Abstract
The aminohydroxylation reaction of olefins is a key organic transformation reaction, typically carried out homogeneously with toxic and expensive osmium (Os) catalysts. Therefore, heterogenisation of this reaction can unlock its industrial potential by allowing reusability of the catalyst. Os–Zn–Al hydrotalcite-like compounds (HTlcs), as potential heterogeneous aminohydroxylation catalysts, were synthesised by the co-precipitation method and characterised by several techniques. Reaction parameters (i.e., solvent system, reaction temperature, and catalyst structure) were optimized with cyclohexene, styrene, and hexene as substrates. The different classes of olefins (aliphatic, aromatic, and functionalised) that were tested gave >99% conversion and high selectivity (>97%) to the corresponding β-amino alcohol. The catalyst HTlc structure had a significant effect on the reaction time and yield of the β-amino alcohols. Under the same testing conditions, a heat treated catalyst (non-HTlc) showed a shorter reaction time, but drop in the yield of β-amino alcohols and rise in diol formation was observed. Leaching tests showed that 2.9% and 3.4% of Os (inactive) leached from the catalyst to the reaction solution when MeCN/water (1:1 v/v) and t-BuOH/water (1:1 v/v), respectively, were used as the solvent system. Recycling studies showed that the catalyst can be reused at least thrice, with no significant difference in the yield of the β-amino-alcohol.
Highlights
Olefins are considered one of the most important starting materials in organic synthesis
The Os–hydrotalcite-like compounds (HTlcs) catalyst was prepared via the co-precipitation method with a target ratio of 0.3 Os/Al, based on previously reported studies [19,20]
The three metals present (Os, Zn, and Al) in the prepared materials were quantitatively determined by means of inductively coupled plasma optical emission spectroscopy (ICP-OES) (Table 1)
Summary
Olefins are considered one of the most important starting materials in organic synthesis They are cheap and widely available, and the double bond allows 1,2-functionalisation via face selective oxidation [1,2]. Olefins are utilised as starting materials to produce diols and vicinal amino alcohols during the dihydroxylation (DH) and aminohydroxylation (AH) reaction, respectively. Both reactions (i.e., DH and AH) are catalysed by OsO4 and are considered to be highly selective and reliable organic transformations, because OsO4 reacts with a wide spectrum of olefins and does not react or reacts very slowly with other organic functional groups present [3]. Osmium is a toxic metal, there have been investigations on the Catalysts 2018, 8, 547; doi:10.3390/catal8110547 www.mdpi.com/journal/catalysts
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