2, 2.025(5); Re−N3, 1.950(5);
O1Al1, 1.781(5); Al1−O1−Re1, 171.7(3).
those of the previously reported (C6F5)3B·(O)Re(MesDAP)-
(Ph).7 The O−Al bond length in 5 is 1.780 Å. In contrast, the
O−B bond length in (C6F5)3B·(O)Re(MesDAP)(Ph) is 1.534
Å. As a result, the structure of 5 is considerably more crowded
than that of its boron analogue.14
Dynamic NMR Behavior of 5. The longer bond to
aluminum is a result of its larger size (covalent radius) in
1
comparison to that of boron. This is also evident in the H
NMR spectrum, as the difference in chemical shifts between the
two diastereotopic protons of the MesDAP ligand is quite large
(∼0.83 ppm) in all ReO/B(C6F5)3 adducts, indicating the
tight association of the Lewis acid to the oxorhenium Lewis
base. This tight association results in substantially different
chemical environments for the two diastereotopic protons. On
the other hand, the difference in diastereotopic chemical shifts
in the ReO/Al(C6F5)3 adducts is substantially smaller (0.69
ppm), which suggests that the aluminum Lewis acid is further
away and therefore exhibits a lesser effect on the diastereotopic
protons. This is also consistent with the observation in the solid
state that the aluminum complexes are less sterically crowded
than the analogous boron complexes (see Supporting
Information).
From Figure 6, it is evident that the sterics of the X-type
ligand affect the rate of hydrogenation. For example, 5 was
approximately 2−3 times faster as a catalyst than 6 and 7, and
about 15 times faster than the unsubstituted hydride catalyst 8.
Side reactions observed for the corresponding borane adducts
(vide supra) were not observed for 8 and thus allowed for an
examination of the steric effects of the hydride ligand. In
addition, the overall rate of the reaction was approximately 3
times faster than the corresponding reaction with the boron
system and roughly 2 times faster than the ReO/Piers’
borane system.
Taft’s linear free energy relationship outlined in Taft’s
equation (eq 4) was utilized in order to examine the steric
effects.6
1
When the H NMR spectrum of 5 was examined from −80
to +100 °C, significant changes were observed (Figure 5). In
the aromatic region, peaks were observed to sharpen and shift
with increasing temperature. In addition, a peak at 5.4 ppm was
observed to broaden and coalesce at approximately −15 °C
ppm. This dynamic behavior is attributed to rotation of the
⎛
⎜
⎝
⎞
⎟
⎠
k
log
= ρ*σ* + δE
s
k
(4)
Me
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with the Lewis basic ReO and facilitates its dissociation to
form a frustrated Lewis pair.
Thus, these data highlight critical design features in ReO/
Lewis acid FLPs. (1) Tight association of the Lewis acid/base
should be avoided in order to allow facile access to an FLP,
which is the catalytically active species. (2) Increased Lewis
acidity appears to result in higher catalytic activity, as
exemplified by the increased reaction rates with Al(C6F5)3
and Piers’ borane. (3) The nature of the central group 13 atom
appears to be important. It appears that the larger covalent
radius of aluminum in comparison to that of the boron
analogues is critical. As noted above, aluminum analogues
appear to be significantly less crowded sterically (see the
Supporting Information). This is critical not only in allowing
easy access to the catalytically active FLP but also in minimizing
deactivation pathways that are available with the boron
analogues.
The Lewis acid Al(C6F5)3 is more thermodynamically stable
than B(C6F5)3 and is less prone to previously observed
deactivation pathways, such as protonation of the C6F5
fragment of the Lewis acid.16 For example, little to no
pentafluorobenzene (C6F5H) as a result of proton transfer
from the acidic ReOH moiety was observed in the aluminum
system. The robustness of the Al-C6F5 fragments was noted in
computational studies by Frenking and Timoshkin, where the
formation of C6F5H as a result of intramolecular protonation
was shown to be strongly exergonic for boron (ΔG° = −22.2
kcal/mol) but only slightly exergonic for the corresponding
aluminum Lewis acid (ΔG° = −1.4 kcal/mol).17 The
elimination of these side reactions can account for the
increased stability of the ReO/Al(C6F5)3 system.
Figure 6. Conversions determined by 1H NMR spectroscopy by
integrating the ratio of tert-butyl singlets of the reactant and product.
Conditions: Re complexes 5−8 (0.0046 mmol), Lewis acid (0.0046
mmol), and olefin (0.092 mmol) in a J. Young tube. kobs(5) = [10(3)]
× 10−2 min−1; kobs(6) = [5(1)] × 10−3 min−1; kobs(7) = [3.7(5)] ×
10−3 min−1; kobs(8) = [6.6(2)] × 10−4 min−1.
