Cerium complexes reactivity

Reaction of ammonium hexanitrocerate and cyclopentadienylsodium under inert conditions gives tris(cyclopentadienyl)cerium and sodium nitrate, removed by filtration before evaporation of solvent [1]. When the filtration step was omitted, and the evaporated solid mixture was heated to 75°C, a violent explosion occurred. This may have involved complexes of the type Ce(N03)Cp2.NaN03[2], but a direct redox reaction between the reactive CeCp3and the oxidant is also possible. 

Organoytterbium chemistry has been developed in the last 20 years, although the development rate is much slower than the other lanthanides like samarium or cerium. Dianionic complexes that are produced from the reaction of ytterbium with diaryl ketones react with various kinds of electrophiles including carbon-heteroatom unsaturated bonds.35 Phenylytterbium iodide, a Grignard-type reagent, is known to have reactivity toward carbon dioxide,36 aldehydes, ketones,37,37 and carboxylic acid derivatives38,3811 to form the corresponding adducts respectively. 

Not until similar information is obtained on the effect of cerium(III), hydrogen ion, and sulfate ion on the oxidations of Cr(C2C>4)3 3 and Cr(OH2)4C204+, will it be appropriate to discuss the relative reactivities of the three oxalato complexes toward cerium(IV). 

Uranium(IV) and thorium(IV) form true sandwich cyclooctatetraenide complexes uranocene U(CgH8)2 and Th(C8H8)2 with the average 180) of the sixteen U-C distances 2.647 A and Th-C distances 2.701 A. Cerium(III) forms a far more reactive anion Ce(C8H8)2- with the quite high average 2.74 A of the sixteen Ce-C distances. 

Electrophilic aromatic substitution of 5-hydroxy-2,4-dimethoxy-3-methylaniline by reaction with the iron complex salts affords the corresponding aryl-substituted tricarbonyliron-cyclohexadiene complexes. O-Acetylation followed by iron-mediated arylamine cydization with concomitant aromatization provides the substituted carbazole derivatives. Oxidation using cerium(IV) ammonium nitrate (CAN) leads to the carbazole-l,4-quinones. Addition of methyllithium at low temperature occurs preferentially at C-1, representing the more reactive carbonyl group, and thus provides in only five steps carbazomycin G (46 % overall yield) and carbazomycin H (7 % overall yield). 

In a nice illustration of the impact of metal coordination upon the reactivity of phospholes, a methodology for the functionalization of these heterocycles in the /3-position has been described (see also Scheme 22) <2001JOM105>. Here, coordination of both the P-lone pair and the cyclic diene system was undertaken. The resulting multimetallic complex 79 was treated with lithium diisopropylamide (LDA) to afford the lithium salt 350 (Scheme 118). This readily undergoes nucleophilic substitution with a variety of electrophiles to afford the corresponding substituted phosphole complexes 351-353. The free phospholes can be isolated following decomplexation with cerium(iv) ammonium nitrate (CAN). 

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

The product distribution depends on the isocyanide used. Only with aromatic (R = o-tolyl or 2,6-xylyl), and not with aliphatic isocyanides (R = Bu , Bu or benzyl) are isonitrile insertion products 24 formed. Aryl isocyanide insertion is obviously very fast since the competing formation of 23 from CO insertion is not observed complexes 22 are only minor products. With aliphatic isocyanides, the thpp complexes 22, from double cycloaddition of dmad cf. Section 3.1.1), are the major products (70 to >95% of the product mixture) and indicate a strongly increased 1,3-dipolar reactivity, i.e. the intermolecular second cycloaddition is preferred to the intramolecular CO insertion. Compared with the ruthenium compound 17, the thpp in 22 is strongly bound to the metal and can only be decomplexed oxidatively with cerium(iv), or under 80 bar of CO. 

From the work of Taube and King, it was suspected that the different reactivities of these presumably identical compounds were caused by catalytic amounts of [Pt(en)2]. This was confirmed both by the addition of the catalyst and also by the addition of cerium(iv) to destroy the catalyst. Addition of five mole per cent [Pt(en)2] results in complete reaction within five minutes of a complex which would otherwise react in the dark at 25 °C with = 44 min. 

The preparation of a stable irontricarbonyl complex of unsubstituted cyclobutadiene [9] provided the real opening for the study of cyclobutadiene since this complex is oxidised by the cerium(IV) cation to cyclobutadiene [10,11]. Although a highly reactive species it was trapped by dienophiles. Since then other methods have also been used to prepare cyclobutadiene, and, in addition to chemical trapping, it has been prepared and studied in matrices at low temperature. These results are discussed in more detail in the body of this chapter. 

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