Article pubs.acs.org/Organometallics
Palladium(II)-Catalyzed Rearrangement and Oligomerization Reactions of cis-Bicyclo[4.2.0]oct-7-ene Katherine Curran,*,† Wilhelm Risse,*,† Mark Hamill,† Patrick Saunders,† Jimmy Muldoon,† Roberto Asensio de la Rosa,† and Incoronata Tritto‡ †
School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland Istituto per lo Studio delle Macromolecole (ISMAC), CNR, Via E. Bassini, 15, I-20133 Milano, Italy
‡
S Supporting Information *
ABSTRACT: In the presence of several Pd(II) catalysts, cis-bicyclo[4.2.0]oct-7-ene (4a) was found to undergo olefin isomerization (“ring walking”) and oligomerization, resulting in the formation of cis-bicyclo[4.2.0]oct-2-ene (4d) and the low-molecular-mass cycloaliphatic oligomers 5a−d, respectively. The catalysts studied are [Pd(NCEt)4][BF4]2 (1), [(η3-allyl)Pd(solv)2][SbF6] (2; solv = CH2Cl2), [(1,10-phenanthroline)Pd(CH3)(NC(CH2)6CH3)][SbF6] (6), and [(2,9-dimethyl-1,10-phenanthroline)Pd(CH3)(NC(CH2)6CH3)][SbF6] (7). Isomerization included the formation of both 4d and the olefinic end groups of 5a−d and ranged from 94% using catalyst 7 to 29% employing catalyst 2. Ab initio and DFT calculations at the LMP2/6-31G** and B3LYP/6-31G** levels show that the thermodynamic stabilities of the bicyclo[4.2.0]octene isomers increase in the order 7-ene 4a < 1(8)-ene 4b < 1-ene 4c ≈ 1(6)-ene 4e ≈ 3-ene 4f < 2-ene 4d. A mechanism of isomerization via subsequent β-hydride eliminations and olefin reinsertions is proposed. These results are in contrast to the reactions of bicyclo[3.2.0]hept-6-ene (3a) catalyzed by 1, 2, and 7 and the reaction of 4a catalyzed by Cp2ZrCl2/ MAO (Cp = η5-C5H5), all of which produced polymers in good yields (73−99%).
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INTRODUCTION A wide range of Pd(II) complexes have been shown to catalyze the olefin addition polymerization of mono- and polycyclic olefins.1−5 For example, high yields of high-molecular-mass polymers of norbornene (eq 1 in Scheme 1) and norbornene
would also undergo addition polymerization with Pd(II) complexes.
Scheme 1. Pd(II)-Catalyzed Addition Polymerizations of Norbornene and 3,3-Dialkylcyclopropene Derivatives
However, high-molecular-mass products could not be isolated. Instead, migration of the carbon−carbon double bond to the 2-position of the six-membered ring occurs, in parallel with the formation of small to moderate quantities of low-molecular-mass oligomeric material. These studies are presented here in this contribution. We are proposing that the reaction mechanism for the double-bond migration involves a series of migratory insertions and β-hydride eliminations. Detailed mechanistic investigations of Pd(II)-catalyzed reactions involving migratory insertion and β-hydride elimination steps have been reported by Brookhart and Johnson et al.8 They showed that Pd(II) complexes with bulky diimine ligands convert ethylene into a polyethylene with a highly branched structure. The process leading to branch formation is called “chain running” (Scheme 2).
derivatives have been achieved with [Pd(NCEt)4][BF4]2 (1),2 [(η3-allyl)Pd(solv)2][SbF6] (2; solv = CH2Cl2),3 and a large number of other catalysts.1,4,5 The bicyclic structure of the monomer is retained during polymerization. Poly(3,3-dialkylcyclopropenes) represent examples of cycloaliphatic polymers that are derived from monocyclic olefins (eq 2 in Scheme 1). These polymers have been synthesized with the aid of Pd(II) complexes bearing (−)-sparteine and 4-substituted 2,2′-isopropylidenebisoxazoline ligands.6 More recently, we extended studies on Pd(II)-catalyzed polymerizations to the four-membered ring of 3a.7 Again, polymerization occurs with retention of the monomer ring structure (eq 3).We anticipated that cis-bicyclo[4.2.0]oct-7-ene (4a) © 2012 American Chemical Society
Received: September 18, 2011 Published: January 19, 2012 882
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Scheme 2. Mechanism for the Pd(II) Diimine Catalyzed Polymerization of Ethylene (Including Chain Running) As Proposed by Brookhart et al.8
Further, McLain et al.9 demonstrated that this class of Pd(II) diimine catalysts yields a 1,3-enchained polymer of cyclopentene (eq 4), which is formed by a sequence of insertion and β-hydride elimination steps.10
Scheme 3. Pd(II)-Catalyzed Rearrangement of cis-Bicyclo[4.2.0]oct-7-ene (4a) to the 2-ene 4d Accompanied by Oligomerization
We present here experimental and computational investigations into the Pd(II)-catalyzed reactions of cis-bicyclo[4.2.0]oct-7-ene (4a).
