Ethenolysis reactions catalyzed by imido alkylidene monoaryloxide monopyrrolide (MAP) complexes of molybdenum

Article abstract of DOI:10.1021/ja904786y

(Chemical Equation Presented) Monoaryloxide-pyrrolide (MAP) olefin metathesis catalysts of molybdenum that contain a chiral bitetralin-based aryloxide ligand are efficient for ethenolysis of methyl oleate, cyclooctene, and cyclopentene. Ethenolysis of 5000 equiv of methyl oleate produced 1-decene (1D) and methyl-9-decenoate (M9D) with a selectivity of >99%, yields up to 95%, and a TON (turnover number) of 4750 in 15 h. Tungstacyclobutane catalysts gave yields approximately half those of molybdenum catalysts, either at room temperature or at 50 °C, although selectivity was still >99%. Ethenolysis of 30000 equiv of cyclooctene to 1,9-decadiene could be carried out with a TON of 22500 at 20 atm (75% yield), while ethenolysis of 10000 equiv of cyclopentene to 1,6-heptadiene could be carried out with a TON of 5800 at 20 atm (58% yield). There is no reason to propose that the efficiency of ethenolysis has been maximized with the most successful catalyst reported here.

Full text of DOI:10.1021/ja904786y

Published on Web 07/20/2009
Ethenolysis Reactions Catalyzed by Imido Alkylidene Monoaryloxide
Monopyrrolide (MAP) Complexes of Molybdenum
Smaranda C. Marinescu,† Richard R. Schrock,*,† Peter Mu¨ller,† and Amir H. Hoveyda‡
Department of Chemistry 6-331, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467
Received June 11, 2009; E-mail: rrs@mit.edu
We have reported recently that monoaryloxide-pyrrolide (MAP)
olefin metathesis catalysts 1 and 2, which can be prepared through
addition of a phenol to a bispyrrolide,1 can be especially efficient
for enantioselective and/or Z-selective reactions.2-4 In the process
ized.9 Metallacyclobutanes that have a TBP structure are proposed to
lose an olefin more readily than an SP species.8 When ethylene is
removed above a solution of Mo(NAr)(CH2CH2CH2)(Me2Pyr)(OBitet),
mixtures of the two diastereomers of Mo(NAr)(CH2)(Me2Pyr)(OBitet)
(δHR ) 12.35 and 12.13 ppm, 12.94 and 12.24 ppm) are observed.6 The
two diastereomers of Mo(NAr)(CH2)(Me2Pyr)(OBitet) decompose to a
significant degree in the absence of ethylene over a period of 1-2 days.
of studying related tungsten MAP complexes5 we noticed that some
methylidene species could be unusually stable, yet highly reactive.
For example, a 0.04 M solution of W(NAr)(CH2)(O-2,3,5,6-
Ph4C6H)(Me2Pyr) (Me2Pyr ) 2,5-dimethylpyrrolide) in toluene-d8
could be heated to 80 °C without significant decomposition of the
tungsten species after ∼1 h. The stability of methylidene species
is likely to be an important feature of MAP species that are
especially efficient in a reaction in which ethylene is present. Long-
lived, reactive methylidene species and the lability of unsubstituted
metallacyclobutane intermediates5,6 suggest that efficient ethenolysis
of internal linear (eq 1) or cyclic olefins may be possible. Efficient
ethenolysis of a natural product such as methyl oleate is attractive
as a method for obtaining useful chemicals from biomass.7 We show
here that ethenolysis reactions that employ molybdenum-based
MAP species can be highly efficient at room temperature and
readily accessible pressures of ethylene.
Figure 1. POV-ray drawing of Mo(NAr)(C3H6)(Me2Pyr)(OBitet). Thermal
ellipsoids are displayed at 50% probability level. Hydrogen atoms are
omitted. Selected distances (Å) and angles (deg): Mo-C(1) ) 2.056(7);
Mo-C(3) ) 2.052(5); Mo-C(2) ) 2.332(6); C(1)-C(2) ) 1.609(11);
C(2)-C(3) ) 1.568(10); Mo-C(1)-C(2) ) 78.0(4); Mo-C(3)-C(2) )
79.0(4); C(1)-C(2)-C(3) ) 119.2(5); C(1)-Mo-C(3) ) 83.7(3).
