Article pubs.acs.org/Organometallics
Synthesis of Methylidene Complexes that Contain a 2,6Dimesitylphenylimido Ligand and Ethenolysis of 2,3Dicarbomethoxynorbornadiene Laura C. H. Gerber and Richard R. Schrock* Department of Chemistry 6-331, Massachusetts Institute of Technology, 77 Massachusetts, 6-331, Cambridge, Massachusetts 02139, United States S Supporting Information *
ABSTRACT: Monoalkoxide pyrrolide (MAP) complexes that contain a 2,6dimesitylphenylimido (NAr*) ligand react with ethylene to yield unsubstituted metallacyclobutanes that are in equilibrium with methylidene complexes, W(NAr*)(CH2)(Me2Pyr)(OR) (R = t-Bu, OCMe(CF3)2, SiPh3, or 2,6-Me2C6H3). Polymerization of 2,3-dicarbomethoxynorbornadiene (DCMNBD) with MCHCMe2Ph (M = Mo or W) initiators is slow as a consequence of a slow propagation step. However, W(NAr*)(CH2)(Me2Pyr)(OR) (R = SiPh3 or 2,6-dimethylphenyl) complexes react readily with 1 equiv of DCMNBD to give a monoinsertion product. The facile reaction between the monoinsertion product and ethylene then allows these complexes to be catalyts for the ring-opening cross-metathesis (ethenolysis) of DCMNBD and DCMNBE (2,3-dicarbomethoxynorbornene) with minimal formation of polymer.
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INTRODUCTION Monoalkoxide pyrrolide (MAP) imido alkylidene complexes of molybdenum and tungsten have been the focus of recent developments in olefin metathesis with high oxidation state catalysts. The most important has been the synthesis of catalysts for Z-selective olefin metathesis reactions such as ringopening metathesis polymerization (ROMP),1 homocoupling of terminal olefins,2 ring-opening/cross-metathesis,3 ethenolysis,4 formation of natural products through ring-closing metathesis,5 and cross metathesis with vinyl boronates.6 The catalysts in these Z-selective reactions in all cases contain a sterically demanding alkoxide such as (R)-3,3′-dibromo-2′-(tertbutyldimethylsilyloxy)-5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-binaphthyl-2-ol) (OBr2Bitet; see Figure 1), O-2,6-(2,4,6Me3C6H2)2C6H3 (OHMT), or O-2,6-(2,4,6-i-Pr3C6H2)2C6H3 (OHIPT; see Figure 1). A sterically demanding aryloxide encourages addition of a substituted olefin to a syn alkylidene
(the substituents on the alkylidene point toward the imido ligand in a syn alkylidene) to yield substituted metallacyclobutanes in which all substituents point away from the aryloxide ligand. Ru-based catalysts have also been found that behave as Z-selective olefin metathesis catalysts, in part as a consequence of the sterically demanding or constrained Nheterocyclic carbene ligands directing addition of a substituted olefin to give a metallacyclobutane intermediate in which all substituents point away from the NHC.7 A second interesting feature of several MAP complexes that contain a sterically demanding aryloxide is that methylidene complexes can be isolated (see Figure 1). Examples of isolated 14 electron MAP species include W(NAr)(CH2)(OBr2Bitet)(Me2Pyr) (Ar = 2,6-i-Pr2C6H3, Me2Pyr = 2,5-dimethylpyrrolide),8a Mo(NAr)(CH2)(OHIPT)(Pyr) (Pyr = pyrrolide),8b W(NAr)(CH 2 )(OTPP)(Me 2 Pyr) (OTPP = O-2,3,5,6Ph4C6H),8a and Mo(NAr)(CH2)(OTPP)(Me2Pyr).8c It has been proposed that sterically demanding aryloxides inhibit bimolecular decomposition of methylidene complexes, which are the most susceptible to bimolecular decomposition.9 Consequently, observation or isolation of methylidenes is also an indication that active metathesis catalysts of that type are likely to be longer-lived. The utility of sterically demanding aryloxides encouraged us to explore the synthesis of compounds that contain a bulky terphenyl imido ligand. Previously reported imido ligand precursors in this category include N-2,6-(2,4,6-iPr3C6H2)2C6H3 and N-2,6-(2,4,6-Me3C6H2)2C6H3 (NAr*).10 The NAr* ligand was chosen for the initial studies. Because a
Figure 1. Isolated four-coordinate MAP methylidene complexes of Mo and W. © XXXX American Chemical Society
Received: August 22, 2013
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dx.doi.org/10.1021/om400844k | Organometallics XXXX, XXX, XXX−XXX
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methylidene complexes. Application of a vacuum to a solution of 2cW cleanly converts all 2cW to 3cW. Compound 3cW can be isolated readily from pure 2cW in this manner. It is most desirable to form 3cW from isolated 2cW in order to avoid possible complications that result from the back reaction of CH2CHCMe2Ph with 3cW to give 1cW. In the proton NMR spectrum of 3cW in C6D6, the methylidene proton resonances are found at 9.56 and 9.00 ppm with 2JHH = 9 Hz. Compound 2cW crystallizes in the space group P21/n with two independent molecules in the asymmetric unit. A drawing of the structure of 2cW is shown in Figure 3. The overall
typical intermediate such as M(NAr*)2(CH2CMe2Ph)2 could not be prepared, presumably for steric reasons, new syntheses that do not require formation of bisNAr* complexes as intermediates had to be designed for both Mo and W.11,12 A selection of four-coordinate neophylidene MAP complexes that have been synthesized is shown in Figure 2.
Figure 2. MAP complexes containing the NAr* ligand.
In this paper, we report the reactions of several of the complexes 1a−1d with ethylene to give metallacyclobutane and methylidene complexes and a preliminary study of ring-opening cross metathesis (ethenolysis) reactions of 2,3-dicarbomethoxynorbornadiene (DCMNBD) catalyzed by NAr* complexes.
