Combinatorial screening of an in situ generated library of tungsten oxyhalide and imido complexes for olefin metathesis

Article abstract of DOI:10.1021/co500087b

A series of substituted tungsten(VI) halides with general formula WECl₄ (E = O or 'NR (imido)) were screened via a high throughput study to identify potential new olefin metathesis catalysts. The tungsten species were treated with a series of aluminum alkyl activators and modifier ligands to generate active catalyst species in situ. Ring-opening metathesis polymerization (ROMP) of cyclooctene was used as a primary screen to identify potential metathesis catalysts and active catalysts were subjected to a secondary screen to evaluate tolerance toward polar functional groups. Several combinations from the high throughput campaign yielded active metathesis catalysts for the ROMP of cyclooctene. However, none of the catalysts examined in this study exhibited any evidence of significant polar functional group tolerance as determined by the results of the secondary cyclooctene/butyl acetate screen.

Full text of DOI:10.1021/co500087b

Research Article
pubs.acs.org/acscombsci
Combinatorial Screening of an In Situ Generated Library of Tungsten
Oxyhalide and Imido Complexes for Olefin Metathesis
Duane R. Romer,* Victor J. Sussman, Ken Burdett, Yu Chen, and Kami J. Miller
The Dow Chemical Company, Core R&D, 1776 Building, Midland, Michigan 48674, United States
S
* Supporting Information
ABSTRACT: A series of substituted tungsten(VI) halides
with general formula WECl4 (E = O or −NR (imido)) were
screened via a high throughput study to identify potential new
olefin metathesis catalysts. The tungsten species were treated
with a series of aluminum alkyl activators and modifier ligands
to generate active catalyst species in situ. Ring-opening
metathesis polymerization (ROMP) of cyclooctene was used
as a primary screen to identify potential metathesis catalysts and active catalysts were subjected to a secondary screen to evaluate
tolerance toward polar functional groups. Several combinations from the high throughput campaign yielded active metathesis
catalysts for the ROMP of cyclooctene. However, none of the catalysts examined in this study exhibited any evidence of
significant polar functional group tolerance as determined by the results of the secondary cyclooctene/butyl acetate screen.
KEYWORDS: tungsten(VI) halides, library, olefin metathesis, ring-opening
lefin metathesis refers to a general class of chemical
group VI, and “Grubbs-type” catalysts which consist of
Oreactions that permit the redistribution of substituents phosphane-stabilized ruthenium complexes. The archetypal
complexes of each case are shown in Figure 1.6 While a
about a carbon−carbon double bond. The term “olefin
metathesis” was first used by Calderon and co-workers at the
Goodyear Tire and Rubber Company to describe a series of
tungsten-catalyzed reactions which facilitated the conversion 2-
pentene into an equilibrium mixture of 2-butene and 3-hexene.1
The development of the field was rapid thereafter, both with
ring-opening metathesis polymerization (ROMP) being
described by the mid 1960s and Chauvin’s elucidation of the
mechanism of the metathesis reaction occurring in the
following decade.2
Olefin metathesis can be carried out using either
homogeneous or heterogeneous catalysts. The latter has been
employed industrially in a variety of processes. For example, the
triolefin process, developed by Phillips Petroleum in the 1960s,
has been effectively employed to convert mixtures of ethylene
and 2-butene into propylene using a silica supported tungsten
oxide catalyst, while the Shell higher olefins process (SHOP)
utilizes metathesis after olefin oligomerization.3 Supported
rhenium and cobalt-molybdate catalysts have also been
developed.4 This technology has been further developed by
ABB Lummus which now offers a complete process solution,
OCT or olefin conversion technology, as a route to on demand
propylene.3 Similar units have been in operation on the Gulf
Coast at several sites, starting with a facility in Channelview,
Texas in the 1980s.5 While these routes are attractive as on-
demand propylene solutions, ready availability of a C4 stream is
a clear limitation.