Assuming that the electronics of the oxorhenium species
remain essentially unchanged by altering the alkyl, hydride, or
aryl substituents in the Re-X position, the electronic factors
ρ*σ* will be negligible; thus, the equation simplifies to eq 5.
⎛
⎜
⎝
⎞
⎟
⎠
The results outlined above can be used to rationally design
new catalytic systems for olefin hydrogenation. As outlined in
the next section, we utilize this strategy to develop
unprecedented catalytic systems for olefin hydrogenations
with an FLP.
k
log
= δE
s
k
(5)
Me
Therefore, the plot of log(k/kMe) vs Es will be linear with a
slope of δ, the steric sensitivity factor. The pseudo-first-order
rate constants were successfully correlated with Taft’s steric
parameter Es6 (Figure 7). The negative value of −0.57 obtained
for δ suggests that the reaction is sensitive to the sterics of the
X substituent. This is consistent with the hypothesis that the
reaction rate is indeed accelerated by the increasing sterics in
the Re-X position. High steric crowding in the proximity of the
metal center prevents the tight association of the Lewis acid
3. Rational Design of ReO/LA FLP for Olefin
Hydrogenation. In contrast to polar substrates, the catalytic
hydrogenation of unactivated olefins by FLPs remains a
challenge. For example, Stephan and Paradies reported the
hydrogenation of 1,1-diphenylethylene with a 20 mol % P/B
FLP catalyst, where P = (C6F5)Ph2P and B = B(C6F5)3.4o The
use of pTolNMe2 as a base allowed the catalyst loading to be
reduced to 5 mol %. The Paradies group extended FLP
reductions to β-nitrostyrenes and acrylates using (2,6-
C6H3F2)3B/2,6-lutidine (20 mol %) at 40 °C with H2 (4
bar).4g Earlier this year we expanded the scope of unactivated
olefins that can be hydrogenated with a ReO/B FLP system
using 5 mol % of catalyst at 100 °C.7
In this paper, we have shown that the modular nature of
ReO/B FLPs allows for easily tuned catalytic activity.
Specifically, the sterics of the ligand were altered by installing
bulkier isopropyl groups in the 2,6-positions of the diamido
pyridine ligand. In addition, the sterics and electronics of the
Lewis acid were altered by utilizing Piers’ borane, HB(C6F5)2.
Finally, the nature of the central group 13 Lewis acid in
M(C6F5)3 (M = boron, aluminum) was investigated by
synthesizing adducts with Al(C6F5)3.
With these modifications we examined the catalytic hydro-
genation of tert-butylethylene as depicted in Table 1. For these
reactions we employed (O)Re(MesDAP)(Ph) and (O)Re-
(DippDAP)(Ph) as the Lewis base components. In addition,
B(C6F5), HB(C6F5)2 and Al(C6F5)3 were employed as the
Figure 7. Taft plot of log(kobsX/kobsMe) versus Es.
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a
Table 1. Catalytic Hydrogenation of tert-Butyl Ethylene with ReO/Lewis Acid Frustrated Lewis Pairs
b
entry
1
oxorhenium complex
Lewis acid
temp (°C)
loading (mol %)
conversion (%)
TON
TOF (h−1
)
(O)Re(MesDAP)Ph
(O)Re(MesDAP)Ph
(O)Re(MesDAP)Ph
(O)Re(MesDAP)Ph
(O)Re(MesDAP)Ph
(O)Re(MesDAP)Ph
(O)Re(MesDAP)Ph
(O)Re(MesDAP)Ph
(O)Re(MesDAP)Ph
(O)Re(DippDAP)Ph
(O)Re(DippDAP)Ph
(O)Re(DippDAP)Ph
B(C6F5)3
100
100
100
100
100
100
80
0.6
0.1
0.6
0.1
0.6
0.1
0.6
0.6
0.6
0.6
0.6
0.6
100
6
169
45
4.0
1.2
4.0
3.4
4.0
20
c
2
B(C6F5)3
3
Al(C6F5)3
Al(C6F5)3
HB(C6F5)2
HB(C6F5)2
HB(C6F5)2
HB(C6F5)2
HB(C6F5)2
B(C6F5)3
100
16
169
130
169
840
169
163
25
c
4
5
100
100
100
96
c
6
7
4.0
3.9
0.6
3.7
1.9
3.5
8
60
9
20
20
10
11
12
100
100
100
93
156
81
Al(C6F5)3
HB(C6F5)2
48
87
146
a
Conditions unless specified otherwise: oxorhenium (0.0046 mmol), Lewis acid (0.0092 mmol), and substrate (0.776 mmol) in toluene (0.5 mL) in
a 25 mL Teflon sealed reaction vessel, pressurized with 50 psi of H2. Conversions were determined by 1H NMR spectroscopy. TON values are based
b
on combined hydrogenation and isomerization catalysis. 2,3-Dimethylbut-2-ene (3−5%) was observed as a side product in all catalytic reactions.
c
3.83 mmol of substrate used.