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RESULTS AND DISCUSSION Pd(II) complexes 1, 2, 6, and 7 have previously been shown to catalyze the polymerization of bicyclo[3.2.0]hept-6-ene (3a) to high-molecular-mass poly(6,7-bicyclo[3.2.0]heptylene) at 20 °C (eq 3).7 By comparison, the six-membered ring homologue 4a does not form a high-molecular-mass product with these catalysts. When the (2,9-dimethyl-1,10-phenanthroline)palladium(II) complex 7 is employed (see entries 1 and 2 in Table 1), the
An amount corresponding to 90−94% of the bicyclic 7-ene 4a undergoes double-bond migration (first two entries of Table 1). Most of this rearrangement, i.e. >80%, occurs at the monomeric level, yielding largely the rearranged bicyclic olefin 4d. The remainder of the rearrangement occurs at the oligomeric level, producing the unsaturated bicyclic end groups of 5a−d (primarily n = 2 and n = 3). A small amount of 4a, i.e. 6−10%, is converted into the saturated (nonolefinic) repeating units of these oligomers of 5a−d. This is the percentage oligomerization given in the first two entries of Table 1. The relative amounts of rearrangement and oligomerization were determined from the intensity of the olefin 1 H NMR signals at δ 5.85−5.72 ppm versus δ 6.12 ppm (7-ene 4a) prior to the reaction (internal standard n-heptane). In the case of the three Pd(II) compounds [Pd(NCEt)4][BF4]2 (1), (η3-allyl)Pd(II) complex 2 generated in situ, and
Table 1. Reactions of cis-Bicyclo[4.2.0]oct-7-ene (4a) with Pd(II) Catalysts 1, 2, 6, and 7 entry a
1 2a 3a 4a 5a 6a 7a 8b
cat.c
rearrangement (%)d
oligomerization (%)e
7 7 1 1 2 2 6 6
94 90 61 62 29 34 55 49
6 10 39 38 71 66 45 51
a Conditions: [4a]/[Pd] = 50/1, [4a] = 0.40−0.94 M, dry CH2Cl2, under N2, T = 20 °C for 16−24 h. [4a] contains 23 mol % n-heptane. b Conditions: in air, CD2Cl2, T = 30 °C. cPd(II) catalyst used. d Percentage of 4a that forms 4d and the unsaturated end groups of 5a−d, determined by 1H NMR spectroscopy. ePercentage of 4a that undergoes oligomerization (chain propagation) via migratory insertion to produce the saturated repeating units of 5a−d: i.e., 100% minus the percentage of rearrangement.
main reaction pathway is the rearrangement to 2-ene 4d,11 with the formation of low-molecular-mass oligomers representing the minor pathway (Scheme 3). The repeating units of these oligomers are potentially linked up at the four-membered ring or at both the four- and six-membered rings, as shown in structures 5a−d.12 883
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bicyclic olefin product formed is the unsubstituted cisbicyclo[4.2.0]oct-2-ene (4d). The relative ratio 4d:exo-8a:exo8b is 58:24:18 (via 1H NMR). This demonstrates that Pd−H species are formed during the reaction and that they are more active toward migratory insertion than the original Pd(II) complex 9. The final product mixture contains [Pd(μ-Cl)(η1:η2-C8H13)]2 (10) in addition to unreacted 9. Pd forms a σ bond to one of the carbon atoms of the chelating ligand in 10, which represents further evidence for the presence of palladium hydride intermediates. We propose that the Pd(II)-catalyzed rearrangement of 4a to the 2-ene 4d in all of the reactions above proceeds via the
(1,10-phenanthroline)Pd(II) complex 6, a moderately large proportion of 4a undergoes rearrangement: i.e., between 29 and 62% (entries 3−8, Table 1). These reactions (Scheme 3) were performed at 20 °C in CH2Cl2 (T = 30 °C, CD2Cl2, in the case of entry 8, Table 1), employing a [4a]/[Pd] mole ratio of 50/1 and reaction times of 16−24 h. In all reactions displayed in Table 1, more than 90% of the bicyclic olefin 4a is consumed within 15 min. With [Pd(NCEt)4][BF4]2, catalyst 1, between 61 and 62% rearrangement occurs (entries 3 and 4, Table 1), while 38−39% of 4a undergoes chain propagation.13 A 43% yield of the 2-ene 4d was isolated from the reaction mixture of entry 4 of Table 1. A fraction corresponding to 19% of 4a (i.e., 62% − 43%) is converted to the bicyclic olefin end groups of 5a−d. The amount of oligomerization is greater with (η3-allyl)Pd(II) catalyst 2, i.e., 66 and 71% (entries 5and 6 of Table 1). GPC analysis (Figure 1) demonstrates that the nonvolatile part of the
migration of the double bond around the cis-bicyclo[4.2.0]octene framework, as shown in Scheme 4. This reaction sequence involves the intermediate formation of several coordinated isomers of cis-bicyclo[4.2.0]octene. We suggest that a series of β-hydride eliminations from β-agostic intermediates8b,c and olefin reinsertions occur. It is noteworthy that the olefin coordinated to palladium in B is a bridgehead olefin, i.e. cis-bicyclo[4.2.0]oct-1(8)-ene (4b) (Scheme 4). Subsequently, the coordinated forms of the 1-ene (C) and 2-ene (D) are formed (Scheme 4). The formation of the 8-substituted 2-enes exo-8b and 5b,d suggests that “ring walking” can also proceed via the coordinated 1(6)-ene E (Scheme 5) and/or 3-ene F. Ab initio and DFT calculations at the LMP2/6-31G** and B3LYP/6-31G** levels show that the 2-ene 4d is the thermodynamically most stable isomer of cis-bicyclo[4.2.0]octene. Table 2 gives the relative electronic energies Erel and relative Gibbs free energies Grel of the most stable conformer of each isomer of cis-bicyclo[4.2.0]octene (Scheme 6). These energies are relative to those of the 7-ene, for which Erel and Grel are set to 0 kcal/mol. The calculated stability of these bicyclic olefin isomers increases in the order 7-ene 4a < 1(8)-ene 4b < 1-ene 4c ≈ 1(6)-ene 4e ≈ 3-ene 4f < 2-ene 4d. The 2-ene 4d is the main bicyclic olefin product formed in the Pd(II)-catalyzed rearrangement of 4a (Scheme 4). Trace quantities of the 3-ene 4f and 1(6)-ene 4e are also produced: i.e., 3−4% and 2−3% relative to the amount of 4d, respectively.17 It is interesting to note that the bridgehead 1(8)-ene 4b is 3.7 kcal/mol lower in Grel than the initial 7-ene 4a.18 This value is based on LMP2/6-31G** calculations. The corresponding difference in free energy Grel at the B3LYP/6-31G** level is −5.0 kcal/mol. By comparison, rearrangement of the five-membered-ring homologue bicyclo[3.2.0]hept-6-ene (3a) to the corresponding bridgehead 1(7)-ene 3b (Scheme 7) represents an uphill process with a significant energy difference: Grel = +10.8 kcal/mol (LMP2/6-31G**) or +9.0 kcal/mol (B3LYP/6-31G**) for 3b (Table 3). The olefin structural unit of 3b is more strained than that of 4b: the calculated dihedral angle (H7C7C1C2) in 3b is 29.1° and deviates to a greater extent from planarity than the olefin unit in 4b, for which the calculated dihedral angle (H8C8C1C2) is 11.4° (Scheme 8). Even though the 2-ene 3d is the lowest energy isomer of bicyclo[3.2.0]heptene (Table 3), it is not observed to be formed when 3a is reacted with Pd(II) complexes 1, 2, 6, and
Figure 1. GPC trace of the oligomers formed using catalyst 2 (entry 6, Table 1) after removal of 4d and the predominant part of dimeric 5a,b. Numbers indicate the degree of oligomerization.
