Titanium-Mediated Catalytic Hydrogenation of Monocyclic and Polycyclic Arenes

Article abstract of DOI:10.1002/chem.201905466

Two electron-reduction of the Ti^IV guanidinate complex (Im^DippN)(^Xyketguan)TiCl₂ gives (η⁶-Im^DippN)(^xyketguan)Ti (1^intra) and (Im^DippN)(^Xyketguan

Full text of DOI:10.1002/chem.201905466

A Journal of
Accepted Article
Title: Titanium-Mediated Catalytic Hydrogenation of Monocyclic and
Polycyclic Arenes
Authors: Alejandra Gómez-Torres, J. Rolando Aguilar-Calderón,
Angela M. Encerrado-Manriquez, Maren Pink, Alejandro J.
Metta-Magaña, Wen-Yee Lee, and Skye Fortier
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To be cited as: Chem. Eur. J. 10.1002/chem.201905466
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10.1002/chem.201905466
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COMMUNICATION
Titanium-Mediated Catalytic Hydrogenation of Monocyclic and
Polycyclic Arenes
Alejandra Gómez-Torres, J. Rolando Aguilar-Calderón, Angela M. Encerrado-Manriquez, Maren Pink,
Alejandro J. Metta-Magaña, Wen-Yee Lee, and Skye Fortier*
Abstract: Two electron-reduction of the Ti(IV) guanidinate complex
(ImDippN)(Xyketguan)TiCl2 gives (η6-ImDippN)(xyketguan)Ti (1intra) and
60 °C for 24 h with a catalyst loading of 5 mol % under 59 psi of
H2. Higher hydrogen pressures (470 psi), longer reaction times
(48 h), and higher catalyst loadings (10 mol %) were necessary to
access other more sterically hindered substituted arenes, but the
catalyst is poisoned by heteroarenes such as pyridine.[8]
(ImDippN)(Xyketguan)Ti(η6-C6H6)
(1inter
)
(
xyketguan
=
[(tBuC=N)C(NXylyl)2]-, Xylyl = 2,5-dimethylphenyl) in the absence or
presence of benzene, respectively. These complexes have been
found to hydrogenate monocyclic and polycyclic arenes under
relatively mild conditions (150 psi, 80 °C) – the first example of
catalytic, homogeneous arene hydrogenation with TON > 1 by a
Group IV system.
Muetterties and co-workers also reported the only example of
a homogeneous Group IV arene hydrogenation catalyst.[9] The
Ti(II)-arene complex Ti(η6-C6-H5Me)Al2Cl8 was shown to catalyse
the conversion of benzene to cyclohexane when heated to 125 °C
in the presence of an H2 atmosphere. However, conversion was
low with no turnover due to evident decomposition of the catalyst
at the elevated working temperatures. Very recently, a hybrid
zirconium system which combines characteristics of both
homogeneous and heterogeneous catalytic systems, benzylated
Cp*Zr(H)Bz3 supported on sulfated zirconia, was found able to
catalyse the all-cis hydrogenation of alkyl substituted and
polycyclic arenes to cyclohexanes at room temperature under 102
psi of H2.[10]
The precious metal based homogeneous catalytic
hydrogenation of substituted and polycyclic arenes has been an
intensive area of study as these systems offer greater flexibility
and stereochemical control over heterogeneous processes.[1]
This is nicely exemplified by the Rhodium-CAAC system
developed in recent years by the Glorious group.[2] Their platform
catalyzes the hydrogenation of a wide range of aromatic
substrates including heteroarenes as well as borylated,
fluorinated, and silylated aromatics to give all cis-hydrogenated
heterocycles and substituted cycloalkanes.[2] While these late
metal systems are effective, it does bring into focus the question
of sustainability, ethical sourcing, and the toxicity of precious
metals.[3] To this end, forays into the development of base metal
arene hydrogenation catalysts warrants investigation, particularly
with respect to abundant and non-toxic early metals such as