Lewis acid components. Reactions were performed under H2
(50 psi), and product formation was analyzed by H NMR
Scheme 8. Proposed Mechanism for ReO/Lewis Acid
Catalyzed Olefin Hydrogenation
1
spectroscopy.
In the previous sections, we examined the effect of varying
the sterics and electronics of the oxorhenium complex, as well
as the Lewis acid component on the rate of the catalytic
hydrogenation of tert-butylethylene. In Table 1, it is evident
that these modifications also have an effect on catalyst stability.
Consequently, a TON value of 840 was achieved with the
(O)Re(MesDAP)(Ph)/HB(C6F5)2 catalyst system (entry 6).
In contrast, TON values of 45 and 130 were achieved with
(O)Re(MesDAP)(Ph)/B(C6F5)3 and the (O)Re(MesDAP)-
(Ph)/Al(C6F5)3, respectively. Thus, the trend in TONs is
HB(C6F5)2 > Al(C6F5)3 > B(C6F5)3. This trend appears to
reflect the increased Lewis acidity of HB(C6F5)2 and Al(C6F5)3
and the stability of these Lewis acids with regard to protolytic
cleavage of the C6F5 group to produce pentafluorobenzene.17
To the best of our knowledge, the type of activity reported in
Table 1 is unprecedented for any reported catalytic FLP
hydrogenation with an unactivated olefin. Furthermore, the
TONs with Piers’ borane are at least 5−6 times those of any
reported FLP system thus far.
Modification of the Lewis acid also allows catalysis to be
performed at lower temperatures (entries 7−9) with turnovers
achievable at room temperature (entry 9). As noted earlier,
catalysis was also possible with the DippDAP ligand (entries
10−12), even though Lewis acid/adducts with this ligand were
not isolated. This lends further support to the notion that Lewis
acid/base adducts do not lie on the catalytic cycle.
In all catalytic reactions 2−5% of 2,3-dimethylbut-2-ene was
also observed. The presence of this substrate can be explained
by isomerization of the tert-butylethylene in the presence of the
Lewis acid via a methyl shift mechanism. To account for these
observations, a modified mechanism has been proposed in
Scheme 8. This mechanism begins with the activation of the
In this paper, it was noted: “if redistribution could be
inhibited, the protic cation derived from FLP activation of H2
could react with the transient alkylaluminate to provide an
entry to a catalytic cycle for hydrogenation.”18b In the current
proposed mechanism the acidic species [(DAP)(Ph)Re
OH]+ is generated that can protonate the anion [(alkyl)AR3]−
and generate the product. The barrier for this process was
shown, via computational studies, to be reasonable.7
CONCLUSIONS
■
In conclusion, we have evaluated the steric and electronic
demands of catalytic olefin hydrogenation with oxorhenium/
Lewis acid FLPs. The sterics of the ligand were altered by
installing bulkier isopropyl groups in the 2,6-positions of the
́
olefin by the Lewis acid. This is similar to a report by Menard
and Stephan, where the first Al-olefin complex was isolated and
shown to react with hydrides to form alkyls.18
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diamidopyridine (DAP) ligand. Increased crowding in the DAP
ligand hinders the formation of Lewis acid/base adducts.
However, species incorporating isopropyl groups were still
active in the catalytic hydrogenation of tert-butylethylene. This
is consistent with the hypothesis that Lewis acid/base adducts
of oxorhenium complexes are not on the catalytic cycle and that
the catalytically active species is a frustrated Lewis pair where
noncovalent interactions between the Lewis base (oxorhenium
complexes) and the Lewis acid account for the stability.
The reaction rates were slightly improved with Piers’ borane,
HB(C6F5)2, and the aluminum analogue Al(C6F5)3 with an
overall 3-fold increase in the rate with aluminum attributed to
factors such as (1) the increased Lewis acidity of aluminum, (2)
the ease of Lewis acid dissociation, and (3) the minimization of
decomposition pathways in the case of Al(C6F5)3.
