Maleic anhydride Lewis acid promoted

The discovery that Lewis acids can promote Diels-Alder reactions has become a powerful tool in synthetic organic chemistry. Yates and Eaton [4] first reported the remarkable acceleration of the reactions of anthracene with maleic anhydride, 1,4-benzoquinone and dimethyl fumarate catalyzed by aluminum chloride. The presence of the Lewis-acid catalyst allows the cycloadditions to be carried out under mild conditions, reactions with low reactive dienes and dienophiles are made possible, and the stereoselectivity, regioselectivity and site selectivity of the cycloaddition reaction can be modified [5]. Consequently, increasing attention has been given to these catalysts in order to develop new regio- and stereoselective synthetic routes based on the Diels-Alder reaction. 

Nb is another highly attractive promoter for the VPO catalysts. Volta et al. [129,130] have demonstrated that Nb is an effective promoter of the activity of the VPO catalysts in n-butane oxidation to maleic anhydride. Nb increases the number of defects at the surface of (V0)2P207 platelets, e.g. O-vacancies, and facilitates the initial C-H activation of n-butane. These sites mainly correspond to coordinatively unsaturated cations acting as Lewis acid sites of low acidity. Furthermore, Volta et al. proposed that the observed improvement in catal)dic properties resulted from a redox effect of Nb on the V /V balance. They proposed that the incorporation of Nb into the VPO framework occurs via substitution of V by Nb " in VO ", i.e. (V0)2P207 -f- Nb =... 

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