product mixture according to entry 6 of Table 1 (after 48 h at 0.1 Torr, 20 °C) is comprised of a series of oligomers of 5a−d with molar masses lower than 2200 (n < 20), of which trimeric 5a−d represents the main fraction. The 13C NMR spectrum of the oligomers displays two series of signals at δ 131.7−130.5 ppm and at δ 127.0−126.3 ppm. By comparison, the olefinic carbons of 4d produce signals at δ 131.0 ppm (C2) and 126.9 ppm (C3) and those of the methylsubstituted olefins 7-exo-methyl-cis-bicyclo[4.2.0]oct-2-ene (exo-8a) and 8-exo-methyl-cis-bicyclo[4.2.0]oct-2-ene (exo-8b) resonate at δ 131.1 and 126.2 ppm and at δ 130.1 and 126.8 ppm, respectively.14 Accordingly, we assign the olefinic 13C NMR signals of 5a−d to end groups which have the 2-ene structure (Scheme 3). Significant quantities of both 7- and 8-exo-methylbicyclo[4.2.0]oct-2-ene (exo-8a and exo-8b) are obtained when the
bicyclic 7-ene 4a is reacted with (cis,cis-1,5-cyclooctadiene)(methyl)palladium chloride (9) using a low [4a]/[9] ratio of 0.5/1. The stereochemistry of exo-8a,b suggests that also oligomers 5a−d are predominantly exo-linked.15,16 The main 884
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Scheme 4. Proposed Catalytic Cycle for the Formation of cis-Bicyclo[4.2.0]oct-2-ene (4d) from 4a in the Presence of 1, 2, 6, 7, and 9, Involving Successive Migratory Insertion and β-Hydride Elimination Steps
Scheme 5. Proposed Migration of the Olefin Double Bond via the Coordinated 1(6)-Ene and 3-Ene Intermediates E and F, Resulting Also in the Formation of 4d, 8b, and 5b,d with H (R) in the 8-Position of the Bicyclic Structure
7 (at 20 °C). Instead, polymerization to poly(6,7-bicyclo[3.2.0]heptylene) occurs.7
In the case of bicyclic octene 4a, oligomerization is in competition with rearrangement to the 2-ene 4d. Brookhart et al. 885
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Scheme 6. Isomers of cis-Bicyclo[4.2.0]octene
Scheme 7. Isomers of Bicyclo[3.2.0]heptene
Table 3. Relative Gibbs Free Energies Grel (Gas Phase, 298.15 K) and Relative Electronic Energies Erel of Each Isomer of Bicyclo[3.2.0]heptene Determined by Calculations at the LMP2/6-31G** and B3LYP/6-31G** Levelsa
Table 2. Relative Gibbs Free Energies Grel (Gas Phase, 298.15 K) and Relative Electronic Energies Erel of the Lowest Energy Conformers of Each Isomer of cisBicyclo[4.2.0]octene Determined by Calculations at the LMP2/6-31G** and B3LYP/6-31G** Levelsa LMP2/6-31G**
LMP2/6-31G**
B3LYP/6-31G**
bicyclo[4.2.0]octene isomer
Grel (kcal/ mol)
Erel (kcal/ mol)
Grel (kcal/ mol)
Erel (kcal/ mol)
4a (7-ene, boat) 4b (1(8)-ene) 4c (1-ene) 4d (2-ene) 4e (1(6)-ene) 4f (3-ene)
0.0 −3.7 −5.2 −7.9 −5.7 −6.1
0.0 −3.8 −5.4 −8.4 −6.0 −5.7
0.0 −5.0 −7.9 −9.4 −8.6 −7.6
0.0 −5.1 −8.0 −9.7 −8.4 −0.7.1
Grel and Erel of the 7-ene 4a are set as 0 kcal/mol. A complete listing of energy values for all conformers/isomers is given in the Supporting Information.
Erel (kcal/ mol)
Grel (kcal/ mol)
Erel (kcal/ mol)
3a (6-ene, boat) 3a (6-ene, chair) 3b (1(7)-ene) 3c (1-ene) 3d (2-ene) 3e (1(5)-ene)
0.0 3.8 10.8 7.7 −4.1 9.1
0.0 4.0 10.8 8.0 −4.1 9.8
0.0 3.3 9.0 5.2 −5.4 6.0
0.0 3.5 9.5 5.6 −5.2 6.9
Grel and Erel of the 6-ene 3a (boat conformer) are set as 0 kcal/mol.
