Alkyne hydroamination and trimerization with titanium bis(phenolate) pyridine complexes: Evidence for low-valent titanium intermediates and synthesis of an ethylene adduct of titanium(II)

Article abstract of DOI:10.1021/om400080g

A class of titanium precatalysts of the type (ONO)TiX₂ (ONO = pyridine-2,6-bis(4,6-di-tert-butylphenolate); X = Bn, NMe₂) has been synthesized and crystallographically characterized. The (ONO)TiX₂ (X = Bn, NMe₂, X₂ = NPh) complexes are highly active precatalysts for the hydroamination of internal alkynes with primary arylamines and some alkylamines. A class of titanium imido/ligand adducts, (ONO)Ti(L)(NR) (L = HNMe₂, py; R = Ph, ^tBu), have also been synthesized and characterized and provide structural analogues to intermediates on the purported catalytic cycle. Furthermore, these complexes exhibit unusual redox behavior. (ONO)TiBn₂ (1) promotes the cyclotrimerization of electron-rich alkynes, likely via a catalytically active Ti^II species that is generated in situ from 1. Depending on reaction conditions, these Ti^II species are proposed to be generated through Ti benzylidene or imido intermediates. A formally Ti^II complex, (ONO)Ti ^II(η²-C₂H₄)(HNMe₂) (7), has been prepared and structurally characterized.

Full text of DOI:10.1021/om400080g

Article
pubs.acs.org/Organometallics
Alkyne Hydroamination and Trimerization with Titanium
Bis(phenolate)pyridine Complexes: Evidence for Low-Valent
Titanium Intermediates and Synthesis of an Ethylene Adduct of
Titanium(II)
Ian A. Tonks, Josef C. Meier, and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125,
United States
S
* Supporting Information
ABSTRACT: A class of titanium precatalysts of the type
(ONO)TiX2 (ONO = pyridine-2,6-bis(4,6-di-tert-butylpheno-
late); X = Bn, NMe2) has been synthesized and crystallo-
graphically characterized. The (ONO)TiX2 (X = Bn, NMe2, X2
= NPh) complexes are highly active precatalysts for the
hydroamination of internal alkynes with primary arylamines
and some alkylamines. A class of titanium imido/ligand
t
adducts, (ONO)Ti(L)(NR) (L = HNMe2, py; R = Ph, Bu),
have also been synthesized and characterized and provide
structural analogues to intermediates on the purported
catalytic cycle. Furthermore, these complexes exhibit unusual redox behavior. (ONO)TiBn2 (1) promotes the cyclotrimerization
of electron-rich alkynes, likely via a catalytically active TiII species that is generated in situ from 1. Depending on reaction
conditions, these TiII species are proposed to be generated through Ti benzylidene or imido intermediates. A formally TiII
complex, (ONO)TiII(η2-C2H4)(HNMe2) (7), has been prepared and structurally characterized.
precatalysts for intermolecular alkyne hydroamination, we
envisioned that ONO titanium complexes might also be
competent and long-lived catalysts. Investigations of the scope
and mechanism of catalysis revealed that, in addition to alkyne
hydroamination, in the absence of amines these same
precatalysts promote the cyclotrimerization of alkynes, likely
via reductive elimination of arene to afford a transient
(ONO)TiII species. The synthesis and structural character-
ization of the ethylene adduct (ONO)TiII(η2-C2H4)(NHMe2)
provides support for this hypothesis.
INTRODUCTION
■
The inter- and intramolecular hydroamination of alkenes and
alkynes has been catalyzed by a range of complexes of transition
metals, lanthanides, and even main-group elements.1 Among
this broad range of catalysts, group 4 complexes2 have been
found to be excellent catalysts for the intermolecular hydro-
amination of alkynes. Bergman and others have investigated the
mechanism2 of the group 4 catalyzed reaction and have found
that it typically proceeds first by protonolysis by an amine and
subsequent formation of a transient metal imido species,
followed by [2 + 2] addition with an alkyne to generate an
azametallacyclobutene that can be protonolyzed by another 1
equiv of amine to complete the catalytic cycle (Scheme 1). A
common catalyst deactivation pathway involves dimerization of
a low-coordinate Ti−imido intermediate, typically discouraged
by utilizing bulky amines or by operating at high temperatures
to favor the monomer. While the product of this reaction,
either an imine or enamine, might often be more easily formed
via amine condensation with a ketone, alkyne hydroamination
remains a useful synthetic toolparticularly because of its atom
economy.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Precatalysts.
