Z-selective olefin metathesis reactions promoted by tungsten Oxo alkylidene complexes

Article abstract of DOI:10.1021/ja210349m

Addition of LiOHMT (OHMT = O-2,6-dimesitylphenoxide) to W(O)(CH-t-Bu)(PMe₂Ph)₂Cl₂ led to WO(CH-t-Bu)Cl(OHMT)(PMe₂Ph) (4). Subsequent addition of Li(2,5-Me₂C₄H₂N) to 4 yielded yellow W(O)(CH-t-Bu)(OHMT)(Me₂Pyr)(PMe₂Ph) (5). Compound 5 is a highly effective catalyst for the Z-selective coupling of selected terminal olefins (at 0.2% loading) to give product in >75% yield with >99% Z configuration. Addition of 2 equiv of B(C₆F₅)₃ to 5 afforded a catalyst activated at the oxo ligand by B(C₆F ₅)₃. 5·B(C₆F₅)₃ is a highly active catalyst that produces thermodynamic products (～20% Z).

Full text of DOI:10.1021/ja210349m

Communication
pubs.acs.org/JACS
Z-Selective Olefin Metathesis Reactions Promoted by Tungsten Oxo
Alkylidene Complexes
Dmitry V. Peryshkov, Richard R. Schrock,* Michael K. Takase, Peter Mu
†
,†
†
†
‡
̈
ller, and Amir H. Hoveyda
†
Department of Chemistry 6-331, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, United States
‡
*
S Supporting Information
interest in oxo alkylidene complexes in the last 25 years has
been sparse.
The most recent development in Mo and W imido alkylidene
5
ABSTRACT: Addition of LiOHMT (OHMT = O-2,6-
dimesitylphenoxide) to W(O)(CH-t-Bu)(PMe Ph) Cl
2
2
2
led to WO(CH-t-Bu)Cl(OHMT)(PMe Ph) (4). Subse-
2
chemistry has been monoaryloxide monopyrrolide (MAP) com-
6
quent addition of Li(2,5-Me C H N) to 4 yielded yellow
2
4
2
plexes. One of the most interesting discoveries is the ability of
W(O)(CH-t-Bu)(OHMT)(Me Pyr)(PMe Ph) (5). Com-
2
2
some MAP catalysts to promote Z-selective metathesis reactions
as a consequence of the presence of a relatively “large” aryloxide
pound 5 is a highly effective catalyst for the Z-selective
coupling of selected terminal olefins (at 0.2% loading) to
give product in >75% yield with >99% Z configuration.
Addition of 2 equiv of B(C F ) to 5 afforded a catalyst
7
and “small” imido group. The preferred metal for Z-selective
couplings of terminal olefins at this time appears to be tungsten,
and the most successful aryloxide ligand has been O-2,6-(2,4,6-
triisopropylphenyl) C H or OHIPT. (The more active
6
5 3
activated at the oxo ligand by B(C F ) . 5·B(C F ) is a
6
5 3
6 5 3
2
6
3
highly active catalyst that produces thermodynamic
8
molybdenum complexes appear to isomerize the Z product
to E.) It has been proposed that the high steric demands of the
OHIPT ligand force all of the metallacyclobutane substituents
to one side of the metallacycle ring and therefore allow only
Z products to form. Since an oxo ligand is smaller than any NR
ligand, the question arose as to whether MAP versions of
tungsten oxo alkylidene complexes would be useful Z-selective
catalysts.
products (∼20% Z).
arly in the development of olefin metathesis catalysts that
E
contain tungsten, it was shown that metathetically more
active and reproducible systems were produced when tungsten
oxo complexes were deliberately employed or were present as
1
impurities in WCl6. The possibility that oxo alkylidene
We chose to attempt to prepare W(O)(CH-t-Bu)(OHIPT)-
complexes, e.g., W(O)(CHR)X (X = chloride, alkoxide, etc.),
2
(
Me Pyr) (Me Pyr = 2,5-dimethylpyrrolide) from W(O)(CH-
2 2
are the true catalysts in at least some of the “classical” olefin
metathesis systems became more likely when 1 (L = PMe3
t-Bu)(PMe Ph) Cl (1a), hoping that both PMe Ph ligands
2
2
2
2
would dissociate from the metal in the crowded coordination
sphere. The reaction between WO(CH-t-Bu)Cl (PMe Ph)
2
and other phosphines) was prepared and isolated in good yield
2
2
2
(
eq 1). Compound 1 was the first high-oxidation-state tungsten
and LiOHIPT in benzene at 22 °C for 14 h led to isolation
alkylidene complex that would both (i) metathesize terminal
of off-white WO(CH-t-Bu)Cl(OHIPT)(PMe Ph) (2) in 60%
2
and internal olefins (in the presence of a trace of AlCl ) and (ii)
3
yield (eq 2). Two isomers of 2 were present in a 3:2 ratio
produce a new alkylidene that could be observed as a
consequence of olefin metathesis.