titanium.[1b]
These results clearly show that early- and 3d-transition metals
can be effective as homogenous arene hydrogenation catalysts
but also suggests an unrealized potential for Group IV elements,
which is especially relevant as titanium is the second most earth
abundant transition metal. In our laboratory, we have been
endeavoring to develop such catalysts. For instance, we reported
the Ti(II) synthon (η6-ImDippN)(ketguan)Ti (A)
(
ketguan
=
[(tBuC=N)C(NDipp)2]-; ImDippN-
=
1,3-bis(Dipp)imidazolin-2-
iminato, Dipp = 2,6-diisopropylphenyl) which behaves as a
Despite the intrinsic limitations of early- and 3d-transition
transfer hydrogenation catalyst (Scheme 1a).[11]
metals to mimic precious-metal type reactivity, a handful of
Compound A readily binds cyclic olefins to give titanium
metallacycles that eliminate cycloalkanes under mild conditions
(30 psi H2 and 55 °C). A major structural feature of A is the two-
electron reduction and coordination of a peripheral Dipp-ring to
the titanium center, providing an intramolecularly masked metal
species. The hydrogenation of cyclic olefins by A suggests the
Ti-arene coordination is labile; however, no indication of arene
binding to A has been observed. Based on this, we have been
working to modify our ligand platform to favor intermolecular over
intramolecular arene binding to promote the hydrogenation of
aromatic substrates, namely by reducing the steric profile of the
ketimine-guanidinate ligand. Here, we report the synthesis and
4]
homogeneous systems are known.[1a,
The earliest example
reported by Muetterties et al., shows the hydrogenation of a
variety of aromatics to cycloalkanes using the tris(phosphite)
cobalt catalyst (η3-C3H5)Co[P(OCH3)3]3 (1 mol%) at room
temperature in the presence of 15 psi of H2; however, while
exhibiting broad substrate tolerance, the catalyst performance is
low.[5] This work was later followed by Rothwell and co-workers
who developed mixed hydrido-aryloxide supported Group V
tantalum and niobium catalysts capable of hydrogenating
benzene and polycyclic aromatics at high H2 pressures (1200 psi)
and moderate temperatures (80-100 °C).[1b, 6] Later, Fryzuk and
co-workers disclosed
a series of niobium cyclohexadienyl
complexes, which catalyze the room temperature hydrogenation
of benzene with 500 psi of H2, reaching 225 turnover numbers
(TON). Yet, treatment of toluene under the same conditions led to
only 2% conversion to methylcyclohexane.[7]
a. Previous work
More recently, Chirik et al. developed the pyridine(diamine)
molybdenum catalyst (4-tBu-iPrBDI)Mo(CH2SiMe3)2 that is able to
hydrogenate benzene, toluene, and o-xylene upon heating at
[*]
Alejandra Gómez-Torres, Dr. J. Rolando Aguilar-Calderón, Angela
M. Encerrado Manriquez, Dr. Alejandro J. Metta-Magaña, Prof. Dr.
Wen-Yee Lee, Prof. Dr. Skye Fortier
A
b. This work
Department of Chemistry and Biochemistry, University of Texas at
El Paso, El Paso, TX 79968 (USA)
E-mail: asfortier@utep.edu
Homepage: http://utminers.utep.edu/asfortier/
Dr. Maren Pink
1Intra
Indiana University Molecular Structure Center, Indiana University,
Bloomington, Indiana, IN 47405 (USA)
Scheme 1. Steric effects on titanium-based hydrogenation catalysis.
Electronic Supplementary Information (ESI) available: Experimental
procedures, NMR, X-Ray and GC-MS data. See DOI: 10.1039/x0xx00000x
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COMMUNICATION
N5
Ti
N1
N6
Figure 1. Solid-state molecular structure of 1inter
.
tracked through electronic absorption spectroscopy as each
complex has distinctive UV-vis spectral traces (Figure S37).