(ONO)TiBn2 (1; ONO = pyridine-2,6-bis(4,6-di-tert-butylphe-
nolate)) was synthesized via protonolysis of TiBn4 with
(ONO)H2 following a literature procedure3c (Scheme 2). Salt
metathesis of TiCl2(NMe2)2 and the dipotassium salt of the
ligand (ONO)K2 generates the bis(amide) (ONO)Ti(NMe2)2
(2). Similarly, salt metathesis of TiCl2(HNMe2)2(NPh) with
(ONO)K2 yields the phenylimido complex (ONO)Ti-
(HNMe2)(NPh) (3). A series of (ONO)Ti imido complexes
can be synthesized by utilizing differently substituted Ti−imido
starting materials of the type TiCl2(L)2(NR) (L = HNMe2, py,
Recently our group3 and others4 have been investigating
pyridine-linked bis(phenoxide), LX2 type pincer ligands for
early-transition-metal catalysts for olefin polymerization. Early-
transition-metal complexes of these ONO ligands are very
thermally robust due to their rigid, all-aryl backbone. In light of
reports that phenoxide-based5 titanium complexes are
t
R = alkyl, aryl; for 4, R = Bu, L = py). Finally, the pyridine-
Received: January 29, 2013
© XXXX American Chemical Society
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Scheme 1
Scheme 2
stabilized dichloride complex (ONO)TiCl2(py) (6) was
synthesized through aminolysis of TiCl2(NMe2)2 with (ONO)-
H2, followed by treatment with excess pyridine. In the absence
of pyridine, the HNMe2 adduct (ONO)TiCl2(HNMe2) (5) is
obtained, although this complex loses some HNMe2 to form a
mixture of (ONO)TiCl2 and (ONO)TiCl2(HNMe2) upon
workup. We were unable to synthesize (ONO)TiCl2-type
complexes through salt metathesis of TiCl4 or TiCl4(THF)2
with (ONO)K2, presumably because the deprotonated ligand
reduces the titanium starting material.
The crystal structure of the C2-symmetric 1 has been
previously reported.3c Interestingly, in the solid state 2 has a Cs-
symmetric ONO ligand with the two phenolate arms occupying
equatorial sites of a distorted trigonal bipyramid (Figure 1). A
Cs-symmetric structure is likely the electronically preferred3d
geometry for π-loaded five-coordinate ONO complexes,
whereas five-coordinate ONO complexes without additional π
donors prefer C2 arrangements. Due to the small, electron-rich
Ti metal center, neither 1 nor 2 can easily accommodate a sixth
ligand: THF, Et2O, pyridine, and PMe3 do not coordinate to
form adducts, whereas the larger congener (ONO)ZrBn2 easily
coordinates a sixth ligand. On the other hand, the
comparatively electron-poor dichloride, unlike 1 and 2, readily
binds a pyridine to form a six-coordinate, Cs-symmetric
complex in the solid state (Figure 1).
3 is not readily crystallized, but the related complex
(ONO)Ti(py)(NtBu) (4) can be crystallized from a 5/1
pentane/toluene solution at −30 °C. The Cs-symmetric ONO
ligand in 4 occupies two equatorial sites as in 2, and the linear
imido functionality occupies the third equatorial site of the
distorted trigonal bipyramid. In a [2 + 2] hydroamination
mechanism, Ti−imido species are key intermediates along the
catalytic cycle. Thus, 4 could provide a structural analogue to a
catalytically relevant [(ONO)TiNR] intermediate. In this case,
an incoming alkyne would likely occupy the same coordination
site as pyridine in 4.
Catalytic Hydroamination of Alkynes. Table 1 summa-
rizes the hydroamination of alkynes with primary amines as
catalyzed by 1. Mixing 1 with amine and alkyne at room
1
temperature results in no reaction; however, H NMR reveals
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Figure 1. Thermal ellipsoid drawings (50% probability level) of 2 (top), 4 (middle), and 6 (bottom). Selected bond lengths (Å) and angles (deg) are
as follows. Complex 2: Ti1−N1, 2.3982(3); Ti1−N2, 1.9049(2); Ti1−N3, 1.8707(2); Ti1−O1, 1.8734(2); Ti1−O2, 1.8756(2). Complex 4: Ti1−
N1, 2.2192(1); Ti1−N2, 1.6877(1); Ti1−N3, 2.2116(1); Ti1−O1, 1.9236(1); Ti1−O2, 1.9320(1); Ti1−N2−C34, 170.97. Complex 6: Ti1−N1,
2.2941(1); Ti1−N2, 2.2542(1); Ti1−O1, 1.8132(1); Ti1−O2, 1.8487(1). Solvent molecules and H atoms are removed for clarity.
