2-Aminopyridinate titanium complexes for the catalytic hydroamination of primary aminoalkenes

Article abstract of DOI:10.1021/om3012695

A series of mono(2-aminopyridinato)tris(dimethylamido) titanium complexes, ApTi(NMe₂)₃ (where Ap = 2-aminopyridinato), have been prepared via protonolysis, and their reactivity for the hydroamination of primary aminoalkenes has been explored. The Ti complex incorporating N,6-dimesityl-2-aminopyridinate as the supporting ancillary ligand has been shown to yield a catalyst suitable for room-temperature intramolecular hydroamination reactions to give gem-disubstituted five-and six-membered-ring products. The comparison of ApTi(NMe₂)₃ with other group 4 catalysts shows that controlling the steric environment at the metal center is the critical determining factor for hydroamination reactivity. The screening of known challenging primary aminoalkene substrates with the most reactive ApTi(NMe₂)₃ shows good breadth of reactivity for the reaction. This complex is not able to cyclize secondary aminoalkene substrates, suggesting this reaction proceeds via an intermediate imido [2+2] cycloaddition pathway. An Ap-supported Ti imido complex, which also exhibits hydroamination activity, has been prepared and fully characterized from ApTi(NMe ₂)₃ and 2,6-dimethylaniline.

Full text of DOI:10.1021/om3012695

Article
pubs.acs.org/Organometallics
2‑Aminopyridinate Titanium Complexes for the Catalytic
Hydroamination of Primary Aminoalkenes
Eugene Chong,† Sadaf Qayyum,‡ Laurel L. Schafer,*,† and Rhett Kempe‡
†Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1
‡Lehrstuhl fur Anorganische Chemie II, Universitat Bayreuth, 95440 Bayreuth, Germany
̈
̈
S
* Supporting Information
ABSTRACT: A series of mono(2-aminopyridinato)tris(dimethylamido) titanium
complexes, ApTi(NMe2)3 (where Ap = 2-aminopyridinato), have been prepared via
protonolysis, and their reactivity for the hydroamination of primary aminoalkenes has
been explored. The Ti complex incorporating N,6-dimesityl-2-aminopyridinate as the
supporting ancillary ligand has been shown to yield a catalyst suitable for room-
temperature intramolecular hydroamination reactions to give gem-disubstituted five-
and six-membered-ring products. The comparison of ApTi(NMe2)3 with other group
4 catalysts shows that controlling the steric environment at the metal center is the
critical determining factor for hydroamination reactivity. The screening of known
challenging primary aminoalkene substrates with the most reactive ApTi(NMe2)3
shows good breadth of reactivity for the reaction. This complex is not able to cyclize secondary aminoalkene substrates,
suggesting this reaction proceeds via an intermediate imido [2+2] cycloaddition pathway. An Ap-supported Ti imido complex,
which also exhibits hydroamination activity, has been prepared and fully characterized from ApTi(NMe2)3 and 2,6-
dimethylaniline.
Scheme 1. Hydroamination Reactions of Challenging
Aminoalkene Substrates with Group 4 Catalysts Can Give
Mixtures of Hydroamination (HA) and
INTRODUCTION
■
Nitrogen-containing molecules are ubiquitous in biologically
active compounds. As such, efficient synthetic routes to access
small-molecule amine building blocks are highly desired for
agrochemical and pharmaceutical applications. Alkene hydro-
amination,1,2 the addition of an N−H bond across a CC
bond, provides an atom-economical route to higher substituted
amines via the formation of a new C−N bond. Group 4 metals
are particularly attractive for this transformation because of
their synthetic applicability, relatively low cost, and low toxicity.
After the initial reports of group 4-mediated hydroamination in
the early 1990s,3−5 the first breakthrough in alkene hydro-
amination was realized about a decade later: cationic Zr and Ti
systems6,7 and neutral complexes such as the commercially
Hydroaminoalkylation (HAA) Products
development of more selective and active Ti catalyst systems
are a challenge within the field.
In the pursuit of developing an easily accessed, robust, and
effective group 4 hydroamination catalyst, ancillary ligands that
incorporate hard donor atoms such as N,O-chelating
(amidate,42,57 2-pyridonate,38 and ureate35,45,47) ligands have
been shown to be very effective for hydroamination catalysis by
our group and others.18,19,22,31,36 The study of complementary
N,N-chelating (amidinate,58 guanidinate,33,59 and 2-amino-
pyridinate (Ap)20,60−62) ligands for group 4 hydroamination
catalysis has previously revealed reduced hydroamination
reactivity profiles, in comparison with N,O-chelating motifs.
Furthermore, the use of a less sterically demanding Ap Ti
complex (N-methyl-2-aminopyridinate) has been recently
reported by Doye for intermolecular hydroaminoalkylation
catalysis.56 Mixed Zr cyclopentadienyl−guanidinate com-
plexes33 and both Ti and Zr biphenyl-tethered Ap complexes20
8
available Ti(NMe2)4 were found to cyclize secondary and
primary aminoalkenes, respectively. Significant progress has
been made since then,9−48 but a limitation in Ti- and Zr-
catalyzed aminoalkene hydroamination (HA) is that they can
undergo α-C−H bond functionalization adjacent to the amino
group, resulting in a new C−C bond and hydroaminoalkylation
side products (Scheme 1).15,49−56 Ti hydroamination cata-
lysts8−26 display limited substrate scope, sluggish reactivity, and
unwanted byproduct formation during cyclohydroamination of
alkenes.15,50 To date, the most broadly useful group 4 alkene
hydroamination catalysts have been reported to use Zr metal
centers.14,18−22,35−38,40,43−46 The smaller ionic radius of Ti and
its established utility as a hydroaminoalkylation catalyst may
rationalize the paucity of Ti hydroamination catalyst develop-
ment. Thus, development of a chemoselective catalyst for
hydroamination reactivity over hydroaminoalkylation and the
Received: January 2, 2013
Published: February 28, 2013
© 2013 American Chemical Society
1858
dx.doi.org/10.1021/om3012695 | Organometallics 2013, 32, 1858−1865

Organometallics
Article
have been tested for unactivated aminoalkene cyclohydroami-
nation, where slow catalysis was observed for both of these Zr
systems and no reactivity was observed for the latter Ti system.
These N,N-chelating ligands remain underexplored and,
considering the dramatically different reactivity reported thus
far, are an important set of complexes for developing an
understanding of hydroamination reactivity trends with group 4
metals.