Figure 1. Grubbs (left) and Schrock (right) metathesis catalysts.
comprehensive account of the role of this transformation in
synthetic chemistry is beyond the scope of the present work, a
thorough review by Trinka and Grubbs chronicles the pivotal
advances in the initial discovery and development of catalysts
for this reaction while a more recent review highlights that this
area remains a rich and challenging field of study.7
While the Schrock-type catalysts were the first homogeneous
catalysts studied, these catalysts are oxophilic and consequently
somewhat more reactive toward oxygen-containing olefins.
Consequently, they may be less well-suited to some
applications such as the use of seed oils as a substitute to
hydrocarbon derived olefin feedstocks. These materials
generally consist of fatty acid triglycerides, and through genetic
modification, plants may be induced to express oils containing a
The homogeneous catalysts which are more closely related to
the subject of this paper fall broadly into two main categories,
so-called “Schrock carbenes” which tend to consist of high
valent early transition metal alkylidene complexes, primarily of
Received: June 3, 2014
Revised: August 25, 2014
Published: September 3, 2014
© 2014 American Chemical Society
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Figure 2. Examples of the single and dual step method for converting FAMES into feedstocks. In route (a) direct ethenolysis, Grubbs catalyst may
be employed albeit at reduced effectiveness in the presence of ethylene. In (b) indirect ethenolysis, Grubbs catalyst may be used in conjunction with
the olefin shown to produce the oxygenated and nonoxygenated products shown (step 1). The 18 carbon olefin produced in this way can be
subsequently converted into a useful α-olefin with an ethylene tolerant metathesis catalyst (step 2).
Figure 3. Catalysts examined as part of the study described in ref 11. For the catalysts examined, only D exhibited significant activity where M = Mo,
although C was also reported to shown some activity toward the acetyl substituted norbornene.
high degree of unsaturation offering a potential “green” route to
hydrocarbons typically derived from crude oil. Following a
transesterification step to produce fatty acid methyl esters
(FAMES), these materials can then be subjected to either direct
or indirect ethenolysis with a metathesis catalyst.8
those required for industrial relevance, owing to the ruthenium
species’ intolerance of ethylene. While a two step approach
using Grubbs catalyst to carry out the initial cleavage of the
methyl ester, followed by application of a more conventional
(and ethylene insensitive) metathesis catalyst may lead to
improved performance, it does so at the expense of added
process cost and complexity. (Figure 2)
While this approach to produce α-olefins can be realized with
a Grubbs type catalyst, it typically occurs at rates that are below
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Figure 4. Generic scheme for the synthesis of imido complexes of WOCl4 from isocyanates (Top). The three imido complexes prepared for this
study are shown below along with the parent WOCl4 (Center). With the exception of the WNDIPCl4, all materials were converted to their etherates
for improved solubility in toluene. The bromide analogue of WOCl4 (WOBr4) was also screened. The abbreviations shown for each compound are
used throughout this document. These parent tungsten halide species were modified with ligands (A1-C3) and activated using three aluminum alkyl
species (Bottom).
Consequently, research efforts in our laboratories have
focused primarily on catalyst discovery, as opposed to process
modifications. The goals of this approach have been 2-fold: To
identify catalysts that exhibit polar functional group tolerance
and to develop a catalyst that does not include ruthenium as the
active metal. This latter goal was initially driven by historic
highs in ruthenium prices during the past decade.9
To this end, this report describes efforts to develop a
metathesis catalyst utilizing tungsten as the active metal. In
particular, this effort has been stimulated by a series of reports
made by Muetterties and co-workers where it was initially
believed that tungsten hexachloride was a highly active catalyst
for the metathesis of internal olefins when activated with
ethanol and ethylaluminum dichloride.10 In a subsequent series
of communications, it was revealed that WOCl4 is in fact the
active transition metal component in these catalyst systems.11
Furthermore, the authors specify propyl acetate as an additive
for the WOCl4-based catalyst, suggesting the potential for polar
functional group tolerance. In the final work in this series of
papers, a methyl substituted tungsten oxychloride, H3CWOCl3,
functions as a catalyst via the formation of a transient
methylidene.10 Recently, Lehtonen and co-workers reported
a
on the use of aminophenol substituted metal oxyhalides of
group VI (Mo and W) as catalysts for the ring opening
metathesis polymerization (ROMP) of norbornene and
dicyclopentadiene in the presence of an aluminum alkyl12
(Figure 3).