1
13
31
according to H, C, and P NMR spectra. Both were
determined to be syn-alkylidenes on the basis of JCαH values for
the alkylidene of 123 Hz (major isomer) and 117 Hz (minor
isomer). The phosphine remains bound to tungsten on the
NMR time scale (JPW = 420 and 379 Hz, respectively) at 22 °C.
An X-ray crystal structure [see the Supporting Information
(
SI)] revealed a distorted square-pyramidal geometry with the
neopentylidene ligand in the apical position and the phosphine
ligand trans to chloride. The alkylidene was found to be
disordered over the syn and anti orientations in a 91:9 ratio.
By the time 1 was discovered, tantalum alkylidene complexes
had been turned into functional olefin metathesis catalysts
3
through use of alkoxides as ligands. Therefore, although some
attempts were made to prepare a W(O)(CH-t-Bu)(OR)2
species from 1, no bisalkoxide species could be isolated. In
view of the synthetic problems encountered upon attempted
alkylation of oxo complexes, including removal of the oxo
1
i
ligand entirely, and to protect alkylidenes against bimolecular
decomposition, attention turned to the synthesis of imido
alkylidene complexes of W and Mo, especially those containing
Received: November 3, 2011
Published: November 22, 2011
4
a phenylimido ligand such as N(2,6-i-Pr C H ). Consequently,
2
6
3
©
2011 American Chemical Society
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dx.doi.org/10.1021/ja210349m | J. Am. Chem.Soc. 2011, 133, 20754−20757

Journal of the American Chemical Society
Communication
The other isomer of 2 could be (for example) one in which the
OHIPT and Cl ligands (eq 2) have switched positions.
Treatment of 2 with Li(Me Pyr) in benzene at 60 °C for
2
1
6 h led to formation of yellow W(O)(CH-t-Bu)(OHIPT)-
(
Me Pyr) (3) in 80% isolated yield. An X-ray structure of
2
3
showed it to have a pseudotetrahedral geometry, a syn-alky-
1
lidene, and an η -Me Pyr ligand (Figure 1). We had considered
2
Figure 2. Thermal ellipsoid plot (50% probability) of syn-W(O)(CH-
t-Bu)(OHMT)(Me Pyr)(PMe Ph) (5). H atoms have been omitted
2
2
for clarity. Solvent molecules are not shown. Selected bond distances
(
Å) and angles (deg): W1−C1 = 1.900(3), W1−O2 = 1.717(2), W1−
O1 = 1.964(2), W1−N1 = 2.074(2), W1−P1 = 2.580(1), W1−O1−
C21 = 159.8(2), W1−C1−C2 = 141.0(2).
Figure 1. Thermal ellipsoid plot (50% probability) of syn-W(O)(CH-
t-Bu)(OHIPT)(Me Pyr) (3). H atoms have been omitted for clarity.
2
Selected bond distances (Å) and angles (deg): W1−C1 = 1.886(3),
W1−O2 = 1.695(3), W1−O1= 1.868(2), W1−N1 = 2.001(2), W1−
O1−C21 = 166.9(2), W1−C1−C2 = 136.7(3).
temperature. This value corresponds to 57% dissociation of
phosphine in a 20 mM solution of 5 in C D .