Considering the asymmetric binding mode and known
hemilability of previously reported N,O-chelating amidate
complexes, the Ap ligands offer similar asymmetry while
affording improved opportunities for tailoring the steric bulk
about the metal center (Scheme 2).57,63−67 Furthermore, there
with aryl Grignard reagents, followed by Pd-catalyzed
Buchwald−Hartwig amination with aniline derivatives. The
variation of methyl and/or isopropyl substituents on the Ap
proligand allows the examination of the influence of the steric
environment at the metal center.77−86
The direct synthesis of Ti Ap complexes 4−6 is easily
accomplished using a protonolysis reaction between proligands
1−3 and commercially available Ti(NMe2)4.60,61,66−73 The
resulting products are isolated in high yields and are easily
isolated and purified (Scheme 4). For example, 4 is
Scheme 4. Synthesis of Mono(2-
aminopyridinato)tris(dimethylamido) Titanium Complexes
Scheme 2. Binding Mode of Amidate (Left),
Aminopyridinate (Center), and Amidinate (Right) Ligands
a
a
[M] = group 4 metal complex; R, R′ = substituent.
quantitatively obtained as an analytically pure, yellow solid
after the removal of the reaction solvent under vacuum, without
any further purification. In the synthesis of 5 or 6, trace
impurities are present upon completion of the reaction;
however purification is easily accomplished by recrystallization
from a solution of hexanes at −35 °C to afford yellow crystals
of 5 or orange crystals of 6.
is precedent for the preparation of monoligated Ap Ti metal
centers,66,67 while we have been unsuccessful to date in
preparing mono(amidate)-ligated group 4 complexes that are
resistant to ligand redistribution. Most importantly, there has
not been an investigation of such monoligated Ap Ti systems
for group 4-catalyzed hydroamination. Titanium Ap com-
plexes60,61,66−73 have been investigated by several research
groups in the past decade, with a focus on olefin polymer-
ization. Two Ti Ap complexes have been examined for
hydroamination, both as a bis-ligated systems; however no
alkene cyclohydroamination was disclosed.20,60,61 Currently,
there is not an effective or versatile Ti catalyst for alkene
cyclohydroamination. Herein, we report the preparation of
ApTi(NMe2)3 complexes with varying steric bulk, and the
finding of a significantly more reactive Ti catalyst with
enhanced substrate scope. The observed trends from catalytic
investigations reveal the importance of steric tuning to promote
enhanced reactivity. We also demonstrate that selective
hydroamination over hydroaminoalkylation can be achieved
through ligand design.
The solid-state molecular structures of 4−6 reveal a common
C1-symmetric structure with distorted trigonal bipyramidal
coordination about the Ti center (Figure 1, Table 1), with N1,
N3, and N4 being in the equatorial plane (∑ of the angles in
the equatorial plane are 358° (4), 355° (5), and 354° (6)).
Taking structure 5 as a representative example, there is a small
bite angle [N1−Ti1−N2 58.84(6)°] for the Ap ligand, in
agreement with similar, previously reported complexes.66,67 The
binding of this N,N-chelating ligand is best described as a
monoanionic amido/neutral pyridine bonding motif; the amido
N1 at the equatorial site binds to Ti at a much shorter distance
[Ti1−N1 2.0548(16) Å] than the pyridine N2 that occupies
the axial site [Ti1−N2 2.44478(17) Å]. This Ti1−N2 bond
length is unusually long compared to known Ti Ap systems
(2.107−2.349 Å),65 and the presence of three dimethylamido
ligands most likely promotes the loose coordination of the
pyridine N2 due to the π-donation and steric bulk of the NMe2
ligands (vide infra). The sum of the angles around the
dimethylamido N atoms is consistent with trigonal planar sp2-
hybridization, and their short bond lengths [1.8929(16)−
1.9154(17) Å] confirm the presence of multiple-bond
character. The bond lengths and angles of 4 and 6 are also
shown in Table 1 for comparison. There are no major
discernible differences in Ti−N bond lengths between 4 and 5.
However, the Ti1−N2 bond length of 6 is shorter than both 4
and 5, which can be attributed to the differing methyl and
isopropyl substituents on opposite sides of the Ap ligand.
ApTi(NMe2)3 complexes are best described as 16 e− species,
with each of the Ap and amido ligands acting as 4 e− donors to
the Ti4+ metal center. Most importantly, the different methyl
and isopropyl substituents of the ligands vary the steric
shielding of the Ti center.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of ApTi(NMe2)3. The
Ap proligands 1−3 (Scheme 3) can be synthesized from 2,6-
dibromopyridine in two steps according to literature
procedures.74−76 The bulky aryl substituents are installed
onto 2,6-dibromopyridine by Ni-catalyzed Kumada coupling
Scheme 3. Bulky 2-Aminopyridine Proligands
1859
dx.doi.org/10.1021/om3012695 | Organometallics 2013, 32, 1858−1865

Organometallics
Article
1
The H NMR spectra of complexes 4−6 in C6D6 show a
large and broad singlet for the dimethylamido protons in the
region δ 3.04−3.07. The dimethylamido signal integrates to 18
protons relative to the respective proton signals of the bound
Ap ligand, consistent with a monoligated complex. The
presence of steric congestion surrounding the metal center is
evident by the observation of hindered rotation of the aryl rings
on the Ap ligand when isopropyl substituents are present. In
complex 6, for example, two doublets (δ 1.33 and 1.21) are
observed for the two isopropyl substituents at R3, whereas one
singlet (δ 2.15) is observed for the two methyl substituents at
R2. The comparison of 13C NMR spectra of 4, 5, and 6 shows
that these complexes are electronically similar, as the Ap C1
signals are observed at comparable chemical shifts at δ 170.2,
168.9, and 170.3, respectively. Furthermore, the dimethylamido
carbon signals are present at nearly the same chemical shift at
ca. δ 45.9, confirming the electronic similarities between these
complexes. These complexes are thermally robust and show no
decomposition in d8-toluene when heated at 110 °C for one
week or at 145 °C over two days.
In contrast to known bis(Ap)-ligated Ti complexes, attempts
to prepare bis-ligated systems with these bulky ligands have not
been successful. The bis-ligated analogue of 4 cannot be
prepared, even when excess proligand 1 and Ti(NMe2)4 are
heated in noncoordinating solvent at 100 °C for extended
reaction times. In a similar experiment, when sterically less
bulky proligand 2 and the prepared complex 5 in a 1:1 ratio are
heated to 100 °C for one day, the reaction color changes from
1
orange to red. Inspection of the H NMR spectrum of the
reaction indicates the presence of 5 and a slight formation of
what may be a bis-ligated analogue of 5, as determined by the
integration of relative peaks. However, this species could not be
isolated.
