Zerovalent [Pd(NHC)(alkene)1,2] complexes bearing expanded-ring N-heterocyclic carbene ligands in transfer hydrogenation of alkynes

Article abstract of DOI:10.1021/om300930w

In search of more active catalysts for the transfer hydrogenation of alkynes, a series of [Pd(NHC)(MA)_1,2] (8-14) and [Pd(NHC)(dvtms)] complexes (1-7), in which the NHC ancillary ligands are expanded-ring N-heterocyclic carbenes (erNHC's), have been prepared. These very bulky, strong σ-donor ligands impart a highly constrained geometry on the complexes and in some cases enable the isolation of coordinatively and electronically unsaturated complexes (10 and 14). Their strong σ-donor character is reflected in a decrease in IR stretching frequency for the C=O bond of the maleic anhydride ligands (8-14) in comparison to their five-membered counterparts. Significantly enhanced catalytic activity in the transfer hydrogenation of 1-phenyl-1-propyne is observed using [Pd(erNHC)(dvtms)] complexes (1-7) as precatalysts. The catalysts show high initial selectivity toward (Z)-alkene. However, double-bond isomerization and over-reduction to the corresponding alkane occur when all the alkyne substrate is consumed; this feature reflects the very high efficiency of these catalysts in the transfer hydrogenation of alkynes as well as alkenes.

Full text of DOI:10.1021/om300930w

Article
pubs.acs.org/Organometallics
Zerovalent [Pd(NHC)(Alkene) ] Complexes Bearing Expanded-Ring
1,2
N‑Heterocyclic Carbene Ligands in Transfer Hydrogenation of
Alkynes
Peter Hauwert,†,⊥ Jay J. Dunsford, Dorette S. Tromp, Jan J. Weigand, Martin Lutz,
‡,#
†
§
∥
,
‡
,†
Kingsley J. Cavell,* and Cornelis J. Elsevier*
†Molecular Inorganic Chemistry, Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, Postbus
9
4157, 1090 GD Amsterdam, The Netherlands
‡
School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, United Kingdom
§
̈ ̈ ̈
̈ ̈
Institut fur Anorganische und Analytische Chemie, Westfalische Wilhelms-Universitat Munster, Corrensstrasse 30, 48149 Munster,
Germany
∥
Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Faculty of Science, Utrecht University, Padualaan 8,
584 CH Utrecht, The Netherlands
3
*
S Supporting Information
ABSTRACT: In search of more active catalysts for the transfer hydrogenation of alkynes, a series of [Pd(NHC)(MA) ] (8−
1
,2
1
4) and [Pd(NHC)(dvtms)] complexes (1−7), in which the NHC ancillary ligands are expanded-ring N-heterocyclic carbenes
(
erNHC’s), have been prepared. These very bulky, strong σ-donor ligands impart a highly constrained geometry on the
complexes and in some cases enable the isolation of coordinatively and electronically unsaturated complexes (10 and 14). Their
strong σ-donor character is reflected in a decrease in IR stretching frequency for the CO bond of the maleic anhydride ligands
(
8−14) in comparison to their five-membered counterparts. Significantly enhanced catalytic activity in the transfer
hydrogenation of 1-phenyl-1-propyne is observed using [Pd(erNHC)(dvtms)] complexes (1−7) as precatalysts. The catalysts
show high initial selectivity toward (Z)-alkene. However, double-bond isomerization and over-reduction to the corresponding
alkane occur when all the alkyne substrate is consumed; this feature reflects the very high efficiency of these catalysts in the
transfer hydrogenation of alkynes as well as alkenes.
INTRODUCTION
elaborate experimental setup and strict monitoring of the
hydrogen uptake to prevent over-reduction. Also, unsolicited
partial Z/E isomerization, double-bond shifts, and problems
with reproducibility occur. Only very few homogeneous
palladium catalysts are known to catalyze the desirable, selective
■
Catalytic transfer hydrogenation is an effective tool for the
reduction of carbonyl compounds, using alcohols or
ammonium formate as hydrogen donors and Ru(II), Ir(I), or
10
1
Rh(I) complexes as catalysts. Notably, a plethora of transfer
hydrogenation reactions involving ketones and imines have
been reported, since polar double bonds are easily reduced.
semihydrogenation of alkynes to (Z)-alkenes. So far, only
2
−5
2
[
(
Pd(Ar-Bian)(η -alkene)] complexes, containing a bis-
arylimino)acenaphthene ligand utilizing molecular hydrogen,
In contrast, transfer hydrogenation of carbon−carbon multiple
bonds has been scarcely reported, as it is far more difficult to
and [Pd(NHC)L ] (L = alkene, solvent; n = 1, 2), containing
1
b,5−8
n
achieve.
While several homogeneous catalytic systems are
an N-heterocyclic carbene ligand employing transfer hydro-
genation, have been reported to effectively hydrogenate a
variety of alkynes to the (Z)-alkenes with good to excellent
known for transfer hydrogenation of alkenes, only three have
been reported for transfer semihydrogenation of alkynes to give
7,9
alkenes (Scheme 1). Especially α-arylalkynes have proven to
be difficult substrates, in both transfer hydrogenation and
8
,9
chemo- and stereoselectivity.
8
,9
hydrogenation using dihydrogen.
Semihydrogenation of alkynes is traditionally performed with
Received: October 3, 2012
Lindlar’s catalyst and dihydrogen, which requires a rather
Published: December 19, 2012
©
2012 American Chemical Society
131
dx.doi.org/10.1021/om300930w | Organometallics 2013, 32, 131−140

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Article
Scheme 1. Transfer Semihydrogenation of Alkynes, Using Triethylammonium Formate as Hydrogen Donor
Recently, the mechanism for the fully stereo- and chemo-
selective transfer hydrogenation of alkynes to cis-alkenes
employing [Pd(IMes)(MA)(solvent)] complexes and triethyl-
ammonium formate as the hydrogen donor has been
elucidated, along with an explanation for the high chemo-
Scheme 2. Expanded Ring NHC Pd(0) Complexes of the
General Formula [Pd(erNHC)(dvtms)] (1−7), Previously
27c
Employed in Mizoroki−Heck Cross-Coupling
11
specificity for alkynes over alkenes. It was found that the
electron density on the metal center positively correlates with
the reaction rate and inversely correlates with chemoselectivity.
For alkynes with electron-releasing substituents, such as bis(4-
methoxyphenyl)acetylene, the chemoselectivity is high but the
reaction rate is lower in comparison with that for aliphatic or
other aromatic alkynes. We have ascribed this behavior to the
lower affinity of electron-rich alkynes for coordination to
12
electron-rich Pd(0)−NHC complexes. In light of this, we
decided to investigate the impact of strongly donating and
sterically demanding ligands such as expanded-ring N-
heterocyclic carbenes (erNHC’s) on the conversion of
electron-rich and/or sterically hindered substrates.