6
6
Both 3 and 5 react with ethylene to give an unsubstituted
metallacyclobutane complex (and t-butylethylene) that has a
square pyramidal structure (presumably with the oxo ligand in
the apical position) on the basis of chemical shifts of metallacycle
the possibility that PMe Ph would be lost from the
2
coordination sphere because of binding of the pyrrolide ligand
5
in an η fashion, thereby producing an electron count of 18 at
9
protons in the range 0.7−4.5 ppm. The reaction of 3 with
the metal. However, other steric factors alone appeared to be
sufficient to cause 14-electron 3 to be formed.
The analogous reaction between 1 and LiOHMT (OHMT =
O-2,6-dimesitylphenoxide) in benzene at 22 °C for 3 h led to
ethylene is relatively slow and what we propose is an inter-
mediate square pyramidal β-t-butylmetallacyclobutane complex
can be observed before free t-butylethylene is formed. In the case
of compound 5, a methylidene complex is formed in addition to
the unsubstituted square pyramidal metallacycle. In both systems
the unsubstituted metallacycles slowly decompose over a period
of 24 h to unidentified products. A square pyramidal metalla-
cyclobutane made from imido alkylidenes has been proposed to
be further from the transition state for olefin loss than is the
the isolation of yellow WO(CH-t-Bu)Cl(OHMT)(PMe Ph)
2
1
(
4) in 70% yield (eq 3). As with 2, the H NMR spectrum of
the product contained two alkylidene doublet resonances
corresponding to two isomers of 4 in a 87:13 ratio. The values
1
of J (122 and 116 Hz) suggested that both isomers are syn-
CH
10a
alkylidenes. Addition of Li(Me Pyr) to 4 in toluene at −30 °C
2
alternative TBP metallacycle. Extensive calculations have been
performed on several high oxidation state metathesis systems
followed by stirring the mixture at 22 °C for 10 h afforded
yellow W(O)(CH-t-Bu)(OHMT)(Me Pyr)(PMe Ph) (5) in
10b,c
that include metallacyclobutane complexes.
2
2
7
0% isolated yield. An X-ray structure of 5 (Figure 2) showed it
Both 3 and 5 serve as initiators for the polymerization of 5,6-
dicarbomethoxynorbornadiene (DCMNBD). The polymeriza-
tion of 50 equiv of DCMNBD was relatively slow (hours) with 3,
and propagation was faster than initiation. The resulting polymer
was >99% cis, 90% syndiotactic. The polymerization of 50 equiv
of DCMNBD with 5 was relatively fast (minutes), and all of the
initiator was consumed. The resulting polymer was >99% cis,
to be a square pyramid with the syn-neopentylidene in the
apical position and the phosphine bound trans to the pyrrolide.
7
b
98% syndiotactic. These results suggest that the steric crowding
is significantly greater in 3 than in 5 after phosphine is lost and is
in fact too great to allow the formation of highly regular
cis,syndiotactic-poly(DCMNBD) from 3.
Homocoupling of neat terminal olefins with 3 took place
slowly (hours) at room temperature. In contrast, 5 was found
to be highly active and highly Z-selective (Table 1). A catalyst
loading as low as 0.2 mol % yielded up to 86% conversion in
The PMe Ph ligand in 5 is partially dissociated at room
2
temperature and rapidly exchanging on and off the metal. The
alkylidene resonance is broad, and its chemical shift is
concentration-dependent (8.57−9.14 ppm for 4−48 mM
1
31
solutions in C D ). Variable-temperature H and P NMR
6
6
6
h for several of the six chosen substrates. No trans product
studies of a 20 mM solution of 5 in CD Cl showed that the
2
2
1
could be observed in the H NMR spectra of the Z products
phosphine is “bound” below −30 °C, as indicated by a sharp
P signal corresponding to the coordinated ligand (1.80 ppm,
1JPW = 289 Hz). On the basis of the chemical shift for free and
coordinated phosphine, the value of the equilibrium constant
for phosphine dissociation was estimated as 0.015 M at room
(
see the SI).