The rate of catalytic hydrogenation was shown to strongly
correlate with the size of the alkyl, aryl, or hydride ligand. This
was confirmed by a linear Taft plot with the steric sensitivity
factor δ = −0.57, which suggests that reaction rates are faster
with a larger X-type ligand. Thus, high steric crowding in the
proximity of the metal center prevents the tight association of
the Lewis acid with the Lewis basic ReO and facilitates its
dissociation to form a frustrated Lewis pair.
These data were used to develop a catalyst ((MesDAP)Re-
(O)(Ph)/HB(C6F5)2) that achieved a TON value of 840 for
the hydrogenation of tert-butylethylene at mild temperatures
(100 °C) and pressures (50 psi of H2). Tuning the oxorhenium
catalysts also resulted in the hydrogenation of tert-butylethylene
at room temperature. These results, to the best of our
knowledge, are unprecedented for the catalytic hydrogenation
of an unactivated olefin by an FLP. This lends further support
to the notion that the modular nature of the oxorhenium
complexes allows for efficient tuning of catalytic activity.19
deuterated toluene. tert-Butylethylene (0.776 mmol, 100 μL or
3.88 mmol, 500 μL) was then added to the resulting solution,
and the storage tube was degassed via three freeze−pump−
thaw cycles. The tube was pressurized with H2 (50 psi, 99.996
purity grade), and the resulting solutions were placed in a
corresponding oil bath and stirred for 42 h. After it was cooled,
the tube was depressurized and the reaction mixture was
analyzed by 1H NMR spectroscopy. Conversions were
determined by integrating the ratios of the tert-butyl singlet
of the product with respect to the reactant.
Synthesis of Pyridine-2,6-diylbis(methylene) Bis(4-
methylbenzenesulfonate). In a 50 mL round-bottom flask,
pyridine-2,6-dimethanol (700 mg, 5.04 mmol) was dissolved in
10 mL of a 1/1 mixture of THF and water and was cooled to 0
°C using an ice bath. NaOH (440 mg, 11.1 mmol) was then
slowly added to the stirred reaction mixture, which was
followed by the dropwise addition of a solution of TsCl in THF
(1.92 g, 10.1 mmol, 8 mL of solution). The resulting cloudy
mixture was stirred for 6 h at 0 °C. After 6 h, the reaction was
warmed to room temperature and the product was extracted
with 15 mL of CH2Cl2. The organic layer was then dried over
anhydrous Na2SO4 and concentrated to give a white solid.
Isolated yield: 1.95 g (86%). 1H NMR (CDCl3): δ 7.80 (d, J =
8.0 Hz, 4H, tosyl aromatic); 7.80 (d, J = 8.0 Hz, 4H, tosyl
aromatic); 7.68 (t, J = 7.7 Hz, 1H, pyridine para-H); 7.32 (d, J
= 8.0 Hz, 6H, tosyl aromatic/pyridine meta-H); 5.04 (s, 4H,
Pyr CH2); 2.44 (s, 6H, tosyl para-CH3).
Synthesis of Pyridine-2,6-bis(diisopropylamine)
(DippDAP). Following standard air-free techniques, in a 100
mL Schlenk flask, 2,6-diisopropylaniline (9.08 mmol, 1.7 mL)
was dissolved in 20 mL of dry THF. To the cooled reaction
mixture at −78 °C was added n-BuLi (9.08 mmol, 3.6 mL)
dropwise, and the reaction mixture was warmed to room
temperature. The reaction mixture was then cooled to −78 °C.
The solution of tosylated pyridine (4.54 mmol, 2.03 g) in dry
THF was then cannula-transferred to a solution of deproto-
nated aniline; this was instantly accompanied by a color change
from yellow to dark orange. The resulting solution was stirred
for 16 h. A saturated solution of NH4Cl (30 mL) was then
placed in the flask, and the product was extracted with ether (2
× 30 mL). The combined organic extracts were dried over
Na2SO4 and then concentrated under vacuum to afford a yellow
EXPERIMENTAL SECTION
General Considerations. Ligands and oxorhenium chlor-
ide, phenyl, hydride, and methyl precursors were prepared as
■
10
9
previously reported.7,11,12b Al(C6F5)3 and HB(C6F5)2 were
synthesized according to modified literature procedures.
B(C6F5)3 was purchased from Strem Inc. and sublimed before
use. All manipulations were performed using standard Schlenk
and glovebox techniques. 1H NMR spectra were recorded on a
Varian Mercury 300 MHz spectrometer at room temperature.
13C and 19F NMR spectra were recorded on a Varian Mercury
400 MHz spectrometer at room temperature. X-ray data were
collected by Dr. Roger Sommer (NCSU). FTIR spectra were
recorded on a JASCO FT/IR-4100 instrument. Elemental
analyses were performed by Atlantic Micro Laboratories, Inc.