Catalytic Hydroamination/Cyclization Reactions. To
investigate the catalytic activity of ApTi(NMe2)3 4−6, 2,2-
diphenyl-5-hexenyl-1-amine was chosen as the substrate for
screening experiments (Table 2). This substrate is a good test
substrate, as it is known to give both hydroamination and
hydroaminoalkylation products at 105 °C with Ti(NMe2)4.15
When the substrate and 10 mol % of 4−6 are left standing at
room temperature for 24 h, we were surprised to find room-
temperature activity for six-membered-ring hydroamination
Table 2. Screening of Complexes 4−6 and Group 4
Tetrakis(dimethylamido) Complexes for Intramolecular
Hydroamination/Cyclization
Figure 1. ORTEP representation of the solid-state molecular
structures of 4 (top), 5 (middle), and 6 (bottom) plotted with 50%
probability ellipsoids. All hydrogen atoms are omitted for clarity.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Complexes 4−6
4
5
6
Ti1−N1
Ti1−N2
Ti1−N3
Ti1−N4
2.0678(17)
2.4274(18)
1.901(3)
2.0548(16)
2.4478(17)
1.9154(17)
1.8929(16)
1.9037(16)
58.84(6)
2.081(4)
2.337(4)
a
b
entry
catalyst
yield (%) (HA:HAA)
1
2
3
4
5
6
4
5
6
5 (1:0)
86 (1:0)
20 (1:0)
50 (9:1)
27 (1:0)
<2 (1:0)
1.893(4)
1.900(12)
1.990(5)
1.911(4)
Ti1−N5
1.923(4)
Ti(NMe2)4
Zr(NMe2)4
Hf(NMe2)4
N1−Ti1−N2
N1−Ti1−N3
N1−Ti1−N4
N3−Ti1−N4
N1−Ti1−N5
N2−Ti1−N5
59.03(6)
60.50(14)
131.70(16)
109.65(16)
112.88(19)
98.32(16)
156.05(16)
133.65(12)
116.4(4)
123.09(7)
122.77(7)
109.32(7)
95.10(7)
107.9(3)
a
Combined NMR yield using 1,3,5-trimethoxybenzene as an internal
standard. HA = hydroamination; HAA = hydroaminoalkylation, ratio
93.52(13)
152.40(12)
b
153.94(6)
1
determined by H NMR spectroscopy.
1860
dx.doi.org/10.1021/om3012695 | Organometallics 2013, 32, 1858−1865

Organometallics
Article
(entries 1−3). Room-temperature activity for alkene hydro-
amination among group 4 systems is uncommon, as only a few
neutral Zr systems have been recently reported for such
reactivity.37,43,45 The activity of the ApTi(NMe2)3 system
increases with a decreasing amount of steric congestion (5 > 6
> 4) present at metal center, where the subtle difference
between the methyl and isopropyl substituents in proximity to
the metal is crucial. In addition, the commercially available
M(NMe2)4 (M = Ti, Zr, and Hf) have been tested for
comparison, and room-temperature activity is also observed
(entries 4−6). Among the group 4 metals of M(NMe2)4, Ti is
the most active for hydroamination, and the activity decreases
down the group (Ti > Zr > Hf). However, the formation of
hydroaminoalkylation product is observed when Ti(NMe2)4 is
utilized (entry 4). In contrast to Ti(NMe2)4, there is no
observable formation of hydroaminoalkylation product in the
reactions catalyzed by 4−6. Moreover, 5 is a much better
hydroamination catalyst than Ti(NMe2)4 (entry 2), but the
activities of 4 and 6 are significantly lower (entries 1 and 3).
These results demonstrate that too much steric congestion at
the metal center decreases the catalyst’s activity. Complex 5
provides a favorable amount of steric accessibility while
presumably sufficient steric bulk to inhibit undesired aggregate
formation and less reactive bis-ligated complexes via ligand
redistribution.
Table 3. Catalytic Hydroamination of Primary Aminoalkenes
by Complex 5
Previous investigations with a variety of bis-ligated
systems10,11,14,18−22 have shown that Zr complexes have
improved reactivity over their Ti congeners. Using in situ
catalyst preparation, we have shown that the cyclohydroami-
nation of 2,2-diphenyl-5-hexenyl-1-amine catalyzed by the
prepared complex 5 has the same reactivity as the in situ
reaction for Ti (eq 1); at 60 °C in 4 h, the reaction goes to
a
Reaction conditions: substrate (1 mmol), 5 (5 mol %), d8-toluene (1
b
c
d
mL). Isolated yield. 10 mol % catalyst. Isolated yield following
derivitization with TsCl. Isolated yield of the major isomer. cis/trans
e
f
dr.
good yield (entry 3). The Thorpe−Ingold effect87 is not
required, as 4-pentenyl-1-amine is cyclized, but high temper-
ature and increased catalyst loading are needed (entry 4).
Catalyst 5 also mediates the formation of α,α′-disubstituted
piperidines with excellent diastereoselectivity (cis/trans = 18:1),
with the preference of positioning methyl and the phenyl
substituent equatorially to minimize the 1,3-diaxial interaction
on the chairlike cyclization transition state (entry 5).88 Among
group 4 systems, only a few Zr catalysts have been reported for
hydroamination with unactivated internal alkenes,35,38 and here
both the trans- and cis-aminoalkenes can undergo hydro-
amination with 5 (entries 6 and 7). The combination of Ti with
the increased accessibility of the metal center due to the
monoligation of 5 results in reactivity that exceeds anything
previously observed for Ti and compares favorably with leading
Zr catalyst systems.14,18−22,35−38,40,43−46
completion by both methods. The in situ preparation involves
stirring 10 mol % of 2 with an equimolar amount of M(NMe2)4
for 5 min in d8-toluene, prior to the addition of substrate.
Equation 1 shows that the combination of this ligand with the
larger metals, Zr and Hf, is not favorable and the reactivity
significantly decreases as one increases the ionic radius. These
results are in agreement with the observed trends in the group
4 tetrakis(dimethylamido) complexes in Table 2.