Scheme 8. Dihedral Angles at Olefin Double Bonds in 4b and 3b
have previously shown that the catalyst resting states in ethylene polymerization with Pd(II)(α-diimine) catalysts are Pd(II)(alkyl)(olefin) complexes.8,19 With reference to their studies, we propose general structure 11 to represent the resting state in the current reactions of 4a (Scheme 9). The fact that the quantity of 4d formed from the Pd(II)catalyzed reactions of bicyclo[4.2.0]oct-7-ene (4a) is of the same order of magnitude as that of the oligomers obtained indicates that the overall activation barrier for the formation of 4d is similar to that of oligomerization. General structure 12 represents other potential resting states which would lead to the formation of oligomer repeating units of 5c,d, which are enchained at both the four- and six-membered rings. In contrast to the rearrangement and oligomerization chemistry occurring with the Pd(II) complexes described above, Cp2ZrCl2 (13) in the presence of methylaluminoxane (MAO) catalyzes polymerization of 4a to give a polymeric product. Poly(cis-bicyclo[4.2.0]octylene) with Mn = 4100 and Mw = 7200 is obtained in 99% yield after 72 h at 70 °C employing a [4a]:[Al]: [Zr] ratio of 250:200:1.20,21 1H NMR and 13C NMR spectra (δ 46.0−22.5 ppm) display very broad signals and indicate an irregular polymer microstructure with predominant 7,8-enchainment. A 1H NMR shoulder at δ 1.3−1.2 ppm suggests that repeating units with linkages between the four- and six-membered rings are also present. These structures result from ring walking during polymerization. The glass transition temperature (Tg) of this product is 223 °C (DSC).
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Grel (kcal/ mol)
a
a
B3LYP/6-31G**
bicyclo[3.2.0]heptene isomer
A reaction mechanism of “ring walking” is proposed for the formation of the 2-ene structure. This involves migration of Pd(II) around the bicyclic hydrocarbon framework. Pd(II) migration is suggested to proceed via an alternating sequence of migratory insertion and β-hydride elimination steps. Further work is currently in progress to map a comprehensive energy profile for this reaction.
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EXPERIMENTAL SECTION
General Procedures. All work involving air- and/or moisturesensitive compounds was performed using standard high-vacuum Schlenk or drybox techniques. 1H and 13C NMR spectra were recorded on a high-resolution 400 MHz Bruker Avance instrument and on both a Varian Inova 500 MHz spectrometer and a Varian Inova 600 MHz spectrometer. Gel permeation chromatographic (GPC) analysis of poly(bicyclo[4.2.0]octylene) (synthesized with the aid of zirconocene dichloride 13) utilized solutions of polymer in o-dichlorobenzene at 145 °C on three Styragel HT (6 × 10−6E-2) columns and one F (2 μm) column. GPC analyses of oligomeric samples were performed on two Oligopore columns and one F (2 μm) column using solutions of oligomer in THF at 35 °C. DSC analyses were performed on a TA Instruments DSC 2010 differential scanning calorimeter. Materials. Dichloromethane was stirred over CaH2 and then distilled under nitrogen and degassed. Heptane was dried in the same way. Toluene was distilled from sodium under a nitrogen atmosphere. cis-Bicyclo[4.2.0]oct-7-ene (4a) was prepared employing a procedure similar to that reported by Liu.22 It contained 23 mol % residual n-heptane, which served as an internal standard for NMR analysis.
CONCLUSION
cis-Bicyclo[4.2.0]oct-7-ene (4a) displays a propensity for rearrangement to cis-bicyclo[4.2.0]oct-2-ene (4d) in the presence of several Pd(II) complexes. Low-molecular-mass cycloaliphatic oligomers are formed simultaneously. 886
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Scheme 9. Two Competing Reaction Pathways for Bicyclic Olefin 4a: (a) Oligomerization and (b) Rearrangement
Dimer Analysis. Pure dimer was isolated by leaving a vial containing the volatile mixture open in the fume hood for 15 h. 1 H NMR (CDCl3, 30 °C, 500 MHz): δ 5.83−5.72 (broad multiplet), 2.72−0.96 ppm (broad multiplet, peak maxima at δ 2.52, 2.15, 1.98, 1.53, and 1.39 ppm). MS: m/z calcd for C16H24 216.1878, found 216.1883. The nonvolatile residue from above was submitted to GPC analysis. A similar procedure was used for the oligomerization of 4a using [(1,10-phenanthroline)Pd(CH3)(NC(CH2)6CH3)][SbF6] (6): extent of rearrangement 49%, extent of oligomerization 51%, determined by 1H NMR spectroscopy as described above. A similar procedure was employed for the reaction of 4a with [(2,9dimethyl-1,10-phenanthroline)Pd(CH3)(NC(CH2)6CH3)][SbF6] (7): 94% rearrangement, 6% oligomerization (entry 1, Table 1). In the case of [Pd(η3-C3H5)(solv)2][SbF6] (2), the catalyst was prepared in situ by the addition of a solution of [Pd(η3-C3H5)Cl]2 (5 mg, 0.014 mmol) in dry dichloromethane (1 mL) to a solution of silver hexafluoroantimonate (10 mg, 0.028 mmol) in dichloromethane (1 mL) in a glovebox. This was stirred for 15 min and then filtered to remove the silver chloride precipitate using a Millipore filter (0.45 μm). A solution of 4a (151 mg, 1.40 mmol) in dry CH2Cl2 (1 mL) was added to the catalyst solution, and the resulting yelloworange solution was stirred overnight. The workup was carried out as described above: extent of rearrangement 29%, extent of oligomerization 71%, determined by 1H NMR spectroscopy as described above. Oligomer Analysis (Nonvolatile Residue). 