Catalysis by Lanthanides and Related Periodic Elements

The Lewis acidity of lanthanide complexes has been known for a long time. It was exploited extensively in their use as NMR shift reagents, mainly Eu(fod)3. They show strong affinity toward carbonyl oxygens and, therefore, have been widely used as catalysts for cycloaddition of dienes with aldehydes [25]. Moreover, the ability of catalytic amounts of lanthanide compoimds to activate coordinating nitriles as well as imines has also been recognized [26]. In recent years lanthanide (III) complexes have demonstrated clear effectiveness in catalyzing not only hetero-Diels-Alder reactions, but also Michael, aldol, Strecker and Friedel-Crafts acylation reactions [27]. 

In addition to lanthanides, yttrium and scandium salts, which appear in the same column of Mendelejew periodic table, were also shown to act as Lewis acid 

8-heptafluoro-3,5-octanedionatoj) and triflate (OTf) (trifluoromethane-sulfonato). A very interesting property of lanthanide and scandium triflates is their stability and conservation of their catalytic power in aqueous solutions. They present another advantage in that they can be nearly 100 % recovered after reaction work-up with water and re-used several times without loss of their catalytic properties. Very recently, a highly efficient polymer-supported scandium catalyst has been designed to operate in micellar systems [29]. This makes lanthanide and scandium triflates promising catalysts in the future development of clean and environmentally-friendly processes. 

The synthetic potential of lanthanide catalysis has been recognized in high pressure chemistry mainly during the last decade although earlier work was concerned with the beneficial effect of the biactivation mode to overcome steric hindrance in hetero-Diels-Alder reactions [30]. In this way, alkyl pyruvates and aldehydes considered as dienophiles could react with 1-methoxy-l,3-butadiene yielding adducts which could serve as synthons in sugar synthesis [31]. 

The advantage of using a catalyst is obvious. When one reactive center of the keto compound is substituted by a methyl or phenyl group (Rz) the catalyst effect is more enhanced. Mesityl oxide (Rj = Rz = R3 = Me) requires very high pressure and efhcient lanthanide catalysis. This reaction involving a sterically congested unsaturated ketone is a prominent example emphasizing the requirement of biactivation to overcome steric hindrance. It should be added that, apart from the mesityl oxide reaction, a modest pressure (300 MPa) is sufficient to induce reactivity in these catalyzed reactions which were shown to proceed only at pressures in excess of 1200 MPa in an uncatalyzed version [33]. 

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