RESULTS AND DISCUSSION
In order to explore the steric and electronic effects of
exchanging IMes by erNHC’s, a series of six- and seven-
■
o
membered amidinium salts bearing 2-methylphenyl (Tol ),
The application of N-heterocyclic carbenes (NHC’s) as
13
2
,4,6-trimethylphenyl (Mes), 2,6-diisopropylphenyl (DIPP),
ligands for homogeneous catalysts has led to enhanced
p
1
4
and 4-methylphenyl (Tol ) N-aryl substituents were prepared,
opportunities, and their use in the past decade has grown
exponentially, in parallel with an improved understanding of
the strong donor ability and metal−ligand bond character of
26a
following previously reported procedures. Initially, attempts
were made to prepare complexes of the general formula
1
4e,15,16,17
[Pd(erNHC)(MA) ], following the approach employed for the
2
32
this class of ligand.
This has led to several new sets of
previously reported IMes-based derivatives: i.e., by in situ
18
carbene architectures, such as remote and abnormal NHC’s,
generation of the free carbene followed by addition of
Pd(tBuDAB)(MA)] in toluene (Scheme 3). However, no
reaction was observed, and after addition of an extra 1 equiv of
maleic anhydride to the solution the Pd(0) precursor was
recovered in more than 90% yield. As the N−C −N bond
angle in erNHC’s is notably larger than in the five-membered
derivatives (IMes, 101°; 6-Mes, 114°; 7-Mes, 116°),
1
5f,19
20
acyclic carbenes,
bioxazoline-based NHC’s, and benzan-
32
[
2
1
nulated NHC’s. A very recent addition to the NHC ligand
armory has been the development of a series of expanded-ring
NHC’s with large σ-donor capacity and substantial steric
demands due to an inherent increase in the N−CNHC−N bond
angle. The simplicity and high modularity of the synthetic
route allow facile adjustment of donating capacity and steric
influence through variation of heterocycle ring size and N
substituents. Additional donor functionalities can also be
NHC
2
2
33,26a
the N
substituents are thrust in toward the carbene carbon, hindering
the approach of the sterically encumbered [Pd(tBuDAB)(MA)]
precursor. When the less hindered Pd(0) precursor [Pd(nbd)-
22b,23
24
34
introduced on the N substituents
or on the backbone,
(MA)] was employed, a reaction was observed upon addition
1
of the in situ generated free carbene (Scheme 3). However, H
and this methodology has led to a variety of novel erNHC
13
1
2
2a,b,24
23,24c
25
26
and C{ H} NMR data for the isolated product demonstrated
the presence of substantial amounts of unidentifiable by-
products; therefore, an alternate approach was necessary.
In search of a suitable alternate approach for the synthesis of
complexes with metals such as Rh,
Ir,
Recently, the list was
extended to include complexes of zerovalent palladium,
Pd(erNHC)(dvtms)] (Scheme 2), which were found to
generate highly active catalysts for Mizoroki−Heck cross-
Pt, Ag,
2
7
28
29
26b,30
22c,31
Pd, Ru, Ni, Cu,
and Au.
[
[
Pd(erNHC)(MA) ] complexes, the previously described
2
27c
2
7c
[Pd(erNHC)(dvtms)] complexes (Scheme 2)
were em-
coupling. Iridium complexes bearing erNHC ligands have
also recently been shown to be extremely effective catalysts for
the transfer hydrogenation of ketones with iPrOH as the source
ployed as well-defined Pd(0) parent precursors in a coligand
displacement protocol. The addition of 2 equiv of maleic
anhydride to a toluene suspension of the [Pd(erNHC)-
dvtms)] complexes resulted in the formation of the
corresponding [Pd(erNHC)(MA)1,2] complexes 8−14
Scheme 4). The complexes were isolated in low to moderate
2
3
of hydrogen.
(
Building on our previous hydrogenation and synthetic
studies, we report here the synthesis and full characterization
(
of novel [Pd(erNHC)(MA) ] complexes, along with the X-ray
1
,2
yields, depending on the steric demand of the NHC ligand.
crystal structure of an air-stable 14-electron Pd(0) species, and
probe their suitability as catalysts in the selective transfer
hydrogenation of alkynes to cis-alkenes.
This is reflected in the low yield of 11 (∼5%) and an inability
p
to isolate the [Pd(7-Tol )(MA) ] complex. Coligand displace-
2
ment could also be achieved through the in situ generation of
1
32
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Organometallics
Scheme 3. Attempted Synthesis of [Pd(erNHC)(MA) ] Complexes from [Pd( BuDAB)(MA)] and [Pd(nbd)(MA)]
Article
t
2
Scheme 4. Synthesis of [Pd(erNHC)(MA) ] Complexes 8−14 via Coligand Exchange from Parent [Pd(erNHC)(dvtms)]
1
,2
Complexes
1
13
1
the parent [Pd(erNHC)(dvtms)] complexes followed by
maleic anhydride addition, obviating the need for isolation of
the parent [Pd(erNHC)(dvtms)] complex (see the Exper-
imental Section for details). All complexes after their formation
were found to be air-stable in the solid state. Complexes 8−11
bearing six-membered NHC ligands are also stable in solution,
whereas complexes 12−14 with seven-membered NHC ligands
slowly decompose in halogenated solvents, as previously
Table 1. H and C{ H} NMR and FT-IR Data for
[Pd(erNHC)(MA) ] Complexes 8−14
1
,2
13C (NNHC
ppm
),
1
H/ C (alkene),
13
ν(CO),
‑1
entry
complex
ppm
cm
1
2
3
4
5
6
7
8
8
207.2
209.0
206.8
204.7
a
4.05/65.8
3.79/68.0
2.90/48.2
3.78/65.5
5.08/62.9
3.83/68.9
2.81/28.1
1813, 1753
1847, 1777
1798, 1730
c
9
10
11
12
13
14
2
7c
noted.
To the best of our knowledge, this is the first
1773, 1733
1779, 1711
1794, 1726
1816, 1761
35
reported example of [Pd(NHC)(dvtms)] complexes being
employed in coligand exchange reactions involving replacement
of dvtms to generate new NHC organometallic complexes
226.3
214.9
b
b
[Pd(IMes)
(MA)
193.1
3.53/64.7
3
6
(
Scheme 4).