1H NMR (CDCl3, 30 °C, 600 MHz): δ 5.85−5.72 (broad multiplet), 2.71−0.98 ppm (broad multiplets, partially overlapped, peak maxima at δ 2.36, 2.26, 2.19, 2.11, 1.96, 1.79, 1.57, and 1.45 ppm). 13C NMR (CDCl3, 30 °C, 600 MHz): δ 131.7−130.4 ppm (series of partially overlapped signals with peak maxima at δ 131.7, 131.6, 131.5, 131.4, 131.3, 131.2, 131.0, 130.8, 130.6, and 130.4 ppm), 127.0−126.2 (series of partially overlapped signals with peak maxima at δ 127.0, 126.9, 126.8, 126.7, 126.5, 126.4, 126.3, and 126.2 ppm), large series of signals between δ 48.1 and 21.1 ppm with more than 50 peaks, some of which merge into broad signals, notable maxima are observed at δ 47.1, 41.4, 37.6, 29.8, 27.2, 22.8, and 21.8 ppm. The large number of signals indicate a stereoirregular oligomer microstructure. Reaction of cis-Bicyclo[4.2.0]oct-7-ene (4a) with (cis,cis-1,5Cyclooctadiene)(methyl)palladium Chloride (9). cis-Bicyclo[4.2.0]oct-7-ene (4a; 44 mg, 0.41 mmol) was added to a solution of (cis,cis-1,5-cyclooctadiene)(methyl)palladium chloride (9; 268 mg, 1.01 mmol) in dry dichloromethane (0.4 mL). The yellow solution was stirred at room temperature for 4 h. Vacuum was then applied gently, and a mixture of exo-7-methyl-cis-bicyclo[4.2.0]oct-2-ene (exo-8a), exo-8-methyl-cis-bicyclo[4.2.0]oct-2-ene(exo-8b), and cis-bicyclo[4.2.0]oct-2-ene (4d) was collected in a cold trap. The mole ratio 4d:exo-8a:exo-8b was 58:24:18, via 1H NMR intensities of the olefinic signals at δ 5.83−5.80 and 5.77−5.74 and the signals at δ 1.05 of exo-8a (3H, CH3) and 1.14 ppm of exo-8b (3H, CH3). NMR assignments were made with the aid of gCOSY and HSQC spectroscopy and using tabulated chemical shift increments.25 exo-7-Methyl-cis-bicyclo[4.2.0]oct-2-ene (exo-8a). 1H NMR (CDCl3, 30 °C, 500 MHz): δ 5.76 (m, 2H, H2, H3), 2.58 (m, 1H, H1),
[Pd(NCEt) 4 ][BF 4 ] 2 , [Pd(η 3 -C 3 H 5 )(solv) 2 ][SbF 6 ], [(1,10phenanthroline)Pd(CH3)(NC(CH2)6CH3)][SbF6], [(2,9-dimethyl1,10-phenanthroline)Pd(CH3)(NC(CH2)6CH3)][SbF6], and (cis,cis1,5-cyclooctadiene)(methyl)palladium chloride were prepared according to procedures by Schramm,23 Mathew,3 Curran,7 and Rulke,24 respectively. Zirconocene dichloride was used as received from Boulder Scientific Co. Methylaluminoxane (MAO) was received from Aldrich as a 10 wt % solution in toluene and was dried (50 °C, 3 h, 0.1 mmHg) before use. Reaction of cis-Bicyclo[4.2.0]oct-7-ene (4a) with Pd(II) Catalysts 1, 2, 6, and 7. Dichloromethane (3.8 mL) and cisbicyclo[4.2.0]oct-7-ene (4a; 382 mg, 3.53 mmol) were added to [Pd(NCEt)4][BF4]2 (1; 34 mg, 0.07 mmol) (glovebox). The yelloworange solution that formed was stirred for 1 h at room temperature. The volatile components were collected in a cold trap (liquid nitrogen) under vacuum (0.5 mbar, 48 h, 20 °C). Dichloromethane was removed from the volatile mixture by distillation (atmospheric pressure). The residue was placed in an open sample vial in a fume hood for 4 h to remove any remaining dichloromethane Yield: 165 mg (43%) of 4d. The final product (412 mg total weight) collected in the sample vial also contained 92 mg of dimer and 155 mg of n-heptane, as determined by 1H NMR spectroscopy. A similarly prepared sample was submitted to microdistillation, and small quantities of 4d were isolated at 60 °C (50 Torr) for characterization by mass spectrometry and NMR spectroscopy. MS: m/z calcd for C8H12 108.0939, found 108.0935.
1 H NMR (CDCl3, 30 °C, 500 MHz): δ 5.82 (1 H, m, H3), 5.76 (1 H, m, H2), 2.68−2.66 (1 H, m, H1), 2.60 (1 H, m, H6), 2.21 (1 H, m, H8b), 2.13 − 2.07 (1 H, m, H4a), 2.01−1.96 (1 H, m, H4b), 1.91−1.87 (1 H, m, H7a), 1.76 (1 H, m, H7b), 1.64−1.59 (1 H, m, H8a), 1.55−1.47 ppm (2 H, m, H5a, H5b). 13C NMR (CDCl3, 30 °C, 500 MHz): δ 131.0 (C2), 126.9 (C3), 33.1 (C1), 32.8 (C6), 27.9 (C8), 24.1 (C5), 22.5 (C7), 21.6 ppm (C4). Very small signals at δ 2.45/31.5 and 5.92/127.8 ppm (1H/13C NMR) correspond to four-membered-ring CH2 units of bicyclo[4.2.0]oct-1(6)ene (4e; 2−3%) and olefin CH of bicyclo[4.2.0]oct-3-ene (4f; 3−4%), respectively. A similar reaction was performed employing 100 mg (0.924 mmol) of 4a and 9 mg (0.018 mmol) of 1. 1H NMR spectroscopic analysis showed that 62% of 4a had rearranged while the remaining 38% had oligomerized (integration of signals at δ 5.83−5.80 and 5.77−5.74 ppm after reaction and δ 6.12 ppm prior to reaction (internal standard n-heptane)). Volatile product components were collected at 0.1 Torr and 20 °C (23 h).