Hydroamination Substrate Scope of 5. Encouraged by
the room-temperature hydroamination activity, the substrate
scope of 5 has been explored using 5 mol % catalyst loading
(Table 3). Catalyst 5 has a dramatically improved breadth of
reactivity over Ti(NMe2)4 and other known Ti systems.8−26
The hydroamination of five- and six-membered rings is readily
feasible in the presence of gem-disubstituents on the amino-
alkene backbone at room temperature or at 60 °C (entries 1
and 2). More importantly, this Ti complex can effectively
cyclize known challenging aminoalkene substrates (entries 3−
7). The selective formation of azepane without the formation of
α-alkylated product is known to be difficult, especially for Ti
systems that have been reported to be prone to unwanted
hydroaminoalkylation side product formation (vide supra).8,15,50
Here the formation of the seven-membered ring is achieved in
In an effort to probe the mechanism of hydroamination
catalysis it was noted that 5 is unable to promote the cyclization
of a secondary aminoalkene substrate at room temperature or
higher temperatures (eq 2). When N-methyl-2,2-diphenyl-4-
pentenyl-1-amine, in the presence of 5, is heated at 110 °C for
24 h, there is no formation of the N-substituted pyrrolidine
product. This result is in stark contrast to that of the primary
aminoalkene analogue (Table 3, entry 1) that undergoes
1861
dx.doi.org/10.1021/om3012695 | Organometallics 2013, 32, 1858−1865

Organometallics
Article
cyclization at room temperature. This is also significantly
different from several recently reported Zr catalysts, including
Zr(NMe2)4, that can indeed mediate secondary aminoalkene
cyclohydroamination.14,30,35,43−46 Notably, the Sadow group
has shown that with their zwitterionic Zr system secondary
aminoalkene cyclohydroamination can only be realized upon
the addition of a substoichiometric amount of primary amine.43
This is proposed to be due to the role of primary amine in the
proton-assisted C−N bond formation.43 However, a similar
experiment with 10 mol % of both aminopyridinate precatalyst
5 and n-hexylamine with N-methyl-2,2-diphenyl-4-pentenyl-1-
amine (modified eq 2) shows no reactivity at room temperature
nor with heating at 110 °C for 24 h. Since the formation of
imido species is inaccessible with a secondary amine substrate,
and 5 is catalytically inactive for such substrates even in the
presence of a primary amine additive, this observation suggests
that intermediate imido species are involved in the C−N bond
formation step of the catalytic cycle (i.e., [2+2] cycloaddition),
as has been previously reported for Ti hydroamination
catalysts.89,90
Synthesis and Characterization of an Imido Complex
from 5. Guanidinate-supported imido complexes have been
isolated and crystallographically characterized and have been
used as catalytically active complexes, albeit sluggish, for alkyne
hydroamination.59 To investigate whether 5 is able to support
such imido species, a stoichiometric reaction of 5 with 2,6-
dimethylaniline (2 equiv) has been carried out in the presence
of excess pyridine as a trapping agent (Scheme 5). Upon
Figure 2. ORTEP representation of the solid-state molecular structure
of 7 plotted with 50% probability ellipsoids for the non-hydrogen
atoms. Selected bond lengths (Å) and angles (deg): Ti1−N1,
2.1524(13); Ti1−N2, 2.1937(13); Ti1−N3, 1.7728(13); Ti1−N4,
1.9777(13); Ti1−N5, 2.2230(13); C24−N3−Ti1, 175.64(11); N1−
Ti1−N2, 61.90(5); N1−Ti1−N5, 90.51(5); N2−Ti1−N4, 99.37(5);
N3−Ti1−N4, 100.45(6); N4−Ti1−N5, 97.34(5); Ti1−N4−H1,
108.4(14).
to Ti [Ti1−N1, 2.1524(13) Å; Ti1−N2, 2.1937(13) Å],
presumably because of the absence of the electronically
saturated and sterically demanding dimethylamido ligands.
The isolated imido complex 7 is a competent catalyst for
alkene hydroamination, as shown in Table 4. In 24 h at room
Scheme 5. Synthesis of an Imido Complex 7 from 5 and 2,6-
Dimethylaniline
Table 4. Comparison of 5 and 7 for Intramolecular
Hydroamination
a
entry
catalyst
yield (%)
1
2
3
4
5
5
7
86
57
b
5
38
c
5
83
d
heating the reaction in benzene at 75 °C overnight and after the
removal of the reaction solvent under vacuum, the terminal
imido complex 7 is obtained. After recrystallization from
benzene/pentane, 7 is obtained as an analytically pure, orange
7
>98
a
1
Yield determined by H NMR spectroscopy using 1,3,5-trimethox-
ybenzene as an internal standard. 2,6-Dimethylaniline (0.20 equiv)
added. Pyridine (0.20 equiv) added. At 60 °C, 4 h.
b
c
d
1
crystalline solid in 77% yield. The H NMR spectrum of 7 in
C6D6 reveals a well-defined monoligated complex that is bound
by two 2,6-dimethylaniline (imido and amido) and one
pyridine; no dimer formation or equilibria with dimeric species
are observed.45 These characteristics are reminiscent of the
known cyclopentadienyl-supported Ti imido complex that is
active in allene hydroamination.91 An X-ray diffraction study of
single crystals of 7, grown from benzene via slow diffusion of
pentane, shows a C1-symmetric, distorted square pyramidal
structure (Figure 2). The imido TiN linkage is confirmed by
its short bond length [Ti1−N3 1.7228(13) Å] and the close to
linear bond angle [C24−N3−Ti1 175.64(11)°]. The second
aniline is bound by an amido linkage [Ti1−N4 1.9777(13) Å],
as seen by its longer bond length and its bent nature. In
contrast to 5, the Ap ligand of 7 is rather symmetrically bound
temperature, the comparison of 5 and 7 initially shows that 7 is
less active than 5 (entries 1 and 2). Upon closer examination
however, it is revealed that the presence of 2,6-dimethylaniline
dramatically slows the reaction catalyzed by 5 (entry 3),
whereas the presence of added pyridine does not inhibit
catalysis (entry 4). The slower activity in the presence of 2,6-
dimethylaniline can be explained by competitive metal binding
of the cyclizable aminoalkene substrate and the noncyclizable
2,6-dimethylaniline.40 This explanation is confirmed by heating
the reaction, to aid in the amido exchange processes, and here
the reaction catalyzed by 7 goes to completion in 4 h (entry 5),
in agreement with 5 (Table 3, entry 2).
1862
dx.doi.org/10.1021/om3012695 | Organometallics 2013, 32, 1858−1865

Organometallics
Article
Proposed Catalytic Cycle. Three different mechanisms
have been postulated for group 4-mediated alkene hydro-
amination: the imido [2+2] cycloaddition,10,32,40 the amido σ-
bond insertion mechanism,30 and the closely related proton-
assisted C−N bond formation.43,45,48 Due to the fact that 5 is
unreactive with secondary amines and is able to readily access a
catalytically active terminal imido species 7, the hydroamination
catalytic cycle of 5 is postulated to proceed via the imido [2+2]
cycloaddition pathway, and a plausible mechanism is proposed
(Scheme 6). In the presence of excess primary aminoalkene
the intense impact of steric bulk for such hydroamination
catalysts and points toward the importance of rapidly
assembled and easily varied tunable ligand frameworks for the
optimization of catalytic activity. However, the in situ screening
of 2 with Zr and Hf has shown that these larger metals are less
reactive than the smaller Ti. These results highlight the
importance of controlling steric accessibility at the metal center,
through judicious selection of ligand steric bulk and choice of
metal. The chemoselectivity between hydroamination over
hydroaminoalkylation has been realized, in contrast to other Ti
hydroamination catalysts, and the development of more
reactive and stereoselective Ti systems is presently under
investigation.