Notably, the successful isolation of 1inter evidently shows that
subtle ligand modifications, in this case substitution of the Dipp-
guanidinate groups in A for less encumbering Xylyl substituents,
can have a significant effect on coordination properties and
molecular dynamics at the metal center. Indeed, whereas A is
thermally unstable in solution at room temperature, leading to
intramolecular C-H oxidative-addition, 1intra and 1inter are
indefinitely stable in such solutions. This, combined with the
interconvertibility of 1intra and 1inter, signaled to us system
characteristics amenable for arene hydrogenation catalysis. This
is especially true as arene capture and reduction has been
proposed as an important first step in the hydrogenation
process.[1b, 12]
Scheme 2. Synthesis and interconversion of 1intra and 1inter
.
characterization of the new, masked titanium species (η6-
ImDippN)(Xyketguan)Ti (1intra) and (ImDippN)(Xyketguan)Ti(η6-C6H6)
(1inter
)
(
Xyketguan
=
[(tBuC=N)C(NXylyl)2]-, Xylyl
=
2,5-
‘dimethylphenyl) and describe their use in the catalytic
hydrogenation of monocyclic and polycyclic arenes (Scheme 1b).
Similar to A, 1intra is synthesized via the two-electron reduction
of the Ti(IV) dihalide (ImDippN)(Xyketguan)TiCl2 with 2.2 equiv of KC8
in THF solution at -78 °C (Scheme 2). Interestingly, when the
reduction is carried out in the presence of a small amount of C6H6,
benzene-capped 1inter forms as the favored product (Scheme 2).
Complexes 1intra and 1inter are highly soluble in aromatic solvents,
and partially soluble in Et2O and non-polar solvents such as
hexanes and pentanes. The molecular structures of 1intra and
1inter were inferred from 1H, 13C{1H}, COSY and HSQC NMR
spectral analyses (Figures S12-S19) with further confirmation
provided by X-ray crystallography.
Accordingly, pressurizing solutions of 1intra in C6D6 (0.036
mmol) with H2 (150 psi) at 40 °C results in the formation of
cyclohexane-d6 – confirmed by NMR and GC/MS spectroscopies
(Figures S25 and S36) – as the sole hydrogenation product over
the course of several hours. Examination of the 13C{1H} NMR
spectrum of the cyclohexane-d6 product (Figure S26) indicates
the presence of several isotopomers,[8] while the 2D NMR
spectrum shows no signs of H/D scrambling into the ligands
(Figure S27). Raising the temperature to 80 °C enhances the rate
of catalysis, giving a modest turn over number of 3 after 80 hours
(Figure S25). During this period, the onset of catalyst
decomposition (~50%) is observed along with diminished catalytic
performance occurring over time. Not surprisingly, conducting the
hydrogenation reaction in C6D12 with 20 equiv of benzene results
in a significantly slower rate with TON < 0.5 after 20 h (Figure
S32).
Light-brown crystals of 1intra or 1inter can be grown from
concentrated toluene or THF/Et2O (10:1) solutions stored at -
35 °C for 4 to 7 days. As anticipated, the solid-state molecular
structure of 1intra is structurally analogous to A as it features η6-
coordination of a peripheral Dipp-ring to the titanium metal center;
however, poor X-ray diffraction data precludes in-depth analysis
of its metrical parameters (Figure S2). On the other hand, the
solid-state structure of 1inter is well-resolved and inspection of the
coordinated benzene moiety reveals clear structural distortions
consistent with dearomatization and reduction to
a
cyclohexadienide dianion (Figures 1 and S3), similar to those
found for the coordinated ring of A.[11]
In solution, the masking interaction of 1intra is clearly
maintained on the NMR timescale as the 1H NMR spectra
displays tell-tale resonances with a multiplet at 3.14 ppm and a
triplet at 2.51 ppm, in a 2:1 ratio, attributable to the upfield-shifted
signals of the dearomatized Dipp ring protons (Figure S12). In
contrast, the protons of the reduced benzene ring in 1inter appear
as a single peak at 3.97 ppm (Figure S16), suggesting resonance-
form delocalization and free rotation of the coordinated ring in
solution.