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Article
2.17 ppm (m, 1H, H7a), 2.13 (m, 1H, H6), 2.03 (m, 2H, H,4a H4b), 1.77 (m, 2H, H,8a H8b), 1.55 (m, 1H, H5a or H5b), 1.43 (m, 1H, H5a or H5b), 1.05 ppm (d, 3H, CH3). 13C NMR (CDCl3, 30 °C, 125 MHz): δ 131.1 ppm (C2), 126.2 (C3), 40.6 (C6), 35.7 (C8), 30.0 (C7), 29.3 (C1), 22.1 (C5), 21.4 (C4), 20.6 ppm (CH3). exo-8-Methyl-cis-bicyclo[4.2.0]oct-2-ene (exo-8b). 1H NMR (CDCl3, 30 °C, 500 MHz): δ 5.77 (m, 2H, H2, H3), 2.52 (m, 1H, H6),
resulting from ring walking during polymerization. 13C NMR (1,2,4- Cl3C6D3, 95 °C, 400 MHz): δ 46.0−38.5 (broad, C7, C8), 36.5−33.5 (broad, C1, C6), 30.3−26.5 (broad, C2, C5), 24.5−22.5 ppm (broad, C3, C4). Two additional polymerization reactions employing 4a as the monomer and 13/MAO as the catalyst were performed at 70 °C (24 h reaction time) and 50 °C (72 h reaction time) using otherwise similar reaction conditions.19 Computational Methods. Calculations were performed using the Jaguar (version 3.5) program.26 The hybrid density functional theory method B3LYP27 and the pseudospectral local MP2 method LMP228 were employed, and geometry optimizations and energy calculations were carried out using the 6-31G** basis set. Electronic energies were calculated for the isomers and conformers of bicyclo[4.2.0]oct-7-ene and bicyclo[3.2.0]hept-6-ene. Vibrational frequency calculations were performed at both B3LYP/6-31G** and LMP2/6-31G* levels*, and the Gibbs free energies were calculated for the gas phase (1 atm and 298.15 K). See the Supporting Information for the Cartesian coordinates, electronic energies, and free energies of all isomers and conformers of 3a−e and 4a−f.
2.23 (m, 1H, H1), 2.06 (m, 2H, H,8a H4a), 1.97 (m, 1H, H4b), 1.85 (m, 1H, H7a), 1.61 (m, 1H, H5a or H5b), 1.53 (m, 2H, H7b, H5a or H5b), 1.14 ppm (d, 3H, CH3). 13C NMR (CDCl3, 30 °C, 125 MHz): δ 130.1 (C2), 126.8 (C3), 40.5 (C1), 36.8 (C8), 30.5 (C7), 29.0 (C6), 25.3 (C5), 22.2 (C4), 21.2 ppm (CH3). Dry dichloromethane was added to the remaining residue, and this was filtered through a pipet containing Celite into a vial. This solution was found to contain unreacted catalyst and [Pd(μ-Cl)(η1:η2-C8H13)]2 (10). These were separated by preparative TLC on silica plates using dichloromethane as the elution solvent. The product/silica mixture was treated with a mixture of water, dichloromethane, and methanol for 10 min (room temperature). The silica was removed by filtration, and the solvents were removed using a rotary evaporator. The product was dried on a vacuum line for 6 h at room temperature. Anal. Calcd (found): C, 38.27 (37.76); H, 5.23 (5.29). 1H NMR (CDCl3, 30 °C, 600 MHz): δ 6.04 (2H, dt, J1,2 = 6.2 Hz, J1,3a = J1,3b =
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ASSOCIATED CONTENT
S Supporting Information *
Figures, text, and tables giving experimental details for the preparation of 4a and oligomers for GPC analysis, enchainment modes of 5c,d, 1H, 13C, and HSQC NMR spectra of 4d and 1 H, 13C, and gCOSY NMR spectra of oligomeric 5a−d, 1H and 13 C NMR spectra of poly(7,8-bicyclo[4.2.0]octylene), GPC traces of 5a−d samples, data for zirconocene-catalyzed polymerizations of 4a, and computational details for all isomers and conformers of 3a−e and 4a−f. This material is available free of charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENTS K.C. acknowledges funding from the Irish Research Council for Science, Engineering and Technology (IRCSET) with gratitude. We thank Ms. Geraldine Fitzpatrick and Dr. Yannick Ortin for NMR spectroscopic analysis and staff of the Istituto per lo Studio delle Macromolecole (ISMAC) in Milan for polymer NMR, GPC, and GC-MS analysis. We also thank Prof. Hans-Herbert Brintzinger, University of Konstanz (Germany), Prof. Gerhard Fink, Max Planck Institut Mülheim (Germany), and Dr. Noel Fitzpatrick, University College Dublin, for valuable advice and assistance.