that upon heating at 90 °C the precatalyst is rapidly consumed,
producing 2 equiv of toluene that result from protonolysis of
the benzyl ligands by amine. For reactions with 2-butyne or 1-
phenyl-1-propyne, 20 turnovers (based on amine) are achieved
in less than 1 h, while reactions with the electron-poor
diphenylacetylene are substantially slower. Mixtures of the E
and Z imine isomers are obtained. In the case of the
hydroamination of the unsymmetrically substituted 1-phenyl-
1-propyne, 1-phenylpropan-2-ylidene imines were obtained
exclusively. Electron-rich, sterically unencumbered arylamines
are particularly good substrates, and even some relatively bulky
arylamines (such as o-toluidine) undergo reaction. Substrates
with excessive steric bulk in the ortho position or with strongly
electron withdrawing groups are unreactive. Bulky alkylamines
such as iPrNH2 react, albeit extremely slowly, in comparison to
the arylamines. Entry 11 indicates that 1 is significantly more
effective for the hydroamination of 1-phenyl-1-propyne than
the similar Beller bulky bis(aryloxide)Ti-catalyzed5 hydro-
amination system (48 h, 130 °C, 10 turnovers, in comparison
to 1 h, 90 °C, >20 turnovers for 1).
Sterically demanding amines such as 2,6-dimethylaniline are
usually among the better substrates6 for many Ti-based
catalysts (such as that derived from Cp2TiMe2), because the
increased steric bulk prevents catalyst deactivation through
imido dimerization (Scheme 1). For 1, however, sterically
demanding arylamines (such as 2,6-dimethylaniline) are
unreactive. Apparently the bulky, tridentate ONO ancillary
ligand prevents coordination of bulky amines. While the steric
bulk of the ONO ligand hampers reactivity with bulky amines,
it also inhibits imido dimerization in smaller arylamines so that
1 exhibits high activities for these substrates.7 Reactions of
substrates with no ortho or meta aryl substitution (entries 1−3
and 5, for example) precipitate an unknown Ti species
(possibly the imido dimer) from solution near the end of the
reaction as the substrate concentrations decrease. These
reactions likely do not proceed through a heterogeneous
process upon precipitation, as the filtrate obtained after filtering
the reaction mixture is still catalytically competent. For
reactions with bulkier amines (entries 2 and 7) the reaction
solutions remain homogeneous.
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Table 1. Catalytic Hydroamination of Alkynes with
a
(ONO)TiBn2 (1)
entry
R1
R2
R3
time (h)
yield, % (E:Z)
1
Me
Me
Ph
1
1
100 (5:1)
100 (5.6:1)
100 (5:1)
0
2
o-CH3C6H4
p-CH3C6H4
2,6-Me2C6H3
p-MeOC6H4
p-CF3C6H4
3,5-tBu2C6H3
2,4-tBu2C6H3
Bn
3
1
4
1
5
1
100 (5.2:1)
0
6
1
7
1
100 (4.9:1)
0
8
1
9
1
0
10
11
12
13
14
15
16
iPr
140
1
95 (5:1)
100 (3:1)
100 (3.6:1)
100 (3:1)
100 (3.6:1)
100 (2.3:1)
0
Figure 2. Reaction time course of diphenylacetylene hydroamination
with 3,5-di-tert-butylaniline catalyzed by 1−3 at 75 °C.
Me
Ph
Ph
Ph
Ph
o-CH3C6H4
p-CH3C6H4
p-MeOC6H4
3,5-tBu2C6H3
Bn
1
1
1
should be the same for all three. While 1 and 2 do proceed at
roughly the same rate, 3 catalyzes the reaction at roughly half
the rate of 1. Because the reaction using 2 proceeds at the same
rate as that using 1 and also produces HNMe2 over the course
of the reaction, we do not believe that HNMe2 inhibition is the
cause for lower reaction rates with 3. Rather, we suspect that
inefficient catalyst activation lowers the overall rate: in order to
enter the catalytic cycle, 3 must lose HNMe2 to form a sterically
unencumbered, four-coordinate phenylimido species that, as
evidenced in earlier reactions, could easily dimerize and
deactivate. Supporting this hypothesis, 3 generates a sub-
stoichiometric amount of the expected product of activation, N-
(1,2-diphenylethylidene)aniline, indicating that some of the
precatalyst never undergoes a [2 + 2] reaction with alkyne.
Although its activity is lower, the competency of 3 for
hydroamination and the observation of the aniline-hydro-
aminated product N-(1,2-diphenylethylidene)aniline give fur-
ther evidence that these [(ONO)Ti]-catalyzed reactions
proceed through a typical imido + alkyne [2 + 2] addition
mechanism.
