Some of the most important industrial processes using homogeneous transition metal catalysts are the hydroformylation of olefins and the alkoxycarbonylation.[9] The latter reaction converts the olefin into an ester by employing carbon monoxide in the presence of an alcohol and a suitable catalyst. In this regard, the methoxycarbonylation of ethylene to methyl propionate and polyketones, respectively are remarkable reactions (Scheme 2). This reaction is catalyzed by diphosphine based Pd(II) complexes. Polyketones (AKROTEK® PK, AKRO-PLASTIC GmbH) are used inter alia as thermoplastic materials in mechanical engineering,[10] methyl propionate (MP) is a precursor of methyl methacrylate (MMA), which in turn is used for the production of acrylic plastic like Plexiglas®.[11]
Scheme 2. Carbonylation reaction to methyl methacrylate and thermoplastics.
The carbonylation reaction outlined in Scheme 2 is sensitive for the kind of catalyst precursor used.
The nature of phosphine ligands employed, can direct the reaction either to polyketones or methyl propionate.[12] It was proposed that cationic Pd(II) catalysts in the presence of excess monodentate phosphine (e.g. PPh3) and Brønsted acids of weakly coordinating anions favor hydroesterfication
of ethylene to afford methyl propionate, whereas bidentate ligands almost exclusively lead to polyketones. Further investigations showed that the chemoselectivity of the reaction originates from the bite angle and the steric bulk of the bidentate ligand.[13,14]
1.2.1 Ethylene / CO-Copolymerization
The development of new bidentate phosphines for specific catalytic applications has been subject of research for several decades. Since the late 1980’s Shell developed new bidentate phosphine ligands which lead selectively to polyketones. Drent et al. found that among the ligands of the series Ph2P(CH2)nPPh2 (n = 1-6), the one with n = 3 is the most effective in terms of both productivity and molecular weight of the polymers. It was proposed that the energy barrier for insertion reactions is lower for a diphosphine with a natural bite angle close to 90°, which is better met by the (CH2)3 backbone (n=3).[12,15]
Scheme 3. Diphosphine ligand (dppp) which is selective for polyketone production.
1.2.2 Methyl Propionate Formation
Later, Drent found that the substitution of the phenyl groups by sterically demanding tert-butyl groups (dtbpp) lead to a change in selectivity of the reaction in favor of the production of methyl propionate (Scheme 4) with 97.4 % yield at 120 °C, 40 bar, CO:ethylene = 2:1, TOF = 25.000 h-1 (turn over frequency).[13] The most effective ligand up to now was developed in 1990. The diphosphine ligand (dtbpx) consists of an -xylene backbone and sterically demanding tert-butyl substituents.[16] The catalyst is generated in-situ from Pd(dba)2, the dtbpx ligand and methanesulfonic acid. The production process provides methyl propionate with a high production rate of TOF = 50.000 h-1 and selectivities greater than 99.9 % under mild conditions (80°C, 10 bar, pressure, CO:ethylene, 1:1).
Scheme 4. Diphosphine ligands (dtbpp and dtbpx) which are selective for methyl propionate production.
Recently, Lucite International (Mitsubishi Rayon) developed a MMA process which exhibits as a key step the low-pressure ethylene carbonylation to methyl propionate with the homogeneous dtbpx diphosphine based catalytic system. This is followed by reaction of the methyl propionate formed with formaldehyde in the gase phase over a fixed-bed heterogeneous catalyst in the presence of methanol to produce MMA and water. This so-called Alpha process is claimed to reduce the total production cost by 40 % and operates at mild conditions. It was first commercialized at the Alpha 1 plant in Singapore in 2008 with an annual capacity of 250kte.[17]
The mechanism of the methoxycarbonylation of ethylene as outlined in Scheme 5 was studied intensively by NMR and infrared spectroscopic techniques.[12,18,19] In principle, there are two possible pathways widely accepted today: The hydride cycle (A) involving a Pd(II) hydride species and the methoxy cycle (B) involving a methoxy Pd species.
Scheme 5. Mechanism of the methoxycarbonylation of ethylene to methyl propionate (v, MP) or polyketones (vii, PK), respectively.
The hydride cycle (A) starts with the formation of the Pd-H (i) species in which ethylene is inserted (ii). Further coordination (step ii-iii) and migratory-insertion of a CO molecule (step iii-iv) into the Pd-C bond of the Pd-ethyl complex (ii) generates the Pd-acyl complex (iv). Subsequent inter- or intramolecular nucleophilic attack of methanol at the Pd-acyl species irreversibly leads to the formation of methyl propionate (v, MP) under regeneration of the Pd-hydride species (i).
Depending on the diphosphine ligand of the Pd(II) catalyst, instead of competing methanolysis termination, a further ethylene molecule can coordinate to the Pd-acyl complex (iv) forming vi and thus leading via subsequent chain growth to polyketones (vii, PK).
Starting point of the methoxy cycle (B) is the CO insertion to the Pd-OMe (viii) bond leading to the formation of the methoxy-carbonyl complex (ix). Subsequent coordination and insertion of ethylene in combination with the methanolysis yields the final product methyl propionate (v, MP).
For both mechanisms all individual steps have been observed in model compounds and depending on the catalytic system and reaction conditions both mechanisms might in principle occur.
However, up to now it seems that the hydride mechanism is the major operative pathway, which is based on mechanistical evidence.[20,21,22 ]
The reaction rate and chemoselectivity towards methylpropionate are mainly controlled by steric factors and thus the bite angle and the steric demand of the bidentate ligands are important issues.
DFT investigations (density functional theory) showed a strong influence of the steric bulk and the bite angle on the methanolysis as rate-determing step. The high electron density caused by the -donor diphosphine stabilizes the 14e--η2-acyl intermediate (iv) and suppresses at the same time the formation of the 16e- complex (e.g. the ethylene complex (vi)). By increasing the steric demand of the catalyst, the resting states of the preceding insertion of the olefin are destabilized, thus easing the rate-determining step. The bulky tert-butyl groups restrict the available space around the other two coordination sites of the square-planar Pd(II) center and consequently inhibit the ethylene coordination forming vi which leads, after subsequent insertion into the resulting Pd-acyl complex, to a growing chain (vii). The chemoselectivity of this reaction is mainly a steric problem. After insertion of just one molecule of each monomer, fast methanolysis of the Pd-acyl iv intermediate occurs with selective formation of methyl propionate (v, MP).[23]
The key role in governing the selectivity of the carbonylation of ethylene is the steric hindrance of the catalytic system. The steric environment in metal complexes is often considered to be more important than electronic factors in determining the structure and the reactivity of complexes. In this regard the steric demand of phosphines can be quantified by the concept of Tolman’s cone angle θ (Figure 1).[24]
Figure 1. Tolman’s cone angle θ and half cone angle φ of diphosphine ligands.
The cone angle is defined as the apex angle of a cylindrical cone centred 2.28 Å from the center of the phosphorus atom and just touching the van der Waals radii of the outermost atoms of the
molecule. We have transferred the principle of Tolman’s cone angle to diphosphine ligands and quantified the catalytic pocket around the palladium center by measurement of the half cone angle
. For this purpose we measured the angles between a vector V defined by the center of the two phosphorus atoms and the Pd(II) center, and four vectors R1, R2, R3, and R4 defined by the Pd(II) center and the substituents on the phosphorus moiety (see right side of Figure 1). The arithmetic average of these angles is the ‘half cone angle’ (for details cf. Exp. Section).[25]