Monoaryloxide pyrrolide (MAP) imido alkylidene complexes of molybdenum and tungsten that contain 2,6-bis(2,5-R2-pyrrolyl) phenoxide (R = i-Pr, Ph) ligands and an unsubstituted metallacyclobutane on its way to losing ethylene

Article abstract of DOI:10.1021/om400114h

We report the synthesis of Mo and W MAP complexes that contain O-2,6-(2,5-R₂-pyrrolyl)₂C₆H₃ (2,6-dipyrrolylphenoxide or ODPP^R) ligands in which R = i-Pr, Ph. W(NAr)(CH-t-Bu)(Pyr)(ODPP^Ph) (4a; Ar = 2,6-disopropylphenyl, Pyr = pyrrolide) reacts readily with ethylene to yield a metallacyclobutane complex, W(NAr)(C₃H₆)(Pyr)(ODPP^Ph) (5). The structure of 5 in the solid state shows that it is approximately a square pyramid with the WC₄ ring spanning apical and basal positions. This SP′ structure, which has never been observed as an actual intermediate, must now be regarded as an integral feature of the metathesis reaction.

Full text of DOI:10.1021/om400114h

Communication
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Monoaryloxide Pyrrolide (MAP) Imido Alkylidene Complexes of
Molybdenum and Tungsten That Contain 2,6-
Bis(2,5‑R2‑pyrrolyl)phenoxide (R = i‑Pr, Ph) Ligands and an
Unsubstituted Metallacyclobutane on Its Way to Losing Ethylene
Michael R. Reithofer, Graham E. Dobereiner, Richard R. Schrock,* and Peter Muller
̈
Department of Chemistry, Massachusetts Institute of Technology 6-331, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139,
United States
S
* Supporting Information
ABSTRACT: We report the synthesis of Mo and W MAP complexes
that contain O-2,6-(2,5-R2-pyrrolyl)2C6H3 (2,6-dipyrrolylphenoxide or
ODPPR) ligands in which R = i-Pr, Ph. W(NAr)(CH-t-Bu)(Pyr)-
(ODPPPh) (4a; Ar = 2,6-disopropylphenyl, Pyr = pyrrolide) reacts
readily with ethylene to yield a metallacyclobutane complex,
W(NAr)(C3H6)(Pyr)(ODPPPh) (5). The structure of 5 in the solid
state shows that it is approximately a square pyramid with the WC4
ring spanning apical and basal positions. This SP′ structure, which has
never been observed as an actual intermediate, must now be regarded
as an integral feature of the metathesis reaction.
n the last several years sterically demanding phenoxide
ligands have been employed to make Mo- and W-based
I
MAP (monoaryloxide pyrrolide) catalysts for stereoselective
olefin metathesis reactions. One of the first was OBr2Bitet, an
enantiomerically pure monophenoxide ligand that yielded
diastereomeric mixtures of MAP catalysts (R′ = H, Me) for
enantioselective ring-opening/cross-metathesis reactions.1 In
the process, it was found that the reaction was not only
enantioselective but also Z-selective. The search for other
suitable sterically demanding phenoxides led to terphenoxides
such as O-2,6-(2,4,6-i-Pr3C6H2)2C6H3 (OHIPT)2 and O-2,6-
(mesityl)2C6H3 (OHMT),3 which were employed to produce
Z-selective catalysts for ROMP4 and homometathesis of
terminal olefins.5 Decafluoroterphenoxide (O-2,6-
(C6F5)2C6H3 = ODFT) has now been added to the list of
2,6-terphenoxides.6 Recently it also has been possible to make
bisaryloxide complexes that are especially efficient in certain
stereoselective reactions, one example being Mo(NC6F5)-
(CHCMe2Ph)(OF2Bitet)2, where OF2Bitet is a fluorinated
relative of OBr2Bitet.7 For all of the above reasons we felt it
desirable to prepare and use other sterically demanding
aryloxides in monoaryloxide or bisaryloxide olefin metathesis
catalysts. Here we describe the synthesis of complexes that
contain O-2,6-(2,5-R2-pyrrolyl)2C6H3 (2,6-dipyrrolylphenoxide
or ODPPR) ligands in which R = i-Pr, Ph.
(DPPiPrOH) or isopropyl alcohol. (See the Supporting
Information for full details.)
Addition of 1 equiv of DPPPhOH or DPPiPrOH to
Mo(NAd)(CHCMe2Ph)(Pyr)2, Mo(NAd)(CHCMe2Ph)-
(Me2Pyr)2, Mo(NAr)(CHCMe2Ph)(Pyr)2, and Mo(NAr)-
(CHCMe2Ph)(Me2Pyr)2 (Ad = 1-adamantyl, Ar = 2,6-i-
2-Methoxy-1,3-diaminobenzene was prepared from 2-bromo-
1,3-dinitrobenzene, as shown in eq 1. The pyrrolyl groups were
then constructed by employing the desired γ-diketone in a
Paal−Knorr condensation followed by deprotection with BBr3.