1
1
b
17
Ph
8
66 (1:0)
83 (1:0)
77 (1:0)
76 (1:0)
100 (1:0)
b
18
o-CH3C6H4
p-CH3C6H4
p-MeOC6H4
3,5-tBu2C6H3
8
b
19
8
b
20
8
b
21
1
a
Conditions: 0.5 mmol of alkyne, 0.5 mmol of amine, 0.025 mmol of
1, 0.75 mL of C6D6, 0.25 mmol of SiMe4. Yield and E:Z ratio
b
1
determined by H NMR. 1 mL rather than 0.75 mL of C6D6 was
used.
Moreover, with diphenylacetylene the reaction mixture with
3,5-di-tert-butylaniline (entry 21) is substantially faster and
contains significantly less precipitate than for those with less
bulky arylamines (entries 17−20). Possibly the steric protection
provided by the arylimido intermediate could be important for
effective catalysis, much as for many previously reported
systems.6 The electron-poor diphenylacetylene likely undergoes
[2 + 2] addition with a transient imido species more slowly
than dimethyl- or methylphenylacetylene, and consequently the
arylimido intermediate builds in concentration and dimerizes.
As a result, reactions with sterically unencumbered amines,
while efficient with electron-rich alkynes, suffer from significant
catalyst deactivation and lower effective turnover numbers
when paired with diphenylacetylene.
Complexes 2 and 3 were also tested as catalysts for the
hydroamination of diphenylacetylene with 3,5-di-tert-butylani-
line. The reactions were carried out at 75 °C under more dilute
conditions in order to monitor the rate of the reaction via H
NMR.
For the reaction using 2, 2 equiv of HNMe2 was generated
per equivalent of Ti, indicating complete consumption of
precatalyst 2 under the reaction conditions. For the reaction
using 3, a substoichiometric amount of the expected imine
product of precatalyst activation, N-(1,2-diphenylethylidene)-
aniline, was generated, indicating that not all of the precatalyst
was activated prior to consumption of substrates.
Figure 2 shows the time course of the hydroaminations
catalyzed beginning with precatalysts 1−3. Assuming reactions
catalyzed using 1, 2, or 3 proceed through the same
mechanism, the active species (and, as a result, reaction rate)
(ONO)TiII-Catalyzed Alkyne Cyclotrimerization. When
(ONO)TiBn2 (1) was allowed to react under catalytic
conditions for 14 h with 10 equiv of 3,5-di-tert-butylaniline
and 30 equiv of dimethylacetylene, GC-MS revealed parent ion
peaks for hexamethylbenzene and N-(3,5-di-tert-butylphenyl)-
2,3,4,5-tetramethylpyrrole in addition to that for the hydro-
aminated product (Scheme 3 and Figure 3). Both of these side
products were generated in low yield (<5%). The N-
aryltetramethylpyrrole is likely generated by insertion of a
second equivalent of dimethylacetylene8 into an azatitanocy-
clobutene intermediate, followed by reductive elimination of
pyrrole with generation of a TiII species. TiII complexes are
well-known catalysts9 for alkyne cyclotrimerization, and it is
likely that the resultant TiII species generated from the pyrrole
reductive elimination also carries out the cyclotrimerization.
Surprisingly, 1 is also a precatalyst for alkyne cyclo-
trimerization in the absence of any primary amine. When 20
equiv of dimethylacetylene was treated with 1 at 90 °C for 1 h,
88% of the dimethylacetylene was converted to hexamethyl-
benzene (eq 1). Unlike the case for the hydroamination
experiments, the precatalyst 1 was not entirely consumed over
the course of the reaction. Instead, roughly 50% of the
precatalyst remained after the reaction was complete. The
1
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Scheme 3
alkyne adduct of “(ONO)TiII” resulted only in alkyne
trimerization. However, when (ONO)Ti(HNMe2)Cl2 (5) was
treated with 1% Na/Hg under an atmosphere of ethylene, the
TiII−ethylene adduct (ONO)Ti(HNMe2)(C2H4) (7) was
isolated in very low yield (eq 2). X-ray-quality crystals of 7
were grown from a concentrated pentane solution cooled to
−30 °C (Figure 4). 7 is a rare example11 of a crystallo-
graphically characterized Ti−ethylene adduct. On the basis of
the C29−C30 bond length of 1.4216(1) Å, 7 is best described
as a resonance hybrid of roughly equal contributions of TiII(η2-
ethylene) and titana(IV)cyclopropane.
