ARTICLE pubs.acs.org/Organometallics
DFT Study of Pd(PMe3)/NMe3-Catalyzed Butadiene Telomerization of Methanol Amir Jabri and Peter H. M. Budzelaar* Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada, R3T 2N2
bS Supporting Information ABSTRACT:
A detailed DFT study of Pd(PMe3)/NMe3-catalyzed butadiene telomerization of methanol predicts that the rate- and selectivitydetermining step of the catalytic cycle is the external nucleophilic attack of methoxide on the π-allyl-Pd fragment of the cationic (PMe3)Pd(η3:η2-octadienyl)þ (3a). A crucial factor affecting the regio- and chemoselectivity appears to be the equilibrium between the chelated 3a and the dechelated species (PMe3)Pd(η3-octadienyl)(L0 )þ (L0 = butadiene 3x/3y, or PMe3 3p): 3a is predicted to convert highly selectively to the linear telomer 8-methoxy-1,6-octadiene, whereas dechelated 3x/3y and 3p should be the major sources of both the branched telomer 3-methoxy-1,7-octadiene and byproduct 1,3,7-octatriene (via a novel β-hydrogen elimination mechanism). Insights into base effects were gained by comparing LiOMe and NMe3 as cocatalysts.
’ INTRODUCTION Palladium-catalyzed telomerization is the simultaneous dimerization/functionalization of a 1,3-diene with an acidic partner (EX = HOR, HNR2, etc.) to give mainly linear octadienes as well as some branched octadienes and 1,3,7-octatriene as major byproducts (Figure 1). Since its discovery 40 years ago,1 palladiumcatalyzed telomerization has attracted significant interest due to its robustness and versatility in the production of a wide variety of valuable products.2 Telomerization is a highly “green” and 100% atom-efficient reaction, and recently several highly efficient telomerization processes were developed for upgrading readily available crude biofeedstocks (e.g., for starch,3a glycerol,3b or myrcene3c), which have the advantage of not requiring any costly purifications or waste disposal due to the high robustness and selectivity of the palladium catalyst. Highly active palladium catalysts, supported by monodentate phosphine ligands in combination with excess trialkylamine base cocatalysts, have been recently commercialized for the telomerization of butadiene with both methanol4a (50 000 mta) under homogeneous conditions and water (5000 mta) under biphasic conditions.4b While current catalysts may be adequate for production of fine chemicals, large improvements in catalyst performance are needed for more widespread applications in large-scale processes. Detailed mechanistic information may aid progress in the rational development and commercial application of new and more efficient telomerization catalyst systems for a larger variety of products. The main issues here are (a) regioselectivity, i.e., formation of the desired linear telomer (e.g., 8-methoxy-1,6-octadiene, 8-MOD) vs the undesired branched r 2011 American Chemical Society
Figure 1. Butadiene telomerization.
isomer (e.g., 3-methoxy-1,7-octadiene, 3-MOD) and byproduct 1,3,7-octatriene (OT), and (b) catalyst productivity. Previous studies of the mechanism by Jolly et al. have led to the generally accepted basic catalytic cycle shown in Figure 2, which was deduced from the isolation and characterization of several catalytic intermediates (complexes 2, 3, and 4).5 These welldefined species were confirmed to lie on the proposed catalytic pathway by following their transformations via low-temperature NMR. The catalytic mechanism appears to involve the following successive steps (for methanol): Step 1: Oxidative butadiene coupling at bis-butadiene Pd(0) complex 1 generates Pd(II)(η3:η1-octadienediyl) species 2. Step 2: Protonation of 2 generates cationic Pd(II)(η3:η2octadienyl)þ complex 3. Received: September 6, 2010 Published: March 04, 2011 1374
dx.doi.org/10.1021/om1008617 | Organometallics 2011, 30, 1374–1381
Organometallics
Figure 2. Simplified proposed mechanism for the butadiene telomerization of methanol.4
ARTICLE
cationic intermediate 3 in telomerization catalysis,8 even at a moderately basic pH.9 As part of our own research interest in the development of accurate descriptors10a for predicting ligand electronic10b and steric effects on catalytic performance, we undertook an initial DFT study of the energy profile in telomerization catalysis via the generally accepted mechanism (now taking into account the important solvent effects). We chose to carefully evaluate the Pd(PMe3) catalyst with a trialkylamine cocatalyst (modeled by NMe3) for the full catalytic cycle, because the most detailed experimental mechanistic data for the complete catalytic cycle have been previously reported for this system.5 This choice allowed us to make direct comparisons between our theoretical model and experimental data. A few key geometries were also evaluated with the LiOMe cocatalyst to gain mechanistic insight into base effects on the rate-limiting step(s).