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RESULTS AND DISCUSSION Reaction of Tungsten MAP Complexes with Ethylene. Addition of 1 atm of ethylene to a degassed solution of 1cW in C6D6 yields the metallacyclobutane complex, W(NAr*)(C3H6)(Me2pyr)(OSiPh3) (2cW, eqs 1 and 2). Running the
Figure 3. Thermal ellipsoid drawing of 2cW at the 50% probability level with hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (deg): W1−N2 = 2.0597(17), W1−C1 = 2.0628(19), W1−C2 = 2.3658(19), W1−C3 = 2.0500(19), C1−C2 = 1.593(3), C2−C3 = 1.600(3), C3−W1−C1 = 83.15(8), C2−C1−W1 = 79.52(10), C2−C3−W1 = 79.77(10), C1−C2−C3 = 117.49(15), Si1−O1−W1 = 165.48(8).
reaction between 1cW and ethylene in a mixture of pentane and diethyl ether followed by cooling the mixture to −25 °C overnight allowed 2cW to be isolated. The proton NMR spectra of 2cW in C6D6 are typical of a trigonal bipyramidal (TBP) metallacyclobutane complex, although about 10% of a methylidene complex, W(NAr*)(CH2)(Me2pyr)(OSiPh3) (3cW, eq 2), and ethylene typically are present in samples under N2 atmosphere as a consequence of the equilibrium between unsubstituted metallacyclobutane complexes and
structure of 2cW is approximately midway between a trigonal bipyramid and a square pyramid; the τ value is 0.60 (where τ = 0 for a square pyramid (SP) and τ = 1 for a trigonal bipyramid (TBP)).13 If we choose to call the geometry of this complex a distorted TBP, the imido and siloxide ligands are in the apical sites, while the pyrrolide and metallacycle are in the equatorial plane of the (distorted) TBP. The N2−W1−C1 angle is 141.31(7)° and the N2−W1−C3 angle is 133.35(7)°, which is a measure of the degree of distortion toward a SP geometry. A metallacyclobutane, W(NAr)(C3H6)(Pyr)(ODPPPh) (Ar = 2,6diisopropylphenyl, ODPPPh = 2,6-di(2,5-diphenylpyrrolyl)phenoxide) that is closer to a square pyramid has been isolated recently in which τ = 0.26, N2−W1−C1 = 150.64(8)° and N2−W1−C1 = 125.03(8)°.14 Movement of the pyrrolide ligand in the equatorial plane from one side to the other is proposed to be an intimate feature of the mechanism of olefin metathesis in which TBP metallacyclobutane complexes are intermediates.15 Compound 3cW crystallizes in space group P21/c with one molecule per asymmetric unit. A drawing of the structure can be found in Figure 4. The tungsten atom, pyrrolide ligand, methylidene ligand, N1, and O1 are disordered over two positions, with the major component representing 90% of the electron density. The two components of the disorder are enantiomers at tungsten. The geometry at tungsten is a distorted tetrahedron. When the molecule is viewed looking down the N1−W1 axis, one mesityl group is above the siloxide B
dx.doi.org/10.1021/om400844k | Organometallics XXXX, XXX, XXX−XXX
Organometallics
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Scheme 1. Reaction of 1aW with Ethylene
Figure 4. Thermal ellipsoid (50%) representation of 3cw. Hydrogen atoms except those on C1, and the minor component of disorder are omitted for clarity. Selected bond lengths (Å) and angles (deg): W1− N1 = 1.7404(15), W1−O1 = 1.853(4), W1−C1 = 1.892(2), W1−N2 = 2.0092(19), N1−C2 = 1.403(2), N1−W1−O1 = 118.41(17), N1− W1−C1 = 102.09(8), O1−W1−C1 = 111.66(17), N1−W1−N2 = 109.55(8), O1−W1−N2 = 111.15(18), C1−W1−N2 = 102.51(8), C2−N1−W1 = 175.38(14), Si1−O1−W1 = 147.9(3).
When this mixture sat for 2 h under an ethylene atmosphere, followed by removal of the volatiles in vacuo, the 1H NMR spectrum in C6D6 under N2 atmosphere showed a 2:1 mixture of 3aW and the ethylene complex 4aW, the ratio of which does not change over 24 h. When 1 atm of ethylene is added to a degassed C6D6 solution of 1aW and the reaction monitored over time, complete conversion to the ethylene complex is observed. Propylene is also observed, the product of rearrangement of the unsubstituted metallacyclobutane complex. These experiments indicate that there is an equilibrium in solution between 2aW and 3aW, that 2aW decomposes to the ethylene complex with concomitant extrusion of propylene, and that in the absence of ethylene 3aW does not decompose to the ethylene complex in a bimolecular coupling reaction. Complex 3aW can be isolated by exposure of a degassed solution of 1aW in toluene to 1 atm ethylene for 20 min, followed by removal of the volatiles in vacuo. The short reaction time prevents any significant amount of the ethylene complex being formed. Isolation of methylidene and unsubstituted metallacyclobutane complexes in general is more difficult for Mo than W analogues. While tungstacyclobutane complexes can be isolated for W MAP species that contain OCMe(CF3)2, OSiPh3, or OAr′ ligands, in analogous Mo systems formation of an ethylene complex prevents isolation of the metallacyclobutane complex in pure form. A M(IV) ethylene complex is formed readily for both Mo and W when OR is t-butoxide. The more electron-rich metal center in O-t-Bu complexes encourages reduction to M(IV), either through bimolecular decomposition, or if bimolecular decomposition is sterically blocked, as is believed to be the case with NAr* complexes, through rearrangement of the metallacyclobutane to propylene. Polymerization of DCMNBD. DCMNBD is polymerized slowly by compounds 1a−1d (1% in C6D6) at room temperature for both Mo and W. The results are shown in Table 1. No initiator yields a polymer with a stereoregular structure either in terms of CC bond isomers or tacticity. Trans contents vary between 19% and 70%. No clear trends are apparent. Compounds 3cW and 3dW react with DCMNBD at a rate that is relatively fast compared to polymerization, as can be shown through a reaction between 1 equiv of DCMNBD and 3cW or 3dW to give the first insertion products, 5cW and 5dW (eq 3). In the case of 5cW, the eight protons in the alkylidene portion of the first insertion product could be assigned through 2D gCOSY NMR methods (see Supporting Information).