CONCLUSIONS
■
Bis(phenolate)pyridine titanium pincer complexes have been
shown to be active catalysts for the hydroamination of internal
alkynes by arylamines. Unlike many other catalytic systems,
sterically unencumbered arylamines are particularly good
substrates, while bulky ortho-substituted amines are unreactive.
In addition to the hydroamination catalysis, under the reaction
conditions these Ti complexes undergo unusual redox
chemistry to generate a TiII complex. Under hydroamination
conditions with excess alkyne, the TiII species is generated via
the reductive elimination of pyrroles and can then catalytically
cyclotrimerize dimethylacetylene. Active [(ONO)TiII] species
can also be generated in the absence of amine through a
benzylidene intermediate, allowing for cyclotrimerization with-
out an added reductant. Finally, a stabilized [(ONO)TiII]
complex has been trapped by ethylene, giving a rare example of
a crystallographically characterized Ti−ethylene adduct and
providing some structural plausibility for the intermediacy of
TiII during these reactions.
Figure 3. GC trace of the product mixture of the reaction of 2-butyne
with 3,5-di-tert-butylaniline catalyzed by 1, showing side products
(determined by GC-MS) resulting from TiII species.
presence of small (<5% yield based on dimethylacetylene)
amounts of phenyltetramethylcyclopentadiene suggests that 1
initially α-eliminates 1 equiv of toluene and undergoes [2 + 2]
reaction of dimethylacetylene with [(ONO)TiCHPh],10
followed by a second addition of dimethylacetylene and
reductive elimination to form the active [(ONO)TiII] catalyst
for dimethylacetylene cyclotrimerization, analogous to the
mechanism for pyrrole formation when amine is present
(Scheme 4).
EXPERIMENTAL SECTION
■
General Considerations and Instrumentation. All air- and
moisture-sensitive compounds were manipulated using standard high-
vacuum and Schlenk techniques or manipulated in a glovebox under a
nitrogen atmosphere. Solvents for air- and moisture-sensitive reactions
were dried over sodium benzophenone ketyl and stored over
titanocene where compatible or dried by the method of Grubbs.12
(ONO)TiII−Ethylene Adduct. Because the (ONO)TiII
species in the alkyne trimerization catalysis is apparently
somewhat persistent, we sought to trap an (ONO)TiII complex
with a π-acceptor ligand. Unfortunately, attempts to isolate an
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Scheme 4
7.43 (d, J = 2.4 Hz, 2 H, aryl-H); 7.48 (d, J = 2.4 Hz, 2 H, aryl-H); 7.57
3
(d, J = 7.9 Hz, 2 H, 3,5-NC5H-H2); 7.86 (t, J = 8.1 Hz, 1 H, 4-
NC5H2-H). 13C NMR (125 MHz, CD2Cl2; δ, ppm): 30.0, 32.0, 34.9,
35.8, 44.9 (alkyl); 122.8, 124.7, 126.4, 126.5, 138.5, 138.7, 141.5,
157.2, 157.7 (aryl).
Synthesis of (ONO)Ti(HNMe2)(NPh) (3). (ONO)H2 (243 mg,
0.5 mmol, 1 equiv) and KBn (130 mg, 1.0 mmol, 2 equiv) were mixed
in 10 mL of C6H6. The mixture was stirred for 1 h to generate a yellow
solution of (ONO)K2, and then a 5 mL C6H6 solution of
(HNMe2)2TiCl2(NPh) (150 mg, 0.5 mmol, 1 equiv) was added
dropwise. The reaction mixture was stirred overnight and then filtered
to remove KCl. C6H6 was then lyophilized, and the resultant orange
solid was washed with 5 mL of pentane to remove any remaining free
ligand, yielding 301 mg (90%) of 3 as a yellow powder. 1H NMR (300
MHz, C6D6; δ, ppm): 1.44 (s, 18 H, C(CH3)3); 1.82 (s, 18 H,
C(CH3)3); 2.54 (d, J = 6.3 Hz, 6 H, NH(CH3)2); 3.69−3.86 (br m, 1
H, NH(CH3)2); 6.25 (d, J = 7.9 Hz, 2 H, 2,6-C6H3-H2); 6.45 (d, J =
7.6 Hz, 1 H, 2,6-C6H4-H); 6.73 (t, J = 7.7 Hz, 2 H, 3,5-C6H3-H2); 7.05
(t, J = 8.0 Hz, 1 H, 4-NC5H2-H); 7.32 (d, J = 8.0 Hz, 2 H, 4-NC5H-
H2); 7.44 (d, J = 2.4 Hz, 2 H, aryl-H); 7.82 (d, J = 2.4 Hz, 2 H, aryl-
H). 1H NMR (300 MHz, CD2Cl2; δ, ppm): 1.39 (s, 18 H, C(CH3)3);
1.65 (s, 18 H, C(CH3)3); 2.94 (d, J = 6.4 Hz, 6 H, NH(CH3)2); 3.79−
3.94 (br m, 1 H, NH(CH3)2); 5.82 (d, J = 8.1 Hz, 2 H, 2,6-C6H3-H2);
6.41 (t, J = 8.1 Hz, 1 H, 4-C6H4-H); 6.61 (t, J = 8.3 Hz, 2 H, 3,5-C6H3-
H2); 7.41 (d, J = 2.3 Hz, 2 H, aryl-H); 7.55 (d, J = 2.3 Hz, 2 H, aryl-
H); 7.70 (d, J = 8.3 Hz, 2 H, 3,5-NC5H-H2); 7.98 (t, J = 8.3 Hz, 1 H,
4-NC5H2-H). 13C NMR (125 MHz, CD2Cl2; δ, ppm): 30.4, 32.0, 34.8,
35.9, 40.6 (alkyl); 119.8, 123.1, 124.0, 126.1, 126.8, 127.2, 127.9,
137.0, 139.6, 140.9, 158.8, 159.5, 169.7 (aryl).