8.7 Hz, H2), 5.56 (2H, ddd, J1,2 = 6.2 Hz, J1,8a = 8.6 Hz, J1,8b = 2.2 Hz, H1), 3.71 (2H, dt, J5,4a = 7.4 Hz, J5,6a = J5,6b = 3.3 Hz, H5), 2.50−2.41 (4H, m, H,4a H8a or H8b), 2.33−2.30 (2H, m, H8a or H8b), 2.22−2.11 (4H, m, H3a,b), 2.00−1.93 (4H, m, H7a,7b), 1.81−1.75 (2H, m, H6b), 1.02 (2H, dd, J4b,4a = 14.6 Hz, J4b,3a = 7.4 Hz, H4b), 0.89−0.70 ppm (2H, m, H6a). 13C NMR (CDCl3, 30 °C, 600 MHz): δ 105.5 (C2), 101.0 (C1), 57.1 (C5), 40.2 (C4), δ 35.6 (C6), 28.9 (C8), 27.0 (C7), 26.3 ppm (C3). These assignments were made with the aid of gCOSY, TOCSY, and HSQC spectra and using the Karplus equation.25b Polymerization of cis-Bicyclo[4.2.0]oct-7-ene with Zirconocene Dichloride (13)/MAO. A 50 mL flask with a side arm was charged with a solution of MAO in toluene (4 M solution, 1 mL). The monomer 4a (500 mg, 4.63 mmol) was added using a syringe. A solution of zirconocene dichloride (13) in toluene (6 mM solution, 3 mL) was added and the mixture stirred in an oil bath at 70 °C for 72 h. Acidified ethanol was added to quench the reaction and to precipitate the polymer. The polymer was collected by filtration and dried under vacuum at 50 °C for 5 h. Yield: 500 mg (99%). Mn (GPC) = 4100; Mw = 7200. Tg (DSC) = 223 °C. Main Structural Motif. 1H NMR (C6D5Br, 25 °C, 300 MHz): δ 3.17−1.18 (broad m, partially overlapped, peak maxima at
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REFERENCES
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δ 2.32 (4H, H1, H6, H7, H8) and 1.81 ppm (8H, H2a, H2b, H3a, H3b, H4a, H4b, H5a, H5b), broad signals and a shoulder at δ 1.2−1.3 ppm suggest an irregular polymer microstructure including structures 888
dx.doi.org/10.1021/om200877p | Organometallics 2012, 31, 882−889
Organometallics
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δ 2.02, 2.35, and 2.53 ppm, which are assigned to the allylic methylene and bridgehead protons (adjacent to the olefinic protons) of 5a−d (see the Supporting Information). (15) The two 1H NMR doublets at δ 1.05 and 1.14 ppm showcorrelations withthe 13C NMR signals (HSQC spectroscopy) at δ 20.6and 21.2 ppm, respectively, and are assigned to the two exo-linked CH3 groups of exo-8a and exo-8b, respectively.By comparison, the methyl group of endo-8b resonates significantly upfield at δ 0.84/16.7ppm (1H/13C NMR): Bogle, X. S.; Leber, P. A.; McCullough, L. A.; Powers, D. C. J. Org. Chem. 2005, 70, 8913.
1595. (b) Jung, I. G.; Lee, Y. T.; Choi, S. Y.; Choi, D. S.; Kang, Y. K.; Chung, Y. K. J. Organomet. Chem. 2009, 694, 297. (c) Thirupathi, N.; Amoroso, D.; Bell, A.; Protasiewicz, J. D. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 103. (d) Myagmarsuren, G.; Lee, K. S.; Jeong, O. Y.; Ihm, S. K. Polymer 2005, 46, 3685. (e) Funk, J. K.; Andes, C. E.; Sen, A. Organometallics 2004, 23, 1680. (f) Taniélian, C.; Kiennemann, A.; Osparpucu, T. Can. J. Chem. 1979, 57, 2022. (g) Schultz, R. G. J. Polym. Sci., Part B: Polym. Lett. 1966, 4, 541. (5) Vinylic addition polymerizations of norbornene can also be carried out with catalysts based on Ti,5a,b Zr,5c,d Hf,5e Cr,5f Co,5g−k and Ni.5l−s References below are selected examples. A polymer containing 2,7linkages was obtained with one of the Zr-based catalysts:5c (a) Sartori, G.; Ciampelli, F. C.; Camelli, N. Chim. Ind. (Milan) 1963, 45, 1478. (b) Tsujino, T; Saegusa, T.; Furukawa, J. Makromol. Chem. 1965, 85, 71. (c) Karafilidis, C.; Angermund, K.; Gabor, B.; Ska, A. R.; Mynott, R. J.; Breitenbruch, G.; Thiel, W.; Fink, G. Angew. Chem., Int. Ed. 2007, 46, 3745. (d) Kaminsky, W.; Bark, A.; Arndt, M. Makromol. Chem., Macromol. Symp. 1991, 47, 83. (e) Brekner, M. J.; Decker, H.; Osan, F. (Hoechst Inc.) Eur. Patent 683,797, 1994. (f) Peucker, U.; Heitz, W. Macromol. Chem. Phys. 2001, 202, 1289. (g) Chen, J. X.; Huang, Y. B.; Li, Z. S.; Zhang, Z. C.; Wei, C. X.; Lan, T. Y.; Zhang, W. J. J. Mol. Catal. A: Chem. 2006, 259, 133. (h) B. L. Goodall, B. L.; L. H. McIntosh, L. H. III; L. F. Rhodes, L. F. Macromol. Symp. 1995, 89, 421. (i) Alt, F. P.; Heitz, W. Macromol. Chem. Phys. 1998, 199, 1951. (j) Yasuda, H.; Y. Nakayama, Y.; Sato, Y. J. Organomet. Chem. 2004, 689, 744. (k) Tarte, N. H.; Woo, S. I.; Cui, L. Q.; Gong, Y. D.; Hwang, Y. H. J. Organomet. Chem. 2008, 693, 729. (l) Kong, Y.; Cheng, M.; Ren, H.; Xu, S.; Song, H.; Yang, M.; Liu, B.; Wang, B. Organometallics 2011, 30, 1677. (m) He, F. P.; Chen, Y. W.; He, X. H.; Chen, M. Q.; Zhou, W. H.; Wu, Q. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 3990. (n) Wang, R. P.; Groux, L. F.; Zargarian, D. Organometallics 2002, 25, 5531. (o) Wang, H. Y.; Jin, G.-X. Eur. J. Inorg. Chem. 2005, 9, 1665. (p) Gao, H.; Guo, W.; Bao, F.; Gui, G.; Zhang, J.; Zhu, F.; Wu, Q. Organometallics 2004, 23, 6273. (q) Huang, Y. B.; Tang, G. R.; Jin, G. Y.; Jin, G.