ligand and the other is between the methylidene and pyrrolide ligands. Addition of 1 atm ethylene to a degassed solution of 1dW in C6D6 produces the metallacyclobutane complex, W(NAr*)(C3H6)(Me2Pyr)(OAr′) (OAr′ = O-2,6-Me2C6H3, 2dW), which can be isolated in a manner similar to that employed to isolate 2cW. Solutions of 2dW again contain a small amount of W(NAr*)(CH2)(Me2Pyr)(OAr′) (3dW) and ethylene according to NMR spectroscopy. Dissolution of crystals of 2dW in toluene and removal of the volatiles in vacuo provides 3dW, which is isolated in 70% yield. Compound 3dW shows only one methylidene resonance in the 1H NMR spectrum in C6D6 at 9.69 ppm. The single resonance in C6D6 is accidental, as two methylidene protons resonances are found in a spectrum of 3dW in CD2Cl2 at 9.55 and 9.76 ppm. The proton that gives rise to the 9.55 ppm resonance is coupled to tungsten with JWH = 17 Hz, while the proton that gives rise to the 9.76 ppm resonance is coupled to tungsten with JWH = 7 Hz. The 1JCH value is 166 Hz for the proton yielding the 9.55 resonance and 131 Hz for the proton yielding the 9.76 resonance, as determined by proton NMR spectra. These data are consistent with the 9.55 ppm resonance being assigned to the syn proton (1JCH = 166 Hz) and the 9.76 ppm resonance being assigned to the anti proton (1JCH = 131 Hz). The smaller 1JCH value is a consequence of an agostic interaction between CHanti and W. W(NAr*)(C3H6)(Me2Pyr)[OCMe(CF3)2] (2bW) can be isolated through addition of ethylene to a MeCN solution of 1bW. However, so far when a vacuum is applied to a solution of 2bW, clean conversion to W(NAr*)(CH2)(Me2Pyr)[OCMe(CF3)2] is not observed. Instead, several unidentified alkylidene resonances typically are observed in the 1H NMR spectrum. One possibility is that they are various acetonitrile adducts. Upon addition of 1 atm of ethylene to a degassed C6D6 solution of W(NAr*)(CHCMe2Ph)(Me2pyr)(O-t-Bu) (1aW), a 1:1 mixture of methylidene W(NAr*)(CH2)(Me2pyr)(O-t-Bu) (3aW) and metallacycle W(NAr*)(C3H6)(Me2pyr)(O-t-Bu) (2aW) is observed in the 1H NMR spectrum (see Scheme 1). C
dx.doi.org/10.1021/om400844k | Organometallics XXXX, XXX, XXX−XXX
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Table 1. Polymerization of DCMNBD with Initiators 1a−1d in Toluene-d8 at 22 °C catalyst 1aMo 1aW 1bMo 1bW 1cMo 1cW 1dMo 1dW
time 6 2 2 7 1 5 1 5
h d d d d d d d
% trans 59 54 57 30a 70 19a 34 38a
a
All reactions are complete at the stated time except those employing 1bW, 1cW, and 1dW. In those cases, the polymer that formed precipitated from toluene-d8 and the extent of completion was not measured. The % trans in all cases was determined by proton NMR methods in CDCl3.
Therefore it is clear that steric factors prevent rapid propagation of the polymerization of DCMNBD. X-ray quality crystals were obtained in the process of recrystallizing 5cW from acetonitrile over a period of days. An X-ray study showed this product to be an acetonitrile adduct but of a compound resulting from a 1,3 proton shift of the doubly allylic proton on the C5 carbon atom in the alkylidene ligand in 5cW to the olefinic C3 carbon atom (eq 4 and Figure 5).
Figure 5. X-ray crystal structure of 5cW′ shown with 50% probability ellipsoids. Hydrogen atoms, solvent molecules, and minor components of disorder are omitted for clarity. The bottom shows the alkylidene ligand and first coordination sphere of W. Selected bond lengths (Å) and angles (deg): W1−N1 = 1.769(2), W1−C1 = 1.895(3), W1−O1 = 1.942(6), W1−N2 = 2.088(2), W1−N3 = 2.189(2), C4−C5 = 1.519(4), C5−C6 = 1.351(4), N1−W1−C1 = 101.00(11), N1−W1− O1 = 147.7(4), C1−W1−O1 = 110.3(3), N1−W1−N2 = 97.39(9), C1−W1−N2 = 102.03(10), O1−W1−N2 = 84.0(4), N1−W1−N3 = 93.70(9), C1−W1−N3 = 90.81(10), O1−W1−N3 = 78.5(4), N2− W1−N3 = 161.05(9), C9−C4−C5 = 113.1(3), C9−C4−C2 = 114.0(3), C5−C6−C7 = 127.3(3), C7−C6−C3 = 122.2(3).
