Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

2.3.2.2 Epoxidation of styrene
CHAPTER 2
Contrary to cyclohexene, no allylic oxidation can occur with styrene as substrate for epoxidation. A
dominant side reaction is the C=C bond cleavage affording the corresponding carbonyl compounds.
The formation of benzaldehyde can complicate the mechanistic investigation of aerobic
epoxidations, as aldehydes in connection with O2 can form carboxylperoxo radicals being the actual
epoxidizing species (cf. Scheme 2‐7). The most often studied substrate styrene can additionally
undergo oligomerization, especially in the presence of radicals. Oligomers are not detectable via GC
analysis and so the use of an internal standard is mandatory in order to obtain reliable results. As for
cyclohexene, Au catalysts were also described to epoxidize styrene with molecular oxygen as the sole
oxidant. While in the studies by Hutching’s group for cyclohexene [97] a small amount of radical
initiator was necessary, gold clusters consisting of 55 atoms deposited on TiO2 oxidized styrene to
benzaldehyde, acetophenone and styrene epoxide at 100 °C without any radical initiators (Table 2‐3,
entry 1) [129]. Benzoic acid was not observed ruling out that epoxidation occurs via a
peroxocarboxylic acid intermediate. Hence, epoxidation is assumed to occur directly on the Au
surface of nanoparticles smaller than 3.5 nm. Further mechanistic studies (e.g. by attenuated total
reflection‐infra red spectroscopy [ATR‐IR]) are necessary to test this assumption. Note that in the
previously described paper by the group of Hutchings styrene oxide selectivities of up to 97 % were
reported. Lignier et al. investigated the epoxidation of (E)‐stilbene with Au/TiO2 and other gold
catalysts where methylcyclohexane was mandatory as a solvent (Table 2‐3, entry 2) [130‐133]. 1,3‐
Dimethylcyclohexane served as an activator of molecular oxygen by forming peroxo radicals thereby
of course being consumed, rendering the reaction less attractive compared to the previous reports.
Epoxidation is facilitated not directly by the metal but by a free radical autoxidation mechanism as it
is mostly the case when organic sacrificial reductants are involved and is exemplary shown in Scheme
2‐10. The radical mechanism is initiated by t BuOO∙ radicals (step 1). Abstraction of H∙ (step 2) and
addition of O2 (step 3) to the solvent molecules affords a peroxy radical capable of epoxidizing the
olefin (step 3 and 4). The alkoxy radical activates another solvent molecule (step 5). The exact role of
the catalyst was not elucidated.
Two examples of Cu catalyzing aerobic epoxidation of styrene are the previously described
low‐surface area phosphates Cu2(OH)PO4 and Cu4O(PO4)2 [60, 128]. Selectivities to the epoxide
reached 20 % for styrene (TOF 0.25 h ‐1 at 80 °C); the use of a standard was not reported.
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