Figure 4. Thermal ellipsoid drawing (50% probability level) of 7.
Selected bond lengths (Å): Ti−N1, 2.2314(2); Ti−N2, 2.2766(2);
Ti−O1, 1.8994(2); Ti−O2, 1.8929(2); Ti−C34, 2.1080(2); Ti−C35,
2.1414(3); C34−C35, 1.4216(1). H atoms and solvent are removed
for clarity.
Liquid amines and alkynes were degassed and passed through a
column of activated alumina or distilled from CaH2 prior to use.
TiBn4 ,1 3 TiCl2 (NMe2 )2 ,1 4 TiCl2 (HNMe2 )2 (NPh), 1 5
3c
TiCl2(py)2(NtBu),15 (ONO)H2,3c and (ONO)TiBn2 were prepared
following literature procedures. Benzene-d6 was purchased from
Cambridge Isotopes and sequentially dried over sodium benzophe-
none ketyl and titanocene. 1H and 13C spectra were recorded on
Varian Mercury 300 or Varian INOVA 500 spectrometers, and
chemical shifts are reported with respect to residual protio solvent
1
impurity for H (s, 7.16 ppm for C6D5H; t, 5.32 ppm for CDHCl2)
and solvent carbons for 13C (t, 128.39 for C6H6; p, 53.84 for CD2Cl2).
Integrated and assigned 1H NMR spectra are available in the
Supporting Information (Figures S1−S4).
Synthesis of (ONO)Ti(py)(NtBu) (4). (ONO)H2 (154 mg, 0.316
mmol, 1 equiv) and KBn (82.5 mg, 0.634 mmol, 2 equiv) were mixed
in 10 mL of C6H6. The mixture was stirred for 1 h to generate a yellow
solution of (ONO)K2, and then a 5 mL C6H6 solution of
(py)2TiCl2(NtBu) (110.3 mg, 0.316 mmol, 1 equiv) was added
dropwise. The reaction mixture was stirred overnight and then filtered
to remove KCl. C6H6 was then lyophilized, and the resultant yellow
solid was washed with 5 mL of pentane to remove any remaining free
ligand, yielding 195 mg (90%) of 4 as a yellow powder. X-ray-quality
crystals were grown from a saturated pentane solution of 4 cooled to
−30 °C. 1H NMR (500 MHz, C6D6; δ, ppm): 0.76 (s, 9 H, N(CH3)3);
1.47 (s, 18 H, C(CH3)3); 1.57 (s, 18 H, C(CH3)3); 6.64 (m, 1 H, py)
6.97 (t, J = 7.9 Hz, 1 H, 4-NC5H2-H); 7.21 (m, 2 H, py); 7.47 (d, J =
7.9 Hz, 2 H, 3,5-NC5H-H2); 7.61 (d, J = 2.5 Hz, 2 H, aryl-H); 7.78 (d,
J = 2.5 Hz, 2 H, aryl-H); 8.76 (m, 2 H, py). 13C NMR (125 MHz,
C6D6; δ, ppm): 29.8, 30.1, 31.8 (C(CH3)3); 34.1, 35.1, 35.9
(C(CH3)3); 118.8, 122.5, 123.7, 124.9, 135.2, 138.2, 139.1, 149.7,
150.7, 159.2, 160.2, 164.2 (aryl).