Late-transition-metal complexes bearing erNHC’s have been
]
2
a
b
c
Not observed. Spectra measured at −50 °C; values from ref 9a. Not
determined.
noted to exhibit N−C −N resonances which are shifted
NHC
significantly to higher frequency compared with those of five-
membered-ring derivatives; these range from 204 to 209 ppm
for complexes of 6-NHCs (8−11) and 215−226 ppm for
complexes of 7-NHCs (12−14) (Table 1). Interestingly, in the
related [Pd(erNHC)(dvtms)] complexes 1−7 this signal
occurs at even higher frequency: for example, 226 and 247
erNHC and maleic anhydride coligands are present in a 1:2
ratio, whereas the integrals of compounds 10 and 14 indicate a
1
:1 ratio. Seemingly, the 2,6-diisopropylphenyl (DIPP)
substituents are so sterically demanding that coordination of
only one maleic anhydride is possible. This feature is observed
not only in the integrals of the peaks in H NMR but also in the
27c
ppm for 1 (6-Mes) and 5 (7-Mes), respectively. The shift to
lower frequency for complexes containing maleic anhydride
coligands indicates that there is possibly an increase in π back-
1
high-frequency shift of the maleic anhydride alkene resonance
1
13
1
by 0.8 and 20 ppm in H and C{ H} NMR, respectively. The
structures of these 14-electron Pd(0) complexes have been
confirmed by single-crystal X-ray structure determination of
complex 14, [Pd(7-DIPP)(MA)] (Figure 4).
37
donation from palladium to maleic anhydride. In complex 8,
bearing N-Tolo substituents, the maleic anhydride signals
appear as sharp doublets at 4.05 and 3.64 ppm, respectively,
while for the seven-membered analogue 12 only one sharp
singlet is observed at 5.05 ppm. Interestingly, the methylene
groups in the backbone of the carbene are all found to be
inequivalent. For compounds 9 and 13, bearing mesityl
substituents, the maleic anhydride resonances appear as a
broad singlet, similar to the case for the related [Pd(IMes)-
The increased Lewis basicity of the erNHC’s relative to their
five-membered counterparts is also reflected in the IR spectra;
increased back-donation into the π* orbital of the maleic
anhydride decreases the strength of the CO bond. Indeed,
for complexes of erNHC’s CO stretching frequencies of the
maleic anhydride carbonyls occur at wavenumbers significantly
11
32
(
MA) ] complex. For compounds 8, 9, and 11−13 the
lower than those for [Pd(IMes)(MA) ] (Table 1), and the
2
2
1
33
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Organometallics
Article
shift to lower wavenumbers increases with an increase in
heterocycle ring size from six- to seven-membered rings.
However, for a set of complexes with a single ring size, the C
O stretching frequencies for maleic anhydride shift to higher
wavenumbers with increasing steric bulk of the aryl substituents
p
o
(
Tol < Tol < Mes). As the steric bulk around the palladium
center increases, the Pd−alkene bond distances increase,
resulting in less efficient π back-donation. However, for
complexes with DIPP substituents (10 and 14) carbonyl
stretching frequencies are significantly lower. This observation
may be rationalized on the basis that in the mono(maleic
anhydride) complexes (10 and 14) the metal donates more
electron density per alkene, and the single alkene can now
approach the metal center more closely.
Solid-State Analysis. The solid-state structures of
complexes 8, 9, 11, and 14 were determined by single-crystal
X-ray diffraction: 8, 9, and 11 (Figures 1−3) are confirmed as
p
Figure 3. X-ray structure of [Pd(6-Tol )(MA) ] (11). Displacement
2
ellipsoids are drawn at the 50% probability level. Hydrogens have been
omitted for clarity.
o
Figure 1. X-ray structure of [Pd(6-Tol )(MA) ] (8). Displacement
2
ellipsoids are drawn at the 50% probability level. Hydrogens have been
omitted for clarity.
Figure 4. X-ray structure of [Pd(6-DIPP)(MA)] (14). Displacement
ellipsoids are drawn at the 50% probability level. Hydrogens have been
omitted for clarity.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
[
Pd(erNHC)(MA) ] Complexes 8, 9, 11, and 14
1,2
8
9
11
14
Pd−CNHC
Pd−CCcentroid
2.113(2)
2.018,
2.138(2)
2.1109(15)
1.971
2.086(3)
2.039,
2.037
2.0274(12),
2.0214(12)
2
.020
N−CNHC−N
tilt angle θ
118.2
115.8(2)
117.17(14)
120.7(2)
2.1(3)
a
63.2(3),
55.0(3),
59.0(3)
54.45(19),
60.58(19)
6
3.0(3)
b
twist angle φ
70.5(4)
9.7(4)
76.6(4),
71.5(4)
56.89(17),
64.57(17)
88.9(4),
87.5(3)
6
a
The tilt angle θ is defined as the angle between the carbene N−
NHC
b
C
−N plane and the Pd−alkene coordination plane. The twist
angle φ is defined as the angle between the carbene N−CNHC−N plane
and the plane of the aromatic ring.
Figure 2. X-ray structure of [Pd(6-Mes)(MA) ] (9). Displacement
2
ellipsoids are drawn at the 50% probability level. Hydrogens and
solvent molecules have been omitted for clarity.
As previously noted in earlier studies, we could not isolate
either the 2,6-diisopropylphenyl (DIPP)- or 2,6-diethylphenyl
(
DEP)-substituted analogues of the [Pd(erNHC)(dvtms)]
27c
1
6-electron [Pd(NHC)(MA) ] complexes, and 14 is a
complexes. It is evident that the combination of chelating
dvtms coligand and erNHC’s with these large substituents is
too sterically demanding. dvtms is a relatively bulky, bidentate
ligand, and employing just one of its alkene moieties would not
2
coordinatively unsaturated 14-electron [Pd(NHC)(MA)]
complex (Figure 4). Selected bond lengths, bond angles, and
dihedral angles are provided in Table 2.
1
34
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Organometallics
Article
provide sufficient electron-withdrawing capacity to stabilize the
coordinatively unsaturated geometry dictated by these bulky
substituents. Maleic anhydride is less sterically demanding and
is a better electron acceptor, providing sufficient stabilization
for the 14e [Pd(7-DIPP)(MA)] species 14 (Figure 4).
4, and 5 in transfer hydrogenation. This comparison of both
NHC ring size and olefin coligand allows a number of
important conclusions to be drawn, and these are discussed
below.
−1
When TOF (h ) values and percentage conversions after 24
h are compared for the complexes of the type [Pd(NHC)-
(MA)1,2], it is evident that increasing NHC ring size is
accompanied by a significant decrease in catalytic activity
(Table 3, entries 2−4 (5-, 6-, and 7-Mes) and 5−7 (5-, 6-, and
The most notable molecular structure reported here is that of
the highly coordinatively unsaturated, 14e complex 14, in which
the palladium center interacts with the ipso carbon of one of
the aryl substituents (Figure 4). The 2,6-diisopropyl (DIPP)
groups on the aryl ring are so sterically demanding that only
one maleic anhydride is able to coordinate, rendering the
complex coordinatively and electronically unsaturated. The
observed interaction with the aryl substituent alleviates the
electronic unsaturation, leading to a highly distorted geometry.