identical to that obtained fortuitously upon attempted recrystallization of 5cW from acetonitrile (except for CD3CN versus CH3CN). How a crystal of 5cW′ was formed during the attempted recrystallization of 5cW from acetonitrile in the absence of triethylamine remains unexplained. Preliminary Studies of Ethylenolysis of 2,3-Dicarbomethoxynorbornadiene (DCMNBD) and 2,3-Dicarbomethoxynorbornene (DCMNBE). To prevent polymerization of norbornenes in ring-opening cross metathesis (ROCM) reactions, norbornenes with substituents in the 7-position are generally required, at least for the reported Mo and W catalysts.16 In certain cases, ROCM of substituted norbornenes with an excess of the cross-partner employing Ru-based metathesis catalysts has been observed,17 as has ROCM of silyl-substituted norbornenes with ethylene.18 Ethenolysis of cyclooctene to yield 1,9-decadiene and cyclopentene to yield 1,6-hexadiene have also been reported, and those reactions are in competition with polymerization of each monomer.4a The observations in the prior section suggest that NAr* methylidene complexes might catalyze ROCM between norbornenes or norbornadienes and ethylene without polymerization of the norbornene or norbornadiene (eq 5 for DCMNBD or exo,endo,rac-2,3-dicarbomethoxynorbornene (DCMNBE)). Although ethenolysis of norbornenes can be
Compound 5cW′ is a distorted square pyramid with the alkylidene ligand at the apical site and a τ value of 0.22. The W1−N1−C13 bond is bent (165.2(2)°) in response to the steric demands of the alkylidene. The hydrogen shift is evident by the essentially tetrahedral geometry at C4 and the planar geometry at C6 (Figure5). Additionally, the C4−C5 bond length (1.519(4)) is indicative of a C−C single bond, while the C5−C6 length (1.351(4)) is characteristic of a CC double bond. To our knowledge, 1,3 hydrogen shifts of this type have not been documented before in polymerization of substituted norbornadienes. Although 5cW appears to be relatively stable in CD3CN, addition of 1 equiv of NEt3 to a solution of 5cW in CD3CN led to conversion of 5cW to 5cW′, consistent with a base-promoted 1,3-proton shift. The gCOSY spectrum of 5cW′ prepared in this manner is clearly different from that due to 5cW (see Supporting Information). An abbreviated X-ray study of 5cW′ synthesized as shown in eq 4 confirmed that the product is D
dx.doi.org/10.1021/om400844k | Organometallics XXXX, XXX, XXX−XXX
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slow as a consequence of a relatively slow propagation and is unselective for formation of cis vs trans double bonds. Whether polymers can be formed stereoselectively from NAr* catalysts and monomers that are less sterically hindered than DCMNBD is not yet known. However, methylidene complexes react rapidly with DCMNBD to give a first-insertion product in high yield and the first-insertion product reacts more readily with ethylene than with DCMNBD. This circumstance allows an ethenolysis reaction to be carried out with little or no polymer formation and minimal optimization of reaction conditions. It seems plausible to expect that ethenolysis reactions of other polymerizeable monomers could be designed through approaches analogous to those reported here, the primary requirements being a slow propagation step in a hypothetical polymerization along with the formation of relatively stable, but reactive methylidene complexes. Imido alkylidene complexes that contain an even more sterically hindered N-2,6-(2,4,6-iPr3C6H2)2C6H3 group remain to be explored.
engineered to some degree in order to avoid formation of polymer instead of diene, methylidene complexes that are biased to react readily with monomer, but not to polymerize the monomer readily, should be best suited for clean ethenolysis reactions. Selected results for ethenolysis of DCMNBD (1% catalyst) are shown in Table 2. Table 2. ROCM Results for DCMNBD Employing 1% Catalyst
a
entry
catalyst
time (h)
°C
P (atm)
% dienea
% polya
1 2 3 4 5 6 7 8 9 10 11 12 13 14
3cW 3cW 3cW 3dW 3dW 3dW 3dW 3dW 3dW 3dW 3dW 3dW 3dW 3dW
16 16 16 16 16 22 48 16 22 24 21 48 93 21
20 20 20 20 20 20 20 20 20 20 60 60 60 80
1 3.7 20 1 3.7 3.7 3.7 20 1 1 1 1 1 1
22 24 3 25 41 47 53 24 45 63 85 70 >98 52
0 4 2 0 0 0 0 0 0 5 0 0 0 8
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EXPERIMENTAL SECTION
General Considerations. All air-sensitive manipulations were performed under nitrogen atmosphere in a glovebox or an air-free dual-manifold Schlenk line. All glassware was oven-dried and allowed to cool under vacuum before use. NMR spectra were obtained on 300, 500, or 600 MHz spectrometers. 1H and 13C NMR spectra are reported in δ (parts per million) relative to tetramethylsilane and referenced to residual 1H/13C signals of the deuterated solvent (1H (δ) benzene 7.16, chloroform 7.27, methylene chloride 5.32, toluene 2.09, acetonitrile 1.94; 13C (δ) benzene 128.39, chloroform 77.23, methylene chloride 54.00, toluene 20.40). 19F NMR spectra are reported in δ (parts per million) relative to trichlorofluoromethane and referenced using an external standard of fluorobenzene (δ −113.15). Diethyl ether, toluene, tetrahydrofuran, pentane, benzene, dichloromethane, dimethoxyethane, and MeCN were sparged with nitrogen and passed through activated alumina. All solvents were stored over 4 Å molecular sieves. Liquid reagents were degassed, brought into the glovebox and stored over 4 Å molecular sieves. Mo(NAr*)(CHCMe2Ph)(Me2pyr)(O-t-Bu) (1aMo),11 W(NAr*)(CHCMe2Ph)(Me2pyr)(O-t-Bu) (1aW),12 W(NAr*)(CHCMe2Ph)(Me2pyr)[OCMe(CF3)2] (1bW),12 W(NAr*)(CHCMe2Ph)(Me2pyr)(OSiPh3) (1cW),12 and W(NAr*)(CHCMe2Ph)(Me2pyr)(OAr′) (1cW)12 were prepared according to literature procedures. All other reagents were used as received. Observation of W(NAr*)(C3H6)(Me2Pyr)(O-t-Bu) (2aW). W(NAr*)(CHCMe2Ph)(Me2pyr)(O-t-Bu) (10.2 mg, 12.6 μmol) was dissolved in 0.5 mL C6D6 in a J. Young NMR tube. The solution was freeze−pump−thawed twice. The tube was refilled with ethylene, and a 1H NMR spectrum was obtained which showed a 1:1 mixture of W(NAr*)(C 3 H 6 )(Me 2 Pyr)(O-t-Bu) (2a W ), W(NAr*)(CH 2 )(Me2pyr)(O-t-Bu) (3aW), and CH2CHCMe2Ph. 1H NMR (C6D6) for 2aW: δ 6.898 (overlapping signals, 3H, ArH), 6.790 (s, 4H, MesH), 6.050 (s, 2H, PyrH), 3.826 (m, 4H, CαH), 2.273 (s, 6H, CH3), 2.127 (s, 6H, CH3), 2.067 (s, 12H, Mes(CH3)ortho), 0.752 (s, 9H, OC(CH3)3), −1.110 (m, 2H, CβH). Synthesis of W(NAr*)(C3H6)(Me2Pyr)[OCMe(CF3)2] (2bW). W(NAr*)(CHCMe2Ph)(Me2pyr)[OCMe(CF3)2] (53.6 mg, 58.3 μmol) was dissolved in 3 mL of a 2:1 mixture of acetronitrile and diethyl ether in a 50 mL Schlenk bomb. The volume of the solution was reduced to 1 mL and the solution degassed by exposure to a vacuum. The flask was refilled with 1 atm of ethylene and the solution stirred for 10 m before the flask was sealed, brought into the glovebox, and cooled to −25 °C for 16 h. The mother liquor was removed from the crystals with a pipet. The crystals were dissolved in pentane, and all volatiles were then removed in vacuo to leave a yellow solid; yield 26.0 mg (54%). 1H NMR (C6D6): δ 6.832 (s, 4H, MesH), 6.750 (t, 1H, ArHpara), 6.675 (d, 2H, ArHmeta), 5.916 (s, 2H, PyrH), 4.073 (dt, JHH = 12 Hz, JHH = 4 Hz, 2H, WCHα), 3.916 (dt, JHH = 11 Hz, JHH = 4 Hz,
Determined by 1H NMR spectroscopy.
