Europium catalyst

The combination of europium catalysts and high pressure (15 kbar) makes amino aldehydes available as heterodienophiles in 4 + 2] cycloadditions (Scheme 7.4) [7]. 

Similarly, pyran rings are a common early-stage synthetic intermediate in a variety of syntheses. In Kutay and Gademann s synthesis of anguinomycin C, they prepare the heterocyclic portion of the target by combination an electron-rich diene with an unsaturated aldehyde in the presence of Jacobsen s chromium (III) catalyst. Ghosh used the same asymmetric catalyst to promote the reaction of an aldehyde and an electron-rich diene in his synthesis of brevisamide. Rawal synthesized a pyranone for use in his synthesis of pederin by combination of a chiral dienophile with Danishefsky s diene. In his synthesis of phorboxazole B, Burke treated Brassard diene 119 with chiral aldehyde 118 and a europium catalyst to yield pyranone 120. ... 

Scheme 24 Asymmetric Mukaiyama aldol reactions using a chiral europium catalyst...

This gives rise to dual valency state (+3 and +4) (23). As to the activity of lanthanide based catalysts we confirm a singular behavior that has been already reported by Chinese scientists (22) and that is summarized in Fig. 9. The activity of lanthanides in promoting the polymerization of butadiene and isoprene shows a large maximum centered on neodymium, the only exception being represented by samarium and europium that are not active, reasonably because they are reduced to bivalent state by aluminum alkyls, as pointed out by Tse-chuan and associates (22). 

Asymmetric Diels-Alder reactions have also been achieved in the presence of poly(ethylene glycol)-supported chiral imidazohdin-4-one [113] and copper-loaded silica-grafted bis(oxazolines) [114]. Polymer-bound, camphor-based polysiloxane-fixed metal 1,3-diketonates (chirasil-metals) (37) have proven to catalyze the hetero Diels-Alder reaction of benzaldehyde and Danishefsky s diene. Best catalysts were obtained when oxovanadium(lV) and europium(III) where employed as coordinating metals. Despite excellent chemical yields the resulting pyran-4-ones were reported to be formed with only moderate stereoselectivity (Scheme 4.22). The polymeric catalysts are soluble in hexane and could be precipitated by addition of methanol. Interestingly, the polymeric oxovanadium(III)-catalysts invoke opposite enantioselectivities compared with their monomeric counterparts [115]. 

There has been to some degree the belief that Mossbauer spectroscopy, although in principle an ideal technique for catalyst studies, for practical purposes can only be applied to problems in catalysis if the catalyst contains either iron or tin. Therefore, one of the main purposes of this review is to show how Mossbauer spectroscopy can be directly extended to many additional Mossbauer atoms or isotopes (such as antimony, europium, nickel, ruthenium, gold, and tungsten) and, perhaps more importantly, how the technique can be extended to obtain information about systems that do not contain a Mossbauer atom. ... 

The success of the correlation of catalytic behavior with bulk Mossbauer parameters by Skalkina et al. is also reflected in the work of Tops0e and Boudart (96). As discussed earlier, these authors found a decrease in the isomer shift of the octahedral iron ions in a lead-promoted Cr-Fe304 carbon monoxide shift catalyst, indicative of an increased covalency of these ions. Schwab et al. (203) have proposed a correlation of the activity for CO oxidation by ferrites with the octahedral ions in these materials, and the electron transfer required for this catalytic process may be facilitated by an increased covalency of the metal ions (204). In view of these suggestions, the lead-promoted catalyst is expected to possess a higher catalytic activity for the CO shift reaction than an unpromoted catalyst, as evidenced by the Mossbauer parameters of these two samples. This has in fact been shown experimentally to be the case (96). For the reverse CO shift reaction over supported europium (176), the success of the correlation between catalytic activity and the Mossbauer parameters (in this case the reducibility) has already been noted in Section III, A, 4. 

Metals. Lanthanide metals are also considered as valuable precursors. For example, alkoxides derived from cheap and low-boiling-point alcohols have been alternatively synthesized from metals in the presence of HgQ2 as catalyst [133]. Representative and specific methods of preparation include transmetalla-tion reactions (Eq. 7-9) [134], using ammonia solutions of ytterbium and europium as synthetic reagents (Eq. 10) [135] and the generation of thiolate complexes from disulfides (Eq. 11) [136],... 

Two main components were used in the model catalysts described in this paper. One component was a europium exchanged ammonium Y zeolite (EuNH-Y). The other component was an amorphous aluminosilicate containing about 75% Si 0 and 25% Al203 (AAA-alumina). All materials were artificially V-contaminated by impregnation with vanadyl naphthenate solutions in benzene. Tetraphenyl tin (in hot toluene) was the passivating agent used. It was added either before or after loading vanadium on the zeolite (EuNH-Y), on the gel or on a gel-zeolite mixture. 

Cyclocondensation with carbonyl compounds. This diene undergoes condensation with both aldehydes and ketones in the presence of 1 cquiv. of zinc chloride or BF, etherate to afford unstable orthoester adducts, whii h are hydrolyzed to 8-lactones (equation I). The europium(lll) catalysts Eu(fod), and the chiral Eu(hfc), are also excellent catalysts,... 

Use of the ehiral europium(III) derivative, tris(3-(heptafluoropropyIhydroxymethylene)-d-camphorato]europium(IlI) [EuChfcij], as the catalyst results in moderate asymmetric induction (18-36% ee). ... 

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