General Procedure for Kinetic Analysis. In an oven-
dried J. Young tube the rhenium complexes (0.0046 mmol) and
the corresponding Lewis acids (0.0092 mmol, 2 equiv with
respect to rhenium) were dissolved in 0.2 mL of deuterated
toluene. 3,3-Dimethyl-1-butene (0.092 mmol) was then added
to the J. Young tube via microsyringe, and the tube was
degassed via three freeze−pump−thaw cycles. The J. Young
tube was then pressurized with H2 (50 psi, 99.996 purity grade)
and placed in an oil bath. Conversions were determined by 1H
NMR spectroscopy by integrating the ratios of the tert-butyl
singlet of the product with respect to the reactant.
1
oil (1.28 g, 61% yield). H NMR (CDCl3): δ 7.68 (t, J = 7.9
Hz, 1H, Pyr para-H); 7.30 (d, J = 7.9 Hz, 2H, Pyr meta-H);
7.18 (m, J = 7.7 Hz, 1H, pyridine para-H); 7.32 (d, J = 8.0 Hz,
6H, tosyl aromatic/pyridine meta-H); 5.04 (s, 4H, Pyr CH2−);
2.44 (s, 6H, tosyl para-CH3).
Synthesis of (DippDAP)Re(O)Cl. In a 250 mL round-
bottom flask, (SMe2)(OPPh3)Re(O)Cl3 (450 mg, 0.693 mmol)
and the DippDAP ligand (1.5 equiv, 1.04 mmol) were dissolved
in 100 mL of absolute EtOH. 2,6-Lutidine (6.93 mmol, 0.8 mL)
was added to the reaction mixture, which was stirred overnight
at room temperature. Filtration afforded 151.6 mg (32% yield)
of a green powder. Vapor diffusion of pentane into a
concentrated solution of methylene chloride resulted in crystals
suitable for X-ray analysis. 1H NMR (CD2Cl2): δ 8.23 (t, J = 7.9
Hz, 1H, NC2H2CH); 7.87 (d, J = 7.7 Hz, 2H, NC2H2CH);
7.21−7.14 (m, 6H, aromatic); 5.60 (s, 4H, MesNCH2); 3.88
(spt, J = 6.8 Hz, 2H, iPr methine); 2.28 (spt, J = 6.8 Hz, 2H, iPr
methine); 1.30 (d, J = 6.9 Hz, 6H, iPr CH3); 1.23 (d, J = 6.9
Hz, 6H, iPr CH3); 1.02 (d, J = 6.9 Hz, 6H, iPr CH3); 0.92 (d, J
=6.9 Hz, 6H, iPr CH3). 13C NMR (CD2Cl2): δ 168.16, 153.94,
147.30, 146.31, 143.73, 126.26, 124.02, 123.61, 117.30, 82.40,
General Procedure for Catalytic Reactions Described
in Table 1. In an oven-dried 25 mL storage tube, the rhenium
complex (0.0046 mmol) and the Lewis acid (0.0092 mmol, 2
equiv with respect to rhenium) were dissolved in 0.5 mL of
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Research Article
27.96, 27.38, 25.42, 25.03, 24.30. Anal. Calcd for
C31H41ClN3ORe: C, 53.70; N, 6.06, H, 5.9., Found: C, 53.83,
N, 6.07, H, 6.13.
NC2H2CH); 7.72 (d, J = 7.7 Hz, 2H, NC2H2CH); 7.20−7.11
(m, 6H, aromatic); 5.64 (d, J = 20.2 Hz, 2H, −NCH2 Pyr); 5.55
(d, J = 20.2 Hz, 2H, −NCH2 Pyr); 3.90 (septet, J = 5.8 Hz, 2H,
iPr methine); 2.18 (septet, J = 5.8 Hz, 2H, iPr methine); 2.13
(s, 3H, Re-Me); 1.31 (d, J = 7.7 Hz, 6H, iPr CH3); 1.18 (d, J =
Synthesis of (DippDAP)Re(O)Ph. In a 25 mL scintillation
vial (DippDAP)Re(O)Cl (91.0 mg, 0.131 mmol) was dissolved
in 5 mL of CH2Cl2. In a glovebox, PhMgBr (2 equiv, 0.09 mL,
0.263 mmol) was added dropwise to the reaction mixture,
which resulted in an immediate color change from green to
brown. The reaction mixture was stirred under an inert
atmosphere for 1 h, after which it was taken outside of the
glovebox and quenched with 15 mL of DI water. The organic
layer was separated, dried over Na2SO4, and concentrated to
approximately 1 mL. Addition of excess pentane resulted in a
brown precipitate, which was collected on a filter frit and dried
under vacuum. Isolated yield: 53.1 mg (55%). Vapor diffusion
of pentane into a concentrated CH2Cl2 solution resulted in X-
i
i
7.7 Hz, 6H, Pr CH3); 1.01 (d, J = 7.7 Hz, 6H, Pr CH3); 1.00
i
(d, J = 7.7 Hz, 6H, Pr CH3). 13C NMR (CD2Cl2): δ 168.50,
153.06, 147.90, 146.10, 141.30, 125.66, 124.17, 124.12, 123.40,
116.40, 81.74, 27.47, 27.03, 25.81, 25.60, 25.14, 23.71, 16.48.