On closer inspection it is seen that the Pd center is coordinated
perpendicularly to the aryl plane (Pd−N−aryl plane angle 85°),
centered directly above the ipso carbon of the aromatic ring.
This interaction heavily influences the structure, as the tilt angle
7
-DIPP)). While increasing ring size leads to an increase in σ-
donor capacity of the erNHC ligand, it is also accompanied by
increased steric demand, which may have an impact on catalytic
performance. For example, when using complex 9 (with the 6-
Mes ancillary ligand) as catalyst for transfer hydrogenation of
bis(4-methoxyphenyl)acetylene, under analogous conditions,
no reaction was observed. The very bulky erNHC ligand is too
large to allow coordination of the more bulky substrate to the
palladium center, thus preventing transfer hydrogenation from
occurring. It appears the enhanced σ-donor capacity of the six-
and seven-membered erNHC ligands is outweighed by the
increasing steric demand of the ligands. Alternatively, and
consistent with the spectroscopic data above, the enhanced σ-
donor capacity of the six- and seven-membered erNHC ligands
results in increased back-donation to the hence more strongly
bound MA coligand, which competes with the alkyne for vacant
sites, thus affecting catalytic performance. Hence, the chemo-
selectivity for alkenes over alkanes for these catalysts remains
excellent, although it is noted for several catalysts (Table 3,
entries 3−6) that an increased amount of allylbenzene is
formed. However, the concept of apparent “selectivity” for
these catalysts is discussed in more detail in the following
paragraphs.
(
θ) is very acute (2.2(3)°) in comparison to those of 8 (63°)
and 9 (55−59°). This feature means that the NHC ring is
almost) in the coordination plane, while for the other
(
complexes the carbene N−CNHC−N plane is twisted away
from the coordination plane (defined by the Pd(alkene)2
moiety) by 63.1(4)° (8) and 57.0(4)° (9), respectively.
The structures of the six-membered ring NHC complexes 8,
9
, and 11 reveal bond lengths and angles which are similar to
literature values for erNHC’s, but with slightly elongated M−
bond lengths of 2.09−2.14 Å. As can be seen in Figures
C
NHC
1
−3 and the data in Table 2, the o-tolyl substituents are
oriented with the methyl group directed away from the metal
center (Figure 1), while the aromatic rings of the mesityl
(
Figure 2) and p-tolyl (Figure 3) substituents are facing the
The application of [Pd(erNHC)(dvtms)] complexes 1−7 as
catalysts might be expected to yield improved reaction rates, as
the metal center is more electron rich than in the related
palladium center. This feature induces more effective back-
bonding to the alkenes in 8, and hence the CC distances are
somewhat longer in 8 than in 9 and 11. Interestingly, the
increased π back-donation to the alkene in 14 is not reflected in
the CC bond distance to the extent that it is in the CO
and the Pd−alkene bond distances. For other M(erNHC)
complexes it has been noted that the carbene carbon exhibits
[
Pd(erNHC)(MA) ] complexes 8−14 (as noted from
1
,2
13
1
C{ H} NMR), and dvtms, albeit chelating, is a more weakly
−1
bound ligand. This is indeed observed: TOF (h ) values are
exceptionally high, and quantitative conversion of the 1-phenyl-
1
3
-propyne substrate occurred between 45 min and 2 h (Table
, entries 8−11). It appears that double-bond isomerization and
pyramidal distortion, measured by the distance of C
out of
NHC
24b
the N−Pd−N plane.
For the present complexes this
over-reduction of the alkene also increases for these catalyst
systems. Nevertheless, a consideration of plots of conversion
versus time for [Pd(erNHC)(dvtms)] complexes 1, 2, 4, and 5
distortion is barely evident, the aforementioned distance
ranging from 0.015 Å in 8 down to 0.007 Å for 14.
Transfer Hydrogenation Catalysis. The catalytic per-
formances of novel [Pd(erNHC)(MA) ] complexes 8−14
(
Figures 5 and 6 and Figures S1 and S2 in the Supporting
1
,2
Information) shows that double-bond isomerization and over-
reduction largely occur only when all the alkyne substrate is
consumed. In a mixture of alkyne substrate and alkene product,
the alkyne is preferentially (and strongly) coordinated and only
when all alkyne is consumed will the alkene coordinate, leading
to the observed double-bond isomerization and over-reduction.
Chemoselectivity is influenced by the lifetime of the [Pd-
were evaluated in the transfer hydrogenation of 1-phenyl-1-
propyne (Scheme 5) and compared to that of the related five-
1
1
membered analogues [Pd(5-NHC)(MA)2]
(Table 3).
Furthermore, to provide additional comparisons with the
Pd(NHC)(MA) ] complexes, we have also tested the
[
1
,2
27c
previously described [Pd(erNHC)(dvtms)] complexes 1, 2,
11
(
NHC)(product alkene)] species; more strongly donating
Scheme 5. Product Distribution for the Transfer
Hydrogenation of 1-Phenyl-1-propyne
bulky ligands should destabilize this species, and hence, it may
be expected that increasing NHC ring size should lead to lower
amounts of alkane being formed.
An interesting NHC ligand, wherein one of the N-aryl
substituents is an o-anisidyl group (Oans) (see Scheme 2,
27c
complex 4), is thought to stabilize catalytic intermediates and
improve catalytic performance. The ligand was previously
applied in iridium-catalyzed transfer hydrogenation (leading to
2
2a,24a
significantly enhanced activities)
catalyzed Mizoroki−Heck cross-coupling
and in palladium-
27c
(complex 4 is
1
35
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Table 3. Transfer Hydrogenation of 1-Phenyl-1-propyne Utilizing [Pd(erNHC)(MA) ] Complexes 8, 9, 10, 13, and 14 and
1
,2
a
[Pd(erNHC)(dvtms)] Complexes 1, 2, 4, and 5
c
selectivity (Z/E/allyl/alkyl)
TOF, h−
1.06
32.4
1
b
conversion after 24 h, %
after 0.5 h
after 24 h
entry
complex
o
1
2
3
4
5
6
7
8
9
[Pd(6- Tol)(MA) ] (8)
33
98/2/tr/tr
97/2/0/0
80/3/17/1
85/2/11/2
74/9/11/7
86/4/7/3
94/4/1/1.4
2/8/0/91
1/4/0/96
<1/3/0/96
0/2/0/98
2
[Pd(IMes)(MA)2]
75
[Pd(6-Mes)(MA) ] (9)
4.3
46
2
[Pd(7-Mes)(MA) ] (13)
0.32
7
2
[Pd(5-DIPP)(MA)2]
47.6
100
[Pd(6-DIPP)(MA)] (10)
[Pd(7-DIPP)(MA)] (10)
[Pd(6-Mes)(dvtms)] (1)
[Pd(7-Mes)(dvtms)] (5)
[Pd(6-Mes/Oans)(dvtms)] (4)
7.5
40.84
72.7
73
19
∼100 (2 h)
∼100 (1 h)
∼100 (2 h)
∼100 (45 min)
81/10/6/3
86/3/2/8
87/3/0/10
88/3/0/9
97.6
1
0
1
91.1
o
1
[Pd(6-Tol )(dvtms)] (2)
142.3
a
b
Reaction conditions: 1-phenyl-1-propyne (150 mmol), HCO H/NEt (440/600 mmol), and catalyst (1.5 mmol) in refluxing MeCN for 24 h.
