Cerium-carbon bonds

Cationic polymerization of alkenes and alkene derivatives has been carried out frequently in aqueous media.107 On the other hand, the reaction of simple olefins with aldehydes in the presence of an acid catalyst is referred to as the Prins reaction.108 The reaction can be carried out by using an aqueous solution of the aldehyde, often resulting in a mixture of carbon-carbon bond formation products.109 Recently, Li and co-workers reported a direct formation of tetrahydropyranol derivatives in water using a cerium-salt catalyzed cyclization in aqueous ionic liquids (Eq. 3.24).110... 

Imamoto, T., Kusumoto, T., Tawarayama, Y., Sugiura, Y., Mita, T., Hatanaka, Y., Yokoyama, M. Carbon-carbon bond-forming reactions using cerium metal or organocerium(lll) reagents. J. Org. Chem. 1984, 49, 3904-3912. 

Fukuzawa, S., Fujinami, T., Sakai, S. Carbon-carbon bond formation between a-halo ketones and aldehydes promoted by cerium(lll) iodide or cerium(lll) chloride-sodium iodide. J. Chem. Soc., Chem. Common. 1985, 777-778. 

Similarly, the specific oxidation of glycols and related compounds has been found to proceed through the formation of intermediate chelates. For example, in the oxidation of 2,3-butanediol in nitric acid solution by cerium (IV) ions, the metal ion probably forms an intermediate chelate with the glycol, extracts an electron from the ligand, and is reduced to its lower oxidation state. A carbon-carbon bond in the glycol is broken in this process with the resultant formation of acetaldehyde (71). 

In this article the term organometallic compound includes alkyl and aryl derivatives of the rare earths—the transition metals of group III, scandium, yttrium, lanthanum and the lanthanides cerium to liitetium with covalent metal-to-carbon a-bonds, as well as the so-called 77-complexes with more than monohapto metal-to-carbon bonds, for example cyclopentadienyl and olefin complexes, metal acetylides, but not carbonyls, cyanides and isocyanide complexes. Derivatives of scandium, yttrium and lanthanum are included and discussed together with the compounds of the lanthanides, because of many similarities in the synthesis and the chemistry of these organometallic derivatives of the rare earths. 

Xu, X., Jiang, R., Zhou, X., liu, Y., Ji, S., and Zhang, Y. (2009). Cerium ammonium nitrate an efficient catalyst for carbon-carbon bond formation from ferrocenyl alcohol substrate. Tetrahedron, 65, 877-882. 

As part of a study of the cleavage of carbon-carbon bonds with a hard acid/soft nucleophile system, several lanthanide chlorides were evaluated to see if their Lewis acidity would promote a reaction similar to Luche s aldehyde acetalization (Fuji et al., 1981). Unfortunately, the reaction of lanthanum, ytterbium and cerium chlorides with a-cyano-a, )8-unsaturated esters yielded varying amounts of Michael adduct and unreacted starting material after several days of reaction. 

Another new cerium(IV) reagent, ceric triethylammonium nitrate (CTEAN), has been prepared for use in the mild high-yield oxidation of benzyUc alcohols and a-hydroxy ketones to the corresponding carbonyl compounds (Firouzabadi and Iranpoor, 1983). This CAN analog is stable, soluble in methylene chloride, acetonitrile, acetone, alcohols and water it gives high yields in simple, nonacidic solvents and produces minimal secondary products from carbon-carbon bond cleavage. 

Imamoto et al. (1984c) reported a detailed investigation of carbon-carbon bond formation promoted by cerium amalgam and by organocaium(III) reagents gener-... 

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. 

Diethyl malonate has been proposed for use as a proton source in these cyclization reactions [124], It is not a sufficiently strong acid to protonate the radical-anion rapidly. However it irreversibly protonates the enol intermediate generated after carbon-caibon bond formation. In one case, control of stereochemistry in favour of the traHS-sunstituted five membered ring 39 was achieved by the addition of cerium(Ill) ions [124],... 

Under appropriate experimental conditions, hydrogen telluride, phenyl tellurol, and sodium hydrogen telluride perform the selective reduction of double bonds of a,/3-unsaturated carbonyl systems.89 90 This reaction with a,j3-unsaturated ketones complements the cerium trichloride/sodium borohydride method, which reduces the carbonyl group, keeping the carbon-carbon double bond intact92 (Scheme 18). 

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. ° ... 

---