Initial reactions employing 3cW (entries 1−3) suggest that 20 atm of ethylene slows the rate of formation of the diene product. A slower rate of formation of diene or polymer would be expected as a consequence of stabilization of the metallacyclobutane (2cW) at higher ethylene pressures. It is not obvious why a small amount polymer is formed at pressures greater than 1 atm but not at 1 atm. Reactions employing 3dW (entries 4−8) suggest that conversion to diene is faster than reactions employing 2cW and more selective for formation of diene. Although the percent yield of diene is similar at 1 and 3.7 atm (compare entries 6 and 9), the amount of diene formed at 20 atm again decreases (compare entries 5 and 8). Reactions run at 1 atm and 60 °C (entries 11−13) appear to give optimum results, while a reaction at 80 °C appears to give a lower yield of diene and some polymer. In view of the variation from one experiment to another (see entries 9 and 10, for example), we hesitate to attempt to rationalize small differences. The extent of catalyst decomposition under a variety of conditions is also unknown but likely to be an important factor in reactions of this general type. Reactions between ethylene (1 atm) and DCMNBE in the presence of 2% 3cW at 60 °C produced only diene in essentially >98% yield in 24 h.
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CONCLUSION The steric protection provided by the NAr* ligand allows methylidene complexes to be isolated, especially those that contain tungsten. Polymerization of DCMNBD is relatively E
dx.doi.org/10.1021/om400844k | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Article
2H, WCHα), 2.214 (s, 6H, ArCH3), 2.018 (s, 6H, ArCH3), 1.958 (s, 12H, Mes(CH3)meta), 0.847 (s, 1H, OC(CH3)(CF3)2), −0.763 (m, 1H, Hβ), −1.169 (m, 1H, Hβ). 13C{1H} NMR (C6D6): δ 142.2, 137.8, 137.4, 136.8, 132.0, 130.0, 129.7, 128.7, 127.5, 108.5 (Aryl), 99.6 (Cα, JCW = 32 Hz), 28.7, 23.1, 21.5 (MesMe), 21.2 (MesMe), 15.7, 14.7, 3.8 (Cβ). 19F NMR (C6D6): δ −77.81. Anal. Calcd for C37H42F6N2OW: C, 53.63; H, 5.11; N, 3.38. Found: C, 53.67; H, 5.04; N, 3.48. Synthesis of W(NAr*)(C3H6)(Me2pyr)(OSiPh3) (2cW). W(NAr*)(CHCMe2Ph)(Me2pyr)(OSiPh3) (36.9 mg, 0.0364 mmol) was dissolved in 10 mL of a 10:1 mixture of pentane and diethyl ether in a 50 mL Schlenk bomb. The solution was degassed by application of a vacuum for several seconds. The flask was refilled with 1 atm ethylene, and the mixture was stirred for 2 h. The flask was brought into the drybox and cooled to −25 °C for 16 h, over which time yellow crystals formed. The mother liquor was removed with a pipet, and the crystals were collected on a glass frit and washed with 0.5 mL of cold pentane; yield 12.0 mg (36%). 1H NMR (C6D6): δ 7.389 (d, 6H, ArH), 7.136−7.071 (overlapping signals, 9H, ArH), 6.768 (s, 4H, MesH), 6.742−6.701 (overlapping signals, 3H, ArH), 6.082 (s, 2H, NMe2C4H2), 3.998 (dt, JHH (d) = 11 Hz, JHH (t) = 4 Hz, 2H, CαH), 3.999 (dt, JHH (d) = 11 Hz, JHH (t) = 4 Hz, 2H, CαH), 2.158 (s, 6H, Me), 2.150 (s, 6H, Me), 1.990 (s, 12H, MesMe), −1.117 (m, 1H, CβH), −1.270 (m, 1H, CβH). 13C{1H} NMR (C6D6): δ 142.3, 137.9, 137.4, 137.3, 136.6, 135.9, 135.8, 132.6, 129.9, 129.7, 129.2, 128.7, 128.3, 126.9, 108.9, 99.3 (Cα), 21.5, 21.4, 16.5, 4.4 (Cβ). Anal. Calcd for C51H54N2OSiW: C, 66.37; H, 5.90; N, 3.04. Found: C, 66.12; H, 6.10; N, 2.90. Synthesis of W(NAr*)(C3H6)(Me2Pyr)(OAr′) (2dW). A sample of W(NAr*)(CHCMe2Ph)(Me2pyr)(OAr′) (78 mg, 0.091 mmol) was dissolved in pentane in a Schlenk bomb. The solution was degassed by applying vacuum for a few seconds. The flask was refilled with 1 atm ethylene and stirred for 5 m. The flask was cooled to −25 °C and orange crystals formed over 16 h. The supernatant was removed by pipet, and the crystals were washed with cold pentane and dried in vacuo; yield 40 mg (57%). 1H NMR (C6D6): δ 6.828−6.806 (overlapping signals, 9H, ArH), 6.621 (t, JHH = 7 Hz, 1H, ArHpara), 5.901 (s, 2H, NMe2C4H2), 3.920 (m, 4H, CαH), 2.217 (s, 6H, CH3), 2.210 (s, 6H, CH3), 2.040 (s, 12H, Mes(CH3)ortho), 1.769 (s, 6H, CH3), −1.219 (m, 2H, CαH). 