31
Only a small increase in conversion was found for reaction
times >6 h, which suggests that the majority of the catalyst has
decomposed at this stage. Decomposition of a catalyst prior to
isomerization of the Z product to E can be a desirable feature of
2
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Journal of the American Chemical Society
Communication
Table 1. Conversions (%) of Neat Terminal Olefins to
Homocoupled >99% Z Metathesis Products Promoted by 5
a
substrate /catalyst loading
time
S1/0.2 % S2/0.2 % S3/0.2 % S4/0.2 % S5/0.2 % S6/1 %
1
3
1
6
2
0 min
0 min
h
28
39
47
66
72
44
67
79
86
88
65
75
75
−
−
−
2
28
39
−
−
10
47
b
h
−
11
73
−
59
4 h
−
−
a
S1 = 1-octene, S2 = allylbenzene, S3 = allylboronic acid pinacolate
ester, S4 = allylSiMe , S5 = 1-decene, S6 = methyl-10-undecenoate.
3
b
The aliquot was taken after 7 h.
the coupling reaction. The reactions were run on a 200 mg
scale in a closed vessel with a volume of ∼20 mL. Homo-
coupling of 1-decene at 0.5 Torr did not show a significant
increase in turnover in comparison with the reaction carried out
under 1 atm nitrogen. We ascribe the relatively low turnover in
the case of allyl-TMS (S4) to steric issues and that in the case
of methyl-10-undecenoate (S6, at 1% catalyst loading) to ester
binding to W. These results should be compared with those
Figure 3. Thermal ellipsoid plot (50% probability) of W(O)(B-
(
C F ) )(CH-t-Bu)(OHMT)(Me Pyr) [5·B(C F ) ]. H atoms have
6 5 3 2 6 5 3
been omitted for clarity. Selected bond distances (Å) and angles (deg):
W1−C1 = 1.868(2), W1−O2 = 1.759(2), W1−O1 = 1.860(2), W1−
N1 = 1.968(2), B1−O2 = 1.571(3), W1−O1−C21 = 150.9(1), W1−
C1−C2 = 155.4(2).
7f
obtained employing a W(N-3,5-Me C H ) catalyst system.
2
6
3
olefins, and decomposition of the active catalyst under the
conditions employed.
A long-standing question in classical olefin metathesis
catalyst systems based on tungsten has been the role of a
Lewis acid. One might expect that a Lewis acid in 5 could bind
to the oxo ligand and thereby create a more electrophilic metal
center and more reactive catalysts. Indeed, we found that
addition of Lewis acids to 5 significantly speeds up metathesis
reactions. For example, addition of 2 equiv of B(C F ) to 5
ASSOCIATED CONTENT
Supporting Information
■
*
S
Experimental details for all compounds and crystal parameters,
data acquisition parameters, and CIF files for complexes 2, 3, 5,
and 5·B(C F ) . This material is available free of charge via the
6
5 3
6 5 3
resulted in a catalyst that converted 90% of 1-octene to
-tetradecene in 1 h at 22 °C (0.2 mol % loading). However,
the 7-tetradecene was only 20% Z. Since pure (Z)-7-
tetradecene (in C D ) is isomerized to a 78:22 mixture of
Internet at http://pubs.acs.org.
7
AUTHOR INFORMATION
Corresponding Author
rrs@mit.edu
■
6
6
(E)- and (Z)-tetradecene by 1 mol % 5 in the presence of 2
equiv of B(C F ) in 15 min, any (Z)-7-tetradecene that is
6
5 3
formed initially in the homocoupling reaction should be
isomerized rapidly to a 4:1 E:Z mixture.
ACKNOWLEDGMENTS
■
We are grateful to the National Science Foundation (CHE-
0841187 and CHE-1111133 to R.R.S.) and the National
Institutes of Health (Grant GM-59426 to R.R.S. and A.H.H.)
for financial support. We thank the National Science
Foundation for departmental X-ray diffraction instrumentation
(CHE-0946721).
Addition of 2 equiv of B(C F ) to 5 led to the formation of
6
5 3
(
Me PhP)[B(C F ) ] and 5·B(C F ) . The B(C F ) in
2 6 5 3 6 5 3 6 5 3
5
·B(C F ) is labile at room temperature, as demonstrated by
6 5 3
1
a broadened alkylidene signal in the H NMR spectrum at
1
7
5
7
.30 ppm. The H NMR spectrum of a 45 mM solution of
·B(C F ) at −60 °C shows a sharp alkylidene resonance at
.06 ppm. An X-ray structure of 5·B(C F ) showed that
6
5 3
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■
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Communication
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dx.doi.org/10.1021/ja210349m | J. Am. Chem.Soc. 2011, 133, 20754−20757