Scheme 6. Proposed Mechanism for the Catalytic
Cyclohydroamination Using 5
EXPERIMENTAL SECTION
■
General Considerations. All reactions were conducted in oven-
dried glassware using standard Schlenk line and glovebox techniques
under an atmosphere of dry dinitrogen, unless described otherwise.
Benzene, hexanes, and pentane were purified and dried by passage
through a column of activated alumina and sparged with dinitrogen.
Ti(NMe2)4 (Sigma-Aldrich), Zr(NMe2)4 (Strem), and Hf(NMe2)4
(Strem) were used as received. d6-Benzene and d8-toluene were
degassed by three freeze−pump−thaw cycles and dried over activated
4 Å molecular sieves. The Ap proligands 1−3 were prepared according
to literature procedure74−76 and sublimed under heat and high vacuum
before use. All amine substrates92 for catalytic reactions were distilled
over CaH2 and degassed by three freeze−pump−thaw cycles before
1
use. H and 13C NMR spectra were recorded on a Bruker 400 MHz
Avance spectrometer at ambient temperature, and chemical shifts are
given relative to the corresponding residual protio-residual solvent.
Mass spectra were recorded on a Kratos MS-50 spectrometer using an
electron impact (70 eV) source or a Bruker Esquire LC spectrometer
using an electrospray ionization source. Elemental analyses were
recorded on a Carlo Erba EA 1108 elemental analyzer. Single-crystal
X-ray structure determinations were performed on a Bruker X8 APEX
II diffractometer at the Department of Chemistry, University of British
Columbia, by Mr. Jacky Yim.
Synthesis of 4. N-(2,6-Diisopropylphenyl)-6-(2,4,6-triisopropyl-
phenyl)-2-aminopyridine (1; 0.274 g, 0.600 mmol) in benzene (∼2
mL) was treated with a solution of Ti(NMe2)4 (0.135 g, 0.600 mmol)
in benzene (∼2 mL), upon which the reaction instantly turned orange.
The reaction was stirred overnight at room temperature, during which
time the ligand dissolved to give an orange solution. The reaction
solvent was removed under vacuum to afford analytically pure 4 as a
yellow solid (>98%). Single crystals for X-ray structure determination
were obtained by recrystallization from a solution of hexanes at −35
°C. 1H NMR (400 MHz, C6D6): δ 7.25−7.16 (m, 3H, Ar−H), 7.14 (s,
2H, Ar−H), 6.87 (dd, 1H, J = 8.4, 7.2 Hz, Ap−H), 6.26 (d, 1H, J = 7.2
Hz, Ap−H), 5.60 (d, 1H, J = 8.4 Hz, Ap−H), 3.55 (septet, 2H, J = 6.9
Hz, −CH(CH3)2), 3.04 (s, 18H, −N(CH3)2), 2.95 (septet, 2H, J = 6.8
Hz, −CH(CH3)2), 2.84 (septet, 1H, J = 6.9 Hz, −CH(CH3)2), 1.36
(d, 6H, J = 6.8 Hz, −CH(CH3)2), 1.32 (d, 6H, J = 6.9 Hz,
−CH(CH3)2), 1.25 (d, 6H, J = 6.9 Hz, −CH(CH3)2), 1.20 (d, 6H, J =
6.8 Hz, −CH(CH3)2), 1.16 (d, 6H, J = 6.8 Hz, −CH(CH3)2). 13C
NMR (100 MHz, C6D6): δ 170.2, 158.0, 149.3, 147.1, 145.1, 144.9,
138.9, 136.8, 126.0, 124.3, 120.8, 114.2, 104.6, 45.9, 35.2, 31.0, 28.9,
27.0, 25.5, 24.7, 24.3, 23.3. MS (EI): m/z = 635 (M+), 591 (M+ −
NMe2), 547 (M+ − 2NMe2), 503 (M+ − 3NMe2). Anal. Calcd for
C38H61N5Ti: C, 71.79; H, 9.67; N, 11.02. Found: C, 71.96; H, 9.63; N,
10.93.
substrate, dimethylamine is liberated by amido exchange
reactions to generate A. A five-coordinate imido species B,
analogous to 7, is then generated by α-hydrogen elimination.
Under catalytic conditions, a neutral amine donor is in place of
the pyridine. This neutral donor ligand is then displaced by
coordinated alkene, to promote a [2+2] cycloaddition, via a
chairlike transition state C to yield azametallacyclobutane
intermediate D. Finally, the protonation of D and an amido
exchange with the substrate affords the cyclized product, and A
is regenerated for the next catalytic cycle.
SUMMARY AND CONCLUSIONS
■
Ap proligands 1−3 with varying amounts of steric bulk have
been utilized, and a series of ApTi(NMe2)3 4−6, as well as
catalytically active titanium Ap-supported imido complex 7,
have been prepared and fully characterized. Investigation of
these complexes for alkene cyclohydroamination at room
temperature has identified 5, which employs an Ap ligand of
moderate steric bulk (2) as a supporting ligand. This complex is
a very active Ti alkene hydroamination catalyst and illustrates
the catalytic potential of this inexpensive, abundant first-row
transition element. The investigations of various ligands show
Synthesis of 5. N,6-Dimesityl-2-aminopyridine (2; 0.661 g, 2.00
mmol) in benzene (∼3 mL) was treated with a solution of Ti(NMe2)4
(0.448 g, 2.00 mmol) in benzene (∼3 mL), upon which the reaction
instantly turned orange. The reaction was stirred at room temperature
for 4 h, during which time the ligand dissolved to give an orange
solution. The reaction solvent was removed under vacuum, and the
resulting compound was recrystallized from a solution of hexanes at
−35 °C to give 5 as yellow crystals (0.965 g, 95%). A sample from
1863
dx.doi.org/10.1021/om3012695 | Organometallics 2013, 32, 1858−1865

Organometallics
Article
1
these crystals was used for X-ray structure determination. H NMR
(400 MHz, C6D6): δ 6.95 (s, 2H, Ar−H), 6.94 (dd, 1H, J = 8.4, 7.2
Hz, Ap−H), 6.83 (s, 2H, Ar−H), 6.08 (d, 1H, J = 7.2 Hz, Ap−H), 5.62
(d, 1H, J = 8.4 Hz, Ap−H), 3.07 (s, 18H, −N(CH3)2), 2.32 (s, 6H,
−CH3), 2.26 (s, 3H, −CH3), 2.18 (s, 6H, −CH3), 2.16 (s, 3H, −CH3).
