1.3 Styrene epoxidation
1.3.2 Oxidation of styrene by group nine transition metals
Extensive research has been done on second and third row transition metals, which are used as catalysts forming the active oxo species, which are capable of performing oxidative cleavage.112,113 Ruthenium based complexes have gained much interest over the years in various fields such as photomolecular devices, probes for biological macromolecules and artificial photosynthesis due to their wide range of chemical accessible oxidation states (Ru-2 to Ru+8), which make them versatile as energy transfer and electron transfer compounds.114,115 One of the key steps in oxidation reactions is the formation of an intermediate ruthenium-oxo species through the mediation of a suitable oxidant.116-121 Ruthenium catalysts have been widely explored in epoxidation reactions using different oxidants and ligand systems.93,112-134
In this review, attention is drawn to the group 9 transition metals, Co, Rh and Ir, in the oxidation of styrene, since they are the focus of this thesis.
M R
OH2
M OH R
M OH
R
O R c1
c2 -H+
O M O
O M O
O M
O R
O R
O M
HO R
M OOH R
d2 d1
R
H+ -H+
1.3.2.1 Co, Rh and Ir
One of the earliest studies that was carried out was with cobalt salen complexes in the oxidation of styrene by Zombeck et al. in 1982.135 The reaction was performed in different solvent systems and was greatly enhanced when RhCl(PPh3)3 was added.135 Drago et al.
synthesized [bis(salcylidene-γ-iminopropyl)methylamine cobalt(II), CoSMDPT, complexes for the oxidation of isoeugenol in the formation vanillin and acetophenone.136 High TONs of 562 were obtained.136
Figure 1.5. The oxidation of isoeugenol by cobalt SMDPT complexes in the formation of vanillin.136
Cobalt salts, CoBr2, CoCl2, Co(acac)3, Co(OAC)2.4H2O and CoF2 were used as catalysts in the oxidation of methyl styrene in a t-BuOH solvent system with O2 as the oxidant.137 The highest conversion (91%) was obtained using CoCl2 as a catalyst. When the same salt was used as a catalyst in the oxidation of styrene, a 30 % conversion was obtained with high yields to benzoic acid.137
Catalytically active Co(III) ions occupying sites in molecular sieves (AlPO-36, microporous aluminophosphate number 36) were highly active catalysts in the oxidation of styrene, with 46% conversion and good selectivity to the epoxide (34%) and diol (59%). 138 Tang et al.
also reported the epoxidation of styrene using cobalt containing molecular sieves, Co- Faujasite zeolite and Co- MCM-41.139 Depending on the solvent system, high conversions were obtained using acetylacetone (69%), with lower expoxide selectivity (30%). Co
OH
OCH3
OH
OCH3
O
O
H3CO
CH3
O
OCH3
OH (1) Co(SMDPT) + O2
25 oC, toluene
25 oC (2) O2
vanillin
dehydroiisoeugenol isoeugenol
+
encapusulated in zeolite Y gave conversions of 63% with a higher selectivity to benzaldehyde (59%).140 In a DMF (N,N dimethylformamide) and DMA (N,N dimethylacetamide) solvent system, lower conversions (44%; 45%) were found, but better selectivity to the epoxide (60%; 74%).139 Mesoporous silica (MCM-41) functionalized with Co(II) salen showed conversions of 56.5% and a TOF of 36 in the oxidation of styrene with hydrogen peroxide as an oxidant.141 The homogenous Co(II) salen complex system showed lower conversion (41.7%) and TOF (15).141
When CoO is used as a catalyst with TBHP as an oxidant, 47% conversion is obtained with highest selectivity to styrene oxide (73%) and low selectivity to benzaldehyde (0.1%).142 Cobalt substituted Keggin-type polyoxoxmetalates impregnated on a Schiff base modified SBA-15 were also used as effective catalysts in styrene oxidation with hydrogen peroxide as an oxidant under mild conditions.143 A 41% conversion was obtained with a 39% yield to benzaldehyde.143 The non-impregnated catalyst showed a 54 % conversion with a 37 % selectivity to benzaldehyde.143 A similar study of Co2+ adsorbed onto functionalized SBA-15 showed conversions of 92% with high epoxide selectivity (63 %) when oxygen was used as an oxidant.144 The selectivity to the epoxide increased when TBHP was used as an oxidant (70%) and was slightly lower when air and hydrogen peroxide were used.144 Schiff base polymer-cobalt complexes used in the oxidation of styrene with TBHP as an oxidant were also highly selective catalysts to styrene oxide (72%).145
Both Rh and Ir complexes have been studied extensively in the oxidation of alkenes88, however, very limited research is carried out using styrene as a substrate. One of the earliest studies was carried out by Takao and co workers using RhCl(PPh3)3 and RhCl3 as catalysts in the oxidation of styrene under oxygen atmosphere.146 When RhCl(PPh3)3 was used high yields to benzaldehyde were obtained at 80 °C with dioxane as a solvent. Trace conversion was noted when the solvent system was changed to ethanol, pyridine or acetic acid. However, RhCl3 used a catalyst gave much higher yield to benzaldehyde and styrene oxide in toluene as compared to when RhCl(PPh3)3 was used.146 A similar study was carried out by the same authors on iridium Vaska complexes IrXCO(PPh3)3 where X = Cl, Br and I.147 The activity of the catalysts increases in the order: Cl < I < Br. They have postulated that the oxidation using these Vaska complexes proceeds via the coordination of both the oxygen and triphenylphosphine as in Fig 1.6.147
Figure 1.6. Ir Vaska complexes used in the oxidation studies.147
Farrar et al. have shown that the coordination of oxygen and styrene is essential for the epoxidation reaction catalyzed by [(RhCl(C2H4)2)2].148 Bis(pyridylimino)isoindolato-iridium complexes gave a 55% conversion over a 48 h period with a 50 % yield to styrene oxide.149
More recently Turlington et al. used an half sandwich Ir complex (Fig 1.7) in the oxidation of styrene with PhIO as an oxidant and they obtained low yields to benzaldehyde (11%) and phenylacetaldehyde (11%).150
Figure 1.7. Half sandwich compound used by Turlington et al. in the oxidation of styrene.150