TOF values determined after 50% conversion or after 24 h if 50% conversion was not achieved. Selectivity, conversion, and TOF (h ) determined
2
3
c
−1
by GC.
Figure 5. Transfer hydrogenation of 1-phenyl-1-propyne with [Pd(6-Mes)(dvtms)] (1).
Figure 6. Transfer hydrogenation of 1-phenyl-1-propyne with [Pd(7-Mes)(dvtms)] (5).
highly active in Mizoroki−Heck coupling). Consequently, it
was considered that the o-anisidyl substituent could have a
similar impact on catalytic performance in the transfer
hydrogenation of alkynes. Indeed, for complex 4 the reaction
rate increased significantly relative to those observed for
complexes 1 and 5 (over-reduction to alkanes occurred after
the alkyne substrate had been consumed; see Figure S2 in the
Supporting Information). However, and most interestingly, by
far the most active catalyst overall was complex 2 (Scheme 2),
When complexes of the type [Pd(6-NHC)(dvtms)] (1, 2
and 4) are compared, catalytic activity appears to decrease
significantly as the steric bulk of the 6-NHC ligand increases (2
>
4 > 1; Table 3, entries 8, 10, and 11), which is consistent with
the trend noted above for complexes containing MA as the
coligand. However, in contrast, complex 5 (containing the
bulky 7-Mes ligand) appears to be significantly more active than
the analogous system 1, with the less sterically demanding 6-
Mes ancillary ligand. In fact, the complex [Pd(7-Mes)(dvtms)]
(
5) was the bulkiest system we were able to prepare. It is
which has the least sterically demanding N-aryl substituents
therefore possible that steric pressure exerted by strongly
bound 7-Mes was sufficient to lead to rapid generation of the
active catalyst and subsequently provide steric protection for a
highly coordinatively unsaturated active species.
o
(
Tol ); however, as previously observed, over-reduction rapidly
occurs following alkyne consumption (see Figure S2 in the
Supporting Information).
1
36
dx.doi.org/10.1021/om300930w | Organometallics 2013, 32, 131−140

Organometallics
Article
Previous mechanistic studies have shown that the presence of
was evident. After this period, the reaction mixture was again
heated to reflux for several more hours, whereupon catalysis
recommenced and continued at approximately the same rate
previously noted. It is therefore evident that the active catalyst
is stable for prolonged periods of time when in the presence of
unreacted alkyne (or product alkene).
maleic anhydride appears to have a positive influence on the
32
chemoselectivity of the transfer hydrogenation reaction.
Therefore, maleic anhydride (1 equiv relative to Pd) was
added to reaction mixtures containing complexes 1−3 and 5−7
Table 4). As expected, addition of maleic anhydride led to an
(
CONCLUSIONS
Table 4. Transfer Hydrogenation of 1-Phenyl-1-propyne
■
with [Pd(erNHC)(dvtms)] Complexes 1−3 and 5−7 in the
In the search for electron-rich catalysts for the transfer
hydrogenation of alkynes, we have synthesized a series of
novel palladium(0) complexes containing expanded-ring N-
heterocyclic carbene ligands and one or two maleic anhydride
coligands. The complexes were characterized using IR and
NMR spectrometry, and their solid-state structures were
investigated using single-crystal X-ray diffraction. The struc-
tures displayed interesting geometries; due to the increased N−
a
Presence of Maleic Anhydride
bc
selectivity , (Z/E/
TOF ,
−1
b
conversion
init
entry
1
complex
h
after 5 h, %
allyl/alkyl)
[Pd(6-Mes)
dvtms)] (1)
24.3
3.4
32
28
40
35
45
35
61
79.2/2.2/17.8/0.2
90.1/2.6/5.9/1.3
98.6/1.3/0/0.1
98.6/1.4/0/0
(
2
3
4
5
6
7
[Pd(7-Mes)
(
dvtms)] (5)
o
[Pd(6-Tol )
12.2
4.3
C
−N bond angles of the carbene, the complexes with very
NHC
(
dvtms)] (2)
bulky 2,6-diisopropyl (DIPP) N-aryl substituents are able to
incorporate only one alkene moiety, leading to electronically
and coordinatively unsaturated 14-electron [Pd(erNHC)-
(MA)] complexes. Applying the [Pd(erNHC)(dvtms)] com-
plexes to the transfer hydrogenation of 1-phenyl-1-propyne
with triethylammonium formate/triethylamine as the hydrogen
donor demonstrates that bulky, electron-rich erNHC ligands
result in significantly increased activities, albeit with lower
selectivities in comparison to those for traditional [Pd(5-
o
[Pd(7-Tol )
(
dvtms)] (6)
[Pd(6-Xyl)
dvtms)] (3)
9.4
82.0/2.2/15.6/0.3
85.8/3.4/8.8/2.0
91.9/2.0/0/5.5
(
[Pd(7-Xyl)
dvtms)] (7)
9.6
(
[Pd(IMes)(MA)
MeCN)2]
7.4
(
a
Reaction conditions: 1-phenyl-1-propyne (150 mmol), HCO H/
2
NEt (440/600 mmol), catalyst (1.5 mmol), and maleic anhydride (1.5
3
b
NHC)L ] complexes. We are currently exploring possibilities to
n
mmol) in refluxing MeCN for 24 h. TOF values determined after 1 h.
Selectivity determined at end of experiment (5 h reflux, 40 h at
c
obtain higher stereoselectivity in the hydrogenation while
keeping the increased activity.
ambient temperature overnight, 2 h reflux).
EXPERIMENTAL SECTION
apparent improvement in selectivity to the (Z)-alkene, but with
decreased activity. However, as noted above, a plot of
conversion versus time (Figure 7) reveals that the addition of
MA has simply slowed down the overall rate of transfer
hydrogenation, including that for the conversion of alkene to
alkane. Therefore, as demonstrated for all other catalysts
discussed here, over-reduction of the (Z)-alkene to alkane only
occurs after all the alkyne has been consumed, but in this case
at a lower rate.