Synthesis of (DAP)Re(Ph)(O)Al(C6F5)3. A 100 mL Schlenk
flask was charged with (DAP)Re(O)Ph (100 mg, 0.154 mmol)
and Al(C6F5)3·(toluene) (95.5 mg, 0.154 mmol, 1 equiv) and
dissolved in 15 mL of toluene under an inert atmosphere. The
resulting brown solution was stirred at room temperature for 1
h. The solvent volume was then reduced to ∼5 mL in vacuo,
and excess hexanes (30 mL) was cannula-transferred into the
flask, which precipitated a brown powder. The resulting mixture
was stirred for another 30 min. Excess solvent was then
cannula-filtered, leaving the brown powder in the flask, which
was further dried in vacuo for 3 h. Isolated yield: 105 mg (0.089
1
ray-quality crystals. H NMR (CD2Cl2): δ 8.18 (t, J = 7.9 Hz,
1H, NC2H2CH); 7.76 (d, J = 7.7 Hz, 2H, NC2H2CH); 6.92−
6.88 (m, 6H, aromatic); 6.43 (t, J = 8.3 Hz, 2H, Ph-ortho); 6.02
(t, J = 7.3 Hz, 1H, Ph-para); 5.87 (d, J = 20.2 Hz, 2H, −NCH2-
Pyr) 5.77 (d, J = 20.2 Hz, 2H, −NCH2-Pyr); 4.09 (spt, J = 6.8
1
mmol, 58%). H NMR (C6D6): δ 6.91 (t, J = 8.0 Hz, 1H,
i
i
Hz, 2H, Pr methine); 2.26 (spt, J = 6.8 Hz, 2H, Pr methine);
NC2H2CH); 6.62 (t, J = 8.0 Hz, 2H, Ph meta-H); 6.54 (s, 2H,
Mes meta-H); 6.53 (d, J = 8.0 Hz, 2H, NC2H2CH); 6.34 (s, 2H,
Mes meta-H); 6.07 (t, J = 7.0 Hz, 1H, Ph para-H); 5.62 (d, J =
21.0 Hz, 2H, MesNCH2); 4.96 (d, J = 21.0 Hz, 2H,
MesNCH2); 2.21 (s, 6H, Mes CH3); 1.92 (s, 6H, Mes CH3);
1.34 (s, 6H, Mes CH3). 13C NMR (C6D6): δ 166.24, 153.77,
142.06, 135.35, 133.80, 132.25, 129.69, 128.67, 124.93, 122.00,
116.30, 80.85, 20.51, 18.74, 18.04. 19F NMR (C6D6): δ
−122.58 (dd, J = 15.3 Hz, J = 12.2 Hz, 2F); −154.30 (t, J =
19.1 Hz, 1F); −162.6 (overlapping spt, J =12.2 Hz, 2F).
Elemental analysis was not attempted on this molecule because
of its instability.
i
i
1.20 (d, J = 6.9 Hz, 6H, Pr CH3); 1.13 (d, J = 6.9 Hz, 6H, Pr
i
CH3); 1.08 (d, J = 6.9 Hz, 6H, Pr CH3); 1.05 (d, J = 6.9 Hz,
6H, Pr CH3). 13C NMR (CD2Cl2): δ 167.27, 153.67, 147.02,
i
145.37, 142.03, 133.04, 125.43, 125.27, 123.46, 123.02, 121.53,
116.66, 81.25, 27.77, 27.00, 26.93, 26.73, 24.26, 22.71, 22.57.
Anal. Calcd for C37H46N3ORe·H2O: C, 59.02; H, 6.43; N, 5.58.
Found: C, 59.11; H, 6.41; N, 5.39.
