Homogeneous catalytic transfer dehydrogenation of alkanes with a group 10 metal center

Article abstract of DOI:10.1039/b913319d

Unambiguous catalytic homogeneous alkane transfer dehydrogenation was observed with a group 10 metal complex catalyst, LPt^II(cyclo-C ₆H₁₀)H, supported by a lipophilic dimethyl-di(4-tert- butyl-2-pyridyl)borate anionic ligand and tert-butylethene as the sacrificial hydrogen acceptor.

Full text of DOI:10.1039/b913319d

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Homogeneous catalytic transfer dehydrogenation of alkanes with
a group 10 metal centerw
Eugene Khaskin, Daniel L. Lew, Shrinwantu Pal and Andrei N. Vedernikov*
Received (in Cambridge, UK) 6th July 2009, Accepted 20th August 2009
First published as an Advance Article on the web 3rd September 2009
DOI: 10.1039/b913319d
Unambiguous catalytic homogeneous alkane transfer dehydro-
genation was observed with a group 10 metal complex catalyst,
LPtII(cyclo-C6H10)H, supported by a lipophilic dimethyl-di(4-tert-
butyl-2-pyridyl)borate anionic ligand and tert-butylethene as the
sacrificial hydrogen acceptor.
our novel dimethyl-bis(4-tert-butyl-2-pyridyl)borate ligand,
dpbtBu, (Scheme 2) can be prepared in a virtually quantitative
yield by a stoichiometric alkane dehydrogenation at room
temperature and, importantly, can serve as a homogeneous
catalyst for the dehydrogenation of various alkanes with TBE
as a sacrificial hydrogen acceptor.
When analyzing the low efficiency of our previous
dpb-based system in stoichiometric alkane dehydrogenation10
we envisioned that it might originate from the low solubility in
alkanes of the presumed Pt(IV) hydride intermediates. To solve
the solubility problem we decided to introduce lipophilic tert-butyl
groups onto the ligand scaffold (dpbtBu ligand). Gratifyingly,
sodium dimethylplatinate(II) complex Na[(dpbtBu)PtIIMe2], 4,
derived from our new lipophilic dimethyl-bis(4-tert-butyl-2-
pyridyl)borate ligand, in contrast to Na[(dpb)PtIIMe2], 1,
proved to be soluble both in cyclohexane and n-pentane.
In both cases, upon addition of water, a fast CH activation
reaction occurred to give platinum(II) olefin hydride products
in virtually quantitative yield (Scheme 2). In the case of
n-pentane selective (495%) formation of a 1-pentene deri-
vative was observed initially when the reaction was stopped
after five minutes; a 1-pentene bound complex 5 was obtained
(Scheme 2).z Complex 5 isomerized after 1 day in cyclohexane
solution towards products derived from cis- and trans-2-
pentenes,y implying that multiple reversible olefin insertion
into Pt–H–b-hydride elimination reactions had occurred.
The alkane dehydrogenation to olefins is a viable CH bond
functionalization strategy.1–8 In particular, the well-defined
homogeneous group 9,2–7 group 77,8 and group 67 organometallic
species have proven to be active in the catalytic transfer
dehydrogenation (eqn (1))2–4,8 and acceptorless dehydrogenation
of alkanes (eqn (2)):5–7
RCH2CH2R0 + R00CHQCH2 - RCHQCH2R0 + R00CH2CH3
(1)
RCH2CH2R0 - RCHQCHR0 + H2 (2)
The acceptorless homogenous systems based on iridium
complexes supported by rigid PCP pincer ligands developed
by Goldman et al.6 operate at high temperatures (150–200 1C),
but are effective in achieving large turnover numbers
(TON 4 103). Analogous PCP–iridium systems that utilize
tert-butylethene (TBE) as a sacrificial hydrogen acceptor exhibit
lower TONs (o300)3 and operate at lower temperatures
(150 1C). Introducing a platinum-based catalyst would add a
group 10 metal into the arsenal of catalysts available for this
important transformation. Previously, soluble platinum-based
complexes have been shown to stoichiometrically dehydrogenate
alkanes,9–11 and while there has been one recent report of
catalytic dehydrogenation,12 TONs were limited to 1.1–1.3.
These latter reactions were accompanied by a fast decomposition
of the catalyst towards platinum black.
A
similar isomerization phenomenon was observed by
Templeton et al.,11 and also by Bercaw, Labinger et al.13 in
the activation of linear alkanes by Pt(II) complexes. Though
the isolation of internal olefin complexes was reported
only, it was inferred from deuterium labeling studies that
initial activation took place at the terminal carbon.
If dehydrogenation of a symmetrical substrate, cyclohexane,
by complex 4 is carried out in the presence of water, a single
product 3 forms which is stable for at least several weeks in
cyclohexane solution.