■
General Remarks. The synthesis of all complexes was carried out
in dried glassware using standard Schlenk techniques under an
atmosphere of purified nitrogen. Solvents were dried according to
38
39
standard procedures. All formamidines, amidinium tetrafluorobor-
22a,26a
40
41
ates,
(
[Pd(tBuDAB)(MA)], [Pd(nbd)(MA)], [Pd(IMes)-
3
2
27c
MA) ], and [Pd(erNHC)(dvtms)] complexes 1−7
were
2
prepared according to literature procedures. Maleic anhydride was
recrystallized from hot dichloromethane to remove maleic acid and
stored in a Schlenk flask under nitrogen. Potassium tert-butoxide was
evacuated overnight and stored in a Schlenk flask under nitrogen.
Other compounds were ordered from commercial sources and used
without further purification. Gas chromatographic analyses were run
on a Carlo Erba HRGC 8000 Top instrument with a DB-5 column and
p-xylene as internal standard. NMR spectra were recorded on a Bruker
The following experiment, which was undertaken to test
overall catalyst stability, is of particular interest. During the
transfer hydrogenation of 1-phenyl-1-propyne with the catalyst
o
system {[Pd(6-Tol )(dvtms)] + MA}, using 0.1 mol % catalyst
loading, the reaction mixture was refluxed in MeCN for 5 h (at
this point approximately 30% conversion of alkyne to alkene
was noted; see Figure S3 in the Supporting Information). The
reaction mixture was then left at ambient temperature for an
additional 40 h, during which time no further hydrogenation
1
13
1
DRX300 spectrometer ( H, 300.11 MHz; C{ H}, 75.47 MHz), a
1
13
1
Bruker ARX400 spectrometer ( H, 400.13 MHz; C{ H}, 100.61
1
MHz), a Varian Mercury 300 spectrometer ( H, 300.13 MHz;
13
1
1
C{ H}, 75.48 MHz), or a Varian Inova 500 spectrometer ( H,
499.86 MHz; 13C{ H}, 125.70 MHz). Positive chemical shifts (ppm)
1
Figure 7. Transfer hydrogenation of 1-phenyl-1-propyne with {[Pd(6-Mes)(dvtms)] (1) + 1 equiv of maleic anhydride}.
1
37
dx.doi.org/10.1021/om300930w | Organometallics 2013, 32, 131−140

Organometallics
Article
denoted in the H and 13C NMR data are reported relative to TMS
1
CC), 48.34 (NCH ), 21.82 (NCH CH ), 21.19 (p-CH ), 18.17 (o-
2
2
2
3
1
13
and were determined by reference to residual H and C solvent
resonances. IR spectra were recorded on a Shimadzu 8400s FT-IR
spectrophotometer.
CH ) ppm. IR: 1847, 1777 (CO), 1713, 1556, 1528 (CC→M)
3
−
1
cm . MS (FAB+): m/z observed 426.1300 for C H N Pd [M − 2
2
2
28
2
+
+
MA] /497.14 for C H N Pd [M − MA − CO + H] .
22
28
2
General Procedure for the Synthesis of [Pd(erNHC)-
(1,3-Bis(2,6-diisopropylphenyl)tetrahydropyrimidin-2(1H)-
2
(
[
alkene)1,2] Complexes 8−14 via Coligand Displacement. The
ylidene)(η -maleic anhydride)palladium (10). The product was
1
Pd(erNHC)(dvtms)] complexes (1 mmol) were suspended in
obtained as an orange powder in 29% yield. H NMR (400 MHz,
CD Cl ): δ 7.26 (t, 2H J = 8.0 Hz, p-C H), 7.16 (d, 4H, J = 8.0 Hz,
m-C H), 3.30 (t, 4H, J = 6.0 Hz, NCH ), 3.02 (septet, 4H, J = 6.9
3
3
toluene (20 mL) with stirring at ambient temperature. To this stirred
suspension was added maleic anhydride (2 mmol) in one portion
under a cone of nitrogen, resulting in a color change to orange, red, or
brown, depending on the carbene N-aryl substituents. The reaction
mixture was stirred for 1−2 h at room temperature, upon which a
yellow to red solid precipitated. The supernatant was taken off and the
solid washed in n-pentane, furnishing the desired complexes. For the
seven-membered-ring NHC complexes 12−14, it was necessary to
concentrate the toluene suspension before adding pentane. The
obtained crude product was then dissolved in dichloromethane and
isolated by precipitation with diethyl ether or pentane. For seven-
membered-ring NHC complexes it can be necessary to quickly filter
the dichloromethane solution over a bed of dried Celite before
precipitation, to remove Pd black.
2
2
A
r
3
3
Ar
2
3
Hz, CHMe ), 2.90 (s, 2H, HCCH), 2.25 (p, 2H, J = 6.6 Hz,
2
3
3
NCH CH ), 1.31 (d, 12H, J = 6.9 Hz), 1.17 (d, 12H, J = 6.9 Hz)
2
2
13
ppm. C NMR (100 MHz, CD Cl ): δ 206.83 (NCN), 169.70 (C
2
2
O), 146.06 (o-C ), 134.52 (i-C ), 129.21 (p-C ), 124.99 (m-C ),
Ar
Ar
Ar
Ar
48.20 (CC), 46.71 (NCH ), 29.36 (CHMe ), 25.32 (CH ), 25.02
2
2
3
(CH ), 21.03 (NCH CH ) ppm. IR: 1798, 1730 (CO), 1552, 1512
3
2
2
−
1
(CC→M) cm . MS (FAB+): m/z observed 510.2241 for
+
C H N Pd [M − MA] /581.2372 for C H N O Pd [M + CO +
2
8
40
2
31 43
2
2
+
H] .
(1,3-Bis(4-methylphenyl)tetrahydropyrimidin-2-(1H)-ylidene)bis-
2
(η -maleic anhydride)palladium (11). This product gave difficulties
in workup and could only be isolated in low yields (±5%). By
concentration of the reaction mixture, addition of diethyl ether, and
standing for several weeks, small yellow crystals were obtained that
General Procedure for the in Situ Synthesis of [Pd(erNHC)-
(
(
alkene)1,2] Complexes 8−14. The azolium tetrafluoroborate salts
t
1
1 mmol) and KO Bu (1.6 mmol) were suspended in toluene (20 mL)
were suitable for crystallographic and spectroscopic analysis. H NMR
3
and stirred at room temperature for 2 h (six-membered rings) or
overnight (seven-membered rings) to generate the free carbene. This
solution was either filtered via a cannula or decanted over a bed of
dried Celite into a dried Schlenk (the Celite was flushed with 2 × 10
mL of toluene), to remove potassium salts. The solution of Pd(dvtms)
in dvtms (10.74% Pd, 1 mL, 1 mmol) was added dropwise to the free
carbene solution, and this clear yellow solution was stirred overnight at
room temperature, resulting in a color change to bright yellow-orange.
