Evaluation of ligand modified palladium catalysts in the wacker oxidation of alkenes
The industrial application of Wacker oxidation of terminal olefins in aqueous aerobic mixtures with PdCl2 and CuCl/CuCl2 has largely been limited to shorter chain alkenes, that is, ethylene. As the alkene chain length increases, so do the challenges that render the reaction inapplicable for large scale production. Longer chain alkenes tend to isomerize due to the limited solubility in organic-aqueous mixtures. More so, the use of co-oxidants such as CuCl or CuCl2 in stoichiometric amounts results in the formation of toxic chlorinated by-products which make the system corrosive. Pd0 aggregation from the PdII active state, is also pertinent in these reactions hence the use of large amounts of a co-oxidant. Small TONs and TOFs have subsequently been reported. As one of the approaches to curb these challenges, ligand support and modification has recently been viewed with interest because it promises efficient stabilization of Pd0, wherein the efficiency of O2 to re-oxidize the Pd0 species is relied upon thereby avoiding Pd0 aggregation. Ligand support can also be used to alter the electronic environment of the PdII centre thereby affecting its activity and selectivity. The application of phosphorus-palladium complexes in this study is not only a new approach in Wacker oxidation but the utilization of the π-accepting and or σ-donating abilities of phosphorus compounds was also advantageous in altering the PdII electronic environment. No co-oxidants were used in this study w.r.t. the oxidation of 1-octene and the complexes evaluated were comparable to those reported in literature with PdCl2/DMA systems under similar conditions. Since oxygen is the preferred oxidant in all oxidation reactions because of its natural abundance, its reported enhanced selectivity and ease of separation from products, it was decided to evaluate the utilization of this reagent as first choice in the current investigation of ligand supported palladium catalysts in the Wacker oxidation. Due to the fact that the phosphite based palladium catalyst, PdCl2[P(OPh)3]2, is readily soluble in DMA, it was determined that no pre-stirring as for PdCl2 was required for this catalyst. In order to obtain the optimum reaction conditions for oxygen as oxidant with this catalyst, conditions like solvent, reaction temperature, O2 pressure and water, catalyst, and substrate concentration were varied. The optimized conditions were determined to be 0.5 mol% of catalyst in DMA:H2O (6:1) under 9 atm of O2 at 80°C, while the optimum substrate concentration was found to be 0.2M. PdCl2[P(OPh)3]2 showed the highest activity of the catalysts evaluated and gave a TOF of >1370 (mol/mol/hr), which compared favourably with other known catalysts like PdCl2, PdCl2(CH3CN)2 Pd(OAc)2, and Pd(CF3SO3)2 where TOF’s of 1429, 1420, 817 and 524 respectively, were obtained under the conditions optimized for PdCl2[P(OPh)3]2. While the palladium metallocycle [Pd(u-Cl)(C6H4O)(OC6H6)2]2 gave TOF’s (1380 mol/mol/hr) virtually the same as PdCl2[P(OPh)3]2, total conversion for the latter catalyst was only 93%, so it can be regarded as the second best of all the catalysts evaluated. The monomers thereof, PdCl[(C6H4O)(C6H6O)2P(OPh3)] and PdCl[(C6H4O)(C6H6O)2(PPh3)], revealed the least basic P(OPh3) to be more reactive (TOF >900 mol/mol/hr) than the TPP containing analogue, where the latter showed no activity within the first hour of reaction. While all the active catalysts showed good selectivities of >80%, the metallocycle [Pd(u-Cl)(C6H4O)(OC6H6)2]2 proved to be the best with a selectivity of 89%. Catalyst recyclability was also observed to at least 3 cycles, with selectivities maintained above 80%. No Pd0 ‘fall-out’ or aggregation was observed with any of the catalysts evaluated. For the palladium phosphinite catalysts 1,2-Ph(OPPh2)2PdCl2 and 1,3-Ph(OPPh2)2PdCl it was found that both were active in the Wacker oxidation of 1-octene albeit with very low rates for the latter complex (1,3-Ph(OPPh2)2PdCl). The low reactivity of 1,3-Ph(OPPh2)2PdCl was similar to that of the phosphines (PPh3)2PdCl2 and (3,5-CF3-PPh2Cl)2PdCl2 where (PPh3)2PdCl2 showed some conversion only after 3 hours and (3,5-CF3-PPh2Cl)2PdCl2 gave only 53% conversion after an hour. Through a comparison of the reactivity of 1,2-Ph(OPPh2)2PdCl2 with that of the hydrolyzed equivalent [μ-ClPd(PPh2OH)(PPh2O)]2, it seemed as if the phosphinite catalysts are prone to hydrolysis under the prevailing conditions as the final conversion of both these catalysts were almost the same (85 and 79% respectively). Hydrogen peroxide and tert-butylhydroperoxide (TBHP) were also evaluated as alternative oxidants with PdCl2[P(OPh)3]2 as catalyst and H2O2 was found to be the better of the two oxidants with conversion (99%), selectivity (86%), and TOF (1220) almost as good as those found for oxygen (100, 82% and 1370 respectively). In addition, the catalyst could also be recycled three times although degradation of the H2O2 was observed and additional peroxide (12 eq.) had to be added with each cycle of substrate. TBHP, however, suffered from moderate selectivities of only 60-65%, while the catalysts was deactivated during the first oxidation cycle and could therefore not be recycled at all. Although all phosphite catalysts promoted isomerization to internal 1-octene isomers to some extent, the cyclopalladated [Pd(u-Cl)(C6H4O)(OC6H6)2]2 catalysts proved to be the best in this aspect of the reaction w.r.t. oxygen as oxidant and led to very low quantities of isomerised products being observed (3 - 4%). It was also evident that the type and amount (for H2O2 and TBHP) of oxidant played a crucial role in enhancing or suppressing isomerization and hydrogen peroxide (at only 2% isomerization) was found to be the best oxidant in this regard followed by oxygen (13%).