Synthesis of (ONO)Ti(py)Cl2 (6). (ONO)H2 (194.5 mg, 0.400
mmol, 1 equiv) and TiCl2(NMe2)2 (83 mg, 0.400 mmol, 1 equiv) were
mixed in 6 mL of C6H6. A dark orange solid immediately precipitated.
After 15 min, 1 mL of pyridine was added, which dissolved the
precipitate to give a dark red solution. The solution was stirred for 1 h
and then filtered and the benzene removed by lyophilization. The
orange-red powder was isolated and washed with 10 mL of pentane,
yielding 5 as an orange-red solid (232 mg, 85%). X-ray-quality crystals
X-ray Crystal Data: General Procedure. Crystals were removed
quickly from a scintillation vial to a microscope slide coated with
Paratone N oil. Samples were selected and mounted on a glass fiber
with Paratone N oil. Data collection was carried out on a Bruker
KAPPA APEX II diffractometer with a 0.71073 Å Mo Kα source. The
structures were solved by direct methods. All non-hydrogen atoms
were refined anisotropically. Details regarding refined data and cell
parameters are available in Table S1 (Supporting Information).
Synthesis of (ONO)Ti(NMe2)2 (2). (ONO)H2 (274 mg, 0.564
mmol, 1 equiv) and KBn (147 mg, 1.127 mmol, 2 equiv) were mixed
in 10 mL of C6H6. The mixture was stirred for 1 h to generate a yellow
solution of (ONO)K2, and then a 5 mL C6H6 solution of
TiCl2(NMe2)2 (117 mg, 0.564 mmol, 1 equiv) was added dropwise.
The reaction mixture was stirred overnight and filtered to remove KCl,
and C6H6 was then lyophilized. The resultant orange powder was
washed with a small amount of cold pentane to remove any remaining
free ligand, yielding 326 mg (91%) of 2 as a light orange powder. X-
ray-quality crystals of 2 were grown from a saturated solution of 2 in
50/50 pentane/ether cooled to −30 °C. 1H NMR (300 MHz, C6D6; δ,
ppm): 1.41 (s, 18 H, C(CH3)3); 1.76 (s, 18 H, C(CH3)3); 3.03 (s, 12
H, N(CH3)2); 7.07 (t, J = 7.9 Hz, 1 H, 4-NC5H2-H); 7.26 (d, J = 7.8
Hz, 2 H, 3,5-NC5H-H2); 7.50 (d, J = 2.5 Hz, 2 H, aryl-H); 7.76 (d, J =
1
2.5 Hz, 2 H, aryl-H). H NMR (500 MHz, CD2Cl2; δ, ppm): 1.37 (s,
18 H, C(CH3)3); 1.51 (s, 18 H, C(CH3)3); 2.94 (s, 12 H, N(CH3)2);
F
dx.doi.org/10.1021/om400080g | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
were grown from slow evaporation of a concentrated solution of 6 in
benzene. 1H NMR (300 MHz, C6D6; δ, ppm): 1.32 (s, 18 H,
C(CH3)3); 2.01 (s, 18 H, C(CH3)3); 6.09 (br s, C5H5N); 6.86 (t, J =
7.9 Hz, 1 H, p-NC5H2-H); 7.02 (br s, C5H5N); 7.30 (d, J = 7.9 Hz, 2
H, 4-NC5H-H2); 7.32 (d, J = 2.3 Hz, 2 H, aryl-H); 7.79 (d, J = 2.3 Hz,
2 H, aryl-H); 8.42 (br s, C5H5N). 13C NMR (125 MHz, C6D6; δ,
ppm): 30.5, 31.4 (C(CH3)3); 34.4, 35.7 (C(CH3)3); 122.6, 123.5,
125.4, 126.3, 128.2, 135.1, 136.8, 138.4, 143.2, 149.9, 157.1, 157.5
(aryl).
Attempted Synthesis of (ONO)TiCl2(HNMe2) (5) and Crys-
tallographic Characterization of (ONO)Ti(HNMe2)(C2H4) (7).
(ONO)H2 (194.5 mg, 0.400 mmol, 1 equiv) and TiCl2(NMe2)2 (83
mg, 0.400 mmol, 1 equiv) were mixed in 6 mL of C6H6. A dark orange
solid immediately precipitated. This dark orange-brown solid was
isolated, washed with 5 mL of pentane, and dried in vacuo, yielding a
mixture of presumably 5 and the base-free (ONO)TiCl2. This mixture
was then used for subsequent reductions that resulted in the
crystallographic characterization of 7.