Although the ring strain of these olefins is expected to result
in a facile ROMP process, the behavior of two of the catalysts
examined was particularly intriguing with regard to potential
stability in the presence of polar functional groups. For catalysts
C and D (where M = Mo), activity for ROMP was found when
using 2-norbornen-5-yl acetate.11 This latter result from
Lehtonen was particularly surprising in light of the similarity
that these types of catalysts bear to catalysts developed by
Schrock and co-workers, which are somewhat more reactive
toward olefins with oxygen functionality. However, Schrock and
his colleages do report that these catalysts display similar
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Figure 5. ROMP screening of WOCl4/ligands with TEA activation.
metathesis activity for ROMP of trifluoromethyl substituted
norbornenes using a molybdenum catalyst.13
screened at ligand to metal precursor molar ratios of 1:1 and
2:1, while the difunctional ligands (A3, C3) were screened at a
1:1 ratio. For the 2:1 ratios, ligands were screened either with
two equivalents of the same ligand or single equivalents of two
different ligands. All possible combinations were examined.
Aluminum activators A1−A3 were used at an Al/W ratio of
1.02:1 for all experiments. A tungsten to olefin ratio of 1:200
was used in all experiments. The known metathesis catalysts
(Grubbs-II and Schrock, Figure 1) were included as positive
controls.
In initial experiments to validate the protocol, WOCl4 was
combined with modifier ligands and triethylaluminum (TEA)
as the activator in the presence of cyclooctene. The results of
this screen are shown in Figure 5. The reproducibility of the
data demonstrates good control of the liquid handling
parameters used in the robotic protocol. Additionally, both
Grubbs and Schrock catalysts polymerize cyclooctene com-
pletely, matching the expected theoretical yield (170 mg) of
polyoctene, demonstrating the protocol gives positive results
with known metathesis catalysts.
This experiment also successfully reproduced the initial
screening hit which led to this work. One equivalent of 2,6-di-
iso-propylphenol (A2) with WOCl4 gives nearly the theoretical
amount of polyoctene under these conditions. Another positive
result in this initial screen is the combination of 2,6-di-iso-
propylphenol (A2) with hexafluoro-2-methylpropanol (A1)
and WOCl4.
With the screening protocol established and validated, the
remaining combinations of ligands, tungsten precursors and
activators were screened. A total of 768 unique experiments
(1536 including replicates) were performed in this initial study
to screen for metathesis activity with cyclooctene.
For each of the libraries screened, a positive hit for
metathesis activity was defined as any metal/ligand/activator
combination which showed residual polymer (nonvolatile
product) with a mass greater than the approximate mass of
residual metal precursor, activator and ligand used in that
experiment. For example, in Figure 5, addition of the ligand A2
improved the efficiency of the catalyst system to approximately
that of the positive controls (Grubbs-II/Schrock). Likewise, the
A1,A2 combination also showed activity equal to that of the
controls. Other ligands and ligand combinations showed
intermediate activity. For example, two equivalents of A2, the
A1,B2 combination, and the A2,C2 combination gave residual
polymer equal to or slightly better than the base WOCl4
system. The screening results for all of the tungsten precursors,
In light of these results, this chemistry was explored in
greater detail. A high-throughput study was initiated to
systematically explore the effect that different alkoxide ligands,
aluminum alkyl activators, and imido groups bound to the
metal center had on catalyst activity and potential functional
group tolerance. Tungsten oxychloride was converted to its
etherate salt to improve its solubility in the hydrocarbon
solvent used for the screening reactions. The imido analogues
shown below were prepared via a literature procedure from
their corresponding isocyanates.14 (Figure 4)
For this work, a homogeneous screening protocol for the
metathesis activity of these catalysts was developed in which the
ring opening metathesis polymerization (ROMP) of cyclo-
octene was used, while tolerance for polar functionality was
tested on hits from this primary screen by attempts to carry out
the ROMP of cyclooctene in the presence of butyl acetate.