1
Monitoring the reaction by H NMR spectroscopy reveals the
presence of both 1intra and 1inter in solution in a relative 3:7 ratio
after heating for 20 hours at 80 °C. This is notable as this
phenomenon is not observed in the absence of H2 (vide supra),
and the origin of the interconversion of 1intra and 1inter under these
conditions is not presently known. Additionally, heating solutions
of 1intra under H2 in C6D12 in the absence of aromatic substrates
gives no indication for the formation of hydride intermediates but
begins to slowly decompose over several days under these
conditions (Figure S24). Nonetheless, the observation of 1intra and
1inter in these solutions indicates that each plays a pivotal role as
catalyst resting states. As such, starting with 1intra/inter (1) as the
catalyst does not alter performance. Moreover, photolyzing the
reactions has no effect on the outcome. Finally, hydrogenation of
the coordinating Dipp-ring of 1intra is not observed but could play
a role in the catalyst decomposition pathway.[6a]
Heating C6D6 solutions of 1intra at 80 °C for extended periods
does not lead to any observable change as indicated by 1H NMR
spectroscopy. Conversely, heating C6D6 solutions of 1inter at 80 °C
slowly leads to the near quantitative formation of 1intra over the
course of two weeks (Figure S21). During this time, C6H6 for C6D6
exchange is observed by the rapid loss of signal intensity of the
masking ring protons in the 1H NMR spectrum of 1inter concomitant
with the appearance of a new, corresponding signal at 3.98 ppm
in the 2D NMR spectrum indicative of C6D6 binding (Figure S23).
Interestingly, the reverse reaction can be achieved by
photolysis of 1intra (0.044 mM) using blue LED light (365 nm) in
C6H6 solution to generate 1inter, a conversion that can be nicely
Gratifyingly, this reactivity demonstrates successful
implementation of
a Group 4 titanium-based system for
homogeneous arene hydrogenation under relatively mild
conditions. In order to determine the catalytic scope of 1, the
hydrogenation of a variety of other aromatic substrates was
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screened including toluene, naphthalene, anthracene, and
styrene (Table 1) (Scheme 3). All reactions were performed
full consumption of the naphthalene. While Rothwell’s niobium
catalysts are capable of hydrogenating tetralin to decalin under
under 150 psi of H2 at 80 °C with ferrocene as an internal standard. 1200 psi of H2,[1b] the same was not observed for Chirik’s
homogeneous molybdenum system, even when increasing the H2
pressure from 59 to 470 psi.[8]
With respect to the hydrogenation of anthracene by 1, its
relative rate is similar to that of naphthalene and selectively forms
1,2,3,4,5,6,7,8-octahydroanthracene (Table 1) (Figures S30 and
S40), though it must be noted that the rate of reaction is severely
limited by the poor solubility of the anthracene under the reaction
conditions.
