Oxygen-free phase-transfer conditions

Determination of the optimal conditions for the reaction of dibromocarbene with alkenes is more difficult than for the corresponding reaction of dichlorocarbene. This is due to the high reactivity of dibromocarbene which enters into other competitive reactions, particularly hydrolysis under the conditions of phase-transfer catalysis and, in the case of alkenes of low reactivity, its precursor bromoform forms products of free radical reactions if the reaction system is not protected from air and oxygen and from light. These processes have been studied and are described in detail in Houben-Weyl, Vol.E19b, pp 1609-1612. 

The holy grail of oxidation chemistry is the design of catalytic systems capable of mediating oxygen transfer from dioxygen, without the need for a sacrificial reductant, i.e. a Mars-van Krevelen mechanism [18] in the liquid phase. Indeed, the confinement of substrate molecules in the micropores of molecular sieves might be expected to create quasi gas phase conditions conductive to such a mechanism at the expense of free radical chain autoxidation (we note, however, that a mechanism involving two metal centres as shown in eq. 12 would be difficult to achieve in a molecular sieve). 

In industry, this reaction is conducted at elevated pressure (10-30 bar) and temperature (80-250 °C). Under such conditions, the content of oxygen must be kept low to avoid the explosive regime. Also, the cyclohexane conversion is kept low (<5%) to avoid the formation of side products. In a microreactor, composition and temperature of the gas phase far above the flammability limit of 4% are possible. A selectivity of 88% was observed. However, it is shown that despite the narrow channels, gas-liquid mass transfer still limits the rate of reaction so that further engineering of a reactor is required that will enable mass transfer free operations and oxygen refilling. 

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