Lanthanum compounds addition reactions

The same authors (77) also investigated the Michael addition of nitromethane to a,/l-unsaturated carbonyl compounds such as methyl crotonate, 3-buten-2-one, 2-cyclohexen-l-one, and crotonaldehyde in the presence of various solid base catalysts (alumina-supported potassium fluoride and hydroxide, alkaline earth metal oxides, and lanthanum oxide). The reactions were carried out at 273 or 323 K the results show that SrO, BaO, and La203 exhibited practically no activity for any Michael additions, whereas MgO and CaO exhibited no activity for the reaction of methyl crotonate and 3-buten-2-one, but low activities for 2-cyclohexen-l-one and crotonaldehyde. The most active catalysts were KF/alumina and KOH/alumina for all of the Michael additions tested. 

Better results were obtained in the methyl methacrylate polymerization reactions (Scheme 12). 153-156 showed high catalytic activity with a strong dependence on the ionic radius of the center metal. The lanthanum complex 154 was the most active catalyst and initiated the polymerization without any cocatalyst. Addition of small amounts of AlEts as cocatalyst increased the yield significantly. Polymerization initiated by 154 depended on the temperature and a low temperature (—78°C) was required to afford almost quantitative yields. The resulting polymers were basically syndiotactic and exhibited high molar masses and narrow polydispersities. The catalytic reaction with the lanthanum compound 157 showed no increase of catalytic activity but led to a larger fraction of atactic poly(methyl methacrylate). Moreover, the catalytic activity of all utilized initiators was solvent dependent. 153, 155, and 156 only showed catalytic activity by the addition of a cocatalyst. 153 afforded lower yield after changing the solvent from toluene into THF. 

Finally, a fascinating development in the field of lanthanum-BINOL complexes remains to be mentioned [25]. These compounds so far have proved to catalyze enantioselectively hydrophos-phonylations of imines [26], nitroaldol reactions [27], Michael additions [28] and cpoxidations of... 

Divalent complexes can also be synthesised by the reductive pathway. This method is less direct, since it involves two steps instead of one for the metathesis reaction (a) synthesis of a suitable trivalent complex as precursor and (b) chemical reduction of this trivalent complex. This method has a number of advantages, however, since, in addition to organothu-lium(II) complexes, it has permitted to isolate compounds that could not be synthesised by the metathesis pathway, such as lanthanum(II),... 

The behaviour of a Pt-based catalyst on a metallic monolith support washcoated with alumina, with the addition of lanthanum and cerium, was studied by Musialik-Piotrowska and Mendyka. The activity of the catalyst was tested in the oxidation of ChB and DCE alone and in two-component mixtures with toluene, -hexane, acetone, ethanol, and ethyl acetate. The influence of non-chlorinated compounds on ChB oxidation differed from one compound to another. Over the whole range of reaction temperatures, ethanol enhanced the conversion of ChB by 10%. The addition of both hydrocarbons also slightly improved ChB destruction, while DCE conversion was inhibited in the presence of each non-chlorinated compound that was added. Both chlorinated hydrocarbons not only inhibited catalytic destruction of each of the non-chlorinated compounds added, but also increased the reaction selechvity and concentration of the intermediate yielded, the first of which was acetaldehyde. 

Further kinetic results relevant to the ligand replacement reactions discussed in this section are those for dissociation of lanthanide(m)-cydta complexes. Nearly all the tervalent rare-earth cations were included in this investigation, along with the closely related complexes of scandium(ra) and yttrium(m). Dissociation rates vary with pH, but are unaffected by the addition of copper(n) ions. Thus there is no direct transfer of cydta from Ln + to Cu + in the manner of equation (7) (next section). The rate constants for the acid-dependent dissociation path show a large variation with the nature of the lanthanide cation they vary from 1291 mol s (at 25 °C, fjb = 0.1 mol 1 ) for lanthanum(ra) to 0.0171 mols for lutecium(m) the trend of rate constants is one of regular decrease as the ionic radius decreases. The rate constant of 0.019 1 mol s for dissociation of the scandium(in) complex is, however, much higher than would be expected from the ionic radius of Sc +. Activation parameters for dissociation of these cydta complexes are reported for the La", Gd, Dy Tm ", Lu , and Y compounds. ... 

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