In the course of our recent CH activation studies in biphasic
alkane–water systems with an anionic PtII complex
1
supported by dimethyl-di(2-pyridyl)borate ligand (dpb),10
we showed that various alkanes can be stoichiometrically
dehydrogenated under very mild conditions to produce PtII
olefin hydride complexes, (dpb)PtII(olefin)H (see Scheme 1 for
an example; 2, olefin = cyclo-C6H10). The yields of these
borato platinum(II) olefin hydrides, however, were limited to a
low 30–40% and their reactivity was not characterized. In this
communication, we disclose that the more lipophilic analogue
of 2, platinum(II) cyclohexene hydride complex 3 supported by
Importantly, it was possible to substitute the sacrificial
olefin TBE for bound cyclohexene or pentene. In particular,
the yield of 6 produced from 3 with 5 equivalents of TBE was
quantitative after 30 minutes at 20 1C. Such reactivity is one of
the prerequisites for a possible catalytic alkane transfer
University of Maryland Chemistry & Biochemistry, College Park,
MD 20742, USA. Tel: +1-301-405-2784; E-mail: avederni@umd.edu
w Electronic supplementary information (ESI) available: Synthesis of
all complexes and spectral information; description of catalytic and
olefin substitution experiments. See DOI: 10.1039/b913319d
Scheme 1 Alkane dehydrogenation with Na[(dpb)PtIIMe2].
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loading was found to be optimal, as higher amounts of TBE
relative to the catalyst were detrimental to the yield of product.
Unlike the case of a previous report by Templeton et al.,12
we were unable to observe transfer dehydrogenation of oxygenated
substrates, such as diethyl ether, with our system.
Based on our observations and the results of DFT-modeling
of the transfer dehydrogenation of cyclohexane with TBE as a
sacrificial substrate, we suggest the mechanism of the catalytic
reaction presented in Scheme 3. The first step, an olefin
exchange 3-1, occurs readily at room temperature. The equilibrium
involving cyclohexene complex 3 is shifted toward the TBE
adduct 6 already with 5 equivalents of TBE, consistent with
the results of our DFT calculations that show that the reaction
3-1 is thermodynamically favorable. Subsequent TBE insertion
into a Pt–H bond, reaction 3-2, leads to a b-CH agostic
complex I and is facile (DGa298 = 5.1 kcal molꢁ1). Coordination
of a cycloalkane to I, reaction 3-3, leads to a relatively high-
energy s-complex II. The cycloalkane oxidative addition 3-4
to form a PtIV dialkyl hydride III is followed by the reductive
coupling 3-5 (DGa298 = 29.7 kcal molꢁ1) which is the reaction
(3) rate determining step (RDS). Similarly, a C–H reductive
elimination from a d6 iridium(III) dialkyl hydride intermediate
was suggested by Goldman et al. to be the RDS in a
(PCP)Ir-catalyzed transfer dehydrogenation of cyclooctene
with TBE as a sacrificial olefin.4 A sequence of the subsequent
neohexane dissociation 3-6 from IV and b-hydride elimination
3-7 from a cycloalkyl intermediate V leads back to the
beginning of the catalytic cycle.
Scheme 2 New lipophilic dpbtBu—complexes 3, 4, 5 and 6. Alkane
dehydrogenation and olefin substitution reactivity.
dehydrogenation. An alkane transfer dehydrogenation was
attempted with complex 3 as the catalyst.
Experiments with TBE as the sacrificial olefin and a cyclo-
alkane as both the substrate and the solvent showed that
catalysis was already possible at 60 1C (eqn (3)):
ð3Þ
For cyclohexane, two catalyst turnovers were achieved after
1 day of reaction time at 60 1C with two equivalents of TBE.
The yield of reaction products, a cycloalkene and neohexane
(NH), was determined using an NMR integration of signals of
olefinic hydrogens of the cycloalkene and the hydrogen atoms
of the tert-butyl group of neohexane, respectively, with anisole
as an internal standard that was added after reaction completion.
The optimal conditions for the catalytic reaction were found
eventually to be 1 day of heating at 100 1C in a closed Schlenk
flask with 20 equivalents of TBE (560 mM) with regard to
the catalyst (28 mM). These conditions gave higher catalyst
turnover though resulted in a faster decomposition of 3
towards undetermined organometallic species after 1 day.
Larger amounts of TBE led to lower TONs, with the reaction
being completely suppressed when 100 equivalents of TBE
were used. Similar behavior was observed by Goldman et al. in
a (PCP)Ir-catalyzed transfer dehydrogenation of cyclooctane
with TBE as a sacrificial olefin.4 In turn, lower amounts of
TBE limited the TON to the maximum amounts of TBE
utilized. For instance, when 2 equivalents of TBE were used,
2 catalyst turnovers were observed.
The key CH-agostic intermediate I derived from TBE might
be trapped by an excess olefin present in solution to form
putative alkyl olefin complex VI, thereby inhibiting the target
dehydrogenation reaction. To suppress formation of species
such as VI, bulkier olefins, trimethyl- and tetramethylethylene,
might be useful. At the same time, olefin steric bulk might have
a negative impact on the reaction steps 3-1–3-4.