13C NMR (100 MHz, C6D6): δ 168.9, 158.0, 144.6, 140.3, 138.2,
137.4, 136.4, 134.0, 133.6, 129.8, 128.5, 112.2, 102.6, 46.0, 21.5, 21.4,
20.8, 19.6. MS (EI): m/z = 509 (M+), 465 (M+ − NMe2), 421 (M+ −
2NMe2), 377 (M+ − 3NMe2). Anal. Calcd for C29H43N5Ti: C, 68.36;
H, 8.51; N, 13.74. Found: C, 68.55; H, 8.50; N, 13.45.
bath at the specified temperature. Once >95% conversion was achieved
as monitored by 1H NMR spectroscopy, the tube was opened and the
contents were diluted with diethyl ether. When the mixture clarified
upon standing at room temperature, it was filtered through Celite, and
the volatiles were removed under reduced pressure. The amines were
purified by flash chromatography on silica gel.
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files of complexes 4−7 and NMR spectra of new
compounds and hydroamination products. This material is
available free of charge via the Internet at http://pubs.acs.org.
Synthesis of 6. N-(2,6-Diisopropylphenyl)-6-(2,6-dimethylphen-
yl)-2-aminopyridine (3; 0.179 g, 0.500 mmol) in benzene (∼2 mL)
was treated with a solution of Ti(NMe2)4 (0.112g, 0.500 mmol) in
benzene (∼2 mL), upon which the reaction instantly turned orange.
The reaction was stirred overnight at room temperature, during which
time the ligand dissolved to give an orange solution. The reaction
solvent was removed under vacuum, and the resulting compound was
recrystallized from a solution of hexanes at −35 °C to give 6 as orange
crystals (0.214 g, 80%). A sample from these crystals was used for X-
ray structure determination. 1H NMR (400 MHz, C6D6): δ 7.27−7.16
(m, 3H, Ar−H), 7.09 (t, 1H, J = 7.5 Hz, Ar−H), 6.99 (d, 2H, J = 7.5
Hz, Ar−H), 6.87 (dd, 1H, J = 8.5, 7.1 Hz, Ap−H), 5.96 (d, 1H, J = 7.1
Hz, Ap−H), 5.54 (d, 1H, J = 8.5 Hz, Ap−H), 3.55 (septet, 2H, J = 6.9
Hz, −CH(CH3)2), 3.05 (s, 18H, −N(CH3)2), 2.15 (s, 6H, −CH3),
1.33 (d, 6H, J = 6.9 Hz, −CH(CH3)2), 1.21 (d, 6H, J = 6.9 Hz,
−CH(CH3)2). 13C NMR (100 MHz, C6D6): δ 170.3, 157.5, 145.0,
144.7, 140.9, 139.9, 136.5, 127.7, 125.9, 124.3, 111.6, 104.3, 45.9, 28.9,
25.4, 24.2, 20.8. MS (EI): m/z = 537 (M+), 493 (M+ − NMe2), 449
(M+ − 2NMe2), 405 (M+ − 3NMe2). Anal. Calcd for C31H47N5Ti: C,
69.26; H, 8.81; N, 13.03. Found: C, 69.26; H, 8.76; N, 12.81.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: schafer@chem.ubc.ca.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support of the
University of British Columbia, the Natural Sciences and
Engineering Research Council of Canada, and Boehringer
Ingelheim. We thank Jacky Yim and Dr. Brian O. Patrick for
assistance with X-ray crystallography. We also thank Vanessa
Tam for work on the preparation of aminoalkene substrates.
Synthesis of 7. To 5 (0.102 g, 0.200 mmol) dissolved in benzene
(∼1 mL) were added 2,6-dimethylaniline (0.0485 g, 49.2 μL, 0.400
mmol) and pyridine (0.0633 g, 64.7 μL, 0.800 mmol). The resulting
red solution was heated at 75 °C for 18 h, and the reaction solvent was
removed under vacuum. The residue was dissolved in benzene (∼1
mL), layered with pentane (∼4 mL), and left at room temperature
overnight, during which time crystals formed. The mother liquor was
decanted, and volatiles were removed under vacuum to give 7 as an
orange crystalline solid (0.107 g, 77%). Single crystals for X-ray
structure determination were obtained by slow diffusion of pentane
REFERENCES
■
(1) Muller, T. E.; Beller, M. Chem. Rev. 1998, 98, 675.
̈
(2) Muller, T. E.; Hultzsch, K. C.; Yus, M.; Foubelo, F.; Tada, M.
̈
Chem. Rev. 2008, 108, 3795.
(3) Walsh, P. J.; Baranger, A. M.; Bergman, R. G. J. Am. Chem. Soc.
1992, 114, 1708.
(4) McGrane, P. L.; Jensen, M.; Livinghouse, T. J. Am. Chem. Soc.
1992, 114, 5459.
(5) McGrane, P. L.; Livinghouse, T. J. Am. Chem. Soc. 1993, 115,
11485.
1
into a solution of 7 in benzene at room temperature. H NMR (400
MHz, C6D6): δ 9.77 (br s, 1H, −NH−); 8.53 (m, 2H, py−Hortho), 7.02
(s, 1H, Ar−H), 6.99 (d, 2H, J = 7.4 Hz, Ar−H), 6.94 (d, 2H, J = 7.4
Hz, Ar−H), 6.91 (dd, 1H, J = 8.5, 7.1 Hz, Ap−H), 6.86 (s, 1H, Ar−
H), 6.74 (t, 1H, J = 7.4 Hz, Ar−H), 6.69 (s, 1H, Ar−H), 6.67 (t, 1H, J
= 7.4 Hz, Ar−H), 6.51 (s, 1H, Ar−H), 6.48 (m, 1H, py−Hpara), 6.09
(m, 2H, py−Hmeta), 5.94 (d, 1H, J = 7.1 Hz, Ap−H), 5.75 (d, 1H, J =
8.5 Hz, Ap−H), 2.74 (s, 3H, −CH3), 2.54 (s, 6H, −CH3), 2.40 (s, 3H,
−CH3), 2.29 (s, 3H, −CH3), 2.20 (s, 3H, −CH3), 2.10 (s, 6H, −CH3),
1.84 (s, 3H, −CH3), 1.75 (s, 3H, −CH3). 13C NMR (100 MHz,
C6D6): δ 167.0, 161.2, 158.7, 154.1, 150.8, 144.1, 141.1, 138.6, 137.1,
137.0, 136.9, 136.2, 135.3, 134.6, 133.4, 132.9, 130.6, 129.9, 128.8,
128.3, 128.0, 124.4, 122.7, 120.2, 118.1, 109.9, 104.3, 21.5, 21.4, 21.1,
20.8, 20.7, 20.5, 19.8, 19.6. Anal. Calcd for C44H49N5Ti: C, 75.96; H,
7.10; N, 10.07. Found: C, 76.04; H, 7.37; N, 9.94.
(6) Knight, P. D.; Munslow, I.; O’Shaughnessy, P. N.; Scott, P. Chem.