Both DPPPhOH and DPPiPrOH were purified by employing
column chromatography and recrystallized from hexane
Received: February 8, 2013
Published: May 2, 2013
© 2013 American Chemical Society
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Pr2C6H3, Pyr = pyrrolide; Me2Pyr = 2,5-dimethylpyrrolide)
produced MAP complexes8 1a,b, 2a,b, and 3a,b.
The reaction to give 1a required heating the mixture for 1 h
at 80 °C, whereas the reaction to give 1b was complete at 22 °C
(∼20 mM) within 4 h. For steric reasons, the reactions to give
2a,b are slower than those that yield 1a,b. It should be noted,
for comparison, that both Mo(NAd)(CHCMe2Ph)(Pyr)-
(OHIPT)5a and Mo(NAr)(CHCMe2Ph)(Pyr)(OHIPT)9 have
been prepared (the latter in situ) from Mo(NR)(CHCMe2Ph)-
(Pyr)2 (R = Ad, Ar) and 1 equiv of HIPTOH. Therefore,
ODPPPh and ODPPiPr behave approximately like the OHIPT
ligand in terms of the synthesis of MAP species through
protonation of bispyrrolides, although apparently small steric
differences between ligands can have profound consequences.
The X-ray structure of Mo(NAr)(CHCMe2Ph)(Me2Pyr)-
(ODPPPh) (2b) is shown in Figure 1. The dihedral angles
Figure 2. Thermal ellipsoid representation of the structure of 3b at the
50% probability level. The solvent molecule and hydrogen atoms are
omitted for clarity.
C6H6. Sonication was not required for the synthesis of
W(NAr)(CH-t-Bu)(Pyr)(ODPPiPr) (4b).
Reactions of MAP complexes with ethylene are becoming
routine means of assessing the stability of metallacyclobutane
and methylidene complexes. For example, compound 4a reacts
readily with ethylene to yield a metallacyclobutane complex,
W(NAr)(C3H6)(Pyr)(ODPPPh) (5). According to proton and
carbon NMR data, 5 has a TBP geometry. Surprisingly, the
structure of 5 in the solid state (Figure 3) is closer to a square
pyramid than a TBP, according to the τ value (0.26), which for
an SP is 0 and for a perfect TBP is 1.10 The metallacyclobutane
carbon atom in approximately the apical position (W−C1 =
2.035(2) Å) is closer to the metal than is the carbon atom in
the basal position (W−C2 = 2.083(2) Å) by a statistically
significant amount (Figure 4). The Cα−Cβ bond lengths
(1.590(3) and 1.603(3) Å) are statistically essentially the same
but vary in the direction which implies that an ethylene that
contains C2 and C3 is approaching or leaving the CNO face of
W(NAr)(CH2)(Pyr)(ODPPPh) approximately trans to the
pyrrolide (Figure 4). The W−C(2) distance is 2.370(2) Å,
which is 0.1−0.2 Å longer than a typical W−C single bond.
Since the structure of 5 is different from that of a typical square-
pyramidal complex (SP in Figure 5), in which the imido group
is in the apical position and the metallacyclic ring in basal
positions, another type that has been observed in the solid state
and in solution,11 we will call the structure of 5 an SP′
metallacyclobutane.
Figure 1. Thermal ellipsoid representation of the structure of 2b at the
50% probability level. The solvent molecule and hydrogen atoms are
omitted for clarity.
between the phenyl ring in ODPPPh and the pyrrolyl rings are
83.7(3)° (C41−C42−N3−C47) and 68.7(3)° (C41−C46−
N4−C67). In the structure of Mo(NAr)(CHCMe2Ph)(Pyr)-
(ODPPiPr) (3b) (Figure 2) the dihedral angles between the
phenyl ring in ODPPiPr and the pyrrolyl rings are 70.3(2)°
(C21−C22−N1−C30) and 80.8(2)° (C21−C26−N2−C37).
The Mo−O−C angle is larger in 3b (167.42(9)°) than in 2b
(153.7(1)°), consistent with the steric demand of the ODPPiPr
ligand system being greater than that of the ODPPPh ligand
system. Other bond distances and angles in the two structures
can be found in the Supporting Information.
