Mechanistic studies on the epoxidation of alkenes with molecular oxygen and aldehydes catalysed by transition metal–β-diketonate complexes

Abstract

The scope, mechanism and kinetics of the aerobic epoxidation of alkenes with an aldehyde and substituted β-diketonate-transition metal complexes as catalysts were studied. β-Diketonate complexes of nickel() proved to be among the best catalysts for this reaction. The epoxidation is not dependent on substrate concentration and is first order in aldehyde, catalyst concentration and oxygen partial pressure. It was shown by reactivity studies and EPR experiments that the reaction is radical in nature. Additional evidence for this was obtained from stereochemical investigations. The metal catalyst is not only an efficient initiator of the reaction, but is also believed to enhance the reactivity of intermediate species in the oxidation process by allowing these to co-ordinate to the metal center. A mechanism is proposed for the catalytic reaction.Molecular oxygen as a cheap, clean and readily available oxidant has received much attention in recent years. 1 Mukaiyama and co-workers 2-7 and others 8-11 have reported that molecular oxygen can be used as the terminal oxidant in the epoxidation of alkenes with an aldehyde or primary alcohol as coreactant and a metal β-diketonate as a catalyst (Scheme 1). There has been discussion in the literature about the mechanism of the 'Mukaiyama' catalytic system and the role of the transitionmetal catalyst in it, which can be omitted as was shown by Kaneda et al. 12 Since peroxyacids, which are formed in the autoxidation of aldehydes, are powerful epoxidizing reagents, the reaction in Scheme 1 might proceed through the peroxyacid as the actual epoxidizing agent. The only role of the transitionmetal catalyst in this scenario is to catalyse the formation of the peracid as shown in Scheme 2.Another possible mechanism proposed in the literature 10 is the formation of a metal-oxygen complex which reacts to form an oxometal species, similar to species described for manganese or vanadium. 13,14 In Scheme 3 this mechanism is outlined for a transition-metal()-β-diketonate complex.A combination of the mechanisms in Schemes 2 and 3 was considered by Nam et al. 15 They investigated the 'Mukaiyama' system using cyclam-type transition-metal complexes and concluded from indirect evidence that the epoxidation reaction in their system is radical in nature. The peroxyacid and the oxometal mechanisms in Schemes 2 and 3 were believed to play no role. An acylperoxy radical, rather than a peroxyacid, was proposed to react with the alkene to form an epoxide. Alternatively, the acylperoxy radical could co-ordinate to the metal first, and this complex subsequently epoxidizes the alkene. The same authors reported shortly after 16 that cyclam complexes of Ni II are inhibitors of this radical reaction. These complexes were believed to be sufficiently good reducing agents to react with an acylperoxy radical and form an unreactive acylperoxy anion and a nickel() complex.In the present paper we further explore the scope, kinetics and mechanism of the epoxidation of alkenes by the 'Mukaiyama' system. The mos...

Alcohol Oxidation by Dioxygen and Aldehydes Catalysed by Square-Planar Cobalt(III) Complexes of Disubstituted Oxamides and Related Ligands

“…[49] The latter pathway, with a nickel acylperoxide involved as the active oxidant, has been suggested recently by Nolte and coworkers for olefin epoxidation, catalysed by nickel(II) β-diketonate complexes, with dioxygen and aldehydes. [20] In contrast, with the square-planar nickel(II)Ϫcyclam complex, an outer-sphere electron transfer process occurs, leading to oxidation of Ni II to Ni III , with concomitant formation of the unreactive acylperoxide anion RC(O)OO Ϫ from the acylperoxyl radical RC(O)OO · . [26b] Clearly, this difference in reactivity arises from the stronger reducing power of the latter nickel(II) complex; the formal potentials for the redox couple Ni III /Ni II in CH 3 CN at 25°C are 1.24 and 0.67 vs. Ag ϩ /Ag for Ni(acac) 2 (acac ϭ acetylacetonate) [50] and Ni(cyclam) 2ϩ complexes, [33b] respectively.…”

“…[12Ϫ18] One possible alternative involves the use of an external reductant for initial activation of dioxygen (reductive O 2 activation). Aldehydes have been successfully used by Mukaiyama [19] and others [20] in this way (i.e., through the in situ generation of peracid from the aldehyde and O 2 ) for the aerobic epoxidation of olefins with a metal β-diketonate complex as catalyst. [21] Hence, during the last decade, a significant number of first row transition metal complexes with different ligand types (e.g., β-oxo iminate, β-oxo esterate, Schiff base, porphyrin, tetraazamacrocycle, oxamate and oxamidate) have been found to be effective catalysts for the oxidation of a variety of organic substrates using dioxygen in the presence of an aldehyde as oxidant.…”

Biomimetic Epoxidation of Olefins Catalyzed by Metalloporphyrins with Molecular Oxygen

“…As reported previously, the oxygenation of organic substrates catalyzed by metalloporphyrin complexes plus aldehyde is usually considered to involve a radical and high-valent metal intermediate mechanism. [94][95] 2,6-Di-tert-butyl-4-methylphenol as a radical trap has been used in the epoxidation of cyclohexene, and it was found that the reaction was completely inhibited. Therefore, the aerobic epoxidation should involve radical species.…”

Novel Manganese Complex as an Efficient Catalyst for the Isobutyraldehyde-Mediated Epoxidation of Cyclic Alkenes with Dioxygen

“…[15,16] In addition, in those cases where the dioxygen source was changed to air instead of molecular oxygen (entries 10 and 11), the conversion decreased significantly. It is possible that the dioxygen concentration influenced the rate of the formation of the acylperoxy radical, which was a rate-determining step [17] in the catalytic cycle of the epoxidations of alkenes. These results are consistent with the speculations proposed by Haber, [15] Valentine, [16] and Nolte [17] using prophyrin, cyclam or b-diketonate complexes of metal as catalyst.…”

“…It is possible that the dioxygen concentration influenced the rate of the formation of the acylperoxy radical, which was a rate-determining step [17] in the catalytic cycle of the epoxidations of alkenes. These results are consistent with the speculations proposed by Haber, [15] Valentine, [16] and Nolte [17] using prophyrin, cyclam or b-diketonate complexes of metal as catalyst. The role of the metal complex in the catalytic process is still under investigation.…”