The alkene (2 mmol) was added in one portion, resulting in a color
change to orange, red, or brown, depending on the carbene N-aryl
substituents. The reaction mixture was stirred for 1−2 h at room
temperature, upon which a yellow to red solid precipitated. The
supernatant was taken off and the solid washed in n-pentane,
furnishing the desired complexes. For the seven-membered-ring
NHC complexes 12−14, it was necessary to concentrate the toluene
suspension before adding pentane. The obtained crude product was
then dissolved in dichloromethane and isolated by precipitation with
diethyl ether or pentane. For seven-membered-ring NHC complexes it
can be necessary to quickly filter the dichloromethane solution over a
bed of dried Celite before precipitation, to remove Pd black.
(300 MHz, CD Cl ): δ 7.08 (dd, 8H, J = 6.x Hz, C H), 3.78 (broad,
2
2
Ar
8H, NCH + HCCH), 2.40 (quintet, 2H, NCH CH ), 2.31 (s, 6H,
2
2
2
13
CH ) ppm. C NMR (300 MHz, CD Cl ): δ 204.72 (NCN), 168.13
3
2
2
(CO), 145.59 (i-C ), 138.32 (CHMe), 130.08 (o-C ), 125.65 (m-
Ar
Ar
C ), 65.49 (CC), 48.64 (NCH ), 21.40 (NCH CH ), 20.72 (CH )
Ar
2
2
2
3
+
ppm. MS (FAB ): m/z observed 441.0803 for C H N O Pd [M −
21
22
2
2
+
MA − CO + H] .
2
(1,3-Bis(2-methylphenyl)-1,3-diazepan-2-ylidene)bis(η -maleic
anhydride)palladium (12). The product was obtained as a dark purple
1
3
powder in 11% yield. H NMR (400 MHz, CD Cl ): δ 7.48 (d, 2H, J
2
2
= 6.8 Hz, o-C H), 7,33 (br s, 6H, m-C H and p-C H), 5.08 (br s,
Ar
Ar
Ar
2H, HCC), 4.26 (br s, 4H, NCH ), 2.42 (br s, 4H, NCH CH ) 2.40
2
2
2
13
(br s, 6H, o-CH ) ppm. C NMR (100 MHz, CD Cl ): δ 167.86
3
2
2
(CO), 142.93 (i-C ), 133.58 (o-C Me), 132.32 (o-C H), 130.54
Ar
Ar
Ar
and 128.55 (2 m-C ), 127.25 (p-C ), 62.90 (CC), 55.48 (NCH ),
Ar
Ar
2
25.61 (NCH CH ), 17.19 (o-CH ) ppm. IR: 1773, 1733, 1654 (CO),
2
2
3
−1
1490 (CC→M) cm . MS (FAB+): m/z observed 385.15, but
product is too poorly soluble for exact mass measurement.
2
(1,3-Bis(2,4,6-trimethylphenyl)-1,3-diazepan-2-ylidene)bis(η -
maleic anhydride)palladium (13). The product was obtained as a
bright red powder in 45% yield. H NMR (400 MHz, CD Cl ): δ 7.02
1
(
1,3-Bis(2-methylphenyl)tetrahydropyrimidin-2-(1H)-ylidene)bis-
η -maleic anhydride)palladium (8). The product was obtained as a
red-brown powder in 63% yield. Crystals were obtained by slow
diffusion of diethyl ether into a saturated dichloromethane solution.
H NMR (400 MHz, CD Cl ): δ 7.32 (d, 2H, J = 7.5 Hz, m-C H),
.24 (t, 2H, J = 7.5 Hz, p-C H), 7.13 (t, 2H, J = 7.2 Hz, m-C H),
2
2
2
(
(s, 4H, C H), 4.18 (br s, 4H, NCH ), 3.83 (br s, 4H, HCCH), 2.57
(br. s, 8H, p-CH ), 2.47 (br s, 12H, NCH CH + o-CH ) ppm. C
3 2 2 3
Ar
2
1
3
NMR (100 MHz, CD Cl ): δ 226.32 (NCN), 168.22 (CO), 145.39
2
2
1
3
2
2
Ar
(p-C ), 138.57 (o-C ), 135.37 (i-C ), 130.05 (m-C ), 68.90 (C
Ar Ar Ar Ar
3
3
7
6
3
C), 57.79 (NCH ), 25.32 (NCH CH ), 21.02 (p-CH ), 18.59 (o-CH )
Ar
Ar
2 2 2 3 3
3
3
−1
.79 (d, 2H, J = 7.7 Hz, o-C H), 4.05 (d, 2H, J = 4.6 Hz, HCC),
ppm. IR: 1779, 1711 (CO), 1653, 1517, 1478 cm (CC→M). MS
Ar
3
3
+
.79 (dt, 2H, J = 12.5 Hz, 6.0 Hz, NCH ), 3.64 (d, 2H, J = 4.6 Hz,
(FAB+): m/z observed 440.1443 for C H N Pd [M − 2 MA] .
2
23 30
2
3
2
HCC), 3.59 (m, 2H, J = 6.0 Hz, NCH ), 2.65 (s, 6H, o-CH ), 2.47
(1,3-Bis(2,6-diisopropylphenyl)-1,3-diazepan-2-ylidene)(η -maleic
anhydride)palladium (14). The product was obtained as a brown
powder in 34% yield. Crystals were obtained by slow diffusion of
2
3
3
13
(
p, 2H, J = 6.0 Hz, NCH CH ) ppm. C NMR (100 MHz, CD Cl ):
2
2
2
2
δ 207.17 (NCN), 169.44 and 167.93 (2 CO), 146.80 (i-C ), 134.10
Ar
1
(
o-C Me), 132.00 (o-C H), 129.31 and 129.06 (2 m-C ), 128.17 (p-
diethyl ether into a saturated dichloromethane solution. H NMR (400
Ar
Ar
Ar
C ), 65.84 and 65.45 (2 CC), 48.20 (NCH ), 21.34 (o-CH ), 18.36
MHz, CD
3.26 (septet, 4H, J = 6.9 Hz, CHMe
2
2
Cl
2
): δ 7.38−7.26 (m, 6H, CArH), 3.89 (br t, 4H, NCH
2
),
Ar
2
3
−1
3
(
NCH CH ) ppm. IR: 1813, 1753 (CO), 1505 (CC→M) cm .
), 2.81 (s, 2H, HCCH), 2.31
2
2
3
3
MS (FAB+): m/z observed 469.0759 for C H N O Pd [M − MA +
(br dt, 4H, NCH
2
CH
2
), 1.40 (d, 12H, J = 6.9 Hz), 1.29 (d, 12H, J =
6.9 Hz) ppm. C NMR (100 MHz, CD Cl ): δ 214.86 (NCN),
170.01 (CO), 144.75 (i-CAr), 135.59 (o-CAr), 128.57 (p-CAr),
125.23 (m-CAr), 55.00 (NCH ),
), 48.07 (CC), 29.66 (CHMe
26.40 (NCH CH ), 25.55 (CH ), 24.87 (CH ) ppm. IR: 1794, 1726
33
37
2
8
+
13
H] . Anal. Calcd for C H O N Pd: C, 55.09; H 4.27; N, 4.94.