Tungsten analogues of 3a,b were also prepared, since
tungsten complexes are emerging as more desirable for Z-
selective reactions.5 The reaction between W(NAr)(CH-t-
Bu)(Pyr)2(dme) and 1 equiv of DPPPhOH led to W(NAr)-
(CH-t-Bu)(Pyr)(ODPPPh) (4a). The reaction was performed
in a sonicator bath due to the limited solubility of DPPPhOH in
Selected distances and angles (averages) in the structures of
five MAP unsubstituted tungstacyclobutane complexes with
TBP structures, which have τ values from 0.47 to 0.68, are
shown in Figure 5,4a,5b,12 including both complexes in the
asymmetric unit of the structure of W(NAr)(C3H6)(MePyr)-
(OBr2Bitet).12 In the one β-substituted SP structure (W(NAr)-
[CH2CH(Ph)CH2](Pyr)(OHIPTNMe2),11 τ = 0.06. It should
be pointed out that τ values for the TBP metallacyclobutane
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Communication
2.370(2) Å in 5 is essentially what is found in the TBP
structures.
Calculations concerning metallacyclobutanes made from
MAP alkylidenes13 suggest that the SP structure is further
from the transition state for olefin loss than a TBP structure,
and an SP′ structure is the closest. All three can be
interconverted readily through five-coordinate rearrange-
ments.14 According to calculations the olefin approaches the
more “open” CNO (imido/alkylidene/OR) face “trans” to the
pyrrolide and forms an SP′ metallacyclobutane structure,
without olefin binding to the metal, to give an intermediate
alkylidene/olefin complex. The SP′ structure becomes a TBP
when the O−M−Nimido angle opens to ∼180° and the pyrrolide
moves into an equatorial position where the N1−M−C3 and
N1−M−C1 angles are equal. A continuation of the movement
of N2, N1, and O leads to a second SP′ structure in which the
metallacyclobutane again spans apical (now C3) and basal
(now C1) sites and the ethylene that is leaving the coordination
sphere contains C1 and C2. Both from experiments and in
terms of calculations the barrier for interconversion of TBP and
SP forms is relatively low.14 TBP and SP′ metallacyclobutane
structures would seem to be even more easily interconverted,
since minimal movement of the imido and aryloxide ligands is
required. The SP′/TBP/SP′ sequence is proposed to be the
intimate mechanism of metathesis by a MAP catalyst, and the
SP structure is a relatively low energy sink.
It is somewhat surprising that the SP′ structure, of which 5 is
the first example to our knowledge, can be observed, but it is
not clear why in this particular case. At this stage we can only
offer that the energy difference between the SP′ and TBP
structures is so low that intramolecular steric forces and/or
packing forces in the crystal tip the balance in favor of SP′. So
far there is no evidence for the SP′ structure in solution NMR
spectra of 5. Evidence would consist of a loss of mirror
symmetry in the metallacylobutane ring at low temperatures.
It should be noted that in NMR studies of Mo and W
metallacyclobutane species9,12 it was found necessary to invoke
a “methylidene/ethylene” intermediate in order that the kinetic
scheme be self-consistent. However, no ethylene/methylidene
has been found to be an intermediate through calculations.13
Therefore, an important question is whether the intermediate
observed in the NMR studies is an SP′ metallacycle instead of
an “ethylene/methylidene” complex.
Figure 3. Thermal ellipsoid representation of the structure of 5 at the
50% probability level. The minor component of the tungsten disorder
and the hydrogen atoms are omitted for clarity. Selected bond lengths
(Å) and angles (deg): W(1)−O(1) = 1.986(2), W(1)−N(1) =
2.031(1), W(1)−N(2) = 1.752(2), W(1)−C(1) = 2.035(2), W(1)−
C(3) = 2.083(2), W···C2 = 2.370(2), C(1)−C(2) = 1.603(3), C(2)−
C(3) = 1.590(3); O(1)−W(1)−N(1) = 84.03(7), N(2)−W(1)−O(1)
= 166.13(7), N(2)−W(1)−C(3) = 93.16(9), N(2)−W(1)−C(1) =
97.96(9), N(1)−W(1)−C(2) = 165.65(8), W(1)−C(3)−C(2) =
79.1(1), C(3)−C(2)−C(1) = 117.5(2), C(2)−C(1)−W(1) = 80.3(1),
C(1)−W(1)−C(3) = 83.02(9), N(1)−W(1)−N(2) = 91.27(8),
N(1)−W(1)−C(3) = 150.64(8).
Figure 4. Thermal ellipsoid drawing (50%) of the metallacyclobutane
moiety in 5 with bond lengths (Å) and angles (deg).