ACKNOWLEDGMENTS
■
We thank Lawrence Henling and Dr. Michael Day for
assistance with the X-ray studies. The Bruker KAPPA APEXII
X-ray diffractometer was purchased via an NSF CRIF:MU
award to the California Institute of Technology (CHE-
0639094). This work has been supported by USDOE Office
of Basic Energy Sciences (Grant No. DE-FG03-85ER13431)
and by the KAUST Center-In-Development at King Fahd
University of Petroleum and Minerals (Dhahran, Saudi Arabia).
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In an inert-atmosphere glovebox, 15.8 mg of the reaction mixture of
5 (0.024 mmol, 1 equiv based on 100% 5) was dissolved in 3 mL of
toluene and placed in a 50 mL flask fitted with a Kontes valve. A 200
equiv amount of 1% Na/Hg was added to the flask, and the flask was
then evacuated on a high-vacuum line. Ethylene (1 atm) was
introduced into the flask, and the reaction mixture was stirred
overnight under a constant 1 atm of ethylene. Over the course of the
reaction, the color changed from red to green and finally to a green-
brown. The reaction mixture was decanted away from the amalgam
and filtered to remove NaCl, and the solvent was removed in vacuo.
The 1H NMR of this mixture revealed the presence of significant
amounts of paramagnetic material. The mixture of products was
extracted into 1 mL of pentane, and red X-ray-quality crystals of 7
were obtained in very low yield after cooling the solution to −30 °C.
Typical Hydroamination Setup. A 250 μL portion of a 2 M
C6D6 solution of alkyne was added to a J. Young NMR tube, followed
by 250 μL of a 2 M C6D6 solution of amine, and finally 250 μL of a 1
M C6D6 solution of catalyst. The solution was spiked with SiMe4 as an
internal standard. The tube was sealed and inverted to mix the three
1
solutions, and a H NMR spectrum was taken. The reaction mixture
was then heated to 90 °C in an oil bath for the desired amount of time.
1
After the reaction was complete, the products were analyzed by H
NMR, and then the reaction mixture was passed through a plug of
silica gel before GC-MS analysis.
Typical Cyclotrimerization Setup. A 250 μL portion of a 2 M
C6D6 solution of alkyne was added to a J. Young NMR tube, followed
by 250 μL of C6D6, and finally 250 μL of a 1 M C6D6 solution of
catalyst. The solution was spiked with SiMe4 as an internal standard.
The tube was sealed and inverted to mix the three solutions, and a 1H
NMR spectrum was taken. The reaction mixture was then heated to 90
°C in an oil bath for the desired amount of time. After the reaction was
complete, the products were analyzed by 1H NMR, and then the
reaction mixture was passed through a plug of silica gel before GC-MS
analysis.
ASSOCIATED CONTENT
* Supporting Information
■
(10) Golisz, S. A.; Labinger, J. A.; Bercaw, J. E. Organometallics 2010,
29, 5026.
S
Full tables of bond lengths, angles, and anisotropic displace-
ment parameters for 2, 4, 6, and 7 and CIF files giving X-ray
crystallographic data for 2, 4, 6, and 7. This material is available
free of charge via the Internet at http://pubs.acs.org. The
crystallographic data for 2, 4, 6 and 7 have also been deposited
with the Cambridge Crystallographic Data Center and can be
obtained by requesting the deposition numbers.
(11) For other crystallographically characterized Ti−C2H4 adducts,
see: (a) Cohen, S. A.; Auburn, P. A.; Bercaw, J. E. J. Am. Chem. Soc.
1983, 105, 1136. (b) Pinkas, J.; Cisarova, I.; Gyepes, R.; Kubista, J.;
Horacek, M.; Mach, K. Organometallics 2012, 31, 5478.
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Timmers, F. J. Organometallics 1996, 15, 1518.
(13) Hamaed, A.; Trudeau, M.; Antonelli, D. M. J. Am. Chem. Soc.
2008, 130, 6992.
(14) Benzing, E.; Kornicker, W. Chem. Ber 1961, 94, 2263.
(15) Adams, N.; Bigmore, H. R.; Blundell, T. L.; Boyd, C. L.;
Dubberley, S. R.; Sealey, A. J.; Cowley, A. R.; Skinner, M. E. G.;
Mountford, P. Inorg. Chem. 2005, 44, 2882.
AUTHOR INFORMATION
Corresponding Author
*E-mail for J.E.B.: bercaw@caltech.edu.
Notes
■
The authors declare no competing financial interest.
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dx.doi.org/10.1021/om400080g | Organometallics XXXX, XXX, XXX−XXX