Ethenolysis (employing 99.5% pure ethylene) of methyl oleate
(Table 1) initiated by 1a at room temperature yields essentially
only 1-decene (1D) and methyl-9-decenoate (M9D) with a selectiv-
ity of >99% and yields up to 95% (entries 1-4). (The other possible
products are 1,18-dimethyl-9-octadecenedioate and 9-octadecene.)
The highest turnovers are found at the higher pressures (compare
entry 3 vs 4). All results are consistent with time dependent catalyst
decomposition and a (low) solubility of ethylene in methyl oleate
that limits conversion at low pressures. The catalysts shown in
entries 5-7 produce products with lower selectivities and yields.
An OBitet catalyst that contains the adamantylimido ligand (1b,
entry 8) is almost as successful as 1a, but two catalysts closely
related to 1a and 1b (entries 9 and 10) gave no product; the reasons
are not yet known. A run employing 10 000 equiv of methyl oleate
at 20 atm of ethylene led to only ∼10% conversion.
RCHdCHR′ + CH2dCH2 f RCHdCH2 + R′CHdCH2
(1)
Exposure of 1a to 1 atm of ethylene has been shown to lead to
mixtures that contain the two diastereomers of 1a, the two diastere-
omers of Mo(NAr)(CH2)(Me2Pyr)(OBitet), the unsubstituted molyb-
dacyclobutane complex, Mo(NAr)(CH2CH2CH2)(Me2Pyr)(OBitet), and
CH2dCHCMe2Ph, along with ethylene (OBitet is the aryloxide in 1).6
A reaction between Mo(NAr)(CHCMe3)(Me2Pyr)(OBitet) and ethylene
allowed the molybdacyclobutane complex, Mo(NAr)(CH2CH2CH2)-
(Me2Pyr)(OBitet), to be isolated at -30 °C in the presence of ethylene
(1 atm). An X-ray structural study reveals it to have the TBP structure
shown in Figure 1, one that is virtually identical to the structure found
for W(NAr)(CH2CH2CH2)(Me2Pyr)(OBitet).5 Molybdacyclobutane
species are especially rare because they lose an olefin readily.8 To the
best of our knowledge only one other molybdacyclobutane, a square
pyramidal bis-tert-butoxide species, has been structurally character-
Tungstacyclobutane catalysts5 (entries 11 and 12) produced
results that were inferior to molybdenum catalysts in yield, either
at room temperature or at 50 °C, although selectivity was still >99%.
Since it is likely that the rate-limiting step in ethenolysis is loss of
ethylene from an unsubstituted metallacyclobutane, one possible
† Massachusetts Institute of Technology.
‡ Boston College.
9
10840 J. AM. CHEM. SOC. 2009, 131, 10840–10841
10.1021/ja904786y CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Ethenolysis of Methyl Oleate (MO)
entry
catalyst
equiv
P (atm)
time (h)
% conva
% selectb
% yieldc
TONd
1
2
3
4
5
6
7
8
Mo(NAr)(CHCMe2Ph)(Me2pyr)(OBitet) (1a)
1a
500
1000
5000
5000
500
500
500
500
50
4
4
4
10
4
4
4
4
4
4
1
20
15 or 48
15
2
20
1
18
1
1
17
18
94
80
58
95
83
87
86
96
0
>99
>99
>99
>99
76
86
92
98
0
0
94
80
58
95
63
75
79
94
0
470
800
2900
4750
315
325
395
470
0
1a
1a
Mo(NAr)(CHCMe2Ph)[OCMe(CF3)2]2
Mo(NAr)(CHCMe2Ph)(Me2pyr)(TPP)
Mo(NAr)(CHCMe2Ph)(Me2pyr)(OSiPh3)
Mo(NAd)(CHCMe2Ph)(Me2pyr)(OBitet) (1b)
Mo(NAd)(CHCMe2Ph)(Pyr)(HIPTO) (2a)
Mo(NAd)(CHCMe2Ph)(Me2pyr)(TPP)
W(NAr)(C3H6)(Me2pyr)(OBitet)
W(NAr)(C3H6)(Me2pyr)(OBitet) (50 °C)
9
10
11
12
50
500
500
0
48
62
0
48
62
0
240
310
4
4
>99
>99
a Conversion ) 100 - [(final moles of MO) × 100/(initial moles of MO)]. b Selectivity ) (1D + M9D) × 100/(total products). c Yield ) (1D or
M9D) × 100/(initial moles of MO). d TON ) % yield[(moles of MO)/(moles of catalyst)].