Commun. 2004, 894.
(7) Gribkov, D. V.; Hultzsch, K. C. Angew. Chem., Int. Ed. 2004, 43,
5542.
(8) Bexrud, J. A.; Beard, J. D.; Leitch, D. C.; Schafer, L. L. Org. Lett.
2005, 7, 1959.
(9) Muller, C.; Loos, C.; Schulenberg, N.; Doye, S. Eur. J. Org. Chem.
̈
2006, 2499.
(10) Thomson, R. K.; Bexrud, J. A.; Schafer, L. L. Organometallics
2006, 25, 4069.
(11) Watson, D. A.; Chiu, M.; Bergman, R. G. Organometallics 2006,
25, 4731.
(12) Lee, A. V.; Schafer, L. L. Organometallics 2006, 25, 5249.
(13) Marcsekova, K.; Loos, C.; Rominger, F.; Doye, S. Synlett 2007,
2564.
General Procedure for Monitoring Intramolecular Hydro-
amination/Cyclization Reactions (eq 2, Tables 2 and 4). Catalyst
(0.0150 mmol, 10 mol %) and 1,3,5-trimethoxybenzene (1.25 M in d8-
toluene, 40 μL, 0.0500 mmol) were dissolved in d8-toluene (460 μL)
in a small vial. The substrate 2,2-diphenyl-5-hexenyl-1-amine (1.50 M
in d8-toluene, 100 μL, 0.150 mmol) was then added and mixed with a
Pasteur pipet. The resulting solution was transferred to a J. Young
NMR tube and either left at room temperature or heated to 60 °C for
(14) Majumder, S.; Odom, A. L. Organometallics 2008, 27, 1174.
(15) Muller, C.; Saak, W.; Doye, S. Eur. J. Org. Chem. 2008, 2731.
̈
(16) Graebe, K.; Pohlki, F.; Doye, S. Eur. J. Org. Chem. 2008, 4815.
(17) Muller, C.; Koch, R.; Doye, S. Chem.Eur. J. 2008, 10430.
̈
(18) Zi, G.; Liu, X.; Xiang, L.; Song, H. Organometallics 2009, 28,
1127.
1
(19) Zi, G.; Zhang, F.; Xiang, L.; Chen, Y.; Fang, W.; Song, H. Dalton
Trans. 2010, 39, 4048.
(20) Zi, G.; Zhang, F.; Liu, X.; Ai, L.; Song, H. J. Organomet. Chem.
2010, 695, 730.
(21) Xiang, L.; Zhang, F.; Zhang, J.; Song, H.; Zi, G. Inorg. Chem.
Commun. 2010, 13, 666.
the specified time. The reaction progress was monitored by H NMR
spectroscopy.
General Procedure for Catalytic Hydroamination of Amino-
alkenes by Complex 5 (Table 3). Complex 5 (0.0510 g, 0.0500
mmol) and the aminoalkene (1.00 mmol) were dissolved in d8-toluene
(1 mL) by mixing with a Pasteur pipet in a small vial. The resulting
solution was transferred to a J. Young NMR tube and heated in an oil
(22) Wang, Q.; Song, H.; Zi, G. J. Organomet. Chem. 2010, 695, 1583.
1864
dx.doi.org/10.1021/om3012695 | Organometallics 2013, 32, 1858−1865

Organometallics
Article
(23) Janssen, T.; Severin, R.; Diekmann, M.; Friedemann, M.; Haase,
D.; Saak, W.; Doye, S.; Beckhaus, R. Organometallics 2010, 29, 1806.
(24) Ayinla, R. O.; Schafer, L. L. Dalton Trans. 2011, 40, 7769.
(25) Helgert, T. R.; Hollis, T. K.; Valente, E. J. Organometallics 2012,
31, 3002.
(26) Jaspers, D.; Saak, W.; Doye, S. Synlett 2012, 23, 2098.
(27) Kim, H.; Lee, P. H.; Livinghouse, T. Chem. Commun. 2005,
5205.
(28) Kim, H.; Kim, Y. K.; Shim, J. H.; Kim, M.; Han, M.;
Livinghouse, T.; Lee, P. H. Adv. Synth. Catal. 2006, 348, 2609.
(29) Wood, M. C.; Leitch, D. C.; Yeung, C. S.; Kozak, J. A.; Schafer,
L. L. Angew. Chem., Int. Ed. 2007, 46, 354.
(30) Stubbert, B. D.; Marks, T. J. J. Am. Chem. Soc. 2007, 129, 6149.
(31) Gott, A. L.; Clarke, A. J.; Clarkson, G. J.; Scott, P.
Organometallics 2007, 26, 1729.
(32) Gott, A. L.; Clarke, A. J.; Clarkson, G. J.; Scott, P. Chem.
Commun. 2008, 1422.
(33) Gott, A. L.; Clarkson, G. J.; Deeth, R. J.; Hammond, M. L.;
Morton, C.; Scott, P. Dalton Trans. 2008, 2983.
(34) Cho, J.; Hollis, K. T.; Helgert, T. R.; Valente, E. J. Chem.
Commun. 2008, 5001.
(61) Smolensky, E.; Kapon, M.; Eisen, M. S. Organometallics 2007,
26, 4510.
(62) Bertolasi, V.; Boaretto, R.; Chierotti, M. R.; Gobetto, R.;
Sostero, S. Dalton Trans. 2007, 5179.
(63) Kempe, R.; Noss, H.; Irrgang, T. J. Organomet. Chem. 2002, 647,
12.
(64) Kempe, R. Eur. J. Inorg. Chem. 2003, 791.
(65) Deeken, S.; Motz, G.; Kempe, R. Z. Anorg. Allg. Chem. 2007,
633, 320.
(66) Noor, A.; Kretschmer, W. P.; Glatz, G.; Kempe, R. Inorg. Chem.
2011, 50, 4598.
(67) Hafeez, M.; Kretschmer, W. P.; Kempe, R. Eur. J. Inorg. Chem.
2011, 5512.
(68) Talja, M.; Luhtanen, T.; Polamo, M.; Klinga, M.; Pakkanen, T.;
Leskela, M. Inorg. Chim. Acta 2008, 361, 2195.
(69) Shen, H.; Chan, H. S.; Xie, Z. Organometallics 2007, 26, 2694.
(70) Fandos, R.; Hernandez, C.; Otero, A.; Rodriguez, A.; Ruiz, M. J.
J. Organomet. Chem. 2005, 690, 4828.
(71) Noor, A.; Kempe, R. Eur. J. Inorg. Chem. 2008, 2377.
(72) Smolensky, E.; Kapon, M.; Woollins, J. D.; Eisen, M. S.
Organometallics 2005, 24, 3255.