Synthesis of (DippDAP)Re(O)H. In a 25 mL pressure
vessel (DippDAP)Re(O)Cl (100 mg, 0.14 mmol) was
dissolved in 5 mL of dry THF in the glovebox. Tributyltin
hydride (0.72 mmol, 0.2 mL) was added, and the reaction
mixture was stirred at 80 °C for 2 days. When the mixture was
cooled, solvent was removed under vacuum and the remaining
residue was taken up in excess hexanes to precipitate the
product as a red powder, which was filtered and dried under
Synthesis of (DAP)Re(Cl)(O)Al(C6F5)3. A 100 mL Schlenk
flask was charged with (DAP)Re(O)Cl (100 mg, 0.164 mmol)
and Al(C6F5)3·(toluene) (102 mg, 0.164 mmol, 1 equiv) and
dissolved in 15 mL of toluene under an inert atmosphere. The
resulting orange solution was stirred at room temperature for 1
h. The solvent volume was then reduced to ∼5 mL in vacuo,
and excess hexanes (30 mL) was cannula-transferred in the
flask, which precipitated an orange powder. The resulting
mixture was stirred for another 30 min. Excess solvent was then
cannula-filtered, leaving the yellow powder in the flask, which
was further dried in vacuo for 3 h. Isolated yield; 135 mg (0.119
1
vacuum. Isolated yield: 70.5 mg (76%). H NMR (CD2Cl2): δ
8.15 (t, J = 7.9 Hz, 1H, NC2H2CH); 7.70 (d, J = 7.7 Hz, 2H,
NC2H2CH); 7.29 (s, 1H, Re-H); 7.14−7.09 (m, 6H, aromatic);
5.63 (d, J = 20.2 Hz, 2H, −NCH2 Pyr); 5.52 (d, J = 20.2 Hz,
2H, −NCH2 Pyr); 3.86 (spt, J = 5.8 Hz, 2H, iPr methine); 2.56
(spt, J = 5.8 Hz, 2H, iPr methine); 1.31 (d, J = 7.7 Hz, 6H, iPr
i
CH3); 1.18 (d, J = 7.7 Hz, 6H, Pr CH3); 1.01 (d, J = 7.7 Hz,
i
i
1
6H, Pr CH3); 1.00 (d, J = 7.7 Hz 6H, Pr CH3). 13C NMR
(CD2Cl2): δ 169.0, 159.17, 147.10, 144.52, 141.49, 125.50,
124.04, 123.70, 116.66, 80.87, 27.54, 27.27, 25.36, 25.29, 24.33,
24.12. FTIR (KBr pellet): ν(Re-H) 2034 cm−1. Anal. Calcd for
C31H42N3ORe: C, 56.51; H, 6.43; N, 6.38. Found: C, 55.97; H,
6.49; N, 6.26.
mmol, 72%). H NMR (C6D6): δ 6.80 (t, J = 8.0 Hz, 1H,
NC2H2CH); 6.71 (s, 2H, Mes meta-H); 6.62 (s, 2H, Mes meta-
H); 6.60 (d, J = 8.0 Hz, 2H, NC2H2CH); 5.43 (d, J = 21.0 Hz,
2H, Mes NCH2); 4.83 (d, J = 21.0 Hz, 2H, Mes NCH2); 2.19
(s, 6H, Mes CH3); 2.06 (s, 6H, Mes CH3); 1.16 (s, 6H, Mes
CH3). 19F NMR (C6D6): δ −122.8 (m, 2F), −153.5 (m, 1F),
−162.3 (m, 2F). The complex was not characterized by 13C
NMR due to its very poor solubility in benzene or toluene.
Elemental analysis was not attempted on this molecule because
of its instability.
Synthesis of (DAP)Re(Me)(O)Al(C6F5)3. A 100 mL
Schlenk flask was charged with (DAP)Re(O)Me (100 mg,
0.170 mmol) and Al(C6F5)3·(toluene) (105 mg, 0.170, 1 equiv)
and dissolved in 15 mL of toluene under an inert atmosphere.
The resulting purple solution was stirred at room temperature
for 1 h. The solvent volume was then reduced to ∼5 mL in
vacuo, and excess hexanes (30 mL) was cannula-transferred in
the flask, which precipitated an orange powder. The resulting
mixture was stirred for another 30 min. Excess solvent was then
Synthesis of (DippDAP)Re(O)Me. In a 25 mL scintillation
vial (DippDAP)Re(O)Cl (60.0 mg, 0.087 mmol) was dissolved
in 5 mL of THF under an inert atmosphere. MeMgBr (2 equiv,
0.06 mL, 0.173 mmol) was added dropwise to the reaction
mixture, which resulted in an immediate color change from
green to purple. The reaction mixture was stirred under an inert
atmosphere for 1 h, after which it was taken outside of the
glovebox and quenched with 15 mL of DI water. The
compound was extracted with 2 × 10 mL of CH2Cl2, and the
organic layer was separated and dried over Na2SO4. Solvent was
removed in vacuo, and addition of excess pentane resulted in a
purple precipitate, which was collected on a filter frit and dried
1
under vacuum. H NMR (CD2Cl2): δ 8.08 (t, J = 7.9 Hz, 1H,
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Research Article
was removed by cannula filtration, leaving the light purple
powder in the flask, which was further dried in vacuo for 3 h.