’ COMPUTATIONAL METHODS
Figure 3. Proposed mechanisms for the formation of the major undesired byproducts.
Step 3: Nucleophilic attack of methoxide on 3 generates the MOD still chelated to Pd(0) (complex 4). Step 4: Olefin exchange of the chelating MOD ligand of 4 with two butadiene molecules regenerates 1 and completes the catalytic cycle. Very recently, a study of the factors controlling catalyst selectivity was reported by Beller et al., which strongly suggests that the chelated intermediate 3 is in equilibrium with the dechelated intermediate 30 (Figure 3, L0 = butadiene or phosphine).6 Reaction of nucleophiles with such dechelated intermediates are poorly selective and produce more of the undesired branched telomer products. The octatriene byproduct has commonly been proposed to form via the deprotonation of the β-hydrogen from intermediate 3 by the base cocatalyst (as depicted in Figure 3), but as of yet no experimental evidence is available to support this hypothesis. To a much smaller extent, the byproducts 4-vinylcyclohexene and polybutadiene also form in a thermal reaction during telomerization catalysis; this can be suppressed in practice by using highly active catalysts and by adding inexpensive stabilizers. A DFT study of telomerization catalysis, comparing the popular PPh3 and IMes ligands, has recently appeared in the literature.7 Only the transition states for step 1, 1 f 2, were examined in detail. However, no energy profiles were calculated for the remaining steps to determine which steps are rate-limiting and what the relative energies of the catalytic intermediates are. In the present work we are focusing mainly on steps 2 and 3, which involve reactions of charged species and are therefore more difficult to model. Solvation is expected to play an important role, as polar solvents are experimentally established to stabilize cationic allyl-Pd complexes that are closely related to
Initial conformational searches were performed using the semiempirical PM3 method11 as implemented in Spartan’06.12 The located lowenergy conformers were taken as input structures for geometry optimization with Turbomole13 coupled to PQS OPTIMIZE14 as an external optimizer. Geometries were optimized at the b3-lyp15 level, using a small-core pseudopotential for Pd.16 Vibrational analyses (analytical second derivatives) were carried out to check the nature of all stationary points and also to calculate the thermal corrections (enthalpy and entropy) for 273 K, 1 bar, gas phase.17 Entropies of dissolved species were scaled by 0.67 to reflect the reduced amount of freedom of such species compared to the gas phase.18 Improved single-point energies were calculated at the b3-lyp/TZVPP19 level. Solvent polarity corrections to the gas-phase energies were modeled with COSMO,20 using a dielectric constant of 33 (methanol). These solvent-corrected electronic energies were then combined with the scaled TZVP thermal corrections to produce the final free energy values mentioned in the text. For lists of energies and optimized geometries, see the Supporting Information.