Other researchers have reported the high throughput synthesis
and screening of heterogeneous olefin metathesis catalysts,15 as
well as strategies for immobilization of ruthenium catalysts on
silica gel or monolithic sol−gel.16 In this work, we give details
for a homogeneous workflow for high throughput screening of
olefin metathesis catalysis, as well as the syntheses of the
precursor tungsten complexes.
RESULTS AND DISCUSSION
■
Researchers have previously used the ROMP reaction of
norbornene as a probe for olefin metathesis activity.12
However, for this study, the ROMP reaction of cyclooctene
was selected as a more realistic probe to eliminate the potential
for false-positive results if a strained bicyclic olefin was used.
Although no polar functionality was initially included to probe
the robustness of the metathesis system, any hits identified in
the nonpolar cyclooctene ROMP screen were subjected to a
secondary screen to identify candidates with polar tolerance.
The initial ROMP screen made no attempt to determine the
maximum turnover numbers for the catalyst. Instead, the
ROMP reaction was simply run for 2 h at 60 °C with a 200:1
monomer to catalyst ratio.
The tungsten precursors used in this study are shown in
Figure 4. The imido compounds WNIPACl4, WNDMPCl4 and
WNDIPCl4 were prepared via literature procedure from
WOCl4 and the corresponding isocyanates.17 The modifier
ligands used in the screening experiments, also shown in Figure
4, included acidic, neutral and phosphane donors. The
monofunctional ligands (A1, A2, A4, B1−B3, C1, C2) were
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activators and ligand combinations are shown in the Supporting
Information.
II and Schrock controls. However, when butyl acetate is added
to the mixture in an equimolar amount relative to cyclooctene,
both the Grubbs and Schrock catalysts retain their activity,
while all metathesis activity is lost for any of the WOCl4/ligand
combinations using either TEA and TIBA. Of the remaining
metal/ligand/activator combinations screened for polar toler-
ance (Table 1), none showed any significant olefin metathesis
activity when butyl acetate was present in the reaction mixture.
Complete results for these screens are shown in the Supporting
Information. It is interesting to note that the activity of the
more oxophilic Schrock catalyst was not significantly
diminished by the addition of butyl acetate. However, this
may be the result of these screening reactions being run at a
200:1 monomer/metal ratio, which is a very low number of
possible catalyst turnover numbers. However, it was sufficient
to rule out any of the other formulations as possible polar
group tolerant catalysts.
Of the ∼768 unique combinations run in the initial nonpolar
ROMP screen with cyclooctene as the monomer, 51 ligand/
tungsten precursor combinations showed some level of ROMP
activity as defined above. In general, WOCl4 and WNDIPCl4
proved to be far superior metal precursors than the others used
in the screen. Of the other metal precursors, WNDMPCl4
showed activity with many ligand/activator combinations, but
all of the active combinations were much lower in activity than
the controls. On the other hand, both WNIPACl4 and WOBr4
did not show any significant activity with any of the ligand/
activator combinations.
With the completion of the nonpolar ROMP screen of
potential tungsten/ligand/activator combinations, we turned
our attention to exploring whether any of the in situ generated
catalysts would demonstrate metathesis activity in the presence
of polar functionality. To examine this problem in a high
throughput fashion, a 1:1 molar ratio mixture of cyclooctene
and butyl acetate were used as the monomer source. We
reasoned that this would serve as a comparable model system
for a monomer which would contain both functionalities as one
would expect in methyl oleate, for example, while retaining the
ease and speed of a gravimetric determination of activity.