The ROMP polymerization of 50 equiv of 5,6-dicarbome-
thoxynorbornadiene was chosen as an initial measure of the
stereoselectivity of the six MAP catalysts described earlier. All
polymers were found to have a >99% cis,syndiotactic structure,
the same structure observed when the initiator is Mo(NAd)-
(CHCMe2Ph)(Pyr)(OHIPT).4a
In contrast, compounds 4a,b show markedly different
behavior in the homometathesis of 1-octene (Table 1). For
comparison, W(NAr)(C3H6)(pyr)(OHIPT)4a (6) was em-
ployed under identical conditions. Catalyst 4a initially provides
the product at a faster rate than 4b or 6, but selectivity for the Z
product erodes over time, with 62% Z product being observed
after 400 min. Catalyst 4b is much slower, providing 83%
conversion over 400 min, but the Z configuration of the
product is maintained, as it is with 6. The difference in
performance between ODPPiPr and ODPPPh highlights the
extreme sensitivity of activity and Z-selectivity of MAP
complexes to steric factors associated with the aryloxide.
Now that an SP′ metallacycle has been structurally
characterized, a persistent question that remains is the degree
Figure 5. Selected distances and angles in five TBP structures
(average) and one SP structure.
complexes would never approach 1, as a consequence of the
constraints inherent in a complex that contains a metal-
lacylobutane ring in the equatorial position; the maximum τ
value is ∼0.68. It should be noted that the W−C(2) distance of
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a
Muller, P.; Takase, M. K.; Hoveyda, A. H. Organometallics 2011, 30,
1780.
̈
Table 1. Homocoupling of 1-Octene with 4a,b
catalyst
t (min)
conversn (%)
Z (%)
(6) Yuan, J.; Schrock, R. R.; Muller, P.; Axtell, J. C.; Dobereiner, G. E.
̈
Organometallics 2012, 31, 4650.
4a
4a
4a
4a
4b
4b
4b
4b
6
10
40
62
72
88
>95
24
36
59
83
5
>95
90
84
62
-
(7) Wang, C.; Haeffner, F.; Schrock, R. R.; Hoveyda, A. H. Angew.
Chem., Int. Ed. 2013, 52, 1939.
(8) Hock, A. S.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc.
2006, 128, 16373.
110
400
10
(9) Schrock, R. R.; King, A. J.; Marinescu, S. C.; Simpson, J. H.;
40
-
Muller, P. Organometallics 2010, 29, 5241.
̈
110
400
10
>95
94
-
(10) Addison, A. W.; Rao, T. J.; Reedijk, J.; van Rijn, J.; Verschoor, G.
C. J. Chem. Soc., Dalton Trans. 1984, 1349.
(11) Yuan, J.; Townsend, E. M.; Schrock, R. R.; Goldman, A. S.;
6
40
16
46
93
-
Muller, P.; Takase, M. Adv. Synth. Catal. 2011, 353, 1985.
̈
(12) Jiang, A. J.; Simpson, J. H.; Muller, P.; Schrock, R. R. J. Am.
̈
6
110
400
>95
>95
Chem. Soc. 2009, 131, 7770.
6
(13) (a) Solans-Monfort, X.; Clot, E.; Coperet, C.; Eisenstein, O. J.
Am. Chem. Soc. 2005, 127, 14015. (b) Poater, A.; Solans-Monfort, X.;
Clot, E.; Coperet, C.; Eisenstein, O. J. Am. Chem. Soc. 2007, 129, 8207.
a
Conditions: 25 °C, 4 mol % catalyst loading, 0.3 M in C6H6. See the
Supporting Information for details.
(c) Solans-Monfort, X.; Coper
2010, 132, 7750. (d) Solans-Monfort, X.; Coper
Organometallics 2012, 31, 6812.
(14) For relatively recent studies see: (a) Moberg, C. Angew. Chem.,
Int. Ed. 2011, 50, 10290. (b) Couzijn, E. P. A.; Slootweg, J. C.; Ehlers,
A. W.; Lammertsma, K. J. Am. Chem. Soc. 2010, 132, 18127.
́
et, C.; Eisenstein, O. J. Am. Chem. Soc.
́
et, C.; Eisenstein, O.
to which the structures and dynamics of unsubstituted
metallacyclobutanes differ from the structures and dynamics
of substituted metallacyclobutanes. Since substituted metal-
lacyclobutanes are inherently more labile toward loss of olefin
than unsubstituted metallacyclobutanes, obtaining answers to
this question through experimental studies is likely to remain
challenging.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, and CIF files giving experimental details for the
synthesis of all compounds and details of X-ray structural
studies. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for R.R.S.: rrs@mit.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Science Foundation (CHE-
1111133 to R.R.S. and CHE-0650456 as part of the Center for
Enabling New Technologies through Catalysis (CENTC)) for
financial support. The X-ray diffractometer was purchased with
the help of funding from the National Science Foundation
(NSF) under Grant Number CHE-0946721. M.R.R. is grateful
to the Austrian Science Foundation for an Erwin-Schroedinger
fellowship (Grant J2822-N19).
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