reason why tungsten is slower than molybdenum is that tungsta-
cyclobutanes release ethylene more slowly than molybdacyclobu-
tanes.5 Another possibility is that the ester carbonyl binds to
tungsten more strongly than it does to molybdenum and inhibits
turnover to a more significant degree.
Ethenolysis of 30 000 equiv of cyclooctene to give 1,9-decadiene
with 1a as the catalyst proceeded with a TON of 22 500 (75% yield)
at 20 atm (Table 2). Initiation of polymerization of cyclooctene
with 1a is slow, so little 1a is consumed before it reacts with
ethylene to yield Mo(NAr)(CH2)(Me2Pyr)(OBitet), and ethenolysis
then proceeds rapidly. At 1 atm of ethylene in an NMR scale
reaction, poly(cyclooctene) can be observed, but the amount of
polymer decreases substantially upon addition of more ethylene.
Essentially the same result as shown in entry 5 of Table 2 was
observed when commercial 99.995% ethylene was employed.
Therefore any potentially harmful impurities in the 99.5% ethylene
(e.g., water or oxygen) do not limit the TON.
successful. The number of possible catalyst variations is large, so
we expect that fine-tuning ultimately should lead to even more
efficient catalysts for ethenolysis.
Ethenolysis of methyl oleate by ruthenium catalysts has been
studied extensively.7 Conversion to products at 10 atm of ethylene
usually is incomplete and/or unselective to 1D and M9D, and no
efficient reactions have been reported at room temperature. In the
most successful case reported here, routinely purified methyl oleate
can be converted virtually completely to 1D and M9D at room
temperature and 10 atm of ethylene.
Acknowledgment. This research was funded by the National
Science Foundation (CHE-0554734 to R.R.S.). We thank Materia,
Inc. for a gift of purified methyl oleate and Margaret Flook and
Annie Jiang for gifts of catalyst samples.
Supporting Information Available: Experimental details for the
synthesis of all compounds and the X-ray structural study. Supporting
Information is available free of charge via the Internet at http://pubs.acs.org.
Table 2. Ethenolysis of Cyclooctene with 1a
References
entry
equiv
P (atm)
T (h)
% conv
% yld
TON
1
2
3
4
5
5000
10 000
10 000
20 000
30 000
10
10
20
20
20
16
20
20
16
20
98
98
93
88
75
90
80
93
88
75
4500
8000
9300
17 600
22 500
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Ethenolysis of 5000 equiv of cyclopentene at 20 atm of 99.5%
ethylene led to an 84% conversion to 1,6-heptadiene in 79% yield in
15 h (TON 3950). In a run employing 10 000 equiv of cyclopentene
and 99.995% ethylene, the yield is 58% and TON 5800 in 20 h.
It currently is not known why Mo(NAr)(CH2)(Me2Pyr)(OBitet)
methylene species are relatively long-lived. Although the main mode
of methylidene decomposition is bimolecular coupling to give
ethylene, some evidence in the literature suggests that ethylene
promotes rearrangement of a metallacyclobutane to an olefin in
certain circumstances.10 If the latter mode of decomposition is
operating in the ethenolysis reactions described here, further
engineering of the catalyst may sterically prevent formation of a six-
coordinate ethylene adduct that leads to metallacycle rearrangement.
The results in Table 1 suggest that a key to high selectivity is
relatively slow productive metathesis to yield 1,18-dimethyl-9-
octadecenedioate and 9-octadecene (for steric reasons), i.e., slow
reaction between a substituted alkylidene and 1D, M9D, or methyl
oleate (the slowest) under the conditions employed. We have no
reason to expect that the turnover numbers have been maximized
with 1a. The chirality of the OBitet ligand seems unlikely to have
much, if anything, to do with its efficiency as an ethenolysis catalyst,
since catalysts such as those shown in entries 6 and 7 are relatively
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