Zirconia and Immobilized Zirconium Complexes

The chemoselective reduction of nitro and carbonyl functions with /-PrOH/ KOH over nickel-stabilized zirconia has recently been reported by Upadhya et al. [10]. This catalytic system selectively reduced 4-nitroacetophenone and 

4-nitrobenzophenone to 4-aminoacetophenone and 4-aminobenzophenone, respectively, without reduction of the carbonyl function. It was assumed that Ni both stabilizes the cubic zirconia structure and actively participates in the reaction by promoting the dehydrogenation of i-PrOH. It was proposed that the Ni species formed, still incorporated in the Z1O2 bulk structure, had only moderate activity, thus enabling chemoselective reduction. 

Leyrit et al. reported the synthesis of silica-anchored mononuclear (tris)isopro-poxyzirconium, (=SiO)Zr(Oi-Pr)3, as a true heterogeneous catalyst in MPVO reactions [11,12]. It is worthy of note that dissolved tetraisopropoxyzirconium is not active in MPVO reactions. The solid catalyst was prepared by reacting partially dehydroxylated silica with tetra(neopentyl)zirconium (Zr(Np)4) to form a mono- 

4-pentanolate product rapidly. In the reverse reaction, 4-methyl-2-pentanol was readily oxidized to the corresponding alcohol by employing acetophenone as hydrogen acceptor. The catalyst proved to be fully recyclable and stable against leaching. Activity in successive runs was essentially identical and no Zr was found in solution. Apparently, the =SiO-Zr bond is stable against alcoholysis. 

Kaspar et al. demonstrated the reduction of a,/5-unsaturated ketones to allylic alcohols with i-PrOH in the gas phase, over MgO as fixed bed catalyst at 250 °C 

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