In separate experiments it was found that trimethylethylene
and tetramethylethylene can displace cyclohexene from 3.
The numbers of equivalents of a cycloolefin and NH
(eqn (3)) produced under these conditions are in a reasonable
agreement, 7.5 and 8 (n = 4), 15 and 13 (n = 2) and 11 and 7
(n = 1), respectively. Formation of 1 equivalent of free
cyclohexene originating from 3 was observed (n = 1, 4) or
accounted for (n = 2) in all the cases.
Analysis of the dark yellow residues obtained upon
completion of reaction after 1 day and removal of solvent
revealed the presence of a complex mixture of pyridine
derivatives as determined by the multitude of aromatic signals,
and a single platinum hydride in low concentration. Stopping
the catalytic reaction after 6 hours revealed that the starting
complex 3 was still present in the mixture. No platinum black
was observed to form during the reaction. A 5% catalyst
Scheme
3
Proposed mechanism of catalytic alkane transfer
dehydrogenation. DFT-calculated Gibbs energies for the case of
n = 2, DGo298, are in kcal molꢁ1 and given in parentheses.
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1
The reactions were monitored by means of H NMR spectro-
Notes and references
scopy at room temperature. In both cases the Pt–H signal of
1
z Another possible diastereomeric 1-pentene complex could not be
identified because of the low fraction of other PtII hydrides
(5% combined) present in the reaction mixtures.
y Addition of TBE to any of these solutions resulted in the clean
formation of one predominant diastereomer of complex 6.
the new complexes was observed (d ꢁ22.19, JPtH = 1313 Hz
1
and d ꢁ21.89, JPtH = 1280 Hz, respectively) whose intensity
was a function of the initial concentrations of 3 and the olefin.
These results suggest that sterics are not a crucial factor in
olefin binding to the Pt atom in dpb complexes (reaction 3-1).
Ready olefin substitution in borate supported Pt complexes
may be the feature that enables catalysis in this system and not
in others.9,11
1 Handbook of C–H Transformations, ed. A.S. Goldman and
R. Ghosh, G. Dyker, Wiley-VCH, Weinheim, 2005, vol 2,
section 2.1, pp. 614–622.
2 R. H. Crabtree, J. M. Mihelcic and J. M. Quirk, J. Am. Chem.
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3 J. A. Maguire and A. S. Goldman, J. Am. Chem. Soc., 1991, 113,
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Soc., 1992, 114, 9492; F. Liu, E. B. Pak, B. Singh, C. M. Jensen and
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To test the effect of the olefin steric bulk on the other steps
of the proposed catalytic cycle we attempted to use trimethyl-
and tetramethylethylene as the sacrificial olefins in the transfer
dehydrogenation of cyclohexane. Trimethylethylene showed
lower TONs than TBE as sacrificial olefin (2–3 TONs at the
optimized conditions). An isomerization to the terminal olefin
position, towards isopropyl ethylene (IPE), was evident in a
large part of the remaining, unhydrogenated sacrificial olefin.
Hence, the actual dihydrogen acceptor here might be IPE
whereas the excessive steric bulk of trimethylethylene itself
might be detrimental for the subsequent reaction steps, 3-2,
3-3 and 3-4. Finally, with tetramethylethylene, we were unable
to observe clear cut cases of catalysis.
In conclusion, we were able to show an unambiguous case
of alkane transfer dehydrogenation that could be carried out
with a group 10 metal acting as a homogenous catalyst:
complex 3. We were able to demonstrate a facile olefin-
for-olefin exchange involving platinum(II)-bound and free
olefins in solution in this system, which plays a significant role
in the overall catalytic success of the system. Subsequent study
of the catalyst deactivation pathways might lead to a next
generation, more efficient Pt-based borate systems for alkane
dehydrogenation.
9 M. W. Holtcamp, L. M. Henling, M. W. Day, J. A. Labinger and
J. E. Bercaw, Inorg. Chim. Acta, 1998, 270, 467; U. Fekl,
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Chem. Commun., 2003, 358.
10 E. Khaskin, P. Y. Zavalij and A. N. Vedernikov, J. Am. Chem.
Soc., 2006, 128, 13054.
We thank the University of Maryland, the Donors of the
American Chemical Society Petroleum Research Fund, and
the National Science Foundation (CHE-0614798) for financial
support of this work as well as Mr Larry M. Kraft for
synthesis of some of the complex 3 used in this work.
11 C. N. Kostelansky, M. G. MacDonald, P. S. White and
J. L. Templeton, Organometallics, 2006, 25, 2993.
12 N. M. West, P. S. White and J. L. Templeton, J. Am. Chem. Soc.,
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13 G. S. Chen, J. A. Labinger and J. E. Bercaw, Proc. Natl. Acad. Sci.
U. S. A., 2007, 104, 6915.
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