13C{1H} NMR (C6D6): δ 160.2, 150.9, 142.4, 137.3, 137.0, 136.4, 132.8, 129.6, 129.1, 129.0, 127.5, 127.0, 120.3, 109.0, 99.1 (Cα, 1JCW = 250 Hz), 21.5, 21.4, 18.7, 16.8, 3.6 (Cβ). Anal. Calcd for C41H48N2OW: C, 64.06; H, 6.29; N, 3.64. Found: C, 64.32; H, 6.46; N, 3.58. W(NAr*)(CH2)(Me2pyr)(O-t-Bu) (3aW). W(NAr*)(CHCMe2Ph)(Me2pyr)(O-t-Bu) (51 mg, 73 μmol) was dissolved in 10 mL of toluene in a 50 mL Schlenk bomb. The solution was degassed by applying a vacuum for a few seconds. The flask was filled with 1 atm ethylene, and the solution was stirred for 10 m. The volatiles were removed in vacuo to give an orange oil. The oil was extracted with pentane, and the solution was filtered and the pentane removed in vacuo. The oil was dissolved in a mixture of acetonitrile and diethyl ether, and the solution was cooled to −25 °C. The mother liquor was removed from the orange crystals formed with a pipet. The crystals were washed with 3 × 0.3 mL cold MeCN and dried under a vacuum; yield 30 mg (68%). 1H NMR (C6D6): δ 9.601 (d, 2JHH = 9 Hz, 1JHW = 15 Hz, 1H, WCH2), 9.510 (d, 2JHH = 9 Hz, 1JHW = 6 Hz, 1H, W CH2), 6.964 (overlapping signals, 3H, ArH), 6.843 (s, 2H, MesH), 6.794 (s, 2H, MesH), 6.083 (s, 2H, NMe2C4H2), 2.204 (s, 6H, CH3), 2.193 (s, 6H, CH3), 2.121 (s, 6H, CH3), 1.970 (s, 6H, CH3), 0.934 (s, 9H, OC(CH3). 13C{1H} NMR (C6D6): δ 235.1 (WCH2), 157.9, 153.9, 138.2, 137.5, 137.5, 137.1, 136.8, 136.8, 136.6, 136.6, 136.3, 136.2, 134.8, 129.4, 129.2, 129.0, 128.8, 126.1, 124.8, 110.0, 109.8, 105.8, 83.5, 31.8, 31.7, 24.9, 21.6, 21.6, 21.4, 21.4, 21.3, 21.0, 17.8, 17.7. Synthesis of W(NAr*)(CH2)(Me2pyr)(OSiPh3) (3cW). W(NAr*)(CHCMe2Ph)(Me2pyr)(OSiPh3) (55.0 mg, 54.3 μmol) was added to 6 mL of a 5:1 mixture of pentane and diethyl ether. The solution was degassed in vacuo and the flask was refilled with 1 atm ethylene and the mixture was stirred for 2 h. The flask was brought into the glovebox and cooled to −25 °C for 16 h, over which time yellow
crystals formed. The mother liquor was removed with a pipet, and the crystals were washed with three times with 0.5 mL of cold pentane. The crystals were dissolved in 10 mL toluene, and the volatiles were removed in vacuo to yield 30.4 mg of yellow powder (63%). 1H NMR (C6D6): δ 9.557 (d, 1H, WCH2, 2JHH = 9 Hz), 8.998 (d, 1H, W CH2, 2JHH = 9 Hz), 7.458 (d, 7 Hz, ArH), 7.189−7.175 (overlapping signals, 2H, ArH), 7.152−7.123 (overlapping signals, 5H, ArH), 6.988 (m, 4H, ArH), 6.841 (s, 2H, ArH), 6.530 (s, 2H, ArH), 5.982 (s, 2H, ArH), 2.209 (s, 6H, CH3), 2.103 (s, 6H, CH3), 1.998 (s, 6H, CH3), 1.885 (s, 6H, CH3). 13C{1H} NMR (C6D6): δ 237.8 (WCH2, 1JCW = 180 Hz), 154.0, 138.6, 137.0, 136.9, 136.5, 135.8, 135.7, 134.9, 130.9, 129.2, 129.1, 110.2, 21.7, 21.4, 20.7, 17.0. Anal. Calcd for C49H50N2OSiW: C, 65.77; H, 5.63; N, 3.13. Found: C, 65.48; H, 5.48; N, 3.07. Synthesis of W(NAr*)(CH2)(Me2Pyr)(OAr′) (3dW). W(NAr*)(CHCMe2Ph)(Me2pyr)(OAr′) (135 mg, 0.158 mmol) was dissolved in pentane in Schlenk bomb. The solution was degassed by reducing the pentane volume to ∼3 mL under vacuum. The flask was refilled with 1 atm of ethylene, and the solution was stirred for 10 m. An orange precipitate formed. The flask was cooled to −25 °C for 16 h, and the solid was collected on a frit and washed with cold pentane. The solid was dissolved in 10 mL of toluene and the volatiles removed in vacuo. The oil was dissolved in pentane and the volatiles were removed in vacuo to yield a yellow solid; yield 82 mg (70%). 1H NMR (C6D6): δ 9.693 (s, 2H, WCH2), 6.963 (s, 3H, ArH), 6.839 (d, 2H, ArH), 6.748 (overlapping signals, 3H, ArH), 6.562 (s, 2H, ArH), 6.602 (NMe2C4H2), 2.157 (s, 6H, CH3), 2.089 (s, 6H, CH3), 2.044 (s, 6H, CH3), 1.995 (s, 6H, CH3), 1.922 (s, 6H, CH3). 1H NMR (C7D8, alkylidene resonance): δ 9.677 (d, 2JHH = 9 Hz, WCH2), 9.644 (d, 2 JHH = 9 Hz, WCH2). 1H NMR (CD2Cl2, alkylidene resonance): δ 9.761 (d, 2JHH = 9 Hz, JHW = 7 Hz, 1JCH = 131 Hz, WCH2), 9.550 (d, 2JHH = 9 Hz, JHW = 17 Hz, 1JCH = 166 Hz, WCH2). 