Surprisingly, there is no indication of the presence of any
monocyclic-hydrogenated 1,2,3,4-tetrahydroanthracene in the
reaction mixture, indicating that it is immediately hydrogenated
upon formation. In comparison, Rothwell’s catalysts give access
to both 1,2,3,4-tetrahydroanthracene and 1,2,3,4,5,6,7,8-
octahydroanthracene
(1200
psi,
80
°C),[1b]
while
perhydroanthracene is the major product of Mutterties’ cobalt
catalyzed reactions.[5a]
Curiously, 1 does not catalyze the aromatic hydrogenation of
styrene to ethylcyclohexane under the general conditions of our
experiments. Instead, it acts as an olefin hydrogenation catalyst
that selectively hydrogenates styrene to ethylbenzene (TON = 9.0,
20 hours) without the formation of ethylcyclohexane (Figures S31
and S41). This transformation takes place at room temperature
and occurs at a higher relative rate than the hydrogenation of
aromatic substrates (Table 1). Upon full consumption of the
styrene, formation of ethylcyclohexane from the hydrogenation of
ethylbenzene is not observed even upon heating the sample at
80 °C over the course of 7 days (Figure S31). Similarly, starting
with ethylbenzene in place of styrene results in no change after
80 h at 80 °C under H2 (150 PSI) (Figure S34). In comparison, (4-
tBu-iPrBDI)Mo(CH2SiMe3)2 also hydrogenates styrene to
ethylbenzene (59 psi), while higher pressures (470 psi) leads to
ethylcyclohexane exclusively.[8] Regardless, when compared
against A, 1 was found to be a superior hydrogenation catalyst as
it gives significantly higher turnovers of ethylbenzene under
comparable conditions (80 °C, 150 psi H2, 18 h, and 8.0 mM in
C6D12), TON = 6 (1) vs TON = 2.5 (A), possibly due to the more
favorable steric profile of 1.
It should be noted that in the case of anthracene and styrene,
the formation of intermediates are observed by 1H NMR
spectroscopy (Figures S30 and S31), which we putatively assign
as the catalytically active species (ImDippN)(Xyketguan)Ti(η6-C12H14)
and (ImDippN)(Xyketguan)Ti(η2-C2H3C6H5), lending credence to the
steps of our proposed catalytic cycle (Scheme 3). In contrast, the
toluene and naphthalene analogs are not seen suggesting these
intermediates are rapidly hydrogenated upon formation.
Finally, attempts to hydrogenate substituted arenes such as
Me3SiPh, phenyl boronic acid pinacol ester, anisole, and aniline
were not met with success, either leading to no observed
reactivity (Me3SiPh) or catalyst decomposition (pinacolborane,
aniline, and anisole). The decomposition of our system by N- and
O-atom containing heterocycles is not surprising as we have
shown our masked Ti-complexes to be potent reductants that are
highly reactive towards heteroatom donors.[13] This suggests that
ligand modifications will be necessary to temper the reactivity to
mitigate such unwanted chemistries.
Scheme 3. Proposed catalytic cycle.
As compared to C6D6, neat toluene-d8 is more rapidly
hydrogenated with nearly twice the TON over the first 20 hours
(Table 1) (Figures S28 and S38). This is surprising, as in the case
of the macrocyclic phosphine amide R[P2N2]Nb(CH2SiMe3), the
catalytic competency of the niobium complex drops precipitously
from benzene to toluene.[7]
Rothwell’s niobium arene
hydrogenation catalysts also show a decrease in relative rate from
benzene to toluene, though, no other correlation between
hydrogenation rates and sterics can be drawn from his system.[1b]
Though, it should be noted, that when using fewer equivalents of
toluene-h8 (20 equiv) in C6D12 with 1, the decrease in the rate of
hydrogenation is pronounced (TON < 0.7 after 20 h) (Figure S33).