To show that any loss of activity was solely the result of the
presence of butyl acetate, the screening plates incorporated
tubes containing cyclooctene alone, as well as the cyclooctene/
butyl acetate mixture. Each plate also included positive controls
(both Grubbs-II and Schrock with cyclooctene and cyclo-
octene/butyl acetate), as well as negative controls (cyclooctene
and cyclooctene/butyl acetate with only the activators).
The metal precursors, ligands, and activator combinations
included in the polar screen are shown in Table 1. As described
above, only combinations which gave reasonable activity in the
nonpolar cyclooctene screen were included in these experi-
ments.
CONCLUSIONS
■
A series of heteroleptic tungsten(VI) halide materials were
screened via high throughput techniques for ring opening
metathesis polymerization (ROMP) activity using the polymer-
ization of cyclooctene as a primary screening probe. From the
768 unique experiments carried out (1536 including replicates),
several active catalysts for the ROMP of cyclooctene were
identified. In particular, tungsten oxychloride diethyl etherate
and 2,6-di-iso-propylphenylimido tungsten tetrachloride ex-
hibited the highest degree of activity.
In general, the use of tri-iso-butylaluminum as an activator
gave superior ROMP activity with all of the metal precursors.
Of the ligands screened, and with WOCl4·OEt2 in particular, a
single equivalent each of 2,6-di-iso-propylphenol (A2) and
hexafluoro-tert-butanol (A1) as modifiers gave superior ROMP
activity with cyclooctene, approaching the activity of Grubbs-II
under the conditions of this screen. Additionally the bifunc-
tional ligand 2,2′-bisphenol (A3), and a 1:1 molar ratio of A1
and A2 also demonstrated highly efficient ROMP activity with
WOCl4 and cyclooctene. It should be noted that the actual
active catalyst species generated from these metal/ligand/
activator combinations are hitherto unidentified.
Universally, addition of butyl acetate to the reaction mixtures
has the effect of eliminating all catalyst activity for the tungsten
metal precursor/ligand/activator combinations studied. Natu-
rally, the Grubbs catalyst employed as a control exhibits activity
in the presence of butyl acetate, consistent with its reported
polar functional group tolerance.
Interestingly, the Schrock catalyst employed as a standard
exhibits some tolerance toward butyl acetate. This is consistent
with the results reported by Lehtonen et al., who noted that the
diacetyl substituted norbornene was most readily polymerized
by structurally similar molybdenum catalysts with tungsten
species being largely inactive.12 Indeed, examination of
molybdenum analogues of the materials described here would
be a logical extension of this work. However, this study would
need to be made with care for two reasons: The anticipated
lesser robustness of the molybdenum analogues of these
tungsten species,17 and a need to ensure that any catalysts
generated are sufficiently differentiated from the well-
established Schrock catalysts. To achieve this end, additional
characterization of the catalyst species generated in situ in this
study would be necessary. Another potential avenue for
differentiating the results described herein could include an
Table 1. Metal Precursor/Ligands/Activator Combinations
for Cyclooctene/Butyl Acetate Polars Screen
metal
precursor
activator
TEA
ligands
WOCl4
none, A1, A2, A1−A2, A1−B1, A1−B2, A1−A4,
A2−C2, A2−A4
WOCl4
TIBA
none, A1, A2, A1−A2, A1−B1, A1−B2, A1−A4,
A2−A4, A2−C2
WOCl4
EADC none, A1, A2, A1−A2, A1−C2, A1−C2, A4, C1,
C2
WNDIPCl4
WNDIPCl4
TEA
none, A1, A1−A2, A1−A4, A1−B2, A1−B3, A2,
A2−B3, A4, A4−B3, B3
TIBA
none, A1, A1−A2, A1−A4, A1−B3, A2, A2−B3,
A4, A4−B3, B3
WNDMPCl4
WNIPACl4
TIBA
TEA
none, A1−A2, A1−C1, A2−B3, B3
none, A1−C1, A2−C1, A4−B3, C1
A ratio of 200:1 monomer/metal precursor (with respect to
cyclooctene) was maintained, and additional toluene was added
to the wells containing only cyclooctene to maintain consistent
concentration of metal precursors and ligands relative to
cyclooctene. Results of the WOCl4 combinations with TEA and
TIBA are shown in Figure 6.