13C{1H} NMR (C6D6): δ 241.4, 138.5, 137.2, 136.8, 136.7, 136.5, 135.2, 129.7, 129.4, 129.1, 129.0, 128.9, 127.6, 126.7, 126.0, 122.8, 110.4, 21.6, 21.4, 20.8, 17.8, 17.7. Anal. Calcd for C39H44N2OW: C, 63.25; H, 5.99; N, 3.78. Found: C, 63.34; H, 6.10; N, 3.44. Observation of Mo(NAr*)(CH2CH2)(Me2Pyr)(O-t-Bu). A solution of Mo(NAr*)(CHCMe2Ph)(Me2Pyr)(O-t-Bu) in 0.5 mL C6D6 in a J.Young NMR tube was degassed with two freeze−pump−thaw cycles. The tube was refilled with 1 atm ethylene, sealed, and inverted to mix. After 16 h, a 1H NMR spectrum showed conversion to Mo(NAr*)(CH2CH2)(Me2Pyr)(O-t-Bu) as the only Mo-based product. 1H NMR (C6D6): δ 6.896 (m, 1H, ArH), 6.833−6.808 (overlapping signals, 6H, ArH), 6.127 (s, 2H, pyrH), 2.448−2.358 (overlapping m, 2H, CH2CH2), 2.297 (s, ArCH3, 6H), 2.227 (s, ArCH3, 6H), 2.074 (s, ArCH3, 6H), 2.023 (s, ArCH3, 6H), 1.921 (m, 1H, CH2CH2), 1.752 (m, 1H, CH2CH2), 0.880 (s, 9H, OC(CH3)3). Observation of 5cW. To a solution of W(NAr*)(CH2)(Me2pyr)(OSiPh3) (27 mg, 30 μmol) in 0.6 mL of C6D6 in a Teflon-stoppered NMR tube was added DCMNBD (5.0 μL, 29 μmol).
1 H NMR (C6D6): δ 8.988 (d, 1H, 3JHH = 2 Hz, 1JCH = 154 Hz, W CH), 7.757 (d, 6H, ArH), 7.222 and 7.215 (overlapping signals, 8H, ArH), 6.930 (s, 2H, ArH), 8.898−8.868 (overlapping signals, 5H, ArH), 6.130 (br s, 1H, (NMe2C4H2), 5.918 (NMe2C4H2), 5.619 (m, 1H, HF), 5.085 (m, 1H, HB), 5.008 (d, 3JHH = 17 Hz, HI), 4.929 (d, 3 JHH = 10 Hz, HG), 3.310 (m, 1H, HE), 3.239 (s, 3H, CO2CH3), 2.714 (s, 3H, CO2CH3), 2.297 (s, 6H, MesCH3), 2.230 (s, 6H, MesCH3), 2.119 (s, 6H, MesCH3), 2.055 (br s, 3H, NMe2C4H2), 1.848 (m,
F
dx.doi.org/10.1021/om400844k | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Article
(3) (a) Ibrahem, I.; Yu, M.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 3844. (b) Yu, M.; Ibrahem, I.; Hasegawa, M.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2012, 134, 2788. (4) (a) Marinescu, S. C.; Schrock, R. R.; Müller, P.; Hoveyda, A. H. J. Am. Chem. Soc. 2009, 131, 10840. (b) Marinescu, S. C.; Levine, D.; Zhao, Y.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2011, 133, 11512. (5) (a) Wang, C.; Yu, M.; Kyle, A. F.; Jacubec, P.; Dixon, D. J.; Schrock, R. R.; Hoveyda, A. H. Chem.Eur. J. 2013, 19, 2726. (b) Wang, C.; Haeffner, F.; Schrock, R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2013, 52, 1939. (6) Kiesewetter, E. T.; O’Brien, R. V.; Yu, E. C.; Meek, S. J.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2013, 135, 6026. (7) (a) Endo, K.; Grubbs, R. H. J. Am. Chem. Soc. 2011, 133, 8525. (b) Keitz, B. K.; Endo, K.; Herbert, M. B.; Grubbs, R. H. J. Am. Chem. Soc. 2011, 133, 9686. (c) Keitz, B. K.; Endo, K.; Patel, P. R.; Herbert, M. B.; Grubbs, R. H. J. Am. Chem. Soc. 2012, 134, 693. (d) Liu, P.; Xu, X.; Dong, X.; Keitz, B. K.; Herbert, M. B.; Grubbs, R. H.; Houk, K. N. J. Am. Chem. Soc. 2012, 134, 1464. (e) Khan, R. K. M.; O’Brien, R. V.; Torker, S.; Li, B.; Hoveyda, A. H. J. Am. Chem. Soc. 2012, 134, 12774. (f) Occhipinti, G.; Hansen, F. R.; Törnroos, K. W.; Jensen, V. R. J. Am. Chem. Soc. 2013, 135, 3331. (g) Kashif, R.; Khan, M.; Torker, S.; Hoveyda, A. H. J. Am. Chem. Soc. 2013, 135, 10258. (h) Mann, T. J.; Speed, A. W. H.; Schrock, R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2013, 52, 8395. (i) Cannon, J. S.; Grubbs, R. H. Angew. Chem., Int. Ed. 2013, 52, 9001. (8) (a) Jiang, A. J.; Simpson, J. H.; Müller, P.; Schrock, R. R. J. Am. Chem. Soc. 2009, 131, 7770. (b) Schrock, R. R.; Jiang, A. J.; Marinescu, S. C.; Simpson, J. H.; Müller, P. Organometallics 2010, 29, 5241. (c) Townsend, E. M.; Kilyanek, S. M.; Schrock, R. R.; Müller, P.; Smith, S. J.; Hoveyda, A. H. Organometallics 2013, 32, 4612. (9) (a) Arndt, S.; Schrock, R. R.; Müller, P. Organometallics 2007, 26, 1279. (b) Schrock, R. R. in Braterman, P. R., Ed. Reactions of Coordinated Ligands; Plenum: New York, 1986; p 221. (c) Schrock, R. R. Chem. Rev. 2002, 102, 145. (d) Schrock, R. R. Chem. Rev. 