(73) Westmoreland, I.; Munslow, I. J.; O’Shaughnessy, P. N.; Scott,
P. Organometallics 2003, 22, 2972.
(74) Scott, N. M.; Schareina, T.; Tok, O.; Kempe, R. Eur. J. Inorg.
(35) Leitch, D. C.; Payne, P. R.; Dunbar, C. R.; Schafer, L. L. J. Am.
Chem. Soc. 2009, 131, 18246.
(36) Reznichenko, A. L.; Hultzsch, K. C. Organometallics 2010, 29,
24.
(37) Manna, K.; Ellern, A.; Sadow, A. D. Chem. Commun. 2010, 46,
Chem. 2004, 3297.
(75) Kretschmer, W. P.; Meetsma, A.; Hessen, B.; Scott, N. M.;
Qayyum, S.; Kempe, R. Chem.Eur. J 2006, 12, 8969.
(76) Schmalz, T.; Burkhardt, J.; Irrgang, T.; Kempe, R. Z.
Kristallogr.New Cryst. Struct. 2005, 220, 405.
(77) Scott, N. M.; Kempe, R. Eur. J. Inorg. Chem. 2005, 1319.
(78) Guillaume, S. M.; Schappacher, M.; Scott, N. M.; Kempe, R. J.
Polym. Sci., Part A: Polym. Chem. 2007, 45, 3611.
(79) Kretschmer, W. P.; Hessen, B.; Noor, A.; Scott, N. M.; Kempe,
R. J. Organomet. Chem. 2007, 692, 4569.
339.
(38) Bexrud, J. A.; Schafer, L. L. Dalton Trans. 2010, 39, 361.
(39) Hu, Y. C.; Liang, C. F.; Tsai, J. H.; Yap, G. P. A.; Chang, Y. T.;
Ong, T. G. Organometallics 2010, 29, 3357.
(40) Allan, L. E. N.; Clarkson, G. J.; Fox, D. J.; Gott, A. L.; Scott, P. J.
Am. Chem. Soc. 2010, 132, 15308.
(41) Antunes, M. A.; Munha, R. F.; Alves, L. G.; Schafer, L. L.;
Martins, A. M. J. Organomet. Chem. 2011, 696, 2.
(42) Ayinla, R. O.; Gibson, T.; Schafer, L. L. J. Organomet. Chem.
2011, 696, 50.
(43) Manna, K.; Xu, S.; Sadow, A. D. Angew. Chem., Int. Ed. 2011, 50,
1865.
(80) Noor, A.; Wagner, F. R.; Kempe, R. Angew. Chem., Int. Ed. 2008,
47, 7246.
(81) Noor, A.; Glatz, G.; Muller, R.; Kaup, M.; Demeshko, S.;
̈
Kempe, R. Nat. Chem. 2009, 1, 322.
(82) Doring, C.; Kretschmer, W. P.; Kempe, R. Eur. J. Inorg. Chem.
̈
(44) Mukherjee, A.; Nembenna, S.; Sen, T. K.; Sarish, S. P.; Ghorai,
P. K.; Ott, H.; Stalke, D.; Mandal, S. K.; Roesky, H. W. Angew. Chem.,
Int. Ed. 2011, 50, 3968.
2010, 2853.
(83) Kretschmer, W. P.; Bauer, T.; Hessen, B.; Kempe, R. Dalton
Trans. 2010, 6847.
(45) Leitch, D. C.; Platel, R. H.; Schafer, L. L. J. Am. Chem. Soc. 2011,
133, 15453.
(46) Wang, X.; Chen, Z.; Sun, X.-L.; Tang, Y.; Xie, Z. Org. Lett. 2011,
13, 4758.
(47) Payne, P. R.; Bexrud, J. A.; Leitch, D. C.; Schafer, L. L. Can. J.
(84) Schwarzmaier, C.; Noor, A.; Glatz, G.; Zabel, M.; Timoshkin, A.
Y.; Cossairt, B. M.; Cummins, C. C.; Kempe, R.; Scheer, M. Angew.
Chem., Int. Ed. 2011, 50, 7283.
(85) Haas, I.; Kretschmer, W. P.; Kempe, R. Organometallics 2011,
30, 4854.
(86) Noor, A.; Tamne, E. S.; Qayyum, S.; Bauer, T.; Kempe, R.
Chem.Eur. J. 2011, 17, 6900.
Chem. 2011, 89, 1222.
(48) Tobisch, S. Inorg. Chem. 2012, 51, 3786.
(49) Kubiak, R.; Prochnow, I.; Doye, S. Angew. Chem., Int. Ed. 2009,
48, 1153.
(50) Bexrud, J. A.; Eisenberger, P.; Leitch, D. C.; Payne, P. R.;
Schafer, L. L. J. Am. Chem. Soc. 2009, 131, 2116.
(51) Prochnow, I.; Kubiak, R.; Frey, O. N.; Beckhaus, R.; Doye, S.
ChemCatChem 2009, 1, 162.
(52) Roesky, P. W. Angew. Chem., Int. Ed. 2009, 48, 4892.
(53) Kubiak, R.; Prochnow, I.; Doye, S. Angew. Chem., Int. Ed. 2010,
49, 2626.
(87) Jung, M. E.; Piizzi, G. Chem. Rev. 2005, 105, 1735.
(88) Molander, G. A.; Dowdy, E. D.; Pack, S. K. J. Org. Chem. 2001,
66, 4344.
(89) Pohlki, F.; Doye, S. Chem. Soc. Rev. 2003, 32, 104.
(90) Severin, R.; Doye, S. Chem. Soc. Rev. 2007, 36, 1407.
(91) Johnson, J. S.; Bergman, R. G. J. Am. Chem. Soc. 2001, 123,
2923.
(92) See Supporting Information.
(54) Prochnow, I.; Zark, P.; Muller, T.; Doye, S. Angew. Chem., Int.
̈
Ed. 2011, 50, 6401.
(55) Ramanathan, B.; Odom, A. L. J. Am. Chem. Soc. 2006, 128, 9344.
(56) Dorfler, J.; Doye, S. Angew. Chem., Int. Ed. 2013, 52, 1806.
̈
(57) Lee, A. V.; Schafer, L. L. Eur. J. Inorg. Chem. 2007, 2243.
(58) Weitershaus, K.; Ward, B. D.; Kubiak, R.; Muller, C.; Wadepohl,
̈
H.; Doye, S.; Gade, L. H. Dalton Trans. 2009, 4586.
(59) Ong, T. G.; Yap, G. P. A.; Richeson, D. S. Organometallics 2002,
21, 2839.
(60) Smolensky, E.; Kapon, M.; Eisen, M. S. Organometallics 2005,
24, 5495.
1865
dx.doi.org/10.1021/om3012695 | Organometallics 2013, 32, 1858−1865