MPVO Reactions Catalyzed by Metal Oxides

Posner et al. have used y-Al203 in the reduction of unsaturated carbonyl compounds by j-PrOH [3]. Dehydrated alumina deprotonated /-PrOH forming the alkoxide in situ. High temperatures were needed (up to 300 °C) and only low yields of alcohol were obtained, because of the occurrence of several side-reactions which are catalyzed by weak Brpnsted acid sites present on the alumina surface. 

Horner and Kaps have used chlorinated y-Al203 in combination with a small amount of Al(Oz-Pr)3 as the catalyst for the reduction of benzaldehyde, cyclohexanone, and acetophenone by z-PrOH [4]. In the absence of Al(Oi-Pr)3, no reaction occurred. The addition of a small amount of strong base was found to enhance the reaction rate. Analogous phenomena have been observed in the Oppenauer oxidation of several secondary alcohols. Strong bases presumably assist the deprotonation of alumina-surface co-ordinated i-PrOH, thereby forming the required alkoxide surface species. The modified alumina, which contained ca 85 mmol chloride/ 100 g alumina, was obtained by heating dry alumina in thionyl chloride for 24 h. The chloride at the surface increases the Lewis acidity of the aluminum ions and the addition of the base facilitates the deprotonation of z-PrOH. 

Wismeijer et al. studied the liquid-phase transfer-hydrogenation of 4-tg/t-butyl-cyclohexanone by z-PrOH at 83 °C over activated y-Al203 as the catalyst [5]. The activity of the catalyst was found to increase with increasing activation temperature. Selective poisoning experiments indicated that co-ordinatively unsaturated Al surface ions (Lewis acid sites), formed upon dehydroxylation, were essential 

Gargano et al. investigated the pretreatment of the alumina catalyst with hydrogen at 270 °C [6]. This enabled the use of much lower reaction temperatures, thus increasing the selectivity towards the alcohols required. Several other metal oxides have been tested and La203 proved to be the best catalyst, with regard to both conversion and selectivity. 

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