2009, 109, 3211. (10) (a) Gavenonis, J.; Tilley, T. D. J. Am. Chem. Soc. 2002, 124, 8536. (b) Gavenonis, J.; Tilley, T. D. Organometallics 2002, 21, 5549. (c) Gavenonis, J.; Tilley, T. D. Organometallics 2004, 23, 31. (d) Iluc, V. M.; Hillhouse, G. L. J. Am. Chem. Soc. 2010, 132, 15148. (e) Laskowski, C. A.; Miller, A. J. M.; Hillhouse, G. L.; Cundari, T. R. J. Am. Chem. Soc. 2011, 133, 771. (f) Iluc, V. M.; Miller, A. J. M.; Anderson, J. S.; Monreal, M. J.; Mehn, M. P.; Hillhouse, G. L. J. Am. Chem. Soc. 2011, 133, 13055. (11) Gerber, L. C. H.; Schrock, R. R.; Müller, P.; Takase, M. K. J. Am. Chem. Soc. 2011, 133, 18142. (12) Gerber, L. C. H.; Schrock, R. R.; Müller, P. Organometallics 2013, 32, 2373. (13) Addison, A. W.; Rao, T. N.; Van Rijn, J. J.; Veschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349. (14) Reithofer, M. R.; Dobereiner, G. E.; Schrock, R. R.; Müller, P.; Smith, S. J. Organometallics 2013, 32, 2489. (15) (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. (c) Solans-Monfort, X.; Copéret, C.; Eisenstein, O. J. Am. Chem. Soc. 2010, 132, 7750. (d) Solans-Monfort, X.; Copéret, C.; Eisenstein, O. Organometallics 2012, 31, 6812. (16) (a) La, D. S.; Sattely, E. S.; Ford, J. G.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2001, 123, 7767. (b) Pilyugina, T. S.; Schrock, R. R.; Müller, P.; Hoveyda, A. H. Organometallics 2007, 26, 831. (17) (a) Hartung, J.; Grubbs, R. H. J. Am. Chem. Soc. 2013, 135, 10183. (b) Wakamatsu, H.; Blechert, S. Angew. Chem., Int. Ed. 2002, 41, 2403. (c) Kannenberg, A.; Rost, D.; Eibauer, S.; Tiede, S.; Blechert, S. Angew. Chem., Int. Ed. 2011, 50, 3299. (d) Tiede, S.; Schlesiger, D.; Rost, D.; Lühl, A.; Blechert, S. Angew. Chem., Int. Ed. 2010, 49, 3972. (e) Liu, Z.; Rainier, J. D. Org. Lett. 2005, 7, 131. (f) Krause, J. O.; Nuyken, O.; Wurst, K.; Buchmeiser, M. R. Chem.Eur. J. 2004, 10, 777. (g) Katayama, H.; Nagao, M.; Ozawa, F. Organometallics 2003,
HC/D), 1.778 (br s, NMe2C4H2, 4H integrated with previous peak), 0.914 (m, 1H, HC/D). 13C{1H} NMR (C6D6): δ 251.9, 170.5, 166.1, 153.4, 150.2, 138.5, 138.1, 137.9, 137.7, 137.3, 136.7, 136.3, 135.5, 130.1, 129.8, 129.7, 128.9, 128.7, 128.2, 125.6, 117.7, 108.4 (br), 107.2 (br), 54.7, 52.5, 51.8, 50.7, 41.8, 21.8, 21.7, 21.1, 20.0 (br), 14.5 (br) (see Supporting Information for further details). Observation of 5cW′. DCMNBD (4.0 μL, 23 μmol) was added to a stirred solution of W(NAr*)(CH2)(Me2Pyr)(OSiPh3) (21.4 mg, 23.9 μmol) in 2 mL of benzene. After 5 m, NEt3 (3.3 μL, 24 μmol) was added. The volatiles were removed in vacuo, and NMR spectra were obtained in CD3CN.
H NMR (CD3CN): δ 11.583 (br s, 1H, HA), 7.559 (dd, 1H, JHH = 11 Hz, JHH = 18 Hz), 7.377−7.335 (overlapping m, 6H, ArH), 7.285 (t, 6H, JHH = 7 Hz, ArH), 7.069 (dd, 6H, JHH = 1.5 Hz, JHH = 8 Hz), 6.897−6.883 (overlapping signals, 4H), 6.733 (s, 2H, pyrH), 5.605 (dd, 1H, JHH = 11 Hz, JHH = 1 Hz, HG), 5.554 (dd, 1H, JHH = 18 Hz, JHH = 1 Hz, HI), 4.955 (pseudo quintet, 1H, JHH = 4 Hz, HB), 3.866 (dd, 1H, JHH = 9 Hz, JHH = 1 Hz, HE), 3.683 (s, 3H, CO2CH3), 3.427 (s, 3H, CO2CH3), 2.360 (m, 1H, HC/D), 2.252 (s, 6H, MesCH3), 2.140 (m, 1H, HC/D), 2.008 (s, 6H, MesCH3), 1.679 (s, 6H, MesCH3), 1.646 (br s, 6H, Pyr(CH3)2) (see Supporting Information for further details.) 1
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ASSOCIATED CONTENT
S Supporting Information *
General experimental details, crystallographic tables (CIF), NMR data, and other experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 617-253-1596. Fax: 617-253-7670. E-mail: rrs@mit. edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Science Foundation (CHE-1111133) and the Department of Energy (DE-FG02-86ER13564) for research support. The MIT Department of Chemistry thanks the NSF for funds to purchase an x-ray diffractometer (CHE0946721).
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REFERENCES
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dx.doi.org/10.1021/om400844k | Organometallics XXXX, XXX, XXX−XXX