2
2
26
24
6
2
Found: C, 55.04; H, 4.27; N, 4.95.
1,3-Bis(2,4,6-trimethylphenyl)tetrahydropyrimidin-2-(1H)-
ylidene)bis(η -maleic anhydride)palladium (9). The product was
obtained as a yellow powder in 47% yield. Crystals were obtained by
slow diffusion of diethyl ether into a saturated dichloromethane
(
2
2
2
2
2
3
3
−1
(CO), 1509, 1458 (CC→M) cm . MS (FAB+): m/z observed
+
595.2526 for C H N O Pd [M + MA − CO + H] .
32
45
2
2
1
solution. H NMR (400 MHz, CD Cl ): δ 6.92 (s, 4H, C H), 3.79
General Procedure for Transfer Hydrogenation of 1-Phenyl-
1-propyne. A stock solution was made of 1-phenyl-1-propyne (150
mM), p-xylene (150 mM), and triethylammonium formate (450 mM
of HCO H and NEt ) in dry MeCN, which was divided over separate
2
2
Ar
3
(
br s, 4H, HCCH), 3.49 (t, 4H, J = 6.0 Hz, NCH ), 2.41 (m, 2H,
2
13
NCH CH ), 2.30 (s, 12H, o-CH ), 2.25 (s, 6H, p-CH ) ppm.
C
2
2
3
3
NMR (100 MHz, CD Cl ): δ 208.97 (NCN), 168.33 (CO), 143.54
2
2
2
3
(
i-C ), 139.01 (p-C ), 135.58 (o-C ), 130.18 (m-C ), 68.04 (br s,
reaction vessels (13 mL/reaction) and heated to 80 °C. The catalysts
Ar
Ar
Ar
Ar
1
38
dx.doi.org/10.1021/om300930w | Organometallics 2013, 32, 131−140

Organometallics
Article
(
1.5 mM final concentration; ±12 mg) were weighed into an alkaloid
REFERENCES
■
tube and dissolved in 1 mL of MeCN before addition to the reaction
vessel; for reactions in which maleic anhydride was added, this was
mixed with the catalyst prior to addition of MeCN. Immediately after
addition of catalyst an aliquot (0.05 mL) of the reaction mixture was
taken, and this was diluted in a GC vial with 1 mL of EtOH to quench
the reaction in the sample. Sampling was repeated at regular time
intervals, in order to monitor the reaction by GC. Rates were
determined for the initial linear part of the reaction profiles. The
reaction rates varied over an order of magnitude for the various
catalysts; thus, in order to compare these, TOFs at 50% conversion are
depicted in Table 3.
Crystallographic Details. Suitable single crystals for all
compounds were coated with Paratone-N oil, mounted using a glass
fiber pin, and frozen in the cold nitrogen stream of the goniometer. X-
ray diffraction data were collected on a Bruker AXS APEX CCD
diffractometer equipped with a rotating anode using graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å) with a scan
width of 0.3°, and the generator setting was 50 kV and 180 mA.
Diffraction data were collected over the full sphere and were corrected
for absorption. Compound 11 was measured on a Bruker Kappa
ApexII diffractometer with sealed tube and Triumph monochromator
(
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λ = 0.71073 Å) using φ and ω scans. The data reduction was
42
(
2
performed with the Bruker SAINT program package. For further
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1999, 1821.
43
2
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ASSOCIATED CONTENT
■
1
*
S
Supporting Information
Figures, tables, and CIF files providing kinetic data for the
(
transfer hydrogenation of 1-phenyl-1-propene with complexes
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2
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AUTHOR INFORMATION
■
G. R. J. Angew. Chem., Int. Ed. 1995, 34, 2371. (b) Scholl, M.; Trnka,
T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron Lett. 1999, 40, 2247.
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Corresponding Author
E-mail: cavellKJ@cf.ac.uk (K.J.C.); C.J.Elsevier@uva.nl
C.J.E.).
*
(
b) Crabtree, R. H. J. Organomet. Chem. 2005, 690, 5451. (c) Hahn, F.
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Present Addresses
⊥
Frames Gas Processing, Dr. A.D. Sacaharovlaan 2, 2405 WB
Alphen aan den Rijn, The Netherlands.
(
#
School of Chemistry, University of Nottingham, University
Park, Nottingham NG7 2RD, U.K..
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
L.; Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc. 2005, 1227, 2485.
(c) Gusev, D. G. Organometallics 2009, 28, 763. (d) Gusev, D. G.
Organometallics 2009, 28, 6458. (e) Kelly, R. A.; Clavier, H.; Giudice,
S.; Scott, N. M.; Stevens, E. D.; Bordner, J.; Samardjiev, I.; Hoff, C. D.;
Cavallo, L.; Nolan, S. P. Organometallics 2008, 27, 202. (f) Khramov,
D. M.; Rosen, E. L.; Er, J. A.; Vu, P. D.; Lynch, V. M.; Bielawski, C. W.
Tetrahedron 2008, 64, 6853. (g) Leuthausser, S.; Schwarz, D.; Plenio,
H. Chem. Eur. J. 2007, 13, 7195. (h) Nolan, S. P. N-Heterocyclic
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Notes
The authors declare no competing financial interest.
(
i) Dro
(
(
̈
ge, T.; Glorius, F. Angew. Chem., Int. Ed. 2010, 49, 6940.
16) Crudden, C. M.; Allen, D. P. Coord. Chem. Rev. 2004, 248, 2247.
17) Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L.
ACKNOWLEDGMENTS
■
The work was funded by the National Research School
Combination Catalysis in The Netherlands. We also gratefully
acknowledge the EPSRC and the Cardiff Catalysis Institute for
J.J.D.’s Ph.D. studentship.
Coord. Chem. Rev. 2009, 253, 687.
(18) Schuster, O.; Yang, L. R.; Raubenheimer, H. G.; Albrecht, M.
Chem. Rev. 2009, 109, 3445.
1
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36) This coligand displacement protocol has also been utilized to
̈
(
access expanded-ring NHC Pd(II) complexes via oxidation of the
parent [Pd(erNHC)(dvtms)] precursors; these results will be reported
shortly.
(
37) The 13C signal of the carbene carbon in [Pd(7-Tol )(MA) ]
o
2
could not be observed. For some of the previously reported
3
6
[
Pd(erNHC)(dvtms)] complexes, this signal has proven to be very
13
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1
40
dx.doi.org/10.1021/om300930w | Organometallics 2013, 32, 131−140