’ RESULTS AND DISCUSSION Step 1: Butadiene Coupling. As mentioned in the Introduction, the first proposed step of the catalytic cycle is the oxidative coupling of the two butadiene ligands at Pd(0) in 1 to form the η3:η1-octadienediyl ligand on Pd(II) in 2 (Figure 2). While 1 could not be isolated or characterized experimentally, several closely related thermally stable Pd(0)(PR3) complexes of chelating diolefins are reported to catalyze butadiene telomerization.5,21 The product 2d has been isolated in high yields from Pd(0) diolefin precursors at temperatures from -80 to -20 °C.5 Isostructural nickel analogues of 1 have been studied extensively and are established to exhibit low barriers for butadiene coupling to generate structural analogues of 2.22 A relatively low barrier for the butadiene coupling reaction by 1 can also be inferred indirectly from the typical pseudo-zeroorder rate dependence of catalysis on butadiene concentration.2 While 2d is thermally robust in the presence of a high concentration of butadiene, it is known to decompose rapidly at ambient temperature in the absence of excess butadiene ligand.5 This behavior suggests that 2d is not highly stable due to a low barrier for interconversion between 2 and 1. As 1 can readily dissociate one butadiene ligand to generate an unsaturated intermediate, rapid decomposition is expected in the absence of excess free butadiene. The idea that 2 can revert to 1 by C-C cleavage is also supported by the observation that the reaction of 2d with excess 1375
dx.doi.org/10.1021/om1008617 |Organometallics 2011, 30, 1374–1381
Organometallics
Figure 4. Favored butadiene coupling pathways.
Figure 5. Selected geometries for butadiene coupling, bond lengths in Å.
phosphine results in the formation of free butadiene and a palladium-butadiene bis-phosphine complex.5a To model the energy profile of this reaction using DFT, we evaluated all six possible conformational isomers of 1 (1a-1f, see SI), similar to the structures reported by Huo.7 Each can lead via its own transition state to six conformational isomers of intermediate 2 (2a-2f); calculated barriers for these reactions are low (1000
3x/3y
1.2 0.01
3p 6
concentration. One recently proposed mechanistic explanation for this observation discussed in the Introduction is that nonchelated analogues of 3 would have lower regioselectivity upon nucleophilic attack (Figure 3).6 Experimental confirmation for this hypothesis is found in the reaction of methoxide with an isolated bis-phosphine analogue of 3, which produces a significantly higher ratio of the branched telomer than 3a.6 We evaluated the regioselectivity generated from the two plausible nonchelating species (Figures 9 and 11), one with an additional phosphine ligand (3p) and one with an extra butadiene (3x/3y; see SI for full details). These nonchelating isomers are readily formed from 3* during catalysis and are always present in small amounts depending on butadiene and phosphine concentrations. The results of our calculations (Figure 10, Table 1) show a significantly lower selectivity by the opened isomers 3p and 3x/ 3y than 3a. We tentatively attribute the higher regioselectivity of 3a to the differential development of ring strain during nucleophilic attack on the chelated octadienyl ligand. In summary, both experimental reports (high regioselectivity from pure 3, low regioselectivity from 3p) and our own calculations suggest that the branched telomer 3-MOD derives mainly from nonchelated isomers of 3 (3x/3y, 3p, Table 1), which are favored at high butadiene and/or phosphine concentrations. Thus, lowering the concentration of butadiene and phosphine ligands logically leads to higher selectivity, as is established experimentally. Proper choice of phosphine ligand steric and electronic properties may also further disfavor the undesired intermediate 3p, which is one important consideration for rational ligand design and catalyst development for the future. The main reason for high selectivity by 3 is likely to be the ring strain, which lowers the barrier to formation of the linear product and significantly raises the barrier to the formation of the branched product. Step 4: Olefin Exchange. Intermediate 4 regenerates intermediate 1 by exchanging the coordinated MOD ligand for two molecules of butadiene, completing the catalytic cycle. As mentioned earlier, 1 cannot be isolated, but 4a is instantly converted to 2d upon exposure to butadiene at -10 °C.5b The two possible mechanistic pathways for re-formation of intermediate 1 from 4 are via either associative or dissociative olefin exchange at the metal center. As we could not locate any stable four-coordinatePd(0) intermediates, we rule out associative pathways. Stable Pd(0) intermediates along a dissociative exchange pathway from 4a to 1a were located (Figure 12): two-coordinate η1-MOD complex 5a (-4.7 kcal/mol), mixed η1-MOD/butadiene complex 5b (-3.1 kcal/mol), and monobutadiene complex 5c (-6.5 kcal/mol). The calculated energy barrier for dissociative exchange was thus very low (