-X. Organometallics 2008, 27, 259. (r) Mast, C.; Krieger, M.; Dehnicke, K.; Greiner, A. Macromol. Rapid Commun. 1999, 20, 232. (s) Barnes, D. A.; Benedikt, G. M.; Goodall, B. L.; Huang, S. S.; Katamarides, H. A.; Lenhard, S.; McIntosh, L. H.; Selvy, K. T.; Shick, R. A.; Rhodes, L. S. Macromolecules 2003, 36, 2623. (t) Maezawa, H.; Matsumoto, J.; Aiura, H.; Asahi, S. (Idemitsu Kosan Ltd., Jpn.) Eur. Pat. Appl. 445,755 A2, 1991. (6) (a) Rush, S.; Reinmuth, A.; Risse, W. J. Am. Chem. Soc. 1996, 118, 12230. (b) Rush, S.; Reinmuth, A.; Risse, W. Macromolecules 1997, 30, 7375. (c) Mathews, N.; Häger, H.; Rush, S.; Risse, W. Polym. Mater. Sci. Eng. 1999, 80, 435. (7) Curran, K.; Risse, W.; Boggioni, L.; Tritto, I. Macromol. Chem. Phys. 2008, 209, 707. (8) (a) Johnson, L. K.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414. (b) Shultz, L. H.; Tempel, D. J.; Brookhart, M. J. Am. Chem. Soc. 2001, 123, 11539. (c) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 2000, 122, 6686. (9) McLain, S.; Feldman, J.; McCord, E. F.; Gardner, K. H.; Teasley, M. F.; Coughlin, E. B.; Sweetman, K. J.; Johnson, L. K.; Brookhart, M. Macromolecules 1998, 31, 6705. (10) Similar cyclopentene polymers are also formed with zircononcene complexes; however, the molecular weights are generally lower. See: (a) Kaminsky, W.; Bark, A.; Arndt, M. Makromol. Chem., Macromol. Symp. 1991, 47, 83. (b) Collins, S.; Kelly, W. M. Macromolecules 1992, 25, 233. (c) Kelly, W. M.; Taylor, N. J.; Collins, S. Macromolecules 1994, 27, 4477. (11) Bicyclo[4.2.0]oct-7-ene(4a) has been reported to rearrange to the 2-ene 4d in the presence of Fe2(CO)9: Cann, K.; Barborak, J. C. Chem. Commun. 1975, 190. (12) See the Supporting Information for the individual structures of 5c,d. (13) GPC analysis shows that the extent of oligomerization and the oligomer distribution obtained with 1 can be described by a Schulz− Flory distribution (see the Supporting Information). (14) Furthermore, a gCOSY spectrum of the oligomers 5a−d displays correlations between the olefinic signals and the signals at
(16) We have previously shown that the polymer of bicyclo[3.2.0] hept-6-ene (3a) is exo-enchained (eq 3).7 (17) The four-membered-ring CH2 protons of 4e give rise to an 1H NMR signal at δ 2.45 ppm (m), which correlates with a 13C NMR signal at δ 31.5 ppm. The olefin CH of 4f resonates at δ 5.92/127.8 ppm (1H/13C NMR). Literature on 4e,f: (a) Baldwin, J. E.; Chang, G. E. C. J. Org. Chem. 1982, 47, 848. (b) Casadevall, E.; Largeaux, C.; Mareau, P. Bull. Soc. Chim. Fr. 1968, 4, 1514. (18) Bridgehead compound 4b contains a trisubstituted double bond, in comparison to the disubstituted double bond in 4a. The energy gained upon higher double bond substitution more than compensates for the torsional and angular strains. (19) Ittel, S. D.; Johnson, L. K. Chem. Rev. 2000, 100, 1169. (20) More details on the zirconocene-catalyzed reactions of 4a are available in the Supporting Information. (21) Weak 1H NMR signals at δ 5.7−5.8 and signals at 0.95−1.10 ppm indicate the presence of olefin and Me end groups, respectively. They suggest β-H elimination or β-H transfer to monomer and chain transfer to Al and the latter as the predominant mode of chain transfer. Accordingly, the product Mn is significantly lower than the theoretical minimum Mn of 27 000 (for [4a]:Zr = 250). For references on chain transfer to Al in zirconocene/MAO-catalyzed olefin polymerizations see: (a) Lieber, S.; Brintzinger, H. H. Macromolecules 2000, 33, 9192. (b) Byun, D.-J.; Kim, S. Y. Macromolecules 2000, 33, 1921. (c) Leino, R.; Luttikhedde, H. J. K.; Lehmus, P.; Wilén, C.-E.; Sjöholm, R.; Lehtonen, A.; Seppälä, J. V.; Näsman, J. H. Macromolecules 1997, 30, 3477. (d) Barsties, E.; Schaible, S.; Prosenc, M. H.; Rief, U.; Roll, W.; Weyand, O.; Dorer, B.; Brintzinger, H. H. J. Organomet. Chem. 1996, 520, 63. (22) Procedure similarto that reported in: Liu, R. S. H. J. Am. Chem. Soc. 1967, 89, 112 (see the Supporting Information). (23) Schramm, R. F. J. Chem. Soc., Chem. Commun. 1968, 898. (24) Rulke, R. E.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.; van Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769. (25) (a) Kalinowski, H.-O.; Berger, S.; Braun, S. Carbon-13 NMR Spectroscopy; Wiley: Chichester, U.K., 1988. (b) http://www. jonathanpmiller.com/Karplus.html. (26) Jaguar 3.5 Computational Program; Schrödinger, Inc., Portland, OR, 1998. (27) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. 1988, 37, 785. (28) Murphy, R. B.; Beachy, M. D.; Friesner, R. A.; Ringnalda, M. N. J. Chem. Phys. 1995, 103, 1481.
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