The hydrogenation of naphthalene by
1 occurs at a
significantly faster initial rate than the pseudo first order C6D6 or
C7D8 reactions, cleanly producing 1,2,3,4-tetrahydronaphthalene
(i.e., tetralin) (Table 1) (Figures S29 and S39). Consecutive
hydrogenation of tetralin to decalin was not observed even after
Table 1. Catalytic hydrogenation of select unsaturated aromatics.[a]
TON TOF[d]
(20 h)
Total TON[e]
(60 h)
Substrate
Product
(h-1)
0.05
[b]
C6D6
cyclohexane-d6
1.0
2.0
[b]
0.09
0.22
Toluene-d8
Naphthalene
Anthracene
Styrene[c]
methylcyclohexane-d8
1.8
4.4
4.6
9.0
2.9
7.4
tetralin
In closing, we have previously shown that our arene-masked
Ti(II) synthon (A) affords access to novel and distinctive late-metal
type reactivity, namely transfer hydrogenation of cyclic olefins.[11]
Building upon this success through subtle but judicious ligand
octahydroanthracene
0.23
0.45
6.5
13.9[c]
ethylbenzene
Pinacolborane
Me3SiPh
–
–
–
–
modification, we have now reported a new platform, 1intra and 1inter
,
with improved thermal stability. Importantly, the steric profile of
1intra and 1inter allows for the intermolecular capture and activation
of aromatic hydrocarbons at the titanium center, catalyzing the
hydrogenation of monocyclic (benzene and toluene) and
polycyclic (naphthalene and anthracene) arenes under relatively
mild conditions (150 psi, 80 °C). Moreover, our new system also
selectively hydrogenates the olefinic substituent of styrene to give
Anisole
Aniline
[a] Reactions conducted using solutions of 1intra (0.036 mmol,1.0 mL) with 20
equiv of substrate at 80 °C under H2 (150 psi) in C6D12 with Fc (0.0269 mmol)
as internal standard. [b] Substrate as solvent. [c] Room temperature reaction,
60 h. [d] Average turnover frequency over 20 hours. [e] Averaged over three
runs.
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performance of 1intra and 1inter is admittedly modest, we present
[12]
[13]
F. J. Hirsekorn, M. C. Rakowski, E. L. Muetterties, J. Am.
Chem. Soc. 1975, 97, 237-238.
J. R. Aguilar-Calderón, J. Murillo, A. Gómez-Torres, C.
Saucedo, A. Jordan, A. J. Metta-Magaña, M. Pink, S.
ethylbenzene at room temperature.
While the catalytic
here
a
rare example of base-metal-mediated arene
hydrogenation catalysis.[1]
Our chemistry also shows that
Fortier.
Organometallics,
ASAP.
challenging chemical transformations, viz. homogenous arene
hydrogenation, can be carried out using inexpensive, abundant,
and non-toxic base metals such as titanium. We are continuing
our efforts to optimize the catalytic outputs of 1 while targeting
other late-metal type reactivity with our arene-masked titanium
platforms.
https://doi.org/10.1021/acs.organomet.9b00637
Acknowledgments
We are grateful to the Welch Foundation (AH-1922-20170325),
ACS PRF (57132-DNI3) and the NSF PREM Program (DMR-
1827745) for financial support of this work. S.F. is an Alfred P.
Sloan Foundation research fellow and is thankful for their support.
We wish to acknowledge the NSF-MRI program (CHE-1827875)
for providing funding for the purchase of an X-ray diffractometer.
We also thank the Advanced Photon Source (Office of Science,
U.S. Department of Energy under DE-AC02-06CH11357) and
NSF's ChemMatCARS Sector 15 (supported by the Divisions of
Chemistry (CHE) and Materials Research (DMR), National
Science Foundation, under CHE-1834750).
Conflicts of interest
There are no conflicts to declare.
Keywords: Arene hydrogenation ‧ titanium ‧early transition
metal ‧homogeneous catalysis ‧ late-metal type reactivity
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COMMUNICATION
COMMUNICATION
Alejandra Gómez-Torres, J. Rolando
Aguilar-Calderón, Angela M. Encerrado
Manriquez, Maren Pink, Alejandro J.
Metta-Magaña, Wen-Yee Lee, and Skye
Fortier *
Δ
Page No. – Page No.
Titanium-Mediated Catalytic
Hydrogenation of Monocyclic and
Polycyclic Arenes
Ti don’t know but Ti been told, hydrogenation of arenes is pretty bold: The
newly synthesized Ti-masked complex (η6-ImDippN)(xyketguan)Ti undergoes
intermolecular conversion under photolysis to give the benzene-capped complex
(ImDippN)(Xyketguan)Ti(η6-C6H6). Both of these compounds are effective at the
hydrogenation of monocyclic and polycyclic aromatic hydrocarbons – a first for Group
IV homogeneous catalysis.
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