Selective Catalytic Oxidation of Cyclohexene with Molecular Oxygen: Radical Versus Nonradical Pathways.

Ilse M Denekamp,Martijn Antens,Thierry K Slot,Gadi Rothenberg

doi:10.1002/cctc.201701538

Abstract

We study the allylic oxidation of cyclohexene with O2 under mild conditions in the presence of transition‐metal catalysts. The catalysts comprise nanometric metal oxide particles supported on porous N‐doped carbons (M/N:C, M=V, Cr, Fe, Co, Ni, Cu, Nb, Mo, W). Most of these metal oxides give only moderate conversions, and the majority of the products are over‐oxidation products. Co/N:C and Cu/N:C, however, give 70–80 % conversion and 40–50 % selectivity to the ketone product, cyclohexene‐2‐one. Control experiments in which we used free‐radical scavengers show that the oxidation follows the expected free‐radical pathway in almost all cases. Surprisingly, the catalytic cycle in the presence of Cu/N:C does not involve free‐radical species in solution. Optimisation of this catalyst gives >85 % conversion with >60 % selectivity to the allylic ketone at 70 °C and 10 bar O2. We used SEM, X‐ray photoelectron spectroscopy and XRD to show that the active particles have a cupric oxide/cuprous oxide core–shell structure, giving a high turnover frequency of approximately 1500 h−1. We attribute the high performance of this Cu/N:C catalyst to a facile surface reaction between adsorbed cyclohexenyl hydroperoxide molecules and activated oxygen species.

Highlights

The allylic oxidation of alkenes is an important chemical reaction
Building on our preliminary communication on alcohol oxidation,[12] we designed a set of metal oxide catalysts supported on N-doped carbons
We try to resolve the different pathways that lead to allylic oxidation, with the goal to gain a better understanding of this important reaction

Summary

Introduction

It allows us to keep the double bond and at the same time create a new alcohol or carbonyl function.[1, 2] As such, it is useful across the board, from bulk chemicals and agrochemicals[3, 4] all the way to fine chemicals and fragrances.[5,6,7] In theory, allylic oxidation is a straightforward exothermic reaction It requires only O2, a free, eco-friendly and widely available reagent. Cyclohexene is a good model compound for two reasons: first, it is a small and symmetric molecule, similar to many starting compounds in chemical synthesis It is itself industrially important and participates in the synthesis cycles of key C6 chemicals such as adipic acid and caprolactone.[13, 14] Building on our preliminary communication on alcohol oxidation,[12] we designed a set of metal oxide catalysts supported on N-doped carbons. We try to resolve the different pathways that lead to allylic oxidation, with the goal to gain a better understanding of this important reaction

Results and Discussion

Conclusions

Conflict of interest