Isolated yield: 107 mg (0.096 mmol, 56%). 1H NMR (C6D6): δ
6.84 (t, J = 8.0 Hz, 1H, NC2H2CH); 6.70 (s, 2H, Mes meta-H);
6.66 (s, 2H, Mes meta-H); 6.47 (d, J = 8.0 Hz, 2H, NC2H2CH);
5.42 (d, J = 21.0 Hz, 2H, Mes NCH2); 4.78 (d, J = 21.0 Hz, 2H,
Mes NCH2); 3.81 (s, 3H, Re-CH3); 2.18 (s, 6H, Mes CH3);
2.07 (s, 6H, Mes−CH3); 1.05 (s, 6H, Mes CH3). 13C NMR
(C6D6): δ 166.45, 152.85, 141.71, 135.88, 134.67, 132.84,
130.01, 129.49, 129.18, 115.92, 81.06, 20.66, 18.12, 17.25. 19F
NMR (C6D6, δ): −123.2 (dd, J = 15.1 Hz, J = 12.1 Hz, 2F);
−154.3 (t, J = 19.1 Hz, 1F); −162.6 (overlapping m, 2F).
Elemental analysis was not attempted on this molecule because
of its instability.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b03313.
■
S
Additional NMR data and X-ray data for 2 and 5 (PDF)
Crystallographic data (CIF)
Crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for E.A.I.: eaison@ncsu.edu.
ORCID
■
Elon A. Ison: 0000-0002-2902-2671
Synthesis of (DAP)Re(Et)(O)Al(C6F5)3. A 100 mL Schlenk
flask was charged with (DAP)Re(O)Et (101 mg, 0.165 mmol)
and Al(C6F5)3·(toluene) (104 mg, 0.170, 1 equiv) and
dissolved in 15 mL of toluene under an inert atmosphere.
The resulting purple solution was stirred at room temperature
for 1 h. The solvent volume was then reduced to ∼5 mL in
vacuo, and excess hexanes (30 mL) was cannula-transferred in
the flask, which precipitated an orange powder. The resulting
mixture was stirred for another 30 min. Excess solvent was then
was removed by cannula filtration, leaving the light purple
powder in the flask, which was further dried in vacuo for 3 h.
Isolated yield: 134 mg (0.117 mmol, 70%). 1H NMR (C6D6): δ
6.99 (t, J = 8.0 Hz, 1H, NC2H2CH); 6.87 (s, 2H, Mes meta-H);
6.70 (d, J = 8.0 Hz, 2H, NC2H2CH); 5.47 (d, J = 21.0 Hz, 2H,
Mes NCH2); 4.89 (d, J = 21.0 Hz, 2H, Mes NCH2); 2.48 (bs,
6H, Mes CH3); 2.26 (s, 6H, Mes CH3); 1.35 (bs, 6H, Mes
CH3); 1.21 (bs, 3H, Re-CH2CH3). The methylene group of the
Re-Et fragment was not observed, presumably due to the rapid
bond rotation. 13C NMR (C6D6): δ 166.20, 152.05, 148.80,
141.11, 137.51, 135.89, 134.74, 131.03, 129.18, 128.93, 128.16,
127.90, 127.65, 127.41, 125.29, 115.53, 81.03, 21.02, 20.436,
17.85, 17.05. 19F NMR (C6D6, δ): −122.9 (dd, J = 15.1 Hz, J =
12.1 Hz, 2F); −154.2 (t, J = 19.1 Hz, 1F); −162.4 (overlapping
m, 2F). Elemental analysis was not attempted on this molecule
because of its instability.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge North Carolina State University and the
National Science Foundation via a CAREER Award (CHE-
0955636) for funding.
REFERENCES
■
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yield: 22 mg (0.020 mmol, 34%). H NMR (C6D6): δ 11.50
(bs, 1H, Re-H); 6.80 (t, J = 8.0 Hz, 1H, NC2H2CH); 6.70 (s,
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1179
DOI: 10.1021/acscatal.6b03313
ACS Catal. 2017, 7, 1170−1180

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