Confirming the results of the earlier screen, ligand
combinations, such as A1 and A1−A2, show olefin metathesis
activity with cyclooctene alone approaching that of the Grubb-
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Figure 6. Results for the cyclooctene and cyclooctene/butyl acetate screens for WOCl4 with TEA and TIBA.
examination of the activity of these catalysts for the ROMP of
cyclooctene at temperatures greater than 60 °C.
into the Tecan drybox, and installed on the J-KEM heated
mixer on the Tecan deck. The tungsten precursor, ligand,
activator, and standards (Grubbs-II, Schrock) solutions were
placed on the reagent station, as was the cyclooctene. For the
polar tolerance screen, a 1:1 (mol/mol) mixture of cyclo-
octene/butyl acetate was used. Using the liquid handler, a
makeup volume of toluene (so that all of the resulting
experimental mixtures were of equal concentration relative to
catalyst and monomer) was added to each vial, followed by the
experimental ligand solution and tungsten precursor. The
orbital mixer was started and the reagents were allowed to form
a complex for 60 min at ambient temperature. The activator
solutions (TEA, TIBA, or EADC) were added to the wells,
followed by the standards. Cyclooctene (or the cyclooctene/
butyl acetate mixture) was added to the wells, and the orbital
mixer was started and the temperature was increased to 60 °C.
The reaction was allowed to proceed for 120 min at 60 °C, and
then all of the wells were quenched by adding 100 μL of a 1.0
M solution of BHT and benzoic acid in toluene. The MTP
containing the experimental mixtures were then removed from
the drybox and evaporated to dryness on a ThermalSavant
Centrifugal Evaporator (80 °C, 0.1 Torr, 8 h). The vials were
then reweighed on the robotic deck in order to determine the
yield of the resulting polymer.
To summarize, several new metathesis catalysts have been
prepared in situ by the addition of a tungsten halide precursor,
an aluminum alkyl activator, and a modifier ligand drawn from
the lists shown in Table 1. These catalysts displayed activity for
the ring opening metathesis polymerization of cyclooctene,
demonstrating that their activity is independent of the ring
strain imparted by a more reactive olefin such as norbornene.
However, none of the catalysts screened in this study exhibited
tolerance toward the presence of polar functional groups,
suggesting that these materials are unlikely to provide the
requisite performance for conversion of FAMES to feedstocks
or other applications where polar functional group tolerance is
critical.
EXPERIMENTAL PROCEDURES
■
General Experimental Procedures. All experiments were
carried out in an inert atmosphere in a nitrogen-filled drybox.
The toluene and cyclooctene were passed through A-2 Alumina
and Q-5 (Engelhard) for moisture and oxygen removal prior to
use. Butyl acetate was distilled, under nitrogen, from calcium
hydride, and then passed through activated alumina. Stock
solutions of all of the materials (tungsten precursors, ligands,
activators) were prepared by weighing the appropriate amount
of material into a 20 mL scintillation vial, then diluting with
toluene to give a 0.075 M solution. Grubbs-II and Schrock’s
catalyst were similarly prepared as 0.05 M solutions in toluene.
All screening experiments were performed on a Cavro Tecan
RSP-9692 Liquid Handler robotic deck installed in a nitrogen
filled drybox and equipped with appropriate reagent stations
and a J-KEM heated orbital mixer.
ASSOCIATED CONTENT
* Supporting Information
■
S
Procedures for the synthesis of starting tungsten complexes,
detailed experimental procedures for high throughput screen-
ing, and complete screening results are available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: Drromer2@dow.com.
■
Representative Experimental Procedure for Olefin
Metathesis Screening. Glass 1200 μL shell vials contained in
a 96-well aluminum microtiter plate (MTP) were weighed by a
Tecan automated weigh station. The MTP plates containing
the vials were then dried overnight at 140 °C, then transferred
Notes
The authors declare no competing financial interest.
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